阅读说明：本技术 一种汽车钢及其制造方法 (Automobile steel and manufacturing method thereof ) 是由 黄明欣 何斌斌 于 2018-01-05 设计创作，主要内容包括：一种强力且可延展的汽车钢,其包含8-11重量％的Mn、0.1-0.35重量％的C、1-3重量％的Al、0.05-0.5重量％的V以及余量的Fe。通过调节奥氏体稳定剂的量,可以在室温下获得具有适当相分数的马氏体和奥氏体的双相显微组织。马氏体分配C进入残留的奥氏体晶粒。马氏体基体可以确保更高的汽车钢强度,而具有不同机械稳定性的残留奥氏体晶粒可以改善汽车钢的延展性,同时达到1500MPa的强度和20％的延展性。该汽车钢的制造方法避免了常规Q&P钢的高淬火温度,且因此降低了制造价格并且易于批量制造。(A strong and ductile automotive steel comprising 8-11 wt.% Mn, 0.1-0.35 wt.% C, 1-3 wt.% Al, 0.05-0.5 wt.% V, and the balance Fe. By adjusting the amount of austenite stabilizer, a dual-phase microstructure of martensite and austenite with appropriate phase fractions can be obtained at room temperature. Martensite partitions C into retained austenite grains. The martensitic matrix ensures a higher strength of the automotive steel, while the retained austenite grains with different mechanical stability improve the ductility of the automotive steel, while achieving a strength of 1500MPa and a ductility of 20%. The manufacturing method of the automotive steel avoids the high quenching temperature of the conventional Q & P steel, and thus reduces the manufacturing price and is easy to manufacture in mass.)

1. A strong and ductile automotive steel comprising manganese in the range of 8-11 wt.%, carbon in the range of 0.1-0.35 wt.%, aluminum in the range of 1-3 wt.%, vanadium in the range of 0.05-0.5 wt.%, and the balance iron, based on the weight of the automotive steel.

2. The automotive steel of claim 1, wherein the automotive steel comprises 9.5-10.5 wt.% Mn, 0.18-0.22 wt.% C, 1.8-2.2 wt.% Al, 0.08-0.12 wt.% V, and the balance iron.

3. The automotive steel of claim 1, further comprising 10 wt.% Mn, 0.2 wt.% C, 2 wt.% Al, 0.1 wt.% V, and the balance Fe.

4. The automotive steel of claim 1, wherein the automotive steel further comprises at least one of the following elements: nickel in the range of 0.1-2.0 wt.%, chromium in the range of 0.2-2.0 wt.%, molybdenum in the range of 0.1-0.5 wt.%, silicon in the range of 0.3-2.0 wt.%, boron in the range of 0.0005-0.005 wt.%, niobium in the range of 0.02-0.10 wt.%, titanium in the range of 0.05-0.25 wt.%, copper in the range of 0.25-0.50 wt.%, and rhenium in the range of 0.002-0.005 wt.%.

5. A method of manufacturing an automotive steel, comprising: preparing a steel ingot comprising manganese in the range of 8-11 wt% and the balance Fe; providing a steel plate from a steel ingot; isothermally holding a steel sheet to form austenite; cooling the steel plate to room temperature; tempering the steel plate at the temperature of 300-400 ℃; and quenching the steel plate to room temperature.

6. A method as set forth in claim 5 wherein the step of providing the steel sheet from the ingot is performed by at least one of casting, hot rolling, forging, and cold rolling.

7. The method of claim 5, wherein the isothermal hold is performed at a temperature of Ac3-20 ℃ to Ac3+100 ℃, where Ac3 is the temperature at which ferrite completely transforms into austenite.

8. The method of claim 5, wherein the isothermal hold is performed for 5-20 minutes.

9. The method of claim 5, wherein the cooling is performed at a first cooling rate higher than 0.5 ℃/s.

10. The method of claim 5, wherein the step of tempering the steel sheet is performed for 5-10 minutes.

11. The method of claim 5, wherein the step of quenching the steel sheet is performed at a second cooling rate higher than 0.5 ℃/s.

