Nano-precipitate strengthened ultra-high strength steel and manufacturing method thereof

Abstract

A nano-precipitate strengthened ultra-high strength steel and a method for producing the same, wherein the ultra-high strength steel contains the following components in weight percent: C: 0.15-0.21%, Si: ≦0.50%, Mn: 0.60-1.60%, Ti: 0.051-0.15%, V: 0.040-0.12%, Cr: 0.20-1.20%, B: 0.0005-0.0030%, Al: 0.02-0.06%, Ca: 0.0005-0.004%, N: ≦0.005%, P: ≦0.020%, S: ≦0.0050%, O: ≦0.0040%, Ti+V≧0.11%, the balance being Fe and unavoidable impurities, and the nano-precipitate control index NPI is 5-26. The present invention also provides a method for producing the above ultra-high strength steel.

Description

[Technical Field]

[0001] The present invention relates to the technical field of ultra-high strength steel, and more particularly to nano-precipitate strengthened ultra-high strength steel and a manufacturing method thereof. [Background technology]

[0002] Hot-rolled ultra-high-strength steel for engineering machinery with a yield strength of 960 MPa is primarily used in the manufacture of components such as crane booms and pump truck placement booms, which place high demands on the steel's strength, plasticity, low-temperature toughness, and fatigue performance. Conventional 960 MPa ultra-high-strength steel is typically manufactured using offline quenching and tempering or online quenching and tempering processes, resulting in a tempered martensite structure. However, tempered martensite exhibits low plasticity, leading to the risk of cracking during subsequent processing, such as bending and hole expansion. For example, the cold bending performance of 960 MPa ultra-high-strength steel manufactured using conventional processes can only meet D = 5-6a at 90° and elongation ≤ 16%. High-temperature tempering can enhance the plasticity of ultra-high-strength steel, but at the expense of reduced strength.

[0003] Chinese patent CN103014538B discloses an ultra-high tensile steel with a yield strength of 960 MPa, which is produced by an online quenching + tempering process at 510-550°C and has a tempered martensite structure.

[0004] Chinese Patent CN102134680A discloses a method for producing a high-tensile steel with a yield strength of 960 MPa, which has a low carbon content and a high Cr content of 0.07-0.09% C and 1.05-1.15% Cr. The steel does not contain micro-alloying elements such as Nb, Ti, or V, but has a relatively high Cr content.

[0005] Chinese patent CN102560274A developed an ultra-high-strength steel with a yield strength of 1000 MPa through offline heat treatment, and the microstructure is tempered martensitic. The main components are Cr: 0.30-0.50%, Mo: 0.30-0.50%, Ni: 0.20-0.50%, and V: 0.030-0.050%.

[0006] Chinese patent CN102505096A discloses that tempered martensitic ultra-high strength steel is obtained by online quenching and tempering at 460~520℃. Summary of the Invention

[0007] overview In order to solve the above technical problems existing in the prior art, the present invention provides a nano-precipitate strengthened ultra-high strength steel and a manufacturing method thereof. Preferably, the ultra-high strength steel has a microstructure of tempered sorbite + numerous nano-precipitates. More preferably, the steel has a yield strength of ≥ 1000 MPa, a tensile strength of ≥ 1050 MPa, and an elongation A of ≥ 19%. 50 , and has an impact energy at -60°C of ≥ 90J.

[0008] A first aspect of the present invention is a steel sheet containing the following components in weight percent: C: 0.15 to 0.21%, Si: ≦0.50%, Mn: 0.60 to 1.60%, Ti: 0.051 to 0.15%, V: 0.040 to 0.12%, Cr: 0.20 to 1.20%, B: 0.0005 to 0.0030%, Al: 0.02 to 0.06%, Ca: 0.0005 to 0.004%, N: ≦0.005%, P: ≦0.020%, S: ≦0.0050%, O: ≦0.0040%, with the balance being Fe and unavoidable impurities; and The following formula: NPI = (Mo + W + 2.3 * a nanoprecipitate control index NPI of 5 to 26, calculated by the formula (Ti+V) / (Ti+V) where each chemical element in the formula represents the numerical value before the percent sign of the mass percent of the corresponding chemical element; and Ti+V≧0.11% Provide steel.

[0009] Preferably, the balance of the chemical composition of the steel is Fe and unavoidable impurities.

