1500mpa grade high plasticity high fracture strain antioxidant hot forming steel and preparation method thereof

Abstract

The application belongs to the technical field of steel alloy materials, and particularly relates to a 1500MPa-grade high-plasticity high-fracture-strain oxidation-resistant hot forming steel and a preparation method thereof. According to mass percentages, the composition and content of the hot forming steel are as follows: C 0.26%-0.28%, Mn 3.6%-3.9%, Si 0.4%-0.7%, Al 1.4%-1.7%, Nb 0.01%-0.03%, V 0.04%-0.07%, S<0.002%, P<0.002%, O<0.002%, N<0.003%, and the balance is Fe. The microstructure of the hot forming steel provided by the application is composed of ferrite, martensite and residual austenite. The yield strength of the hot forming steel is 830-870MPa, the tensile strength is 1520-1570MPa, the elongation after fracture is 13%-18%, the three-point bending angle is 60-90 degrees, and the hot forming steel has high oxidation resistance, does not need atmosphere protection during hot stamping forming, and the thickness of the oxidation layer after hot stamping forming is 2-4 microns, so that the shot blasting process can be omitted.

Description

Technical Field

[0001] This invention belongs to the field of steel alloy materials technology, specifically relating to a 1500MPa grade high plasticity, high fracture strain, oxidation-resistant hot-formed steel and its preparation method. Background Technology

[0002] Whether for traditional gasoline-powered vehicles or rapidly developing new energy vehicles, lightweighting of the body-in-white is crucial for achieving green and low-carbon development in the automotive industry. Hot stamping offers technical advantages such as low springback, low stamping load, and high formability, and the tensile strength of stamped parts can reach over 1500 MPa. Therefore, hot-formed steel is an important material for lightweighting the body-in-white. Currently, the most widely used hot-formed steel is 22MnB5 steel (available in bare sheet and aluminum-silicon coated sheet) with a tensile strength of 1500 MPa. However, its total elongation is only ~7%, and bare sheet is prone to oxidation and decarburization during hot stamping, requiring shot peening of the stamped parts. Aluminum-silicon coated sheets have high raw material and process costs, and commercial production is limited by ArcelorMittal's patent protection. Therefore, there is an urgent need to develop oxidation-resistant, uncoated hot-formed steel. In addition, most body structural components undergo bending deformation during a car collision. At this time, the deformation state of the structural components is close to plane strain bending. The current automotive industry generally uses the VDA 238-100 three-point bending test standard to measure the fracture strain of hot-formed steel. Therefore, hot-formed steel should also have good fracture strain.

[0003] The patent application with publication number CN 110643909A proposes an anti-oxidation ultra-high strength steel plate for hot stamping and its low-temperature hot forming process. Its alloy composition contains 6%~8% Mn and 2%~5% Cr. Mn and Cr are expensive, so the alloy cost is relatively high. In addition, the high Mn content is not conducive to welding performance.

[0004] The patent application with publication number CN 114045440A proposes a high-strength, high-plasticity hot-formed steel for automobiles with oxidation resistance and a hot-forming process. It improves the high-temperature oxidation resistance by using elements such as Cr and Mo, and the alloy composition design has a relatively low Si content, which is conducive to obtaining good surface quality of the steel sheet.

[0005] The patent application with publication number CN 114561591A proposes an uncoated reinforced high-temperature oxidation-resistant hot stamping steel with added Y element. Its alloy composition design contains Cr and Si antioxidant elements. In addition, the addition of Y element inhibits the formation of oxide scale and improves the adhesion of oxide scale.

[0006] The patent with authorization announcement number CN 113913700B discloses a method for producing 1700MPa grade hot-formed steel. The alloy composition design contains high levels of Si, Cr and Mn elements, with contents of 0.8%~1%, 2.5%~3% and 5.5%~6% respectively. The hot-formed strip is prepared using a twin-roll thin strip continuous casting process.

