Ultra-high strength and toughness steel plate and method for producing the same

Abstract

This application discloses an ultra-high strength and toughness steel plate and its production method. In the steel plate, C 0.18~0.21%, Si 0.36~0.42%, Mn 0.86~0.94%, Cr 0.32~0.38%, Ni 0.52~0.58%, Nb 0.012~0.019%, Ti 0.012~0.019%, Alt 0.021~0.049%, Ca 0.0022~0.0052%, B 0.0016~0.0021%, Zr 0.012~0.019%, La 55~100ppm, N 22~42ppm, O≤15ppm, H≤1.5ppm, P≤0.008%, S≤0.0012%, 0.12≤(Nb+Ti) / (C+N)≤0.18, 1≤La / (O+N+S)≤2.

Description

Technical Field

[0001] This application belongs to the field of steel alloy materials technology, and relates to an ultra-high strength and toughness steel plate and its production method. Background Technology

[0002] With the development of infrastructure and the improvement of mechanization, the performance requirements for equipment in fields such as mining machinery, agricultural machinery, power machinery, building materials, and railway transportation are becoming increasingly stringent. High-strength steel, due to its excellent load-bearing capacity and weight reduction effect, is widely used in key components of these fields. For example, the blades and buckets of bulldozers and loaders, electric shovels used in coal mining, the carriages of automatic loading and unloading mining trucks, and the bottom linings of scraper conveyors all require high-strength steel plates to withstand the severe impact and wear of materials during operation.

[0003] Especially in mining and material transportation operations, equipment components are subjected to harsh environments of high stress and high impact for extended periods. For example, electric shovel buckets and mine cars must not only withstand the enormous impact loads from falling ore but also resist the severe wear caused by material sliding. This requires the steel plates used to possess extremely high strength and excellent wear resistance to extend the service life of the components. Summary of the Invention

[0004] The purpose of this application is to provide an ultra-high strength and toughness steel plate and its production method to solve the problem of poor comprehensive performance of existing high-strength steel.

[0005] To achieve the aforementioned objectives, one embodiment of this application provides a steel plate whose chemical composition, by mass percentage, comprises: C 0.18~0.21%, Si 0.36~0.42%, Mn 0.86~0.94%, Cr 0.32~0.38%, Ni 0.52~0.58%, Nb 0.012~0.019%, Ti 0.012~0.019%, Al 0.021~0.049%, Ca 0.0022~0.0052%, B 0.0016~0.0021%, Zr 0.012~0.019%, La 55~100ppm, N 22~42ppm, O≤15ppm, H≤1.5ppm, P≤0.008%, S≤0.0012%, the remainder being Fe and unavoidable impurities, and satisfying: 0.12≤(Nb+Ti) / (C+N)≤0.18, 1≤La / (O+N+S)≤2, carbon equivalent CEV is 0.40~0.50%, Mn equivalent ∑Mn is 1.30~1.40%, hardenability index J≥2.1;

[0006] CEV=C+Mn / 6+(Cr+Mo+V) / 5+(Cu+Ni) / 15; ∑Mn=Mn+3.28Mo+0.64Cr+0.5W+0.37Ni+0.23Cu+0.03Si+0.1(Alt+V+Ti+Nb); J = (1.24C - 0.95C) 2 )×(0.7Si+1)×(3.33Mn+1)×(2.16Cr+1)×(3Mo+1)×(0.36Ni+1)×(0.37Cu+1)×(1.73V+1)×(1.55Alt+1).

[0007] In one embodiment, the microstructure of the steel plate is a composite microstructure of lath martensite, acicular martensite, ferrite, and retained austenite, wherein the sum of the proportions of lath martensite and acicular martensite is 90-94%, the proportion of ferrite is 3-5%, and the proportion of retained austenite is 3-7%.

[0008] In one embodiment, the thickness a of the steel plate is 6~30mm, the yield strength is ≥1050MPa, the tensile strength is ≥1350MPa, the hardness is 420~460HBW, the elongation is ≥12%, and the impact energy AKV at -40℃ is ≥50J; the steel plate is cold-bent 180° with a bending mandrel diameter D=3a, and there are no cracks on the surface of the steel plate.

[0009] In one embodiment, the strength-ductility product of the steel plate is ≥16.5 GPa·%.

[0010] To achieve the aforementioned objectives, one embodiment of this application provides a method for producing steel plates. The method includes, in sequence, a steel smelting process, a continuous casting process, a heating process, a rolling process, a cooling process, a quenching process, and a tempering process. In the heating process, the heating temperature is (T) Nb(C,N) +30)℃~(T Nb(C,N) +60)℃, heat preservation time is 0.1d~0.5dmin; In the rolling process, the continuously cast billet is sequentially subjected to rough rolling and finish rolling to form a steel plate. The initial rolling temperature for rough rolling is (T). Nb(C,N) -130)℃~(T Nb(C,N) -50)℃, the initial rolling temperature for finishing rolling is (T nr -20)℃~(T nr +10)℃, final rolling temperature is (A) r3 +60)℃~(A r3 +100)℃; In the cooling process, the steel plate is cooled at a rate of (V). B +5) ℃ / s~(V B +10)℃ / s; In the quenching process, the steel plate that has undergone the cooling process is heated and then rapidly cooled, with the heating temperature being (A). c3 -30)℃~(A c3 -10)℃, heating time is (1.5a+20)~(1.5a+30)min, cooling rate is (V B +15) ℃ / s~(V B +20)℃ / s; In the tempering process, the heating temperature is (Ms-235)℃~(Ms-210)℃, where Ms is the martensitic transformation start temperature, Ms=561-474C-33Mn-17Cr-17Ni-21Mo; Among them, T Nb(C,N) =6770 / (2.26 lg((C+12 / 14N)×Nb))-273; recrystallization temperature T nr =887+464C+6445Nb-644 +890Ti+363Alt-357Si; A r3 A is the temperature at which austenite precipitates ferrite during the cooling process. r3 =910-310C-80Mn-20Cu-15Cr-55Ni-80Mo; A c3 A is the austenitizing completion temperature. c3 =912-250C-16Mn+48Si-2Cr-16Ni+95V+96Ti+210Alt-10Cu; a is the thickness of the steel plate; V B V is the critical cooling rate for bainite formation. B =e (13.08-8.8C-1.07Mn-0.7Ni-0.57Cr-9.2Mo-366B) / 3600.

[0011] In one embodiment, during the heating process, the continuously cast billet is fed into a heating furnace and heated to the heating temperature, and then kept at that temperature in the heating furnace. The total time the continuously cast billet is in the furnace is 1.1d to 1.5dmin.

[0012] In one embodiment, during the cooling process, the steel plate is water-cooled, and the water immersion temperature of the steel plate is ≥ (A). r3 -20)℃, the final cooling temperature of the steel plate is (Bs-10)℃~ (Bs-30)℃, where Bs is the bainite transformation start temperature, Bs=630-45Mn-40V-35Si-30Cr-25Mo-20Ni-15W.

[0013] In one embodiment, in the tempering process, the steel plate that has undergone the quenching process is sent into a heating furnace and heated to the heating temperature, and then kept at the temperature in the heating furnace. The total time the steel plate is in the furnace is (5a+10)~(5a+30) min.

[0014] In one embodiment, in the continuous casting process, the molten steel obtained from the steel smelting process is continuously cast into a continuous casting billet. The superheat of the molten steel during casting is 8~20℃. Light reduction is applied at the end of the continuous casting process, with a reduction rate of 1.2~1.4 mm / m. The continuous casting speed v satisfies: 200×L / F-0.05≤v≤200×L / F+0.05, where L is the perimeter of the continuous casting billet obtained in the continuous casting process, in mm, and F is the cross-sectional area of ​​the continuous casting billet, in mm². 2 .

