A martensite-bainite multilayer gradient biomimetic structure steel and a preparation method thereof

Abstract

This invention discloses a martensitic-bainitic multilayer gradient biomimetic structural steel and its preparation method. The multilayer gradient biomimetic structural steel has a structure in which hard phase layers and soft phase layers are arranged alternately to form a hardness gradient distribution. The hard phase layer is based on tempered martensite. The soft phase layer contains a continuous matrix of lower bainitic ferrite, interspersed with secondary quenched martensite and grain boundary film-like retained austenite. The martensitic-bainitic multilayer gradient biomimetic structural steel of this invention has a multi-level biomimetic structure from macroscopic to microscopic. Macroscopically, it presents a clear layered configuration of alternating hard and soft phase layers, forming a significant gradient in mechanical properties. Microscopically, the hard phase layer is based on tempered martensite, and the soft phase layer contains a continuous matrix of lower bainitic ferrite laths, interspersed with island-like secondary quenched martensite and grain boundary film-like retained austenite. Ultimately, this biomimetic structural steel exhibits excellent comprehensive mechanical properties, surpassing traditional ultra-high strength steels of the same strength level.

Description

Technical Field

[0001] This invention relates to the field of metallurgical technology, and in particular to a martensitic-bainitic multilayer gradient biomimetic structural steel and its preparation method. Background Technology

[0002] Currently, the technological path to achieve a commercial yield strength of ≥1000MPa heavily relies on the classic paradigm of martensitic transformation strengthening, namely, obtaining a microstructure dominated by high-dislocation-density lath martensite through quenching. However, this paradigm has always been plagued by a long-standing dilemma of "strength-plasticity / toughness inversion." Specifically, the uniform elongation of quenched martensitic steel is generally suppressed to below 7%, resulting in low fracture toughness and significant macroscopic brittleness. Although subsequent tempering can improve plasticity to some extent, it inevitably leads to a strength loss of up to 20-30%, while also increasing process complexity and energy consumption. Essentially, the performance control window of this technology system, which relies on a single phase transformation strengthening, is approaching saturation.

[0003] Driven by this, biomimetic structural design provides a revolutionary perspective for breaking through the performance limits of metallic materials. In the steel material system, the regulation of the multiphase structure of martensite (hard phase) and bainite (soft phase) is recognized as one of the most promising biomimetic approaches for engineering applications. Meanwhile, research has confirmed that appropriately shaped and distributed lower bainite can effectively segment the original austenite grains, refine the martensite lath bundles, and, during deformation, act as plastic units to coordinate strain and absorb energy, thereby endowing the material with considerable toughness reserves while maintaining high strength.

[0004] However, transforming biomimetic concepts into stable, efficient, and scalable industrial manufacturing technologies faces severe engineering and scientific challenges. Existing technologies mainly suffer from three major bottlenecks: First, the paradox of microstructure control caused by phase transformation kinetic coupling. For traditional medium- and high-carbon alloy steels with uniform chemical composition, the martensitic transformation (below the Ms point) and the isothermal transformation of lower bainite (usually in the 300-400℃ temperature range) are strongly coupled and compete thermodynamically and kinetically. This results in a microstructure that is mostly a dispersed and random mixture of martensite, bainite, and retained austenite, far removed from the macroscopic layered alternation and ordered gradient design pursued by biomimetic science, and the strengthening and toughening effect has a clear upper limit. Second, insufficient "soft-hardness contrast" limits the synergistic effect. Within a single-component system, the intrinsic hardness difference between martensite and lower bainite obtained through process control is limited (the micro Vickers hardness difference is usually less than 150 HV). This insufficient "mechanical property contrast" results in inadequate plastic flow of the soft phase (bainite) under load, failing to maximize energy dissipation; the hard phase (martensite) also provides a weak barrier against crack propagation. Ultimately, the synergistic mechanism of "soft phase energy dissipation and hard phase crack inhibition," upon which the biomimetic structure relies for its effectiveness, is not fully activated, and the performance improvement falls far short of theoretical expectations. Third, the process window is extremely narrow, leading to poor engineering feasibility. To obtain the target multiphase microstructure in homogeneous materials, the cooling process must be precisely controlled within an extremely narrow phase transition temperature window (often within ±10℃). This severely weakens its competitiveness in high-end equipment applications requiring high reliability and low-cost mass production.

[0005] Therefore, it is urgent to optimize the alloy element composition of dissimilar steels and improve current preparation methods. Summary of the Invention

[0006] The purpose of this invention is to design heterogeneous steel grades with complementary phase transformation characteristics, and to utilize the difference in phase transformation kinetics during hot rolling to achieve spatial decoupling and ordered distribution of martensite and lower bainite. This allows for the one-step direct construction of a "soft matrix encapsulating hard phase" structure with a significant hardness gradient, macroscopic layered order, and interfacial metallurgical bonding, thereby constructing a high-performance martensite-bainite multilayer gradient biomimetic structural steel.

[0007] To achieve the above objectives, in a first aspect, the present invention provides a martensitic-bainitic multilayer gradient biomimetic structural steel, which is a structure in which hard phase layers and soft phase layers are arranged alternately to form a hardness gradient distribution; The hard phase layer is based on tempered martensite; The soft phase layer contains a continuous matrix of lower bainitic ferrite, interspersed with secondary quenched martensite and grain boundary film-like retained austenite.

[0008] Furthermore, the difference in microhardness between the hard phase layer and the soft phase layer is ≥150 HV; The microhardness of the soft phase layer is 300~380HV.

[0009] Secondly, the present invention also provides a method for preparing martensitic-bainitic multilayer gradient biomimetic structural steel, comprising, A composite billet is formed by stacking hard phase steel and soft phase steel in sequence, wherein the hard phase steel and the soft phase steel are martensitic steel and bainitic steel, respectively, and the phase transformation temperature difference between the hard phase steel and the soft phase steel is ≥150℃. The composite billet is rolled and heat-treated to obtain a martensitic-bainitic multilayer gradient biomimetic structural steel with alternating hard and soft phase layers forming a hardness gradient distribution. The hard phase layer is based on tempered martensite; The soft phase layer contains a continuous matrix of lower bainitic ferrite, interspersed with secondary quenched martensite and grain boundary film-like retained austenite.

