Delayed fracture resistant hot-stamped formed member and method of making

Abstract

The application relates to the technical field of steel materials, in particular to a delayed cracking resistant hot stamping forming component and a preparation method. The preparation method comprises a smelting process, a rolling process, a plating process and a hot stamping process. The components of the steel are controlled according to the weight percentage as follows: C: 0.42%-0.52%, Mn: 0.6%-0.9%, Si: 0.15%-0.4%, (Al+Co): 0.5%-1.2%, Mo<=1.0%, Ti: 0.03%-0.3%, V<=0.4%, Cr: 0.1%-0.4%, Ni: 0.1%-0.5%, Sn<=0.08%, 0.01%<=La+Ce<=0.06%, B<=0.003%, N<=0.003%, wherein the weight fraction ratio of the elements is: Al / Co>=5, (Ti+V) / (C-0.4%)>=2, and the rest of the elements are Fe and inevitable impurities. The method realizes excellent delayed cracking resistance while obtaining super-high strength through innovative alloy design, fine precipitation phase control and slow cooling self-tempering process below the Ms point.

Description

Technical Field

[0001] This invention belongs to the field of steel materials technology, specifically relating to a hot stamping forming component resistant to delayed cracking and its preparation method. Background Technology

[0002] Hot stamping technology is a crucial method for manufacturing ultra-high-strength automotive structural components such as A-pillars, B-pillars, and crash beams. With the development of lightweight vehicles, the use of hot-stamped steel is gradually increasing, and its strength grade is also gradually improving. Currently, some domestic OEMs have achieved tensile strengths of 1500MPa or even 2000MPa and above by rapidly quenching steel sheets to the austenitic region to obtain martensitic structures. With the continuous improvement of automotive lightweighting and collision safety requirements, the development of hot-stamped steel sheets with tensile strength ≥2200MPa, combined with good plasticity and resistance to delayed cracking, has become a hot topic in the industry. Simultaneously, there is a demand for hot-stamped steel with strengths of 2400MPa and above. However, ultra-high-strength hot-stamped steels generally face the risk of hydrogen-induced delayed cracking (also known as hydrogen embrittlement). Hydrogen atoms penetrate the steel matrix during quenching or under service conditions, and under stress, accumulate at grain boundaries, inclusions, or second-phase interfaces, causing components to fracture suddenly without obvious warning. Therefore, how to effectively suppress hydrogen-induced delayed cracking while obtaining ultra-high strength is a technical problem that urgently needs to be solved in this field.

[0003] To address the aforementioned technical problems, various solutions have been proposed in the existing technology. For example, Chinese invention patent CN119571207B discloses an Al-Si coated hot-stamped steel sheet and its preparation method. The steel sheet substrate uses the following chemical composition by mass percentage: C 0.29~0.42%, Si 0.50~0.90%, Mn 0.30~0.70%, Al 0.10~0.40%, V 0.05~0.20%, B 0.001~0.01%, Cr 0.01~0.40%, Cu 0.10~0.30%, Mg 0.002~0.005%, with the balance being Fe and unavoidable impurities. The martensitic phase transformation initiation temperature is controlled by the formula Ms=500-320[C]-50[Mn]-30[Cr]-5([Cu]+[Si]), requiring Ms≥335℃. This scheme uses an Al-Si coating, and the cooling rate after hot stamping is controlled at 40~60℃ / s, with a demolding temperature ≤200℃. Precipitation strengthening is achieved by forming VC carbides through V element, which can obtain hot stamped components with yield strength ≥1250MPa, tensile strength ≥2000MPa, and VDA238-100 three-point bending angle ≥40°. However, this scheme has the following shortcomings: First, the Si content in the steel plate matrix is ​​as high as 0.50~0.90%. During hot rolling and annealing, oxides of Si, Mn and Al are easily formed on the surface and grain boundaries of the steel plate. These oxides will significantly reduce the adhesion between the Al-Si coating and the steel substrate, leading to defects such as incomplete coating and coating peeling. This provides a channel for hydrogen atom intrusion and increases the risk of hydrogen-induced delayed cracking. Second, it is rapidly cooled below the Ms point at a cooling rate of 40~60℃ / s, which easily leads to high-density dislocations and micro-stress concentration. At the same time, it promotes the formation of brittle twinned martensite and increases the sensitivity to quenching cracks and hydrogen-induced cracking.

[0004] For example, Chinese invention patent application CN119956218A discloses a hot-formed steel sheet resistant to delayed cracking. The steel sheet matrix chemical composition by mass percentage is: C 0.28~0.38%, Si 0.2~0.3%, Mn 0.08~1.5%, Ti 0.02~0.05%, V 0.08~0.2%, Nb 0.02~0.05%, Cr 0.12~0.2%, B 0.001~0.003%, Als≤0.01%, Ca 0.003~0.01%, O 0.035~0.04%, with the balance being Fe and unavoidable impurities. This scheme increases the oxygen content to 0.035~0.04%, forming composite oxide inclusions in the steel, and using oxide / MnS composite inclusions with a size <2000nm as hydrogen traps. The inclusion density is controlled at 20~50 inclusions / mm². 2Simultaneously, by modifying the morphology of inclusions through Ca treatment and applying an Al-Si coating, hot-stamped components with a tensile strength of 2000 MPa can be obtained. However, this approach also has significant drawbacks: First, oxide inclusions, while acting as hydrogen traps, can also become preferential sites for hydrogen-induced crack initiation, thus exacerbating the risk of delayed cracking; second, the higher oxygen content design further aggravates the risk of intergranular oxidation, leading to an increase in oxides on the steel plate surface and at grain boundaries, which not only reduces coating adhesion but also increases the channels for hydrogen intrusion and diffusion.

