High-fracture-strain component formed via hot stamping, steel plate for hot stamping, and hot stamping process

Abstract

A high-fracture-strain component formed via hot stamping, a steel plate for hot stamping, and a hot stamping process. The composition of the steel for hot stamping is C: 0.055-0.080%, Mn: 1.1-1.4%, Si: 0.02-0.15%, Nb: 0.020-0.045%, and Ti: 0.025-0.060%, the remaining elements being Fe and unavoidable impurities, and the element mass fraction ratio being: Ti / (C+N)≥0.6, 0.045%≤Nb+Ti≤0.105%, and C+Mn / 20+Si / 30+2P+4S≤0.21%. Ti and Nb are combined with C and N atoms in the steel, consuming solid-solution C / N atoms, and forming nanoscale second-phase particles which are uniformly dispersed and distributed on a matrix, reducing the difference in hardness at the interface of ferrite and martensite, and improving the plasticity and toughness of the material. By means of innovative alloy components and an innovative hot stamping process, the structure and mechanical properties of the steel after hot stamping are regulated and controlled; the component formed via hot stamping has a tensile strength of 600-850 MPa, an elongation rate of 13-28%, and a VDA bending angle of 100°-150°.

Description

High fracture strain hot stamping component, hot stamping steel plate and hot stamping process

[0001] This application claims priority to the Chinese patent application filed with the China Patent Office on November 21, 2023, with application number 202311561911.7 and invention name “High fracture strain hot stamping formed components, steel plates for hot stamping and hot stamping processes”, the entire contents of which are incorporated by reference into this application. Technical Field

[0002] The present invention belongs to the technical field of steel materials, and relates to a boron-containing steel composition suitable for manufacturing complex phase steel and hot forming steel and a production process of its coating products, specifically high fracture strain hot stamping formed components, hot stamping steel plates and hot stamping processes. Background Art

[0003] With the rapid development of science and technology, in order to reduce the weight of the vehicle body and improve its safety performance, the development of high-strength steel for automobiles has become an inevitable trend.

[0004] Hot stamping technology involves heating a steel sheet blank or preformed part to austenitizing temperature, holding it at that temperature for a specified period, and then rapidly transferring it to a die using a robot. Then, it is rapidly stamped and formed in a press. After quenching and holding in the die for a specified period, the resulting ultra-high-strength stamped part is produced. At high temperatures, the steel sheet is in an austenitized state, achieving a strength of approximately 200 MPa and high plasticity. Under very low press pressure, the part is initially formed and then hardened to a martensitic structure, effectively resolving the trade-off between strength and formability. In recent years, hot-stamped steel has been increasingly used. This technology is crucial for both lightweighting and ensuring crash safety in automobiles. To ensure crash safety, automotive components must have sufficient resistance to collision intrusion while also absorbing the energy of a collision through excellent deformation. Therefore, the development of hot-stamped steel with high fracture strain (high VDA bending resistance) for energy-absorbing areas is particularly important. Furthermore, the development of integrated door rings has further spurred the development of this type of hot-stamped steel.

[0005] In fact, most automotive body components undergo bending deformation during a collision, with the deformation state of the components approaching plane strain. Previous studies have shown that the collision performance of hot-stamped steel components is related to their bending properties under plane strain. In the VDA 238-100 three-point bend test standard developed by the German Association of the Automotive Industry, the width of the steel sheet undergoing bending deformation is much greater than its thickness, and the radius of the bending punch is extremely small, resulting in the steel sheet also experiencing a plane strain stress state. For this reason, this method is currently widely used in the automotive industry to quickly measure the fracture strain of hot-stamped steel. When the bending load reaches its peak, cracks begin to appear on the outer surface of the material due to the tensile stress, and the bending angle at this point also reaches its maximum value (i.e., the maximum bending angle αmax). Because αmax can be used to evaluate the bending performance of a material, many international automotive companies use αmax, along with tensile properties, as indicators of material mechanical properties. Furthermore, at the bending angle αmax, the strain on the outer surface of the material reaches the maximum strain it can withstand under plane strain without fracture, i.e., the bending fracture strain. Flexural fracture strain (the equivalent strain on the material's outer surface corresponding to αmax) more directly evaluates the material's fracture limit under plane strain conditions. This result can be used directly as the material's fracture limit strain in vehicle CAE crash analysis and becomes an important parameter for determining whether a component will experience collision fracture failure. A higher αmax indicates a higher flexural fracture strain.

