A hydrogen embrittlement resistant automotive steel and method of manufacturing the same

Abstract

The present application belongs to the technical field of automobile steel, and particularly relates to a hydrogen embrittlement resistant automobile steel and a manufacturing method thereof. The composition comprises C: 0.18%-0.25%, Si: 0.15%-0.35%, Mn: 0.80%-1.50%, Mo: 0.5%-0.8%, Cr: 0.3%-0.6%, Ti: 0.01%-0.03%, Als: 0.02%-0.06%, the mass percentage of Mo and Cr satisfies 0.9%<=Mo+Cr<=1.4%, and Mo / Cr=1-2, and the balance is Fe and inevitable impurities; the automobile steel with both ultrahigh strength and excellent hydrogen embrittlement resistance is obtained through alloy composition design and process control.

Description

Technical Field

[0001] This invention belongs to the field of automotive steel technology, specifically relating to a hydrogen embrittlement resistant automotive steel and its manufacturing method. Background Technology

[0002] With the automotive industry's increasing demands for lightweighting and safety, high-strength steel is being used more and more extensively in vehicle body structures and safety components. However, as the strength level of steel increases (e.g., tensile strength greater than 1000 MPa), its susceptibility to hydrogen embrittlement also increases significantly. Hydrogen embrittlement refers to the brittle fracture phenomenon that occurs in metallic materials under the combined action of stress and hydrogen, seriously threatening the service safety of components.

[0003] Currently, common methods for improving the hydrogen embrittlement resistance of steel include: 1) adding alloying elements such as Nb, V, Mo, Cu, and Ni to increase matrix traps; and 2) using heat treatment to control the microstructure. However, these methods often have limitations: simply adding alloying elements is costly, has limited effect on strength improvement, and increases the difficulty of metallurgical production; conventional heat treatment processes are not precise enough in controlling hydrogen traps, making it difficult to achieve a balance between high strength and hydrogen embrittlement resistance. Therefore, how to enable steel to achieve ultra-high strength while possessing excellent hydrogen embrittlement resistance, which is conducive to large-scale stable production, is a problem that needs to be overcome in this field. Summary of the Invention

[0004] To overcome the shortcomings of the prior art, the present invention provides a hydrogen embrittlement resistant automotive steel and its manufacturing method. The present invention provides an automotive steel that has both ultra-high strength and excellent hydrogen embrittlement resistance. The alloy composition is simple, easy to carry out metallurgical production, and suitable for large-scale industrial stable production.

[0005] This invention is specifically achieved through the following technical solutions: The first objective of this invention is to provide a hydrogen-resistant automotive steel, comprising the following components by weight percentage: C: 0.18%~0.25%, Si: 0.15%~0.35%, Mn: 0.80%~1.50%, Mo: 0.5%~0.8%, Cr: 0.3%~0.6%, Ti: 0.01%~0.03%, Als: 0.02%~0.06%, P≤0.012%, S≤0.003%, N≤0.005%, O≤0.002%, balance being Fe and unavoidable impurities, totaling 100%; The mass percentages of Mo and Cr satisfy the following conditions: 0.9% ≤ Mo + Cr ≤ 1.4%, and Mo / Cr = 1~2.

[0006] The microstructure of hydrogen embrittlement resistant automotive steel consists of stabilized retained austenite and nanoscale carbides distributed within a martensitic matrix.

[0007] The second objective of this invention is to provide a method for manufacturing hydrogen-resistant automotive steel, comprising the following steps: Step 1: According to the mass ratio of the above elements, smelt and refine each component and cast it into a slab.

[0008] Step 2: Heat the slab to allow all elements to be fully dissolved.

[0009] Step 3: The heated slab is rolled in two stages to obtain a steel plate of the target thickness.

[0010] Step 4: Cool the steel plate to the bainitic transformation zone to generate bainitic structure; obtain partial bainitic structure, refine the grains of the final structure, and prepare a specific initial structure for subsequent processes.

[0011] Step 5: Reheat the cooled steel plate to between Ac1 and Ac3, where the bainite melts. Then quench it to room temperature to retain some austenite clusters in the microstructure and form martensite, thus obtaining a mixed microstructure of martensite and carbon-rich undissolved austenite, which lays the foundation for the formation of stable austenite in the subsequent tempering.

