Production method of annealing-free high-strength steel for fasteners

Abstract

A production method of annealing-free high-strength steel for fasteners comprises the technological processes of converter smelting, LF furnace refining, RH vacuum treatment, continuous casting pouring, controlled rolling and controlled cooling, and the steel comprises, by weight, 0.30%-0.33% of C, 0.10%-0.30% of Si, 0.60%-0.70% of Mn, 0.40%-0.50% of Cr, 0.10%-0.20% of Mo, 0.05%-0.08% of V, smaller than or equal to 0.020% of P, smaller than or equal to 0.010% of S and 0.02%-0.06% of Al. The annealing-free high-strength fastener steel with the diameter of 8-30 mm is produced, H removal control is enhanced in the smelting process, the quality of a casting blank is guaranteed through reasonable optimization of the superheat degree, the pulling speed and tail end electromagnetic stirring parameters in the continuous casting process, and the structure is homogenized through a high-temperature diffusion technology in the rolling process; good finished product quality and high-precision wire rod size are obtained through a controlled rolling and controlled cooling process, the tensile strength of the steel is 1000 MPa or above, the yield strength of the steel is 970 MPa or above, and according to the cold heading performance, a customer does not need to conduct annealing treatment before cold heading production.

Description

Technical Field

[0001] The present invention belongs to the technical field of metal materials and relates to a method for producing annealing-free high-strength steel for fasteners. Background Art

[0002] At present, the production process of high-strength fasteners mainly adopts the process of annealing-drawing-cold heading. This production process has the pain points of high energy consumption, complex production procedures and long production cycle. The electricity consumption of spheroidizing annealing of steel is about 120 to 150 kWh per ton of steel, accounting for about 20% of the production cost. After annealing, pickling is required to remove the iron oxide scale, which produces wastewater and waste slag. Annealing is time-consuming, which restricts the delivery efficiency of fasteners. The EU CBAM carbon tariff requires: after 2034, the carbon footprint of imported steel products shall be ≤1.3 tons of CO2 / ton of steel. The traditional process with existing technology is 1.8 tons. The annealing-free process reduces CO2 emissions by 0.8 tons per ton of steel, which is the key to breaking through green trade barriers.

[0003] Conventional annealing-free steels, such as 10B21, currently have low strength levels and are not suitable for producing high-strength fasteners with strengths of 1000 MPa and above. Currently, demand for fasteners across various industries is shifting towards lightweighting and high strength. Therefore, a solution for annealing-free high-strength fastener steel that combines high strength, excellent cold heading performance, and process feasibility is needed to meet future demands for carbon reduction and lower production costs. Summary of the Invention

[0004] In response to the above-mentioned technical difficulties, the present invention aims to provide a method for producing annealing-free high-strength fastener steel, which has low life cycle cost, strong process adaptability, can be implemented on existing steel plant production lines, and performance indicators fully meet the requirements of high-strength fasteners. The annealing process can be eliminated during the processing, and the cold heading performance of the obtained steel allows customers to avoid annealing treatment before cold heading production.

[0005] The technical solution of the present invention: The process includes converter smelting, LF furnace refining, RH vacuum treatment, continuous casting, controlled rolling and controlled cooling. The characteristics are: the chemical composition of the steel is C=0.30%-0.33%, Si=0.10%-0.30%, Mn=0.60%-0.70%, Cr=0.40%-0.50%, Mo=0.10%-0.20%, V=0.05%-0.08%, P≤0.020%, S≤0.010%, Al=0.02%-0.06%, and the remainder is Fe and unavoidable impurities. The key process steps for producing annealing-free high-strength fastener steel with a diameter of 8-30 mm include: (1) Smelting: Control the target composition of the final molten steel to be C = 0.10% ~ 0.20%, P ≤ 0.015%, prohibit slag, and add Si, Mn, Cr, Mo, and V alloys during the steelmaking process.

