Hot-formed steel sheet resistant to delayed cracking, hot-stamped formed member and manufacturing method

Abstract

The present application relates to the technical field of steel material, and particularly relates to a hot forming steel plate resistant to delayed cracking, a hot stamping forming component and a preparation method. The preparation method comprises the following steps: a smelting process: controlling the components of the steel according to the percentage by weight: 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%, B≤0.003%, N≤0.003%, wherein the weight fraction ratio of the elements is: Al / Co≥5, (Ti+V) / (C-0.4%)≥2, the rest elements are Fe and inevitable impurities; a rolling-annealing process; a hot stamping process: heating the hot forming steel plate to austenitization and holding, then transferring to a die with a temperature of 80~280 DEG C for quenching to 100~250 DEG C, thereby obtaining the hot stamping forming component with the metastable carbide size of 1~50 nm. The method reduces the risk of delayed cracking of the hot stamping forming component and effectively improves the mechanical properties by reasonably regulating the compound addition amount of micro-alloying elements and cooperating with the hot stamping process parameters.

Description

Technical Field

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

[0002] With the development of automotive lightweighting, the use of hot stamping steel is gradually increasing, and the strength grade is also gradually improving. At present, some domestic OEMs have applied 2200MPa grade hot stamping steel, and at the same time, they have put forward application requirements for 2400MPa and above grade hot stamping steel. However, such high strength combined with good toughness has become a problem for alloy design.

[0003] For the martensite of Fe-C alloys, lath (dislocation) martensite forms when the carbon content is <0.3%; when the carbon content is >0.6%, plate-like (twinned) martensite is dominant, with a volume fraction >50%; when the carbon content is between 0.3% and 0.6%, plate-like and lath-like martensite often appear mixed. As the C content increases, the martensitic transformation initiation point M... s The lower the carbon content, the greater the amount of lamellar twinned martensite. The toughness of martensite is mainly determined by its substructure. Low-carbon dislocation-type martensite has relatively high strength and good toughness; while high-carbon twinned martensite has very poor toughness. This is not only due to the brittleness caused by internal microcracks, but may also be related to the presence of twinned substructures and the uneven distribution of carbides precipitating along the twin planes during tempering. Some also believe it may be related to the segregation of C atoms at twin boundaries.

[0004] With the development of lightweighting in automobiles, 2200MPa grade hot-stamped steel has been applied to body structural reinforcement components. For low-alloy hot-stamped steel, when its quenched microstructure strength reaches 2200MPa, its carbon content generally exceeds 0.36%. Without any measures, the final microstructure will inevitably contain a large amount of twinned martensite. To ensure the toughness of hot-stamped steel materials of this strength grade, common methods include tempering heat treatment or utilizing microalloying and carbon element reactions to form a second phase, such as the second phase precipitation formed by combining V, Ti, Nb microalloying elements and C elements mentioned in patent 201610535069.3, which consumes the solid solution C content to reduce the amount of twinned martensite formed. However, when the carbon content is higher than 0.4%, the strategy of microalloying second phase precipitation requires a large amount of V, Ti, Nb microalloying elements, which is costly and difficult to achieve in industrial production. Therefore, it is not feasible to improve the toughness of hot-stamped steel with a C content >0.4% by a single means. It is necessary to use one or more other methods to keep most of the martensitic substructure dislocation-type so that it can have both strength and toughness.

[0005] Therefore, how to combine toughness and resistance to delayed cracking to further improve the strength grade of hot stamping steel products has become a technical problem that urgently needs to be solved in this field. Summary of the Invention

[0006] To address the shortcomings of existing technologies, the main objective of this invention is to provide a hot-formed steel sheet resistant to delayed cracking, a hot-stamped component, and a preparation method thereof. By rationally controlling the amount of microalloying elements added and coordinating with the hot-stamping process parameters, the risk of delayed cracking in the hot-stamped component is reduced, while simultaneously improving its mechanical properties.

