Super horse-shellfish multiphase structure and strengthening and toughening thermal forming method

Abstract

The invention provides a super horse-shellfish complex phase structure and strengthening and toughening hot forming method which comprises the technological processes of flash heating, high-temperature stamping forming, medium-temperature short-time dynamic partitioning and low-temperature step quenching. The structure of the ultrahigh-strength steel plate is transformed into a nanoscale superfine martensite-bainite complex-phase structure with dispersed nano carbides and interlayer retained austenite films for the first time, and the dispersed and uniformly distributed nano carbides and the superfine and dense martensite-bainite complex-phase structure can achieve complex-phase structure enhancement; the ultra-fine dense structure with a small amount of low-temperature bainite and the intergranular nano-thickness retained austenite film can realize toughening of a complex-phase structure, so that the strength and the toughness are increased on the premise of'texturing 'of the material, and the collision performance and the service reliability of a hot-formed component are remarkably improved; and a reliable technical support is provided for light weight and safety of the automobile.

Description

Technical Field

[0001] The invention relates to the field of hot forming, and in particular to a super Ma-Bay complex phase structure and a strengthening and toughening hot forming method. Background Art

[0002] Driven by the green manufacturing concept of "energy conservation, emission reduction, low carbon and environmental protection", automobile lightweighting has become an important trend in global industrial development. Hot-formed steel has become one of the key materials for achieving automobile lightweighting due to its significant advantages such as high strength, low rebound, high yield rate and low cost. At present, 22MnB5 hot-formed steel with a tensile strength of about 1.5GPa and an elongation of about 6% has been widely used in automobile safety structural parts. In order to further promote the process of automobile lightweighting, the global automobile and steel industries are actively developing a new generation of hot-formed steel with a tensile strength of 2GPa and above. However, the classic inverted relationship between strength and toughness in steel materials shows that an increase in strength is often accompanied by a decrease in toughness, which may lead to fracture failure and delayed cracking problems in automobile safety structural parts, making it difficult to meet collision energy absorption indicators, thereby limiting the application of high-strength hot-formed steel.

[0003] Therefore, ensuring the lightweight of the car while maintaining ultra-high strength and giving the material high toughness, high anti-intrusion performance and excellent collision energy absorption effect has become a key technical problem that needs to be urgently solved in the application of hot-formed steel components. Summary of the invention

[0004] The purpose of the present invention is to provide a method for strengthening and toughening the hot forming of ultra-high strength steel Mazda-B composite phase structure, which can solve the technical problems of poor toughness and difficulty in matching strength and toughness in existing hot formed components with strength levels of 2 GPa and above.

[0005] In order to solve the above technical problems, the present invention provides a super martensite-bainite multiphase structure and strengthening and toughening hot forming method, which constructs a multi-phase and multi-scale synergistic strengthening and toughening structure with "martensite / bainite nano-scale ultrafine matrix structure + nano-carbide + metastable film-like residual austenite" through a thermomechanical coupling phase transformation path control strategy consisting of "flash heating-high temperature stamping-medium temperature short-time dynamic partitioning-low temperature graded quenching", which specifically includes the following steps: S10, flash heating the ultra-high strength steel plate to ensure that the microstructure of the ultra-high strength steel plate has completed austenite transformation and has uniform composition; S20, transferring the heated ultra-high strength steel plate to a mold for high temperature stamping and pressure holding treatment; S30, a medium temperature short-time dynamic partitioning treatment is performed on the ultra-high strength steel plate to ensure that the microstructure of the ultra-high strength steel plate is transformed into nano-scale low-temperature bainite with a mass content of 1% to 35% and a size of 20 to 300 nm and carbon-rich undercooled austenite; S40, perform the first-stage low-temperature rapid quenching treatment in the low-temperature step quenching process for the ultra-high strength steel plate to ensure that the microstructure transformation of the ultra-high strength steel plate forms a nano-scale ultra-fine martensite-bainite duplex structure with nano-scale martensite grains sized 10 - 150 nm and an intergranular retained austenite film; S50, perform the second-stage low-temperature slow quenching treatment in the low-temperature step quenching process for the ultra-high strength steel plate and cool it to room temperature to ensure that the microstructure transformation of the ultra-high strength steel plate forms a nano-scale ultra-fine martensite-bainite duplex structure with dispersed nano-carbides and an interlayer retained austenite film. Preferably, in step S10: after rapidly heating the ultra-high strength steel plate to a temperature of 800 - 1000 °C, hold it for 1 - 300 s, and the heating rate is 30 - 200 °C / s.

