A high-toughness and high-strength steel

Abstract

This invention belongs to the field of iron and steel metallurgy and high-strength structural steel technology, and discloses a high-toughness and high-strength steel. The austenitic transformation point temperature of the steel plate is increased by the combined addition of Al and Cr elements; the hardenability of the steel plate is improved by C, Mn, and Cr elements; in the smelting and continuous casting stages, LF-VD combined refining is used to ensure that the oxygen content (O) in the molten steel is ≤0.0040wt% and the nitrogen content (N) is ≤0.0050wt%; in the continuous casting stage, light pressure reduction or electromagnetic stirring is used to improve the low-magnification quality of the continuously cast billet, and the maximum width of the macroscopic segregation band visible on the cross-section of the continuously cast billet is ≤5mm. Uniform heating, rolling in the austenitic single-phase region and rolling in the two-phase region, and offline tempering are used to achieve the regulation of the two-phase microlayer structure and the control of strength and toughness. The prepared steel plate has a yield strength ≥1000MPa, tensile strength ≥1250MPa, elongation after fracture ≥8%, and impact energy Akv ≥80J at -40℃.

Description

Technical Field

[0001] This invention relates to the field of iron and steel metallurgy and high-strength structural steel technology, specifically to a high-toughness, high-strength steel. Background Technology

[0002] High-strength structural steel plates are widely used in engineering machinery and other fields due to their combination of high strength, good plasticity, and relatively low alloy cost. However, with the increasing demand for specialized engineering machinery used in harsh environments such as extremely low temperatures and high-altitude permafrost, existing high-strength steels have revealed deficiencies in low-temperature impact toughness. This directly affects the impact resistance and service life of these devices under frigid conditions, hindering their widespread adoption. Therefore, developing a high-strength, high-toughness steel that possesses both high strength and excellent impact toughness at low temperatures, while maintaining a controllable cost, has become an urgent technical problem to be solved in this field.

[0003] Currently, high-strength steel with a yield strength of 1000 MPa is widely used in key load-bearing components of mining machinery and engineering machinery, demonstrating excellent strength and formability. To ensure high hardness and high strength, the microstructure of existing high-strength steel with a yield strength of 1000 MPa is typically lath martensite. Although lath martensite enhances strength through high dislocation density and fine substructure, it also increases crack susceptibility, leading to a decrease in impact toughness. Under low-temperature service conditions, low-toughness high-strength steel plates are more prone to cracking under impact loads, significantly reducing equipment safety. Therefore, improving the low-temperature impact toughness of high-strength structural steel remains a key technology that needs to be mastered. To this end, this invention proposes a high-toughness high-strength steel that differs significantly from existing high-strength structural steel technologies in terms of alloy composition design, preparation process, and microstructure control.

[0004] Chinese patent CN118639142A discloses a method for producing 1000MPa grade steel for engineering structures. Its composition by mass percentage is as follows: C: 0.08%~0.15%, Si: 0.06%~0.15%, Mn: 0.5%~1.0%, P≤0.01%, S≤0.015%, Nb: 0.010%~0.040%, Ti: 0.010%~0.020%, Als: 0.02%~0.05%, V: 0.010%~0.040%, B: 0.0005%~0.001%, N≤0.0035%, Cr: 0.5%~0.8%, Ni: 0.5%~1.0%, Cu: 0.20%~0.4%, Ceq: 0.45%~0.52%, Ceq=C+Mn / 6+(Cr+Mo+V) / 5+(Cu+Ni) / 15, with the balance being Fe and unavoidable impurities, and the total amount of impurity elements ≤0.05%. The steel plate manufacturing process includes steelmaking, ladle refining, RH vacuum degassing, micro-Ti treatment, B alloying, continuous casting, billet reheating, controlled rolling and cooling, hot straightening, stacking slow cooling, and quenching. The steel plate described in this patent has a significantly low Al content, does not further limit the contents of C, Al, Ni, and Cr, and the manufacturing process does not involve two-phase rolling.

[0005] Chinese Patent Publication No. CN108018503B discloses a layered ultrafine-grained dual-phase ferritic / martensitic steel and its preparation method. The typical composition by mass percentage is: C: 0.15-0.25%, Cr: 0.5-2.0%, Mn: 1.5-3.0%, Si: 0.7-2.50%, P: <0.05%, S: <0.06%, with the balance being Fe. The microstructure of the layered ultrafine-grained dual-phase steel is characterized by alternating layers of ferrite and martensite, with the ferrite exhibiting an equiaxed shape and an average grain size of approximately 1 μm. The volume fraction of martensite is 10-50%, the martensite lamellar thickness is 0.2-2 μm, and the lamellar spacing is 0.2-2 μm. The Cr and Al content of the steel plate described in this patent is significantly low, and no further limitations are made on the content of C, Al, Ni, and Cr.