[0010] Furthermore, the composition of the hard phase steel grade, by mass percentage, includes: C: 0.25%~0.40%, Si: 0.2%~1.5%, Mn: 1.0%~3.0%, Cr: 0.5%~2.0%, Mo: 0.1%~0.5%, Ni: 0.5%~2.0%, B: 0.001%~0.005%, V / Nb: 0.02%~0.1%, with the balance being Fe and unavoidable impurities.

[0011] Furthermore, the composition of the soft phase steel grade, by mass percentage, includes: C: 0.15%~0.25%, Si: 0.3%~1.0%, Mn: 7.0%~10.0%, Al: 1.0%~2.0%, Cr: 0.5%~1.5%, Mo: 0.1%~0.3%, V / Nb: 0.02%~0.1%, with the balance being Fe and unavoidable impurities.

[0012] Furthermore, the phase transformation initiation temperature of the hard phase layer steel is 300~400℃.

[0013] Furthermore, the hard phase steel and the soft phase steel are plate-shaped with a thickness of 0.5~1.0 mm; The thickness of the composite blank is 3~9mm.

[0014] Furthermore, the rolling is carried out in multiple passes at a temperature of 700~900℃, with the temperature decreasing as the number of passes increases, and the total deformation of the final composite billet is 85%±1%.

[0015] Furthermore, the heat treatment sequentially includes a low-temperature salt bath at 180~200℃ for 1~5s, isothermal tempering at 370~450℃ for 30~60s, and water quenching to room temperature.

[0016] Furthermore, after the composite billet is formed, it is subjected to austenitizing treatment at 900~1000℃.

[0017] Compared with the prior art, the present invention has the following beneficial effects: The martensitic-bainitic multilayer gradient biomimetic structural steel of this invention has a multi-level biomimetic structure from macro to micro. Macroscopically, it exhibits a clear layered configuration with alternating hard and soft phase layers, forming a significant gradient in mechanical properties. Microscopically, the hard phase layer is based on tempered martensite, while the soft phase layer has a continuous matrix of lower bainitic ferrite laths, interspersed with island-shaped secondary quenched martensite and grain boundary film-like retained austenite, forming a microscopic multiphase composite structure of "soft matrix encapsulating hard phase". Ultimately, this biomimetic structural steel exhibits excellent comprehensive mechanical properties, surpassing traditional ultra-high strength steels of the same strength level. Attached Figure Description

[0018] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0019] Figure 1 The temperature transition diagram of the preparation method of the martensitic-bainitic multilayer gradient biomimetic structural steel of the present invention is shown; Figure 2 An optical microscope image of the hard phase layer of the martensitic-bainitic multilayer gradient biomimetic structural steel prepared in Example 1 is shown. Figure 3 An optical microscope image of the soft phase layer of the martensitic-bainitic multilayer gradient biomimetic structural steel prepared in Example 1 is shown. Figure 4 An optical microscope characterization of the phase structure of the steel prepared in Comparative Example 1 is shown. Detailed Implementation

[0020] Breaking through the physical limitations of phase transformation in single alloy systems, achieving macroscopically ordered arrangement of soft and hard phases, and possessing a wide and controllable process window, a novel method for preparing ultra-high-strength steel has become an urgent technical need to resolve the strength-toughness contradiction in steels used in high-end equipment and to promote the upgrading of lightweight materials technology. The concept of this invention lies in using heterogeneous (iron-based) steels with deliberately differentiated phase transformation characteristics as raw materials, and actively and directly constructing a biomimetic gradient structure of martensite / bainite with clear interfaces and macroscopic alternating layers in a one-step process during the interaction of rolling deformation and solid-state phase transformation.

[0021] In view of this, on the one hand, the present invention designs a martensitic-bainitic multilayer gradient biomimetic structural steel, which has a multi-level biomimetic structure from macroscopic to microscopic. Macroscopically, it exhibits a clear layered configuration with alternating hard phase layers (with tempered martensite as the matrix, microhardness ≥550HV) and soft phase layers (with bainite as the matrix, microhardness 300~380HV), with an interlayer hardness difference ≥150HV, forming a significant mechanical property gradient; microscopically, the hard phase layers are composed of tempered martensite... The matrix consists of a soft phase layer with a continuous matrix of lower bainitic ferrite laths, interspersed with island-shaped secondary quenched martensite and grain boundary film-like retained austenite, forming a microscopic multiphase composite structure of "soft matrix encapsulating hard phase". Ultimately, this structural steel exhibits excellent comprehensive mechanical properties: yield strength ≥1750MPa, tensile strength ≥2400MPa, elongation ≥12%, and room temperature impact energy ≥120J, comprehensively surpassing traditional ultra-high strength steels of the same strength level.

[0022] On the other hand, the present invention also provides a method for preparing martensitic-bainitic multilayer gradient biomimetic structural steel, comprising the following steps: (1) Alloy composition design of dissimilar steel grades Two alloy steel grades with distinct phase transformation kinetic objectives were designed: a hard-phase layer steel of martensitic steel and a soft-phase layer steel of bainitic steel. Both have Fe as their matrix, and their phase transformation temperature windows were deliberately separated and decoupled through the differentiation and synergistic regulation of alloying elements.

[0023] In some preferred embodiments, the chemical composition of the hard-phase steel, by mass percentage, includes: C: 0.25%~0.40%, Si: 0.2%~1.5%, Mn: 1.0%~3.0%, Cr: 0.5%~2.0%, Mo: 0.1%~0.5%, Ni: 0.5%~2.0%, B: 0.001%~0.005%, V / Nb: 0.02%~0.1%, with the balance being Fe and unavoidable impurities. Through alloy composition design, its martensitic transformation start temperature (Ms) is placed in a relatively high range (e.g., 300~400℃), its martensitic transformation end temperature (Mf) is ≥200℃, and its bainitic transformation is significantly suppressed, ensuring that it preferentially and completely transforms into martensite in subsequent processes.

