2000MPa grade high-strength, plastic and oxidation-resistant hot-formed steel and its production method by continuous casting and rolling of thin slabs

Abstract

This invention discloses a method for producing 2000MPa grade high-strength, plastic, and oxidation-resistant hot-formed steel using thin slab continuous casting and rolling, belonging to the field of ultra-high-strength steel for automotive body-in-white. The chemical composition, by mass percentage, is: C 0.3%–0.4%, Mn 1.0%–1.6%, Cr 1.5%–3.0%, Si 1.0%–3.0%, V 0%–0.2%, with the remainder being Fe and unavoidable impurity elements. The production method includes smelting, thin slab continuous casting and rolling, laminar flow cooling, coiling, pre-hot forming heating to austenitization, die stamping, and quenching. This invention is based on a thin slab continuous casting and rolling process, which can directly produce thin-gauge hot-formed steel, realizing hot production instead of cold production. The production process is simple and efficient, and no expensive alloying elements are added, resulting in low manufacturing costs. After hot forming, the product has a yield strength ≥1400MPa, tensile strength ≥2000MPa, and total elongation ≥8%, exhibiting good strength-plasticity matching. The surface oxidation of the hot-formed product is low, eliminating the need for subsequent shot peening treatment, thus avoiding part deformation caused by shot peening and reducing production costs.

Description

Technical Field

[0001] This invention belongs to the technical field of ultra-high strength steel for automotive body-in-white, specifically relating to a production method of 2000MPa grade high-strength, plastic and oxidation-resistant hot-formed steel based on a thin slab continuous casting and rolling process. Background Technology

[0002] Hot-formed steel is the most representative ultra-high-strength steel for automobiles and has already achieved commercial application. Traditional hot-formed steel has several problems: First, the production process is long, using a traditional hot continuous rolling + cold rolling method. Second, existing commercially available hot-formed steel is mainly divided into Al-Si coated sheets and uncoated sheets (bare sheets). Al-Si coated sheets have high production costs and hinder hydrogen diffusion, easily leading to hydrogen embrittlement. Bare sheets are prone to surface oxidation during hot forming, requiring subsequent shot peening, which can cause part deformation and affect part usability. Third, as the strength of hot-formed steel continues to increase, from 1500MPa to 2000MPa, the material's plasticity decreases, showing a clear inverse relationship between strength and plasticity.

[0003] To shorten the manufacturing process of hot-formed steel, Chinese patents CN106086685A, CN106086683B, CN106086684B, and CN106086686A have proposed manufacturing methods for producing 1500MPa, 1700MPa, 1900MPa, and 2000MPa grade hot-formed steel based on a thin slab continuous casting and rolling process. These methods enable the production of thin-gauge hot-formed steel, which can replace traditional cold-rolled products. However, the composition design of the aforementioned patented hot-formed steel mainly uses the Mn-B series, requiring shot peening after hot forming. Furthermore, the strength-plasticity matching of these materials is relatively poor; for example, the elongation of the 2000MPa grade hot-formed steel is only 5.0%.

[0004] To address the oxidation problem of hot-formed steel during the hot forming process, Chinese patent CN114045440B proposes a high-strength, high-ductility hot-formed steel for automobiles with oxidation resistance and a hot forming process. It employs a traditional Mn-B composition system, adding 1.50%-3.20% Cr and 0.10%-0.30% Mo to achieve oxidation resistance. This allows the hot-formed steel to be heated and held at temperatures without a protective atmosphere, and it can be directly processed into subsequent steps without shot peening. The tensile strength can reach 2000 MPa. However, the addition of the expensive alloying element Mo results in high alloy costs, and the hot-rolling + cold-rolling production process is lengthy. Chinese patent CN113846272B proposes a hot-rolling preparation method for 1700 MPa grade high-Cr-Si thin-gauge hot-formed steel. This method improves the oxidation resistance of the product surface during hot forming by adding 0.8-2.8% Si and 1.5-3.9% Cr. The problem is that the traditional hot rolling production method has limited production capacity for thin-gauge products. At the same time, under the conditions of high temperature and long heating (>1250℃, >120min) in the traditional hot rolling process, the iron oxide scale on the surface is difficult to remove due to the high Si content, making it difficult to control the surface quality of the product. Summary of the Invention

[0005] To address the aforementioned technical problems, this invention provides a method for producing 2000MPa grade high-strength, plastic, and oxidation-resistant hot-formed steel using thin slab continuous casting and rolling. The aim is to shorten the manufacturing process of hot-formed steel, improve the strength-plasticity balance of hot-formed steel, and eliminate the need for shot peening after hot forming of the bare slab, thereby reducing energy consumption and CO2 emissions during production, increasing production efficiency, and lowering production costs. The technical solution of this invention is as follows:

[0006] A method for producing 2000MPa grade high-strength, plastic, and oxidation-resistant hot-formed steel by continuous casting and rolling of thin slabs, characterized in that the hot-formed steel, by mass percentage, has the following chemical composition: C: 0.3%–0.4%, Mn: 1.0%–1.6%, Cr: 1.5%–3.0%, Si: 1.0%–3.0%, V: 0%–0.2%, with the remainder being Fe and unavoidable impurity elements; the hot-rolled steel sheet can be produced using either of the following two processes;

[0007] The first production process is: the high-temperature continuous casting billet produced by thin slab continuous casting is directly fed into the roller hearth heating furnace for reheating without rough rolling. After exiting the heating furnace, it is descaled by high-pressure water and then hot-rolled, then cooled in laminar flow and coiled to obtain thin-gauge hot-rolled steel plates.

[0008] The second production process is as follows: the high-temperature continuous casting billet produced by thin slab continuous casting is directly subjected to 2-3 passes of rough rolling to obtain intermediate billet. Then, the intermediate billet is reheated and subjected to high-pressure water descaling, followed by 5 passes of finish rolling, laminar flow cooling and coiling to obtain thin-gauge hot-rolled steel plate.

[0009] Furthermore, the production steps of the first production process are as follows:

[0010] (1) Thin slab continuous casting is adopted, with a slab thickness of 50-90mm, a continuous casting speed of 4.5-6.0m / min, a cooling rate of 10-100℃ / s during the solidification process of molten steel, and an outlet temperature of 900-1050℃ for the slab continuous casting machine.

[0011] (2) The high-temperature continuous casting billet is directly fed into the roller hearth furnace for reheating. The heating temperature is 1100-1200℃, the furnace time is 15-30min, and the furnace atmosphere is controlled to be a weak oxidizing atmosphere.

[0012] (3) After the continuously cast billet exits the heating furnace, it is descaled by high-pressure water and then hot rolled in 5-7 passes. The reduction rate of the first two passes is controlled at 45-60%, the final rolling temperature is controlled at 860-900℃, and the thickness of the strip at the rolling exit is 0.7-2.0mm.

[0013] (4) After the strip exits the rolling mill, it is subjected to laminar flow cooling and coiling. The cooling rate is 10-50℃ / s, and the coiling temperature is controlled at 650-720℃ to obtain thin-gauge hot-rolled steel plates.

[0014] Further, the heating temperature in step (2) is preferably 1150-1180℃, and the furnace time is 20-30 min; the winding temperature in step (4) is preferably 660-680℃.

[0015] Furthermore, the production steps of the second production process are as follows:

[0016] (1) Thin slab continuous casting is adopted, with a slab thickness of 70-130mm, a continuous casting speed of 4.5-6.5m / min, a cooling rate of 10-100℃ / s during the solidification process of molten steel, and a slab outlet temperature of 1100-1200℃.

[0017] (2) The high-temperature continuous casting billet is directly subjected to rough rolling in 2-3 passes with a reduction rate of 50-60% per pass. The temperature of the intermediate billet at the exit of the rough rolling mill is >900℃ and the thickness of the intermediate billet is 8-30mm.

[0018] (3) Electromagnetic induction heating is used to quickly reheat the intermediate billet for a short time. The heating rate is 10-30℃ / s, and the temperature of the intermediate billet exiting the electromagnetic induction heating furnace is controlled at 1050-1150℃.

[0019] (4) After the intermediate billet exits the electromagnetic induction heating furnace, it is descaled by high-pressure water and then hot rolled in 5 passes. The reduction rate of the first two passes is controlled at 35-50%, the final rolling temperature is controlled at 840-880℃, and the thickness of the strip at the rolling exit is 0.7-2.0mm.

[0020] (5) After the strip exits the rolling mill, it is subjected to laminar flow cooling and coiling. The cooling rate is 10-50℃ / s, and the coiling temperature is controlled at 650-720℃ to obtain thin-gauge hot-rolled steel plates.

[0021] Furthermore, the winding temperature in step (5) is preferably 660-680℃.

[0022] Furthermore, the final forming process of the hot-formed steel is as follows:

[0023] (1) The obtained thin-gauge hot-rolled steel sheet is reheated to 840-950℃ and held for 180-300s. The furnace atmosphere can be air, nitrogen or other protective atmosphere.

[0024] (2) After the steel plate is taken out of the heating furnace, it is stamped and rapidly cooled in a hot stamping die. The cooling rate is controlled at 15-80℃ / s and the cooling temperature is controlled below 200℃.

