An advanced hot stamping forming method of high Cr-si alloying free coating hot forming steel

Abstract

The application discloses an advanced hot stamping forming method of high Cr-Si alloyed plating-free hot forming steel, and the method is characterized in that: a high Cr-Si alloy composition system is innovatively adopted to avoid adding expensive alloy elements such as Ni and Mo, and the material and energy of an Al-Si plating process link are saved; an advanced hot stamping forming method of induction heating and conventional heating furnace rapid heating is adopted, induction heating is used to heat the sheet to above 400 DEG C in a short time, bare plates can be directly used for hot forming, the cost is low, the process is simple, and industrialization is easy to realize; the hot forming steel product is excellent in plasticity, smooth and bright in surface, and thin in surface oxide skin, and the shot blasting and oxide skin removing process is not needed, and the hot forming production efficiency is greatly improved; the problems of the prior art hot forming steel plate, such as the insufficient plasticity, strength and toughness and quenching depth of the full martensite structure formed by using B element to improve the quenching depth, and the serious oxidation during the hot forming of the bare plate, are solved; and the production of the hot forming steel with a thickness of more than 6 mm is realized.

Description

Technical Field

[0001] This invention belongs to the field of metal processing technology, specifically relating to an advanced hot stamping forming method for high Cr-Si alloyed coating-free hot-formed steel. Background Technology

[0002] The production and consumption of automobiles involve many fields such as energy, environment, and safety. The problems exposed in these areas have gradually constrained the development of the automotive industry. People are trying different methods to solve these contradictions, such as improving fuel economy, developing alternative energy sources, and building intelligent transportation systems. From the perspective of automotive design and manufacturing, achieving lightweighting through material optimization while ensuring body strength and safety is an important way to meet the requirements of green development in the transportation sector. Statistics show that for every 10% reduction in vehicle weight, fuel consumption can be reduced by 6%–8%, and exhaust emissions by 5%–6%. The application of advanced high-strength automotive steel in the body-in-white can simultaneously meet safety and lightweighting requirements. Automotive steel that achieves weight reduction without compromising safety has very high performance requirements, needing not only high strength but also good formability. Advanced high-strength automotive steel, due to its excellent characteristics such as high strength, high toughness, high energy absorption, and high resistance to intrusion, can well meet the needs of current automobile production.

[0003] Against the backdrop of automotive lightweighting and improved passenger safety, advanced high-strength steel, as a key structural material for vehicle bodies, is trending towards thinner profiles and higher strength. Currently, the application level of advanced high-strength steel has reached 1180 MPa, and even higher strength levels (≥1500 MPa) have been developed. While traditional cold stamping technology is inexpensive, the ultra-high strength of the steel sheets presents certain challenges, such as high springback, low formability, and die damage. Hot forming technology can effectively solve or avoid these problems.

[0004] However, high-temperature oxidation is unavoidable for bare steel during hot forming. While stainless steel offers excellent high-temperature oxidation resistance, its high cost and poor weldability limit its large-scale application. Therefore, even when produced under nitrogen conditions, commercially available hot-formed bare steel still suffers from severe high-temperature oxidation. After hot forming, the resulting oxide scale requires shot blasting for removal, increasing costs and significantly impacting processing accuracy. Al-Si coatings are widely used to address oxidation protection during hot forming. However, coated steel also presents several problems: ArcelorMittal holds the patent for the widely used Al-Si coating, incurring substantial patent fees; the raw materials and processing costs are high, resulting in long production cycles; coated hot-formed steel exhibits slow heat transfer efficiency and low production rates; the coating can stick to the rolls and affect the steel's weldability.

[0005] Furthermore, the traditional hot-formed steel 22MnB5 suffers from insufficient toughness and energy absorption due to its alloy composition limitations. This boron steel relies on boron (Boron) to improve hardenability, resulting in a lath martensitic structure after quenching. Consequently, its elongation and bending properties are poor, affecting its safety performance during collisions. Moreover, the existing 22MnB5 hot-formed steel has insufficient hardenability, making it impossible to hot-form steel plates thicker than 6mm. Therefore, current technology cannot produce hot-formed steel with a thickness exceeding 6mm.

