阅读说明：本技术 高Mn钢及其制造方法 (High Mn steel and method for producing same ) 是由 泉大地 木津谷茂树 植田圭治 中岛孝一 于 2019-07-31 设计创作，主要内容包括：本发明提供低温韧性和表面性状优良的高Mn钢。该高Mn钢具有以质量％计含有C：0.100％以上且0.700％以下、Si：0.05％以上且1.00％以下、Mn：20.0％以上且35.0％以下、P：0.030％以下、S：0.0070％以下、Al：0.010％以上且0.070％以下、Cr：0.50％以上且5.00％以下、N：0.0050％以上且0.0500％以下、O：0.0050％以下、Ti：0.005％以下和Nb：0.005％以下、余量为Fe和不可避免的杂质的成分组成,并且具有以奥氏体作为基底相的显微组织,该显微组织中的Mn富集部的Mn浓度为38.0％以下并且KAM值的平均值为0.3以上,屈服强度为400MPa以上,-196℃下的夏比冲击试验的吸收能vE_(-196)为100J以上并且脆性断面率小于10％。(The invention provides a high Mn steel excellent in low-temperature toughness and surface properties. The high Mn steel has a composition containing, in mass%, C: 0.100% or more and 0.700% or less, Si: 0.05% or more and 1.00% or less, Mn: 20.0% or more and 35.0% or less, P: 0.030% or less, S: 0.0070% or less, Al: 0.010% to 0.070%, Cr: 0.50% or more and 5.00% or less, N: 0.0050% or more and 0.0500% or less, O: 0.0050% or less, Ti: 0.005% or less and Nb: 0.005% or less, and the balance Fe and inevitable impurities, and has a microstructure having austenite as a base phase, the compositionThe Mn concentration of the Mn-enriched part in the microstructure is 38.0% or less, the average value of KAM value is 0.3 or more, the yield strength is 400MPa or more, and the absorption energy vE of Charpy impact test at-196 DEG C ‑196 100J or more and a brittle fracture ratio of less than 10%.)

1. A high Mn steel having a composition containing, in mass%, C: 0.100% or more and 0.700% or less, Si: 0.05% or more and 1.00% or less, Mn: 20.0% or more and 35.0% or less, P: 0.030% or less, S: 0.0070% or less, Al: 0.010% to 0.070%, Cr: 0.50% or more and 5.00% or less, N: 0.0050% or more and 0.0500% or less, O: 0.0050% or less, Ti: 0.005% or less and Nb: 0.005% or less, and the balance Fe and inevitable impurities, and has a microstructure having austenite as a base phase, wherein the Mn concentration of a Mn-enriched portion in the microstructure is 38.0% or less, the average value of KAM (mean orientation difference of cores) values is 0.3 or more, the yield strength is 400MPa or more, and the absorption energy vE of a Charpy impact test at-196 ℃ is_-196100J or more and a brittle fracture ratio of less than 10%.

2. The high Mn steel according to claim 1, wherein the composition further contains, in mass%, a metal selected from the group consisting of Cu: 0.01% to 0.50% inclusive, Mo: 2.00% or less, V: 2.00% or less and W: 2.00% or less.

3. The high Mn steel according to claim 1 or 2, wherein the composition further contains, in mass%, a component selected from the group consisting of Ca: 0.0005% or more and 0.0050% or less, Mg: 0.0005% or more and 0.0050% or less and REM: 0.0010% or more and 0.0200% or less.

4. A method for producing a high Mn steel, wherein a steel material having the composition according to claim 1, 2 or 3 is heated to a temperature range of 1100 ℃ or higher and 1300 ℃ or lower, and then hot-rolled under conditions of a rolling completion temperature of 800 ℃ or higher and a total rolling reduction of 20% or higher, and descaling is performed during the hot-rolling.

5. A method for producing a high Mn steel, wherein a steel material having the composition according to claim 1, 2 or 3 is heated to a temperature range of 1100 ℃ or higher and 1300 ℃ or lower, then subjected to a first hot rolling under conditions of a rolling completion temperature of 1100 ℃ or higher and a total rolling reduction of 20% or higher, and then subjected to a second hot rolling under conditions of a rolling completion temperature of 700 ℃ or higher and lower than 950 ℃, and further subjected to descaling treatment in the second hot rolling.

6. A method for producing a high Mn steel, wherein a steel material having the composition of claim 1, 2 or 3 is heated to a temperature range of 1100 ℃ or higher and 1300 ℃ or lower, then subjected to a first hot rolling under conditions in which a rolling completion temperature is 800 ℃ or higher and lower than 1100 ℃ and a total reduction ratio is 20% or higher, then subjected to a reheating of 1100 ℃ or higher and 1300 ℃ or lower, subjected to a second hot rolling under conditions in which the rolling completion temperature is 700 ℃ or higher and lower than 950 ℃, and subjected to a descaling treatment in the second hot rolling.