[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%, B≤0.003%, N≤0.003%, wherein 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 then continuously cast into plates. The billet is subjected to hot rolling and selective pickling, cold rolling, and continuous annealing to obtain a hot-formed steel plate with a thickness of 0.5~3.0 mm. The hot stamping process involves heating the hot-formed steel plate to austenitization and holding it at that temperature, then transferring it to a mold at a temperature of 80~280℃ and quenching it to 100~250℃ to obtain a hot-stamped component with a metastable carbide size of 1~50 nm. The total heating time is equal to the material thickness in mm × 115 + (50~200) s, the transfer time is 3~12 s, the hot stamping pressure is not greater than 15 MPa, and the holding time is 3~50 s. 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 an embodiment of the present invention, the rolling annealing 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 840~910℃, 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 500~650℃.

[0009] According to one embodiment of the present invention, the rolling annealing process includes: pickling treatment: passing the hot-rolled plate through an acidic medium to remove the surface oxide layer to obtain a pickled plate, wherein the temperature of the acidic medium is 65~90℃ and the pickling speed is 50~180m / min.

[0010] According to one embodiment of the present invention, the rolling annealing process includes: cold rolling treatment: rolling the pickled plate through multiple passes to obtain a cold-rolled plate with a thickness of 0.5~3.0mm, wherein the cold rolling reduction rate is 35%~70%, and the cold rolling reduction rate gradually decreases as the thickness of the cold-rolled plate increases.

[0011] According to one embodiment of the present invention, the rolling annealing process includes: continuous annealing treatment: heating the cold-rolled sheet to A C1 Keep the temperature above the set temperature, then cool it slowly at a rate of 1-5℃ / s to 600-670℃, followed by rapid cooling at a rate of 5-15℃ / s to 360-400℃, and finally cool it to below 180℃.

[0012] According to one embodiment of the present invention, during the continuous annealing process, the cold-rolled sheet is heated to A. C1 The heating process above the specified temperature and maintaining the temperature includes: First stage heating: including a preheating section, which uses the recovered heat from the heated waste gas to heat the cold-rolled sheet; the temperature of the cold-rolled sheet at the outlet of the preheating section is 120~180℃; Second stage heating: including a first heating stage, which uses radiant tubes to heat the cold-rolled sheet to 680~710℃; Third stage heating: including a second heating stage and a third heating stage, which uses radiant tubes to heat the cold-rolled sheet to 730~850℃ and then maintains the temperature for 1~8 minutes; During the heating stage, the unit speed is controlled at 60~130 m / min.

[0013] According to one embodiment of the present invention, during the continuous annealing process, the unit speed is controlled at 80~140m / min during the cooling stage, wherein the unit speed is reduced by 30m / min for every 0.4mm increase in the thickness of the cold-rolled plate.

[0014] According to one embodiment of the present invention, during the continuous annealing process, the flattening elongation ranges from 0.35% to 0.65%, wherein the flattening elongation decreases by 0.08% for every 0.4 mm increase in the thickness of the cold-rolled sheet.

[0015] According to another 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 contains more than 50% self-tempered martensite, ≤6% retained austenite volume fraction, and the size of metastable carbides is 1~50 nm, wherein the metastable carbides include at least one of ε-carbides and η-carbides.

[0016] According to another aspect of the present invention, a hot-formed steel sheet resistant to delayed cracking is provided, selected from hot-rolled sheet, pickled sheet, cold-rolled sheet, and continuously annealed sheet prepared by the method described in any of the above embodiments, wherein the hot-formed steel sheet is composed of the following chemical composition 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%, 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.