[0006] Currently, the conventional hot stamping process for hot-formed steel involves heating boron steel sheets to an austenitic state in a roller furnace, rapidly transferring them to a die for stamping, and simultaneously quenching the parts within the die at a cooling rate greater than 27°C / s under pressure. This pressure-holding quenching process yields ultra-high-strength steel parts with a uniform martensitic structure. However, Al-Si coated sheets are limited by the alloy layer formation mechanism, requiring heating to the austenitic temperature at a rate less than 15°C / s. Furthermore, the Al-Si layer on the surface of Al-Si coated sheets prevents rapid heating via induction heating. For bare sheets, severe oxidation occurs during hot forming, necessitating shot blasting after hot forming to meet the surface requirements for subsequent part fabrication. Summary of the Invention

[0007] To overcome the shortcomings of existing technologies, this invention addresses the problems of insufficient plasticity and toughness in the fully martensitic structure formed by using boron (B) to improve hardenability in existing hot-formed steel sheets, as well as the severe oxidation of bare hot-formed steel sheets during hot forming, and the complex and costly production process of Al-Si coated sheets that can prevent oxidation. This invention provides a low-cost high-Cr-Si alloy composition system and an advanced hot stamping forming method for producing coating-free hot-formed steel using induction heating and conventional furnace heating for rapid heating. Induction heating can instantly heat the sheet material, raising it to over 400°C in a short time according to different product thicknesses, before placing it in a furnace for secondary heating to the target temperature and holding it at that temperature. Utilizing a combination of induction heating and conventional furnace heating simplifies the preparation process, resulting in hot-formed steel products with excellent plasticity, a smooth and bright surface, and a thin oxide scale, eliminating the need for subsequent shot blasting to remove the oxide scale, and significantly improving the production efficiency of hot forming.

[0008] To achieve the above-mentioned objectives, this invention provides an advanced hot stamping forming method for high-Cr-Si alloyed coating-free hot-formed steel. The hot-formed steel composition system is as follows (by mass percentage): C: 0.15~0.35%, Mn: 0.8~3.2%, Si: 0.8~2.8%, S: <0.01%, P: <0.015%, Al: 0.01~0.05%, Cr: 1.5~3.9%, Nb: 0.01~0.05%, V: 0.01~0.05%, Ti: 0.01~0.03%, Cu: 0.05~0.15%, with the balance being Fe and other unavoidable impurities.

[0009] This invention employs high-Cr-Si alloyed steel with the addition of small amounts of microalloying elements. C is an austenite stabilizing element; its combination with microalloying elements can provide precipitation strengthening. However, excessive C content deteriorates weldability; therefore, the mass percentage of carbon used is 0.15~0.35%. Mn significantly increases hardenability, but high manganese content increases carbon equivalent, worsening weldability and reducing high-temperature oxidation resistance. Therefore, the mass percentage of manganese used is 0.8~3.2%. Cr significantly improves hardenability, refines quenched martensite laths, and greatly enhances high-temperature oxidation resistance during hot forming. Considering the performance and alloy cost of 1500MPa grade hot-formed steel, the mass percentage of chromium used is 1.5~3.9%. Si provides solid solution strengthening and effectively inhibits the formation of coarse carbides. Si also enhances high-temperature oxidation resistance; however, excessive Si content leads to brittleness. Therefore, the mass percentage of silicon used is 0.8~2.8%. Niobium (Nb) inhibits austenite recrystallization during rolling, significantly refining high-temperature austenite grains and enabling hot rolling to replace cold rolling. However, excessive Nb content deteriorates the surface quality of continuously cast billets; therefore, the mass percentage of niobium used is 0.01–0.05%. Vanadium (V) refines quenched martensite laths, and V and Nb together precipitate as a precipitation strengthening phase, improving resistance to hydrogen embrittlement; therefore, the mass percentage of vanadium used is 0.01–0.05%. Titanium (Ti) refines the original austenite grains; trace amounts of Ti fix N atoms to form precipitates, inhibiting abnormal growth of austenite grains in the coarse-grained heat-affected zone of welds; therefore, the mass percentage of titanium used is 0.01–0.03%. Copper (Cu) improves corrosion resistance; however, excessive Cu induces embrittlement; therefore, the mass percentage of copper used is 0.05–0.15%. Al is mainly used for deoxidation and grain refinement, and to a certain extent improves the uniformity of the microstructure and properties of steel. Therefore, the mass percentage of aluminum used is 0.01~0.05%. S and P are impurity elements in steel and should be controlled within a certain range.