[0025] Furthermore, the reheating temperature in step (1) is preferably 850-870°C.

[0026] Furthermore, the microstructure of the hot-formed steel is entirely martensitic, with a yield strength of 1400-1600 MPa, a tensile strength of 2000-2200 MPa, and a total elongation of 8-12%. Simultaneously, the oxidation rate of the hot-formed steel within the hot-forming temperature range is less than 2.6 g / m³. 2 .

[0027] Compared with the prior art, the present invention has at least the following beneficial effects:

[0028] (1) Thin-gauge hot-formed steel with a thickness of 0.7-2.0 mm can be directly produced using a thin slab continuous casting and rolling process, eliminating the need for cold rolling and annealing, making the manufacturing process simpler and more efficient. At the same time, the rapid solidification characteristic of thin slab continuous casting can effectively suppress the segregation of alloying elements in hot-formed steel, laying a good foundation for the uniformity control of product microstructure and properties; the characteristics of short-time low-temperature reheating or direct rolling of thin slab continuous casting and rolling can avoid the problem of difficult surface scale removal that is common in the shot-peening-free hot-formed steel of this invention due to its high silicon content (1.0-3.0%).

[0029] (2) This invention adopts a C-Mn-Si-Cr-V composition system, which differs from the traditional Mn-B system of hot-formed steel, thus reducing the occurrence of "boron embrittlement" defects. Through the coupling effect of Si and Cr elements alone, without the need to add expensive alloying elements such as Mo, the oxidation resistance of hot-formed steel can be significantly improved, achieving shot peening-free treatment and significantly reducing production costs. The addition of the microalloying element V can significantly improve the hydrogen embrittlement resistance of hot-formed steel. Simultaneously, the precipitation of the second phase of V, combined with the significantly improved stability of cementite in the steel by Cr elements, effectively inhibits the growth of the original austenite grains and refines the martensite structure during hot forming, while providing precipitation strengthening effects of over 150 MPa, thus improving the strength-plasticity balance of the material.

[0030] (3) This invention provides a hot-formed steel with high strength, plasticity, and oxidation resistance, with a yield strength of 1400–1600 MPa, a tensile strength of 2000–2200 MPa, and a total elongation of 8–12%. The thickness of the iron oxide scale after hot forming is less than one-tenth that of traditional Mn-B series hot-formed steel. More importantly, compared with other existing technologies, the manufacturing process of this invention is simpler and more efficient, while having lower production costs and better application prospects. Attached Figure Description

[0031] Figure 1 These are typical microstructure photographs of the product after thermoforming according to the present invention.

[0032] Figure 2 This is a typical tensile stress-strain curve of the product after thermoforming according to the present invention.

[0033] Figure 3 This invention compares the oxidation rates of the product of this invention with those of traditional Mn-B series hot-formed steel at different hot-forming temperatures. Detailed Implementation

[0034] The following embodiments describe the disclosure of this application in more detail. These embodiments are merely illustrative and are not intended to limit the invention or its application or use in any way.

[0035] Example 1:

[0036] Using the chemical composition shown in Embodiment 1 of Table 1, a continuously cast billet with a thickness of 50 mm was obtained through smelting and casting at a casting speed of 6.0 m / min. After exiting the casting machine, the billet was directly heated in a roller hearth soaking furnace at 1150℃ for 20 min. After exiting the furnace, the billet underwent high-pressure water descaling, followed by seven passes of rolling to a strip thickness of 1.2 mm at a final rolling temperature of 880℃. This was followed by laminar flow cooling and coiling at a temperature of 700℃. The resulting thin-gauge hot-rolled steel sheet was reheated to 850℃ and held for 300 s in an air atmosphere. After being removed from the furnace, the steel sheet was stamped and rapidly cooled in a hot stamping die at a rate controlled at 20℃ / s and a temperature below 200℃. The mechanical properties and surface oxide scale thickness of the hot-formed steel obtained using the above process are shown in Embodiment 1 of Table 2.

[0037] Example 2:

[0038] Using the chemical composition shown in Embodiment 2 of Table 1, a continuously cast billet with a thickness of 70 mm was obtained through smelting and casting at a casting speed of 5.0 m / min. After exiting the casting machine, the billet was directly heated in a roller hearth soaking furnace at 1170℃ for 25 min. After exiting the furnace, the billet underwent high-pressure water descaling, followed by seven passes of rolling to a strip thickness of 1.4 mm and a final rolling temperature of 860℃. This was followed by laminar flow cooling and coiling at a temperature of 680℃. The resulting thin-gauge hot-rolled steel sheet was reheated to 870℃ and held for 240 s in an air atmosphere. After being removed from the furnace, the steel sheet was stamped and rapidly cooled in a hot stamping die at a rate controlled at 30℃ / s and a temperature below 200℃. The mechanical properties and surface oxide scale thickness of the hot-formed steel obtained using the above process are shown in Embodiment 2 of Table 2.