[0005] In summary, how to simultaneously avoid intergranular oxidation, resist delayed cracking, and further improve the strength grade of hot-stamped steel products has become a pressing technical problem to be solved in this field. Summary of the Invention

[0006] In view of the shortcomings of the prior art, the main objective of this invention is to provide a hot stamping forming component and its preparation method that is resistant to delayed cracking. Through innovative alloy design, refined control of precipitated phases and slow cooling self-tempering process below Ms point, excellent resistance to delayed cracking is achieved while obtaining ultra-high strength.

[0007] To solve at least one of the above-mentioned technical problems, the present invention adopts the following technical solution: According to one aspect of the present invention, a method for preparing a hot-stamped forming component resistant to delayed cracking is provided, comprising the following steps: smelting process: controlling the steel composition by weight percentage as follows: C: 0.42%~0.52%, Mn: 0.6%~0.9%, Si: 0.15%~0.4%, (Al+Co): 0.5%~1.2%, Mo≤1.0%, Ti: 0.03%~0.3%, V≤0.4%, Cr: 0.1%~0.4%, Ni: 0.1%~0.5%, Sn≤0.08%, 0.01%≤La+Ce≤0.06%, B≤0.003%, N≤0.003%, wherein the element weight fraction ratio is: Al / Co≥5, (Ti+V) / (C-0.4%)≥2, and the remaining elements are Fe and unavoidable impurities, and the components are smelted and continuously cast into a slab; rolling process: sequentially performing hot rolling treatment and pickling treatment on the slab. The process involves obtaining hot-rolled pickled steel sheets, and selectively cold-rolling the hot-rolled pickled steel sheets according to the final thickness and / or surface quality requirements of the hot-stamped components to obtain cold-rolled strip steel; a coating process: using the hot-rolled pickled steel sheets or the cold-rolled strip steel as substrates, a pre-coating process is performed to form a zinc-based coating or an aluminum-based coating on the surface of the substrate to obtain a coated sheet; a hot stamping process: the coated sheet is heated to austenitization and held at that temperature, then transferred to a temperature of 9... Quenching at 0~200℃ to 100~220℃ in a mold yields a hot-stamped component with metastable carbide size of 1~50nm. The total heating time is equal to the material thickness × 110 + (60~190) s, the transfer time is 0~15s, the hot stamping pressure is not greater than 12MPa, and the holding time is 1~50s. The holding time increases with the increase of the material thickness. The metastable carbide includes at least one of ε-carbide and η-carbide.

[0008] According to one embodiment of the present invention, the rolling process includes: hot rolling treatment: the slab is heated, descaled, rough rolled, finish rolled and laminar flow cooled to obtain a hot rolled plate, wherein the final rolling temperature is 860~900℃, the laminar flow cooling adopts a front-stage cooling method, the cooling rates of the upper and lower manifolds are 40%~60% and 70%~85% respectively, and the coiling temperature is 490~570℃; pickling treatment: the hot rolled plate is passed through an acidic medium to remove the surface oxide layer to obtain a pickled plate, wherein the temperature of the acidic medium is 70~88℃ and the pickling speed is 40~160m / min.

[0009] According to one embodiment of the present invention, the rolling process includes: cold rolling treatment: rolling the pickled plate through multiple passes to obtain a cold-rolled plate with a thickness of 0.6~2.5mm, wherein the cold rolling reduction rate is 30%~65%, and the cold rolling reduction rate gradually decreases as the thickness of the cold-rolled plate increases.

[0010] According to one embodiment of the present invention, in the coating process, the pre-coating treatment is a zinc-based coating treatment, which includes: heating the substrate to 300°C, 600-700°C and 730-830°C respectively at segmented heating rates of 10-20°C / s, 3-10°C / s and 0.4-3°C / s, holding it at the same temperature for 30-100s, slowly cooling it to 600-680°C, and then rapidly cooling it to 445-468°C, immersing it in a zinc bath for hot-dip galvanizing treatment, with an immersion time of 1-30s, and cooling it to room temperature at a cooling rate of ≥5°C / s after removing it from the zinc bath to obtain a hot-dip galvanized sheet.

[0011] According to one embodiment of the present invention, in the coating process, the pre-coating treatment is a zinc-based coating treatment, comprising: using an electroplating method, depositing a zinc layer or a zinc-iron alloy layer on the surface of the substrate in an electroplating solution with the substrate as the cathode, at a current density of 5~80 A / dm. 2 The electroplating solution temperature is 20~50℃ to obtain electroplated zinc sheet or zinc-iron electroplated sheet.

[0012] According to one embodiment of the present invention, in the coating process, the pre-coating treatment is an aluminum-based coating treatment, comprising: heating the substrate to 300°C, 600-700°C and 730-830°C respectively at segmented heating rates of 10-20°C / s, 3-10°C / s and 0.4-3°C / s, holding the substrate at the same temperature for 30-100s, cooling it to 625-680°C, immersing it in an Al-Si bath or an Al-Si-Mg bath for hot-dip coating treatment, with an immersion time of 2-25s, and then cooling it to room temperature at a cooling rate of ≥3°C / s to obtain an Al-Si coated plate or an Al-Si-Mg coated plate.

