A production method of a low-nitrogen and low-oxygen steel for automobile industry welding

Abstract

This invention discloses a method for producing low-nitrogen, low-oxygen welding steel for the automotive industry, comprising converter smelting, LF refining, and continuous casting. In the converter smelting, the carbon content of the molten steel is controlled at 0.04–0.06% at the converter endpoint, and the steel temperature is ≥1620℃. Argon is blown into the ladle, and refined lime, barium-silicon aluminum, low-carbon low-phosphorus alloy, and low-aluminum ferrosilicon are added sequentially when 1 / 3 of the steel is tapped from the converter. In the LF refining, hot refining slag and fluorite are added before the refined steel enters the station. During the refining process, silicon carbide and a carburizing agent are added in multiple batches for slag surface deoxidation. This invention employs a method of weak deoxidation at tapping, recycling of hot refining slag, and protective casting to avoid or mitigate nitrogen absorption problems during converter tapping, refining arc initiation, and continuous casting, achieving low oxygen and nitrogen content control ([N] ≤ 0.0030%, total oxygen content ≤ 0.0030%), improving billet quality, and yielding significant economic and social benefits.

Description

Technical Field

[0001] This invention belongs to the field of metallurgical technology, specifically relating to a method for producing low-nitrogen, low-oxygen welding steel for the automotive industry. Background Technology

[0002] With the widespread adoption of welding robots and the development of automated welding technology both domestically and internationally, manufacturing enterprises are increasingly demanding welding materials suitable for automated welding. Argon-shielded welding wire is an arc welding technology that uses argon as a shielding gas. During welding, it isolates the welding area from gases such as oxygen and nitrogen, thus preventing oxidation and contamination. It is widely used in the precision welding of various metal materials and plays a vital role in modern industrial manufacturing. Its lower total oxygen and nitrogen content improves spatter and welding performance. The weld metal exhibits good plasticity and toughness, easy arc initiation and re-initiation, a stable and gentle arc, and aesthetically pleasing weld formation, making it the preferred steel for automated robotic welding in the automotive manufacturing process both domestically and internationally.

[0003] The argon-shielded welding wire smelting process mainly presents the following problems: 1) The converter tapping temperature is high, and the molten steel is in direct contact with air. According to the nitrogen-oxygen balance theory and Sievert's law, deep deoxidation will cause severe nitrogen absorption in the molten steel, while no deoxidation will increase the oxygen content in the molten steel and the number of inclusions. 2) The addition of slag-forming materials such as lime, wollastonite, and fluorite during the refining process results in a slow slag formation rate. The refining slag cannot quickly cover the surface of the molten steel, making it extremely easy to absorb oxygen and nitrogen during this process, affecting the quality of the molten steel. Summary of the Invention

[0004] The purpose of this invention is to provide a production method for low-nitrogen, low-oxygen welding steel for the automotive industry. This method uses a combination of weak deoxidation and hot slag recycling to achieve low-nitrogen and low-oxygen control, reduce nitrogen and oxygen absorption during the tapping, refining, and casting processes, improve continuous casting performance, reduce production costs, and improve billet quality.

[0005] To address the above issues, the present invention adopts the following technical solution: A method for producing low-nitrogen, low-oxygen welding steel for the automotive industry includes converter smelting, LF refining, and continuous casting; details are as follows: (1) Converter smelting Argon is blown continuously in ladles 1-4 of the converter bottom-blown system. After ladle 5 is equipped with refining slag recycling capabilities, the nitrogen-argon switching ratio is switched from 0% to 40%. At the end of the converter smelting process, the carbon content in the molten steel is controlled at 0.04-0.06%, and the steel temperature is ≥1620℃. Argon is blown into the ladle bottom. When the converter tapps 1 / 3 of the steel, refining lime, silicon-barium-aluminum alloy, low-carbon low-phosphorus alloy, and low-aluminum ferrosilicon are added sequentially. The purpose of adding silicon-barium-aluminum alloy at tapping is to perform weak deoxidation.

[0006] After the steel is tapped, the silicon content in the steel is controlled at 0.50-0.65%, and the manganese content is controlled at 1.10-1.20%.

[0007] (2) LF refining Before the refined molten steel enters the station, hot refining slag and fluorite are added; during the refining and energizing process, silicon carbide and carbon raiser are added in multiple batches to deoxidize the slag surface; samples are taken for testing, and medium-carbon ferromanganese and low-aluminum ferrosilicon are added according to the test results to adjust the composition to the target value.

