1Gpa low-cost high-quality hot-rolled complex-phase steel and production method thereof

Abstract

The invention discloses 1Gpa low-cost high-quality hot-rolled complex-phase steel and a production method thereof, and belongs to the technical field of steel production. The complex phase steel comprises the following chemical components in percentage by mass: 0.08 to 0.12 percent of C, 1.5 to 1.9 percent of Mn, less than or equal to 0.015 percent of S, less than or equal to 0.022 percent of P, 0.2 to 0.3 percent of Si, 0.02 to 0.06 percent of Als, 0.5 to 0.7 percent of Cr, less than or equal to 0.007 percent of N and the balance of Fe and inevitable impurities. The low-C, low-Mn and low-Cr component design is adopted, the cooling process adopts a two-stage cooling mode, a uniform ferrite + martensite + bainite microstructure is obtained, the alloy cost can be reduced, meanwhile, the mechanical property of the steel is effectively improved, the tensile strength of the obtained complex-phase steel is larger than or equal to 1 GPa, the yield strength is larger than or equal to 640 MPa, the percentage elongation after fracture A80 is larger than or equal to 8%, and the machining performance is excellent.

Description

Technical Field

[0001] This invention belongs to the field of steel production technology and relates to a low-cost, high-quality hot-rolled multiphase steel with a production rate of 1 GPa and its production method. Background Technology

[0002] With growing environmental and safety awareness, advanced high-strength automotive steel is increasingly being adopted by the automotive industry due to its inherent advantages in lightweighting, energy conservation, emission reduction, and improved collision safety. However, current advanced high-strength steels often incorporate higher levels of alloying elements such as Nb, V, Ti, and Mo, resulting in higher production costs and negatively impacting weldability. Currently, the microstructure of hot-rolled high-strength dual-phase steel is primarily ferrite + martensite. While reducing the addition of alloying elements and adjusting the rolling process can achieve the required tensile yield strength, the processing performance is often unsatisfactory, leading to cracking issues during stamping, rolling, and bending processes.

[0003] If, while reducing the amount of alloy added, a new rolling process is designed to introduce bainitic structure and form a multiphase steel of ferrite + martensite + bainite, it is possible to effectively increase the processing performance of the strip steel while maintaining high strength. This can be widely used in automotive chassis suspension, bumpers, A and B pillars and other automotive parts, and has good market prospects. Summary of the Invention

[0004] To address the aforementioned technical problems, this invention provides a 1Gpa low-cost, high-quality hot-rolled multiphase steel and its production method, which effectively improves the strength of the multiphase steel and enhances its processing performance while reducing alloy costs.

[0005] To achieve the above-mentioned objectives, the technical solution adopted by this invention is as follows: A low-cost, high-quality hot-rolled multiphase steel with a strength of 1 GPa has the following chemical composition and mass percentage: C: 0.08–0.12%, Mn: 1.5–1.9%, S≤0.015%, P≤0.022%, Si: 0.2–0.3%, Als: 0.02–0.06%, Cr: 0.5–0.7%, N≤0.007%, with the balance being Fe and unavoidable impurities.

[0006] Furthermore, the thickness of the multiphase steel is 2.0–8.0 mm.

[0007] Furthermore, the microstructure of the multiphase steel consists of ferrite, bainite, and martensite, wherein the volume content of ferrite is 10-15%, the volume content of martensite is 20-30%, and the remainder is bainite.

[0008] Furthermore, the multiphase steel has a tensile strength ≥1 GPa, a yield strength ≥640 MPa, and an elongation A. 80 ≥8%.

[0009] On the other hand, the present invention also provides a method for producing the above-mentioned 1Gpa low-cost high-quality hot-rolled multiphase steel, which includes steelmaking, continuous casting, heating, rolling and cooling processes. The cooling process employs a two-stage cooling process. The first stage involves water cooling at a rate of 30–50°C / s, cooling the water to 740–750°C, followed by air cooling for 3.0–4.0 seconds. The second stage involves water cooling at a rate of 25–15°C / s, cooling the water to 510–530°C.

