Hot-stamped steel sheet and hot-stamped member

Abstract

The present invention relates to a hot stamping steel sheet and a hot stamped component. The chemical composition of the steel substrate satisfies: Mn eq ≥ 1.85%, Ac3 ≤ 880°C, Ac1 ≥ 730°C and F b‑s ≤ 3.2%, Mn eq = Mn + 1.29Cr + 3.28Mo + 0.46Cu + 0.37Ni + 0.07Si; Ac3 = 902 - 255C - 11Mn + 19Si - 5Cr + 13Mo - 20Ni + 55V; Ac1 = 754.83 - 32.25C - 17.76Mn + 23.23Si + 17.3Cr + 4.5Mo + 15.62Ni; and F b‑S = 1.4Mn + 1.1Cr + 1.4Si + 1.3Ni + 1.6Mo - 0.78Al, which is beneficial to the flatness before the subsequent hot stamping process, and can reduce the anisotropy of toughness while obtaining the target strength of the hot stamped component.

Description

Technical Field

[0001] This invention relates to a hot-stamping steel sheet and a 1000 MPa-grade high-toughness hot-stamping component made therefrom. Background Technology

[0002] In modern automotive manufacturing, hot-stamped steel sheets have become crucial materials for achieving lightweighting and improving safety performance in automobiles due to their excellent formability, oxidation resistance, and high strength. However, while the currently available 1000 MPa grade hot-stamped steel sheets achieve high strength and toughness through the hot-stamping process, some issues still exist.

[0003] CN115652218A discloses a low-carbon, high-toughness hot-stamped component and steel plate. The steel plate matrix of the hot-stamped component contains, by mass percentage, 0.053%≤C≤0.10%, 0.05%≤Si≤0.30%, 1.81%≤Mn≤2.7%, 0.01%≤Cr≤0.7%, 0.01%≤Al≤0.5%, 0.0005%≤B≤0.005%, 0.015%≤Ti≤0.05%, 0≤Nb+V≤0.2%, 0.001%≤P≤0.100%, 0.0001%≤S≤0.100%, Fe≥95%, and unavoidable impurities, with Mn+0.26Si+1.3Cr≥2.20%. By area percentage, the microstructure of the steel plate matrix includes less than 5% bainite, less than 3% austenite, less than 3% ferrite, and less than 0.2% Nb-V-Ti carbides, with the remainder being martensite. The high Mn content in this patent results in a higher amount of martensite in the room temperature microstructure of the annealed or coated steel sheet, leading to greater work hardening capacity and hindering subsequent processing. Furthermore, the addition of a high Mn content easily causes segregation, resulting in banded structures in the steel sheet during production. This leads to a more pronounced directional variation in mechanical properties, affecting the service safety of subsequently hot-stamped components.

[0004] CN107810281A provides a press-hardened steel component, wherein the chemical composition of the steel, by weight, comprises: 0.062% ≤ C ≤ 0.095%, 1.4% ≤ Mn ≤ 1.9%, 0.2% ≤ Si ≤ 0.5%, 0.020% ≤ Al ≤ 0.070%, 0.02% ≤ Cr ≤ 0.1%, wherein: 1.5% ≤ (C + Mn + Si + Cr) ≤ 2.7%, 0.040% ≤ N b ≤ 0.060%, 3.4 × N ≤ Ti ≤ 8 × N, wherein: 0.044% ≤ (Nb + Ti) ≤ 0.090%, 0.0005% ≤ B ≤ 0.004%, 0.001% ≤ N ≤ 0.009%, 0.0005% ≤ S ≤ 0.003%, 0.001% ≤ P ≤ 0.020%, optionally: 0.0001% ≤ Ca ≤ 0.003%, with the remainder being Fe and unavoidable impurities. However, this patent teaches that the Cr content should not exceed 0.1% to ensure that the microstructure of the part contains less than 40% bainite by surface fraction, and to avoid the formation of a fully martensitic structure in the deformation region of the hot-stamped part, thus achieving high toughness and flexibility of the press-hardened part through microstructure control. However, low Cr content cannot effectively inhibit the formation of austenite in the microstructure during the heating process of steel plate annealing or annealing coating production, which makes it difficult to control the occurrence of martensitic phase transformation during cooling. This results in higher work hardening ability of the steel plate after annealing and reduced cold workability of the steel plate.

[0005] CN115261742B provides a hot-stamped component with a tensile strength of 1000 MPa, whose chemical composition by weight percentage is: C: 0.05~0.20%, Si: 0.02~1.00%, Mn: 0.5~2.0%, P≤0.10%, S≤0.05%, Al: 0.01~0.30%, Nb: 0.01~0.04%, Ti: 0.01~0.06%, Cr: 0.12~0.50%, B: 0.001~0.05%, with the remainder being Fe and other unavoidable impurities, and must simultaneously satisfy: 0.24%≤C+Mn / 6≤0.45%; 0.05%≤Nb+Ti+B×10≤0.15%. The patent teaches that in order to obtain high-toughness hot stamping parts, the annealing temperature of the steel plate is controlled at 720-780℃ during the production process, and the proportion of fine martensite and carbide particles dispersed along the ferrite grain boundaries in its microstructure is 10-40%. However, this will result in the steel plate having a high work hardening ability and poor cold working performance after annealing.

[0006] CN116815068A relates to a hot-forming steel and its preparation method. The chemical composition of the steel matrix, by weight fraction, is: C: 0.05-0.15%, Mn: 0.6-2.0%, Si≤0.20%, S≤0.005%, P≤0.020%, Al≤0.08%, Nb: 0.01-0.60%, Ti: 0.02-0.07%, V: 0.002-0.05%, N≤0.01%, Ni≤0.100%, Cr: 0.1-0.3%, Mo≤0.100%, B: 0.0003-0.003%, O≤0.003%; and simultaneously satisfies: 1.2%≤[Cr]+[Mn]≤2.0%, 0.04%≤[Ti]+[Nb]+[V]≤0.15%. However, as mentioned earlier, the design with low Si and Cr content cannot suppress the formation of austenite during the heating process of steel sheet annealing or plating production, which makes it difficult to control the occurrence of martensitic phase transformation during cooling. This results in high work hardening properties of the steel sheet after annealing, which is not conducive to subsequent processing.

[0007] In view of the above, there is still a need to develop a 1000 MPa grade hot-stamping steel sheet and hot-stamping components with high manufacturability and processability and high toughness after hot stamping. Summary of the Invention

[0008] This invention was made in view of the above-mentioned problems existing in the prior art.

[0009] One of the objectives of this invention is to provide a steel sheet for hot stamping that, while ensuring the strength of the hot stamping components manufactured from it, is easy to level after annealing or annealing plating to improve its manufacturability, especially in high-precision laser welding processes; and has reduced banded segregation structure to improve the bending fracture strain of the hot stamping components made from it in the direction of poor toughness, thereby improving the overall impact deformation capability of the component.

[0010] To achieve the above objectives, the present invention discloses a hot-stamping steel sheet, wherein, by mass percentage, the chemical composition of the steel matrix of the hot-stamping steel sheet comprises: 0.060%≤C≤0.100%, 1.30%≤Mn≤1.70%, 0.10%≤Si≤0.70%, 0.15%≤Cr≤0.70%, 0.01%≤Al≤0.10%, 0.01%≤Ti≤0.1%, 0.001%≤B≤0.01%, 0.01%≤Nb≤0.1%, and at least one of the following: 0≤N≤0.010%. 0≤P≤0.020%, 0≤S≤0.010%, 0≤V≤0.10%, 0≤Ni≤0.10%, 0≤Mo≤0.15%, 0≤W≤0.20%, 0≤Cu≤0.20%, 0≤Co≤0.50%, 0≤Ta≤0.10%, 0≤Sn≤0.05%, 0≤Sb≤0.05%, 0≤As≤0.05%, 0≤Ca≤0.010%, 0≤Mg≤0.010%, 0≤Zr≤0.10%, 0≤O≤0.02%, and 0≤REM≤0.05%, with the balance being Fe and impurities.

[0011] Wherein, the steel matrix of the hot stamping steel plate satisfies: Mn eq ≥1.85%, Ac3≤880℃, Ac1≥730℃ and F b-s ≤3.2%,

[0012] Among them, Mn eq =Mn+1.29Cr+3.28Mo+0.46Cu+0.37Ni+0.07Si;

[0013] Ac3=902-255C-11Mn+19Si-5Cr+13Mo-20Ni+55V;

[0014] Ac1 = 754.83 - 32.25C - 17.76Mn + 23.23Si + 17.3Cr + 4.5Mo + 15.62Ni; and

[0015] F b-s =1.4Mn+1.1Cr+1.4Si+1.3Ni+1.6Mo-0.78Al.

[0016] Optionally, Mn eq ≥1.90%.

[0017] Optionally, Ac3 ≤ 875℃.

[0018] Optionally, Ac1 ≥ 735℃; Optionally, Ac1 ≥ 738℃; Optionally, Ac1 ≥ 745℃.

[0019] Optionally, F b-s ≤3.1%; optionally, F b-s ≤3.0%; optionally, F b-s ≤2.9%.

[0020] Optionally, the hot-stamping steel sheet has the following properties: tensile strength TS≤800 MPa, work hardening index n 1~3% The tensile strength TS of the hot-stamping steel sheet is ≤0.200, and the elongation after fracture is ≥17.0%. Optionally, the tensile strength TS of the hot-stamping steel sheet is ≤750 MPa. Optionally, the work hardening index n of the hot-stamping steel sheet is... 1~3% ≤0.180. Optionally, the work hardening index n of the hot-stamping steel sheet is... 1~3% ≤0.170.

[0021] Optionally, by area percentage, the room temperature microstructure of the steel matrix is ​​mainly composed of ferrite and pearlite, with martensite accounting for no more than 8.0%, optionally no more than 4.0%, and further optionally, the room temperature microstructure does not contain martensite.

[0022] Optionally, 0.065%≤C≤0.090% or 0.15%≤Si≤0.55% or 1.35%≤Mn≤1.65% or 0.20%≤Cr≤0.60%.

[0023] Optionally, Si+Cr≤0.90%.

[0024] Optionally, the chemical composition of the steel matrix, by mass percentage, comprises at least one of the following: 0.0001%≤N≤0.010%, 0.001%≤P≤0.020%, 0.0001%≤S≤0.010%, 0.01%≤V≤0.10%, 0.01%≤Ni≤0.10%, 0.01%≤Mo≤0.15%, 0.001%≤W≤0.20%, and 0.001%≤Cu≤0.20%. The following concentrations are specified: 0.01%≤Co≤0.50%, 0.001%≤Ta≤0.10%, 0.001%≤Sn≤0.05%, 0.001%≤Sb≤0.05%, 0.001%≤As≤0.05%, 0.001%≤Ca≤0.010%, 0.001%≤Mg≤0.010%, 0.001%≤Zr≤0.10%, O≤0.006%, and 0.0001%≤REM≤0.05%. Optionally, 0.001%≤W+Mo+Ni+Cu+Co≤0.30%. Further optionally, 0.001%≤Mo+W+Cu+Co+Sn+Sb+Ca+Mg+Zr+REM≤0.30%.

[0025] Optionally, the degree of banded segregation in the room temperature microstructure of the steel matrix of the hot stamping steel sheet according to the present invention does not exceed grade 2.5A in the standard rating chart of GB / T 34474.1-2017, and optionally does not exceed grade 2A.

[0026] Optionally, a hot-dip galvanized aluminum alloy pre-plating layer or a zinc alloy pre-plating layer is coated on at least one surface, preferably two surfaces, of the steel substrate. Optionally, the average thickness of the aluminum alloy pre-plating layer is 5 to 33 µm.