[0008] After the composition and temperature are suitable, the nitrogen content in the steel is tested to be ≤0.0030%, and the steel is soft-blown for 15 minutes before being poured on the continuous casting machine; the superheat of the continuous casting furnace is 20-45℃.

[0009] (3) Continuous casting Argon gas is used for protective casting, and a carbon-free covering agent is used.

[0010] Further, in step (1) of this invention, the amount of refined lime added is 2-4 kg per ton of molten steel; the amount of silicon-barium-aluminum added is 0-0.2 kg / t per ton of molten steel. This invention uses silicon-aluminum-barium as a deoxidizer because it has multi-element synergistic deoxidation characteristics: Si element rapidly pre-deoxidizes, generating SiO2 and reducing the initial oxygen content; Al element strongly deoxidizes, fixing residual oxygen to generate Al2O3, achieving deep deoxidation; Ba element has a strong affinity for oxygen, generating low-melting-point BaO, assisting in deep deoxidation. The low-melting-point composite oxides generated by this deoxidizer easily aggregate and grow, quickly floating into the slag layer, improving the fluidity of molten steel, significantly improving steel quality and continuous casting smoothness, while also improving the impact toughness, fatigue life, and cold / hot working performance of the steel.

[0011] Further, in step (1) of the present invention, the refined lime has a CaO content ≥90%; the low-carbon low-phosphorus alloy has a carbon content ≤0.50% and a manganese content of 60.0~67.0%; the low-aluminum ferrosilicon has a carbon content ≤0.20%, a silicon content ≥72.0%, and an aluminum content ≤0.10%; and the silicon-barium-aluminum alloy has a silicon content ≥30.0%, a barium content ≥12.0%, and an aluminum content ≥15.0%.

[0012] Furthermore, in step (2) of the present invention, the amount of hot refining slag added is 4 to 11 kg per ton of molten steel; the amount of fluorite added is 0 to 3.0 kg per ton of molten steel.

[0013] Furthermore, in step (2) of the present invention, the amount of silicon carbide added is 0.4 to 0.7 kg per ton of molten steel, and the amount of carbon raiser added is 0.4 to 0.7 kg per ton of molten steel.

[0014] Furthermore, in step (2) of the present invention, the carbon content of the medium-carbon ferromanganese is ≤2.0%, and the manganese content is 75.0-82.0%; the R of the hot refining slag is 1.4-1.6.

[0015] Furthermore, in step (3) of the present invention, the carbon-free covering agent has a C content of ≤0.5%, a CaO content of 40-50%, a SiO2 content of ≤10.0%, a MgO content of ≤5.0%, a F content of ≤0.50%, and a Fe2O3 content of ≤2.0%.

[0016] Furthermore, the chemical composition of the welding steel for the automotive industry produced by the method of the present invention, by weight percentage, is as follows: C: 0.06-0.10%, Si: 0.50-0.75%, Mn: 1.00-1.40%, P≤0.022%, S: ≤0.022%, N: ≤0.0030%, Ca: ≤0.0003%, Cr: ≤0.030%, Ni: ≤0.030%, Cu: ≤0.030%, total oxygen: ≤0.0030%, with the balance being Fe and unavoidable impurities.

[0017] This invention employs aluminum killing and weak deoxidation to reduce the oxygen content in steel to below 30 ppm, and reduces the nitrogen content in steel to below 30 ppm through hot refining slag recycling; thus reducing nitrogen and oxygen absorption problems during smelting, refining and casting processes.

[0018] The beneficial effects of this invention are as follows: This invention employs a weak deoxidation + hot refining recycling method. Specifically, during the converter blowing process, argon gas is blown back into the furnace bottom, and nitrogen in the molten pool is removed through argon gas circulation. During the tapping process, precipitation deoxidation and diffusion deoxidation are used to reduce the oxygen content of the molten steel, while simultaneously preventing nitrogen absorption by the molten steel. The hot refining slag is reused to increase the thickness of the refining slag layer, preventing nitrogen absorption by the molten steel during arc ignition. Simultaneously, a weakly reducing refining slag forms an oxidizing atmosphere on the slag surface to isolate nitrogen, further reducing nitrogen absorption by the molten steel. This achieves precise control of nitrogen and oxygen content, reduces the problem of nitrogen and oxygen absorption during casting, improves billet quality, and lowers production costs. Detailed Implementation

[0019] The technical solution of the present invention will be further described in detail below through embodiments.

