A method for preparing ultra-pure stainless steel by combined vacuum arc remelting and secondary refining

Abstract

The present application relates to the metallurgical technology field, especially relate to a kind of off-oven refining combined vacuum arc remelting preparation ultra-pure stainless steel method.The preparation method of the present application is by AOD station to transfer the crude molten steel obtained by EAF smelting to AOD refining furnace to blow oxygen refining, pre-reduction refining slag is CaO-CaF2-SiO2-MgO slag system;After pre-reduction treatment, reduction treatment is carried out, and the reduction refining slag is CaO-CaF2-SiO2-MgO slag system;The molten steel after reduction treatment is transferred to VD furnace for vacuum treatment, and the treated molten steel is transferred to ladle, then poured into mold, and ingot is obtained;After ingot is forged into electrode, ultra-pure stainless steel is prepared by VAR, with ultra-high cleanliness, coarse system inclusions is 0 level, fine system A+B+C+D inclusions≤0.5 level, effectively realize the accurate low control of aluminum content in steel, and simultaneously reduce the oxygen content and non-metallic inclusion content in steel, finally prepare high-quality ultra-pure stainless steel.

Description

Technical Field

[0001] This invention relates to the field of metallurgical technology, and in particular to a method for preparing ultrapure stainless steel by ladle refining combined with vacuum arc remelting. Background Technology

[0002] Ultra-pure stainless steel, as a high-end metallurgical material with extremely low levels of impurity elements (such as S, P, O, N, and harmful trace elements) and precisely controlled non-metallic inclusions, possesses irreplaceable application value in key fields such as semiconductor equipment, aerospace, high-end equipment manufacturing, precision electronics, and the nuclear industry due to its excellent mechanical properties, corrosion resistance, fatigue life, and processability. With the continuous improvement of modern industry's requirements for component reliability, service life, and adaptability to extreme operating conditions, more stringent standards have been placed on the control of impurity element content, inclusion morphology, and size in ultra-pure stainless steel, posing a severe challenge to traditional smelting processes.

[0003] Currently, the preparation of ultrapure stainless steel typically employs a dual-vacuum process (VIM+VAR), which is costly. However, ladle refining technology still faces several technical bottlenecks: First, at the AOD station, although segmented blowing technology is used, the blowing time for each segment lacks quantitative constraints. Operators mainly rely on lagging signals such as molten steel temperature, flame characteristics, or sampling analysis to subjectively judge the timing of stage transitions. Different operational experiences lead to inconsistent decarburization processes, easily resulting in "over-blowing" (severe chromium oxidation, reducing chromium recovery) or "under-blowing" (carbon content not meeting the target, requiring extended refining time). This causes significant fluctuations in the final carbon content and chromium recovery rate of the molten steel, making it difficult to meet the narrow composition control requirements of high-grade stainless steel. Second, in the traditional AOD refining process, the pre-reduction and reduction stages often use a single slag formula, making it difficult to achieve a synergistic effect of decarburization, desulfurization, deoxidation, dealuminization, and inclusion removal. The existing MgO-Al2O3-CaO-SiO2 slag system suffers from drawbacks such as high melting point, weak desulfurization capacity, and high cost, limiting its stable application in high-quality steel grades such as ultra-pure stainless steel. Furthermore, traditional VD (vacuum arc remelting) stations easily lead to deterioration of steel cleanliness (increased total oxygen content, excessive inclusions), increased slag entrapment defects, decreased temperature hit rate, argon waste, and poor process adaptability. These problems ultimately manifest as reduced steel fatigue life, increased scrap rate during flaw detection, higher production costs, and lagging automation levels, becoming key bottlenecks restricting the stable production of high-quality steel. Finally, in the subsequent vacuum arc remelting (VAR) process, there is a lack of clear guidance on setting parameters such as melting rate and filling ratio for different steel grades, making it difficult to reduce the gas content in the steel to the target level.