[0027] Another object of the present invention is a hot-stamped component that improves the anisotropy of its toughness by increasing the fracture strain in the direction where the toughness was originally poor, thereby improving the overall fracture resistance of the hot-stamped component. The hot-stamped component is made of the aforementioned hot-stamping steel sheet according to the present invention and includes a steel matrix. Since the hot-stamping process does not cause a change in the composition of the steel matrix, the composition of the steel matrix of the hot-stamped component is consistent with the composition of the steel matrix of the hot-stamping steel sheet from which it is made; therefore, the above-mentioned limitations on the steel matrix also apply to the steel matrix of the hot-stamped component.

[0028] The hot-stamped component according to an embodiment of the present invention includes a steel matrix, the chemical composition of which, by mass percentage, comprises: 0.060%≤C≤0.100%, 1.30%≤Mn≤1.70%, 0.10%≤Si≤0.70%, 0.15%≤Cr≤0.70%, 0.01%≤Al≤0.10%, 0.01%≤Ti≤0.1%, 0.001%≤B≤0.01%, 0.01%≤Nb≤0.1%, and at least one of the following: 0≤N≤0.010%, 0≤P≤0. 0.20%, 0≤S≤0.010%, 0≤V≤0.10%, 0≤Ni≤0.10%, 0≤Mo≤0.15%, 0≤W≤0.20%, 0≤Cu≤0.20%, 0≤Co≤0.50%, 0≤Ta≤0.10%, 0≤Sn≤0.05%, 0≤Sb≤0.05%, 0≤As≤0.05%, 0≤Ca≤0.010%, 0≤Mg≤0.010%, 0≤Zr≤0.10%, 0≤O≤0.02%, and 0≤REM≤0.05%, with the balance being Fe and impurities.

[0029] Wherein, the steel matrix satisfies: Mn eq ≥1.85%, Ac3≤880℃, Ac1≥730℃ and F b-s ≤3.2%,

[0030] Where Mn eq =Mn+1.29Cr+3.28Mo+0.46Cu+0.37Ni+0.07Si;

[0031] Ac3=902-255C-11Mn+19Si-5Cr+13Mo-20Ni+55V;

[0032] Ac1 = 754.83 - 32.25C - 17.76Mn + 23.23Si + 17.3Cr + 4.5Mo + 15.62Ni; and

[0033] F b-s =1.4Mn+1.1Cr+1.4Si+1.3Ni+1.6Mo-0.78Al.

[0034] Optionally, the hot-stamped component has a tensile strength TS ≥ 1100 MPa and an elongation after fracture ≥ 5.0%. The fracture strain of the hot-stamped component in the rolling direction (RD) and in the transverse direction perpendicular to the rolling direction (TD) is not less than 0.75, and the difference between the fracture strain in the RD direction and the fracture strain in the TD direction is not greater than 0.14, effectively ensuring the collision safety of automotive parts.

[0035] Optionally, the hot-stamped component has a tensile strength TS ≥ 1115 MPa, an elongation after fracture ≥ 6.0%, and a fracture strain of not less than 0.76 in both the RD and TD directions, and the difference in fracture strain between the RD and TD directions is not greater than 0.13.

[0036] Optionally, the hot-stamped component has a tensile strength TS ≥ 1115 MPa, an elongation after fracture ≥ 6.0%, and a fracture strain of not less than 0.80 in both the RD and TD directions, and the difference in fracture strain between the RD and TD directions is not greater than 0.11.

[0037] Optionally, the room temperature microstructure of the steel matrix of the hot-stamped component is mainly martensite, and the proportion of ferrite and bainite by area is not greater than 5.0%, optionally not greater than 3.0%, optionally not greater than 2.0%, further optionally not containing ferrite, and even more optionally not containing ferrite and bainite.

[0038] Optionally, the hot-stamped component includes a metal coating on at least one surface, preferably two surfaces, of the steel substrate. Optionally, the metal coating may be an aluminum alloy coating, such as an aluminum-silicon coating, or a zinc alloy coating. Optionally, the average thickness of the aluminum-silicon coating is 10–50 µm.

[0039] On the one hand, to ensure the strength of hot-stamped components, i.e., to maximize the amount of martensite in their room-temperature microstructure, a large amount of alloying elements, such as high levels of Mn and Mo, are added to hot-stamped steel sheets to ensure hardenability. However, while the addition of high levels of Mn and Mo ensures the hardenability of the material, it is highly prone to segregation. This leads to the formation of banded structures along the rolling direction (RD direction) during the production process, resulting in carbon and alloy element-depleted and enriched bands within the microstructure. The inventors have discovered that carbon and alloy element-depleted bands form low-strength martensite after hot stamping, while enriched bands form hard and brittle martensite. In other words, the presence of banded structures leads to stratification of the martensite microstructure within the room-temperature microstructure of the hot-stamped component. The banded distribution of martensite with varying hardness results in stress-strain distribution between martensite layers during steel sheet deformation, easily causing localized stress concentration and ultimately reducing the bending toughness of the steel sheet. Furthermore, the banded distribution of martensite itself leads to significant anisotropy in the mechanical properties of the final hot-stamped components. Generally, the toughness of the material does not change significantly in the direction parallel to the banded structure (i.e., the rolling RD direction), and still exhibits good toughness. However, the toughness decreases significantly in the direction perpendicular to the banded structure (i.e., the transverse TD direction). However, when using hot-stamping steel sheets, to improve material utilization and production convenience, the optimization of layout utilization is the core focus, and the directionality of material mechanical properties cannot be considered. In this case, the difference in toughness of hot-stamped components caused by banded structures in different directions is highly likely to lead to premature component failure, endangering the safety of occupants. Therefore, the design criteria for part performance are based on the mechanical properties in the direction with the worst performance. Therefore, this invention needs to resolve the contradiction between adding alloying elements to ensure sufficient hardenability and reducing the addition of alloying elements to avoid banded structures.

[0040] On the other hand, hot-stamped steel sheets of different strengths and / or thicknesses are typically joined together by laser welding. Laser welding places strict requirements on the flatness of the steel sheets, generally requiring a warpage of less than 3 mm to ensure the quality of the welded sheets and the smooth progress of subsequent processing. To ensure the steel sheets achieve the desired flatness before laser welding, a leveling process is required after the steel coil is uncoiled. This leveling process mainly involves applying small plastic deformation (typically 0.5~1%) to the steel sheet to release and balance internal stresses, thereby eliminating deformation stress and making the steel sheet flatter. Without considering equipment limitations, the strength of the steel sheet does not necessarily affect this process; that is, high-strength steel sheets can achieve the desired flatness, while low-strength steel sheets may not. This is because the inventors have discovered that the work hardening capacity of the steel sheet significantly affects its leveling performance. Specifically, even with lower steel sheet strength, a better work hardening capacity makes leveling more difficult. This is because even under small deformations, such as 1% plastic deformation, the steel plate already hardens in the deformation zone, easily causing uneven performance between the deformation and non-deformation zones. This results in uneven distribution of residual stress within the steel plate after leveling, leading to poor plate shape, such as warping. Consequently, the steel plate becomes unsuitable for laser welding, or it may deviate from its designated position in the furnace during hot stamping, making it impossible for the robotic arm to grasp and transfer it to the mold after exiting the furnace, ultimately rendering the steel plate unusable. Therefore, to improve the leveling performance of hot stamping steel plates, this invention proposes reducing the work hardening capacity of annealed steel plates.

[0041] To at least alleviate the phenomena mentioned above, this invention aims to improve the toughness in the transverse direction while ensuring the target strength of the hot-stamped component, thereby reducing the difference in toughness in different directions, and simultaneously improving the leveling performance, thereby improving collision safety and yield. That is, it simultaneously controls the Mn content of the steel plate. eq Ac3, Ac1, and F b-s The specific reasons are detailed below.

[0042] First, to ensure the strength of the hot-stamped components, that is, to ensure that their room-temperature microstructure contains as much martensite as possible, the inventors, when formulating the alloy composition, required that Mn... eq (i.e., Mn) eq=The content of Mn+1.29Cr+3.28Mo+0.46Cu+0.37Ni+0.07Si (Equation 1) is not less than 1.85% to promote the formation of martensite during hot stamping and ensure the expected strength of the hot stamped component. On the other hand, due to the addition of lower carbon content, the Ac3 temperature increases, which may prevent complete austenitization. Therefore, Ac3 (i.e., Ac3=902-255C-11Mn+19Si-5Cr+13Mo-20Ni+55V (Equation 2)) is required to be no greater than 880℃ to ensure full austenitization during hot stamping, thereby obtaining as much martensite as possible during cooling and ensuring the strength of the hot stamped component.

[0043] Secondly, ensuring the strength (i.e., Mn) of the manufactured hot-stamped component. eq With a content of not less than 1.85%, this invention proposes to reduce the work hardening capacity of hot-stamping steel sheets used in the manufacture of hot-stamped components to improve the leveling performance of the steel sheets. Existing technologies have not considered the influence of material microstructure and properties on the work hardening capacity of hot-stamping steel sheets. The inventors realized that controlling the room temperature microstructure of annealed or annealed coated hot-stamping steel sheets is particularly important for reducing their work hardening capacity. This is because the martensitic phase has a significant hardness difference with other constituent phases, meaning that when the room temperature microstructure contains a large amount of martensitic phase, the resulting steel sheet will have a high work hardening capacity (e.g., the characteristic of duplex steel as commonly known), thereby reducing the leveling performance of the steel sheet. Therefore, the inventors propose to adjust the work hardening capacity at small deformations by reducing the proportion (by area) of the martensitic phase in the room temperature microstructure of annealed and annealed coated steel sheets, thereby improving their leveling performance.

[0044] Annealing of hot-stamping steel sheets typically involves heating the sheet to 720-850°C and holding it at that temperature for a period of time, then slowly cooling it to a temperature above 600°C before rapidly cooling it to 200-400°C. Annealing plating treatments, such as hot-dip galvanizing for aluminum-silicon coatings, involve slowly cooling the heated hot-stamping steel sheet to approximately 650°C and immersing it in a molten aluminum-silicon bath for plating, followed by rapid cooling to below 300°C. Generally, heating to 740-840°C is considered two-phase annealing, meaning the annealing temperature is higher than Ac1. This temperature allows for the full release of residual stress from cold rolling deformation, resulting in a microstructure of ferrite + pearlite + martensite, and also yields a final product with lower yield strength and higher elongation. However, through careful research, the inventors discovered that when using low-temperature annealing in the two-phase region, although only a small portion of the steel sheet undergoes austenitization, carbon's high solubility in austenite and extremely low solubility in ferrite leads to a significant increase in carbon content within the austenite due to its high solubility in austenite and extremely low solubility in ferrite. Simultaneously, manganese (Mn), as an austenite stabilizing element, also accumulates in the austenite, making it even more difficult for the carbon-rich austenite to undergo ferrite or pearlite phase transformation during subsequent cooling to the over-aging temperature or plating temperature, ultimately resulting in a martensite transformation. The formation of high-hardness martensite within the microstructure significantly increases work hardening and increases the tensile strength of the steel sheet, which is detrimental to the leveling properties of hot-stamping steel sheets. Although the proportion of austenite in the high-temperature microstructure increases with the increase of annealing temperature, which alleviates the carbon enrichment of austenite and makes subsequent ferrite or pearlite phase transformation easier, and the proportion of martensite obtained in the room temperature microstructure decreases, the high content of alloying elements such as Mn / Cr / Si added to 1000 MPa grade steel also means that the steel plate surface is more prone to oxidation, which will lead to serious surface quality problems in the final product.

