High-strength hydrogen-embrittlement-resistant press hardening steel and preparation method therefor and use thereof

Abstract

The present application relates to the technical field of alloy materials, and provides high-strength hydrogen-embrittlement-resistant press hardening steel and a preparation method therefor and a use thereof. The high-strength hydrogen-embrittlement-resistant press hardening steel comprises the following components in percentage by weight: 0.22-0.28% of C, 0.8-1.4% of Mn, 0.01-0.08% of Nb, 0.2-0.8% of Cr, 0.001-0.005% of B, 0.01-0.06% of Ti, 0-0.5% of V, 0-0.5% of Si, 0-1.0% of Mo, 0-2.0% of Ni, 0-1.0% of W, and the balance being Fe and inevitable impurities, wherein the ratio of the weight percentage of Cr to the weight percentage of Nb is greater than 8. The high-strength hydrogen-embrittlement-resistant press hardening steel can have excellent strength and hydrogen embrittlement resistance, so that the safety performance of vehicles can be greatly improved, and the purpose of light weight is achieved.

Description

A high-strength hydrogen embrittlement-resistant hot-formed steel and its preparation method and application

[0001] This application claims priority to the Chinese patent application filed with the Patent Office of China on September 19, 2023, with application number 202311206733.6 and application name “A high-strength, hydrogen-embrittlement-resistant hot-formed steel and its application”, and the Chinese patent application filed with the Patent Office of China on September 19, 2023, with application number 202311211073.0 and application name “A hot-formed steel and its preparation method and application”, the entire contents of which are incorporated by reference into this application. Technical Field

[0002] The present application belongs to the technical field of alloy materials, and specifically relates to a high-strength, hydrogen-embrittlement-resistant hot-formed steel and a preparation method and application thereof. Background Art

[0003] Due to its excellent properties, such as high strength, low springback, and high dimensional accuracy, hot-formed steel is widely used in vehicle safety-related structural components, reducing vehicle weight and energy consumption while improving vehicle safety. The production process for hot-formed steel primarily involves smelting, casting, hot rolling, pickling, cold rolling, annealing, and hot stamping. The hot stamping process generally involves billet heating, stamping, pressure-holding quenching, and laser trimming.

[0004] Currently, 22MnB5 hot-formed steel with a strength of 1500 MPa is the most widely used in vehicle body safety-related structural components, primarily due to its excellent combination of strength and resistance to hydrogen embrittlement-induced delayed cracking. While ensuring safety performance requirements, lightweighting vehicles has become one of the most direct and effective solutions for energy conservation and emission reduction. Consequently, research is underway into hot-formed steels with higher strength grades (1600-2000 MPa) in the hope of partially or completely replacing 1500 MPa-grade hot-formed steels. However, despite their high strength, these 1600-2000 MPa-grade hot-formed steels carry the risk of hydrogen embrittlement cracking and fail to meet the SEP1970 standard, which requires no cracking for 120 hours under four-point bending loading conditions, acid immersion (0.1 mol / L HCl), and a preset yield strength of 100%. If used in vehicle safety-related structural components, vehicle safety performance cannot be guaranteed. Therefore, how to improve the strength of hot-formed steel while also ensuring its resistance to hydrogen embrittlement is an urgent issue that needs to be addressed.

[0005] Application Contents

[0006] The present application aims to solve the difficult problems in the related art at least to a certain extent. To this end, the embodiments of the present application provide a high-strength hydrogen embrittlement-resistant hot-formed steel and its preparation method and application.

[0007] In a first aspect, the high-strength, hydrogen embrittlement-resistant hot-formed steel of an embodiment of the present application comprises the following components, by weight percentage: C 0.22-0.28%, Mn 0.8-1.4%, Nb 0.01-0.08%, Cr 0.2-0.8%, B 0.001-0.005%, Ti 0.01-0.06%, V 0-0.5%, Si 0-0.5%, Mo 0-1.0%, Ni 0-2.0%, W 0-1.0%, and the balance is Fe and unavoidable impurities. The ratio of the weight percentage of Cr to the weight percentage of Nb is greater than 8.

[0008] The advantages and technical effects brought by the hot-formed steel of the embodiment of the present application are as follows:

[0009] (1) The hot-formed steel of the embodiment of the present application contains specified amounts of C, Mn, Cr, Mo and B, which can improve the hardenability of the hot-formed steel, reduce the critical cooling rate, and ensure that the final structure contains more than 95% martensite, thereby improving the tensile strength of the hot-formed steel.

[0010] (2) The hot-formed steel of the embodiment of the present application contains a specified amount of Cr and Nb at the same time, and the ratio between the weight percentage of Cr and the weight percentage of Nb is greater than 8. Based on the limited content of Nb carbide precipitation, by adding a small amount of Cr, the precipitation of Nb-containing composite carbides can be promoted to form composite carbides of Cr and Nb, which can make up for the insufficient Nb content and still ensure that a large number of nano-scale precipitated particles are present in the structure, thereby significantly improving the precipitation strengthening effect; at the same time, more importantly, a large amount of Cr and Nb composite carbides can act as hydrogen traps to improve the hydrogen embrittlement resistance of the hot-formed steel, ensuring that the hydrogen embrittlement resistance is not lost while the strength is improved. It is worth noting that the Cr content cannot be lower than 0.2% or higher than 0.8%. A Cr content lower than 0.2% will reduce the Cr-promoted precipitation kinetics of the composite carbides of Cr and Nb, while a Cr content higher than 0.8% will cause Cr to precipitate alone in the form of Cr-rich carbides, reducing the content of the composite carbides of Cr and Nb, thereby reducing the hydrogen embrittlement resistance of the hot-formed steel.

[0011] (3) The hot-formed steel of the embodiment of the present application contains a specified amount of Ti. The main function of Ti is to fix the nitrogen element to form TiN, and to prevent N from forming BN with B, thereby giving full play to the role of B in improving hardenability.

[0012] (4) The hot-formed steel of the embodiment of the present application also contains a specified amount of V. V can refine the grain size, making the original austenite grain size of the hot-formed steel smaller, ensuring that its toughness does not decrease or even increases while the strength is improved; at the same time, its carbides can also act as hydrogen traps, increasing the hydrogen embrittlement resistance of the hot-formed steel.

[0013] (5) The hot-formed steel in the embodiment of the present application also contains a specified amount of Si. During self-tempering after hot stamping and die quenching, Si can hinder carbon diffusion, inhibit the rate of martensite decomposition and carbide aggregation and growth, and significantly improve the tempering stability and strength of the hot-formed steel.

[0014] (6) The hot-formed steel of the embodiment of the present application also contains a specified amount of Ni, which can ensure that the strength of the hot-formed steel is improved without significantly reducing its toughness;

[0015] (7) The hot-formed steel of the embodiment of the present application also contains a specified amount of W. W forms carbides, which can improve the strength of the hot-formed steel while improving its toughness.

[0016] In some embodiments, 710 MPa < 3000 MPa×weight percent of C + 65 MPa×weight percent of Mn < 900 MPa.

[0017] In some embodiments, the weight percentage of Mn is 0.8-1.3%.

[0018] In some embodiments, the ratio between the weight percent of Cr and the weight percent of Nb is greater than 12.

[0019] In some embodiments, the weight percentage of Nb is 0.02-0.04%.

[0020] In some embodiments, the content of impurity N is less than 100 ppm, and the content of impurities P, S, and O are all less than 200 ppm.

[0021] In some embodiments, the critical cooling rate of the hot-formed steel is below 50° C. / s, and / or the metallographic structure of the hot-formed steel is above 95% martensite, preferably 100% martensite.

[0022] In some embodiments, the hot-formed steel has a prior austenite grain size of 3-15 μm, and / or, in a VDA238-100 standard test, a bending angle of a 1.4 mm thick steel plate of the hot-formed steel is greater than 50°.

[0023] In some embodiments, the tensile strength of the hot-formed steel is above 1600 MPa. At the same time, the hot-formed steel does not crack for 120 hours in the SEP1970 standard test under four-point bending loading conditions and preset 100% yield strength.

[0024] In addition, the embodiments of the present application also provide the application of high-strength hydrogen embrittlement-resistant hot-formed steel in vehicles.

[0025] The advantages and technical effects brought about by the application of the embodiments of the present application are:

[0026] Since the hot-formed steel in the embodiment of the present application has excellent strength and resistance to hydrogen embrittlement, it can be used in vehicle parts such as seat crossbeams, A / B pillars, door anti-collision beams, front and rear bumpers, door sill beams and center channels, which can greatly improve the safety performance of the vehicle and achieve the purpose of lightweighting.