7. The method of manufacturing high Mn steel according to claims 5 and 6, wherein in the first hot rolling, descaling is performed.

8. The method for producing high Mn steel according to claims 4 to 7, wherein after the final hot rolling, a cooling treatment is performed at an average cooling rate of 1.0 ℃/sec or more from a temperature of not less than (rolling end temperature-100 ℃) to a temperature range of not less than 300 ℃ and not more than 650 ℃.

Technical Field

The present invention relates to a high Mn steel having excellent toughness particularly at low temperatures, which is suitable for structural steels used in extremely low temperature environments such as liquefied gas storage tanks, and a method for producing the same.

Background

Since the use environment of a structure such as a tank for liquefied gas storage tank is extremely low temperature, in order to use a hot-rolled steel sheet for the structure, the steel sheet is required to have not only excellent strength but also excellent toughness at extremely low temperature. For example, in the case of a hot rolled steel sheet used for a storage tank for liquefied natural gas, it is necessary to ensure excellent toughness in a temperature range lower than the boiling point of liquefied natural gas, i.e., -164 ℃. If the steel plate used for the structure for the cryogenic tank has poor low-temperature toughness, the safety of the structure for the cryogenic tank may not be maintained, and therefore, the steel plate to be used is strongly required to have improved low-temperature toughness.

In response to this demand, austenitic stainless steel, 9% Ni steel, or 5000-series aluminum alloy, which has austenite as a steel sheet structure and does not exhibit brittleness at extremely low temperatures, has been conventionally used. However, since the alloy cost and the manufacturing cost are high, a steel material which is inexpensive and excellent in low-temperature toughness is desired.

In addition, for a structure such as a tank for a liquefied gas storage tank, coating is required for rust prevention and corrosion prevention of a steel sheet, and it is important to make the appearance after the coating beautiful in terms of environmental compatibility. Therefore, the hot-rolled steel sheet used in the storage tank for liquefied natural gas is required to have excellent properties of the surface of the steel sheet serving as a coating base, that is, to have less unevenness on the surface of the steel sheet.

Therefore, as a new steel material replacing conventional steel for extremely low temperature, for example, patent document 1 proposes to use high Mn steel containing a large amount of Mn, which is a relatively inexpensive austenite stabilizing element, as structural steel for extremely low temperature environment. Patent document 1 proposes the following technique: the stacking fault energy is controlled so that the low-temperature toughness is excellent and surface unevenness is not generated.

Documents of the prior art

Patent document

Patent document 1: japanese Kokai publication Hei-2017-507249

Disclosure of Invention

Problems to be solved by the invention

The technique described in patent document 1 can provide a high Mn steel having excellent surface quality without surface unevenness after working such as drawing, but does not mention the surface roughness of the hot-rolled steel sheet to be produced. That is, the hot-rolled steel sheet after production is generally shipped after the surface is made uniform by shot blasting. When the surface of the steel sheet after the shot blasting is rough, local rust is generated, and therefore, it is necessary to modify the surface properties by a grinding machine or the like, which causes a problem of lowering productivity.

Accordingly, an object of the present invention is to provide a high Mn steel excellent in low-temperature toughness and surface properties. Furthermore, it is an object of the present invention to provide an advantageous method for manufacturing such a high Mn steel. Here, the phrase "excellent low-temperature toughness" means that the absorption energy vE in the Charpy impact test at-196 ℃ is_-196100J or more and a brittle fracture ratio of less than 10%, and "excellent surface properties" means that the surface roughness Ra after the general shot blasting treatment is 200 μm or less.

Means for solving the problems

In order to solve the above problems, the present inventors have conducted extensive studies on various factors that determine the composition and structure of a steel sheet for high Mn steel, and have arrived at the following findings a to d.

a. It has been found that in austenitic steels with a high Mn content, if Mn-enriched portions with an Mn concentration of more than 38.0 mass% are formed, the brittle fracture ratio at low temperatures is 10% or more, resulting in deterioration of low temperature toughness. Therefore, in order to improve the low-temperature toughness of the high Mn steel, it is effective to set the Mn concentration in the Mn-enriched portion to 38.0 mass% or less.

b. In the austenitic steel having a high Mn content, if Cr is added in an amount exceeding 5.00 mass%, descaling during hot rolling becomes insufficient, and a rough surface having a surface roughness Ra of more than 200 μm is formed after shot blasting is applied to the hot-rolled sheet. Therefore, in order to improve the surface properties of high Mn steel, the amount of Cr added needs to be 5.00 mass% or less.

c. If hot rolling and descaling are performed under appropriate conditions, the above a and b can be realized, and the manufacturing cost can be suppressed.

d. It is effective to impart a high dislocation density by hot rolling under appropriate conditions for improving the yield strength.