[0045] When the annealing temperature is chosen to be lower than the Ac1 temperature (the temperature at which austenite begins to form), no phase transformation to austenite occurs within the microstructure, thus avoiding the formation of martensite during subsequent cooling. Instead, residual stress from the cold rolling process is released only through recrystallization. Consequently, the microstructure of the material remains essentially unchanged during subsequent cooling, resulting in a recrystallized annealed microstructure of the cold-rolled material, i.e., a ferrite + pearlite microstructure in the annealed state. However, for annealing and annealed plating processes, excessively low annealing temperatures will not achieve sufficient recrystallization, resulting in steel plates with relatively high strength but poor ductility and toughness.

[0046] Given the above, the choice of annealing temperature significantly affects the proportion of martensite phase in the final room temperature microstructure of annealed and annealed coated steel sheets. However, annealing is a common process in steel sheet production, and its adjustment is slow, with continuous adjustments between different products affecting production efficiency. Therefore, this invention aims to improve the work hardening ability of annealed and annealed coated steel sheets without changing the conventional annealing process. Thus, while ensuring the strength (i.e., Mn) of the hot-stamped components... eq Based on the premise that the content of Ac1 is not less than 1.85% and Ac3 is not greater than 880℃, the inventors propose to control the temperature of Ac1 (i.e., Ac1 = 754.83 - 32.25C - 17.76Mn + 23.23Si + 17.3Cr + 4.5Mo + 15.62Ni (Equation 3)) to be not less than 730℃ through alloy proportioning. The increase in Ac1 temperature enables recrystallization annealing to be achieved at a higher temperature. Therefore, compared with the conventional low Ac1, at the same annealing temperature, the increased Ac1 can reduce the degree of austenitization of the steel plate, thereby reducing the proportion of martensite phase in the final room temperature microstructure, thus reducing the strength and work hardening ability of the annealed and annealed coated steel plate.

[0047] Finally, in order to improve the toughness in the transverse direction perpendicular to the rolling direction, thereby improving the anisotropic difference in toughness and achieving an overall improvement in the collision safety of hot-stamped components, the inventors propose to control the formation of banded structures in the room temperature structure of hot-stamped components.

[0048] Since segregation of alloying elements generally occurs during the liquid-phase solidification process in alloy smelting, to quantitatively determine the influence of elements such as Mn, Cr, Si, Ni, and Mo on banded structure formation, the inventors input the alloy composition of this invention into the thermodynamic calculation software Thermo-Calc to obtain the corresponding equilibrium phase diagram and the content of each element in the liquid phase and δ-ferrite phase at different temperatures. Then, based on the phase diagram, the temperature corresponding to the maximum δ-ferrite content was found, and the content of each element in the liquid phase at that temperature was compared with the content in the already formed δ-ferrite phase to obtain the influence coefficient of each element on segregation formation, thereby quantitatively clarifying the influence of banded structure forming elements Mn, Cr, Si, Ni, and Mo. It is worth noting that this invention considers the influence of Al on segregation because the inventors found that it exhibits an effect of reducing the performance degradation caused by segregation. The average banded structure sensitivity factor obtained from the above theoretical calculations for the alloy composition of this invention is [value missing]. F b-s =1.4Mn+1.1Cr+1.4Si+1.3Ni+1.6Mo-0.78Al (Equation 4). The inventors discovered that when the segregation factor... F b-sAt a concentration not exceeding 3.2%, the formation of banded structures caused by the segregation of alloying elements can be effectively controlled, keeping the segregation level within 2.5A. This improves the uniformity of the martensitic structure in the hot-stamped component, thus enhancing toughness in the transverse direction. Consequently, it reduces the anisotropy of toughness in the hot-stamped component, resulting in better impact energy absorption performance overall. It should be understood that, as previously noted, toughness in the rolling direction is not significantly affected by banded structures and still exhibits good toughness. Therefore, although only improving toughness in the transverse direction is proposed here, the reduction of banded structures also benefits toughness in the rolling direction, although it will not significantly improve toughness in the transverse direction.

[0049] As can be seen from the above discussion, regardless of Mn eq Control, or Ac3, Ac1, and F b-s The control of these parameters is achieved through appropriate alloy proportions, meaning that adjusting one parameter will inevitably affect the others. Therefore, the adjustment of these parameters needs to be considered holistically; that is, the alloy composition of the hot-stamping steel sheet and the hot-stamping components made therefrom according to the present invention must simultaneously meet the following conditions:

[0050] (1) Mn eq ≥1.85%;

[0051] (2) Ac3≤880℃;

[0052] (3) Ac1 ≥ 730℃; and

[0053] (4) F b-s ≤3.2%.

[0054] According to the present invention, the martensite content in the room temperature microstructure of the hot-stamping steel sheet after annealing or annealing and plating treatment does not exceed 10%, thereby ensuring that the tensile strength of the steel sheet is not greater than 800 MPa and reducing the work hardening ability of the steel sheet. 1~3% The strength is no greater than 0.200, which improves the leveling performance of the steel plate and thus helps to increase the yield of subsequent production. At the same time, the hot-stamped components made from this steel plate have both high strength (tensile strength TS not less than 1100 MPa) and improved toughness in the transverse direction (not less than 0.75), and reduced anisotropy of toughness (the difference in toughness in the RD and TD directions is within 0.14), thereby improving the overall collision safety of the hot-stamped components.

[0055] In this document, all descriptions of chemical element content (%) refer to mass percentage. Unless otherwise specified, preferred embodiments can be freely combined as needed. Those skilled in the art will understand that various factors in this invention, such as Mn, are not necessarily related to chemical element content. eq Ac1, Ac3, and F b-s In the calculations, each element is calculated as a percentage of its mass. For example, in Equation 4, 0.78Al, assuming an Al content of 0.05%, is calculated as 0.05% Al, i.e., 0.78Al = 0.78 × 0.05% = 0.039%. Furthermore, any range or specific value mentioned herein is applicable to this invention. For example, 0.060~0.100% includes any subrange and any specific value within it, such as any value like 0.068%, 0.080%, or any range like 0.069~0.088%, 0.062~0.092%. Unless explicitly stated otherwise, all ranges include end values. Attached Figure Description

[0056] Figure 1 The true stress-true strain curves of annealed steel plate T1 after engineering stress-engineering strain transformation according to an embodiment of the present invention are shown, and the true stress-true strain curves for n are also shown. 1~3% The fitted curve for the interval;

[0057] Figure 2 A schematic diagram of a notched specimen for fracture strain testing according to an embodiment of the present invention is shown;

[0058] Figure 3 Typical room temperature microstructure of the steel matrix after annealing of CT1 steel plate is shown for comparison.

[0059] Figure 4 and Figure 5 Typical room temperature microstructures of steel substrates after T3 annealing and T6 annealing and plating treatments according to embodiments of the present invention are shown respectively.

[0060] Figure 6 , Figure 7 , Figure 8 and Figure 9 The hot-stamped component CT2 for comparison is shown separately. P and CT3 P And the hot-stamped component T7 according to an embodiment of the present invention. P and T3 P Typical room temperature microstructure morphology of steel matrix;

[0061] Figure 10 and Figure 11Typical room temperature microstructures of the steel substrates after T7 annealing according to an embodiment of the present invention and after CT4 annealing and plating for comparison are shown respectively.

[0062] Figure 12 Typical room temperature microstructure of the steel matrix after T1 annealing of steel plates according to embodiments of the present invention is shown; and

[0063] Figure 13 An embodiment according to the present invention is shown. F b-s and ε fRD - ε fTD The relationship. Detailed Implementation

[0064] The present invention will now be described in more detail with reference to exemplary embodiments. The following embodiments or experimental data are intended to illustrate the present invention, and those skilled in the art should understand that the present invention is not limited to these embodiments or experimental data. Unless otherwise specified, preferred embodiments can be freely combined as needed.

[0065] The chemical composition of the steel matrix of the hot-stamped component according to the present invention is consistent with the chemical composition of the steel matrix of the hot-stamped steel sheet to which it is made. By mass percentage, the chemical composition of both steel matrices includes: 0.060%≤C≤0.100%, 1.30%≤Mn≤1.70%, 0.10%≤Si≤0.70%, 0.15%≤Cr≤0.70%, 0.01%≤Al≤0.10%, 0.01%≤Ti≤0.1%, 0.001%≤B≤0.01%, 0.01%≤Nb≤0.1%, and at least one of the following: 0≤N≤0.010%, 0≤P≤0.01%. The steel matrix of the hot-stamping steel plate contains 0.020%, 0≤S≤0.010%, 0≤V≤0.10%, 0≤Ni≤0.10%, 0≤Mo≤0.15%, 0≤W≤0.20%, 0≤Cu≤0.20%, 0≤Co≤0.50%, 0≤Ta≤0.10%, 0≤Sn≤0.05%, 0≤Sb≤0.05%, 0≤As≤0.05%, 0≤Ca≤0.010%, 0≤Mg≤0.010%, 0≤Zr≤0.10%, 0≤O≤0.02%, and 0≤REM≤0.05%, with the balance being Fe and impurities. The steel matrix of the hot-stamping steel plate satisfies the following condition: Mn eq ≥1.85%, Ac3≤880℃, Ac1≥730℃ and F b-s ≤3.2%,

[0066] Among them, Mn eq=Mn+1.29Cr+3.28Mo+0.46Cu+0.37Ni+0.07Si;

[0067] Ac3=902-255C-11Mn+19Si-5Cr+13Mo-20Ni+55V;

[0068] Ac1 = 754.83 - 32.25C - 17.76Mn + 23.23Si + 17.3Cr + 4.5Mo + 15.62Ni; and

[0069] F b-s =1.4Mn+1.1Cr+1.4Si+1.3Ni+1.6Mo-0.78Al.

[0070] The chemical composition of the steel matrix of the hot-stamping steel sheet or hot-stamping component according to the present invention is described in detail below:

[0071] C: 0.060~0.100%

[0072] Carbon (C) is the most commonly used alloying element in steel to improve its strength. Higher C content results in better hardenability and higher strength. However, as C content increases, the fracture strain of the steel plate decreases, leading to a reduction in toughness. Therefore, this invention requires that the C content be controlled to not exceed 0.100% to ensure the hot-stamped components possess the desired toughness. Simultaneously, C significantly affects the phase transformation characteristics of steel; a decrease in C content will significantly increase the Ac3 temperature of the steel, which is detrimental to complete austenitization during the hot-stamping process. Therefore, this invention requires that the C content be not less than 0.060%. Optionally, the C content can be in the range of 0.065% to 0.090%.

[0073] Mn: 1.30~1.70%

[0074] Manganese (Mn) improves the hardenability of steel by stabilizing austenite and expanding the austenite phase region. When the Mn content is below approximately 1.30%, its improvement on the hardenability of the material is not significant, and it cannot guarantee that the steel will obtain sufficient martensite during hot stamping, thus making it difficult to guarantee the high strength and hardness of the material. However, when the Mn content is too high, Mn will cause severe banded segregation, resulting in a significant difference in toughness in the rolling RD direction and transverse TD direction after hot stamping. In addition, Mn is an austenite stabilizing element, which increases the stability of austenite while lowering the Ac1 temperature, resulting in more martensite in the room temperature microstructure of annealed or annealed coated plates, which is detrimental to the cold working performance of annealed or annealed coated plates. Therefore, the present invention sets the upper limit of Mn content to 1.70%. Optionally, the Mn content is in the range of 1.35% to 1.65%.