[0010] Preferably, the steel further contains one or more elements selected from Nb, Mg, Ni, Cu, Mo, and W, where Nb: 0-0.060%, Mg: 0-0.003%, Ni: 0-0.30%, Cu: 0-0.40%, Mo: 0-0.40%, and W: 0-0.30%.

[0011] Preferably, the steel according to the invention has a microstructure of tempered sorbite and nanoprecipitates; wherein the tempered sorbite has a prior austenite grain size (i.e., initial austenite grain size) of 5 to 10 μm, the nanoprecipitates include TiC and VC precipitates, and the nanoprecipitates have a size of 2 to 5 nm.

[0012] Preferably, the steel according to the invention has a yield strength of ≥ 1000 MPa, a tensile strength of ≥ 1050 MPa, an elongation A of ≥ 19% 50 and an impact energy at -60°C of ≥ 90 J. More preferably, the steel has the following properties: yield strength of 1000-1115 MPa, tensile strength of 1050-1172 MPa, elongation A of 19-25%. 50 and an impact energy at -60°C of 90 to 129 J.

[0013] Unless otherwise specified, the content of each element in the steel composition of the present invention is expressed in weight percent. In the composition design of the steel according to the present invention, the effect of each element is as follows:

[0014] Carbon: C exerts a solid solution strengthening effect and can adjust the strength, plasticity, and toughness of the martensite structure. Experimental results show that the relationship between the tensile strength and carbon content of low-carbon martensite after reheating and quenching is as follows: Rm = 2510 × C + 790 (empirical formula, the value before the percent sign for the weight percent of C is replaced by the weight percent of the steel for calculation, unit: MPa). After quenching, tempering is applied to further adjust the strength, plasticity, and toughness. Excessive C content increases the overall C equivalent, making the steel more susceptible to cracking during welding. The C content in this invention is 0.15-0.21%.

[0015] Silicon: A certain amount of Si can play a better role in deoxidation, and at the same time can inhibit the precipitation of carbides and improve the toughness of steel in the tempering process. Excessive Si can cause the formation of red iron scale. Therefore, the Si content in the present invention is ≦0.50%.

[0016] Manganese: A Mn content of 0.6% or more can enhance the hardenability of the steel. However, a content of more than 1.6% can lead to segregation and the formation of inclusions such as MnS, which deteriorate the toughness of martensitic high-strength steels. Therefore, the Mn content in the steel of the present invention is 0.60-1.60%, preferably 1.20-1.60%.

[0017] Titanium: As a microalloying element, Ti can form numerous nanoscale precipitates with elements such as C and N through controlled rolling and cooling processes. On the one hand, it can strongly inhibit the growth of austenite grains during heat treatment, and on the other hand, it can retain numerous nanoprecipitates after heat treatment and play a role in precipitation strengthening. The Ti content in the steel of the present invention is 0.051-0.15%, preferably 0.051-0.09%, to ensure the strength of the steel while reducing production costs.

[0018] Vanadium: As a microalloying element, V can form nanoscale precipitates with C. A large number of nanoscale V precipitates are produced during heat treatment and tempering. The V content in the steel of the present invention is 0.040-0.12%, preferably 0.04-0.08%, to ensure the strength of the steel while reducing the manufacturing cost.

[0019] Niobium: As a microalloying element, Nb can form nanoscale precipitates with C, which can effectively inhibit the growth of austenite grains during hot rolling, thereby refining the structure after phase transformation. The Nb content in the steel of the present invention is 0-0.060%, preferably 0.03% or less.

[0020] Magnesium: By controlling the steelmaking and continuous casting processes, trace amounts of magnesium form fine MgO precipitate particles during steelmaking. As a result, TiN adheres to the MgO to form a composite precipitate, MgO-TiN, which modifies the shape of the cubic TiN, making the composite precipitate closer to a spherical shape. At the same time, the growth of harmful TiN can be controlled, thereby reducing the number of large TiN particles. The maximum size of TiN can be reduced from the conventional 8-10 μm to 5 μm or less to improve the toughness and plasticity of the steel. The magnesium content in the steel of the present invention is 0-0.003%.

[0021] Chromium: A Cr content of 0.2% or more can enhance the hardenability of the steel, which leads to the formation of a fully martensitic structure during quenching. Cr forms Cr carbides during tempering, which has the effect of resisting temper softening. A Cr content of more than 1.20% can cause excessive sparks during welding, which can adversely affect the quality of the weld. Therefore, the Cr content in the steel of the present invention is 0.20-1.20%, preferably 0.20-0.80%.