[0024] The following lists the roles of various alloying elements in hard-phase steel: Carbon (C) is one of the most powerful elements for improving hardenability, but it drastically lowers the Ms point. A medium-carbon design is employed here, providing sufficient hardenability to ensure martensite formation while avoiding Ms points falling below the target range due to excessive carbon content.

[0025] Mn and Ni, as strong austenite stabilizing elements, primarily enhance the stability of supercooled austenite by significantly shifting the "C-curve" of medium- and high-temperature transformations such as ferrite, pearlite, and bainite to the right, creating a time window to avoid these transformations. They also lower the Ms point to some extent, therefore their content is controlled within the upper limit required to achieve hardenability targets.

[0026] Mo and B are key to suppressing non-martensitic transformation, especially bainitic transformation. Mo effectively separates the pearlite and bainite transformation regions, forming a very gentle "tail" in the bainite transformation region, significantly delaying the onset of bainite transformation. Trace amounts of B segregate at austenite grain boundaries, preferentially and strongly inhibiting ferrite nucleation at these boundaries, thus highly efficiently delaying the high-temperature transformation.

[0027] Cr can help improve hardenability and further enhance the alloying effect.

[0028] The main function of V / Nb is to refine grains and improve the strength and toughness of materials, while its influence on the dominant phase transformation pathway is relatively minor.

[0029] In some preferred embodiments, the chemical composition of the soft-phase steel grade, by mass percentage, includes: C: 0.15%~0.25%, Si: 0.3%~1.0%, Mn: 7.0%~10.0%, Al: 1.0%~2.0%, Cr: 0.5%~1.5%, Mo: 0.1%~0.3%, V / Nb: 0.02%~0.1%, with the balance being Fe and unavoidable impurities. Through alloy composition design, the martensite initiation temperature (Ms≤180℃) is significantly reduced, while simultaneously widening and stabilizing the isothermal transformation temperature range of the lower bainite (350~550℃), thereby creating a phase transformation temperature window difference of at least 150℃ compared to the hard-phase steel grade.

[0030] The following lists the roles of various alloying elements in soft-phase steel: Mn is the most crucial and impactful design element. Such a high Mn content of 7.0% to 10.0% has two decisive effects: It drastically lowers the Ms point: making austenite extremely stable and forcibly reducing the martensitic transformation initiation temperature to below 180°C, ensuring from the outset that martensitic transformation does not occur prematurely in subsequent processes. It widens and stabilizes the bainite region: it strongly delays all diffusion-type transformations, causing the bainite transformation's "C-curve" to shift significantly to the right and become flatter, thus forming a broad transformation plateau between 350 and 550°C that facilitates isothermal treatment.

[0031] C, Si, and Al form a synergistic combination. A relatively low carbon content of 0.15%–0.25% helps maintain a relatively high upper limit for the bainitic transformation temperature. Silicon and aluminum are both strong ferrite stabilizers and strongly inhibit carbide precipitation. During the bainitic transformation, they delay cementite formation, thus significantly postponing the completion of the transformation. This effect effectively broadens the time and temperature window for the bainitic transformation, making the isothermal process easier to control and resulting in a bainitic microstructure with superior toughness.

[0032] Cr, Mo, and V / Nb mainly help stabilize austenite, suppress high-temperature transformation, and refine grains, serving the overall design goal of the main controlling elements (Mn, Si, Al).

[0033] (2) Independent preparation of single steel plates: The raw materials of the two steel grades designed in step (1) are vacuum induction melted, and then precision casting, multi-directional forging, controlled rolling and cooling and cold rolling and continuous annealing processes are used to prepare single steel plates with clean surface and uniform structure.

[0034] In some preferred embodiments, the final veneer thickness is controlled between 0.5 and 1.0 mm. This thickness range is an optimized choice that balances process stability and interface quality: if it is too thin, defects may occur in subsequent lamination due to oxidation or uneven deformation; if it is too thick, the rolling force requirement increases dramatically and the uniformity of core temperature is difficult to guarantee.

[0035] (3) Vacuum encapsulation of composite preforms The two single-element steel plates obtained in step (2) are precision wire-cut into a set size and then stacked alternately in the manner of "hard phase steel / soft phase steel / hard phase steel..." to form a composite billet with a thickness of 3~9mm; the composite billet is placed in a sealed body made of special alloy steel.

[0036] In some preferred embodiments, the bottom of the sealing body and the cover plate are coated with an isolation coating. This isolation coating is used to prevent the steel from sticking to the container at high temperatures, ensuring the workpiece can be removed. The isolation coating is prepared from conventional raw materials and does not require strict limitation; it only needs to achieve the corresponding demolding effect and does not involve substantial improvement of the technical solution. For example, the raw materials for the isolation coating are nano-SiO2, nano-Al2O3, and anhydrous ethanol mixed in a mass ratio of 2:1:6, and the thickness of the isolation coating after drying is 50~100μm. The SiO2-Al2O3 composite isolation coating in the specific embodiments uses the same formula, and the specific components are not described in detail in the embodiments.

[0037] After the composite blank is placed in a sealed body made of special alloy steel, it is sealed using carbon dioxide gas shielded welding. During the welding process, high-purity argon gas is introduced through pre-reserved vents as a protective gas. After welding, a vacuum pump is used to evacuate the inner cavity of the box to an absolute pressure ≤30Pa, and a hydraulic clamping system is used to ensure long-term sealing reliability under a pressure of 15-20MPa, completely preventing high-temperature oxidation.

[0038] The vacuum sealing in this step is to prevent high-temperature oxidation of the composite billet from causing interface defects that could affect subsequent vacuum hot rolling. If the process allows for vacuum preservation of the composite billet and direct transfer to subsequent vacuum hot rolling, the listed vacuum sealing method is unnecessary. Vacuum sealing does not involve any substantial improvement to the technical solution.

[0039] (4) Vacuum austenitization and multi-pass gradient hot rolling of composite billets: The sealed body is placed in a special vacuum heat treatment furnace and austenitized at 900~1000℃. The holding time is determined based on the total thickness, with a reference of 30~60 minutes. That is, when the total thickness of the composite billet is 3mm, the holding time is 30 minutes. For every 1mm increase in thickness, the holding time is extended by about 5 minutes. Subsequently, the hot composite billet is quickly transferred to a hot rolling mill for multi-pass rolling. The rolling is carried out at a temperature of 700~900℃, and the temperature decreases as the number of passes increases. The final total deformation of the composite billet is 85%±1%.