[0020] In the following embodiments, the refined lime in step 1 has a CaO content ≥90%; the low-carbon, low-phosphorus alloy has a carbon content ≤0.50% and a manganese content of 60.0-67.0%; the low-aluminum ferrosilicon has a carbon content ≤0.20%, a silicon content ≥72.0%, and an aluminum content ≤0.10%; the silicon-barium-aluminum alloy has a silicon content ≥30.0%, a barium content ≥12.0%, and an aluminum content ≥15.0%. In step 2, the medium-carbon ferromanganese has a carbon content ≤2.0% and a manganese content of 75.0-82.0%; the refining slag has an R value of 1.4-1.6. Example 1

[0021] A method for producing low-nitrogen, low-oxygen welding steel for the automotive industry includes the following steps: 1) Smelting in a 120t top-and-bottom combined blowing converter, with 0% nitrogen-argon switching for bottom blowing, a final carbon content of 0.051%, a tapping temperature of 1632℃, bottom blowing of argon gas into the ladle, and a tapping rate of 124.5t. When the converter has tapped 1 / 3 of the steel, 311kg of refined lime, 0.12kg / t of silicon-barium-aluminum alloy, 2055kg of low-carbon and low-phosphorus alloy, and 620kg of low-aluminum ferrosilicon are added sequentially.

[0022] After the steel is produced, the silicon content in the steel is 0.63%, and the manganese content is controlled at 1.13%.

[0023] 2) LF refining process: After the molten steel enters the LF furnace, 1100 kg of hot refining slag and 50 kg of fluorite are added; the refining slag-forming time is 8 minutes; during the refining power-on process, 60 kg of silicon carbide and 60 kg of carbon raiser are added in multiple batches to deoxidize the slag surface; samples are taken for testing and 20 kg of medium carbon ferromanganese and 30 kg of low aluminum ferrosilicon are added according to the test results to adjust the composition to the target value.

[0024] After the composition and temperature were suitable, samples were taken for testing. The nitrogen content in the steel was 0.0021% and the oxygen content was 0.0015%. After 15 minutes of soft blowing, the steel was poured on the continuous casting machine. This batch consisted of 11 heats, and this was the 4th heat in the continuous casting. The superheat of the tundish was 34℃. 3) The billet is produced using 165mm*165mm continuous casting small square billets. Argon gas protection is used for continuous casting, and a carbon-free covering agent is used. The carbon-free covering agent contains 0.4% C, 45% CaO, 8.5% SiO2, 3.0% MgO, 0.30% F, and 1.3% Fe2O3.

[0025] The chemical composition of the welding steel for the automotive industry produced in this embodiment, by weight percentage, is as follows: C: 0.07%, Si: 0.60%, Mn: 1.15%, P: 0.010%, S: 0.007%, N: 0.0021%, Ca: 0.0001%, Cr: 0.015%, Ni: 0.0055%, Cu: 0.0064%, total oxygen: 0.0015%, with the balance being Fe and unavoidable impurities. Example 2

[0026] A method for producing low-nitrogen, low-oxygen welding steel for the automotive industry includes the following steps: 1) Smelting in a 120t top-and-bottom combined blowing converter, with 10% nitrogen-argon switching for bottom blowing, a final carbon content of 0.055%, a tapping temperature of 1630℃, bottom blowing of argon gas into the ladle, and a tapping rate of 124.3t. When the converter has tapped 1 / 3 of the steel, 305kg of refined lime, 0.12kg / t of silicon-barium-aluminum alloy, 2050kg of low-carbon and low-phosphorus alloy, and 630kg of low-aluminum ferrosilicon are added sequentially.

[0027] After the steel is produced, the silicon content in the steel is 0.64%, and the manganese content is controlled at 1.12%.

[0028] 2) LF refining process: After the molten steel enters the LF furnace, 1008 kg of hot refining slag is added and 30 kg of fluorite is added; the refining slag time is 8 minutes. During the refining power-on process, 50 kg of silicon carbide and 40 kg of carbon raiser are added in multiple batches to deoxidize the slag surface. Samples are taken for testing and 30 kg of medium carbon ferromanganese and 20 kg of low aluminum ferrosilicon are added according to the test results to adjust the composition to the target value.

[0029] After the composition and temperature were suitable, samples were taken for testing. The nitrogen content in the steel was 0.0017% and the oxygen content was 0.0018%. After 15 minutes of soft blowing, the steel was poured on the continuous casting machine. This batch consisted of 11 heats, and this was the 5th heat in the continuous casting. The superheat of the tundish was 32℃. 3) The product is produced using 165mm*165mm continuous casting small square billets. Argon gas protection is used for continuous casting, and a carbon-free covering agent is used. The carbon-free covering agent contains 0.32% C, 44% CaO, 8.0% SiO2, 4.0% MgO, 0.40% F, and 1.7% Fe2O3.

