A hot-stamped component with excellent resistance to low-temperature brittleness and its manufacturing method

Abstract

This invention discloses a hot-stamped component with excellent resistance to low-temperature brittleness, containing Fe and unavoidable impurities, and also containing the following chemical elements in the following mass percentages: C: 0.26–0.40%, Si: 0.1–1.5%, Mn+Cr: 0.5–3.0%, Al: 0.01–0.50%, Nb+Ti: 0.04–0.25%, Mo+Ni: 0.1–1.0%, B: 0.001–0.005%. The matrix of the microstructure of the hot-stamped component has lath-shaped tempered martensite with a volume ratio higher than 85%. The microstructure of the hot-stamped component also has precipitates, wherein the amount of Nb and Ti precipitates accounts for more than 50% of the total mass fraction of these two elements, and the Nb and Ti precipitates are dispersed in the matrix in a granular manner. Accordingly, this invention also discloses a method for manufacturing the above-mentioned hot-stamped component.

Description

Technical Field

[0001] This invention relates to a hot-stamped component and its manufacturing method, and more particularly to a hot-stamped component resistant to low-temperature brittleness and its manufacturing method. Background Technology

[0002] In recent years, with the gradual increase in the number of cars on the road worldwide, the resulting environmental pollution and energy consumption have become increasingly serious. Lightweight technology for automobiles can effectively alleviate these problems, and therefore, automobile manufacturers have conducted a great deal of research on lightweighting of automobiles.

[0003] Research has found that when ultra-high-strength steel is used to manufacture automotive structures, it can reduce vehicle weight while ensuring safety. However, in the actual production of ultra-high-strength steel products with tensile strengths exceeding 1180 MPa, cold forming is extremely difficult, and springback control is insufficient. In contrast, components produced by hot stamping have the characteristics of ultra-high strength, ease of forming, and high dimensional accuracy. Therefore, this type of hot-formed ultra-high-strength steel product has gradually become one of the important technical solutions for vehicle body lightweighting.

[0004] Currently, with the gradual upgrading of collision safety regulations, the market demand for hot-stamping steel with a tensile strength exceeding 1700MPa is also increasing. However, research has found that hot-stamping steel products with a tensile strength exceeding 1700MPa perform poorly in terms of bending and impact properties, especially at -60℃. This leads to a risk of brittle cracking in hot-stamped components during service in extremely cold regions, which could cause incalculable losses to users' lives and property.

[0005] Therefore, to ensure the service life of hot-stamped parts in extreme low-temperature environments, the low-temperature brittleness of materials must be considered when manufacturing ultra-high-strength hot-stamped parts. The currently used VDA238-100 bending performance test is relatively close to the actual collision failure mode of parts, so the evaluation of low-temperature VDA (bending angle) is particularly important.

[0006] However, while hot-stamping steels with a strength of 1700MPa have been published to achieve toughness, they also have several shortcomings, and related studies have not paid attention to or explained the low-temperature brittleness problem under planar deformation conditions.

[0007] For example, Chinese patent document CN110423953A, published on November 8, 2019, entitled "A hot-formed component with excellent bending properties and a tensile strength of 1800 MPa or higher and its preparation method," discloses a hot-formed component with excellent bending properties and a tensile strength of 1800 MPa or higher, comprising the following chemical composition by weight percentage: C: 0.29-0.35%, Si≤0.5%, Mn: 0.5-1.5%, P≤0.020%, S≤0.010%, Cr≤0.50%, Al: 0.01-0.06%, Nb: 0.01-0.06%, V: 0.01-0.06%, Mo≤0.5%, with the remainder being Fe and unavoidable impurities; the surface layer of the hot-formed component is a soft-phase ferrite structure, and the inner layer is a martensitic structure. In this technical solution, the bending performance of the material is improved by using a surface softening phase. However, surface softening inevitably leads to a reduction in the impact resistance of the component. Furthermore, this technical solution does not address the bending performance under low-temperature conditions.

[0008] For example, Chinese patent document CN106460115A, published on February 22, 2017, entitled "Heat-Treated Steel and Manufacturing Method Thereof," discloses a heat-treated steel with a tensile strength of over 1.8 GPa and excellent toughness and weldability. Its chemical composition is designed as follows: C: 0.05–0.30%, Mn: 2.0%–10.0%, Cr: 0.01%–1.0%, Ti: 0.01%–0.0%. 0.1%, B: 0.001%~0.01%, Si: less than 0.08%, P: less than 0.05%, S: less than 0.05%, N: less than 0.01%, Ni: 0%~2.0%, Cu, Mo, V: 0%~1.0% respectively, the remainder being Fe and unavoidable impurities; when the C equivalent is [C] and the Mn equivalent is [Mn], it satisfies "4612×[C]+102×[Mn]+605≥1800". The microstructure of this heat-treated steel has a martensite structure with a volume ratio of more than 90%, and the dislocation density in the martensite is 9.0×10 15 m -2 That's all. In this technical solution, high strength of parts is achieved by using low carbon and high manganese, but the high manganese design makes steelmaking more difficult and is prone to segregation defects that worsen toughness.

