A method of controlling sulphides in a sulphur-containing gear steel

Abstract

This invention discloses a method for controlling sulfides in sulfur-containing gear steel, belonging to the field of metallurgical technology, including converter, LF refining, RH refining, and continuous casting processes. Converter process: molten steel tapping temperature 1630±20℃, carbon-oxygen product controlled below 0.0020; LF refining process: aluminum content in molten steel controlled at 0.035-0.045%, LF slag composition adjusted to R: 1.5-2.5, FeO+MnO: 1.0-2.0%; RH refining process: ferrotitanium added in one step to adjust the composition appropriately; Continuous casting process: argon-blown stopper rods and submerged entry nozzles are used for continuous casting, and 150-200 kg of low-basicity tundish covering agent is added to the tundish. The sulfur-containing gear steel produced by this invention can be continuously drawn for more than 10 heats, and the level of Class A inclusions can be stably controlled at A coarse ≤ 1.0 grade and A fine ≤ 1.5 grade, with a pass rate of over 99.9%.

Description

Technical Field

[0001] This invention belongs to the field of metallurgical technology, specifically relating to a method for controlling sulfides in sulfur-containing gear steel. Background Technology

[0002] With the rapid development of my country's machinery manufacturing industry, especially the automobile manufacturing industry, the demand for steel for gear transmission components, which are key parts of automobiles and construction machinery, is increasing rapidly. The stresses experienced by these transmission components during power transmission and speed change are complex and variable, and they also experience contact wear. Therefore, the quality requirements for the special steel used are high and stringent. It must not only possess good strength, toughness, and wear resistance, and be able to withstand impact, bending, and contact stresses effectively, but also exhibit minimal deformation and high precision to reduce noise.

[0003] In actual production, to meet the requirements of high-end gear steel, aluminum is added to refine the grains and strengthen the toughness, giving the steel good strength and toughness; sulfur is added to improve machinability and meet the high machining accuracy requirements of the workpiece.

[0004] Sulfur in steel mainly exists in the form of MnS. Pure MnS inclusions appear as light black, elongated strips, which can increase the machinability of steel by disrupting the continuity of the matrix. Sulfide inclusions (i.e., type A inclusions) are small in size, numerous, and diffusely distributed, effectively ensuring that the sulfides have a low aspect ratio after rolling, which is key to improving the machinability of steel. In addition, calcium treatment of molten steel can form various calcium aluminates. During solidification, certain types of calcium aluminates can serve as effective nucleation sites for MnS. Utilizing the low deformation capacity of calcium aluminates, low aspect ratio composite near-spindle-shaped MnS composite inclusions can be achieved after rolling. These composite sulfide inclusions can effectively improve the machinability of steel.

[0005] However, the smooth production of sulfur-containing gear steel and the control of sulfide inclusions are key challenges. Firstly, CaS and Al2O3 inclusions easily form after the calcium wire is applied, which can clog the nozzle during continuous casting, causing stopper rods to rise, nozzles to become blocked and flow to drop, or even accidents leading to shutdowns. This disrupts the normal production rhythm of the steel plant and keeps production costs high. Secondly, controlling the low aspect ratio and fine, dispersed morphology of single sulfides or calcium aluminate-based composite sulfides in the billet is crucial.

[0006] The inclusion requirements for sulfur-containing gear steel are shown in the table below:

[0007] Table 1: Requirements for Non-metallic Inclusion Grades in Sulfur-Containing Gear Steel

[0008]

[0009]

[0010] Therefore, it is necessary to develop a method for controlling sulfides in sulfur-containing gear steel produced by small billet continuous casting machines. Summary of the Invention

[0011] The technical problem to be solved by the present invention is to provide a method for controlling sulfides in sulfur-containing gear steel, so as to ensure smooth production of sulfur-containing gear steel and good machinability.