12. A method for manufacturing a strong and ductile automotive steel, comprising the steps of:

(1) providing a steel ingot comprising 8-11 wt.% Mn, 0.1-0.35 wt.% C, 1-3 wt.% Al, 0.05-0.5 wt.% V, and the balance Fe;

(2) forging and rolling the ingot to provide a steel plate having a thickness of 4-6mm, and cooling the steel plate;

(3) batch annealing at 500-750 deg.C for 5-10 hr;

(4) pickling to remove an oxide layer in the steel sheet;

(5) cold-rolling the steel sheet to provide a cold-rolled steel sheet having a final thickness of 0.8-2 mm;

(6) treating a steel sheet by hot working to obtain a dual-phase microstructure in which austenite is embedded in a martensitic matrix, and cooling the steel sheet to room temperature at a cooling rate of more than 0.5 ℃/s, wherein the hot working route comprises holding the steel sheet isothermally at a temperature of Ac3-20 ℃ to Ac3+100 ℃ for 5-20 minutes to form part or all of austenite, wherein Ac3 is the temperature at which ferrite is completely transformed into austenite; and

(7) the steel plate is tempered at the temperature of 300-400 ℃ for 5-20 minutes and quenched to room temperature at a cooling rate higher than 0.5 ℃/s.

13. The method of claim 12, wherein, in step (7), the volume fraction of martensite after quenching to room temperature is 70% -90%.

Technical Field

The present invention generally relates to a strong and ductile automotive steel, and a method for manufacturing such an automotive steel.

Background

Lightweight automobiles are desirable for energy conservation, less greenhouse gas emissions, and other environmental friendliness. Therefore, lightweight automotive vehicles are an irreversible trend for the global automotive industry. This can be achieved by the extensive use of Advanced High Strength Steels (AHSS) in the automotive industry. AHSS is mainly used for manufacturing structural parts of automobiles such as B-pillars. Due to the high strength, AHSS including Dual Phase (DP) steel and quench distribution (Q & P) steel can use thinner plates than conventional steel, thereby achieving lighter vehicle weight without sacrificing passenger safety.

Currently, DP steel is the most widely used AHSS in the automotive industry. DP steels may be classified into different grades such as DP 580, DP780, and DP 980, depending on ultimate tensile strength. Thus, the strength of DP steel has reached a limit (<1 GPa). In other words, the contribution of DP steel to the weight reduction of automobiles has reached its limit. The root cause of the limited strength of DP steel is due to its soft ferritic matrix. In contrast, the hard martensitic matrix in Q & P steels may overcome this drawback. Therefore, Q & P steel has been the subject of intense research in the AHSS field. Currently, there are two commercial grades of Q & P steel, including Q & P980 and Q & P1180. The development of Q & P steels has made it possible to further reduce the weight of automobiles.

Currently, the aim of researchers is to further improve the strength of Q & P steels, for example to the 1500MPa level (Q & P1500). The manganese (Mn) content of current commercial Q & P steels is relatively low. For example, the Mn content in both Q & P980 and Q & P1180 is 3 wt% or less. It is well known that Mn element is a strong austenite stabilizer. Due to the low Mn content, the optimal quenching temperature ranges for both Q & P980 and Q & P1180 are 200-300 ℃. The dispensing temperature is generally higher than the quenching temperature. Therefore, the Q & P concept initially encounters significant difficulties in existing steel production lines. Furthermore, the strengths of Q & P980 and Q & P1180 are also close to their limits. Therefore, increasing the strength of Q & P steels is the next step in the steel industry. Alloy design plays a key role in improving the properties of Q & P steels. Currently, researchers tend to increase the Mn element and the carbon (C) element in Q & P steels. However, the Mn content in the proposed Q & P steel is mostly below 5 wt.%. As a result, researchers still cannot circumvent the high quench temperatures in Q & P steels.