[0009] In summary, existing hot-stamping steels with tensile strengths above 1700 MPa primarily aim for ultra-high strength and room-temperature toughness, but are not relevant to or have no connection to resistance to low-temperature bending brittle fracture. Therefore, to address the problem of brittle fracture in ultra-high-strength hot-stamped components in extremely cold regions, the inventors aim to provide a new hot-stamped component with superior resistance to low-temperature brittleness and its manufacturing method to effectively meet market demands. Summary of the Invention

[0010] One objective of this invention is to provide a hot-stamped component with excellent resistance to low-temperature brittleness. This hot-stamped component possesses ultra-high strength while also exhibiting excellent resistance to low-temperature brittle cracking. Its room temperature tensile strength is greater than 1700 MPa, and the ratio of its bending performance at -60°C to that at room temperature (20°C) (i.e., the ratio of the bending angle at -60°C to that at 20°C) is greater than 0.85. Simultaneously, the low-temperature strength-toughness product at -60°C (room temperature tensile strength × -60°C bending angle) is not less than 8 × 10⁻⁶. 4 The use of this hot-stamped component can effectively solve the problem of brittle fracture of ultra-high-strength hot-stamped components in extremely cold regions. It has good application prospects and can be widely used in the automotive, shipbuilding, and machinery industries.

[0011] To achieve the above objectives, the present invention provides a hot-stamped component with excellent resistance to low-temperature brittleness, which contains Fe and unavoidable impurities, and also contains the following chemical elements in the following mass percentages:

[0012] C: 0.26~0.40%, Si: 0.1~1.5%, Mn+Cr: 0.5~3.0%, Al: 0.01~0.50%, Nb+Ti: 0.04~0.25%, Mo+Ni: 0.1~1.0%, B: 0.001~0.005%;

[0013] The microstructure of the hot-stamped component has a matrix of lath-shaped tempered martensite with a volume ratio higher than 85%.

[0014] The microstructure of the hot-stamped component also has precipitated phases, wherein the amount of Nb and Ti precipitates accounts for more than 50% of the total mass fraction of these two elements, and the Nb and Ti precipitates are dispersed in the matrix in a particulate form.

[0015] Furthermore, in the hot-stamped component with excellent resistance to low-temperature brittleness described in this invention, the mass percentage content of each chemical element is as follows:

[0016] C: 0.26–0.40%, Si: 0.1–1.5%, Mn+Cr: 0.5–3.0%, Al: 0.01–0.50%, Nb+Ti: 0.04–0.25%, Mo+Ni: 0.1–1.0%, B: 0.001–0.005%; balance is Fe and other unavoidable impurities.

[0017] In the hot-stamped component with excellent resistance to low-temperature brittleness described in this invention, the design principles of each chemical element are as follows:

[0018] C: In the hot-stamped components with excellent resistance to low-temperature brittleness described in this invention, carbon (C) is a key element for achieving ultra-high strength in hot-stamped steel. When the C content in the steel is below 0.26%, it is difficult to achieve the 1700 MPa strength target for hot-stamped components. However, it should be noted that the C content in the steel should not be too high either. As the C content in the steel increases, the low-temperature bending performance and weldability of the hot-stamped components will significantly deteriorate. Therefore, considering the influence of C content on the performance of hot-stamped components, in the hot-stamped components with excellent resistance to low-temperature brittleness described in this invention, the mass percentage of C is specifically controlled between 0.26% and 0.40%.

[0019] Si: In the hot-stamped parts with excellent resistance to low-temperature brittleness described in this invention, the addition of a certain amount of Si can effectively improve the strength of the hot-stamped parts and help deoxidize the steel. Studies have found that when the Si content in the hot-stamped steel is below 0.1%, its deoxidation effect is poor; however, when the Si content in the hot-stamped steel exceeds 1.5%, it affects the platingability of the steel sheet. Therefore, in order to maximize the beneficial effects of Si, the mass percentage of Si in the hot-stamped parts with excellent resistance to low-temperature brittleness described in this invention is controlled between 0.1% and 1.5%.

[0020] Mn and Cr: In the hot-stamped components with excellent resistance to low-temperature brittleness described in this invention, the addition of certain amounts of Mn and Cr can effectively improve the hardenability, strength, and hardness of the hot-stamped steel, with similar effects. Studies have found that when the Mn+Cr content in the hot-stamped steel is below 0.5%, the contribution of Mn and Cr elements to the steel's strength is low, failing to achieve the purpose of this invention; while when the Mn+Cr content in the hot-stamped steel is above 3.0%, it deteriorates the steel's manufacturability and weldability. Therefore, in the hot-stamped components with excellent resistance to low-temperature brittleness described in this invention, the sum of the mass percentages of Mn and Cr elements, "Mn+Cr", is specifically controlled between 0.5% and 3.0%.

[0021] Al: In the hot-stamped component with excellent resistance to low-temperature brittleness described in this invention, Al acts as a deoxidizing element. Therefore, to ensure that Al exerts its deoxidizing effect, at least 0.01% Al needs to be added to the hot-stamping steel. However, it should be noted that the Al content in the hot-stamping steel should not be too high. When the steel contains too much Al, it will cause the formation of large inclusions and lead to poor manufacturability. Therefore, in the hot-stamped component with excellent resistance to low-temperature brittleness described in this invention, the mass percentage of Al is controlled between 0.01% and 0.50%.