[0039] Example 3:

[0040] Using the chemical composition shown in Embodiment 3 of Table 1, a continuously cast billet with a thickness of 90 mm was obtained through smelting and casting at a casting speed of 4.5 m / min. After exiting the casting machine, the billet was directly heated in a roller hearth soaking furnace at 1200℃ for 30 min. After exiting the furnace, the billet underwent high-pressure water descaling, followed by seven passes of rolling to a strip thickness of 2.0 mm and a final rolling temperature of 860℃. This was followed by laminar flow cooling and coiling at a temperature of 660℃. The resulting thin-gauge hot-rolled steel sheet was reheated to 900℃ and held for 180 s in a nitrogen protective atmosphere. After being removed from the furnace, the steel sheet was stamped and rapidly cooled in a hot stamping die at a rate controlled at 50℃ / s and a temperature below 200℃. The mechanical properties and surface oxide scale thickness of the hot-formed steel obtained using the above process are shown in Embodiment 3 of Table 2.

[0041] Example 4:

[0042] Using the chemical composition shown in Embodiment 4 of Table 1, a continuously cast billet with a thickness of 95 mm was obtained through smelting and casting at a casting speed of 5.5 m / min. After exiting the casting machine, the billet underwent three passes of rough rolling, resulting in an intermediate billet thickness of 10 mm and an exit temperature of 980℃. The intermediate billet was then reheated using electromagnetic induction heating at 1080℃, followed by high-pressure water descaling and five passes of finish rolling at a final rolling temperature of 840℃, resulting in a strip thickness of 0.7 mm. Laminar flow cooling and coiling were then performed, with the coiling temperature controlled at 650℃. The obtained thin-gauge hot-rolled steel sheet was reheated to 840℃ and held for 180 s in an air atmosphere. After being removed from the furnace, the steel sheet was stamped and rapidly cooled using hot stamping dies at a rate controlled at 80℃ / s and a temperature controlled below 200℃. The mechanical properties and surface oxide scale thickness of the hot-formed steel obtained using the above process are shown in Embodiment 4 of Table 2.

[0043] Example 5:

[0044] Using the chemical composition shown in Implementation 5 of Table 1, a continuously cast billet with a thickness of 110 mm was obtained through smelting and casting at a continuous casting speed of 5.0 m / min. After exiting the casting machine, the billet underwent three passes of rough rolling, resulting in an intermediate billet thickness of 12 mm and an exit temperature of 1000 °C. The intermediate billet was then reheated using electromagnetic induction heating at 1150 °C, followed by high-pressure water descaling and five passes of finish rolling at a final rolling temperature of 840 °C, resulting in a strip thickness of 1.6 mm. Subsequently, laminar flow cooling and coiling were performed, with the coiling temperature controlled at 650 °C. The thin-gauge hot-rolled steel sheet obtained above was reheated to 870 °C and held for 240 s in an air atmosphere in the heating furnace. After being removed from the heating furnace, the steel sheet was stamped and rapidly cooled in a hot stamping die at a cooling rate controlled at 60 °C / s and a cooling temperature controlled below 200 °C. The mechanical properties and surface iron oxide scale thickness of the hot-formed steel obtained by the above process are shown in Implementation 5 in Table 2.

[0045] Table 1 shows the chemical composition (wt.%) of each embodiment of the present invention;

[0046] Table 2 shows the mechanical properties of each embodiment of the present invention after thermoforming.

[0047] Table 1 shows the chemical composition (wt.%) of each embodiment of the present invention.

[0048]

[0049] Table 2. Strip thickness and mechanical properties after thermoforming in various embodiments of the present invention.

