Hot-pressed parts with excellent resistance to low-temperature embrittlement and manufacturing method thereof

Abstract

A hot-pressed part having excellent low-temperature embrittlement resistance contains Fe and unavoidable impurities, and further contains 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%, and B: 0.001-0.005%, the microstructure of the hot-pressed part having a matrix with more than 85% by volume of lath-like tempered martensite, and the microstructure of the hot-pressed part also has a precipitate phase, with the precipitated amounts of Nb and Ti elements accounting for 50% or more of the total mass fraction of these two elements, and the Nb and Ti precipitate phases being particulate and dispersedly distributed in the matrix. A method for manufacturing the hot-pressed part is also disclosed.

Description

[Technical Field]

[0001] Technical Field The present invention relates to a hot-pressed part and a manufacturing method thereof, and more particularly to a hot-pressed part having excellent resistance to low-temperature embrittlement and a manufacturing method thereof. [Background technology]

[0002] Background technology In recent years, with the gradual increase in the number of automobiles owned worldwide, environmental pollution and energy consumption caused by automobiles have become increasingly serious. However, lightweight automobile technology can effectively alleviate these problems, and automobile manufacturers are currently conducting extensive research into lightweight automobiles.

[0003] Research has shown that using ultra-high strength steel sheets to manufacture automobile structures can achieve the goal of reducing automobile weight while ensuring safety. However, when actually manufacturing ultra-high strength steel products with tensile strengths exceeding 1180 MPa, their cold forming is extremely difficult and their rebound control ability is insufficient. However, parts manufactured by hot stamping have the characteristics of ultra-high strength, easy forming, and high dimensional accuracy, so such hot-formed ultra-high strength steel products are increasingly becoming an important technological solution for reducing the weight of automobile bodies.

[0004] Currently, with the gradual strengthening of collision safety regulations, the market demand for hot-pressed steel with a tensile strength of over 1700 MPa is also increasing day by day. However, research has shown that hot-pressed steel products with a tensile strength of over 1700 MPa have very poor bending and impact performance, especially at low temperatures of -60°C, which means that hot-pressed parts used in extremely cold regions are at risk of brittle cracking, causing immeasurable damage to the safety of users' lives and property.

[0005] Therefore, to ensure the service life of hot-pressed parts in extremely low-temperature environments, it is necessary to consider the low-temperature embrittlement resistance of the material when manufacturing ultra-high-strength hot-pressed parts.The currently used VDA238-100 bending performance test is relatively close to the impact failure mode of actual parts, so evaluation of the low-temperature VDA (bending angle) is particularly important.

[0006] However, the hot press steels with a strength of 1700 MPa that have been released so far have some drawbacks, even though they have achieved toughness, and the related research has not paid attention to or explained the problem of low-temperature embrittlement under planar deformation conditions.

[0007] For example, a Chinese patent document with publication number CN110423953A, publication date November 8, 2019, and title "Hot-formed part with excellent bending performance and tensile strength of 1800 MPa or more and manufacturing method thereof" discloses a hot-formed part with excellent bending performance and tensile strength of 1800 MPa or more, which contains the following chemical components in the following weight percentages: 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 balance being Fe and unavoidable impurities; the surface layer of the hot-formed part is a ferrite structure as a softened phase, and the inner layer is a martensite structure. In this technical solution, the softened phase in the surface layer improves the bending performance of the material, but the softening of the surface layer inevitably reduces the impact deformation resistance of the part, and the technical solution does not mention the bending performance under low temperature conditions.

[0008] For example, a Chinese patent document with publication number CN106460115A, publication date February 22, 2017, and title "Heat-treated steel material and manufacturing method thereof" discloses a heat-treated steel material with a tensile strength of 1.8 GPa or more and excellent toughness and weldability, and its chemical composition is: C: 0.05-0.30%, Mn: 2.0%-10.0%, Cr: 0.01%-1.0%, Ti: 0.0 The components are 1% to 0.1%, B: 0.001% to 0.01%, Si: 0.08% or less, P: 0.05% or less, S: 0.05% or less, N: 0.01% or less, Ni: 0% to 2.0%, Cu, Mo, and V: 0% to 1.0% each, and the balance is Fe and unavoidable impurities; where the carbon equivalent is [C] and the manganese equivalent is [Mn], the formula satisfies "4612 x [C] + 102 x [Mn] + 605 ≥ 1800." The microstructure of the heat-treated steel has a martensite structure that accounts for 90% or more by volume, and the dislocation density in the martensite is 9.0 x 10 15 m -2 In this technical solution, high strength of the parts is achieved by using low carbon and high manganese, but the high manganese design makes steelmaking difficult and is prone to segregation defects that deteriorate toughness.

[0009] In summary, conventional hot-press steels with a tensile strength of 1700 MPa or more mainly provide ultra-high strength and room-temperature toughness, but do not or are not related to resistance to low-temperature brittle cracking. Therefore, in order to solve the above-mentioned brittle fracture problem of ultra-high strength hot-pressed parts currently found in extremely cold regions, the present inventors hope to provide a new hot-pressed part with excellent low-temperature brittle cracking resistance and a manufacturing method thereof that can effectively meet market demands. Summary of the Invention [Problem to be solved by the invention]

