Super martensite-bainite composite microstructure and toughened hot forming method

Abstract

The application provides a super martensite-bainite complex phase organization and toughening hot forming method, which first changes the structure of an ultra-high strength steel plate into a nanoscale super fine martensite-bainite complex phase organization with dispersed nanometer carbide and interlayer residual austenite film through a process flow of flash heating-high temperature stamping forming-middle temperature short time dynamic distribution-low temperature grading quenching, wherein the dispersed nanometer carbide and the super fine martensite-bainite complex phase organization can realize complex phase organization strengthening, the super fine and dense structure with a small amount of low temperature bainite and the intergranular nanometer thickness residual austenite film can realize complex phase organization toughening, so that the strength and toughness are both increased under the premise of material "simplification", the crash performance and service reliability of the hot forming component are significantly improved, and reliable technical support is provided for automobile light weight and safety.

Description

Technical Field

[0001] This invention relates to the field of thermoforming, and more particularly to a super Marble-Bai multiphase microstructure and a toughening thermoforming method. Background Technology

[0002] Driven by the green manufacturing concept of "energy conservation, emission reduction, low carbon, and environmental protection," automotive lightweighting has become a significant trend in global industrial development. Hot-formed steel, with its significant advantages such as high strength, low springback, high yield, and low cost, has become one of the key materials for achieving automotive lightweighting. Currently, 22MnB5 hot-formed steel, with a tensile strength of approximately 1.5 GPa and an elongation of approximately 6%, is widely used in automotive safety structural components. To further advance automotive lightweighting, the global automotive and steel industries are actively developing a new generation of hot-formed steel with tensile strengths reaching 2 GPa and above. However, the classic strength-toughness inverse relationship in steel materials indicates that an increase in strength is often accompanied by a decrease in toughness. This can lead to fracture failure and delayed cracking in automotive safety structural components, making it difficult to meet collision energy absorption standards and thus limiting the application of high-strength hot-formed steel.

[0003] Therefore, ensuring lightweight vehicles while maintaining ultra-high strength and endowing materials with high toughness, high intrusion resistance, and excellent collision energy absorption has become a key technical challenge that urgently needs to be solved in the application of hot-formed steel components. Summary of the Invention

[0004] The purpose of this invention is to provide a method for strengthening and toughening ultra-high strength steel with Marble-Bayer multiphase structure through hot forming, 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 2GPa and above.

[0005] To address the aforementioned technical problems, this invention provides a super martensite / bainite multiphase microstructure and a toughening thermoforming method. This method utilizes a thermo-mechanically coupled phase transformation path control strategy consisting of "flash heating – high-temperature stamping – medium-temperature short-time dynamic partitioning – low-temperature graded quenching" to construct a multiphase, multi-scale synergistically toughened structure with "martensite / bainite nanoscale ultrafine matrix + nano-carbide + metastable thin-film retained austenite." The specific steps include:

[0006] S10 is used to flash heat the ultra-high strength steel plate to ensure that the microstructure of the ultra-high strength steel plate completes the austenitic transformation and has a uniform composition.

[0007] S20 involves transferring the heated ultra-high-strength steel plate to a mold for high-temperature stamping and pressure holding.

[0008] S30 involves applying a medium-temperature, short-time dynamic partitioning treatment to ultra-high-strength steel plates to ensure that the microstructure of the ultra-high-strength steel plates transforms into nanoscale low-temperature bainite and carbon-rich supercooled austenite with a mass content of 1% to 35% and a size of 20 to 300 nm.

[0009] S40 is the first stage of low-temperature rapid quenching in the low-temperature graded quenching process of ultra-high strength steel plate, to ensure that the microstructure of ultra-high strength steel plate is transformed into nanoscale ultrafine martensite-bainite complex structure with a size of 10~150nm and intergranular residual austenite film.

[0010] S50 involves performing the second stage of low-temperature slow cooling quenching in a low-temperature graded 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 into a nanoscale ultrafine martensite-bainite multiphase structure with dispersed nano-carbide and interlayer retained austenite film. Preferably, in step S10: the ultra-high-strength steel plate is flash-heated to a temperature of 800-1000℃ and then held at that temperature for a short time of 1-300s, with a heating rate of 30-200℃ / s.