[0006] Preferably, the rapid heating method in step S10 includes any one of induction heating, resistance heating, contact heating, electro-thermal hybrid heating, and heating using a heating furnace with a temperature-raising function; the grain size of austenite in the ultra-high strength steel plate is less than 2 μm.

[0007] Preferably, in step S20: the transfer time of the ultra-high strength steel plate to the mold is 2 - 10 s, the amount of deformation in stamping forming is 1% - 60%, the temperature when entering the mold is 600 - 900 °C, the stamping speed is 30 - 200 mm / s, and the pressure holding time is 2 - 60 s.

[0008] Preferably, in step S30: the temperature adjustment method for the medium-temperature short-time dynamic partitioning treatment is dynamic fluctuation adjustment or isothermal adjustment; among them, the medium-temperature short-time dynamic partitioning temperature range is (M s - 30 °C) to (M s + 150 °C), the partitioning time is 1 - 300 s, and M s is the martensite phase transformation start temperature of the ultra-high strength steel plate.

[0009] Preferably, in step S40: the quenching rate is greater than 35 °C / s, the quenching end point temperature is (M f - 100 °C) to (M f - 50 °C), M f is the martensite phase transformation end temperature of the ultra-high strength steel plate, the mass content of retained austenite is 2 - 12%, and the thickness is less than 50 nm; in step S50: the quenching rate is 1 - 20 °C / s.

[0010] Preferably, use a mold with fast cooling and fast heating functions to perform short-time slow cooling treatment and the first-stage low-temperature rapid quenching treatment on the ultra-high strength steel plate; the material of the mold is hot work die steel or nickel-based superalloy.

[0011] Preferably, the cooling medium loaded in the mold in step S40 includes at least one of ice water, nanofluid, and liquid nitrogen, and the nanofluid includes graphene nanofluid with water as the dispersion medium or carbon nanotube nanofluid with water as the dispersion medium.

[0012] Preferably, in step S50: when the ultra-high strength steel plate is subjected to the second-stage low-temperature slow cooling quenching treatment with air, the air cooling rate is 0.1~5°C / s; when the ultra-high strength steel plate is subjected to the second-stage low-temperature slow cooling quenching treatment with quenching oil, the oil cooling rate is 1~20°C / s.

[0013] Preferably, in steps S10 to S50, a non-contact digital infrared thermometer and a contact thermometer are used to monitor and control the temperature of the ultra-high strength steel plate in real time.

[0014] The beneficial effects of the present invention are as follows: Different from the prior art, the present invention provides a method for strengthening and toughening the martensite-bainite duplex structure of ultra-high strength steel, which forms a nano-scale ultra-fine martensite-bainite duplex structure with dispersed nano-carbides and interlayer retained austenite films through the technological process of flash heating - hot stamping forming - medium-temperature short-time dynamic partitioning - low-temperature step quenching. The dispersed and evenly distributed nano-carbides and the ultra-fine and dense nano-scale martensite-bainite duplex structure can achieve the strengthening of the duplex structure, and the ultra-fine and dense structure with a small amount of low-temperature bainite and the nano-scale intergranular retained austenite film can achieve the toughening of the duplex structure, thereby achieving the double increase of strength and toughness on the premise of "elementalization" of the material, significantly improving the collision performance and service reliability of the hot-formed components, and providing reliable technical support for automotive lightweight and safety. At the same time, the above-mentioned strengthening and toughening hot forming method forms hot-formed components with high strength and high toughness with low production cost and simple and efficient production process, achieving the double increase of strength and toughness compared with traditional hot-formed components. Description of the Drawings

[0015] Figure 1 It is a flow chart of the method for strengthening and toughening the martensite-bainite duplex structure of ultra-high strength steel provided by the present invention; Figure 2 It is a process route diagram of the method for strengthening and toughening the martensite-bainite duplex structure of ultra-high strength steel provided in Embodiment 1 of the present invention; Figure 3a It is a scanning electron microscope schematic diagram of the microstructure of the hot-formed component prepared in Embodiment 1 of the present invention; Figure 3b It is a scanning electron microscope schematic diagram of the martensite laths in the microstructure of the hot-formed component prepared in Embodiment 1 of the present invention; Figure 4 It is a schematic diagram of the comparison of the mechanical properties of the hot-formed component prepared in Embodiment 1 of the present invention and the hot-formed component prepared by the traditional process. Detailed implementation manners

[0016] The technical solutions in the embodiments of the present invention will be clearly and completely described below in conjunction with the embodiments of the present invention. Obviously, the described embodiments are only a part of the embodiments of the present invention, rather than all the embodiments. All other embodiments obtained by those of ordinary skill in the art based on the embodiments of the present invention without creative efforts shall fall within the protection scope of the present invention.