[0010] Contents of the invention One object of the present invention is to provide a hot-pressed part that has excellent resistance to low-temperature embrittlement, and the hot-pressed part has both ultra-high strength and excellent resistance to low-temperature embrittlement cracking. The hot-pressed part has a tensile strength at room temperature of more than 1700 MPa (e.g., 1700 to 2200 MPa), a ratio of bending performance at a low temperature of -60°C to bending performance at room temperature (20°C) (i.e., the ratio of bending angles at -60°C and 20°C) of more than 0.85 (e.g., 0.86 to 0.93), and a -60°C low-temperature toughness product (tensile strength at room temperature × bending angle at -60°C) of 8 × 10 4 or more (e.g., 8.8×10 4 ~11.3×10 4 The use of this hot stamping part can effectively solve the problem of brittle fracture of ultra-high strength hot stamping parts currently existing in extreme cold regions, and has good application prospects, and can be widely used in industries such as automobiles, ships, and machinery. [Means for solving the problem]

[0011] To achieve the above object, the present invention provides a hot-pressed part having excellent resistance to low-temperature embrittlement, which contains Fe and unavoidable impurities, and further contains the following chemical elements in the following mass percentages: Contains: 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-pressed part has greater than 85% by volume of lath-tempered martensite; The microstructure of the hot-pressed part further has a precipitate phase, in which the precipitate amounts of Nb and Ti elements account for 50% or more of the total mass fraction of these two elements, and the precipitate phases of Nb and Ti are particulate and dispersedly distributed in the matrix.

[0012] Furthermore, in the hot-pressed part having excellent resistance to low-temperature embrittlement according to the present invention, the mass percentage content of each chemical element is: 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 remainder being Fe and other unavoidable impurities.

[0013] Furthermore, in the hot-pressed part having excellent resistance to low-temperature embrittlement according to the present invention, the mass percentage content of each chemical element is: C: 0.26-0.40%; Si: 0.1-1.5%; Mn: ≦2.5%, Cr≦2%, and Mn+Cr: 0.5-3.0%; Al: 0.01-0.50%; Nb: 0.01-0.1%, Ti: 0.02-0.15%, and Nb+Ti: 0.04-0.25%; Mo: ≦0.6%, Ni: ≦0.8%, and Mo+Ni: 0.1-1.0%; B: 0.001-0.005%; the balance being Fe and other unavoidable impurities.

[0014] In the hot-pressed part having excellent resistance to low-temperature embrittlement according to the present invention, the design principles of each chemical element are as follows.

[0015] C: In the hot-pressed parts with excellent low-temperature embrittlement resistance of the present invention, C is a key element for achieving ultra-high strength in the hot-press steel. If the C content in the steel is below 0.26%, it becomes difficult to achieve the target strength of 1700 MPa in the hot-pressed parts. However, as the C content in the steel increases, the low-temperature bending and welding performance of the hot-pressed parts significantly deteriorate, so it is important to note that the C content in the steel should not be too high. Therefore, considering the impact of the C content on the performance of the hot-pressed parts, the mass percentage of C in the hot-pressed parts with excellent low-temperature embrittlement resistance of the present invention is specifically controlled to 0.26 to 0.40%.

[0016] Si: In the hot-pressed part having excellent low-temperature embrittlement resistance according to the present invention, adding a certain amount of Si can effectively improve the strength of the hot-pressed part and contribute to deoxidation of the steel. Research has shown that when the Si content in the hot-pressed steel is 0.1% or less, the deoxidation effect is weakened, while when the Si content in the hot-pressed steel is greater than 1.5%, the galvanic properties of the steel sheet are affected. Therefore, to maximize the beneficial effects of Si, the mass percentage of Si in the hot-pressed part having excellent low-temperature embrittlement resistance according to the present invention is controlled to 0.1-1.5%. In some embodiments, the mass percentage of Si is controlled to 0.2-0.8%.

[0017] Mn, Cr: In the hot-pressed part having excellent low-temperature embrittlement resistance according to the present invention, the addition of Mn and Cr in predetermined amounts effectively improves the hardenability of the hot-pressed steel and the strength and hardness of the steel, with the effects being similar. Research has shown that if the Mn+Cr content in the hot-pressed steel is less than 0.5%, the contribution of the Mn and Cr elements to the steel strength is reduced, preventing the objectives of the present invention from being achieved. However, if the Mn+Cr content in the hot-pressed steel exceeds 3.0%, the manufacturability and weldability of the steel deteriorate. Therefore, in the hot-pressed part having excellent low-temperature embrittlement resistance according to the present invention, the total mass percentage content of the Mn and Cr elements (Mn+Cr) is specifically controlled to 0.5-3.0%. In some embodiments, the Mn content is ≦2.5%. In some embodiments, the Cr content is ≦2%. In some embodiments, the Mn content is 0.2-2.5%, the Cr content is 0.2-2%, and the Mn+Cr content is 0.5-3.0%. In some embodiments, the Mn content is 0.3-1.5%, the Cr content is 0.2-1.2%, and the Mn+Cr content is 0.5-2.7%.

[0018] Al: In the hot-pressed part having excellent low-temperature embrittlement resistance according to the present invention, Al acts as a deoxidizing element and exhibits a deoxidizing effect. Therefore, to ensure that the deoxidizing effect of the Al element itself is exerted, it is necessary to add 0.01% or more Al to the steel for hot press-forming. However, since a large amount of Al contained in the steel can cause the formation of coarse inclusions in the steel and lead to deterioration of manufacturability, it is important to note that the Al content in the steel for hot press-forming should not be too high. Therefore, in the hot-pressed part having excellent low-temperature embrittlement resistance according to the present invention, the mass percentage content of the Al element is controlled to 0.01 to 0.50%. In some embodiments, the mass percentage content of the Al element is controlled to 0.01 to 0.10%.