[0006] CN108707825A discloses a method for producing 550 MPa-grade high-plasticity steel sheet for hot stamping. The alloy composition is C: 0.08-0.10%, Si: 0.25-0.40%, Mn: 1.10-1.50%, P ≤ 0.02%, S ≤ 0.01%, Als: 0.01-0.10%, N ≤ 0.005%, and Nb: 0.025-0.05%. Due to the lack of grain-refining Ti in the alloy composition, the grains are relatively coarse, resulting in the production of hot-stamped components with relatively low strength (below 620 MPa). Furthermore, the high coiling temperature involved in this invention makes hot rolling prone to grain boundary oxidation, making it unsuitable for the production of pre-coated steel sheet.

[0007] CN113957349B discloses a 600MPa-grade hot-formed steel and its production method. Its composition is C: 0.015-0.04%; Si: 0.30-0.50%; Mn: 1.6-2.0%; Cr: 0.2-0.4%; Nb: 0.06-0.10%; Al≤0.003%; P≤0.020%; S≤0.004%; N≤0.005%; O: 0.003-0.006%, with the remainder being Fe and unavoidable impurities. Based on a thin strip casting and rolling process, this invention provides a 600MPa-grade hot-formed steel without banding or decarburization, addressing the shortcomings of existing hot-formed steels, including the banding and decarburization of existing bare plate hot-formed steels. From the composition point of view, the alloy contains relatively high content of Mn and Nb, which is costly and difficult to control. In addition, the decarburization layer compensation described in the invention is unrealistic in the existing hot stamping process. Under the existing industrial hot stamping conditions, all bare plates will inevitably produce a decarburization layer of a certain thickness after hot stamping (unless the atmosphere is extremely good and the furnace is extremely airtight, which is actually unrealistic in industrial production lines).

[0008] Summary of the Invention

[0009] In order to overcome the defects of the above-mentioned existing technologies, the present invention uses innovative alloy composition, production process and hot stamping process to flexibly control the microstructure and mechanical properties after hot stamping, improve the fracture strain of hot-formed steel to ensure the energy absorption effect, and well meet the soft zone energy absorption requirements of automotive materials, and more friendly achieve collision safety energy management.

[0010] In order to achieve the above-mentioned object of the invention, the present invention provides a preparation process of high fracture strain hot stamping steel and hot stamping formed components.

[0011] The chemical composition of the high fracture strain hot stamping steel has a mass percentage range of C: 0.055-0.080%, Mn: 1.1-1.4%, Si: 0.02-0.15%, Nb: 0.020-0.045%, and Ti: 0.025-0.060%. The remaining elements are Fe and unavoidable impurities. The mass fraction ratios of these elements meet the following conditions: Ti / (C+N) ≥ 0.6, 0.045% ≤ Nb+Ti ≤ 0.105%, and C+Mn / 20+Si / 30+2P+4S ≤ 0.21%. Ti and Nb combine with carbon and nitrogen atoms in the steel, consuming dissolved carbon and nitrogen atoms to form nanoscale second-phase particles that are evenly dispersed throughout the matrix. This reduces the hardness difference at the interface between ferrite and martensite, effectively improving the material's plasticity and toughness.

[0012] The high fracture strain hot stamping steel is commonly used in the soft zone performance parts of automobiles. The tensile strength after hot stamping is 600-850MPa and the elongation A50 The bending angle of VDA238-100 under the maximum load is 100-150° (1.6 mm), and its organizational characteristics are a large amount of ferrite + a small amount of martensite.