[0027] In a second aspect, a method for preparing hot-formed steel according to an embodiment of the present application comprises the following steps: smelting, casting, hot rolling, coiling, pickling, cold rolling, continuous annealing, and hot stamping, wherein the continuous annealing has a holding temperature of 740°C or higher and a cooling rate of less than 40°C / s; and the epitaxial ferrite content in the microstructure after continuous annealing is greater than 10%.

[0028] The hot-formed steel includes the following components by weight: C 0.22-0.28%, Mn 0.8-1.4%, Si 0.1-0.5%, Nb 0.01-0.08%, Cr 0.2-0.8%, B 0.001-0.005%, Ti 0.01-0.06%, V 0-0.5%, Mo 0-1.0%, Ni 0-2.0%, W 0-1.0%, Cu 0-2.0%, and the balance is Fe and unavoidable impurities.

[0029] The advantages and technical effects of the method for preparing hot-formed steel in the embodiment of the present application are as follows:

[0030] (1) The hot-formed steel of the embodiment of the present application contains specified amounts of C, Mn, Cr, Mo and B, which can improve the hardenability of the hot-formed steel, reduce the critical cooling rate, and ensure that the final structure contains more than 95% martensite, thereby improving the tensile strength of the hot-formed steel.

[0031] (2) The hot-formed steel of the embodiment of the present application contains a specified amount of Nb, which improves the strength of the hot-formed steel through the precipitation strengthening effect of Nb; at the same time, the precipitation of nano-scale Nb-containing carbides can effectively hinder the growth of the original austenite grain boundaries, refine the microstructure, and thus improve the bending toughness of the hot-formed steel.

[0032] (3) The hot-formed steel of the embodiment of the present application contains a specified amount of Ti. The main function of Ti is to fix the nitrogen element to form TiN, and to prevent N from forming BN with B, thereby giving full play to the role of B in improving hardenability.

[0033] (4) The hot-formed steel of the embodiment of the present application also contains a specified amount of V, which can refine the grain size, making the original austenite grain size of the hot-formed steel smaller, ensuring that the strength is improved while its toughness is not reduced or even improved.

[0034] (5) The hot-formed steel in the embodiment of the present application also contains a specified amount of Si. During self-tempering after hot stamping and die quenching, Si can hinder carbon diffusion, inhibit the rate of martensite decomposition and carbide aggregation and growth, significantly improve the tempering stability of the hot-formed steel, and ensure that the strength is almost not lost.

[0035] (6) The hot-formed steel of the embodiment of the present application also contains a specified amount of Ni, which can ensure that the strength of the hot-formed steel is improved without significantly reducing its toughness.

[0036] (7) The hot-formed steel of the embodiment of the present application also contains a specified amount of W. W forms carbides, which can improve the strength of the hot-formed steel while improving its toughness.

[0037] (8) The hot-formed steel of the embodiment of the present application also contains a specified amount of Cu. The role of Cu is to enhance the strength of the hot-formed steel through precipitation strengthening.

[0038] (9) Based on the chemical composition of the hot-formed steel in the embodiment of the present application, the preparation method in the embodiment of the present application optimizes the initial microstructure by regulating the holding temperature of the continuous annealing to be above 740°C and the cooling rate of the continuous annealing to be lower than 40°C / s, and introduces more than 10% of epitaxial ferrite rich in C, Mn and Si, so as to provide more austenite nucleation points during the hot stamping process, refine the original austenite grain size, and achieve improved bending toughness of the high-strength hot-formed steel.

[0039] (10) The preparation method of the embodiment of the present application produces higher strength grade hot-formed steel (1.6-1.9GPa) without changing the existing production process, ensuring that the strength is improved without losing toughness. It is expected to replace the traditional 1.5GPa hot-formed steel, achieve vehicle weight reduction, ensure passenger safety and better improve the vehicle's endurance.

[0040] In some embodiments, the continuous annealing is performed at a holding temperature of 740-880°C.

[0041] In some embodiments, the continuous annealing is performed at a holding temperature of 740-860°C.

[0042] In some embodiments, the continuous annealing is performed at a holding temperature of 780-880°C.

[0043] In some embodiments, the cooling rate of the continuous annealing is below 35° C. / s, preferably 10-35° C. / s.

[0044] In some embodiments, the cooling rate of the continuous annealing is less than 25° C. / s, preferably 10-25° C. / s.

[0045] In some embodiments, the epitaxial ferrite content in the microstructure after continuous annealing is greater than 12%.

[0046] In some embodiments, the epitaxial ferrite content in the microstructure after continuous annealing is greater than 15%.

[0047] In some embodiments, the hot-formed steel has a prior austenite grain size of 3-15 μm.

[0048] In addition, embodiments of the present application also provide applications of hot-formed steel in vehicles.

[0049] The advantages and technical effects brought about by the application of the embodiments of the present application are:

[0050] Since the hot-formed steel in the embodiment of the present application has excellent strength and bending toughness, it can be used in vehicle parts such as seat crossbeams, A / B pillars, door anti-collision beams, front and rear bumpers, door sill beams and center channels, which can greatly improve the safety performance of the vehicle and achieve the purpose of lightweighting.

[0051] The above description is only an overview of the technical solution of the present application. In order to more clearly understand the technical means of the present application, it can be implemented in accordance with the contents of the specification. In order to make the above and other purposes, features and advantages of the present application more obvious and easy to understand, the specific implementation methods of the present application are listed below. BRIEF DESCRIPTION OF THE DRAWINGS

[0052] In order to more clearly illustrate the embodiments of the present application or the technical solutions in the prior art, a brief introduction will be given below to the drawings required for use in the embodiments or the description of the prior art. Obviously, the drawings described below are some embodiments of the present application. For ordinary technicians in this field, other drawings can be obtained based on these drawings without any creative work.

[0053] The drawings described herein are used to provide a further understanding of the present application and constitute a part of the present application. The illustrative embodiments of the present application and their descriptions are used to explain the present application and do not constitute an improper limitation on the present application. In the drawings:

[0054] FIG1 is a schematic flow chart of a method for preparing high-strength, hydrogen embrittlement-resistant hot-formed steel according to the present invention;

[0055] FIG2 is a schematic flow chart of the hot stamping forming steps of the high-strength hydrogen embrittlement-resistant hot-formed steel of the present application;

[0056] FIG3 is a hot stamping process diagram of high-strength hydrogen embrittlement-resistant hot-formed steel according to Example 3 of the present application;

[0057] FIG4 is a transmission electron microscope image and energy spectrum diagram of the high-strength hydrogen embrittlement-resistant hot-formed steel of Example 3 of the present application; the circled substance in the left figure is the composite carbide of Cr and Nb;

[0058] FIG5 is a scanning electron microscope image of the hot-formed steel of Example 1;

[0059] FIG6 is a metallographic diagram of the hot-formed steel of Example 1;

[0060] FIG7 is a scanning electron microscope image of the hot-formed steel of Example 2;

[0061] FIG8 is a metallographic image of the hot-formed steel of Example 2;

[0062] FIG9 is a scanning electron microscope image of the hot-formed steel of Example 3;

[0063] FIG10 is a metallographic image of the hot-formed steel of Example 3;

[0064] FIG11 is an engineering stress-strain curve diagram of the Al-Si coating plate in Comparative Example 3. Specific embodiments

[0065] To make the purpose, technical solutions, and advantages of the embodiments of this application more clear, the technical solutions in the embodiments of this application will be clearly and completely described below in conjunction with the drawings in the embodiments of this application. Obviously, the described embodiments are part of the embodiments of this application, not all of the embodiments. Based on the embodiments in this application, all other embodiments obtained by ordinary technicians in this field without making creative efforts are within the scope of protection of this application.

[0066] In order to make the technical problems, technical solutions and beneficial effects to be solved by this application more clearly understood, this application is further described in detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are only used to explain this application and are not intended to limit this application.

[0067] The terms "first", "second", "third", "fourth", etc. (if any) in the specification and claims of the present application and the above-mentioned drawings are used to distinguish similar objects and are not necessarily used to describe a specific order or sequential order. It should be understood that the numbers used in this way can be interchangeable where appropriate, so that the embodiments of the present application described herein can, for example, be implemented in an order other than those illustrated or described herein. In addition, the terms "including" and "having" and any variations thereof are intended to cover non-exclusive inclusions, for example, a process, method, system, product or device that includes a series of steps or units is not necessarily limited to those steps or units clearly listed, but may include other steps or units that are not clearly listed or inherent to these processes, methods, products or devices.