[0007] In summary, the development of oxidation-resistant hot-formed steel is one of the key directions in the research and development of automotive steel, but there are still some technical problems, and there is a lack of relevant reports on the fracture strain performance of oxidation-resistant hot-formed steel in currently published patents. Summary of the Invention

[0008] To address the problem of oxidation and decarburization that easily occur in traditional 1500MPa grade 22MnB5 steel bare plates during hot stamping, this invention provides a 1500MPa grade high plasticity, high fracture strain, oxidation-resistant hot-formed steel and its preparation method. The hot-formed steel has high oxidation resistance, does not require atmosphere protection during hot stamping, and the oxide layer thickness after hot stamping is 2~4μm, which eliminates the need for shot blasting.

[0009] The technical solution of this invention is:

[0010] A 1500MPa grade high plasticity, high fracture strain, oxidation-resistant hot-formed steel, with the following composition and content by mass percentage: C 0.26%~0.28%, Mn 3.6%~3.9%, Si 0.4%~0.7%, Al 1.4%~1.7%, Nb 0.01%~0.03%, V 0.04%~0.07%, S<0.002%, P<0.002%, O<0.002%, N<0.003%, with the balance being Fe.

[0011] The 1500MPa grade high plasticity, high fracture strain, oxidation resistant hot-formed steel, by mass percentage, must have the following relationship: Si+Al≤2.1%, Si+Mn≤4.4%.

[0012] The 1500MPa grade high plasticity, high fracture strain, oxidation-resistant hot-formed steel, by mass percentage, requires the addition of Nb and V to satisfy the relationship Nb+V≥0.07%.

[0013] The preparation method of the 1500MPa grade high plasticity, high fracture strain, oxidation-resistant hot-formed steel is carried out according to the following steps:

[0014] (1) Melt steel according to the set composition;

[0015] (2) The molten steel is used to prepare slabs through continuous casting equipment. The slab thickness is 150~200mm, the billet pulling speed is 1.4~1.5m / min, and the superheat of the molten steel in the ladle is 20~30℃ during continuous casting.

[0016] (3) Heat the slab in a heating furnace, and then heat it to 1210~1215℃ for 4~5 hours;

[0017] (4) After the slab is taken out of the heating furnace, it is quickly descaled. The temperature of the slab before descaling is 1190~1200℃ and the descaling water pressure is ≥30MPa. The slab is subjected to three rough rolling passes. The initial rolling temperature is 1150~1170℃ and the reduction rate of each rough rolling pass is 20%~25%. Then it is finished rolled to 1.4~1.8mm and the final rolling temperature is 800~810℃. After finishing rolling, the steel plate is coiled and air-cooled to the coiling temperature of 300~330℃ to obtain hot-rolled thin strip steel.

[0018] (5) Pickling is performed on the hot-rolled thin strip steel;

[0019] (6) The pickled hot-rolled thin strip steel is hot-stamped into shape according to the following steps:

[0020] a. Cut the hot-rolled thin strip steel into the desired shape of steel plate, and then heat it to 790~810℃ at 50~80℃ / s and hold it for 3~4 minutes;

[0021] b. Using a robotic arm, the heated steel plate from step a is transferred to a hot stamping die for deformation and cooling in 3-5 seconds. The cooling rate is ≥5℃ / s. The plate is cooled to 310-340℃ and then held under pressure for 30-40 seconds. After holding the pressure, the formed part is removed and air-cooled to room temperature to obtain hot-formed steel.

[0022] In the preparation method of the 1500MPa grade high plasticity, high fracture strain, oxidation resistant hot-formed steel, in step (4), the microstructure of the hot-rolled thin strip steel consists of lath martensite, twinned martensite and retained austenite. The volume fraction of lath martensite is 82%~94%, the volume fraction of twinned martensite is 1%~3%, the volume fraction of retained austenite is 5%~15%, the morphology of retained austenite is lath-shaped, and the lath thickness of retained austenite is 15~200nm.

[0023] In the preparation method of the 1500MPa grade high plasticity high fracture strain oxidation resistant hot forming steel, in step (6), the steel plate does not require atmosphere protection during the heating and holding process and hot stamping process, and the hot forming steel does not need to be shot blasted after hot stamping.

[0024] The preparation method of the 1500MPa grade high plasticity, high fracture strain, oxidation resistant hot-formed steel, the hot-formed steel obtained in step (6) consists of ferrite, martensite and retained austenite, the volume fraction of ferrite is 25%~35%, the volume fraction of martensite is 45%~55%, the volume fraction of retained austenite is 13%~20%, and the ferrite is lath-shaped, the retained austenite is lath-shaped and block-shaped; in the retained austenite, the volume ratio of lath-shaped retained austenite is 90%~95%.