[0022] Nb and Ti: In the hot-stamped components with excellent resistance to low-temperature brittleness described in this invention, Nb and Ti are both important microalloying elements in steel. Some Nb and Ti can play a role in solid solution strengthening. In addition, Nb and Ti have extremely strong bonding forces with C and N, and can combine to precipitate stable carbides, nitrides, and carbonitrides, thereby inhibiting the growth of austenite grains during hot stamping and thus refining the grains. Furthermore, the precipitated phases formed by the combination can also act as hydrogen traps, which can reduce the sensitivity of hot-stamped steel to hydrogen-induced delayed cracking and significantly improve the low-temperature toughness of the steel. Moreover, Ti is also a good deoxidizing and nitrogen-fixing effective element, which can not only reduce oxide inclusions in steel, but also prevent B from combining with N to form BN. Therefore, in order to give full play to the beneficial effects of Nb and Ti, in the hot-stamped components described in this invention, the sum of the mass percentages of Nb and Ti, "Nb+Ti", is specifically controlled between 0.04% and 0.25%. When the Nb+Ti content is less than 0.04%, the amount of precipitated phase formed is insufficient, and the beneficial effect is limited; while when the Nb+Ti content is higher than 0.25%, the effect will be saturated, and blocky nitrides will easily form, which will deteriorate the low-temperature bending performance of the product.

[0023] Mo and Ni: In the hot-stamped components with excellent resistance to low-temperature brittleness described in this invention, both Mo and Ni elements can significantly improve the bending performance of the hot-stamped components, especially enhancing their resistance to low-temperature brittleness. Ni element can lower the ductile-brittle transition temperature of hot-stamped steel, which is of great significance for improving the bending toughness of the steel. Mo element can significantly improve the hardenability of the steel; its carbides can not only refine austenite grains but also suppress tempered martensite brittleness, thereby comprehensively improving the strength and toughness of the hot-stamped components. Therefore, adding a certain amount of Mo and Ni can give the hot-stamped components excellent resistance to low-temperature cracking. However, considering the alloy cost of steel and the saturation of element effects, in this invention, the sum of the mass percentages of Mo and Ni elements, "Mo + Ni", is specifically controlled between 0.1% and 1.0%.

[0024] B: In the hot-stamped parts with excellent resistance to low-temperature brittleness described in this invention, adding a certain amount of element B can greatly increase the hardenability of the steel. However, it should be noted that the content of element B in the steel has an optimal range; when the content of element B in the steel exceeds a certain amount, the effect of increasing hardenability is not significant. Therefore, in this invention, the mass percentage content of element B is specifically controlled between 0.001% and 0.005%.

[0025] It should be noted that quenching of medium- and high-carbon hot-stamping steel results in hot-stamped parts with high dislocation density and phase transformation internal stress, and they are prone to forming twinned martensite. Twinned martensite is hard and brittle, and its bending performance is significantly lower than that of lath martensite. Although the initial martensite formed during hot stamping quenching to the high-temperature zone undergoes self-tempering to form tempered martensite and precipitate carbides, the overall performance of the parts is still poor. Therefore, this invention studies how to suppress the formation of twinned martensite in the matrix through composition and process control, and achieves a matrix with a volume ratio of lath-shaped tempered martensite higher than 85%.

[0026] Studies have shown that the size of the precipitated phases of Nb, Ti, and Mo has a significant impact on the bending performance of hot-stamped parts in this invention. Therefore, it is necessary to optimize the entire process to ensure that the Nb+Ti precipitation ratio accounts for more than 50% of the total mass fraction, and to reduce the size of the precipitated phases to achieve a dispersed distribution. Specifically, the method for detecting the Nb and Ti precipitation amounts involves using chemical electrolytic extraction to obtain residues containing precipitated carbides, nitrides, and carbonitrides. Then, inductively coupled plasma atomic absorption spectrometry (ICP-AES) is used to determine the mass fraction of Nb+Ti precipitation.

[0027] Furthermore, in the hot-stamped part with excellent resistance to low-temperature brittleness described in this invention, among the unavoidable impurities, P≤0.03%, S≤0.01%, N≤0.004%, and O≤0.004%.

[0028] In the above technical solution, P, S, N, O and H are all impurity elements in steel. When technical conditions permit, in order to obtain steel with better performance and higher quality, the content of impurity elements in the material should be reduced as much as possible.

[0029] P and S: In this invention, both P and S are harmful elements. P segregation leads to cold brittleness in steel; while S segregation and sulfides such as MnS reduce the toughness of steel, causing hot brittleness at high temperatures. The purpose of this invention is to improve the resistance of hot-stamped parts to low-temperature brittleness. To this end, the mass percentage of P and S elements in the steel must be strictly controlled, specifically: P ≤ 0.03%, S ≤ 0.01%.

[0030] Nitrogen (N): In this invention, nitrogen (N) is an impurity element in hot-stamping steel. It has a strong affinity for elements such as Ti, Al, and B, and the resulting TiN, AlN, and BN are hard phase inclusions, which are the initiation sites for brittle cracking. Therefore, the N content in hot-stamping steel must be strictly controlled, ensuring that the N content is ≤0.004%, preferably further controlled to ≤0.003%.