[0068] The technical solution of the present application is described in detail below in conjunction with specific embodiments. The following specific embodiments can be combined with each other, and the same or similar concepts or processes may not be described in detail in some embodiments.

[0069] An embodiment of the present application provides a high-strength, hydrogen embrittlement-resistant hot-formed steel, which comprises the following components, by weight percentage: C 0.22-0.28%, Mn 0.8-1.4%, Nb 0.01-0.08%, Cr 0.2-0.8%, B 0.001-0.005%, Ti 0.01-0.06%, V 0-0.5%, Si 0-0.5%, Mo 0-1.0%, Ni 0-2.0%, W 0-1.0%, and the balance is Fe and unavoidable impurities. The ratio between the weight percentage of Cr and the weight percentage of Nb is greater than 8.

[0070] In the hot-formed steel of the embodiments of the present application, the weight percentage of C is 0.22-0.28%, for example, 0.22%, 0.23%, 0.24%, 0.25%, 0.26%, 0.27%, 0.28%, etc. The role of C is to improve the strength and hardenability of hot-formed steel. When the weight percentage of C is lower than 0.22%, the strength of the hot-formed steel is lower than 1600 MPa, and the strength cannot be improved compared with the commonly used 22MnB5 hot-formed steel. When the weight percentage of C is higher than 0.28%, the toughness of the hot-formed steel is significantly reduced. Considering the matching of strength and toughness, the weight percentage of C is selected to be 0.22-0.28%. Preferably, in some embodiments, the weight percentage of C is 0.24-0.28%.

[0071] In the hot-formed steel of the embodiments of the present application, the weight percentage of Mn is 0.8-1.4%, for example, 0.8%, 0.9%, 1.0%, 1.1%, 1.2%, 1.3%, 1.4%, etc. Mn can increase the hardenability of steel and lower the austenite formation temperature. When the weight percentage of Mn is less than 0.8%, the hardenability of the steel is poor and the austenite formation temperature is high; when the weight percentage of Mn is greater than 1.4%, welding is not conducive. Therefore, the weight percentage of Mn is 0.8-1.4%. Preferably, in some embodiments, the weight percentage of Mn is 0.8-1.3%.

[0072] In some embodiments, 710 MPa < 3000 MPa × weight percentage of C + 65 MPa × weight percentage of Mn < 900 MPa. C and Mn have a solid solution strengthening effect. In the aforementioned relationship, 3000 MPa is the solid solution strength of C, 65 MPa is the solid solution strength of Mn, 3000 MPa × weight percentage of C is the contribution from the solid solution strengthening effect of C, and 65 MPa × weight percentage of Mn is the contribution from the solid solution strengthening effect of Mn. The C and Mn contents satisfying the aforementioned relationship can improve strength while controlling the weld carbon equivalent, thereby avoiding deterioration of welding performance.

[0073] In the hot-formed steel of the embodiments of the present application, the weight percentage of Nb is 0.01-0.08%, for example, 0.01%, 0.02%, 0.03%, 0.04%, 0.05%, 0.06%, 0.07%, 0.08%, etc. During the hot stamping process, Nb forms carbides, which can refine the grain size, reducing the original austenite grain size of the hot-formed steel and increasing the toughness of the hot-formed steel. Carbides also act as hydrogen traps, increasing the hot-formed steel's resistance to hydrogen embrittlement. When the weight percentage of Nb is less than 0.01%, the toughness and hydrogen embrittlement resistance of the hot-formed steel are both poor. When the weight percentage of Nb is greater than 0.08%, the grain refinement and precipitation strengthening effects are insignificant. Therefore, the weight percentage of Nb is 0.01-0.08%. Preferably, in some embodiments, the weight percentage of Nb is 0.02-0.04%.

[0074] In the hot-formed steel of the embodiments of the present application, the weight percentage of Cr is 0.2-0.8%, for example, 0.2%, 0.3%, 0.4%, 0.5%, 0.6%, 0.7%, 0.8%, etc. Hot-formed steel requires a relatively low critical cooling rate to ensure complete transformation from austenite to martensite during the cooling process. However, reducing the original austenite grain size increases the critical cooling rate, so a specified amount of Cr is added to improve the steel's hardenability and reduce its critical cooling rate. When the weight percentage of Cr is less than 0.2%, the critical cooling rate of the steel is high, potentially leading to the formation of other soft phases such as ferrite and pearlite, significantly reducing the strength of the hot-formed steel. When the weight percentage of Cr is greater than 0.8%, dense Cr2O3 oxide scale is produced during the hot rolling process, making it difficult to clean by pickling and affecting the hot-dip coating effect. Therefore, the weight percentage of Cr is 0.2-0.8%. Preferably, in some embodiments, the weight percentage of Cr is 0.4-0.7%.

[0075] The present applicant has discovered that when Cr and Nb are within their respective defined ranges and the ratio of the weight percentage of Cr to the weight percentage of Nb is greater than 8 (e.g., 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, etc.), Cr and Nb form fine composite carbides, which can have a precipitation strengthening effect and improve the mechanical properties of hot-formed steel. Furthermore, the composite carbides of Cr and Nb can also act as hydrogen traps to improve the hydrogen embrittlement resistance of the hot-formed steel. The synergistic effects of Cr and Nb, combined with the above two effects, enhance the mechanical properties and hydrogen embrittlement resistance of the hot-formed steel, resulting in a tensile strength of the hot-formed steel exceeding 1600 MPa and no cracking for 120 hours at 100% yield strength.

[0076] If there is only Cr or only Nb, it is impossible to form a composite carbide of Cr and Nb, which affects the mechanical properties and hydrogen embrittlement resistance of the hot-formed steel. If the Cr content is higher than the range of the embodiment of the present application, Cr is mainly formed. 23 Coarse carbides (over 100nm or even micrometers) such as C6 and Cr7C3 deteriorate mechanical properties, reducing both toughness and strength. They also consume significant amounts of carbon, reducing strength. Simultaneously, the content of Cr and Nb composite carbides is significantly reduced, making hydrogen capture less effective and making hot-formed steel susceptible to hydrogen embrittlement cracking at 100% yield strength. If the ratio of the weight percentage of Cr to the weight percentage of Nb is less than or equal to 8, the hot-formed steel's hydrogen embrittlement resistance will also be deteriorated, making it susceptible to hydrogen embrittlement cracking at 100% yield strength.

[0077] Preferably, in some embodiments, the ratio of the weight percentage of Cr to the weight percentage of Nb is greater than 12. When the weight percentage ratio of Cr to Nb is preferably within the above range, it is beneficial to further improve the mechanical properties and hydrogen embrittlement resistance of the hot-formed steel.

[0078] In the hot-formed steel of the embodiments of the present application, the weight percentage of B is 0.001-0.005%, such as 0.001%, 0.002%, 0.003%, 0.004%, 0.005%, etc. The role of B is to improve the hardenability of steel and reduce the critical cooling rate of steel, so that the steel material transforms from the austenite phase to the martensite phase during the hot stamping step, thereby improving the strength of the hot-formed steel. When the weight percentage of B is lower than 0.001%, the critical cooling rate of steel is high, and other soft phases such as ferrite and pearlite may be produced, which greatly reduces the strength of the hot-formed steel; when the weight percentage of B is higher than 0.005%, the hardenability effect is not significant, so the weight percentage of B is 0.001-0.005%. Preferably, in some embodiments, the weight percentage of B is 0.002-0.004%.

[0079] In the hot-formed steels of the present application, the weight percentage of Ti is 0.01-0.06%, for example, 0.01%, 0.02%, 0.03%, 0.04%, 0.05%, 0.06%, etc. The addition of Ti forms precipitates in the form of TiN, which can fix the nitrogen element and avoid consuming the boron element, thereby fully utilizing the boron's effect of improving hardenability. Preferably, in some embodiments, the weight percentage of Ti is 0.02-0.05%.

[0080] During the hot stamping process, V forms carbides, which refine the grain size, reducing the original austenite grain size of the hot-formed steel and increasing its toughness. Furthermore, its carbides act as hydrogen traps, enhancing the hot-formed steel's resistance to hydrogen embrittlement. When the weight percentage of V exceeds 0.5%, grain refinement and precipitation strengthening are insignificant, so the weight percentage of V is preferably between 0 and 0.5%. Preferably, in some embodiments, the weight percentage of V is between 0 and 0.05%.