[0025] The method for preparing the 1500MPa grade high plasticity, high fracture strain, oxidation resistant hot-formed steel, the yield strength of the hot-formed steel obtained in step (6) is 830~870MPa, the tensile strength is 1520~1570MPa, and the elongation after fracture is 13%~18%.

[0026] The preparation method of the 1500MPa grade high plasticity high fracture strain oxidation resistant hot forming steel, the three-point bending angle of the hot forming steel obtained in step (6) is 60°~90°, and this three-point bending angle is determined according to the VDA238-100 three-point bending test standard formulated by the German Association of the Automotive Industry.

[0027] The preparation method of the 1500MPa grade high plasticity high fracture strain oxidation resistant hot forming steel, the oxide layer thickness of the hot forming steel obtained in step (6) is 2~4μm.

[0028] The design concept of this invention is:

[0029] This invention strictly controls the content of C, Mn, Al, and Si elements in the composition, wherein C and Mn elements can play a role in stabilizing austenite. Adding Al and Si elements can effectively improve the austenite content of the material. e1 and A e3The heating temperature of the steel sheet during hot forming is maintained within the ferrite + austenite two-phase temperature range, resulting in a microstructure composed of ferrite, martensite, and retained austenite in the hot-formed steel. Al and Si elements form dense SiO2 and Al2O3 oxide films on the matrix surface at high temperatures, improving the steel's resistance to high-temperature oxidation. However, the mass percentage of Si should be strictly controlled to ≤0.7%, as excessive Si content leads to poor surface quality in hot-rolled thin strip steel. The mass percentage of Mn should be strictly controlled to ≤3.9%, as excessive manganese content deteriorates weldability. Furthermore, this invention strictly controls the mass percentages of Si, Al, and Mn in the molten steel to meet the requirements of Si+Al≤2.1% and Si+Mn≤4.4%. This ratio ensures that the iron oxide scale in the hot-rolled thin strip steel is easily removed by pickling, resulting in good steel sheet surface quality. Simultaneously, this invention strictly controls the mass percentages of Nb and V in the molten steel to meet the requirement of Nb+V≥0.07%, achieving a yield strength of 830~870 MPa for the hot-formed steel.

[0030] In the process of preparing 1500MPa grade high plasticity, high fracture strain, oxidation-resistant hot-formed steel, the present invention strictly controls the heating rate of the steel plate during hot stamping at 50~80℃ / s, which helps to reduce the thickness of the oxide layer.

[0031] The advantages and beneficial effects of this invention are as follows:

[0032] 1. The alloy chemical composition and hot stamping forming process provided by this invention can obtain 1500MPa grade high plasticity and high fracture strain hot-formed steel with an elongation after fracture of 13%~18% and a three-point bending angle of 60°~90°, which is expected to replace traditional 22MnB5 steel.

[0033] 2. This invention improves the high-temperature oxidation resistance of steel plates by matching Si and Al elements. The steel plates do not require gas protection during hot stamping, and the stamped parts do not require shot peening.

[0034] 3. The heating temperature of the steel plate during hot stamping of this invention is significantly lower than that of traditional 22MnB5 steel, which has the advantages of low carbon, environmental protection and energy saving.

[0035] 4. The addition of aluminum element at a mass percentage of 1.4% to 1.7% in the alloy composition of this invention contributes to the lightweighting of the body-in-white. Attached Figure Description

[0036] Figure 1 This is a secondary electron image of the hot-formed steel structure obtained in Embodiment 1 of the present invention;

[0037] Figure 2The figure shows the engineering stress-strain curve of the hot-formed steel obtained in Example 1 of the present invention; in the figure, the horizontal axis represents engineering strain (%) and the vertical axis represents engineering stress (MPa). Detailed Implementation

[0038] In this embodiment of the invention, the strength and elongation tests were conducted according to GB / T228.1-2010. The gauge length of the tensile sample was 25 mm, the test was conducted at room temperature, and the tensile rate was 2 mm / min. In this embodiment of the invention, the three-point bending angle was determined according to the VDA 238-100 three-point bending test standard established by the German Association of the Automotive Industry.

