A light gauge steel hot rolled coil and a method of producing the same

Abstract

This invention relates to the field of metallic materials and their preparation technology, specifically a lightweight hot-rolled steel coil and its production method. The chemical composition and weight percentage content of the coil are as follows: C: 0.6~1.5%, Si: 0.03~1.0%, Mn: 15~30%, Al: 4.0~10.0%, Ni: ≤5.0%, P: ≤0.020%, S: ≤0.010%, Nb+V+Ti ≤0.25%, with the remainder being Fe and unavoidable impurities. The production process involves smelting and continuous casting or ingot casting into billets, billet heating, descaling, temperature-controlled rolling, rapid cooling, and coiling. The material of this invention has a yield strength of 867~960 MPa, a tensile strength of 1060~1230 MPa, and an elongation of 32~48%, achieving industrialized production of lightweight hot-rolled steel coils and laying the foundation for further application and promotion of the material in automobiles and special engineering machinery.

Description

Technical Field

[0001] This invention relates to the field of metallic materials and their preparation technology, specifically to a lightweight hot-rolled steel coil and its production method. Background Technology

[0002] Lightweight steel is produced by adding lightweight elements such as carbon (C), al (Al), and silicon (Si) to steel, thereby reducing its density. The degree of density reduction depends on the amount of C and Al elements added, with Al having the greatest impact on the material's density; adding 1% Al can reduce the steel's density by 0.101 g / cm³. 3 In recent years, lightweight steel has received increasing attention due to its significant weight reduction effect and broad application prospects. Based on different phase compositions, lightweight steel currently exists in ferritic, austenitic, and duplex forms. Among them, ferritic steel has relatively low strength, duplex steel is difficult to control during hot working, and austenitic steel has excellent formability and can simultaneously achieve both strength and toughness, thus attracting considerable attention.

[0003] Austenitic lightweight steels typically incorporate large amounts of austenite-stabilizing elements such as Mn to lower the transformation temperature of austenite to ferrite, allowing hot-rolled steel strips to retain a significant amount of austenite at room temperature. However, as the alloy content increases, the material's hot working properties and deformation resistance change dramatically. Achieving stable control of the hot rolling process within a narrow process window to avoid defects is one of the main problems in current industrial production and a major obstacle limiting the widespread application of these materials.

[0004] Patent CN111663085A, entitled "A Hot-Rolled Austenitic Low-Density Steel with Ultra-High Strength and Plasticity and its Production Method," describes a production method involving smelting and continuous casting into billets; heating the continuous casting; rough rolling; water spray cooling; finish rolling; rapid heating; coiling; and natural cooling to room temperature. This document primarily uses thin-gauge billets of 55-75mm, with low-temperature control throughout the rolling process. This results in high mill loads, significant production difficulties, and an increased risk of rolling cracks.

[0005] Patent CN112760568A, entitled "A High-Strength, High-Plasticity, Low-Density Steel and Its Preparation Method," describes a production method that includes smelting ingots in a vacuum induction furnace, homogenizing and forging the ingots to obtain forged bars or plates, and then subjecting the forged bars or plates to solution treatment and aging treatment. The production process and cycle described in this document are lengthy and unsuitable for industrial application.

[0006] This invention realizes the industrial production of hot-rolled coils of lightweight steel by implementing processes such as heating, rolling, and cooling of lightweight steel billets with designed composition, laying the foundation for the further application and promotion of lightweight steel materials in automobiles and special engineering machinery. Summary of the Invention

[0007] The purpose of this invention is to address the aforementioned problems by providing a lightweight hot-rolled steel coil and its production method. This objective is achieved as follows: A lightweight hot-rolled steel coil has the following chemical composition and weight percentage content: C: 0.6~1.5%, Si: 0.03~1.0%, Mn: 15~30%, Al: 4.0~10.0%, Ni: ≤5.0%, P: ≤0.020%, S: ≤0.010%, Nb+V+Ti ≤0.25%, with the remainder being Fe and unavoidable impurities.

