Hot stamping formed component with high fracture strain, steel sheet for hot stamping, and hot stamping process

Abstract

The present invention discloses a high fracture strain hot stamping forming component, a steel sheet for hot stamping, and a hot stamping process. Through innovative alloy compositions and hot stamping processes, the structure and mechanical properties after hot stamping are flexibly regulated, and the fracture strain of the hot-formed steel is increased to ensure the energy absorption effect. By using Ti and Nb elements to combine with C and N atoms in the steel, the dissolved C / N atoms are consumed, and nanoscale second-phase particles are formed and uniformly dispersed on the matrix, reducing the hardness difference at the ferrite-martensite interface and effectively improving the plasticity and toughness of the material; the materials involved are relatively simple, and the alloy cost is low. The preparation process of the steel sheet for hot stamping and the hot stamping process window are wide. Based on the control of the transfer time of the hot stamping blank, the flexible control of the content of epitaxial ferrite (oriented attached ferrite) is realized, and finally the preparation of hot stamping forming components with different strength grades after hot stamping is realized, which is very beneficial to the popularization of integrated door rings and will effectively contribute to automotive lightweighting and safety energy absorption management.

Description

Technical Field

[0001] The present invention belongs to the technical field of steel materials, and relates to a boron-containing steel composition suitable for manufacturing dual-phase steel and hot-forming steel and a production process of its coating product, specifically high fracture strain hot stamping forming components, hot stamping steel plates and hot stamping processes. Background Art

[0002] With the rapid development of science and technology, in order to reduce the body weight of vehicles while improving their safety performance, the development of automotive steels towards high strength has become an inevitable trend.

[0003] Hot stamping forming technology refers to a new forming process in which a steel plate blank or a preformed part blank is heated to the austenitizing temperature, held for a certain period of time, then quickly transferred to a die by a manipulator, and then quickly stamped into shape by a press, and an ultra-high strength stamped part is obtained after quenching and holding pressure in the die for a certain period of time. At high temperatures, the steel plate is in the austenitized state, with a strength of about 200 MPa and high plasticity. It is first formed under a very small press pressure and then hardened into a martensite structure, perfectly solving the contradiction between strength and formability. In recent years, hot-forming steels have been gradually applied. It is very important for achieving vehicle lightweight and ensuring collision safety. To ensure collision safety, automotive components need to have sufficient ability to resist collision intrusion, and at the same time rely on good deformation ability to absorb the energy brought by the collision. Therefore, it is particularly necessary to develop hot stamping steels with high fracture strain (high VDA bending performance) for automotive energy-absorbing areas. In addition, the development of integrated door rings has further promoted the research and development of such hot stamping steels.

[0004] In fact, the vast majority of automotive body structural components undergo bending deformation during a collision, and at this time, the deformation state endured by the components is close to plane strain. Existing research has shown that the collision performance of hot-stamped steel components is related to their bending performance under plane strain. In the VDA238-100 three-point tip bending test standard formulated by the German Automotive Industry Association, the width of the steel plate undergoing bending deformation is much larger than its thickness, and the radius of the bending punch is extremely small, resulting in the steel plate also enduring a stress state of plane strain. For this reason, this method is commonly used in the current 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 under the influence of tensile stress, and at this time, the bending angle also reaches its maximum value (i.e., the maximum bending angle αmax). Since αmax can evaluate the bending performance of the material, many automotive enterprises internationally use αmax and tensile performance as indicators to measure the mechanical properties of materials. In addition, when the bending angle is αmax, the strain endured by the outer surface of the material reaches the maximum strain value that it can withstand without fracture under plane strain conditions, that is, the bending fracture strain. The bending fracture strain (the equivalent strain on the outer surface of the material corresponding to αmax) more directly evaluates the fracture limit of the material under plane strain conditions. At the same time, this result can be directly used as the fracture limit strain of the material for the CAE collision analysis of the entire vehicle and becomes an important parameter for judging whether a component undergoes collision fracture failure. A higher αmax indicates that the material has a higher bending fracture strain.

[0005] CN108707825A discloses a production method for a high-plasticity steel plate for hot stamping forming of 550 MPa grade, which gives the alloying components as 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%, Nb: 0.025 - 0.05%. Since the alloying components lack the grain-refining Ti element, the grains are relatively coarse, and only hot-stamped forming components with a relatively low strength level (below 620 MPa) can be produced. In addition, the coiling temperature involved in this invention is relatively high, and grain boundary oxidation is likely to occur during hot rolling, which is not suitable for the production of pre-coated steel plates.