[0081] In the hot-formed steel of the embodiments of the present application, the weight percentage of Si is 0-0.5%, for example, 0, 0.1%, 0.2%, 0.3%, 0.4%, 0.5%, etc. The role of Si is to improve the resistance to self-tempering during hot stamping and prevent the reduction of strength. When the weight percentage of Si is higher than 0.5%, more SiO2 is easily formed in the iron oxide scale, which is not conducive to improving the surface quality of the hot-rolled strip. Therefore, the weight percentage of Si is 0-0.5%. Preferably, in some embodiments, the weight percentage of Si is 0-0.4%.

[0082] In the hot-formed steel of the embodiments of the present application, the weight percentage of Mo is 0-1.0%, for example, 0, 0.1%, 0.2%, 0.3%, 0.4%, 0.5%, 0.6%, 0.7%, 0.8%, 0.9%, 1.0%, etc. The function of Mo is to improve hardenability. When the weight percentage of Mo exceeds 1.0%, the hardenability is not significant, so the weight percentage of Mo is 0-1.0%. Preferably, in some embodiments, the weight percentage of Mo is 0-0.5%.

[0083] In the hot-formed steel of the embodiments of the present application, the weight percentage of Ni is 0-2.0%, for example, 0, 0.2%, 0.4%, 0.6%, 0.8%, 1.0%, 1.2%, 1.4%, 1.6%, 1.8%, 2.0%, etc. The role of Ni is to improve strength while ensuring that toughness is not significantly reduced. When the weight percentage of Ni is higher than 2.0%, the strength improvement is not significant, so the weight percentage of Ni is 0-2.0%. Preferably, in some embodiments, the weight percentage of Ni is 0-0.2%.

[0084] In the hot-formed steel of the embodiments of the present application, the weight percentage of W is 0-1.0%, for example, 0, 0.1%, 0.2%, 0.3%, 0.4%, 0.5%, 0.6%, 0.7%, 0.8%, 0.9%, 1.0%, etc. W serves to improve hardenability and provide solid solution strengthening. When the weight percentage of W exceeds 1.0%, the strength improvement is insignificant, so the weight percentage of W is 0-1.0%. Preferably, in some embodiments, the weight percentage of W is 0-0.3%.

[0085] Preferably, in some embodiments, the impurity N content is less than 100 ppm, and the impurities P, S, and O content are all less than 200 ppm. N, P, S, and O are unavoidable impurities, and their presence can reduce the toughness of hot-formed steel. Therefore, the lower the N, P, S, and O content, the better. In the embodiments of the present application, the N content is controlled to less than 100 ppm, and the P, S, and O content is controlled to less than 200 ppm to prevent a reduction in the toughness of the hot-formed steel.

[0086] Preferably, in some embodiments, the critical cooling rate of the hot-formed steel is 50°C / s or less, and / or the metallographic structure of the hot-formed steel is at least 95% martensite. When the critical cooling rate of the hot-formed steel is too high, the transformation of austenite to martensite is hindered, which in turn hinders the improvement of the hot-formed steel's strength. When the proportion of martensite in the metallographic structure of the hot-formed steel is too low, the corresponding proportion of other soft phases such as ferrite and pearlite is too high, which is also detrimental to improving the hot-formed steel's strength.

[0087] Preferably, in some embodiments, the original austenite grain size of the hot-formed steel is 3-15 μm; and / or the bending angle of the hot-formed steel for a 1.4 mm thick plate is greater than 50° in the VDA 238-100 standard test. When the original austenite grain size of the hot-formed steel is too large, the bending angle decreases, indicating a decrease in toughness.

[0088] Preferably, in some embodiments, the tensile strength of the hot-formed steel is above 1600 MPa, for example, 1600 MPa, 1660 MPa, 1700 MPa, 1750 MPa, 1770 MPa, 1780 MPa, 1800 MPa, etc. At the same time, the hot-formed steel does not crack for 120 hours in the SEP1970 standard test under four-point bending loading conditions and preset 100% yield strength.

[0089] In addition, an embodiment of the present application also provides a method for preparing high-strength hydrogen embrittlement-resistant hot-formed steel, as shown in Figures 1 and 2, comprising the following steps: smelting, casting, hot rolling, coiling, pickling, cold rolling, continuous annealing and hot stamping, wherein the hot stamping step comprises the following steps: blank heating, stamping and pressure-holding quenching.

[0090] In some embodiments, the final rolling temperature in the hot rolling step is 780-900° C., and / or the coiling temperature is 450-600° C., so as to obtain a hot rolled coil with no or low internal oxidation.

[0091] In some embodiments, the dew point of the continuous annealing is -16-31°C, and / or the temperature of the continuous annealing is 740-800°C, so as to obtain uniform structure and properties.

[0092] In some embodiments, coating is further performed between continuous annealing and hot stamping. After the bare steel plate is obtained by continuous annealing, an aluminum-silicon layer or a zinc layer can be coated on the surface of the bare steel plate in advance to prevent oxidation of the bare steel plate during the subsequent hot stamping process.

[0093] In addition, the embodiments of the present application also provide the application of the high-strength, hydrogen embrittlement-resistant hot-formed steel of the embodiments of the present application in vehicles.

[0094] Since the hot-formed steel in the embodiment of the present application has excellent strength and resistance to hydrogen embrittlement, it can be used in vehicle parts such as seat crossbeams, A / B pillars, door anti-collision beams, front and rear bumpers, door sill beams and center channels, which can greatly improve the safety performance of the vehicle and achieve the purpose of lightweighting.

[0095] The present application is described in detail below with reference to the embodiments and drawings.

[0096] Example 1

[0097] A high-strength hydrogen embrittlement-resistant hot-formed steel, the chemical composition of which is shown in Table 1.

[0098] The preparation method of the hot-formed steel comprises the following steps:

[0099] (1) Smelting: The steel is prepared according to the chemical composition of the hot-formed steel and smelted into molten steel; (2) The molten steel is cast to obtain a steel billet; (3) The steel billet is hot-rolled at a final rolling temperature of 870°C to obtain a 4 mm thick steel plate; (4) The hot-rolled thick steel plate is coiled at a coiling temperature of 550°C to obtain a hot-rolled coil; (5) The hot-rolled coil is pickled to obtain a pickled plate; (6) The pickled plate is then cold-rolled at room temperature to obtain a 1.4 mm thick steel plate. mm thick thin steel plate; (7) continuously annealing the thin steel plate at a dew point of -23°C and a temperature of 780°C for 10 minutes to obtain a bare steel plate; (8) coating the bare steel plate with an aluminum silicon layer; (9) heating the steel plate with the aluminum silicon layer to 920°C in a roller bottom heating furnace, and then placing it in a hot stamping die, and then hot stamping it under a constant pressure of about 30°C, and directly quenching it in the hot stamping die to obtain the final stamped part.

[0100] Comparative Example 1

[0101] A hot-formed steel, the chemical composition of which is shown in Table 1. The preparation method of the hot-formed steel is the same as that of Example 1.

[0102] Comparative Example 2

[0103] A hot-formed steel, the chemical composition of which is shown in Table 1. The preparation method of the hot-formed steel is the same as that of Example 1.

[0104] Example 2

[0105] A high-strength hydrogen embrittlement-resistant hot-formed steel, the chemical composition of which is shown in Table 1. The preparation method of the hot-formed steel is the same as that of Example 1.

[0106] Comparative Example 3

[0107] A hot-formed steel, the chemical composition of which is shown in Table 1. The preparation method of the hot-formed steel is the same as that of Example 1.

[0108] Example 3

[0109] A high-strength hydrogen embrittlement-resistant hot-formed steel, the chemical composition of which is shown in Table 1. The preparation method of the hot-formed steel is the same as that of Example 1.

[0110] Comparative Example 4

[0111] A hot-formed steel, the chemical composition of which is shown in Table 1. The preparation method of the hot-formed steel is the same as that of Example 1.

[0112] Example 4

[0113] A high-strength hydrogen embrittlement-resistant hot-formed steel, the chemical composition of which is shown in Table 1. The preparation method of the hot-formed steel is the same as that of Example 1.

[0114] Comparative Example 5

[0115] A hot-formed steel, the chemical composition of which is shown in Table 1. The preparation method of the hot-formed steel is the same as that of Example 1.

[0116] Comparative Example 6

[0117] A hot-formed steel, the chemical composition of which is shown in Table 1. The preparation method of the hot-formed steel is the same as that of Example 1.