[0030] The chemical composition of the welding steel for the automotive industry produced in this embodiment, by weight percentage, is as follows: C: 0.07%, Si: 0.62%, Mn: 1.17%, P: 0.010%, S: 0.006%, N: 0.0017%, Ca: 0.0001%, Cr: 0.013%, Ni: 0.0053%, Cu: 0.0063%, total oxygen: 0.0018%, with the balance being Fe and unavoidable impurities. Example 3

[0031] A method for producing low-nitrogen, low-oxygen welding steel for the automotive industry includes the following steps: 1) Smelting in a 120t top-and-bottom combined blowing converter, with 30% nitrogen-argon switching for bottom blowing. The final carbon content of the converter is 0.057%, the tapping temperature is 1628℃, and argon is blown into the ladle at the bottom. The steel output is 123.9t. When the converter has tapped 1 / 3 of the steel, 302kg of refined lime, 0.2kg / t of silicon-barium-aluminum, 2042kg of low-carbon and low-phosphorus alloy, and 611kg of low-aluminum ferrosilicon are added sequentially.

[0032] After the steel is tapped, the silicon content is 0.61% and the manganese content is controlled at 1.16%. 2) LF refining process: After the molten steel enters the LF furnace, 1008 kg of hot refining slag is added and 30 kg of fluorite is added; the refining slag time is 8 min; during the refining power supply process, 40 kg of silicon carbide and 29 kg of carbon raiser are added in multiple batches to deoxidize the slag surface; samples are taken for testing and 40 kg of medium carbon ferromanganese and 30 kg of low aluminum ferrosilicon are added according to the test results to adjust the composition to the target value.

[0033] After the composition and temperature were suitable, samples were taken for testing. The nitrogen content in the steel was 0.0019% and the oxygen content was 0.0020%. After 15 minutes of soft blowing, the steel was poured on the continuous casting machine. This batch consisted of 11 heats, and this was the 6th heat in a continuous casting process. The superheat of the tundish was 33℃. 3) The billet is produced using 165mm*165mm continuous casting small square billets. Argon gas protection is used for continuous casting, and a carbon-free covering agent is used. The carbon-free covering agent contains 0.22% C, 43% CaO, 7.5% SiO2, 3.0% MgO, 0.30% F, and 1.6% Fe2O3.

[0034] The chemical composition of the welding steel for the automotive industry produced in this embodiment, by weight percentage, is as follows: C: 0.07%, Si: 0.65%, Mn: 1.19%, P: 0.010%, S: 0.007%, N: 0.0022%, Ca: 0.0001%, Cr: 0.017%, Ni: 0.0073%, Cu: 0.0058%, total oxygen: 0.0020%, with the balance being Fe and unavoidable impurities. Example 4

[0035] A method for producing low-nitrogen, low-oxygen welding steel for the automotive industry includes the following steps: 1) Smelting in a 120t top-and-bottom combined blowing converter, with 40% nitrogen-argon switching for bottom blowing. The final carbon content of the converter is 0.051%, the tapping temperature is 1632℃, and argon is used for bottom blowing in the ladle. The steel output is 124t. When the converter has tapped 1 / 3 of the steel, 305kg of refined lime, 0kg / t of silicon-barium-aluminum, 2050kg of low-carbon and low-phosphorus alloy, and 620kg of low-aluminum ferrosilicon are added sequentially.

[0036] After the steel is produced, the silicon content in the steel is 0.64%, and the manganese content is controlled at 1.19%.

[0037] 2) LF refining process: After the molten steel enters the LF furnace, 1100 kg of hot refining slag and 350 kg of fluorite are added; the refining slag time is 8 minutes. During the refining power-on process, 50 kg of silicon carbide and 30 kg of carbon raiser are added in multiple batches to deoxidize the slag surface. Samples are taken for testing, and 30 kg of medium carbon ferromanganese and 20 kg of low aluminum ferrosilicon are added according to the test results to adjust the composition to the target value.

[0038] After the composition and temperature were suitable, samples were taken for testing. The nitrogen content in the steel was 0.0023% and the oxygen content was 0.0024%. After 15 minutes of soft blowing, the steel was poured on the continuous casting machine. This batch consisted of 11 heats, and this was the 7th heat in the continuous casting. The superheat of the tundish was 36℃. 3) The billet is produced using 165mm*165mm continuous casting small square billets. Argon gas protection is used for continuous casting, and a carbon-free covering agent is used. The carbon-free covering agent contains 0.27% C, 44% CaO, 8.0% SiO2, 4.0% MgO, 0.40% F, and 1.2% Fe2O3.