[0017] Aiming at the key problems of poor toughness and difficult strength-ductility matching of ultra-high strength steel hot forming components at 2 GPa and above, as well as problems such as insufficient collision energy absorption and delayed cracking of components caused thereby, the present invention proposes an innovative solution: adopting a thermo-mechanical coupling phase transformation path regulation strategy to construct a multi-phase and multi-scale synergistic strengthening and toughening structure of "martensite / bainite nanoscale ultrafine matrix structure + nano-carbide + metastable thin-film retained austenite" to break through the problem of strength-ductility inversion. The specific process of the ultra-high strength steel martensite-bainite duplex phase structure strengthening and toughening hot forming method provided by the present invention is as follows: flash heating - hot stamping forming - medium temperature short-time dynamic partitioning (near M s point bainite quenching - short-time partitioning) - low temperature step quenching (the first stage: quenching with rapid cooling in the martensite transformation region (M s ~M f ), the second stage: slow cooling quenching below the M f point). Under the premise of "elementalization" of the material, the strength and toughness are both increased, significantly improving the collision performance and service reliability of the component, and providing reliable technical support for automotive lightweight and safety; where M s is the starting temperature of martensite transformation, and M f is the ending temperature of martensite transformation.

[0018] Please refer to Figure 1 , Figure 1 which is the flow chart of the ultra-high strength steel martensite-bainite duplex phase structure strengthening and toughening hot forming method provided by the present invention; among them, the above-mentioned strengthening and toughening hot forming method includes the following steps: S10, perform flash heating treatment on the ultra-high strength steel plate to ensure that the organizational structure of the ultra-high strength steel plate completes austenite transformation and the composition is uniform.

[0019] Specifically, the step S10 specifically includes: First, perform blanking and punching on the ultra-high strength steel billet to obtain an ultra-high strength steel plate with a suitable shape, and the thickness of the ultra-high strength steel plate is 0.5 - 10 mm; among them, the blanking and punching treatment includes: grinding and deburring the edge of the ultra-high strength steel billet to avoid problems such as uneven heating caused by poor contact with the electrode during the electro-treatment process.

[0020] Secondly, the steel plate is heated to an appropriate austenitizing temperature by flash heating (the heating termination temperature in general heat treatment is about 100 °C higher than the complete austenite transformation temperature), and short-time heat preservation is carried out to ensure obtaining fine prior austenite grains and achieving complete austenitization. Among them, the heating rate is 30 - 200 °C / s, the austenitizing temperature is 880 - 970 °C, and the heat preservation time is 5 - 300 s.

[0021] Specifically, the flash heating method in step S10 includes induction heating, resistance heating, contact heating, electrical hybrid heating, or a heating furnace with a high-speed heating function, etc.; compared with conventional heating, it has the advantages of fast heating speed, high heating efficiency, and small occupied space. In addition, for components with small sizes, tests can be carried out through a continuous annealing testing machine.

[0022] Specifically, the size of the prior austenite grains obtained in step S10 < 2 μm. The fine austenite grain boundaries provide more nucleation sites for bainite transformation. At the same time, based on the tissue heredity effect, the grain boundaries hinder the growth of bainite ferrite, thus obtaining a nano-bainite structure.

[0023] S20, transfer the heated ultra-high strength steel plate to a mold for stamping forming and pressure holding treatment.

[0024] Specifically, step S20 also includes: Quickly transfer the austenitized ultra-high strength steel plate to a mold for stamping forming to obtain a component with the required shape; the transfer time is 2 - 10 s, the temperature when entering the mold is 600 - 900 °C, the stamping speed is 30 - 200 mm / s, the deformation amount in stamping forming is 1% - 60%, and the pressure holding time is 2 - 60 s.

[0025] Specifically, the ultra-high strength steel plate in step S20 is formed at high temperature, and the kinetic energy of metal atoms increases significantly, promoting the recovery and recrystallization process of the material. Recrystallization can effectively eliminate the work hardening phenomenon, significantly reduce the deformation resistance of the metal, and at the same time greatly improve its plastic deformation ability, thereby reducing the pressure required for stamping forming and reducing the risk of defects generated during the forming process.

[0026] Specifically, by applying an appropriate high-temperature deformation amount to the ultra-high strength steel plate in step S20, a large number of dislocations are introduced into the austenite. These dislocations rearrange and annihilate at high temperature to form sub-grain boundaries, which can be used as preferred positions for bainite nucleation, significantly increasing the density of bainite nucleation sites. At the same time, the deformation promotes local carbon diffusion, forming a "carbon-depleted zone" in the deformed austenite, thereby increasing the driving force for bainite nucleation and further accelerating the formation of bainite.