[0031] O: In this invention, oxygen readily forms oxide inclusions with non-metals in steel, which significantly deteriorates the bending properties of the steel. Therefore, the steelmaking deoxidation process must be strictly controlled, and the mass percentage of oxygen must be controlled to meet the requirement of O ≤ 0.004%. Of course, in some preferred embodiments, O can be further controlled to ≤ 0.0025%.

[0032] Furthermore, in the hot-stamped component with excellent resistance to low-temperature brittleness described in this invention, N ≤ 0.003% and / or O ≤ 0.0025%.

[0033] Furthermore, in the hot-stamped component with excellent resistance to low-temperature brittleness described in this invention, its chemical elements also include at least one of the following: Cu: 0.01-1.0%, W: 0.01-0.5%, V: 0.01-0.5%.

[0034] In the above technical solution of the present invention, in order to further optimize the performance of the prepared hot stamping part, Cu, W and V elements can be added more preferably when designing the chemical composition of the hot stamping part.

[0035] Cu, W, and V: In the hot-stamped parts with excellent resistance to low-temperature brittleness described in this invention, trace amounts of Cu can effectively refine the grains and improve the strength and toughness of the hot-stamped steel. Cu also enhances the material's corrosion resistance. W and V in the steel achieve grain refinement through the precipitation of fine carbonitrides, which effectively improves the toughness of the hot-stamped steel. However, considering the alloying cost of steel and the saturation of elemental effects, in this invention, it is specifically preferred to add one or more of Cu, W, and V, and specifically control their content to meet the following: Cu: 0.01–1.0%, W: 0.01–0.5%, V: 0.01–0.5%.

[0036] Furthermore, in the hot-stamped component with excellent resistance to low-temperature brittleness described in this invention, the precipitated phase of Ti element includes TiN, the aspect ratio of TiN is less than 6, and the density of TiN with a major axis greater than 2 μm, calculated by area ratio, is less than 50 particles / mm². 2 .

[0037] In this invention, research shows that the nitrogen content in hot-stamped parts affects the shape, size, and quantity of TiN. Long, strip-shaped TiN, especially those with a string-like morphology, and TiN inclusions with a long axis greater than 2 μm significantly deteriorate low-temperature bending performance; therefore, their quantity must be controlled to be below 50 inclusions / mm. 2 .

[0038] Furthermore, in the hot-stamped component with excellent resistance to low-temperature brittleness described in this invention, the particle diameters of TiC, Nb(C,N), and MoC in the precipitated phase are less than 80 nm.

[0039] Furthermore, in the hot-stamped component with excellent resistance to low-temperature brittleness described in this invention, the matrix of its microstructure also has one or more of ferrite, pearlite, bainite, martensite, and retained austenite.

[0040] Furthermore, in the hot-stamped component with excellent resistance to low-temperature brittleness described in this invention, its tensile strength is greater than 1700 MPa, its bending performance ratio at -60°C to that at room temperature is greater than 0.85, and its strength-toughness product at -60°C is not less than 8 × 10⁻⁶. 4 .

[0041] It should be noted that in this invention, the bending performance of the hot-stamped component at -60℃ and at room temperature (20℃) is tested according to the VDA238-100 test method. Specifically, when conducting the low-temperature test to determine the bending performance at -60℃, the sample is placed in an alcohol cooling bath to cool to the target temperature, held at that temperature for 20 minutes, and then quickly transferred to the VDA bending equipment, ensuring that the bending test is completed within 1 minute to obtain the corresponding bending performance at -60℃.

[0042] Furthermore, in the hot-stamped component with excellent resistance to low-temperature brittleness described in this invention, the diffuse H content in the hot-stamped component is less than 0.15 ppm.

[0043] Accordingly, another objective of the present invention is to provide a method for manufacturing the above-mentioned hot stamping component with excellent resistance to low-temperature brittleness. This manufacturing method is simple to produce and has a reasonable process design. The hot stamping component obtained has ultra-high strength and excellent resistance to low-temperature brittle cracking. It has good application prospects and can be widely used in the automotive, shipbuilding, and machinery industries.

[0044] To achieve the above objectives, the present invention provides a method for manufacturing the aforementioned hot-stamped component with excellent resistance to low-temperature brittleness, comprising the following steps:

[0045] (1) Smelting and casting;

[0046] (2) Hot rolling, coiling, pickling: The temperature of the slab exiting the heating furnace is controlled at 1220-1280℃, and the final rolling temperature is 880-940℃; the coiling temperature is controlled at 580-680℃, and after coiling, it is cooled to below 300℃ at a rate of 0.3-1℃ / s.

[0047] (3) Cold rolling and annealing: control the total reduction of cold rolling to 30-60%; control the annealing temperature to 680-750℃, and cool to room temperature at an average rate of 1-15℃ / s after annealing;

[0048] (4) Hot stamping: Control the reheating temperature of the steel plate to 850-950℃ and the reheating time to 2-10min. Then quickly transfer the steel plate to the mold to complete the hot stamping. Then cool it to 100-200℃ at an average rate of 10-100℃ / s, and then air cool it to room temperature.