[0010] Furthermore, in the steelmaking process, the blast furnace molten iron is first smelted in a top-and-bottom combined blowing converter, and then refined in an LF refining furnace. The chemical composition and mass percentage of the molten steel are as follows: C: 0.08-0.12%, Mn: 1.5-1.9%, S≤0.015%, P≤0.022%, Si: 0.2-0.3%, Als: 0.02-0.06%, Cr: 0.5-0.7%, N≤0.007%, with the balance being Fe and unavoidable impurities.

[0011] Furthermore, in the heating process, the slab is heated to a furnace temperature of 1170–1230°C.

[0012] Furthermore, in the rolling process, after heating, the slab is descaled, and after two roughing stands perform 3+3 six-pass roughing, it enters the hot coil box with a total roughing reduction of ≥75%. After the roughing is uncoiled, it is descaled and then subjected to seven-stand hot continuous finishing rolling with a total finishing reduction of ≥75%. The finishing rolling start temperature is 1030~1070℃ and the finishing rolling temperature is 870~890℃.

[0013] The mechanism and function of controlling the content of various chemical components in the multiphase steel of this invention are as follows: Both carbon (C) and manganese (Mn) are relatively inexpensive solid solution strengthening elements, and appropriate levels of C and Mn can ensure the strength grade of steel. However, increasing the C content will adversely affect the weldability of the material. The solid solution strengthening and austenite stabilization effects are strongest when the mass fraction of Mn is between 1.0% and 2.0%. Exceeding this range easily leads to segregation, while below this range, the solid solution strengthening and grain refinement strengthening effects weaken. Therefore, a low-carbon, low-manganese composition design is adopted, controlling the C content of the material at 0.08–0.12% and the manganese content at 1.5–1.9%.

[0014] Si promotes the enrichment of C into austenite and the formation of proeutectoid ferrite, expanding the process window for ferrite formation. It is easier to obtain polygonal ferrite structure in the intermediate air cooling section, which is beneficial to improving the elongation of the product. However, excessive Si addition will affect the surface quality of the material. Therefore, the Si content is controlled at 0.2-0.3%.

[0015] Cr and Fe can form a continuous solid solution, and Cr and C can form various carbides. Cr has a greater affinity for carbon than Fe and Mn, which can shrink the austenite phase region, promote bainite formation, slow down the decomposition rate of austenite, and significantly improve the hardenability of steel. With increasing Cr content, the ductile-brittle transition of steel is significantly enhanced, and impact toughness decreases sharply at higher Cr contents. Therefore, the Cr content is controlled at 0.5–0.7%.

[0016] Both phosphorus (P) and sulfur (S) are impurity elements in steel. P tends to segregate at grain boundaries, reducing the plasticity and toughness of steel. S readily forms MnS inclusions with manganese (Mn), which reduces the transverse tensile properties of the steel plate. Therefore, it is advisable to minimize the P and S content of the material while considering cost. In this invention, S is controlled to be ≤0.015% and P ≤0.022%.

[0017] The beneficial effects of adopting the above technical solution are as follows: Based on the principles of automotive lightweighting and low alloying, this invention achieves a low-cost advantage by using a low-C, low-Mn, and low-Cr composition design, without adding high-cost alloying elements such as Nb, V, Ti, and Mo. Simultaneously, through a two-stage water-cooling process, a microstructure composed of ferrite, martensite, and bainite is obtained, with ferrite comprising 10-15% by volume, martensite comprising 20-30% by volume, and the remainder being bainite. The hot-rolled multiphase steel obtained by this invention has a tensile strength ≥1 GPa, a yield strength ≥640 MPa, and an elongation A... 80 With a strength of ≥8%, it achieves excellent processing performance while maintaining high strength, and can be used to manufacture automotive parts such as chassis suspension, bumpers, and A / B pillars. Attached Figure Description

[0018] Figure 1 This is a microstructure diagram of the multiphase steel obtained in Example 1 of the present invention. Detailed Implementation

[0019] The present invention will be further described in detail below with reference to specific embodiments.