[0015] In one embodiment, in the continuous casting process, after the continuously cast billet exits the crystallizer, it is cooled in the secondary cooling zone, and the specific water volume in the secondary cooling zone is controlled to be 0.5~0.7L / kg.

[0016] In one embodiment, the thickness of the continuously cast billet obtained by the continuous casting process is 220~320mm, the center segregation level of the continuously cast billet is ≤C1.5 or B1.0, and there is no A-type segregation, the center porosity level is ≤0.5, the levels of A, B and C type inclusions are all ≤0.5, and the levels of D and Ds type inclusions are all ≤1.

[0017] In one embodiment, the steelmaking process includes sequential steps of hot metal pretreatment, converter smelting, LF refining, and RH refining. In the converter smelting step, the molten pool in the converter is subjected to top and bottom blowing, with oxygen blown from the top and argon blown from the bottom. The flow rate of the bottom-blown argon is 0.8~1.2 Nm³. 3 / (min·t), lime and dolomite are used for slag formation, and the slag basicity is controlled at 3.0~3.5. The P content in the molten steel at the end of the converter is less than 0.005% by mass percentage; In the LF refining step, the slag basicity is controlled at 4.5~5.0, and the S content in the molten steel is controlled to be less than 0.0010% by mass. After alloying, 100~300m / furnace of aluminum wire is fed in and stirred for 3~5min. The O content in the molten steel is controlled to be less than 0.0040% by mass, and the tapping temperature is 1650~1670℃. In the RH refining step, after the molten steel is allowed to stand under vacuum, calcium wire is fed in after the vacuum is broken to control the Ca, O, and S content in the molten steel to meet the following condition: 2.0 ≤ (Ca - 3.5O) / S ≤ 2.5. Then, the mixture is stirred for 2-3 minutes. Lanthanum-iron alloy is added to control the La content in the molten steel to 55-100 ppm, and the mixture is gently stirred with bottom-blown argon gas for 5-8 minutes, with an argon flow rate ≤ 1 Nm³.3 / h; Add zirconium ferroalloy to adjust the Zr content in the molten steel to 0.012~0.016%, and gently stir with bottom-blown argon for 5~8 minutes, with an argon flow rate ≤1Nm 3 / h, then stir for 15~20min.

[0018] In one embodiment, in the pre-desulfurization step of the molten iron, magnesium powder, lime powder and fluorite powder are mixed and then injected into the molten iron to control the S content in the molten iron after desulfurization to be less than 0.0012% by mass percentage; wherein, the amount of magnesium powder injected is 0.35~0.45 kg per ton of molten iron.

[0019] In one embodiment, during the LF refining step, the heating and alloying process is carried out at a rate of 3~4 Nm for the first 10 minutes. 3 Argon gas was bottom-blown at a flow rate of / h, followed by 0.6~1.2Nm 3 Bottom-blown argon gas at a flow rate of / h.

[0020] In one embodiment, the RH refining step, the step of vacuum settling the molten steel includes: settling the molten steel under a vacuum of ≤30Pa for 25~30min to control the O content in the molten steel to be 0.001~0.003% by mass percentage.

[0021] In one embodiment, during the RH refining step, the mass percentage of La in the lanthanum-iron alloy is 30-40%, and the mass percentage of Zr in the zirconium-iron alloy is 30-50%.

[0022] Compared with the prior art, the beneficial effects of this application are as follows: (1) In terms of chemical composition, the steel plate of this application generates high-melting-point, high-sphericity rare earth sulfides and oxides by adding rare earth La, which replaces traditional long strip MnS and other brittle inclusions, and controls the aspect ratio of the inclusions to ≤1.6, thereby achieving spheroidization modification of the inclusions, eliminating stress concentration sources, and reducing the risk of crack initiation; through Nb-Ti microalloying, the precipitated Nb(C,N) particles pin the grain boundaries and inhibit the growth of austenite grains; Ti can also fix free N; by adding an appropriate amount of Ni, the austenite phase region is expanded, hardenability is improved, martensite formation is promoted, and the ductile-brittle transition temperature is reduced, thus improving low-temperature toughness; by adding a trace amount of B, the grain boundary energy is reduced, the premature precipitation of ferrite is inhibited, and the phase transformation kinetics are regulated; through Zr microalloying, nano-sized Zr(C,N) particles are generated, which pin the grain boundaries and subgrain boundaries, inhibit the coarsening of austenite grains, avoid the formation of coarse martensite during welding, and maintain the toughness of the weld joint. Thus, the chemical composition design scheme of this application can achieve a balance between high strength and high toughness in steel plates.

[0023] (2) The steel plate production method combines the aforementioned chemical composition design scheme, and uses rare earth microalloying to modify the inclusions to achieve spheroidization modification of the inclusions; and through the synergistic effect of Nb and Ti microalloying and rolling process, it inhibits the growth of austenite grains and promotes deformation-induced precipitation, thereby achieving the effect of grain refinement; combined with the two-phase quenching process, it controls the ratio of soft and hard phases of martensite, ferrite and retained austenite, so that the steel plate has both high strength and excellent plasticity and toughness; combined with the quenching and tempering process to eliminate residual stress, and utilizes the pinning effect of Zr carbonitride composite particles to inhibit grain coarsening. Attached Figure Description

[0024] Figure 1 This is a metallographic diagram of the steel plate of Embodiment 1 of this application; Figure 2 This is a metallographic image of the residual austenite in the steel plate of Embodiment 1 of this application; Figure 3 This is a metallographic diagram of the steel plate of Embodiment 2 of this application. Detailed Implementation

[0025] To make the objectives, technical solutions, and advantages of this invention clearer, the technical solutions of this invention will be clearly and completely described below in conjunction with specific embodiments and corresponding drawings. Obviously, the described embodiments are only a part of the embodiments of this invention, and not all of them. Based on the embodiments of this invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of this invention.

[0026] In response to the application requirements of high-strength steel, one embodiment of this application provides a steel plate and a method for producing the same.

[0027] Regarding the chemical composition, the steel plate comprises, by mass percentage: C 0.18~0.21%, Si 0.36~0.42%, Mn 0.86~0.94%, Cr 0.32~0.38%, Ni 0.52~0.58%, Nb 0.012~0.019%, Ti 0.012~0.019%, Al 0.021~0.049%, Ca 0.0022~0.0052%, B 0.0016~0.0021%, Zr 0.012~0.019%, La 55~100ppm, N 22~42ppm, O≤15ppm, H≤1.5ppm, P≤0.008%, S≤0.0012%, the remainder being Fe and unavoidable impurities, and satisfying: 0.12≤(Nb+Ti) / (C+N)≤0.18, 1≤La / (O+N+S)≤2, carbon equivalent CEV is 0.40~0.50%, Mn equivalent ∑Mn is 1.30~1.40%, hardenability index J≥2.1; CEV=C+Mn / 6+(Cr+Mo+V) / 5+(Cu+Ni) / 15; ∑Mn=Mn+3.28Mo+0.64Cr+0.5W+0.37Ni+0.23Cu+0.03Si+0.1(Alt+V+Ti+Nb); J = (1.24C - 0.95C) 2 )×(0.7Si+1)×(3.33Mn+1)×(2.16Cr+1)×(3Mo+1)×(0.36Ni+1)×(0.37Cu+1)×(1.73V+1)×(1.55Alt+1).

[0028] It is understood that the calculation formulas are only applicable to the numerical values ​​of the corresponding indicators, and the units of each indicator will be explained separately. The same applies to the formulas mentioned later. For example, the units of CEV and ∑Mn here are both percentages.