[0049] (5) Low temperature tempering: Keep warm in a baking oven at 150-250℃ for 10-40 minutes, then remove and air cool to room temperature.

[0050] In the above technical solution of the present invention, in the smelting process of step (1), it is necessary to smelt according to the chemical composition designed in the present invention, and after denitrification and deoxidation in a converter, it is further cast into a slab.

[0051] It should be noted that the present invention can remove some nitrogen during the oxygen blowing decarburization process of smelting. Extending the argon blowing time in the later stage can effectively ensure that the N content in the steel is less than 0.004%, and more preferably less than 0.003%. Correspondingly, the subsequent refining deoxidation can promote the vacuum reaction of C and O. Adding Al in the later stage for stabilization can reduce the O content in the steel to O≤0.004%, and more preferably O≤0.0025%.

[0052] Accordingly, in step (2) of the manufacturing method described in this invention, the furnace exit temperature of the hot-rolled slab should be relatively high to ensure sufficient dissolution of microalloying elements, so as to facilitate their precipitation during the subsequent hot rolling coiling process. Based on the consideration of the amount of precipitation and the avoidance of coarsening of the precipitated phases, this invention specifically controls the furnace exit temperature of the slab between 1220 and 1280°C, and the final rolling temperature between 880 and 940°C. Furthermore, the inventors' research indicates that controlling the coiling temperature T within the range of 580 to 680°C during the coiling process facilitates the formation of precipitated phases. Moreover, adopting a reduced cooling rate after coiling, at a rate of 0.3 to 1°C / s, can ensure that the steel coil remains within the coiling temperature range of T to 300°C for an extended period, thereby further promoting the precipitation of microalloying elements such as Nb and Ti carbides, nitrides, and carbonitrides.

[0053] Furthermore, in step (3) of the manufacturing method described in this invention, the total cold rolling reduction needs to be controlled between 30% and 60%. If the total cold rolling reduction of the steel plate is less than 30%, the grain refinement effect of the steel plate is not obvious; if the total cold rolling reduction of the steel plate is greater than 60%, the residual stress inside the steel plate is large and there are more banded structures, which is not conducive to subsequent production, and the banded structures will significantly deteriorate the toughness of the subsequent hot-stamped parts.

[0054] In addition, in step (3), annealing has the effect of improving the non-equilibrium structure of the rolled steel sheet, which can reduce the segregation of elements such as C and Mn in the banded structure. In this invention, the annealing temperature is controlled between 680 and 750°C, which is beneficial to optimizing the uniformity of the steel sheet composition; and after annealing, cooling to room temperature at an average cooling rate of 1 to 15°C / s can promote the precipitation of some microalloying elements and avoid the banded structure and compositional segregation formed by rapid cooling.

[0055] In this technical solution designed in this invention, the steel plate after annealing needs to undergo hot stamping forming in step (4). In this invention, the steel plate is specifically reheated to 850-950°C and the heating time is controlled to be 2-10 minutes. The above range can ensure that the steel plate is fully austenitized and avoid the coarsening of austenite grains.

[0056] Accordingly, after heating, the steel plate needs to be quickly transferred to the mold to complete the hot stamping, and then cooled to 100-200°C at an average rate of 10-100°C / s. When the average cooling rate is below 10°C / s, the strength of the hot-stamped part cannot be guaranteed; while when the average cooling rate is above 100°C / s, the following problems arise: firstly, rapid cooling leads to the formation of twinned martensite; secondly, the martensite dislocation density is high and the internal stress is large; and thirdly, it is not conducive to the formation of precipitates. All three of these points deteriorate the low-temperature bending performance of the hot-stamped part. Furthermore, cooling the hot-stamped part to below 200°C is below the end point of the low-carbon martensite phase transformation. If the final cooling temperature is below 100°C, it will be beneficial to the formation of medium- and high-carbon twinned martensite. Therefore, in this invention, the cooling temperature is specifically controlled between 100-200°C.

[0057] In this invention, the hot-stamped parts prepared after hot stamping treatment need to be tempered at a low temperature in a low temperature baking oven at 150-250°C, and the tempering holding time needs to be controlled at 10-40 minutes. The main purpose is to reduce the proportion of high dislocation density martensite so as to ensure that the proportion of lath tempered martensite in the final hot-stamped parts is higher than 85%.

[0058] It should be noted that during tempering, the removal of supersaturated carbon from martensite promotes carbide precipitation, including but not limited to microalloyed carbide precipitates. Furthermore, hydrogen (H) is a harmful element in this invention; the aggregation of free diffused hydrogen in hot-stamped parts deteriorates toughness and increases the risk of cracking. Low-temperature tempering removes diffused hydrogen from hot-stamped parts, ensuring that the diffused hydrogen content in the finished hot-stamped parts is ≤0.15ppm, which helps reduce the risk of brittle cracking and further improves the low-temperature bending performance of the hot-stamped parts.