[0075] Si: 0.10~0.70%

[0076] The addition of Si can increase the Ac1 temperature of the material, thereby expanding the ferrite phase region. Correspondingly, this reduces austenitic phase transformation during annealing and promotes ferrite or pearlite transformation during post-annealing cooling, ensuring that the final annealed or annealed coated sheet has less martensite in its room temperature microstructure. Furthermore, a certain amount of Si can strengthen the steel matrix through solid solution strengthening. However, excessive Si makes the surface of the steel substrate prone to oxidation, leading to poor quality of subsequent coatings and excessive oxide scale during hot stamping, among other surface quality issues. Moreover, since Si promotes ferrite formation, excessive Si addition will cause ferrite to form in areas with poor fit and inadequate cooling, affecting the performance of the final component. Based on the above analysis, this invention controls the Si content within the range of 0.10~0.70%. Optionally, the Si content is within the range of 0.15~0.55%, and more preferably within the range of 0.15~0.50%.

[0077] Cr: 0.15~0.70%

[0078] First, by reducing the decomposition rate of austenite, Cr can improve the hardenability of steel. Furthermore, considering that Mn is also an element that easily leads to banded segregation, Cr, an economical alloying element that is less prone to banded segregation, is used to replace Mn to reduce the adverse effects of banded segregation, thus reducing the Mn content while ensuring hardenability. In addition, Cr, as a ferrite-stabilizing element, can also increase the Ac1 temperature of the steel plate, making it more difficult for the material to undergo austenite transformation during annealing. This reduces the proportion of austenite in the steel plate microstructure at the same annealing temperature, ultimately reducing the proportion of martensite phase in the room temperature microstructure of annealed or anneal-coated hot-stamping steel plates. However, the addition of Cr makes it difficult to remove the oxide scale formed on the surface of the hot-rolled coil. Simultaneously, the steel plate surface is prone to oxidation during annealing, affecting the surface quality of the final product. In summary, the Cr content in this invention is controlled within the range of 0.15% to 0.70%, optionally within the range of 0.20% to 0.60%.

[0079] Optionally, Si+Cr≤0.90%

[0080] Both Si and Cr are easily oxidized elements. When added together, they will form an oxide film on the surface of the steel plate, making it difficult to pickle and thus affecting the quality of subsequent annealing and / or coating. Therefore, the total amount of the two elements added should not exceed 0.90%.

[0081] B: 0.001~0.01%

[0082] During hot stamping, boron (B) can segregate at the austenite grain boundaries, inhibiting ferrite formation and significantly improving the hardenability of steel. However, excessive B content can lead to boron embrittlement. Therefore, to promote hardenability and avoid boron embrittlement, this invention requires the B content to be in the range of 0.001 to 0.01%.

[0083] Al: 0.01~0.10%

[0084] Al is a strong deoxidizing element and is therefore often used as a deoxidizer in steel smelting. However, excessive Al can cause increased resistance at the crystallizer inlet during continuous casting. Therefore, this invention requires the Al content to be in the range of 0.01% to 0.10%.

[0085] Nb: 0.01~0.1%, Ti: 0.01~0.1%

[0086] The addition of Nb and Ti can form carbides, nitrides, or carbonitrides with carbon and nitrogen. The uniform and fine precipitation, which anchors the original austenite grain boundaries, refines the grains, thus improving the strength and toughness of the steel. Simultaneously, the precipitates are dispersed within the steel matrix, resulting in precipitation strengthening. Therefore, the components described in this invention can be added in appropriate amounts. Furthermore, Ti and N have a strong bonding force. Adding a certain amount of Ti to hot-stamping steel sheets can form TiN with the N in solid solution in the steel, preventing the combination of N and B elements, thus consolidating N and protecting B, ensuring hardenability. However, excessive addition of the above microalloying elements will lead to a significant increase in cost. Therefore, this invention requires the Nb and Ti contents to be in the range of 0.01% to 0.1%, respectively.

[0087] V: 0~0.10%

[0088] The role of V in precipitation strengthening and grain refinement is similar to that of Nb and Ti. Optionally, the amount of V added does not exceed 0.10%, and is optionally in the range of 0.01 to 0.10%.

[0089] P: 0~0.020%

[0090] In steel, phosphorus (P) is an unavoidable impurity element. On the one hand, P, as a solid solution strengthening element, can relatively inexpensively improve the strength of steel plates. On the other hand, when the P content exceeds 0.0200%, P will segregate at grain boundaries, leading to significant adverse effects such as a decrease in toughness. Therefore, the upper limit of P content is no more than 0.020%, preferably no more than 0.010%. Optionally, considering that keeping the P content below 0.001% would increase smelting costs, the lower limit of P content can be no less than 0.001%, and optionally no less than 0.004%.

[0091] S: 0~0.010%

[0092] Similar to phosphorus (P), sulfur (S) is an unavoidable impurity element in steel. It reacts with manganese (Mn) in the steel to form MnS, becoming inclusions in the steel. When the S content exceeds 0.010%, a large amount of MnS will significantly impair the ductility and toughness of the steel, leading to a deterioration in workability. Therefore, the upper limit of the S content is no more than 0.010%, preferably no more than 0.005%. Similarly, optionally, considering that keeping the S content below 0.0001% would increase smelting costs, the lower limit of the S content can be no less than 0.0001%, preferably no less than 0.0005%, and more preferably no less than 0.001%.

[0093] N: 0~0.010%

[0094] Nitrogen (N) is an unavoidable impurity element in steel, especially for steel containing boron (B). Nitrogen combines with B, significantly reducing B's effect on improving hardenability. Therefore, it is necessary to minimize the N content. Thus, the N content in this invention is controlled to not exceed 0.010%. However, in industrial production, all N removal processes imply increased production costs. Therefore, considering production costs, the lower limit for N content can be no less than 0.0001%.

[0095] Ni: 0~0.10%

[0096] Ni is a hardenability-improving element, and it can also improve the low-temperature toughness of steel. Therefore, its lower limit value can be optionally set to 0.01%. F b-s With a factor coefficient of 1.3, the large addition of Ni will inevitably lead to an increase in alloy cost and result in... F b-s The content of Ni is increased. Therefore, the Ni content is set to no more than 0.10%.

[0097] Mo: 0~0.15%

[0098] Mo can improve the hardenability of steel plates and prevent temper brittleness. Preferably, it can be added at 0.01% or more. However, excessive Mo addition will lead to the formation of a large number of carbides within the steel plate microstructure and their segregation at grain boundaries. F b-s The factor coefficient is 1.6, which is much higher than that of Mn. Therefore, excessive addition of Mo reduces the toughness of hot-stamped steel and increases costs. Therefore, the Mo content is set to be below 0.15%.

[0099] W: 0~0.20%

[0100] The addition of W can significantly increase the strength of steel plates, and W-containing precipitates can serve as hydrogen capture sites; therefore, it is preferable to add more than 0.001%. However, W is a strong carbide-forming element, and when the W content exceeds 0.20%, coarse and unevenly distributed carbides will be formed, which will seriously impair the performance of hot-stamped components. Therefore, the W content is set to below 0.20%.

[0101] Cu: 0~0.20%

[0102] Cu increases the hardenability of steel sheets, thereby improving the strength of hot-stamped components. To achieve this effect, the Cu content is preferably set to 0.001% or more. However, Cu tends to segregate at grain boundaries and form copper embrittlement, reducing the hot workability of the steel sheet; therefore, the Cu content is set to 0.20% or less.

[0103] Co: 0~0.50%

[0104] The addition of Co can increase the martensitic transformation temperature, which is beneficial for the self-tempering of martensite and thus improves its toughness. Therefore, its lower limit can preferably be set at 0.01%. However, Co is a rare and valuable element, and excessive addition will increase the cost of steel sheets. Moreover, Co will promote the graphitization of carbon in steel, which will damage the toughness of hot-stamped components. Therefore, the addition of Co is set to below 0.50%; alternatively, the Co content is set to 0.01% ≤ Co ≤ 0.50%.

[0105] Ta: 0~0.10%

[0106] Ta, as a strong carbide-forming element, can form stable carbides with C, thereby refining the grain size and enhancing the strength and toughness of the steel plate. Therefore, to achieve this effect, the Ta content is preferably not less than 0.001%. Furthermore, when the Ta content exceeds 0.10%, a large amount of carbides will precipitate and easily accumulate at grain boundaries, leading to a decrease in the toughness of the steel plate. Therefore, the Ta content is set to not exceed 0.10%.

[0107] Sn: 0~0.05%

[0108] The addition of Sn can refine the grain size and improve the machinability of steel plates by combining with elements such as S. Preferably, the Sn content is not less than 0.001%. However, Sn tends to segregate at grain boundaries, increasing the brittleness of the steel. Therefore, the Sn content is set to not exceed 0.05%.

[0109] Sb: 0~0.05%

[0110] Sb also has the effect of refining grains, so to achieve this effect, the preferred Sb addition is not less than 0.001%. However, excessive segregation at grain boundaries can also have an adverse effect on the properties of steel, especially its plasticity and toughness, so the Sn content is set to not exceed 0.05%.

[0111] As: 0-0.05%

[0112] As can strengthen steel to a certain extent, increasing its strength. However, as tends to accumulate at grain boundaries, leading to increased brittleness and deteriorated weldability. Therefore, the as content is set at 0 ≤ As ≤ 0.05%.

[0113] Mg: 0~0.010%, Ca: 0~0.010%

[0114] Mg and Ca are commonly used for deoxidation during steel smelting and can also form sulfides with S, improving the quantity and morphology of inclusions in the steel sheet structure. Therefore, their addition amount is preferably not less than 0.001%. However, when the content of both Mg and Ca exceeds 0.010%, large-sized inclusions are formed in the steel sheet structure, damaging the toughness of hot-stamped components. Therefore, the content of Mg and Ca is set to not exceed 0.010%.

[0115] Zr: 0~0.10%

[0116] Zr is a strong carbide-forming element, with effects similar to those of V, Nb, and Ti. Adding a small amount can refine the grain size and improve the low-temperature toughness of the steel plate. Preferably, the Zr content is not less than 0.001%. However, when the Zr content exceeds 0.10%, large-sized carbides form within the steel plate structure, leading to a decrease in the toughness of the hot-stamped components. Therefore, the Zr content is set to 0 ≤ Zr ≤ 0.10%.

[0117] O: 0~0.02%

[0118] O forms coarse oxides in steel, which is detrimental to the toughness of hot-stamped components. Therefore, a lower O content is preferred, and the O content should not exceed 0.02%, more preferably 0.006% or less.

[0119] REM: 0~0.05%

[0120] REM (refined oxidizing agent) improves deformability and toughness of hot-stamped components by inhibiting oxide formation. To achieve this effect, the REM content is preferably set to 0.001% or more. However, the REM content should be controlled to not exceed 0.05% to avoid the formation of coarse oxides. In this invention, REM refers to a total of 17 elements including Sc, Y, and La series elements, and the REM content refers to the total content of these elements.

[0121] Optionally, 0 ≤ Mo + W + Cu + Co + Sn + Sb + Ca + Mg + Zr + REM ≤ 0.30%.

[0122] Elements such as Mo and W can improve the hardenability of steel, but the addition of these alloying elements increases the cost of the alloy. Therefore, they should only be added in appropriate amounts to the material. Meanwhile, Mo can combine with Cr and V to precipitate in combination, consuming C in the steel matrix, thus playing a role in precipitation strengthening and improving the toughness of the martensitic matrix. Therefore, considering the economic efficiency of the material alloy, the total amount of the above alloying elements should be controlled to not exceed 0.30%.

[0123] It should be noted that in the steel sheet of the exemplary embodiment of the present invention, the remaining portion other than the components described above consists of Fe and impurities. Impurities are components that are mixed into the steel sheet during industrial manufacturing using raw materials such as ore and scrap iron, and include substances that are not intentionally added relative to the steel sheet according to the embodiment of the present invention (so-called unavoidable impurities). Furthermore, impurities are elements other than the components described above, and also include elements contained in the steel sheet at a level that does not affect the characteristics of the steel sheet of the embodiment of the present invention due to the unique effects of that element. For example, sometimes unavoidable impurity elements such as As are mixed in from scrap, but if it is within the normal range, it will not affect the characteristics of the steel matrix of the hot-stamping steel sheet.