[0006] CN113957349B discloses a 600MPa grade hot forming steel and its production method. The chemical composition is as follows: 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%, and the balance is Fe and inevitable impurity elements. Based on the thin strip casting and rolling process, the invention provides a 600MPa grade hot forming steel without banded structure and decarburized layer, which makes up for the shortcomings of banded structure in existing hot forming steels and the shortcomings of decarburized layer in existing bare plate hot forming steels. In terms of composition, the alloy composition contains relatively high contents of Mn and Nb, with high cost and great control difficulty. In addition, it is unrealistic to make up for the decarburized layer in the existing hot stamping process described in the invention. Under the existing industrial hot stamping conditions, all bare plates will inevitably produce a certain thickness of decarburized layer after hot stamping (unless the atmosphere is extremely good and the furnace sealing is extremely good, which is actually unrealistic in industrial production lines). Summary of the Invention

[0007] In order to overcome the defects existing in the above-mentioned prior art, the present invention flexibly regulates the structure and mechanical properties after hot stamping through innovative alloy composition, production process and hot stamping process, improves the fracture strain of the hot forming steel to ensure the energy absorption effect, well meets the requirements of the energy absorption part in the soft area of automotive materials, and more friendly realizes the collision safety energy management.

[0008] In order to achieve the above-mentioned invention purpose, the present invention provides a steel for hot stamping with high fracture strain and a preparation process of a hot stamping formed component.

[0009] The mass percentage range of the chemical composition of the steel for hot stamping with high fracture strain is as follows: 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 inevitable impurities; among them, the mass fraction ratio of elements meets the following conditions: Ti / (C + N) ≥ 0.6, 0.045% ≤ Nb + Ti ≤ 0.105%, C + Mn / 20 + Si / 30 + 2P + 4S ≤ 0.21%. By using Ti and Nb elements to combine with C and N atoms in the steel grade, consuming the solid solution C / N atoms, forming nano-scale second-phase particles and uniformly dispersing them on the matrix, the hardness difference at the interface between ferrite and martensite is reduced, effectively improving the plasticity and toughness of the material.

[0010] The steel for hot stamping with high fracture strain is commonly used in the performance parts of the soft area of automobiles. After hot stamping, the tensile strength is 600 - 850MPa, and the elongation A 50It is 13 - 28%, the bending angle corresponding to the maximum load of VDA238 - 100 is 100 - 150° (1.6mm), and its tissue feature is a large amount of ferrite + a small amount of martensite.

[0011] A preparation process of the above high fracture strain hot stamping steel and a hot stamping formed component, the process comprising the following steps:

[0012] ① Smelting process: Smelt according to the chemical composition of the high fracture strain hot stamping steel, and cast into a slab.

[0013] ② Hot rolling process: After heating, descaling, rough rolling, finish rolling and laminar cooling of the slab, a hot rolled coil is obtained; the descaling process is strictly controlled in the rough rolling and finish rolling stages, and its finish rolling temperature is 850 - 930°C; the laminar cooling adopts the front - stage cooling method, the cooling rates of the upper and lower headers are 45 - 60% and 70 - 85% respectively, and the coiling temperature is 530 - 620°C.

[0014] ③ Pickling and cold rolling process: After pickling the hot rolled coil, it is cold rolled into a thin strip steel with a thickness of 0.8 - 3.0mm, and its cold rolling reduction rate is 45 - 75%, and the cold rolling reduction rate gradually decreases as the cold rolling thickness of the material increases.

[0015] ④ Continuous annealing process: The cold rolled thin strip steel is first heated to 710°C and 810 - 860°C at heating rates of 3 - 8°C / s and 0.5 - 5°C / s respectively in sections, held for 50 - 170s, then slowly cooled to 730 - 760°C at a rate of 1 - 5°C / s and rapidly cooled to 300 - 380°C in sequence, over - aged and held for 400 - 800s and then cooled to room temperature, and the temper rolling elongation range is 0.30 - 0.55%; a continuously annealed sheet material is obtained.

[0016] 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 in sections; after soaking and holding for 30 - 100s, it is cooled to 635 - 670°C, held for a period of time for equilibrium and then enters the Al - Si bath for Al - Si plating treatment, and the time is 2 - 25s. After coming out of the Al - Si bath, it is cooled to room temperature at a speed of ≥3°C / s, and the unit speed is 60 - 110m / min. The temper rolling elongation range is 0.15 - 1.0%; a plated Al - Si sheet material is obtained.