[0013] According to one embodiment of the present invention, in the coating process, the Al-Si-Mg bath further contains Zn, and the mass percentage of Zn in the bath is 0.5%~5.0%, thereby obtaining an Al-Si-Mg-Zn composite coating plate.

[0014] According to one embodiment of the present invention, the single-sided coating weight of the zinc-based coating or the aluminum-based coating is 15~120g / m², and the coating thickness is 5~50μm.

[0015] According to one embodiment of the present invention, after the pre-coating process, the process further includes: The plated plate is flattened, and the flattening elongation range is 0.2% to 1.5%.

[0016] According to one aspect of the present invention, a hot-stamped forming component resistant to delayed cracking is provided, which is prepared by the method described in any of the above embodiments, wherein the microstructure of the hot-stamped forming component has a self-tempered martensite content of more than 50%, a retained austenite volume fraction of ≤6%, and a metastable carbide size of 1~50 nm, wherein the metastable carbide includes at least one of ε-carbide and η-carbide.

[0017] By adopting the above technical solution, the present invention has at least one of the following advantages compared with the prior art: (1) Increase the Ms / Mf temperature of martensitic phase transformation: Add appropriate amounts of austenite destabilizing elements such as Al and Co to increase the Ms / Mf temperature, reduce / avoid the formation of brittle twinned martensite, reduce the internal stress generated during martensitic phase transformation, and inhibit the generation of quenching cracks. At the same time, after the Mf point is increased, the amount of untransformed residual austenite at room temperature is reduced, which can effectively ensure the strength of martensite.

[0018] (2) Precisely control the content of Ti+V microalloying element composite addition, Ti: 0.03~0.3%, (Ti+V) / (C-0.4%)≥2. Among them, the Ti alloy addition form is sponge titanium with a purity ≥99.5%. During the steelmaking process, the N element content is controlled to below 0.003% while strong cooling with secondary cooling water is carried out to avoid the occurrence of large TiN particles that are detrimental to toughness. At the same time, by controlling the continuous casting, hot rolling, cold rolling and hot stamping processes, the precipitation of the second particle is achieved at different stages, thereby achieving grain refinement and microstructure control at different stages. The ultimate goal is to obtain the martensitic microstructure after hot stamping. The volume fraction of microalloyed carbonitrides, nitrides or carbides is >0.3%, the volume fraction of precipitates in the 0~15nm range is ≥0.2%, the volume fraction of precipitates in the 15~100nm range is ≥0.1%, and the volume fraction of precipitates in the 100~500nm range is ≤0.15%; the fine microalloyed precipitates play a dual role of grain refinement and hydrogen trapping.

[0019] (3) Special hot stamping process: The coated plate is heated to austenitization and held at that temperature, and then transferred to a mold at a temperature of 90~200℃ and quenched to 100~220℃. The cooling rate of the hot stamping steel sheet after austenitization is precisely controlled to be ≤10℃ / s below the Ms point temperature. The control methods are not limited to adjusting the mold surface temperature (100℃~Ms temperature point), the stamping pressure of the press (≤12MPa), the holding time, and the mold design. The final microstructure of the hot stamping component has a self-tempered martensite content of more than 50% and a residual austenite volume fraction of ≤6%. The high proportion of self-tempered martensite effectively releases the quenching internal stress and reduces the driving force of hydrogen-induced cracking. The metastable carbides uniformly dispersed in the matrix include ε-carbides and / or η-carbides with a size of 1~50nm. The uniformly dispersed ε-carbides and / or η-carbides provide a secondary hardening effect, playing a strong dispersion strengthening and precipitation strengthening role.

[0020] (4) The intergranular oxidation control element Sn ≤ 0.08%, optimally 0.06%. When added to steel, on the one hand, it is enriched on the surface and surface layer of hot-rolled steel plate at high coiling temperature after hot rolling, preventing oxygen from diffusing into the interior of steel, thereby inhibiting the formation of internal oxides composed of Si, Mn, Al, Fe, etc. in the grain boundaries and grains inside the steel; on the other hand, the addition of Sn element significantly improves the adhesion between the coating and the steel substrate; at the same time, the composite addition of Al / Co increases the Ms / Mf temperature point, reduces or avoids the formation of brittle twin martensite, reduces the internal stress generated during martensitic phase transformation, and inhibits the generation of quenching cracks; rare earth elements La and / or Ce can modify large-sized, sharp brittle inclusions in steel into fine, spherical, and dispersed composite inclusions, thereby significantly reducing the risk of hydrogen-induced crack initiation and purifying grain boundaries, further improving the resistance to hydrogen embrittlement.

[0021] (5) Based on innovative alloy design and reasonable composition ratio of C, Al, Co, Ti, V, etc., and the implementation of special hot stamping process, the yield strength ≥1500MPa, tensile strength ≥2400MPa, elongation ≥5%, and hydrogen embrittlement risk index (HERI) ≤2.5% after hot stamping at -20℃ dew point. Attached Figure Description

[0022] 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 only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained from these drawings without creative effort.