[0012] Step 6: Perform the first stage of tempering on the quenched steel plate to precipitate nano-sized carbides. The precipitated nano-sized carbides with high density and strong binding energy form hydrogen traps. They act like "anchors" to firmly lock hydrogen atoms and prevent them from diffusing.

[0013] Step 7: Then, a second stage of tempering is carried out to stabilize the austenite clusters. The stable residual austenite phase can act as a benign hydrogen trap, effectively capturing diffusible hydrogen in martensitic high-strength steel.

[0014] Preferably, the heat treatment refers to a heating temperature of 1200℃~1250℃ and a holding time of 2h~3h to ensure that all elements are dissolved into austenite.

[0015] Preferably, in the two-stage rolling process, the first stage is rough rolling, carried out in the recrystallization zone, with an initial rolling temperature ≥1100℃ and a cumulative reduction rate of 50%~60%, rolling the slab to an intermediate thickness; the second stage is finish rolling, carried out in the non-recrystallization zone, with an initial rolling temperature ≤980℃ and a final rolling temperature of 850~900℃, ultimately rolling into a hot-rolled steel plate of the target thickness (2.0mm~4.0mm). Rolling in the non-recrystallization zone accumulates deformation energy, providing numerous nucleation sites for subsequent phase transformation and refining the final microstructure.

[0016] Preferably, during cooling, the cooling rate is 25℃ / s~30℃ / s, the temperature of the bainitic transformation zone is 300℃~350℃, and the holding time at this temperature is 5min~15min. It should be further noted that in conventional methods, to obtain martensite and thus improve the strength of the steel, it is generally obtained directly by high-temperature holding followed by quenching. However, this invention aims not only to improve the strength of the steel but also to simultaneously improve its resistance to hydrogen embrittlement. Therefore, in the process of obtaining martensite, its formation process has been improved: not only is martensite obtained, but also preconditions for better hydrogen trap formation are considered during the martensite formation process. Specifically, the hydrogen traps of this invention include two types: nanoscale carbides (Type A) and stabilized retained austenite (Type B). Before forming martensite, this invention pre-prepares a bainitic structure, the formation of which is a "carbon removal" process: when austenite transforms into bainitic ferrite, carbon is removed and enriched in the surrounding untransformed austenite. These highly carbon-enriched austenite regions remain stable even after subsequent quenching to room temperature, becoming retained austenite. In contrast, the martensite obtained by direct quenching has a relatively uniform carbon distribution and lacks these natural, highly carbon-rich austenite regions, resulting in insufficient stable retained austenite in the final structure and reducing the hydrogen embrittlement resistance of the Type B hydrogen traps. Furthermore, the fine bainitic ferrite laths, during their formation, divide the original austenite grains into smaller regions. These fine bainite and carbon-rich austenite regions transform into even finer, more dispersed new austenite grains during subsequent critical region heating. After quenching, these fine new austenite grains result in extremely fine martensite lath bundles. This fine-grained structure not only has higher strength but also better toughness and hinders hydrogen diffusion. Therefore, this invention, by pre-obtaining a bainitic structure during the martensite preparation process, not only facilitates the formation of hydrogen traps and improves the steel's resistance to hydrogen embrittlement but also promotes better martensite formation, thus contributing to increased steel strength.

[0017] Preferably, the temperature of Ac1 is 710℃~730℃, the temperature of Ac3 is 850℃~870℃, and the holding time is 3min~10min.

[0018] Preferably, the temperature of the first stage tempering is 320℃~380℃, and the holding time is 30min~90min.

[0019] Preferably, the temperature for the second-stage tempering is 220℃~280℃, and the holding time is 1.5h~4h.

[0020] Compared with the prior art, the present invention has the following beneficial effects: This invention, through alloy composition design, microstructure, and process control, forms stable retained austenite and nanoscale carbide strong hydrogen traps in a martensitic matrix, thereby capturing and fixing hydrogen atoms and preventing their diffusion to stress concentration areas, fundamentally inhibiting hydrogen embrittlement. Specifically:

[0021] This invention improves the alloy composition by adding Mo, Cr, and Ti elements to the traditional C, Si, and Mn elements in steel. While Mn is a commonly used element to improve hardenability, high Mn content in high-strength steel can promote segregation, which is detrimental to hydrogen embrittlement. This invention reduces the Mn content and adds Mo and Cr elements to the alloy. By controlling the Cr and Mo content, hardenability is ensured, guaranteeing the formation of a uniform and fine martensite structure during quenching and cooling. During the first stage of tempering, Mo and Cr promote the precipitation of fine nanoscale carbides formed by Ti and C. Simultaneously, Mo and Cr also generate corresponding carbides. These carbides act as "hydrogen traps," effectively capturing hydrogen atoms and preventing hydrogen diffusion to grain boundaries or stress concentration areas, thereby reducing hydrogen embrittlement sensitivity. To achieve the above-mentioned control effect, the Mo + Cr content needs to be controlled to 0.9% ≤ Mo + Cr ≤ 1.4%. When the total Mo + Cr content is low, hardenability is insufficient, which may lead to uneven microstructure and reduced strength; at the same time, insufficient carbide hydrogen trap density will significantly reduce resistance to hydrogen embrittlement. When the total Mo + Cr content is too high, excess Cr will promote the formation of coarse carbides at high temperatures, and these coarse carbides themselves may become hydrogen embrittlement crack initiation sites. Besides promoting hardenability, Mo can significantly suppress first-stage temper embrittlement. This allows the present invention to confidently and effectively implement the crucial first-stage medium-temperature tempering without worrying about toughness degradation. Therefore, when adding both Mo and Cr, a preference is given to adding more Mo. When Mo / Cr = 1~2, hardenability, the formation of uniform and fine martensite, the precipitation of fine nanoscale carbides, and the suppression of first-stage temper embrittlement can be ensured simultaneously. When Mo / Cr < 1, the Mo content is low, leading to first-stage temper embrittlement and decreased strength. To maintain the total Mo + Cr content within the range of 0.9%~1.4%, excess Cr is added, which again leads to the formation of coarse carbides and a decrease in hydrogen embrittlement resistance. When Mo / Cr > 2, Mo is an expensive element, and excessive addition will significantly increase costs. Therefore, in alloy composition design, based on the above mechanism and a large amount of experimental data, this invention concludes that by controlling the mass percentage of Mo and Cr to meet the following conditions: 0.9% ≤ Mo + Cr ≤ 1.4%, and Mo / Cr = 1~2, the strength and resistance to hydrogen embrittlement of steel can be improved.

[0022] From a microstructure perspective, the hydrogen embrittlement-resistant automotive steel exhibits a microstructure consisting of stabilized retained austenite and nanoscale carbides within a martensitic matrix. The nanoscale carbides precipitate through the synergistic effect of specific Mo and Cr contents, acting as "anchors" to firmly lock hydrogen atoms in place and prevent their diffusion. The stabilized retained austenite possesses a high irreversible hydrogen binding energy, enabling it to absorb large amounts of hydrogen and further prevent its diffusion. This microstructure allows the steel to possess both ultra-high strength and excellent resistance to hydrogen embrittlement.

[0023] In terms of manufacturing process, this invention first cools the rolled steel plate to the bainitic transformation zone to generate a bainitic structure. Then, through a "critical zone quenching + segmented partitioning tempering" process, the original bainitic structure is transformed into martensite, improving the strength of the steel. The two types of hydrogen traps mentioned above are prepared within the martensite structure. The secondary heating temperature is precisely controlled between (Ac1~Ac3), rather than completely above Ac3, aiming to intentionally retain a small amount of carbon-rich austenite clusters, laying the microstructure foundation for the subsequent formation of stable retained austenite. The first-stage tempering promotes the precipitation of nanoscale carbides, while the second-stage tempering stabilizes these austenite clusters, releases residual stress, and further adjusts the distribution of hydrogen traps.

[0024] The alloy composition of this invention is simple and readily available, and its manufacturing process is simple and easy to implement. The resulting steel has ultra-high strength and excellent resistance to hydrogen embrittlement. Attached Figure Description

[0025] Figure 1 This is a microstructure diagram of the steel prepared in Example 3 of the present invention. Detailed Implementation

[0026] To enable those skilled in the art to better understand and implement the technical solutions of the present invention, the present invention will be further described below with reference to specific embodiments and accompanying drawings. However, the embodiments described are not intended to limit the present invention. Unless otherwise specified, the experimental methods and detection methods described in the following embodiments are conventional methods; unless otherwise specified, the reagents and materials described are commercially available.