[0008] A method for producing lightweight hot-rolled steel coils includes the following steps: Step 1: Smelting: Continuous casting or ingot casting to form billets, with the billet thickness controlled at 150~230mm; Step 2: Rolling: Heating the billet at a temperature controlled at 1180~1270℃ for 10mm / 12~20min; After exiting the heating furnace, high-pressure water descaling is performed; then, roughing is carried out using 1~2 stands, with 5~7 passes, a single pass reduction rate ≤15% and a single pass reduction amount ≤20mm, and the billet exit temperature at the last stand of the roughing mill is 1000~1150℃; intermediate billets are fed into the edge processing section. Heater, ensuring edge temperature 950~1100℃, finally using 6~7 stands for finishing rolling, single pass reduction rate 10~30%, finishing mill exit speed controlled at 3~15m / s, finishing mill final rolling temperature controlled at 850~950℃; Step 3: Two cooling processes can be used after rolling: Process 1, after rolling, rapid cooling to ≤350℃ before coiling, cooling rate 10~50℃ / s; Process 2, after rolling, rapid cooling to 600~700℃ before coiling, cooling rate 5~20℃ / s, after coiling, placed in heat preservation pit for slow cooling, cooling rate 5~30℃ / h.

[0009] The beneficial effects of this invention are: 1. This process enables the industrial production of lightweight hot-rolled steel coils.

[0010] 2. The steel has a high alloy content, high high-temperature rolling force and poor thermoplasticity. The rolling reduction is controlled in stages during roughing and finishing to ensure the surface quality of the coil. In addition, an edge heater is used in the intermediate billet to avoid edge cracking caused by rapid temperature drop at the edge.

[0011] 3. Design two cooling process paths to achieve differentiated control of material properties. Detailed Implementation

[0012] The lightweight steel in this invention features a fully austenitic microstructure with a ferrite content ≤3%. Due to the high alloy content, high-temperature controlled rolling is employed throughout to reduce rolling load. Two cooling processes are designed after the steel strip is coiled, based on performance requirements. The design process is as follows: heating temperature is 1180℃~1270℃, holding time is 10mm / 12-20min to ensure uniform and thorough heating of the billet. The rolling reduction rate is controlled in stages during roughing and finishing rolling to avoid excessive rolling load or single-pass reduction, which could lead to surface defects. An edge heater is used on the intermediate billet after roughing to ensure the temperature of both edges and prevent edge cracking. The finishing rolling temperature of the steel strip is required to be ≥850℃, followed by direct water cooling at a rate ≥10℃ / s, with a final cooling temperature ≤350℃ to prevent the precipitation and growth of κ-phase carbides. To improve material strength, the steel strip can be wound at a high temperature of 600-700℃ and then placed in a slow cooling pit for slow cooling to promote the aging precipitation of κ-phase carbides and achieve a secondary improvement in material strength.

[0013] The composition and weight percentage content of a high-strength lightweight steel hot-rolled coil are as follows: C: 0.6~1.5%, Si: 0.03~1.0%, Mn: 15~30%, Al: 4.0~10.0%, Ni: ≤5.0%, P: ≤0.020%, S: ≤0.010%, Nb+V+Ti≤0.25%, with the remainder being Fe and unavoidable impurities.

[0014] A method for producing high-strength lightweight steel hot-rolled coils, comprising the following steps: (1) smelting and continuous casting or ingot casting to form a billet, with the billet thickness controlled at 150~230mm; (2) direct heating of the billet, with the heating temperature controlled at 1180~1270℃ and the heating time at 10mm / 12-20min; (3) descaling with high-pressure water after exiting the heating furnace; (4) rough rolling using 1~2 stands, with 5-7 rough rolling passes, and a single pass reduction rate ≤15%. (5) Use edge heaters for intermediate billets to ensure edge temperature of 950~1100℃ and avoid edge cracking during rolling; (6) Use 6~7 stands for finishing rolling, with a single-pass reduction rate of 10-30%, and control the exit speed of the finishing mill at 3~15m / s, and control the finishing rolling temperature at 850-950℃; (7) Two cooling processes can be used after rolling according to performance requirements. Process 1: After rolling, quickly cool to ≤350℃ and then coil, with a cooling rate of 10-50℃ / s; Process 2: After rolling, quickly cool to 600-650℃ and then coil, with a cooling rate of 5-20℃ / s, and then place the coiled billet in the heat preservation pit for slow cooling, with a cooling rate of 5-30℃ / h. Example 1

[0015] 1. Smelting: The chemical composition and weight percentage content are as follows: C: 0.8%, Si: 0.20%, Mn: 25%, Al: 6.0%, P: 0.012%, S: 0.003%, Nb: 0.15%. The billet is formed by continuous casting or ingot casting, with a billet thickness of 200mm.