[0059] Of course, in practical implementation, the low-temperature tempering process designed in this invention is not limited to offline baking in an oven; it can also be completed online through mold induction heating or online furnace heat treatment. Furthermore, this invention specifically controls the tempering holding time between 10 and 40 minutes because: if the low-temperature tempering time is too short, less than 10 minutes, short-time tempering cannot achieve the aforementioned effect; while if the low-temperature tempering time is too long, exceeding 40 minutes, high-temperature long-term tempering may reduce strength and potentially enter the tempering brittleness range, leading to a weakening of both the strength and low-temperature bending performance of the final hot-stamped part.

[0060] Furthermore, in the manufacturing method described in this invention, after the annealing step in step (3), a step of plating or coating the steel plate is also included.

[0061] In this technical solution designed in this invention, the hot-stamped steel sheet obtained after annealing can be an uncoated bare sheet, or it can be further coated or plated to reduce the iron oxide scale on the steel sheet and improve the corrosion resistance of the steel sheet.

[0062] Compared with the prior art, the hot-stamped component with excellent resistance to low-temperature brittleness and its manufacturing method described in this invention have the following advantages and beneficial effects:

[0063] This invention, through rational design of chemical composition and optimized process, develops a hot-stamped component with excellent resistance to low-temperature brittleness. While possessing ultra-high strength, it also exhibits excellent resistance to low-temperature brittle cracking. The tensile strength of this hot-stamped component is greater than 1700 MPa, and the ratio of its bending performance at -60℃ to that at room temperature (20℃) is greater than 0.85. Simultaneously, the strength-toughness product at -60℃ (room temperature tensile strength × -60℃ bending angle) is not less than 8 × 10⁻⁶. 4 The use of this hot-stamped component can effectively solve the problem of brittle fracture of ultra-high-strength hot-stamped components in extremely cold regions. It has good application prospects and can be widely used in the automotive, shipbuilding, and machinery industries.

[0064] To achieve the aforementioned beneficial effects, the inventors made the following improvements:

[0065] 1. The inventors improved the chemical composition and process design of hot-stamped parts by strictly controlling the N and O content in the steel to suppress the formation of large-scale nitride inclusions of TiN, AlN, and BN, thereby reducing non-metallic oxide inclusions in the steel. These nitride and oxide hard phase inclusions are the initiation points for brittle fracture and significantly deteriorate low-temperature bending performance. However, the precipitation of hard inclusions such as TiN is unavoidable. Therefore, this invention requires reducing the content per unit area while employing process measures to suppress their growth, controlling the ratio of TiN's major axis to minor axis to be less than 6, and ensuring that the density of TiN with a major axis greater than 2 μm, calculated by area ratio, is less than 50 inclusions / mm². 2 .

[0066] 2. By controlling the hot rolling and annealing processes, the inventors can effectively promote the precipitation of Nb+Ti, ensuring that the precipitated amounts of Nb and Ti elements account for more than 50% of the total mass fraction of these two elements. The precipitated Nb and Ti elements can inhibit the growth of austenite grains during hot stamping and reheating, refining the grains and effectively improving low-temperature bending performance. Furthermore, the precipitated Nb and Ti elements can reduce the risk of hydrogen embrittlement. Therefore, this invention, while meeting the required precipitation amount, also effectively controls the size of the Nb and Ti precipitates, ensuring that the particle diameters of TiC, Nb(C,N), and MoC in the precipitates are less than 80 nm, thereby forming a dispersed nanoscale precipitate phase and further enhancing low-temperature bending performance.

[0067] 3. This invention promotes the precipitation of microalloyed carbides, reduces the C content in the matrix, and suppresses the formation of twinned martensite by controlling the quenching cooling rate and the cooling end temperature. During the tempering process, the hot-stamped parts also undergo low-temperature tempering to ensure that the volume ratio of lath tempered martensite in the final microstructure is higher than 85%, thereby reducing the martensite dislocation density and improving low-temperature bending performance.

[0068] 4. By adding trace amounts of Mo and Ni, this invention can prevent the aforementioned low-temperature tempering brittleness and inhibit low-temperature crack propagation, and effectively improve the low-temperature bending performance of hot-stamped parts. Attached Figure Description

[0069] Figure 1 The image shows the microstructure of the hot-stamped component from Example 6 under a scanning electron microscope. Detailed Implementation

[0070] The following will further explain and illustrate the excellent low-temperature brittleness resistance of the hot-stamped component and its manufacturing method according to the present invention with reference to specific embodiments. However, such explanation and illustration do not constitute an improper limitation on the technical solution of the present invention.

[0071] Examples 1-10

[0072] The hot-stamped parts in Examples 1-10 were all manufactured using the following steps:

[0073] (1) Smelting and casting according to the chemical composition shown in Table 1 below: In actual operation, smelting is carried out according to the chemical composition designed in Table 1, and then denitrification, refining and deoxidation are carried out in a converter, and then casting into slabs.

[0074] (2) Hot rolling, coiling, and pickling: The slab obtained from casting is hot rolled and the temperature of the slab exiting the heating furnace is controlled at 1220-1280℃, and the final rolling temperature is controlled at 880-940℃; after hot rolling, it is coiled and the coiling temperature is controlled at 580-680℃. After coiling, it is cooled to below 300℃ at a rate of 0.3-1℃ / s; then it is pickled to obtain the hot-rolled slab.

