A method for producing high purity molten steel

Abstract

The application discloses a preparation method of high-purity molten steel, which comprises the following steps: controlling the oxygen content of molten steel to be less than or equal to 500 ppm after converter blowing is finished; adding metal aluminum into the molten steel during the molten steel discharging process; controlling the acid-soluble aluminum content in the molten steel to be 0.020%-0.040% after deoxidation; controlling the refining slag basicity to be 3.5-5.0 and the slag quantity to be 5-8 kg per ton of steel; adding calcium wire into the molten steel during the refining process; controlling the calcium content in the molten steel to be 5-10 ppm after calcium treatment; then performing 5-10 min weak argon blowing stirring; performing RH vacuum treatment on the molten steel after calcium treatment, wherein the circulating gas is argon, and the flow rate is 1300-1450 L / min; after the RH vacuum treatment, performing static stirring treatment on the molten steel, adopting argon soft blowing, and finally statically placing the molten steel to obtain high-purity molten steel. The application has the advantages that the quantity of inclusions in the molten steel is effectively controlled, the purity of the molten steel is improved, and stable production of high-purity molten steel with the inclusion quantity less than 10 / mm 2 is realized.

Description

Technical Field

[0001] This invention belongs to the field of iron and steel metallurgy technology, and particularly relates to a method for preparing high-purity molten steel. Background Technology

[0002] As the steel industry develops towards high-end and refined production, aerospace, automobile manufacturing, semiconductor equipment, and high-end bearings are placing increasingly higher demands on the purity of steel. Extensive metallurgical research and production practice have shown that non-metallic inclusions in molten steel are a key factor restricting the improvement of steel performance. These inclusions mainly originate from deoxidation reaction products, slag entrainment, and refractory material erosion, and their quantity, size, morphology, and distribution directly affect the plasticity, toughness, fatigue strength, and cold working properties of the steel. When the number of inclusions in molten steel exceeds a certain threshold, the steel's pass rate drops significantly, making it difficult to guarantee its reliability.

[0003] However, existing processes generally suffer from problems such as complex procedures, insufficient deoxidation, poor inclusion removal, and difficulty in controlling process parameters, making it difficult to consistently achieve the purity of molten steel that meets the production standards for high-end steel. Specifically, these include: In the deoxidation process, some processes use a single deoxidizer or deoxidation method, such as using only aluminum for final deoxidation. Although this can quickly reduce the oxygen content in the molten steel, the Al2O3 inclusions generated have a high melting point and small particle size, making them difficult to remove from the molten steel by natural floating. A large number of fine inclusions remain in the molten steel, becoming a potential problem for the subsequent performance of the steel.

[0004] In the refining slag-forming process, although some processes employ refining slag-forming technology, the control of key parameters such as slag basicity, oxidizing properties, and fluidity is unreasonable. When the slag basicity is too low, the slag's adsorption capacity for acidic inclusions is insufficient; when the basicity is too high, the slag's fluidity deteriorates, preventing it from fully contacting the molten steel, resulting in low inclusion removal efficiency. Furthermore, some processes do not fully consider the interfacial reaction between slag and molten steel during slag-forming, failing to effectively promote the aggregation and growth of inclusions, further reducing the inclusion removal effect.

[0005] In the control of inclusion morphology, some processes lack an effective synergistic mechanism between calcium treatment and vacuum refining. While calcium treatment can modify high-melting-point Al2O3 inclusions into low-melting-point calcium aluminates and improve their morphology, improper control of the timing of calcium treatment, the amount of calcium added, and the wire feeding speed may lead to the formation of high-melting-point CaS inclusions, which would worsen the purity of the molten steel. Simultaneously, during vacuum refining, if parameters such as vacuum level, evacuation time, and stirring intensity are not properly controlled, it is impossible to effectively remove gases and inclusions from the molten steel, and it is also difficult to achieve sufficient modification and flotation removal of inclusions.

[0006] Currently, the number of inclusions in molten steel produced by existing processes often exceeds 10 per mm.2 Furthermore, the inclusions are mostly irregularly shaped and angular, which seriously affects the overall performance of the steel and makes it impossible to meet the stringent requirements of high-end fields such as aerospace and automobile manufacturing. Summary of the Invention

[0007] The purpose of this invention is to address the problems of insufficient purity, excessive inclusions, difficulty in control, and low product reliability in existing molten steel. It provides a method for preparing high-purity molten steel by precisely controlling key parameters in each process step, effectively controlling the number of inclusions in the molten steel, improving its purity, and achieving an inclusion count of less than 10 inclusions / mm. 2 Stable production of high-purity molten steel.

