High-strength and high-toughness steel with bimodal heterogeneous structure and hot forming method thereof

Abstract

This invention relates to a high-strength, high-toughness steel with a bimodal heterogeneous structure and its hot-forming method, comprising the following steps: heating a first hot-formed steel to a temperature above Ac3 for austenitization treatment to obtain a second hot-formed steel; cooling the second hot-formed steel to the bainite transformation temperature range to obtain a third hot-formed steel; subjecting the third hot-formed steel to pressure treatment within the bainite transformation temperature range, completing the isothermal transformation of bainite under stress to obtain a fourth hot-formed steel; and cooling the fourth hot-formed steel to room temperature at a cooling rate of ≥200℃ / s to obtain a high-strength, high-toughness steel. This invention uses applied stress to preferentially orient and nucleate and grow bainite, increasing the bainite grain size; the resulting coarse bainite divides the austenite grains into multiple small, isolated regions, forming refined martensite under ultra-high cooling rates, resulting in a hot-formed steel with both martensite and bainite coexisting and large grain size differences, possessing high strength, high plasticity, and high toughness.

Description

Technical Field

[0001] This invention belongs to the field of ultra-high strength steel component forming and manufacturing, specifically relating to a high-strength and high-toughness steel with a bimodal heterogeneous structure and its hot forming method. Background Technology

[0002] Hot-formed steel is widely used in the automotive manufacturing industry due to its cost advantages and excellent high-temperature forming performance. As automotive body structures continue to evolve towards lightweighting and higher safety performance, key structural components face increasingly stringent requirements for the synergistic improvement of material strength and plasticity, posing new challenges to the strengthening and toughening design of hot-formed steel. Currently, utilizing martensitic phase transformation combined with precipitation strengthening and grain refinement strengthening by microalloying elements such as Nb, Mo, and V has become the main technical path to improve the strength of hot-formed steel. Among them, quenched martensite exhibits excellent strength performance due to its ultra-fine microstructure and high dislocation density. However, single martensite structures typically exhibit low uniform elongation and poor impact toughness, limiting its application potential in higher-performance scenarios.

[0003] In fact, in the hot stamping forming-quenching process, in addition to directly quenching to below the martensitic transformation initiation temperature to obtain a fully martensitic structure, a multiphase structure containing martensite, ferrite, lower bainite and retained austenite can also be obtained by quenching after a short period of holding in the austenitic region, bainitic region or martensitic region.

[0004] In recent years, the theory of heterogeneous deformation-induced stress has provided a new perspective for understanding the strengthening and toughening mechanism of multiphase microstructures. Within this theoretical framework, martensite, as the hard phase, and bainite, as the soft phase, exhibit strain gradients due to their different mechanical properties during plastic deformation. This induces the accumulation of geometrically necessary dislocations at the phase interface, resulting in back stress, or HDI stress. This stress field can effectively delay local stress concentration and postpone necking, thereby achieving a synergistic improvement in both strength and plasticity.

[0005] However, traditional isothermal quenching processes have significant limitations when treating martensitic / bainitic multiphase steels. Under conventional process conditions, the grain size difference between martensite and bainite is small, and the two phases are on similar orders of magnitude, making it difficult to form a significant bimodal scale distribution. Studies have shown that the strengthening effect of HDI stress is closely related to the size difference between the soft and hard phases: when the size difference between the two phases is small, the strain gradient region is limited, the accumulation efficiency of geometrically necessary dislocations is low, and the back stress effect is not fully activated, limiting the potential for strengthening and toughening the multiphase structure. Therefore, developing a heat treatment method that can effectively increase the size difference between martensite and bainite and fully utilize the strengthening effect of HDI stress is of significant theoretical and engineering application value for further improving the synergistic strength and plasticity properties of hot-formed steels. Summary of the Invention

[0006] The purpose of this invention is to overcome the above-mentioned technical deficiencies and provide a high-strength and high-toughness steel with a bimodal heterostructure and its hot forming method, thereby solving the technical problem that the small difference in the size of martensite and bainite grains in the preparation of hot-formed steel in the prior art leads to the limited HDI stress strengthening effect.