[0075] (3) Cold rolling and annealing: The hot-rolled slab is cold-rolled and the total reduction of cold rolling is controlled to be 30-60%; the cold-rolled steel plate needs to be further annealed and the annealing temperature is controlled to be 680-750℃. After annealing, it is cooled to room temperature at an average rate of 1-15℃ / s; after annealing, the steel plate can also be coated or plated to obtain a steel plate with a coating or plate.

[0076] (4) Hot stamping: Control the reheating temperature of the steel plate to 850-950℃ and the reheating time to 2-10min. Then quickly transfer the steel plate to the mold to complete the hot stamping. Then cool it to 100-200℃ at an average rate of 10-100℃ / s, and then air cool it to room temperature to obtain the corresponding hot stamped parts.

[0077] (5) Low-temperature tempering: The prepared hot stamping parts are placed in a baking oven at 150-250°C and kept at that temperature for 10-40 minutes. Then they are taken out and air-cooled to room temperature to obtain a finished hot stamping part with a final thickness of 1.4 mm.

[0078] In this technical solution designed by the present invention, the chemical composition design and related processes of the hot stamping parts of Examples 1-10 prepared by the present invention all meet the design specifications of the present invention.

[0079] Table 1 lists the mass percentage of each chemical element in the hot-stamped parts of Examples 1-10.

[0080]

[0081] Accordingly, Tables 2-1 and 2-2 list the specific process parameters of the hot stamped parts of Examples 1-10 in the above process steps (1)-(5).

[0082] Table 2-1.

[0083]

[0084] Table 2-2.

[0085]

[0086] In this invention, after completing the above manufacturing process, samples of the hot-stamped parts of Examples 1-10 can be taken respectively, and the microstructure of the hot-stamped parts samples of Examples 1-10 can be observed. It was found that the matrix of the microstructure of the hot-stamped parts samples of Examples 1-10 has lath-shaped tempered martensite with a volume ratio of more than 85%. At the same time, the microstructure of the hot-stamped parts of each example also contains one or more of ferrite, pearlite, bainite, martensite, and retained austenite. The specific observation and analysis results are listed in Table 3 below.

[0087] Correspondingly, when observing the microstructure of the hot-stamped component samples of Examples 1-10, it is not difficult to find that the microstructure of the hot-stamped component also has precipitated phases, in which the amount of Nb and Ti elements precipitated accounts for more than 50% of the total mass fraction of these two elements, and the Nb and Ti precipitated phases are dispersed in the matrix in a particulate form.

[0088] Further analysis of these precipitates revealed that, in Examples 1-10, the particle diameters of TiC, Nb(C,N), and MoC in the precipitates were all less than 80 nm. Furthermore, the Ti-element precipitates specifically included TiN, with all TiN particles having a major-to-minor axis ratio less than 6, and the density of TiN particles with a major axis greater than 2 μm (calculated by area ratio) less than 50 particles / mm². 2 .

[0089] Table 3 lists the observation and analysis results of the microstructure of the hot-stamped components of Examples 1-10.

[0090] Table 3.

[0091]

[0092] Accordingly, after completing the above observation and analysis of the microstructure, in order to further illustrate that the hot-stamped parts prepared by the present invention have excellent mechanical properties, the inventors sampled the hot-stamped parts of Examples 1-10 again based on the obtained hot-stamped parts of these examples and tested the strength and resistance to low-temperature brittleness of the hot-stamped parts of Examples 1-10. The relevant test results are listed in Table 4 below.

[0093] In this invention, the detection methods used in Examples 1-10 are described as follows:

[0094] Tensile test: The room temperature tensile strength was tested in accordance with the requirements of GB / T 228 Metallic Materials Tensile Test Method at Room Temperature, so as to determine the room temperature tensile strength of the hot stamping parts of Examples 1-10.

[0095] Bending performance test: The bending performance of the hot-stamped parts of Examples 1-10 at -60℃ and room temperature (20℃) was tested according to the VDA238-100 test method. When testing the bending performance at -60℃, the sample was placed in an alcohol cooling bath to cool to the target temperature and held for 20 minutes. Then, the sample was quickly transferred to the VDA bending equipment, and the bending test was completed within 1 minute to obtain the corresponding bending performance at -60℃. Based on the measured bending performance of the hot-stamped parts of Examples 1-10 at -60℃ and room temperature (20℃), the bending performance ratio (i.e., the ratio of the bending angle at -60℃ to the bending angle at 20℃) can be further obtained.

[0096] Accordingly, based on the above tests, the low-temperature strength product (room temperature tensile strength × -60°C bending angle) of the hot-stamped parts of Examples 1-10 can be further obtained.

[0097] Furthermore, the diffusion H content in the hot-stamped parts of Examples 1-10 was measured using a Bruker G4 diffusion H analyzer, and the diffusion H content in the hot-stamped parts of each example was further measured.

[0098] Table 4 lists the test results of the mechanical properties of the hot-stamped parts from Examples 1-10.

[0099] Table 4.

[0100]

[0101] As can be seen from Table 4 above, the hot-stamped components of Examples 1-10 prepared using the technical solution designed in this invention exhibit extremely high strength and excellent resistance to low-temperature brittleness. Their tensile strength is between 1729 and 2191 MPa, and the ratio of their bending performance at -60°C to that at room temperature (20°C) is between 0.86 and 0.93. Furthermore, their low-temperature strength-toughness product at -60°C is specifically 8.8 × 10⁻⁶. 4 -11.3×10 4 between.