[0006] The material and process system proposed in this invention is applicable to high-toughness and high-strength steel with a yield strength of 1000MPa. It has broad application potential in extremely cold and low-temperature service environments such as engineering equipment that are in service in low-temperature environments. It can effectively make up for the performance defects of traditional high-toughness and high-strength steel in low-temperature service environments. Summary of the Invention

[0007] (I) The technical problems to be solved; The purpose of this invention is to overcome the shortcomings of existing technologies and address the problem of low-temperature impact toughness in existing 1000MPa grade high-strength structural steel, which leads to cracking during service and insufficient safety. This invention provides a high-toughness, high-strength steel. Based on low-alloy high-strength structural steel, the invention employs the synergistic regulation of the two-phase transformation temperature using C, Al, Ni, and Cr elements to provide a reasonable process window for two-phase rolling. By combining microalloying, two-phase hot rolling, and online quenching processes, and designing corresponding two-phase rolling temperatures according to the alloy composition, the high-strength steel achieves a martensitic and ferrite two-phase microlayered morphology. While ensuring high strength and hardness, the layered structure toughening mechanism gives it extremely high low-temperature impact toughness. The manufacturing process is simple and easy to promote, effectively solving the problem of insufficient safety of high-strength steel plates in extremely low-temperature and high-altitude service conditions, and promoting the development of engineering machinery and equipment for special service environments.

[0008] (II) Technical Solution; To achieve the above objectives, the present invention adopts the following technical solution: A high-toughness, high-strength steel has the following chemical composition by mass percentage: C: 0.18%~0.22%, Si: 0.10%~0.16%, Mn: 0.7%~1.2%, Cr: 0.5%~1.0%, Al: 0.9%~1.5%, Ni: 0.8%~1.2%, B: 0.001%~0.003%, Nb: 0.01%~0.03%, Ti: 0.01%~0.03%, V: 0.02%~0.06%, Ca: 0.001%~0.003%, S≤0.003%, P≤0.010%, N≤0.005%, O≤0.004%, with the balance being Fe and unavoidable impurities.

[0009] A method for preparing high-toughness and high-strength steel, the specific steps of which are as follows: Step S1: Smelting and continuous casting to obtain a continuously cast billet; Step S2, rolling in the single-phase region and the two-phase region of austenite: The continuously cast billet is heated to 1150℃~1250℃ and held for 2h~3.5h, ensuring the temperature difference between the core and surface of the billet is ≤50℃. The held billet is then rolled in the austenitic single-phase region to obtain an intermediate billet. The final rolling temperature is 980℃~1020℃, and the roughing reduction is ≥60%. After hot rolling, the intermediate billet is air-cooled to the two-phase region of austenite and ferrite for two-phase rolling. The final rolling temperature is T. γ / α The following relationship must be satisfied: 890-203 W (C) 1 / 2 +60 W (Al)+15.2 W (Ni)≥T γ / α≥860-203 W (C) 1 / 2 +60 W (Al)+15.2 W (Ni); The finishing rolling reduction rate is ≥60%, and the hot-rolled steel plate is quenched online to room temperature by water cooling. Step S3, Heat treatment process: The steel plate after online quenching is subjected to offline low-temperature tempering for heat treatment.

[0010] The mass percentages of C, Cr, Al, and Ni satisfy the following relationship: 910-203 W (C) 1 / 2 +60 W (Al)+15.2 W (Ni)≥870; 723-16.9 W (Ni)+16.9 W (Cr)+40 W (Al)≥710; in, W (C) represents the mass percentage of element C, expressed as % . W (Al) represents the mass percentage of element Al, expressed as % . W (Ni) represents the mass percentage of element Ni, expressed as % . W (Cr) represents the mass percentage of element Cr, in percentages.

[0011] The high-toughness, high-strength steel plate has a two-phase microlayered morphology of martensite and ferrite, with a martensite volume fraction V. α' Satisfying 70%≤V α' ≤85%, ferrite volume fraction V α Satisfying 15%≤V α ≤30%; the ratio of the rolling direction dimension to the thickness direction dimension of the original austenite grain boundary of martensite is ≥3, and the ratio of the rolling direction dimension to the thickness direction dimension of the ferrite grain boundary is ≥3.

[0012] The high-toughness, high-strength steel has a yield strength ≥1000MPa, tensile strength ≥1250MPa, elongation after fracture ≥8%, and impact energy Akv ≥80J at -40℃.

[0013] The smelting and continuous casting specifically refer to: The molten iron is desulfurized to control the sulfur content to below 0.008%, and then the treated molten iron is smelted in a converter. After desulfurization, the molten iron is smelted into steel in a converter. During the converter smelting process, slag-forming agents and deoxidizers are added to reduce the sulfur content in the molten steel and ensure that the sulfur content is below 0.005%. The LF-VD combined refining process was employed, with a total refining time of ≥60 min. Ca was added to the molten steel at the end of the LF-VD combined refining process, and the reaction was carried out at 1580℃-1650℃ for 20-30 min. After LF-VD combined refining, the oxygen content (O) in the molten steel was ≤0.0040%, and the nitrogen content (N) was ≤0.0050%. Molten steel is poured under full protection to form a continuous casting billet; the traction speed of the continuous casting machine is 0.5m / min-1.5m / min; light pressure or electromagnetic stirring is used in the secondary cooling section of the continuous casting machine.

[0014] The total reduction rate under light pressure is 0.3%-3% of the thickness of the continuous casting billet before reduction; the electromagnetic stirring frequency is 0.5Hz–5Hz.

[0015] The offline low-temperature tempering specifically involves heating the online-quenched steel plate to 160℃-200℃ and holding it at that temperature for a duration t that satisfies the following relationship: t≥1.5×a; Where t is the heat preservation time in minutes; a is the thickness of the steel plate in mm; and the steel plate is air-cooled to room temperature after heat preservation.