[0019] Nb, Ti: In the hot-pressed parts of the present invention with excellent low-temperature embrittlement resistance, Nb and Ti are important micro-alloying elements in the steel. Not only do they partially contribute to solid solution strengthening, but they also have extremely strong bonding strength with C and N, forming stable carbides, nitrides, and carbonitrides, which inhibit austenite grain growth and refine grains during hot-pressing. Furthermore, the precipitates formed by these bondings also function as hydrogen traps, reducing the susceptibility of hot-pressed steel to hydrogen-induced delayed cracking and significantly improving the low-temperature toughness of the steel. Ti also has excellent deoxidation and nitrogen fixation properties, reducing oxide inclusions in the steel and preventing the combination of B and N to form BN. Therefore, in order to maximize the beneficial effects of Nb and Ti, the total mass percentage content of Nb and Ti in the hot-pressed part of the present invention, "Nb + Ti," is specifically controlled to 0.04 to 0.25%. However, if the Nb + Ti content is below 0.04%, the number of precipitates formed is insufficient, limiting the beneficial effects. However, if the Nb + Ti content exceeds 0.25%, the effect saturates, and blocky nitrides are easily formed, resulting in a deterioration in the low-temperature bending performance of the product. In some embodiments, the Nb content is 0.01 to 0.1%. In some embodiments, the Ti content is 0.02 to 0.15%.

[0020] Mo, Ni: In the hot-pressed part having excellent low-temperature embrittlement resistance according to the present invention, both Mo and Ni significantly improve the bending performance of the hot-pressed part, particularly its low-temperature embrittlement resistance. Ni can lower the ductile-brittle transition temperature of hot-pressed steel, which is important for improving the bending toughness of the steel. Meanwhile, Mo significantly improves the hardenability of the steel, and its carbides not only refine the austenite grains but also suppress the brittleness of tempered martensite, thereby improving the overall strength and toughness of the hot-pressed part. Therefore, while the addition of Mo and Ni in predetermined amounts can provide hot-pressed parts with excellent cold cracking resistance, taking into consideration the alloy cost of the steel and the saturation level of the elemental effects, the present invention specifically controls the total mass percentage content of Mo and Ni (Mo + Ni) to 0.1 to 1.0%. In some embodiments, the Mo content is ≦0.6%. In some embodiments, the Ni content is ≦0.8%. In some embodiments, the Mo content is 0.1 to 0.6%, the Ni content is 0.1 to 0.8%, and the total mass percentage content of Mo and Ni elements "Mo+Ni" is controlled to 0.1 to 1.0%.

[0021] B: In the hot-pressed parts according to the present invention, which have excellent resistance to low-temperature embrittlement, adding a certain amount of B element can significantly improve the hardenability of the steel. However, if the B element content in the steel exceeds a certain amount, the effect of improving hardenability becomes less pronounced, so it is important to note that there is an optimum range for the B element content in the steel. Therefore, in the present invention, the mass percentage content of B element is specifically controlled to 0.001 to 0.005%.

[0022] Hot-pressed parts made from medium- and high-carbon hot-pressed steels have high dislocation densities and transformation internal stresses, making them prone to twinned martensite formation. However, twinned martensite is hard and brittle, and its bending properties are significantly lower than those of lath martensite. Although some of the initial martensite formed by hot-pressing at high temperatures undergoes self-tempering to form tempered martensite and precipitate carbides, the overall performance of the part remains poor. Therefore, in this study, we suppressed the formation of twinned martensite in the matrix through composition and process control, and the matrix contains 85% or more lath-tempered martensite by volume. In some embodiments, the microstructure of the hot-pressed part of this invention has a matrix with a volume fraction of 88-98% lath-tempered martensite.

[0023] Research has shown that the size of the precipitates of elements such as Nb, Ti, and Mo has a significant impact on the bending performance of hot-pressed parts. Therefore, the entire process must be optimized to ensure that the Nb+Ti precipitation ratio accounts for 50% or more (e.g., 55-75%) of the total mass fraction, and that the precipitates are small in size and dispersedly distributed. Specifically, the method for measuring the amount of Nb and Ti precipitates is to obtain the precipitated carbide, nitride, and carbonitride residues through chemical electrolytic extraction, and then use inductively coupled plasma testing to determine the Nb+Ti precipitate mass fraction.

[0024] Furthermore, in the hot-pressed part having excellent resistance to low-temperature embrittlement according to the present invention, the unavoidable impurities are P≦0.03%, S≦0.01%, N≦0.004%, and O≦0.004%.

[0025] In the above technical solutions, P, S, N, O and H are all impurity elements in steel, and as far as technically permissible, the content of impurity elements in the material should be reduced as much as possible to obtain steel products with better performance and better quality.

[0026] P, S: In the present invention, both P and S are harmful elements, and segregation of P element causes low-temperature embrittlement of steel, while segregation of S element and sulfides such as MnS reduce the toughness of steel and cause high-temperature embrittlement at high temperatures. Since the purpose of the present invention is to improve the low-temperature embrittlement resistance of hot-pressed parts, it is necessary to strictly control the mass percentage contents of P and S elements in steel, specifically, P≦0.03% and S≦0.01%.

[0027] N: In the present invention, N is an impurity element in the steel for hot pressing. It has a strong affinity with elements such as Ti, Al, and B, and the TiN, AlN, and BN formed by bonding are hard phase inclusions that are the source of brittle cracking. Therefore, the N content in the steel for hot pressing must be strictly controlled and ensured to satisfy N≦0.004%, and preferably, can be further controlled to N≦0.003%.