[0029] In the calculation formula, the symbols of each element represent the mass percentage of the corresponding element. That is, C, Mn, Cr, Mo, V, Cu, Ni, W, Si, Alt, and Nb in the calculation formula represent the mass percentages of C, Mn, Cr, Mo, V, Cu, Ni, W, Si, Alt, and Nb, respectively.

[0030] For example, if the mass percentage of carbon in the steel plate is 0.20%, then the C in the calculation formula represents the mass percentage of 0.20.

[0031] Of course, if the chemical composition of the steel plate does not include a certain element, the mass percentage of that element in the calculation formula shall be calculated as 0.

[0032] The element symbols in the calculation formulas below will follow the same pattern, and will not be repeated hereafter.

[0033] The functions of each chemical component and the details of content control are explained below.

[0034] Carbon (C) is the most economical strengthening element in steel, exhibiting significant solid solution strengthening. It substantially improves the strength and hardness of steel by pinning dislocation movement. The higher the C content, the more pronounced the solid solution strengthening effect. However, excessive C addition will form coarse cementite, leading to a sharp decrease in toughness and weldability. Therefore, in this application, the C content is limited to 0.18~0.21%, and the carbon equivalent (CEV) is limited to 0.40~0.50%.

[0035] Si: Silicon plays a solid solution strengthening role in steel, and can also inhibit carbide decomposition and improve tempering stability. Si combines with impurity elements such as P and S in steel to form high-melting-point oxides, thus acting as nucleation cores to refine grains in heterogeneous formation. If too much Si is added, Si will form a low-melting-point FeS-SiO2 composite phase with S, increasing the tendency for hot cracking in the weld. Therefore, the Si content is limited to 0.36~0.42% in this application.

[0036] Mn: Manganese plays a solid solution strengthening role in steel, which can improve hardenability, enhance wear resistance, and lower the austenite decomposition temperature. However, if too much Mn is added, the grains in the hardened austenite zone (HAZ) will coarsen during high-temperature holding, and upon cooling, coarse ferrite or pearlite will form, thereby reducing impact toughness and increasing the tendency for cold cracking in welds. Therefore, in this application, the Mn content is limited to 0.86~0.94%, and the Mn equivalent ∑Mn is limited to 1.30~1.40%.

[0037] Cr: In steel, chromium can form stable carbides with carbon, pinning grain boundaries and inhibiting austenite grain growth, thus refining the matrix structure and improving the strength and toughness of the steel. However, if too much Cr is added, the hard martensite structure easily forms in the weld zone (HAZ) during cooling, leading to increased brittleness of the weld joint and delayed cracking. Therefore, the Cr content in this application is limited to 0.32~0.38%.

[0038] Ni: Nickel is an austenite stabilizing element that can lower the martensite transformation temperature and promote the formation of fine acicular or lath martensite, thereby reducing brittleness and increasing low-temperature toughness. However, Ni is an expensive alloy, and adding too much will significantly increase costs. Therefore, the Ni content is limited to 0.52-0.58% in this application.

[0039] Nb and Ti: Nb and Ti combine with C and N to form carbonitride particles, which pin austenite grain boundaries during steel heating processes such as rolling or welding thermal cycles, inhibiting grain coarsening at high temperatures and reducing the risk of cracking. Furthermore, Ti can react with N in the steel to form stable TiN precipitates, thereby reducing the reaction between B and N and protecting B. However, excessive addition of Nb and Ti can form coarse carbonitrides, which become crack initiation sites. Therefore, in this application, the content of both Nb and Ti is limited to 0.012~0.019%.

[0040] Al: Aluminum is a deoxidizing element in steel, which can significantly reduce the oxygen content in molten steel. Al can combine with nitrogen to form AlN particles, which can inhibit austenite grain growth, thus refining the base metal grains and inhibiting weld grain coarsening. However, excessive Al addition will form coarse Al2O3 inclusions, which can become crack initiation sites. Therefore, in this application, the Al content is limited to 0.021~0.049%.

[0041] Ca: Calcium has a strong affinity for O and S (thus protecting Zr and La and improving their yield), and can form high-melting-point calcium aluminates, calcium silicates, calcium sulfides, and other complex inclusions. Ca can reduce free oxygen and oxide inclusions in steel, reducing the oxidizing properties of molten steel; it can also reduce sulfide segregation at grain boundaries, improving the hot working properties and resistance to hot cracking of steel; and it can modify inclusions, transforming them into spherical or ellipsoidal calcium aluminates and calcium silicates, reducing the sharpness of inclusions and interfacial stress concentration. However, excessive Ca addition will form high-melting-point calcium aluminate inclusions, which are prone to agglomeration in the weld and promote the formation of brittle phases under welding thermal cycling, increasing the risk of cold cracking. Therefore, the Ca content in this application is limited to 0.0022~0.0052%.

[0042] B: Boron can segregate at austenite grain boundaries, lower grain boundary energy, inhibit ferrite nucleation, and significantly improve the hardenability of steel. However, excessive B addition can lead to the precipitation of a large amount of boron nitrides, which in turn promotes ferrite nucleation. Therefore, the B content is limited to 0.0016~0.0021% in this application.

[0043] Zirconium has a strong affinity for carbon and nitrogen, forming stable carbonitrides. These compounds are finely dispersed in the steel, acting as grain boundary pinning agents and reducing the formation of coarse microstructures. However, excessive Zr addition will result in coarse ZrC and ZrN particles or intermetallic compounds with Fe, increasing brittleness. Therefore, the Zr content in this application is limited to 0.012~0.019%.

[0044] Lanthanum (La): Lanthanum has a strong affinity for harmful elements such as phosphorus (P), sulfur (S), and oxygen (O), forming high-melting-point, low-plasticity rare earth compounds that improve the purity of steel. La can modify inclusions, improving their morphology and forming spherical or short rod-shaped rare earth inclusions that are evenly distributed, thereby reducing the cutting effect of inclusions on the matrix and improving the toughness and wear resistance of steel. By fixing elements such as sulfur (S) and oxygen (O), La can reduce the formation of low-melting-point eutectics and decrease the susceptibility to hot cracking. However, excessive addition of La can form coarse inclusions, which can easily clog the nozzle during continuous casting. Therefore, the La content is limited to 55-100 ppm in this application.

[0045] Nitrogen (N) can form high-hardness nitrides with Nb, Ti, and Zr, acting as dispersed second-phase particles that hinder dislocation movement and grain growth, thereby improving resistance to abrasive wear and weldability. However, excessive N content can easily lead to porosity, HAZ embrittlement, and uneven microstructure during welding. Therefore, the N content is controlled at 22-42 ppm in this application.

[0046] H: Hydrogen is a harmful element in steel, easily accumulating at grain boundaries and defects to form diffusible hydrogen, which, together with stress, causes hydrogen-induced cracking. Therefore, the H content in this application is controlled to ≤1.5ppm.

[0047] Phosphorus (P) is a harmful element in steel, easily segregating at grain boundaries and increasing susceptibility to cold cracking during welding. During welding, localized high temperatures can exacerbate P segregation, forming low-melting-point phases and initiating hot cracking. Therefore, the P content in this application is controlled to ≤0.008%.

[0048] Sulfur is a harmful element in steel. During welding, sulfur tends to segregate at the solidification front of the weld metal, forming a low-melting-point eutectic liquid film that hinders intergranular bonding and increases the tendency for cold cracking in the weld. Therefore, the sulfur content is controlled to ≤0.0012% in this application.