[0006] (2) Refining: Lime is added to LF to make white slag for deoxidation, and the alkalinity is controlled at 6.0-8.0. Aluminum wire is fed before the end of LF furnace refining to adjust the aluminum content in the molten steel to 0.030%-0.070%; calcium wire is fed to the molten steel for calcium treatment before leaving the station, of which the calcium wire feeding amount of the first continuous casting furnace is 100 meters and the feeding amount of the continuous casting furnace is 50 meters, and then a covering agent is added to protect the molten steel; the RH is vacuumed to below 67Pa and the vacuum time is maintained for 15 minutes, and then the air is broken to determine the hydrogen content of the molten steel and the hydrogen content is controlled to ≤1.5ppm; after the RH vacuum treatment is completed, argon soft blowing treatment is carried out, with an argon flow rate of 50L / min and a soft blowing time of not less than 12 minutes.

[0007] (3) Continuous casting: Weak cooling is adopted, the water content is 0.20-0.25 L / kG, the target value of the tundish superheat is 15-25°C, the casting speed is 0.8-1.2 m / min, the end electromagnetic stirring current is 300-500 A, and the frequency is 3-8 Hz; the light reduction process at the end of continuous casting is adopted with a reduction of 10-12 mm; the billet size is 150 mm × 150 mm × 12.0 m, and the billet is placed in a slow cooling pit for slow cooling for 24 hours.

[0008] (4) Rolling: The billet is heated at 1080-1120℃, and the heating time does not exceed 150min; the billet is descaled by high-pressure water after being discharged from the furnace, and the descaling pressure is not less than 20Mpa. The reduction ratio of the first and second passes is greater than 60%; the rough rolling temperature is 950-1000℃, and water penetration controlled rolling is carried out. The finishing rolling temperature is 750-780℃.

[0009] (5) Slow cooling after rolling: After rolling, the wire rods are cooled slowly on an air-cooled roller conveyor, and the roller conveyor is covered with an insulation cover; the speed of the first section of the roller conveyor is 0.06 m / s to ensure that the wire rods can be densely packed and slowly cooled on the roller conveyor, thereby obtaining high-strength steel for fasteners with a tensile strength of more than 1000 MPa and a yield strength of more than 970 MPa.

[0010] Furthermore, the chemical composition of the optimized steel is as follows: C=0.30%~0.32%, Si=0.10%~0.30%, Mn=0.60%~0.70%, Cr=0.40%~0.48%, Mo=0.10%~0.18%, V=0.05%~0.07%, P≤0.020%, S≤0.010%, Al=0.02%~0.06%, and the rest is Fe and unavoidable impurities.

[0011] Furthermore, in step (2) refining, aluminum is used for deoxidation, and when the Al content is 0.02% to 0.06%, the AlN precipitation temperature is greater than 1100°C.

[0012] Furthermore, step (3) continuous casting: the billet size is 150 mm × 150 mm × 12.0 m.

[0013] Furthermore, step (4) rolling: using a KOCKS rolling mill + a final rolling and sizing unit for rolling, the ovality of the wire rod size is controlled within 0.10 mm.

[0014] Principle of the invention: 1) Low-carbon, high-purity design: C = 0.30% to 0.33% balances strength and ductility, while P / S limit control improves toughness. Cr = 0.40% to 0.50% improves hardenability, Mo = 0.10% to 0.20% refines grains, and V = 0.05% to 0.08% promotes precipitation strengthening, synergistically increasing strength to ≥1000 MPa.

[0015] 2) AlN refinement technology: When the Al content is 0.02% to 0.06%, the AlN precipitation temperature is greater than 1100°C, which effectively inhibits the growth of austenite grains. At the same time, low-temperature rolling is used to improve the plasticity of the product.

[0016] 3) Eliminate the risk of delayed fracture of fasteners through RH vacuum dehydrogenation ([H] ≤ 1.5ppm), and improve billet segregation by superheating at 15 to 25°C and end-stage electromagnetic stirring (400A / 6Hz).