[0007] To achieve the above-mentioned technical objectives, the technical solution provided by this invention is as follows: In a first aspect, the present invention provides a hot forming method for high-strength and high-toughness steel with a bimodal heterostructure, comprising the following steps: S1, heating a first hot-formed steel to a temperature above Ac3 for austenitization treatment to obtain a second hot-formed steel; S2, cooling the second hot-formed steel to the bainite transformation temperature range to obtain a third hot-formed steel; S3, subjecting the third hot-formed steel to pressure treatment within the bainite transformation temperature range, and completing the isothermal transformation of bainite under stress to obtain a fourth hot-formed steel; S4, cooling the fourth hot-formed steel to room temperature at a cooling rate of ≥200℃ / s to obtain high-strength and high-toughness steel.

[0008] Secondly, the present invention provides a high-strength, high-toughness steel obtained by the above-mentioned thermoforming method.

[0009] Compared with the prior art, the beneficial effects of the present invention include: This invention first obtains an austenitic microstructure with uniform composition and appropriate grain size at high temperature, providing a uniform parent phase for subsequent phase transformation; then it cools to the bainite transformation temperature range, and breaks the isotropy of the austenite crystals by applying external stress, causing bainite to preferentially nucleate and grow along an orientation with a specific crystallographic relationship to the principal stress direction, increasing the bainite grain size; at the same time, the formed coarse bainite effectively divides the austenite grains into multiple small isolated regions, and finally, at an ultra-high cooling rate of ≥200℃ / s, the divided austenite grains form refined martensite, thereby obtaining hot-formed steel with the coexistence of martensite and bainite and large differences in grain size, effectively enhancing the HDI stress strengthening effect. The hot-formed steel obtained by this invention has high strength (tensile strength above 2400 MPa), high plasticity (elongation above 10%), and high toughness (impact toughness above 34 J). Attached Figure Description

[0010] Figure 1 This is a microstructure diagram of the high-strength and high-toughness steel obtained in Example 1 of the present invention. Detailed Implementation

[0011] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are merely illustrative and not intended to limit the invention.

[0012] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention pertains; the terminology used herein is for the purpose of describing particular embodiments only and is not intended to limit the invention; the terms “comprising” and “having”, and any variations thereof, in the specification, claims, and foregoing description of the invention, are intended to cover non-exclusive inclusion.

[0013] In the description of the embodiments of this application, technical terms such as "first" and "second" are used only to distinguish different objects and should not be construed as indicating or implying relative importance or implicitly specifying the number, specific order, or primary and secondary relationship of the indicated technical features. In the description of the embodiments of this application, "multiple" means two or more, unless otherwise explicitly defined.

[0014] In this document, the term "embodiment" means that a particular feature, structure, or characteristic described in connection with an embodiment may be included in at least one embodiment of the invention. The appearance of this phrase in various places throughout the specification does not necessarily refer to the same embodiment, nor is it a separate or alternative embodiment mutually exclusive with other embodiments. It will be explicitly and implicitly understood by those skilled in the art that the embodiments described herein can be combined with other embodiments.

[0015] To address the shortcomings of existing technologies in the preparation of hot-formed steel, such as small differences in the grain size of martensite and bainite, similar order of magnitude of the two phases, and difficulty in forming a significant bimodal scale distribution, which limits the HDI stress strengthening effect, this invention provides a high-strength and high-toughness steel with a bimodal heterostructure and its hot-forming method. Based on the synergistic control of thermal, mechanical, and cold multi-fields, a martensite / lower bainite multiphase microstructure is constructed. Based on the synergistic deformation behavior between the two phases, the resulting martensite / lower bainite multiphase microstructure can significantly improve the material's plasticity and toughness while maintaining a high strength level. Simultaneously, through precise design of the integrated hot stamping and heat treatment process, selective coarsening of bainite and effective refinement of martensite are achieved, constructing a heterostructure with a significant bimodal scale distribution. This fully activates the HDI stress strengthening effect, breaking through the limits of the traditional multiphase microstructure's synergistic improvement in strength and plasticity, ultimately achieving a significant leap in the strength and plasticity of the hot-formed steel.