[0118] Example 5

[0119] A high-strength hydrogen embrittlement-resistant hot-formed steel, the chemical composition of which is shown in Table 1. The preparation method of the hot-formed steel is the same as that of Example 1.

[0120] Comparative Example 7

[0121] A hot-formed steel, the chemical composition of which is shown in Table 1. The preparation method of the hot-formed steel is the same as that of Example 1.

[0122] Performance Testing

[0123] Bending angle test: According to the VDA238-100 test standard, the bending angles of 1.4mm thick hot-formed steel plates in the TD and RD directions are tested, and the minimum value is taken.

[0124] Hydrogen embrittlement resistance: According to the SEP1970 test standard, the sample is immersed in acid (0.1 mol / L HCl) under four-point bending loading conditions, preset to 100% yield strength, and observed for cracking within 120 hours. If no cracking occurs after 120 hours at 100% yield strength due to hydrogen embrittlement, the hydrogen embrittlement resistance is qualified; if cracking occurs at 100% yield strength due to hydrogen embrittlement, the hydrogen embrittlement resistance is unqualified.

[0125] Table 1. Chemical composition of hot-formed steels of Examples 1-5 and Comparative Examples 1-7

[0126] Table 2. Properties of hot-formed steels of Examples 1-5 and Comparative Examples 1-7

[0127] It can be seen from Table 1 and Table 2 that the hot-formed steels of Examples 1-5 of the present application have excellent tensile strength and hydrogen embrittlement resistance. Specifically, the tensile strength is above 1600 MPa, and the 100% yield strength is not cracked after 120 hours of hydrogen embrittlement.

[0128] From the comparison between Example 1 and Comparative Example 1, the comparison between Example 2 and Comparative Example 3, the comparison between Example 3 and Comparative Example 4, the comparison between Example 4 and Comparative Example 5, and the comparison between Example 5 and Comparative Example 7, it can be seen that the Cr content of Comparative Examples 1, 3, 4, 5, and 7 is lower than the range defined in the present application, and the ratio between the weight percentage of Cr and the weight percentage of Nb is less than 8, and the obtained hot-formed steel suffers from hydrogen embrittlement cracking at 100% yield strength.

[0129] From the comparison between Example 4 and Comparative Example 6, it can be seen that the ratio between the weight percentage of Cr and the weight percentage of Nb in Comparative Example 6 is less than 8, and the obtained hot-formed steel suffers from hydrogen embrittlement cracking at 100% yield strength.

[0130] A comparison of Example 1 and Comparative Example 2 shows that the Cr content in Comparative Example 2 exceeds the specified range of this application, and the resulting hot-formed steel suffers from hydrogen embrittlement cracking at 100% yield strength. Furthermore, increasing the Cr content also increases the production cost of hot-formed steel.

[0131] The present application provides a method for preparing hot-formed steel, comprising the following steps: smelting, casting, hot rolling, coiling, pickling, cold rolling, continuous annealing, and hot stamping, wherein the continuous annealing has a holding temperature of 740°C or higher and a cooling rate of less than 40°C / s; and the epitaxial ferrite content in the microstructure after continuous annealing is greater than 10%.

[0132] The hot-formed steel includes the following components by weight: C 0.22-0.28%, Mn 0.8-1.4%, Si 0.1-0.5%, Nb 0.01-0.08%, Cr 0.2-0.8%, B 0.001-0.005%, Ti 0.01-0.06%, V 0-0.5%, Mo 0-1.0%, Ni 0-2.0%, W 0-1.0%, Cu 0-2.0%, and the balance is Fe and unavoidable impurities.

[0133] The preparation method of the hot-formed steel in the embodiment of the present application optimizes the initial microstructure obtained after cold rolling by regulating the holding temperature and cooling rate of continuous annealing without changing the conventional process of hot-formed steel, and introduces more epitaxial ferrite rich in C, Mn and Si to provide more austenite nucleation points in the subsequent hot stamping process, refine the original austenite grain size, and provide more residual austenite to be retained to room temperature due to the presence of rich Mn, thereby achieving an improvement in the bending toughness of high-strength hot-formed steel above 1.6 GPa.

[0134] In the method for preparing hot-formed steel according to the embodiment of the present application, the microstructure obtained after the cold rolling step, given the composition of the hot-formed steel, is ferrite and pearlite. By controlling the holding temperature and / or cooling rate during continuous annealing, the microstructure is ensured to consist of retained ferrite, epitaxial ferrite, pearlite, and other minor hard phases (bainite and martensite).

[0135] The holding temperature of the continuous annealing is above 740°C, for example, 740°C, 750°C, 760°C, 770°C, 780°C, 790°C, 800°C, 810°C, 820°C, 830°C, 840°C, 850°C, 860°C, 870°C, 880°C, etc. The cooling rate of the continuous annealing is lower than 40°C / s, for example, 10°C / s, 15°C / s, 20°C / s, 25°C / s, 30°C / s, 35°C / s, 39°C / s, etc., to optimize the initial microstructure state and ensure that the epitaxial ferrite content in the microstructure after continuous annealing is above 10%. C, Mn, and Si are enriched in the epitaxial ferrite, and austenitization is first formed here, thereby providing more nucleation points. The amount of original austenite formed in the subsequent hot stamping process increases, the size is reduced, and the bending toughness of the hot-formed steel is significantly improved. When the holding temperature of continuous annealing is lower than 740°C and / or the cooling rate is greater than or equal to 40°C / s, it is not conducive to the formation of epitaxial ferrite. When the content of epitaxial ferrite is lower than 10%, more hard phase structures such as bainite and martensite will be formed, and the bending toughness of the hot-formed steel will not be significantly improved.

[0136] The preparation method of the hot-formed steel in the embodiment of the present application is to regulate the holding temperature and cooling rate of continuous annealing based on the composition of the hot-formed steel, so that the microstructure after continuous annealing contains more than 10% epitaxial ferrite, and finally obtain a bending angle of the hot-formed steel in the TD direction of more than 42°.

[0137] In some embodiments, a hot-dip coating process may be included between the continuous annealing and the hot stamping process. For example, a hot-dip coating of an aluminum-silicon layer or a zinc layer may be performed to prevent oxidation of the bare steel sheet during the subsequent hot stamping process. When the hot-dip coating process is included, the cooling rate of the continuous annealing process refers to the cooling rate from the continuous annealing holding temperature to the hot-dip coating temperature. When the hot-dip coating process is not included, the cooling rate of the continuous annealing process refers to the cooling rate from the continuous annealing holding temperature to room temperature.

[0138] In some embodiments, the billet heating temperature in the hot rolling step is 1200-1250°C, the final rolling temperature in the hot rolling step is 870-890°C, and the coiling temperature is 450-600°C to prevent internal oxidation in the middle of the hot-rolled coil in the width direction, resulting in incomplete pickling of the oxide layer during the subsequent pickling process, affecting the hot dip plating effect of the plate.

[0139] In some embodiments, the holding temperature of the continuous annealing is 740-880°C, and the cooling rate of the continuous annealing is less than 35°C / s. The holding temperature and cooling rate of the continuous annealing are within the above ranges, which is conducive to further optimizing the initial organizational state, ensuring that the epitaxial ferrite content in the microstructure after continuous annealing is more than 12%, and further improving the bending toughness of the hot-formed steel. When the holding temperature of the continuous annealing is too low, it is not conducive to increasing the epitaxial ferrite content in the microstructure after continuous annealing; when the holding temperature of the continuous annealing is too high, it is not conducive to cost reduction and efficiency improvement. When the cooling rate of the continuous annealing is too high, it is not conducive to increasing the epitaxial ferrite content in the microstructure after continuous annealing. Taking into account the limitations of the current continuous annealing equipment and cooling means themselves, preferably, the holding temperature of the continuous annealing is 740-860°C, and the cooling rate of the continuous annealing is 10-35°C / s.

[0140] Preferably, in some embodiments, the continuous annealing temperature is 780-880°C, and the cooling rate is 25°C / s or less. The epitaxial ferrite content in the microstructure after continuous annealing is 15% or more, which further improves the bending toughness of the hot-formed steel. Considering the limitations of current continuous annealing equipment and cooling methods, the continuous annealing temperature is preferably 780-860°C, and the cooling rate is 10-25°C / s.

