A 1700mpa grade hot forming steel with high cold bending performance and a preparation method and application thereof

Abstract

The application provides a 1700MPa-grade hot forming steel with high cold bending performance and a preparation method and application thereof. A surface layer of the hot forming steel has soft ferrite structure, a subsurface layer has mixed structure of soft ferrite and martensite, and a core layer has carbon-poor 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 is 650-700 DEG C; (B) the pickling speed is less than or equal to 150 m / min; (C) the temperature of continuous annealing is 780-830 DEG C; (D) the atmosphere in the furnace is a mixture of nitrogen and hydrogen; (E) the dew point temperature is-35 DEG C to-17 DEG C. Through fine component design and process integration, the application successfully solves the contradiction between material strength and toughness, realizes double guarantee of high strength and high toughness, simultaneously meets the light weight requirement, and improves the competitiveness of existing products.

Description

Technical Field

[0001] This invention belongs to the field of alloy materials technology, and relates to a hot-formed steel, particularly a hot-formed steel with high cold bending performance of 1700MPa and its preparation method and application. Background Technology

[0002] 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.

[0003] In terms of composition design, the toughness of hot-formed steel is improved through the addition of microalloying elements or composite additions. For example, CN106399837A improves toughness by adding vanadium (V) microalloying to refine the grains and by using precipitated particles to consume carbon (C) to reduce the proportion of brittle twinned martensite. CN106947919A improves toughness by adding manganese (Mn) to ensure a small amount of retained austenite in the final microstructure and by using niobium (Nb) and titanium (Ti) composite additions to refine the grains. Although the above strategies have played a certain role in improving toughness, the effect is limited and still does not fully meet the technical requirements.

[0004] In terms of process optimization, CN115058650A employs a rapid heating method to obtain a multiphase microstructure of martensite matrix-submicron retained austenite-nanocarbides by controlling the cementite in the steel, thus enabling the hot-formed steel to possess both high strength and high toughness. CN116174558A uses a two-step heating method to significantly improve bending toughness. First, the material is heated to 950-1100℃ to form a 5-100μm ferrite diffusion layer. Then, to prevent austenite grain growth during high-temperature heating, a second low-temperature heating (800-870℃) is performed to refine the original austenite grain size, thereby improving bending toughness. Although both methods can significantly improve bending toughness, their complex processes and the inherent fluctuations in material properties make large-scale application difficult.

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

[0006] The purpose of this invention 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, the contradiction between material strength and toughness is successfully resolved, achieving a dual guarantee 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 invention employs the following technical solution:

[0008] In a first aspect, the present invention 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 invention modifies the distribution characteristics of traditional all-martensitic microstructure by introducing 5-40 μm ferrite and a mixed microstructure of ferrite and martensite into the surface layer of hot-formed steel. By leveraging the synergistic effect between the ferrite and martensitic microstructures 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 commercially available 1500 MPa hot-formed steel, the bending toughness is comparable. This invention 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.

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

[0013] Preferably, the core layer also contains ferrite structure, and the ferrite structure accounts for ≤5wt% of the core layer.

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

[0015]

[0016] The balance includes Fe and unavoidable impurities.

[0017] Preferably, by mass percentage, C+Al+Si≥1.0%, Cr+Mn≤3.0%, and (Cr+Mn) / (C+Al+Si)≤1.5%.

[0018] Preferably, Nb+V ≤ 0.05% by mass percentage.

[0019] Preferably, the welding carbon equivalent CE, calculated as a percentage by mass, is C + Mn / 6 + (Cr + Mo + V) / 5 + (Ni + Cu) / 15 < 0.5%.

[0020] Preferably, Si+Cr ≤ 1.2% by mass percentage.

[0021] In a second aspect, the present invention 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:

[0022] (A) The coiling temperature after hot rolling is 650-700℃;

[0023] (B) Pickling speed ≤ 150 m / min;

[0024] (C) The continuous annealing temperature is 780-830℃;

[0025] (D) The atmosphere inside the furnace is a mixture of nitrogen and hydrogen;

[0026] (E) Dew point temperature is -35°C to -17°C.

[0027] This invention achieves thorough decarburization and refined grain structure by reasonably adjusting the key preparation conditions of hot-formed steel, which further improves the toughness of the material. It can replace safety parts with low strength and high toughness, thus achieving the goal of weight reduction and lightweighting.

[0028] Preferably, the continuous annealing time is 8-12 minutes.

[0029] Thirdly, the present invention provides an application of hot-formed steel as described in the first aspect, 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.

[0030] This invention, utilizing hot-formed steel, not only maintains or enhances 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.