[0028] O: In the present invention, since the O element is very likely to form oxide inclusions with non-metallic elements in the steel, which significantly deteriorates the bending properties of the steel, it is necessary to strictly control the deoxidation process in steelmaking and ensure that the mass percentage content of the O element satisfies O≦0.004%. Of course, in some embodiments, it may be further controlled to O≦0.0025%.

[0029] Furthermore, in the hot-pressed part having excellent resistance to low-temperature embrittlement according to the present invention, N≦0.003% and / or O≦0.0025%.

[0030] Furthermore, the chemical elements of the hot-pressed part having excellent resistance to low-temperature embrittlement according to the present invention further include at least one of Cu: 0.01 to 1.0%, W: 0.01 to 0.5%, and V: 0.01 to 0.5%.

[0031] In the above technical solution of the present invention, in order to further optimize the performance of the prepared hot-pressed part, it is preferable to further add Cu, W, and V elements to the hot-pressed part during chemical composition design.

[0032] Cu, W, V: In the hot-pressed parts of the present invention with excellent low-temperature embrittlement resistance, trace amounts of Cu effectively refine grains and improve the toughness of the steel for hot pressing. Cu also has the effect of improving the corrosion resistance of the material. W and V in the steel refine grains through the precipitation of fine carbonitrides, effectively improving the toughness of the steel for hot pressing. However, taking into consideration the alloy cost of the steel and the saturation level of the elemental effects, the present invention preferably adds one or more of Cu, W, and V, and controls their contents to specifically satisfy the following: Cu: 0.01-1.0%, W: 0.01-0.5%, V: 0.01-0.5%.

[0033] Furthermore, in the hot-pressed part of the present invention having excellent resistance to low-temperature embrittlement, the precipitated phase of Ti element contains TiN, and the ratio of the long axis to the short axis of TiN is less than 6 (for example, 2.8 to 5.8), and when calculated in terms of area ratio, the density of TiN having a long axis exceeding 2 μm is 50 pieces / mm 2 Less than (e.g., 15 to 50 pieces / mm 2 )

[0034] In the present invention, research has shown that the content of N element in hot-pressed parts affects the shape, size, and number of TiN. TiN with long, especially string-like, morphology and TiN inclusions with a major axis exceeding 2 μm significantly deteriorate low-temperature bending performance, so the number of TiN inclusions is set to 50 / mm 2 It is necessary to control it to less than

[0035] Furthermore, in the hot-pressed part having excellent resistance to low-temperature embrittlement according to the present invention, the particle diameters of TiC, Nb(C,N), and MoC in the precipitate phase are less than 80 nm.

[0036] Furthermore, in the hot-pressed part having excellent resistance to low-temperature embrittlement according to the present invention, the matrix of the microstructure further contains one or more of ferrite, pearlite, bainite, martensite, and retained austenite.

[0037] Furthermore, the hot-pressed part of the present invention, which has excellent resistance to low-temperature embrittlement, has a tensile strength of more than 1700 MPa, a ratio of bending performance at a low temperature of -60°C to bending performance at room temperature of more than 0.85, and a -60°C low-temperature toughness product of 8×10 4 That's all.

[0038] In the present invention, the bending performance of the hot-pressed parts of the present invention at a low temperature of -60°C and at room temperature (20°C) is measured in accordance with the VDA238-100 test method. However, when performing a low-temperature test to measure the bending performance at -60°C, specifically, the sample is placed in an alcohol cooling bath to cool to the target temperature, and after keeping the temperature for 20 minutes, the sample is quickly transferred to a VDA bending machine, and the bending test is completed within 1 minute to obtain the corresponding bending performance at a low temperature of -60°C.

[0039] Furthermore, in the hot-pressed part having excellent resistance to low-temperature embrittlement according to the present invention, the content of diffused H in the hot-pressed part is less than 0.15 ppm, for example, 0.04 to 0.15 ppm.

[0040] Accordingly, another object of the present invention is to provide a method for manufacturing the above-mentioned hot-pressed parts having excellent low-temperature embrittlement resistance, which is simple to produce and has a reasonable process design. The obtained hot-pressed parts have ultra-high strength and excellent low-temperature embrittlement cracking resistance, and have good application prospects and can be widely used in industries such as automobiles, ships, and machinery.

[0041] In order to achieve the above object, the present invention provides a method for producing a hot-pressed part having excellent resistance to low-temperature embrittlement, comprising the following steps: (1) Smelting and casting; (2) Hot rolling, coiling, and pickling: However, the temperature at which the slab is removed from the heating furnace is controlled to 1220-1280°C, and the temperature at the end of rolling is controlled to 880-940°C; the coiling temperature is controlled to 580-680°C, and after coiling, it is cooled to 300°C or less (for example, 200-300°C) at a rate of 0.3-1°C / s; (3) Cold rolling and annealing: The total reduction in cold rolling is controlled to 30-60%; the annealing temperature is controlled to 680-750°C, and after annealing, the material is cooled to room temperature at an average rate of 1-15°C / s; (4) Hot press forming: The steel plate is reheated to a temperature of 850-950°C for 2-10 minutes, and then the steel plate is quickly transferred to a mold to complete the hot press. Then, the steel plate is cooled to 100-200°C at an average rate of 10-100°C / s, and then air-cooled to room temperature. (5) Low-temperature tempering: Keep in an oven at 150 to 250°C for 10 to 40 minutes, then remove and air-cool to room temperature.

[0042] 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 according to the present invention, and after undergoing converter denitrification and refining deoxidation, it is further cast to produce slabs.