[0010] This composition of hot-formed steel has strong high-temperature oxidation resistance. Therefore, when using this composition of steel sheet for hot forming, there is no need to add an anti-oxidation Al-Si coating. The bare sheet can be used directly for hot forming. After hot forming, the oxide scale of the bare sheet is thin, and there is no need to perform shot blasting or other oxide scale removal treatments.

[0011] An advanced hot stamping forming method for high Cr-Si alloyed coating-free hot-formable steel, the method comprising the following steps:

[0012] ① Heating process: 1.2~10mm steel plates with the above composition system are first induction heated to 400-680℃ within 10 seconds; then heated to 880-980℃ in a furnace and held at that temperature for 2-15 minutes. The furnace atmosphere is either air or nitrogen, or a mixture of nitrogen and methane. The heating process yields a fully austenitized high-temperature microstructure with an original austenite grain size of 4-50µm.

[0013] The high-Cr-Si alloyed steel sheet of the aforementioned composition can be produced by hot rolling of continuously cast billets of this composition. During the coiling and bell-type furnace annealing processes after rolling, a certain amount of Cr-containing carbides will be formed. During the austenitization process in hot forming, the dissolution rate of Cr carbides is relatively slow. The pinning effect of the precipitated phase and the dragging effect of Cr atoms can inhibit the growth of austenite grains. Compared with the traditional hot-formed steel 22MnB5, the austenite grain size is significantly refined. Furthermore, the high-Cr-Si alloyed steel sheet of the aforementioned composition can be hot-formed without coating. Compared with traditional Al-Si coated steel sheets, the high heat transfer rate during hot stamping can accelerate the austenitization process, shorten the isothermal time, and improve production efficiency.

[0014] By changing the heating temperature and isothermal time in the furnace, the size, morphology, and elemental distribution of the original austenite grains can be altered. This also causes changes in the martensitic transformation temperature and the content of retained austenite, ultimately affecting the final microstructure and properties. Based on the characteristics of the steel grade, this invention selects appropriate induction heating and furnace heating temperatures and isothermal times.

[0015] In order to slow down the high-temperature oxidation rate of steel plates during hot forming, the protective atmosphere in the heating furnace is nitrogen or a mixture of nitrogen and methane. By controlling the atmosphere in the furnace, the high-temperature oxidation behavior of the bare plate during hot forming and the thickness of the final surface oxide scale can be controlled to a certain extent.

[0016] ② Transfer process: The steel plate heated in step ① is transferred in the air to a hot stamping forming press. The transfer process takes 5-18 seconds, and the temperature after the transfer is 720-860℃.

[0017] ③ Hot stamping forming process: The steel sheet is hot-stamped in a mold with an internal cooling system and held under pressure of 3-25 MPa. It is then rapidly quenched (die quenching) at a cooling rate of 15-200℃ / s to below the martensitic completion temperature (approximately 200℃) to obtain high-strength hot-formed steel. After die quenching, the microstructure of the steel sheet consists of martensite and 1-8% retained austenite, with a yield strength of 1200-1400 MPa, tensile strength of 1500-1800 MPa, elongation of 8-14%, a bending angle exceeding 60°, and a surface oxide scale thickness of 0.2-1.5 μm.

[0018] Traditional 22MnB5 hot-formed steel relies on boron (Cr) to improve hardenability, resulting in a lath martensite structure after quenching. This leads to poor elongation and bending performance, negatively impacting safety during impacts. In contrast, the high-Cr-Si alloyed hot-formed steel of this invention utilizes Cr to significantly stabilize austenite. By adjusting the pressure during hot forming, the cooling rate during die quenching can be controlled. Besides forming lath martensite, a certain amount of retained austenite is also obtained. Under strain, the TRIP effect occurs, significantly improving plasticity and bending properties.

[0019] ④ Baking process: Place the hot-formed steel obtained in step ③ into a heat treatment furnace at 170℃ and hold for 20 minutes. The baking process can increase the yield strength of the hot-formed steel plate, reduce the tensile strength, and slightly increase the bending angle and elongation.

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

[0021] ①Low cost. On the one hand, the steel composition design does not include expensive alloying elements such as Ni and Mo, and the use of Cr-Si alloying reduces costs from the source. On the other hand, the process is simplified, saving materials and energy in the intermediate links of Al-Si coating, thus reducing costs in the process. The coating-free hot-formed steel has good oxidation resistance and can be directly hot-formed from bare plates, eliminating the cost of shot blasting to remove iron oxide scale after hot forming.