[0049] Based on this, the steel plate of this application, in terms of chemical composition, generates high-melting-point, high-sphericity rare earth sulfides and oxides by adding rare earth element La, replacing traditional long strip-shaped brittle inclusions such as MnS, controlling the aspect ratio of the inclusions to ≤1.6, achieving spheroidization modification of the inclusions, eliminating stress concentration sources, and reducing the risk of crack initiation; through Nb-Ti microalloying, the precipitated Nb(C,N) particles pin the grain boundaries, inhibiting austenite grain growth; Ti can also fix free N; by adding an appropriate amount of Ni, the austenite phase region is expanded, hardenability is improved, martensite formation is promoted, and the ductile-brittle transition temperature is lowered, improving low-temperature toughness; by adding trace amounts of B, the grain boundary energy is reduced, the premature precipitation of ferrite is inhibited, and the phase transformation kinetics are regulated; through Zr microalloying, nano-sized Zr(C,N) particles are generated, pinning grain boundaries and subgrain boundaries, inhibiting austenite grain coarsening, avoiding the formation of coarse martensite during welding, and maintaining the toughness of the weld joint. Thus, the chemical composition design scheme of this application can achieve a balance between high strength and high toughness in steel plates.

[0050] The metallographic structure of the steel plate is a composite structure of lath martensite, acicular martensite, ferrite, and retained austenite. The sum of the proportions of lath martensite and acicular martensite is 90-94%, the proportion of ferrite is 3-5%, and the proportion of retained austenite is 3-7%.

[0051] Here, the metallographic structure of the steel plate can be obtained by performing microstructure testing according to GB / T 13298-2015 "Metallic Microstructure Examination Method".

[0052] According to YB / T 4676-2018 "Analysis of Precipitated Phases in Steel by Transmission Electron Microscopy", nanoscale carbides are distributed on the matrix of the steel plate.

[0053] Measurements show that the steel plate has a thickness (a) of 6-30 mm, a yield strength ≥1050 MPa, a tensile strength ≥1350 MPa, a hardness of 420-460 HBW, an elongation ≥12%, and an impact energy (AKV) ≥50 J at -40℃. When cold-bent 180° with a bending mandrel diameter D = 3a, the steel plate surface shows no cracks.

[0054] The measurements of yield strength, tensile strength, and elongation mentioned above can be performed according to GB / T 228.1-2021 "Metallic materials, tensile testing—Part 1: Test method at room temperature". The measurements of hardness mentioned above can be performed according to GB / T231.1-2018 "Metallic materials, Brinell hardness testing—Part 1: Test method". The measurements of impact energy at -40℃ mentioned above can be performed according to GB / T 229-2020 "Metallic materials, Charpy pendulum impact test method". The measurements of cold bending performance mentioned above can be performed according to GB / T 232-2024 "Metallic materials, bending test method".

[0055] The strength-ductility product of the steel plate is ≥16.5 GPa·%.

[0056] The above measurement of strength-ductility product can be carried out in accordance with GB / T 228.1-2021 "Metallic materials, tensile testing - Part 1: Test method at room temperature".

[0057] One embodiment of this application also provides a method for producing steel plates.

[0058] The aforementioned steel plate was produced using this production method.

[0059] The steel plate production method includes the following sequential processes: steel smelting, continuous casting, heating, rolling, cooling, quenching, and tempering.

[0060] In the heating process, the heating temperature is (T) Nb(C,N) +30)℃~(T Nb(C,N) +60)℃, holding time is 0.1d~0.5dmin, T Nb(C,N) =6770 / (2.26 lg((C+12 / 14N)×Nb))-273. Where, T Nb(C,N) is the solid solution temperature of Nb carbonitrides.

[0061] By controlling the heating temperature and holding time before rolling, it is possible to ensure that the carbonitrides of Nb are fully dissolved, avoid undissolved particles from affecting toughness, and prevent excessive temperature from causing coarsening of austenite grains and increased oxidation and burn-off of the continuous casting billet.

[0062] In the rolling process, the continuously cast billet is sequentially subjected to rough rolling and finish rolling to form a steel plate. The initial rolling temperature for rough rolling is (T). Nb(C,N) -130)℃~(T Nb(C,N) -50)℃, the initial rolling temperature for finishing rolling is (T nr -20)℃~(T nr +10)℃, final rolling temperature is (A) r3 +60)℃~(A r3 +100)℃; where the recrystallization temperature T nr =887+464C+6445Nb-644 +890Ti+363Alt-357Si; A r3 A is the temperature at which austenite precipitates ferrite during the cooling process. r3 =910-310C-80Mn-20Cu-15Cr-55Ni-80Mo.

[0063] By rolling in the non-recrystallized austenite region during the rough rolling stage, the austenite recrystallization is suppressed by strain energy, and the strain-induced precipitation of Nb carbonitrides is promoted, thus refining the grains. During the finish rolling stage, the austenite recrystallization region or critical region is rolled. By controlling the rolling temperature, the microstructure before phase transformation can be optimized, ensuring that a fine-grained ferrite + bainite composite microstructure is obtained after phase transformation.

[0064] In the cooling process, the steel plate is cooled at a rate of (V). B +5) ℃ / s~(V B +10)℃ / s, V B V is the critical cooling rate for bainite formation. B =e (13.08-8.8C-1.07Mn-0.7Ni-0.57Cr-9.2Mo-366B) / 3600.

[0065] The cooling process controls the cooling rate to form a partial acicular ferrite + lath bainite structure, thereby obtaining a good precursor structure, which ensures improved toughness and weldability after quenching.

[0066] In the quenching process, the steel plate that has undergone the cooling process is heated and then rapidly cooled, with the heating temperature being (A). c3 -30)℃~(A c3 -10)℃, heating time is (1.5a+20)~(1.5a+30)min, cooling rate is (V B +15) ℃ / s~(V B +20)℃ / s, where A c3 A is the austenitizing completion temperature. c3 =912-250C-16Mn+48Si-2Cr-16Ni+95V+96Ti+210Alt-10Cu; a is the thickness of the steel plate.

[0067] The quenching process controls the heating temperature to reach the incomplete austenitization region, retaining some undissolved ferrite. This refines the austenite grains and reduces quenching stress. As the grain boundary area increases, the steel plate's resistance to crack propagation is improved. Furthermore, rapid cooling ensures that the supercooled austenite directly crosses the bainite transformation region, suppressing the formation of non-martensite structures, especially coarse MA islands, to obtain a martensite + retained austenite structure, thereby improving toughness.

[0068] In the tempering process, the heating temperature is (Ms-235)℃~(Ms-210)℃, where Ms is the martensitic transformation start temperature, Ms=561-474C-33Mn-17Cr-17Ni-21Mo.

[0069] Rapid cooling during steel plate quenching can form fine carbides on the steel plate matrix. These fine carbides do not grow after low-temperature tempering, thus allowing fine carbides to precipitate on the martensitic skeleton. At the same time, the degree of lattice distortion is reduced, thereby effectively eliminating quenching stress, reducing the brittleness of the steel plate, improving impact resistance, and avoiding failure due to stress concentration during use.

[0070] Thus, the steel plate production method, combined with the aforementioned chemical composition design scheme, modifies inclusions through rare earth microalloying to achieve spheroidization modification of inclusions; and through the synergistic effect of Nb and Ti microalloying and rolling process, it inhibits austenite grain growth and promotes deformation-induced precipitation, achieving the effect of grain refinement; combined with the two-phase quenching process, it controls the ratio of soft and hard phases of martensite, ferrite, and retained austenite, so that the steel plate has both high strength and excellent plasticity and toughness; combined with quenching and tempering processes to eliminate residual stress, and utilizing the pinning effect of Zr carbonitride composite particles to inhibit grain coarsening.