[0004] Therefore, there is an urgent need to develop a smelting process that can achieve synergistic optimization of decarburization, desulfurization, deoxidation, degassing, and inclusion control through ladle refining combined with vacuum arc remelting, in order to produce ultrapure stainless steel with high purity, uniform structure, and stable performance. Summary of the Invention

[0005] In view of the above-mentioned technical problems, the present invention provides a method for preparing ultrapure stainless steel by combining ladle refining with vacuum arc remelting, in order to solve the problems of the existing segmented blowing technology, which mainly relies on lagging signals such as molten steel temperature, flame characteristics or sampling analysis to subjectively judge the timing of stage transition, and is prone to "overblowing" or "underblowing" phenomena; and the existing MgO-Al2O3-CaO-SiO2 slag system has high melting point and weak desulfurization capacity; while the lack of traditional VD station can easily lead to deterioration of molten steel cleanliness, increase of slag entrapment defects, decrease of temperature hit rate, argon waste and poor process adaptability.

[0006] To achieve the above-mentioned objective, this invention provides a method for preparing ultrapure stainless steel by ladle refining combined with vacuum arc remelting, comprising the following steps: Step 1: The steelmaking raw materials are smelted through EAF to obtain crude molten steel; Step 2: The crude molten steel is transferred to an AOD furnace for oxygen refining, which consists of three stages: (1) Main decarbonization period: The target carbon content is 0.2%, the gas ratio is O2:Ar=4:1~3:1, and the blowing time is determined according to Equation 1: Formula 1; (2) Deceleration and decarburization period: The target carbon content is 0.06%~0.08%, the gas ratio is O2:Ar=2:1~1:1, and the blowing time is determined according to formula 2: Formula 2; (3) Refining and decarburization period: The target carbon content is 0.02%, the gas ratio is O2:Ar=1:2~1:3, and the blowing time is determined according to formula 3: Formula 3; In formulas 1, 2, and 3, C0 represents the initial carbon content during the main decarburization period; t1 represents the blowing time during the main decarburization period (in minutes); t2 represents the blowing time during the deceleration decarburization period (in minutes); t3 represents the blowing time during the refining decarburization period (in minutes); m represents the total mass of molten steel in the AOD furnace (in kg); η1 represents the oxygen refining efficiency during the main decarburization period; η2 represents the oxygen refining efficiency during the deceleration decarburization period; and η3 represents the oxygen refining efficiency during the refining decarburization period. Specifically, η1, η2, and η3 are determined based on the furnace conditions and the ratio of oxygen to argon introduced. The oxygen blowing rate is determined by the ratio of oxygen to argon introduced, and the unit is Nm³. 3 / min; Step 3: After oxygen blowing refining, pre-reducing refining slag is added for pre-reduction treatment; Step 4: After the pre-reduction treatment, remove the pre-reduction refining slag floating on the surface of the molten steel, and add reducing refining slag for reduction treatment; Step 5: After adjusting the composition, the molten steel with the acceptable composition is introduced into the ladle and transferred to the VD furnace for degassing treatment. The soft blowing rate is determined by Equation 4: Equation 4; In formula 4, Q soft The soft blowing rate is expressed in L / min. d p ρ is the diameter of the soft-blow air inlet, in meters (m); l This refers to the density of molten steel, expressed in kg / m³. 3 ;ρ g This refers to the density of the soft-blown gas, expressed in kg / m³. 3 g is the acceleration due to gravity, with units of kg / m. 3 ; h slag denoted as slag layer height in meters (m); r is the surface tension coefficient, typically taken as 0.3 to 0.5.

[0007] Step 6: After degassing, the molten steel with the required composition is introduced into the ladle, transported to the casting area, poured into the mold, and solidified and cooled to obtain the ingot. Step 7: Forge the ingot into electrodes, controlling the filling ratio to be 0.3~0.6; remelt the electrodes using vacuum arc remelting, controlling the vacuum degree to be ≤10. -3 Pa; When the solidification range of the smelting alloy is T L - T S At ≤60℃, the melting rate is determined by Equation 5: Formula 5; When the solidification range of the smelting alloy is T L - T S When the temperature is >60℃, the melting rate is determined by Equation 6: Formula 6; In formulas 5 and 6, T L The liquidus temperature of the steel grade is expressed in °C. T S The solidus temperature of the steel grade is expressed in °C. vD is the melting rate of the vacuum arc remelting consumable electrode, in kg / min; D is the diameter of the vacuum arc remelting crystallizer, in mm. Step 8: After vacuum arc remelting, multi-stage current reduction and shrinkage are carried out during the refining capping period to obtain ultrapure stainless steel.

