Dual-phase steel and its manufacturing method

Abstract

The present invention discloses a dual-phase steel containing 90% or more Fe and unavoidable impurities, as well as the following mass percents: C: 0.09%-0.11%, Si: 0.1%-0.3%, Mn: 1.4%-1.6%, Al: 0.01%-0.03%, Nb: 0.01%-0.03%, Ti: 0.01%-0.03%, and B: 0.0020%-0.0030%. By rationally controlling the chemical composition of the steel, the present invention has obtained a dual-phase steel that combines low cost with high mechanical performance. The present invention also discloses a method for producing the above dual-phase steel.

Description

[Technical Field]

[0001] The present invention relates to the field of metallurgy, and in particular to dual-phase steels and methods for producing same. [Background technology]

[0002] With the global energy crisis and intensifying environmental issues, energy conservation and safety have become the main development directions of the automobile manufacturing industry. Among them, reducing vehicle weight is one of the measures to save energy and reduce emissions. High-strength duplex stainless steel has good mechanical properties and service performance, so it is widely used in the production of vehicle structural parts.

[0003] With the development of ultra-high strength steels and the changes in the current market, ultra-high strength steels are expected to combine low cost with high performance. Currently, 780DP (duplex) steel is still the mainstream applied steel, accounting for about 60% of all duplex steels, and is widely used in various types of structural and safety components.

[0004] Canadian Patent Document CA2526488 (published on December 2, 2004) discloses a cold-rolled steel sheet having the following chemical composition: C: 0.05-0.09%; Si: 0.4-1.3%; Mn: 2.5-3.2%; Mo: 0.05-0.5% or Ni: 0.05-2% may be optionally added; P: 0.001-0.05%; S≦0.08*Ti-3.43*N+0.004; N≦0.006%; Al: 0.005-0.10%; Ti: 0.001-0.045%; Nb≦0.04% or B: 0.0002-0.0015% may be optionally added, and Ca may be added for processing; the remainder being Fe and unavoidable impurities. The steel plate had a bainite content of over 7% and a Pcm of 0.3. It was hot-rolled at temperatures above Ar3, coiled at temperatures below 700°C, cold-rolled, annealed between 700 and 900°C, and rapidly cooled from 550 to 700°C, resulting in a high-strength steel with a final strength of 780 MPa or higher. This steel is characterized by strong local deformation capacity and low hardness in the weld zone. However, the steel's high Mn content in the design caused severe banding and resulted in inconsistent mechanical properties. Furthermore, the addition of high Mn content also resulted in a relatively high Si content, which was detrimental to the surface quality and weldability of the steel.

[0005] US Patent Document US20050167007 (published on August 4, 2005) discloses a high-strength steel sheet having the following chemical composition: C: 0.05-0.13%, Si: 0.5-2.5%, Mn: 0.5-3.5%, Cr: 0.05-1%, Mo: 0.05-0.6%, Al≦0.1%, S≦0.005%, N≦0.01%, P≦0.03%; and at least one of Ti: 0.005-0.05%, Nb: 0.005-0.05%, and V: 0.005-0.2% may be selectively added. The steel was hot-rolled at a temperature of Ar3 or higher, coiled at 450-700°C, annealed, cooled and quenched at a rate of 100°C / s between 700-900°C, and tempered at 180-450°C, resulting in a high-strength steel with a tensile strength of 780 MPa and a hole expansion ratio of over 50%. The main problem with this steel is that the total alloy content is too high, resulting in a high Si content, which is unfavorable for the weldability and phosphate treatability of the steel.

[0006] Chinese Patent Document CN101363099A (published February 11, 2009) discloses an ultra-high-strength dual-phase steel containing 0.14-0.21% C, 0.4-0.9% Si, 1.5-2.1% Mn, ≤0.02% P, ≤0.01% S, 0.001-0.05% Nb, 0.001-0.02% V, and the balance being Fe and unavoidable impurities. This high-strength dual-phase steel was obtained by hot-rolling and cold-rolling, followed by a temperature hold between 760 and 820°C, a cooling rate of 40-50°C / s, and an aging time of 180-300 seconds at 240-320°C. However, the carbon equivalent of this steel was designed to be high, and it lacked the characteristics of well-balanced performance.

