阅读说明：本技术 超高强度耐候钢及其高摩擦轧制 (Ultrahigh-strength weather-resistant steel and high-friction rolling thereof ) 是由 T.王 K.米什拉 于 2019-09-20 设计创作，主要内容包括：本文中公开了包括0.5％和1.5％之间的镍的轻型超高强度耐候钢板。本文中还公开了不含原奥氏体晶界并且具有抹平图案的高摩擦轧制的碳合金钢带。本文中还进一步公开了如下的高摩擦轧制的碳合金钢带：其已经被表面均匀化以提供不含抹平图案的薄铸钢带。(Disclosed herein are lightweight ultra-high strength weathering steel sheets including between 0.5% and 1.5% nickel. Also disclosed herein is a high friction rolled carbon alloy steel strip free of prior austenite grain boundaries and having a smoothed pattern. Further disclosed herein are high friction rolled carbon alloy steel strips as follows: which has been surface homogenized to provide a thin cast steel strip free of a flattened pattern.)

1. A lightweight ultra-high strength weathering steel sheet comprising:

a carbon alloy steel strip cast at a casting thickness of less than or equal to 2.5mm having a composition comprising:

(i) between 0.20% and 0.35% carbon, less than 1.0% chromium, between 0.7% and 2.0% manganese, between 0.10% and 0.50% silicon, between 0.1% and 1.0% copper, less than or equal to 0.12% niobium, less than 0.5% molybdenum, between 0.5% and 1.5% nickel, and sedated with silicon containing less than 0.01% aluminum, and

(ii) the balance being iron and impurities resulting from the smelting;

wherein in said composition the inclusion of nickel shifts the peritectic point away from the carbon region and/or raises the transition temperature of the peritectic point to form a carbon alloy steel strip having a microstructure with at least 75% by volume martensite or martensite plus bainite, a yield strength between 700 and 1600MPa, a tensile strength between 1000 and 2100MPa, and an elongation between 1% and 10%.

2. A light weight ultra-high strength weathering steel plate of claim 1, further comprising a corrosion index of 6.0 or greater.

3. The manufacturing method of the light ultrahigh-strength weather-resistant steel plate comprises the following steps:

(a) preparing a molten steel melt comprising:

(i) between 0.20% and 0.35% carbon, less than 1.0% chromium, between 0.7% and 2.0% manganese, between 0.10% and 0.50% silicon, between 0.1% and 1.0% copper, less than or equal to 0.12% niobium, less than 0.5% molybdenum, between 0.5% and 1.5% nickel, silicon killed with less than 0.01% aluminum, and

(ii) the balance being iron and impurities resulting from the smelting;

(b) forming the melt into a casting pool supported on the casting surfaces of a pair of cooled casting rolls having a nip therebetween;

(c) counter-rotating the casting rolls and melting the melt at greater than 10.0MW/m²Is solidified into a steel sheet having a thickness of less than 2.5mm delivered downwardly from the nip and the sheet is cooled in a non-oxidizing atmosphere to below 1100 ℃ and above the Ar3 temperature at a cooling rate of greater than 15 ℃/s; and

(d) rapidly cooling to form a steel sheet having a microstructure with at least 75% by volume martensite or martensite plus bainite, a yield strength between 700 and 1600MPa, a tensile strength between 1000 and 2100MPa, and an elongation between 1% and 10%, wherein the inclusion of nickel shifts the peritectic point away from the carbon region and/or raises the transition temperature of the peritectic point to inhibit defect formation in the high strength martensitic steel sheet.

4. The method of claim 3, further comprising the steps of:

(e) the steel sheet is hot rolled to a hot rolled thickness at a reduction of between 15% and 50% of the cast thickness before rapid cooling.

5. The method of claim 3, further comprising the steps of:

(e) the steel sheet is hot rolled to a hot rolled thickness at a reduction of between 15% and 35% of the cast thickness before rapid cooling.

6. An ultra-high strength, weather resistant, thin cast steel strip comprising:

an as-cast thickness of less than or equal to 2.5mm, having a composition comprising:

between 0.20% and 0.40% carbon, less than 1.0% chromium, between 0.7% and 2.0% manganese, between 0.10% and 0.50% silicon, between 0.1% and 1.0% copper, less than or equal to 0.12% niobium, less than 0.5% molybdenum, between 0.5% and 1.5% nickel, and sedated with silicon containing less than 0.01% aluminum, and the balance iron and impurities resulting from melting, by weight;

a pair of opposing surfaces free of prior austenite grain boundary pits and having a smoothed pattern; and

a microstructure having at least 75% martensite by volume, a yield strength between 700 and 1600MPa, a tensile strength between 1000 and 2100MPa, and an elongation between 1% and 10%.

7. The ultra-high strength, weather resistant, thin cast steel strip as claimed in claim 6 further comprising a corrosion index of 6.0 or greater.

8. The ultra-high strength, weather resistant, thin cast steel strip as claimed in claim 6 wherein the pair of opposed surfaces are substantially free of prior austenite grain boundary pits.

9. The ultra-high strength, weather resistant, thin cast steel strip as claimed in claim 6 wherein the pair of opposed surfaces is substantially free of prior austenite grain boundary pits.

10. The ultra-high strength, weather resistant, thin cast steel strip as claimed in claim 6 wherein the martensite in the steel sheet is derived from austenite having a grain size greater than 100 μm.

11. The ultra-high strength, weather resistant, thin cast steel strip as claimed in claim 6 wherein the martensite in the steel sheet is derived from austenite having a grain size greater than 150 μm.

12. The ultra-high strength, weather resistant, thin cast steel strip as claimed in claim 6 further comprising a hot rolled thickness at a reduction between 15% and 50% of the as-cast thickness.

13. The ultra-high strength, weather resistant, thin cast steel strip as claimed in claim 6 wherein the steel strip passes the 3T 180 degree bend test.

14. The manufacturing method of the ultra-high strength weather-resistant steel strip comprises the following steps:

(a) preparing a molten steel melt comprising:

between 0.20% and 0.40% carbon, less than 1.0% chromium, between 0.7% and 2.0% manganese, between 0.10% and 0.50% silicon, between 0.1% and 1.0% copper, less than or equal to 0.12% niobium, less than 0.5% molybdenum, between 0.5% and 1.5% nickel, and sedated with silicon containing less than 0.01% aluminum, and the balance iron and impurities resulting from melting, by weight;

(b) forming the melt into a casting pool supported on the casting surfaces of a pair of cooled casting rolls having a nip therebetween;

(c) bringing the casting rolls to greater than 10.0MW/m²Counter-rotating and solidifying the molten melt into a steel strip having a thickness of less than or equal to 2.5mm delivered downwardly from the nip, wherein there is thermal etching in the hot box, and cooling the strip to below 1080 ℃ and Ar in a non-oxidizing atmosphere at a cooling rate of greater than 15 ℃/s₃Above the temperature;

(d) the thin cast steel strip is high friction rolled to a hot rolled thickness at a reduction between 15% and 50% of the as-cast thickness resulting in a hot rolled steel strip free of prior austenite grain boundary pits and having a trowelled pattern.

15. The method of claim 14, further comprising the steps of:

(e) rapidly cooling the thin cast steel strip to between 100 and 200 ℃ at a rate of more than 100 ℃/s after the high friction rolling step, wherein the high friction hot rolled steel strip comprises a microstructure having at least 75% martensite or at least 75% martensite plus bainite by volume, a yield strength between 700 and 1600MPa, a tensile strength between 1000 and 2100MPa, and an elongation between 1% and 10%.

16. The method of claim 14 wherein the thin cast steel strip comprises a corrosion index of 6.0 or greater.

17. The method of claim 14, wherein the step of high friction rolling is performed with a coefficient of friction equal to or greater than 0.20.

18. The method of claim 14, wherein the step of high friction rolling occurs after hot etching in a hot box to remove prior austenite grain boundary pits formed by the hot etching.

19. The method of claim 14, wherein the step of high friction rolling is performed with or without lubrication with a coefficient of friction equal to or greater than 0.20.

20. The method of claim 14, wherein the step of high friction rolling further comprises a work roll force between 600 and 2500 tons.

21. The method of claim 14 wherein the thin cast steel strip passes the 3T 180 degree bend test.

22. The method of claim 14 wherein the thin cast steel strip exhibits no defects after 120 hours of corrosion testing.

23. An ultra-high strength, weather resistant, thin cast steel strip comprising:

an as-cast thickness of less than or equal to 2.5mm, having a composition comprising:

between 0.20% and 0.40% carbon, less than 1.0% chromium, between 0.7% and 2.0% manganese, between 0.10% and 0.50% silicon, between 0.1% and 1.0% copper, less than or equal to 0.12% niobium, less than 0.5% molybdenum, between 0.5% and 1.5% nickel, and sedated with silicon containing less than 0.01% aluminum, and the balance iron and impurities resulting from melting, by weight;

a pair of opposing surfaces that have been high friction hot rolled to form a troweling pattern and that have been further surface homogenized to remove the troweling pattern; and

a microstructure having at least 75% martensite or at least 75% martensite plus bainite by volume, a yield strength between 700 and 1600MPa, a tensile strength between 1000 and 2100MPa, and an elongation between 1% and 10%.

24. The ultra-high strength, weather resistant, thin cast steel strip as claimed in claim 23 comprising a microstructure having at least 90% martensite or at least 90% martensite plus bainite by volume.

25. The ultra-high strength, weather resistant, thin cast steel strip as claimed in claim 23 comprising a microstructure having at least 95% martensite or at least 95% martensite plus bainite by volume.

26. The ultra-high strength, weather resistant, thin cast steel strip as claimed in claim 23 wherein the pair of opposed surfaces are free of prior austenite grain boundary pits.

27. The ultra-high strength, weather resistant, thin cast steel strip as claimed in claim 23 wherein the pair of opposed surfaces are substantially free of prior austenite grain boundary pits.

28. The ultra-high strength, weather resistant, thin cast steel strip as claimed in claim 23 wherein the pair of opposed surfaces is substantially free of prior austenite grain boundary pits.

29. The ultra-high strength, weather resistant, thin cast steel strip as claimed in claim 23 wherein the martensite in the steel strip is derived from austenite having a grain size greater than 100 μm.

30. The ultra-high strength, weather resistant, thin cast steel strip as claimed in claim 23 wherein the martensite in the steel strip is derived from austenite having a grain size greater than 150 μm.

31. The ultra-high strength, weather resistant, thin cast steel strip as claimed in claim 23 further comprising a hot rolled thickness at a reduction between 15% and 50% of the as-cast thickness.

32. The manufacturing method of the ultra-high strength weather-resistant steel strip comprises the following steps:

(a) preparing a molten steel melt;

(b) forming the melt into a casting pool supported on the casting surfaces of a pair of cooled casting rolls having a nip therebetween;

(c) bringing the casting rolls to greater than 10.0MW/m²And solidifying the molten melt into a steel strip having a thickness of less than or equal to 2.5mm delivered downwardly from the nip, and cooling the strip to below 1080 ℃ and Ar in a non-oxidizing atmosphere at a cooling rate of greater than 15 ℃/s₃Above the temperature;

(d) high friction rolling the thin cast steel strip to a hot rolled thickness at a reduction between 15% and 50% of the as-cast thickness to produce a hot rolled steel strip having a surface free of prior austenite grain boundary pits and having a flattened pattern; and

(e) the high friction hot rolled steel strip is surface homogenized to eliminate a floating pattern on the surface.

33. The method of claim 32, wherein the step of high friction rolling occurs after hot etching occurs in a hot box to remove prior austenite grain boundary pits formed by the hot etching.

34. The method of claim 32, further comprising the steps of:

(f) rapidly cooling the thin cast steel strip to between 100 and 200 ℃ at a rate greater than 100 ℃/s after the step of high friction rolling, wherein the high friction hot rolled steel strip comprises a microstructure having at least 75% martensite or at least 75% martensite plus bainite by volume, a yield strength between 700 and 1600MPa, a tensile strength between 1000 and 2100MPa, and an elongation between 1% and 10%.