[0016] The maximum width of the macrosegregation band on the cross-section of the continuously cast billet prepared according to step S1 is ≤5mm.

[0017] The roles of each element in the steel plate of this invention are as follows: Carbon (C) is one of the most fundamental and important alloying elements for improving the strength of steel. When C dissolves into the ferrite lattice, it causes lattice distortion, hindering dislocation movement through solid solution strengthening, thereby increasing the yield strength and tensile strength of the steel. Studies have shown that when the mass fraction of C varies within the range of 0.10% to 0.29%, the yield strength and tensile strength of lath martensitic steel exhibit an approximately linear relationship with the mass fraction of carbon. Simultaneously, under appropriate cooling conditions, as the C content increases, the hardenability and structural strengthening ability of the steel also improve, which is beneficial for obtaining a high-strength microstructure. However, excessively high C content can adversely affect the toughness and weldability of the material. On the one hand, higher C content easily promotes the formation of hard and brittle microstructures, reducing the low-temperature impact toughness of the steel and increasing its quenching crack susceptibility; on the other hand, increased carbon content leads to a decrease in the Ac3 temperature of high-strength steel, which is unfavorable for the implementation of two-phase rolling processes. Therefore, considering both strength and low-temperature impact toughness, the carbon content is designed to be 0.18% to 0.22%.

[0018] Si: Si is a common solid solution strengthening element in high-strength steel, and its appropriate addition can effectively improve the strength of the steel. Si atoms can dissolve infinitely in ferrite in the form of substitutional solid solution, causing lattice distortion and hindering dislocation movement, thereby increasing the strength of the steel. However, excessive Si content will reduce the toughness and weldability of the steel. Therefore, the Si content is designed to be 0.10%~0.16%.

[0019] Mn: As an austenite stabilizing element, manganese (Mn) plays a crucial role in low-alloy wear-resistant steels. It not only significantly reduces the martensitic transformation temperature but also decreases the retained austenite content. With increasing Mn content, the hardenability of the steel plate improves, contributing to the formation of a high-strength martensitic structure. However, the addition of Mn also brings some negative effects, such as increased temper brittleness and a tendency to produce segregation and banded structures. Furthermore, Mn has a high overheat sensitivity in steel, easily promoting rapid austenite grain growth; therefore, considering all factors, the Mn content is designed to be 0.7%~1.2%.

[0020] Cr: Cr can significantly improve the hardenability of steel, enabling it to obtain high-strength structures such as martensite or bainite even under relatively low cooling rates. In this application, Cr plays a role in regulating the temperature of the two-phase region. An appropriate amount of Cr can increase the Ac1 temperature, which is beneficial for controlling the ferrite volume fraction during the two-phase rolling process. Therefore, the Cr content is designed to be 0.50%~1.0%.

[0021] Ni: Because the atomic radius and volume of Ni are very close, it is uniformly distributed in the matrix. At the same time, by increasing the stacking fault energy, it makes it easier for dislocations to cross-slip at low temperatures, avoiding stress concentration and improving low-temperature impact toughness. However, Ni is relatively expensive. In actual production, the Ni content should be reduced as much as possible while ensuring that the steel plate has sufficient low-temperature impact toughness. Therefore, the Ni content is designed to be 0.8%~1.2%.

[0022] Al: Al can significantly expand the temperature range of the ferrite single-phase region and the austenite-ferrite two-phase region in the equilibrium phase diagram of high-strength steel, increase the Ac3 temperature, and expand the hot rolling process window in the two-phase region. Therefore, adding an appropriate amount of Al is beneficial for the implementation of hot rolling in the two-phase region. However, when the Al content is too high, δ-ferrite forms in the steel, which seriously deteriorates the impact toughness of high-strength steel. Therefore, the Al content in this application is designed to be 0.9%~1.5%.

[0023] In the design of the alloy composition of this steel plate, the constraints of the hot rolling process window in the two-phase region are fully matched. The process temperature range of hot rolling in the two-phase region is limited to the characteristic temperature of steel phase transformation Ac1 to Ac3. This is the core process for preparing the two-phase microlayer structure. The steel is subjected to plastic load in the Ac1 to Ac3 range to complete the hot rolling deformation. The austenite phase and α-ferrite phase grains in the matrix are simultaneously flattened and elongated under the action of rolling stress. The two phases are elongated along the rolling direction, and finally the characteristic two-phase microlayer structure is formed.

[0024] The temperatures of Ac1 to Ac3 directly determine the feasibility of rolling in the two-phase region and the microstructure and properties of the finished product. Temperatures that are too high or too low in Ac1 and Ac3 will have an adverse effect on rolling stability and the microstructure of the steel plate. On the one hand, if the temperature of alloy Ac1 and Ac3 is too high, the rolling temperature of the two-phase region will increase accordingly. The high temperature condition will induce dynamic recrystallization of deformed austenite. The deformed austenite grains that have been flattened and elongated during the rolling process will re-nucleate and grow through recrystallization, eventually transforming into regular equiaxed grains, completely destroying the two-phase microlayer morphology, making it impossible to obtain a two-phase microlayer structure, resulting in the toughness not meeting the requirements.