[0124] The chemical composition of the steel substrate of the aforementioned hot-stamped steel sheet and hot-stamped components can be determined using general analytical methods. For example, it can be determined using ICP-AES (Inductively Coupled Plasma-Atomic Emission Spectrometry). If the surface of the hot-stamped component has a coating or paint film, the chemical composition can be analyzed after removing the coating or paint film by mechanical grinding.

[0125] To achieve the desired technical effect, the present invention further synergistically adjusts the alloy ratio within the above-mentioned component ranges to simultaneously satisfy the following:

[0126] Mn eq =Mn + 1.29Cr + 3.28Mo + 0.46Cu + 0.37Ni + 0.07Si and Mn eq ≥1.85%

[0127] Mn equivalent, Mn eq This characterizes the contribution of alloying elements other than carbon to hardenability. For 1000 MPa grade steel plates with low carbon design, Mn is the main alloying element affecting hardenability. eqTo ensure the strength of hot-stamped components, the microstructure must produce as much martensite as possible during the hot-stamping process. Therefore, to ensure hardenability, Mn is required to... eq Not less than 1.85%. Optionally, Mn eq Not less than 1.90%.

[0128] Ac3=902-255C-11Mn+19Si-5Cr+13Mo-20Ni+55V, Ac3≤880℃

[0129] Ac3 represents the temperature at which the material achieves complete austenitization. Increasing the Ac1 temperature by adjusting the alloying elements such as Si, Cr, and Mn will also change the Ac3 temperature. However, in order to ensure the final strength of the hot-stamped material, the material needs to achieve complete austenitization during hot stamping heating, and then obtain as much martensite as possible during cooling. Therefore, this invention controls the Ac3 temperature to be no higher than 880°C, and optionally, Ac3 ≤ 875°C.

[0130] Ac1=754.83-32.25C-17.76Mn+23.23Si+17.3Cr+4.5Mo+15.62Ni, Ac1≥730℃

[0131] Ac1 represents the temperature at which austenitization begins in the steel sheet during annealing heating. A higher Ac1 temperature means that less austenite is formed in the steel sheet at the same heating temperature. For 1000 MPa grade steel sheets, due to their low-carbon design, they inherently have a high Ac1. The inventors have optimized the proportions of alloying elements such as Si, Cr, and Mn to increase Ac1 to no less than 730℃, making austenite transformation in the steel sheet more difficult to occur at commonly used annealing temperatures. In this case, the material only undergoes recovery recrystallization or a small amount of austenitic phase transformation, thereby minimizing the transformation of supercooled austenite to martensite during subsequent cooling, and thus reducing the proportion of martensite phase in the room temperature microstructure of the annealed or annealed coated steel sheet, thereby improving the work hardening ability of the steel sheet and reducing its strength. Optionally, Ac1 ≥ 735℃. Optionally, Ac1 ≥ 738℃. Optionally, Ac1 ≥ 745℃.

[0132] Segregation factor F b-s =1.4Mn+1.1Cr+1.4Si+1.3Ni+1.6Mo-0.78Al, F b-s ≤3.2%

[0133] Because 1000 MPa grade hot-stamping steel sheets are designed with low carbon, alloying elements such as Mn, Si, Cr, Ni, and Mo are added to improve hardenability. However, while the addition of a large number of alloying elements can ensure the hardenability of the material, it also easily leads to segregation, resulting in banded structures within the steel sheet microstructure. This, in turn, causes significant directional differences in the properties of the hot-stamping steel sheet. Therefore, to improve the banded segregation of hot-stamping steel sheets, reduce the weakening of toughness in the transverse direction, reduce the anisotropic differences in toughness, and thus ensure the overall impact deformation capability of hot-stamped components, the influence of banded segregation-forming elements must be comprehensively considered in the alloy design. Since segregation of alloying elements generally occurs during the liquid-phase solidification of alloy smelting, in order to quantitatively determine the influence of elements such as Mn, Cr, Si, Ni, and Mo on the formation of banded structures, the inventors input the alloy composition of this invention into the thermodynamic calculation software Thermo-Calc to obtain the corresponding equilibrium phase diagram and the content of each element in the liquid phase and δ-ferrite phase at different temperatures. Then, based on the phase diagram, the temperature corresponding to the maximum δ-ferrite content was found, and the content of each element in the liquid phase at that temperature was compared with the content in the already formed δ-ferrite phase to obtain the influence coefficient of each element on segregation formation, thereby quantitatively clarifying the influence of banded structure forming elements Mn, Cr, Si, Ni, and Mo. In subsequent composition design, while ensuring hardenability, the amount of each alloying element added is controlled by adjusting the amount added, replacing alloying elements that easily cause segregation with alloying elements that have low or low segregation rates, such as replacing Mn with Cr. This achieves economic efficiency while controlling the segregation of the structure to a low level. Therefore, this invention controls the segregation factor. F b-s Not higher than 3.2%. Optionally, F b-s Not higher than 3.1%. Optionally, F b-s Not higher than 3.0%. Optional, F b-s Not higher than 2.9%.

[0134] The measurement or fitting method involved in this invention is as follows:

[0135] Band segregation evaluation method

[0136] According to the specifications in GB / T 34474.1-2017, samples were taken at multiple locations along the RD direction from each obtained steel plate. Then, the degree of banding was evaluated for each steel plate sample. Specifically, for each sample, a metallographic sample was prepared from a cross-section coplanar in the thickness and RD directions according to the specifications in the standard. Then, photographs were taken at least three locations on the cross-section using an optical microscope at a magnification of 500x, resulting in a field of view of approximately 290 μm × 195 μm. The photographs were then compared with a standard rating chart.

[0137] Microstructure measurement methods

[0138] For the measurement of room temperature microstructures of samples, such as martensite (M) and ferrite (F), for each sample, a section perpendicular to the coplanar planes of the RD and TD directions is used as the observation surface. This is further prepared as a metallographic sample and observed using a metallographic microscope or scanning electron microscope (SEM). Typically, under a metallographic microscope, the microstructure is magnified to 500x, at which point the field of view is approximately 290 μm × 195 μm. Under SEM, the microstructure is magnified to 5000x, at which point the field of view is approximately 56 μm × 42 μm. For the field of view to be measured, the proportion of each phase within the microstructure is obtained using the area method: the area occupied by the identified microstructure within the field of view is divided by the total area of ​​that field of view to obtain the proportion (%) of that microstructure. At least three fields of view are measured for each sample to obtain the average value.

[0139] Tensile property test method

[0140] Tensile properties were tested according to GB / T 228.1 Metallic materials, tensile testing—Part 1: Tests at room temperature. For annealed and annealed coated specimens of hot-stamped steel sheets, P6 specimens (L0=80 mm, b0=20 mm) were used. For the mechanical property testing of hot-stamped components in the hot-stamped state and the mechanical property testing of baked components after coating and baking treatment, P5 specimens (L0=50 mm, b0=12.5 mm) were used. All specimens were taken in the rolling direction. Test results may include: yield strength YS (MPa), tensile strength TS (MPa), elongation after fracture A (%), and engineering stress-engineering strain curves. At least three parallel samples were tested for each group of tests to obtain the average value.

[0141] Work hardening test method

[0142] In this invention, work hardening capacity refers to the degree of work hardening of a material under small deformation (elongation in the range of 1-3%), using the work hardening index n. 1~3% Characterization. 1~3% Calculated according to the following process:

[0143] 1. Obtain engineering stress using the tensile property tests described above. σ eng and engineering strain ε eng According to the conversion formula σ true = σ eng *(1+ ε eng )and ε true =ln(1+ ε eng True stress was obtained respectively. σ true Harmony and True Response ε true Then, the true stress is plotted. σ true -True Response ε true curve;

[0144] 2. Using exponential curves σ =K* ε n Fitting True Stress σ true -True Response ε true The curve, where n is the work hardening index, and the fitting range is 1~3% engineering strain, based on... ε true =ln(1+ ε eng The range corresponding to the true strain is 0.0100~0.0296. For example, the true stress-true strain curve obtained from the tensile test of annealed steel plate T1 is shown below. Figure 1 As shown, the function σ =1174* ε^0.170 This refers to the fitted curve corresponding to the true strain range of 0.0100 to 0.0296 (corresponding to 1% to 3% engineering strain), where 0.170 is the fitted n. 1~3% ;

[0145] 3. Each test group should have at least 3 parallel samples, and the average value should be used as the final n. 1~3% .

[0146] Fracture strain testing methods

[0147] Using wire cutting, flat rectangular notched specimens were cut from the hot-stamped component in the RD direction and in the TD direction perpendicular to the RD direction, respectively. Their geometric structures are schematically shown as follows: Figure 2As shown. The notched specimen has: long cut edges 11 and 12 along the length direction L, with a length of approximately 140 mm; short cut edges 13 and 14 at both ends of the long cut edges 11 and 12 and perpendicular to them, with a width of approximately 20 mm; and notches 15 respectively provided on each of the two long cut edges, with their centers located at L / 2, such that the notches are symmetrical in both the length direction L and the width direction W perpendicular to the length direction, and have a radius R of 5 mm.

[0148] After the notched specimens were prepared, they were tested using a standard tensile testing machine at a tensile rate of 0.3 mm / min. The following measurements were taken using vernier calipers:

[0149] a) Thickness of the notched section 20 of the notched specimen before tensile testing t 0. The notched section 20 is the portion of the specimen between the two notches 15. The dashed lines therein are only schematically illustrating this notched section and do not represent any structure. Specifically, at least five thickness measurements were performed within the notched section, approximately 5 mm from the center longitudinal axis of the specimen in the width direction, and the average value was taken.

[0150] b) Thickness of the notched specimen at the fracture surface after tensile fracture t f Specifically, at least five thickness measurements were performed at the fracture site within a width direction of approximately 5 mm from the center longitudinal axis of the specimen, and the average value was then taken.

[0151] Then, the equivalent fracture strain is calculated using the following equation (5). ε f .

[0152] (Equation 5)

[0153] Each test group should have at least three parallel samples, and the average value should be used as the final result. ε f The results in the RD direction and the TD direction are denoted as follows: ε fRD and ε fTD Then, the difference in fracture strain between the sample in the rolling direction and the transverse direction can be calculated. ε fRD – ε fTD .

[0154] The degree of material deformation is often measured by strain. Strain is the ratio of the difference between the deformed and undeformed dimensions of the deformed region to the undeformed dimension. Therefore, strain is a relative indicator. It is common knowledge in the industry that even for materials of different thicknesses, the strain at the moment of fracture remains consistent, indicating that fracture strain is an indicator independent of thickness and only related to the intrinsic properties of the material.

[0155] The method for manufacturing steel sheets according to exemplary embodiments of the present invention uses materials within the above-described composition range and undergoes hot rolling and cold rolling, annealing, or annealing plating processes. Hereinafter, an example of a method for manufacturing hot-stamping steel sheets according to embodiments of the present invention will be described, but it will be understood that the method for manufacturing hot-stamping steel sheets of the present invention is not limited to the following approach.

[0156] Materials with the components shown in Table 1 were prepared into test steel plates using the following process:

[0157] a) Steelmaking: Steelmaking is carried out by vacuum induction furnace, electric furnace or converter according to the composition in Table 1, and billets are produced by continuous casting technology or thin slab continuous casting and rolling process is directly adopted.