[0027] Example 1 A hydrogen-resistant automotive steel, comprising the following components by weight percentage: C: 0.2%, Si: 0.3%, Mn: 1.5%, Mo: 0.6%, Cr: 0.3%, Ti: 0.01%, Als: 0.02%, P≤0.012%, S≤0.003%, N≤0.005%, O≤0.002%, with the balance being Fe and unavoidable impurities; wherein the mass percentages of Mo and Cr satisfy: Mo + Cr = 0.9%, Mo / Cr = 2. The microstructure of the hydrogen embrittlement resistant automotive steel is: stabilized retained austenite and nanoscale carbides distributed in a martensitic matrix.

[0028] The manufacturing method of the above-mentioned hydrogen embrittlement resistant automotive steel includes the following steps: (1) According to the mass ratio of the above elements, each component is smelted, refined, and cast into slabs. The specific steps are as follows: the primary smelting is carried out in an electric arc furnace (EAF), and the tapping temperature of the primary smelting should be controlled at 1600℃. Then, ladle refining (LF furnace) is carried out. During the refining process, the temperature of the molten steel is controlled at 1550℃. Alloying elements are added according to the target composition for fine adjustment, and white slag operation is performed to maintain a reducing atmosphere to ensure alloy yield and reduce oxide inclusions. In order to minimize the gas content (especially hydrogen and oxygen) in the molten steel, the refined molten steel needs to be subjected to vacuum circulation degassing (RH) treatment. The temperature at the beginning of vacuum degassing is maintained at 1520℃, the vacuum degree is ≤0.5 Torr, and the treatment time is 20 minutes. The molten steel after vacuum treatment is continuously cast, the superheat of the molten steel is controlled at 35℃, electromagnetic stirring is used in the crystallizer, and secondary cooling is carried out in a weak cooling mode to obtain slabs.

[0029] (2) Heat the slab to 1200℃ and hold it for 2 hours to allow all elements to be fully dissolved.

[0030] (3) The heated slab is rolled in two stages. The first stage rolling temperature is 1100℃ and the cumulative reduction rate is 50%. The second stage rolling temperature is 980℃ and the final rolling temperature is 850℃ to obtain a steel plate with a thickness of 2.0mm.

[0031] (4) Cool the steel plate to the 300℃ bainitic transformation zone at a cooling rate of 25℃ / s and hold for 10 min to generate bainitic structure.

[0032] (5) The cooled steel plate is reheated to 800°C and held for 5 minutes. Then it is quenched to room temperature at a cooling rate of 30°C / s to retain some austenite clusters in the microstructure and form martensite.

[0033] (6) The quenched steel plate is subjected to a first-stage tempering at a temperature of 350℃ for 60 minutes. Then, a second-stage tempering is performed at a temperature of 250℃ for 2 hours to obtain hydrogen embrittlement resistant automotive steel.

[0034] Example 2 A hydrogen-resistant automotive steel, comprising the following components by weight percentage: C: 0.18%, Si: 0.35%, Mn: 1.2%, Mo: 0.8%, Cr: 0.4%, Ti: 0.01%, Als: 0.06%, P≤0.012%, S≤0.003%, N≤0.005%, O≤0.002%, with the balance being Fe and unavoidable impurities; wherein the mass percentages of Mo and Cr satisfy: Mo + Cr = 1.2%, Mo / Cr = 2. The microstructure of the hydrogen embrittlement resistant automotive steel is: stabilized retained austenite and nanoscale carbides distributed in a martensitic matrix.

[0035] The manufacturing method of the above-mentioned hydrogen embrittlement resistant automotive steel includes the following steps: (1) According to the mass ratio of the above elements, each component is smelted, refined, and cast into slabs. The specific steps are as follows: the primary smelting is carried out in an electric arc furnace (EAF), and the tapping temperature of the primary smelting should be controlled at 1580℃. Then, ladle refining (LF furnace) is carried out. During the refining process, the temperature of the molten steel is controlled at 1550℃. Alloying elements are added according to the target composition for fine adjustment, and white slag operation is performed to maintain a reducing atmosphere to ensure alloy yield and reduce oxide inclusions. In order to minimize the gas content (especially hydrogen and oxygen) in the molten steel, the refined molten steel needs to be subjected to vacuum circulation degassing (RH) treatment. The temperature at the beginning of vacuum degassing is maintained at 1520℃, the vacuum degree is ≤0.5 Torr, and the treatment time is 20 minutes. The molten steel after vacuum treatment is continuously cast, the superheat of the molten steel is controlled at 35℃, electromagnetic stirring is used in the crystallizer, and secondary cooling is carried out in a weak cooling mode to obtain slabs.