35. The method of claim 32, wherein the molten steel melt comprises, by weight, between 0.20% and 0.40% carbon, less than 1.0% chromium, between 0.7% and 2.0% manganese, between 0.10% and 0.50% silicon, between 0.1% and 1.0% copper, less than or equal to 0.12% niobium, less than 0.5% molybdenum, between 0.5% and 1.5% nickel, and is killed by silicon containing less than 0.01% aluminum, and the balance being iron and impurities resulting from melting.

36. The method of claim 32, wherein the step of high friction rolling is performed with a coefficient of friction equal to or greater than 0.20.

37. The method of claim 32, wherein the step of high friction rolling is performed with a coefficient of friction equal to or greater than 0.20 using lubrication.

38. The method of claim 32, wherein the step of high friction rolling is performed without the use of lubrication with a coefficient of friction equal to or greater than 0.20.

39. The method of claim 32, further comprising adjusting the percent reduction to achieve a coefficient of friction equal to or greater than 0.20.

40. The method of claim 32, wherein the step of high friction rolling further comprises a work roll force between 600 and 2500 tons.

Technical Field

The present invention relates to a thin cast steel strip, a method for high friction rolling of a thin cast steel strip, and a steel product made from the thin cast steel strip and by the method.

Background

In a twin roll caster, molten metal is introduced between a pair of counter-rotating internally cooled casting rolls such that metal shells solidify on the moving roll surfaces and are brought together at the nip therebetween to produce a solidified strip product, which is delivered downwardly from the nip between the casting rolls. The term "nip" is used herein to refer to the general area: at this region, the casting rolls are closest together. Pouring molten metal from a ladle through a metal delivery system comprising a tundish and a core nozzle located above the nip to form a molten metal casting pool supported on the casting surfaces of the rolls above the nip and extending along the nip length. The casting pool is typically confined between refractory side plates or dams held in sliding engagement with the end surfaces of the rolls to dam the two ends of the casting pool against outflow.

To achieve the desired thickness, the thin steel strip may be passed through a rolling mill to hot roll the thin steel strip. When hot rolling, the thin steel strip is typically lubricated to reduce roll gap friction, which in turn reduces rolling load and roll wear, as well as providing a smoother surface finish. Lubrication is used to provide low friction conditions. The low friction condition is defined as a friction condition in which the friction coefficient (μ) of the roll gap is less than 0.20. After hot rolling, the thin steel strip undergoes a cooling process. Under low friction conditions, large prior austenite grain boundary pits have been observed on the hot rolled outer surface of the cooled thin steel strip after undergoing a pickling or acid etching process to remove the scale. Specifically, although thin steel strip tested using the dye penetrant technique appears defect-free, prior austenite grain boundaries are acid etched to form prior austenite grain boundary pits after acid pickling of the same thin steel strip. The etching may further cause defect phenomena to occur along the etched prior austenite grain boundaries and resulting pits. The resulting defects and spaces (which are more commonly referred to as spaces) can extend to a depth of at least 5 microns, and in some cases, to a depth of 5-10 microns.

Also suitable for use in the present disclosure is that the weathering steel is typically a high strength low alloy steel that is resistant to atmospheric corrosion. In the presence of moisture and air, low alloy steels oxidize at a rate that depends on the level of exposure to oxygen, moisture, and atmospheric contaminants for the metal surface. When steel oxidizes, it can form an oxide layer often referred to as rust. As the oxidation process proceeds, the oxide layer forms a barrier to the ingress of oxygen, moisture and contaminants, and the rate of rusting slows. In the case of weathering steels, the oxidation process is initiated in the same way, but the specific alloying elements in the steel produce a stable protective oxide layer that adheres to the base metal and is much less porous than oxide layers typically formed in non-weathering steels. The result is a much lower corrosion rate than would be found on ordinary non-weatherable structural steel.

Weathering steel is defined in ASTM A606High strength, low alloy, hot and cold rolled steel sheet with improved atmospheric corrosion resistance Standard Specification for Steel, plate and strip (Sheet) and Strip,High Strength,Low-Alloy,HotRolled and Cold Rolled with Improved Atmospheric Corrosion Resistance)。Weathering steels are supplied in two types: type 2, which contains at least 0.20% copper (minimum 0.18% Cu for product inspection) based on casting or melting analysis (heat analysis); and type 4, which contains additional alloying elements to provide a composition as provided by ASTM G101Standard for evaluating atmospheric corrosion resistance of low alloy steel South (Standard Guide for Estimating the Atmospheric resonance Resistance of Low- Alloy Steels)A calculated corrosion index of at least 6.0 and provides a significantly better level of corrosion resistance than that of carbon steel with or without copper additions.

Prior to the present invention, weathering steels were typically limited to yield strengths less than 700MPa and tensile strengths less than 1000 MPa. Also, prior to the present invention, the strength properties of weathering steels were typically achieved by age hardening. U.S. patent No.10,174,398 (incorporated herein by reference) is an example of weathering steel achieved by age hardening.

Disclosure of Invention

In one set of examples, the present disclosure sets forth providing a lightweight ultra-high strength weathering steel formed by shifting the peritectic point away from the carbon region and/or increasing the transition temperature of the peritectic point of the composition. Specifically, shifting the peritectic point away from the carbon region and/or increasing the transition temperature of the peritectic point of the composition appears to suppress defects and result in a high strength martensitic steel sheet that is defect free. In the present example, the addition of nickel is relied upon for this purpose, where the addition of nickel must be sufficient to shift the 'peritectic point' away from the carbon region that would otherwise be present in the same composition without the addition of nickel. Also disclosed are the following products made from the ultra-high strength weathering steel: it is of various shapes (as otherwise disclosed herein) and has improved strength properties not previously obtainable.

In another set of examples, the present disclosure addresses the elimination of prior austenite grain boundary pits, but maintains a smeared (smear) pattern. In this set of examples, the thin cast steel strip is subjected to high friction rolling conditions in which grain boundary pits form a floating pattern at least at the surface of the thin cast steel strip. In particular, the present example addresses the formation of a flattened pattern of prior austenite grain boundary pits when the prior austenite grain boundary pits are eliminated from the surface and the formability of the steel strip or steel product is improved. By improving the formability of the steel strip, previously unavailable products with various shapes (as otherwise disclosed herein) and with improved strength properties become available. This example is applicable not only to the aforementioned ultra-high strength weathering steels, but may additionally be applicable to martensitic steels, other weathering steels, and/or steel strips or products that exhibit prior austenite grain boundary pits.

In addition, however, in another set of examples, the present disclosure addresses the elimination of grain boundary pits and the smearing patterns formed therefrom. In this set of examples, the thin cast steel strip undergoes surface homogenization to eliminate the floating pattern. As a result, the thin cast steel strip has a surface that is free of not only prior austenite grain boundary pits, but also, in addition, a floating pattern produced as a result of the high friction rolling conditions to provide a thin cast steel strip surface having a surface roughness (Ra) of not more than 2.5 μm in some examples. This example is applicable not only to the aforementioned ultra-high strength weathering steels, but may additionally be applicable to martensitic steels, other weathering steels, and/or steel strips or products that exhibit prior austenite grain boundary pits.

Super-strength weather-resistant steel

First, a lightweight ultra-high strength weathering steel plate manufactured by the steps comprising: (a) preparing a molten steel melt comprising: (i) between 0.20% and 0.35% carbon, less than 1.0% chromium, between 0.7% and 2.0% manganese, between 0.10% and 0.50% silicon, between 0.1% and 1.0% copper, less than or equal to 0.12% niobium, less than 0.5% molybdenum, between 0.5% and 1.5% nickel, and sedated with silicon containing less than 0.01% aluminum, by weight, and (ii) the balance iron and impurities resulting from smelting; (b) at a molecular weight of more than 10.0MW/m²Is solidified into a steel sheet having a thickness of 2.5mm or less and the sheet is subjected to rapid cooling and/or hot rolling before hot rolling when hot rollingCooling to below 1080 ℃ and Ar in non-oxidizing atmosphere at a cooling rate of more than 15 ℃/s₃Above the temperature; and (c) rapidly cooling to form a steel sheet having a microstructure with at least 75% martensite by volume, a yield strength between 700 and 1600MPa, a tensile strength between 1000 and 2100MPa, and an elongation between 1% and 10%.

Here and elsewhere in this disclosure, elongation means total elongation. By "rapid cooling" is meant cooling at a rate greater than 100 deg.c/s to between 100 and 200 deg.c. Rapid cooling of the nickel-added composition of the invention achieves steel strip up to more than 95% martensite phase. In one example, the rapid cooling forms a steel sheet having a microstructure with at least 95% martensite by volume. The addition of nickel must be sufficient to shift the 'peritectic point' away from the carbon region that would otherwise be present in the same composition without nickel added. Specifically, it is believed that the inclusion of nickel in the composition helps to shift the peritectic point away from the carbon region and/or increase the transition temperature of the peritectic point of the composition, which appears to suppress defects and results in a high strength martensitic steel sheet that is free of defects. In one example, the lightweight ultra-high strength weathering steel sheet may also be hot rolled to between 15% and 50% reduction before rapid cooling.

The carbon level in the steel sheet of the present invention is preferably not 0.20% or less to suppress peritectic cracking of the steel sheet. The addition of nickel is provided to further inhibit peritectic cracking of the steel sheet, but does so independent of relying on carbon composition alone. The effect of nickel on corrosion index is reflected in the following equation for determining the result of the corrosion index calculation: cu 26.01+ Ni 3.88+ Cr 1.2+ Si 1.49+ P17.28-Cu Ni 7.29-Ni P9.1-Cu 33.39 (wherein the elements are in weight percent).

The molten melt may be brought to a molecular weight of greater than 10.0MW/m²Is solidified into a steel sheet having a thickness of less than 2.5mm, and the sheet may be cooled to 1080 ℃ or below and Ar in a non-oxidizing atmosphere at a cooling rate of more than 15 ℃/s before rapid cooling and/or before hot rolling when hot rolling₃Above the temperature. The non-oxidizing atmosphere is an atmosphere of typically an inert gas such as nitrogen or argon or mixtures thereof, containing less than about by weight5% oxygen. In another example, the sheet may be cooled to below 1100 ℃ and Ar in a non-oxidizing atmosphere at a cooling rate of greater than 15 ℃/s prior to rapid cooling and/or prior to hot rolling when hot rolled₃Above the temperature.

In some examples, the martensite in the steel sheet may be formed from austenite having a grain size greater than 100 μm. In other examples, the martensite in the steel sheet may be formed of austenite having a grain size greater than 150 μm.

The steel sheet is rapidly cooled to form a steel sheet having a microstructure with at least 75% martensite, a yield strength between 700 and 1600MPa, a tensile strength between 1000 and 2100MPa, and an elongation between 1% and 10%. In other examples, the steel sheet is rapidly cooled to form a steel sheet having a microstructure with at least 75% martensite plus bainite. In a particular example, the rapid cooling forms a steel sheet having a microstructure with at least 95% martensite plus bainite by volume.

In some examples, the steel sheet may be hot rolled to between 15% and 35% reduction before rapid cooling. In other examples, the steel sheet may be hot rolled to between 15% and 50% reduction before rapid cooling.

The molten steel used to make the ultra-high strength weathering steel plate is silicon killed (i.e., silicon deoxidized) comprising between 0.10% and 0.50% by weight silicon. The steel sheet may further include less than 0.008% aluminum or less than 0.006% aluminum by weight. The molten melt may have a free oxygen content of between 5 and 70ppm or between 5 and 60 ppm. The steel sheet may have a total oxygen content of greater than 50 ppm. The inclusions comprise MnOSiO typically with 50% of them being less than 5 μm in size₂And has the potential to enhance the microstructure evolution (evolution) and hence the mechanical properties of the tape.