[0013] A process for preparing the above-mentioned high fracture strain hot stamping steel and hot stamping formed components, the process comprising the following steps:

[0014] ① Smelting process: Smelting is performed according to the chemical composition of the high fracture strain hot stamping steel, and the steel is cast into a slab.

[0015] ② Hot rolling process: The slab is heated, descaled, rough rolled, finished rolled and laminar cooled to obtain hot rolled coils; the descaling process is strictly controlled in the rough rolling and finishing rolling stages, and the final rolling temperature is 850-930℃; the laminar cooling adopts the front-stage cooling method, and the upper and lower header cooling rates are 45-60% and 70-85% respectively, and the coiling temperature is 530-620℃.

[0016] ③ Pickling process: After pickling, the hot-rolled coil is cold-rolled into a thin strip steel of 0.8 to 3.0 mm. The cold rolling reduction rate is 45 to 75%. As the cold-rolled thickness of the material increases, the cold rolling reduction rate gradually decreases.

[0017] ④Continuous annealing process: The cold-rolled thin strip is first heated in sections to 710°C and 810-860°C at heating rates of 3-8°C / s and 0.5-5°C / s, respectively. After holding for 50-170s, it is slowly cooled to 730-760°C and rapidly cooled to 300-380°C at rates of 1-5°C / s and 35-50°C / s, respectively. After overaging and holding for 400-800s, it is cooled to room temperature. The flat elongation range is 0.30-0.55%; and a continuously annealed sheet is obtained.

[0018] Hot-dip Al-Si process: The cold-rolled thin strip is first heated in stages to 300°C, 700°C to 730°C, and 800°C to 850°C at heating rates of 10-20°C / s, 3-10°C / s, and 0.4-3°C / s, respectively. After being held at this temperature for 30-100 seconds, the strip is cooled to 635-670°C. After a period of equalization, the strip is placed in an Al-Si bath for Al-Si plating, which lasts for 2-25 seconds. After exiting the bath, the strip is cooled to room temperature at a rate of ≥3°C / s. The line speed is 60-110 m / min. The flattening elongation ranges from 0.15% to 1.0%, resulting in an Al-Si-coated sheet.

[0019] ⑤ Hot stamping process: The continuously annealed sheet or Al-Si plated sheet obtained in step ④ is heated to austenitization (890-960°C), held at this temperature for a certain period of time (total heating time = material thickness * 115 + 50-100s), and then quickly transferred to the mold for quenching. The transfer time is (3-12s). After being transferred to the mold, it is quenched to 20-180°C, at a pressure of 5-20MPa, and the holding time is 5-20s (depending on the thickness of the sheet). The furnace can be either a box furnace or a roller hearth furnace. The furnace for heating the continuously annealed sheet is filled with nitrogen and a small amount of natural gas or only pure nitrogen. The furnace atmosphere for heating the Al-Si plated sheet is dry compressed air with a dew point controlled at -30--5°C.

[0020] The above hot stamping process can realize the preparation of hot stamped steel formed components with different strength grades by adjusting the sheet transfer time. When the sheet transfer time is short, the strength after quenching is high; when the sheet transfer time is long, the strength after quenching is reduced.

[0021] The yield strength of the hot stamping steel continuous annealing sheet obtained after quenching by adjusting the hot stamping heat treatment process under the above alloy system is 350-650MPa, the tensile strength is 600-840MPa, and the elongation A 50 The bending angle of the bare plate after hot stamping (grinding the surface decarburized layer) with a thickness of 1.6mm is 100-148° under the maximum bending load of VDA238-100. The yield strength of the Al-Si plate obtained after adjusting the hot stamping heat treatment process and quenching is 350-660MPa, the tensile strength is 600-850MPa, and the elongation A is 0. 50 The relative humidity is 13-28%. The corresponding bending angle under the maximum bending load of 1.6mm thick Al-Si coated sheet VDA238-100 is 100-145°. Whether it is continuously annealed sheet or Al-Si coated sheet quenched after hot stamping, the microstructure is composed of a large amount of ferrite (composed of proeutectoid ferrite + epitaxial ferrite / epitaxial ferrite) + a small amount of martensite (ferrite>70%), and nano-scale microalloyed precipitates are present on the matrix.