[0031] The numerical range described in this invention 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 invention will not exhaustively list all the specific point values ​​included in the range.

[0032] Compared with the prior art, the present invention has the following beneficial effects:

[0033] This invention modifies the distribution characteristics of traditional all-martensitic structure by introducing 5-40μm ferrite and a mixed structure of ferrite and martensite into the surface layer of hot-formed steel. Simultaneously, it rationally adjusts the key preparation conditions of hot-formed steel, achieving thorough decarburization and grain refinement. The result is hot-formed steel with high cold bending performance of 1700MPa grade. While maintaining a strength 200MPa higher than commercially available 1500MPa grade 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

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

[0035] Figure 2 These are micrographs of the hot-formed steel obtained in Example 1;

[0036] Figure 3 This is a comparison chart of the tensile and bending properties of the hot-formed steel obtained in Example 1 and the traditional 22MnB5 hot-formed steel. Detailed Implementation

[0037] The technical solution of the present invention will be further illustrated below through specific embodiments. Those skilled in the art should understand that the embodiments described are merely illustrative of the present invention and should not be construed as limiting the invention in any way.

[0038] In order to combine high strength and high toughness while meeting the requirements of lightweighting, the present invention 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.

[0039] 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.

[0040] The tensile strength of the 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 is not limited to the listed values. Other unlisted values ​​within this range are also applicable.

[0041] This invention modifies the distribution characteristics of traditional all-martensitic microstructure by introducing 5-40 μm ferrite and a mixed microstructure of ferrite and martensite into the surface layer of hot-formed steel. By leveraging the synergistic effect between the ferrite and martensitic microstructures 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 commercially available 1500 MPa hot-formed steel, the bending toughness is comparable. This invention 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.

[0042] 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.

[0043] In some embodiments, the core layer also contains ferrite, and the ferrite accounts for ≤5 wt% of the core layer, for example, 0.5 wt%, 1 wt%, 1.5 wt%, 2 wt%, 2.5 wt%, 3 wt%, 3.5 wt%, 4 wt%, 4.5 wt%, or 5 wt%, but is not limited to the listed values; other unlisted values ​​within this range are also applicable.

[0044] In some embodiments, the chemical composition of the hot-formed steel, by weight percentage, includes:

[0045]

[0046] The balance includes Fe and unavoidable impurities.

[0047] 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.

[0048] In this invention, 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%, more preferably 0.20-0.28%.

[0049] In this invention, 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 oxide content is detrimental to pickling removal and the coating effect of aluminum-silicon coatings, the Si content is preferably no higher than 0.40%. During the smelting stage, in order to remove as much O as possible from the molten steel, the Si content is no less than 0.05%. Therefore, the Si content is determined to be 0.05-0.40%, and more preferably 0.10-0.40%.

[0050] In this invention, Mn has both solid solution strengthening properties and can lower the austenitizing temperature; therefore, the Mn content must be at least 0.50%. However, considering that excessively high Mn content can easily lead to severe segregation bands, which are 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%.

[0051] In this invention, 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%.

[0052] In this invention, 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 beneficial for delayed hydrogen embrittlement cracking; therefore, a minimum Nb content of 0.01% is preferred, and V may not be added.

[0053] In this invention, boron (B) helps improve the hardenability of steel, reduces the critical cooling rate at which martensite begins to form, and ensures that the final microstructure is a fully martensitic structure. Since adding a small amount of B can significantly improve hardenability, the B content is determined to be 0.003-0.020%.

[0054] In this invention, Ti helps reduce the amount of nitrogen (N) in the steel, preventing N from combining with boron (B) to form BN, which weakens the hardenability of boron. Since the N content in the steel is not very high, the Ti content is determined to be 0.01-0.05%.

[0055] In this invention, Ni helps to lower the low-temperature brittle transition temperature of high-strength steel. Generally speaking, when the Ni content is less than 0.30%, the brittle transition temperature is below -100℃. Therefore, the Ni content is determined to be 0.00-0.30%.

[0056] In this invention, Mo helps delay the transformation of austenite to pearlite, improving the hardenability of austenite, and also helps delay the transformation of ε-carbides 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%.

[0057] In this invention, 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%.

[0058] 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.

[0059] In this invention, 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%.

[0060] 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.

[0061] In this invention, in order to ensure that the microalloying elements can be fully extracted, the Nb+V content is specifically limited to ≤0.05% based on the selection of C content.

[0062] 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.

[0063] This invention specifically limits the welding carbon equivalent (CE) to below 0.5%, which helps to further improve welding performance.

[0064] 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.

[0065] In this invention, 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%.