The present invention has been completed based on the above findings, and the gist thereof is as follows.

1. A high Mn steel having a composition containing, in mass%, C: 0.100% or more and 0.700% or less, Si: 0.05% or more and 1.00% or less, Mn: 20.0% or more and 35.0% or less, P: 0.030% or less, S: 0.0070% or less, Al: 0.010% to 0.070%, Cr: 0.50% or more and 5.00% or less, N: 0.0050% or more and 0.0500% or less, O: 0.0050% or less, Ti: 0.005% or less and Nb: 0.005% or less, and the balance Fe and inevitable impurities, and has a microstructure having austenite as a base phase, wherein the Mn concentration of a Mn-enriched portion in the microstructure is 38.0 mass% or less, the Average value of KAM (Kernel Average Misorientation) values is 0.3 or more, the yield strength is 400MPa or more, and the absorption energy vE of a Charpy impact test at-196 ℃ is_-196100J or more and a brittle fracture ratio of less than 10%.

2. The high Mn steel according to claim 1, wherein the composition further contains, in mass%, a component selected from the group consisting of Cu: 0.01% to 0.50% inclusive, Mo: 2.00% or less, V: 2.00% or less and W: 2.00% or less.

3. The high Mn steel according to claim 1 or 2, wherein the above composition further contains, in mass%, a component selected from the group consisting of Ca: 0.0005% or more and 0.0050% or less, Mg: 0.0005% or more and 0.0050% or less and REM: 0.0010% or more and 0.0200% or less.

4. A method for producing a high Mn steel, wherein a steel material having the composition of 1, 2 or 3 is heated to a temperature range of 1100 ℃ or higher and 1300 ℃ or lower, then hot-rolled under conditions of a rolling completion temperature of 800 ℃ or higher and a total rolling reduction of 20% or higher, and descaling is performed during the hot-rolling.

Here, the temperature range and the temperature are the surface temperature of the steel material or the steel sheet, respectively.

5. A method for producing a high Mn steel, wherein a steel material having the composition of 1, 2 or 3 is heated to a temperature range of 1100 ℃ or higher and 1300 ℃ or lower, then subjected to a first hot rolling under conditions of a rolling completion temperature of 1100 ℃ or higher and a total rolling reduction of 20% or higher, and then subjected to a second hot rolling under conditions of a rolling completion temperature of 700 ℃ or higher and lower than 950 ℃, and further subjected to descaling treatment in the second hot rolling.

6. A method for producing a high Mn steel, wherein a steel material having the composition of 1, 2 or 3 is heated to a temperature range of 1100 ℃ or higher and 1300 ℃ or lower, then subjected to a first hot rolling under conditions in which a rolling completion temperature is 800 ℃ or higher and lower than 1100 ℃ and a total reduction ratio is 20% or higher, then subjected to a reheating of 1100 ℃ or higher and 1300 ℃ or lower, subjected to a second hot rolling under conditions in which the rolling completion temperature is 700 ℃ or higher and lower than 950 ℃, and subjected to a descaling treatment in the second hot rolling.

7. The method for producing high Mn steel according to any one of the above 5 and 6, wherein descaling is performed in the first hot rolling.

8. The method for producing a high Mn steel according to any one of the above 4 to 7, wherein after the final hot rolling, a cooling treatment is performed at an average cooling rate of 1.0 ℃/sec or more from a temperature of not less than (rolling end temperature-100 ℃) to a temperature range of not less than 300 ℃ and not more than 650 ℃.

Effects of the invention

According to the present invention, a high Mn steel excellent in low-temperature toughness and surface properties can be provided. Therefore, the high Mn steel of the present invention is industrially significant in contributing to improvement in safety and life of a steel structure used in an extremely low temperature environment, such as a liquefied gas storage tank. In addition, the production method of the present invention can provide a method excellent in economy since it does not cause a reduction in productivity or an increase in production cost.

Drawings

FIG. 1 is a graph showing the results of measurement of Mn concentration in the Mn rich portion and the absorption energy in the Charpy impact test at-196 ℃.

Detailed Description

Hereinafter, the high Mn steel of the present invention will be described in detail.

[ composition of ingredients ]

First, the composition of the high Mn steel of the present invention and the reasons for the limitation thereof will be described. Unless otherwise specified, the expression "%" in the component composition means "% by mass".

C: 0.100% or more and 0.700% or less

C is an inexpensive austenite stabilizing element and is an important element for obtaining austenite. In order to obtain the effect, C needs to be contained by 0.100% or more. On the other hand, if the content exceeds 0.700%, Cr carbide is excessively generated, and the low-temperature toughness is lowered. Therefore, C is set to 0.100% or more and 0.700% or less. Preferably, the content is set to 0.200% or more and 0.600% or less.

