A method for controlling inclusions in ultra-high strength spring steel

Abstract

The invention discloses an inclusion control method for ultra-high strength spring steel. In the process of steel tapping from a converter, the molten steel is deoxidized while slag washing and desulfurization are performed with high basicity slag. Then, the desulfurized slag generated by the slag washing in the steel tapping process is removed. After the slag is removed, the ladle is hoisted into an LF refining station, lime and river sand are added to perform slag making operation. In the refining process, 85SiC particles are diffused to form refined slag with low melting point, good fluidity and low oxidizability. The inclusions in the steel are controlled to be low melting point, small size, small number and small influence on fatigue life. The rating of each type of inclusions does not exceed level 1.0, and the total oxygen content TO in the steel does not exceed 15ppm.

Description

Technical Field

[0001] The invention belongs to the technical field of steel smelting and relates to an inclusion control method for ultra-high strength spring steel. Background Art

[0002] As industries such as automobiles and high-speed trains enter the stage of high-quality development, railway speed-up and automobile lightweighting have become one of the main directions of high-quality development. The design stress of springs used in vehicles in these industries continues to increase, which requires not only good strength and toughness of spring steel, but also long fatigue life. Non-metallic inclusions in steel are one of the most important internal defects of steel materials and one of the main factors affecting the long fatigue life of spring steel.

[0003] The presence of inclusions in steel destroys the continuity and uniformity of the steel matrix, easily forms stress concentration, and reduces the fatigue life of the steel. Therefore, in order to make spring steel strong and tough and have excellent fatigue resistance, it is necessary to understand the formation and change mechanism of inclusions during the spring steel smelting process, so as to better control and remove it to improve the cleanliness and homogeneity of the steel.

[0004] At present, most steel companies need to pre-treat and desulfurize molten iron before smelting silicon-manganese deoxidation. In the converter smelting process, if the slag, scrap steel and other raw and auxiliary materials are not controlled, the sulfur content in the steel will easily exceed 0.01%, and the sulfur content will increase during subsequent refining. At the same time, in the LF refining process, the basicity of the refined slag of spring steel using silicon-manganese deoxidation is controlled at around 2.0, and the refined slag system also changes. Some steel mills use aluminum deoxidation for spring steel, and the basicity of the refined slag is controlled higher, and the refined slag system is completely different. Therefore, in view of the high fatigue life requirements of spring steel, the production cost must be low at the same time. Summary of the invention

[0005] The purpose of the present invention is to provide a method for controlling inclusions in ultra-high strength spring steel, and to obtain an out-of-furnace refining process control technology for ultra-high cleanliness spring steel with a sulfur content not exceeding 0.01% and a rating of various inclusions not exceeding grade 1.0 without using molten iron pretreatment, so as to control the sulfur, oxygen and inclusions in the steel at a very low level, thereby reducing costs and fully meeting the requirements of fatigue life of spring steel.

[0006] The technical solution of the present invention:

[0007] A method for controlling inclusions in ultra-high strength spring steel, the key process steps include:

[0008] (1) The converter smelting is operated according to the normal smelting process, and the converter endpoint is controlled to be O≤350ppm, C≥0.08%, and the endpoint temperature≥1600℃;

[0009] (2) During the steel-making process, silicon-manganese alloy, ferrosilicon and medium-carbon ferromanganese are added to deoxidize and alloy the molten steel. During the steel-making process, 300-400 kg of CaO and 100-200 kg of Al2O3 are added to the ladle. During the steel-making process, the slag thickness is ≤20 mm, the slag basicity is ≥4, and the molten steel is slag-washed and desulfurized;

[0010] (3) After the converter is finished tapping, the ladle uses large argon gas to stir the molten steel for 3 to 5 minutes for further desulfurization, removing the sulfur content to below 0.005%;

[0011] (4) After desulfurization, the ladle is hoisted to the slag removal station to remove the desulfurization slag. After the slag removal is completed, the ladle is hoisted to the LF refining station for refining;