[0023] Figure 1 This is a flowchart of a method for preparing a hot-stamped component resistant to delayed cracking according to the present invention. Detailed Implementation

[0024] To make the objectives, technical solutions, and advantages of the present invention clearer, the embodiments of the present invention will be further described in detail below with reference to specific examples and the accompanying drawings.

[0025] It should be understood that the embodiments of the invention shown in the exemplary embodiments are merely illustrative. Although only a few embodiments have been described in detail in this invention, those skilled in the art will readily recognize that various modifications are possible without substantially departing from the teachings of the invention. Accordingly, all such modifications should be included within the scope of the invention. Other substitutions, modifications, variations, and deletions can be made to the design, operating conditions, and parameters of the following exemplary embodiments without departing from the spirit of the invention.

[0026] According to one aspect of the present invention, embodiments of the present invention provide a method for preparing a hot-stamped component resistant to delayed cracking, such as... Figure 1 As shown, its overall structure may include the following steps: S100: Smelting process, the steel composition is controlled by weight percentage: C: 0.42%~0.52%, Mn: 0.6%~0.9%, Si: 0.15%~0.4%, (Al+Co): 0.5%~1.2%, Mo≤1.0%, Ti: 0.03%~0.3%, V≤0.4%, Cr: 0.1%~0.4%, Ni: 0.1%~0.5%, Sn≤0.08%, 0.01%≤La+Ce≤0.06%, B≤0.003%, N≤0.003%, where the element weight fraction ratio is: Al / Co≥5, (Ti+V) / (C-0.4%)≥2, the remaining elements are Fe and unavoidable impurities, and the components are smelted and continuously cast into slabs; S200: Rolling process, wherein the slab is subjected to hot rolling and pickling treatment in sequence to obtain hot-rolled pickled plate, and the hot-rolled pickled plate is selectively subjected to cold rolling treatment according to the final thickness and / or surface quality requirements of the hot stamping formed component to obtain cold-rolled strip steel; S300: Coating process, using the hot-rolled pickled plate or the cold-rolled strip as a substrate, performing a pre-coating process to form a zinc-based coating or an aluminum-based coating on the surface of the substrate, to obtain a coated plate. S400: Hot stamping process: The plated sheet is heated to austenitization and held at that temperature, then transferred to a mold at a temperature of 90~200℃ and quenched to 100~220℃ to obtain a hot stamped component with metastable carbide size of 1~50nm. The total heating time is = material thickness × 110 + (60~190)s, the transfer time is 0~15s, the hot stamping pressure is not greater than 12MPa, and the holding time is 1~50s. The holding time increases with the increase of material thickness. The metastable carbide includes at least one of ε-carbide and η-carbide.

[0027] In step S100, the chemical composition of the slab is precisely controlled, achieving multiple synergistic effects.

[0028] Co and Al can act as austenite destabilizing elements to raise the Ms / Mf temperature point, reduce / avoid the production of brittle twinned martensite, lower the internal stress generated during martensitic phase transformation, and inhibit the formation of quenching cracks. Meanwhile, M... f The reduction in the amount of untransformed residual austenite at room temperature after the point enhancement effectively ensures the strength of martensite. The combination of Co and Al effectively increases the Ms and Mf points, promoting sufficient self-tempering during the martensitic phase transformation and reducing the formation of twinned martensite. Simultaneously, Co directly dissolves in the matrix, enhancing interatomic bonding and increasing matrix hardness, effectively ensuring the performance of the martensite after tempering. In the embodiments of this invention, Al and Si are used as the main deoxidizers and metastable carbide morphology control elements to stabilize the carbide morphology; Co is used in a lower content to assist in increasing the tendency to form lath martensite during quenching. Preferably, the Al / Co content can be controlled to satisfy Al / Co≥5 and Si: 0.15%~0.4%, to precisely regulate the martensitic substructure and the microscopic characteristics of metastable carbides.

[0029] In embodiments of the present invention, the content of Ti+V microalloying element composite addition can be precisely controlled. Ti can fix and bind N under high-temperature conditions, ensuring the effectiveness of B solid solution. Furthermore, the precipitates formed by Ti at high temperatures can inhibit microstructure growth during hot rolling and heating of the billet in the hot rolling furnace, and the total solid solution temperature of these precipitates is relatively high. In contrast, V alone has a lower total solid solution temperature (~900℃), allowing V to precipitate in large quantities and at finer sizes at temperatures below 900℃. The simultaneous addition of Ti and V often results in Ti+V composite precipitates in the steel, which have a higher total solid solution temperature. These precipitates are less likely to re-dissolve during hot stamping austenitization, thus enabling the application of nanoscale precipitation. Additionally, this composite addition can obtain V4C3 and (Ti,V)C with atomic vacancies. These precipitates have a stronger hydrogen trapping effect, which is beneficial in combating hydrogen-induced delayed cracking. Preferably, the contents of both can be controlled to meet the following conditions: Ti: 0.03%~0.3%, V≤0.4%, and (Ti+V) / (C-0.4)≥2, to ensure the formation of sufficient quantity and volume fraction of nano-carbides in the alloy. Combined with dispersed nano-scale ε / η metastable carbides, these phases can not only pin dislocations to achieve precipitation strengthening, but also act as high-density hydrogen traps, fundamentally solving the core problem of ultra-high strength steel's high sensitivity to hydrogen-induced delayed cracking, and greatly improving the service safety of components.