[0008] Furthermore, the impurity element content in the preparation of ultrapure stainless steel is as follows: Al≤0.0035%, O≤0.0008%, S≤0.0005%, N≤0.005%, H≤0.0001%; ​​inclusion rating: coarse inclusions are grade 0, and fine inclusions of type A+B+C+D are ≤0.5.

[0009] Furthermore, in step 3, the basicity of the pre-reduced refining slag does not exceed 2.

[0010] Furthermore, in step 3, the pre-reduction refining slag is a CaO-CaF2-SiO2-MgO slag system, with the following composition by mass percentage: CaF2 10%–20%, CaO 35%–45%, MgO 3%–7%, SiO2 25%–35%, and the balance being impurities with an impurity content ≤1%.

[0011] Furthermore, in step 4, the basicity of the reducing refining residue is not less than 3.

[0012] Furthermore, in step 4, the reducing refining slag is a CaO-CaF2-SiO2-MgO slag system, with the following composition by mass percentage: CaF2 10%–20%, CaO 45%–55%, MgO 3%–7%, SiO2 12%–20%, and the balance being impurities with an impurity content ≤1%.

[0013] Furthermore, in step 5, during the vacuum degassing process in the VD furnace, the vacuum degree is 50 Pa to 100 Pa, and the degassing time is 15 min to 30 min.

[0014] Furthermore, in step 6, the oxygen content of the ingot is ≤0.002%.

[0015] Furthermore, in step 7, the inlet water temperature of the vacuum arc remelting cleaner is 25℃~35℃, and the outlet water temperature does not exceed 55℃.

[0016] Furthermore, in step 8, the multi-stage current reduction compensation time is 30 min to 50 min, and the compensation cycle is 4 to 6 times.

[0017] Compared with the prior art, the beneficial effects of the present invention are as follows: 1. The present invention provides a method for preparing ultrapure stainless steel using a duplex smelting process of ladle refining (AOD+VD) + vacuum arc remelting (VAR), which enables controlled aluminum production and deep removal of oxygen and sulfur, as well as effective modification of inclusions in 316L austenitic stainless steel. The prepared ultrapure 316L stainless steel has extremely high purity: Al≤0.0035%, O≤0.0008%, S≤0.0005%, N≤0.005%, H≤0.0001%. Inclusion rating: coarse inclusions are grade 0, and fine inclusions (A+B+C+D) are ≤0.5.

[0018] 2. Step 2 of this invention specifically divides the oxygen-argon mixing stage of oxygen refining into three blowing stages. By matching and limiting the oxygen-argon ratio and oxygen blowing time in different blowing stages, the decarburization efficiency is maximized and the oxidation of chromium is suppressed, achieving "decarburization and chromium preservation". Precise control of steel temperature and composition shortens the smelting cycle and reduces costs such as ferrosilicon and electricity consumption. Through the oxidation reaction of oxygen with carbon, silicon, manganese, and other elements in the steel, excess carbon and easily oxidized elements are rapidly removed, providing a low-impurity foundation for subsequent refining and improving cleanliness.

[0019] 3. Step 5 of this invention clarifies the soft blowing rate of the VD process, maximizing the removal of minute inclusions in the molten steel, homogenizing the composition and temperature, and ensuring that inclusions that have floated to the slag-steel interface during vacuum treatment are fully adsorbed by the slag layer, while avoiding violent turbulence that could lead to secondary oxidation or slag entrapment in the molten steel. A reasonable soft blowing time (usually 10-20 minutes, depending on the steel grade) can significantly reduce the total oxygen content (T[O]) and inclusion rating in the steel, improve the purity of the molten steel, homogenize the composition and temperature of the molten steel, and reduce segregation.

[0020] 4. Vacuum arc remelting (VAR) can efficiently remove harmful impurities and gases such as oxygen and sulfur from molten steel, reduce the number and size of inclusions, significantly improve the uniformity and density of material composition, refine solidified grains, and optimize microstructure, thereby effectively improving the mechanical properties, corrosion resistance and other key performance characteristics of metallic materials.