[0027] S30, perform medium-temperature short-time dynamic partitioning treatment on the ultra-high strength steel plate to ensure that the microstructure transformation of the ultra-high strength steel plate forms nano-scale low-temperature bainite with a mass content of 1% - 35% and a size of 20 - 300 nm and carbon-rich supercooled austenite.

[0028] Specifically, the S30 step further includes: After stamping, perform medium-temperature short-time dynamic partitioning treatment on the ultra-high strength steel plate. The temperature adjustment method of the medium-temperature short-time dynamic partitioning treatment is dynamic fluctuation adjustment or isothermal adjustment; among them, the dynamic partitioning start temperature is (Ms + 30°C) - (Ms + 150°C), the slow cooling rate < 10°C, the slow cooling time is 3 - 60 s, and the dynamic partitioning end point temperature is (M s - 30°C) - (M s + 30°C), where M s is the martensite transformation start temperature of the ultra-high strength steel plate to ensure that the microstructure transformation of the ultra-high strength steel plate forms nano-scale low-temperature bainite with a mass content of 1% - 35% and a size of 20 - 300 nm and carbon-rich supercooled austenite.

[0029] Specifically, the dynamic partitioning start temperature in the S30 step is slightly higher than the martensite transformation starting point temperature M s , and a lower slow cooling start temperature is beneficial to the formation of lower bainite (i.e., low-temperature bainite) with excellent mechanical properties, and avoids the formation of upper bainite or granular bainite structures with poor mechanical properties in a higher temperature range. At the same time, a lower slow cooling start temperature can provide a larger supercooling degree, significantly improve the nucleation driving force of bainite transformation, and increase the nucleation rate in the early stage of bainite. During the transformation from austenite to bainite, the high-density dislocations introduced by deformation will hinder the migration of the phase transformation interface, enhance the stability of austenite, and thus inhibit the growth of bainite. In addition, at a lower temperature, the strength of supercooled austenite is higher, and the interaction between adjacent grains further restricts the size of bainite laths. Finally, a nano-scale low-temperature bainite structure with high strength, high toughness and fine grains is obtained.

[0030] Specifically, the dynamic partitioning end point temperature in the S30 step is close to the martensite transformation starting point temperature M s , and a lower dynamic partitioning end point temperature is beneficial to the further formation of low-temperature bainite with excellent mechanical properties, and avoids the formation of upper bainite or granular bainite structures with poor mechanical properties in a higher temperature range.

[0031] Specifically, the dynamic partitioning time in step S30 can be preferably 6 to 15 seconds; within this time range, an appropriate amount of low-temperature bainite (4% to 12% by mass) can be formed to ensure that it has no significant negative impact on the strength of the component. If the slow cooling time is too long, the increase in the low-temperature bainite content will reduce the subsequent generation of martensite, while the grain growth will lead to a decrease in strength, and brittle carbides (such as cementite) may precipitate, which will have an adverse effect on the toughness of the organization, thereby reducing the comprehensive mechanical properties of the material.

[0032] Specifically, in step S30: the preferentially formed nanoscale low-temperature bainite structure divides the original austenite grains into multiple small areas, significantly limiting the growth space of martensite, thereby making the martensite lath bundles formed subsequently smaller. In addition, the carbon-rich supercooled austenite produced during the partitioning process is retained to room temperature after quenching to form a film-like residual austenite. This residual austenite, as a tough phase, can alleviate the stress concentration at the crack tip through the transformation induced plasticity (TRIP) effect when subjected to impact loads, effectively hindering crack propagation, thereby significantly improving the toughness and impact resistance of the material.

[0033] S40, the ultra-high strength steel plate is subjected to the first stage of low temperature quenching in the low temperature graded quenching process to ensure that the microstructure of the ultra-high strength steel plate is transformed into a nano-scale martensite grain with a size of 10-150nm and a nano-scale ultra-fine Martensite-Austenite composite structure of intergranular residual austenite film. Specifically, step S40 also includes: After a short period of slow cooling, the ultra-high strength steel plate is subjected to a first-stage low-temperature chilling quenching treatment, and the quenching rate is greater than 50°C / s to ensure that the microstructure of the ultra-high strength steel plate is transformed into a nano-scale martensite grain with a size of 10-150nm and a nano-scale ultra-fine martensite-Beckmann complex phase structure of intergranular retained austenite film; the mass content of the retained austenite is 2-8%, and the thickness is less than 50nm; wherein, the quenching rate of the first-stage low-temperature chilling quenching treatment is greater than 50°C / s, and the quenching end point temperature is (M s -100℃)~(M s -50℃), M s It is the end point temperature of martensitic transformation of ultra-high strength steel plate.