[0011] Preferably, the flash heating method in step S10 includes any one of induction heating, resistance heating, contact heating, electric 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 2μm.

[0012] Preferably, in step S20: the transfer time of the ultra-high strength steel plate to the mold is 2~10s, the deformation in the stamping process is 1%~60%, the mold entry temperature is 600~900℃, the stamping speed is 30~200mm / s, and the holding time is 2~60s.

[0013] Preferably, in step S30: the temperature adjustment method for the medium-temperature short-time dynamic allocation treatment is dynamic fluctuation adjustment or isothermal adjustment; wherein, the medium-temperature short-time dynamic allocation temperature range is (M s -30℃~(M) s +150℃), dispensing time is 1~300s, M s The temperature at which the martensitic phase transformation begins in the ultra-high strength steel plate is denoted as .

[0014] Preferably, in step S40: the quenching rate is greater than 35℃ / s, and the quenching end temperature is (M f -100℃~(M f -50℃), M f The temperature at which the martensitic phase transformation of the ultra-high strength steel plate ends is 2-12% by mass, and the thickness is less than 50 nm; in step S50: the quenching rate is 1-20℃ / s.

[0015] Preferably, a mold with rapid cooling and heating function is used to perform short-term slow cooling treatment and first-stage low-temperature quenching treatment on ultra-high strength steel plates; the mold material is hot work die steel or nickel-based high-temperature alloy.

[0016] 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.

[0017] Preferably, in step S50: when air is used to perform the second-stage low-temperature slow cooling quenching treatment on the ultra-high strength steel plate, the air cooling rate is 0.1~5℃ / s; when quenching oil is used to perform the second-stage low-temperature slow cooling quenching treatment on the ultra-high strength steel plate, the oil cooling rate is 1~20℃ / s.

[0018] 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.

[0019] The beneficial effects of this invention are as follows: Unlike existing technologies, this invention provides a method for strengthening and toughening ultra-high strength steel martensite-bainite multiphase microstructure through a process flow of flash heating – high-temperature stamping – medium-temperature short-time dynamic partitioning – low-temperature graded quenching. This method, for the first time, transforms the microstructure of ultra-high strength steel sheets into a nanoscale ultrafine martensite-bainite multiphase microstructure with dispersed nano-carbide and interlaminar retained austenite films. The uniformly distributed nano-carbide and ultrafine nanoscale martensite-bainite multiphase microstructure achieve multiphase reinforcement, while the ultrafine microstructure with a small amount of low-temperature bainite and the intergranular nano-thickness retained austenite film achieves multiphase toughening. Thus, under the premise of material "refinement," both strength and toughness are increased, significantly improving the collision performance and service reliability of hot-formed components, providing reliable technical support for automotive lightweighting and safety. Simultaneously, this strengthening and toughening hot-formed method, with lower production costs and a simple and efficient production process, produces hot-formed components with high strength and high toughness, achieving a dual increase in strength and toughness compared to traditional hot-formed components. Attached Figure Description

[0020] Figure 1 Flowchart of the method for strengthening and toughening thermoforming of ultra-high strength steel Marble-Bayer multiphase structure provided by the present invention;

[0021] Figure 2 This is a process route diagram for the hot forming method of strengthening and toughening ultra-high strength steel Marble-Bayer multiphase structure provided in Embodiment 1 of the present invention;

[0022] Figure 3a A scanning electron microscope schematic diagram of the microstructure of the thermoformed component prepared in Embodiment 1 of the present invention;

[0023] Figure 3b This is a scanning electron microscope schematic diagram of the martensitic laths in the microstructure of the thermoformed component prepared in Example 1 of the present invention.

[0024] Figure 4 This is a schematic diagram comparing the mechanical properties of the thermoformed component prepared in Example 1 of the present invention with those of the thermoformed component prepared by conventional processes. Detailed Implementation

[0025] The technical solutions of the present invention will be clearly and completely described below with reference to the embodiments of the present invention. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative effort are within the scope of protection of the present invention.