[0008] To achieve the above objectives, the present invention adopts the following technical solution: A method for preparing high-purity molten steel, specifically comprising: (1) Converter tapping control: After the converter blowing is completed, the oxygen content of the tapped steel is controlled to be ≤500ppm. During the tapping process, slag blocking operation is adopted to prevent converter slag from entering the ladle and avoid secondary oxidation of molten steel. The tapping temperature is controlled at 1620~1680℃ and the tapping speed is controlled at 3~5t / min to ensure that the molten steel flows into the ladle smoothly.

[0009] (2) Fully deoxidizing aluminum: During the tapping process, metallic aluminum is added to the molten steel for full deoxidation. The amount of metallic aluminum added is 1.2~1.8 kg / ton of steel. The addition method is to add it evenly with the steel flow to ensure that the aluminum reacts fully with the oxygen in the molten steel. After deoxidation, the acid-soluble aluminum content in the molten steel is controlled at 0.020%~0.040%. The main deoxidation product is Al2O3, which lays the foundation for the subsequent removal of inclusions.

[0010] (3) Refining to produce high-basicity slag: The deoxidized molten steel is transferred to the refining furnace for refining and slag production. The raw materials for slag production are lime, fluorite and bauxite. The amount of lime added is 8~12 kg / ton of steel, the amount of fluorite added is 1~2 kg / ton of steel, and the amount of bauxite added is 2~3 kg / ton of steel. The basicity of the refining slag CaO / SiO2 is controlled at 3.5~5.0, the amount of slag is controlled at 5~8 kg / ton of steel, the refining temperature is controlled at 1580~1620℃, and the refining time is 30~45 min. Through the adsorption effect of high-basicity slag, inclusions such as Al2O3 in the molten steel are removed, and the oxidizing properties of the steel slag are reduced. The FeO+MnO content in the slag is controlled at ≤0.9%.

[0011] (4) Calcium treatment control: During the refining process, calcium wire is added to the molten steel for calcium treatment. The calcium wire used is calcium silicate wire with a calcium content ≥30%. The wire feeding speed is 120~150m / min, and the feeding amount is adjusted according to the composition of the molten steel to ensure that the calcium content in the molten steel is stable at 5~10ppm after calcium treatment. The role of calcium is to transform the high melting point and brittle Al2O3 inclusions in the molten steel into low melting point and spherical calcium aluminate composite inclusions, so as to avoid defects caused by inclusions in subsequent processing. At the same time, it promotes the flotation and removal of inclusions. After calcium treatment, weak argon blowing and stirring are performed for 5~10min to ensure that the calcium is evenly distributed in the molten steel and the reaction is sufficient.

[0012] (5) RH Vacuum Treatment: The calcium-treated molten steel is transferred to the RH vacuum refining unit for vacuum treatment. The vacuum degree is controlled at ≤40Pa, and the vacuum treatment time is 15~25min. Argon is used as the circulating gas, and the flow rate is controlled at 1300~1450L / min. The insertion depth of the immersion tube is 150~200mm. Through RH vacuum treatment, gaseous elements such as hydrogen and nitrogen in the molten steel are efficiently removed. At the same time, residual inclusions in the molten steel are promoted to float, further improving the purity of the molten steel. During the vacuum treatment process, the temperature drop of the molten steel is controlled to not exceed 30℃.

[0013] (6) Static stirring treatment: After the RH vacuum treatment, the molten steel is subjected to static stirring treatment. The static stirring is carried out by argon soft blowing, with the argon flow rate controlled at 30~50L / min and the static stirring time being 15~25min. During the static stirring process, the surface of the molten steel is kept covered, and sufficient ladle covering agent is added to prevent air from entering and causing secondary oxidation of the molten steel. Through static stirring, the remaining fine inclusions in the molten steel are fully floated to the steel slag and adsorbed and removed. At the same time, the composition and temperature of the molten steel are homogenized. After the static stirring is completed, the molten steel is allowed to stand for 5~10min to obtain high-purity molten steel.

[0014] Furthermore, in step (1), the molten steel needs to be sampled and tested before tapping from the converter to ensure that the oxygen content of the tapped steel is ≤500ppm. If the oxygen content exceeds the standard, supplementary blowing is required. The supplementary blowing time is controlled at 1~2 minutes to avoid excessive blowing that causes fluctuations in the composition of the molten steel.

[0015] Furthermore, in step (2), the aluminum metal is aluminum granules with a purity of ≥99.5% and a particle size of 5~10mm, which are added evenly with the steel flow. When adding, avoid concentrated addition to prevent excessive local deoxidation and the generation of bubble defects, which would affect the quality of the molten steel.