[0020] Example 1 The production method of 1Gpa low-cost, high-quality hot-rolled multiphase steel in this embodiment includes steelmaking, continuous casting, heating, rolling, and cooling processes. The specific steps are as follows: (1) Steelmaking process: The blast furnace molten iron is first smelted in a top and bottom blowing converter, and then refined in an LF refining furnace. The chemical composition and mass percentage of the molten steel are as follows: C: 0.08%, Mn: 1.52%, S: 0.002%, P: 0.013%, Si: 0.2%, Als: 0.02%, Cr: 0.5%, N: 0.0036%, with the balance being Fe and unavoidable impurities; (2) Continuous casting process: The molten steel is continuously cast, and the continuous casting billet is 1750mm wide and 230mm thick; (3) Heating process: The slab is heated to 1224℃ when it exits the furnace; (4) Rolling process: After heating, the slab is descaled and then subjected to 3+3 six-pass rough rolling before entering the hot coil box. The total reduction rate of rough rolling is 75.7%. After the rough rolling is uncoiled, it is descaled and then subjected to seven-stand hot continuous finishing rolling. The total reduction rate of finishing rolling is 91.6%. The starting temperature of finishing rolling is 1039℃ and the final rolling temperature is 879℃. (5) Cooling process: After finishing rolling, two stages of water cooling are performed. The first stage water cooling rate is 37℃ / s, and the water cooling reaches 750℃. Then, the water cooling is performed for 3s within this temperature range. Finally, the second stage water cooling is performed to 521℃ for coiling. The second stage water cooling rate is 22℃ / s.

[0021] The multiphase steel obtained in this embodiment was sampled at one-quarter of its width for performance testing. The test results are shown in Table 1.

[0022] Metallographic samples were etched with 4% nitric acid alcohol, and images were acquired using an optical microscope. The large B-phase silver-gray area contains ferrite, the white strip-shaped areas contain martensite, and the remainder is bainite. The metallographic structure of the multiphase steel is shown in [reference needed]. Figure 1 (Metallographic microstructure diagrams of the multiphase steel obtained in Examples 2-5 and...) Figure 1 Similar, therefore omitted and not listed again. From Figure 1 It can be seen that in the microstructure of the multiphase steel obtained in this embodiment, the volume content of ferrite is 12%, the volume content of martensite is 20%, and the remainder is bainite.

[0023] Example 2 The production method of 1Gpa low-cost, high-quality hot-rolled multiphase steel in this embodiment includes steelmaking, continuous casting, heating, rolling, and cooling processes. The specific steps are as follows: (1) Steelmaking process: The blast furnace molten iron is first smelted in a top and bottom blowing converter, and then refined in an LF refining furnace. The chemical composition and mass percentage of the molten steel are as follows: C: 0.12%, Mn: 1.9%, S: 0.001%, P: 0.015%, Si: 0.3%, Als: 0.06%, Cr: 0.7%, N: 0.003%, with the remainder being iron and unavoidable impurities; (2) Continuous casting process: The molten steel is continuously cast, and the continuous casting billet is 1650mm wide and 230mm thick; (3) Heating process: The slab is heated to 1213℃ when it exits the furnace; (4) Rolling process: After heating, the slab is descaled and then subjected to 3+3 six-pass rough rolling before entering the hot coil box. The total reduction rate of rough rolling is 84.7%. After the rough rolling is uncoiled, it is descaled and then subjected to seven-stand hot continuous finishing rolling. The total reduction rate of finishing rolling is 86.1%. The starting temperature of finishing rolling is 1043℃ and the final rolling temperature is 886℃. (5) Cooling process: After finishing rolling, two stages of water cooling are performed. The first stage water cooling rate is 44℃ / s, and the water cooling reaches 741℃. Then, the water cooling is performed for 4s within this temperature range. Finally, the second stage water cooling is performed to 520℃ for coiling. The second stage water cooling rate is 19℃ / s.

[0024] The multiphase steel obtained in this embodiment was sampled at one-quarter of its width for performance testing. The test results are shown in Table 1.

[0025] After etching the metallographic sample with 4% nitric acid alcohol, the image was acquired using an optical microscope. In the microstructure of the multiphase steel obtained in this embodiment, the volume content of ferrite was 15%, the volume content of martensite was 28%, and the remainder was bainite.