[0017] By adopting the above technical solution, the present invention has at least one of the following advantages compared with the prior art: (1) Enhance the martensitic phase transformation M s / M f Temperature point: Adding appropriate amounts of austenitic stabilizing elements such as Al and Co increases M. s / M f Temperature point, reducing / avoiding the production of brittle twinned martensite, reducing internal stress generated during martensitic phase transformation, suppressing the formation of quenching cracks, and simultaneously M f After the point is raised, the amount of residual austenite that has not been transformed at room temperature is reduced, which can effectively ensure the strength of martensite. (2) Precisely control the content of Ti+V microalloying element composite addition, Ti: 0.03~0.3%, V≤0.4%. Among them, the Ti alloy addition form is sponge titanium with a purity ≥99.5%. The steelmaking process controls the N element content to below 0.003% while carrying out secondary cooling water strong cooling control to avoid the appearance of liquid precipitated TiN large particle inclusions that are not conducive to toughness. At the same time, by controlling the continuous casting, hot rolling, annealing and hot stamping processes, the second particle is precipitated at different stages, and then the grain refinement and structure control are achieved at different stages. The most important thing is to obtain the martensite structure after hot stamping. (Microalloyed carbonitrides, nitrides or carbides) volume fraction > 0.3%, 0~15nm precipitate volume fraction ≥ 0.2%, 15~100nm precipitate volume fraction ≥ 0.1%, 100~500nm precipitate volume fraction ≤ 0.15%.

[0018] (3) Special hot stamping process: Precise control of the hot stamping steel sheet after austenitization at M s The cooling rate below the specified temperature is ≤10℃ / s, and its control methods are not limited to adjusting the mold surface temperature (100℃-M). sThe key factors include temperature point, press stamping pressure (≤15MPa), holding time, and die design. The final microstructure of hot-stamped components exhibits a self-tempered martensite content exceeding 50%, a retained austenite volume fraction ≤6%, and a uniformly dispersed metastable carbide matrix, including ε-carbides and / or η-carbides with sizes ranging from 1 to 50 nm, which provide strong dispersion strengthening and precipitation strengthening. Furthermore, the crucial hot stamping process effectively reduces quenching stress and further controls the formation of twinned martensite.

[0019] (4) 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% after hot stamping at -20℃ dew point. Attached Figure Description

[0020] 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.

[0021] Figure 1 A flowchart of a method for preparing a hot-stamped component resistant to delayed cracking according to the present invention; Figure 2 The stress-strain curve of the hot-rolled plate obtained in Example 1 of this invention after hot stamping is shown. Figure 3 The stress-strain curve of the cold-rolled sheet obtained in Example 7 of this invention after hot stamping is shown. Figure 4 The stress-strain curve of the steel plate obtained in Example 13 of this invention after hot stamping is shown. Figure 5 The stress-strain curve of the hot-rolled plate obtained in Comparative Example 1 of this invention after hot stamping is shown. Figure 6 The image shows the metallographic structure of the hot-rolled plate obtained in Embodiment 1 of the present invention after hot stamping. Figure 7 The image shows the metallographic structure of the cold-rolled sheet obtained in Example 7 of this invention after hot stamping. Figure 8 The image shows the metallographic structure of the continuous annealing plate obtained in Embodiment 13 of the present invention after hot stamping. Figure 9 This is a transmission electron microscope (TEM) image of the steel plate obtained in Example 6 of the present invention after being stamped. Detailed Implementation

[0022] 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.

[0023] 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.

[0024] According to one aspect of the present invention, an embodiment of the present invention provides a hot-formed steel sheet resistant to delayed cracking, which may be composed of the following chemical composition 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%, 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.

[0025] Co and Al can be used as austenite destabilizing elements to increase M. s / M f Temperature point, reducing / avoiding the production of brittle twinned martensite, reducing internal stress generated during martensitic phase transformation, suppressing the formation of quenching cracks, and simultaneously 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 can effectively increase the amount of M. s and M f The method promotes sufficient self-tempering during the martensitic phase transformation, 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.

[0026] 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 the 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℃. When Ti and V are added simultaneously, Ti+V composite precipitates often appear in the steel, with a higher total solid solution temperature. These precipitates are less likely to re-dissolve during hot stamping austenitization, thus enabling the nanoscale precipitation to play a practical role. 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.