[0039] The present invention will be further described in detail below through embodiments.

[0040] Example 1

[0041] In this embodiment, the preparation method of 1500MPa grade high plasticity, high fracture strain, oxidation-resistant hot-formed steel is carried out according to the following steps:

[0042] (1) The molten steel is smelted according to the set composition, which is as follows by mass percentage: C 0.26%, Mn 3.6%, Si 0.4%, Al 1.4%, Nb 0.01%, V 0.07%, S 0.0012%, P 0.0011%, O 0.0012%, N 0.0013%, with the balance being Fe;

[0043] (2) The molten steel is used to prepare slabs through continuous casting equipment. The slab thickness is 150 mm, the billet pulling speed is 1.4 m / min, and the superheat of the molten steel in the ladle is 20℃ during continuous casting.

[0044] (3) Heat the slab in a heating furnace, and then heat it to 1210℃ for 4 hours;

[0045] (4) After the slab is taken out of the heating furnace, it is quickly descaled. The temperature of the slab before descaling is 1190℃ and the descaling water pressure is 30MPa. The slab is subjected to three rough rolling passes. The initial rolling temperature is 1150℃ and the reduction rate of each rough rolling pass is 20%. Then it is finished rolled to 1.4mm and the final rolling temperature is 800℃. After finishing rolling, the steel plate is coiled and air-cooled to the coiling temperature of 300℃ to obtain hot-rolled thin strip steel. The microstructure of the obtained hot-rolled thin strip steel consists of lath martensite, twinned martensite and retained austenite. The volume fraction of lath martensite is 94%, the volume fraction of twinned martensite is 1%, the volume fraction of retained austenite is 5%, the morphology of retained austenite is lath-shaped, and the lath thickness of retained austenite is 15nm.

[0046] (5) Pickling is performed on the hot-rolled thin strip steel;

[0047] (6) The pickled hot-rolled thin strip steel is hot-stamped into shape according to the following steps:

[0048] a. Cut the steel plate to the desired shape, then heat it to 790℃ at 50℃ / s and hold for 3 minutes;

[0049] b. Using a robotic arm, the heated steel plate from step a is transferred to a hot stamping die for deformation and cooling over 3 seconds. The cooling rate is 5°C / s. The plate is cooled to 310°C and held under pressure for 30 seconds. After holding, the formed part is removed and air-cooled to room temperature to obtain hot-formed steel. No protective atmosphere is required during the heating, holding, and hot stamping processes. The hot-formed steel does not require shot blasting after hot stamping.

[0050] In this embodiment, the obtained hot-formed steel microstructure consists of ferrite, martensite, and retained austenite. The volume fraction of ferrite is 25%, the volume fraction of martensite is 55%, and the volume fraction of retained austenite is 20%. The ferrite morphology is lath-like, and the retained austenite morphology is both lath-like and blocky. Among the retained austenite, the volume percentage of lath-like retained austenite is 90% (secondary electron microscopy of the hot-formed steel microstructure is shown in the image). Figure 1 (As shown). The yield strength of the hot-formed steel is 857 MPa, the tensile strength is 1533 MPa, and the elongation after fracture is 13.4% (the engineering stress-strain curve of the hot-formed steel is shown in Figure 1). Figure 2 (As shown). The three-point bending angle of the hot-formed steel is 60°, and the oxide layer thickness of the hot-formed steel is 3.3 μm.

[0051] Example 2

[0052] In this embodiment, the preparation method of 1500MPa grade high plasticity, high fracture strain, oxidation-resistant hot-formed steel is carried out according to the following steps:

[0053] (1) The molten steel is smelted according to the set composition, which is as follows by mass percentage: C 0.28%, Mn 3.7%, Si 0.7%, Al 1.4%, Nb 0.03%, V 0.04%, S 0.0016%, P 0.0014%, O 0.0012%, N 0.0023%, with the balance being Fe;

[0054] (2) The molten steel is used to prepare slabs with a thickness of 200 mm and a casting speed of 1.5 m / min. The superheat of the molten steel in the ladle is 30℃ during the continuous casting process.