[0034] Specifically, the quenching cooling rate in step S40 is controlled at >50°C / s; a higher quenching rate significantly refines the martensite grain size by increasing the degree of supercooling, improving the martensite nucleation rate and inhibiting grain growth.

[0035] Specifically, the medium-temperature short-time dynamic partitioning - first-stage low-temperature rapid quenching process in steps S30 to S40 can be carried out in a mold with fast heating and cooling functions. The mold material is H13 hot work die steel or nickel-based superalloy, which has excellent thermal conductivity, wear resistance and high-temperature resistance. High-power heating elements (such as resistance heating rods, induction coils, etc.) are installed inside the mold, which can achieve short-time rapid heating, and the maximum heating rate can reach 200 °C / s. Efficient cooling channels are designed near the surface of the mold, and the cooling medium uses high-thermal conductivity media such as ice water (cooling rate > 50 °C / s), nanofluids (water + graphene nanofluids, water + carbon nanotube nanofluids, etc., cooling rate > 150 °C / s) or liquid nitrogen (cooling rate > 300 °C / s). The mold is equipped with a multi-point temperature monitoring and feedback control system to ensure uniform temperature distribution on the mold surface.

[0036] Specifically, the medium-temperature short-time dynamic partitioning - first-stage low-temperature rapid quenching process in steps S30 to S40 can be carried out in stages outside the mold: First, the ultra-high strength steel plate after hot forming is quickly transferred to a salt bath furnace at a set temperature for short-time partitioning to obtain a certain content of nano low-temperature bainite structure and carbon-rich supercooled austenite. Subsequently, the sheet is quickly placed in a liquid nitrogen bath assisted by an ultrasonic field for rapid quenching, and the vibration effect of ultrasonic waves is used to break the gas film, thereby obtaining nano martensite laths and thin-film-like retained austenite.

[0037] S50, perform the second-stage low-temperature slow cooling quenching treatment in the low-temperature step quenching process on the ultra-high strength steel plate and cool it to room temperature to ensure that the microstructure transformation of the ultra-high strength steel plate forms a nano-scale ultra-fine martensite-bainite duplex structure with dispersed nano-carbides and interlayer retained austenite thin films. Specifically, step S50 also includes: After the first-stage low-temperature rapid quenching treatment, the ultra-high strength steel plate is subjected to the second-stage low-temperature slow cooling quenching to room temperature, and the slow cooling rate is controlled at 1 - 20 °C / s. By the second-stage low-temperature slow cooling quenching (i.e., slowing down the cooling rate of the component on the basis of the second-stage low-temperature slow cooling quenching), the self-tempering effect can be induced in the formed martensite matrix to form nano-scale self-tempered martensite and uniformly dispersed nano ε-carbides, and at the same time effectively suppress the adverse effects of brittle and hard twin martensite. Finally, a nano-scale ultra-fine martensite-bainite duplex structure with dispersed nano-carbides and interlayer retained austenite thin films (referred to as the super martensite-bainite duplex structure for short) is obtained, which significantly improves the strength-ductility matching and comprehensive mechanical properties of the material.

[0038] Specifically, the second-stage low-temperature slow-cooling quenching process in step S50 can be completed within the mold. The specific process includes: First, the temperature of the ultra-high-strength steel plate is monitored in real time through a temperature monitoring and feedback control system. When the temperature drops to 180-250°C, the heating system is turned on. By precisely adjusting the current density, the temperature of the mold surface layer is moderately increased to ensure that the component cools slowly at a rate of 1-20°C / s, thereby achieving the self-tempering effect and optimizing the tissue performance.

[0039] Specifically, the second-stage low-temperature slow-cooling quenching process in step S50 can be completed outside the mold. The specific process includes: transferring the ultra-high-strength steel plate to the air, and air cooling can also achieve the self-tempering effect of the ultra-high-strength steel plate, with an air cooling rate of 0.1-5°C / s; in order to obtain finer carbides, the component can also be placed in high-speed quenching oil for oil quenching, with an oil cooling rate of 1-20°C / s.

[0040] Specifically, in steps S10 to S50, a non-contact digital infrared thermometer and a contact thermometer are used to monitor and control the temperature of the ultra-high-strength steel plate in real time.

[0041] The above-mentioned hot forming method for strengthening the martensite-bainite duplex structure of ultra-high-strength steel will be described in detail below through specific examples.