Summary of The Invention

The invention relates to a new and advantageous automotive steel comprising an increased Mn content, and to a simple method for producing a strong and ductile automotive steel.

In one aspect of the present invention, there is provided a strong and ductile automotive steel including manganese (Mn) in a range of 8 to 11 wt%, carbon (C) in a range of 0.1 to 0.35 wt%, aluminum (Al) in a range of 1 to 3 wt%, vanadium (V) in a range of 0.05 to 0.5 wt%, and the balance iron (Fe), based on the weight of the automotive steel.

Preferably, the automotive steel comprises 9.5-10.5 wt.% Mn, 0.18-0.22 wt.% C, 1.8-2.2 wt.% Al, 0.08-0.12 wt.% V, and the balance Fe.

In an exemplary embodiment, a strong and ductile automotive steel comprises, in weight percent: 10 wt% Mn, 0.2 wt% C, 2 wt% Al, 0.1 wt% V and the balance Fe.

Preferably, the automotive steel further comprises at least one of the following elements: nickel (Ni) in the range of 0.1-2.0 wt.%, chromium (Cr) in the range of 0.2-2.0 wt.%, molybdenum (Mo) in the range of 0.1-0.5 wt.%, silicon (Si) in the range of 0.3-2.0 wt.%, boron (B) in the range of 0.0005-0.005 wt.%, niobium (Nb) in the range of 0.02-0.10 wt.%, titanium (Ti) in the range of 0.05-0.25 wt.%, copper (Cu) in the range of 0.25-0.50 wt.%, and rhenium (Re) in the range of 0.002-0.005 wt.%.

In another aspect of the present invention, there is provided a method of manufacturing an automotive steel, comprising: preparing a steel ingot containing manganese (Mn) in a range of 8-11 wt% and the balance Fe; providing a steel plate from a steel ingot; isothermally holding a steel sheet to form austenite; cooling the steel plate to room temperature; tempering the steel plate at the temperature of 300-400 ℃; and quenching the steel plate to room temperature.

Preferably, the step of providing a steel sheet is performed by at least one of casting, hot rolling, forging, and cold rolling.

Preferably, the isothermal holding is performed at a temperature of Ac3-20 ℃ to Ac3+100 ℃, where Ac3 is the temperature at which ferrite is completely transformed into the austenitic form.

Preferably, the step of isothermally holding is performed for 5 to 20 minutes.

Preferably, the room temperature is in the range of 10 ℃ to 40 ℃.

Preferably, the cooling step is performed by at least one of air, oil and water.

Preferably, the cooling step is performed at a first cooling rate higher than 0.5 ℃/s.

Preferably, the step of tempering the steel sheet is performed for 5 to 10 minutes.

Preferably, the step of quenching the steel sheet is performed at a second cooling rate higher than 0.5 ℃/s.

Preferably, the steel ingot further includes carbon (C) in the range of 0.1 to 0.35 wt%, aluminum (Al) in the range of 1 to 3 wt%, and vanadium (V) in the range of 0.05 to 0.5 wt%.

More preferably, the automotive steel comprises 9.5-10.5 wt.% Mn, 0.18-0.22 wt.% C, 1.8-2.2 wt.% Al, 0.08-0.12 wt.% V, and the balance Fe, based on the weight of the automotive steel.

Preferably, the steel ingot further includes at least one of nickel (Ni), chromium (Cr), molybdenum (Mo), silicon (Si), boron (B), niobium (Nb), titanium (Ti), copper (Cu), and rhenium (Re).