[0055] (3) Heat the slab in a heating furnace, and then heat it to 1215℃ for 5 hours;

[0056] (4) After the slab is taken out of the heating furnace, it is quickly descaled. The slab temperature before descaling is 1200℃ and the descaling water pressure is 40MPa. The slab is subjected to three rough rolling passes. The initial rolling temperature is 1170℃ and the reduction rate of each rough rolling pass is 25%. Then it is finished rolled to 1.8mm and the final rolling temperature is 810℃. After finishing rolling, the steel plate is coiled and air-cooled to the coiling temperature of 330℃ to obtain hot-rolled thin strip steel. The microstructure of the obtained hot-rolled thin strip steel consists of lath martensite, twinned martensite and retained austenite. The volume fraction of lath martensite is 82%, the volume fraction of twinned martensite is 3%, and the volume fraction of retained austenite is 15%. The morphology of retained austenite is lath-shaped and the lath thickness of retained austenite is 200nm.

[0057] (5) Pickling is performed on the hot-rolled thin strip steel;

[0058] (6) The pickled hot-rolled thin strip steel is hot-stamped into shape according to the following steps:

[0059] a. Cut the steel plate to the desired shape, then heat it to 810℃ at 80℃ / s and hold for 4 minutes;

[0060] b. Using a robotic arm, the heated steel plate from step a is transferred to a hot stamping die for deformation and cooling over 5 seconds. The cooling rate is 13℃ / s. The plate is cooled to 340℃ and then held under pressure for 40 seconds. After holding the pressure, the formed part is removed and air-cooled to room temperature to obtain hot-formed steel. No protective atmosphere is required during the heating, holding, and hot stamping processes. The hot-formed steel does not require shot blasting after hot stamping.

[0061] In this embodiment, the obtained hot-formed steel microstructure consists of ferrite, martensite, and retained austenite. The volume fraction of ferrite is 35%, the volume fraction of martensite is 45%, and the volume fraction of retained austenite is 20%. The ferrite morphology is lath-like, and the retained austenite morphology is both lath-like and blocky. Among the retained austenite, the volume proportion of lath-like retained austenite is 95%. The yield strength of the hot-formed steel is 870 MPa, the tensile strength is 1570 MPa, and the elongation after fracture is 18%. The three-point bending angle of the hot-formed steel is 90°, and the oxide layer thickness of the hot-formed steel is 4 μm.

[0062] Example 3

[0063] In this embodiment, the preparation method of 1500MPa grade high plasticity, high fracture strain, oxidation-resistant hot-formed steel is carried out according to the following steps:

[0064] (1) The molten steel is smelted according to the set composition, which is as follows by mass percentage: C 0.27%, Mn 3.9%, Si 0.4%, Al 1.7%, Nb 0.02%, V 0.06%, S 0.0019%, P 0.0012%, O 0.0012%, N 0.0023%, with the balance being Fe;

[0065] (2) The molten steel is used to prepare slabs through continuous casting equipment. The slab thickness is 170 mm, the billet pulling speed is 1.45 m / min, and the superheat of the molten steel in the ladle is 25℃ during continuous casting.

[0066] (3) The slab is heated in a heating furnace, and the reheating temperature is 1211℃, and the holding time is 4.5 hours;

[0067] (4) After the slab is taken out of the heating furnace, it is quickly descaled. The temperature of the slab before descaling is 1195℃ and the descaling water pressure is 35MPa. The slab is subjected to three rough rolling passes. The initial rolling temperature is 1160℃ and the reduction rate of each rough rolling pass is 22%. Then it is finished rolled to 1.5mm and the final rolling temperature is 805℃. After finishing rolling, the steel plate is coiled and air-cooled to the coiling temperature of 320℃ to obtain hot-rolled thin strip steel. The microstructure of the obtained hot-rolled thin strip steel consists of lath martensite, twinned martensite and retained austenite. The volume fraction of lath martensite is 90%, the volume fraction of twinned martensite is 2%, and the volume fraction of retained austenite is 8%. The morphology of retained austenite is lath-shaped and the lath thickness of retained austenite is 150nm.