[0040] In some preferred embodiments, the rolling process employs a three-stage precision temperature-controlled rolling procedure: First pass: Performed at 840~860℃ with a reduction rate of 45±2%, immediately returned to a vacuum furnace for holding at the same temperature for 5~8 minutes after rolling. Second pass: Performed at 790~810℃ with a reduction rate of 45±2%, held for 3~5 minutes after rolling. Final pass: Performed at 750~790℃, using a large reduction rate (50±2%) to complete the finishing rolling, and introducing a tension coefficient of 0.8~1.2 for micro-tension control.

[0041] In some preferred embodiments, the temperature is monitored by an infrared temperature measurement system throughout the rolling process, and the temperature fluctuation between each stage does not exceed ±15℃. The final total deformation reaches 85±1%, resulting in a high-quality composite plate with a thickness deviation of no more than ±0.05mm and fully diffused metallurgical bonding at the interface.

[0042] (5) Preferential quenching formation of martensite layer The rolled composite sheet is rapidly placed in a low-temperature salt bath furnace at 180-200℃ for short-term isothermal quenching, with a dwell time of 1-5 seconds. The purpose of this step is to rapidly transform the undercooled austenite in the hard-phase steel (target martensite layer) into quenched martensite. This allows the soft-phase steel (target bainite layer), due to its extremely low Ms point (≤180℃), to remain in an undercooled austenite state at this salt bath temperature, thus achieving the initial decoupling of the two-phase transformation. The appropriate dwell time facilitates short-range diffusion of carbon in the martensite of the hard-phase steel, creating conditions for the formation of a small amount of thin-film retained austenite and initially improving toughness.

[0043] (6) Isothermal transformation of bainite layer and tempering of martensite The composite plate treated in step (5) is quickly transferred to another salt bath furnace with a higher temperature for isothermal tempering at 370~450℃ for 30~60 seconds. The purpose of this step is to partially transform the supercooled austenite in the soft phase layer steel (target bainite layer) into acicular lower bainite with excellent strength and toughness within this temperature range, while effectively inhibiting the formation of unfavorable structures such as upper bainite. At the same time, the quenched martensite already formed in the hard phase layer steel undergoes short-term self-tempering during the isothermal process, precipitating fine carbides, releasing internal stress, significantly improving toughness, and evolving into tempered martensite. This is a secondary decoupling and performance synergistic optimization of the two-phase transformation.

[0044] (7) Final quenching and gradient structure shaping The composite plate, after isothermal tempering, is rapidly immersed in room temperature water for final quenching. This step aims to transform the untransformed residual supercooled austenite in the soft phase layer steel into secondary quenched martensite. It quickly establishes a multiphase composite gradient structure consisting of tempered martensite (hard phase layer), lower bainite (soft phase layer), secondary quenched martensite, and intergranular thin-film retained austenite.

[0045] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to specific embodiments and accompanying drawings. 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 skilled in the art without creative effort are within the scope of protection of the present invention. The endpoints and any values ​​of the ranges disclosed in the present invention are not limited to the precise ranges or values, and these ranges or values ​​should be understood to include values ​​close to these ranges or values. For numerical ranges, the endpoint values ​​of each range, the endpoint values ​​of each range and individual point values, and individual point values ​​can be combined with each other to obtain one or more new numerical ranges, and these numerical ranges should be considered as specifically disclosed in the present invention.

[0046] Example 1 A method for preparing martensitic-bainitic multilayer gradient biomimetic structural steel includes the following steps: Step (1): Based on the design principle of decoupling phase transformation thermodynamics and kinetics, JMatPro and Thermo-Calc software were used for collaborative calculation and optimization to finally determine the composition (mass fraction) of the two components. The composition of the hard phase layer element, by mass percentage, includes: C: 0.35%, Si: 1.2%, Mn: 2.5%, Cr: 1.8%, Mo: 0.3%, Ni: 1.5%, B: 0.003%, V: 0.05%, with the balance being Fe. The software simulation predicts that its martensitic transformation initiation temperature (Ms) is approximately 320℃, and the bainitic transformation incubation period is significantly prolonged, indicating a tendency to preferentially undergo complete martensitic transformation. The composition of the soft phase layer element, by mass percentage, includes: C: 0.20%, Si: 0.8%, Mn: 8.5%, Al: 1.5%, Cr: 1.0%, Mo: 0.2%, Nb: 0.06%, with the balance being Fe. Simulations show that its Ms point is suppressed to approximately 160°C, and a broad lower bainite transformation plateau exists in the 350–500°C temperature range. The difference in phase transformation temperature windows between the two phases exceeds 160°C, providing a theoretical basis for achieving stepwise phase transformation.

[0047] Step (2): Based on the hard phase layer and soft phase layer designed in step (1), the raw materials are batched and vacuum induction melted separately, and then cast into ingots. After homogenization treatment at 1200℃ for 120 minutes, the steel ingots are forged at 1100℃, with a final forging temperature not lower than 850℃ and a forging ratio controlled at 8:1, followed by air cooling. Then, hot rolling is performed: the temperature is heated to 1200℃ and held for 2 hours, and rolling is completed in the temperature range of 1050℃ to 880℃. After rolling, the temperature is controlled to be cooled to 650℃ at a cooling rate of 15℃ / s and then air cooled. Finally, after cold rolling and continuous annealing at 750℃ for 5 minutes, hard phase layer steel plates and soft phase layer steel plates with smooth surface, uniform structure and thickness of (1.0±0.02) mm are obtained.

[0048] Step (3): The hard phase steel plate and the soft phase steel plate are processed to a length of 150 mm and a width of 100 mm, and then stacked alternately in a specially made alloy sealing box in a "hard / soft / hard / soft / hard" sequence to form a composite billet. To ensure the purity of the interface at high temperature, a SiO2-Al2O3 composite isolation coating with a thickness of (80±5) μm is prepared on the inner surface of the box by atmospheric plasma spraying. Subsequently, under the protective atmosphere of continuous introduction of 10 L / min high-purity argon gas (purity ≥99.999%), the box is sealed and welded using ER307Si welding wire. After sealing, a rotary vane vacuum pump is used to reduce the pressure inside the box to (25±5) Pa, and 20 MPa of hydraulic pressure is applied to maintain the sealing state, completely isolating the composite billet from oxidation risk in subsequent processes.