[0017] 4) High-precision rolling combined with slow cooling after rolling reduces the hardness of the wire rod while reducing the processing strengthening in subsequent processing, providing direct conditions for annealing-free drawing.

[0018] The innovative points and beneficial effects of the present invention are as follows: Cr, Mo, and V elements are used to improve the strength of the material, and an appropriate amount of Al is added to refine the grains. The smelting process strengthens the control of H removal, and the continuous casting process ensures the quality of the ingot by rationally optimizing the superheat, drawing speed, and terminal electromagnetic stirring parameters. The rolling process uses a high-temperature diffusion process to achieve uniform organization, and a controlled rolling and controlled cooling process is used to obtain good finished product quality and high-precision wire rod size. After a small amount of drawing, the required fine wire size is achieved and the strength does not increase too much. The whole life cycle cost is low, the process adaptability is strong, and it can be implemented in the existing steel plant production line. The performance indicators fully meet the requirements of high-strength fasteners. The tensile strength of the steel is above 1000MP, the yield strength is above 970MPa, and the annealing process can be eliminated during the processing. The cold heading performance of the obtained steel allows customers to avoid annealing before cold heading production. DETAILED DESCRIPTION Example 1

[0019] A method for producing annealing-free high-strength fastener steel follows a converter-refining-continuous casting-rolling process. The chemical composition of the steel is shown in Table 1, with the remainder being Fe and unavoidable impurities. Key process steps include: (1) The C content at the converter end point is 0.12%.

[0020] (2) Refining: The basicity of the final slag of the LF furnace refining slag is 6.9; RH vacuum is maintained for 16 minutes, RH soft blowing is performed for 15 minutes, and the H value at the exit is 1.1ppm.

[0021] (3) Continuous casting: secondary cooling water volume 0.20L / kg, tundish superheat 24℃, pulling speed 1.0m / min, end electromagnetic stirring current 350A, frequency 5Hz; end light pressure down 10mm, the billet is placed in the slow cooling pit for slow cooling for 24h.

[0022] (4) Rolling: The temperature of the high temperature section of the billet heating is controlled at 1100°C, the heating time is 115 min, and the finishing rolling temperature is 760°C. The wire rod size is 20.01~20.08mm.

[0023] (5) Slow cooling after rolling: The fans of the air-cooled roller conveyor are all turned off, and the speed of the first roller conveyor is 0.06 m / s. Example 2

[0024] A method for producing annealing-free high-strength fastener steel follows a converter-refining-continuous casting-rolling process. The chemical composition of the steel is shown in Table 1, with the remainder being Fe and unavoidable impurities. Key process steps include: (1) Smelting: The C content at the converter end point is 0.14%.

[0025] (2) Refining: The basicity of the final slag of the LF furnace refining slag is 6.6; RH vacuum is maintained for 17 minutes, RH soft blowing is performed for 17 minutes, and the H value at the exit is 1.2ppm.

[0026] (3) Continuous casting: secondary cooling water volume 0.20L / kg, tundish superheat 22℃, pulling speed 1.0m / min, end electromagnetic stirring current 350A, frequency 5Hz; end light pressure down 10mm, and the billet is placed in a slow cooling pit for slow cooling for 24h.

[0027] (4) Rolling: The temperature of the high temperature section of the billet heating is controlled at 1110°C, the heating time is 110 min, and the finishing rolling temperature is 770°C. The wire rod size is 10.02~10.10mm.

[0028] (5) Slow cooling after rolling: The fans of the air-cooled roller conveyor are all turned off, and the speed of the first roller conveyor is 0.06 m / s.

[0029] Table 1 Chemical composition of steel produced in Example (wt%) .

[0030] Table 2 Customer processing technology for fastener steel produced in the embodiment and test results of finished fastener performance .