[0007] Chinese Patent Document CN103060703A discloses a 780 MPa grade cold-rolled dual-phase steel sheet, the microstructure of which is a fine equiaxed ferrite matrix with uniformly distributed martensite islands on the ferrite matrix, with the following chemical element contents (by mass): C: 0.06-0.1%, Si≦0.28%, Mn: 1.8-2.3%, Cr: 0.1-0.4%; if Cr≧0.3%, no Mo is added; if Cr<0.3%, Mo=0.3-Cr; Al: 0.015-0.05%; at least one of Nb and Ti is used, with Nb+Ti being in the range of 0.02-0.05%, and the balance being Fe and unavoidable impurities. The 780 MPa grade cold-rolled dual-phase steel sheet has high strength, good elongation, good phosphating properties, and small mechanical property anisotropy. However, the alloy of the invention is designed to contain a large amount of alloying elements such as Cr and Mo, which is disadvantageous for cost reduction.

[0008] For this reason, while some conventional dual-phase stainless steels have excellent formability, they contain high C or Si contents, or high alloy contents such as Cr, Ni, and Mo, which result in poor weldability, surface quality, and phosphate treatment performance, and are expensive. On the other hand, some steels with high Si contents have a high hole expansion ratio and good bending performance, but a high yield ratio and poor press performance. To date, no 780 MPa dual-phase stainless steel has been developed that combines high mechanical performance with low cost. Summary of the Invention [Problem to be solved by the invention]

[0009] In order to solve the above problems, the present invention provides a dual-phase steel that achieves both low cost and excellent mechanical properties through rational element composition design. [Means for solving the problem]

[0010] In a first aspect of the present invention, there is provided a dual-phase steel containing, in mass percent, 90% or more of Fe and inevitable impurities, as well as 0.09% to 0.11% of C, 0.1% to 0.3% of Si, 1.4% to 1.6% of Mn, 0.01% to 0.03% of Al, 0.01% to 0.03% of Nb, 0.01% to 0.03% of Ti, and 0.0020% to 0.0030% of B. In a second aspect of the present invention, there is provided a dual-phase steel containing, in mass percent, 0.09% to 0.11% C, 0.1% to 0.3% Si, 1.4% to 1.6% Mn, 0.01% to 0.03% Al, 0.01% to 0.03% Nb, 0.01% to 0.03% Ti, 0.01% to 0.03% B, and the balance being Fe and unavoidable impurities. Preferably, the duplex stainless steel of the present invention is free of Mo and Cr.

[0011] The chemical composition of the dual-phase steel of the present invention is primarily C-Si-Mn, with trace amounts of the high-hardenability element B added to further reduce the Mn content, and trace amounts of Nb and Ti added to suppress austenite grain growth and effectively refine the crystal grains. By using the above-described appropriate chemical composition design, the present invention can obtain dual-phase steel with 80 kgf strength that combines low cost with excellent mechanical properties without the addition of valuable alloying elements such as Mo and Cr.

[0012] In the present invention, the design principles of each chemical element are as follows: C: The addition of carbon can increase the strength of steel and the hardness of martensite in steel. If the carbon content in steel is less than 0.09%, the strength of the steel sheet is affected and the amount and stability of austenite are adversely affected. On the other hand, if the carbon content in steel exceeds 0.11%, the hardness of martensite becomes too high and the grain size becomes coarse, which is adverse to the formability of the steel sheet. Furthermore, the carbon equivalent becomes too high, which is adverse to welding use of the steel. Therefore, in the present invention, the carbon content is controlled to be between 0.09% and 0.11%.

[0013] Si: Adding Si to steel improves hardenability. Furthermore, Si dissolved in steel affects dislocation interactions, increases the work hardening rate, and moderately increases the elongation of dual-phase stainless steel, helping to achieve good formability. However, it should be noted that too high a Si content in steel can be detrimental to surface quality control. Therefore, in the present invention, the Si content is controlled to between 0.1% and 0.3%.

[0014] Mn: The addition of Mn contributes to improving the hardenability of steel and effectively increases the strength of steel sheets. If the Mn content in steel is less than 1.4%, the strength of the steel sheet is insufficient; if the Mn content in steel is more than 1.6%, the strength of the steel sheet is too high, resulting in poor formability. Therefore, in the present invention, the Mn content is controlled to be between 1.4% and 1.6%.

[0015] B: The addition of B contributes to improving the hardenability of steel and effectively increases the strength of steel sheets. If the B content in steel is less than 0.0020%, the strength of the steel sheet will be insufficient; if the B content in steel is more than 0.0030%, the strength of the steel sheet will be too high, resulting in reduced formability. Therefore, in the present invention, the B content is controlled to be between 0.0020% and 0.0030%.