[0025] On the other hand, if the temperatures of alloy Ac1 and Ac3 are too low, the rolling temperature in the two-phase region will be too low, and the high-temperature plasticity of the steel plate will decrease sharply. Under plastic deformation conditions, a large number of dislocation entanglements and dislocation pile-ups will be generated inside the two-phase grains, which will significantly increase the yield strength of the matrix, but will induce stress concentration within the grains, damage the toughness of the material, and cause the impact toughness to fail to meet the requirements. At the same time, a large number of dislocation pile-ups inside the grains will further lead to an increase in hot rolling deformation, a sharp increase in rolling load, and a high risk of inducing rolling cracks, making it difficult to stably implement the hot rolling process in the two-phase region.

[0026] This application identifies that, within the alloy composition system, C, Al, Ni, and Cr elements significantly influence the phase transformation point of the steel plate. Increased Al and Ni content leads to a significant increase in both Ac3 and Ac1 temperatures, increased Cr content significantly increases the Ac1 temperature, and increased C content significantly decreases the Ac3 temperature. Based on the influence of these key elements on the phase transformation point, this application employs the principle of controlling a single variable, designing multiple alloy compositions and performing thermodynamic calculations to determine the phase transformation point temperature of the steel plate under different alloy compositions. Based on the thermodynamic calculation results, the alloy composition variable and the phase transformation point temperature are fitted, determining the phase transformation point Ac3 of the steel plate in this application to be 910-203. W (C) 1 / 2 +60 W (Al)+15.2 W (Ni), Ac1 = 723 - 16.9 W (Ni)+16.9 W (Cr)+40 W(Al). Based on the alloy composition range of this application, multiple heats of high-toughness and high-strength steel were smelted. The phase transformation point temperature of the steel plate was measured using a static phase transformation instrument. The phase transformation point temperature of this application was verified to conform to the calculation results of the formula. To ensure the smooth implementation of hot rolling in the two-phase region, the phase transformation point temperature of the steel plate is Ac3≥870℃, and Ac1≥710℃.

[0027] During the hot rolling process in the two-phase region, as the final rolling temperature decreases, the volume fraction of ferrite in the steel plate gradually increases, while the volume fraction of martensite gradually decreases. The strength and hardness of ferrite in the steel plate are significantly lower than those of martensite. Therefore, when the volume fraction of ferrite is too high, the strength of the steel plate decreases significantly, failing to meet the mechanical property requirements of this application. Thus, the volume fraction of ferrite in the steel plate, V, is determined. α Must satisfy 15%≤V α ≤30%. To achieve ferrite volume fraction control, the final rolling temperature T during hot rolling of the two-phase region of the steel plate is... γ / α Must meet 890-203 W (C) 1 / 2 +60 W (Al)+15.2 W (Ni)≥T γ / α ≥860-203 W (C) 1 / 2 +60 W (Al)+15.2 W (Ni).

[0028] To ensure that martensite and ferrite exhibit a microlayered morphology, the roughing reduction rate in the two-phase hot rolling region is ≥60%. The rolling deformation effect of the two-phase hot rolling region flattens the grains along the thickness direction and elongates them along the rolling direction. To ensure that the two-phase microlayered morphology formed by hot rolling in the two-phase region can be retained to room temperature, and to prevent the formation of equiaxed new phases during cooling that could disrupt the layered structure, the hot-rolled steel sheet must undergo an online quenching process and be water-cooled to room temperature. This prevents diffusive phase transformations in the steel sheet and effectively suppresses the formation of equiaxed grains. To achieve synergistic optimization of high strength and excellent low-temperature impact toughness in low-cost high-strength steel with a yield strength of 1000MPa, this invention addresses the problem of insufficient low-temperature impact toughness in high-grade high-strength steel. It develops a complete process design from alloy composition to smelting and continuous casting, fully utilizing the integrated design concept of alloy composition and rolling process. It increases the Ac3 temperature by adding C, Al, Ni, and Cr elements in a composite manner, and obtains a martensitic and ferrite two-phase microlayered structure through two-phase rolling and online quenching processes. The toughening effect of the layered structure significantly improves the low-temperature impact toughness of the high-strength steel plate in the direction perpendicular to the plate surface, ensuring that the high-strength steel possesses both high strength and high toughness, with a yield strength ≥1000MPa, tensile strength ≥1250MPa, elongation after fracture ≥8%, and impact energy Akv ≥80J at -40℃.

[0029] (III) Beneficial Effects; As can be seen from the above technical solution, compared with the prior art, the beneficial effects of the present invention are as follows: (1) Significantly improves the low-temperature impact toughness of high-strength steel; Compared with traditional high-strength steel products, this invention achieves significant improvement in low-temperature impact toughness (impact energy ≥80J at -40℃) while ensuring high strength, making it particularly suitable for the development of engineering machinery and equipment in extreme and harsh environments such as polar regions and plateaus.

[0030] (2) Constructing an integrated design concept for alloy design and preparation process; By adding C, Al, Ni, and Cr in combination, the temperature range of the two-phase region of high-strength steel is expanded and the Ac3 temperature is increased, providing a process window for deformation in the two-phase region. By combining online quenching and offline low-temperature tempering processes, a two-phase microlayered structure with both high strength and high toughness is obtained, which significantly improves the overall service performance of the steel.