[0022] Compared with the prior art, the present invention has the following beneficial effects:

[0023] The materials involved in this invention are relatively simple, with low alloy costs. They utilize Ti and Nb elements to combine with C and N atoms in steel, consuming dissolved C / N atoms to form nanoscale second-phase particles that are evenly dispersed throughout the matrix, reducing the hardness difference at the ferrite-martensite interface and effectively improving the material's plasticity and toughness. The hot stamping steel preparation process and hot stamping process window are wide. By controlling the hot stamping sheet transfer time, flexible control of the epitaxial ferrite (oriented epitaxial ferrite) content is achieved, ultimately enabling the production of hot stamped components with varying strength grades after hot stamping. This is highly beneficial for the promotion of integrated door rings and will effectively contribute to vehicle lightweighting and safe energy absorption management. BRIEF DESCRIPTION OF THE DRAWINGS

[0024] FIG1 is a diagram of the matrix structure and coating morphology of an Al-Si plated sheet after hot stamping in an embodiment;

[0025] FIG2 is a photograph of the matrix structure of the Al-Si plated sheet before hot stamping in the embodiment;

[0026] FIG3 is the coating morphology of the Al-Si plated sheet before hot stamping in the embodiment. DETAILED DESCRIPTION

[0027] To facilitate understanding of the present application, a more comprehensive description of the present application will be provided below with reference to the accompanying drawings. The accompanying drawings illustrate preferred embodiments of the present application. However, the present application may be implemented in many different forms and is not limited to the embodiments described herein. Rather, these embodiments are provided to provide a more thorough and comprehensive understanding of the disclosure of the present application.

[0028] The present invention will be further described below with reference to specific examples, but the present invention is not limited in any way. To avoid redundancy, the raw materials in the following examples are all commercially available products unless otherwise specified, and the methods used are all conventional methods unless otherwise specified.

[0029] Example

[0030] This embodiment provides three groups (numbered A, B, and C) of alloy compositions of high fracture strain hot stamping steels, as shown in Table 1.

[0031] Table 1 Chemical composition (wt.%) of high fracture strain hot stamping steel of Example 3

[0032] The specific process steps for preparing the above-mentioned high fracture strain hot stamping steel and hot stamping formed components are as follows:

[0033] ① Smelting process: Smelt the steel according to the chemical composition of the high fracture strain hot stamping steel in Table 1 and cast it into slabs.

[0034] Hot Rolling: Hot-rolled coils are obtained after the slab undergoes heating, descaling, rough rolling, finishing rolling, and laminar cooling. Descaling is strictly controlled during the roughing and finishing rolling stages, and the final rolling temperature is 850-930°C. Laminar cooling utilizes a front-end cooling method, with upper and lower header cooling rates of 45-60% and 70-85%, respectively. The coiling temperature is 530-620°C. Specific hot rolling process parameters are shown in Table 2.

[0035] Table 2 Main process parameters of hot rolling

[0036] ③ Pickling process: After pickling, the hot-rolled coil is cold-rolled into a thin strip steel of 0.8 to 3.0 mm. The cold rolling reduction rate is 45 to 75%. As the cold-rolled thickness of the material increases, the cold rolling reduction rate gradually decreases.

[0037] ④Continuous annealing process: The cold-rolled thin strip is first heated in sections to 710°C and 810-860°C at heating rates of 3-8°C / s and 0.5-5°C / s, respectively. After holding for 50-170s, it is slowly cooled to 730-760°C and rapidly cooled to 300-380°C at rates of 1-5°C / s and 35-50°C / s, respectively. After overaging and holding for 400-800s, it is cooled to room temperature. The flat elongation range is 0.30-0.55%; and a continuously annealed sheet is obtained.