[0021] 5. The ultrapure stainless steel prepared using the method of this invention can meet the ultrapure requirements of key stainless steel materials in fields such as semiconductor equipment and nuclear power, and can thus be applied to key components of semiconductor equipment (such as high-purity gas transmission pipelines and joints) and nuclear power equipment (such as main pipelines, magnet support plates, and bolts). This method not only provides key technical support for the industrial mass production of high-end ultrapure stainless steel, but also effectively meets the urgent needs of aerospace, high-end equipment manufacturing and other fields for high-performance steel. Attached Figure Description

[0022] Figure 1 Metallographic image of ultrapure stainless steel prepared in Example 1 of this invention; Figure 2Metallographic image of ultrapure stainless steel prepared in Example 2 of this invention; Figure 3 Metallographic image of ultrapure stainless steel prepared in Example 3 of this invention; Figure 4 Metallographic image of ultrapure stainless steel prepared in Example 4 of this invention; Figure 5 Metallographic image of stainless steel prepared in Comparative Example 1 of this invention; Figure 6 This is a metallographic image of the stainless steel prepared in Comparative Example 2 of the present invention; Figure 7 Metallographic image of stainless steel prepared in Comparative Example 3 of this invention; Figure 8 The image shows the metallographic image of the stainless steel prepared in Comparative Example 4 of this invention. Figure 9 This is a metallographic image of the stainless steel prepared in Comparative Example 5 of the present invention; Figure 10 The images show typical inclusion morphologies and elemental distributions in the ultrapure stainless steel prepared in Example 1 of this invention (Figure a shows MgO inclusions and Figure b shows CaO inclusions). Detailed Implementation

[0023] This embodiment takes an ultrapure 316L austenitic stainless steel as an example, which, by mass percentage, includes the following elements: C ≤ 0.02%; Cr 16.00%~18.00%; more preferably 16.50%~17.30%; Ni 14.00%~16.00%; more preferably 11.50%~12.50%; Mo 2.00%~3.00%; more preferably 2.40%~2.60%. N ≤ 0.005%; Si 0.30%~0.75%; more preferably 0.50%~0.60%; Mn ≤ 0.50%; P ≤ 0.003%; S ≤ 0.0005%; Al ≤ 0.0035%; O ≤ 0.0008%; H ≤ 0.0001%; ​​the remainder is Fe; the coarse inclusions in ultrapure 316L austenitic stainless steel are grade 0, and the fine inclusions (A+B+C+D) are ≤ 0.5 grade. In this invention, Al, P, S, and O are impurity elements, and the lower the content, the better.

[0024] The coarse inclusions in the above-mentioned ultrapure 316L austenitic stainless steel are grade 0, and the fine inclusions (A+B+C+D) are ≤ grade 0.5.

[0025] This invention provides a method for preparing the above-mentioned ultrapure 316L austenitic stainless steel, comprising the following steps: Step 1: The steelmaking raw materials are smelted through EAF to obtain crude molten steel.

[0026] Step 2: The crude molten steel is transferred to an AOD furnace for oxygen refining, which consists of three stages: (1) Main decarbonization period: The target carbon content is 0.2%, the gas ratio is O2:Ar=4:1~3:1, and the blowing time is determined according to Equation 1: Formula 1; (2) Deceleration and decarburization period: The target carbon content is 0.06%~0.08%, the gas ratio is O2:Ar=2:1~1:1, and the blowing time is determined according to formula 2: Formula 2; (3) Refining and decarburization period: The target carbon content is 0.02%, the gas ratio is O2:Ar=1:2~1:3, and the blowing time is determined according to formula 3: Formula 3; In the stainless steel smelting process, formulas 1, 2, and 3 above define the blowing time for each stage based on the total mass of molten steel, oxygen refining efficiency, and oxygen blowing rate. The main function of the AOD process is decarburization, with the key being "decarburization and chromium preservation." By matching and limiting the oxygen-argon ratio and oxygen blowing time at different blowing stages, decarburization efficiency can be maximized, chromium oxidation can be suppressed, steel temperature and composition can be precisely controlled, the smelting cycle can be shortened, and costs such as ferrosilicon and electricity consumption can be reduced. In addition, it can also improve the purity of molten steel (e.g., reduce oxygen content to 50ppm-80ppm and sulfur content to below 10ppm), reduce composition fluctuations, thereby ensuring quality stability and slowing down the erosion of the furnace lining by high temperatures.