[0043] In the present invention, the smelting oxygen blowing decarburization process can remove a portion of the nitrogen, and by extending the argon gas blowing time in the later stage, it is possible to effectively ensure that the N element content in the steel is less than 0.004%, preferably less than 0.003%; correspondingly, it is necessary to note that the refining deoxidation performed in the later stage can promote the vacuum reaction between C and O, and by adding Al for stabilization in the later stage, it is possible to reduce the O element content in the steel to O≦0.004%, preferably O≦0.0025%.

[0044] Accordingly, in step (2) of the manufacturing method of the present invention, the discharge temperature of the hot-rolled slab must be high to ensure sufficient dissolution of the microalloy elements and allow them to precipitate sufficiently during the subsequent hot rolling and coiling process. To avoid coarsening of the precipitated phases, coupled with the number of precipitations, the present invention specifically controls the discharge temperature at which the slab is removed from the heating furnace to 1220-1280°C, and the rolling end temperature to 880-940°C. Furthermore, according to the inventors' research, controlling the coiling temperature T during the coiling process to a range of 580-680°C contributes to the formation of precipitated phases. Furthermore, by adopting a reduced cooling rate after coiling, at a rate of 0.3-1°C / s, it is possible to ensure that the steel coil remains between the coiling temperature T and 300°C for a long period of time, thereby further promoting the precipitation of carbides, nitrides, and carbonitrides of microalloy elements such as Nb and Ti.

[0045] Furthermore, in step (3) of the manufacturing method according to the present invention, the total cold rolling reduction must be controlled to 30% to 60%. If the total cold rolling reduction of the steel sheet is less than 30%, the effect of refining the crystal grains of the steel sheet becomes insignificant, but if the total cold rolling reduction of the steel sheet is more than 60%, the internal residual stress of the steel sheet increases and the band-like structure increases, which is not only disadvantageous for subsequent production but also significantly deteriorates the toughness of the subsequent hot-pressed part.

[0046] In step (3), annealing improves the non-equilibrium structure of the rolled steel sheet and reduces the segregation of elements such as C and Mn in the band-like structure. In the present invention, controlling the annealing temperature to 680-750°C contributes to optimizing the uniformity of the steel sheet's composition and structure; after annealing, cooling to room temperature at an average cooling rate of 1-15°C / s promotes the precipitation of some of the micro-alloying elements, while avoiding the band-like structure and compositional segregation that would otherwise occur due to rapid cooling.

[0047] In this technical solution designed according to the present invention, the steel sheet after annealing needs to be subjected to hot press forming in step (4). Specifically, in the present invention, the steel sheet is reheated to 850-950°C and the heating time is controlled to 2-10 minutes, which not only ensures sufficient austenitization of the steel sheet but also prevents coarsening of austenite grains.

[0048] Accordingly, after heating, the steel sheet must be quickly transferred to a mold to complete the hot pressing, and then cooled to 100-200°C at an average rate of 10-100°C / s. However, if the average cooling rate is below 10°C / s, the strength of the resulting hot-pressed part cannot be ensured. However, if the average cooling rate is above 100°C / s, the following problems arise: first, twinned martensite is formed due to rapid cooling; second, the high dislocation density of martensite results in large internal stress; and third, it is unfavorable for the formation of precipitated phases. All three of these issues degrade the low-temperature bending performance of the resulting hot-pressed part. Furthermore, cooling the resulting hot-pressed part below 200°C already lowers the temperature below the end point of low-carbon martensite transformation. However, cooling below 100°C contributes to the formation of medium- or high-carbon twinned martensite. Therefore, in the present invention, cooling is specifically controlled to 100-200°C.

[0049] In the present invention, the hot-pressed parts prepared after the hot pressing process must be further tempered at low temperature in a low-temperature oven at 150 to 250°C, and the tempering temperature retention time must be controlled to 10 to 40 minutes. The main purpose of this is to reduce the proportion of high dislocation density martensite, thereby ensuring that the proportion of lath-shaped tempered martensite in the final hot-pressed part exceeds 85%.

[0050] During the tempering process, the removal of supersaturated carbon from martensite promotes the precipitation of carbides, including but not limited to microalloyed carbide precipitates. Furthermore, it is important to note that H is a harmful element in the present invention, and the aggregation of free diffused H in hot-pressed parts can deteriorate toughness and increase the risk of cracking. Low-temperature tempering removes diffused H from the hot-pressed parts, reducing the diffused H content in the finished hot-pressed parts to ≦0.15 ppm, thereby reducing the risk of brittle cracking and further improving the low-temperature bending performance of the hot-pressed parts.

[0051] Of course, in specific implementation, the low-temperature tempering process designed according to the present invention is not limited to being completed offline in an oven, but can also be completed online by heat treatment using mold induction heating or an online heating furnace. The reason why the tempering temperature retention time is specifically controlled to 10 to 40 minutes in the present invention is that if the low-temperature tempering time is too short, such as less than 10 minutes, the above-mentioned effects cannot be achieved with short-term tempering, but if the low-temperature tempering time is too long, such as more than 40 minutes, long-term high-temperature tempering will reduce strength and may even fall into the temper embrittlement range, which will result in reduced strength and low-temperature bending performance of the final hot-pressed part.

[0052] Furthermore, the manufacturing method according to the present invention further comprises a step of plating or coating the steel sheet after the annealing step of step (3).

[0053] In this technical solution designed according to the present invention, the hot-pressed steel sheet obtained after annealing can be a bare sheet without plating, but the steel sheet can also be further plated or coated to reduce the heated oxidation scale of the steel sheet and improve the corrosion resistance of the steel sheet.