[0016] In a first aspect, the present invention provides a hot forming method for high-strength and high-toughness steel with a bimodal heterostructure, comprising the following steps: S1, the first hot-formed steel is heated to a temperature above Ac3 and subjected to austenitization treatment to obtain the second hot-formed steel; S2, the second hot-formed steel is cooled to the bainite transformation temperature range to obtain the third hot-formed steel; S3, within the bainite transformation temperature range, the third hot-formed steel is subjected to pressure treatment, and the isothermal transformation of bainite is completed under stress to obtain the fourth hot-formed steel; S4, the fourth type of hot-formed steel, is cooled to room temperature at a cooling rate of ≥200℃ / s to obtain high-strength and high-toughness steel.

[0017] This invention heats a first hot-formed steel to a temperature above Ac3 to obtain an austenitic microstructure with uniform composition and appropriate grain size, providing a uniform parent phase for subsequent phase transformation; then cools it to the bainite transformation temperature range, and breaks the isotropy of the austenite crystals by applying external stress, causing bainite to preferentially nucleate and grow along an orientation with a specific crystallographic relationship to the principal stress direction, increasing the grain size; at the same time, with the formation of coarse bainite, the original austenite grains are effectively divided by bainite bundles, laying the microstructure foundation for subsequent martensite refinement; finally, refined martensite is formed at an ultra-high cooling rate of ≥200℃ / s.

[0018] In some embodiments, in step S1, the composition of the first hot-formed steel, by mass percentage, includes: C 0.2-0.5%, Si 0.15-0.4%, Mn 0.8-1.3%, P 0.01-0.02%, S 0.001-0.005%, Cr 0.1-0.3%, Ti 0.03-0.07%, Nb 0.03-0.07%, with the balance being Fe.

[0019] In some embodiments, the austenitizing treatment conditions in step S1 include: holding at a temperature of 900–950°C for 5–15 minutes under a protective atmosphere. Excessive austenitizing temperature or time will result in coarse austenite grains, which is detrimental to the final microstructure refinement; insufficient temperature or time will lead to incomplete dissolution of alloying elements, affecting the uniformity of subsequent bainite transformation and hindering bainite growth.

[0020] In some embodiments, the cooling rate in step S2 is 5–15 °C / s. The present invention employs a slow cooling rate of 5–15 °C / s, which is sufficient to suppress the precipitation of high-temperature ferrite, avoid the formation of proeutectoid ferrite, and ensure that supercooled austenite directly enters the bainite transformation region.

[0021] In some embodiments, in step S2, the cooling method includes slow cooling with the furnace and optional air cooling.

[0022] It should be noted that the present invention can adjust the air cooling intensity through a PID controller to ensure a stable cooling rate.

[0023] In some embodiments, the bainite transformation temperature range in step S2 is 400–450°C. This invention employs a moderate lower bainite transformation temperature (400–450°C) to provide the necessary temperature conditions for lower bainite coarsening: this ensures sufficient bainite nucleation, which is beneficial for long-range carbon atom diffusion and provides kinetic conditions for the growth and merging of bainite laths, avoiding microstructure refinement due to excessive supercooling; simultaneously, it does not exceed 450°C to prevent the formation of upper bainite, which is detrimental to the microstructure.