[0039] The chemical composition of the welding steel for the automotive industry produced in this embodiment, calculated by weight percentage, is as follows: C: 0.08%, Si: 0.66%, Mn: 1.21%, P: 0.011%, S: 0.008%, N: 0.0023%, Ca: 0.0001%, Cr: 0.019%, Ni: 0.0065%, Cu: 0.0053%, total oxygen: 0.0024%, with the balance being Fe and unavoidable impurities.

[0040] As can be seen from Examples 1-4, the present invention adopts a method of weak deoxidation + hot refining and recycling, which achieves precise control of nitrogen and oxygen content, reduces the problem of nitrogen and oxygen being inhaled during the casting process, improves the quality of the cast billet, and reduces production costs.

[0041] The above description is only a preferred embodiment of the present invention. It should be noted that for those skilled in the art, several improvements and modifications can be made without departing from the principle of the present invention, and these improvements and modifications should also be considered within the scope of protection of the present invention.

Claims

1. A method for producing low-nitrogen, low-oxygen welding steel for the automotive industry, characterized in that, It includes converter smelting, LF refining and continuous casting; in the converter smelting, the carbon content of the molten steel is controlled at 0.04-0.06% at the end of the converter, and the temperature of the molten steel is ≥1620℃; argon gas is blown into the bottom of the ladle, and refined lime, silicon barium aluminum, low carbon low phosphorus alloy and low aluminum ferrosilicon are added in sequence when 1 / 3 of the steel is tapped from the converter.

2. The method for producing low-nitrogen, low-oxygen welding steel for the automotive industry according to claim 1, characterized in that, In the converter smelting process, the bottom blowing of the intermediate ladle 1-4 is carried out with argon blowing throughout the process. After the fifth intermediate ladle is equipped with the conditions for recycling refining slag, the nitrogen-argon switching is changed to 0-40%.

3. The method for producing low-nitrogen, low-oxygen welding steel for the automotive industry according to claim 1, characterized in that, The amount of refined lime added is 2-4 kg per ton of molten steel; the amount of silicon-barium-aluminum added is 0-0.2 kg / t per ton of molten steel.

4. The method for producing low-nitrogen, low-oxygen welding steel for the automotive industry according to claim 1, characterized in that, After the converter smelting is completed, the silicon content in the steel is controlled at 0.50-0.65%, and the manganese content is controlled at 1.10-1.20%.

5. The method for producing low-nitrogen, low-oxygen welding steel for the automotive industry according to claim 1, characterized in that, In the LF refining process, hot refining slag and fluorite are added before the molten steel enters the station; during the refining energization process, silicon carbide and carburizing agent are added in multiple batches to deoxidize the slag surface.

6. The method for producing low-nitrogen, low-oxygen welding steel for the automotive industry according to claim 5, characterized in that, The hot refining slag is added at a rate of 4-11 kg per ton of molten steel; the fluorite is added at a rate of 0-3.0 kg per ton of molten steel.

7. A method for producing low-nitrogen, low-oxygen welding steel for the automotive industry according to claim 5 or 6, characterized in that, The R value of the hot refining slag is 1.4 to 1.

6.

8. The method for producing low-nitrogen, low-oxygen welding steel for the automotive industry according to claim 1, characterized in that, The continuous casting process employs argon-protected pouring and uses a carbon-free covering agent.

9. A method for producing low-nitrogen, low-oxygen welding steel for the automotive industry according to claim 1, characterized in that, The carbon-free covering agent has a C content of ≤0.5%, a CaO content of 40-50%, a SiO2 content of ≤10.0%, a MgO content of ≤5.0%, a F content of ≤0.50%, and a Fe2O3 content of ≤2.0%.

10. A method for producing low-nitrogen, low-oxygen welding steel for the automotive industry according to claim 1, characterized in that, The chemical composition of the welding steel for the automotive industry produced by the method, by weight percentage, is as follows: C: 0.06–0.10%, Si: 0.50–0.75%, Mn: 1.00–1.40%, P ≤ 0.022%, S: ≤ 0.022%, N: ≤ 0.0030%, Ca: ≤ 0.0003%, Cr: ≤ 0.030%, Ni: ≤ 0.030%, Cu: ≤ 0.030%, total oxygen: ≤ 0.0030%, with the balance being Fe and unavoidable impurities.