Also discloses a manufacturing method of the light ultrahigh-strength weather-resistant steel plate, which comprises the following steps: (a) preparing a molten steel melt comprising: (i) between 0.20% and 0.35% carbon, less than 1.0% chromium, between 0.7% and 2.0% manganese, between 0.10% and 0.50% silicon, between 0.1% and 1.0% copper, less than 0.12% niobium, less than 0.5% molybdenum, between 0.5% and 1.5% by weightAnd is silicon killed containing less than 0.01% aluminum, and (ii) the balance is iron and impurities resulting from smelting; (b) forming the molten melt into a casting pool supported on the casting surfaces of a pair of cooled casting rolls having a nip therebetween; (c) counter-rotating the casting rolls and at greater than 10.0MW/m²Is solidified, thereby producing a steel sheet having a thickness of less than 2.5mm, and the sheet is cooled to below 1080 ℃ and Ar before rapid cooling and/or when hot rolled in a non-oxidizing atmosphere before hot rolling at a cooling rate of more than 15 ℃/s₃Above temperature, and (d) rapidly cooling to form a steel sheet having a microstructure with at least 75% martensite, a yield strength between 700 and 1600MPa, a tensile strength between 1000 and 2100MPa, and an elongation between 1% and 10%. In a particular example, the rapid cooling forms a steel sheet having a microstructure with at least 95% martensite plus bainite by volume. The sheet may be cooled to below 1100 ℃ and Ar before rapid cooling and/or when hot rolled in a non-oxidizing atmosphere prior to hot rolling at a cooling rate of greater than 15 ℃/s₃Above the temperature. The steel sheet composition cannot be made to have a carbon level below 0.20% because it does not contribute to peritectic cracking of the steel sheet. In one example, a lightweight ultra-high strength weathering steel sheet may be hot rolled to between 15% and 50% reduction before rapid cooling.

Further, the method for manufacturing the lightweight ultra-high strength weathering steel plate may include the step of tempering the steel plate at a temperature between 150 ℃ and 250 ℃ for 2-6 hours.

The molten melt may have a free oxygen content of between 5 and 70ppm or between 5 and 60 ppm. The steel sheet may have a total oxygen content of greater than 50 ppm. The molten melt may be brought to a molecular weight of greater than 10.0MW/m²Is solidified into a steel sheet having a thickness of less than 2.5mm, and is cooled to 1080 ℃ or less and Ar is reduced in a non-oxidizing atmosphere at a cooling rate of more than 15 ℃/s before rapid cooling and/or before hot rolling in a non-oxidizing atmosphere when hot rolling₃Above the temperature. In another example, the sheet may be cooled to below 1100 ℃ and Ar in a non-oxidizing atmosphere at a cooling rate of greater than 15 ℃/s prior to rapid cooling and/or prior to hot rolling when hot rolled₃Above the temperature.

In some embodiments, the martensite in the steel sheet may be derived from austenite having a grain size greater than 100 μm. In other embodiments, the martensite in the steel sheet may be derived from austenite having a grain size greater than 150 μm.

The method of manufacturing a light-weight ultra-high strength weathering steel sheet may further include hot rolling the steel sheet to a reduction ratio of between 15% and 35%, and then rapidly cooling to form a steel sheet having a microstructure with at least 75% by volume martensite, a yield strength of between 700 and 1600MPa, a tensile strength of between 1000 and 2100MPa, and an elongation of between 1% and 10%. In some embodiments, the method of manufacturing a light-weight ultra-high strength steel sheet may further comprise hot rolling the steel sheet to a reduction between 15% and 50%, and then rapidly cooling to form a steel sheet having a microstructure with at least 75% by volume martensite plus bainite, a yield strength between 700 and 1600MPa, a tensile strength between 1000 and 2100MPa, and an elongation between 1% and 10%. Further, a method of manufacturing a hot rolled light ultra high strength steel sheet may include hot rolling the steel sheet to a reduction between 15% and 35% and then rapidly cooling to form a steel sheet having a microstructure with at least 75% by volume martensite plus bainite, a yield strength between 700 and 1600MPa, a tensile strength between 1000 and 2100MPa, and an elongation between 1% and 10%. In the above specific example, the hot rolled steel sheet and then rapidly cooled forms a steel sheet having a microstructure having at least 95% martensite plus bainite by volume.

Also disclosed is a steel pile comprising one or more flanges (flanges) and a web (web) formed from a carbon alloy steel sheet having the composition: which comprises, by weight, between 0.20% and 0.35% carbon, less than 1.0% chromium, between 0.7% and 2.0% manganese, between 0.10% and 0.50% silicon, between 0.1% and 1.0% copper, less than or equal to 0.12% niobium, less than 0.5% molybdenum, between 0.5% and 1.5% nickel, and is silicon killed containing less than 0.01% aluminum, wherein the carbon alloy steel sheet has a microstructure with at least 75% by volume martensite or martensite plus bainite, a yield strength between 700 and 1600MPa, a tensile strength between 1000 and 2100MPa, an elongation between 1% and 10%, and has a corrosion index of 6.0 or more.

High-friction rolled high-strength weathering steel

Second, in one set of examples, thin cast carbon alloy steel strip having an as cast thickness of less than or equal to 2.5mm is presently disclosed. These examples are applicable not only to the aforementioned ultra-high strength weathering steels, but may additionally be applicable to martensitic steels, other weathering steels, and/or steel strips or products that exhibit prior austenite grain boundary pits. The carbon alloy thin cast steel strip may include between 0.20% and 0.40% carbon, less than 1.0% chromium, between 0.7% and 2.0% manganese, between 0.10% and 0.50% silicon, between 0.1% and 1.0% copper, less than or equal to 0.12% niobium, less than 0.5% molybdenum, between 0.5% and 1.5% nickel, and is silicon killed with less than 0.01% aluminum, and the balance iron and impurities resulting from smelting, by weight. After high friction hot rolling, the thickness of the carbon alloy thin cast steel strip is reduced by 15-50% of the as-cast thickness. The hot rolled steel strip includes a pair of opposed high friction hot rolled surfaces that are substantially free, or free of prior austenite grain boundary pits and have a troweled pattern. In some embodiments, the steel strip comprises a microstructure having at least 75% martensite or at least 75% martensite plus bainite by volume, a yield strength between 700 and 1600MPa, a tensile strength between 1000 and 2100MPa, and an elongation between 1% and 10%. In some examples, the steel strip is a weathering steel having a corrosion index of 6.0 or greater.

In some examples, the pair of opposing high friction hot rolled surfaces are substantially free of prior austenite grain boundary pits. In some examples, the pair of opposing high friction hot rolled surfaces are substantially free of prior austenite grain boundary pits.

Also disclosed is a method of making a hot rolled carbon alloy steel strip comprising by weight between 0.20% and 0.40% carbon, less than 1.0% chromium, between 0.7% and 2.0% manganese, between 0.10% and 0.50% silicon, between 0.1% and 1.0% copper, less than or equal to 0.12% niobium, less than 0.5% molybdenum, between 0.5% and 1.5% nickel, and killed by silicon containing less than 0.01% aluminum, with the balance being iron and impurities resulting from smelting, the method comprising the steps of:

(a) preparing a molten steel melt;

(b) forming the melt into a casting pool supported on the casting surfaces of a pair of cooled casting rolls having a nip therebetween;

(c) counter-rotating the casting rolls and melting the melt at greater than 10.0MW/m²Is solidified into a steel strip having a thickness of less than or equal to 2.5mm delivered downwards from the nip, and the strip is cooled to below 1080 ℃ and Ar in a non-oxidizing atmosphere at a cooling rate of greater than 15 ℃/s₃Above the temperature;

(d) high friction hot rolling a thin cast steel strip to a hot rolled thickness at a reduction between 15% and 50% of the as-cast thickness produces a hot rolled steel strip that is substantially free, or free of prior austenite grain boundary pits and has a screeded pattern.

The high friction hot rolled thin cast steel strip having a smear pattern that is substantially free, or free of prior austenite grain boundary pits may be a weathering steel having a corrosion index of 6.0 or greater. Moreover, the high friction hot rolled steel strip may comprise a microstructure having at least 75% martensite or at least 75% martensite plus bainite by volume, a yield strength between 700 and 1600MPa, a tensile strength between 1000 and 2100MPa, and an elongation between 1% and 10%.

High friction rolled high strength martensitic steel

Third, in yet another set of examples, a thin cast carbon alloy steel strip is presently disclosed that includes a pair of opposed high friction hot rolled surfaces that have been surface homogenized while having been high friction rolled. These inventive examples are applicable not only to the ultra-high strength weathering steels previously described, but may additionally be applicable to martensitic steels, other weathering steels, and/or steel strips or products that exhibit prior austenite grain boundary pits. The pair of opposing high friction hot rolled surfaces, when surface homogenized, are free of the previously formed flattened grain boundary pits as a result of the high friction rolling process. In some embodiments, the carbon alloy thin cast steel strip may further include a microstructure having at least 75% martensite or at least 75% martensite plus bainite by volume and a yield strength between 700 and 1600MPa, a tensile strength between 1000 and 2100MPa, and an elongation between 1% and 10%. In some embodiments, the steel strip includes a microstructure having at least 90% martensite or at least 90% martensite plus bainite by volume. In some embodiments, the steel strip of claim 1 comprises a microstructure having at least 95% martensite or at least 95% martensite plus bainite by volume.

Exemplary homogenized steel strip within the scope of the present disclosure may include, by weight, between 0.20% and 0.40% carbon, less than 1.0% chromium, between 0.7% and 2.0% manganese, between 0.10% and 0.50% silicon, between 0.1% and 1.0% copper, less than or equal to 0.12% niobium, less than 0.5% molybdenum, between 0.5% and 1.5% nickel, and silicon killed with less than 0.01% aluminum, and the balance iron and impurities resulting from melting.

A method of making a hot rolled carbon alloy steel strip is also disclosed. The method may comprise the steps of:

(a) preparing a molten steel melt;

(b) forming the melt into a casting pool supported on the casting surfaces of a pair of cooled casting rolls having a nip therebetween;

(c) counter-rotating the casting rolls and melting the melt at greater than 10.0MW/m²Is solidified into a steel strip having a thickness of less than or equal to 2.5mm delivered downwards from the nip, and the strip is cooled to below 1080 ℃ and Ar in a non-oxidizing atmosphere at a cooling rate of greater than 15 ℃/s₃Above the temperature;

(d) high friction rolling the thin cast steel strip to a hot rolled thickness at a reduction between 15% and 50% of the as-cast thickness to produce a hot rolled steel strip free of prior austenite grain boundary pits and having a trowelled pattern; and

(e) the high friction hot rolled steel strip is surface homogenized to eliminate the floating pattern.

The high friction hot rolled homogenized thin cast steel strip may comprise a microstructure having at least 75% martensite or at least 75% martensite plus bainite by volume, a yield strength between 700 and 1600MPa, a tensile strength between 1000 and 2100MPa, and an elongation between 1% and 10%, thereby providing a high strength martensitic steel. Further, the high friction hot rolled homogenized steel strip may include between 0.20% and 0.40% carbon, less than 1.0% chromium, between 0.7% and 2.0% manganese, between 0.10% and 0.50% silicon, between 0.1% and 1.0% copper, less than or equal to 0.12% niobium, less than 0.5% molybdenum, between 0.5% and 1.5% nickel, and is silicon killed with less than 0.01% aluminum, and the balance iron and impurities resulting from smelting, by weight.

Drawings

The present invention may be more fully described and explained with reference to the accompanying drawings, in which:

FIG. 1 illustrates a strip casting plant with hot rolling mill and coiler on the lead-in line.

FIG. 2 illustrates details of a twin roll strip caster.

Fig. 3 is a photomicrograph of a steel sheet having a microstructure with at least 75% martensite.

Fig. 4 is a phase diagram illustrating the effect of nickel in shifting the peritectic point away from the carbon region.