[0027] Furthermore, after oxygen blowing and decarburization in step 2, a sample of molten steel can be taken for composition analysis. Based on the analysis results, steelmaking raw materials (such as Cr, Si, Mn, etc.) can be added to adjust the composition.

[0028] Step 3: After oxygen blowing refining, pre-reduction refining slag is added for pre-reduction treatment. The pre-reduction refining slag is a CaO-CaF2-SiO2-MgO slag system. The composition, by mass percentage, is 10%-20% CaF2, 35%-45% CaO, 3%-7% MgO, 25%-35% SiO2, and the balance is impurities with an impurity content ≤1%. The basicity of the slag system does not exceed 2, and the preferred basicity of the slag system is 1.2-2.

[0029] Step 4: After the pre-reduction treatment, remove the pre-reducing refining slag floating on the surface of the molten steel, and add reducing refining slag for reduction treatment. The reducing refining slag is a CaO-CaF2-SiO2-MgO slag system. The composition, by mass percentage, is 10%-20% CaF2, 45%-55% CaO, 3%-7% MgO, 12%-20% SiO2, and the balance is impurities with an impurity content ≤1%. The basicity is greater than 3, preferably 3-5.

[0030] Furthermore, after step 4, a sample of molten steel can be taken for composition analysis, and steelmaking raw materials (such as Cr, Si, Mn, etc.) can be added again to adjust the composition based on the analysis results.

[0031] Step 5: Introduce the molten steel with the required composition into the ladle, transfer it to the VD furnace, and perform degassing treatment. The vacuum degree is 50 Pa to 100 Pa, the degassing time is 15 min to 30 min, and the soft blowing rate is determined by Equation 4. Equation 4; In Formula 4, the surface tension coefficient r is 0.3 in this embodiment; g is 9.81 kg / m³. 3 .

[0032] Formula 4 above clarifies the soft blowing rate of the VD process, thereby maximizing the removal of minute inclusions in the molten steel, homogenizing composition and temperature, and ensuring that inclusions that have floated to the slag-steel interface during vacuum treatment are fully adsorbed by the slag layer, while avoiding violent turbulence that could lead to secondary oxidation of the molten steel or slag entrapment. A reasonable soft blowing time (usually 10-20 minutes, depending on the steel grade) can significantly reduce the total oxygen content (T[O]) and inclusion rating in the steel, improve the purity of the molten steel, homogenize the composition and temperature of the molten steel, and reduce segregation. In addition, precise control of the soft blowing time is also beneficial for stabilizing the production rhythm, reducing energy consumption, and avoiding insufficient purification due to excessively short soft blowing or excessively long soft blowing that could lead to excessive temperature drop and affect casting.

[0033] Step 6: After degassing, the molten steel with the required composition is introduced into the ladle, transported to the casting area, poured into the mold, and solidified and cooled to obtain the ingot. The oxygen content of the ingot is ≤0.002%.

[0034] Step 7: Forge the ingot into electrodes, controlling the filling ratio to be 0.3~0.6; remelt the electrodes using vacuum arc remelting, controlling the vacuum degree to be ≤10. -3 Pa; The determination of the melting rate is related to the solid-liquid phase temperature of the steel grade and the diameter of the crystallizer; When the smelting alloy (such as 304, 316, etc.) is T L - T S At temperatures ≤60℃, the solidification range is relatively narrow, and the melting rate is determined by Equation 5: Formula 5; When the smelting alloy (such as S32654, Inconel 718, etc.) is T L - T S When the temperature is >60℃, the solidification range is wider, and the melting rate is determined by Equation 6: Formula 6; Formulas 5 and 6 above, in vacuum arc remelting (VAR), clearly define the relationship between steel grade, crystallizer diameter, and optimal melting rate. Precisely matching the melting rate optimizes the shape and depth of the molten pool and the heat transfer conditions at the solidification front, promoting the flotation and separation of non-metallic inclusions and reducing macroscopic inclusions remaining in the ingot. Simultaneously, a suitable melting rate effectively suppresses the segregation of impurity elements (such as sulfur and phosphorus) caused by excessively fast or slow local solidification, preventing the enrichment of impurity elements between dendrites or grain boundaries, and reducing the content of gases such as hydrogen and nitrogen. These constraints not only significantly improve the purity and microstructure uniformity of the molten steel but also reduce the precipitation of harmful phases or the formation of secondary inclusions caused by inappropriate melting rates, ultimately achieving a high degree of consistency in purity, density, and mechanical properties in the prepared ultrapure stainless steel material.

