1700 mpa grade hot-formed steel having excellent cold bending performance, preparation method therefor and use thereof

Abstract

Provided in the present disclosure are a 1700 MPa grade hot-formed steel having excellent cold bending performance, a preparation method therefor, and the use thereof. A surface layer of the hot-formed steel has a soft-phase ferrite structure, a sub-surface layer has a mixed structure of soft-phase ferrite and martensite, and a core layer has a low-carbon martensite structure. The preparation method comprises any one or a combination of at least two of the following conditions: (A) the coiling temperature after hot rolling being 650-700°C; (B) the pickling speed being ≤ 150 m / min; (C) the continuous annealing temperature being 780-830°C; (D) the furnace atmosphere being a mixed gas of nitrogen and hydrogen; and (E) the dew point temperature being -35°C to -17°C.

Description

A hot-formed steel with high cold bending performance of 1700MPa, its preparation method and application

[0001] Cross-reference to related applications

[0002] This application claims priority to Chinese Patent Application No. 202411708859.8, filed in China on November 26, 2024, the entire contents of which are incorporated herein by reference. Technical Field

[0003] This disclosure belongs to the field of alloy materials technology, and relates to a hot-formed steel, and more particularly to a hot-formed steel with high cold bending performance of 1700MPa and its preparation method and application. Background Technology

[0004] Hot stamping is a promising manufacturing process for high-strength steel parts. It significantly improves material strength through martensitic phase transformation and, compared to cold stamping, more effectively reduces springback. Currently, the strength of hot-formed steel is mainly concentrated in the range of 500-2000 MPa, with material selection following the principle of "using the right material in the right place." For example, the upper part of a B-pillar typically requires high strength to provide sufficient resistance to intrusion, while the lower part requires high toughness to absorb impact energy generated during a collision. However, in recent years, with the increasing demand for lightweighting, manufacturers have proposed using ultra-high-strength steel in energy-absorbing areas to achieve weight reduction. It is worth noting that the material in the energy-absorbing area must possess sufficiently high bending toughness to avoid premature failure during use. To address this issue, industry researchers have conducted in-depth exploration and research on composition design and process improvement.

[0005] How to provide a hot-formed steel with high cold bending performance and its preparation method, which has both high strength and high toughness, and meets the requirements of lightweighting, has become an urgent problem to be solved by those skilled in the art. Summary of the Invention

[0006] The purpose of this disclosure is to provide a hot-formed steel with high cold bending performance of 1700MPa, its preparation method and application. Through precise composition design and process integration, it achieves dual protection of high strength and high toughness, while meeting the requirements of lightweighting and improving the competitiveness of existing products.

[0007] To achieve this objective, the present disclosure adopts the following technical solution:

[0008] In a first aspect, this disclosure provides a hot-formed steel with high cold bending performance of 1700MPa, wherein the surface layer of the hot-formed steel has a soft phase ferrite structure, the subsurface layer has a mixed structure of soft phase ferrite and martensite, and the core layer has a carbon-depleted martensite structure.

[0009] The thickness of the surface layer is 0-20 μm, and the thickness of the subsurface layer is 5-20 μm.

[0010] The tensile strength of the hot-formed steel is 1650-1750 MPa.

[0011] This disclosure modifies the distribution characteristics of traditional all-martensitic structure by introducing 5-40 μm ferrite and a mixed structure of ferrite and martensite into the surface layer of hot-formed steel. By leveraging the synergistic effect between the ferrite and martensitic structures and limiting the thickness range of each layer, a hot-formed steel with high cold bending performance of 1700 MPa is obtained. While ensuring that the strength is 200 MPa higher than that of commercial 1500 MPa hot-formed steel, the bending toughness is comparable, achieving a dual guarantee of high strength and high toughness. At the same time, it meets the requirements of lightweighting and improves the competitiveness of existing products.

[0012] In some embodiments, the carbon content of the carbon-depleted martensite structure is 50%-90% of the target carbon content in the hot-formed steel.

[0013] In some embodiments, the core layer also contains ferrite tissue, and the ferrite tissue accounts for ≤5 wt% of the core layer.

[0014] In some embodiments, the chemical composition of the hot-formed steel, by weight percentage, includes:

[0015] The balance includes Fe and unavoidable impurities.