Fig. 5 is a flow diagram of a process according to one or more aspects of the present disclosure.

Fig. 6 is an image showing the surface of a steel strip hot-rolled under high friction conditions after a surface uniformization process.

FIG. 7 is an image showing the surface of a hot rolled steel strip having a trowelled pattern subjected to high friction conditions that has not been homogenized.

Fig. 8 is a friction coefficient model chart created for measuring the friction coefficient for a specific pair of work rolls, the rolling mill specific force, and the corresponding reduction ratio.

Fig. 9 is a Continuous Cooling Transformation (CCT) diagram of steel.

Detailed Description

Lightweight ultra-high strength weathering steel panels are described herein in one example. The lightweight ultra-high strength weathering steel plate may be made from a molten melt. The molten melt may be processed through a twin roll caster. In one example of the above-described method,the lightweight ultrahigh-strength weathering steel plate may be manufactured by the steps comprising: (a) preparing a molten steel melt comprising: (i) between 0.20% and 0.35% carbon, less than 1.0% chromium, between 0.7% and 2.0% manganese, between 0.10% and 0.50% silicon, between 0.1% and 1.0% copper, less than or equal to 0.12% niobium, less than 0.5% molybdenum, between 0.5% and 1.5% nickel, and sedated with silicon containing less than 0.01% aluminum, and (ii) the balance iron and impurities resulting from smelting, by weight; (b) at a molecular weight of more than 10.0MW/m²Is solidified, thereby producing a steel sheet having a thickness of less than 2.5mm, and is cooled to 1080 ℃ or below and Ar in a non-oxidizing atmosphere at a cooling rate of more than 15 ℃/s before rapid cooling and/or before hot rolling when hot rolling₃Above the temperature; and (c) rapidly cooling to form a steel sheet having a microstructure possessing at least 75% by volume martensite or martensite plus bainite, a yield strength between 700 and 1600MPa, a tensile strength between 1000 and 2100MPa, and an elongation between 1% and 10%. In one example, the lightweight ultra-high strength weathering steel sheet may also be hot rolled to between 15% and 50% reduction before rapid cooling. The sheet may be cooled to below 1100 ℃ and Ar before rapid cooling and/or when hot rolled in a non-oxidizing atmosphere before hot rolling at a cooling rate of greater than 15 ℃/s₃Above the temperature. Ar (Ar)₃The temperature is the temperature at which austenite begins to transform to ferrite during cooling. That is, Ar₃The temperature is the austenite transformation point. In various examples, the inclusion of nickel shifts the peritectic point away from the carbon region and/or increases the transition temperature of the peritectic point of the steel sheet composition to provide a defect-free steel sheet. The effect of nickel on corrosion index is reflected in the following equation for determining the result of the corrosion index calculation: cu 26.01+ Ni 3.88+ Cr 1.2+ Si 1.49+ P17.28-Cu Ni 7.29-Ni P9.1-Cu 33.39 (wherein the elements are in weight percent).

Also described herein is a thin cast steel strip having a hot rolled exterior side surface as follows: the exterior side surface is characterized by being substantially free, or free of prior austenite grain boundary pits, but having a slick, or elongated surface structure, such as in the example of high friction rolled high strength martensitic steel. Methods or processes for making the same are also described herein. These examples are applicable not only to the aforementioned ultra-high strength weathering steels, but may additionally be applicable to martensitic steels, other weathering steels, and/or steel strips or products that exhibit prior austenite grain boundary pits.

Further described herein is a thin steel strip having a hot rolled outer side surface as follows: the exterior side surface is characterized by being substantially free, or free of prior austenite grain boundary pits and free of a slick, or elongated, surface structure, such as in the example of high friction rolled high strength weathering steel. Methods or processes for making the same are also described herein. These examples are applicable not only to the aforementioned ultra-high strength weathering steels, but may additionally be applicable to martensitic steels, other weathering steels, and/or steel strips or products having prior austenite grain boundary pits.

As used herein, predominantly free means that less than 50% of each opposing hot rolled exterior side surface contains prior austenite grain boundaries or prior austenite grain boundary pits after acid etching (pickling). By at least substantially free of all prior austenite grain boundaries or prior austenite grain boundary pits is meant that 10% or less of each opposing hot rolled exterior side surface contains prior austenite grain boundary pits or prior austenite grain boundary pits after acid etching (pickling). The pits form etched grain boundary pits after acid etching (also known as pickling) making the prior austenite grain boundaries visible at 250x magnification. In other cases, free means that each opposing hot rolled outer side surface is free (i.e., completely free) of prior austenite grain boundary pits, including being free of any prior austenite grain boundary pits after acid etching. It is emphasized that prior austenite grain boundaries may still be present within the material of the hot rolled strip, where grain boundary pits and spaces on the surface have been by the techniques described herein (e.g., where hot rolling is at a)_r3A temperature above the temperature occurs using a roll gap friction coefficient equal to or greater than 0.20).

Fig. 1 and 2 illustrate successive components of a strip casting machine for continuously casting steel strip or plate according to the invention. Twin roll caster 11 continuously produces cast steel strip 12 which is transported in a transport path 10 through a guide table 13 to a pinch roll stand 14 having pinch rolls 14A. The strip immediately after leaving the pinch roll stand 14 is passed to a hot rolling mill 16 having a pair of work rolls 16A and back rolls 16B, where the cast strip is hot rolled to a desired reduction thickness in the hot rolling mill 16. The hot rolled strip is conveyed onto a run-out table 17 where the strip enters an intensive cooling section via water jets 18 (or other suitable means). The rolled and cooled strip is then passed through a pinch roll stand 20 comprising a pair of pinch rolls 20A and then to a coiler 19.

As shown in FIG. 2, twin roll caster 11 comprises a main machine frame 21 which supports a pair of laterally positioned casting rolls 22 having casting surfaces 22A. Molten metal is supplied during a casting operation from a ladle (not shown) to a tundish 23, through a refractory brick sheath (shroud)24 to a distributor or removable tundish 25, and then from the distributor or removable tundish 25 through a metal delivery nozzle 26 to between the casting rolls 22 above a nip 27. The molten metal delivered between the casting rolls 22 forms a casting pool 30 supported on the casting rolls above the nip. The casting pool 30 is bounded at the ends of the casting rolls by a pair of side dams or plates 28, which side dams or plates 28 may be urged against the ends of the casting rolls by a pair of pushers (not shown) comprising hydraulic cylinder units (not shown) connected to side plate holders. The upper surface of the casting pool 30 (commonly referred to as the "meniscus" type level) is generally above the lower end of the delivery nozzle so that the lower end of the delivery nozzle is submerged within the casting pool 30. The casting rolls 22 are internally water cooled so that the shells solidify on the moving casting roll surfaces as they pass through the casting pool and are brought together between them at the nip 27 to produce the cast strip 12 which is delivered downwardly from the nip between the casting rolls.

The twin roll caster may be of the type illustrated and described in considerable detail in U.S. Pat. Nos. 5,184,668, 5,277,243, 5,488,988 and/or U.S. patent application No.12/050,987 published as U.S. publication No. 2009/0236068A 1. For appropriate constructional details of twin roll casters that may be used in the present examples, reference is made to these patents and publications, which are incorporated by reference.

After the thin steel strip is formed (cast) using any desired process, such as the strip casting process described above in connection with fig. 1 and 2, the strip may be hot rolled and cooled to form the desired thin steel strip having opposing hot rolled exterior side surfaces that are at least predominantly free, substantially free, or free of prior austenite grain boundary pits. As illustrated in fig. 1, the in-line hot rolling mill 16 provides 15% to 50% reduction of the strip from the caster. On the run-out table 17, the cooling may include water cooling sections for controlling the cooling rate of the austenitic transformation to achieve the desired microstructure and material properties.

Fig. 3 shows a micrograph of a steel sheet having a microstructure with at least 75% martensite from prior austenite having a grain size of at least 100 μm. In some examples, the steel sheet is rapidly cooled to form a steel sheet having a microstructure with at least 90% by volume martensite or martensite and bainite. In another example, the steel sheet is rapidly cooled to form a steel sheet having a microstructure with at least 95% by volume martensite or martensite and bainite. In each of these examples, the steel sheet may additionally be hot rolled to between 15% and 50% reduction before rapid cooling.

Referring back to fig. 1, the hot box 15 is illustrated. After the strip has been formed, it may be transferred to an environmentally controlled box, referred to as hot box 15, where it continues to be passively cooled before being hot rolled to its final gauge by hot rolling mill 16, as shown in FIG. 1. An environmentally controlled box with a protective atmosphere is maintained until entry into the hot rolling mill 16. Within the hot box, the strip is moved on a guide table 13 to a pinch roll stand 14. In examples of the present disclosure, undesirable thermal etching may occur in the hot box 15. Based on whether the thermal etching has taken place in the hot box or not, the strip may be hot rolled under high friction rolling conditions based on parameters defined in more detail below.

In certain instances, the method of forming a thin steel strip further comprises hot rolling the thin steel strip using a pair of counter-rotating work rolls that produce an increased coefficient of friction (μ) sufficient to produce the following counter-hot rolled exterior side surfaces of the thin steel strip: the exterior side surface is characterized by being substantially free, or free of prior austenite grain boundary pits, and by having a shape that is formable by plastic deformation under shearThe surface smoothing pattern of (a) is associated with the elongated surface structures. In some cases, the pair of opposing work rolls is at A_r3A temperature above the temperature produces a coefficient of friction (μ) equal to or greater than 0.20, 0.25, 0.268, or 0.27, each with or without lubrication. It is recognized that the coefficient of friction may be increased by increasing the surface roughness of the work roll surface, eliminating the use of any lubrication, reducing the amount of lubrication used, and/or selecting the particular type of lubrication to be used. Other mechanisms for increasing the coefficient of friction, as may be known to one of ordinary skill, may be used in addition to or separately from the previously described mechanisms. The above process is generally referred to herein as high friction rolling.

As mentioned above, it is recognized that high friction rolling may be achieved by increasing the surface roughness of the surface of one or more work rolls. This is generally referred to herein as work roll surface texturing. The work roll surface texturing can be varied and measured by various parameters used in high friction rolling applications. For example, the average roughness (Ra) of the work roll profile may provide a reference point for generating the coefficient of friction necessary for the roll gap as noted above in the examples. To achieve high friction rolling by way of work roll surface texturing, in one example, the freshly ground and textured work roll may have an Ra between 2.5 μm and 7.0 μm. The newly ground and textured work roll is more generally referred to herein as a new work roll. In a specific example, the new work roll may have an Ra of between 3.18 μm and 4.0 μm. The roughness average of the new work roll may decrease during use or upon wear. Thus, the high friction rolling conditions described above can also be produced depending on the used work rolls, as long as the used work rolls have an Ra between 2.0 μm and 4.0 μm in one example. In a specific example, the used work rolls may have an Ra between 1.74 μm and 3.0 μm while still achieving the high friction rolling conditions described above.

Additionally or alternatively, the mean surface roughness depth (Rz) of the work roll profile may also be relied upon as an indicator to achieve the high friction rolling conditions described above. The new work roll may have an Rz of between 20 μm and 41 μm. In one particular example, the new work roll may have an Rz of between 21.90 μm and 28.32 μm. The high friction rolling conditions for the above may in one example be dependent on the used work rolls as long as they maintain an Rz of between 10 μm and 20 μm before out of service. In one particular example, the used work roll has an Rz of between 13.90 μm and 20.16 μm before out of service.

In addition, however, the above parameters may be further defined by the average spacing (Sm) between peaks throughout the profile. The new work rolls relied upon to create high friction rolling conditions may include between 90 μm and 150 μm of Sm. In one particular example, the new work rolls relied upon to produce high friction rolling conditions include Sm between 96 and 141 μm. The high friction rolling conditions for the above may in one example be dependent on the used work rolls as long as they maintain Sm between 115 μm and 165 μm.