[0068] (5) Pickling is performed on the hot-rolled thin strip steel;

[0069] (6) The pickled hot-rolled thin strip steel is hot-stamped into shape according to the following steps:

[0070] a. Cut the steel plate to the desired shape, then heat it to 800℃ at 60℃ / s and hold for 3.5 minutes;

[0071] b. Using a robotic arm, the heated steel sheet from step a is transferred to a hot stamping die for deformation and cooling over 4 seconds. The cooling rate is 20°C / s. The temperature is lowered to 320°C, and pressure is maintained for 35 seconds. After pressure maintenance, the formed part is removed and air-cooled to room temperature to obtain hot-formed steel. No protective atmosphere is required during the heating, holding, and hot stamping processes. The hot-formed steel does not require shot blasting after hot stamping.

[0072] In this embodiment, the obtained hot-formed steel microstructure consists of ferrite, martensite, and retained austenite, wherein the volume fraction of ferrite is 32%, the volume fraction of martensite is 55%, and the volume fraction of retained austenite is 13%. The ferrite morphology is lath-like, and the retained austenite morphology is both lath-like and blocky. Among the retained austenite, lath-like retained austenite accounts for 93% of the volume. The hot-formed steel has a yield strength of 850 MPa, a tensile strength of 1550 MPa, and an elongation after fracture of 15%. The three-point bending angle of the hot-formed steel is 70°, and the oxide layer thickness is 3 μm.

[0073] Example 4

[0074] In this embodiment, the preparation method of 1500MPa grade high plasticity, high fracture strain, oxidation-resistant hot-formed steel is carried out according to the following steps:

[0075] (1) The molten steel is smelted according to the set composition, which is as follows by mass percentage: C 0.272%, Mn 3.8%, Si 0.5%, Al 1.6%, Nb 0.025%, V 0.067%, S 0.0012%, P 0.0014%, O 0.0011%, N 0.0013%, with the balance being Fe;

[0076] (2) The molten steel is used to prepare slabs through continuous casting equipment. The slab thickness is 160 mm, the billet pulling speed is 1.4 m / min, and the superheat of the molten steel in the ladle is 26℃ during continuous casting.

[0077] (3) Heat the slab in a heating furnace, and then heat it to 1212℃ for 4 hours;

[0078] (4) After the slab is taken out of the heating furnace, it is quickly descaled. The temperature of the slab before descaling is 1195℃ and the descaling water pressure is 50MPa. The slab is subjected to three rough rolling passes. The initial rolling temperature is 1170℃ and the reduction rate of each rough rolling pass is 25%. Then it is finished rolled to 1.7mm and the final rolling temperature is 802℃. After finishing rolling, the steel plate is coiled and air-cooled to the coiling temperature of 310℃ to obtain hot-rolled thin strip steel. The microstructure of the obtained hot-rolled thin strip steel consists of lath martensite, twinned martensite and retained austenite. The volume fraction of lath martensite is 85%, the volume fraction of twinned martensite is 3%, and the volume fraction of retained austenite is 12%. The morphology of retained austenite is lath-shaped and the lath thickness of retained austenite is 100nm.

[0079] (5) Pickling is performed on the hot-rolled thin strip steel;

[0080] (6) The pickled hot-rolled thin strip steel is hot-stamped into shape according to the following steps:

[0081] a. Cut the steel plate to the desired shape, then heat it to 800℃ at 70℃ / s and hold for 3 minutes;

[0082] b. Using a robotic arm, the heated steel plate from step a is transferred to a hot stamping die for deformation and cooling over 5 seconds. The cooling rate is 30°C / s. The plate is cooled to 320°C and then held under pressure for 38 seconds. After holding the pressure, the formed part is removed and air-cooled to room temperature to obtain hot-formed steel. No protective atmosphere is required during the heating, holding, and hot stamping processes. The hot-formed steel does not require shot blasting after hot stamping.

[0083] In this embodiment, the obtained hot-formed steel microstructure consists of ferrite, martensite, and retained austenite, wherein the volume fraction of ferrite is 31%, the volume fraction of martensite is 54%, and the volume fraction of retained austenite is 15%. The ferrite morphology is lath-like, and the retained austenite morphology is both lath-like and blocky; among the retained austenite, lath-like retained austenite accounts for 90% of the volume. The hot-formed steel has a yield strength of 860 MPa, a tensile strength of 1530 MPa, and an elongation after fracture of 14%. The three-point bending angle of the hot-formed steel is 80°, and the oxide layer thickness of the hot-formed steel is 2 μm.