[0017] ⑤Hot stamping process: Heat the continuously annealed sheet or Al-Si plated sheet obtained in step ④ to austenitization (890 - 960 °C), hold for a certain time, the total heating time = sheet thickness * 115 + 50 - 100 s, then quickly transfer it to the die for quenching, the transfer time is (3 - 12 s), transfer it to the die and quench to 20 - 180 °C, the pressure is 5 - 20 MPa, and the pressure holding time is 5 - 20 s (determined according to the sheet thickness). Either a box furnace or a roller hearth furnace can be used for the furnace. Nitrogen and a small amount of natural gas are filled in the furnace for heating the continuously annealed sheet, or only pure nitrogen is filled. The atmosphere of the furnace for heating the Al-Si plated sheet is dry compressed air, and the dew point is controlled at -30 to -5 °C.

[0018] For the above hot stamping process, by adjusting the sheet transfer time, the preparation of hot stamping steel forming components with different strength grades can be realized. Under the condition of a short sheet transfer time, the strength after quenching is high; under the condition of a long sheet transfer time, the strength after quenching decreases.

[0019] For the continuously annealed sheet of hot stamping steel under the above alloy system, the yield strength of the material obtained after quenching by adjusting the hot stamping heat treatment process is 350 - 650 MPa, the tensile strength is 600 - 840 MPa, and the elongation A 50 is 13 - 28%. The bending angle corresponding to the maximum bending load of the 1.6 mm thick hot stamped bare sheet (after grinding the surface decarburized layer) in VDA238 - 100 is 100 - 148°. For the Al-Si plated sheet, the yield strength of the material obtained after quenching by adjusting the hot stamping heat treatment process is 350 - 660 MPa, the tensile strength is 600 - 850 MPa, and the elongation A 50 is 13 - 28%. The bending angle corresponding to the maximum bending load of the 1.6 mm thick Al-Si coated sheet in VDA238 - 100 is 100 - 145°. Whether it is the continuously annealed sheet or the Al-Si plated sheet, the quenched structure after hot stamping is composed of a large amount of ferrite (consisting of proeutectoid ferrite + oriented attached ferrite / epitaxial ferrite) + a small amount of martensite (ferrite > 70%), and nano-scale microalloy precipitation phases exist on the matrix.

[0020] Compared with the prior art, the beneficial effects of the present invention are:

[0021] The materials involved in the present invention are relatively plain, with low alloy costs. By using Ti and Nb elements to combine with C and N atoms in steel, the dissolved C / N atoms are consumed to form nanoscale second-phase particles, which are uniformly and dispersedly distributed on the matrix, reducing the hardness difference at the ferrite-martensite interface and effectively improving the plasticity and toughness of the materials. The preparation process of the steel for hot stamping forming and the hot stamping process window are wide. Based on the control of the transfer time of the hot stamping sheet, the flexible control of the content of epitaxial ferrite (oriented attached ferrite) is realized, and finally the production and preparation of hot stamping forming components with different strength grades after hot stamping are achieved, which is very beneficial to the popularization of integrated door rings and will effectively contribute to vehicle lightweight and safety energy absorption management. Description of the Drawings

[0022] Figure 1 It is the matrix structure and coating morphology after hot stamping of the Al-Si plated sheet in the embodiment;

[0023] Figure 2 It is a photo of the matrix structure of the Al-Si plated sheet before hot stamping in the embodiment;

[0024] Figure 3 It is the coating morphology of the Al-Si plated sheet before hot stamping in the embodiment. Detailed Embodiments

[0025] The present invention will be further described below in conjunction with specific embodiments, but it is not limited to the present invention in any way. To avoid repetition, in the following embodiments, the raw materials are all commercially available products without special instructions, and the methods used are all conventional methods without special instructions.

[0026] Embodiment

[0027] This embodiment provides the alloy compositions of 3 groups (numbered A, B, C) of steel for hot stamping with high fracture strain, as shown in Table 1 specifically.

[0028] Table 1 Chemical compositions (wt.%) of 3 groups of steel for hot stamping with high fracture strain in the embodiment

[0029] Example number C Si Mn Nb Ti P S N Als A 0.055 0.15 1.4 0.035 0.025 0.022 0.0020 0.0029 0.026 B 0.068 0.10 1.2 0.035 0.050 0.019 0.0012 0.0032 0.032 C 0.080 0.08 1.1 0.039 0.060 0.017 0.0010 0.0030 0.031

[0030] The preparation process of the above-mentioned steel for hot stamping with high fracture strain and the hot stamping forming component is as follows in specific process steps:

[0031] ① Smelting process: Smelting is carried out according to the chemical compositions of the steel for hot stamping with high fracture strain in Table 1, and the slab is formed by casting.