Table 1 below illustrates test data measured as a function of position on the work rolls for work roll surface texturing to produce high friction rolling conditions and further provides a comparison between new work roll parameters and used work roll parameters before the used work rolls are going out of service:

"OS Qtr" is operator side quarter; and "Avg" is an average value

"Ctr" is the center of the band; and "Avg" is an average value

The DS Qtr is a drive side quarter; and "Avg" is an average value

Determining whether high friction rolling is suitable for use in examples of the present disclosure may depend on whether hot etching has occurred in the hot box. Hot etching is a side effect or consequence of the casting process that exposes prior austenite grain boundary pits at the surface of the steel strip. As noted above, prior austenite grain boundary pits may tend to cause the aforementioned defect phenomenon along etched prior austenite grain boundary pits upon further acid etching. Specifically, when the steel is exposed to high temperatures, such as a hot box, in an inert atmosphere, the hot etching reveals prior austenite grain boundary pits in the steel strip by forming grooves at the intersections between the prior austenite grain boundary pits and the surface. These grooves make the prior austenite grain boundary pits visible at the surface. Thus, the present examples of the process identify high friction rolling as the step that produces the desired steel properties when hot etched in a hot box. Regardless of the presence or absence of hot etching and evidence of prior austenite grain boundary pitting, high friction rolling can be provided to increase recrystallization of the thin steel strip.

FIG. 5 is a flow chart illustrating a process for applying high friction rolling and/or surface homogenization. In this example, determining whether a steel strip or steel product should undergo high friction rolling depends on whether undesirable thermal etching has occurred in the hot box 510. If hot etching has not occurred in the hot box, high friction rolling is not needed and not undertaken to (1) smooth out prior austenite grain boundary pits, (2) increase formability of steel products, such as ultra-high strength weathering steels, for example, and/or (3) improve hydrogen resistance (H)₂) Embrittlement. However, even if thermal etching has not occurred in the hot box, high friction rolling may still be pursued in order to achieve recrystallization 520 or to produce a microstructure as otherwise disclosed herein. If hot etching has occurred in hot box 510, high friction rolling 530 is performed to (1) smooth prior austenite grain boundary pits, (2) increase formability of the ultra-high strength weathering steel, and/or (3) improve hydrogen (H) resistance by removing prior austenite grain boundary pits and eliminating weak points formed as defects after 120 hours corrosion testing₂) Embrittlement. In one example of the present disclosure, an ultra-high strength weathering steel 550 having a trowelled pattern is produced. In another embodiment of the present disclosure, the trowel pattern is removed, thereby improving the pitting corrosion resistance 540, such as that required in automotive applications. Such an embodiment produces, for example, a high strength martensitic steel 560. Floating patternCan be removed by means of a surface homogenization process. Fig. 5 additionally illustrates a surface homogenization process 540. The applicability of the surface homogenization process is discussed in more detail below with respect to the present disclosure. Representative examples are also discussed in more detail below.

Super-strength weather-resistant steel

In some embodiments, the lightweight ultra-high strength weathering steel sheet may be made from a molten melt. The molten melt may be processed through a twin roll caster. In one example, the lightweight ultra-high strength weathering steel plate may be made by the steps comprising: (a) preparing a molten steel melt comprising: (i) between 0.20% and 0.35% carbon, less than 1.0% chromium, between 0.7% and 2.0% manganese, between 0.10% and 0.50% silicon, between 0.1% and 1.0% copper, less than or equal to 0.12% niobium, less than 0.5% molybdenum, between 0.5% and 1.5% nickel, and sedated with silicon containing less than 0.01% aluminum, by weight, and (ii) the balance iron and impurities resulting from smelting; (b) at a molecular weight of more than 10.0MW/m²Is solidified, thereby producing a steel sheet having a thickness of less than 2.5mm and is cooled to 1080 ℃ or below and Ar in a non-oxidizing atmosphere at a cooling rate of more than 15 ℃/s before rapid cooling and/or before hot rolling when hot rolling₃Above the temperature; and (c) rapidly cooling to form a steel sheet having a microstructure possessing at least 75% by volume martensite or martensite plus bainite, a yield strength between 700 and 1600MPa, a tensile strength between 1000 and 2100MPa, and an elongation between 1% and 10%. In one example, the lightweight ultra-high strength weathering steel sheet may also be hot rolled to between 15% and 50% reduction before rapid cooling. The sheet may be cooled to below 1100 ℃ and Ar before rapid cooling and/or when hot rolled in a non-oxidizing atmosphere prior to hot rolling at a cooling rate of greater than 15 ℃/s₃Above the temperature. Ar (Ar)₃The temperature is the temperature at which austenite begins to transform to ferrite during cooling. That is, Ar₃The temperature is the austenite transformation point. In various examples, the inclusion of nickel shifts the peritectic point away from the carbon region and/or increases the transition temperature of the peritectic point of the steel sheet composition to provide a defect-free steel sheet. The influence of nickel on the corrosion indexIn the following equation for determining the corrosion index calculation: cu 26.01+ Ni 3.88+ Cr 1.2+ Si 1.49+ P17.28-Cu Ni 7.29-Ni P9.1-Cu 33.39 (wherein the elements are in weight percent).

Examples of the steel sheet of the present invention provide for the addition of nickel to further prevent peritectic cracking while maintaining or improving hardenability. In particular, between 0.5% and 1.5% by weight of nickel is added. The addition of nickel is believed to prevent warping of the belt shell caused by the volume change of the peritectic zone during phase transformation on the casting rolls and thus enhance uniform heat transfer during belt solidification. It is believed that the addition of nickel shifts the peritectic point away from the carbon zone and/or raises the transition temperature of the peritectic point of the composition to form a defect free steel sheet. The phase diagram of fig. 4 illustrates this. Specifically, the phase diagram of fig. 4 illustrates the effect of each of 0.0 wt% nickel 100, 0.2 wt% nickel 110, and 0.4 wt% nickel 120. As illustrated in FIG. 4, the peritectic point P is found at the intersection of the liquid + delta phase 90, the delta + gamma phase 50, and the liquid + gamma phase 60₁₀₀、P₁₁₀And P₁₂₀A lower mass percentage of carbon (C) is transferred to a higher temperature with increasing nickel. Otherwise, the carbon content makes the steel strip susceptible to defects at lower temperatures in steel strips with high yield strength. The addition of nickel shifts the peritectic point away from the carbon zone and/or raises the transformation temperature of the peritectic point of the steel sheet to provide a defect-free martensitic steel strip with high yield strength.

The effect of nickel on corrosion index is reflected in the following equation for determining the result of the corrosion index calculation: cu 26.01+ Ni 3.88+ Cr 1.2+ Si 1.49+ P17.28-Cu Ni 7.29-Ni P9.1-Cu 33.39 (wherein the elements are in weight percent).

Table 2 below shows several composition examples of the lightweight ultra-high strength weathering steel sheet of the present disclosure.

TABLE 2

In Table 2, LecoN is the weight percent nitrogen (N) measured₂) And CEAWS is measuredWeight percent Carbon Equivalent (CE).

Other elements that are dependent on hardenability produce the opposite effect by moving the peritectic point closer to the carbon region. Such elements include chromium and molybdenum which are dependent for increased hardenability but ultimately lead to peritectic cracking. By the addition of nickel, hardenability is improved and peritectic cracking is reduced to provide a fully quenched martensitic grade steel strip with high strength.

In the compositions of the present invention, the addition of nickel may be combined with a limited amount of chromium and/or molybdenum, as described herein. As a result, nickel mitigates any effect these hardening elements may have in creating peritectic cracking. However, in one example, the additional nickel is not combined with the intentional addition of boron. Boron was intentionally added at 5ppm or more. That is, in one example, the addition of nickel will be used in combination with substantially no boron or less than 5ppm boron. In addition, the lightweight ultra-high strength weathering steel plate may be manufactured by further tempering the steel plate at a temperature between 150 ℃ and 250 ℃ for 2-6 hours. Tempering the steel sheet provides improved elongation with minimal loss of strength. For example, after tempering as described herein, a steel sheet having a yield strength of 1250MPa, a tensile strength of 1600MPa, and an elongation of 2% is improved to a yield strength of 1250MPa, a tensile strength of 1525MPa, and an elongation of 5%.

The lightweight ultra-high strength weathering steel plate may be silicon killed, containing less than 0.008% aluminum or less than 0.006% aluminum by weight. The molten melt may have a free oxygen content of between 5 and 70ppm or between 5 and 60 ppm. The steel sheet may have a total oxygen content of greater than 50 ppm. The inclusions comprise MnOSiO typically with 50% of them being less than 5 μm in size₂And has the potential to enhance the microstructure evolution and hence the mechanical properties of the strip.

The molten melt may be greater than 10.0MW/m²Is solidified into a steel sheet having a thickness of less than 2.5mm, and is cooled to 1080 ℃ or less and Ar in a non-oxidizing atmosphere at a cooling rate of more than 15 ℃/s₃Above the temperature. The non-oxidizing atmosphere is an atmosphere of typically an inert gas such as nitrogen or argon or mixtures thereof, which contains less than about 5% by weight oxygen.

In some embodiments, the martensite in the steel sheet may be formed of austenite having a grain size greater than 100 μm. In other embodiments, the martensite in the steel sheet may be formed of austenite having a grain size greater than 150 μm. At a power of more than 10MW/m²The rapid solidification of the heat flux enables the production of austenite grain sizes in response to controlled cooling to achieve defect-free sheet manufacture.

The steel sheet may additionally be hot rolled to between 15% and 50% reduction and then rapidly cooled to form a steel sheet having a microstructure with at least 75% martensite plus bainite, a yield strength between 700 and 1600MPa, a tensile strength between 1000 and 2100MPa, and an elongation between 1% and 10%. Further, the steel sheet may be hot rolled to a reduction between 15% and 35% and then rapidly cooled to form a steel sheet having a microstructure with at least 75% martensite plus bainite, a yield strength between 700 and 1600MPa, a tensile strength between 1000 and 2100MPa, and an elongation between 1% and 10%. In one example, a steel sheet is hot rolled to between 15% and 50% reduction and then rapidly cooled to form a steel sheet having a microstructure with at least 90% by volume martensite or martensite and bainite. In even yet another example, the steel sheet is hot rolled to a reduction of between 15% and 50% and then rapidly cooled to form a steel sheet having a microstructure with at least 95% by volume martensite or martensite and bainite.

Many products can be made from lightweight ultra-high strength weathering steel sheets of the type described herein. One example of a product that can be made from lightweight ultra-high strength weathering steel sheet includes steel piles. In one example, a steel pile comprises a web formed from a strip of carbon alloy steel of the kind described above and one or more flanges. The steel pile may further comprise a length, wherein the web and the one or more flanges extend the length. In use, the length of the steel piles is forced into the ground or soil to provide a structural foundation. The steel piles are forced into the ground or soil using a ram such as a piston or hammer. The ram may be part of and driven by the pile driver. The ram impacts or impacts the steel pile, forcing the steel pile into the ground or soil. Due to the impact, the previous steel piles may warp or deform under the impact of the ram. To avoid buckling or damage to the previous steel pile, the RPM or force of the pile driver is maintained below a damage threshold. The present steel pile has demonstrated the ability to increase the RPM or force applied to the steel pile compared to previous steel piles without buckling or damaging the steel pile, as reflected by the strength properties of the steel pile. Specifically, as tested, prior steel piles of comparable dimensional characteristics were driven and structurally damaged, with the steel piles of the present disclosure providing 25% RPM increase. Moreover, previous steel piles are not additionally weathering steel. Therefore, previous steel piles are susceptible to corrosion due to their placement in external conditions, including ground and soil conditions. Again, the present invention piling provides the corrosion index necessary to withstand these conditions. The strength properties and corrosion properties of the present invention have not previously been seen in combination for such products.