[0016] In some embodiments, by mass percentage, C+Al+Si≥1.0%, Cr+Mn≤3.0%, and (Cr+Mn) / (C+Al+Si)≤1.5%.

[0017] In some embodiments, Nb+V ≤ 0.05% by mass percentage.

[0018] In some embodiments, the welding carbon equivalent CE, by mass percentage, is C + Mn / 6 + (Cr + Mo + V) / 5 + (Ni + Cu) / 15 < 0.5%.

[0019] In some embodiments, Si+Cr ≤ 1.2% by mass percentage.

[0020] Secondly, this disclosure provides a method for preparing hot-formed steel as described in the first aspect, wherein the preparation method satisfies any one or a combination of at least two of the following conditions:

[0021] (A) The coiling temperature after hot rolling is 650-700℃;

[0022] (B) Pickling speed ≤ 150 m / min;

[0023] (C) The continuous annealing temperature is 780-830℃;

[0024] (D) The atmosphere inside the furnace is a mixture of nitrogen and hydrogen;

[0025] (E) Dew point temperature is -35°C to -17°C.

[0026] This disclosure achieves full decarburization and refined grain structure by reasonably adjusting the key preparation conditions of hot-formed steel, which further improves the toughness of the material, making it a replacement for safety parts with low strength and high toughness, thus achieving the goal of weight reduction and lightweighting.

[0027] In some embodiments, the continuous annealing time is 8-12 minutes.

[0028] Thirdly, this disclosure provides an application of hot-formed steel as described in the first aspect, said hot-formed steel being used to manufacture automotive body structural components, including seat beams, A-pillars, B-pillars, door anti-collision beams, front and rear bumpers, sill beams, or center tunnels.

[0029] This disclosure utilizes hot-formed steel, which not only maintains or improves the safety and reliability of vehicle body structural components but also significantly reduces vehicle weight and improves fuel efficiency, thereby reducing carbon emissions. This innovation will drive the automotive industry towards greater efficiency and environmental friendliness, achieving a dual breakthrough in technology and materials.

[0030] The numerical range described in this disclosure includes not only the point values ​​listed above, but also any point values ​​within the numerical ranges not listed above. Due to space limitations and for the sake of brevity, this disclosure will not exhaustively list all the specific point values ​​included in the range.

[0031] Compared with the prior art, this disclosure has the following beneficial effects:

[0032] This disclosure introduces 5-40 μm ferrite and a mixed ferrite-martensite microstructure into the surface layer of hot-formed steel by altering the distribution characteristics of the traditional all-martensite microstructure. Simultaneously, it rationally modulates the key preparation conditions for hot-formed steel, achieving thorough decarburization and grain refinement. This results in a hot-formed steel with high cold bending performance of 1700 MPa. While maintaining a strength 200 MPa higher than commercially available 1500 MPa hot-formed steel, it achieves comparable bending toughness, successfully resolving the contradiction between material strength and toughness. This provides a dual guarantee of high strength and high toughness, while also meeting lightweight requirements and enhancing the competitiveness of existing products. Attached Figure Description

[0033] Figure 1 is a micrograph of the decarburized layer of the hot-formed steel obtained in Example 1;

[0034] Figure 2 is a micrograph of the hot-formed steel obtained in Example 1;

[0035] Figure 3 is a comparison of the tensile and bending properties of the hot-formed steel obtained in Example 1 and the traditional 22MnB5 hot-formed steel. Detailed Implementation

[0036] The technical solutions of this disclosure will be further illustrated below through specific embodiments. Those skilled in the art should understand that the embodiments described are merely illustrative of this disclosure and should not be construed as specific limitations thereof.

[0037] In order to combine high strength and high toughness while meeting the requirements of lightweighting, this disclosure provides a hot-formed steel with high cold bending performance of 1700MPa. The surface layer of the hot-formed steel has a soft phase ferrite structure, the sub-surface layer has a mixed structure of soft phase ferrite and martensite, and the core layer has a carbon-depleted martensite structure.

[0038] The thickness of the surface layer is 0-20 μm, for example, it can be 0 μm, 2 μm, 4 μm, 6 μm, 8 μm, 10 μm, 12 μm, 14 μm, 16 μm, 18 μm or 20 μm. The thickness of the subsurface layer is 5-20 μm, for example, it can be 5 μm, 6 μm, 7 μm, 8 μm, 9 μm, 10 μm, 11 μm, 12 μm, 13 μm, 14 μm, 15 μm, 16 μm, 17 μm, 18 μm, 19 μm or 20 μm, but it is not limited to the listed values. Other unlisted values ​​within this range are also applicable.