[0054] The hot-pressed part having excellent resistance to low-temperature embrittlement and the method for manufacturing the same according to the present invention have the following advantages and beneficial effects compared to the prior art: The present invention has developed a hot-pressed part that has both ultra-high strength and excellent resistance to low-temperature brittle cracking through a combination of rational chemical composition design and an optimized process. The hot-pressed part has a tensile strength of over 1700 MPa, a ratio of bending performance at -60°C to bending performance at room temperature (20°C) of over 0.85, and a -60°C low-temperature toughness product (tensile strength at room temperature × bending angle at -60°C) of 8 x 10 4 That's all. The use of this hot stamped part can effectively solve the problem of brittle fracture of ultra-high strength hot stamped parts currently existing in extreme cold regions, and has good application prospects, and can be widely used in industries such as automobiles, ships, and machinery.

[0055] In order to achieve the above beneficial effects, the inventors have made the following several improvements: 1. The inventors improved the chemical composition and process design of hot-pressed parts and strictly controlled the N and O content in the steel, thereby suppressing the formation of large-sized nitride inclusions such as TiN, AlN, and BN, and reducing non-metallic oxide inclusions in the steel. These nitride and oxide hard phase inclusions are the source of brittle cracking and significantly degrade low-temperature bending performance. However, since the precipitation of hard inclusions such as TiN is inevitable, the present invention reduces the content per unit area and suppresses their growth through process measures. This results in a TiN long-to-short axis ratio of less than 6, and the density of TiN with a long axis exceeding 2 μm is 50 pieces / mm. 2 It is necessary to control it so that it is less than

[0056] 2. By controlling the hot rolling and annealing processes, the inventors were able to effectively promote the precipitation of Nb and Ti so that the precipitation amounts of Nb and Ti accounted for more than 50% of the combined mass fraction of these two elements. The Nb and Ti precipitates suppressed the coarsening of austenite grains during reheating after hot rolling and refined the grains, effectively improving low-temperature bending performance. Furthermore, the Nb and Ti precipitates also reduced the risk of hydrogen embrittlement. Therefore, while satisfying the precipitation number requirement, the present invention also effectively controlled the size of the Nb and Ti precipitates so that the particle diameters of TiC, Nb(C,N), and MoC in the precipitates were less than 80 nm, forming dispersed nanoscale precipitates and further improving low-temperature bending performance.

[0057] 3. The present invention promotes the precipitation of micro-alloyed carbides, reduces the carbon content of the matrix, and controls the quenching cooling rate and cooling end temperature to suppress the formation of twin martensite. During the tempering process, the hot-pressed parts are specifically tempered at low temperatures to ensure that the volume ratio of lath-shaped tempered martensite in the final microstructure exceeds 85%, reducing the dislocation density of martensite and improving low-temperature bending performance.

[0058] 4. The present invention adds trace amounts of Mo and Ni to prevent the embrittlement caused by low-temperature tempering, inhibit the expansion of cold cracking, and effectively improve the low-temperature bending performance of hot-pressed parts. [Brief explanation of the drawings]

[0059] [Figure 1] FIG. 1 is a photograph of the microstructure of the hot-pressed part according to Example 6 taken by a scanning electron microscope. DETAILED DESCRIPTION OF THE INVENTION

[0060] Specific Embodiments Hereinafter, the hot-pressed part having excellent resistance to low-temperature embrittlement and the manufacturing method thereof according to the present invention will be further explained based on specific examples, but the explanations and interpretations are not intended to unduly limit the technical solutions of the present invention. [Example]

[0061] Examples 1 to 10 The hot-pressed parts according to Examples 1 to 10 were all manufactured by the following process: (1) Smelting and casting were carried out according to the chemical composition shown in Table 1 below: In actual operation, smelting was carried out according to the chemical composition designed in Table 1, and then the slabs were produced by converter denitrification, refining deoxidation, and further casting.

[0062] (2) Hot rolling, coiling, and pickling: The slab obtained by casting was hot rolled, and the temperature at which the slab was removed from the heating furnace was controlled to 1220-1280°C, and the rolling end temperature was controlled to 880-940°C; after hot rolling, the slab was coiled, and the coiling temperature was controlled to 580-680°C, and after coiling, it was cooled to 300°C or less at a rate of 0.3-1°C / s; and then pickled to obtain a hot-rolled slab.

[0063] (3) Cold rolling, annealing: The hot-rolled slab was cold-rolled, and the total cold-rolling reduction was controlled to 30-60%; the cold-rolled steel sheet needed to be further annealed, and specifically, the annealing temperature was controlled to 680-750°C, and after annealing, it was cooled to room temperature at an average rate of 1-15°C / s; however, after annealing was completed, the steel sheet could also be further plated or coated to obtain a plated or coated steel sheet.

[0064] (4) Hot press forming: The reheating temperature of the steel plate is controlled to 850-950°C, and the reheating time is controlled to 2-10 minutes. Then, the steel plate is quickly transferred to a mold to complete the hot press; it is then cooled to 100-200°C at an average rate of 10-100°C / s, and then air-cooled to room temperature to obtain the corresponding hot press part.

[0065] (5) Low-temperature tempering: The prepared hot-pressed parts were transferred to an oven at 150-250°C and kept at that temperature for 10-40 minutes, then removed and air-cooled to room temperature to obtain finished hot-pressed parts with a final thickness of 1.4 mm.

[0066] In this technical solution designed by the present invention, the chemical composition designs and related processes of the hot-pressed parts according to Examples 1 to 10 prepared as described above in the present invention all met the requirements of the design specifications of the present invention.

[0067] The mass percentage ratios of the chemical elements in the hot-pressed parts according to Examples 1 to 10 are shown in Table 1.