One example of a steel pile is a steel pile comprising a web and one or more flanges formed from a carbon alloy steel strip having a composition comprising: between 0.20% and 0.35% carbon, less than 1.0% chromium, between 0.7% and 2.0% manganese, between 0.10% and 0.50% silicon, between 0.1% and 1.0% copper, less than or equal to 0.12% niobium, less than 0.5% molybdenum, between 0.5% and 1.5% nickel, and silicon killed containing less than 0.01% aluminum, by weight, wherein the carbon alloy steel strip has a microstructure with at least 75% by volume martensite or martensite plus bainite, a yield strength between 700 and 1600MPa, a tensile strength between 1000 and 2100MPa, an elongation between 1% and 10% and has a corrosion index of 6.0 or more. In one example, the steel pile may be formed from a carbon alloy steel strip cast at a casting thickness of less than or equal to 2.5 mm. In another example, the steel piles may be formed of steel strips of less than or equal to 2.0 mm. In even yet another example, the steel pile may be formed of a steel plate having a thickness of between 1.4mm and 1.5mm or 1.4mm or 1.5 mm. The steel piles may be channel-shaped (channel) such as C-channel, box (box) channel, double channel, etc. The steel piles may additionally or alternatively be i-shaped members, angles (angles), structural T-shapes, hollow structural section shapes (hollow structural sections), double angles, S-shapes, tubes, etc. Also, many of these components may be joined together, for example welded together, to form a single steel pile. It is recognized herein that additional products can be made from lightweight ultra-high strength weathering steel sheets. Further, it is recognized herein that additional products may be made from ultra-high strength weathering steels that are not manufactured by a twin roll caster, but rather ultra-high strength products may be made by other methods.

Additional examples of ultra-high strength weathering steels are provided below:

a lightweight ultra-high strength steel sheet comprising: a carbon alloy steel strip cast at a casting thickness of less than or equal to 2.5mm having a composition comprising:

(i) between 0.20% and 0.35% carbon, less than 1.0% chromium, between 0.7% and 2.0% manganese, between 0.10% and 0.50% silicon, between 0.1% and 1.0% copper, less than or equal to 0.12% niobium, less than 0.5% molybdenum, between 0.5% and 1.5% nickel, and sedated with silicon containing less than 0.01% aluminum, and

(ii) the balance being iron and impurities resulting from the smelting;

wherein inclusion of nickel in the composition shifts the peritectic point away from the carbon region and/or increases the transformation temperature of the peritectic point to form a defect-free carbon alloy steel strip having a microstructure with at least 75% by volume martensite or martensite plus bainite, a yield strength between 700 and 1600MPa, a tensile strength between 1000 and 2100MPa, and an elongation between 1% and 10%.

In one example above, the lightweight ultra-high strength steel sheet has a microstructure having at least 75% by volume martensite. In another example above, the lightweight ultra-high strength steel sheet has a microstructure having at least 90% by volume martensite. In yet another example of the above, the light-weight ultrahigh-strength steel sheet has a microstructure having at least 95% martensite.

In one example above, the lightweight ultra-high strength steel sheet includes less than 5ppm boron.

In one example above, the lightweight ultra-high strength steel sheet comprises between 0.05% and 0.12% niobium.

In one example of the above, the martensite in the steel sheet comes from austenite having a grain size of more than 100 μm.

In one example of the above, the martensite in the steel sheet comes from austenite having a grain size of more than 150 μm.

In one example above, the steel sheet may additionally be hot rolled to between 15% and 50% reduction before rapid cooling.

In one example above, a carbon alloy steel sheet is hot rolled to a hot rolled thickness at a reduction of between 15% and 35% of the cast thickness before rapid cooling.

In one example above, the steel sheet is a weathering steel having a corrosion index of 6.0 or greater.

The manufacturing method of the light ultrahigh-strength weather-resistant steel plate comprises the following steps:

(a) preparing a molten steel melt comprising:

(i) between 0.20% and 0.35% carbon, less than 1.0% chromium, between 0.7% and 2.0% manganese, between 0.10% and 0.50% silicon, between 0.1% and 1.0% copper, less than or equal to 0.12% niobium, less than 0.5% molybdenum, between 0.5% and 1.5% nickel, silicon killed with less than 0.01% aluminum, and

(ii) the balance being iron and impurities resulting from the smelting;

(b) forming the melt into a casting pool supported on the casting surfaces of a pair of cooled casting rolls having a nip therebetween;

(c) counter-rotating the casting rolls and melting the melt at greater than 10.0MW/m²Solidifying the heat flux of (a) into a steel sheet having a thickness of less than 2.5mm conveyed downwardly from the nip and cooling the sheet in a non-oxidizing atmosphere to below 1100 ℃ and above the Ar3 temperature at a cooling rate of greater than 15 ℃/s; and

(d) rapidly cooling to form a steel sheet having a microstructure with at least 75% by volume martensite or martensite plus bainite, a yield strength between 700 and 1600MPa, a tensile strength between 1000 and 2100MPa, and an elongation between 1% and 10%, wherein the inclusion of nickel shifts the peritectic point away from the carbon region and/or raises the transition temperature of the peritectic point to inhibit crack or defect formation in the high strength martensitic steel sheet.

In one example above, the microstructure has at least 75% martensite by volume. In another example above, the microstructure has at least 90 volume% martensite. In yet another example above, the microstructure has at least 95 volume% martensite.

In one example above, a carbon alloy steel sheet is formed having less than 5ppm boron.

In one example above, the carbon alloy steel sheet includes between 0.05% and 0.12% niobium.

In one example of the above, the martensite in the steel sheet comes from austenite having a grain size of more than 100 μm.

In one example of the above, the martensite in the steel sheet comes from austenite having a grain size of more than 150 μm.

In one example above, the steel sheet is hot rolled to a hot rolled thickness at a reduction of between 15% and 50% of the cast thickness before rapid cooling.

In one example above, the steel sheet is hot rolled to a hot rolled thickness at a reduction of between 15% and 35% of the cast thickness before rapid cooling.

In one example above, the high strength steel sheet is defect free.

Also disclosed is a steel pile comprising a web formed of a carbon alloy steel plate cast at a casting thickness of less than or equal to 2.5mm and one or more flanges, the carbon alloy steel plate having a composition comprising: between 0.20% and 0.35% carbon, less than 1.0% chromium, between 0.7% and 2.0% manganese, between 0.10% and 0.50% silicon, between 0.1% and 1.0% copper, less than or equal to 0.12% niobium, less than 0.5% molybdenum, between 0.5% and 1.5% nickel, and silicon killed containing less than 0.01% aluminum, by weight, wherein the carbon alloy steel sheet has a microstructure possessing at least 75% by volume of martensite or martensite plus bainite, a yield strength between 700 and 1600MPa, a tensile strength between 1000 and 2100MPa, an elongation between 1% and 10% and is defect-free.

In one example above, the lightweight ultra-high strength steel sheet has a microstructure having at least 75% by volume martensite. In another example above, the lightweight ultra-high strength steel sheet has a microstructure having at least 90% by volume martensite. In yet another example of the above, the light-weight ultrahigh-strength steel sheet has a microstructure having at least 95% martensite.

In one example above, the carbon alloy steel sheet of the steel pile comprises less than 5ppm boron.

In one example above, the carbon alloy steel sheet of the steel pile comprises between 0.05% and 0.12% niobium.

In one example of the above, the martensite in the steel pile comes from austenite having a grain size greater than 100 μm.

In one example of the above, the martensite in the steel pile comes from austenite having a grain size greater than 150 μm.

In one example above, the steel sheet may additionally be hot rolled to between 15% and 50% reduction before rapid cooling.

In one example above, a carbon alloy steel sheet is hot rolled to a hot rolled thickness at a reduction of between 15% and 35% of the cast thickness before rapid cooling.

In one example above, the carbon alloy steel sheet is a weathering steel having a corrosion index of 6.0 or more.

High-friction rolled high-strength weathering steel

In the following examples, high friction rolled high strength weathering steel sheets are disclosed. An example of the ultra-high strength weathering steel plate is made by the steps comprising: (a) preparing a molten steel melt comprising: (i) between 0.20% and 0.40% carbon, less than 1.0% chromium, between 0.7% and 2.0% manganese, between 0.10% and 0.50% silicon, between 0.1% and 1.0% copper, less than or equal to 0.12% niobium, less than 0.5% molybdenum, between 0.5% and 1.5% nickel, and sedated with silicon containing less than 0.01% aluminum, by weight, and (ii) the balance iron and impurities resulting from smelting; (b) at a molecular weight of more than 10.0MW/m²Is solidified to be less than or equal to2.5mm thick and cooling the plate to 1080 ℃ or below and Ar in a non-oxidizing atmosphere at a cooling rate of greater than 15 ℃/s prior to rapid cooling₃Above the temperature; (c) high friction rolling the thin cast steel strip to a hot rolled thickness at a reduction of between 15% and 50% of the as-cast thickness to produce a hot rolled steel strip that is substantially free, or free of prior austenite grain boundary pits and has a screeded pattern; and (d) rapidly cooling to form a steel sheet having a microstructure possessing at least 75% martensite or at least 75% martensite plus bainite by volume, a yield strength between 700 and 1600MPa, a tensile strength between 1000 and 2100MPa, and an elongation between 1% and 10%. Here and elsewhere in this disclosure, elongation means total elongation. By "rapid cooling" is meant cooling at a rate greater than 100 ℃/s to between 100 and 200 ℃. The rapid cooling of the composition according to the invention with the addition of nickel achieves up to more than 95% of the martensitic phase steel strip. In one example, the rapid cooling forms a steel sheet having a microstructure with at least 95% martensite or at least 95% martensite plus bainite by volume. The addition of nickel must be sufficient to shift the 'peritectic point' away from the carbon region that would otherwise be present in the same composition without nickel added. In particular, the inclusion of nickel in the composition is believed to help shift the peritectic point away from the carbon region and/or increase the transition temperature of the peritectic point of the composition, which appears to suppress defects and produce ultra-high strength weathering steel sheets free of defects.

The formability of the ultrahigh-strength weathering steel is further improved by high-friction rolling of the ultrahigh-strength weathering steel. A measure of formability is set forth by ASTM a370 bend test standard. In embodiments, the ultra-high strength weathering steel of the present disclosure will pass the 3T 180 degree bend test and will do so consistently. In particular, high friction rolling produces screeding from prior austenite grain boundary pits by plastic deformation under shear. These elongated surface structures characterized by a trowelled pattern are desirable for the properties of ultra-high strength weathering steels. In particular, formability of ultra-high strength weathering steel is improved due to the floating pattern.

The steel strip may further comprise greater than 0.005% by weight niobium or niobiumAt 0.01% or 0.02% niobium. The steel strip may comprise greater than 0.05% molybdenum or greater than 0.1% or 0.2% molybdenum by weight. The steel strip may be silicon killed, containing less than 0.008% aluminium or less than 0.006% aluminium by weight. The molten melt may have a free oxygen content of between 5 and 70 ppm. The steel strip may have a total oxygen content greater than 50 ppm. The inclusions comprise MnOSiO typically with 50% of them being less than 5 μm in size₂And has the potential to enhance the microstructure evolution and hence the mechanical properties of the strip.

The molten melt may be greater than 10.0MW/m²Is solidified into a steel strip having a thickness of less than 2.5mm and is cooled to 1080 ℃ or less and Ar in a non-oxidizing atmosphere at a cooling rate of more than 15 ℃/s₃Above the temperature. The non-oxidizing atmosphere is an atmosphere of typically an inert gas such as nitrogen or argon or mixtures thereof, which contains less than about 5% by weight oxygen.

In some embodiments, the martensite in the steel strip may be derived from austenite having a grain size greater than 100 μm. In other embodiments, the martensite in the steel strip may be derived from austenite having a grain size greater than 150 μm. At a power of more than 10MW/m²The rapid solidification of the heat flux enables the production of austenite grain sizes responsive to controlled cooling after subsequent hot rolling to achieve defect-free strip manufacture.