In a preferred embodiment, a method for manufacturing a strong and ductile automotive steel comprises the steps of:

(1) providing a steel ingot comprising 8-11 wt.% Mn, 0.1-0.35 wt.% C, 1-3 wt.% Al, 0.05-0.5 wt.% V, and the balance Fe;

(2) forging and rolling the ingot to provide a steel plate having a thickness of 4-6mm, and cooling the steel plate;

(3) batch annealing at 500-750 deg.C for 5-10 hr;

(4) pickling to remove an oxide layer in the steel sheet;

(5) cold-rolling the steel sheet to provide a cold-rolled steel sheet having a final thickness of 0.8-2 mm;

(6) treating the steel sheet by hot working to obtain a dual-phase microstructure with austenite embedded in a martensitic matrix, and cooling the steel sheet to room temperature at a cooling rate of more than 0.5 ℃/s, wherein the hot working route comprises isothermally holding the steel sheet in a temperature range of Ac3-20 ℃ to Ac3+100 ℃ for a period of 5-20 minutes to form part or all of the austenite; and

(7) the steel sheet is tempered in the temperature range of 300-400 ℃ for a period of 5-20 minutes and quenched to room temperature at a cooling rate of more than 0.5 ℃/s.

Preferably, the volume fraction of martensite after quenching to room temperature is in the range of 70% to 90%. The volume fraction (f) of martensite can be determined by the following equation f 1-exp (-C1(Ms-T)), where C1 is an empirical parameter, Ms is the martensite start temperature, and T is the temperature below the Ms temperature. T here is room temperature (10-40 ℃). The Ms temperature can be determined by the following equation: ms 539-423C-30.4Mn-17.7Ni-12.1Cr-7.5Mo-7.5Si (. degree. C.), wherein the elements in the equation are in mass percent.

Preferably, the steel sheet is cooled to room temperature by air, oil or water.

Preferably, the steel sheet is cooled to room temperature by water.

According to the invention, the quenching temperature is lowered to room temperature by increasing the Mn content in the proposed Q & P steel, while conventional low temperature tempering is employed to promote C partitioning. Thus, a strong and ductile Q & P steel is obtained. It would be a great advance in the automotive industry to produce strong and ductile Q & P steels by a simple room temperature quenching and low temperature tempering process.

Brief Description of Drawings

Many aspects of this embodiment can be better understood with reference to the following drawings. The components in the figures are not necessarily to scale, emphasis instead being placed upon clearly illustrating the principles of the present embodiments. Moreover, in the drawings, all the views are schematic and like reference numerals designate corresponding parts throughout the several views.

FIG. 1 is a schematic representation of a thermomechanical processing route for an automotive steel having a chemical composition of Fe-10Mn-0.2C-2Al-0.1V (in weight%), according to one embodiment of the present invention.

Fig. 2 shows an engineering stress-strain curve of an automotive steel according to an exemplary embodiment when isothermally held at 800 ℃ for 10 minutes in an air furnace.

Fig. 3 shows an engineering stress curve of an automotive steel according to an embodiment of the present invention when isothermally held at 850 ℃ for 10 minutes in an air furnace.

Fig. 4 shows an engineering stress curve of an automotive steel according to an embodiment of the present invention when isothermally held at 900 c for 10 minutes in an air furnace.

Detailed description of the invention

The present disclosure is illustrated by way of example, and not by way of limitation, in the figures of the accompanying drawings and in which like reference numerals refer to similar elements. It should be noted that references to "an" or "one" embodiment in this disclosure are not necessarily to the same embodiment, and such references may mean "at least one".

According to the invention, the strong and ductile automotive steel comprises, in weight percent: 8-11 wt% Mn, 0.1-0.35 wt% C, 1-3 wt% Al, 0.05-0.5 wt% V, and the balance iron.

In an exemplary embodiment, the strong and ductile automotive steel comprises, in weight percent: 10 wt% Mn, 0.2 wt% C, 2 wt% Al, 0.1 wt% V and the balance Fe.

According to the present invention, element C is effective in improving the strength of automotive steel. Meanwhile, C is a strong austenite stabilizer. In the present invention, the C content is selected to be 0.1 wt% or more to obtain these effects. However, when the C content is more than 0.35 wt%, the weldability of the automobile steel is lowered. Therefore, the C content is selected in the range of 0.1 to 0.35 wt%.

