A high-temperature oxidation resistant 1800 mpa hot-formed steel and a method of making the same

Abstract

The application discloses a kind of anti-high temperature oxidation 1800MPa hot forming steel and preparation method thereof.The hot forming steel is prepared by specific chemical composition ratio, through smelting and continuous casting, hot rolling, cold rolling, annealing, quenching and other processes, the chemical composition of the hot forming steel is as follows: C 0.30-0.33%, Si: 0.5-0.7%, Mn 1.3-1.6%, P≤0.019%, S≤0.003%, Alt: 0.041-0.61%, Cr: 0.21-0.23%, Ti: 0.037-0.057%, B 0.002-0.005%, the balance is Fe and impurities.The application precisely controls alloy composition and process parameters, so that the quenched microstructure of the hot forming steel is martensite, the yield strength is greater than or equal to 1150MPa, the tensile strength is greater than or equal to 1800MPa, and the surface oxide scale thickness is less than or equal to 5μm, without shot blasting treatment, it can be directly coated, solves the problem of high temperature oxidation, insufficient mechanical properties or high production cost of traditional hot forming steel, the preparation process is simple, suitable for industrial production, and can be widely used in the field of automobile lightweight.

Description

Technical Field

[0001] This invention belongs to the field of iron and steel smelting technology, and particularly relates to a 1800MPa hot-formed steel resistant to high-temperature oxidation and its preparation method. Background Technology

[0002] Currently, 1800MPa grade hot-formed steel, as a key material for automotive lightweighting, faces severe high-temperature oxidation challenges during the hot processing of automobile manufacturing. Although traditional aluminum-silicon coating technology can block oxygen to some extent and reduce high-temperature oxidation, during austenitization heating, the coating and matrix elements are prone to interdiffusion, leading to the formation of a hard and brittle phase at the interface, significantly reducing the material's plasticity. More seriously, under the combined effect of thermal stress and coating, liquid metal embrittlement may be induced, posing a safety risk of sudden fracture of parts during collisions.

[0003] Uncoated processes require specialized atmosphere-protected heating furnaces, resulting in significant equipment costs and high energy consumption, severely hindering production efficiency and cost control. Current technologies, whether coated or uncoated, struggle to simultaneously achieve the oxidation resistance, mechanical safety, and economic efficiency of hot-formed steel, failing to meet the demands of next-generation automobiles for high-safety, lightweight, and low-cost hot-formed steel. Therefore, there is an urgent need to develop a novel bulk alloying solution. By precisely controlling the content and combined effects of antioxidant elements in the steel, a continuous and dense composite oxide film can be generated on the substrate surface, fundamentally enhancing the material's intrinsic oxidation resistance. This would allow for effective suppression of iron oxide scale formation during heating in conventional air, while ensuring the final formed parts maintain a high strength of 1800 MPa and a good balance of toughness, providing a technological breakthrough for next-generation high-safety hot-formed steel. Summary of the Invention

[0004] The present invention aims to overcome the shortcomings of the prior art and provide a high-temperature oxidation resistant 1800MPa hot-formed steel and its preparation method. The steel has excellent high-temperature oxidation resistance without relying on surface coating, which can effectively extend the service life of hot forming molds. The resulting parts have excellent surface quality and can be directly coated without shot blasting. While ensuring the high strength mechanical properties of the material, it significantly reduces production costs and process complexity.