[0158] b) Hot rolling: The billet is heated to 1200℃ and held for 2 hours, then hot rolled at 800℃~1200℃, and coiled at 500~650℃ to form a hot-rolled steel coil. The hot-rolled coil is then pickled to remove the oxide scale generated during the hot rolling process.

[0159] c) Cold rolling: The hot-rolled coil that has been pickled is cold-rolled with a reduction of 30% to 70% to obtain a cold-rolled steel sheet with a thickness of 1.4 mm.

[0160] d) Annealing or annealing plating process:

[0161] d1) Heating step: Heat the cold-rolled steel sheet to 750°C and hold for 180 seconds; optionally, heat the cold-rolled steel sheet to a temperature in the range of 720~850°C;

[0162] d2) Cooling steps:

[0163] After heating step d1), the steel plate is slowly cooled (e.g., by a fan) to 600-700°C, then rapidly cooled (e.g., by air cooling) to 200-400°C, followed by slow cooling, i.e., aging treatment. After completion, it is air-cooled to room temperature to form the final annealed steel plate product, denoted as T1, T3, T5, T7 and CT1, CT3; or

[0164] d2*) Coating steps:

[0165] After heating step d1), the steel plate is slowly cooled to 650°C and then immersed in the heated plating bath for 2-7 seconds, during which the set temperature of the plating bath is maintained at 650°C; optionally, the set temperature of the plating bath is maintained in the range of 630-670°C, and optionally, 640-660°C. Then, after the steel plate leaves the plating bath and before the plating bath on at least one surface solidifies, excess plating bath on at least one surface is removed by air knife purging to control the coating thickness on said at least one surface; the steel plate is then air-cooled to room temperature to obtain a coated steel plate with an aluminum alloy coating, i.e., the final coated steel plate product, denoted as T2, T4, T6 and CT2, CT4, with average pre-coating thicknesses of 7.5 μm, 13.5 μm, 11.3 μm and 8.9 μm, 15.2 μm, respectively. Optionally, the plating bath used in this invention contains, by mass percentage: 9%-12% Si, less than 4% Fe, the balance being Al and impurities. Optionally, the average thickness of the aluminum alloy pre-coating obtained after coating is 5 to 33 µm. Optionally, the Si content in the plating bath is 9.2% to 11.2% by mass. It will be understood that other pre-coatings, such as zinc alloy pre-coatings, can also be applied to cold-rolled sheets.

[0166] T1-T7 are embodiments according to the present invention, and CT1-CT4 are comparative examples. eq (%), Ac3 (°C), Ac1 (°C), and F b-s (%) are calculated according to Equations 1-4 of the present invention, and are specifically listed in Table 1.

[0167] Table 1 Chemical composition of the steel matrix of hot stamping steel sheet (%, balance is Fe and impurities)

[0168]

[0169] Using the aforementioned methods for evaluating banded segregation, measuring microstructure, testing tensile properties, and fitting work hardening capacity, the proportion of martensite, the degree of banded segregation, and the work hardening index n of each of the experimental steel plates T1-T7 and CT1-CT4 were obtained. 1~3% The tensile properties (YS, TS and A) are shown in Table 2.

[0170] Table 2. Microstructure and property results of each test steel plate

[0171] serial number Martensite percentage, % Band segregation rating <![CDATA[n 1~3% ]]> YS, MPa TS, MPa A, % T1 3.5 2A 0.170 449 668 20.0 T2 5.7 2A 0.172 476 695 21.0 T3 7.2 2A 0.174 466 680 22.0 T4 2.4 2.5A 0.164 488 709 18.0 T5 2.5 2.5A 0.165 530 735 18.0 T6 0 2.5A 0.158 518 748 17.5 T7 3.0 2.5A 0.168 507 727 18.6 CT1 23 2A <![CDATA[ 0.234 ]]> 429 <![CDATA[ 880 ]]> <![CDATA[ 14.1 ]]> CT2 2.9 1.5A 0.167 460 676 20.0 CT3 0 <![CDATA[ 3A ]]> 0.155 468 687 19.8 CT4 2.2 <![CDATA[ 3A ]]> 0.160 486 715 20.4

[0172] Performance testing of hot stamped components

[0173] All test steel plates T1-T7 and CT1-CT4 were subjected to hot stamping process simulation. The specific process is as follows:

[0174] (1) Heating process: Heat the steel plate to 930℃ and hold for 240 s;

[0175] (2) Stamping process: The sample is then transferred to the press, the transfer time is 5~8 s, and then hot stamping is performed;

[0176] (3) Cooling process: After hot stamping, the parts are cooled to below 200°C to obtain the corresponding hot stamped components. In order to simulate the situation where some parts of the hot stamped components do not fit the mold well enough, resulting in a slightly slower cooling of the hot stamped components, the demolding temperature of the hot stamped components is controlled during the hot stamping process simulation. For example, it is controlled at 160°C. The hot stamped components obtained from T4 steel plates at this time are denoted as T4. p The ejection temperature of the remaining hot-stamped components is controlled at 100℃, and

[0177] (4) Optionally, to simulate the painting and baking process of automotive parts, the hot-stamped components are subjected to baking treatment (i.e., tempering treatment): the baking process can be set to 130~300℃ for 5~30 min. For example, hot-stamped components obtained from T1, T5, T6, T7 and CT1-CT4 steel plates are denoted as T1 after baking treatment at 170℃ for 20 min. p T5 p T6 p T7 p and CT1 p -CT4 p A hot-stamped component obtained from T2 steel plate, after being baked at 150℃ for 30 minutes, is denoted as T2. p A hot-stamped component obtained from T3 steel plate, after being baked at 250℃ for 7 minutes, is designated as T3. p ;

[0178] In addition, the above hot-stamped component T2 p T4 p T6 p and CT2 p CT4 p The average thicknesses of the aluminum-silicon coatings were 11.2 μm, 17.8 μm, 15.5 μm and 12.4 μm, 20.6 μm, respectively.

[0179] It will be understood that the method for manufacturing hot-stamped components using the hot-stamping steel sheet of the present invention and the thickness of the coating are not limited to the specific examples described above. Optionally, at least one surface of the steel substrate of the hot-stamped component is covered with an aluminum-silicon coating with an average thickness of 10-50 µm. Knowing the expected performance, those skilled in the art can select an appropriate production method from conventional methods as needed, for example, to soften the martensite structure, tempering at 130-300°C after hot stamping; or to control the demolding temperature of the hot-stamped component to enhance the effect of martensite self-tempering and thus toughen the martensite, so that the expected performance can be obtained even without subsequent baking (which is equivalent to tempering), for example, controlling the demolding temperature of the hot-stamped component at 100-200°C. It will be understood that for materials of the 1000 MPa grade, due to their high martensitic phase transformation temperature, the difference in performance due to coating and baking treatment is small. In addition, during the cooling process, by controlling the demolding temperature or adjusting the cooling rate, performance comparable to that after coating and baking can also be obtained, such as T4. p As demonstrated by test results of other samples that have undergone a painting and baking process (see Table 3). Therefore, the relevant properties of the hot-stamped components submitted in this invention can be obtained directly after the cooling process or after the painting and baking process of automotive parts.

[0180] Based on the aforementioned tensile property testing method and fracture strain testing method, the hot-stamped component T1 was tested. p -T7 p and CT1 p -CT4 p The test was conducted, and the specific results are shown in Table 3.

[0181] Table 3. Room temperature microstructure and properties of hot-stamped components

[0182] serial number YS, MPa TS, MPa A, % <![CDATA[ ε fRD ]]> <![CDATA[ ε fTD ]]> <![CDATA[ ε fRD - ε fTD ]]> Microstructure <![CDATA[T1 p ]]> 905 1124 7.2 0.95 0.85 0.10 Full Martensite <![CDATA[T2 p ]]> 984 1136 6.7 0.93 0.82 0.11 Full Martensite <![CDATA[T3 p ]]> 916 1115 6.4 1.02 0.94 0.08 Full Martensite <![CDATA[T4 p ]]> 916 1187 6.3 0.88 0.76 0.12 <![CDATA[Martensite +2.8% ferrite / bainite > <![CDATA[T5 p ]]> 1012 1182 6.5 0.90 0.80 0.11 Full Martensite <![CDATA[T6 p ]]> 980 1133 6.2 0.92 0.81 0.11 Full Martensite <![CDATA[T7 p ]]> 994 1153 6.5 0.92 0.79 0.13 <![CDATA[Martensite +4.6% ferrite / bainite > <![CDATA[CT1 p ]]> 967 1164 6.3 0.91 0.83 0.08 Full Martensite <![CDATA[CT2 p ]]> <![CDATA[ 750 ]]> <![CDATA[ 960 ]]> 7.8 not applicable not applicable not applicable <![CDATA[ Martensite + > 5.0% ferrite / bainite ]]> <![CDATA[CT3 p ]]> <![CDATA[ 761 ]]> <![CDATA[ 986 ]]> 7.5 not applicable not applicable not applicable <![CDATA[ Martensite + >5.0% ferrite ]]> <![CDATA[CT4 p ]]> 989 1187 6.6 <![CDATA[ 0.86 ]]> <![CDATA[ 0.69 ]]> <![CDATA[ 0.17 ]]> Full Martensite

[0183] "Not Applicable" Explanation: CT2 P and CT3 P Since the strength failed to meet the target requirements, although strength and toughness are contradictory, and lower strength can achieve higher fracture strain, the control of its structure, especially the generation of a large amount of ferrite and bainite, leads to poor stability of the structure of the final hot-stamped component. Consequently, the performance of the hot-stamped component is prone to large fluctuations, which is not the focus of this invention. Therefore, its toughness was not further measured.

[0184] Figure 3This image shows the typical room-temperature microstructure of the steel matrix after annealing CT1 steel plate. Due to the alloy design of CT1 (high Mn and low Cr), the Ac1 temperature is relatively low, only 725℃. Therefore, the room-temperature microstructure of this annealed CT1 sample not only contains ferrite and pearlite but also a large amount of martensite, accounting for approximately 23%. This results in significantly higher strength and work hardening capacity of the CT1 steel plate, achieving a tensile strength TS of up to 880 MPa, an elongation after fracture of only 14.1%, and an inductance of 0.234. 1~3% This not only increases the difficulty of leveling the steel plate CT1, but also results in an uneven distribution of residual stress inside the sample after leveling, which adversely affects the yield of subsequent processing.

[0185] In contrast, Figure 4 This image shows the typical room-temperature microstructure of steel plate T3 after annealing. Compared to CT1, to achieve a higher Ac1, the alloy design of steel plate T3 reduced the Mn content and increased the Cr content, resulting in an Ac1 temperature of 730℃. Under these conditions, the room-temperature microstructure of steel plate T3 is dominated by ferrite and pearlite, with martensite accounting for approximately 7.2%, a significant decrease compared to the approximately 69% martensite content in CT1. Correspondingly, the tensile strength TS of steel plate T3 is 680 MPa. 1~3% With a value of 0.174 and an elongation after fracture (A) as high as 22.0%, it exhibits excellent leveling performance. Therefore, in order to achieve high manufacturability of the steel plate and realize n 1~3% The value is not greater than 0.200. This application first requires Ac1 ≥ 730℃.