[0022] Molybdenum: A certain amount of Mo can enhance the hardenability of steel, which leads to the formation of a fully martensite structure during quenching. Mo reacts with C during high-temperature tempering to form carbide particles, which have the effect of resisting temper softening and softening of welded joints. Excessive Mo content increases the carbon equivalent and deteriorates weld performance. Meanwhile, as a precious metal, high Mo usage increases costs. Therefore, the Mo content in the steel of the present invention is 0 to 0.40%, preferably 0.25% or less.

[0023] Tungsten: W can increase the hardenability of the steel, form carbide particles during tempering, and have significant resistance to temper softening and temper embrittlement. The W content in the steel of the present invention is 0-0.30%, preferably 0.22% or less.

[0024] Nickel: A certain amount of Ni has the role of refining the martensite structure and improving the toughness of the steel. Excessive Ni increases the carbon equivalent and deteriorates the welding performance. At the same time, as a precious metal, the use of high amounts of Ni increases the cost. Therefore, the Ni content in the steel of the present invention is 0 to 0.30%, preferably 0.20% or less.

[0025] Copper: Cu contributes to precipitation strengthening during tempering. Furthermore, a certain amount of Cu can enhance the corrosion resistance in ultra-high strength steel for engineering machinery. The Cu content of the present invention is 0-0.40%, preferably 0.25% or less.

[0026] Boron: A small amount of B can enhance the hardenability and strength of steel. However, a B content of more than 0.0030% is prone to segregation and the formation of carbon-boron compounds, which severely deteriorates the toughness of the steel. Therefore, the B content in the steel of the present invention is 0.0005 to 0.0030%, preferably 0.0005 to 0.0020%.

[0027] Aluminum: An Al content of 0.2% or more serves as a deoxidizer on the one hand, and the resulting traces of Al2O3 can refine particles during slab heating and further enhance microstructural refinement in rolled steel sheets. However, an Al content of more than 0.06% is prone to producing Al-oxide-mediated defects. The Al content in the steel of the present invention is 0.02-0.06%, preferably 0.02-0.04%.

[0028] Calcium: Trace amounts of Ca can purify molten steel during smelting and modify the shape and size of inclusions such as MnS, thereby improving the toughness of the steel. A Ca content of more than 0.004% tends to form larger Ca compounds, which in turn deteriorates the toughness. Therefore, the Ca content in the steel of the present invention is 0.0005 to 0.004%, preferably 0.0015 to 0.0035%.

[0029] Nitrogen: The steel of the present invention contains a relatively high Ti content, which tends to form large cubic TiN particles with N, which deteriorates the plasticity and toughness of the plate. In the present invention, on the one hand, the content of N in the steel is strictly controlled to 0.0050% or less, preferably 0.0030% or less throughout the refining process, and on the other hand, by adding trace amounts of rare earth elements in combination with a certain overheating in the steelmaking and molten steel solidification processes, the size and amount of TiN formed can be effectively reduced.

[0030] Phosphorus, sulfur, and oxygen: P, S, and O are impurity elements in steel that affect the plasticity and toughness of the steel. The contents of these elements in the steel of the present invention are strictly controlled to P≦0.020%, S≦0.0050%, and O≦0.0040%; preferably, at least one of the following is satisfied: P≦0.012%, S≦0.0035%, and O≦0.0035%.