[0024] In some embodiments, the conditions for pressurization in step S3 include: a holding pressure of 50–300 MPa and a holding time of 10–40 seconds. This invention provides additional mechanical driving force for the bainitic transformation by applying external stress, thereby lowering the energy barrier for bainitic nucleation, enabling the bainitic transformation to begin earlier, accelerating the transformation rate, and shortening the isothermal time.

[0025] In some embodiments, the cooling method in step S4 includes high-pressure water spray cooling, with a spray pressure of 2-3 MPa and a spray density of 10-20 L / (m³). 2 ·s).

[0026] Secondly, the present invention provides a high-strength, high-toughness steel obtained by the above-mentioned thermoforming method.

[0027] In some embodiments, the high-strength and high-toughness steel includes a bainitic and martensitic dual-phase structure, wherein the average grain size of the bainite is denoted as DB, the average grain size of the martensite is denoted as DM, and DB / DM≥3.

[0028] The main mechanism of action and advantages of this invention are as follows: (1) The present invention uses austenitization treatment to fully dissolve carbides and moderately increase the size of austenite grains, providing a uniform parent phase for subsequent phase transformation; after austenitization treatment, high temperature slow cooling is used to suppress the precipitation of high temperature ferrite, ensuring that the supercooled austenite directly enters the bainite transformation region.

[0029] (2) The applied stress field breaks the isotropy of austenite crystals, causing bainite to preferentially nucleate and grow along orientations that have a specific crystallographic relationship with the principal stress direction. This orientation selectivity leads to the following results: bainite variants that match the stress direction grow preferentially, and variant selection is enhanced; adjacent bainite laths with the same orientation are easy to merge, forming large bainite bundles; under multiple bundles, bainite grows in parallel along the stress direction, and the width of the bainite bundles increases significantly.

[0030] Simultaneously, under stress, the untransformed supercooled austenite undergoes slight plastic deformation, introducing crystal defects such as dislocations. These defects have the following effects: serving as rapid diffusion channels for carbon atoms, accelerating bainite growth; and becoming preferential nucleation sites for subsequent martensite, increasing the nucleation density.

[0031] With the formation of coarse bainite, the original austenite grains are effectively segmented by bainite bundles. According to grain refinement theory, the refinement effect of the second phase segmenting the parent phase grains is closely related to the volume fraction and distribution morphology of the second phase. In this invention, coarse bainite bundles penetrate the austenite grains, dividing them into multiple small, isolated regions, laying the microstructural foundation for subsequent martensite refinement.

[0032] (3) In the traditional quenching and cooling process, the martensite laths formed first may undergo slight self-tempering during subsequent cooling, resulting in lath coarsening. In this invention, after the pressure holding is completed, the pressure is released, and the steel plate is immediately cooled to room temperature at an ultra-high cooling rate of ≥200℃ / s. Under extreme supercooling, martensite nucleates instantly, increasing the nucleation rate by several orders of magnitude. However, the simultaneous nucleation of a large amount of martensite restricts growth. Therefore, this invention uses this ultra-fast cooling method to allow the steel plate to pass through the tempering temperature range instantly, completely suppressing self-tempering. At the same time, ultra-fast cooling "freezes" the high-density dislocations generated during the martensitic transformation process in the microstructure, avoiding dislocation recovery and rearrangement, and further strengthening the martensite.

[0033] Meanwhile, the coarse bainite formed during the isothermal transformation of bainite has divided the original austenite grains into multiple small, isolated regions. These small, undercooled austenite regions have the following characteristics: size effect: the smaller the size of the austenite region, the higher its stability and the lower the martensite transformation initiation temperature (Ms); interface effect: the small austenite is surrounded by bainite, and the phase interface is dense, providing more favorable sites for martensite nucleation; constraint effect: the surrounding bainite mechanically constrains the austenite, further inhibiting the growth of martensite laths.