[0068] [Table 1]

[0069] Accordingly, the specific process parameters in the above process steps (1) to (5) of the hot-pressed parts according to Examples 1 to 10 are shown in Tables 2-1 and 2-2.

[0070] [Table 2-1]

[0071] [Table 2-2]

[0072] In the present invention, after the completion of the above manufacturing process, samples were taken from each of the hot-pressed parts of Examples 1 to 10, which were the obtained finished products, and the microstructures of the hot-pressed part samples of Examples 1 to 10 were observed. It was found that the matrix of the microstructure of the hot-pressed part samples of Examples 1 to 10 had a volume ratio of more than 85% lath-shaped tempered martensite; at the same time, the microstructure of the hot-pressed part of each example further had one or more of ferrite, pearlite, bainite, martensite, and retained austenite, and the specific observation and analysis results are shown in Table 3 below.

[0073] Accordingly, as can be easily seen from observing the microstructures of the samples of hot-pressed parts according to Examples 1 to 10, the microstructures of the hot-pressed parts further have precipitate phases, with the precipitate amounts of Nb and Ti elements accounting for 50% or more of the total mass fraction of these two elements, and the precipitate phases of Nb and Ti being particulate and dispersedly distributed in the matrix.

[0074] Further analysis of these precipitates reveals that in Examples 1 to 10, the particle diameters of TiC, Nb(C,N), and MoC in the precipitates are all less than 80 nm, and the precipitates of Ti element may specifically include TiN, and the ratios of the major axis to the minor axis of TiN are all less than 6. Calculated by area ratio, the density of TiN with a major axis exceeding 2 μm is all 50 particles / mm 2 is less than.

[0075] The results of observation and analysis of the microstructures of the hot-pressed parts according to Examples 1 to 10 are shown in Table 3.

[0076] [Table 3]

[0077] Accordingly, after the above observations and analyses of the microstructure were completed, in order to further demonstrate that the hot-pressed parts prepared according to the present invention have excellent mechanical performance, the inventors resampled the hot-pressed parts according to Examples 1 to 10 and measured the strength and low-temperature embrittlement resistance of the hot-pressed parts according to Examples 1 to 10, and the relevant measurement results are shown in Table 4 below.

[0078] In the present invention, the measurement means applied to Examples 1 to 10 were as follows: Tensile test: The tensile strength of the hot-pressed parts according to Examples 1 to 10 was measured at room temperature according to the room temperature test method of GB / T 228, Tensile test for metallic materials.

[0079] Bending performance test: The bending performance of the hot-pressed parts of Examples 1 to 10 at low temperatures of -60°C and at room temperature (20°C) was measured according to the VDA238-100 measurement method. However, when conducting a low-temperature test to measure bending performance at -60°C, the sample was placed in an alcohol cooling bath to cool to the target temperature, and after 20 minutes of incubation, the sample was quickly transferred to a VDA bending machine. The bending test was completed within 1 minute to obtain corresponding bending performance at low temperatures of -60°C. The bending performance ratio (i.e., the ratio of the bending angle at -60°C to the bending angle at 20°C) for the hot-pressed parts of Examples 1 to 10 at low temperatures of -60°C and at room temperature (20°C) was also obtained.

[0080] Correspondingly, based on the above test, the -60°C low temperature toughness products (tensile strength at room temperature x bending angle at -60°C) of the hot-pressed parts according to Examples 1 to 10 were also obtained.

[0081] Furthermore, the diffused H content in the hot-pressed parts prepared according to Examples 1 to 10 was measured using a Bruker G4 diffused H analyzer, and the diffused H content in the hot-pressed parts according to each example was also measured.

[0082] Table 4 shows the test results of the mechanical properties of the hot-pressed parts according to Examples 1 to 10.

[0083] [Table 4]

[0084] As can be seen from Table 4 above, the hot-pressed parts of Examples 1 to 10 specifically prepared by the technical solution designed according to the present invention have ultra-high strength and excellent low-temperature embrittlement resistance. Their tensile strength is 1729 to 2191 MPa, the ratio of bending performance at low temperature (-60°C) to bending performance at room temperature (20°C) is 0.86 to 0.93, and the -60°C low-temperature toughness product is specifically 8.8 × 10 4 ~11.3×10 4 It was.

[0085] Moreover, the final diffused H content of the hot-pressed parts according to Examples 1 to 10 prepared by this technical solution of the present invention was ≦0.15 ppm, specifically 0.04 to 0.15 ppm.

[0086] In summary, the hot-pressed parts produced according to the present invention have ultra-high strength and excellent resistance to low-temperature brittle cracking. The use of these hot-pressed parts can effectively solve the problem of brittle fracture of ultra-high strength hot-pressed parts currently existing in extremely cold regions, and has good application prospects, being widely applicable in industries such as automobiles, ships, and machinery.

[0087] FIG. 1 is a photograph of the microstructure of the hot-pressed part according to Example 6 taken by a scanning electron microscope.

[0088] As shown in FIG. 1, the microstructure of the hot-pressed part according to Example 6 is shown, and as can be easily seen from FIG. 1, the ratio of lath-tempered martensite therein exceeded 85%.

[0089] Furthermore, the combinations of the technical features in this application are not limited to the combinations described in the claims of this application or the combinations described in the specific examples, and as long as there are no contradictions, all technical features described in this application can be freely combined or combined in any form.

[0090] Furthermore, it should be noted that the above-mentioned embodiments are merely specific embodiments of the present invention, and the present invention is not limited to the above-mentioned embodiments, and it is clear that any similar changes or modifications that can be directly derived or easily conceived by those skilled in the art from the disclosure of the present invention are included in the protection scope of the present invention.