[0031] In particular, the steel composition of the present invention also comprises: (1) The following formula: NPI = (Mo + W + 2.3 *a nano-precipitation control index NPI of 5 to 26, calculated by (Ti+V) / (Cr) / (Ti+V), where each chemical element in the formula represents the numerical value before the percent sign of the mass percent of the corresponding chemical element. Preferably, the NPI is 7 to 15, and an NPI in this value range allows better steel performance, such as higher strength and greater elongation; Mo, W, and Cr are all strong carbide-forming elements that inhibit carbon diffusion. When steel is coiled for a long period at 560-680°C, the size of the precipitated TiC and VC is relatively large, reaching 6-15 nm, resulting in a relatively weak precipitation strengthening effect, with only a 50-100 MPa increase in steel strength. Furthermore, in the subsequent heat treatment process, when the steel is heated to 900°C and held there for 5-10 minutes, the 6-15 nm TiC partially dissolves and the VC completely dissolves, making it difficult for nanoprecipitates to exert their strengthening effect on the steel. In the present invention, by adding a certain amount of Mo, W, and / or Cr in combination, especially when the NPI is 5≦NPI≦26, it is possible to inhibit the growth of TiC and control the dissolution rate of TiC within an appropriate range, so that the average size of TiC is in the micro-nano scale of 2-5 mm. Furthermore, when the steel is subsequently heated to 900°C, quenched, and then tempered at 500-600°C and held for 10-30 minutes, VC and TiC precipitate again. The addition of Mo, W, and / or Cr can control the VC and TiC precipitates within the micro-nano size range of about 2-5 nm. If the NPI is too low, the VC and TiC become coarse, and if the NPI is too high, the VC and TiC do not precipitate sufficiently. Therefore, in the present invention, the NPI is controlled to be 5-26; and (2) Ti+V≧0.11%; Ti+V≧0.11% allows full utilization of the precipitation strengthening effect from TiC and VC during heat treatment. When Ti+V≧0.11%, sufficient micro-nano precipitate strengthening can be achieved. Preferably, in combination with NPI and heat treatment processes, the TiC and VC micro-nano precipitates of the present invention can produce a precipitation strengthening effect of 180-280 MPa. Preferably, the Ti+V content in the steel is 0.12-0.24%. must be satisfied.

[0032] The present invention also relates to a method for producing a method for manufacturing a semiconductor device comprising the steps of: 1) Smelting and Casting smelting and refining steel using a converter or electric furnace according to the above composition, and then casting it into slabs; 2) Heating The slab is placed in a heating furnace at 1220-1300°C for heating, and the temperature of the slab is held after the center temperature of the slab reaches the furnace temperature, wherein the holding time is >30 minutes, such as 30-200 minutes, preferably 100-200 minutes; 3) Rolling The slab is rolled to a target thickness by single-stand reversing rolling or multi-stand continuous hot rolling to obtain a steel plate, wherein the finish rolling temperature is 820 to 920°C; 4) Cooling The hot-rolled steel sheet is cooled to 560-680°C at a cooling rate of 10-30°C / s to obtain a ferrite and pearlite structure; in the ferrite, a large number of TiC precipitates with a size of 6-15 nm are produced; 5) Quenching + tempering heat treatment Quenching heat treatment: heating the steel plate to Ac3+(20~50)℃, holding for 5~10 minutes, and then quenching to room temperature at a cooling rate of ≧150℃ / s, such as 170~300℃ / s, preferably 190~250℃ / s; Tempering heat treatment: heating the steel plate to 500-600°C, holding for 10-30 minutes, and then air-cooling to room temperature; where Ac3 is the temperature at which the austenite transformation is completed; Ac3 = 955-350C-25Mn+51Si+106Nb+100Ti+68Al-11Cr-33Ni-16Cu+67Mo (wherein each chemical element in the formula represents the numerical value before the percent symbol of the mass percent of the corresponding chemical element). The present invention relates to a method for producing the steel, comprising:

[0033] The method for producing the ultra-high strength steel of the present invention comprises the steps of:

[0034] When magnesium is introduced during steelmaking, the superheat of molten steel in the converter is controlled to within 15°C to form fine MgO precipitates, while the secondary cooling water flow in continuous casting is moderately increased to increase the solidification rate of the molten steel. TiN adheres to the magnesium oxide to form composite precipitates, MgO-TiN, which modify the shape of the cubic TiN to become closer to spheres. Meanwhile, the composite precipitates are relatively dispersed, which helps control the growth of harmful TiN and reduces the amount of large TiN particles. The maximum size of TiN can be reduced from the conventional 8-10 μm to 5 μm or less to improve the toughness and plasticity of the steel.

[0035] In the slab heating process, controlling the heating temperature between 1220 and 1300°C with a central holding time of >30 minutes ensures complete dissolution of TiC precipitates formed during continuous casting. When the heating temperature exceeds 1300°C, it causes excessive growth of austenite grains, resulting in weakened grain boundary cohesion that is prone to cracking during rolling.

[0036] In the manufacturing method of the present invention, the finish rolling temperature is 820 to 920°C, and austenite particles can be refined by austenite recrystallization. After hot rolling, the steel sheet is cooled to 560 to 680°C at a cooling rate of 10 to 30°C / s to obtain ferrite, pearlite, and nano-precipitates. Here, the size of the precipitated TiC is 6 to 15 nm.