[0026] To address the critical challenges of poor toughness and difficulty in matching strength and toughness in hot-formed components made of ultra-high-strength steel with a strength of 2 GPa and above, and the resulting problems such as insufficient energy absorption during component impact and delayed cracking, this invention proposes an innovative solution: employing a thermo-coupled phase transformation path control strategy to construct a multi-phase, multi-scale synergistic toughening structure of "martensite / bainite nanoscale ultrafine matrix + nano-carbides + metastable thin-film retained austenite," thus overcoming the problem of strength-toughness inversion. The specific process of the hot-forming method for toughening ultra-high-strength steel with martensite / bainite multiphase structure provided by this invention is as follows: flash heating – high-temperature stamping – medium-temperature short-time dynamic partitioning (near M... s Point bainitic quenching - short-time partitioning) - low-temperature staged quenching (first stage: martensitic transformation zone (M s ~M f Rapid quenching, Level 2: M f The following slow cooling and quenching process, under the premise of material "simplification," achieves a dual increase in strength and toughness, significantly improving the collision performance and service reliability of components, and providing reliable technical support for automotive lightweighting and safety; among them, M s M is the temperature at which the martensitic phase transformation begins. f This is the temperature at which the martensitic phase transformation ends.

[0027] Please see Figure 1 , Figure 1 A flowchart of the hot forming method for strengthening and toughening ultra-high strength steel with Marble-Bayer multiphase microstructure provided by the present invention; wherein the above-mentioned hot forming method for strengthening and toughening includes the following steps:

[0028] S10 is used to flash heat-treat ultra-high strength steel plates to ensure that the microstructure of the ultra-high strength steel plates completes the austenitic transformation and has a uniform composition.

[0029] Specifically, step S10 includes:

[0030] First, the ultra-high strength steel billet is blanked and punched to obtain an ultra-high strength steel plate with a suitable shape. The thickness of the ultra-high strength steel plate is 0.5-10mm. The blanking and punching process includes grinding the edges of the ultra-high strength steel billet to remove burrs, so as to avoid uneven heating caused by poor contact with the electrodes during the electro-treatment process.

[0031] Secondly, the steel plate is heated to a suitable austenitizing temperature using flash heating (generally, the heating termination temperature of heat treatment is about 100°C higher than the complete austenite transformation temperature), and then held at that temperature for a short time to ensure the acquisition of fine proto-austenite grains and achieve complete austenitization. The heating rate is 30–200°C / s, the austenitizing temperature is 880–970°C, and the holding time is 5–300s.

[0032] Specifically, the flash heating method in step S10 includes induction heating, resistance heating, contact heating, electric hybrid heating, or heating furnaces with high-speed heating functions; compared with conventional heating, it has the advantages of fast heating speed, high heating efficiency and small footprint. In addition, smaller components can be tested using a continuous annealing test machine.

[0033] Specifically, the original austenite grain size obtained in step S10 is <2μm. The fine austenite grain boundaries provide more nucleation sites for the bainitic phase transformation, and at the same time, based on the microstructure inheritance effect, the grain boundaries hinder the growth of bainitic ferrite, thereby obtaining a nanobainitic microstructure.

[0034] S20 involves transferring the heated ultra-high strength steel plate to a mold for stamping and pressure holding.

[0035] Specifically, step S20 also includes:

[0036] The austenitized ultra-high strength steel plate is rapidly transferred to a die for stamping to obtain a component of the desired shape; the transfer time is 2~10s, the die entry temperature is 600~900℃, the stamping speed is 30~200mm / s, the deformation during stamping is 1%~60%, and the holding time is 2~60s.

[0037] Specifically, in step S20, the ultra-high-strength steel sheet is formed at high temperature, which significantly increases the kinetic energy of metal atoms, promoting the material's recovery and recrystallization processes. Recrystallization can effectively eliminate work hardening, significantly reduce the metal's deformation resistance, and greatly improve its plastic deformation capacity, thereby reducing the pressure required for stamping and lowering the risk of defects during the forming process.

[0038] Specifically, in step S20, a large number of dislocations are introduced into the austenite by applying appropriate high-temperature deformation to the ultra-high-strength steel plate. These dislocations rearrange and annihilate at high temperatures, forming subgrain boundaries. These subgrain boundaries can serve as preferred sites for bainite nucleation, significantly increasing the density of bainite nucleation sites. At the same time, deformation promotes local carbon diffusion, forming "carbon-depleted regions" in the deformed austenite, thereby increasing the driving force for bainite nucleation and further accelerating bainite formation.