Claims

1. A hot-pressed part having excellent resistance to low-temperature embrittlement, It contains Fe and unavoidable impurities, and further contains the following chemical elements in the following mass percentage contents: Contains C: 0.26 to 0.40%, Si: 0.1 to 1.5%, Mn + Cr: 0.5 to 3.0%, Al: 0.01 to 0.50%, Nb + Ti: 0.04 to 0.25%, Mo + Ni: 0.1 to 1.0%, B: 0.001 to 0.005%; the matrix of the microstructure of the hot pressed part has greater than 85% by volume of lath tempered martensite; The microstructure of the hot-pressed part further has a precipitate phase, wherein the precipitate amounts of Nb and Ti elements account for 50% or more of the total mass fraction of these two elements, and the precipitate phases of Nb and Ti are particulate and dispersedly distributed in the matrix. A hot-pressed part having excellent resistance to low-temperature embrittlement, characterized by:

2. The mass percentage content of each chemical element is: 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 being Fe and other unavoidable impurities.

2. A hot-pressed part having excellent resistance to low-temperature embrittlement according to claim 1.

3. The mass percentage content of each chemical element is: C: 0.26-0.40%; Si: 0.1-1.5%; Mn: ≦2.5%, Cr≦2%, and Mn+Cr: 0.5-3.0%; Al: 0.01-0.50%; Nb: 0.01-0.1%, Ti: 0.02-0.15%, and Nb+Ti: 0.04-0.25%; Mo: ≦0.6%, Ni: ≦0.8%, and Mo+Ni: 0.1-1.0%; B: 0.001-0.005%; the balance being Fe and other unavoidable impurities.

2. A hot-pressed part having excellent resistance to low-temperature embrittlement according to claim 1.

4. The hot-pressed part having excellent low-temperature embrittlement resistance according to any one of claims 1 to 3, characterized in that the inevitable impurities are P ≦ 0.03%, S ≦ 0.01%, N ≦ 0.004%, and O ≦ 0.004%.

5. 5. The hot-pressed part having excellent resistance to low-temperature embrittlement according to claim 4, characterized in that N≦0.003% and / or O≦0.0025%.

6. The chemical elements further include at least one of Cu: 0.01 to 1.0%, W: 0.01 to 0.5%, and V: 0.01 to 0.5%. A hot-pressed part having excellent low-temperature embrittlement resistance according to any one of claims 1 to 3.

7. The precipitated phase of Ti element contains TiN, and the ratio of the long axis to the short axis of TiN is less than 6. When calculated by area ratio, the density of TiN having a long axis exceeding 2 μm is 50 pieces / mm ２ The hot-pressed part having excellent low-temperature embrittlement resistance according to any one of claims 1 to 3, wherein the hardness is less than 100%.

8. The hot-pressed part having excellent low-temperature embrittlement resistance according to any one of claims 1 to 3, characterized in that the particle diameters of TiC, Nb(C,N), and MoC in the precipitation phase are less than 80 nm.

9. The hot-pressed part having excellent low-temperature embrittlement resistance according to any one of claims 1 to 3, characterized in that the matrix of the microstructure further contains one or more of ferrite, pearlite, bainite, martensite, and retained austenite.

10. Its tensile strength exceeds 1700 MPa, the ratio of bending performance at low temperature (-60°C) to bending performance at room temperature exceeds 0.85, and the strength-toughness product at low temperature (-60°C) is 8×10 ４ The hot-pressed part having excellent low-temperature embrittlement resistance according to any one of claims 1 to 3, characterized in that

11. Its tensile strength is 1700-2200 MPa, the ratio of bending performance at -60°C to bending angle at room temperature is 0.86-0.93, and the -60°C low temperature toughness product is 8.8 × 10 ４ ~11.3 x 10 ４ The hot-pressed part having excellent resistance to low-temperature embrittlement according to claim 10, characterized in that

12. The hot-pressed part having excellent low-temperature embrittlement resistance according to any one of claims 1 to 3, characterized in that the diffused H content in the hot-pressed part is less than 0.15 ppm.

13. The method for manufacturing a hot-pressed part having excellent low-temperature embrittlement resistance according to any one of claims 1 to 13, comprising the following steps: (1) Smelting and casting; (2) Hot rolling, coiling, and pickling: the temperature at which the slab is removed from the heating furnace is controlled to 1220-1280°C, and the rolling end temperature is controlled to 880-940°C; the coiling temperature is controlled to 580-680°C, and after coiling, the slab is cooled to 300°C or less at a rate of 0.3-1°C / s; (3) Cold rolling, annealing: the total reduction in cold rolling is controlled to 30-60%; the annealing temperature is controlled to 680-750°C, and after annealing, the steel is cooled to room temperature at an average rate of 1-15°C / s; (4) Hot press forming: The reheating temperature of the steel sheet is controlled to 850-950°C, and the reheating time is controlled to 2-10 minutes. Then, the steel sheet is quickly transferred to a mold to complete the hot press; then, it is cooled to 100-200°C at an average rate of 10-100°C / s, and then air-cooled to room temperature; (5) Low-temperature tempering: The material is kept in an oven at 150 to 250°C for 10 to 40 minutes, then removed and air-cooled to room temperature.

14. The method according to claim 13, further comprising the step of plating or coating the steel sheet after the annealing step (3).

15. The method according to claim 13, wherein the film is cooled to 200 to 300°C at a rate of 0.3 to 1°C / s after winding.

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