[0071] In one embodiment, during the heating process, the continuously cast billet is fed into a heating furnace and heated to the heating temperature, then held at that temperature. The total time the billet spends in the furnace is 1.1 to 1.5 days. That is, after the billet is fed into the heating furnace, it is first heated to the heating temperature and then held at that temperature. Thus, by controlling the total time spent in the furnace and the holding time, the time to reach the heating temperature can be controlled, i.e., the heating rate can be controlled.

[0072] In one embodiment, the cooling process involves water cooling of the steel plate, i.e., cooling the steel plate using water cooling. The water immersion temperature of the steel plate is ≥ (A). r3 -20)℃, the final cooling temperature of the steel plate is (Bs-10)℃~(Bs-30)℃, where Bs is the bainite transformation start temperature, Bs=630-45Mn-40V-35Si-30Cr-25Mo-20Ni-15W.

[0073] In one embodiment, during the tempering process, the steel plate that has undergone the quenching process is fed into a heating furnace and heated to the heating temperature, then held at that temperature in the furnace. The total time the steel plate spends in the furnace is (5a+10) to (5a+30) minutes. That is, after the steel plate is fed into the heating furnace, it is first heated to the heating temperature and then held at that temperature. Thus, by controlling the total time spent in the furnace, the heating rate and holding time can be controlled.

[0074] In one embodiment, in the continuous casting process, the molten steel obtained from the steel smelting process is continuously cast into a continuous casting billet. The superheat of the molten steel during casting is 8~20℃, and a light reduction is applied at the end of the continuous casting process, with a reduction rate of 1.2~1.4 mm / m. The continuous casting speed v satisfies: 200×L / F-0.05≤v≤200×L / F+0.05, where L is the cross-sectional perimeter of the continuous casting billet obtained in the continuous casting process, in mm, and F is the cross-sectional area of ​​the continuous casting billet, in mm². 2 Thus, by controlling the casting of molten steel with low superheat, the reduction rate, and the casting speed, the quality of the continuously cast billet can be improved, and the center segregation and center porosity of the continuously cast billet can be controlled.

[0075] It is understandable that the chemical composition of the continuously cast billet is roughly the same as that of the molten steel obtained from the steel smelting process and the final steel plate product, all of which conform to the chemical composition of the steel plate mentioned above, and will not be repeated here.

[0076] The thickness of the continuously cast billet is 220~320mm.

[0077] Measurements showed that the center segregation level of the continuously cast billet was ≤C1.5 or ≤B1.0, with no Class A segregation. Class B segregation was discontinuous linear segregation, and Class C segregation was dotted segregation. The center porosity level was ≤0.5, and the levels of Class A, B, and C inclusions were all ≤0.5. The levels of Class D and Ds inclusions were all ≤1.

[0078] The above measurements of center segregation level, center porosity level, and inclusion level of continuously cast billets can be based on YB / T 4003. The 2016 "Low-Magnification Microstructure Defect Rating Chart for Continuously Cast Steel Slabs" was developed.

[0079] In this process, the molten steel obtained from the steelmaking process is cast into a continuously cast billet on a continuous casting machine. Specifically, the molten steel is first distributed to the crystallizer via an tundish, where it solidifies to form a billet shell. The billet shell still contains molten steel. After exiting the crystallizer, it is cooled in a secondary cooling zone using air mist cooling, with a water concentration of 0.5~0.7 L / kg. This rapid cooling through a strong cooling mode prevents inclusions from agglomerating during the solidification process, thereby reducing the inclusion level.

[0080] In one embodiment, the steelmaking process includes sequential steps of molten iron pretreatment, converter smelting, LF refining, and RH refining.

[0081] In the converter smelting step, the molten pool in the converter is subjected to top and bottom blowing, with oxygen blown from the top and argon blown from the bottom. The flow rate of the bottom-blown argon is 0.8~1.2 Nm³. 3 / (min·t), lime and dolomite are used for slag making, and the slag basicity is controlled at 3.0~3.5. The P content in the molten steel at the end of the converter is less than 0.005% by mass percentage.

[0082] In the LF refining step, the slag basicity is controlled at 4.5~5.0, and the sulfur content in the molten steel is controlled to be less than 0.0010% by mass. After alloying, 100~300m / furnace of aluminum wire is fed in and stirred statically for 3~5 minutes, controlling the oxygen content in the molten steel to be less than 0.0040% by mass, and the tapping temperature is 1650~1670℃. Thus, by controlling the slag basicity to 4.5~5.0, deep desulfurization can be achieved, and by controlling the timing and amount of aluminum wire feeding, deep deoxidation can be achieved.

[0083] Among them, the LF refining process uses an LF furnace to refine molten steel, with a steel loading of 180t per furnace.

[0084] In the RH refining step, after the molten steel is allowed to stand under vacuum, calcium wire is fed in after the vacuum is broken to control the Ca, O, and S content in the molten steel to meet the following condition: 2.0 ≤ (Ca - 3.5O) / S ≤ 2.5. Then, the mixture is stirred for 2-3 minutes. Lanthanum-iron alloy is added to control the La content in the molten steel to 55-100 ppm, and the mixture is gently stirred with bottom-blown argon gas for 5-8 minutes, with an argon flow rate ≤ 1 Nm³. 3 / h; Add zirconium ferroalloy to adjust the Zr content in the molten steel to 0.012~0.016%, and gently stir with bottom-blown argon for 5~8 minutes, with an argon flow rate ≤1Nm 3 / h, then stir for 15~20min.

[0085] Thus, by controlling the content of calcium wire fed into the molten steel, Ca in the molten steel can preferentially react with O to form CaO, and Ca can be completely combined with S, avoiding the formation of low-melting-point FeS. After feeding the calcium wire, static stirring can promote the aggregation and growth of CaO and CaS inclusions, which is convenient for subsequent flotation and removal. Furthermore, by adding La, which has a strong affinity for O and S, rare earth oxides (La2O3), rare earth sulfides (La2S3), and rare earth sulfur oxides (La2O2S) can be generated. This can modify brittle Al2O3 and MnS inclusions and promote their spheroidization, making the aspect ratio of the sulfides ≤1.6 to improve impact toughness. By controlling the amount and timing of La addition, it is possible not only to ensure that there is enough La to react with the residual O and S to form stable rare earth inclusions and completely modify brittle Al2O3 and MnS inclusions, but also to avoid excessive La from causing increased viscosity and deterioration of fluidity in the molten steel, or the formation of coarse rare earth compounds. By gently stirring with argon gas after adding La, rare earth inclusions can be evenly distributed in the molten steel, while preventing vigorous stirring from entrapping air or exacerbating inclusion breakage. Adding zirconium-iron alloy allows Zr to preferentially react with O to form high-melting-point ZrO2, further reducing oxygen activity and achieving deep purification of the molten steel in conjunction with rare earth elements. Furthermore, Zr can react with C and N in the molten steel to form compounds such as ZrN and ZrC, which act as heterogeneous nucleation sites, inhibiting grain growth and improving the steel's strength, toughness, and weldability. Gentle stirring with argon gas after adding Zr promotes uniform dissolution of Zr, followed by prolonged static stirring, allowing inclusions to float fully and reducing the number and size of inclusions in the finished steel.

[0086] In the lanthanum-iron alloy, the mass percentage of La is 30-40%. In the zirconium-iron alloy, the mass percentage of Zr is 30-50%.

[0087] In one embodiment, in the pre-desulfurization step of the molten iron, magnesium powder, lime powder and fluorite powder are mixed and then injected into the molten iron to control the S content in the molten iron after desulfurization to be less than 0.0012% by mass percentage; wherein, the amount of magnesium powder injected is 0.35~0.45 kg per ton of molten iron.