[0034] Therefore, through the process of this invention, a martensite / bainite multiphase structure with significant bimodal scale distribution characteristics is finally obtained, wherein: bainite: coarsens under thermo-mechanical coupling to form a large soft phase region, with an average grain size denoted as DB; martensite: formed by ultrafast cooling transformation in the segmented and refined austenite region, with a small size, with an average grain size denoted as DM; size difference: satisfying DB / DM≥3, forming a significant bimodal scale distribution characteristic.

[0035] The present invention will be further described in detail below through specific embodiments. It should be noted that the embodiments described below are exemplary and are only used to explain this application, and should not be construed as limiting this application. Where specific techniques or conditions are not specified in the embodiments, they shall be performed in accordance with the techniques or conditions described in the literature in this field or in accordance with the product instructions. Reagents or instruments used that do not specify the manufacturer are all conventional products that can be obtained commercially.

[0036] To ensure comparability, the following examples and comparative examples all used commercially available microalloyed hot-formed steel sheets of the same batch and with the same specifications and composition as the test steel raw materials, in order to accurately isolate and demonstrate the technical effects brought about by the process steps of the present invention. The chemical composition of the test steel (mass percentage, wt%) is as follows: C 0.35 wt.%, Si 0.26 wt.%, Mn 1.1 wt.%, P 0.015 wt.%, S 0.003 wt.%, Cr 0.2 wt.%, Ti 0.05 wt.%, Nb 0.05 wt.%, Fe balance.

[0037] Example 1 A hot forming method for high-strength and high-toughness steel with a bimodal heterostructure includes the following steps: (1) Austenitizing treatment: The test steel plate (first hot-formed steel) was placed in a box-type resistance furnace with nitrogen atmosphere protection and heated to 950°C at a heating rate of 10°C / s. The temperature was held for 10 minutes to fully dissolve the carbides and form a uniform austenitic structure to obtain the second hot-formed steel.

[0038] (2) High temperature slow cooling: The second hot-formed steel is cooled from 950°C to the bainitic transformation temperature zone of 420°C by a combination of slow cooling in the furnace and air cooling at a cooling rate of 10°C / s to obtain the third hot-formed steel.

[0039] (3) Medium-temperature isothermal forming: The third hot-formed steel, whose temperature has dropped to 420℃, is immediately transferred to a 200-ton servo press for medium-temperature isothermal forming. The transfer time is controlled within 3 seconds to ensure that the temperature drop does not exceed 50℃. The servo press is equipped with a heatable mold, and the mold temperature is set to 420±5℃. After the third hot-formed steel is bonded to the mold, pressure is applied in three stages: 50 MPa is rapidly applied within 0-2 seconds to make the steel plate bond completely with the mold, and 150 MPa is applied within 2-5 seconds. The pressure is held for 20 seconds under 150 MPa pressure and 420℃ temperature. During the holding period, the temperature fluctuation is controlled within ±5℃ and the pressure fluctuation is controlled within ±10 MPa to form a certain proportion of lower bainite structure (excessive temperature fluctuation can easily lead to the formation of martensite, while appropriate pressure can promote bainite nucleation) to obtain the fourth hot-formed steel.

[0040] (4) Low-temperature extreme cooling: After the pressure holding is completed, the pressure is quickly released within 1-3 seconds, and the high-pressure water spray quenching system in the mold is immediately started to cool the fourth hot-formed steel ultra-fast. The time interval between pressure release and quenching start is less than 0.5 seconds. The high-pressure water spraying system uses a nozzle with a diameter of 0.3 mm, a spray pressure of 2.5 MPa, and a spray density of 15 L / (m²·s). The cooling rate is monitored in real time by a fine wire thermocouple embedded in the steel plate. The results show that the average cooling rate from 420℃ to 20℃ is 200℃ / s, which causes the remaining untransformed supercooled austenite to undergo martensitic transformation under ultra-high cooling rate, forming an ultra-fine martensitic structure, and obtaining a high-strength and high-toughness steel with bainite and martensite dual phase structure.