[0031] (3) The process is highly feasible and the industrialization path is clear; All process parameters can be achieved under industrial production conditions, without relying on special equipment or high-cost processing methods, and have excellent prospects for promotion and industrialization value. Attached Figure Description

[0032] Figure 1 This is a schematic diagram of the rolling and heat treatment process of the steel plate in Example 1. Detailed Implementation

[0033] The technical solutions in the embodiments of the present invention will be clearly and completely described below. Obviously, the described embodiments are only a part of the embodiments of the present invention, and not all of them. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0034] Example 1 A 20mm thick high-toughness, high-strength steel is smelted according to its chemical composition, which is as follows by weight percentage: C: 0.18%, Si: 0.16%, Mn: 1.2%, Al: 0.9%, Cr: 1.0%, Ni: 1.2%, B: 0.003%, Nb: 0.01%, Ti: 0.01%, V: 0.02%, Ca: 0.003%, S: 0.002%, P: 0.010%, N: 0.0035%, O: 0.0025%, with the balance being Fe and unavoidable impurities; Its preparation methods include smelting and continuous casting, rolling of austenitic single-phase region and two-phase region, and heat treatment processes: Step S1, Smelting and Continuous Casting: The steel plates were smelted according to their chemical composition and content. After desulfurization treatment, the sulfur content in the molten iron was controlled to be below 0.008%. The molten iron was smelted in a converter and refined using a combined LF-VD process. The total LF-VD refining time was 80 minutes. The LF-VD refining process used Al as the initial deoxidizer, and Ca was added to the molten steel at the end of the LF-VD refining process and reacted at 1630℃ for 25 minutes. The molten steel was cast under full protection to form a continuous casting billet. The traction speed of the continuous casting machine was 1 m / min. In the secondary cooling section of the continuous casting machine, a light reduction was used to improve the quality of the continuous casting billet. The total reduction rate of the light reduction was 1.8%.

[0035] Step S2, rolling in the single-phase region and the two-phase region of austenite: The continuously cast billet was heated to 1200℃ and held for 2.5 hours, with a temperature difference of 40℃ between the core and surface of the billet. After holding, the billet was rolled in the austenitic single-phase region at a final rolling temperature of 990℃ and a roughing reduction of 65%. The intermediate billet was cooled to 860℃ and rolled in the two-phase region at a final rolling temperature of 860℃ and a finishing reduction of 60%. The hot-rolled steel plate was quenched online and then water-cooled to room temperature. Step S3, Heat treatment process: The steel plate after online quenching is subjected to offline low-temperature tempering heat treatment. The steel plate after online quenching is heated to 180℃ and held for 40 minutes. After holding, the steel plate is air-cooled to room temperature.

[0036] Testing revealed that the high-toughness, high-strength steel plate has a dual-phase microlayered structure of martensite and ferrite. The ratio of the rolling direction dimension to the thickness dimension of the original austenite grain boundary of martensite and the rolling direction dimension to the thickness dimension of ferrite grain boundary are both 4.2. The volume fraction of ferrite is 23%, the volume fraction of martensite is 77%, the yield strength is 1100 MPa, the tensile strength is 1320 MPa, the impact energy at -40℃ perpendicular to the plate surface is 92 J, and the elongation after fracture is 11%.

[0037] Example 2 A 20mm thick high-toughness, high-strength steel is smelted according to its chemical composition, which is as follows by weight percentage: C: 0.22%, Si: 0.10%, Mn: 0.7%, Al: 1.5%, Cr: 0.5%, Ni: 0.8%, B: 0.001%, Nb: 0.03%, Ti: 0.03%, V: 0.06%, Ca: 0.001%, S: 0.003%, P: 0.010%, N: 0.0035%, O: 0.0025%, with the balance being Fe and unavoidable impurities; The preparation methods include smelting and continuous casting, rolling of austenitic single-phase region and two-phase region, and heat treatment processes: Step S1, Smelting and Continuous Casting: The steel plates were smelted according to their chemical composition and content. After desulfurization treatment, the sulfur content in the molten iron was controlled to be below 0.008%. The molten iron was smelted in a converter and refined using a combined LF-VD process. The total time for the combined LF-VD process was 80 minutes. The combined LF-VD process used Al for initial deoxidation, and Ca was added to the molten steel at the end of the combined LF-VD process and reacted at 1630℃ for 25 minutes. The molten steel was cast under full protection to form a continuous casting billet. The traction speed of the continuous casting machine was 1 m / min. Electromagnetic stirring was used in the secondary cooling section of the continuous casting machine to improve the quality of the continuous casting billet, with an operating frequency of 0.8 Hz.

[0038] Step S2, rolling in the single-phase region and the two-phase region of austenite: The continuously cast billet was heated to 1200℃ and held for 2.5 hours, with a temperature difference of 42℃ between the core and surface of the billet. After holding, the billet was rolled in the austenitic single-phase region at a final rolling temperature of 990℃ and a roughing reduction of 65%. The intermediate billet was cooled to 880℃ and rolled in the two-phase region at a final rolling temperature of 875℃ and a finishing reduction of 60%. The hot-rolled steel plate was quenched online and then water-cooled to room temperature. Step S3, Heat treatment process: The steel plate after online quenching is heat-treated by offline low-temperature tempering process. The steel plate after online quenching is heated to 180℃ and held for 40 minutes. After holding, the steel plate is air-cooled to room temperature.

[0039] Testing revealed that the high-toughness, high-strength steel plate has a dual-phase microlayered structure of martensite and ferrite. The ratio of the rolling direction dimension to the thickness dimension of the original austenite grain boundary of martensite and the rolling direction dimension to the thickness dimension of ferrite grain boundary is 4.1. The volume fraction of ferrite is 28%, the volume fraction of martensite is 72%, the yield strength is 1080 MPa, the tensile strength is 1300 MPa, the impact energy at -40℃ perpendicular to the plate surface is 102 J, and the elongation after fracture is 11.1%.