According to the invention, the Mn element is also a strong austenite stabiliser. Similarly, Mn element may provide solid solution strengthening to improve the strength of automotive steel. In order to obtain the appropriate amount of martensite and austenite volume fractions after quenching to room temperature, the Mn content in the automotive steel is selected to be 8 wt.% or more. However, the Mn content should not be higher than 11 wt.%, since a higher Mn content does not lead to a suitable amount of martensite and thus to the desired mechanical properties. Therefore, the Mn content is selected in the range of 8 wt% to 11 wt%.

According to the present invention, the element V can increase the strength of the automobile steel. Meanwhile, the V element can refine the austenite grain size, and the generated V precipitation can improve the delayed fracture resistance of the automobile steel. The amount of V is selected to be 0.05 wt% or more to obtain the above-described effects. However, the addition of V will increase the price of the steel. For the reasons described above, the V content is selected to be 0.05 wt% or more, but preferably 0.5 wt% or less.

According to the present invention, Al element can suppress cementite precipitation during the tempering process. In order to achieve this effect, the Al content is selected to be 1 wt% or more. However, if the Al content is more than 3 wt%, there is a high possibility of having large oxide inclusions and-ferrite, and resulting in poor ductility of the automobile steel. For the above reasons, the Al content is selected to be 1 wt% or more but 3 wt% or less.

In addition, the automotive steel may further comprise at least one of the following elements to improve properties: ni (0.1-2.0 wt%), Cr (0.2-2.0 wt%), Mo (0.1-0.5 wt%) and B (0.0005-0.005 wt%). These elements may be included to improve hardenability and low temperature toughness of the automotive steel. To achieve these effects, the amount of Ni and Mo should be higher than 0.1 wt%, the amount of Cr should be higher than 0.2 wt%, and the amount of B should be higher than 0.0005 wt%. However, when the Ni content or the Cr content is higher than 2 wt%, or when the Mo content is higher than 0.5 wt% or when the B content is higher than 0.005 wt%, a saturation effect occurs and the price of the automobile steel will also increase. Therefore, the amount of these elements should be kept below the above upper limit.

According to the invention, Nb (0.02-0.1 wt%) and Ti (0.05-0.25 wt%) may also be added to refine the prior austenite grain size. Ti can form TiN and suppress the formation of BN, so B atoms can improve the hardenability of the automotive steel. Preferably, the amount of Nb is higher than 0.02 wt% and the amount of Ti is higher than 0.05 wt%. However, when the Nb content is higher than 0.1 wt% or when Ti is higher than 0.25 wt%, a saturation effect occurs and the price of the automobile steel will also increase. Therefore, the amount of these elements should be kept below the above upper limit.

According to the present invention, Cu (0.25-0.50 wt%) is added to improve the strength of the automotive steel. In order to achieve this effect, the amount of Cu is selected to be 0.25 wt% or more. However, when the amount of Cu is more than 0.5 wt%, the steel will have poor hot rolling properties and the weldability will decrease. Therefore, the amount of Cu should be kept below the above upper limit.

According to the present invention, Si (0.3-1.0 wt%) is added to improve oxidation resistance and corrosion resistance of the automotive steel. Si element can also inhibit precipitation of cementite during the tempering process. To achieve this effect, the amount of Si is selected to be 0.3 wt% or more. However, when the amount of Si is above 1.0 wt.%, the steel will have a strong oxide layer, which will embed into the surface during the hot rolling process. Thus, the surface quality, hot ductility, weldability and fatigue properties will decrease. Therefore, the amount of Si should be kept below the above upper limit.

According to the invention, Re (0.25 to 0.50 wt.%) is added to improve the morphology and size distribution of the particles in the automotive steel. To achieve this effect, the amount of Re is selected to be 0.002 wt% or more. However, when the amount of Re is higher than 0.005 wt%, a saturation effect will occur and the price of automobile steel will also increase. Therefore, the amount of Re should be kept below the above upper limit.