[0039] The tensile strength of hot-formed steel is 1650-1750 MPa, for example, it can be 1650 MPa, 1660 MPa, 1670 MPa, 1680 MPa, 1690 MPa, 1700 MPa, 1710 MPa, 1720 MPa, 1730 MPa, 1740 MPa or 1750 MPa, but it is not limited to the listed values. Other unlisted values ​​within this range also apply.

[0040] This disclosure modifies the distribution characteristics of traditional all-martensitic structure by introducing 5-40 μm ferrite and a mixed structure of ferrite and martensite into the surface layer of hot-formed steel. By leveraging the synergistic effect between the ferrite and martensitic structures and limiting the thickness range of each layer, a hot-formed steel with high cold bending performance of 1700 MPa is obtained. While ensuring that the strength is 200 MPa higher than that of commercial 1500 MPa hot-formed steel, the bending toughness is comparable. This successfully resolves the contradiction between material strength and toughness, achieving a dual guarantee of high strength and high toughness, while also meeting lightweight requirements and improving the competitiveness of existing products.

[0041] In some embodiments, the carbon content of the carbon-depleted martensite structure is 50%-90% of the target carbon content in the hot-formed steel, for example, it can be 50%, 55%, 60%, 65%, 70%, 75%, 80%, 85% or 90%, but is not limited to the listed values, and other unlisted values ​​within this range are also applicable.

[0042] In some embodiments, the core layer also contains ferrite, and the proportion of ferrite in the core layer is ≤5wt%, for example, it can be 0.5wt%, 1wt%, 1.5wt%, 2wt%, 2.5wt%, 3wt%, 3.5wt%, 4wt%, 4.5wt%, or 5wt%, but is not limited to the listed values, other unlisted values ​​within this range are also applicable.

[0043] In some embodiments, the chemical composition of the hot-formed steel, by weight percentage, includes:

[0044] The balance includes Fe and unavoidable impurities.

[0045] The C content is 0.20-0.30%, for example, it can be 0.20%, 0.21%, 0.22%, 0.23%, 0.24%, 0.25%, 0.26%, 0.27%, 0.28%, 0.29%, or 0.30%; the Si content is 0.05-0.40%, for example, it can be 0.05%, 0.10%, 0.15%, 0.20%, 0.25%, 0.30%, 0.35%, or 0.40%; and the Mn content is 0.50-2.00%, for example, it can be 0.50%, 0.60%, 0.70%, 0.80%, 0.90%, 1.00%, 1.10%, 1.20%, 1.30%, 1.40%, or 1.50%. The content of Cr is 0.20-1.00%, for example, it can be 0.20%, 0.25%, 0.30%, 0.35%, 0.40%, 0.45%, 0.50%, 0.55%, 0.60%, 0.65%, 0.70%, 0.75%, 0.80%, 0.85%, 0.90%, 0.95%, or 1.00%, and the content of Al is 0.00-0.50%, for example, it can be 0.00%, 0.05%, 0.10%, 0.15%, 0.20%, 0.25%, 0.30%, 0.35%, 0.40%, 0.45%, or 0.50%, and the content of Nb is... The content of nitrogen (B) is 0.01-0.05%, for example, 0.01%, 0.02%, 0.03%, 0.04%, or 0.05%; the content of phosphorus (B) is 0.003-0.020%, for example, 0.003%, 0.004%, 0.006%, 0.008%, 0.010%, 0.012%, 0.014%, 0.016%, 0.018%, or 0.020%; the content of titanium (Ti) is 0.01-0.05%, for example, 0.01%, 0.02%, 0.03%, 0.04%, or 0.05%; and the content of nitrogen (Ni) is 0.00-0.30%, for example, 0.00%, 0.02%, 0.04%, 0.06%, 0.08%, or 0.10%. The content of the following components is specified: 0.12%, 0.14%, 0.16%, 0.18%, 0.20%, 0.22%, 0.24%, 0.26%, 0.28%, or 0.30%; the content of Mo is 0.00-0.05%, for example, 0.00%, 0.01%, 0.02%, 0.03%, 0.04%, or 0.05%; the content of V is 0.00-0.05%, for example, 0.00%, 0.01%, 0.02%, 0.03%, 0.04%, or 0.05%; and the content of Cu is 0.00-1.50%, for example, 0.00%, 0.02%, 0.04%, 0.06%, 0.08%, 0.10%, 0.12%, 0.14%, or 0.30%.The content of sulfur (S) is 0.00-0.01%, for example, it can be 0.00%, 0.001%, 0.002%, 0.003%, 0.004%, 0.005%, 0.006%, 0.007%, 0.008%, 0.009%, or 0.01%. The content of phosphorus (P) is 0.00-0.08%, for example, it can be 0.00%, 0.01%, 0.02%, 0.03%, 0.04%, 0.05%, 0.06%, 0.07%, or 0.08%, but is not limited to the listed values; other unlisted values ​​within this range also apply.