[0037] In the heat treatment process, the steel sheet is heated to Ac3+ (20-50)°C and the holding time is controlled to 5-10 minutes. In the present invention, by adding a certain amount of Mo, W and / or Cr in combination so that 5≦NPI≦26, the dissolution rate of TiC can be controlled within an appropriate range during the heat treatment process so that the size of TiC is controlled to a micro-nanoscale size of 2-5 nm. During this period, the nanoscale TiC can strongly inhibit the growth of austenite and refine the austenite and structure after quenching.

[0038] During tempering (holding at 500-600°C for 10-30 minutes), the steel sheet develops a tempered sorbite structure. VC and TiC undergo secondary precipitation during the tempering process, with their precipitate sizes controlled within the micro-nanoscale range of approximately 2-5 nm. The combined nano-precipitation of TiC and VC can produce a precipitation strengthening effect of 180-280 MPa.

[0039] The steel of this invention achieves a tempered sorbite + nano-precipitate microstructure through quenching and high-temperature tempering. The large amount of nano-precipitates ensures that a strength of 1000 MPa or more can still be achieved after high-temperature tempering. By using rare earth elements to purify the molten steel and control the size and shape of the inclusions, cracks caused by inclusions during deformation are reduced and the plasticity of the steel plate is improved.

[0040] The beneficial effects of the present invention are as follows:

[0041] The present invention achieves a steel with a microstructure of tempered sorbite and nano-precipitates through controlled rolling, cooling, and heat treatment processes. The steel of the present invention has numerous nano-precipitates of TiC and VC, which ensures that the steel plate has a strength of 1000 MPa or more after high-temperature tempering. The tempered sorbite obtained from high-temperature tempering improves the plasticity of the steel plate. By using rare earth elements to purify the molten steel and control the size and shape of the inclusions, cracks caused by the inclusions during deformation are reduced and the plasticity of the steel plate is improved.

[0042] Compared with the prior art, the present invention obtains steel with a microstructure of tempered sorbite + nano-precipitates, and by combining rare earth purification of the molten steel and controlling the size and shape of the inclusions, an ultra-high strength steel with higher plasticity and toughness is obtained. [Brief explanation of the drawings]

[0043] [Figure 1] FIG. 1 is a photograph of the metal structure of the ultra-high tensile steel after rapid heat treatment in Example 3 of the present invention, taken with an optical microscope. [Figure 2] FIG. 2 is a photograph of the metal structure of the ultra-high tensile steel after rapid heat treatment in Example 3 of the present invention, taken with a scanning electron microscope. DETAILED DESCRIPTION OF THE INVENTION

[0044] Detailed Description The invention will be further described in conjunction with the examples and the accompanying drawings.

[0045] Table 1 shows the compositions and corresponding NPIs of the inventive and comparative examples, with the balance being Fe and unavoidable impurities. The inventive and comparative steels were produced using the method of the present invention described above, with the process parameters shown in Table 2. The corresponding performance of the inventive and comparative steels is shown in Table 3.

[0046] Comparative Examples 1-4 were produced using essentially the same method as Inventive Example 3. The differences are that the NPI of Comparative Examples 1 and 2 is not within the range defined by the present invention, the Ti+V of Comparative Example 3 is not within the range defined by the present invention, and the Cr content in Comparative Example 4 is not within the range defined by the present invention.

[0047] The yield strength, tensile strength, and elongation of the steels in Table 3 were tested according to standard GB / T 228.1-2021 "Metallic materials - Tensile tests - Part 1: Test methods at room temperature." The impact energy at -60°C was determined according to GB / T 229-2020 "Metallic materials - Charpy pendulum impact test method."

[0048] The average austenite grain size was measured according to standard GB / T 6394-2017 "Method for determining the average grain size of metals." The polished samples were etched with picric acid, and 10 or more microstructural photographs were taken by optical microscope to statistically determine the average austenite grain size.

[0049] The size of the nanoprecipitates was measured by observing thin film samples using a transmission electron microscope (TEM) to statistically determine the size of the precipitates.

[0050] Figures 1 and 2 show optical and scanning electron micrographs, respectively, of Example 3 of the present invention. As shown in Figure 1, the steel of Example 3 exhibits a tempered sorbite structure after heat treatment. As shown in Figure 2, the steel of Example 3 has numerous micro- and nano-precipitates in its structure after heat treatment.