[0039] S30 involves applying a short-time dynamic partitioning treatment at medium temperature to ultra-high strength steel plates to ensure that the microstructure of the ultra-high strength steel plates transforms into nanoscale low-temperature bainite and carbon-rich supercooled austenite with a mass content of 1% to 35% and a size of 20 to 300 nm.

[0040] Specifically, step S30 also includes:

[0041] After stamping, the ultra-high strength steel plate undergoes a short-term dynamic distribution treatment at a medium temperature. The temperature regulation method for this treatment is either dynamic fluctuation regulation or isothermal regulation. Specifically, the starting temperature for dynamic distribution is (Ms+30℃) to (Ms+150℃), the slow cooling rate is <10℃, the slow cooling time is 3–60 seconds, and the ending temperature for dynamic distribution is (Ms+30℃) to (Ms+150℃). s -30℃~(M) s +30℃), M s This is the starting temperature for the martensitic phase transformation of ultra-high strength steel plates, ensuring that the microstructure of ultra-high strength steel plates transforms into nanoscale low-temperature bainite with a mass content of 1% to 35% and a size of 20 to 300 nm, as well as carbon-rich supercooled austenite.

[0042] Specifically, the dynamic partitioning start temperature in step S30 is slightly higher than the martensitic phase transformation initiation temperature M. s A lower, slower cooling onset temperature favors the formation of lower bainite (i.e., low-temperature bainite) with excellent mechanical properties, avoiding the formation of upper bainite or granular bainite with poor mechanical properties at higher temperatures. Simultaneously, the lower cooling onset temperature provides greater undercooling, significantly increasing the nucleation driving force for bainite transformation and raising the early nucleation rate of bainite. During the austenite-to-bainite transformation, the high-density dislocations introduced by deformation hinder the migration of the phase transformation interface, enhancing the stability of austenite and thus inhibiting bainite growth. Furthermore, at lower temperatures, the undercooled austenite exhibits higher strength, and the interaction between adjacent grains further restricts the size of the bainite laths. Ultimately, a nanoscale low-temperature bainite microstructure with high strength, high toughness, and fine grains is obtained.

[0043] Specifically, the dynamic partitioning end temperature in step S30 is close to the martensitic phase transformation initiation temperature M. sA lower dynamic distribution end point temperature is conducive to the further formation of low-temperature bainite with excellent mechanical properties, avoiding the formation of upper bainite or granular bainite with poor mechanical properties in the higher temperature range.

[0044] Specifically, the dynamic partitioning time in step S30 is preferably 6–15 seconds. Within this time range, an appropriate amount of low-temperature bainite (4%–12% by mass) can be formed, ensuring that it has no significant negative impact on the strength of the component. If the slow cooling time is too long, the increased content of low-temperature bainite will reduce the amount of subsequent martensite formation. At the same time, grain growth will lead to a decrease in strength and may precipitate brittle carbides (such as cementite), which will adversely affect the toughness of the microstructure, thereby reducing the overall mechanical properties of the material.

[0045] Specifically, in step S30: the preferentially formed nanoscale low-temperature bainitic structure divides the original austenite grains into multiple fine regions, significantly limiting the growth space of martensite, thus making the subsequently formed martensite lath bundles even finer. Furthermore, the carbon-rich supercooled austenite generated during the partitioning process is retained at room temperature after quenching, forming a thin film of retained austenite. This retained austenite, as a toughening phase, can alleviate stress concentration at the crack tip through the transformation-induced plasticity (TRIP) effect under impact loads, effectively hindering crack propagation and thus significantly improving the material's toughness and impact resistance.

[0046] S40 involves the first stage of a low-temperature graded quenching process for ultra-high-strength steel plates, ensuring that the microstructure of the ultra-high-strength steel plate transforms into a nanoscale ultrafine martensite-bainite composite microstructure with nanoscale martensite grains of 10~150nm size and intergranular retained austenite films. Specifically, step S40 also includes:

[0047] After a short period of slow cooling, the ultra-high strength steel plate undergoes a first-stage low-temperature quenching treatment with a quenching rate >50℃ / s to ensure that the microstructure of the ultra-high strength steel plate transforms into a nanoscale ultrafine martensite-bainite complex structure with nanoscale martensite grains of 10~150nm in size and intergranular retained austenite films; the mass content of retained austenite is 2~8%, and the thickness is less than 50nm; wherein, the quenching rate of the first-stage low-temperature quenching treatment is >50℃ / s, and the quenching end temperature is (M s -100℃~(M s -50℃), M s This is the temperature at which the martensitic phase transformation of ultra-high strength steel plate ends.