[0046] In this disclosure, carbon (C) is the most effective element for improving the strength of steel through interstitial solid solution strengthening. To ensure sufficient precipitation of C along with microalloying elements Nb and V, the C content cannot be too low and must be higher than 0.2%. However, excessively high C content can easily lead to the formation of brittle twinned martensite and deteriorate weldability. Therefore, the C content is determined to be 0.20-0.30%, and in some embodiments, the C content is 0.20-0.28%.

[0047] In this disclosure, Si is a solid solution strengthening element. Considering that a high Si content directly affects the production of dense iron oxide scale on the surface of steel during hot rolling and annealing, and that Si oxides are detrimental to pickling removal and the coating effect of aluminum-silicon coatings, the Si content is kept as low as possible, not exceeding 0.40%. During the smelting stage, in order to remove as much O as possible from the molten steel, the Si content is not less than 0.05%. Therefore, the Si content is determined to be 0.05-0.40%, and in some embodiments, the Si content is 0.10-0.40%.

[0048] In this disclosure, Mn has both solid solution strengthening effect and can lower the austenitizing temperature; therefore, the Mn content is at least 0.50%. However, considering that excessively high Mn content can easily lead to severe segregation bands, which is extremely detrimental to the bending angle and hydrogen embrittlement resistance in the TD direction, the Mn content is determined to be 0.50-2.00%.

[0049] In this disclosure, Cr helps improve the hardenability of steel, reduces the critical cooling rate for martensitic transformation, and ensures that the core layer microstructure of hot-formed steel is fully martensitic. Considering that the iron oxide scale formed by Cr is difficult to remove during the pickling stage, thus affecting coating quality and surface decarburization, the Cr content is determined to be 0.20-1.00%.

[0050] In this disclosure, Nb and V ensure the required strength is achieved under suitable C content conditions, while simultaneously reducing the welding carbon equivalent. The addition of microalloying elements results in fine precipitates, achieving precipitation strengthening. Furthermore, these fine precipitates effectively refine grain size, improving the steel's bending toughness and hydrogen embrittlement resistance. Moreover, compared to V-containing precipitates, Nb-containing precipitates are more favorable for delayed hydrogen embrittlement cracking; therefore, in some embodiments, the Nb content is as low as 0.01%, and V may not be added.

[0051] In this disclosure, B helps improve the hardenability of the steel, reduces the critical cooling rate at which martensite begins to form, and ensures that the final microstructure is a fully martensitic microstructure. Since adding a small amount of B can significantly improve hardenability, the B content is determined to be 0.003-0.020%.

[0052] In this disclosure, Ti helps reduce the amount of N in the steel, preventing N from combining with B to form BN, which would weaken the hardenability of B. Since the N content in the steel is not very high, the Ti content is determined to be 0.01-0.05%.

[0053] In this disclosure, Ni helps to lower the low-temperature brittle transition temperature of high-strength steel. Generally, based on the fact that the brittle transition temperature is below -100°C when the Ni content is less than 0.30%, the Ni content is determined to be 0.00-0.30%.

[0054] In this disclosure, Mo helps delay the transformation of austenite to pearlite, improving the hardenability of austenite, and also helps delay the transformation of ε-carbide to cementite, suppressing the temper brittleness of martensite. Considering that various elements with good hardenability, such as Cr and B, are added to the steel, the Mo content is determined to be 0.00-0.05%.

[0055] In this disclosure, Al, P and S are impurity elements in this alloy composition system, so it is necessary to minimize the introduction of these elements. The selection of each alloying element is as follows: Al: 0.00-0.50%; P: 0.00-0.08%; S: 0.00-0.01%.