Si: 0.05% to 1.00%

Si functions as a deoxidizing material, is essential for steel making, and has the effect of being solid-dissolved in steel to increase the strength of a steel sheet by solid-solution strengthening. In order to obtain such an effect, Si needs to be contained by 0.05% or more. On the other hand, if the content exceeds 1.00%, the low-temperature toughness and weldability deteriorate. Therefore, Si is set to 0.05% or more and 1.00% or less. Preferably, the content is set to 0.07% or more and 0.50% or less.

Mn: 20.0% to 35.0%

Mn is a relatively inexpensive austenite stabilizing element. In the present invention, the element is an important element for achieving both strength and low-temperature toughness. In order to obtain this effect, Mn needs to be contained by 20.0% or more. On the other hand, if the content exceeds 35.0%, the low-temperature toughness deteriorates. Therefore, Mn is set to 20.0% or more and 35.0% or less. Preferably, the content is set to 23.0% or more and 32.0% or less.

P: less than 0.030%

When the content of P exceeds 0.030%, the low-temperature toughness deteriorates, and segregation occurs in grain boundaries, which becomes a starting point of stress corrosion cracking. Therefore, the upper limit is 0.030%, and the lower limit is preferably as small as possible. Therefore, P is set to 0.030% or less. Since an excessive reduction in P increases the refining cost and is economically disadvantageous, it is preferably set to 0.002% or more. Preferably, the content is set to 0.005% or more and 0.028% or less, and more preferably, 0.024% or less.

S: 0.0070% or less

Since S deteriorates the low-temperature toughness and ductility of the base material, S is preferably reduced as much as possible with an upper limit of 0.0070%. Therefore, S is set to 0.0070% or less. Since an excessive reduction in S increases the refining cost and is economically disadvantageous, it is preferable to set S to 0.0010% or more. Preferably, the content is set to 0.0020% to 0.0060%.

Al: 0.010% to 0.070%

Al functions as a deoxidizer and is most widely used in a molten steel deoxidation process for steel sheets. In order to obtain such an effect, Al needs to be contained by 0.010% or more. On the other hand, if the content exceeds 0.070%, the content is set to 0.070% or less because the content is mixed into the weld metal portion during welding and the toughness of the weld metal is deteriorated. Preferably, the content is set to 0.020% or more and 0.060% or less.

Cr: 0.50% or more and 5.00% or less

Cr is an element effective for stabilizing austenite by adding an appropriate amount and improving low-temperature toughness and base metal strength. In order to obtain such an effect, Cr needs to be contained by 0.50% or more. On the other hand, if the content exceeds 5.00%, the low-temperature toughness and the stress corrosion cracking resistance are lowered due to the formation of Cr carbide. In addition, descaling during hot rolling is insufficient, and the surface roughness deteriorates. Therefore, Cr is set to 0.50% or more and 5.00% or less. Preferably, the content is set to 0.60% or more and 4.00% or less, and more preferably, 0.70% or more and 3.50% or less. In particular, in order to improve the stress corrosion cracking resistance, it is preferably 2.00% or more, and more preferably more than 2.70%.

N: 0.0050% or more and 0.0500% or less

N is an austenite stabilizing element and is an element effective for improving low-temperature toughness. In order to obtain such an effect, N needs to be contained by 0.0050% or more. On the other hand, if the content exceeds 0.0500%, the nitride or carbonitride coarsens and the toughness decreases. Therefore, N is set to 0.0050% or more and 0.0500% or less. Preferably, the content is set to 0.0060% to 0.0400%.

O: 0.0050% or less

O deteriorates low-temperature toughness due to the formation of oxides. Therefore, O is set to a range of 0.0050% or less. Preferably 0.0045% or less. The lower limit of the content is not particularly limited, and it is preferable to set the content to 0.0010% or more because excessive reduction of O increases the refining cost and is economically disadvantageous.

The contents of Ti and Nb are respectively suppressed to be below 0.005 percent

Ti and Nb form high melting point carbonitrides in steel to suppress coarsening of crystal grains, and as a result, serve as starting points of fracture and paths through which cracks propagate. In particular, in high Mn steel, the control of the structure for improving the low temperature toughness and the ductility is hindered, and therefore, it is necessary to intentionally suppress the control. That is, Ti and Nb are components inevitably mixed from raw materials and the like, and are usually represented by Ti: greater than 0.005% and 0.010% or less and Nb: more than 0.005% and not more than 0.010% are mixed. Therefore, it is necessary to suppress the content of each of Ti and Nb to 0.005% or less by avoiding the inevitable mixing of Ti and Nb as much as possible by the method described later and the like. By suppressing the contents of Ti and Nb to 0.005% or less, respectively, the adverse effect of the carbonitride can be eliminated, and excellent low-temperature toughness and ductility can be ensured. The content of each of Ti and Nb is preferably set to 0.003% or less.