[0016] Al: Adding Al element to steel can exert deoxidizing effect and grain refinement effect. On the other hand, the lower the Al content, the more advantageous it is for smelting malleability. In the present invention, the Al content is controlled to be between 0.01% and 0.03%.

[0017] Nb: Nb is a strong carbide-forming element that refines grains. Adding a small amount of Nb to microalloy steels during controlled rolling significantly reduces the recrystallization temperature of deformed austenite through grain pinning and subgrain boundary pinning, providing nucleation sites and significantly affecting grain refinement. During continuous annealing and austenitization, the soaked undissolved carbide / nitride sites prevent grain coarsening through the grain boundary pinning mechanism, thereby effectively refining grains. However, the Nb content should not be too high, otherwise production costs will increase. Therefore, in this invention, the Nb content is controlled between 0.01 and 0.03%, preferably between 0.015 and 0.025%.

[0018] Ti: Similar to the effect of Nb, Ti, a strong carbide-forming element added to steel, also has the effect of strongly suppressing the growth of austenite grains at high temperatures, contributing to grain refinement. Furthermore, adding a large amount of Ti increases production costs. Therefore, in the present invention, the Ti content is controlled between 0.01 and 0.03%, preferably between 0.015 and 0.025%.

[0019] The present invention, by designing the composition of the above-mentioned dual-phase steel, eliminates the need to add valuable alloying elements such as Mo and Cr, ensuring economic efficiency. The dual-phase steel of the present invention requires sufficient alloying elements of C, Mn, and B to provide the dual-phase steel with sufficient hardenability and ensure high strength of 80 kgf / cm2 at continuous annealing gas cooling rates of 40-100°C / s. However, the contents of alloying elements C, Mn, and B in the dual-phase steel must not be too high, otherwise the resulting dual-phase steel will have poor weldability and formability.

[0020] Preferably, in the dual-phase steel of the present invention, the Nb content is 0.015% to 0.025% and / or the Ti content is 0.015% to 0.025%, but if the Nb and Ti contents are too low, the corresponding grain refinement effect is not significant, but if the Nb and Ti contents are too high, the cost increases.

[0021] Preferably, the hardenability factor Y of the duplex steel of the present invention Q satisfies 1.9≦YQ≦2.1, and is calculated by the following formula: Y Q = Mn + 200 × B, where Mn and B represent the mass percent content of the corresponding element, preceded by the percent sign. Q indicates the combined effect of b and Mn in the steel, and the hardenability factor Y Q By controlling Y within the above range, it is possible to reduce costs and further improve the mechanical properties, particularly the strength, of the dual-phase stainless steel. Q If the value is less than 1.9, the strength of the resulting steel will not reach the 80 kg class; Q If the value exceeds 2.1, the elongation of the corresponding steel cannot meet the requirement. In addition, since the Mn content is the maximum equivalent that affects the overall cost in alloy design, in the present invention, by adding an appropriate amount of B, the Mn content can be further reduced, which is advantageous for cost reduction, and at the same time, the combined hardenability of Mn and B can be utilized to further improve the mechanical properties of the dual-phase steel, which is also advantageous for improving the processing performance in on-site production, including the rolling stability in hot rolling and cold rolling.

[0022] Preferably, in the dual-phase steel of the present invention, the mass percent contents of the impurity elements satisfy P≦0.015%, S≦0.003%, N≦0.005%.

[0023] P, N, and S are all unavoidable impurity elements in steel, and the lower the P, N, and S element contents in steel, the better the performance of the steel. Specifically, MnS formed by S has a significant effect on forming performance, and N tends to cause cracks and bubbles on the slab surface. Therefore, in the dual-phase steel described in the present invention, the P content is controlled to ≦0.015%, S content ≦0.003%, and N content ≦0.005%.

[0024] In the present invention, the microstructure of the dual-phase steel comprises martensite and ferrite, and preferably the microstructure of the dual-phase steel of the present invention consists of martensite and ferrite, with the volume percent content of martensite being 55% to 85%, preferably 58-80%. The content of martensite in the steel structure of the present invention is directly related to the strength of the dual-phase steel, and in order to harmonize the hard and soft phases during deformation of the steel and improve the overall performance of the steel, some ferrite must be present in the steel.

[0025] Preferably, the average grain size of both martensite and ferrite in the dual-phase steel of the present invention is 5 μm or less, and more preferably, the grain size of both martensite and ferrite in the dual-phase steel is 5 μm or less, which contributes to improving the strength and workability of the steel.