[0141] In some embodiments, the continuous annealing holding time is 6-30 minutes, for example, 6 minutes, 10 minutes, 15 minutes, 20 minutes, 25 minutes, 30 minutes, etc., to facilitate obtaining uniform microstructure and performance. It is understood that when the continuous annealing holding temperature is set higher, the continuous annealing holding time can be shorter; when the continuous annealing holding temperature is set lower, the continuous annealing holding time can be set longer. After continuous annealing under the above conditions, the tensile strength of the material is in the range of 400-900 MPa, which is convenient for coiling and stamping sheet cutting.

[0142] In the preparation method of the hot-formed steel of the embodiment of the present application, the weight percentage of C in the hot-formed steel is 0.22-0.28%, for example, 0.22%, 0.23%, 0.24%, 0.25%, 0.26%, 0.27%, 0.28%, etc. The role of C is to improve the strength and hardenability of the hot-formed steel. When the weight percentage of C is lower than 0.22%, the strength of the hot-formed steel is lower than 1.6 GPa, and the strength cannot be improved compared with the commonly used 22MnB5 hot-formed steel. When the weight percentage of C is higher than 0.28%, the toughness of the hot-formed steel is significantly reduced and the welding performance is deteriorated. Considering the strength-toughness matching and taking into account the welding performance, the weight percentage of C is selected to be 0.22-0.28%.

[0143] In the method for preparing hot-formed steel according to an embodiment of the present application, the weight percentage of Mn in the hot-formed steel is 0.8-1.4%, for example, 0.8%, 0.9%, 1.0%, 1.1%, 1.2%, 1.3%, 1.4%, etc. Mn can increase the hardenability of steel, lower the austenite formation temperature, and provide solid solution strengthening by substitution. When the weight percentage of Mn is less than 0.8%, the hardenability of the steel is poor and the austenite formation temperature is high. When the weight percentage of Mn is greater than 1.4%, welding is not conducive. Therefore, the weight percentage of Mn is 0.8-1.4%.

[0144] In some embodiments, 710 MPa < 3000 MPa × weight percentage of C + 65 MPa × weight percentage of Mn < 900 MPa. C and Mn have a solid solution strengthening effect. In the aforementioned relationship, 3000 MPa is the solid solution strength of C, 65 MPa is the solid solution strength of Mn, 3000 MPa × weight percentage of C is the contribution from the solid solution strengthening effect of C, and 65 MPa × weight percentage of Mn is the contribution from the solid solution strengthening effect of Mn. The C and Mn contents satisfying the aforementioned relationship can improve the strength of hot-formed steel while controlling the welding carbon equivalent and avoiding deterioration of welding performance.

[0145] In the method for preparing hot-formed steel in the embodiment of the present application, the weight percentage of Si in the hot-formed steel is 0.1-0.5%, for example, 0.1%, 0.2%, 0.3%, 0.4%, 0.5%, etc. The role of Si is, on the one hand, to promote the formation of epitaxial ferrite during the cooling process of continuous annealing, thereby improving the toughness of the hot-formed steel; on the other hand, to improve the self-tempering resistance during the hot stamping forming process and prevent the reduction of strength. When the weight percentage of Si is lower than 0.1%, it is not conducive to the formation of epitaxial ferrite during the cooling process of continuous annealing, and further not conducive to improving the toughness of the hot-formed steel. When the weight percentage of Si is higher than 0.5%, it is easy to form more SiO2 in the iron oxide scale formed in the hot rolling process, which is not conducive to improving the surface quality of the hot-rolled strip. Therefore, the weight percentage of Si is 0.1-0.5%.

[0146] In the method for preparing hot-formed steel according to an embodiment of the present application, the weight percentage of Nb in the hot-formed steel is 0.01-0.08%, for example, 0.01%, 0.02%, 0.03%, 0.04%, 0.05%, 0.06%, 0.07%, 0.08%, etc. During the hot stamping process, Nb forms carbides or carbonitrides, which pin the austenite grain boundaries and refine the grain size, thereby reducing the original austenite grain size of the hot-formed steel. This ensures that the original austenite grain size of the material after hot stamping is less than 15mm, thereby improving the toughness of the hot-formed steel. When the weight percentage of Nb is less than 0.01%, the toughness of the hot-formed steel is poor; when the weight percentage of Nb is greater than 0.08%, the grain refinement effect is not significant and the cost is increased. Therefore, the weight percentage of Nb is 0.01-0.08%.

[0147] In the method for preparing hot-formed steel according to the embodiment of the present application, the weight percentage of Cr in the hot-formed steel is 0.2-0.8%, for example, 0.2%, 0.3%, 0.4%, 0.5%, 0.6%, 0.7%, 0.8%, etc. Hot-formed steel requires a lower critical cooling rate to ensure complete transformation from austenite to martensite during the die cooling process after hot stamping, thereby improving the mechanical properties of the hot-formed steel. However, reducing the original austenite grain size increases the critical cooling rate, so a specified amount of Cr is added to improve the hardenability of the steel and reduce its critical cooling rate. When the weight percentage of Cr is less than 0.2%, the critical cooling rate of the steel is high, which may produce soft ferrite and pearlite structures, significantly reducing the strength of the hot-formed steel. When the weight percentage of Cr is higher than 0.8%, dense iron oxide scale (Cr2O3) is produced during the hot rolling process, which is difficult to clean by pickling and affects the subsequent hot-dip plating effect. Therefore, the weight percentage of Cr is 0.2-0.8%.

[0148] In some embodiments, the Cr and Mn contents are in the following relationship: 1% < Cr (by weight) + Mn (by weight) < 2%. This prevents the formation of severe and dense iron oxide scale (Cr2O3) during hot rolling, which can affect pickling and subsequent hot-dip galvanizing. Furthermore, this prevents oxidation of the steel during continuous annealing, which can affect hot-dip galvanizing. When the Cr (by weight) + Mn (by weight) percentage is less than 1%, adverse effects such as insufficient hardenability and excessively high austenitizing temperatures may occur. When the Cr (by weight) + Mn (by weight) percentage is greater than 2%, adverse effects such as poor weldability and poor hot-dip galvanizing may occur.

[0149] In the method for preparing hot-formed steel in the embodiment of the present application, the weight percentage of B in the hot-formed steel is 0.001-0.005%, for example, 0.001%, 0.002%, 0.003%, 0.004%, 0.005%, etc. The role of B is to improve the hardenability of the steel and reduce the critical cooling rate of the steel, so that the steel material transforms from the austenite phase to the martensite phase during the hot stamping step, thereby improving the strength of the hot-formed steel. When the weight percentage of B is lower than 0.001%, the critical cooling rate of the steel is high, and other soft phases such as ferrite and pearlite may be produced, which greatly reduces the strength of the hot-formed steel; when the weight percentage of B is higher than 0.005%, the hardenability effect is not significant, so the weight percentage of B is 0.001-0.005%.

[0150] In the method for preparing hot-formed steel according to the embodiment of the present application, the weight percentage of Ti in the hot-formed steel is 0.01-0.06%, for example, 0.01%, 0.02%, 0.03%, 0.04%, 0.05%, 0.06%, etc. The addition of Ti forms precipitates in the form of TiN, which can fix the nitrogen element and avoid consuming the boron element, thereby fully utilizing the boron's effect of improving hardenability.

[0151] In the method for preparing hot-formed steel according to an embodiment of the present application, the weight percentage of V in the hot-formed steel is 0-0.5%. During the hot stamping process, V forms carbides or carbonitrides, which pin austenite grain boundaries and refine the grain size, resulting in a smaller original austenite grain size in the hot-formed steel. This ensures that the original austenite grain size of the material after hot stamping is less than 15 mm, thereby improving the toughness of the hot-formed steel. When the weight percentage of V is higher than 0.5%, grain refinement and precipitation strengthening are not significant, and production costs may increase. Therefore, the weight percentage of V is 0-0.5%.

[0152] In the method for preparing hot-formed steel according to the embodiment of the present application, the weight percentage of Mo in the hot-formed steel is 0-1.0%, for example, 0, 0.1%, 0.2%, 0.3%, 0.4%, 0.5%, 0.6%, 0.7%, 0.8%, 0.9%, 1.0%, etc. The role of Mo is to improve the hardenability of steel and reduce the critical cooling rate of steel, so that the steel material transforms from the austenite phase to the martensite phase during the hot stamping step, thereby improving the strength of the hot-formed steel. When the weight percentage of Mo is higher than 1.0%, the hardenability is not significant, but the production cost will increase, which is not conducive to cost reduction and efficiency improvement. Therefore, the weight percentage of Mo is 0-1.0%.