[0084] Example 5

[0085] In this embodiment, the preparation method of 1500MPa grade high plasticity, high fracture strain, oxidation-resistant hot-formed steel is carried out according to the following steps:

[0086] (1) The molten steel is smelted according to the set composition, which is as follows by mass percentage: C 0.26%, Mn 3.8%, Si 0.6%, Al 1.45%, Nb 0.03%, V 0.05%, S 0.0012%, P 0.0012%, O 0.0012%, N 0.0023%, with the balance being Fe;

[0087] (2) The molten steel is used to prepare slabs with a thickness of 170 mm and a casting speed of 1.5 m / min. The superheat of the molten steel in the ladle is 30 °C during the continuous casting process.

[0088] (3) Heat the slab in a heating furnace, and then heat it to 1215℃ for 4 hours;

[0089] (4) After the slab is taken out of the heating furnace, it is quickly descaled. The temperature of the slab before descaling is 1190℃ and the descaling water pressure is 60MPa. The slab is subjected to three rough rolling passes. The initial rolling temperature is 1170℃ and the reduction rate of each rough rolling pass is 20%. Then it is finished rolled to 1.8mm and the final rolling temperature is 800℃. After finishing rolling, the steel plate is coiled and air-cooled to the coiling temperature of 300℃ to obtain hot-rolled thin strip steel. The microstructure of the obtained hot-rolled thin strip steel consists of lath martensite, twinned martensite and retained austenite. The volume fraction of lath martensite is 83.5%, the volume fraction of twinned martensite is 1.5%, the volume fraction of retained austenite is 15%, the morphology of retained austenite is lath-shaped, and the lath thickness of retained austenite is 80nm.

[0090] (5) Pickling is performed on the hot-rolled thin strip steel;

[0091] (6) The pickled hot-rolled thin strip steel is hot-stamped into shape according to the following steps:

[0092] a. Cut the steel plate to the desired shape, then heat it to 790℃ at 80℃ / s and hold for 4 minutes;

[0093] b. Using a robotic arm, the heated steel plate from step a is transferred to a hot stamping die for deformation and cooling over 5 seconds. The cooling rate is 25°C / s. The plate is cooled to 320°C and then held under pressure for 33 seconds. After holding, the formed part is removed and air-cooled to room temperature to obtain hot-formed steel. No protective atmosphere is required during the heating, holding, and hot stamping processes. The hot-formed steel does not require shot blasting after hot stamping.

[0094] In this embodiment, the obtained hot-formed steel microstructure consists of ferrite, martensite, and retained austenite, wherein the volume fraction of ferrite is 30%, the volume fraction of martensite is 53%, and the volume fraction of retained austenite is 17%. The ferrite morphology is lath-like, and the retained austenite morphology is both lath-like and blocky. Among the retained austenite, lath-like retained austenite accounts for 92% of the volume. The hot-formed steel has a yield strength of 830 MPa, a tensile strength of 1520 MPa, and an elongation after fracture of 13%. The three-point bending angle of the hot-formed steel is 72°, and the oxide layer thickness is 2 μm.