[0026] Preferably, the duplex stainless steel of the present invention is an 80 kg grade duplex stainless steel having a yield strength of ≥ 420 MPa, preferably ≥ 430 MPa, a tensile strength of ≥ 800 MPa, preferably ≥ 820 MPa, and an A50 gage elongation at break of ≥ 18%, preferably ≥ 19%. In some embodiments, the duplex stainless steel of the present invention has a yield strength of ≥ 450 MPa, a tensile strength of ≥ 820 MPa, and an A50 gage elongation at break of ≥ 20%.

[0027] "80 kg class" as used herein means that the steel of the present invention can withstand a force of 80 kg or more per square centimeter. In some embodiments, the steel of the present invention can withstand a force of 80-90 kg per square centimeter, preferably 83-90 kg per square centimeter.

[0028] Another aspect of the present invention provides a method for producing the above-mentioned dual-phase steel, comprising the steps of: smelting and continuous casting molten steel to obtain a continuously cast slab; hot rolling the continuously cast slab; cold rolling; annealing; tempering; and leveling.

[0029] Preferably, in the annealing step, the annealing soaking temperature is controlled to 825 to 855°C, and the annealing time is controlled to 40 to 200 seconds, and then the steel is cooled at a rate of 3 to 5°C / s to a quenching start temperature of 735 to 760°C, and then quenched at a rate of 40 to 100°C / s (e.g., 40 to 80°C / s), with the quenching end temperature being 220 to 260°C.

[0030] Preferably, the annealing soaking temperature is 830 to 840° C. When the annealing soaking temperature is within the above range, the crystal grain size of the obtained dual-phase steel becomes finer, and the mechanical properties and formability become more excellent.

[0031] In the hot rolling step, the continuously cast slab is first heated to 1160-1190°C and kept at this temperature for 150 minutes or more (for example, 150-210 minutes), then subjected to final hot rolling at 850-890°C, followed by quenching at a rate of 30-80°C / s after rolling; the coiling temperature is controlled to 500-540°C, and the slab is air-cooled after coiling. The 80 kgf / s dual-phase steel of the present invention can meet production requirements without requiring slow cooling or other treatments for the hot rolling rolls.

[0032] Preferably, in the cold rolling step, the reduction ratio of the cold rolling is 50 to 70%. Preferably, the tempering temperature is 220-260°C and the tempering time is 100-400 seconds. A long tempering time is advantageous for reducing the hardness difference between the ferrite and martensite phases of the dual-phase steel. However, the tempering time should not be too long, otherwise the resulting steel will have a strength of less than 80 kgf / cm2.

[0033] Preferably, in the leveling step, the leveling reduction is ≦0.3%. The present invention employs a rational chemical composition design and optimizes the manufacturing process to obtain a dual-phase steel that combines low cost with excellent performance (especially high strength and excellent elongation) without adding Mo or Cr. The dual-phase steel is an 80 kgf / mm² grade dual-phase steel, and its microstructure includes martensite and ferrite. Its yield strength is ≥ 420 MPa, tensile strength is ≥ 800 MPa, and A 50 Gauge elongation at break ≥ 18%. [Brief explanation of the drawings]

[0034] [Figure 1] 1 is a microstructure photograph of the dual-phase steel of Example 1 of the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0035] The dual-phase steel and the manufacturing method thereof according to the present invention will be further described below with reference to specific examples, but the following description is for the purpose of explaining the present invention and is not intended to limit the technical scope of the present invention to the scope of the description. [Example]