[0049] Step (4): Place the encapsulation box in a vacuum heat treatment furnace and heat it to 950℃ at a rate of 10℃ / min, and hold it for 50 minutes to ensure that each component is fully austenitized and that the elements are initially diffused. Subsequently, perform multi-pass gradient rolling compounding on a reversible hot rolling mill with Φ450mm work rolls, with the work roll linear speed kept constant at 20m / min. The specific procedure is as follows: First pass: carried out at (850±5)℃, with a reduction rate of 45%, and immediately returned to the vacuum furnace after rolling to hold at 850℃ for 6 minutes. Second pass: carried out at (800±5)℃, with a reduction rate of 45%, and held at 800℃ for 4 minutes after rolling. Third pass (final rolling): carried out at (760±5)℃, using a large reduction of 50% for finishing, and using a micro-tension control system (tension coefficient 1.0). The entire process is monitored online using infrared thermal imagers and laser thickness gauges to ensure that the temperature difference between passes is no greater than ±10℃, the final rolled plate thickness is (0.755±0.03) mm, and the total deformation reaches 84.5% of the design requirements.

[0050] Step (5), Martensite preferential quenching (first decoupling): Quickly transfer to a KNO3-NaNO2 mixed salt bath at (190±2)℃ and quench for 10 seconds. At this time, due to its higher Ms point, the hard phase layer has most of its undercooled austenite transformed into quenched martensite; while the soft phase layer, due to its Ms point being lower than the salt bath temperature, still basically remains in the state of undercooled austenite.

[0051] Step (6), isothermal transformation of bainite layer and tempering of martensite (secondary decoupling and synergy): Immediately transfer to a salt bath at (400±2)℃ and hold isothermally for 50 seconds. During this stage, the undercooled austenite portion in the soft phase layer transforms into acicular lower bainite; at the same time, the quenched martensite already formed in the hard phase layer undergoes self-tempering and evolves into tempered martensite.

[0052] Step (7), final quenching and structural shaping: rapid water quenching to room temperature, so that the untransformed residual supercooled austenite in the soft phase layer is transformed into secondary quenched martensite, thereby fixing the entire multiphase gradient structure.

[0053] Figure 1 The temperature transitions in the preparation method of martensitic-bainitic multilayer gradient biomimetic structural steel in the above steps are summarized.

[0054] Example 2 A method for preparing martensitic-bainitic multilayer gradient biomimetic structural steel includes the following steps: Step (1): Based on the design principle of decoupling phase change thermodynamics and kinetics, JMatPro and Thermo-Calc software were used for collaborative calculation and optimization to finally determine the composition (mass fraction) of the two components. The composition of the hard phase layer element, by mass percentage, includes: C: 0.38%, Si: 1.0%, Mn: 2.8%, Cr: 2.0%, Mo: 0.4%, Ni: 2.0%, B: 0.004%, Nb: 0.10%, with the balance being Fe; the composition of the soft phase layer element, by mass percentage, includes: C: 0.15%, Si: 1.0%, Mn: 9.5%, Al: 2.0%, Cr: 0.5%, Mo: 0.3%, V: 0.08%, with the balance being Fe. In this embodiment, compared to Embodiment 1, the elemental composition of the hard phase layer is fine-tuned, increasing the contents of C, Cr, Mo, and Ni to further enhance hardenability, solid solution strengthening, and secondary hardening potential. The goal is to obtain a tempered martensitic matrix with higher hardness and strength while maintaining a sufficiently high Ms point (~310℃). The soft phase layer uses a lower C content and higher Mn and Al contents to minimize the Ms point (~150℃), widen the lower bainite transformation range, and simultaneously utilize Al to refine the grains and improve toughness, forming a softer and tougher intermediate layer.

[0055] Step (2): The steel is cast using the same method as in Example 1 to obtain a hard phase steel plate and a soft phase steel plate with a thickness of (0.8±0.02) mm.

[0056] Step (3): The hard phase steel plate and the soft phase steel plate are processed to a length of 150 mm and a width of 100 mm, and then stacked alternately in a special alloy sealing box in a "hard / soft / hard" sequence to form a composite blank. Welding and sealing are completed in the same way as in Example 1.

[0057] Step (4): Place the encapsulation box in a vacuum heat treatment furnace and heat it to 980℃ at a rate of 10℃ / min, and hold it for 40 minutes to ensure that each component is fully austenitized and that the elements are initially diffused. Subsequently, perform multi-pass gradient rolling compounding on a reversible hot rolling mill with Φ450mm work rolls, with the work roll linear speed kept constant at 20m / min. The specific procedure is as follows: First pass: carried out at (860±5)℃, with a reduction rate of 46%, and immediately returned to the vacuum furnace after rolling to hold at 860℃ for 5 minutes. Second pass: carried out at (810±5)℃, with a reduction rate of 46%, and held at 810℃ for 3 minutes after rolling. Third pass (final rolling): carried out at (780±5)℃, using a large reduction of 52% for finishing, and using a micro-tension control system (tension coefficient 1.0). The entire process is monitored online using an infrared thermal imager and a laser thickness gauge to ensure that the temperature difference between passes is no greater than ±10℃, the final rolled plate thickness is (0.336±0.02) mm, and the total deformation reaches 86% of the design requirements. Due to the small number of layers and the thickness of each layer, the process in this embodiment focuses on completing large deformation in a higher temperature range to ensure full bonding between the three layers.

[0058] Step (5), Martensite preferential quenching (first decoupling): Quickly transfer to a KNO3-NaNO2 mixed salt bath at (185±2)℃ and quench for 8 seconds. At this time, due to its higher Ms point, the hard phase layer has most of its supercooled austenite transformed into quenched martensite; while the soft phase layer, because its Ms point is lower than the salt bath temperature, still basically remains in the state of supercooled austenite.