[0016] Furthermore, in step (3), weak stirring is used in the refining and slag-making process, and the argon flow rate is controlled at 80~120L / min to avoid secondary oxidation of molten steel caused by strong stirring. At the same time, it promotes the full reaction of slag and steel and improves the removal efficiency of inclusions. During the refining process, the composition of molten steel and slag system are sampled and tested regularly, and the amount of slag-making raw materials added is adjusted in a timely manner.

[0017] Furthermore, in step (6), after static stirring, the molten steel is inspected for inclusions, and the Aspex index is used to count the number of inclusions to ensure that the number of inclusions in the molten steel is less than 10 per mm. 2 .

[0018] Compared with the prior art, the advantages of the technical solution of the present invention are as follows: (1) The present invention precisely controls the oxygen content of the converter steel to ≤500ppm, reducing the oxygen content in the molten steel from the source, laying the foundation for subsequent deoxidation and inclusion removal, and avoiding insufficient deoxidation and increased inclusions due to excessive initial oxygen content. (2) This invention uses aluminum to fully deoxidize and combines it with high-basicity slag refining to achieve deep removal of oxygen in molten steel and effective adsorption of Al2O3 inclusions. High-basicity slag can fully adsorb oxide inclusions in molten steel, while reducing the oxidizing properties of steel slag and reducing the secondary generation of inclusions. Referring to existing high-basicity slag refining technology, the slag system ratio and process parameters have been further optimized to improve the inclusion removal effect. (3) This invention controls the calcium content of molten steel to 5-10 ppm through calcium treatment, transforming high-melting-point Al2O3 inclusions into low-melting-point calcium aluminate composite inclusions, improving the morphology of inclusions, and promoting the flotation of inclusions. Combined with RH vacuum treatment and static stirring process, deep removal of inclusions is achieved, ultimately reducing the number of inclusions in molten steel to less than 10 per mm. 2 The purity of molten steel has been significantly improved, meeting the requirements for the production of high-end steel products; (4) The process flow of the present invention is reasonable, the parameters of each link are controllable, the operation is simple, no new complex equipment is required, the production cost is low, and the stable batch production of high-purity molten steel can be achieved. Detailed Implementation Example 1

[0019] To make the present invention clearer, a method for preparing high-purity molten steel according to the present invention is further described below. The specific embodiments described herein are only for explaining the present invention and are not intended to limit the present invention.

[0020] This embodiment provides a method for preparing high-purity molten steel, including the following steps: S1, Converter tapping control: After the converter blowing is completed, the oxygen content of the molten steel is sampled and tested to be 420ppm, which meets the requirement of ≤500ppm. During the tapping process, slag blocking operation is adopted, the tapping temperature is controlled at 1650℃, the tapping speed is 4t / min, and the molten steel flows into the ladle steadily. S2, Aluminum fully deoxidized: During the tapping process, aluminum particles with a purity of 99.6% and a particle size of 5~8mm are uniformly added with the steel stream at a rate of 1.5kg / ton of steel. After deoxidation, the acid-soluble aluminum content in the molten steel is tested to be 0.030%, indicating that deoxidation is sufficient. S3, refining to produce high-basicity slag: The deoxidized molten steel is transferred to the refining furnace, and 10 kg / ton of lime, 1.5 kg / ton of fluorite, and 2.5 kg / ton of bauxite are added. The basicity of the refining slag is controlled at 4.2, the slag quantity is 6.5 kg / ton of steel, the refining temperature is controlled at 1600℃, and the refining time is 38 min. Argon gas is used for weak stirring during the refining process at a flow rate of 100 L / min. Samples are taken regularly for testing, and the slag composition is adjusted. After refining, the FeO+MnO content in the slag is tested to be 0.7%. S4, Calcium treatment control: During the refining process, silicon-calcium wire with a calcium content of 32% is fed into the molten steel at a feeding speed of 130m / min. The feeding amount is adjusted according to the composition of the molten steel. After calcium treatment, the calcium content in the molten steel is tested to be 7ppm. After calcium treatment, weak argon blowing and stirring are performed for 8 minutes to ensure uniform calcium distribution. S5, RH vacuum treatment: The calcium-treated molten steel is transferred to the RH vacuum refining unit, the vacuum degree is controlled at 39Pa, the vacuum treatment time is 20min, the circulating argon flow rate is 1350L / min, the immersion tube insertion depth is 180mm, and the temperature of the molten steel drops by 25℃ during the vacuum treatment. S6, Static Stirring Treatment: After RH vacuum treatment, static stirring is performed using argon gas soft blowing at a flow rate of 40 L / min for 20 min. The molten steel surface is kept covered during the stirring process. After stirring, the molten steel is allowed to stand for 8 min, and samples are taken to check for inclusions; the number is 7 inclusions / mm. 2 The requirement is less than 10 pieces / mm. 2 To meet the requirements of obtaining high-purity molten steel. Example 2