The content of Ti and Nb may be reduced to 0% as a matter of course, but since the load during steel making becomes high and is economically disadvantageous, it is preferable to set each of Ti and Nb to 0.001% or more from the viewpoint of economy.

The balance of iron and inevitable impurities other than the above components. The inevitable impurities include H, B and the like, and the total amount thereof is 0.01% or less.

In the present invention, the following elements may be contained as necessary in addition to the above essential elements for the purpose of further improving the strength and the low-temperature toughness.

Cu: 0.01% to 0.50% inclusive, Mo: 2.00% or less, V: 2.00% or less, W: 2.00% or less of one or more

Cu is an element that not only increases the strength of the steel sheet by solid solution strengthening, but also improves the mobility of dislocations and the low-temperature toughness. In order to obtain such an effect, Cu is preferably contained in an amount of 0.01% or more. On the other hand, if the content exceeds 0.50%, the surface properties during rolling deteriorate. Therefore, Cu is preferably set to 0.01% or more and 0.50% or less. More preferably, the content is set to 0.02% or more and 0.40% or less. More preferably, it is set to less than 0.20%.

Mo, V and W contribute to stabilization of austenite and to improvement of base material strength. In order to obtain such effects, it is preferable that Mo, V, and W are each contained by 0.001% or more. On the other hand, if the content exceeds 2.00%, coarse carbonitrides are formed, which become starting points of fracture and increase the production cost. Therefore, when these alloying elements are contained, the content is preferably set to 2.00% or less, respectively. More preferably, it is set to 0.003% or more and 1.70% or less, and still more preferably, it is set to 1.50% or less.

Ca: 0.0005% or more and 0.0050% or less, Mg: 0.0005% or more and 0.0050% or less, REM: 0.0010% or more and 0.0200% or less

Ca. Mg and REM are elements useful for controlling the morphology of inclusions, and may be contained as necessary. The term "morphology control" of inclusions means that the stretched sulfide-based inclusions are changed into granular inclusions. By controlling the morphology of the inclusions, ductility, toughness and sulfide stress corrosion cracking resistance are improved. In order to obtain such effects, Ca and Mg are preferably contained at 0.0005% or more, and REM is preferably contained at 0.0010% or more. On the other hand, if any element is contained in a large amount, the amount of non-metallic inclusions increases, and conversely ductility, toughness, and sulfide stress corrosion cracking resistance may decrease. In addition, it sometimes becomes economically disadvantageous.

Therefore, when Ca and Mg are contained, each of them is preferably set to 0.0005% or more and 0.0050% or less, and when REM is contained, it is preferably set to 0.0010% or more and 0.0200% or less. More preferably, Ca is 0.0010% or more and 0.0040% or less, Mg is 0.0010% or more and 0.0040% or less, and REM is 0.0020% or more and 0.0150% or less.

[ tissue ]

Microstructure with austenite as base phase

In the case where the crystal structure of the steel material is a body-centered cubic structure (bcc), the steel material may be brittle-fractured in a low-temperature environment, and thus is not suitable for use in a low-temperature environment. Here, when it is assumed that the steel material is used in a low-temperature environment, the base phase of the steel material must have an austenite structure having a face-centered cubic structure (fcc). The phrase "austenite is used as a base phase" means that the austenite phase is 90% or more in terms of area percentage. The balance other than the austenite phase is a ferrite phase or a martensite phase. The austenite phase is more preferably 95% or more, and may be 100%.

The Mn concentration of the Mn-enriched part in the microstructure is 38.0 mass% or less

In a hot-rolled steel sheet obtained by hot-rolling a steel material having the above-described composition, Mn-enriched portions are inevitably generated. The Mn enriched portion is a portion having the highest Mn concentration in the microsegregation portion. When a steel material containing Mn is hot-rolled, the Mn-enriched portion is inevitably generated due to band-shaped segregation of Mn.

Therefore, the Mn concentration in the Mn enriched portion and the absorption energy in the charpy impact test at-196 ℃ were measured for steel sheets obtained by subjecting steel materials having the above-described composition to hot rolling under various conditions, and the results are shown in fig. 1. As shown in the figure, when the Mn concentration in the Mn-enriched portion is 38.0 mass% or less after hot rolling under appropriate conditions is performed on the steel material having the above-described composition, the above-described absorption energy of 100J or more can be realized. The Mn concentration in the Mn-enriched portion is preferably 37.0 mass% or less.

The lower limit of the Mn concentration in the Mn-enriched portion is not particularly limited, and is preferably set to 25.0 mass% or more for the reason of ensuring the stability of austenite.