[0051] As can be seen from the metallographic photographs in Figures 1 and 2, the metallographic structure of the finished steel sheet is a uniform equiaxed tempered sorbite with a fine and dense structure, and the average grain size of the primary austenite in the tempered sorbite is about 6 μm. In Figure 2, it can be seen that there are numerous granular carbide precipitates in the steel of the present invention, and that 90% or more of the TiC and TiV precipitates are 2-5 nm in size.

[0052] By adding Mo, V and / or Cr to steels containing specific amounts of Ti and V to make them satisfy the above NPI, the steels of Examples 1 to 8 of the present invention can be obtained with a microstructure of tempered sorbite + nano-precipitates with a size of 2 to 5 nm, which brings about an overall improvement in the strength, plasticity and toughness of the steel.

[0053] It can be seen from Table 3 that, compared with Comparative Examples 1 to 4, the present invention makes it possible to obtain steels with significantly improved strength, plasticity, and toughness by controlling the composition of steel elements.

[0054] In summary, the present invention employs controlled rolling and cooling in combination with an offline heat treatment process, and by controlling the chemical composition design, base metal structure, heating rate, holding time, cooling rate, etc., the steel achieves ultra-high strength while maintaining good elongation and low-temperature impact toughness, etc.

[0055] [Table 1]

[0056] [Table 2]

[0057] [Table 3]

Claims

1. Contains the following ingredients in weight percent: C: 0.15 to 0.21%, Si: ≦0.50%, Mn: 0.60 to 1.60%, Ti: 0.051 to 0.15%, V: 0.040 to 0.12%, Cr: 0.20 to 1.20%, B: 0.0005 to 0.0030%, Al: 0.02 to 0.06%, Ca: 0.0005 to 0.004%, N: ≦0.005%, P: ≦0.020%, S: ≦0.0050%, O: ≦0.0040%, the balance including Fe and inevitable impurities; and The following formula: NPI = (Mo + W + 2.3 ＊ a nanoprecipitation control index NPI of 5 to 26, calculated by the formula (Ti+V) / (Ti+V) / (Cr) / (Ti+V), where each chemical element in the formula represents the numerical value before the percent sign of the mass percent of the corresponding chemical element; and Ti + V ≧ 0.11% is satisfied; steel.

2. 2. The steel according to claim 1, characterized in that the balance is Fe and unavoidable impurities.

3. 3. A steel according to claim 1 or 2, characterized in that the steel further comprises one or more elements selected from Nb, Mg, Ni, Cu, Mo and W, in wt. %: Nb: 0-0.060%, Mg: 0-0.003%, Ni: 0-0.30%, Cu: 0-0.40%, Mo: 0-0.40%, W: 0-0.30%.

4. 4. The steel according to claim 1, wherein the steel has a microstructure of tempered sorbite and nanoprecipitates; wherein the tempered sorbite has a prior austenite grain size of 5 to 10 μm, the nanoprecipitates include TiC and VC precipitates, and the nanoprecipitates have a size of 2 to 5 nm.

5. The steel has a yield strength of ≥ 1000 MPa, a tensile strength of ≥ 1050 MPa, and an elongation A of ≥ 19%. ５０ and an impact energy at -60°C of ≥ 90 J.

6. The following steps: 1) Smelting and Casting smelting and refining the steel using a converter or an electric furnace according to the composition of any one of claims 1 to 3, followed by casting into slabs; 2) Heating Heating the slab in a furnace at 1220-1300°C and holding the slab for >30 minutes after the center temperature of the slab reaches furnace temperature; 3) Rolling The slab is rolled to a target thickness by single-stand reversing rolling or multi-stand continuous hot rolling to obtain a steel plate, wherein the finish rolling temperature is 820 to 920°C; 4) Cooling cooling the hot-rolled steel sheet to a coiling temperature of 560 to 680°C at a cooling rate of 10 to 30°C / sec; 5) Quenching + tempering heat treatment Quenching heat treatment: The steel plate was ３ + Heating to (20-50)°C, holding for 5-10 minutes, then quenching to room temperature at a cooling rate of ≥ 150°C / sec; Tempering heat treatment: heating the steel plate to 500 to 600°C, holding for 10 to 30 minutes, and then air-cooling to room temperature; ３ is the temperature at which the austenite transformation is completed; Ac ３ = 955 - 350C - 25Mn + 51Si + 106Nb + 100Ti + 68Al - 11Cr - 33Ni - 16Cu + 67Mo (wherein each chemical element in the formula represents the numerical value before the percent sign of the mass percent of the corresponding chemical element) The method for producing a steel according to any one of claims 1 to 5, comprising:

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