[0153] In the method for preparing hot-formed steel according to the embodiment of the present application, the weight percentage of nickel in the hot-formed steel is 0-2.0%, for example, 0, 0.2%, 0.4%, 0.6%, 0.8%, 1.0%, 1.2%, 1.4%, 1.6%, 1.8%, 2.0%, etc. The role of nickel is to improve strength while ensuring that toughness is not significantly reduced. When the weight percentage of nickel is higher than 2.0%, the strength improvement is not significant, so the weight percentage of nickel is 0-2.0%.

[0154] In the method for preparing hot-formed steel according to the embodiments of the present application, the weight percentage of W in the hot-formed steel is 0-1.0%, for example, 0, 0.1%, 0.2%, 0.3%, 0.4%, 0.5%, 0.6%, 0.7%, 0.8%, 0.9%, 1.0%, etc. W improves hardenability and provides solid solution strengthening. When the weight percentage of W exceeds 1.0%, the strength improvement is insignificant, so the weight percentage of W is 0-1.0%.

[0155] In the method for preparing hot-formed steel according to an embodiment of the present application, the weight percentage of Cu in the hot-formed steel is 0-2.0%, for example, 0, 0.2%, 0.4%, 0.6%, 0.8%, 1.0%, 1.2%, 1.4%, 1.6%, 1.8%, 2.0%, etc. The role of Cu is to improve the strength of the hot-formed steel through precipitation strengthening of a nanoscale Cu-rich phase. When the weight percentage of Cu exceeds 2.0%, the strength improvement is insignificant and production costs increase. Therefore, the weight percentage of Cu is 0-2.0%.

[0156] In some embodiments, the impurity N content is less than 100 ppm, and the impurities P, S, and O content are all less than 200 ppm. N, P, S, and O are unavoidable impurities, and their presence can reduce the toughness of hot-formed steel. Therefore, the lower the N, P, S, and O content, the better. In the embodiments of the present application, the N content is controlled to less than 100 ppm, and the P, S, and O content is controlled to less than 200 ppm to prevent a reduction in the toughness of the hot-formed steel.

[0157] In some embodiments, the critical cooling rate of the hot-formed steel is 25-50°C / s. If the critical cooling rate of the hot-formed steel is too low, a fully martensite structure is more likely to form during the continuous annealing cooling process, which is not conducive to the formation of epitaxial ferrite. If the critical cooling rate of the hot-formed steel is too high, the transformation of austenite to martensite is not conducive, which in turn is not conducive to improving the strength of the hot-formed steel.

[0158] In some embodiments, the microstructure of the hot-formed steel is greater than 95% martensite, for example, 95%, 96%, 97%, 98%, 99%, 100%, etc. When the proportion of martensite in the microstructure of the hot-formed steel is too low, the corresponding proportions of soft ferrite and pearlite are too high, which is not conducive to improving the strength of the hot-formed steel. Preferably, the microstructure of the hot-formed steel is 100% martensite.

[0159] In some embodiments, the original austenite grain size of the hot-formed steel is 3-15 μm. When the original austenite grain size of the hot-formed steel is too large, the bending angle decreases, which means that the toughness decreases.

[0160] In some embodiments, the tensile strength of the hot-formed steel is greater than 1.6 GPa, such as 1.6 GPa, 1.7 GPa, 1.8 GPa, 1.9 GPa, etc.; in the VDA238-100 standard test, the bending angle of the 1.4 mm thick steel plate of the hot-formed steel is greater than 42°.

[0161] In addition, embodiments of the present application also provide applications of hot-formed steel in vehicles.

[0162] Since the hot-formed steel in the embodiment of the present application has excellent strength and bending toughness, it can be used in vehicle parts such as seat crossbeams, A / B pillars, door anti-collision beams, front and rear bumpers, door sill beams and center channels, which can greatly improve the safety performance of the vehicle and achieve the purpose of lightweighting.

[0163] The present application is described in detail below with reference to the embodiments and drawings.

[0164] Example 6

[0165] A high-strength, high-bending-toughness hot-formed steel has a chemical composition as shown in Table 3. The content of the impurity N is less than 100 ppm, and the contents of the impurities P, S, and O are all less than 200 ppm.

[0166] The preparation method of the hot-formed steel comprises the following steps:

[0167] (1) Smelting: The ingredients are prepared according to the chemical composition of the hot-formed steel and smelted into molten steel;

[0168] (2) casting molten steel to obtain a steel billet;

[0169] (3) hot rolling the steel slab at a heating temperature of 1200°C and a final rolling temperature of 870°C to obtain a 4 mm thick steel plate;

[0170] (4) coiling the hot-rolled thick steel plate at a coiling temperature of 550° C. to obtain a hot-rolled coil;

[0171] (5) pickling the hot-rolled coil to obtain a pickled plate;

[0172] (6) cold rolling the pickled plate at room temperature to obtain a thin steel plate with a thickness of 1.4 mm;

[0173] (7) The thin steel plate was continuously annealed at 770°C for 10 min, then placed in an Al-Si bath at 650°C for hot dip coating at a cooling rate of 25°C / s, and then rapidly cooled to room temperature at a cooling rate of 35°C / s to obtain an Al-Si coated plate;

[0174] (8) The steel plate with Al-Si coating was heated to 920 °C in a roller bottom heating furnace and kept warm for 240 s. The austenitized sheet was then quickly transferred to a hot stamping die and hot stamped under a constant pressure of about 20 MPa. The sheet was then quenched directly in the hot stamping die to obtain the final stamped part.

[0175] Example 7

[0176] The chemical composition and preparation method of the hot-formed steel of Example 7 are the same as those of Example 6, except that continuous annealing is performed at 800°C.

[0177] Example 8

[0178] The chemical composition and preparation method of the hot-formed steel in Example 8 are the same as those in Example 6, except that continuous annealing is performed at 830°C.

[0179] Example 9

[0180] The chemical composition and preparation method of the hot-formed steel of Example 9 are the same as those of Example 6, except that continuous annealing is performed at 860°C.

[0181] Example 10

[0182] The chemical composition of the hot-formed steel of Example 10 is shown in Table 5. The preparation method of Example 10 is the same as that of Example 6.

[0183] Example 11

[0184] The chemical composition and preparation method of the hot-formed steel of Example 11 are the same as those of Example 10, except that continuous annealing is performed at 800°C.

[0185] Example 12

[0186] The chemical composition and preparation method of the hot-formed steel of Example 12 are the same as those of Example 10, except that continuous annealing is performed at 830°C.

[0187] Example 13

[0188] The chemical composition of the hot-formed steel of Example 13 is shown in Table 7. The preparation method of Example 13 is the same as that of Example 1, except that continuous annealing is performed at 740°C.

[0189] Example 14

[0190] The chemical composition and preparation method of the hot-formed steel of Example 14 are the same as those of Example 13, except that continuous annealing is performed at 760°C.

[0191] Example 15

[0192] The chemical composition and preparation method of the hot-formed steel of Example 15 are the same as those of Example 13, except that continuous annealing is performed at 780°C.

[0193] Example 16

[0194] The chemical composition and preparation method of the hot-formed steel of Example 16 are the same as those of Example 13, except that continuous annealing is performed at 800°C.

[0195] Example 17

[0196] The chemical composition and preparation method of the hot-formed steel of Example 17 are the same as those of Example 16, except that after continuous annealing, the steel is placed in an Al-Si liquid pot at 650°C for hot dip plating at a cooling rate of 10°C / s.

[0197] Example 18

[0198] The chemical composition and preparation method of the hot-formed steel of Example 18 are the same as those of Example 16, except that after continuous annealing, the steel is cooled at a rate of 20°C / s and then placed in an Al-Si bath at 650°C for hot dip plating.

[0199] Example 19

[0200] The chemical composition and preparation method of the hot-formed steel of Example 19 are the same as those of Example 16, except that after continuous annealing, the steel is cooled at a rate of 30°C / s and then placed in an Al-Si bath at 650°C for hot dip plating.

[0201] Example 20

[0202] The chemical composition and preparation method of the hot-formed steel of Example 20 are the same as those of Example 16, except that after continuous annealing, the steel is cooled at a rate of 35°C / s and then placed in an Al-Si bath at 650°C for hot dip plating.

[0203] Comparative Example 8

[0204] The chemical composition and preparation method of the hot-formed steel of Comparative Example 8 are the same as those of Example 16, except that after continuous annealing, the steel is cooled at a rate of 40°C / s and then placed in an Al-Si bath at 650°C for hot dip plating.