[0041] Comparative Example 1 (Conventional Hot Stamping Process) (1) Austenitizing treatment: Same as in Example 1, the test steel plate was heated to 950°C and held for 10 minutes.

[0042] (2) High-temperature forming: The austenitized steel plate is directly transferred to the press for high-temperature forming. The transfer time is controlled within 5 seconds, and the temperature of the steel plate is kept above 750℃. The mold temperature is room temperature, the forming pressure is 150 MPa, and the holding time is 10 seconds, so that the steel plate is formed in the mold.

[0043] (3) In-mold quenching: After the pressure holding is completed, the steel plate is kept in the mold to continue cooling. The quenching is carried out by the heat conduction of the mold itself. Since high-pressure water spraying is not used, the actual cooling rate is about 30℃ / s, which causes the supercooled austenite to directly undergo martensite transformation and obtain a full martensite structure.

[0044] Comparative Example 2 (Traditional isothermal quenching process) (1) Austenitizing treatment: Same as in Example 1, the test steel plate was heated to 950°C and held for 10 minutes.

[0045] (2) High-temperature forming: The austenitized steel sheet is directly transferred to a press for high-temperature forming. The transfer time is controlled within 5 seconds, and the temperature of the steel sheet is maintained above 750°C. The mold temperature is set to 420°C (the same isothermal temperature as in Example 1), the forming pressure is 150 MPa, and the holding time is 20 seconds, so that the steel sheet completes forming and partial bainitic isothermal transformation at 420°C.

[0046] (3) Isothermal quenching: After the pressure is removed, the steel plate is transferred to a high-pressure water spray quenching system for cooling. The cooling rate is the same as in Example 1, which is 200℃ / s, so that the remaining untransformed supercooled austenite undergoes martensitic transformation.

[0047] Comparative Example 3 Compared with Example 1, the only difference is that the cooling rate of high temperature slow cooling in step (2) is adjusted to 3 ℃ / s, and the other steps and conditions are the same as in Example 1.

[0048] Comparative Example 4: Compared with Example 1, the only difference is that the cooling rate of high temperature slow cooling in step (2) is adjusted to 20 ℃ / s, and the other steps and conditions are the same as in Example 1.

[0049] Performance testing Microstructure characterization and mechanical property testing were performed on all samples.

[0050] (1) Microstructure characterization: The sample was cut from the central region of the steel plate, and the observation surface was a cross-section perpendicular to the rolling direction. After etching with 4% nitric acid alcohol solution for 8-10 seconds, the microstructure was observed using a Zeiss Sigma 500 scanning electron microscope. Phase analysis and grain size statistics were performed using an Oxford Instruments Symmetry EBSD system with a step size of 50 nm. The data analysis software was Aztec Crystal. The microstructure of Example 1 is as follows. Figure 1 As shown in Table 1, the microstructure characterization results (grain size) of Example 1 and Comparative Examples 1-4 are shown in Table 1.

[0051] Table 1. Statistical results of grain size in Example 1 and Comparative Examples 1-4

[0052] like Figure 1 As shown in Table 1, the microstructure of Example 1 is a bainite / martensite multiphase structure, wherein the average width of the bainite lath bundles is 4.1 μm, the average diameter of the equivalent circle of the martensite region is 1.3 μm, and the calculated size ratio of bainite to martensite, DB / DM, is 3.15.

[0053] The microstructure of Comparative Example 1 is a typical all-martensite structure. Scanning electron microscopy reveals a slender, interwoven distribution of martensite laths, with no bainite present. EBSD analysis shows that the average size of the martensite laths is approximately 2.5 μm.

[0054] The microstructure of Comparative Example 2 is a martensite / bainite multiphase structure with an average width of 2.3 μm for the bainite lath bundles and an average size of 1.8 μm for the martensite region. The calculated size ratio of bainite to martensite, DB / DM, is 1.28, which is significantly lower than 3.15 in Example 1.