[0040] Example 3 A 20mm thick high-toughness, high-strength steel is smelted according to its chemical composition, which is as follows by weight percentage: C: 0.20%, Si: 0.14%, Mn: 1.0%, Al: 1.2%, Cr: 0.6%, Ni: 1.0%, B: 0.002%, Nb: 0.02%, Ti: 0.02%, V: 0.04%, Ca: 0.001%, S: 0.003%, P: 0.010%, N: 0.0035%, O: 0.0030%, with the balance being Fe and unavoidable impurities; The preparation methods include smelting and continuous casting, rolling of austenitic single-phase region and two-phase region, and heat treatment processes: Step S1, Smelting and Continuous Casting: The steel plates were smelted according to their chemical composition and content. After desulfurization treatment, the sulfur content in the molten iron was controlled to be below 0.008%. The molten iron was smelted in a converter and refined using a combined LF-VD process. The total time for the combined LF-VD process was 80 minutes. The combined LF-VD process used Al for initial deoxidation, and Ca was added to the molten steel at the end of the combined LF-VD process and reacted at 1630℃ for 25 minutes. The molten steel was cast under full protection to form a continuous casting billet. The traction speed of the continuous casting machine was 1 m / min. Electromagnetic stirring was used in the secondary cooling section of the continuous casting machine to improve the quality of the continuous casting billet, with an operating frequency of 0.8 Hz.

[0041] Step S2, rolling in the single-phase region and the two-phase region of austenite: The continuously cast billet was heated to 1200℃ and held for 2.5 hours, with a temperature difference of 41℃ between the core and surface of the billet. After holding, the billet was rolled in the austenitic single-phase region at a final rolling temperature of 990℃ and a roughing reduction of 65%. The intermediate billet was cooled to 875℃ and rolled in the two-phase region at a final rolling temperature of 865℃ and a finishing reduction of 60%. The hot-rolled steel plate was quenched online and then water-cooled to room temperature. Step S3, Heat treatment process: The steel plate after online quenching is heat-treated by offline low-temperature tempering process. The steel plate after online quenching is heated to 180℃ and held for 40 minutes. After holding, the steel plate is air-cooled to room temperature.

[0042] Testing revealed that the high-toughness, high-strength steel plate has a dual-phase microlayered structure of martensite and ferrite. The ratio of the rolling direction dimension to the thickness dimension of the original austenite grain boundary of martensite and the rolling direction dimension to the thickness dimension of ferrite grain boundary is 4.2. The volume fraction of ferrite is 29%, the volume fraction of martensite is 71%, the yield strength is 1100 MPa, the tensile strength is 1310 MPa, the impact energy at -40°C perpendicular to the plate surface is 104 J, and the elongation after fracture is 11.7%.

[0043] Comparative Example 1 A 20mm thick high-toughness, high-strength steel was smelted according to its chemical composition, which, by weight percentage, is as follows: C: 0.20%, Si: 0.15%, Mn: 1%, Al: 1%, Cr: 0.9%, Ni: 1%, B: 0.002%, Nb: 0.012%, Ti: 0.011%, V: 0.03%, Ca: 0.002%, S: 0.003%, P: 0.010%, N: 0.0035%, O: 0.0025%, with the balance being Fe and unavoidable impurities. Preparation methods include smelting and continuous casting, hot rolling, and heat treatment processes: Step S1, Smelting and Continuous Casting: The steel plates were smelted according to their chemical composition and content. After desulfurization treatment, the sulfur content in the molten iron was controlled to be below 0.008%. The molten iron was smelted in a converter and refined using a combined LF-VD process. The total time for the combined LF-VD process was 80 minutes. The combined LF-VD process used Al for initial deoxidation, and Ca was added to the molten steel at the end of the combined LF-VD process and reacted at 1630℃ for 25 minutes. The molten steel was cast under full protection to form a continuous casting billet. The traction speed of the continuous casting machine was 1 m / min. In the secondary cooling section of the continuous casting machine, a light reduction was used to improve the quality of the continuous casting billet. The total reduction of the light reduction was 1.8%.

[0044] Step S2, Hot rolling process: The continuously cast billet was heated to 1200℃ and held for 2.5 hours, with a temperature difference of 38℃ between the core and surface of the billet. After holding, the billet was rolled in the austenitic single-phase region at a final rolling temperature of 990℃ and a roughing reduction of 65%. The intermediate billet was cooled to 920℃ and hot rolled at a final rolling temperature of 900℃ and a finishing reduction of 60%. The hot-rolled steel plate was then quenched online and water-cooled to room temperature. Step S3, Heat treatment process: The steel plate after online quenching is heat-treated by offline low-temperature tempering process. The steel plate after online quenching is heated to 180℃ and held for 40 minutes. After holding, the steel plate is air-cooled to room temperature.

[0045] Testing revealed that the high-toughness, high-strength steel plate has a fully martensitic microstructure, with the ratio of the rolling dimension to the thickness dimension of the original austenite grain boundary of martensite being 1.5. The yield strength is 1180 MPa, the tensile strength is 1400 MPa, the impact energy at -40°C perpendicular to the plate surface is 40 J, and the elongation after fracture is 10.2%.