[0026] Example 3 The production method of 1Gpa low-cost, high-quality hot-rolled multiphase steel in this embodiment includes steelmaking, continuous casting, heating, rolling, and cooling processes. The specific steps are as follows: (1) Steelmaking process: The blast furnace molten iron is first smelted in a top and bottom blowing converter, and then refined in an LF refining furnace. The chemical composition and mass percentage of the molten steel are as follows: C: 0.102%, Mn: 1.67%, S: 0.001%, P: 0.016%, Si: 0.24%, Als: 0.036%, Cr: 0.59%, N: 0.004%, with the remainder being iron and unavoidable impurities; (2) Continuous casting process: The molten steel is continuously cast, and the continuous casting billet is 1680mm wide and 230mm thick; (3) Heating process: The slab is heated to 1220℃ when it exits the furnace; (4) Rolling process: After heating, the slab is descaled and then subjected to 3+3 six-pass rough rolling before entering the hot coil box. The total reduction rate of rough rolling is 85.3%. After the rough rolling is uncoiled, it is descaled and then subjected to seven-stand hot continuous finishing rolling. The total reduction rate of finishing rolling is 91.1%. The starting temperature of finishing rolling is 1065℃ and the final rolling temperature is 880℃. (5) Cooling process: After finishing rolling, two stages of water cooling are carried out. The first stage water cooling rate is 40℃ / s, and the water cooling reaches 740℃. Then, it is air-cooled for 3s within this temperature range. Then, the second stage water cooling reaches 522℃ for coiling. The second stage water cooling rate is 22℃ / s.

[0027] The multiphase steel obtained in this embodiment was sampled at one-quarter of its width for performance testing. The test results are shown in Table 1.

[0028] After etching the metallographic sample with 4% nitric acid alcohol, the image was acquired using an optical microscope. In the microstructure of the multiphase steel obtained in this embodiment, the volume content of ferrite was 13%, the volume content of martensite was 29%, and the remainder was bainite.

[0029] Example 4 The production method of 1Gpa low-cost, high-quality hot-rolled multiphase steel in this embodiment includes steelmaking, continuous casting, heating, rolling, and cooling processes. The specific steps are as follows: (1) Steelmaking process: The blast furnace molten iron is first smelted in a top and bottom blown converter, and then refined in an LF refining furnace. The chemical composition and mass percentage of the molten steel are as follows: C: 0.082%, Mn: 1.51%, S: 0.002%, P: 0.013%, Si: 0.24%, Als: 0.058%, Cr: 0.68%, N: 0.0056%, with the remainder being iron and unavoidable impurities; (2) Continuous casting process: The molten steel is continuously cast, and the continuous casting billet is 1640mm wide and 230mm thick; (3) Heating process: The slab is heated to 1227℃ when it exits the furnace; (4) Rolling process: After heating, the slab is descaled and then subjected to 3+3 six-pass rough rolling before entering the hot coil box. The total reduction rate of rough rolling is 87.2%. After the rough rolling is uncoiled, it is descaled and then subjected to seven-stand hot continuous finishing rolling. The total reduction rate of finishing rolling is 96.8%. The starting temperature of finishing rolling is 1039℃ and the final rolling temperature is 879℃. (5) Cooling process: After finishing rolling, two stages of water cooling are performed. The first stage water cooling rate is 37℃ / s, and the water cooling reaches 749℃. Then, the water cooling is performed for 4s within this temperature range. Finally, the second stage water cooling is performed to 510℃ for coiling. The second stage water cooling rate is 23℃ / s.

[0030] The multiphase steel obtained in this embodiment was sampled at one-quarter of its width for performance testing. The test results are shown in Table 1.

[0031] After etching the metallographic sample with 4% nitric acid alcohol, the image was acquired using an optical microscope. In the microstructure of the multiphase steel obtained in this embodiment, the volume content of ferrite was 10%, the volume content of martensite was 30%, and the remainder was bainite.

[0032] Example 5 The production method of 1Gpa low-cost, high-quality hot-rolled multiphase steel in this embodiment includes steelmaking, continuous casting, heating, rolling, and cooling processes. The specific steps are as follows: (1) Steelmaking process: The blast furnace molten iron is first smelted in a top and bottom blown converter, and then refined in a double-refining process of LF refining furnace-RH refining furnace. The chemical composition and mass percentage of the molten steel are as follows: C: 0.105%, Mn: 1.6%, S: 0.003%, P: 0.012%, Si: 0.25%, Als: 0.039%, Cr: 0.6%, N: 0.0037%, with the remainder being iron and unavoidable impurities; (2) Continuous casting process: The molten steel is continuously cast, and the continuous casting billet is 1700mm wide and 230mm thick; (3) Heating process: The slab is heated to 1206℃ when it exits the furnace; (4) Rolling process: After heating, the slab is descaled and then subjected to 3+3 six-pass rough rolling before entering the hot coil box. The total reduction rate of rough rolling is 85.2%. After the rough rolling is uncoiled, it is descaled and then subjected to seven-stand hot continuous finishing rolling. The total reduction rate of finishing rolling is 97%. The starting temperature of finishing rolling is 1040℃ and the final rolling temperature is 873℃. (5) Cooling process: After finishing rolling, two stages of water cooling are carried out. The first stage water cooling rate is 33℃ / s, and the water cooling reaches 745℃. Then, it is air-cooled for 4s within this temperature range. Then, the second stage water cooling reaches 530℃ for coiling. The second stage water cooling rate is 22℃ / s.