[0012] (5) During the LF refining process, lime and river sand are added to make slag, and the basicity of the slag is controlled at 1.0~1.3. During the refining process, 85SiC is used to diffuse and deoxidize the refined slag. The operation time of LF refining, slag making, deoxidation, alloying and heating is ≤40min;

[0013] (6) After the LF refining operation is completed, the molten steel is soft-blown for 20 minutes. The slag surface should be blown open to the size of a bowl mouth in the first 10 minutes of soft blowing. When the soft blowing lasts between 10 and 20 minutes, the argon gas is turned down so that the slag surface is not blown open and the slag surface fluctuates slightly, so as to obtain a spring steel with a total oxygen content of less than 15 ppm and all kinds of inclusions not exceeding grade 1.0.

[0014] Beneficial effects of the invention: In view of the high fatigue life requirements of spring steel, the invention eliminates the molten iron pretreatment during the smelting process and adopts a suitable refining process outside the furnace to control the sulfur, oxygen and inclusions in the steel to a very low level, which not only reduces the cost but also fully meets the fatigue life requirements of spring steel. Through the above process control, the total oxygen content of spring steel can be stably controlled within 15ppm, and all kinds of inclusions do not exceed level 1.0. BRIEF DESCRIPTION OF THE DRAWINGS

[0015] Figure 1 This is a graph showing the changes in the number and size of inclusions in the samples taken during the smelting process of the present invention. DETAILED DESCRIPTION Example 1

[0016] A method for controlling inclusions in ultra-high strength spring steel. The molten iron entering the converter is not pretreated, the S content in the molten iron is 0.035%, the temperature is 1326°C, 121 tons of molten iron and 32 tons of scrap steel are added to the converter, the C content in the molten iron is 4.26%, and the Si content is 0.43%. The key process steps include:

[0017] (1) The converter smelting is operated according to the normal smelting process, the converter end oxygen content O = 338ppm, the converter end carbon content C = 0.08%, and the end temperature is 1623℃;

[0018] (2) During the steel-making process, silicon-manganese alloy, ferrosilicon and medium-carbon ferromanganese are added to deoxidize and alloy the molten steel. According to the condition of converter slag, 80 kg of CaO and 180 kg of Al2O3 (slag material is configured according to slag basicity R=5) are added to the ladle during the steel-making process to wash and desulfurize the molten steel. The slag is strictly controlled during the steel-making process.

[0019] (3) After the converter is finished tapping, the ladle is stirred with large argon gas for 5 minutes to carry out further desulfurization operation. Steel samples and slag samples are taken to analyze the sulfur content and slag composition. The sulfur content is S=0.0036% and the slag basicity is 4.7;

[0020] (4) After desulfurization is completed, the ladle is hoisted to the slag removal station to remove the desulfurization slag. The slag removal is completed when the surface of the molten steel is exposed. Then the ladle is hoisted to the LF refining station for refining;

[0021] (5) During the LF refining process, lime and river sand are added to make slag. The amount of slag added is configured according to the slag basicity of 1.0~1.3. During the refining process, 80kg85SiC, 20kg carbon powder and 20kgCaC2 are added to diffuse and deoxidize the refined slag. The LF refining slag making, deoxidation, alloying and heating operation time is 36min.

[0022] (6) After the LF refining operation is completed, the molten steel is soft-blown for 20 minutes. 10 minutes before the soft blowing, the argon gas is used to blow the slag surface open to the size of a bowl. 10 minutes after the soft blowing, the argon gas is turned down, the slag surface is not blown open, and the slag surface fluctuates slightly.

[0023] Two groups of samples were taken from the test heat wire rods to test the total oxygen content, and the inclusions were classified and tested for rating. The total oxygen content TO of the wire rods was 9.1ppm and 9.9ppm. The inclusion types and ratings are shown in Table 1.

[0024] Table 1 Inclusion types and ratings of the test heats in Example 1

[0025] .