[0016] 2. Rolling: (1) Heating the billet, with the heating temperature controlled at 1250℃ and the heating time at 4h; (2) After exiting the heating furnace, descaling is carried out by high-pressure water once; (3) Roughing is carried out using a 1-stand mill, with 7 passes, a reduction of 10-15mm per pass, an intermediate billet thickness of 70mm, and an outlet temperature of 1090℃ at the last stand of the roughing mill; (4) Edge heaters are used on the intermediate billets to ensure the edge temperature and avoid edge cracking during rolling; (5) Finishing is carried out using a 7-stand mill, with a reduction of 3-15mm per pass, a final rolled product thickness of 10mm, an outlet speed of 4.5m / s at the last stand of the finishing mill, and a finishing mill temperature of 930℃; (6) After rolling, the billet is rapidly cooled to 280℃ and coiled, with a cooling rate of 15℃ / s; (7) The yield strength of the material is 867MPa, the tensile strength is 1060MPa, and the elongation after fracture is 48%. Example 2

[0017] 1. Smelting: The chemical composition and weight percentage content are as follows: C: 1.2%, Si: 0.15%, Mn: 29%, Al: 9.6%, P: 0.010%, S: 0.004%, Ni: 1.5%, V: 0.2%. The billet is produced by continuous casting or ingot casting, with a billet thickness of 150mm.

[0018] 2. Rolling: (1) Heating the billet, the heating temperature is controlled at 1230℃, and the heating time is 4h; (2) After exiting the heating furnace, high pressure water is used for descaling once; (3) One stand roughing is used, with 7 passes, the reduction per pass is 10-15mm, the thickness of the intermediate billet is 50mm, and the outlet temperature of the billet at the last stand of the roughing mill is 1050℃; (4) The intermediate billet is equipped with an edge heater to ensure the edge temperature and avoid edge cracking during rolling; (5) Seven stands finishing is used, with a reduction per pass of 3-15mm, the thickness of the finished product is 5mm, the outlet speed of the last stand of the finishing mill is controlled at 6m / s, and the finishing temperature is 890℃; (6) After rolling, the strip is quickly cooled to 650℃ at a cooling rate of 15℃ / s and then coiled. After coiling, the strip is placed directly in the slow cooling pit for heat preservation, with a cooling rate of about 25℃ / h. After two days, it is hoisted out for performance inspection. (7) The material has a yield strength of 960 MPa, a tensile strength of 1230 MPa, and an elongation after fracture of 32%.

[0019] The above description is only a specific embodiment of the present invention, but the structural features protected by the present invention are not limited thereto. Any changes or modifications made by those skilled in the art within the scope of the present invention are covered by the patent scope of the present invention.

Claims

1. A light gauge steel hot-rolled coil characterized in that: The chemical composition and weight percentage content are as follows: C: 0.6~1.5%, Si: 0.03~1.0%, Mn: 15~30%, Al: 4.0~10.0%, Ni: ≤5.0%, P: ≤0.020%, S: ≤0.010%, Nb+V+Ti≤0.25%, with the remainder being Fe and unavoidable impurities; A method for producing lightweight hot-rolled steel coils includes the following steps: Step 1: Smelting: Continuous casting or ingot casting to form billets, with billet thickness controlled at 150~230mm; Step 2: Rolling: The billet is heated to a temperature of 1180~1270℃ for 10mm / 12~20min. After exiting the furnace, it is descaled with high-pressure water. Then, it is roughed using 1~2 stands with 5~7 passes, a single pass reduction rate ≤15% and a single pass reduction amount ≤20mm. The billet exit temperature at the last stand of the roughing mill is 1000~1150℃. The intermediate billet is heated by edge heaters to ensure an edge temperature of 950~1100℃. Finally, it is finished using 6~7 stands with a single pass reduction rate of 10~30%. The exit speed at the last stand of the finishing mill is controlled at 3~15m / s, and the finishing mill temperature is controlled at 850~950℃. Step 3: Two cooling processes can be used after rolling: Process 1, after rolling, rapidly cool to ≤350℃ and then coil, with a cooling rate of 10~50℃ / s; Process 2, after rolling, rapidly cool to 600~700℃ and then coil, with a cooling rate of 5~20℃ / s, and then place in an insulation pit for slow cooling, with a cooling rate of 5~30℃ / h.