[0088] In one embodiment, during the LF refining step, the heating and alloying process is carried out at a rate of 3~4 Nm for the first 10 minutes. 3 Argon gas was bottom-blown at a flow rate of / h, followed by 0.6~1.2Nm 3 A bottom-blown argon flow rate of / h is used. Thus, in the early stages of refining, a higher stirring intensity can accelerate the deoxidation reaction and the collision and aggregation of inclusions; while in the later stages of refining, a lower stirring intensity can be used to avoid slag entrapment caused by excessive agitation of molten steel, while promoting the flotation and removal of inclusions.

[0089] In one embodiment, the RH refining step, specifically the vacuum settling of molten steel, includes: setting the molten steel under a vacuum of ≤30 Pa for 25-30 minutes to control the O content in the molten steel to be 0.001-0.003% by mass. This allows dissolved O, N, and H in the molten steel to escape due to the reduced partial pressure under high vacuum.

[0090] The following are some specific embodiments to further illustrate the beneficial effects of this application.

[0091] Example 1 The production method of steel plates includes the following steps in sequence: steel smelting, continuous casting, heating, rolling, cooling, quenching, and tempering.

[0092] The steelmaking process includes the sequential steps of hot metal pretreatment, converter smelting, LF refining, and RH refining.

[0093] In the pre-desulfurization step of molten iron, magnesium powder, lime powder and fluorite powder are mixed and then injected into the molten iron to control the sulfur content in the molten iron after desulfurization to be 0.0010%; among them, the amount of magnesium powder injected is 0.4 kg per ton of molten iron.

[0094] In the converter smelting process, the molten pool in the converter is subjected to top and bottom blowing, with oxygen blown from the top and argon blown from the bottom. The flow rate of the bottom-blown argon is 0.9 Nm³. 3 / (min·t), lime and dolomite are used for slag making, the slag basicity is controlled at 3.2, and the P content in the molten steel at the end of the converter is 0.0012%.

[0095] In the LF refining step, an LF furnace is used to refine the molten steel, with a charge of 180t per furnace. During the heating and alloying process, the initial charge is 3.6 Nm. 3 Argon gas was bottom-blown at a flow rate of / h, followed by 0.8 Nm³ / h. 3 Bottom-blown argon gas was supplied at a flow rate of / h. During the refining process, the slag basicity was controlled at 4.8, and the S content in the molten steel was controlled at 0.0008%. After alloying, 260m / furnace of aluminum wire was fed in and stirred statically for 4.5min. The O content in the molten steel was controlled at 0.0032%, and the tapping temperature was 1665℃.

[0096] In the RH refining step, molten steel is allowed to stand under a high vacuum of 28 Pa for 26 minutes to control the O content in the molten steel to 0.0016%. Then, the vacuum is broken and calcium wire is fed in to adjust the Ca content in the molten steel to 0.0075%, followed by stirring for 2.2 minutes. Lanthanum-iron alloy is added to adjust the La content in the molten steel to 70 ppm, and the mixture is then gently stirred with bottom-blown argon gas for 6 minutes at a flow rate of 0.8 Nm³. 3 / h; Zirconium-iron alloy was added to adjust the Zr content in the molten steel to 0.015%, and the mixture was gently stirred with bottom-blown argon for 7 minutes at a flow rate of 0.8 Nm. 3 / h, then stir statically for 18min.

[0097] In the lanthanum-iron alloy, the mass percentage of La is 35%, and in the zirconium-iron alloy, the mass percentage of Zr is 40%.

[0098] After the above steps, the chemical composition of the molten steel obtained from the steelmaking process, by mass percentage, includes: C 0.19%, Si 0.38%, Mn 0.89%, Cr 0.34%, Ni 0.54%, Nb 0.015%, Ti 0.015%, Alt 0.032%, Ca 0.0032%, B 0.0019%, Zr 0.014%, La 85ppm, N 32ppm, O 8ppm, H 1.2ppm, P 0.007%, S 0.0008%, (Nb+Ti) / (C+N)=0.16, La / (O+N+S)=1.77, carbon equivalent CEV=0.44%, Mn equivalent ∑Mn=1.33%, and hardenability index J=2.20.

[0099] In the continuous casting process, the molten steel obtained from smelting is poured into a continuous casting machine to form a continuously cast billet with a thickness of 220 mm. The chemical composition of the billet is approximately the same as that of the molten steel obtained from the smelting process. The superheat of the molten steel during casting is 12°C, the casting speed v is 1.0 m / min, and after exiting the crystallizer, it is cooled in the secondary cooling zone using air mist cooling. The specific water volume in the secondary cooling zone is 0.6 L / kg. A light reduction is applied at the end of the continuous casting process, with a reduction rate of 1.3 mm / m.

[0100] Measurements showed that the center segregation level of the continuously cast billet was C1.0, with no Class A segregation, and the center porosity level was 0.5. Inclusions of Class A, B, and C were all at level 0, and inclusions of Class D and Ds were all at level 0.5.

[0101] In the heating process, the continuous casting billet is sent into the heating furnace and heated to 1180℃ and held for 45 minutes. The total time the continuous casting billet is in the furnace is 265 minutes.

[0102] In the rolling process, the continuously cast billet is first rough rolled to an intermediate billet with a thickness of 65mm. The initial rolling temperature of the rough rolling is 1050℃. Then, it is finished rolled into a steel plate with a thickness of 16mm. The initial rolling temperature of the finish rolling is 880℃, and the final rolling temperature is 830℃.

[0103] During the cooling process, the rolled steel plate immediately enters the ultra-fast cooling system and is rapidly cooled by water cooling. The water temperature of the steel plate is 780℃, the cooling rate is 10℃ / s, and the final cooling temperature of the steel plate is 540℃.

[0104] In the quenching process, the cooled steel plate is sent into the heating furnace for heating and then rapidly water-cooled to room temperature on the quenching machine. The heating temperature is 850℃, the heating time is 50min, and the cooling rate is 22℃ / s.

[0105] In the tempering process, the quenched steel plate is sent into a heating furnace and heated to 200°C. It is then held in the heating furnace to obtain the finished steel plate. The total time the steel plate is in the furnace is 100 minutes.

[0106] Samples of the finished steel plates were taken and tested, and the results were as follows: (1) such as Figure 1 As shown, the metallographic structure of the steel plate is a composite structure of lath martensite, acicular martensite, ferrite, and retained austenite. The combined area of ​​lath martensite and acicular martensite on the metallographic sampling surface is approximately 91%, ferrite accounts for approximately 3%, and retained austenite accounts for approximately 6%. The metallographic image of retained austenite is shown below. Figure 2 As shown.

[0107] (2) The yield strength is 1120MPa, the tensile strength is 1400MPa, the hardness is 435HBW, the elongation is 14%, and the impact energy at -40℃ is AKV 65J; the steel plate is cold-bent 180° with a bending mandrel diameter D=3a and there are no cracks on the surface.

[0108] (3) The strength-ductility product of the steel plate is 19.6 GPa·%.

[0109] Example 2 The production method of steel plates includes the following steps in sequence: steel smelting, continuous casting, heating, rolling, cooling, quenching, and tempering.

[0110] The steelmaking process includes the sequential steps of hot metal pretreatment, converter smelting, LF refining, and RH refining.

[0111] In the pre-desulfurization step of molten iron, magnesium powder, lime powder and fluorite powder are mixed and then injected into the molten iron to control the sulfur content in the molten iron after desulfurization to be 0.0009%; among them, the amount of magnesium powder injected is 0.42 kg per ton of molten iron.