[0036] (2) Heat the slab to 1250℃ and hold it for 3 hours to allow all elements to be fully dissolved.

[0037] (3) The heated slab is rolled in two stages. The first stage rolling temperature is 1200℃ and the cumulative reduction rate is 50%. The second stage rolling temperature is 980℃ and the final rolling temperature is 900℃ to obtain a 2mm steel plate.

[0038] (4) Cool the steel plate to the 350℃ bainitic transformation zone at a cooling rate of 30℃ / s and hold for 5 minutes to generate bainitic structure.

[0039] (5) Reheat the cooled steel plate to 850°C, hold for 10 minutes, and then quench it to room temperature at a cooling rate of 30°C / s.

[0040] (6) The quenched steel plate is subjected to a first-stage tempering at a temperature of 320℃ for 90 minutes. Then, a second-stage tempering is performed at a temperature of ~280℃ for 4 hours to obtain hydrogen embrittlement resistant automotive steel.

[0041] Example 3 A hydrogen-resistant automotive steel, comprising the following components by weight percentage: C: 0.25%, Si: 0.15%, Mn: 0.8%, Mo: 0.7%, Cr: 0.35%, Ti: 0.03%, Als: 0.06%, P≤0.012%, S≤0.003%, N≤0.005%, O≤0.002%, with the balance being Fe and unavoidable impurities; wherein, the mass percentages of Mo and Cr satisfy: Mo + Cr = 1.05%, Mo / Cr = 2. The microstructure of the hydrogen embrittlement resistant automotive steel is: stabilized retained austenite and nanoscale carbides distributed in a bainitic matrix. For example... Figure 1 As shown.

[0042] The manufacturing method of the above-mentioned hydrogen embrittlement resistant automotive steel includes the following steps: (1) According to the mass ratio of the above elements, each component is smelted, refined, and cast into slabs. The specific steps are as follows: the primary smelting is carried out in an electric arc furnace (EAF), and the tapping temperature of the primary smelting should be controlled at 1600℃. Then, ladle refining (LF furnace) is carried out. During the refining process, the temperature of the molten steel is controlled at 1550℃. Alloying elements are added according to the target composition for fine adjustment, and white slag operation is performed to maintain a reducing atmosphere to ensure alloy yield and reduce oxide inclusions. In order to minimize the gas content (especially hydrogen and oxygen) in the molten steel, the refined molten steel needs to be subjected to vacuum circulation degassing (RH) treatment. The temperature at the beginning of vacuum degassing is maintained at 1520℃, the vacuum degree is ≤0.5 Torr, and the treatment time is 20 minutes. The molten steel after vacuum treatment is continuously cast, the superheat of the molten steel is controlled at 35℃, electromagnetic stirring is used in the crystallizer, and secondary cooling is carried out in a weak cooling mode to obtain slabs.

[0043] (2) Heat the slab to 1200℃ and hold it for 2 hours to allow all elements to be fully dissolved.

[0044] (3) The heated slab is rolled in two stages. The first stage rolling temperature is 1100℃ and the cumulative reduction rate is 60%. The second stage rolling temperature is 980℃ and the final rolling temperature is 8500℃ to obtain a 2mm steel plate.

[0045] (4) Cool the steel plate to the 340℃ bainitic transformation zone at a cooling rate of 30℃ / s and hold for 5 minutes to generate bainitic structure.

[0046] (5) Reheat the cooled steel plate to 800°C and hold for 5 minutes, then quench it to room temperature at a cooling rate of 30°C / s.

[0047] (6) The quenched steel plate is subjected to a first-stage tempering at a temperature of 360℃ for 60 minutes. Then, a second-stage tempering is performed at a temperature of 250℃ for 3 hours to obtain hydrogen-resistant automotive steel.

[0048] Example 4 A hydrogen-resistant automotive steel, comprising the following components by weight percentage: C: 0.25%, Si: 0.2%, Mn: 0.9%, Mo: 0.5%, Cr: 0.5%, Ti: 0.03%, Als: 0.06%, P≤0.012%, S≤0.003%, N≤0.005%, O≤0.002%, with the balance being Fe and unavoidable impurities; wherein the mass percentages of Mo and Cr satisfy: Mo + Cr = 1.0%, Mo / Cr = 1. The microstructure of the hydrogen embrittlement resistant automotive steel is: stabilized retained austenite and nanoscale carbides distributed in a martensitic matrix.