[0005] To achieve the above-mentioned objectives, the present invention adopts the following technical solution: The high-temperature oxidation-resistant 1800MPa hot-formed steel of the present invention has the following chemical composition by mass percentage: C: 0.30-0.33%, Si: 0.5-0.7%, Mn: 1.3-1.6%, P≤0.019%, S≤0.003%, Alt: 0.041-0.61%, Cr: 0.21-0.23%, Ti: 0.037-0.057%, B: 0.002-0.005%, with the balance being Fe and impurities. The core idea of ​​the composition design of the present invention is to precisely control the specific ratio of anti-oxidation elements such as Si, Cr, and Al, so that they preferentially undergo selective oxidation over Fe at high temperatures, and form a dense protective film mainly composed of SiO2, Cr2O3, Al2O3 and their composite oxides on the surface of the steel plate, which effectively blocks the inward diffusion of oxygen and the outward diffusion of iron, thereby significantly reducing the oxidation rate. C and Mn are key elements to ensure hardenability and final strength; microalloying of Ti and B helps to refine grains and fix N, improving toughness; extremely low P and S content aims to ensure the purity of steel and improve ductility and toughness.

[0006] This invention provides a method for preparing the above-mentioned high-temperature oxidation-resistant 1800MPa hot-formed steel, specifically including the following steps: (1) Smelting and continuous casting: The whole process of “converter smelting → LF refining → continuous casting” is adopted. During converter smelting, the molten iron is pretreated and then blown from top and bottom. Oxygen is blown to decarburize and raise the temperature. When tapping the steel, aluminum ferrosilicon ferromanganese is added at the same time for deoxidation and alloying. The tapping temperature is controlled to be ≥1630℃. Argon is blown throughout the LF refining process to ensure that the molten steel has uniform composition and meets the purity standards. During continuous casting, the superheat is controlled to be 30-50℃, the casting speed is 1.2-1.8m / min, and special protective slag for hot forming steel is used to cast the continuous casting billet with a thickness of 55-65mm.

[0007] (2) Hot rolling: The continuous casting billet is sent into the heating furnace and heated to 1200-1250℃ and homogenized. The total homogenization time is not less than 10 minutes. After heating, the continuous casting billet is subjected to high-pressure water descaling to remove the iron oxide scale on the surface. Then rough rolling and finish rolling are carried out. The rough rolling adopts the "3+3" pass mode and is rolled to an intermediate billet with a thickness of 40-55mm. The start rolling temperature of finish rolling is controlled at 1050-1100℃, the finish rolling temperature is 880-930℃, and the final thickness is 2.0-5.0mm. After rolling, laminar flow cooling is adopted with a cooling rate of 15-25℃ / s and the coiling temperature is controlled at 600-630℃ to complete the coiling.

[0008] (3) Cold rolling: The hot-rolled steel strip is pickled with hydrochloric acid to remove the surface iron oxide scale, and then sent to a 5-stand continuous rolling mill for cold rolling. The reduction rate is controlled at 50-70%, and finally cold rolled to a thickness of 1.0-2.0 mm.

[0009] (4) Annealing: The cold-rolled steel coil is sent into a bell furnace for full hydrogen annealing. The annealing temperature is controlled at 680-720℃ and held for 8-12 hours. Annealing eliminates the internal stress of cold rolling and improves the uniformity of the steel structure.

[0010] (5) Leveling treatment: Based on the thickness of the strip, a suitable elongation rate is given, and the annealed steel coil is leveled to ensure that the surface of the steel strip is flat and free of waviness, in preparation for the subsequent quenching process.

[0011] (6) Quenching: After the flattened steel strip is cut, it is sent into a pure nitrogen protective atmosphere furnace for austenitization heating at an austenitization temperature of 900-940℃ and held for 3-4 minutes; then it is hot stamped and formed, and simultaneously quenched at a cooling rate of 60-80℃ / s, and cooled to room temperature to obtain an ultra-high strength structure mainly composed of martensite.

[0012] The present invention provides hot-formed steel prepared by the above method, the microstructure of which in the quenched state is mainly martensite, the yield strength Rp0.2≥1150MPa, the tensile strength Rm≥1800MPa, and it also has excellent toughness and high-temperature oxidation resistance.

[0013] The surface oxide scale thickness is ≤5μm, with excellent surface quality, and it can be directly painted without shot blasting. The above-mentioned hot-formed steel can be used to manufacture automotive structural parts, especially for manufacturing body safety components with extremely high strength and safety requirements, such as A-pillars, B-pillars, crash beams, and sill beams.