[0038] Hot-dip Al-Si process: The cold-rolled thin strip is first heated in stages to 300°C, 700°C to 730°C, and 800°C to 850°C at heating rates of 10-20°C / s, 3-10°C / s, and 0.4-3°C / s, respectively. After being held at this temperature for 30-100 seconds, the strip is cooled to 635-670°C. After a period of equalization, the strip is placed in an Al-Si bath for Al-Si plating, which lasts for 2-25 seconds. After exiting the bath, the strip is cooled to room temperature at a rate of ≥3°C / s. The line speed is 60-110 m / min. The flattening elongation ranges from 0.15% to 1.0%, resulting in an Al-Si-coated sheet.

[0039] ⑤ Hot stamping process: The continuously annealed sheet or Al-Si plated sheet obtained in step ④ is heated to austenitization (890-960°C), held at this temperature for a certain period of time (total heating time = material thickness * 115 + 50-100s), and then quickly transferred to the mold for quenching. The transfer time is (3-12s). After being transferred to the mold, it is quenched to 20-180°C, at a pressure of 5-20MPa, and the holding time is 5-20s (depending on the thickness of the sheet). The furnace can be either a box furnace or a roller hearth furnace. The furnace for heating the continuously annealed sheet is filled with nitrogen and a small amount of natural gas or pure nitrogen. The furnace atmosphere for heating the Al-Si plated sheet is dry compressed air with a dew point controlled at -30--5°C.

[0040] 1.60mm thick continuously annealed sheet or Al-Si-coated sheet was heated and austenitized in a roller-hearth furnace, using the process shown in Table 3. After being held at the austenitizing temperature for 280 seconds, the sheet was air-cooled for 3, 7, and 12 seconds, respectively, before being transferred to a flat die (samples A-3, A-7, and A-12; B-3, B-7, and B-12; and C-3, C-7, and C-12, respectively). The microstructure after quenching was ferrite with a small amount of fine martensite. The microstructure is shown in Figure 1, and the mechanical properties are summarized in Table 4.

[0041] Table 3 Roller hearth furnace heating process

[0042] Table 4 Mechanical properties of hot-dip Al-Si steel plates for hot stamping with high fracture strain after hot stamping

[0043] The results show that the high fracture strain hot stamping steel of the present invention has excellent strength-ductility product and three-point bending performance representing bending fracture strain, meeting the high energy absorption requirements of automotive parts; it is suitable for the preparation of hot stamping steel and its corresponding hot stamping parts, the microstructure before hot stamping is ferrite + pearlite structure (as shown in Figure 2), the Al-Si coating thickness is 8-15μm, preferably 10-12μm (as shown in Figure 3); by regulating the hot stamping sheet transfer time, the microstructure and performance after hot stamping are flexibly controlled, the microstructure after hot stamping is ferrite + a small amount of martensite (the size and content of ferrite can be controlled by the air cooling time of sheet transfer), the tensile strength is 600-850MPa, the maximum bending angle of VDA238-100 representing collision performance is above 100-150°, and its plastic toughness is much better than that of 22MnB5 steel (three-point bending angle is about 60°).

[0044] The technical features of the above-mentioned embodiments can be combined arbitrarily. In order to make the description concise, not all possible combinations of the technical features in the above-mentioned embodiments are described. However, as long as there is no contradiction in the combination of these technical features, they should be considered to be within the scope of this specification.

[0045] The above-described embodiments merely represent several implementation methods of the present application. While the descriptions are relatively specific and detailed, they should not be construed as limiting the scope of the present invention. It should be noted that a person skilled in the art could make various modifications and improvements without departing from the spirit of the present application, all of which fall within the scope of protection of the present application. Therefore, the scope of protection of the present patent application shall be determined by the appended claims.