[0048] Specifically, in step S40, the quenching cooling rate is controlled at >50℃ / s; the higher quenching rate significantly refines the martensite grain size by increasing the undercooling, improving the martensite nucleation rate, and suppressing grain growth.

[0049] Specifically, the medium-temperature short-time dynamic distribution – first-stage low-temperature quenching process in steps S30 to S40 can be carried out in a mold with rapid cooling and heating capabilities. The mold material is H13 hot work die steel or nickel-based high-temperature alloy, which has excellent thermal conductivity, wear resistance, and high-temperature resistance. The mold has built-in high-power heating elements (such as resistance heating rods, induction coils, etc.) to achieve rapid heating in a short time, with a maximum heating rate of up to 200℃ / s. The mold is designed with efficient cooling channels near the surface, and the cooling medium uses high thermal conductivity media such as ice water (cooling rate > 50℃ / s), nanofluids (water + graphene nanofluids, water + carbon nanotube nanofluids, etc., cooling rate > 150℃ / s), or liquid nitrogen (cooling rate > 300℃ / s). The mold is equipped with a multi-point temperature monitoring and feedback control system to ensure uniform temperature distribution on the mold surface.

[0050] Specifically, the intermediate-temperature short-time dynamic partitioning – first-stage low-temperature quenching process in steps S30 to S40 can be carried out in stages outside the mold: First, the ultra-high-strength steel plate formed at high temperature is rapidly transferred to a salt bath furnace at a set temperature for short-time partitioning to obtain a certain content of nano-low-temperature bainite and carbon-rich supercooled austenite. Subsequently, the plate is rapidly placed in an ultrasonically assisted liquid nitrogen bath for quenching, utilizing the vibration effect of ultrasound to break the gas film, thereby obtaining nano-martensite laths and thin-film retained austenite.

[0051] S50 involves performing the second stage of low-temperature slow cooling quenching in a low-temperature graded quenching process on ultra-high-strength steel plates, followed by cooling to room temperature. This ensures that the microstructure of the ultra-high-strength steel plates transforms into a nanoscale ultrafine martensite-bainite multiphase microstructure with dispersed nano-carbide and interlaminar retained austenite films. Specifically, the S50 step also includes:

[0052] After the first-stage low-temperature quenching treatment, the ultra-high-strength steel plate undergoes a second-stage low-temperature slow quenching to room temperature, with the slow cooling rate controlled at 1~20℃ / s. This second-stage low-temperature slow quenching (i.e., slowing down the cooling rate of the component based on the second-stage low-temperature slow quenching) induces a self-tempering effect in the already formed martensitic matrix, forming nanoscale self-tempered martensite and uniformly dispersed nano-ε-carbides, while effectively suppressing the adverse effects of brittle and hard twinned martensite. Ultimately, a nanoscale ultrafine martensite-bainite multiphase structure (referred to as super martensite-bainite multiphase structure) with dispersed nano-carbides and interlaminar retained austenite films is obtained, significantly improving the material's strength-toughness balance and overall mechanical properties.

[0053] Specifically, the second-stage low-temperature slow cooling quenching process in step S50 can be completed inside the mold. The specific process includes: firstly, 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℃, the heating system is turned on. By precisely adjusting the current density, the surface temperature of the mold is moderately increased to ensure that the component is slowly cooled at a rate of 1~20℃ / s, thereby achieving the self-tempering effect and optimizing the microstructure and properties.

[0054] 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 into the air for air cooling, which can also achieve the self-tempering effect of the ultra-high strength steel plate. The air cooling rate is 0.1~5℃ / s. In order to obtain finer carbides, the component can also be placed in high-speed quenching oil for oil quenching. The oil cooling rate is 1~20℃ / s.

[0055] 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.

[0056] The above-mentioned method for strengthening and toughening thermoforming of Marble-Bayer multiphase microstructure in ultra-high strength steel is described in detail below through specific embodiments.