As noted above, the steel strip of this set of examples may include a microstructure having martensite or martensite plus bainite. Martensite is formed in carbon steel by rapid cooling or quenching of austenite. Austenite has a particular crystal structure known as Face Centered Cubic (FCC). If allowed to cool naturally, austenite transforms into ferrite and cementite. However, when austenite is rapidly cooled or quenched, face-centered cubic austenite transforms into a highly strained body-centered tetragonal (BCT) form of ferrite that is supersaturated with carbon. The resulting shear deformation produces a large number of dislocations, which is the main strengthening mechanism of the steel. The martensite reaction begins during cooling when the austenite reaches the martensite start temperature and the parent austenite becomes thermodynamically unstable. As the sample is quenched, an increasing percentage of the austenite transforms to martensite until a lower transformation temperature is reached, at which point the transformation is complete.

However, martensitic steels tend to produce large prior austenite grain boundary pits observed on the hot rolled outer surface of a cooled thin steel strip formed from a low friction condition rolled steel. The step of pickling or acid etching amplifies these defects which lead to defects and spaces. High friction rolling is now being introduced as an alternative to overcome the identified problems for low friction condition rolling of martensitic steels. High friction rolling produces a smoothed boundary (grain boundary) pattern. A flattened boundary pattern may be more generally referred to herein as a flattened pattern. Additionally, a flattened boundary pattern may alternatively be referred to as a fish scale pattern.

Just as relying on the above ultra-high strength weathering steels to produce product shapes and configurations such as the piles described above, many products can be produced from high friction rolled high strength weathering steel plates of the type described herein. As above, one example of a product that can be manufactured from high friction rolled high strength weathering steel sheet includes steel piles. In one example, the steel pile includes a web and one or more flanges formed from the various carbon alloy steel strips described above. The steel pile may further comprise a length, wherein the web and the one or more flanges extend the length. In use, the length of the steel pile is forced into the ground or soil to provide a structural base. The steel piles are forced into the ground or soil using a ram such as a piston or hammer. The ram may be part of and driven by the pile driver. The ram impacts or impacts the steel pile, forcing the steel pile into the ground or soil. Due to the impact, the previous steel piles may warp or deform under the impact of the ram. To avoid buckling or damage to the previous steel pile, the RPM or force of the pile driver is maintained below a damage threshold. The present steel pile has demonstrated the ability to increase the RPM or force applied to the steel pile compared to previous steel piles without buckling or damaging the steel pile, as reflected by the strength properties of the steel pile. Specifically, as tested, prior steel piles of comparable dimensional characteristics were driven and structurally damaged, with the steel piles of the present disclosure providing 25% RPM amplification. Moreover, previous steel piles are not additionally weathering steel. Therefore, previous steel piles are susceptible to corrosion due to their placement in external conditions, including ground and soil conditions. Again, the present invention piling provides the corrosion index necessary to withstand these conditions. The strength properties and corrosion properties of the present invention have not previously been seen in combination for such products.

In one example, the steel pile may be formed from a carbon alloy steel strip casting of the present example cast at a casting thickness of less than or equal to 2.5 mm. In another example, the steel pile may be formed of the steel strip of this example of less than or equal to 2.0 mm. In even yet another example, the steel pile may be formed from a steel plate of the present example having a thickness of between 1.4mm and 1.5mm or 1.4mm or 1.5 mm. The steel piles may be channel-shaped such as C-channel, box channel, double channel, etc. The steel pile may additionally or alternatively be an i-shaped member, an angle, a structural T-shape, a hollow structural section, a double angle, an S-shape, a tube, or the like. Also, many of these components may be joined together, e.g., welded together, to form a single steel pile. It is recognized herein that additional products may be made from high friction rolled ultra high strength weathering steel plates.

High friction rolled high strength martensitic steel

In an embodiment of the present disclosure, a high strength martensitic steel sheet is also disclosed. The following examples of high strength martensitic steel sheets may additionally include weathering characteristics. Therefore, the high strength martensitic steel sheet example herein may also be referred to as ultra high strength weathering steel sheet due to such properties. Martensitic steels are increasingly used in applications requiring high strength, such as in the automotive industry. Martensitic steels provide the necessary strength for the automotive industry while reducing energy consumption and improving fuel economy. Martensite is formed in carbon steel by rapid cooling or quenching of austenite. Austenite has a particular crystal structure known as Face Centered Cubic (FCC). If allowed to cool naturally, austenite transforms into ferrite and cementite. However, when austenite is rapidly cooled or quenched, face-centered cubic austenite transforms into a highly strained body-centered tetragonal (BCT) form of ferrite that is supersaturated with carbon. The resulting shear deformation produces a large number of dislocations, which is the main strengthening mechanism of the steel. The martensite reaction begins during cooling when the austenite reaches the martensite start temperature and the parent austenite becomes thermodynamically unstable. As the sample is quenched, an increasing percentage of the austenite transforms to martensite until a lower transformation temperature is reached, at which point the transformation is complete.

However, martensitic steels tend to produce large prior austenite grain boundary pits observed on the hot rolled outer surface of a cooled thin steel strip formed from a low friction condition rolled steel. The pickling or acid etching step amplifies these defects leading to defects and spaces. High friction rolling is now being introduced as an alternative for overcoming the identified problems for rolling martensitic steels under low friction conditions, however, it has also been observed that high friction rolling produces an undesirable surface finish. In particular, high friction rolling produces a smoothed boundary pattern combined with a non-uniform surface finish. The smoothed boundary pattern may be more generally referred to herein as a troweled pattern. Additionally, a flattened boundary pattern may alternatively be referred to as a fish scale pattern. Then, uneven surface finishes with a trowelled pattern (e.g., when thin steel strip is subjected to subsequent acid etching) become prone to trapping acid and/or cause excessive corrosion, resulting in excessive pitting. In view of this, for some steel strips or products, such as martensitic steel sheets used in automotive applications, it is necessary to perform an additional surface treatment to provide the following surfaces: wherein a floating pattern and/or uneven surface finish is removed from the surface.

To reduce or eliminate the screeding pattern and/or uneven surface finish, the thin steel strip is subjected to a surface homogenization process after the hot rolling mill. Examples of surface homogenization processes include abrasive blasting, such as, for example, by using grinding wheels, shot blasting, sand blasting, wet blasting, pressurized application of other abrasives, and the like. One specific example of a surface homogenization process includes environmentally-friendly pickled (eco-pickled) surfaces (referred to herein as "EPS"). Other examples of surface homogenization processes include the forceful application of abrasive media to the steel strip surface to homogenize the steel strip surface. For a powerful application, it may also rely on a pressurized component (assembly). For example, the fluid may propel the abrasive medium. Fluids as used herein include liquids and air. Additionally or alternatively, the mechanical device may provide a forceful application. The surface homogenization process occurs after the thin cast steel strip reaches room temperature. That is, the surface homogenization process does not occur in an on-line process using a hot rolling mill. The surface homogenization process may occur at a location separate from the hot rolling mill and/or the twin casting mill or off-line therefrom. In some examples, the surface homogenization process may occur after coiling.

As used herein, a surface homogenization process changes the surface to be free of or eliminate a floating pattern. The thin steel strip surface that does not contain a troweling pattern or in which the troweling pattern has been eliminated is a surface that passes the 120 hour corrosion test without any surface pitting. The test piece not subjected to the surface homogenization process was cracked (fractured) due to surface corrosion after 24 hours during the 120-hour corrosion test. Fig. 6 is an image showing a high-friction hot-rolled steel strip whose surface is homogenized using EPS. By contrast, fig. 7 is an image showing the surface of a high friction hot rolled steel strip having a trowel pattern that has not been subjected to a surface uniformizing process. As noted above, a smeared pattern, unless it is removed by a surface homogenization process, may trap acid upon acid etching and thus be prone to excessive pitting and/or corrosion. In summary and as used herein, a surface that has undergone surface homogenization is a surface that is free of a previously formed floating pattern by high friction rolling conditions.

After hot rolling, the hot-rolled thin steel strip is cooled. In each embodiment, the steel strip undergoes a surface homogenization process after cooling. It is recognized that cooling may be achieved by any known means. In some cases, when cooling the thin steel strip, the thin steel strip is cooled to equal to or less than the martensite start temperature M_STo thereby form martensite from prior austenite in the thin steel strip.

One embodiment of a high strength martensitic steel sheet is made by the steps comprising: (a) preparing a molten steel melt comprising: (i) between 0.20% and 0.40% carbon, less than 1.0% chromium, between 0.7% and 2.0% manganese, between 0.10% and 0.50% silicon, between 0.1% and 1.0% copper, less than or equal to 0.12% niobium, less than 0.5% molybdenum, between 0.5% and 1.5% nickel, and sedated with silicon containing less than 0.01% aluminum, by weight, and (ii) the balanceIron and impurities resulting from smelting; (b) at a molecular weight of more than 10.0MW/m²Is solidified into a steel sheet having a thickness of 2.5mm or less and the sheet is cooled to 1080 ℃ or less and Ar in a non-oxidizing atmosphere at a cooling rate of more than 15 ℃/s before rapid cooling₃Above the temperature; (c) high friction rolling the thin cast strip to a hot rolled thickness at a reduction between 15% and 50% of the as-cast thickness to produce a hot rolled strip free of prior austenite grain boundary pits; (d) rapidly cooling to form a steel sheet having a microstructure with at least 75% martensite or at least 75% martensite plus bainite by volume, a yield strength between 700 and 1600MPa, a tensile strength between 1000 and 2100MPa, and an elongation between 1% and 10%; and (e) surface homogenizing the high friction hot rolled steel strip to produce a high friction hot rolled steel strip having a pair of opposing high friction hot rolled homogenized surfaces free of a floating pattern. Here and elsewhere in this disclosure, elongation means total elongation. By "rapid cooling" is meant cooling at a rate greater than 100 ℃/s to between 100 and 200 ℃. The rapid cooling of the composition according to the invention with the addition of nickel achieves steel strips with up to more than 95% of the martensitic phase. In one example, the rapid cooling forms a steel sheet having a microstructure with at least 95% martensite or at least 95% martensite plus bainite by volume. The addition of nickel must be sufficient to shift the 'peritectic point' away from the carbon region that would otherwise be present in the same composition without nickel added. Specifically, the inclusion of nickel in the composition is believed to help shift the peritectic point away from the carbon region and/or increase the transformation temperature of the peritectic point of the composition, which appears to suppress defects and results in a high strength martensitic steel sheet that is defect free.

Further variants of the examples of high friction rolled high strength martensitic steel are given below. In some examples, the steel strip may include a pair of opposing high friction hot rolled homogenized surfaces substantially free of prior austenite grain boundary pits and a troweling pattern. In yet another example, the steel strip may further comprise a pair of opposing high friction hot rolled homogenization surfaces that are substantially free of prior austenite grain boundary pits and a troweling pattern. In each of these examples, the surface may have a surface roughness (Ra) of no more than 2.5 μm.

In some examples, the thin steel strip may further be tempered at a temperature between 150 ℃ and 250 ℃ for 2-6 hours. Tempering the steel strip provides improved elongation with minimal loss of strength. For example, after tempering as described herein, a steel strip having a yield strength of 1250MPa, a tensile strength of 1600MPa and an elongation of 2% is improved to a yield strength of 1250MPa, a tensile strength of 1525MPa and an elongation of 5%.

The steel strip may further comprise greater than 0.005% niobium or greater than 0.01% or 0.02% niobium by weight. The steel strip may comprise greater than 0.05% molybdenum or greater than 0.1% or 0.2% molybdenum by weight. The steel strip may be silicon killed, containing less than 0.008% aluminium or less than 0.006% aluminium by weight. The molten melt may have a free oxygen content of 5-70 ppm. The steel strip may have a total oxygen content greater than 50 ppm. The inclusions comprise MnOSiO typically with 50% of them being less than 5 μm in size₂And has the potential to enhance the microstructure evolution and hence the mechanical properties of the strip.