[0042] Example 1: Example 1 of the present invention provides a hot forming method for strengthening the martensite-bainite duplex structure of ultra-high-strength steel. The test object is a 2GPa-grade hot forming steel with a thickness of 2mm. The composition of the steel plate is as follows by mass fraction: C: 0.36%, Si: 0.25%, Mn: 1.20%, Cr: 0.2%, Ti: 0.05%, B: 0.0026%, Mo: 0.2%, Nb: 0.05%, Al: 0.03%, Co: 0.18%, Ni: 0.25, and the rest is Fe and unavoidable impurities. The slab microstructure of the above steel plate consists of about 72% ferrite and about 28% pearlite, with a tensile strength of 470MPa and an elongation of 30%. The specific steps of the above method include: Step (1): Blanking the ultra-high-strength steel plate, and cutting it into a steel plate with a suitable shape through a shearing machine. The edges of the steel plate are polished to remove burrs to avoid uneven heating caused by poor contact with the electrode during the heat treatment process. The steel plate is flash-heated by a contact heating method. In order to avoid decarburization and scale formation on the surface of the steel plate during the heating process, N2 protective gas is introduced, and the heating rate is 100-150°C / s. The surface temperature of the steel plate is measured through a contact thermometer throughout the process. When the temperature of the steel plate rises to 930-950°C, it is kept warm for 15-20s short-term to obtain fine prior austenite grains with a size of 1.4-1.6μm.

[0043] Step (2): The austenitized steel plate is taken out by a manipulator gripper and quickly transferred to a stamping die for stamping to obtain a component with the required shape. Among them, the steel plate transfer time is 3 - 5 s, the die entry temperature is 770°C - 840°C, the stamping speed is 50 - 70 mm / s, the stamping pressure is 40 - 60 MPa, the strain is 12 - 14%, and the pressure holding time is 10 - 20 s. Specifically, when the surface temperature of the austenitized steel plate drops to 480°C, the stamping die needs to be electrically heated in advance; when the stamping die temperature is heated to 370°C for short-term heat preservation, the heat preservation time is 3 - 6 s, so that the die entry temperature of the steel plate is maintained between 770°C and 840°C.

[0044] Step (3): After stamping, a die with rapid cooling and heating functions is used to perform medium-temperature short-term dynamic partitioning on the component. The starting temperature of dynamic partitioning is 380 - 400°C (higher than the martensite transformation start temperature Ms of 347°C), the cooling rate of dynamic partitioning is 3 - 5°C, the dynamic partitioning time is 6 - 9 s, and the end point temperature of dynamic partitioning is 335 - 382°C, so as to obtain 6 - 9% of nano-scale low-temperature bainite. The bainite size is 50 - 150 nm.

[0045] Step (5): At 5 - 7 s of the dynamic partitioning time in step (4), turn on the cooling system with ice water in the die with rapid cooling and heating functions (because after dynamic partitioning, first-stage low-temperature rapid quenching is required, and the temperature of the steel plate will not drop instantaneously at the starting stage of the first-stage low-temperature rapid quenching); after the medium-temperature short-term dynamic partitioning ends, the above-mentioned die is quickly cooled through the cooling system to perform the first-stage low-temperature rapid quenching on the component. The end point temperature of the first-stage low-temperature rapid quenching is <150°C (at this time, the martensite transformation end point temperature M of the component f is 196°C); among them, the die cooling rate is 50°C / s - 70°C / s, so as to obtain a nano-scale ultrafine martensite-bainite duplex structure with nano-scale martensite grains and interlayer retained austenite films. The martensite size is 30 - 100 nm, the retained austenite content is 4 - 6%, and the thickness of the retained austenite film is 30 - 50 nm.

[0046] Step (6): After the first-stage low-temperature rapid quenching ends, use a die with rapid cooling and heating functions to slowly cool the component to room temperature through the second-stage low-temperature slow quenching, and the slow cooling rate is controlled at 5 - 10°C / s, finally obtaining a nano-scale ultrafine martensite-bainite high-strength and tough duplex structure with dispersed nano-carbides and interlayer retained austenite films. Specifically, when the surface temperature of the component drops to 180 - 220°C, turn on the heating system of the die with rapid cooling and heating functions, and moderately increase the surface temperature of the die by precisely adjusting the current density to ensure that the component slowly cools at a rate of 5 - 10°C / s, thereby realizing the self-tempering effect and optimizing the tissue performance.