Claims

1. High fracture strain hot stamping steel, characterized in that: The chemical composition mass percentage range of the hot stamping steel is: C: 0.055-0.080%, Mn: 1.1-1.4%, Si: 0.02-0.15%, Nb: 0.020-0.045%, Ti: 0.025-0.060%, and the remaining elements are Fe and unavoidable impurities; Among them, the mass fraction ratio of elements is: Ti / (C+N)≥0.6, 0.045%≤Nb+Ti≤0.105%, C+Mn / 20+Si / 30+2P+4S≤0.21%; The high fracture strain hot stamping steel has a tensile strength of 600-850 MPa after hot stamping, an elongation of 13-28%, and a corresponding bending angle of 100°-150° under the maximum load of VDA238-100.

2. A process for preparing a hot stamping formed component using the high fracture strain hot stamping steel as claimed in claim 1, characterized in that: The process comprises the following steps: ① Smelting process: smelting according to the chemical composition of the high fracture strain hot stamping steel and casting into a slab; ②Hot rolling process: The slab is heated, descaled, rough rolled, finished rolled and laminar cooled to obtain hot rolled coils; the descaling process is controlled in the rough rolling and finishing rolling stages, and the final rolling temperature is 850-930°C; the laminar cooling adopts the front cooling method, the upper and lower header cooling rates are 45-60% and 70-85% respectively, and the coiling temperature is 530-620°C; ③ Pickling process: After pickling, the hot-rolled coil is cold-rolled into a thin strip steel of 0.8-3.0 mm, and the cold-rolling reduction rate is 45-75%. As the cold-rolled thickness of the material increases, the cold-rolling reduction rate gradually decreases; ④Continuous annealing process: The cold-rolled thin strip is first heated to 710℃ and 810~860℃ in sections at a heating rate of 3~8℃ / s and 0.5~5℃ / s respectively, and then kept warm for 50~170s, and then slowly cooled to 730~760℃ and quickly cooled to 300~380℃ at a rate of 1~5℃ / s and 35~50℃ / s respectively, and then cooled to room temperature after aging and keeping warm for 400~800s. The flattening elongation range is 0.30~0.55%; Producing a continuously annealed sheet; Hot-dip Al-Si process: the cold-rolled thin strip steel is first heated to 300°C, 700-730°C and 800-850°C at heating rates of 10-20°C / s, 3-10°C / s and 0.4-3°C / s respectively; after being evenly heated and kept for 30-100s, it is cooled to 635-670°C, and after being evenly kept for a period of time, it enters the Al-Si pool for Al-Si plating treatment, and the time is 2-25s. After exiting the Al-Si pool, it is cooled to room temperature at a speed of ≥3°C / s, the unit speed is 60-110m / min, and the flattening elongation range is 0.15-1.0%; Al-Si plated sheet is obtained; ⑤Hot stamping process: heat the continuously annealed sheet or Al-Si plated sheet obtained in step ④ to 890-960°C for austenitization, keep it warm for a certain time, the total heating time = material thickness * 115 + 50-100s, and then quickly transfer it to the mold for quenching, the transfer time is 3-12s; transfer it to the mold and quench it to 20-180°C, the pressure is 5-20MPa, and the holding time is 5-20s.

3. The process according to claim 2, characterized in that The heating continuous annealing sheet furnace of the hot stamping process is filled with nitrogen and a small amount of natural gas or only pure nitrogen; the atmosphere of the heating furnace for Al-Si plated sheet is dry compressed air, and the dew point is controlled at -30 to -5°C.

4. The process according to claim 3, characterized in that The yield strength of the hot stamping steel continuous annealing sheet obtained after the hot stamping process in step ⑤ is 350-650MPa, the tensile strength is 600-840MPa, and the elongation A 50 It is 13-28%, and the corresponding bending angle under the maximum bending load of the bare plate VDA238-100 with a thickness of 1.6 mm after hot stamping is 110-150°.

5. The process according to claim 4, characterized in that The yield strength of the material obtained by the hot stamping steel plated Al-Si after the hot stamping process in step ⑤ is 350-660MPa, the tensile strength is 600-850MPa, and the elongation A 50 It is 13-28%, and the corresponding bending angle of 1.6mm thick Al-Si coated plate VDA238-100 under maximum bending load is 100-145°.

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