[0030] Rare earth elements modify large, sharp, brittle inclusions in steel, such as TiN and Al2O3, into fine, spherical, and dispersed composite inclusions, significantly reducing the risk of hydrogen-induced crack initiation and purifying grain boundaries. When the rare earth element content is too low, the inclusion modification effect is not obvious and it is difficult to form an effective spherical and dispersed distribution. Conversely, if the content is too high, it will lead to coarsening of the rare earth inclusions and the formation of excessive rare earth oxides, which will impair toughness and resistance to hydrogen embrittlement. Preferably, the content can be controlled at 0.01% ≤ La + Ce ≤ 0.06% to achieve the best inclusion modification and hydrogen embrittlement resistance effect.

[0031] The addition of Sn inhibits intergranular oxidation during hot rolling and annealing, improving coating adhesion. The mechanism of Sn's action manifests in two stages: after hot rolling at high coiling temperatures, Sn accumulates on the surface and outer layer of the hot-rolled steel sheet, preventing oxygen diffusion into the steel's interior, thereby inhibiting the formation of internal oxides composed of Si, Mn, Al, Fe, etc., within the grain boundaries and grains. After pickling and cold rolling, Sn again accumulates on the surface and outer layer of the steel sheet during high-temperature annealing, preventing the diffusion of oxidizing elements such as Si, Mn, and Al from the steel to the surface, thus inhibiting the formation of oxides composed of Si, Mn, and Al on the annealed steel sheet surface. As a result, the steel sheet surface remains clean and has good wettability, allowing for good metallurgical bonding between zinc-based and aluminum-based coatings and the steel substrate during hot-dip galvanizing, resulting in better adhesion. A complete coating not only avoids surface defects such as incomplete coating and coating peeling but also effectively blocks the intrusion of external hydrogen sources, further enhancing resistance to hydrogen embrittlement. Preferably, the Al / Co content can be controlled to meet Sn≤0.08%, which effectively prevents the formation of intergranular oxidation and surface oxides, significantly improves the adhesion of zinc-based or aluminum-based coatings to the steel substrate, and reduces hydrogen intrusion channels, thereby improving the resistance to delayed cracking.

[0032] In step S100, the preferred form of Ti alloy addition is sponge titanium with a purity ≥99.5%. During the smelting process, the N element content is controlled to below 0.003% while undergoing secondary cooling with strong cooling. To improve cooling intensity, the specific water volume is adjusted to 0.7~0.8 kg / t steel, effectively preventing the formation of large TiN particles that negatively impact toughness. Simultaneously, by controlling continuous casting, and subsequent hot rolling, annealing, and hot stamping processes, [the following is achieved / achieved / etc.]. The precipitation of the second particle at different stages leads to grain refinement and microstructure control at different stages, ultimately resulting in the desired martensitic microstructure after hot stamping. Volume fraction > 0.3%, volume fraction of precipitated particles in the 0~15nm range ≥ 0.2%, volume fraction of precipitated particles in the 15~100nm range ≥ 0.1%, and volume fraction of precipitated particles in the 100~500nm range ≤ 0.15%.

[0033] The rolling process in step S200 may selectively cold roll the pickled steel coil according to the coating type and / or finished product thickness requirements to obtain cold-rolled thin strip steel with a predetermined thickness or directly use the pickled steel coil as a substrate.

[0034] In one embodiment of the present invention, the rolling process in step S200 may include: S210, Hot rolling treatment: The slab is heated, descaled, rough rolled, finish rolled and laminar flow cooled to obtain a hot rolled plate; S220, pickling treatment: The hot-rolled plate is passed through an acidic medium to remove the surface oxide layer, resulting in a pickled plate.

[0035] Specifically, the descaling process is strictly controlled in both the roughing and finishing stages, using high-pressure water to remove surface iron oxide scale. In the roughing stage, the thick billet is thinned into an intermediate billet through multiple rolling passes; in the finishing stage, a multi-stand continuous rolling mill is used, controlling the final rolling temperature at 860~900℃. Laminar flow cooling employs a front-stage cooling method, with the upper and lower manifold cooling rates set to 40%~60% and 70%~85% respectively, and the coiling temperature at 490~570℃. Before pickling, the hot-rolled plate can undergo pretreatment by spraying to remove surface contaminants; subsequently, the hot-rolled plate is immersed in hydrochloric acid or sulfuric acid to dissolve the oxide scale through a chemical reaction; finally, it is washed with water to remove residual acid, neutralized with a weak alkali for corrosion prevention, and dried to prevent rust, resulting in a pickled plate. Preferably, the acidic medium temperature can be set to 70~88℃, and pickling can be performed at a speed of 40~160 m / min.

[0036] In one embodiment of the present invention, the rolling process in step S200 may further include: S230, Cold rolling treatment: The pickled plate is rolled in multiple passes to obtain a cold-rolled plate with a thickness of 0.6~2.5mm.

[0037] Specifically, using pickled hot-rolled steel as raw material, the strip is gradually thinned through multiple rolling passes at room temperature to obtain high-precision, high-surface-quality strip. Preferably, the cold rolling reduction rate can be set to 30%~65%. The cold rolling reduction rate gradually decreases as the thickness of the cold-rolled steel increases.