The Average value of KAM (Kernel Average Misorientation) value is 0.3 or more

The KAM value is as follows: EBSD (Electron Back Scattering Diffraction) analysis was performed on the depth positions 1/4 and 1/2 of the hot-rolled steel sheet from the surface to the thickness in any two visual fields in 500 μm × 200 μm visual fields, and the results were obtained as the average of the differences in orientation between each pixel in the grain and the adjacent pixels. The KAM value reflects a local crystal orientation change due to dislocations in the structure, and a higher KAM value indicates a larger difference in orientation between the measurement point and an adjacent site. That is, the higher the KAM value, the higher the degree of local deformation in the grains, and therefore, the higher the KAM value in the steel sheet after rolling, the higher the dislocation density. When the average value of the KAM value is 0.3 or more, a large number of dislocations are accumulated, and thus the yield strength is improved. Preferably 0.5 or more. On the other hand, if the average value of the KAM value exceeds 1.3, toughness may deteriorate, and therefore, it is preferably set to 1.3 or less.

The hot-rolled sheet having the above-described composition, an Mn-enriched portion having an Mn concentration of 38.0% or less and an average value of KAM values of 0.3 or more has a surface roughness Ra of 200 μm or less after shot blasting by a general method by descaling at least in the final hot rolling. This is because descaling suppresses an increase in surface roughness caused by the biting of scale during rolling, suppresses the occurrence of cooling unevenness during cooling caused by scale, and makes the material surface hardness uniform, thereby suppressing an increase in surface roughness during shot blasting.

In addition, when the surface roughness Ra after shot blasting exceeds 200 μm, not only the appearance after coating is impaired, but also local corrosion progresses in the depressed portions, so that Ra needs to be 200 μm or less. Preferably 150 μm or less, and more preferably 120 μm or less. The lower limit of Ra is not particularly limited, but is preferably set to 5 μm or more in order to prevent an increase in dressing cost.

Further, Mn diffuses from the steel to the surface of the steel sheet in the form of oxides called surface concentrates and precipitates and concentrates on the surface of the steel sheet, and therefore, by setting the Mn concentration of the Mn-enriched portion to 38.0% or less, Ra of 200 μm or less can be achieved.

The high Mn steel of the present invention can be produced by melting the molten steel having the above-described composition by a known melting method using a converter, an electric furnace, or the like. In addition, secondary refining may be performed using a vacuum degassing furnace. In this case, in order to limit Ti and Nb which inhibit proper structure control to the above ranges, it is necessary to avoid inevitable mixing from raw materials and the like and to take measures to reduce the contents thereof. For example, the concentration of Ti and Nb in the final steel slab product is reduced by reducing the basicity of the slag in the refining stage, allowing their alloys to be tapped enriched into the slag. Further, a method of oxidizing by blowing oxygen and separating by flotation an alloy of Ti and Nb at the time of reflux can be used. Then, a steel material such as a billet having a predetermined size is preferably produced by a known casting method such as a continuous casting method.

Further, in order to produce the steel material excellent in low-temperature toughness, the steel material is heated to a temperature range of 1100 ℃ to 1300 ℃, then hot-rolled under conditions of a rolling completion temperature of 800 ℃ to 20% or more of a total rolling reduction, and descaling is performed during the hot-rolling. Hereinafter, each step will be explained.

[ heating temperature of steel material: 1100 ℃ to 1300 ℃ inclusive ]

In order to obtain the high Mn steel having the above-described structure, it is important to heat the steel to a temperature range of 1100 ℃ or more and 1300 ℃ or less, and to perform hot rolling at a rolling completion temperature of 800 ℃ or more and a total rolling reduction of 20% or more. The temperature control here is based on the surface temperature of the steel material.

That is, the heating temperature before rolling is set to 1100 ℃ or higher in order to promote the diffusion of Mn by hot rolling. On the other hand, since the steel may start to melt when the temperature exceeds 1300 ℃, the upper limit of the heating temperature is set to 1300 ℃. Preferably 1150 ℃ or more and 1250 ℃ or less.

[ Hot Rolling: rolling finishing temperature of 800 ℃ or higher and total rolling reduction of 20% or higher ]

Next, in hot rolling, it is important to shorten the distance between the Mn-rich portion and the Mn-lean portion and promote the diffusion of Mn by increasing the total reduction rate at the end of rolling to 20% or more. The total reduction rate is preferably set to 30% or more. The upper limit of the total reduction ratio is not particularly limited, and is preferably set to 98% or less from the viewpoint of improving the rolling effect. Here, the total reduction ratio is a reduction ratio of the thickness of the slab on the inlet side of the first hot rolling at the time when the first hot rolling is finished and a reduction ratio of the thickness of the slab on the inlet side of the second hot rolling at the time when the second hot rolling is finished, and when the two hot rolling is performed, the total reduction ratio is preferably 20% or more at the end of the first hot rolling and 50% or more at the end of the second hot rolling, and when the hot rolling is performed only once, the total reduction ratio is preferably 60% or more.