[0186] As the alloy composition is adjusted to further increase Ac1, the proportion of martensite in the room temperature microstructure of the steel plate gradually decreases, corresponding to n 1~3% It also gradually decreases. As shown in the results of T1 and T4-T7, when Ac1 ≥ 738℃, the proportion of martensite in the room temperature microstructure of the corresponding steel matrix does not exceed 4%, and correspondingly, n 1~3% Not greater than 0.170. Furthermore, when Ac1 ≥ 745℃, the proportion of martensite in the room temperature microstructure of the corresponding steel matrix does not exceed 2.5%, correspondingly, n 1~3% Not greater than 0.165. For example, such as Figure 5 As shown, the Ac1 of steel plate T6 is 748℃, slightly lower than the annealing temperature of 750℃. It should be understood that 748℃ is a theoretical calculation value; the actual phase transformation temperature is usually 5-10℃ higher. Therefore, for steel plate T6, its annealing is carried out below the actual Ac1 temperature. Consequently, the obtained room temperature microstructure consists entirely of ferrite and pearlite, without martensite. Correspondingly, n... 1~3%With a work hardening capacity of only 0.158, the work hardening capacity of this steel plate is further reduced compared to steel plate T3, which further reduces the uneven distribution of residual stress inside the steel plate after leveling, resulting in a better plate shape and facilitating subsequent processing. Therefore, to further improve the leveling performance, Ac1 can be further increased, optionally Ac1≥735℃; optionally Ac1≥738℃; and optionally Ac1≥745℃.

[0187] It will be understood that elongation is related to the work hardening ability of a material during plastic deformation. To a certain extent, sustained and effective work hardening can delay the occurrence of necking, thereby improving the material's plasticity and achieving a high elongation. In other words, good work hardening ability will promote an increase in the material's elongation. Therefore, relative to n... 1~3% In slightly lower cases (such as T4 to T6 steel plates for hot stamping), there is a slightly higher n. 1~3% Hot-stamped steel sheets (such as hot-stamped steel sheets T1 to T3) achieve higher elongation. High work hardening also contributes to improved final strength, but the tensile strength of hot-stamped steel sheets T1 to T3 is still relatively low. This is mainly because their yield strength is inherently lower. The reasons for this are as follows: Firstly, their microstructure contains a certain amount of martensite (≥3.5%), which leads to pre-yielding of ferrite, thus reducing yield strength. Secondly, the content of alloying elements such as C and Si added during the alloy design of T1 to T3 steel sheets is relatively low, resulting in a lower solid solution strengthening effect and consequently, slightly lower ferrite strength. These factors combined lead to low yield strength in steel sheets T1 to T3. Even during subsequent plastic deformation, the steel sheets exhibit high work hardening capacity due to the formation of martensite within their microstructure, but they still fail to achieve higher tensile strength compared to steel sheets T4 to T6.

[0188] As shown in Table 2, the tensile strength TS of the hot-stamping steel plates according to the embodiments of the present invention is not greater than 800 MPa, and the work hardening index n 1~3% The tensile strength TS of the hot-stamping steel sheet is not greater than 0.200, and the elongation after fracture is not less than 17.0%. Further, the tensile strength TS of the hot-stamping steel sheet is not greater than 750 MPa. Optionally, the work hardening index n of the hot-stamping steel sheet is... 1~3% Not greater than 0.180. Optionally, n 1~3% Not greater than 0.170. Optionally, n 1~3% No greater than 0.165.

[0189] It is worth noting that when adjusting the alloy to reduce the work hardening ability of annealed or annealed coated steel sheets, other expected properties should also be guaranteed, especially the target strength and toughness of the final hot-stamped component.

[0190] For hot-stamped components made from hot-stamping steel plates according to the present invention, the expected tensile strength TS is not less than 1100 MPa, the elongation after fracture is ≥5.0%, the fracture strain in both the TD direction and the TD direction is not less than 0.75, and the difference in fracture strain between the RD and TD directions is not greater than 0.14, that is, reducing the anisotropy of toughness, thereby effectively improving the collision safety of hot-stamped components.

[0191] As shown in Table 3, hot-stamped component T1 P To T7 P The tensile strength of all components is greater than 1100 MPa and the elongation after fracture is ≥5.0%. More specifically, the tensile strength is not less than 1115 MPa and the elongation after fracture is ≥6.0%, and the fracture strain in the RD and TD directions and the difference in fracture strain between the two directions meet the above-mentioned expected requirements. In contrast, the hot-stamped component CT2... P and CT3 P The strength of the hot-stamped component CT4 did not meet the standard. P The fracture strain and fracture strain difference in the TD direction both failed to meet the standards. The specific reasons are explained below.

[0192] Figure 6 , Figure 7 , Figure 8 , Figure 9 The hot-stamped component CT2 after baking is shown separately. P CT3 P T7 P and T3 P Typical room temperature microstructure morphology of steel matrix. For example... Figure 6 As shown, hot-stamped component CT2 P The typical room-temperature microstructure includes distinct ferrite and bainite regions (accounting for more than 5.0%), with a tensile strength (TS) of 960 MPa. Hot-stamped component CT3 P A large amount of ferrite was also found in the room temperature tissue, such as Figure 7 As shown, its tensile strength TS is 986 MPa. The presence of ferrite and / or bainite significantly reduces the final strength of the component, failing to meet the expected requirements. In contrast, the hot-stamped component T7... P The ferrite / bainite ratio in the room temperature microstructure is less than 5.0%, allowing for a sufficient amount of martensite (approximately 95.4%), thus achieving a tensile strength (TS) as high as 1153 MPa. Furthermore, as... Figure 9 As shown, hot-stamped component T3 P The room temperature microstructure is entirely martensite, with no obvious ferrite / bainite structure observed.

[0193] Based on the above results, the reasons for the undesirable presence of a large amount of ferrite and / or bainite (totaling greater than 5.0%) in the room temperature microstructure of hot-stamped components are as follows. For hot-stamped component CT2... P Its Mn content is low, which makes Mn eq With a hardenability of only 1.73%, it has poor hardenability. This leads to the easy formation of a certain volume of ferrite and / or bainite regions in the steel matrix during the cooling process of hot stamping. For hot-stamped components CT3... P Although its Mn eq The content was already greater than 1.85%, but ferrite still appeared in its room temperature microstructure. This is because the hot-stamped component CT3... P The steel matrix contains a high Si content to increase the Ac1 temperature, thereby solving the problem of high work hardening caused by the formation of martensite in the microstructure during annealing or annealing plating. However, this high Si content results in an Ac3 temperature as high as 886℃ for the steel matrix of steel plate CT3. This high Ac3 temperature makes the hot-stamped component CT3... P The steel matrix cannot achieve complete austenitization during the heating process of hot stamping, resulting in a ferrite content greater than 5.0% in the room temperature microstructure obtained after hot stamping, which reduces the tensile strength of the hot-stamped component. This also shows that adjusting one parameter can affect other parameters, therefore, the adjustment of these parameters needs to be considered holistically.

[0194] In contrast, although hot-stamped component T7 P manganese equivalent Mn eq With hot-stamped component CT3 P Similar to the previous method, but by adjusting the alloy ratio, its Ac3 temperature was lowered to 870℃, thereby reducing the ferrite / bainite content in the room temperature microstructure obtained after hot stamping to below 5.0%, ensuring the final tensile strength. Furthermore, by adjusting the alloy composition, such as the matching of alloying elements like Mn, Si, and Cr, the manganese equivalent Mn of the material was increased. eq Simultaneously lowering the Ac3 temperature can effectively reduce the formation of ferrite and bainite in the microstructure of hot-stamped components. When the hardenability of the material increases to a certain extent and the Ac3 temperature is effectively reduced, the steel plate can achieve full austenitization during the heating process of hot stamping, and ultimately form a fully martensitic microstructure during the subsequent cooling process, such as the T3 hot-stamped component. P Mn eq The concentration is 2.08%, and the Ac3 temperature is 867℃, increasing the manganese equivalent Mn. eqThis ensures the hardenability of the steel matrix and allows for full austenitization during the heating process of hot stamping, ultimately forming a fully martensitic structure (i.e., without ferrite and / or bainite) during subsequent cooling, further guaranteeing the final tensile strength. Therefore, to ensure that the tensile strength of the hot-stamped component is not less than 1100 MPa, this application not only requires Mn... eq ≥1.85% and Ac3≤880℃. Correspondingly, the room temperature microstructure of the steel matrix of the hot-stamped component is mainly martensite, and the proportion of ferrite and bainite by area does not exceed 5.0%, optionally not more than 3.0%, and further optionally not more than 2.0%.

[0195] Furthermore, comparing the hot-stamped components T5 and T7 obtained from steel plates, P and T7 P Based on the strength, under the same Ac3, it has a higher Mn content. eq Hot-stamped components T5 obtained from steel plate T5 P Its tensile strength is slightly higher. This is because higher Mn content results in slightly higher tensile strength. eq This means better hardenability, ensuring more martensite formation, which may improve the corresponding tensile strength. It should be noted that in Mn... eq With a concentration of ≥1.85%, sufficient hardenability can already be guaranteed, therefore further increasing Mn... eq No significant increase in intensity will be observed. However, given that other parameters are satisfied, it is still desirable to increase Mn. eq To ensure the effective guarantee of achieving the target strength. Optionally, Mn eq ≥1.90%; optionally, Mn eq ≥2.00%; optionally, Mn eq ≥2.10%. For Ac3, the requirement for full austenitization is already met at a temperature not exceeding 880℃. Therefore, further reducing Ac3 will not substantially change the strength of subsequent products, but further reducing Ac3 can ensure the realization of full austenitization. Therefore, Ac3 can optionally be ≤875℃; or Ac3 can optionally be ≤870℃.

[0196] As mentioned earlier, the toughness of hot-stamped components is also an important design parameter. A commonly used method for testing material toughness is the static three-point bending test (i.e., the VDA sharp bending test, VDA 238-100 standard). However, the inventors have found that when performing the VDA sharp bending test on materials with a VDA bending angle of 80° or more, due to the material's good toughness, the material does not fail even after the indenter reaches its limit displacement, making it impossible to detect the maximum bending angle and consequently, the fracture strain. To solve this problem, this invention uses a notched tensile test instead of the VDA sharp bending test. On the one hand, due to the presence of the notch, stress concentration occurs in the area near the notch during tensile deformation, making its stress significantly higher than in other areas. In this case, the work hardening effect of the material itself is insufficient to cause deformation in areas other than the notch. Therefore, the notched tensile test ensures that the test results are not affected by work hardening and only characterize the material's toughness. On the other hand, the notched tensile test determines the strain change by measuring the thickness change of the fracture area after the specimen is broken. Therefore, the measurement range of the notched tensile test is not affected by the toughness of the material and the limitations of the equipment, and it is more suitable for toughness testing of high toughness materials. The measurement results of the notched tensile test are shown in Table 3.

[0197] As shown in Table 3, the obtained hot-stamped component CT4 p The fracture strain in the RD direction is 0.86, while the fracture strain in the TD direction, which is perpendicular to the RD direction, is only 0.69. The difference in fracture strain between these two directions is as high as 0.17, exhibiting significant anisotropy. Furthermore, the lower fracture strain in the TD direction limits the application of the material, severely impacting the impact energy absorption performance of the hot-stamped component when applied without considering the material's directional properties. In contrast, the hot-stamped component T7 obtained from steel plate T7... p The fracture strain in the RD direction is 0.92, and the fracture strain in the TD direction relative to CT4 is... p It increased by approximately 14% to 0.79, and the fracture strain in both directions was greater than 0.75 with a difference of 0.13, compared to CT4. p This reduction of approximately 28% not only increases the fracture strain in the TD direction but also narrows the difference in fracture strain between the TD and RD directions, effectively improving the lower limit of toughness and impact energy absorption performance of hot-stamped components. The reasons for this performance improvement are as follows.

[0198] Combining the results in Tables 1, 2, and 3, it can be seen that the fracture toughness of hot-stamped components and the fracture strain and their difference in the RD and TD directions are affected by the degree of banded segregation of the hot-stamping steel sheet used, and this degree of banded segregation is influenced by the banded microstructure sensitive factor. F b-s The impact.