[0047] Please refer to Figure 2, Figure 2 is the process route diagram of the strengthening and toughening hot forming method for the martensite-bainite complex phase structure provided in Embodiment 1 of the present invention; the specific route is: flash heating - hot stamping forming at high temperature (stage a-b) - medium temperature short-time dynamic partitioning (stage b-c) - the first-stage rapid cooling quenching (stage c-d) - the second-stage slow cooling quenching (stage d- e stage); A C3 is the complete transformation temperature of austenite, and M s is the starting temperature of martensite transformation, and M f is the ending temperature of martensite transformation.

[0048] It can be seen from Figure 2 that in order to avoid the temperature-time curve of the steel plate directly falling into the martensite transformation zone, it is necessary to slow down the cooling rate of the steel plate, that is, in order to shift the temperature-time curve of the steel plate towards the bainite transformation zone, during the medium temperature short-time dynamic partitioning stage, the partitioning starting temperature is slightly higher than the starting temperature M s of martensite transformation, and the partitioning ending temperature of medium temperature short-time dynamic partitioning is close to the starting temperature M s of martensite transformation, so as to realize the non-isothermal slow continuous phase transformation process of bainite at medium temperature and form lower bainite with a certain content, higher strength and good toughness.

[0049] Please refer to Figures 3a to 3b , Figure 3a is the scanning electron microscope schematic diagram of the microstructure of the hot forming component prepared in Embodiment 1 of the present invention; Figure 3b is the scanning electron microscope schematic diagram of the martensite laths in the microstructure of the hot forming component prepared in Embodiment 1 of the present invention; among them, the microstructure of the finally prepared hot forming component of the above B1500HS steel plate consists of bainite with an average size of 100 nm (mass content of 9.3%), martensite with an average size of 70 nm (mass content of 86.1%, as Figure 3b shown), thin-film retained austenite with an average size of 40 nm (mass content of 4.1%) and spherical ε-carbide with an average size of 2 nm dispersed on the matrix (mass content of 0.5%). The finally prepared hot forming component has a tensile strength of 2158 MPa, an elongation rate of 11.31%, and an impact toughness as high as 610 kJ·m -2 .

[0050] Please refer to Figure 4 , Figure 4Schematic diagram for comparing the mechanical properties of the hot-formed component prepared in Example 1 of the present invention and the hot-formed component prepared by the traditional process; among them, it can be seen from the comparison between the high-temperature hot-formed component prepared in Example 1 of the present invention and the hot-formed component made by the traditional hot-forming process method that the hot-formed component prepared in Example 1 of the present invention has better mechanical data in any one of the tensile strength (MPa), yield strength (MPa), elongation rate (%), and impact toughness (KJ*m -2 ).

[0051] Compared with the prior art, the beneficial effects of the present invention are as follows: (1) Through the process flow of flash heating - hot stamping forming - medium-temperature short-time dynamic partitioning (near M s point bainite quenching - short-time partitioning) - low-temperature step quenching (the first stage: quenching by rapid cooling in the martensite transformation region (M s ~M f ), the second stage: slow cooling quenching below the M f point), the present invention obtains for the first time a nano-scale ultra-fine martensite-bainite duplex structure with dispersed nano-carbides and intergranular retained austenite films. Among them, the ultra-fine and dense martensite-bainite duplex structure and the uniformly distributed nano-carbides achieve duplex structure strengthening, and the ultra-fine and dense structure with a small amount of low-temperature bainite and the intergranular nano-thick retained austenite film achieve duplex structure toughening.

[0052] (2) The present invention forms a steel plate component with high strength and high toughness with a low production cost and a simple and efficient production process. Compared with the traditional hot-formed component, the component formed by this process realizes the double increase of strength and toughness.

[0053] (3) Compared with the traditional hot-forming process, the production efficiency of the present invention is greatly increased, and it is suitable for the production of large-scale ultra-high-strength steel hot-formed components.

[0054] It should be noted that the above embodiments all belong to the same inventive concept. The descriptions of the embodiments have their own emphases. For the parts not described in detail in individual embodiments, reference can be made to the descriptions in other embodiments. The above embodiments only express the implementation manners of the present invention, and their descriptions are relatively specific and detailed, but they should not be construed as limiting the scope of the invention patent. It should be pointed out that for those of ordinary skill in the art, without departing from the inventive concept of the present invention, several deformations and improvements can still be made, and these all belong to the protection scope of the present invention. Therefore, the protection scope of the present invention patent shall be subject to the appended claims.