[0035] Step 8: After vacuum arc remelting, multi-stage current reduction and shrinkage are carried out during the refining capping period. The multi-stage current reduction and shrinkage time is 30 min to 50 min, and the shrinkage cycle is 4 to 6 times to obtain ultrapure stainless steel.

[0036] Unless otherwise specified, all raw materials and equipment used in this invention are commercially available.

[0037] The ultrapure 316L austenitic stainless steel provided by the present invention will be described in detail below with reference to Examples 1 to 4. However, the following examples should not be construed as limiting the scope of protection of the present invention.

[0038] The smelting equipment in the following embodiments is a 3t argon-oxygen refining furnace with a total mass of molten steel of 2.6t~2.8t, a 3t vacuum degassing furnace, a 3t vacuum electric arc remelting furnace with an electrode mass of 2.6t~2.8t, and four heats of experimental steel were specifically implemented, corresponding to Examples 1 to 4 respectively.

[0039] Table 1. Impurity composition of raw materials used in smelting in Examples 1 to 4

[0040] Based on the above preparation method, the specific AOD smelting parameters for Examples 1 to 4 are shown in Table 2.

[0041] Table 2. Smelting parameters for preparing stainless steel in Examples 1 to 4

[0042] The compositions of the pre-reduction period slag and the reduction period slag in Examples 1 to 4 are shown in Tables 3 and 4.

[0043] Table 3. Slag composition during the AOD pre-reduction period of Examples 1 to 4

[0044] Table 4. Slag composition during the AOD reduction period in Examples 1 to 4

[0045] The specific VD smelting parameters for Examples 1 to 4 are shown in Table 5.

[0046] Table 5. VD smelting parameters for the preparation of 316L austenitic stainless steel in Examples 1 to 4.

[0047] Since Examples 1 to 4 are for preparing 316L austenitic stainless steel, the solidification range is relatively narrow. Therefore, the melting rate is determined by Equation 5 above, i.e., the melting rate is controlled as follows: .

[0048] Table 6. VAR smelting parameters for the preparation of 316L austenitic stainless steel in Examples 1 to 4

[0049] The impurity element content of the ultrapure 316L austenitic stainless steel obtained by the above method is shown in Table 7. It can be seen that after the process of preparing ultrapure stainless steel by ladle refining combined with vacuum arc remelting, the deoxidation and desulfurization effects are very obvious, and the impurity elements are well controlled.

[0050] Comparative Example 1, based on Example 1, at the AOD station, the main decarburization period blowing time was 50 min, the deceleration decarburization period blowing time was 35 min, and the refining decarburization period blowing time was 20 min; other steps and parameters were the same.

[0051] Comparative Example 2, based on Example 1, had a soft blowing rate of 100 L / min at the VD station, with other steps and parameters being the same.

[0052] Comparative Example 3, based on Example 1, was carried out at a vacuum arc remelting station with a melting speed of 4.5 kg / min, and other steps and parameters were the same.

[0053] Comparative Example 4, based on Example 1, adjusts the composition of the pre-reduction refining slag to a commonly used slag system, specifically including: CaO 48%, CaF2 10%, Al2O3 25%, SiO2 12%, MgO 5%, with other steps and parameters remaining the same.

[0054] Comparative Example 5, based on Example 1, adjusts the reduction to refine the slag into a commonly used slag system, specifically including: CaO 35%, CaF2 10%, Al2O3 25%, SiO2 25%, MgO 5%, with other steps and parameters remaining the same.