[0014] Compared with the prior art, the beneficial effects of the present invention are as follows: 1. Excellent high-temperature oxidation resistance: By precisely controlling the alloy composition, a continuous and dense composite oxide film is generated on the surface of hot-formed steel during high-temperature heating. Without relying on surface coating, the formation of iron oxide scale can be effectively suppressed. The thickness of surface iron oxide scale is ≤5μm. It can be directly coated without shot blasting, significantly reducing production processes and costs.

[0015] 2. Superior mechanical properties: Through optimized composition and full-process process control, the final product achieves a uniform microstructure dominated by martensite, realizing an ideal match between ultra-high strength of over 1800MPa and good toughness, fully meeting the stringent requirements of automotive lightweighting for material performance.

[0016] 3. High safety and reliability: It avoids the risk of liquid metal embrittlement (LME) that may be caused by traditional Al-Si coatings, fundamentally eliminating the hidden danger of sudden fracture due to interface brittle phases, and improving the safety of parts in use.

[0017] 4. Low cost and easy production: This invention does not require expensive Al-Si coating or complex protective atmosphere heating furnace, the alloy cost is low, the preparation process is highly compatible with existing conventional steel production processes, it is suitable for large-scale industrial production, and the economic benefits are significant. Attached Figure Description

[0018] Figure 1 This is a SEM image of the surface oxide layer of hot-formed steel after the leveling process, provided in an embodiment of the present invention.

[0019] Figure 2 The metallographic structure of the hot-formed steel after quenching is shown in the embodiment of the present invention. Detailed Implementation

[0020] The present invention will be further described in detail below with reference to the accompanying drawings and specific embodiments, but the scope of protection of the present invention is not limited thereto.

[0021] Example 1 1. Chemical composition design The target composition (wt%) is: C: 0.31%, Si: 0.5%, Mn: 1.45%, P: 0.015%, S: 0.002%, Alt: 0.041%, Cr: 0.21%, Ti: 0.037%, B: 0.0035%, with the balance being Fe and impurities.

[0022] 2. Preparation process (1) Smelting and continuous casting: After pretreatment, the molten iron is smelted by top and bottom blowing in a converter. The tapping temperature is 1650℃. Ferroaluminum, ferrosilicon, and ferromanganese are added during tapping for deoxidation and alloying. The molten steel is then refined in an LF furnace for 35 minutes to precisely adjust the composition and purify the steel. Subsequently, thin slab continuous casting is carried out. The slab thickness is 60mm, the superheat is controlled at 30℃, and the casting speed is 1.5 m / min.

[0023] (2) Hot rolling: The continuously cast slab is homogenized in a walking beam furnace at 1230℃ for 12 minutes. After descaling with high-pressure water, it is rough rolled (rolled to an intermediate slab of about 45mm) and finish rolled. The initial rolling temperature of the finish rolling is 1070℃, and the final rolling temperature is 900℃, finally rolling it into a 3.0mm thick hot-rolled strip. After rolling, it is cooled in laminar flow at a cooling rate of about 20℃ / s, and the coiling temperature is 620℃.

[0024] (3) Cold rolling: After being pickled with hydrochloric acid, the hot-rolled steel strip is subjected to five-stand cold continuous rolling with a total reduction of about 67% to obtain a 1.0 mm thick cold-rolled steel strip.

[0025] (4) Annealing: The steel coil is heated to 700°C and held for 10 hours using a full hydrogen bell-type annealing furnace.

[0026] (5) Leveling: The annealed steel strip is leveled and the elongation is controlled at 0.8% to obtain a good strip shape.

[0027] (6) Quenching: The flattened steel plate is heated to 920°C in a furnace under pure nitrogen protection and held for 3.5 min to fully austenitize it. Then it is hot stamped and quickly transferred to a mold for quenching. The cooling rate is about 70°C / s.