[0059] Step (6), isothermal transformation of bainite layer and tempering of martensite (secondary decoupling and synergy): Immediately transfer to a salt bath at (380±2)℃ and hold isothermally for 60 seconds. During this stage, the undercooled austenite portion in the soft phase layer transforms into acicular lower bainite; at the same time, the quenched martensite already formed in the hard phase layer undergoes self-tempering and evolves into tempered martensite.

[0060] Step (7), final quenching and structural shaping: rapid water quenching to room temperature, so that the untransformed residual supercooled austenite in the soft phase layer is transformed into secondary quenched martensite, thereby fixing the entire multiphase gradient structure.

[0061] Example 3 A method for preparing martensitic-bainitic multilayer gradient biomimetic structural steel includes the following steps: Step (1): Based on the design principle of decoupling phase transformation thermodynamics and kinetics, JMatPro and Thermo-Calc software were used for collaborative calculation and optimization to finally determine the composition (mass fraction) of the two components. The composition of the hard phase layer element, by mass percentage, includes: C: 0.28%, Si: 0.6%, Mn: 1.5%, Cr: 1.0%, Mo: 0.2%, Ni: 1.0%, B: 0.002%, V: 0.03%, with the balance being Fe; the composition of the soft phase layer element, by mass percentage, includes: C: 0.22%, Si: 0.5%, Mn: 7.5%, Al: 1.0%, Cr: 1.5%, Mo: 0.15%, V: 0.04%, with the balance being Fe. In this embodiment, compared with Example 1, the elemental composition of the hard phase layer element is controlled to moderately reduce the alloy content, aiming to obtain medium-carbon tempered martensite with better toughness while ensuring the Ms point (~350℃) and reducing costs. The soft phase layer uses an alloy composition with a moderate proportion to ensure a low Ms point (~170℃) and a stable lower bainite transformation, while the high Cr content helps to improve corrosion resistance and tempering stability.

[0062] Step (2): The steel is cast using the same method as in Example 1 to obtain a hard phase steel plate and a soft phase steel plate with a thickness of (0.7±0.02) mm.

[0063] Step (3): The hard phase steel plate and the soft phase steel plate are processed to a length of 150 mm and a width of 100 mm, and then stacked alternately in a special alloy sealing box in the sequence of "hard / soft / hard / soft / hard / soft / hard" to form a composite blank. Welding and sealing are completed in the same way as in Example 1.

[0064] Step (4): Place the encapsulation box in a vacuum heat treatment furnace and heat it to 930℃ at a rate of 10℃ / min, and hold it for 65 minutes to ensure that each component is fully austenitized and that the elements are initially diffused. Subsequently, perform multi-pass gradient rolling compound rolling on a reversible hot rolling mill with Φ450mm work rolls, with the work roll linear speed kept constant at 20m / min. The specific procedure is as follows: First pass: carried out at (845±5)℃, with a reduction rate of 44%, and immediately returned to the vacuum furnace after rolling to hold at 860℃ for 8 minutes. Second pass: carried out at (795±5)℃, with a reduction rate of 44%, and held at 810℃ for 5 minutes after rolling. Third pass (final rolling): carried out at (755±5)℃, with a reduction rate of 48%, and the micro-tension control system (tension coefficient 1.0) is activated. The entire process is monitored online using infrared thermal imagers and laser thickness gauges to ensure that the temperature difference between passes is no greater than ±10℃, the final rolled plate thickness is (0.74±0.02) mm, and the total deformation reaches 85% of the design requirements. At this point, the theoretical average thickness of a single layer is approximately 105 μm, forming a fine periodic gradient structure.

[0065] Step (5), Martensite preferential quenching (first decoupling): Quickly transfer to a KNO3-NaNO2 mixed salt bath at (185±2)℃ and quench for 8 seconds. At this time, due to its higher Ms point, the hard phase layer has most of its supercooled austenite transformed into quenched martensite; while the soft phase layer, because its Ms point is lower than the salt bath temperature, still basically remains in the state of supercooled austenite.

[0066] Step (6), isothermal transformation of bainite layer and tempering of martensite (secondary decoupling and synergy): Immediately transfer to a salt bath at (380±2)℃ and hold isothermally for 60 seconds. During this stage, the undercooled austenite portion in the soft phase layer transforms into acicular lower bainite; at the same time, the quenched martensite already formed in the hard phase layer undergoes self-tempering and evolves into tempered martensite.

[0067] Step (7), final quenching and structural shaping: rapid water quenching to room temperature, so that the untransformed residual supercooled austenite in the soft phase layer is transformed into secondary quenched martensite, thereby fixing the entire multiphase gradient structure.

[0068] Comparative Example 1 Compared with Example 1, the difference is that in step (3), only hard phase steel plates are stacked to form a composite billet.

[0069] Comparative Example 2 Compared with Example 1, the difference is that in step (3), only soft phase steel plates are stacked to form a composite billet.

[0070] Comparative Example 3 Compared with Example 1, the difference is that in step (3), the soft phase steel plate is replaced with conventional medium manganese TRIP steel of the same size. The chemical composition of the medium manganese TRIP steel, by mass percentage, includes C: 0.25%, Si: 1.8%, Mn: 5.0%, Al: 0.05%, Cr: 0.2%, with the balance being Fe, and its simulated Ms point is about 280°C.

[0071] Comparative Example 4 Compared with Example 1, the difference is that the order of steps (5) and (6) is reversed. The composite billet to be rolled is first subjected to isothermal treatment at 400°C, then low-temperature quenching at 190°C, and finally water quenching.

[0072] Comparative Example 5 Compared with Example 1, the difference is that step (5) is missing. The composite billet to be rolled is directly subjected to isothermal treatment at 400°C without undergoing low-temperature quenching treatment at 190°C.

[0073] Comparative Example 6 Compared with Example 1, the difference is that step (6) is missing. The composite billet to be rolled is directly water quenched after being subjected to low-temperature quenching treatment at 190°C.