[0027] Based on the same inventive concept, according to another aspect of the present invention, the present invention also provides a hot-stamped member resistant to delayed cracking prepared from the above-mentioned hot-forming steel sheet resistant to delayed cracking, and a method for preparing the same. Figure 1 As shown, the method for preparing a hot-stamped component resistant to delayed cracking specifically includes 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%, 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 and annealing process, hot rolling treatment of slab, and selective pickling treatment, cold rolling treatment, and continuous annealing treatment to obtain hot stamping steel sheet with a thickness of 0.5~3.0mm; S300: Hot stamping process, the hot stamping forming steel plate is heated to austenitization and held at that temperature, and then transferred to a mold at a temperature of 80~280℃ and quenched to 100~250℃ to obtain a hot stamping forming component with metastable carbide size of 1~50nm. The total heating time = material thickness mm × 115 + (50~200) s, the transfer time is 3~12s, the hot stamping pressure is not greater than 15MPa, and the holding time is 3~50s. The holding time increases with the increase of material thickness. The metastable carbide includes at least one of ε-carbide and η-carbide.

[0028] 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%.

[0029] The rolling and annealing process in step S200 may involve hot rolling, as well as selective pickling, cold rolling, and continuous annealing. The resulting hot-rolled plate, pickled plate, cold-rolled plate, and continuously annealed plate can all be used as hot-formed steel plates for hot stamping.

[0030] In one embodiment of the present invention, the rolling annealing process in step S200 may include: S210, hot rolling treatment, is the process of heating, descaling, rough rolling, finish rolling and laminar flow cooling of slabs to obtain hot-rolled plates.

[0031] 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 840~910℃. 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 500~650℃.

[0032] In one embodiment of the present invention, the rolling annealing process in step S200 may further include: S220, pickling treatment, involves passing hot-rolled steel plates through an acidic medium to remove the surface oxide layer, resulting in pickled steel plates.

[0033] Specifically, before pickling, the hot-rolled plate can undergo pretreatment by spraying to remove surface dirt; then, 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, thus obtaining the pickled plate. Preferably, the acidic medium temperature can be set to 65~90℃, and the pickling can be carried out at a speed of 50~180m / min.

[0034] In one embodiment of the present invention, the rolling annealing process in step S200 may further include: S230, cold rolling: cold-rolled sheet with a thickness of 0.5~3.0mm is obtained by rolling pickled sheet through multiple passes.

[0035] 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 steel. Preferably, the cold rolling reduction rate can be set to 35%~70%. The cold rolling reduction rate gradually decreases as the thickness of the cold-rolled steel increases.

[0036] In one embodiment of the present invention, the rolling annealing process in step S200 may include: S240, continuous annealing treatment, heating the cold-rolled sheet to A C1 The temperature is kept above the specified level and then cooled.

[0037] Specifically, the process of heating cold-rolled steel sheets may include: The first stage of heating includes a preheating section, which uses the recovered heat from the heated waste gas to heat the cold-rolled sheet. The temperature of the cold-rolled sheet at the outlet of the preheating section is 120~180℃. The second stage of heating includes a heating stage in which the cold-rolled sheet is heated to 680~710℃ using radiant tubes. The third stage of heating includes the second and third heating stages, which use radiant tubes to heat the cold-rolled sheet to 730~850℃.

[0038] Throughout the heating process, the unit speed is controlled between 60 and 130 m / min, depending on the thickness of the steel plate. The thicker the steel plate, the lower the unit speed. By adjusting the heating speed, the uniformity of the microstructure is maximized. After heating through the above process, the temperature is held for 1 to 8 minutes to eliminate work hardening and optimize the microstructure properties.

[0039] Furthermore, during the continuous annealing process, the temperature is first cooled to 600-670℃ at a slow cooling rate of 1-5℃ / s, then rapidly cooled to 360-400℃ at a rate of 5-15℃ / s, and finally cooled to below 180℃. The unit speed is controlled at 80-140 m / min throughout the cooling stage. Specifically, the unit speed decreases by 30 m / min for every 0.4 mm increase in cold-rolled sheet thickness. Additionally, the leveling elongation ranges from 0.35% to 0.65% during the continuous annealing process. Specifically, the leveling elongation decreases by 0.080% for every 0.4 mm increase in cold-rolled sheet thickness.