[0036] Examples 1 to 5 and Comparative Examples 1 to 14 The dual-phase steels of Examples 1-5 of the present invention were manufactured by the following steps: (1) Based on the composition shown in Table 1 below, molten steel was smelted and continuously cast to produce continuous cast slabs; (2) The continuously cast slab is hot-rolled, where the continuously cast slab is first heated to 1160-1190°C, kept at that temperature for 150 minutes or more, and then hot-rolled to a final rolling temperature of 850-890°C. After rolling, the slab is rapidly cooled at a rate of 30-80°C / s, and then coiled. The coiling temperature is 500-540°C, and the slab is air-cooled after coiling; (3) Cold rolling: The cold rolling reduction was controlled to 50-70%; (4) Annealing: The annealing temperature is 825-855°C, the annealing time is 40-200s, and then the quenching is started at a rate of 3-5°C / s to a quenching start temperature of 735-760°C, and then quenched at a rate of 40-100°C / s to a quenching end temperature of 220-260°C; (5) Tempering: Tempering temperature is 220-260 o C, and the tempering time was set to 100 to 400 seconds. (6) Leveling: Leveling reduction rate ≦0.3%. The steels of Comparative Examples 1 to 14 were also produced in substantially the same manner as the above-described Examples of the present invention, with the compositions shown in Table 1 below, except that the chemical element contents or one or more manufacturing process parameters of Comparative Examples 1 to 14 did not satisfy the requirements of the present invention. The 80 kg class dual phase steel of the present invention means that the microstructure of the steel of the present invention comprises two phases and the steel of the present invention can withstand 80 kg per square centimeter. Table 1 shows the chemical compositions of the dual-phase steels of Examples 1 to 5 and the steels of Comparative Examples 1 to 14 and the corresponding hardenability factors Y Q Indicates the value of

[0037] [Table 1]

[0038] Tables 2-1 and 2-2 show the specific process parameters for the dual-phase steels of Examples 1-5 and the steels of Comparative Examples 1-14.

[0039] [Table 2-1]

[0040] [Table 2-2]

[0041] In Table 2-2, the quenching end temperature for each example and comparative example is the same as the tempering temperature, because in actual process operations, tempering is performed immediately after the quenching operation is completed.

[0042] The resulting dual-phase steels of Examples 1-5 and Comparative Examples 1-14 were each sampled to obtain corresponding samples. Tensile tests and Charpy impact tests were conducted on the resulting steel sample plates to obtain performance data for the steels of the Examples and Comparative Examples. The test results for each of the Examples and Comparative Examples are shown in Table 3.

[0043] Table 3 shows the performance test results of the dual-phase steels of Examples 1-5 and the steels of Comparative Examples 1-14.

[0044] [Table 3]

[0045] As can be seen from Table 3, Examples 1 to 5 of the present invention have excellent overall performance, with yield strengths of 420 MPa or more, tensile strengths of 800 MPa or more, and A 50 The gauge breaking elongation is ≥ 18%. The dual-phase steels of each example have a tensile strength exceeding 800 MPa and good elongation without adding valuable alloying elements such as Mo or Cr. The overall performance of the dual-phase steels of Examples 1-5 of the present invention is clearly superior to that of Comparative Examples 1-14.

[0046] The measurement method for each property in Table 3 is according to GB / T228-2010 room temperature tensile test method for metallic materials, and A 50 The gauge elongation at break refers to a tensile specimen with a parallel length x width of 50 mm x 25 mm.

[0047] As is clear from Table 1-3, compared to the steels of Comparative Examples 1-14, the steels of Examples 1-5 of the present invention have chemical compositions within the required range, and by combining optimized process parameters, dual-phase steels that combine low cost with high performance were obtained.

[0048] For Examples 1-5 and Comparative Examples 1-14, the microstructures of the dual-phase steels etched with 4% (volume fraction) nitric acid alcohol were observed using an optical microscope, and the volume fraction and size of martensite and ferrite in the steel were measured using image analysis software. The microstructure of Example 1 is shown in Figure 1. As can be seen from Figure 1, the microstructure of the dual-phase steel of Example 1 of the present invention contains martensite and ferrite, with the proportion of martensite in the figure being 63%. The structure of the dual-phase steel of the present invention is uniform, and the percentage of martensite shown in any cross-section of the steel can be considered as the volume percentage content of martensite in the steel. That is, the volume fraction of the dual-phase steel in Example 1 is 63%, and the average grain size of martensite and ferrite are both 5 μm or less.

[0049] Table 4 shows the microstructures of the steels of Examples 1-5 and Comparative Examples 1-14, the volume fractions of martensite in the steels, and the average grain sizes of martensite and ferrite in the steels.

[0050] [Table 4]

[0051] TIFF2025528243000007.tif100170

[0052] As is clear from Table 4, the dual-phase steels of Examples 1-5 have martensite and ferrite structures, and the volume fraction of martensite in the steel is within the range of 55% to 85% specified in the present invention, and the average grain size of both martensite and ferrite is 5 μm or less. The steel of Comparative Example 1-14 also has martensite and ferrite structures, but because its chemical composition or manufacturing process does not satisfy the conditions specified in the present invention, the microstructure expected in the present invention is not obtained, and the volume fraction of martensite is outside the range specified in the present invention.