[0074] Test case The microstructure of the composite steels prepared in Example 1 and Comparative Example 1 was observed using an optical microscope. Figure 1 and Figure 2 It can be seen that the hard phase layer of the steel prepared in Example 1 uses tempered martensite as the matrix, and the interior of the soft phase layer is composed of lower bainitic ferrite laths as a continuous matrix, interspersed with island-shaped secondary quenched martensite and grain boundary film-like retained austenite, forming a microscopic multiphase composite structure of "soft matrix encapsulating hard phase". However, Figure 3 The microstructure of the steel prepared in Comparative Example 1 is a typical dispersed mixed structure: fine lath martensite, acicular lower bainite, and blocky / film-like retained austenite are randomly and disorderly interwoven at the microscale. These results also demonstrate that the present invention has successfully achieved a martensite-lower bainite microstructure gradient construction in hot-rolled mixed steel grades.

[0075] To further test the performance of the prepared composite steel, the microhardness of the hard and soft phase layers was tested under a load of 500g. The tensile strength, yield strength, and elongation of the composite steel were tested according to standard GB / T 228.1-2021 "Metallic materials, tensile testing—Part 1: Tests at room temperature," with the strength-ductility product being the product of tensile strength and elongation. The room temperature impact energy of the composite steel was tested according to standard GB / T 229-2020 "Metallic materials, Charpy impact test." These results are shown in Table 1. It should be noted that the composite steels of Comparative Example 1 and Comparative Example 2 were rolled from a single steel plate, and their microhardness is the average hardness measured across all layers.

[0076] Table 1

[0077] As can be seen from the test results in Table 1, Comparative Examples 1 and 2, which did not have interlayer hardness gradient design, are significantly inferior to the composite steels of Examples 1-3 in all aspects of performance. This is because the composite steels of Comparative Examples 1 and 2 exhibit a random and disordered distribution of fine lath martensite, acicular lower bainite, and blocky / film-like retained austenite at the microscale in their microstructure. This prevents them from forming the hard phase layer of the composite steels of Examples 1-3, which uses tempered martensite as the matrix and lower bainitic ferrite laths as the continuous matrix inside the soft phase layer, interspersed with island-shaped secondary quenched martensite and grain boundary film-like retained austenite, thus forming a microscopic multiphase composite structure of "soft matrix encapsulating hard phase".

[0078] Furthermore, compared to Comparative Examples 3-6, only the hard and soft phase layers of Examples 1-3 exhibited a microhardness difference of ≥150 HV. This is because, through the rational design of the steel composition of the hard and soft phase layers, the martensitic transformation start temperature (Ms) of the hard phase layer steel is in a higher range, and the martensitic transformation end temperature (Mf) is ≥200℃; while the soft phase layer steel significantly reduces the martensitic transformation start temperature (Ms≤180℃), and at the same time widens and stabilizes its lower bainitic isothermal transformation temperature range (350~550℃), thus forming a phase transformation temperature window difference of at least 150℃ with the hard phase layer steel. By combining the rolling process with a low-temperature salt bath at 180~200℃ for 1~5s, isothermal tempering at 370~450℃ for 30~60s, and water quenching to room temperature, a hard phase layer with tempered martensite as the matrix is ​​effectively formed. Inside, a soft phase layer with lower bainitic ferrite laths as the continuous matrix and interspersed island-shaped secondary quenched martensite and grain boundary film-like retained austenite is formed.

[0079] In Comparative Example 3, conventional medium-manganese TRIP steel was used as the soft phase layer steel. Its Ms point (~280℃) was higher than the first salt bath temperature (190℃). During the first salt bath, it underwent martensitic transformation simultaneously with the hard phase layer, resulting in the complete failure of the "stepwise phase transformation." Consequently, the final microstructure could not form an alternating gradient of soft (lower bainite) and hard (tempered martensite). The soft phase layer actually transformed into high-hardness tempered martensite. Therefore, compared to Example 1, the difference between the hard and soft phase layers in the composite steel of Comparative Example 3 was only 30 HV, the gradient almost disappeared, the elongation plummeted to 6.5%, and the room temperature impact energy plummeted to 45 J. Its overall performance was far inferior to Example 1, reproducing the "strength-toughness inversion" problem.

[0080] In Comparative Example 4, the rolled composite billet was first subjected to isothermal treatment at 400℃, followed by cryogenic quenching at 190℃, and finally water quenching. At 400℃, the undercooled austenite in both the hard and soft phase layers was above its lower bainite transformation zone. Part of the undercooled austenite in the soft phase layer transformed into the target lower bainite; the undercooled austenite in the hard phase layer did not transform into martensite, but instead underwent medium-high temperature transformations such as pearlite or upper bainite at this temperature, or remained partially unchanged. This is because although its Ms point is high (~320℃), it is far below 400℃, and martensitic transformation cannot occur at this temperature, instead falling into the undesirable medium-temperature transformation zone. During the cryogenic quenching at 190℃, the remaining undercooled austenite in the soft phase layer, due to its low Ms point (~160℃), partially transformed into martensite at 190℃. The microstructure of the hard phase layer, which had undergone medium-temperature transformation in the previous step, remained structurally stable at 190℃, and no further significant phase transformations occurred. Finally, the final microstructure was fixed by water quenching. In the final microstructure, the hard phase layer mainly consists of upper bainite or pearlite with low strength, hardness, and poor toughness, completely losing the high-hardness tempered martensite matrix; the soft phase layer has a mixed microstructure, containing lower bainite formed in the first step and quenched martensite formed in the second step, but lacking the synergistic support of tempered martensite. Therefore, compared with Example 1, the composite steel of Comparative Example 4 has no effective hardness gradient between the hard and soft phase layers, and all key mechanical properties are far lower than those of Example 1, with a particularly large strength loss.