[0040] In step S300, hot-rolled sheet, pickled sheet, cold-rolled sheet, or continuously annealed sheet is used as the steel sheet for hot stamping. The hot-stamping steel sheet is heated to the austenitizing temperature using a box furnace, roller hearth furnace, resistance heating, or induction heating. The austenitized hot-stamping steel sheet is precisely controlled at M... s The cooling rate below the point temperature is ≤10℃ / s, and its control methods are not limited to adjusting the mold surface temperature (100℃-M). s The design parameters include temperature point, press stamping pressure (≤15MPa), holding time (designed according to material thickness), and die design (water channels, air channels, etc.). The resulting hot-stamped components exhibit a final microstructure with self-tempered martensite content exceeding 50%, retained austenite volume fraction ≤6%, and a uniformly dispersed metastable carbide matrix (ε / η) with a size of 1~50nm. The key hot-stamping process effectively reduces quenching stress and controls the formation of twinned martensite. This process further enhances the hydrogen embrittlement resistance of the invented material.

[0041] The hot-stamped quenched plates or components obtained after quenching by the hot-stamping heat treatment process of the above alloy system are composed of martensite with a self-tempered martensite content of more than 50%, or martensite with a self-tempered martensite content of more than 50% + a small amount of ferrite, or martensite with a self-tempered martensite content of more than 50% + bainite, or martensite with a self-tempered martensite content of more than 50% + a small amount of ferrite + bainite, with a retained austenite volume fraction ≤6%, and ε / η metastable carbides uniformly dispersed in the matrix, and also contain a certain amount of nanoscale microalloyed precipitates. 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 ≥38°, and hydrogen embrittlement risk index (HERI) ≤2% after hot stamping at a dew point of -20℃.

[0042] 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.

[0043] S100: The smelting process controls 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%, B≤0.003%, N≤0.003%. The element weight fraction ratios are: Al / Co≥5, (Ti+V) / (C-0.4%)≥2. The remaining elements are Fe and unavoidable impurities. After smelting, each component is continuously cast into slab samples A1-A3 and B1-B2. The specific chemical composition of each sample is shown in Table 1. Table 1 Chemical composition (wt.%) of slab samples

[0044] S200: Rolling and annealing process, which involves hot rolling of the slab and selectively pickling, cold rolling, and continuous annealing to obtain a steel sheet for hot stamping.

[0045] Slab samples A1-A3 and B1-B2 were hot-rolled to obtain hot-rolled plates. Specific hot-rolling process parameters are shown in Table 2. Table 2 Hot rolling process parameters for slabs

[0046] The hot-rolled sheets obtained in Examples 1-6 and Comparative Examples 1-2 were sequentially pickled and cold-rolled to obtain cold-rolled sheets. Specific pickling and cold-rolling process parameters are shown in Table 3. Table 3. Process parameters for pickling and cold rolling of hot-rolled steel plates

[0047] The hot-rolled sheets obtained from Examples 1-6 and Comparative Examples 1-2 were sequentially pickled and continuously annealed to obtain cold-rolled sheets. Specific pickling and continuous annealing process parameters are shown in Table 4. Table 4. Pickling + annealing process parameters for hot-rolled steel plates

[0048] S300: Hot stamping process. The hot stamping forming steel plates obtained in Examples 1-18 and Comparative Examples 1-6 are heated to austenitization and held at that temperature, and then hot stamping is performed. The specific hot stamping process parameters are shown in Table 5. Table 5 Hot stamping process parameters

[0049] The sheet metals obtained in Examples 1, 7, 13, and Comparative Example 1 were subjected to hot stamping quenching at 930℃ for 280s and then low-temperature tempering at 170℃ for 20min, respectively. The measured tensile curves are shown below. Figure 2-5 As shown.

[0050] See Figure 6-8 The microstructure of the hot-stamped and quenched sheet metal or components consists of martensite (self-tempered martensite content higher than 50%), martensite (self-tempered martensite content higher than 50%) + a small amount of ferrite, martensite (self-tempered martensite content higher than 50%) + bainite, or martensite (self-tempered martensite content higher than 50%) + a small amount of ferrite + bainite, with a retained austenite volume fraction ≤6%. For example... Figure 9 As shown, after hot stamping, ε / η metastable carbides are uniformly dispersed on the component matrix, and a certain amount of nanoscale microalloyed precipitates also exist.