[0022] ② Improve thermoforming production efficiency. Induction heating can heat the sheet to over 400°C in a short time, increasing heating efficiency by 40% and shortening heating time by 20%, thus further improving thermoforming production efficiency compared to current conventional thermoforming processes.

[0023] ③ The manufacturing process is simple and easy to industrialize. The hot-formed steel production process includes induction heating, furnace heating, hot stamping, and die quenching. Compared with Al-Si coated hot-formed steel, it has faster heat transfer and higher production efficiency. There is no need to consider the surface damage of parts after Al-Si coating adheres to the rollers or the problem of roller cleaning. The process is easy to control and simple to operate. High properties and high surface quality can be obtained by adjusting the furnace atmosphere and hot stamping pressure, making it easy to achieve industrial production.

[0024] ④ The hot-formed steel exhibits excellent plasticity. The original austenite grains are small in size, and after quenching, uniform lath martensite and a certain amount of retained austenite are obtained. Due to the uniformity of the microstructure and the TRIP effect of the retained austenite, the plasticity is significantly improved. After hot forming, the yield strength is 1200-1400MPa, the tensile strength is 1500-1800MPa, the elongation is 8.0-14%, and the bending angle can reach over 60°.

[0025] ⑤ The hot-formed steel has good surface quality. The thickness of the iron oxide scale on the surface after hot forming is 0.25-1.5 μm.

[0026] ⑥Al-Si coated plates cannot achieve rapid heating through induction heating. Attached Figure Description

[0027] Figure 1 The image shows the microstructure of the steel plate after quenching following hot stamping forming in Example 1.

[0028] Figure 2 The bending performance curve of the hot-formed steel obtained after the baking process in Example 1 is shown (horizontal axis - load displacement / mm, vertical axis - load / N).

[0029] Figure 3 The image shows the thickness of the iron oxide scale on the hot-formed steel obtained after the baking process in Example 1.

[0030] Figure 4 The image shows a cross-sectional SEM image of the hot-formed steel in Comparative Example 1. Detailed Implementation

[0031] The present invention will be further described below with reference to specific embodiments, but this does not limit the invention in any way. To avoid redundancy, unless otherwise specified, the raw materials used in the following embodiments are all commercially available products, and the methods used are all conventional methods unless otherwise specified. In the embodiments, the microstructure and cross-sectional iron oxide scale morphology were observed using a Zeiss Auriga scanning electron microscope, and the mechanical properties were tested using an Instron 5984 tensile testing machine.

[0032] The induction heating equipment used in this embodiment is a high-frequency induction heater, the heating furnace is a box-type resistance furnace, and the thermoforming equipment is a hot stamping forming press.

[0033] In the embodiment, the alloy billet is prepared into a hot-rolled steel sheet for hot stamping according to the following method: the alloy billet (continuously cast billet) is heated to 1150~1280℃ in a heating furnace and held for 1~2h. After one descaling to remove furnace-grown iron oxide scale, the roughing rolling start temperature is 1100~1200℃, and it is roughed in 6 passes with a total reduction rate of 76%~83% and an intermediate billet thickness of 40mm~55mm. After removing secondary iron oxide scale before finishing rolling, it is finished rolled into a steel strip of a specified thickness in 6~7 passes. The finishing rolling start temperature and finish rolling temperature are 1000~1080℃ and 870~930℃, respectively. After hot rolling, it is water-cooled to the coiling temperature at a cooling rate of 5~25℃ / s.

[0034] The steel strip is coiled and then air-cooled to room temperature to obtain hot-rolled steel sheet. The hot-rolled steel sheet is then subjected to bell-type furnace annealing. The bell-type furnace annealing process uses a hydrogen atmosphere to heat the temperature from room temperature to 245℃~400℃ at an average heating rate of 150℃ / hour; then it is heated to the target holding temperature (650℃~770℃) at an average heating rate of 45℃ / hour; it is held at the target temperature for 8~12 hours; then it is cooled from the target temperature to 300℃~500℃ and then rapidly cooled to the furnace exit temperature of 100℃ at an average cooling rate of 35℃ / hour; it is then removed from the furnace when cooled to 100℃.