[0026] Example 2

[0027] A method for controlling inclusions in ultra-high strength spring steel. The molten iron entering the converter is not pretreated, the molten iron sulfur content S=0.032%, the temperature is 1317 ℃, 118 tons of molten iron and 36 tons of scrap steel are added to the converter, the carbon content of the molten iron is 4.18%, and the Si content is 0.45%. The key process steps include:

[0028] (1) The converter smelting is operated according to the normal smelting process, the converter end oxygen content O = 219ppm, the converter end carbon content C = 0.13%, and the end temperature is 1605℃;

[0029] (2) During the steel-making process, silicon-manganese alloy, ferrosilicon and medium-carbon ferromanganese are added to deoxidize and alloy the molten steel. According to the condition of converter slag, 10 kg of CaO3 and 100 kg of Al2O3 are added to the ladle during the steel-making process (the slag is configured according to the slag basicity R=5). During the steel-making process, the slag is strictly controlled, and the slag is washed and desulfurized.

[0030] (3) After the converter is tapped, the ladle is stirred with large argon gas for 3 minutes for further desulfurization. Steel and slag samples are taken to analyze the sulfur content and slag composition. The sulfur content is S=0.0048% and the slag basicity is 4.3;

[0031] (4) After desulfurization is completed, the ladle is hoisted to the slag removal station to remove the desulfurization slag. The slag removal is completed when the surface of the molten steel is exposed. Then the ladle is hoisted to the LF refining station for refining;

[0032] (5) During the LF refining process, lime and river sand are added to make slag. The amount of slag added is configured according to the slag basicity of 1.0~1.3. During the refining process, 80kg85SiC, 20kg carbon powder and 20kgCaC2 are added to diffuse and deoxidize the refined slag. The LF refining slag making, deoxidation, alloying and heating operation time is 40min.

[0033] (6) After the LF refining operation is completed, the molten steel is soft-blown for 20 minutes. 10 minutes before the soft blowing, the argon gas is used to blow the slag surface open to the size of a bowl. 10 minutes after the soft blowing, the argon gas is turned down, the slag surface is not blown open, and the slag surface fluctuates slightly.

[0034] Two groups of samples were taken from the test heat wire rods to test the total oxygen content, and the inclusions were classified and tested for rating. The total oxygen content TO of the wire rods was 8.6ppm and 8.3ppm. The inclusion types and ratings are shown in Table 2.

[0035] Table 2 Inclusion types and ratings of the test heat in Example 2

[0036] .

Claims

1. A method for controlling inclusions in ultra-high strength spring steel, the key process steps comprising: (1) The converter smelting is operated according to the normal smelting process, and the converter endpoint is controlled to be O≤350ppm, C≥0.08%, and the endpoint temperature≥1600℃; (2) During the steel-making process, silicon-manganese alloy, ferrosilicon and medium-carbon ferromanganese are added to deoxidize and alloy the molten steel. During the steel-making process, 300-400 kg of CaO and 100-200 kg of Al2O3 are added to the ladle. During the steel-making process, the slag thickness is ≤20 mm, the slag basicity is ≥4, and the molten steel is slag-washed and desulfurized; (3) After the converter is finished tapping, the ladle uses large argon gas to stir the molten steel for 3 to 5 minutes for further desulfurization, removing the sulfur content to below 0.005%; (4) After desulfurization, the ladle is hoisted to the slag removal station to remove the desulfurization slag. After the slag removal is completed, the ladle is hoisted to the LF refining station for refining; (5) During the LF refining process, lime and river sand are added to make slag, and the basicity of the slag is controlled at 1.0~1.

3. During the refining process, 85SiC is used to diffuse and deoxidize the refined slag. The slag making, deoxidation, alloying and heating operation time of LF refining is ≤40min; (6) After the LF refining operation is completed, the molten steel is soft-blown for 20 minutes. The slag surface should be blown open to the size of a bowl mouth in the first 10 minutes of soft blowing. The argon gas is turned down between 10 and 20 minutes of soft blowing so that the slag surface is not blown open and the slag surface fluctuates slightly, so as to obtain a spring steel with a total oxygen content of less than 15 ppm and all kinds of inclusions not exceeding grade 1.0.

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