[0056] In some embodiments, by mass percentage, C+Al+Si≥1.0%, for example, can be 1.0%, 1.2%, 1.4%, 1.6%, 1.8%, 2.0%, 2.2%, 2.4%, 2.6%, 2.8%, or 3.0%; Cr+Mn≤3.0%, for example, can be 1.0%, 1.2%, 1.4%, 1.6%, 1.8%, 2.0%, 2.2%, 2.4%, 2.6%, 2.8%, or 3.0%; and (Cr+Mn) / (C+Al+Si)≤1.5%, for example, can be 0.2%, 0.4%, 0.6%, 0.8%, 1.0%, 1.2%, 1.4%, or 1.5%, but is not limited to the listed values; other unlisted values ​​within this range also apply.

[0057] In this disclosure, considering that Cr and Mn elements will prevent steel from decarburizing, and that C, Al and Si elements will increase the tendency of steel to decarburize, the following limits are specifically set: C+Al+Si≥1.0%, Cr+Mn≤3.0%, and (Cr+Mn) / (C+Al+Si)≤1.5%.

[0058] In some embodiments, Nb+V ≤ 0.05% by mass percentage, for example, it can be 0.01%, 0.02%, 0.03%, 0.04% or 0.05%, but is not limited to the listed values, other unlisted values ​​within this range also apply.

[0059] In this disclosure, in order to ensure that the microalloying elements can be fully precipitated, the Nb+V content is specifically limited to ≤0.05% based on the selection of C content.

[0060] In some embodiments, the welding carbon equivalent CE = C + Mn / 6 + (Cr + Mo + V) / 5 + (Ni + Cu) / 15 < 0.5% by mass percentage, for example, can be 0.05%, 0.1%, 0.15%, 0.2%, 0.25%, 0.3%, 0.35%, 0.4%, or 0.45%, but is not limited to the listed values; other unlisted values ​​within this range are also applicable.

[0061] This disclosure specifically limits the welding carbon equivalent (CE) to below 0.5%, which helps to further improve welding performance.

[0062] In some embodiments, Si+Cr ≤ 1.2% by mass percentage, for example, it can be 0.1%, 0.2%, 0.3%, 0.4%, 0.5%, 0.6%, 0.7%, 0.8%, 0.9%, 1.0%, 1.1%, or 1.2%, but is not limited to the listed values; other unlisted values ​​within this range also apply.

[0063] In this disclosure, to avoid the iron oxide scale being difficult to remove during the pickling process and to ensure the coating quality, the Si+Cr content is specifically limited to ≤1.2%.

[0064] This disclosure also provides a method for preparing the above-mentioned hot-formed steel, wherein the preparation method satisfies any one or a combination of at least two of the following conditions:

[0065] (A) The coiling temperature after hot rolling is 650-700℃, for example, it can be 650℃, 660℃, 670℃, 680℃, 690℃ or 700℃, but it is not limited to the listed values. Other unlisted values ​​within this range are also applicable.

[0066] (B) Pickling speed ≤150m / min, for example, it can be 20m / min, 40m / min, 60m / min, 80m / min, 100m / min, 120m / min, 140m / min or 150m / min, but is not limited to the listed values. Other unlisted values ​​within this range are also applicable.

[0067] (C) The continuous annealing temperature is 780-830℃, for example, it can be 780℃, 790℃, 800℃, 810℃, 820℃ or 830℃, but it is not limited to the listed values. Other unlisted values ​​within this range are also applicable.

[0068] (D) The atmosphere inside the furnace is a mixture of nitrogen and hydrogen.

[0069] (E) The dew point temperature is from -35°C to -17°C, for example, it can be -35°C, -34°C, -33°C, -32°C, -31°C, -30°C, -29°C, -28°C, -27°C, -26°C, -25°C, -24°C, -23°C, -22°C, -21°C, -20°C, -19°C, -18°C or -17°C, but is not limited to the listed values. Other unlisted values ​​within this range also apply.

[0070] This disclosure specifically limits the coiling temperature after hot rolling to 650-700℃ to ensure that microalloying elements have sufficient driving force for precipitation and surface decarburization; considering that the coiling temperature is too high and the generated iron oxide scale is too thick, the pickling speed is specifically limited to below 150m / min to ensure complete removal of iron oxide scale; the special limitation of continuous annealing temperature, furnace atmosphere and dew point temperature helps to further achieve decarburization.