[0057] Example 1:

[0058] Example 1 of this invention provides a hot forming method for strengthening and toughening ultra-high strength steel with a Marble-Bayan multiphase microstructure. The test object is a 2 GPa grade hot-formed steel with a thickness of 2 mm. The composition of the steel plate by mass fraction is as follows: 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%, with the remainder being Fe and unavoidable impurities. The slab microstructure of the above steel plate consists of approximately 72% ferrite and approximately 28% pearlite, with a tensile strength of 470 MPa and an elongation of 30%. The specific steps of the above method include:

[0059] Step (1): The ultra-high strength steel plate is blanked and cut into a suitable shape using a shearing machine. The edges of the steel plate are ground to remove burrs to avoid uneven heating caused by poor contact with the electrodes during heat treatment. The steel plate is flash-heated using contact heating. To prevent decarburization and oxide scale formation on the steel plate surface during heating, N2 protective gas is introduced. The heating rate is 100~150℃ / s. The surface temperature of the steel plate is measured throughout the process using a contact thermometer. When the steel plate temperature reaches 930~950℃, it is held for 15~20s to obtain fine austenite grains with a size of 1.4~1.6μm.

[0060] Step (2): The austenitized steel sheet is quickly transferred to a stamping die using a robotic gripper to form the desired shape. The transfer time is 3-5 seconds, the die entry temperature is 770℃-840℃, the stamping speed is 50-70 mm / s, the stamping pressure is 40-60 MPa, the deformation is 12-14%, and the holding time is 10-20 seconds. Specifically, when the surface temperature of the austenitized steel sheet drops to 480℃, the stamping die needs to be preheated electrically. When the stamping die temperature is heated to 370℃, it is held for a short time for 3-6 seconds to keep the die entry temperature of the steel sheet between 770℃ and 840℃.

[0061] Step (3): After stamping, a mold with rapid cooling and heating function is used to perform medium-temperature short-time dynamic partitioning on the component. The starting temperature of dynamic partitioning is 380~400℃ (greater than the martensitic phase transformation start temperature Ms of 347℃), the cooling rate of dynamic partitioning is 3~5℃, the dynamic partitioning time is 6~9s, and the ending temperature of dynamic partitioning is 335~382℃, in order to obtain 6~9% nanoscale low-temperature bainite. The bainite size is 50~150nm.

[0062] Step (5): During the dynamic distribution time of 5-7 seconds in step (4), the cooling system with ice water in the mold with rapid cooling and heating function is turned on (because the first stage of low temperature quenching is required after the dynamic distribution, the temperature of the steel plate will not drop instantaneously at the beginning of the first stage of low temperature quenching); after the short-time dynamic distribution at medium temperature is completed, the above mold is rapidly cooled through the cooling system to perform the first stage of low temperature quenching on the component. The end temperature of the first stage of low temperature quenching is <150℃ (at this time, the end temperature of the martensitic transformation of the component is M). f The temperature is 196℃). The mold cooling rate is 50℃ / s to 70℃ / s to obtain a nanoscale ultrafine martensite-bainite complex structure with nanoscale martensite grains and interlayer residual austenite film. The martensite size is 30~100nm, the residual austenite content is 4~6%, and the residual austenite film thickness is 30~50nm.

[0063] Step (6): After the first-stage low-temperature quenching, the component is subjected to a second-stage low-temperature slow quenching to room temperature using a mold with rapid cooling and heating functions. The slow cooling rate is controlled at 5~10℃ / s, ultimately obtaining a nanoscale ultrafine martensite-bainite high-strength and tough multiphase structure with dispersed nano-carbide and interlayer retained austenite film. Specifically, when the surface temperature of the component drops to 180~220℃, the heating system of the mold with rapid cooling and heating functions is turned on. By precisely adjusting the current density, the surface temperature of the mold is moderately increased, ensuring that the component is slowly cooled at a rate of 5~10℃ / s, thereby achieving a self-tempering effect and optimizing the microstructure and properties.

[0064] Please see Figure 2, Figure 2 This is a process route diagram for the hot forming method of strengthening and toughening ultra-high strength steel with Marble-Bayer multiphase structure provided in Embodiment 1 of the present invention; the specific route is as follows: flash heating – high temperature stamping (ab stage) – medium temperature short-time dynamic partitioning (bc stage) – first-stage rapid quenching (cd stage) – second-stage slow quenching (d- e Phase); A C3 M is the temperature at which austenite undergoes a complete transformation. s M is the temperature at which the martensitic phase transformation begins. f This is the temperature at which the martensitic phase transformation ends.