[0049] The manufacturing method of the above-mentioned hydrogen embrittlement resistant automotive steel includes the following steps: (1) According to the mass ratio of the above elements, each component is smelted, refined, and cast into slabs. The specific steps are as follows: the primary smelting is carried out in an electric arc furnace (EAF), and the tapping temperature of the primary smelting should be controlled at 1600℃. Then, ladle refining (LF furnace) is carried out. During the refining process, the temperature of the molten steel is controlled at 1550℃. Alloying elements are added according to the target composition for fine adjustment, and white slag operation is performed to maintain a reducing atmosphere to ensure alloy yield and reduce oxide inclusions. In order to minimize the gas content (especially hydrogen and oxygen) in the molten steel, the refined molten steel needs to be subjected to vacuum circulation degassing (RH) treatment. The temperature at the beginning of vacuum degassing is maintained at 1520℃, the vacuum degree is ≤0.5 Torr, and the treatment time is 20 minutes. The molten steel after vacuum treatment is continuously cast, the superheat of the molten steel is controlled at 35℃, electromagnetic stirring is used in the crystallizer, and secondary cooling is carried out in a weak cooling mode to obtain slabs.

[0050] (2) Heat the slab to 1200℃ and hold it for 2 hours to allow all elements to be fully dissolved.

[0051] (3) The heated slab is rolled in two stages. The first stage rolling temperature is 1100℃ and the cumulative reduction rate is 50%. The second stage rolling temperature is 980℃ and the final rolling temperature is 850℃ to obtain a steel plate with a thickness of 2mm.

[0052] (4) Cool the steel plate to the 300℃ bainitic transformation zone at a cooling rate of 30℃ / s and hold for 15 minutes to generate bainitic structure.

[0053] (5) Reheat the cooled steel plate to between 830°C and hold for 10 minutes, then quench it to room temperature at a cooling rate of 30°C / s.

[0054] (6) The quenched steel plate is subjected to a first-stage tempering at a temperature of 320℃ for 50 minutes. Then, a second-stage tempering is performed at a temperature of 260℃ for 3 hours to obtain hydrogen-resistant automotive steel.

[0055] The steel plates obtained in the above embodiments were subjected to performance tests. Tensile strength was tested according to GB / T 228.1-2021 standard. For the evaluation of its hydrogen-induced delayed fracture susceptibility, the hydrogen-induced delayed fracture threshold stress (σ) was used. th As an evaluation parameter, the test method is as follows: A constant load delayed fracture test is used. Multiple samples are polished and then placed in an HCl solution for electrochemical hydrogen charging at a current density of 1 mA / cm². 2 The test lasted 24 hours. Multiple hydrogen-charged specimens were subjected to constant loads in an indoor atmospheric environment. For a group of five pre-charged specimens with identical geometry, different levels of constant tensile loads were applied. The time taken for each specimen to fracture from loading was recorded (fracture time). If a specimen did not fracture within 200 hours, it was considered "passed." If a specimen fractured within 200 hours, the stress level of the next specimen was lowered by one step; if it did not fracture within 200 hours, the stress level of the next specimen was increased by one step. This test was repeated for multiple specimens until a clear stress boundary between "fracture" and "non-fracture" was observed. σ th The value is the arithmetic mean of the lowest stress of all unbroken specimens and the highest stress of all broken specimens. The critical stress value σ is used as the reference. th With tensile strength R m The ratio (σ) th / R m The hydrogen embrittlement resistance was evaluated, and the higher the ratio, the better the performance. The results are shown in Table 1.

[0056] Table 1. Steel plate performance data obtained from the above embodiments As shown in Table 1, the steel plates prepared in Examples 1 to 4 of this invention have tensile strengths of 1320-1380 MPa and relatively high hydrogen-induced delayed fracture threshold stress ratios, indicating that the steel obtained by this invention possesses ultra-high strength and excellent resistance to hydrogen embrittlement. To further illustrate the performance advantages of the steel plates of this invention, the following comparative examples are also provided.

[0057] Comparative Example 1 Compared with Example 3, the specific component composition is as follows: C: 0.18%, Si: 0.35%, Mn: 1.2%, Mo: 0.2%, Cr: 0.4%, Ti: 0.01%, Als: 0.06%, P≤0.012%, S≤0.003%, N≤0.005%, O≤0.002%, with the balance being Fe and unavoidable impurities; wherein the mass percentages of Mo and Cr satisfy: Mo + Cr = 0.6%, Mo / Cr = 0.5.