[0071] As can be seen, this disclosure achieves full decarburization and grain refinement by reasonably adjusting the key preparation conditions of hot-formed steel, further improving the toughness of the material, making it a replacement for safety parts with low strength and high toughness, thus achieving the goal of weight reduction and lightweighting.

[0072] In some embodiments, the continuous annealing time is 8-12 min, for example, it can be 8 min, 8.5 min, 9 min, 9.5 min, 10 min, 10.5 min, 11 min, 11.5 min or 12 min, but is not limited to the listed values, other unlisted values ​​within this range are also applicable.

[0073] This disclosure also provides an application of the above-mentioned hot-formed steel, which is used to manufacture automotive body structural components, including seat beams, A-pillars, B-pillars, door anti-collision beams, front and rear bumpers, sill beams, or center tunnels.

[0074] This disclosure utilizes hot-formed steel, which not only maintains or improves the safety and reliability of vehicle body structural components but also significantly reduces vehicle weight and improves fuel efficiency, thereby reducing carbon emissions. This innovation will drive the automotive industry towards greater efficiency and environmental friendliness, achieving a dual breakthrough in technology and materials.

[0075] Examples 1-6

[0076] The embodiments in this group each provide a hot-formed steel with high cold bending performance of 1700MPa. The chemical composition of the hot-formed steel by mass percentage is shown in Table 1 below.

[0077] Table 1

[0078] The balance is Fe and unavoidable impurities.

[0079] The content relationship of chemical components in the hot-formed steel provided in Examples 1-6 is shown in Table 2 below:

[0080] Table 2

[0081] In the table above, the welding carbon equivalent CE = C + Mn / 6 + (Cr + Mo + V) / 5 + (Ni + Cu) / 15.

[0082] The hot-formed steel preparation method provided in this set of embodiments includes the following steps:

[0083] (1) Controlled rolling and controlled cooling

[0084] The continuously cast billet is heated to 1200℃ in a heating furnace and held for 2 hours. Then, it undergoes descaling to remove the furnace-grown iron oxide scale. The rough rolling start temperature is set to 1100±100℃, the finish rolling start temperature is set to 1040±40℃, and the final rolling temperature is set to 920±50℃. After hot rolling, it is water-cooled to the initial coiling temperature at a cooling rate of 20℃ / s.

[0085] (2) Winding

[0086] The steel strip is wound into a coil. The coiling temperature of the steel coil in each embodiment is shown in Table 3. Then, it is slowly air-cooled to room temperature to obtain a hot-rolled steel sheet for hot forming after hot rolling and coiling. The hot-rolled steel sheet can be used for hot forming. The microstructure of the hot-rolled and coiled hot-rolled steel sheet is ferrite, pearlite and a small amount of bainite.

[0087] (3) Pickling and cold rolling

[0088] To ensure coating quality and avoid residual iron oxide scale on the hot-rolled plate surface, the pickling speed of each embodiment was controlled according to Table 3, and then multiple rolling passes were performed at room temperature according to the target plate thickness.

[0089] (4) Continuous annealing

[0090] Considering the influence of annealing temperature and dew point on surface decarburization, the continuous annealing temperature, continuous annealing time and dew point temperature of each embodiment were controlled according to Table 3, and the furnace atmosphere was N2+H2.

[0091] In some embodiments, the conditions satisfied by the preparation methods provided in Examples 1-6 are shown in Table 3 below:

[0092] Table 3

[0093] Testing revealed that the surface layer of the hot-formed steels obtained in Examples 1-6 all contained soft ferrite, the subsurface layer contained a mixed structure of soft ferrite and martensite, and the core layer contained carbon-depleted martensite (carbon content of 50%-90% of the target carbon content in the hot-formed steel).

[0094] In some embodiments, taking the hot-formed steel obtained in Example 1 as an example, its decarburized layer is shown in Figure 1 below, and its microstructure is shown in Figure 2 below. The thickness of its surface layer is about 5 μm, the thickness of its subsurface layer is about 12 μm, and the core is carbon-depleted martensite.

[0095] The performance parameters of the hot-formed steels obtained in Examples 1-6 are shown in Table 4 below after testing.