[0050]

Claims

1. A method for producing 2000MPa grade high-strength, plastic, and oxidation-resistant hot-formed steel by continuous casting and rolling of thin slabs, characterized in that, The hot-formed steel, by mass percentage, has the following chemical composition: C: 0.3%–0.4%, Mn: 1.0%–1.6%, Cr: 1.5%–3.0%, Si: 1.0%–2.0%, V: 0%–0.2%, with the remainder being Fe and unavoidable impurity elements; the hot-rolled sheet of the hot-formed steel can be produced using either of the following two processes; The first production process is: the high-temperature continuous casting billet produced by thin slab casting is directly fed into the roller hearth furnace for reheating without rough rolling. After exiting the furnace, it is descaled by high-pressure water and then hot-rolled, then cooled in laminar flow and coiled to obtain thin-gauge hot-rolled steel plates. The second production process is: the high-temperature continuous casting billet produced by thin slab continuous casting is directly subjected to 2-3 passes of rough rolling to obtain intermediate billet. Then, the intermediate billet is reheated and subjected to high-pressure water descaling, followed by 5 passes of finish rolling, laminar flow cooling and coiling to obtain thin-gauge hot-rolled steel plate. The production steps of the first production process are as follows: (1) Thin slab continuous casting is adopted, the slab thickness is 50-90mm, the continuous casting speed is 4.5-6.0m / min, the cooling rate of the molten steel solidification process is 10-100℃ / s, and the outlet temperature of the slab continuous casting machine is 900-1050℃; (2) The high-temperature continuous casting billet is directly fed into the roller hearth furnace for reheating. The heating temperature is 1100-1200℃ and the time in the furnace is 15-30min. The atmosphere in the furnace is controlled to be a weak oxidizing atmosphere. (3) After the continuous casting billet exits the heating furnace, it is descaled by high pressure water and then hot rolled in 5-7 passes. The reduction rate of the first two passes is controlled at 45-60%, the final rolling temperature is controlled at 860-900℃, and the thickness of the strip at the rolling exit is 0.7-2.0mm. (4) After the strip exits the rolling mill, it is subjected to laminar flow cooling and coiling. The cooling rate is 10-50℃ / s, and the coiling temperature is controlled at 650-720℃ to obtain thin-gauge hot-rolled steel plates. The production steps of the second production process are as follows: (1) Thin slab continuous casting is adopted, with a slab thickness of 70-130mm, a continuous casting speed of 4.5-6.5m / min, a cooling rate of 10-100℃ / s during the solidification process of molten steel, and a slab outlet temperature of 1100-1200℃. (2) The high-temperature continuous casting billet is directly subjected to rough rolling in 2-3 passes with a reduction rate of 50-60% per pass. The temperature of the intermediate billet at the exit of the rough rolling mill is >900℃ and the thickness of the intermediate billet is 8-30mm. (3) Electromagnetic induction heating is used to quickly reheat the intermediate billet for a short time. The heating rate is 10-30℃ / s, and the temperature of the intermediate billet exiting the electromagnetic induction heating furnace is controlled at 1050-1150℃. (4) After the intermediate billet exits the electromagnetic induction heating furnace, it is descaled by high-pressure water and then hot rolled in 5 passes. The reduction rate of the first two passes is controlled at 35-50%, the final rolling temperature is controlled at 840-880℃, and the thickness of the strip at the rolling exit is 0.7-2.0mm. (5) After the strip exits the rolling mill, it is subjected to laminar flow cooling and coiling. The cooling rate is 10-50℃ / s, and the coiling temperature is controlled at 650-720℃ to obtain thin-gauge hot-rolled steel plates. The final forming process of the hot-formed steel is as follows: (1) The obtained thin-gauge hot-rolled steel sheet is reheated to 840-950℃ and held for 180-300s. The furnace atmosphere is air, nitrogen or other protective atmosphere. (2) After the steel plate is taken out of the heating furnace, it is stamped and rapidly cooled in a hot stamping die. The cooling rate is controlled at 15-80℃ / s and the cooling temperature is controlled below 200℃.

2. The method for producing 2000MPa grade high-strength, plastic, and oxidation-resistant hot-formed steel by continuous casting and rolling of thin slabs according to claim 1, characterized in that, In the first production process, the heating temperature in step (2) is 1150-1180℃, and the furnace time is 20-30min; the winding temperature in step (4) is 660-680℃.

3. The method for producing 2000MPa grade high-strength, plastic, and oxidation-resistant hot-formed steel by continuous casting and rolling of thin slabs according to claim 1, characterized in that, In the second production process, the winding temperature in step (5) is 660-680℃.

4. The method for producing 2000MPa grade high-strength, plastic, and oxidation-resistant hot-formed steel by continuous casting and rolling of thin slabs according to claim 1, characterized in that, In the final forming process of hot-formed steel, the reheating temperature in step (1) is 850-870℃.

5. The method for producing 2000MPa grade high-strength, plastic, and oxidation-resistant hot-formed steel by continuous casting and rolling of thin slabs according to claim 1, characterized in that, After hot forming, the steel has a fully martensitic microstructure, a yield strength of 1400-1600 MPa, a tensile strength of 2000-2200 MPa, and a total elongation of 8-12%.

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