[0028] 3. Performance Testing (1) Oxidation performance: Take a flat steel plate sample, heat it to 930℃ in air and hold it for 5 minutes to simulate the hot forming heating process. After cooling, observe that only a very thin, dark gray oxide film is formed on the surface, and no significant powdery material falls off when touched by hand. Figure 1 The SEM images show that the oxide layer is continuous, dense, and uniform in thickness, and is well bonded to the substrate.

[0029] (2) Microstructure: Metallographic observation of the quenched sample, such as... Figure 2 As shown, its microstructure is almost entirely composed of fine lath martensite, with a uniform structure and no obvious proeutectoid ferrite or other high-temperature transformation products.

[0030] (3) Mechanical properties: Tensile specimens were prepared and tested according to GB / T 228.1 standard. The results are as follows: yield strength Rp0.2 is 1180 MPa, tensile strength Rm is 1850 MPa, and elongation after fracture A80mm is 6.5%. The mechanical properties fully meet the technical requirements of 1800MPa grade hot-formed steel.

[0031] The above embodiments demonstrate that the chemical composition and preparation method provided by the present invention can successfully obtain hot-formed steel with excellent high-temperature oxidation resistance, mechanical properties of 1800 MPa or higher, and suitable for industrial production.

[0032] The above description is merely a preferred embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any equivalent substitutions or modifications made by those skilled in the art within the scope of the technology disclosed in the present invention, based on the technical solution and inventive concept of the present invention, should be covered within the scope of protection of the present invention.

Claims

1. A method for preparing hot-formed steel resistant to high-temperature oxidation at 1800 MPa, characterized in that, The chemical composition of the hot-formed steel, by mass percentage, is as follows: C: 0.30-0.33%, Si: 0.5-0.7%, Mn: 1.3-1.6%, P≤0.019%, S≤0.003%, Alt: 0.041-0.61%, Cr: 0.21-0.23%, Ti: 0.037-0.057%, B: 0.002-0.005%, with the remainder being Fe and unavoidable impurities; The preparation method includes the following steps: (1) Smelting and continuous casting: The continuous casting billet is obtained by using converter smelting, LF refining and continuous casting processes; (2) Hot rolling: The continuously cast billet is heated to 1200-1250℃ for homogenization, and then rough rolled and finished rolled to a thickness of 2.0-5.0mm. The final rolling temperature is 880-930℃, and the coiling temperature is 600-630℃. (3) Cold rolling: After pickling, cold rolling to 1.0-2.0 mm with a reduction rate of 50-70%; (4) Annealing: Full hydrogen annealing in a bell furnace at a temperature of 680-720℃ for 8-12 hours; (5) Quenching: The flattened steel strip is heated to austenitize at a temperature of 900-940℃, held for 3-4 minutes, and then quenched at a cooling rate of 60-80℃ / s.

2. The method according to claim 1, characterized in that, In step (1): the converter tapping temperature is ≥1630℃, argon is blown throughout the LF refining process; the continuous casting superheat is 30-50℃, the casting speed is 1.2-1.8m / min, and special protective slag for hot forming steel is used.

3. The method according to claim 1, characterized in that, In step (2): the roughing process uses a "3+3" pass mode to roll to a 40-55mm intermediate billet; the finishing process starts at a rolling temperature of 1050-1100℃ and the laminar cooling rate is 15-25℃ / s.

4. The method according to claim 1, characterized in that, In step (5): quenching is carried out in a furnace under a pure nitrogen protective atmosphere, and hot stamping is completed simultaneously.

5. A hot-formed steel resistant to high-temperature oxidation at 1800MPa, characterized in that, Prepared using any one of the methods described in claims 1-4, the quenched microstructure is martensite, with a yield strength ≥1150MPa and a tensile strength ≥1800MPa.

6. The hot-formed steel according to claim 5, characterized in that, Its surface has an iron oxide scale thickness of ≤5μm, and it can be directly coated without shot blasting.

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