[0055] Comparative Example 3 has an average width of 4.4 μm for bainite lath bundles and an average size of 1.2 μm for martensite regions. The calculated size ratio of bainite to martensite, DB / DM, is 3.67, which is higher than 3.15 in Example 1. However, the microstructure of Comparative Example 3 is a mixture of ferrite, bainite, and martensite, and the average width (DF) of the ferrite lath bundles is 5.7 μm.

[0056] The microstructure of Comparative Example 4 is mainly martensite with only a small amount of bainite. The average width of the bainite lath bundles is 2.1 μm, the average size of the martensite region is 1.8 μm, and the size ratio is 1.17, which is significantly lower than 3.15 in Example 1.

[0057] (2) Nanoindentation tests were performed using an Agilent G200 nanoindenter. The load was 5 mN and the holding time was 10 seconds. Twenty points were tested for each phase, and the average value was taken. The hardness difference between martensite and bainite was calculated. The statistical results of nanoindentation of each phase in Example 1 and Comparative Examples 1-4 are shown in Table 2.

[0058] Table 2. Statistical results of nanoindentation of each phase in Example 1 and Comparative Examples 1-4

[0059] As shown in Table 2, the nanoindentation test results of Example 1 show that the average hardness of martensite is 9.2 GPa, the average hardness of bainite is 3.5 GPa, and the hardness difference between the two phases is 5.7 GPa. The soft and hard phase HDI stress strengthening is formed, thereby significantly improving the strength.

[0060] Comparative Example 1, due to its single martensite microstructure, exhibits no interphase hardness difference or dimensional variation. Nanoindentation testing results show that the average hardness of the martensite is 8.0 GPa.

[0061] Comparative 3-nanometer indentation tests showed that the average hardness of ferrite was 2.3 GPa, bainite was 3.4 GPa, and martensite was 9.4 GPa, with a hardness difference of 6.0 GPa between the two phases. However, unlike bainite, due to the excessively low high-temperature cooling rate, ferrite passed through the ferrite phase region during cooling, thus forming a softer phase. Its abundant presence significantly reduced the strength of the sample.

[0062] Comparative Example 4 has a microstructure mainly composed of martensite, and the hardness difference between the martensite and bainite phases is small, only 1.6 GPa.

[0063] (2) Quasi-static uniaxial tensile tests were conducted according to GB / T 228.1 standard, with 3 parallel specimens tested for each sample, and the average value was taken as the final result; room temperature impact tests were conducted according to GB / T 1817-2017 standard, with 3 parallel specimens tested for each sample, and the average value was taken as the final result. The mechanical property test results of Example 1 and Comparative Examples 1-4 are shown in Table 3.

[0064] Table 3 Mechanical property test results

[0065] The tensile test results in Table 3 show that the tensile strength of Example 1 reached 2412 MPa and the elongation was 10.3%, demonstrating excellent strength-plasticity matching.

[0066] The tensile test results of Comparative Example 1 show that the tensile strength is 1869 MPa and the elongation is 6.5%, which shows a high strength level but poor plasticity.

[0067] The tensile test results of Comparative Example 2 showed that the tensile strength was 2027 MPa and the elongation was 9.1%. Its strength-plasticity matching was better than that of Comparative Example 1 but worse than that of Example 1.

[0068] The tensile test results of Comparative Example 3 showed that the tensile strength was 1341 MPa and the elongation was 12.1%, exhibiting high plasticity but poor strength.

[0069] The tensile test results of Comparative Example 4 show that the tensile strength is 1837 MPa and the elongation is 6.1%, which is similar to but slightly lower than the mechanical properties of Comparative Example 1. This is because although it entered the bainitic phase region during the continuous cooling process, the hardness difference between the phases was low, and the HDI stress strengthening was not fully utilized.