Similarly, the rolling completion temperature is set to 800 ℃ or higher from the viewpoint of promoting the diffusion of Mn during rolling and ensuring low-temperature toughness. This is because the rolling completion temperature is much lower than two-thirds of the melting point of Mn (1246 ℃ C.) when it is lower than 800 ℃ C, and thus Mn cannot be sufficiently diffused. The results of the studies by the present inventors led to the following findings: when the rolling completion temperature is 800 ℃ or higher, Mn can be sufficiently diffused. It is considered that the reason is presumably that the Mn diffusion coefficient in austenite is small, and rolling in a temperature range of 800 ℃ or higher is required for sufficient diffusion of Mn. Preferably 950 ℃ or higher, and more preferably 1000 ℃ or higher. From the viewpoint of securing strength, the upper limit of the rolling end temperature is preferably set to 1050 ℃.

Further, it is advantageous to add a second hot rolling satisfying the following conditions after the above hot rolling, as necessary, for promoting the diffusion of Mn. In this case, the second hot rolling may be continued as it is when the finishing temperature of the first hot rolling is 1100 ℃ or more, but reheating at 1100 ℃ or more is performed when the finishing temperature is less than 1100 ℃. Here, since the steel may start to melt when the temperature exceeds 1300 ℃, the upper limit of the heating temperature is set to 1300 ℃. The temperature control is based on the surface temperature of the steel material.

[ second hot rolling: the rolling finishing temperature is 700 ℃ or more and less than 950 ℃)

The second hot rolling is performed at least one time in a temperature range of 700 ℃ or more and less than 950 ℃. That is, by performing rolling at a temperature of less than 950 ℃ in one pass or more, preferably at a rolling reduction of 10% or more per pass, dislocations introduced in the first rolling are difficult to recover and easily remain, so that the KAM value can be further increased. On the other hand, when finish rolling is performed at a temperature of 950 ℃ or higher, the crystal grain size becomes excessively coarse, and the desired yield strength cannot be obtained. Therefore, the finish rolling is performed for more than one pass at a temperature lower than 950 ℃. The upper limit of the rolling completion temperature is preferably 900 ℃ or less, more preferably 850 ℃ or less.

On the other hand, when the rolling end temperature is less than 700 ℃, the toughness deteriorates, so the rolling end temperature is set to 700 ℃ or higher. Preferably 750 ℃ or higher. The total rolling reduction at the time of finishing the second hot rolling is preferably 20% or more, and more preferably 50% or more. However, since the toughness deteriorates when the reduction is performed at more than 95%, the total reduction ratio at the end of the second hot rolling is preferably 95% or less. The total rolling reduction at the time of finishing the second hot rolling is a value calculated by using the thickness before the second hot rolling and the thickness after the second hot rolling.

Further, by performing descaling treatment at least once during hot rolling, a steel sheet having excellent surface properties can be produced. Preferably, the number of times is two or more, and more preferably three or more. The upper limit of the number of times is not particularly limited, and the number of times is preferably 20 or less in terms of operation. Here, the descaling treatment is preferably performed before the first pass of hot rolling. In the descaling treatment, the descaling treatment is performed in the hot rolling when the hot rolling is performed once, and in the case where the hot rolling is performed twice, the descaling treatment is performed in at least the second hot rolling. Further, in the case of performing hot rolling twice, it is more preferable to perform descaling treatment in both the first and second hot rolling.

Next, in the case of one hot rolling, it is preferable to perform cooling treatment under the following conditions after the hot rolling, and in the case of two hot rolling, it is preferable to perform cooling treatment under the following conditions after the second hot rolling.

[ cooling rate in a temperature range from a temperature of (rolling end temperature-100 ℃) or more to a temperature of 300 ℃ or more and 650 ℃: 1.0 ℃/sec or more

After the hot rolling is completed, cooling is preferably performed quickly. When the steel sheet after hot rolling is slowly cooled, the formation of precipitates is promoted, which may result in deterioration of low-temperature toughness. The formation of these precipitates can be suppressed by cooling at a cooling rate of 1.0 ℃/sec or more in a temperature range from a temperature of not less than (rolling end temperature-100 ℃) to a temperature of not less than 300 ℃ and 650 ℃. First, the cooling rate is defined in the temperature range from (rolling end temperature-100 ℃) or higher to 300 ℃ or higher and 650 ℃ or lower because the temperature range corresponds to the precipitation temperature range of carbide. When excessive cooling is performed, the steel sheet is deformed, and productivity is lowered. Particularly, for steel materials having a thickness of 10mm or less, air cooling is preferable. Therefore, the upper limit of the cooling start temperature is preferably set to 900 ℃.

Since the generation of precipitates is likely to be promoted when the average cooling rate in the above temperature range is less than 1.0 ℃/sec, the average cooling rate is preferably set to 1.0 ℃/sec or more. On the other hand, from the viewpoint of preventing deformation of the steel sheet due to excessive cooling, the upper limit of the average cooling rate is preferably set to 15.0 ℃/sec or less. In particular, a steel material having a thickness of 10mm or less is preferably 5.0 ℃/sec or less, and more preferably 3.0 ℃/sec or less.