[0035] The hot-rolled steel sheet annealed in a bell furnace is uncoiled and straightened, and then pickled to remove the oxide scale to below 2μm, forming a hot-rolled steel sheet for subsequent hot stamping.

[0036] Example 1

[0037] An advanced hot stamping forming method for high-Cr-Si alloyed coating-free hot-formable steel is disclosed. The alloy billet has the following chemical composition by weight percentage: C: 0.30%, Mn: 0.8%, Si: 0.8%, S: 0.005%, P: 0.008%, Al: 0.01%, Cr: 3.5%, Nb: 0.05%, V: 0.01%, Ti: 0.03%, Cu: 0.05%, with the balance being Fe and other unavoidable impurities. A high-ductility coating-free hot-formable steel with a thickness of 1.2 mm is prepared using the following process:

[0038] ① Heating process

[0039] The 1.2mm hot-rolled steel plate with the above composition was first placed in an induction heating device and heated to 400°C for 7 seconds. Then it was placed in a heating furnace at 880°C with a nitrogen atmosphere. The furnace was then held for 5 minutes to obtain a fully austenitized high-temperature microstructure with an original austenite grain size of 4.3μm.

[0040] ② Transfer process

[0041] The steel sheet is transferred in the air to a hot stamping forming press. The transfer process takes 8 seconds, and the temperature after the transfer is 720℃.

[0042] ③ Hot stamping forming process

[0043] The steel plate is hot-stamped in a mold with an internal cooling system and held under pressure of 25 MPa. It is then rapidly quenched at a cooling rate of 40-200℃ / s to below the martensitic completion temperature. After mold quenching, the microstructure of the steel plate consists of martensite and 3% retained austenite. Figure 1 As shown.

[0044] ④ Baking process

[0045] The steel plate was placed in a heat treatment furnace at 170℃ and held for 20 minutes. The resulting hot-formed steel had a yield strength of 1400 MPa, a tensile strength of 1790 MPa, an elongation of 8.3%, and a bending angle of 66°. The bending performance curve is shown in the figure below. Figure 2 As shown. The thickness of the surface iron oxide scale is approximately 0.25 μm, as... Figure 3 As shown.

[0046] Example 2

[0047] An advanced hot stamping forming method for high-Cr-Si alloyed coating-free hot-formable steel is disclosed. The alloy billet has the following chemical composition by weight percentage: C: 0.21%, Mn: 2.2%, Si: 1.6%, S: 0.003%, P: 0.01%, Al: 0.03%, Cr: 2.4%, Nb: 0.03%, V: 0.03%, Ti: 0.02%, Cu: 0.12%, with the balance being Fe and other unavoidable impurities. A high-ductility coating-free hot-formable steel with a thickness of 6 mm is prepared. The process steps are as follows:

[0048] ① Heating process

[0049] The 6mm hot-rolled steel plate with the above composition was first placed in an induction heating device and heated to 600°C for 8 seconds. Then it was placed in a heating furnace at 960°C with a nitrogen and methane atmosphere. The furnace was then held for 7 minutes to obtain a fully austenitized high-temperature microstructure with an original austenite grain size of 5.8µm.

[0050] ② Transfer process

[0051] The steel sheet is transferred in the air to a hot stamping forming press. The transfer process takes 8 seconds, and the temperature after the transfer is 830℃.

[0052] ③ Hot stamping forming process

[0053] The steel plate is hot-stamped in a mold with an internal cooling system and held under pressure of 18 MPa. It is then rapidly quenched at a cooling rate of 40-135℃ / s to below the martensite completion temperature. After mold quenching, the microstructure of the steel plate is martensite and 5.5% retained austenite.

[0054] ④ Baking process

[0055] The steel plate was placed in a heat treatment furnace at 170℃ and held for 20 minutes. The resulting hot-formed steel had a yield strength of 1302 MPa, a tensile strength of 1698 MPa, an elongation of 13.2%, a bending angle of 66°, and a surface oxide scale thickness of 0.94 μm.