The molten melt may be melted at greater than 10.0MW/m²Is solidified into a steel strip having a thickness of less than 2.5mm and is cooled to 1080 ℃ or less and Ar in a non-oxidizing atmosphere at a cooling rate of more than 15 ℃/s₃Above the temperature. The non-oxidizing atmosphere is an atmosphere of typically an inert gas such as nitrogen or argon or mixtures thereof, which contains less than about 5% by weight oxygen.

In some embodiments, the martensite in the steel strip may be derived from austenite having a grain size greater than 100 μm. In other embodiments, the martensite in the steel strip may be derived from austenite having a grain size greater than 150 μm. At a power of more than 10MW/m²The rapid solidification of the heat flux enables the production of austenite grain sizes responsive to controlled cooling after subsequent hot rolling to achieve defect-free strip manufacture.

High friction rolled steel sheets may be provided for use in hot stamping applications. Typically, steel sheets relied upon for use in hot stamping applications are stainless steel compositions or require aluminum-silicon corrosion resistant coatings. In hot stamping applications, a corrosion resistant protective layer is desirable while maintaining high strength properties and favorable surface structure characteristics. The high friction rolling compositions of the present invention have achieved the desired properties without relying on stainless steel compositions or otherwise providing aluminum-silicon corrosion resistant coatings. In contrast, the high friction rolling compositions of the present invention rely on a mixture of nickel, chromium and copper as illustrated in the various examples above to improve corrosion resistance. In hot stamping applications, the high friction rolled steel sheet is subjected to austenitizing conditions between 900 ℃ and 930 ℃ for a period of time between 6 minutes and 10 minutes. In one example, a high friction rolled steel sheet is subjected to austenitizing conditions at 900 ℃ for a period of 6 minutes. In another example, a high friction rolled steel sheet is subjected to austenitizing conditions at 900 ℃ for a period of 10 minutes. In yet another example, the high friction rolled steel sheet is subjected to austenitizing conditions at 930 ℃ for a period of 6 minutes. In even yet another example, the high friction rolled steel sheet is subjected to austenitizing conditions at 930 ℃ for a period of 10 minutes. Table 3 below illustrates that the properties of the high friction rolled steel sheet are maintained above the minimum tensile strength of 1500MPa, the minimum yield strength of 1100MPa, and the minimum elongation of 3% for hot stamping applications.

TABLE 3

Austenitizing conditions	Tensile strength (MPa)	Yield strength (MPa)	Elongation (%)
				900 ℃ for 6 minutes	1546.98	1155.06	7.3
900 ℃ for 6 minutes	1576.65	1154.37	7.0
				900 ℃ for 10 minutes	1591.14	1168.86	6.4
900 ℃ for 10 minutes	1578.03	1152.30	6.6
				930 ℃ for 6 minutes	1566.30	1146.09	7.3
930 ℃ for 6 minutes	1566.99	1178.52	6.5
				930 ℃ for 10 minutes	1509.03	1109.52	6.6
930 ℃ for 10 minutes	1521.45	1129.53	6.4

In these examples, the steel sheet provided for use in hot stamping applications may include the composition of any of the examples of steel sheets disclosed above, but is a steel sheet that may remain unquenched. In particular, a steel sheet provided for use in hot stamping applications may be made by steps comprising: (a) preparing a molten steel melt comprising: (i) between 0.20% and 0.40% carbon, less than 1.0% chromium, between 0.7% and 2.0% manganese, between 0.10% and 0.50% silicon, between 0.1% and 1.0% copper, less than or equal to 0.12% niobium, less than 0.5% molybdenum, between 0.5% and 1.5% nickel, and sedated with silicon containing less than 0.01% aluminum, by weight, and (ii) the balance iron and impurities resulting from smelting; (b) at a molecular weight of more than 10.0MW/m²Is solidified into a steel sheet having a thickness of 2.5mm or less and the sheet is cooled to 1080 ℃ or less and Ar in a non-oxidizing atmosphere at a cooling rate of more than 15 ℃/s before rapid cooling₃Above the temperature; (c) high friction rolling the thin cast steel strip to a hot rolled thickness at a reduction of between 15% and 50% of the as-cast thickness to produce a hot rolled steel strip having predominantly no, substantially no, or no prior austenite grain boundary pits and having a screeded pattern; and (d) cooling at less than 100 ℃/s to form a steel sheet having a predominantly bainitic microstructure. That is, the steel sheet provided for use in hot stamping applications may be any of the examples of steel sheets disclosed above except for: the steel sheet is not rapidly cooled and, therefore, a microstructure mainly or substantially having martensite or martensite plus bainite is not formed. In contrast, steel sheets provided for use in hot stamping applications are cooled at less than 100 ℃/s.

Hot rolling, including low friction hot rolling and high friction hot rolling

Hot rolling (and more specifically, low friction rolling and high friction rolling) as relied upon in the above examples of the present disclosure is described further below. The concepts described below can be applied to the examples provided above as needed to achieve the properties of each respective example. Generally, in each hot rolling example, prior to cooling the strip, for example in particular embodiments to a temperature at which austenite in the steel transforms to martensite,the strip is conveyed through a hot rolling mill to reduce the as-cast thickness. In certain cases, the hot solidified strip (cast strip) may be conveyed through a hot rolling mill while at an entry temperature greater than 1050 ℃ and in some cases up to 1150 ℃. After the strip exits the hot rolling mill, the strip is cooled, for example in certain exemplary cases to a temperature at which austenite in the steel transforms to martensite, by cooling to a temperature equal to or less than the martensite start temperature Ms. In some cases, the temperature is 600 ℃ or less, wherein the martensite start temperature M_SDepending on the particular composition. Cooling may be achieved by any known method using any known mechanism, including the mechanisms described above. In some cases, the cooling is fast enough to avoid appreciable ferrite initiation, which is also affected by the composition. In such a case, for example, the cooling is configured to reduce the temperature of the belt at a rate of about 100 ℃ to 200 ℃ per second.

Hot rolling is performed using one or more pairs of counter-rotating work rolls. Work rolls are commonly used to reduce the thickness of a substrate such as a plate or belt. This is accomplished by passing the substrate through a gap disposed between the pair of work rolls, the gap being less than the thickness of the substrate. This gap is also referred to as the roll gap. During thermal processing, a force is applied to the substrate by the work rolls, thereby exerting a rolling force on the substrate to thereby achieve a desired reduction in the thickness of the substrate. In doing so, friction is generated between the substrate and each work roll as the substrate translates through the gap. This friction is called roll gap friction.

Conventionally, it is desirable to reduce the seam friction during hot rolling of steel sheets and strips. By reducing the slot friction (and therefore the coefficient of friction), rolling loads and roll wear are reduced and machine life is extended. Various techniques have been employed to reduce the roll gap friction and coefficient of friction. In certain exemplary cases, thin steel belts are lubricated to reduce roll gap friction. Lubrication may take the form of: oil applied to the rolls and/or the thin steel strip, or scale formed along the outside of the thin steel strip prior to hot rolling. By using lubrication, hot rolling can occur under low friction conditions, where the coefficient of friction (μ) of the roll gap is less than 0.20.

In one example, the coefficient of friction (μ) is determined based on a hot rolling model developed by the HATCH for a particular set of work rolls. The model is shown in FIG. 8, which provides the reduction in thickness of the thin steel strip in percent along the X-axis and the specific force "P" in kN/mm along the Y-axis. The specific force P is the normal (perpendicular) force applied to the substrate by the work roll. The model includes five (5) curves, each representing a coefficient of friction and providing a relationship between reduction and work roll force. For each coefficient of friction, the expected work roll force is obtained based on the measured reduction. In operation, during hot rolling, a target coefficient of friction is preset by adjusting the work roll lubrication, a target reduction is set by the desired strip thickness required at the mill exit to meet a particular customer order, and the actual work roll force will be adjusted to achieve the target reduction. FIG. 8 shows typical forces required to achieve a target reduction for a particular coefficient of friction.

In certain exemplary cases, the coefficient of friction is equal to or greater than 0.20. In other exemplary cases, the coefficient of friction is equal to or greater than 0.25, equal to or greater than 0.268, or equal to or greater than 0.27. It is recognized that these coefficients of friction are sufficient under certain conditions for austenitic steels (which are the steel alloys used in the examples shown in the figures) to at least predominantly or substantially eliminate prior austenite grain boundary pits from the hot rolled surface and to produce elongated surface features that are plastically formed by shear, wherein the steel is austenitic during hot rolling but forms martensite with prior austenite grains and prior austenite grain boundary pits present after cooling. As previously mentioned, various factors or parameters may be varied to achieve a desired coefficient of friction under certain conditions. It is noted that for the friction coefficient values previously described, for a substrate having a thickness of 5mm or less prior to hot rolling, the normal force applied to the substrate during hot rolling may be 600 to 2500 tons at a temperature of greater than 1050 ℃, and in some cases up to 1150 ℃, of the substrate entering the work rolls as the substrate enters the pair of work rolls and translates or advances at a rate of 45-75 meters per minute (m/min). For these coefficients of friction, the work rolls had diameters of 400-600 mm. Of course, variations outside each of these parameter ranges may be used as desired to achieve different coefficients of friction, as may be desired to achieve the surface characteristics of the hot rolling described herein.

In one example, hot rolling is conducted under high friction conditions with a coefficient of friction of 0.25 at 60 meters per minute (m/min) at a 22% reduction with a work roll force of approximately 820 tons. In another example, hot rolling is conducted under high friction conditions with a coefficient of friction of 0.27 at 60 meters per minute (m/min) at a 22% reduction rate with a work roll force of approximately 900 tons.

Hot rolling of thin steel strip as relied upon in the examples of the present disclosure when the thin steel strip is at A_r3At a temperature above the temperature. Ar (Ar)₃The temperature is the temperature at which austenite begins to transform to ferrite during cooling. That is, Ar₃The temperature is the austenite transformation point. Ar (Ar)₃Temperature in ratio A₃A position several degrees lower in temperature. At Ar₃Below the temperature, alpha ferrite is formed. These temperatures are shown in the exemplary CCT diagram in fig. 9. In FIG. 9, A₃170 represents the upper temperature at which ferrite stability ends at equilibrium. Ar (Ar)₃The upper limit temperature at which the ferrite stability ends during cooling. More specifically, Ar₃The temperature is the temperature at which austenite begins to transform to ferrite during cooling. That is, Ar₃The temperature is the austenite transformation point. For comparison, A₁180 denotes the lower limit temperature at which ferrite stability ends at equilibrium.

Referring also to fig. 9, a ferrite curve 220 represents a transformation temperature of a microstructure generating 1% ferrite, a pearlite curve 230 represents a transformation temperature of a microstructure generating 1% pearlite, an austenite curve 250 represents a transformation temperature of a microstructure generating 1% austenite, and a bainite curve (B)_s)240 denotes the transformation temperature resulting in a microstructure of 1% bainite. As described in greater detail previously, the martensite start temperature M_SRepresented by the martensite curve 190, where martensite begins to form from prior austenite in the thin steel strip. Further illustrated in fig. 9 is a 50% martensite curve 200 representing a microstructure having at least 50% martensite. Additionally, FIG. 9 illustrates a 90% martensite microstructure having at least 90% martensiteThe volume curve 210.

In the exemplary CCT plot shown in FIG. 9, the martensite start transition temperature M is shown_S190. The austenite in the strip transforms to martensite when passing through the cooler. In particular, in this case, cooling the strip to below 600 ℃ results in a transformation of coarse austenite, in which a distribution of fine iron carbides is precipitated within the martensite.

While the invention has been illustrated and described in the foregoing drawings and description, the same is to be considered as illustrative and not restrictive in character, it being understood that only illustrative embodiments have been shown and described and that all changes and modifications that come within the spirit of the invention as described by the following claims are desired to be protected. Additional features of the invention will become apparent to those skilled in the art upon consideration of the description. Changes may be made without departing from the spirit and scope of the invention.