The hot-rolled steel sheet manufactured through the above steps has a reduced Mn concentration in the Mn enriched portion in a hot-rolled state, and therefore does not require a subsequent heat treatment.

Examples

The present invention will be described in detail below with reference to examples. The present invention is not limited to the following examples.

Billets having the composition shown in table 1 were produced by the converter-ladle refining-continuous casting method. Next, the obtained slabs were hot-rolled under the conditions shown in Table 2 to prepare steel sheets having a thickness of 6 to 30 mm. The obtained steel sheet was evaluated for tensile properties, toughness and structure in the following manner.

(1) Tensile test Properties

Tensile test pieces of JIS5 were cut out from each of the obtained steel sheets, and tensile test was carried out according to the regulations of JIS Z2241 (1998) to examine the tensile test characteristics. In the present invention, tensile properties are judged to be excellent when the yield strength is 400MPa or more and the tensile strength is 800MPa or more. Further, the elongation of 40% or more was judged to be excellent in ductility.

(2) Low temperature toughness

From the position where the distance from the surface of each steel plate having a plate thickness of more than 20mm was 1/4 or the direction parallel to the rolling direction where the distance from the surface of each steel plate having a plate thickness of 10mm to 20mm was 1/2, Charpy V-notch test pieces were cut out in accordance with the specification of JIS Z2202 (1998), and Charpy impact tests were carried out on each steel plate in accordance with the specification of JIS Z2242 (1998) using three test pieces to determine the absorption energy at-196 ℃ and the toughness of the base metal was evaluated. In addition, for the steel plate with a thickness of less than 10mm, according to the above JIS standard, cut 5mm small-sized Charpy V notch test piece, three test pieces of Charpy impact test, to obtain-196 ℃ absorption energy. Further, the value was changed to 1.5 times, and the toughness of the base material was evaluated. In the present invention, the absorption energies (vE) of three test pieces are measured_-196) The average value of (A) is 100J or more, and the toughness of the base material is excellent. This is because a brittle fracture surface may be included when the value is less than 100J.

(3) Tissue evaluation

Value of KAM

The EBSD (Electron Back Scattering Diffraction) analysis in a field of view of 500 μm. times.200 μm on the polished surface of the cross section in the rolling direction was performed in any two fields of view at the position 1/4 and the position 1/2 of the plate thickness (measurement step: 0.3 μm) for the hot-rolled steel sheet using a Scanning Electron Microscope (SEM) JSM-7001F manufactured by Japan Electron corporation, and the average value of the entire measurement region, which was determined from the results as the average value of the difference in orientation (°) between each pixel and the adjacent pixel within the crystal grain, was taken as the average KAM value.

Mn concentration in Mn-enriched part

Furthermore, EPMA (Electron Probe Micro Analyzer) analysis was performed at the EBSD measurement site of the KAM value to determine the Mn concentration, and the site having the highest Mn concentration was defined as the enrichment site.

Area fraction of austenite

EBSD analysis (measurement step width: 0.3 μm) was performed at the EBSD measurement position, and the austenite area ratio was measured from the obtained phase diagram.

Brittle fracture ratio

After a Charpy impact test was performed at-196 ℃, SEM observation (10 visual fields were performed at 500 Xmagnification) was performed to measure the brittle fracture ratio.

Surface roughness Ra

Further, the surface of the steel sheet after shot blasting treatment using shot blasting material having a vickers Hardness (HV) of 400 or more and a grain size of ASTM E11 sieve No.12 or more was determined for the steel sheet after hot rolling, and the standard length and the evaluation length were determined in accordance with JIS B0633, and the surface roughness Ra was measured. Here, the surface properties are excellent when the surface roughness Ra is 200 μm or less.

The results obtained as described above are shown in table 3.

[ Table 1]

[ Table 2]

[ Table 2]

The total reduction ratio is the rolling ratio of the first and second rolling passes with respect to the thickness of the hot rolled steel sheet at the hot rolling inlet side and the hot rolled steel sheet at the hot rolling outlet side

[ Table 3]

[ Table 3]

Value obtained by multiplying absorption energy of a 5mm small-sized test piece by 1.5

It was confirmed that the high Mn steel according to the present invention satisfies the above-mentioned target properties (yield strength of the base material is 400MPa or more, low temperature toughness to absorb energy (vE)_-196) Has an average value of 100J or more, a brittle fracture ratio of less than 10%, and a surface roughness Ra of 200 μm or less). On the other hand, in the comparative examples which deviate from the scope of the present invention, one or more of the yield strength, low-temperature toughness, and surface roughness cannot satisfy the above-mentioned target properties.