[0012] To solve the above-mentioned technical problems, the present invention adopts the following technical solution: a method for controlling sulfides in sulfur-containing gear steel, the method comprising converter, LF refining, RH refining and continuous casting processes; the processes of each process are as follows:

[0013] (1) Converter process: The iron composition requirement is S≤0.045%. The composition and mass percentage of the molten steel produced by converter smelting are: C: 0.06~0.12%, P≤0.018%, S: 0.025~0.035%. The molten steel temperature is 1630±20℃, the carbon-oxygen product is controlled below 0.0020, and the tapping time is 3.5-6.0min. During the tapping process, the following are added to the ladle in sequence: carbon raiser → ferrosilicon manganese → high carbon ferromanganese → medium carbon ferrochrome. After tapping, the composition of the ladle sample is C: 0.13~0.18%, Si: 0.15~0.20%, Mn: 0.80~0.90%, P≤0.020%, S≤0.035%, Cr: 1.00~1.10%.

[0014] (2) LF refining process: In the early stage of LF refining, add 50-80 kg of silicon carbide, 500-600 kg of lime, and 100-200 kg of fluorite to heat up the slag. After 15 minutes of heating, take a temperature sample and adjust the carbon, silicon, manganese, and chromium elements according to the composition. Before leaving the LF station, feed 200-300 meters of aluminum wire into the molten steel at one time to control the aluminum content of the molten steel at 0.035-0.045%. Add carbonized rice husks for heat preservation. Feeding aluminum wire at one time before leaving the station can effectively improve the aluminum yield and reduce the total amount of Al2O3 inclusions in the molten steel. Adjust the LF slag composition to R: 1.5-2.5, FeO+MnO: 1.0-2.0%.

[0015] (3) RH refining process: After maintaining high vacuum for 3 minutes, add the ferrotitanium that needs to be adjusted at once. After adjusting the composition, continue to maintain vacuum for ≥12 minutes after adding ferrotitanium. After breaking the vacuum, gently blow the molten steel for 10-20 minutes. No aluminum products are added to the molten steel during the RH refining process.

[0016] (4) Continuous casting process: Continuous casting adopts argon blowing stopper rod and submerged nozzle. After the ladle is opened for casting, when the liquid level in the tundish reaches 350mm, 150-200kg of low-alkalinity tundish covering agent is added to the tundish, and then sufficient ordinary covering agent is added to ensure that the molten steel surface is not exposed. After the casting is successfully opened, the stopper rod is opened for argon blowing.

[0017] In the converter process described in this invention, carbon powder and silicon-manganese alloy are used for deoxidation during alloying, without the addition of aluminum products for deoxidation.

[0018] In the LF refining process of this invention, the CaO content in the refining slag is controlled at 45-55%, and the SiO2 content is controlled at 20-30%.

[0019] In the RH refining process described in this invention, high vacuum refers to a vacuum degree ≤100Pa and a vacuum holding time ≥15min; the RH refining process only adjusts the titanium content, while the other elements are adjusted in the LF refining process.

[0020] In the continuous casting process described in this invention, the low-basicity ladle covering agent is added once and not replenished subsequently. It is used to adsorb inclusions in the molten steel. Its composition is CaO: 30-40%, SiO2: 25-30%, Al2O3: 10-15%, MgO: 10-15%, Fe2O3≤6%, C≤2%.

[0021] In the continuous casting process described in this invention, the argon blowing flow rate of the stopper rod is 0.5-0.8 L / min; the submerged entry nozzle adopts... Dashuikou.

[0022] The chemical composition and mass percentage of the sulfur-containing gear steel described in this invention are as follows: C: 0.18-0.21%, Si: 0.17-0.30%, Mn: 0.90-1.05%, Cr: 1.08-1.20%, Ti: 0.04-0.08%, S: 0.015-0.035%, P≤0.025%, Als: 0.015-0.035%, Ni≤0.30%, Cu≤0.20%, Mo≤0.10%, total oxygen content 10-20ppm, total nitrogen content ≤50ppm, and the balance being Fe and unavoidable impurities, wherein the manganese-sulfur ratio ranges from 25 to 70.

[0023] The sulfur-containing gear steel obtained by the control method described in this invention has an A-grade coarse inclusion level of ≤1.0 and an A-grade fine inclusion level of ≤1.5.

[0024] The design principle of this invention is as follows:

[0025] 1. By controlling the carbon-oxygen product to below 0.0020 and the carbon content to 0.06-0.12%, the initial oxygen content of molten steel can be controlled to below 350ppm, reducing the oxidizing properties of molten steel and avoiding the formation of Al2O3 inclusions that easily clog the nozzle.