[0056] Example 3

[0057] An advanced hot stamping forming method for high-Cr-Si alloyed coating-free hot-formable steel is disclosed. The alloy billet has the following chemical composition by weight percentage: C: 0.15%, Mn: 3.0%, Si: 2.5%, S: 0.008%, P: 0.012%, Al: 0.05%, Cr: 1.5%, Nb: 0.01%, V: 0.05%, Ti: 0.01%, Cu: 0.15%, with the balance being Fe and other unavoidable impurities. A high-ductility coating-free hot-formable steel with a thickness of 10 mm is prepared. The process steps are as follows:

[0058] ① Heating process

[0059] The 10mm hot-rolled steel plate with the above composition was first placed in an induction heating device and heated to 650°C for 9 seconds. Then it was placed in a heating furnace at 980°C with an air atmosphere. The furnace was then held for 10 minutes to obtain a fully austenitized high-temperature microstructure with an original austenite grain size of 9.8μm.

[0060] ② Transfer process

[0061] The steel sheet is transferred in the air to a hot stamping forming press. The transfer process takes 8 seconds, and the temperature after the transfer is 860℃.

[0062] ③ Hot stamping forming process

[0063] The steel plate is hot-stamped in a mold with an internal cooling system and held under pressure of 25 MPa. It is then rapidly quenched at a cooling rate of 30-120℃ / s to below the martensite completion temperature. The microstructure of the steel plate after mold quenching is martensite and 8.5% retained austenite.

[0064] ④ Baking process

[0065] The steel plate was placed in a heat treatment furnace at 170℃ and held for 20 minutes. The resulting hot-formed steel had a yield strength of 1310 MPa, a tensile strength of 1725 MPa, an elongation of 15.7%, and a surface oxide scale thickness of about 1.2 μm.

[0066] Comparative Example 1

[0067] The bare sheet of commercially available hot-formed steel grade 22MnB5 was directly subjected to hot stamping as described in Example 1, and the cross-sectional SEM image of the hot-formed steel is shown below. Figure 4 As shown, compared with Embodiment 1 of this application, the existing hot-formed steel 22MnB5 bare sheet will have an oxide layer of 6-60μm on the surface after hot stamping, which needs to be removed by cumbersome processes such as shot blasting.

[0068] For anyone skilled in the art, many possible variations and modifications can be made to the technical solutions of this invention based on the disclosed technical content, or equivalent embodiments can be modified accordingly, without departing from the scope of the technical solutions of this invention. Therefore, any simple modifications, equivalent changes, and modifications made to the above embodiments based on the technical essence of this invention without departing from the content of the technical solutions of this invention should still fall within the protection scope of the technical solutions of this invention.

Claims

1. An advanced hot-stamping forming method of a high Cr-Si alloyed uncoated hot-formed steel, characterized in that, The method comprises the following steps: The high Cr-Si alloying non-coated hot forming steel composition system is as follows: C: 0.15% to 0.35%, Mn: 0.8% to 3.2%, Si: 0.8% to 2.8%, S: <0.01%, P: <0.015%, Al: 0.01% to 0.05%, Cr: 1.5% to 3.9%, Nb: 0.01% to 0.05%, V: 0.01% to 0.05%, Ti: 0.01% to 0.03%, Cu: 0.05% to 0.15%, and the balance of Fe and other inevitable impurities; ① Heating process: the 6-10mm steel plate with the above composition system is first heated by induction, and the steel plate is heated to 400-680℃ within 10s; then heated to 880-980℃ in a heating furnace, and kept in the furnace for 2-15min; The protective atmosphere in the heating furnace during the heating process is nitrogen or nitrogen and methane mixed gas; the high-temperature structure of the completely austenitized steel plate obtained by the heating process has an original austenite grain size of 4-50μm; ② Transfer process: the steel plate heated in step ① is transferred to a hot stamping forming press in air, and the transfer process is 5-18s, and the temperature after the transfer is 720-860℃; ③ Hot stamping forming treatment: the steel plate is hot stamping formed in a die with an internal cooling system, and is in a pressure maintaining state, the pressure is 3-25MPa, then rapidly quenched to below the martensite completion temperature at a cooling speed of 15-200℃ / s to obtain a high-strength hot forming steel; ④ Baking process: the hot forming steel obtained in step ③ is placed in a heat treatment furnace at 170℃ for 20min; The microstructure of the steel plate after the rapid quenching is martensite and 1%-8% residual austenite, the yield strength is 1200-1400MPa, the tensile strength is 1500-1800MPa, the elongation is 8%-14%, the bending angle is above 60°, and the surface iron oxide scale thickness is 0.2-1.5μm.

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