[0058] The preparation method is the same as in Example 3.

[0059] Comparative Example 2 Compared with Example 3, the specific component composition is as follows: C: 0.18%, Si: 0.35%, Mn: 1.2%, Mo: 0.3%, Cr: 0.6%, Ti: 0.01%, Als: 0.06%, P≤0.012%, S≤0.003%, N≤0.005%, O≤0.002%, with the balance being Fe and unavoidable impurities; wherein the mass percentages of Mo and Cr satisfy: Mo + Cr = 0.9%, Mo / Cr = 0.5.

[0060] The preparation method is the same as in Example 3.

[0061] Comparative Example 3 Compared with Example 3, the specific component composition is as follows: C: 0.18%, Si: 0.35%, Mn: 1.2%, Mo: 0.6%, Cr: 1.2%, Ti: 0.01%, Als: 0.06%, P≤0.012%, S≤0.003%, N≤0.005%, O≤0.002%, with the balance being Fe and unavoidable impurities; wherein the mass percentages of Mo and Cr satisfy: Mo + Cr = 1.8%, Mo / Cr = 0.5.

[0062] The preparation method is the same as in Example 3.

[0063] Comparative Example 4 Compared with Example 3, the specific component composition is as follows: C: 0.18%, Si: 0.35%, Mn: 1.2%, Mo: 0.9%, Cr: 0.3%, Ti: 0.01%, Als: 0.06%, P≤0.012%, S≤0.003%, N≤0.005%, O≤0.002%, with the balance being Fe and unavoidable impurities; wherein the mass percentages of Mo and Cr satisfy: Mo + Cr = 1.2%, Mo / Cr = 3.

[0064] The preparation method is the same as in Example 3.

[0065] Comparative Example 5 Compared with Example 3, the specific component composition is as follows: C: 0.18%, Si: 0.35%, Mn: 1.2%, Ti: 0.01%, Als: 0.06%, P≤0.012%, S≤0.003%, N≤0.005%, O≤0.002%, balance Fe and unavoidable impurities.

[0066] Comparative Example 6 Compared with Example 3, in step (5), the cooled steel plate is reheated to 900°C.

[0067] Comparative Example 7 Compared with Example 3, a one-step tempering process was adopted, with a tempering temperature of 250°C and a holding time of 3 hours.

[0068] Comparative Example 8 Compared with Example 3, the difference is that the steel plate obtained in step (3) is directly quenched to room temperature at a cooling rate of 30℃ / s, and then subjected to first-stage tempering and second-stage tempering. The temperature of the first-stage tempering is 360℃ and the holding time is 60min. The temperature of the second-stage tempering is 250℃ and the holding time is 3h.

[0069] The steel plates obtained in the above comparative examples were subjected to performance tests. Tensile strength: tested according to GB / T 228.1-2021 standard, using the above constant load delayed fracture test method, hydrogen was charged in 0.1 mol / L HCl solution, and the ratio of critical stress value σth to tensile strength Rm (σth / Rm) was used to evaluate the hydrogen embrittlement resistance. The higher the ratio, the better the performance. The data of the steel plate in Example 3 were used for comparison, and the results are shown in Table 2.