[0096] Table 4

[0097] In the table above, the tensile strength test refers to GB / T 228.1-2021 Metallic materials, tensile testing—Part 1: Test method at room temperature; the bending angle test refers to GB / T 232-2024 Metallic materials, bending test method; the coating quality evaluation refers to GB / T 34566-2017 Hot stamped steel sheets and strips for automobiles; and the weldability evaluation refers to GB / T 39167-2020 Tensile shear test method for resistance spot welds and projection welds.

[0098] Using traditional 22MnB5 hot-formed steel as a comparative example, the tensile and bending curves of the hot-formed steel obtained in Example 1 are shown in Figure 3 below.

[0099] As shown in Figure 3, the hot-formed steel obtained in Example 1 exhibits significant advantages in tensile and bending properties compared to traditional 22MnB5 hot-formed steel. The tensile and bending properties of the hot-formed steels obtained in Examples 2-6 are similar to those in Example 1, and therefore will not be described in detail here.

[0100] Therefore, this disclosure, by altering the distribution characteristics of the traditional all-martensitic structure, introduces 5-40 μm ferrite and a mixed structure of ferrite and martensite into the surface layer of hot-formed steel. Simultaneously, by rationally adjusting the key preparation conditions of hot-formed steel, it achieves thorough decarburization and grain refinement, ultimately obtaining hot-formed steel with high cold bending performance of 1700 MPa. While ensuring a strength 200 MPa higher than commercially available 1500 MPa hot-formed steel, it maintains comparable bending toughness, successfully resolving the contradiction between material strength and toughness. This achieves a dual guarantee of high strength and high toughness, while also meeting lightweight requirements and enhancing the competitiveness of existing products.

[0101] The applicant declares that the above description is only a specific implementation of this disclosure, but the protection scope of this disclosure is not limited thereto. Those skilled in the art should understand that any changes or substitutions that can be easily conceived by those skilled in the art within the technical scope disclosed in this disclosure fall within the protection and disclosure scope of this disclosure.

Claims

1. A hot-formable steel with high cold bending performance of 1700MPa, wherein, The surface layer of the hot-formed steel has a soft phase ferrite structure, the subsurface layer has a mixed structure of soft phase ferrite and martensite, and the core layer has a carbon-depleted martensite structure. The thickness of the surface layer is 0-20 μm, and the thickness of the subsurface layer is 5-20 μm; The tensile strength of the hot-formed steel is 1650-1750 MPa.

2. The hot-formed steel according to claim 1, wherein, The carbon content of the carbon-depleted martensite structure is 50%-90% of the target carbon content in the hot-formed steel; The core layer also contains ferrite tissue, and the proportion of ferrite tissue in the core layer is ≤5%.

3. The hot-formed steel according to claim 1, wherein, The chemical composition of the hot-formed steel, by weight percentage, includes: The balance includes Fe and unavoidable impurities.

4. The hot-formed steel according to claim 3, wherein, By mass percentage, C+Al+Si≥1.0%, Cr+Mn≤3.0%, (Cr+Mn) / (C+Al+Si)≤1.5%.

5. The hot-formed steel according to claim 3, wherein, Based on mass percentage, Nb+V ≤ 0.05%.

6. The hot-formed steel according to claim 3, wherein, Based on mass percentage, the welding carbon equivalent CE = C + Mn / 6 + (Cr + Mo + V) / 5 + (Ni + Cu) / 15 < 0.5%.

7. The hot-formed steel according to claim 3, wherein, By mass percentage, Si+Cr≤1.2%.

8. A method for preparing hot-formed steel as described in any one of claims 1-7, wherein, The preparation method satisfies any one or at least a combination of two of the following conditions: (A) The coiling temperature after hot rolling is 650-700℃; (B) Pickling speed ≤ 150 m / min; (C) The continuous annealing temperature is 780-830℃; (D) The atmosphere inside the furnace is a mixture of nitrogen and hydrogen; (E) Dew point temperature is -35°C to -17°C.

9. The preparation method according to claim 8, wherein, The continuous annealing time is 8-12 minutes.

10. An application of the hot-formed steel as described in any one of claims 1-7, wherein, The hot-formed steel is used to manufacture automotive body structural components, including seat beams, A-pillars, B-pillars, door anti-collision beams, front and rear bumpers, sill beams, or center tunnels.

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