[0046] Comparative Example 2 A 20mm thick high-toughness and high-strength steel was smelted according to its chemical composition, which is as follows by weight percentage: C: 0.20%, Si: 0.15%, Mn: 1%, Al: 0.5%, Cr: 0.8%, Ni: 1%, B: 0.002%, Nb: 0.012%, Ti: 0.011%, V: 0.03%, Ca: 0.002%, S: 0.003%, P: 0.010%, N: 0.0035%, O: 0.0025%, with the balance being Fe and unavoidable impurities; The preparation methods include smelting and continuous casting, rolling of austenitic single-phase region and two-phase region, and heat treatment processes: Step S1, Smelting and Continuous Casting: The steel plates were smelted according to their chemical composition and content. After desulfurization treatment, the sulfur content in the molten iron was controlled to be below 0.008%. The molten iron was smelted in a converter and refined using a combined LF-VD process. The total time for the combined LF-VD process was 80 minutes. The combined LF-VD process used Al for initial deoxidation, and Ca was added to the molten steel at the end of the combined LF-VD process and reacted at 1630℃ for 25 minutes. The molten steel was cast under full protection to form a continuous casting billet. The traction speed of the continuous casting machine was 1 m / min. In the secondary cooling section of the continuous casting machine, a light reduction was used to improve the quality of the continuous casting billet. The total reduction of the light reduction was 1.8%.

[0047] Step S2, rolling in the single-phase region and the two-phase region of austenite: The continuously cast billet was heated to 1200℃ and held for 2.5 hours, with a temperature difference of 40℃ between the core and surface of the billet. After holding, the billet was rolled in the austenitic single-phase region at a final rolling temperature of 990℃ and a roughing reduction of 65%. The intermediate billet was cooled to 830℃ and rolled in the two-phase region at a final rolling temperature of 814℃ and a finishing reduction of 60%. The hot-rolled steel plate was quenched online and then water-cooled to room temperature. Step S3, Heat treatment process: The steel plate after online quenching is heat-treated by low-temperature tempering process. The steel plate after online quenching is heated to 180℃ and held for 40mm. After holding, the steel plate is air-cooled to room temperature.

[0048] Testing revealed that the high-toughness, high-strength steel plate exhibits a martensitic and ferrite dual-phase microlayered structure. The ratio of the rolling direction dimension to the thickness dimension of the original austenite grain boundaries in martensite, as well as the ratio of the rolling direction dimension to the thickness dimension of the ferrite grain boundaries, is 3.2. The ferrite volume fraction is 23%, the martensite volume fraction is 77%, the yield strength is 1280 MPa, the tensile strength is 1410 MPa, the impact energy at -40°C perpendicular to the plate surface is 31 J, and the elongation after fracture is 7.6%. Due to the relatively low rolling deformation temperature, the dislocation density in both ferrite and martensite is high, but stress concentration occurs, reducing plasticity and toughness.

[0049] Comparative Example 3 A 20mm thick high-toughness and high-strength steel was smelted according to its chemical composition, which is as follows by weight percentage: C: 0.20%, Si: 0.15%, Mn: 1%, Al: 1.9%, Cr: 0.6%, Ni: 1%, B: 0.002%, Nb: 0.012%, Ti: 0.011%, V: 0.03%, Ca: 0.002%, S: 0.003%, P: 0.010%, N: 0.0035%, O: 0.0025%, with the balance being Fe and unavoidable impurities; The preparation methods include smelting and continuous casting, rolling of austenitic single-phase region and two-phase region, and heat treatment processes: Step S1, Smelting and Continuous Casting: The steel plates were smelted according to their chemical composition and content. After desulfurization treatment, the sulfur content in the molten iron was controlled to be below 0.008%. The molten iron was smelted in a converter and refined using a combined LF-VD process. The total time for the combined LF-VD process was 80 minutes. The combined LF-VD process used Al for initial deoxidation, and Ca was added to the molten steel at the end of the combined LF-VD process and reacted at 1630℃ for 25 minutes. The molten steel was cast under full protection to form a continuous casting billet. The traction speed of the continuous casting machine was 1 m / min. In the secondary cooling section of the continuous casting machine, a light reduction was used to improve the quality of the continuous casting billet. The total reduction of the light reduction was 1.8%.

[0050] Step S2, rolling in the single-phase region and the two-phase region of austenite: The continuously cast billet was heated to 1200℃ and held for 2.5 hours, with a temperature difference of 40℃ between the core and surface of the billet. After holding, the billet was rolled in the austenitic single-phase region at a final rolling temperature of 990℃ and a roughing reduction of 65%. The intermediate billet was cooled to 920℃ and rolled in the two-phase region at a final rolling temperature of 910℃ and a finishing reduction of 60%. The hot-rolled steel plate was quenched online and then water-cooled to room temperature. Step S3, Heat treatment process: The steel plate after online quenching is heat-treated by low-temperature tempering process. The steel plate after online quenching is heated to 180℃ and held for 40mm. After holding, the steel plate is air-cooled to room temperature.