[0205] Comparative Example 9

[0206] The chemical composition and preparation method of the hot-formed steel of Comparative Example 9 are the same as those of Example 16, except that after continuous annealing, the steel is cooled at a rate of 50°C / s and then placed in an Al-Si bath at 650°C for hot dip plating.

[0207] Comparative Example 10

[0208] The chemical composition and preparation method of the hot-formed steel of Comparative Example 10 are the same as those of Example 16, except that it is continuously annealed at a temperature of 780°C for 6 minutes, and then enters an Al-Si liquid pot at 650°C for hot dip plating at a cooling rate of 40°C / s.

[0209] Performance Testing

[0210] Bending angle test: According to the VDA238-100 test standard, the bending angles of 1.4mm thick hot-formed steel plates in the TD and RD directions are tested, and the minimum value is taken.

[0211] Table 3. Chemical composition of hot-formed steels of Examples 6-9

[0212] Table 4. Tensile strength and bending properties of hot-formed steels of Examples 6-9

[0213] Table 5. Chemical composition of hot-formed steels of Examples 10-12

[0214] Table 6. Tensile strength and bending properties of hot-formed steels of Examples 10-12

[0215] Table 7. Chemical composition of hot-formed steels of Examples 13-16

[0216] Table 8. Tensile strength and bending properties of hot-formed steels of Examples 13-16

[0217] Table 9. Tensile strength and bending properties of hot-formed steels of Examples 17-20 and Comparative Examples 8-9

[0218] It can be seen from Tables 4, 6, 8 and 9 that the hot-formed steels of Examples 6-21 have both excellent strength and bending toughness.

[0219] From the comparison between Examples 6-9 in Table 4, the comparison between Examples 10-12 in Table 6, and the comparison between Examples 13-16 in Table 8, it can be seen that within the continuous annealing holding temperature range of 740-860°C, the higher the holding temperature, the higher the epitaxial ferrite content after continuous annealing and before hot stamping, and the larger the bending angle of the hot-formed steel.

[0220] A comparison between Example 16 in Table 8, Examples 17-20 in Table 9, and Comparative Examples 8-9 shows that during the continuous annealing cooling process at 10-35°C / s, the lower the cooling rate, the higher the epitaxial ferrite content after continuous annealing and before hot stamping, and the larger the bending angle of the hot-formed steel. However, in Comparative Examples 8 and 9, the continuous annealing cooling rate exceeded the range specified in this application, resulting in an epitaxial ferrite content below 10% after continuous annealing, ultimately significantly reducing the bending angle of the hot-formed steel.

[0221] It should be noted that, for the sake of simplicity, the method embodiments are described as a series of action combinations, but those skilled in the art should be aware that the embodiments of the present application are not limited by the order of the actions described, because according to the embodiments of the present application, certain steps can be performed in other orders or simultaneously. Secondly, those skilled in the art should also be aware that the embodiments described in the specification are all preferred embodiments, and the actions involved are not necessarily required by the embodiments of the present application.

[0222] It should be noted that, in this document, the terms "comprises," "includes," or any other variations thereof are intended to encompass non-exclusive inclusion, such that a process, method, article, or apparatus comprising a series of elements includes not only those elements but also other elements not explicitly listed, or elements inherent to such process, method, article, or apparatus. In the absence of further limitations, an element defined by the phrase "comprising a ..." does not exclude the presence of other identical elements in the process, method, article, or apparatus comprising the element.

[0223] The embodiments of the present application are described above in conjunction with the accompanying drawings, but the present application is not limited to the above-mentioned specific implementation methods. The above-mentioned specific implementation methods are merely illustrative and not restrictive. Under the guidance of this application, ordinary technicians in this field can also make many forms without departing from the purpose of this application and the scope of protection of the claims, all of which are protected by this application.

Claims

1. A high-strength hydrogen embrittlement-resistant hot-formed steel, characterized in that: The invention comprises the following components by weight percentage: C 0.22-0.28%, Mn 0.8-1.4%, Nb 0.01-0.08%, Cr 0.2-0.8%, B 0.001-0.005%, Ti 0.01-0.06%, V 0-0.5%, Si 0-0.5%, Mo 0-1.0%, Ni 0-2.0%, W 0-1.0%, the balance being Fe and unavoidable impurities, and the ratio between the weight percentage of Cr and the weight percentage of Nb is greater than 8.

2. The high-strength hydrogen embrittlement-resistant hot-formed steel according to claim 1, characterized in that: 710MPa<3000MPa×weight percentage of C+65MPa×weight percentage of Mn<900MPa.

3. The high-strength hydrogen embrittlement-resistant hot-formed steel according to claim 1 or 2, characterized in that: The weight percentage of Mn is 0.8-1.3%.

4. The high-strength hydrogen embrittlement-resistant hot-formed steel according to claim 1, characterized in that: The ratio between the weight percent of Cr and the weight percent of Nb is greater than 12.

5. The high-strength hydrogen embrittlement-resistant hot-formed steel according to claim 1 or 4, characterized in that: The weight percentage of Nb is 0.02-0.04%.

6. The high-strength hydrogen embrittlement-resistant hot-formed steel according to claim 1, characterized in that: The content of impurity N is less than 100 ppm, and the content of impurities P, S, and O are all less than 200 ppm.

7. The high-strength hydrogen embrittlement-resistant hot-formed steel according to claim 1, characterized in that: The critical cooling rate of the hot-formed steel is 50° C. / s or less, and / or the metallographic structure of the hot-formed steel is 95% or more martensite, preferably 100% martensite.

8. The high-strength hydrogen embrittlement-resistant hot-formed steel according to claim 1, characterized in that: The original austenite grain size of the hot-formed steel is 3-15 μm, and / or, in a VDA238-100 standard test, the bending angle of a 1.4 mm thick steel plate of the hot-formed steel is greater than 50°.

9. The high-strength hydrogen embrittlement-resistant hot-formed steel according to claim 1, characterized in that: The tensile strength of the hot-formed steel is above 1600 MPa. Meanwhile, in the SEP 1970 standard test, the hot-formed steel is acid-soaked under four-point bending loading conditions and preset with 100% yield strength without cracking for 120 hours.

10. Use of the high-strength hydrogen embrittlement-resistant hot-formed steel according to any one of claims 1 to 9 in vehicles.

11. A method for preparing hot-formed steel, characterized in that: The following steps are involved: Smelting, casting, hot rolling, coiling, pickling, cold rolling, continuous annealing and hot stamping, the continuous annealing holding temperature is above 740°C, the continuous annealing cooling rate is lower than 40°C / s; the epitaxial ferrite content in the microstructure after continuous annealing is above 10%; The hot-formed steel comprises the following components by weight percentage: C 0.22-0.28%, Mn 0.8-1.4%, Si 0.1-0.5%, Nb 0.01-0.08%, Cr 0.2-0.8%, B 0.001-0.005%, Ti 0.01-0.06%, V 0-0.5%, Mo 0-1.0%, Ni 0-2.0%, W 0-1.0%, Cu 0-2.0%, The balance is Fe and inevitable impurities.

12. The method for preparing hot-formed steel according to claim 11, characterized in that: The continuous annealing is carried out at a holding temperature of 740-880°C.

13. The method for preparing hot-formed steel according to claim 12, characterized in that: The continuous annealing is carried out at a holding temperature of 740-860°C.

14. The method for preparing hot-formed steel according to claim 12, characterized in that: The continuous annealing is carried out at a holding temperature of 780-880°C.

15. The method for preparing hot-formed steel according to claim 11 or 12, characterized in that: The cooling rate of the continuous annealing is 35° C. / s or less.

16. The method for preparing hot-formed steel according to claim 15, characterized in that: The cooling rate of the continuous annealing is 10-35°C / s.

17. The method for preparing hot-formed steel according to claim 15, characterized in that: The cooling rate of the continuous annealing is below 25°C / s.

18. The method for preparing hot-formed steel according to claim 17, characterized in that: The cooling rate of the continuous annealing is 10-25°C / s.

19. The method for preparing hot-formed steel according to claim 11 or 12, characterized in that: The epitaxial ferrite content in the microstructure after continuous annealing is more than 12%.

20. The method for preparing hot-formed steel according to claim 19, characterized in that: The epitaxial ferrite content in the microstructure after continuous annealing is more than 15%.

21. The method for preparing hot-formed steel according to claim 11 or 12, characterized in that: The original austenite grain size of the hot-formed steel is 3-15 μm.

22. A hot-formed steel, characterized in that: The invention is prepared by the preparation method according to any one of claims 11 to 21.

23. Use of the hot formed steel according to claim 22 in a vehicle.

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