[0065] Depend on Figure 2 It is known that, in order to prevent the temperature-time curve of the steel plate from falling directly into the martensitic transformation region, it is necessary to slow down the cooling rate of the steel plate. That is, to shift the temperature-time curve of the steel plate towards the bainitic transformation region, during the medium-temperature short-time dynamic partitioning stage, the partitioning start temperature should be slightly higher than the martensitic transformation initiation temperature M. s The end temperature of the short-term dynamic partitioning at medium temperature is close to the martensitic phase transformation initiation temperature M. s This enables a slow, continuous phase transformation process of bainite at medium temperature and non-isothermal temperature, resulting in lower bainite with a certain content of high strength and good toughness.

[0066] Please see Figures 3a to 3b , Figure 3a A scanning electron microscope schematic diagram of the microstructure of the thermoformed component prepared in Embodiment 1 of the present invention; Figure 3b This is a scanning electron microscope (SEM) schematic diagram of the martensitic laths in the microstructure of the hot-formed component prepared in Example 1 of the present invention; wherein, the microstructure of the hot-formed component finally prepared from the above-mentioned B1500HS steel sheet consists of bainite (9.3% by mass) with an average size of 100 nm and martensite (86.1% by mass) with an average size of 70 nm. Figure 3b The thermoformed component consists of a thin film of retained austenite with an average size of 40 nm (mass content 4.1%) and spherical ε-carbides with an average size of 2 nm dispersed on the matrix (mass content 0.5%). The resulting thermoformed component exhibits a tensile strength of 2158 MPa, an elongation of 11.31%, and an impact toughness as high as 610 kJ·m. -2 .

[0067] Please see Figure 4 , Figure 4This is a schematic diagram comparing the mechanical properties of the hot-formed component prepared in Example 1 of the present invention and a hot-formed component prepared by a conventional process. As can be seen from the comparison between the hot-formed component prepared in Example 1 of the present invention and the hot-formed component prepared by a conventional hot-formed process, the hot-formed component prepared in Example 1 of the present invention exhibits superior tensile strength (MPa), yield strength (MPa), elongation (%), and impact toughness (KJ*m). -2 Any one of the mechanical properties in ) has better mechanical data.

[0068] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0069] (1) This invention utilizes flash heating – high-temperature stamping – medium-temperature short-time dynamic distribution (near M) s Point bainitic quenching - short-time partitioning) - low-temperature staged quenching (first stage: martensitic transformation zone (M s ~M f Rapid quenching, Level 2: M f By using a slow cooling quenching process, a nanoscale ultrafine Marcel-Bainite multiphase structure with dispersed nanocarbide and intergranular retained austenite film was obtained for the first time. The ultrafine Marcel-Bainite multiphase structure and the dispersed and uniformly distributed nanocarbide achieve the reinforcement of the multiphase structure, while the ultrafine structure with a small amount of low-temperature bainite and the intergranular nano-thickness retained austenite film achieve the toughening of the multiphase structure.

[0070] (2) This invention produces steel plate components with high strength and high toughness with low production cost and simple and efficient production process. Compared with traditional hot-formed components, the components formed by this process achieve a double increase in strength and toughness.

[0071] (3) Compared with the traditional hot forming process, the production efficiency of the present invention is greatly improved, and it is suitable for the large-scale production of ultra-high strength steel hot forming components.

[0072] It should be noted that all the above embodiments belong to the same inventive concept, and the descriptions of each embodiment have different focuses. Where the description in a particular embodiment is not exhaustive, please refer to the descriptions in other embodiments. The above embodiments only illustrate the implementation 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 skilled in the art, several modifications and improvements can be made without departing from the inventive concept, and these all fall within the protection scope of the present invention. Therefore, the protection scope of this invention patent should be determined by the appended claims.