[0038] The coated plate obtained in step S300 can be a zinc-based coating or an aluminum-based coating, including at least one selected from hot-dip galvanizing, zinc electroplating or zinc-iron electroplating, and hot-dip aluminizing. Specifically, the single-sided coating weight of the zinc-based coating or the aluminum-based coating is 15~120 g / m². 2 The coating thickness is 5~50μm. After the pre-coating treatment, it is preferable to further include: flattening the coated plate, with a flattening elongation range of 0.2%~1.5%.

[0039] In one embodiment of the present invention, the pre-coating treatment is a zinc-based coating treatment, comprising: heating the substrate to 300°C, 600-700°C and 730-830°C respectively at segmented heating rates of 10-20°C / s, 3-10°C / s and 0.4-3°C / s, holding the substrate at the same temperature for 30-100s, slowly cooling it to 600-680°C, then rapidly cooling it to 445-468°C, immersing it in a zinc bath for hot-dip galvanizing treatment, with an immersion time of 1-30s, and cooling it to room temperature at a cooling rate of ≥5°C / s after removing it from the zinc bath to obtain a hot-dip galvanized sheet.

[0040] In one embodiment of the present invention, the pre-coating treatment is a zinc-based coating treatment, comprising: depositing a zinc layer or a zinc-iron alloy layer on the surface of the substrate in an electroplating solution using the substrate as the cathode, with a current density of 5~80 A / dm. 2 The electroplating solution temperature is 20~50℃ to obtain electroplated zinc sheet or zinc-iron electroplated sheet.

[0041] In one embodiment of the present invention, the pre-coating treatment is an aluminum-based coating treatment, comprising: heating the substrate to 300°C, 600-700°C and 730-830°C respectively at segmented heating rates of 10-20°C / s, 3-10°C / s and 0.4-3°C / s, holding the substrate at the same temperature for 30-100s, cooling it to 625-680°C, immersing it in an Al-Si bath or an Al-Si-Mg bath for hot-dip coating treatment for 2-25s, and then cooling it to room temperature at a cooling rate of ≥3°C / s to obtain an Al-Si coated plate or an Al-Si-Mg coated plate.

[0042] In one embodiment of the present invention, the Al-Si-Mg bath further contains Zn, and the mass percentage of Zn in the bath is 0.5% to 5.0%, thereby obtaining an Al-Si-Mg-Zn composite coating plate.

[0043] In the hot stamping process of step S300, a die is quenched at 90~200℃ to 100~220℃. The total heating time is controlled as material thickness × 110 + (60~190) s, and the pressure is ≤12MPa. The holding time increases with the increase of material thickness, which effectively controls the cooling rate below Ms point, promotes the formation of self-tempering martensite, effectively reduces quenching stress, and controls the formation of twinned martensite. This process can further effectively improve the hydrogen embrittlement resistance of the invented material. The resulting component has metastable ε-carbides and / or η-carbides with a size of 1~50nm. These nanoscale carbides are uniformly dispersed in the martensitic matrix, providing both a secondary hardening effect and acting as irreversible hydrogen traps to fix hydrogen atoms. Through the synergistic effect of the above-mentioned components and processes, the present invention achieves excellent comprehensive performance with tensile strength ≥2400MPa, yield strength ≥1500MPa, elongation ≥5%, three-point bending angle ≥32°, and hydrogen embrittlement risk index HERI ≤2.5%.

[0044] According to one aspect of the present invention, embodiments of the present invention also provide a hot-stamped forming component resistant to delayed cracking prepared by the method described in any of the above embodiments. The microstructure of this hot-stamped forming component has a self-tempered martensite content (volume fraction) higher than 50%, a retained austenite volume fraction ≤6%, and metastable carbides with a size of 1~50 nm. The metastable carbides include at least one of ε-carbides and η-carbides. Specifically, the microstructure of the hot-stamped forming component is composed of martensite with a self-tempered martensite content higher than 50%, or martensite with a self-tempered martensite content higher than 50% + a small amount of ferrite, or martensite with a self-tempered martensite content higher than 50% + bainite, or martensite with a self-tempered martensite content higher than 50% + a small amount of ferrite + bainite, with a retained austenite volume fraction ≤6%. ε / η metastable carbides are uniformly dispersed in the matrix, and a certain amount of nanoscale microalloyed precipitates are also present. On the one hand, the hot-stamped components according to the present invention not only effectively improve the matrix strength by forming a composite strengthening mechanism through a self-tempered martensitic matrix and uniformly dispersed nanoscale ε / η metastable carbides, but also effectively reduce / avoid the formation of twinned martensite by reducing the solid solution C content in austenite and increasing the Ms and Mf point temperatures, thereby greatly suppressing the brittleness caused by high strength and achieving a balance between strength and toughness. On the other hand, by utilizing dispersed nanoscale ε / η metastable carbides and V / Ti microalloyed carbonitrides / carbides as high-density hydrogen traps, the core problem of the high sensitivity of ultra-high strength steel to hydrogen-induced delayed cracking is fundamentally solved, greatly improving the service safety of the components. The yield strength of the hot-stamped steel material of 2400MPa and above obtained is ≥1500MPa, tensile strength ≥2400MPa, elongation ≥5%, bending angle αmax ≥32°, and hydrogen embrittlement risk index (HERI) ≤2.5% after hot stamping at a dew point of -20℃.

[0045] The following are specific embodiments and process parameters of the method for preparing hot stamping forming components resistant to delayed cracking according to the present invention.