[0102] Furthermore, the final diffusion H content of the hot-stamped components of Examples 1-10 prepared using this technical solution of the present invention is ≤0.15ppm, specifically between 0.04-0.15ppm.

[0103] In summary, the hot-stamped components produced according to the present invention not only have ultra-high strength but also excellent resistance to low-temperature brittleness. Using this hot-stamped component can effectively solve the problem of brittle fracture of ultra-high strength hot-stamped components in extremely cold regions. It has good application prospects and can be widely used in the automotive, shipbuilding, and machinery industries.

[0104] Figure 1 The image shows the microstructure of the hot-stamped component from Example 6 under a scanning electron microscope.

[0105] like Figure 1 As shown, Figure 1 The microstructure of the hot-stamped component of Example 6 is given, from Figure 1 It is easy to see that the proportion of lath tempered martensite is higher than 85%.

[0106] It should be noted that the combination of the technical features in this case is not limited to the combination methods described in the claims of this case or the combination methods described in the specific embodiments. All technical features described in this case can be freely combined or combined in any way, unless they contradict each other.

[0107] It should also be noted that the embodiments listed above are merely specific embodiments of the present invention. Obviously, the present invention is not limited to the above embodiments, and similar changes or modifications made thereto are those that can be directly derived or easily conceived by those skilled in the art from the content disclosed in the present invention, and should all fall within the protection scope of the present invention.

Claims

1. A hot-stamped component with excellent resistance to low-temperature brittleness, characterized in that, Its mass percentage content of each chemical element is as follows: C: 0.26~0.40%, Si: 0.1~1.5%, Mn+Cr: 0.5~3.0%, Al: 0.01~0.50%, Nb+Ti: 0.04~0.25%, Mo+Ni: 0.1~1.0%, B: 0.001~0.005%; the balance is Fe and other unavoidable impurities. The microstructure of the hot-stamped component has a matrix of lath-shaped tempered martensite with a volume ratio higher than 85%. The microstructure of the hot-stamped component also includes precipitated phases, wherein the precipitated amounts of Nb and Ti elements account for more than 50% of the total mass fraction of these two elements. The Nb and Ti precipitated phases are dispersed in the matrix in a granular form. The particle diameters of TiC, Nb(C,N), and MoC in the precipitated phases are less than 80 nm. The Ti precipitated phase includes TiN, with a major-to-minor axis ratio of less than 6, and the density of TiN with a major axis greater than 2 μm, calculated by area ratio, is less than 50 particles / mm². 2 .

2. The hot-stamped component with excellent resistance to low-temperature brittleness as described in claim 1, characterized in that, In unavoidable impurities, P ≤ 0.03%, S ≤ 0.01%, N ≤ 0.004%, O ≤ 0.004%.

3. The hot-stamped component with excellent resistance to low-temperature brittleness as described in claim 2, characterized in that, N≤0.003% and / or O≤0.0025%.

4. The hot-stamped component with excellent resistance to low-temperature brittleness as described in claim 1, characterized in that, Its chemical elements also include at least one of the following: Cu: 0.01~1.0%, W: 0.01~0.5%, V: 0.01~0.5%.

5. The hot-stamped component with excellent resistance to low-temperature brittleness as described in claim 1, characterized in that, Its microstructure matrix also has one or more of ferrite, pearlite, bainite, martensite, and retained austenite.

6. The hot-stamped component with excellent resistance to low-temperature brittleness as described in claim 1, characterized in that, Its tensile strength is greater than 1700 MPa, its bending performance ratio at -60℃ to that at room temperature is greater than 0.85, and its strength-toughness product at -60℃ is not less than 8×10⁻⁶. 4 .

7. The hot-stamped component with excellent resistance to low-temperature brittleness as described in claim 1, characterized in that, The diffused hydrogen content in the hot-stamped component is less than 0.15 ppm.

8. A method for manufacturing a hot-stamped part with excellent resistance to low-temperature brittleness as described in any one of claims 1-7, characterized in that, It includes the following steps: Smelting and casting; Hot rolling, coiling, and pickling: The temperature of the slab exiting the heating furnace is controlled at 1220~1280℃, and the final rolling temperature is 880~940℃; the coiling temperature is controlled at 580~680℃, and after coiling, it is cooled to below 300℃ at a rate of 0.3~1℃ / s. Cold rolling and annealing: control the total cold rolling reduction to 30~60%; control the annealing temperature to 680~750℃, and cool to room temperature at an average rate of 1~15℃ / s after annealing; Hot stamping: Control the reheating temperature of the steel plate to 850~950℃, the reheating time to 2~10min, and then quickly transfer the steel plate to the mold to complete the hot stamping; then cool it to 100~200℃ at an average rate of 10~100℃ / s, and then air cool it to room temperature; Low-temperature tempering: Hold in a baking oven at 150~250℃ for 10~40 minutes, then remove and air cool to room temperature.

9. The manufacturing method as described in claim 8, characterized in that, The annealing process also includes a step of plating or coating the steel sheet.

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