[0026] 2. The converter uses a combination of recarburizing agent and silicon-manganese deoxidation, which can reduce the total production of Al2O3 in the molten steel. The carbon in the recarburizing agent reacts with oxygen to generate CO and CO2 bubbles, which leave the molten steel in the form of bubbles, thus purifying the molten steel. The bubbles generated on the surface of the molten steel can also reduce the contact between the molten steel surface and the air, thereby reducing secondary oxidation and nitrogen addition in the molten steel. The SiO2 and MnO produced by silicon-manganese oxidation have little impact on the fluidity of the molten steel.

[0027] 3. The LF furnace controls the micro-oxidizing slag, only raises the temperature and does not carry out desulfurization reaction. It utilizes the initial sulfur content in the molten steel. Compared with the production mode of other sulfur-containing gear steels that first desulfurizes and then increases sulfur content, the sulfur content distribution is more uniform.

[0028] 4. The aluminum wire is fed in one go before leaving the LF furnace. This can take advantage of the high oxygen conditions in the molten steel in the early stage to inhibit the increase of nitrogen in the molten steel. It can also reduce the amount of aluminum products added (high absorption rate) and reduce the total amount of Al2O3 inclusions in the molten steel.

[0029] 5. RH utilizes high vacuum to remove Al2O3 inclusions and nitrogen content, and avoids the formation of CaS inclusions in molten steel by not using calcium lines;

[0030] 6. Argon blowing with a stopper rod and a large-sized nozzle can reduce the accumulation of tiny Al2O3 inclusions in the stopper rod, allowing tiny Al2O3 to enter the molten steel evenly. The tiny Al2O3 inclusions can also have the effect of oxide metallurgy.

[0031] 7. The continuous casting machine uses a φ35mm nozzle and employs stopper rod argon blowing to improve the fluidity of the molten steel. A low-basicity tundish covering agent is added during the initial casting to absorb inclusions in the molten steel.

[0032] The beneficial effects of adopting the above technical solution are as follows: 1. The converter controls the endpoint oxidation by using a carbon raiser + silicon-manganese deoxidizer, which can avoid the generation of Al2O3 inclusions; 2. Aluminum is added in the later stage of LF refining, resulting in a lower total amount of Al2O3 in the molten steel, which is easily removed by collision during RH refining; 3. RH does not require a calcium line, avoiding the large-scale generation of CaS inclusions; 4. The process of this invention is simple, easy to operate, and has significant effects. It solves the problem of nozzle blockage caused by Al2O3 and CaS inclusions, achieves stable production of sulfur-containing gear steel, avoids the production of CaS, improves the morphology of sulfides, and its production is simple with minimal impact on the cycle time, ensuring normal production of steel plants, reducing production costs, and improving enterprise efficiency. It has excellent promotion and application value in this industry. This invention allows for the production of sulfur-containing gear steel on a small billet continuous casting machine, increasing the number of continuous casting furnaces to over 10. The sulfide inclusion level can be controlled to A coarse ≤ 1.0 and A fine ≤ 1.5, with a pass rate of over 99.9%. This effectively reduces production accidents, improves product quality, and ensures a stable and controlled production process. Attached Figure Description

[0033] Figure 1 This is a diagram illustrating the distribution pattern of type A inclusions in Case 1. Detailed Implementation

[0034] The specific embodiments of the present invention will be further described below with reference to examples, but the present invention is not limited to the scope of the embodiments described herein.

[0035] Examples 1-12

[0036] The specific process of a method for controlling sulfides in sulfur-containing gear steel is described below.

[0037] 1) The production process is as follows: converter, LF refining, RH refining, and small billet continuous casting. The equipment process parameters are as follows: the converter is a 120t top and bottom blowing converter with a charge of 155±2t and a molten steel volume of 140±2t; the LF refining furnace is a double-station electrode rotary furnace; the RH refining furnace is a double-station furnace; the small billet continuous casting machine is an 8-strand, 200mm×200mm billet continuous casting machine with one tundish. The molten steel weight is 20t when the tundish liquid level is 350mm and 39t when the tundish liquid level is 700mm (normal pouring liquid level).