[0053] As described above, the present invention provides a dual-phase steel that achieves both low cost and high performance by combining a rational chemical composition design with an optimized process.

[0054] It should be noted that all technical features described herein can be freely combined or combined in any manner as long as they are not mutually inconsistent. It will be apparent to those skilled in the art that various modifications and variations can be made to the present invention without departing from the scope of the present invention. For example, features shown or described as part of one embodiment may be used with another embodiment to produce yet another embodiment. Therefore, it is intended that the present invention cover such modifications and variations provided they come within the scope of the appended claims and their equivalents.

Claims

1. A dual-phase steel characterized by containing 90% or more Fe and inevitable impurities, and also containing, in mass percent, C: 0.09% to 0.11%, Si: 0.1% to 0.3%, Mn: 1.4% to 1.6%, Al: 0.01% to 0.03%, Nb: 0.01% to 0.03%, Ti: 0.01% to 0.03%, and B: 0.0020% to 0.0030%.

2. A dual-phase steel comprising, in mass percent, C: 0.09% to 0.11%, Si: 0.1% to 0.3%, Mn: 1.4% to 1.6%, Al: 0.01% to 0.03%, Nb: 0.01% to 0.03%, Ti: 0.01% to 0.03%, B: 0.0020% to 0.0030%, and the balance being Fe and unavoidable impurities.

3. 3. The dual-phase steel according to claim 1, wherein said dual-phase steel does not contain Mo or Cr.

4. The hardenability factor Y of the above dual-phase steel Ｑ is 1.9≦Y Ｑ ≦2.1, and Y Ｑ 3. A duplex steel according to claim 1, characterized in that Mn and B represent the numerical values ​​before the percent sign of the mass percent content of the corresponding element.

5. 3. The dual-phase steel according to claim 1, wherein the mass percent contents of impurity elements satisfy the following: P≦0.015%, S≦0.003%, N≦0.005%.

6. 3. A dual-phase steel according to claim 1 or 2, characterized in that the microstructure of said dual-phase steel comprises, preferably consists of, martensite and ferrite, and more preferably the volume percentage content of said martensite is ≥ 55% and ≤ 85%, preferably 58-80%.

7. 7. The dual-phase steel according to claim 6, wherein the average grain size of the martensite and the average grain size of the ferrite are both 5 μm or less; preferably, the grain sizes of the martensite and the ferrite are both 5 μm or less.

8. The above-mentioned dual-phase steel is an 80 kg class dual-phase steel, with a yield strength of 420 MPa or more, a tensile strength of 800 MPa or more, and an A ５０ 3. The dual-phase steel according to claim 1, characterized in that it has a gauge breaking elongation of ≥ 18%.

9. 3. The duplex stainless steel of claim 1, wherein the duplex stainless steel has a yield strength of ≥ 450 MPa, a tensile strength of ≥ 820 MPa, an A50 gauge elongation at break of ≥ 20%, and is capable of withstanding a force of 83-90 kg per square centimeter.

10. A method for producing a dual-phase steel according to any one of claims 1 to 9, characterized in that it comprises the steps of: 1) Smelting and continuous casting of molten steel to produce continuous cast slabs; 2) hot rolling the continuously cast slab; 3) cold rolling; 4) Annealing; 5) tempering; and 6) By leveling, a dual phase steel is obtained.

11. The method of claim 10, wherein the annealing step is performed at a soaking temperature of 825-855°C for a duration of 40-200 seconds, followed by cooling at a rate of 3-5°C / s to a quenching start temperature of 735-760°C, and then quenching at a rate of 40-100°C / s to a quenching end temperature of 220-260°C; preferably, the soaking temperature is 830-840°C.

12. 12. The method according to claim 10 or 11, wherein in the hot rolling step, the continuously cast slab is first heated to 1160 to 1190°C, kept at that temperature for 150 minutes or more, and then subjected to final hot rolling at 850 to 890°C, and after rolling, is quenched at a rate of 30 to 80°C / s; and then coiled at a coiling temperature of 500 to 540°C, and is air-cooled after coiling.

13. 12. The method according to claim 10, wherein in the cold rolling step, the reduction ratio of the cold rolling is 50 to 70%.

14. The manufacturing method according to claim 10 or 11, wherein in the tempering step, the tempering temperature is 220 to 260°C, and the tempering time is 100 to 400 seconds.

15. 12. The method according to claim 10, wherein in the leveling step, the leveling reduction is ≦0.3%.

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