[0112] In the converter smelting process, the molten pool in the converter is subjected to top and bottom blowing, with oxygen blown from the top and argon blown from the bottom. The flow rate of the bottom-blown argon is 1.0 Nm³. 3 / (min·t), lime and dolomite are used for slag making, the slag basicity is controlled at 3.3, and the P content in the molten steel at the end of the converter is 0.0010%.

[0113] In the LF refining step, an LF furnace is used to refine the molten steel, with a charge of 180t per furnace. During the refining heating and alloying process, the initial temperature is 3.8 Nm. 3 Bottom-blown argon gas at a flow rate of / h; then at 0.7Nm 3 Bottom-blown argon gas was supplied at a flow rate of / h. During the refining process, the slag basicity was controlled at 4.7, and the S content in the molten steel was controlled at 0.0008%. After alloying, 250m / furnace of aluminum wire was fed in and stirred statically for 4.5min. The O content in the molten steel was controlled at 0.0030%, and the tapping temperature was 1668℃.

[0114] In the RH refining step, molten steel is allowed to stand under a high vacuum of 26 Pa for 27 minutes to control the O content in the molten steel to 0.0015%. Then, the vacuum is broken and calcium wire is fed in to adjust the Ca content in the molten steel to 0.0070%, followed by stirring for 2.5 minutes. Lanthanum-iron alloy is added to adjust the La content in the molten steel to 75 ppm, and the mixture is then gently stirred with bottom-blown argon gas for 7 minutes at a flow rate of 0.7 Nm³. 3 / h; Zirconium-iron alloy was added to adjust the Zr content in the molten steel to 0.014%, and the mixture was gently stirred with bottom-blown argon for 6 minutes at a flow rate of 0.9 Nm. 3 / h, then stir statically for 16min.

[0115] In the lanthanum-iron alloy, the mass percentage of La is 35%, and in the zirconium-iron alloy, the mass percentage of Zr is 40%.

[0116] After the above steps, the chemical composition of the molten steel obtained from the steelmaking process, by mass percentage, includes: C 0.20%, Si 0.39%, Mn 0.92%, Cr 0.35%, Ni 0.56%, Nb 0.017%, Ti 0.015%, Alt 0.033%, Ca 0.0032%, B 0.0016%, Zr 0.015%, La 78ppm, N 35ppm, O 8ppm, H 1.1ppm, P 0.008%, S 0.0009%, (Nb+Ti) / (C+N)=0.16, La / (O+N+S)=1.50, carbon equivalent CEV=0.46%, Mn equivalent ∑Mn=1.37%, and hardenability index J=2.41.

[0117] In the continuous casting process, molten steel obtained from smelting is poured into a continuous casting machine to form a continuously cast billet with a thickness of 220 mm. The chemical composition of the billet is approximately the same as that of the molten steel obtained from the smelting process. The superheat of the molten steel during casting is 15°C, the casting speed v is 1.0 m / min, and after exiting the crystallizer, it is cooled in the secondary cooling zone using air mist cooling. The specific water volume in the secondary cooling zone is 0.7 L / kg. A light reduction is applied at the end of the continuous casting process, with a reduction rate of 1.4 mm / m.

[0118] Measurements showed that the center segregation level of the continuously cast billet was C1.0, with no Class A segregation, and the center porosity level was 0.5. Inclusions of Class A, B, and C were all at level 0, and inclusions of Class Ds were all at level 0.5.

[0119] In the heating process, the continuous casting billet is sent into the heating furnace and heated to 1200℃ and held for 48 minutes. The total time the continuous casting billet is in the furnace is 268 minutes.

[0120] In the controlled rolling process, the continuously cast billet is first rough rolled to an intermediate billet with a thickness of 90 mm. The initial rolling temperature of the rough rolling is 1050℃. Then, it is finished rolled into a steel plate with a thickness of 25 mm. The initial rolling temperature of the finish rolling is 875℃, and the final rolling temperature is 840℃.

[0121] During the cooling process, the rolled steel plate immediately enters the ultra-fast cooling system and is rapidly cooled by water cooling. The water temperature of the steel plate is 790℃, the cooling rate is 12℃ / s, and the final cooling temperature of the steel plate is 530℃.

[0122] In the quenching process, the cooled steel plate is sent into the heating furnace for heating and then rapidly water-cooled to room temperature on the quenching machine. The heating temperature is 850℃, the heating time is 60min, and the cooling rate is 20℃ / s.

[0123] In the tempering process, the quenched steel plate is sent into a heating furnace and heated to 200°C. It is then held in the heating furnace to obtain the finished steel plate. The total time the steel plate is in the furnace is 150 minutes.

[0124] Samples of the finished steel plates were taken and tested, and the results were as follows: (1) such as Figure 3 As shown, the metallographic structure of the steel plate is a composite structure of lath martensite, acicular martensite, ferrite, and retained austenite. The sum of the area ratios of lath martensite and acicular martensite in the metallographic sampling surface is approximately 91%, the area ratio of ferrite in the metallographic sampling surface is approximately 3%, and the area ratio of retained austenite in the metallographic sampling surface is approximately 6%.

[0125] (2) The yield strength is 1150MPa, the tensile strength is 1450MPa, the hardness is 438HBW, the elongation is 15%, and the impact energy at -40℃ is AKV 66J; the steel plate is cold-bent 180° with a bending mandrel diameter D=3a and there are no cracks on the surface.

[0126] (3) The strength-ductility product of the steel plate is 21.75 GPa·%.

[0127] It should be understood that although this specification describes embodiments, not every embodiment contains only one independent technical solution. This way of describing the specification is only for clarity. Those skilled in the art should regard the specification as a whole. The technical solutions in each embodiment can also be appropriately combined to form other embodiments that can be understood by those skilled in the art.

[0128] The detailed descriptions listed above are merely specific descriptions of feasible embodiments of the present invention, and are not intended to limit the scope of protection of the present invention. All equivalent embodiments or modifications made without departing from the spirit of the present invention should be included within the scope of protection of the present invention.

Claims

1. A steel plate, characterized in that, Its chemical composition, by mass percentage, includes: C 0.18~0.21%, Si 0.36~0.42%, Mn 0.86~0.94%, Cr 0.32~0.38%, Ni 0.52~0.58%, Nb 0.012~0.019%, Ti 0.012~0.019%, Alt 0.021~0.049%, Ca 0.0022~0.0052%, B 0.0016~0.0021%, Zr 0.012~0.019%, La 55~100ppm, N 22~42ppm, O≤15ppm, H≤1.5ppm, P≤0.008%, S≤0.0012%, the remainder being Fe and unavoidable impurities, and satisfying: 0.12≤(Nb+Ti) / (C+N)≤0.18, 1≤La / (O+N+S)≤2, carbon equivalent CEV is 0.40~0.50%, Mn equivalent ∑Mn is 1.30~1.40%, hardenability index J≥2.1; CEV=C+Mn / 6+(Cr+Mo+V) / 5+(Cu+Ni) / 15; ∑Mn=Mn+3.28Mo+0.64Cr+0.5W+0.37Ni+0.23Cu+0.03Si+0.1(Alt+V+Ti+Nb); J = (1.24C - 0.95C) 2 )×(0.7Si+1)×(3.33Mn+1)×(2.16Cr+1)×(3Mo+1)×(0.36Ni+1)×(0.37Cu+1)×(1.73V+1)×(1.55Alt+1)。 2. The steel plate according to claim 1, characterized in that, The microstructure of the steel plate is a composite microstructure of lath martensite, acicular martensite, ferrite, and retained austenite. The sum of the proportions of lath martensite and acicular martensite is 90-94%, the proportion of ferrite is 3-5%, and the proportion of retained austenite is 3-7%.