[0046] S100: The smelting process controls the steel composition by weight percentage, and the components are smelted and continuously cast into slab samples A1-A3 and control samples B1-B2. The specific chemical composition of each sample is shown in Table 1. Table 1 Chemical composition (wt.%) of slab samples

[0047] S200: Rolling process. Slab samples A1-A3 and B1-B2 are sequentially hot-rolled and pickled to obtain hot-rolled pickled slabs, and selectively cold-rolled to obtain cold-rolled strip steel. Specific hot-rolling process parameters are shown in Table 2. Table 2 Rolling process parameters

[0048] S300: Coating process, using the hot-rolled pickled plate or the cold-rolled strip as a substrate, performing a pre-coating treatment to form a zinc-based coating or an aluminum-based coating on the surface of the substrate, to obtain a coated plate.

[0049] The substrates obtained in Examples 1-3 and Comparative Example 1 were heated to 300°C, 650°C and 780°C respectively at segmented heating rates of 15°C / s, 7°C / s and 2°C / s. After heat homogenization and holding at the temperature for 70s, they were slowly cooled to 610°C and then rapidly cooled to 450°C. They were then immersed in a zinc bath for hot-dip galvanizing treatment for 20s. After being removed from the zinc bath, they were cooled to room temperature at a cooling rate of ≥5°C / s to obtain hot-dip galvanized sheets.

[0050] The substrates obtained in Examples 4-6 and Comparative Example 2 were heated to 300°C, 680°C and 820°C respectively at segmented heating rates of 15°C / s, 7°C / s and 1.5°C / s. After heat soaking and holding at the same temperature for 50s, they were cooled to 650°C and immersed in an Al-Si bath or an Al-Si-Mg bath for hot-dip plating treatment for 2-25s. Afterwards, they were cooled to room temperature at a cooling rate of ≥3°C / s to obtain Al-Si coated plates.

[0051] S400: Hot stamping process. The coated plates obtained in Examples 1-6 and Comparative Examples 1-2 are heated to austenitization and held at that temperature, and then hot stamped. The specific hot stamping process parameters are shown in Table 3. Table 3 Hot stamping process parameters

[0052] The mechanical properties of the obtained hot-stamped components are shown in Table 4: Table 4 Mechanical properties of hot-stamped components

[0053] The hydrogen embrittlement risk index (HERI) is the percentage of the difference between the reference fracture stress and the actual fracture stress, expressed as HERI = (σ0 - σ...). H ) / σ0×100%. Where σ0 is the reference fracture stress, i.e., the fracture stress of the material in the absence of hydrogen, in MPa; σ HHydrogen-induced fracture stress (HERI) refers to the actual fracture stress of a material under specific hydrogen content or hydrogen-filled environments, measured in MPa. This hydrogen embrittlement risk index quantifies the reduction in fracture stress at low strain rates (ensuring hydrogen-dominated fracture) relative to the baseline fracture stress at high strain rates (ensuring non-hydrogen-dominated fracture), sensitively reflecting the degree to which hydrogen weakens the mechanical properties of high-strength steel, thus effectively assessing its risk of hydrogen-induced delayed fracture. The VDA238-100 three-point bending angle evaluation mentioned above can be referenced in VDA238-100-2017 published by the German Association of the Automotive Industry; the hydrogen embrittlement risk index related to delayed cracking evaluation can be referenced in the group standard T / CSAE 049-2024, "Test Method for Hydrogen-Induced Delayed Fracture Risk of Hot-Stamped Steel with Aluminum-Silicon Coated for Automotive Use (1500 MPa and Above)".

[0054] As can be seen from Examples 1-6, the yield strength of hot stamping steel materials of 2400MPa and above obtained by hot stamping heat treatment and quenching under the above alloy system is ≥1500MPa, tensile strength ≥2400MPa, elongation ≥5%, bending angle αmax ≥32° (1.5mm plate thickness), and hydrogen embrittlement risk index (HERI) ≤2.5% after hot stamping at -20℃ dew point.

[0055] The above are merely preferred embodiments of the present invention and are not intended to limit the scope of the present invention. Any modifications or equivalent substitutions made to the present invention without departing from the spirit and scope thereof should be covered within the protection scope of the claims of the present invention.

[0056] Those skilled in the art should understand that the discussion of any of the above embodiments is merely exemplary and is not intended to imply that the scope of the disclosed embodiments of the present invention is limited to these examples; within the framework of the embodiments of the present invention, the technical features of the above embodiments or different embodiments can also be combined, and there are many other variations of different aspects of the embodiments of the present invention as described above, which are not provided in detail for the sake of brevity. Therefore, any omissions, modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the embodiments of the present invention should be included within the protection scope of the embodiments of the present invention.