[0070] In summary, this invention first obtains an austenitic microstructure with uniform composition and appropriate grain size at high temperature. Then, by controlling the cooling rate, it achieves slow cooling from high temperature to the bainite transformation temperature range. Combined with pressure treatment, the isotropic nature of the austenite crystals is broken by applying external stress, causing bainite to preferentially nucleate and grow along an orientation with a specific crystallographic relationship to the principal stress direction, increasing the bainite grain size. At the same time, the formed coarse bainite effectively divides the austenite grains into multiple small isolated regions. Finally, at an ultra-high cooling rate of ≥200℃ / s, the divided austenite grains are transformed into refined martensite, thereby obtaining hot-formed steel with the coexistence of martensite and bainite and large differences in grain size. This effectively enhances the HDI stress strengthening effect. The obtained hot-formed steel has high strength (tensile strength above 2400 MPa), high plasticity (elongation above 10%), and high toughness (impact toughness above 34 J).

[0071] The specific embodiments of the present invention described above do not constitute a limitation on the scope of protection of the present invention. Any other corresponding changes and modifications made in accordance with the technical concept of the present invention should be included within the scope of protection of the claims of the present invention.

Claims

1. A hot forming method for high-strength, high-toughness steel with a bimodal heterostructure, characterized in that, Includes the following steps: S1, the first hot-formed steel is heated to a temperature above Ac3 and subjected to austenitization treatment to obtain the second hot-formed steel; S2, the second hot-formed steel is cooled to the bainite transformation temperature range to obtain the third hot-formed steel; S3, within the bainite transformation temperature range, the third hot-formed steel is subjected to pressure treatment to complete the isothermal transformation of bainite under stress, thereby obtaining the fourth hot-formed steel; S4, the fourth hot-formed steel is cooled to room temperature at a cooling rate of ≥200℃ / s to obtain high-strength and high-toughness steel.

2. The hot forming method for high-strength and high-toughness steel with a bimodal heterostructure according to claim 1, characterized in that, In step S1, the composition of the first hot-formed steel, by mass percentage, includes: C 0.2-0.5%, Si 0.15-0.4%, Mn 0.8-1.3%, P 0.01-0.02%, S 0.001-0.005%, Cr 0.1-0.3%, Ti 0.03-0.07%, Nb 0.03-0.07%, with the balance being Fe.

3. The hot forming method for high-strength and high-toughness steel with a bimodal heterostructure according to claim 1, characterized in that, In step S1, the conditions for the austenitizing treatment include: holding at a temperature of 900–950°C under a protective atmosphere for 5–15 minutes.

4. The hot forming method for high-strength and high-toughness steel with a bimodal heterostructure according to claim 1, characterized in that, In step S2, the cooling rate is 5 to 15 °C / s.

5. The hot forming method for high-strength and high-toughness steel with a bimodal heterostructure according to claim 1, characterized in that, In step S2, the cooling method includes slow cooling with the furnace and optional air cooling.

6. The hot forming method for high-strength and high-toughness steel with a bimodal heterostructure according to claim 1, characterized in that, In step S2, the bainitic transformation temperature range is 400–450°C.

7. The hot forming method for high-strength and high-toughness steel with a bimodal heterostructure according to claim 1, characterized in that, In step S3, the conditions for pressurization include: a holding pressure of 50–300 MPa and a holding time of 10–40 seconds.

8. The hot forming method for high-strength and high-toughness steel with a bimodal heterostructure according to claim 1, characterized in that, In step S4, the cooling method includes high-pressure water spray cooling, with a spray pressure of 2-3 MPa and a spray density of 10-20 L / (m³). 2 ·s).

9. High-strength, high-toughness steel obtained by the hot forming method according to any one of claims 1-8.

10. The high-strength, high-toughness steel according to claim 9, characterized in that, The high-strength and high-toughness steel includes a bainitic and martensitic dual-phase structure, wherein the average grain size of bainite is denoted as DB, the average grain size of martensite is denoted as DM, and DB / DM≥3.

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