[0038] 2) This control method includes converter, LF refining, RH refining and continuous casting processes, and the steps of each process are as follows:

[0039] (1) Converter process: The iron composition requirement is S≤0.045%. The composition and mass percentage of the molten steel produced by converter smelting are: C: 0.06-0.12%, P≤0.025%, S: 0.025-0.035%. The molten steel temperature is 1630±20℃, the carbon-oxygen product is controlled below 0.0020, and the tapping time is 3.5~6.0min. During the tapping process, the following are added to the ladle in sequence: carbon raiser → ferrosilicon manganese → high carbon ferrosilicon manganese → medium carbon ferrochrome. After tapping, the composition of the ladle sample is C: 0.13~0.18%, Si: 0.15~0.20%, Mn: 0.80~0.90%, P≤0.020%, S≤0.035%, Cr: 1.00~1.10%.

[0040] (2) LF refining process: In the early stage of LF refining, 50-80 kg of silicon carbide is added, along with 500-600 kg of lime and 100-200 kg of fluorite to slag and raise the temperature. After 15 minutes of electric heating, the temperature is measured and samples are taken. The carbon, silicon, manganese and chromium elements are adjusted appropriately according to the composition. Before leaving the LF station, 200-300 meters of aluminum wire is fed into the molten steel at one time to control the aluminum content of the molten steel at 0.035-0.045%, and carbonized rice husks are added for heat preservation. The LF slag composition is adjusted to R: 1.5-2.5, FeO+MnO: 1.0-2.0%, CaO: 45-55%, SiO2 = 20-30%.

[0041] (3) RH refining process: After maintaining high vacuum for 3 minutes, add the ferrotitanium that needs to be adjusted at once. After adjusting the composition, continue to maintain vacuum for ≥12 minutes after adding ferrotitanium. After breaking the vacuum, gently blow the molten steel for 10-20 minutes. No aluminum products are added to the molten steel during the RH refining process.

[0042] (4) Continuous casting process: Continuous casting adopts argon blowing stopper rod and φ35mm submerged nozzle. After the ladle starts casting, when the liquid level in the tundish reaches 350mm, 150-200kg of low-alkalinity tundish covering agent is added to the tundish, and then sufficient ordinary covering agent is added to ensure that the steel liquid surface is not exposed. After successful casting, the stopper rod is opened to blow argon, and the argon blowing flow rate is controlled at 0.5-0.8L / min.

[0043] The sulfur-containing gear steel obtained by the above method can be produced in more than 10 consecutive heats, and the sulfide content can be controlled at grade A coarse ≤ 1.0 and grade A fine ≤ 1.5.

[0044] The process parameters of the converter process in each embodiment are shown in Table 1, and the composition of the ladle samples after tapping from the converter process is shown in Table 2; the process parameters of the LF refining process in each embodiment are shown in Table 3; the process parameters of the RH refining and continuous casting processes in each embodiment and the detection data of Class A inclusions in the obtained sulfur-containing gear steel are shown in Table 4.

[0045] Table 1 Process parameters for the converter process

[0046]

[0047]

[0048] Table 2. Composition (wt%) of ladle samples after steel tapping from the converter process.

[0049]

[0050] Table 3 Process parameters for the LF refining process

[0051]

[0052]

[0053] Table 4. Process parameters of RH refining and continuous casting processes and detection data of Class A inclusions in sulfur-containing gear steel.

[0054]

[0055] The attached figures show two images of the distribution patterns of Class A inclusions in Implementation Case 1, both at levels A coarse ≤ 0.5 and A fine ≤ 0.5. (The attached figures for other embodiments are similar and are therefore omitted.)

[0056] The above embodiments are only used to illustrate and not limit the technical solutions of the present invention. Although the present invention has been described in detail with reference to the above embodiments, those skilled in the art should understand that modifications or equivalent substitutions can still be made to the present invention without departing from the spirit and scope of the present invention. Any modifications or partial substitutions should be covered within the scope of the claims of the present invention.