[0051] The mechanical properties of the obtained hot-stamped components are shown in Table 6: Table 6 Mechanical properties of hot-stamped components

[0052] 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 H Hydrogen-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)".

[0053] As can be seen from Examples 1-18, 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 ≥38° (1.5mm plate thickness), and hydrogen embrittlement risk index (HERI) ≤2% after hot stamping at -20℃ dew point.

[0054] 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.

[0055] 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%, 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-annealing process: The slab is hot-rolled and selectively pickled, cold-rolled and continuously annealed to obtain a hot-formed steel plate with a thickness of 0.5~3.0mm; Hot stamping process: The hot-formed steel plate is heated to austenitization and held at that temperature, then transferred to a mold at a temperature of 80~280℃ for hot stamping and quenched to 100~250℃, thus obtaining a hot-stamped component with metastable carbide size of 1~50nm. The total heating time is = material thickness mm × 115 + (50~200) s, the transfer time is 3~12s, the hot stamping pressure is not greater than 15MPa, and the holding time is 3~50s. The holding time increases with the increase of 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-annealing 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 840~910℃, 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 500~650℃.

3. The method for preparing a hot-stamped component resistant to delayed cracking according to claim 2, characterized in that, The rolling-annealing process includes: 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 65~90℃ and the pickling speed is 50~180m / min.

4. The method for preparing a hot-stamped component resistant to delayed cracking according to claim 3, characterized in that, The rolling-annealing process includes: Cold rolling process: The pickled plate is rolled in multiple passes to obtain a cold-rolled plate with a thickness of 0.5~3.0mm, wherein the cold rolling reduction rate is 35%~70%, and the cold rolling reduction rate gradually decreases as the thickness of the cold-rolled plate increases.

5. The method for preparing a hot-stamped component resistant to delayed cracking according to claim 4, characterized in that, The rolling-annealing process includes: Continuous annealing: The cold-rolled sheet is heated to A C1 Keep the temperature above the set temperature, then cool it slowly at a rate of 1-5℃ / s to 600-670℃, followed by rapid cooling at a rate of 5-15℃ / s to 360-400℃, and finally cool it to below 180℃.

6. The method for preparing a hot-stamped component resistant to delayed cracking according to claim 5, characterized in that, During the continuous annealing process, the cold-rolled sheet is heated to A. C1 Temperatures above and heat preservation include: The first stage of heating includes a preheating stage, which uses the recovered heat from the heated waste gas to heat the cold-rolled sheet. The temperature of the cold-rolled sheet at the outlet of the preheating stage is 120~180℃. The second stage of heating includes a heating stage in which the cold-rolled sheet is heated to 680~710℃ using a radiant tube. The third stage of heating includes the second and third stages of heating, in which the cold-rolled plate is heated to 730~850℃ using a radiant tube and then held at that temperature for 1~8 minutes. During the heating phase, the unit speed is controlled at 60~130m / min.

7. The method for preparing a hot-stamped component resistant to delayed cracking according to claim 6, characterized in that, During the continuous annealing process, the unit speed is controlled at 80~140m / min during the cooling stage. For every 0.4mm increase in the thickness of the cold-rolled sheet, the unit speed is reduced by 30m / min.

8. The method for preparing a hot-stamped component resistant to delayed cracking according to claim 6, characterized in that, During the continuous annealing process, the flattening elongation ranges from 0.35% to 0.65%, wherein the flattening elongation decreases by 0.08% for every 0.4mm increase in the thickness of the cold-rolled sheet.

9. A hot-stamped component resistant to delayed cracking, characterized in that, Prepared by the method of any one of claims 1-8, the hot-stamped component has a self-tempered martensite volume fraction higher than 50%, a retained austenite volume fraction ≤6%, and metastable carbides with a size of 1~50 nm, wherein the metastable carbides include at least one of ε-carbides and η-carbides.

10. A hot-formed steel sheet resistant to delayed cracking, characterized in that, The hot-formed steel sheet is selected from the hot-rolled sheet, pickled sheet, cold-rolled sheet, and continuously annealed sheet prepared by the method of any one of claims 1-8, wherein the hot-formed steel sheet is composed of the following chemical composition 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%, 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.

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