[0055] Table 7. Impurity element content of steels in Examples 1 to 4 and Comparative Examples 1 to 5

[0056] The inclusion ratings of the ultrapure stainless steel prepared in the above embodiments are shown in Table 8. It can be seen that the coarse inclusions in the steel of the embodiments are all grade 0, and the fine inclusions (A+B+C+D categories) are all ≤0.5.

[0057] Table 8. Inclusion grades of stainless steels prepared in Examples 1 to 4 and Comparative Examples 1 to 5

[0058] In summary, as shown in Tables 7 and 8, the test results of impurity element content and inclusion ratings for Examples 1 to 4 and Comparative Examples 1 to 5 are listed. Specifically, Comparative Example 1, due to the lack of control over AOD blowing time, resulted in an increase in O content to 0.0011%, leading to a worse inclusion rating; Comparative Example 2, due to the lack of control over VD soft blowing rate, resulted in an increase in O content to 0.0010%, and the inclusions became larger due to slag entrainment; Comparative Example 3, due to the lack of control over VAR melting rate, resulted in an increase in O content to 0.0011%, leading to a worse inclusion rating; Comparative Example 4, due to the lack of control over the composition of AOD pre-reduction refining slag, resulted in an increase in Al content to 0.011% and O content to 0.0013%, leading to a worse inclusion rating; Comparative Example 5, due to the lack of control over the composition of AOD reduction refining slag, resulted in an increase in Al content to 0.021% and O content to 0.0016%, leading to a worse inclusion rating.

[0059] according to Figures 1 to 4 As shown in the metallographic images of the ultrapure stainless steel prepared in Examples 1 to 4, it can be clearly seen that there are no obvious inclusions in the steel under the metallographic microscope, which is consistent with the test results in Table 8. Figures 5 to 9 The metallographic images of stainless steel prepared for comparative examples 1 to 5 show obvious inclusions in the field of view, and the number of inclusions is greater than that of the examples, and the size of the inclusions is larger than that of the examples, which is consistent with the test results in Table 8. Figure 10 The images show typical inclusion morphology and elemental distribution in the ultrapure stainless steel prepared in Example 1. Figure a and the two images to its right show MgO inclusions, while Figure b and the two images to its right show CaO inclusions. It can be seen that the inclusion size is about 1 μm and the shape is spherical, with excellent control effect.

[0060] In summary, this invention employs a method for preparing ultrapure stainless steel using ladle refining combined with vacuum arc remelting, achieving control over the purity of the stainless steel as well as good control over its microstructure and properties.