3. The steel plate according to claim 1, characterized in that, The steel plate has a thickness a of 6~30mm, a yield strength ≥1050MPa, a tensile strength ≥1350MPa, a hardness of 420~460HBW, an elongation ≥12%, and an impact energy AKV ≥50J at -40℃; it is cold-bent 180° with a bending mandrel diameter D=3a, and the steel plate surface is free of cracks.

4. The steel plate according to claim 1, characterized in that, The strength-ductility product of the steel plate is ≥16.5 GPa·%.

5. A method for producing steel plates, characterized in that, The production method includes the following steps performed in sequence: steel smelting, continuous casting, heating, rolling, cooling, quenching, and tempering. In the heating process, the heating temperature is (T) Nb(C,N) +30)℃~(T Nb(C,N) +60)℃, heat preservation time is 0.1d~0.5dmin; In the rolling process, the continuously cast billet is sequentially subjected to rough rolling and finish rolling to form a steel plate. The initial rolling temperature for rough rolling is (T). Nb(C,N) -130)℃~(T Nb(C,N) -50)℃, the initial rolling temperature for finishing rolling is (T nr -20)℃~(T nr +10)℃, final rolling temperature is (A) r3 +60)℃~(A r3 +100)℃; In the cooling process, the steel plate is cooled at a rate of (V). B +5) ℃ / s~(V B +10)℃ / s; In the quenching process, the steel plate that has undergone the cooling process is heated and then rapidly cooled, with the heating temperature being (A). c3 -30)℃~(A c3 -10)℃, heating time is (1.5a+20)~(1.5a+30)min, cooling rate is (V B +15) ℃ / s~(V B +20)℃ / s; In the tempering process, the heating temperature is (Ms-235)℃~(Ms-210)℃, where Ms is the martensitic transformation start temperature, Ms=561-474C-33Mn-17Cr-17Ni-21Mo; Among them, T Nb(C,N) =6770 / (2.26 lg((C+12 / 14N)×Nb))-273; recrystallization temperature T nr =887+464C+6445Nb-644 +890Ti+363Alt-357Si; A r3 A is the temperature at which austenite precipitates ferrite during the cooling process. r3 =910-310C-80Mn-20Cu-15Cr-55Ni-80Mo; A c3 A is the austenitizing completion temperature. c3 =912-250C-16Mn+48Si-2Cr-16Ni+95V+96Ti+210Alt-10Cu; a is the thickness of the steel plate; V B V is the critical cooling rate for bainite formation. B =e (13.08-8.8C-1.07Mn-0.7Ni-0.57Cr-9.2Mo-366B) / 3600.

6. The method for producing steel plates according to claim 5, characterized in that, In the heating process, the continuously cast billet is fed into a heating furnace and heated to the heating temperature, and then kept at that temperature in the heating furnace. The total time the continuously cast billet is in the furnace is 1.1d to 1.5dmin.

7. The method for producing steel plates according to claim 5, characterized in that, In the cooling process, the steel plate is water-cooled, and the water immersion temperature of the steel plate is ≥ (A). r3 -20)℃, the final cooling temperature of the steel plate is (Bs-10)℃~ (Bs-30)℃, where Bs is the bainite transformation start temperature, Bs=630-45Mn-40V-35Si-30Cr-25Mo-20Ni-15W.

8. The method for producing steel plates according to claim 5, characterized in that, In the tempering process, the steel plate that has undergone the quenching process is sent into the heating furnace and heated to the heating temperature, and then kept at the temperature in the heating furnace. The total time the steel plate is in the furnace is (5a+10)~(5a+30) min.

9. The method for producing steel plates according to claim 5, characterized in that, In the continuous casting process, the molten steel obtained from the steel smelting process is continuously cast into a continuous casting billet. The superheat of the molten steel during casting is 8~20℃. Light reduction is applied at the end of the continuous casting process, with a reduction rate of 1.2~1.4 mm / m. The continuous casting speed v satisfies: 200×L / F-0.05≤v≤200×L / F+0.05, where L is the perimeter of the continuous casting billet obtained in the continuous casting process, in mm, and F is the cross-sectional area of ​​the continuous casting billet, in mm². 2 .

10. The method for producing steel plates according to claim 9, characterized in that, In the continuous casting process, after the continuously cast billet exits the crystallizer, it is cooled in the secondary cooling zone, and the specific water volume in the secondary cooling zone is controlled to be 0.5~0.7L / kg.

11. The method for producing steel plates according to claim 9, characterized in that, The thickness of the continuously cast billet obtained by the continuous casting process is 220~320mm. The center segregation level of the continuously cast billet is ≤C1.5 or B1.0, and there is no A-type segregation. The center porosity level is ≤0.

5. The levels of A, B and C type inclusions are all ≤0.

5. The levels of D and Ds type inclusions are all ≤1.

12. The method for producing steel plates according to claim 5, characterized in that, The steelmaking process includes sequential steps of hot metal pretreatment, converter smelting, LF refining and RH refining. In the converter smelting step, the molten pool in the converter is subjected to top and bottom blowing, with oxygen blown from the top and argon blown from the bottom. The flow rate of the bottom-blown argon is 0.8~1.2 Nm³. 3 / (min·t), lime and dolomite are used for slag formation, and the slag basicity is controlled at 3.0~3.

5. The P content in the molten steel at the end of the converter is less than 0.005% by mass percentage; In the LF refining step, the slag basicity is controlled at 4.5~5.0, and the S content in the molten steel is controlled to be less than 0.0010% by mass. After alloying, 100~300m / furnace of aluminum wire is fed in and stirred for 3~5min. The O content in the molten steel is controlled to be less than 0.0040% by mass, and the tapping temperature is 1650~1670℃. In the RH refining step, after the molten steel is allowed to stand under vacuum, calcium wire is fed in after the vacuum is broken to control the Ca, O, and S content in the molten steel to meet the following condition: 2.0 ≤ (Ca - 3.5O) / S ≤ 2.

5. Then, the mixture is stirred for 2-3 minutes. Lanthanum-iron alloy is added to control the La content in the molten steel to 55-100 ppm, and the mixture is gently stirred with bottom-blown argon gas for 5-8 minutes, with an argon flow rate ≤ 1 Nm³. 3 / h; Add zirconium ferroalloy to adjust the Zr content in the molten steel to 0.012~0.016%, and gently stir with bottom-blown argon for 5~8 minutes, with an argon flow rate ≤1Nm 3 / h, then stir for 15~20min.

13. The method for producing steel plates according to claim 12, characterized in that, In the pre-desulfurization step of the molten iron, magnesium powder, lime powder and fluorite powder are mixed and then injected into the molten iron to control the S content in the molten iron after desulfurization to be less than 0.0012% by mass percentage; wherein, the amount of magnesium powder injected is 0.35~0.45 kg per ton of molten iron.

14. The method for producing steel plates according to claim 12, characterized in that, In the LF refining step, during the heating and alloying process, the temperature is 3~4 Nm for the first 10 minutes. 3 Argon gas was bottom-blown at a flow rate of / h, followed by 0.6~1.2Nm 3 Bottom-blown argon gas at a flow rate of / h.

15. The method for producing steel plates according to claim 12, characterized in that, In the RH refining step, the step of vacuum settling the molten steel includes: settling the molten steel under a vacuum of ≤30Pa for 25~30min to control the O content in the molten steel to be 0.001~0.003% by mass percentage.

16. The method for producing steel plates according to claim 12, characterized in that, In the RH refining step, the mass percentage of La in the lanthanum-iron alloy is 30-40%, and the mass percentage of Zr in the zirconium-iron alloy is 30-50%.

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