[0070] Table 2. Steel plate performance data obtained from the above embodiments and comparative examples. As shown in Table 2, in Comparative Example 1, compared to Example 3, the Mo + Cr = 0.6% and Mo / Cr = 0.5, with low additions of Mo and Cr elements, resulted in insufficient hardenability, leading to uneven formation of martensite structure, reduced strength, and hindering the precipitation of fine nanoscale carbides. This also resulted in low hydrogen trap density and decreased resistance to hydrogen embrittlement. In Comparative Example 2, the total amount of Mo and Cr elements was increased, improving hardenability compared to Comparative Example 1. However, due to the low Mo ratio of Mo / Cr = 0.5, the first-stage tempering embrittlement occurred, preventing successful implementation of the first stage, reducing strength, and affecting the precipitation of nanoscale carbides, thus lowering resistance to hydrogen embrittlement. In Comparative Example 3, the total amount of Mo and Cr was further increased, resulting in Mo + Cr = 1.8% and Mo / Cr = 0.5. However, excessive Cr content and a high total Mo + Cr content can cause excessive Cr to promote the formation of coarse carbides at high temperatures. These coarse carbides themselves may become hydrogen embrittlement crack initiation sites, affecting strength and resistance to hydrogen embrittlement. In Comparative Example 4, Mo + Cr = 1.2%, within a reasonable range, slightly improved strength compared to Comparative Example 3, with Mo / Cr = 3. The higher Mo content has a smaller impact on strength and resistance to hydrogen embrittlement. Furthermore, considering cost, this dosage range increases costs; therefore, a maximum Mo / Cr of 2 is sufficient, achieving excellent performance while controlling costs. In Comparative Example 5, no Mo or Cr was added. Because the Mn content used in this invention is low, its hardenability is poor, resulting in decreased strength and hindering the precipitation of nano-sized carbides, reducing hydrogen trap density and decreasing resistance to hydrogen embrittlement. Compared to Example 3, in Comparative Example 6, step (5) involved reheating the cooled steel plate to 900°C. This excessively high temperature resulted in insufficient residual austenite clusters and decreased resistance to hydrogen embrittlement. In Comparative Example 7, compared to Example 3, a one-step tempering process was used, resulting in no nanoscale carbide precipitation and decreased resistance to hydrogen embrittlement. In Comparative Example 8, compared to Example 3, the steel plate obtained in step (3) was directly quenched to room temperature at a cooling rate of 30°C / s, i.e., martensite was obtained using a conventional method without pre-obtaining bainite. This resulted in decreased strength and reduced resistance to hydrogen embrittlement.

[0071] Obviously, those skilled in the art can make various modifications and variations to this invention without departing from its spirit and scope. Therefore, it is intended to include any modifications and variations that fall within the scope of the claims and their equivalents.

Claims

1. A method for manufacturing hydrogen-embrittlement-resistant automotive steel, characterized in that, Includes the following steps: Hydrogen-embrittlement-resistant automotive steel is composed of the following components by weight percentage: C: 0.18%~0.25%, Si: 0.15%~0.35%, Mn: 0.80%~1.50%, Mo: 0.5%~0.8%, Cr: 0.3%~0.6%, Ti: 0.01%~0.03%, Als: 0.02%~0.06%, P≤0.012%, S≤0.003%, N≤0.005%, O≤0.002%, balance being Fe and unavoidable impurities, totaling 100%; The mass percentages of Mo and Cr satisfy the following conditions: 0.9% ≤ Mo + Cr ≤ 1.4%, and Mo / Cr = 1~2; According to the mass ratio of each element, the components are smelted, refined, and cast into slabs. The slab is heated to ensure that all elements are fully dissolved. The heated slab is rolled in two stages to obtain a steel plate of the target thickness. The steel plate is cooled to the bainitic transformation region to generate a bainitic structure; The cooled steel plate is reheated to between Ac1 and Ac3, the bainite melts, and the microstructure undergoes partial austenitization. Then it is quenched to room temperature to form a martensitic matrix and dispersed austenitic clusters. The quenched steel plate is subjected to a first-stage tempering to precipitate nano-scale carbides; then a second-stage tempering is performed to stabilize the austenite clusters and obtain hydrogen embrittlement resistant automotive steel. During cooling, the cooling rate is 25℃ / s~30℃ / s, the temperature of the bainitic phase transformation region is 300℃~350℃, and the holding time is 5min~15min; The temperature for the first stage of tempering is 320℃~380℃, and the holding time is 30min~90min; The second stage of tempering is at a temperature of 220℃~280℃, and the holding time is 1.5h~4h.

2. The manufacturing method according to claim 1, characterized in that, The microstructure of hydrogen embrittlement resistant automotive steel consists of stabilized retained austenite and nanoscale carbides distributed within a martensitic matrix.

3. The manufacturing method according to claim 1, characterized in that, Heat treatment refers to heating at a temperature of 1200℃~1250℃ and holding for 2h~3h.

4. The manufacturing method according to claim 1, characterized in that, In two-stage rolling, the first stage opening rolling temperature is ≥1100℃, and the cumulative reduction rate is 50%~60%; the second stage opening rolling temperature is ≤980℃, and the final rolling temperature is 850℃~900℃.

5. The manufacturing method according to claim 1, characterized in that, The Ac1 temperature is 710℃~730℃, the Ac3 temperature is 850℃~870℃, and the holding time is 3min~10min; the quenching cooling rate is 25℃ / s~30℃ / s.

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