Claims

1. A method for producing annealing-free high-strength fastener steel, the process comprising converter smelting, LF furnace refining, RH vacuum treatment, continuous casting, controlled rolling and controlled cooling, characterized in that: The chemical composition of the steel is as follows: C = 0.30% to 0.33%, Si = 0.10% to 0.30%, Mn = 0.60% to 0.70%, Cr = 0.40% to 0.50%, Mo = 0.10% to 0.20%, V = 0.05% to 0.08%, P ≤ 0.020%, S ≤ 0.010%, Al = 0.02% to 0.06%, and the remainder is Fe and unavoidable impurities; the key process steps for producing annealing-free high-strength fastener steel with a diameter of 8 to 30 mm include: (1) Smelting: Control the target composition of the final molten steel to be C = 0.10% ~ 0.20%, P ≤ 0.015%, prohibit slag, and add Si, Mn, Cr, Mo, and V alloys during the tapping process; (2) Refining: lime is added to LF to make white slag for deoxidation, and the alkalinity is controlled at 6.0-8.

0. Aluminum wire is fed before the end of LF furnace refining to adjust the aluminum content in the molten steel to 0.030%-0.070%. Before leaving the station, calcium wire is fed to the molten steel for calcium treatment, of which the feeding amount of calcium wire for the first continuous casting furnace is 100 meters, and the feeding amount for the continuous casting furnace is 50 meters. Then, a covering agent is added to protect the molten steel; after entering RH, vacuum is pumped to below 67Pa and the vacuum time is maintained for 15 minutes, and then the air is broken to determine the hydrogen content of the molten steel and the hydrogen content is controlled to ≤1.5ppm; after the RH vacuum treatment is completed, argon soft blowing treatment is carried out, with an argon flow rate of 50L / min and a soft blowing time of not less than 12 minutes; (3) Continuous casting: weak cooling is adopted, the water content is 0.20-0.25 L / kg, the target value of the tundish superheat is 15-25°C, the casting speed is 0.8-1.2 m / min, the end electromagnetic stirring current is 300-500 A, and the frequency is 3-8 Hz; the reduction is 10-12 mm using the light reduction process at the end of continuous casting; the billet is placed in a slow cooling pit for slow cooling for 24 hours; (4) Rolling: The billet is heated at 1080-1120°C, and the heating time does not exceed 150 min. The billet is descaled by high-pressure water after leaving the furnace, and the descaling pressure is not less than 20 MPa. The first and second pass reduction ratios are greater than 60%. The rough rolling temperature is 950-1000°C, and water penetration controlled rolling is performed. The finishing rolling temperature is 750-780°C. (5) Slow cooling after rolling: After rolling, the wire rods are cooled slowly on an air-cooled roller conveyor, and the roller conveyor is covered with an insulation cover; the speed of the first section of the roller conveyor is 0.06 m / s to ensure that the wire rods can be densely packed and slowly cooled on the roller conveyor, thereby obtaining high-strength steel for fasteners with a tensile strength of more than 1000 MPa and a yield strength of more than 970 MPa.

2. The method for producing annealing-free high-strength fastener steel according to claim 1, characterized in that: The chemical composition of steel in weight percentage is C=0.30%~0.32%, Si=0.10%~0.30%, Mn=0.60%~0.70%, Cr=0.40%~0.48%, Mo=0.10%~0.18%, V=0.05%~0.07%, P≤0.020%, S≤0.010%, Al=0.02%~0.06%, and the rest is Fe and unavoidable impurities.

3. The method for producing annealing-free high-strength fastener steel according to claim 1, characterized in that Step (2) Refining: Deoxidation with aluminum element, when the Al content is 0.02% to 0.06%, the AlN precipitation temperature is greater than 1100°C.

4. The method for producing annealing-free high-strength fastener steel according to claim 1, characterized in that Step (3) continuous casting: the billet size is 150 mm × 150 mm × 12.0 m.

5. The method for producing annealing-free high-strength fastener steel according to claim 1, characterized in that Step (4) rolling: rolling is performed using a KOCKS rolling mill + a final rolling and sizing unit, and the ovality of the wire rod size is controlled within 0.10 mm.