[0061] 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 preparing ultrapure stainless steel by ladle refining combined with vacuum arc remelting, characterized in that, Includes the following steps: Step 1: The steelmaking raw materials are smelted through EAF to obtain crude molten steel; Step 2: The crude molten steel is transferred to an AOD furnace for oxygen blowing refining, which is divided into three blowing stages: (1) Main decarbonization period: The target carbon content is 0.2%, the gas ratio is O2:Ar=4:1~3:1, and the blowing time is determined according to Equation 1: Formula 1; (2) Deceleration and decarburization period: The target carbon content is 0.06%~0.08%, the gas ratio is O2:Ar=2:1~1:1, and the blowing time is determined according to formula 2: Formula 2; (3) Refining and decarburization period: The target carbon content is 0.02%, the gas ratio is O2:Ar=1:2~1:3, and the blowing time is determined according to formula 3: Formula 3; In Equations 1, 2, and 3, C0 represents the initial carbon content during the main decarburization period; t1 represents the blowing time during the main decarburization period (in minutes); t2 represents the blowing time during the deceleration decarburization period (in minutes); t3 represents the blowing time during the refining decarburization period (in minutes); m represents the total mass of molten steel in the AOD furnace (in kg); η1 represents the oxygen refining efficiency during the main decarburization period; η2 represents the oxygen refining efficiency during the deceleration decarburization period; and η3 represents the oxygen refining efficiency during the refining decarburization period. Oxygen blowing rate, unit: Nm 3 / min; Step 3: After oxygen blowing refining, pre-reduction refining slag is added for pre-reduction treatment. The pre-reduction refining slag is a CaO-CaF2-SiO2-MgO slag system. Step 4: After the pre-reduction treatment, remove the pre-reducing refining slag floating on the surface of the molten steel, and add reducing refining slag for reduction treatment. The reducing refining slag is a CaO-CaF2-SiO2-MgO slag system. Step 5: The reduced-treated molten steel with the required composition is introduced into the ladle and transferred into the VD furnace for degassing. The soft blowing rate is determined by Equation 4. Equation 4; In Equation 4, Q soft The soft blowing rate is expressed in L / min. d p is the diameter of the soft-blow air inlet, in meters (m). ρ l This refers to the density of molten steel, expressed in kg / m³. 3 ; ρ g This refers to the density of the soft-blown gas, expressed in kg / m³. 3 g is the acceleration due to gravity, with units of kg / m. 3 ; h slag is the slag layer height in meters; r is the surface tension coefficient. Step 6: After degassing, the molten steel with the required composition is introduced into the ladle, transported to the casting area, poured into the mold, and solidified and cooled to obtain the ingot. Step 7: Forge the ingot into an electrode, controlling the filling ratio to be 0.3~0.6; remelt the electrode using vacuum arc remelting, controlling the vacuum degree to be ≤10. -3 Pa; When the solidification range of the smelting alloy is T L -T S At ≤60℃, the melting rate is determined by Equation 5: Formula 5; When the solidification range of the smelting alloy is TL - TS When the temperature is >60℃, the melting rate is determined by Equation 6: Formula 6; In equations 5 and 6, T L The liquidus temperature of the steel grade is expressed in °C. T S The solidus temperature of the steel grade is expressed in °C. v D is the melting rate of the vacuum arc remelting consumable electrode, in kg / min; D is the diameter of the vacuum arc remelting crystallizer, in mm. Step 8: After vacuum arc remelting, multi-stage current reduction and shrinkage are carried out during the refining capping period to obtain ultrapure stainless steel.

2. The method for preparing ultrapure stainless steel by ladle refining combined with vacuum arc remelting according to claim 1, characterized in that, The impurity element content in the preparation of ultrapure stainless steel is as follows: Al≤0.0035%, O≤0.0008%, S≤0.0005%, N≤0.005%, H≤0.0001%; Inclusion rating: coarse inclusions are grade 0, and fine inclusions (A+B+C+D) are grade ≤ 0.

5.

3. The method for preparing ultrapure stainless steel by ladle refining combined with vacuum arc remelting according to claim 1, characterized in that, In step 3, the basicity of the pre-reduced refining slag does not exceed 2; the composition of the pre-reduced refining slag, by mass percentage, is 10%–20% CaF2, 35%–45% CaO, 3%–7% MgO, 25%–35% SiO2, with the remainder being impurities, and the impurity content is ≤1%.

4. The method for preparing ultrapure stainless steel by ladle refining combined with vacuum arc remelting according to claim 1, characterized in that, In step 4, the basicity of the reducing refining slag is not less than 3; the reducing refining slag is a CaO-CaF2-SiO2-MgO slag system, and the composition by mass percentage is: CaF2 10%-20%, CaO 45%-55%, MgO 3%-7%, SiO2 12%-20%, with the balance being impurities, and the impurity content ≤1%.

5. The method for preparing ultrapure stainless steel by ladle refining combined with vacuum arc remelting according to claim 1, characterized in that, In step 5, the vacuum degassing process of the VD furnace has a vacuum degree of 50 Pa to 100 Pa and a degassing time of 15 min to 30 min.

6. The method for preparing ultrapure stainless steel by ladle refining combined with vacuum arc remelting according to claim 1, characterized in that, In step 6, the oxygen content of the ingot is ≤0.002%.

7. The method for preparing ultrapure stainless steel by ladle refining combined with vacuum arc remelting according to claim 1, characterized in that, In step 7, the inlet water temperature for the vacuum arc remelting is 25°C to 35°C, and the outlet water temperature does not exceed 55°C.

8. The method for preparing ultrapure stainless steel by ladle refining combined with vacuum arc remelting according to claim 1, characterized in that, In step 8, the multi-stage current reduction compensation time is 30-50 minutes, and the compensation cycle is 4-6 times.

Images