[0033] The multiphase steel obtained in this embodiment was sampled at one-quarter of its width for performance testing. The test results are shown in Table 1.

[0034] After etching the metallographic sample with 4% nitric acid alcohol, the image was acquired using an optical microscope. In the microstructure of the multiphase steel obtained in this embodiment, the volume content of ferrite was 13%, the volume content of martensite was 24%, and the remainder was bainite.

[0035] Table 1. Test results of thickness and properties of multiphase steel in Examples 1-5

Claims

1. A 1 Gpa low cost high quality hot rolled multiphase steel, characterized in that, Its chemical composition and mass percentage are as follows: C: 0.08-0.12%, Mn: 1.5-1.9%, S≤0.015%, P≤0.022%, Si: 0.2-0.3%, Als: 0.02-0.06%, Cr: 0.5-0.7%, N≤0.007%, with the balance being Fe and unavoidable impurities.

2. The 1 Gpa low cost high quality hot rolled multiphase steel according to claim 1, characterized in that, The thickness of the multiphase steel is 2.0 to 8.0 mm.

3. The 1Gpa low-cost, high-quality hot-rolled multiphase steel according to claim 2, characterized in that, The microstructure of the multiphase steel consists of ferrite, bainite, and martensite, wherein the volume content of ferrite is 10-15%, the volume content of martensite is 20-30%, and the remainder is bainite.

4. The 1Gpa low-cost, high-quality hot-rolled multiphase steel according to claim 3, characterized in that, The multiphase steel has a tensile strength ≥1 GPa, a yield strength ≥640 MPa, and an elongation A. 80 ≥8%.

5. A method for producing 1Gpa low-cost, high-quality hot-rolled multiphase steel according to any one of claims 1-4, characterized in that, This includes steelmaking, continuous casting, heating, rolling, and cooling processes; The cooling process employs a two-stage cooling process. The first stage involves water cooling at a rate of 30–50°C / s, cooling the water to 740–750°C, followed by air cooling for 3.0–4.0 seconds. The second stage involves water cooling at a rate of 25–15°C / s, cooling the water to 510–530°C.

6. The method for producing 1Gpa low-cost, high-quality hot-rolled multiphase steel according to claim 5, characterized in that, In the steelmaking process, blast furnace molten iron is first smelted in a top-and-bottom combined blowing converter, and then refined in an LF refining furnace. The chemical composition and mass percentage of the molten steel are as follows: C: 0.08-0.12%, Mn: 1.5-1.9%, S≤0.015%, P≤0.022%, Si: 0.2-0.3%, Als: 0.02-0.06%, Cr: 0.5-0.7%, N≤0.007%, with the balance being Fe and unavoidable impurities.

7. The method for producing 1Gpa low-cost, high-quality hot-rolled multiphase steel according to claim 7, characterized in that, In the heating process, the slab is heated to a furnace temperature of 1170–1230°C.

8. The method for producing 1Gpa low-cost, high-quality hot-rolled multiphase steel according to claim 8, characterized in that, In the rolling process, after heating, the slab is descaled, and after six passes of rough rolling (3+3) on two roughing stands, it enters the hot coil box, with a total roughing reduction rate of ≥75%.

9. The method for producing 1Gpa low-cost, high-quality hot-rolled multiphase steel according to claim 9, characterized in that, The rolling process involves descaling after rough rolling and then hot continuous finishing rolling on seven stands. The total reduction rate of finishing rolling is ≥75%, the starting temperature of finishing rolling is 1030~1070℃, and the finishing rolling temperature is 870~890℃.

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