Claims

1. A method for controlling sulfides in sulfur-containing gear steel, characterized in that, The method includes converter, LF refining, RH refining and continuous casting processes; (1) Converter process: The iron composition requirement is S≤0.045%. The composition and mass percentage of the molten steel produced by converter smelting are: C: 0.06~0.12%, P≤0.018%, S: 0.025~0.035%. The molten steel temperature is 1630±20℃, the carbon-oxygen product is controlled below 0.0020, and the tapping time is 3.5-6.0min. During the tapping process, the following are added to the ladle in sequence: carbon raiser → ferrosilicon → high carbon ferrosilicon → medium carbon ferrochrome. After tapping, the composition of the ladle sample is C: 0.13~0.18%, Si: 0.15~0.20%, Mn: 0.80~0.90%, P≤0.020%, S≤0.035%, Cr: 1.00~1.10%. (2) LF refining process: In the early stage of LF refining, 60-80 kg of silicon carbide is added, along with 500-600 kg of lime and 100-200 kg of fluorite to heat up the slag. After 15 minutes of heating, the temperature is measured and samples are taken. The carbon, silicon, manganese, and chromium elements are adjusted appropriately according to the composition. Before leaving the LF station, 200-300 meters of aluminum wire is fed into the molten steel at one time to control the aluminum content of the molten steel at 0.035-0.045%, and carbonized rice husks are added for heat preservation. The LF slag composition is adjusted to R: 1.9-2.5, FeO+MnO: 1.0-2.0%. (3) RH refining process: After maintaining high vacuum for 3 minutes, add the ferrotitanium that needs to be adjusted at once. After adjusting the composition appropriately, continue to maintain vacuum for ≥12 minutes after adding ferrotitanium. After breaking the vacuum, gently blow the molten steel for 10-20 minutes. No aluminum products are added to the molten steel during the RH refining process. In the RH process, high vacuum refers to a vacuum degree ≤100Pa and a vacuum holding time ≥15min; (4) Continuous casting process: Continuous casting adopts argon blowing stopper rod and submerged nozzle. After the ladle is opened for casting, when the liquid level in the tundish reaches 350mm, 150-200kg of low-alkalinity tundish covering agent is added to the tundish, and then sufficient ordinary covering agent is added to ensure that the molten steel surface is not exposed. After the casting is successfully opened, the stopper rod is opened for argon blowing. In the continuous casting process, the argon blowing flow rate of the stopper rod is 0.5-0.8 L / min; the submerged entry nozzle uses a large φ35 mm nozzle; The sulfur-containing gear steel obtained by the control method has an A-grade coarse inclusion level of ≤1.0 and an A-grade fine inclusion level of ≤1.

5. In the converter process, carbon powder and silicon-manganese alloy are used for deoxidation during alloying, without adding aluminum products for deoxidation.

2. The method for controlling sulfides in sulfur-containing gear steel according to claim 1, characterized in that, In the LF refining process, the CaO content in the refining slag is controlled at 45-55%, and the SiO2 content is controlled at 20-30%.

3. The method for controlling sulfides in sulfur-containing gear steel according to claim 1, characterized in that, The RH process, specifically the RH refining process, only adjusts the titanium content; all other elements are adjusted in the LF refining process.

4. The method for controlling sulfides in sulfur-containing gear steel according to claim 1, characterized in that, In the continuous casting process, the low-basicity ladle covering agent is added once and will not be added again. It is used to adsorb inclusions in the molten steel. Its composition is CaO: 30-40%, SiO2: 25-30%, Al2O3: 10-15%, MgO: 10-15%, Fe2O3≤6%, C≤2%.

5. A method for controlling sulfides in sulfur-containing gear steel according to any one of claims 1-4, characterized in that, The chemical composition and mass percentage of the sulfur-containing gear steel are as follows: C: 0.18-0.21%, Si: 0.17-0.30%, Mn: 0.90-1.05%, Cr: 1.08-1.20%, Ti: 0.04-0.08%, S: 0.015-0.035%, P≤0.025%, Als: 0.015-0.035%, Ni≤0.30%, Cu≤0.20%, Mo≤0.10%, total oxygen content 10-20ppm, total nitrogen content ≤50ppm, and the balance being Fe and unavoidable impurities, wherein the manganese-sulfur ratio ranges from 25 to 70.

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