[0032] ②Hot rolling process: The slab is heated, descaled, rough rolled, finish rolled and laminar cooled to obtain a hot rolled coil; The descaling process is strictly controlled in the rough rolling and finish rolling stages, and its final rolling temperature is 850 - 930 °C; The laminar cooling adopts the front-section cooling method, and the cooling rates of the upper and lower headers are 45 - 60% and 70 - 85% respectively, and the coiling temperature is 530 - 620 °C. The specific hot rolling process parameters are shown in Table 2.

[0033] Table 2 Main hot rolling process parameters

[0034] Example number Initial rolling temperature / °C Final rolling temperature / °C Coiling temperature / °C A 1100 920 530 B 1075 900 570 C 1130 860 620

[0035] ③Pickling and cold rolling process: The hot rolled coil is pickled and then cold rolled into a thin strip steel with a thickness of 0.8 - 3.0 mm, and its cold rolling reduction rate is 45 - 75%. As the cold rolling thickness of the material increases, the cold rolling reduction rate gradually decreases.

[0036] ④Continuous annealing process: The cold rolled thin strip steel is first heated to 710 °C and 810 - 860 °C at heating rates of 3 - 8 °C / s and 0.5 - 5 °C / s respectively in sections, and after holding for 50 - 170 s, it is slowly cooled to 730 - 760 °C at a rate of 1 - 5 °C / s and rapidly cooled to 300 - 380 °C in turn, and after overaging and holding for 400 - 800 s, it is cooled to room temperature. The temper rolling elongation range is 0.30 - 0.55%; The continuous annealed sheet is obtained.

[0037] 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 in sections; After soaking and holding for 30 - 100 s, it is cooled to 635 - 670 °C, and after holding for a certain period of time for equilibration, it enters the Al - Si bath for Al - Si plating treatment, and the time is 2 - 25 s. After coming out of the Al - Si bath, it is cooled to room temperature at a speed of ≥3 °C / s, and the unit speed is 60 - 110 m / min. The temper rolling elongation range is 0.15 - 1.0%; The Al - Si plated sheet is obtained.

[0038] ⑤Hot stamping process: The continuous annealed sheet or Al - Si plated sheet obtained in step ④ is heated to austenitization (890 - 960 °C), held for a certain time, and the total heating time = sheet thickness * 115 + 50 - 100 s. Then it is quickly transferred to the die for quenching, and the transfer time is (3 - 12 s). It is quenched to 20 - 180 °C on the die, with a pressure of 5 - 20 MPa and a pressure holding time of 5 - 20 s (determined according to the sheet thickness). Either a box furnace or a roller hearth furnace can be used for the furnace. Nitrogen and a small amount of natural gas or pure nitrogen are filled in the furnace for heating the continuous annealed sheet, and the furnace atmosphere for heating the Al - Si plated sheet is dry compressed air, and the dew point is controlled at - 30 - - 5 °C.

[0039] The 1.60-mm thick continuous annealing sheet or Al-Si plated sheet is heated in a roller hearth furnace and austenitized, and the process is shown in Table 3. After holding at the austenitizing temperature for 280 s, it is air-cooled for 3, 7, and 12 s and transferred to a flat die (specimen numbers are A-3, A-7, A-12; B-3, B-7, B-12; C-3, C-7, C-12) and the microstructure after quenching is ferrite + a small amount of fine martensite. Its microstructure is as shown in Figure 1 shown, and its mechanical properties are shown in Table 4 below.

[0040] Table 3 Roller hearth furnace heating process

[0041]

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

[0043]

[0044] The results show that the hot stamping steel with high fracture strain of the present invention has excellent strength-ductility product and three-point bending performance representing the bending fracture strain, meeting the requirements of high energy absorption for automotive component materials; it is suitable for the preparation of hot stamping forming steel and its corresponding hot stamping components. The microstructure before hot stamping is ferrite + pearlite (as shown in Figure 2 shown), the thickness of the Al-Si coating is 8-15 μm, preferably 10-12 μm (as shown in Figure 3 ); by regulating the transfer time of the hot stamping sheet, the microstructure and properties after hot stamping are flexibly regulated. The microstructure after hot stamping is ferrite + a small amount of martensite (the size and content of ferrite can be regulated by the air-cooling time of sheet transfer), the tensile strength is 600-850 MPa, the maximum bending angle of VDA238-100 representing the collision performance is 100-150° or more, and its plastic toughness is far better than that of 22MnB5 steel (the three-point bending angle is about 60°).