Claims

1. A super martensite-bainite complex phase structure and a strengthening and toughening hot forming method, characterized in that, It includes the following steps: S10. Flash heating the ultra-high strength steel plate to ensure that the microstructure of the ultra-high strength steel plate completes austenite transformation and the composition is uniform; S20. Transferring the heated ultra-high strength steel plate to a mold for hot stamping forming and pressure holding treatment; S30. Performing medium-temperature short-time dynamic partitioning treatment on the ultra-high strength steel plate to ensure that the microstructure of the ultra-high strength steel plate transforms to form nano-scale low-temperature bainite with a mass content of 1% - 35% and a size of 20 - 300 nm and carbon-rich supercooled austenite; S40. Performing the first-stage low-temperature rapid quenching treatment in the low-temperature step quenching process on the ultra-high strength steel plate to ensure that the microstructure of the ultra-high strength steel plate transforms to form a nano-scale ultra-fine martensite-bainite composite structure with nano-scale martensite grains with a size of 10 - 150 nm and an intergranular residual austenite film; S50. Performing the second-stage low-temperature slow quenching treatment in the low-temperature step quenching process on the ultra-high strength steel plate and cooling it to room temperature to ensure that the microstructure of the ultra-high strength steel plate transforms to form a nano-scale ultra-fine martensite-bainite composite structure with dispersed nano-carbides and an interlayer residual austenite film.

2. The hot forming method for strengthening and toughening the martensite-bainite duplex structure of ultra-high strength steel according to claim 1, characterized in that, In the step S10: The ultra-high strength steel plate is flash heated to a temperature of 800 - 1000 °C, then short-time held for 1 - 300 s, and the heating rate is 30 - 200 °C / s.

3. The hot forming method for strengthening and toughening the martensite-bainite duplex structure of ultra-high strength steel according to claim 1, characterized in that, The flash heating method in the step S10 includes any one of induction heating, resistance heating, contact heating, electrical hybrid heating, and heating using a heating furnace with a heating function; the grain size of austenite in the ultra-high strength steel plate is less than 2 μm.

4. The ultra-high strength steel martensite-bainite complex phase structure strengthening and toughening hot forming method according to claim 1, characterized in that, In the step S20: The transfer time of the ultra-high strength steel plate to the mold is 2 - 10 s, the deformation amount in hot stamping forming is 1% - 60%, the temperature when entering the mold is 600 - 900 °C, the stamping speed is 30 - 200 mm / s, and the pressure holding time is 2 - 60 s.

5. The hot forming method for strengthening and toughening the martensite-bainite duplex structure of ultra-high strength steel according to claim 1, characterized in that, In the step S30: The temperature adjustment method of the medium-temperature short-time dynamic partitioning treatment is dynamic fluctuation adjustment or isothermal adjustment; Among them, the medium-temperature short-time dynamic partitioning temperature range is (M s - 30°C) to (M s + 150°C), the partitioning time is 1 to 300 s, and M s is the martensite start temperature of the ultra-high strength steel plate.

6. The hot forming method for strengthening and toughening the martensite-bainite duplex structure of ultra-high strength steel according to claim 1, characterized in that In the step S40: the quenching rate is greater than 35 °C / s, and the quenching end point temperature is (M f -100 °C) to (M f -50 °C), where M f is the martensite phase transformation end point temperature of the ultra-high strength steel plate, the mass content of the retained austenite is 2-12%, and the thickness is less than 50 nm; in the step S50: the quenching rate is 1-20 °C / s.

7. The ultra-high strength steel martensite-bainite complex phase structure strengthening and toughening hot forming method according to claim 1, characterized in that, Using a mold with a fast cooling and fast heating function to perform the medium-temperature short-time dynamic partitioning treatment and the first-stage low-temperature rapid quenching treatment on the ultra-high strength steel plate; the material of the mold is hot work die steel or nickel-based superalloy.

8. The ultra-high strength steel martensite-bainite duplex structure strengthening and toughening hot forming method according to claim 7, characterized in that, In the steps from S30 to S40, the cooling medium loaded in the mold includes at least one of ice water, nanofluid, and liquid nitrogen, and the nanofluid includes graphene nanofluid with water as the dispersion medium or carbon nanotube nanofluid with water as the dispersion medium.

9. The ultra-high strength steel martensite-bainite duplex phase structure strengthening and toughening hot forming method according to claim 8, characterized in that, In the step S50: When performing the second-stage low-temperature slow quenching treatment on the ultra-high strength steel plate with air, the air cooling rate is 0.1 - 5 °C / s; when performing the second-stage low-temperature slow quenching treatment on the ultra-high strength steel plate with quenching oil, the oil cooling rate is 1 - 20 °C / s.

10. The ultra-high strength steel martensite-bainite complex phase structure strengthening and toughening hot forming method according to claim 1, characterized in that, In the steps from S10 to S50, a non-contact digital infrared thermometer and a contact thermometer are used to monitor and control the temperature of the ultra-high strength steel plate in real time.

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