[0199] Specifically, steel plate T7 and steel plate CT4 F b-s They are 3.2% and 3.5% respectively, as if respectively by Figure 10 (T7) and Figure 11 The typical room temperature microstructure of the steel matrix after annealing and annealing coating treatment in (CT4) is shown. Compared with the room temperature microstructure of CT4, the room temperature microstructure of steel plate T7 has significantly less banded structure. Comparison with standard rating charts shows that the banded segregation ratings for steel plate T7 and steel plate CT4 are 2.5A and 3A, respectively. The reduced banded structure in steel plate T7 compared to CT4 lowers the degree of martensitic delamination in the resulting hot-stamped component, thereby reducing the adverse effect of banded structure on toughness in the TD direction (i.e., increasing fracture strain in the TD direction), and thus narrowing the difference in toughness between different directions in the hot-stamped component.

[0200] Furthermore, steel plate T1 F b-s It is 2.9%. Compared to steel plate T7 (see...) Figure 10 Compared to the previous method, the room temperature microstructure of the steel matrix after T1 annealing of the steel plate showed less banded structure, such as... Figure 12 As shown. After comparison with the standard rating chart, the banded segregation rating of steel plate T1 is 2A, therefore the fracture strain energy in the TD direction is further improved compared to steel plate T7, reaching 0.85, and the difference in fracture strain between its RD and TD directions is 0.10. It can be seen that by further adjusting... F b-s It can reduce the degree of banded segregation in the room temperature structure of steel plate after annealing / plating treatment, thereby improving the uniformity of the martensitic structure of the final hot stamping component, effectively improving the fracture strain in the TD direction, thereby reducing the fracture strain difference between the RD and TD directions of the hot stamping component, and further helping to improve the overall fracture strain of the material, so that the hot stamping component has better impact energy absorption performance.

[0201] To further clarify F b-s The relationship between the fracture strain difference and the obtained results was numerically fitted. For example... Figure 13 As shown, F b-s The relationship between the linear fit of the fracture strain difference and the equation is as follows: ε fRD - ε fTD =0.1131 F b-s -0.2284, with a goodness-of-fit R² of 0.9645, indicating a good linear relationship. Based on this relationship and the data in Tables 1-3, this invention requires...F b-s ≤3.2% so that the banded segregation rating is within 2.5A, the corresponding hot-stamped component has a fracture strain of not less than 0.75 in both the RD direction and the TD direction, and the difference between the fracture strain of the hot-stamped component in the RD direction and the TD direction is not greater than 0.14.

[0202] Optionally, the present invention requires F b-s ≤3.1%, the corresponding hot-stamped component has a fracture strain of not less than 0.76 in both the RD direction and the TD direction, and the difference between the fracture strain of the hot-stamped component in the RD direction and the TD direction is not greater than 0.13.

[0203] Optionally, the present invention requires F b-s ≤3.0%, the corresponding hot-stamped component has a fracture strain of not less than 0.80 in both the RD direction and the TD direction, and the difference between the fracture strain in the RD direction and the TD direction of the hot-stamped component is not greater than 0.11.

[0204] Optionally, in F b-s Based on ≤3.0%, this invention requires Meq ≥ 2.00% and Ac3 ≤ 875℃ to ensure that the room temperature microstructure of the hot-stamped component is a fully martensitic structure, thereby achieving high strength and high toughness in the hot-stamped component. Furthermore, in F b-s Based on the conditions of ≤3.0%, Meq≥2.00%, and Ac3≤875℃, this invention requires Ac1≥738℃ such that n 1~3% Not greater than 0.170, thus the hot-stamped component has high strength and high toughness, while the corresponding steel plate has improved work hardening ability.

[0205] Optionally, the present invention requires F b-s ≤2.9% so that the banded segregation rating is 2A, the corresponding hot-stamped component has a fracture strain of not less than 0.80 in both the RD direction and the TD direction, and the difference between the fracture strain of the hot-stamped component in the RD direction and the TD direction is not greater than 0.10.

[0206] It is worth noting that the above-mentioned Mn eq Ac3, Ac1 and F b-s Control is achieved by adjusting the alloy content, and adjusting the alloy content for one parameter will inevitably affect the values ​​of other parameters. For example, adjusting the alloy to meet the condition of Ac1 may, for example, lead to Ac3 or... Fb-s The condition is not met. Therefore, Mn eq Ac3, Ac1 and F b-s These are interconnected and mutually influential, and should be considered as a whole. Therefore, in order to improve the leveling performance of hot-stamping steel sheets and ensure the strength and toughness of hot-stamped components obtained from hot-stamping steel sheets, the present invention requires that the following conditions be met simultaneously: Mn eq ≥1.85%, Ac3≤880℃, Ac1≥730℃ and F b-s ≤3.2%.

[0207] The above embodiments and experimental data are intended to illustrate the present invention by way of example. Those skilled in the art should understand that the present invention is not limited to these embodiments, and various modifications can be made without departing from the scope of protection of the present invention.

Claims

1. A steel plate for hot stamping, wherein, The chemical composition of the steel matrix of the hot-stamping steel sheet, by mass percentage, includes: 0.060%≤C≤0.100%, 1.35%≤Mn≤1.70%, 0.10%≤Si≤0.70%, 0.15%≤Cr≤0.70%, 0.01%≤Al≤0.10%, 0.01%≤Ti≤0.1%, 0.001%≤B≤0.01%, 0.01%≤Nb≤0.1%, and at least one of the following: 0≤N≤0.010%, 0≤P≤0.020%, 0≤ S≤0.010%, 0≤V≤0.10%, 0≤Ni≤0.10%, 0≤Mo≤0.15%, 0≤W≤0.20%, 0≤Cu≤0.20%, 0≤Co≤0.50%, 0≤Ta≤0.10%, 0≤Sn≤0.05%, 0≤Sb≤0.05%, 0≤As≤0.05%, 0≤Ca≤0.010%, 0≤Mg≤0.010%, 0≤Zr≤0.10%, 0≤O≤0.02%, and 0≤REM≤0.05%, with the balance being Fe and impurities. Wherein, the steel matrix of the hot stamping steel plate satisfies: Mn eq ≥1.85%, Ac3≤880℃, Ac1≥735℃ and F b-s ≤3.2%, Among them, Mn eq =Mn+1.29Cr+3.28Mo+0.46Cu+0.37Ni+0.07Si; Ac3=902-255C-11Mn+19Si-5Cr+13Mo-20Ni+55V; Ac1 = 754.83 - 32.25C - 17.76Mn + 23.23Si + 17.3Cr + 4.5Mo + 15.62Ni; and F b-s =1.4Mn+1.1Cr+1.4Si+1.3Ni+1.6Mo-0.78Al。 2. The hot-stamping steel plate according to claim 1, wherein, Mn eq ≥1.90%。 3. The hot-stamping steel plate according to claim 1, wherein, Mn eq ≥2.00%, and / or Ac3≤875℃.

4. The hot-stamping steel plate according to claim 1, wherein, F b-s ≤3.1%。 5. The hot-stamping steel plate according to claim 1, wherein, Ac1 ≥ 738℃, and / or F b-s ≤3.0%.

6. The hot-stamping steel plate according to claim 1, wherein, Ac1 ≥ 745℃, and / or F b-s ≤2.9%.

7. The hot-stamping steel sheet according to any one of claims 1 to 6, wherein, The hot-stamping steel sheet has the following properties: tensile strength TS≤800 MPa, work hardening index n 1~3% ≤0.20, and elongation after fracture ≥17.0%.

8. The hot-stamping steel sheet according to any one of claims 1 to 6, wherein, The hot-stamping steel sheet has the following properties: tensile strength TS≤750 MPa, work hardening index n 1~3% ≤0.180, and elongation after fracture ≥17.0%.

9. The hot-stamping steel plate according to claim 8, wherein, The hot-stamping steel sheet has the following properties: work hardening index n 1~3% ≤0.

170.

10. The hot-stamping steel sheet according to any one of claims 1 to 6, wherein, In terms of area percentage, the room temperature microstructure of the steel matrix is ​​mainly composed of ferrite and pearlite, with martensite accounting for no more than 8.0%.

11. The hot-stamping steel sheet according to any one of claims 1 to 6, wherein, In terms of area percentage, the room temperature microstructure of the steel matrix is ​​mainly composed of ferrite and pearlite, with martensite accounting for no more than 4.0%.

12. The hot-stamping steel sheet according to any one of claims 1 to 6, wherein, 0.065%≤C≤0.090%, and / or 0.15%≤Si≤0.55%, and / or 1.35%≤Mn≤1.65%, and / or 0.20%≤Cr≤0.60%.

13. The hot-stamping steel sheet according to any one of claims 1 to 6, wherein, The chemical composition of the steel matrix, by mass percentage, comprises at least one of the following: 0.0001%≤N≤0.010%, 0.001%≤P≤0.020%, 0.0001%≤S≤0.010%, 0.01%≤V≤0.10%, 0.01%≤Ni≤0.10%, 0.01%≤Mo≤0.15%, 0.001%≤W≤0.20%, 0.001%≤Cu≤0.20%, 0 0.001%≤Co≤0.50%, 0.001%≤Ta≤0.10%, 0.001%≤Sn≤0.05%, 0.001%≤Sb≤0.05%, 0.001%≤As≤0.05%, 0.001%≤Ca≤0.010%, 0.001%≤Mg≤0.010%, 0.001%≤Zr≤0.10%, O≤0.006%, and 0.0001%≤REM≤0.05%.

14. The hot-stamping steel sheet according to any one of claims 1 to 6, wherein, The degree of banded segregation in the room temperature microstructure of the steel matrix of the hot stamping steel plate does not exceed grade 2.5A of the standard rating chart of GB / T 34474.1-2017.

15. The hot-stamping steel sheet according to any one of claims 1 to 6, wherein, At least one surface of the steel substrate is coated with an aluminum alloy pre-plating layer with an average thickness of 5 to 33 µm.

16. A hot-stamped component made of a hot-stamped steel sheet according to any one of the preceding claims, the hot-stamped component comprising a steel substrate.

17. The hot-stamped component according to claim 16, which has undergone a coating and baking process.

18. The hot-stamped component according to claim 16 or 17, wherein the tensile strength TS≥1100 MPa, the elongation after fracture≥5.0%, the fracture strain in the rolling direction and in the transverse direction perpendicular to the rolling direction is not less than 0.75, and the difference in fracture strain between the hot-stamped component in the rolling direction and in the transverse direction is not greater than 0.

14.

19. The hot-stamped component according to claim 16 or 17, wherein the tensile strength TS≥1115 MPa, the elongation after fracture≥6.0%, the fracture strain in the rolling direction and in the transverse direction perpendicular to the rolling direction is not less than 0.76, and the difference in fracture strain between the rolling direction and the transverse direction is not greater than 0.

13.

20. The hot-stamped component according to claim 16 or 17, wherein the tensile strength TS≥1115 MPa, the elongation after fracture≥6.0%, the fracture strain in the rolling direction and in the transverse direction perpendicular to the rolling direction is not less than 0.80, and the difference in fracture strain between the rolling direction and the transverse direction is not greater than 0.

11.

21. The hot-stamped component according to claim 16 or 17, wherein, The room temperature microstructure of the steel matrix of the hot-stamped component is mainly martensite, with ferrite and bainite accounting for no more than 5.0% of the area.

22. The hot-stamped component according to claim 16 or 17, wherein, The room temperature microstructure of the steel matrix of the hot-stamped component is mainly martensite, with ferrite and bainite accounting for no more than 2.0% of the area.

23. The hot-stamped component according to claim 16 or 17, wherein, At least one surface of the steel substrate of the hot-stamped component is covered with an aluminum-silicon coating with an average thickness of 10 to 50 µm.

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