[0045] For any person skilled in the art, without departing from the scope of the technical solution of the present invention, many possible changes and modifications can be made to the technical solution of the present invention by using the technical content disclosed above, or modified into equivalent embodiments with equivalent changes. Therefore, any simple modification, equivalent change, and modification made to the above embodiments based on the technical essence of the present invention without departing from the content of the technical solution of the present invention shall still fall within the scope of protection of the technical solution of the present invention.

Claims

1. A process for preparing a hot stamping forming component, characterized in that, the process comprises the following steps: ① Smelting process: Smelt according to the chemical composition of the high fracture strain hot stamping steel, and cast into slabs; The mass percentage range of the chemical composition of the hot stamping steel is C: 0.068% - 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 inevitable impurities; Among them, the element mass fraction ratio is: Ti / (C + N) ≥ 0.6, 0.045% ≤ Nb + Ti ≤ 0.105%, C + Mn / 20 + Si / 30 + 2P + 4S ≤ 0.21%; The tensile strength of the high fracture strain hot stamping steel after hot stamping is 600 - 850 MPa, the elongation is 13% - 28%, and the bending angle corresponding to the maximum load of VDA238 - 100 is 100° - 150°; ② Hot rolling process: Heat the slabs, descale, rough roll, finish roll and laminar cooling to obtain hot rolled coils; Control the descaling process in the rough rolling and finish rolling stages, and the final rolling temperature is 850°C - 930°C; The laminar cooling adopts the front section cooling method, and the cooling rates of the upper and lower headers are 45% - 60% and 70% - 85% respectively, and the coiling temperature is 530°C - 620°C; ③ Acid rolling process: After pickling the hot rolled coils, cold roll them into thin strip steel with a thickness of 0.8 - 3.0 mm, and the cold rolling reduction rate is 45% - 75%. As the cold rolling thickness of the material increases, the cold rolling reduction rate gradually decreases; ④ Continuous annealing process: The cold rolled thin strip steel is first heated to 710°C and 810°C - 860°C at heating rates of 3 - 8°C / s and 0.5 - 5°C / s respectively in sections, held for 50 - 170 s, and then slowly cooled to 730°C - 760°C and rapidly cooled to 300°C - 380°C at rates of 1 - 5°C / s and 35 - 50°C / s respectively. After overaging and holding for 400 - 800 s, it is cooled to room temperature, and the temper rolling elongation range is 0.30% - 0.55%; Obtain continuously annealed sheet materials; Hot dip Al - Si process: The cold rolled thin strip steel is first heated to 300°C, 700°C - 730°C and 800°C - 850°C at heating rates of 10 - 20°C / s, 3 - 10°C / s and 0.4 - 3°C / s respectively in sections; After soaking and holding for 30 - 100 s, it is cooled to 635°C - 670°C, held for a period of time for equilibration and then enters the Al - Si bath for Al - Si plating treatment, and the time is 2 - 25 s. After exiting the Al - Si bath, it is cooled to room temperature at a speed of ≥ 3°C / s, the unit speed is 60 - 110 m / min, and the temper rolling elongation range is 0.15% - 1.0%; Obtain Al - Si plated sheet materials; ⑤Hot stamping process: Heat the continuously annealed sheet or Al-Si plated sheet obtained in step ④ to 890°C - 960°C for austenitization, hold for a certain time, the total heating time = sheet thickness * 115 + 50 - 100 s, then quickly transfer it to the die for quenching, the transfer time is 3 - 12 s; transfer it to the die and quench to 20°C - 180°C, the pressure is 5 - 20 MPa, and the pressure holding time is 5 - 20 s; In the furnace for heating the continuously annealed sheet in the hot stamping process, nitrogen and natural gas are filled or only pure nitrogen is filled; the atmosphere in the furnace for heating the Al-Si plated sheet is dry compressed air, and the dew point is controlled at -30°C to -5°C.

2. The process for preparing a hot stamping formed component according to claim 1, characterized in that, The yield strength of the material obtained after the hot stamping process of the hot stamping formed component continuous annealing sheet is 350 - 650 MPa, the tensile strength is 600 - 840 MPa, and the elongation A 50 is 13% - 28%. The bending angle corresponding to the maximum bending load of the 1.6 mm thick hot stamped bare sheet VDA238 - 100 is 110° - 150°.

3. The process for preparing a hot stamping formed component according to claim 1, characterized in that, The yield strength of the material obtained after the hot stamping process in step ⑤ of the hot stamping steel Al-Si plated sheet is 350 - 660 MPa, the tensile strength is 600 - 850 MPa, and the elongation A 50 is 13% - 28%. The bending angle corresponding to the maximum bending load of the 1.6 mm thick Al-Si coated sheet in VDA238-100 is 100° - 145°.

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