Claims

1. A method for preparing a hot-stamped component resistant to delayed cracking, characterized in that, Includes the following steps: Smelting process: The steel composition is controlled by weight percentage as follows: C: 0.42%~0.52%, Mn: 0.6%~0.9%, Si: 0.15%~0.4%, (Al+Co): 0.5%~1.2%, Mo≤1.0%, Ti: 0.03%~0.3%, V≤0.4%, Cr: 0.1%~0.4%, Ni: 0.1%~0.5%, Sn≤0.08%, 0.01%≤La+Ce≤0.06%, B≤0.003%, N≤0.003%. Among them, the element weight fraction ratio is: Al / Co≥5, (Ti+V) / (C-0.4%)≥2, and the remaining elements are Fe and unavoidable impurities. The components are smelted and then continuously cast into slabs. Rolling process: The slab is subjected to hot rolling and pickling treatment in sequence to obtain hot-rolled pickled plate, and the hot-rolled pickled plate is selectively subjected to cold rolling treatment according to the final thickness and / or surface quality requirements of the hot stamping formed component to obtain cold-rolled strip steel. Coating process: Using the hot-rolled pickled plate or the cold-rolled strip as a substrate, a pre-coating process is performed to form a zinc-based coating or an aluminum-based coating on the surface of the substrate to obtain a coated plate. Hot stamping process: The plated sheet is heated to austenitization and held at that temperature, then transferred to a mold at a temperature of 90~200℃ and quenched to 100~220℃ to obtain a hot stamped component with metastable carbide size of 1~50nm. The total heating time is equal to the material thickness × 110 + (60~190) s, the transfer time is 0~15s, the hot stamping pressure is not greater than 12MPa, and the holding time is 1~50s. The holding time increases with the increase of the material thickness. The metastable carbide includes at least one of ε-carbide and η-carbide.

2. The method for preparing a hot-stamped component resistant to delayed cracking according to claim 1, characterized in that, The rolling process includes: Hot rolling process: The slab is heated, descaled, rough rolled, finish rolled and laminar flow cooled to obtain a hot rolled plate. The final rolling temperature is 860~900℃, the laminar flow cooling adopts the front cooling method, the cooling rates of the upper and lower manifolds are 40%~60% and 70%~85% respectively, and the coiling temperature is 490~570℃. Pickling treatment: The hot-rolled plate is passed through an acidic medium to remove the surface oxide layer, and a pickled plate is obtained. The temperature of the acidic medium is 70~88℃ and the pickling speed is 40~160m / min.

3. The method for preparing a hot-stamped component resistant to delayed cracking according to claim 2, characterized in that, The rolling process includes: Cold rolling process: The pickled plate is rolled in multiple passes to obtain a cold-rolled plate with a thickness of 0.6~2.5mm, wherein the cold rolling reduction rate is 30%~65%, and the cold rolling reduction rate gradually decreases as the thickness of the cold-rolled plate increases.

4. The method for preparing a hot-stamped component resistant to delayed cracking according to claim 3, characterized in that, In the coating process, the pre-coating treatment is a zinc-based coating treatment, including: The substrate is heated to 300℃, 600~700℃ and 730~830℃ respectively at segmented heating rates of 10~20℃ / s, 3~10℃ / s and 0.4~3℃ / s. After heat homogenization and holding at the same temperature for 30~100s, it is slowly cooled to 600~680℃ and then rapidly cooled to 445~468℃. It is then immersed in a zinc bath for hot-dip galvanizing treatment for 1~30s. After being removed from the zinc bath, it is cooled to room temperature at a cooling rate of ≥5℃ / s to obtain a hot-dip galvanized sheet.

5. The method for preparing a hot-stamped component resistant to delayed cracking according to claim 3, characterized in that, In the coating process, the pre-coating treatment is a zinc-based coating treatment, including: Electroplating is used, with the substrate as the cathode in the electroplating solution, to deposit a zinc layer or a zinc-iron alloy layer on the surface of the substrate, at a current density of 5~80 A / dm. 2 The electroplating solution temperature is 20~50℃ to obtain electroplated zinc sheet or zinc-iron electroplated sheet.

6. The method for preparing a hot-stamped component resistant to delayed cracking according to claim 3, characterized in that, In the coating process, the pre-coating treatment is an aluminum-based coating treatment, including: The substrate is heated to 300℃, 600~700℃ and 730~830℃ respectively at segmented heating rates of 10~20℃ / s, 3~10℃ / s and 0.4~3℃ / s. After heat soaking and holding at the same temperature for 30~100s, it is cooled to 625~680℃ and immersed in an Al-Si bath or Al-Si-Mg bath for hot-dip plating treatment for 2~25s. Then, it is cooled to room temperature at a cooling rate of ≥3℃ / s to obtain an Al-Si coated plate or an Al-Si-Mg coated plate.

7. The method for preparing a hot-stamped component resistant to delayed cracking according to claim 6, characterized in that, In the coating process, the Al-Si-Mg bath further contains Zn, with the mass percentage of Zn in the bath being 0.5%~5.0%, to obtain an Al-Si-Mg-Zn composite coated plate.

8. The method for preparing a hot-stamped member resistant to delayed cracking according to any one of claims 1-7, characterized in that, The single-sided coating weight of the zinc-based coating or the aluminum-based coating is 15~120g / m², and the coating thickness is 5~50μm.

9. The method for preparing a hot-stamped component resistant to delayed cracking according to claim 8, characterized in that, Following the pre-coating process, the process further includes: The plated plate is flattened, and the flattening elongation range is 0.2~1.5%.

10. A hot-stamped component resistant to delayed cracking, characterized in that, Prepared by the method of any one of claims 1-9, the hot-stamped component has a self-tempered martensite content of more than 50%, a retained austenite volume fraction of ≤6%, and a metastable carbide size of 1~50 nm, wherein the metastable carbide includes at least one of ε-carbide and η-carbide.

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