[0066] The present invention 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:

[0067] (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.

[0068] (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.

[0069] (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.

[0070] (D) The atmosphere inside the furnace is a mixture of nitrogen and hydrogen.

[0071] (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.

[0072] This invention specifically limits the coiling temperature after hot rolling to 650-700℃ to ensure that microalloying elements have sufficient driving force to precipitate and surface decarburize. 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.

[0073] As can be seen, by reasonably adjusting the key preparation conditions of hot-formed steel, this invention achieves full decarburization and refinement of grain structure, further improving the toughness of the material. It can replace safety parts with low strength and high toughness, thus achieving the goal of weight reduction and lightweighting.

[0074] 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.

[0075] The present invention also provides an application of the above-mentioned hot-formed steel, which is used to manufacture automotive body structural components, including seat crossbeams, A-pillars, B-pillars, door anti-collision beams, front and rear bumpers, sill beams, or center tunnels.

[0076] This invention, utilizing hot-formed steel, not only maintains or enhances 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.

[0077] Examples 1-6

[0078] The embodiments in this group 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.

[0079] Table 1

[0080]

[0081]

[0082] The balance is Fe and unavoidable impurities.

[0083] The content relationship of chemical components in the hot-formed steel provided in Examples 1-6 is shown in Table 2 below:

[0084] Table 2

[0085]

[0086] In the table above, the welding carbon equivalent CE = C + Mn / 6 + (Cr + Mo + V) / 5 + (Ni + Cu) / 15.

[0087] The hot-formed steel preparation method provided in this set of embodiments includes the following steps:

[0088] (1) Controlled rolling and controlled cooling

[0089] 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.

[0090] (2) Winding

[0091] 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.

[0092] (3) Pickling and cold rolling

[0093] 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.

[0094] (4) Continuous annealing

[0095] 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.

[0096] Specifically, the conditions satisfied by the preparation methods provided in Examples 1-6 are shown in Table 3 below:

[0097] Table 3

[0098]

[0099]

[0100] 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).

[0101] Specifically, taking the hot-formed steel obtained in Example 1 as an example, its decarburized layer is shown below. Figure 1 The microstructure is shown below. Figure 2 Its surface layer is about 5 μm thick, the subsurface layer is about 12 μm thick, and the core is carbon-depleted martensite.

[0102] The performance parameters of the hot-formed steels obtained in Examples 1-6 are shown in Table 4 below after testing.

[0103] Table 4

[0104]

[0105] 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.

[0106] Using conventional 22MnB5 hot-formed steel as a comparative example, the tensile and bending curves of the hot-formed steel obtained in Example 1 are shown below. Figure 3 .

[0107] Depend on Figure 3 It can be seen that 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.

[0108] Therefore, this invention, by altering the distribution characteristics of traditional all-martensitic structure, introduces 5-40μm 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 1700MPa grade. While ensuring a strength 200MPa higher than commercially available 1500MPa grade 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.

[0109] The applicant declares that the above description is only a specific embodiment of the present invention, but the protection scope of the present invention 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 the present invention fall within the protection and disclosure scope of the present invention.

Claims

1. A hot-formed steel with high cold bending performance of 1700MPa, characterized in that, 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; The chemical composition of the hot-formed steel, by weight percentage, includes: C0.20-0.30%； Si 0.05-0.40%; Mn 0.50-2.00%; Cr0.20-1.00%； Al0.00-0.50%； Nb 0.01-0.05%; B0.003-0.020%； Ti 0.01-0.05%; Ni 0.00-0.30%; Mo 0.00-0.05%; V0.00-0.05%； Cu 0.00-1.50%; S0.00-0.01%； P0.00-0.08%； The balance consists of Fe and unavoidable impurities; By mass percentage, C+Al+Si≥1.0%; (Cr+Mn) / (C+Al+Si)≤1.5; Based on mass percentage, the welding carbon equivalent CE = C + Mn / 6 + (Cr + Mo + V) / 5 + (Ni + Cu) / 15 < 0.5%.

2. The hot-formed steel according to claim 1, characterized in that, The carbon content of the carbon-depleted martensite structure is 50%-90% of the target carbon content in 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, characterized in that, Based on mass percentage, Nb+V≤0.05%.

4. The hot-formed steel according to claim 1, characterized in that, By mass percentage, Si+Cr≤1.2%.

5. A method for preparing hot-formed steel as described in any one of claims 1-4, characterized in that, 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.

6. The preparation method according to claim 5, characterized in that, The continuous annealing time is 8-12 minutes.

7. An application of the hot-formed steel as described in any one of claims 1-4, characterized in that, 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.

Images