[0051] Testing revealed that the high-toughness, high-strength steel plate has a dual-phase structure of martensite and ferrite. The ratio of the rolling direction dimension to the thickness dimension of the original austenite grain boundaries of martensite is 1.6, and the ratio of the rolling direction dimension to the thickness dimension of ferrite grain boundaries is 3.5. The volume fraction of ferrite is 21%, the volume fraction of martensite is 79%, the yield strength is 1050 MPa, the tensile strength is 1280 MPa, the impact energy at -40°C perpendicular to the plate surface is 60 J, and the elongation after fracture is 11.9%.

Claims

1. A high-toughness, high-strength steel, characterized in that, The chemical composition (by mass percentage) of the high-toughness, high-strength steel is as follows: C: 0.18%~0.22%, Si: 0.10%~0.16%, Mn: 0.7%~1.2%, Cr: 0.5%~1.0%, Al: 0.9%~1.5%, Ni: 0.8%~1.2%, B: 0.001%~0.003%, Nb: 0.01%~0.03%, Ti: 0.01%~0.03%, V: 0.02%~0.06%, Ca: 0.001%~0.003%, S≤0.003%, P≤0.010%, N≤0.005%, O≤0.004%, with the balance being Fe and unavoidable impurities; The specific steps for preparing the high-toughness and high-strength steel are as follows: Step S1: Smelting and continuous casting to obtain a continuously cast billet; Step S2, rolling in the single-phase region and the two-phase region of austenite: The continuously cast billet is heated to 1150℃~1250℃ and held for 2h~3.5h, ensuring the temperature difference between the core and surface of the billet is ≤50℃. The held billet is then rolled in the austenitic single-phase region to obtain an intermediate billet. The final rolling temperature is 980℃~1020℃, and the roughing reduction is ≥60%. After hot rolling, the intermediate billet is air-cooled to the two-phase region of austenite and ferrite for two-phase rolling. The final rolling temperature is T. γ / α The following relationship must be satisfied: 890-2 W (C) 1 / 2 +60 W (Al)+15.2 W (Ni)≥T γ / α ≥860-203 W (C) 1 / 2 +60 W (Al)+15.2 W (Ni) The finishing rolling reduction rate is ≥60%, and the hot-rolled steel plate is quenched online to room temperature by water cooling. Step S3, Heat treatment process: The steel plate after online quenching is subjected to offline low-temperature tempering for heat treatment.

2. The high-toughness, high-strength steel according to claim 1, characterized in that, in, The mass percentages of C, Cr, Al, and Ni satisfy the following relationship: 910-2 W (C) 1 / 2 +60 W (Al)+15.2 W (Ni)≥8 723-16.9 W (Ni)+1 W (Cr)+4 W (Al)≥7 in, W (C) represents the mass percentage of element C, expressed as % . W (Al) represents the mass percentage of element Al, expressed as % . W (Ni) represents the mass percentage of element Ni, expressed as % . W (Cr) represents the mass percentage of element Cr, in units of %.

3. The high-toughness, high-strength steel according to claim 1, characterized in that, The high-toughness, high-strength steel plate has a two-phase microlayered morphology of martensite and ferrite, with a martensite volume fraction V. α' Satisfying 70%≤V α' ≤85%, ferrite volume fraction V α Satisfying 15%≤V α ≤30%; the ratio of the rolling direction dimension to the thickness direction dimension of the original austenite grain boundary of martensite is ≥3, and the ratio of the rolling direction dimension to the thickness direction dimension of the ferrite grain boundary is ≥3.

4. The high-toughness, high-strength steel according to claim 1, characterized in that, The high-toughness, high-strength steel has a yield strength ≥1000MPa, tensile strength ≥1250MPa, elongation after fracture ≥8%, and impact energy Akv ≥80J at -40℃.

5. The high-toughness, high-strength steel according to claim 1, characterized in that, The smelting and continuous casting specifically refer to: The molten iron is desulfurized to control the sulfur content to below 0.008%, and then the treated molten iron is smelted in a converter. After desulfurization, the molten iron is smelted into steel in a converter. During the converter smelting process, slag-forming agents and deoxidizers are added to reduce the sulfur content in the molten steel and ensure that the sulfur content is below 0.005%. The LF-VD combined refining process was employed, with a total refining time of ≥60 min. Ca was added to the molten steel at the end of the LF-VD combined refining process, and the reaction was carried out at 1580℃-1650℃ for 20-30 min. After LF-VD combined refining, the oxygen content (O) in the molten steel was ≤0.0040%, and the nitrogen content (N) was ≤0.0050%. Molten steel is poured under full protection to form a continuous casting billet; the traction speed of the continuous casting machine is 0.5m / min-1.5m / min; light pressure or electromagnetic stirring is used in the secondary cooling section of the continuous casting machine.

6. The high-toughness, high-strength steel according to claim 5, characterized in that, The total reduction rate under light pressure is 0.3%-3% of the thickness of the continuous casting billet before reduction; the electromagnetic stirring frequency is 0.5Hz–5Hz.

7. The high-toughness, high-strength steel according to claim 1, characterized in that, The offline low-temperature tempering specifically involves heating the online-quenched steel plate to 160℃-200℃ and holding it at that temperature for a duration t that satisfies the following relationship: t≥1.5×a; Where t is the heat preservation time in minutes; a is the thickness of the steel plate in mm; and the steel plate is air-cooled to room temperature after heat preservation.

8. The high-toughness, high-strength steel according to any one of claims 1-7, characterized in that, The maximum width of the macrosegregation band on the cross-section of the continuously cast billet prepared in step S1 is ≤5mm.

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