According to the present invention, the steel ingot may be cast, hot rolled or cold rolled to manufacture automobile steel. For the casting technique, continuous casting is preferably used to produce slabs. For hot rolling, it is preferable that the slab is heated at a temperature of 1100-1250 ℃ and hot-rolled to a thickness of 50-80mm by 5-20 passes to manufacture a thick hot-rolled plate or has a thin hot-rolled plate by further hot-rolling to a thickness of 4-10mm by 7-10 passes. For cold rolling, batch annealing at a temperature of 500-750 ℃ for 5-10 hours is preferably employed to soften the hot rolled sheet. Cold rolling in 5-12 passes to provide a cold rolled sheet with a final thickness of 0.8mm to 2 mm. If the hot rolled sheet can be cold rolled directly to a target thickness (0.8mm to 2mm) after pickling, the previous batch annealing step can be eliminated to save energy and cost. Other conventional thermomechanical processing techniques in the steel industry, such as forging and galvanizing, may also be used herein to make automotive steel.

After the steel plate is obtained, a hot working route is adopted to obtain a dual-phase microstructure with austenite embedded in a martensite matrix. The steel sheet is isothermally held in a temperature range of Ac3-20 ℃ to Ac3+100 ℃ for a period of 5 to 20 minutes to form part or all of the austenite. Ac3 refers to the temperature at which ferrite completely transforms into austenite. This process may be employed after cooling the hot rolled product to room temperature or directly after the hot rolling process. The plate is then cooled to room temperature at a cooling rate higher than 0.5 ℃/s. The cooling medium may be water, oil, air or other conventional cooling medium in the steel industry. According to the chemical composition of the present invention, after quenching to room temperature, there is a large amount of martensite with some retained austenite and/or a small amount of ferrite.

The steel sheet is then tempered at a temperature in the range of 300 to 400 c for a period of 5-10 minutes and finally quenched to room temperature at a cooling rate higher than 0.5 c/s. The cooling medium may be water, oil, air or other conventional cooling medium in the steel industry. The tempering process serves to distribute C from martensite to retained austenite so that the austenite may have appropriate mechanical stability, and to provide a continuous transformation induced plasticity (TRIP) effect to improve ductility of the automotive steel. In addition, the tempering process facilitates mitigating martensitic transformation induced residual stresses during quenching to room temperature. Zn coatings using dip Galvanized (GI) or alloyed hot dip Galvanized (GA) can be used to make galvanized (galvannealed) or alloyed galvanized (galvannealed) steel sheets for automotive applications. In addition, steel sheets without Zn coating can also be used for automotive applications, according to the requirements of the automotive industry. It is worth mentioning that the chemical composition should be designed to have a volume fraction of martensite of 70% -90% after quenching to room temperature. If the volume fraction of martensite is below 60%, the Mn content should be reduced. Reducing the C content to obtain a larger volume fraction of martensite is undesirable, as reducing the C content will significantly reduce the strength of the martensitic matrix. If the volume fraction of martensite is higher than 90%, the Mn content and/or the C content should be increased. For reasons previously related to the strength of martensite, it is preferred to increase the C content to obtain a less martensitic matrix. After quenching to room temperature, the volume fraction (f) of martensite in automotive steels with different Mn and C contents can be determined by the following equation f 1-exp (-C1(Ms-T)), where C1 is an empirical parameter obtained from a large statistical number and can be selected to be-0.011, Ms is the martensite start temperature, and T is a temperature below the Ms temperature, where T is room temperature (10-40 ℃). The Ms temperature can be determined by the following equation: 539-423C-30.4Mn-17.7Ni-12.1Cr-7.5Mo-7.5Si (. degree. C.), wherein the elements are calculated by mass percent.

Best Mode for Carrying Out The Invention

The following are examples illustrating the practice of the method of the present invention. These examples should not be construed as limiting.