Claims

1. A method for strengthening and toughening thermoforming of ultra-high strength steel with a Marshall-Bay multiphase structure, characterized in that, Includes the following steps: S10, the ultra-high strength steel plate is subjected to flash heating treatment to ensure that the microstructure of the ultra-high strength steel plate completes the austenitic transformation and has a uniform composition. S20, the heated ultra-high strength steel plate is transferred to a mold for high-temperature stamping and pressure holding treatment; S30, the ultra-high strength steel plate is subjected to medium-temperature short-time dynamic distribution treatment to ensure that the microstructure of the ultra-high strength steel plate is transformed into nanoscale low-temperature bainite and carbon-rich supercooled austenite with a mass content of 1% to 35% and a size of 20 to 300 nm. S40, the ultra-high strength steel plate is subjected to the first stage of low-temperature rapid quenching in the low-temperature graded quenching process to ensure that the microstructure of the ultra-high strength steel plate is transformed into a nanoscale ultrafine martensite-bainite complex structure with a size of 10~150nm and intergranular residual austenite film. S50, the ultra-high strength steel plate is subjected to the second stage of low-temperature slow cooling quenching treatment in the low-temperature graded quenching process, and cooled to room temperature to ensure that the microstructure of the ultra-high strength steel plate is transformed into a nanoscale ultrafine martensite-bainite multiphase microstructure with dispersed nano carbides and interlayer retained austenite film. In step S30: the temperature adjustment method for the medium-temperature short-time dynamic allocation treatment is dynamic fluctuation adjustment or isothermal adjustment; the medium-temperature short-time dynamic allocation temperature range is (M s -30℃~(M) s +150℃), dispensing time is 1~300s, M s The temperature at which the martensitic phase transformation begins in the ultra-high strength steel plate is denoted as .

2. The method for strengthening and toughening thermoforming of ultra-high strength steel with Marble-Bayer multiphase microstructure according to claim 1, characterized in that, In step S10: the ultra-high strength steel plate is flash-heated to a temperature of 800-1000℃ and then held at that temperature for 1-300s for a short time, with a heating rate of 30-200℃ / s.

3. The method for strengthening and toughening thermoforming of ultra-high strength steel with Marble-Bayer multiphase microstructure according to claim 1, characterized in that, The flash heating method in step S10 includes any one of induction heating, resistance heating, contact heating, electric 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 2μm.

4. The method for strengthening and toughening thermoforming of ultra-high strength steel with Marble-Bayer multiphase microstructure according to claim 1, characterized in that, In step S20: the transfer time of the ultra-high strength steel plate to the mold is 2~10s, the deformation during stamping is 1%~60%, the mold entry temperature is 600~900℃, the stamping speed is 30~200mm / s, and the holding time is 2~60s.

5. The method for strengthening and toughening thermoforming of ultra-high strength steel with Marble-Bayer multiphase microstructure according to claim 1, characterized in that, In step S40: the quenching rate is greater than 35℃ / s, and the quenching end temperature is (M). f -100℃~(M f -50℃), M f The martensitic transformation end temperature of the ultra-high strength steel plate is 2~12% by mass, and the thickness is less than 50nm; in step S50: the quenching rate is 1~20℃ / s.

6. The method for strengthening and toughening thermoforming of ultra-high strength steel with Marble-Bayer multiphase microstructure according to claim 1, characterized in that, The ultra-high strength steel plate is subjected to medium-temperature short-time dynamic distribution treatment and first-stage low-temperature quenching treatment using a mold with rapid cooling and heating functions; the mold is made of hot work die steel or nickel-based high-temperature alloy.

7. The method for strengthening and toughening thermoforming of ultra-high strength steel with Marble-Bayer multiphase microstructure according to claim 6, characterized in that, The cooling medium loaded in the mold during steps S30 to S40 includes at least one of ice water, nanofluid, and liquid nitrogen, wherein the nanofluid includes graphene nanofluid with water as the dispersion medium or carbon nanotube nanofluid with water as the dispersion medium.

8. The method for strengthening and toughening thermoforming of ultra-high strength steel with Marble-Bayer multiphase microstructure according to claim 7, characterized in that, In step S50: when air is used to perform the second-stage low-temperature slow cooling quenching treatment on the ultra-high strength steel plate, the air cooling rate is 0.1~5℃ / s; when quenching oil is used to perform the second-stage low-temperature slow cooling quenching treatment on the ultra-high strength steel plate, the oil cooling rate is 1~20℃ / s.

9. The method for strengthening and toughening thermoforming of ultra-high strength steel with Marble-Bayer multiphase microstructure according to claim 1, characterized in that, 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.

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