Claims

1. A method for preparing 1500MPa grade high-plasticity, high-fracture-strain, oxidation-resistant hot-formable steel, characterized in that, The composition and content of hot-formed steel by mass percentage are: C 0.26%~0.28%, Mn 3.6%~3.9%, Si 0.4%~0.7%, Al 1.4%~1.7%, Nb 0.01%~0.03%, V 0.04%~0.07%, S<0.002%, P<0.002%, O<0.002%, N<0.003%, with the balance being Fe; The preparation method of the 1500MPa grade high plasticity, high fracture strain, oxidation-resistant hot-formed steel is carried out according to the following steps: (1) Melt steel according to the set composition; (2) The molten steel is used to prepare slabs through continuous casting equipment. The slab thickness is 150~200mm, the billet pulling speed is 1.4~1.5m / min, and the superheat of the molten steel in the ladle is 20~30℃ during continuous casting. (3) Heat the slab in a heating furnace at a temperature of 1210~1215℃ and hold it for 4~5 hours; (4) After the slab is taken out of the heating furnace, it is quickly descaled. The temperature of the slab before descaling is 1190~1200℃ and the descaling water pressure is ≥30MPa. The slab is subjected to three rough rolling passes. The initial rolling temperature is 1150~1170℃ and the reduction rate of each rough rolling pass is 20%~25%. Then it is finished rolled to 1.4~1.8mm and the final rolling temperature is 800~810℃. After finishing rolling, the steel plate is coiled and air-cooled to the coiling temperature of 300~330℃ to obtain hot-rolled thin strip steel. (5) Pickling is performed on the hot-rolled thin strip steel; (6) The pickled hot-rolled thin strip steel is hot-stamped into shape according to the following steps: a. Cut the hot-rolled thin strip steel into the desired shape of steel plate, and then heat it to 790~810℃ at 50~80℃ / s and hold it for 3~4 minutes; b. Using a robotic arm, the heated steel plate from step a is transferred to a hot stamping die for deformation and cooling in 3-5 seconds. The cooling rate is ≥5℃ / s. The plate is cooled to 310-340℃ and then held under pressure for 30-40 seconds. After holding the pressure, the formed part is removed and air-cooled to room temperature to obtain hot-formed steel.

2. The method for preparing 1500MPa grade high-plasticity, high-fracture-strain, oxidation-resistant hot-formed steel according to claim 1, characterized in that, The addition amounts of Si, Al, and Mn, by mass percentage, must satisfy the following relationship: Si + Al ≤ 2.1%, Si + Mn ≤ 4.4%.

3. The method for preparing 1500MPa grade high plasticity, high fracture strain, oxidation-resistant hot-formed steel according to claim 1, characterized in that, The amount of Nb and V added, by mass percentage, must satisfy the relationship Nb+V≥0.07%.

4. The method for preparing 1500MPa grade high-plasticity, high-fracture-strain, oxidation-resistant hot-formed steel according to claim 1, characterized in that, In step (4), the microstructure of the hot-rolled thin strip steel consists of lath martensite, twinned martensite and retained austenite. The volume fraction of lath martensite is 82%~94%, the volume fraction of twinned martensite is 1%~3%, the volume fraction of retained austenite is 5%~15%, the morphology of retained austenite is lath-shaped, and the lath thickness of retained austenite is 15~200nm.

5. The method for preparing 1500MPa grade high-plasticity, high-fracture-strain, oxidation-resistant hot-formable steel according to claim 1, characterized in that, In step (6), the steel plate does not require atmospheric protection during the heating and heat preservation process and hot stamping. After hot stamping, the hot-formed steel does not need to be shot blasted.

6. The method for preparing 1500MPa grade high-plasticity, high-fracture-strain, oxidation-resistant hot-formable steel according to claim 1, characterized in that, The hot-formed steel microstructure obtained in step (6) consists of ferrite, martensite and retained austenite. The volume fraction of ferrite is 25%~35%, the volume fraction of martensite is 45%~55%, and the volume fraction of retained austenite is 13%~20%. The ferrite is lath-shaped, and the retained austenite is lath-shaped and blocky. The volume fraction of lath-shaped retained austenite is 90%~95%.

7. The method for preparing 1500MPa grade high-plasticity, high-fracture-strain, oxidation-resistant hot-formed steel according to claim 1, characterized in that, The yield strength of the hot-formed steel obtained in step (6) is 830~870MPa, the tensile strength is 1520~1570MPa, and the elongation after fracture is 13%~18%.

8. The method for preparing 1500MPa grade high-plasticity, high-fracture-strain, oxidation-resistant hot-formable steel according to claim 1, characterized in that, The three-point bending angle of the hot-formed steel obtained in step (6) is 60°~90°. This three-point bending angle is determined according to the VDA 238-100 three-point bending test standard formulated by the German Association of the Automotive Industry.

9. The method for preparing 1500MPa grade high-plasticity, high-fracture-strain, oxidation-resistant hot-formable steel according to claim 1, characterized in that, The oxide layer thickness of the hot-formed steel obtained in step (6) is 2~4μm.

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