[0081] In Comparative Example 5, the rolled composite billet was directly subjected to isothermal treatment at 400℃ without low-temperature quenching at 190℃. In this way, the supercooled austenite (Ms~320℃) in the hard phase layer does not transform into martensite at 400℃, but has sufficient time for a medium-temperature transformation, mainly forming non-martensitic structures such as upper bainite or granular bainite; the supercooled austenite (Ms~160℃) in the soft phase layer partially transforms into lower bainite at 400℃. The final microstructure after quenching: the matrix of the hard phase layer is upper bainite or granular bainite, with low strength and hardness; the matrix of the soft phase layer is lower bainite plus secondary quenched martensite islands. Although the overall microstructure of the soft phase layer is close to the target, because the hard phase layer fails to form a high-strength martensite / tempered martensite matrix, an effective performance gradient of alternating "soft-hard" phases cannot be formed macroscopically, and the toughening effect of the soft phase layer loses its support. Therefore, compared to Example 1, the composite steel in Comparative Example 5 has a small difference in hardness between the hard phase layer and the soft phase layer, resulting in a comprehensive deterioration in performance and a particularly significant decrease in strength.

[0082] Comparative Example 6: The rolled composite billet was subjected to low-temperature quenching at 190℃ followed by direct water quenching. The hard phase layer, having already transformed into quenched martensite during 190℃ quenching, was then directly water quenched without tempering, resulting in extremely high internal stress, no carbide precipitation, and a brittle state. The soft phase layer, due to its low Ms point during 190℃ quenching, remained as supercooled austenite. After direct water quenching, this portion of supercooled austenite was entirely transformed into high-carbon secondary quenched martensite. In the final microstructure, the high-stress quenched martensite in the hard phase layer was highly brittle. The soft phase layer consisted entirely of quenched martensite (primary + secondary), completely lacking a tough lower bainitic matrix. Overall: The material as a whole became a fully martensitic structure, losing the designed "soft matrix encapsulating hard phase," and macroscopically losing its performance gradient, becoming a homogeneous high-carbon martensitic steel.

[0083] In summary, the present invention has the following advantages: (1) This invention adopts a phase transformation temperature decoupling design, which completely separates the phase transformation temperature ranges of martensitic steel (Mf≥200℃) and bainitic steel (Ms≤180℃) by precisely controlling the alloy composition. This effectively solves the technical problem that it is difficult to achieve precise control of martensite / bainite within the narrow phase transformation temperature window of 300~400℃ for traditional single steel grades.

[0084] (2) The present invention achieves the gradient construction of martensite-lower bainite structure in hot-rolled mixed steel by using the gradient cooling path of "rapid quenching of martensite + isothermal transformation of bainite".

[0085] (3) This invention breaks the limitation of controlling the strength and toughness of a single steel grade. This unique microstructure design enables the material to maintain a tensile strength of ≥2400MPa while achieving an elongation of up to 12%, providing a new solution for the development of a new generation of high-performance structural materials.

[0086] Finally, it should be noted that the above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art can still modify the technical solutions described in the foregoing embodiments or make equivalent substitutions for some of the technical features. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A martensitic-bainitic multilayer gradient biomimetic structural steel, characterized in that, It has a structure in which hard phase layers and soft phase layers are arranged alternately to form a hardness gradient distribution; The hard phase layer is based on tempered martensite; The soft phase layer contains a continuous matrix of lower bainitic ferrite, interspersed with secondary quenched martensite and grain boundary film-like retained austenite.

2. The martensitic-bainitic multilayer gradient biomimetic structural steel according to claim 1, characterized in that, The microhardness difference between the hard phase layer and the soft phase layer is ≥150 HV; The microhardness of the soft phase layer is 300~380HV.

3. A method for preparing martensitic-bainitic multilayer gradient biomimetic structural steel, characterized in that, include, A composite billet is formed by stacking hard phase steel and soft phase steel in sequence, wherein the hard phase steel and the soft phase steel are martensitic steel and bainitic steel, respectively, and the phase transformation temperature difference between the hard phase steel and the soft phase steel is ≥150℃. The composite billet is rolled and heat-treated to obtain a martensitic-bainitic multilayer gradient biomimetic structural steel with alternating hard and soft phase layers forming a hardness gradient distribution. The hard phase layer is based on tempered martensite; The soft phase layer contains a continuous matrix of lower bainitic ferrite, interspersed with secondary quenched martensite and grain boundary film-like retained austenite.

4. The method for preparing martensitic-bainitic multilayer gradient biomimetic structural steel according to claim 3, characterized in that, The composition of the hard phase steel grade, by mass percentage, includes: C: 0.25%~0.40%, Si: 0.2%~1.5%, Mn: 1.0%~3.0%, Cr: 0.5%~2.0%, Mo: 0.1%~0.5%, Ni: 0.5%~2.0%, B: 0.001%~0.005%, V / Nb: 0.02%~0.1%, with the balance being Fe and unavoidable impurities.

5. The method for preparing martensitic-bainitic multilayer gradient biomimetic structural steel according to claim 3, characterized in that, The composition of the soft phase steel grade, by mass percentage, includes: C: 0.15%~0.25%, Si: 0.3%~1.0%, Mn: 7.0%~10.0%, Al: 1.0%~2.0%, Cr: 0.5%~1.5%, Mo: 0.1%~0.3%, V / Nb: 0.02%~0.1%, with the balance being Fe and unavoidable impurities.

6. The method for preparing martensitic-bainitic multilayer gradient biomimetic structural steel according to claim 3, characterized in that, The phase transformation initiation temperature of the hard phase layer steel is 300~400℃.

7. The method for preparing martensitic-bainitic multilayer gradient biomimetic structural steel according to claim 3, characterized in that, The hard phase steel and the soft phase steel are plate-shaped with a thickness of 0.5~1.0mm; The thickness of the composite blank is 3~9mm.

8. The method for preparing martensitic-bainitic multilayer gradient biomimetic structural steel according to claim 3, characterized in that, The rolling process is carried out in multiple passes at a temperature of 700~900℃, with the temperature decreasing as the number of passes increases. The total deformation of the final composite billet is 85%±1%.

9. The method for preparing martensitic-bainitic multilayer gradient biomimetic structural steel according to claim 3, characterized in that, The heat treatment includes, in sequence, a low-temperature salt bath at 180-200°C for 1-5 seconds, isothermal tempering at 370-450°C for 30-60 seconds, and water quenching to room temperature.

10. The method for preparing martensitic-bainitic multilayer gradient biomimetic structural steel according to any one of claims 3 to 9, characterized in that, After the composite billet is formed, it is further subjected to austenitizing treatment at 900~1000℃.

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