[0021] This embodiment provides a method for preparing high-purity molten steel, including the following steps: S1, Converter tapping control: After the converter blowing is completed, the oxygen content of the molten steel is sampled and tested to be 480ppm, which meets the requirement of ≤500ppm. During the tapping process, slag blocking operation is adopted, the tapping temperature is controlled at 1630℃, the tapping speed is 3.5t / min, and the molten steel flows into the ladle steadily. S2, Aluminum fully deoxidized: During the tapping process, aluminum particles with a purity of 99.5% and a particle size of 6~10mm are uniformly added with the steel stream at a rate of 1.3kg / ton of steel. After deoxidation, the acid-soluble aluminum content in the molten steel is found to be 0.025%, indicating that deoxidation is sufficient. S3, refining to produce high-basicity slag: The deoxidized molten steel is transferred to the refining furnace, and 8.5 kg / ton of lime, 1.2 kg / ton of fluorite, and 2.2 kg / ton of bauxite are added. The basicity of the refining slag is controlled at 3.8, the slag quantity is 5.8 kg / ton of steel, the refining temperature is controlled at 1590℃, and the refining time is 35 min. Argon gas is used for weak stirring during the refining process at a flow rate of 90 L / min. Samples are taken regularly for testing, and the slag composition is adjusted. After refining, the FeO+MnO content in the slag is tested to be 0.8%. S4, Calcium treatment control: During the refining process, silicon-calcium wire with a calcium content of 30% is fed into the molten steel at a feeding speed of 125 m / min. The feeding amount is adjusted according to the composition of the molten steel. After calcium treatment, the calcium content in the molten steel is tested to be 5.5 ppm. After calcium treatment, weak argon blowing and stirring are performed for 6 minutes to ensure uniform calcium distribution. S5, RH vacuum treatment: The calcium-treated molten steel is transferred to the RH vacuum refining unit, the vacuum degree is controlled at 38Pa, the vacuum treatment time is 18min, the circulating argon flow rate is 1320L / min, the immersion tube insertion depth is 160mm, and the temperature of the molten steel drops by 28℃ during the vacuum treatment. S6, Static Stirring Treatment: After RH vacuum treatment, static stirring is performed using argon gas soft blowing at a flow rate of 35 L / min for 18 min. The molten steel surface is kept covered during stirring. After stirring, the molten steel is allowed to stand for 6 min, and samples are taken to detect inclusions; the number is 8 inclusions / mm. 2 The requirement is less than 10 pieces / mm. 2 To meet the requirements of obtaining high-purity molten steel. Example 3

[0022] This embodiment provides a method for preparing high-purity molten steel, including the following steps: S1, Converter tapping control: After the converter blowing is completed, the oxygen content of the molten steel is sampled and tested to be 390ppm, which meets the requirement of ≤500ppm. Slag blocking operation is adopted during the tapping process, the tapping temperature is controlled at 1670℃, the tapping speed is 4.5t / min, and the molten steel flows into the ladle steadily. S2, Aluminum fully deoxidized: During the tapping process, aluminum particles with a purity of 99.7% and a particle size of 5~9mm are uniformly added with the steel stream at a rate of 1.7kg / ton of steel. After deoxidation, the acid-soluble aluminum content in the molten steel is measured to be 0.038%, indicating that deoxidation is sufficient.

[0023] S3, refining to produce high-basicity slag: The deoxidized molten steel is transferred to the refining furnace, and lime (11 kg / ton of steel), fluorite (1.8 kg / ton of steel), and bauxite (2.8 kg / ton of steel) are added. The basicity of the refining slag is controlled at 4.8, the slag quantity is 7.2 kg / ton of steel, the refining temperature is controlled at 1610℃, and the refining time is 42 min. Argon gas is used for weak stirring during the refining process at a flow rate of 110 L / min. Samples are taken regularly for testing, and the slag composition is adjusted. After refining, the FeO+MnO content in the slag is tested to be 0.6%. S4, Calcium treatment control: During the refining process, silicon-calcium wire with a calcium content of 35% is fed into the molten steel at a feeding speed of 145 m / min. The feeding amount is adjusted according to the composition of the molten steel. After calcium treatment, the calcium content in the molten steel is tested to be 9.2 ppm. After calcium treatment, weak argon blowing and stirring are performed for 9 minutes to ensure uniform calcium distribution. S5, RH vacuum treatment: The calcium-treated molten steel is transferred to the RH vacuum refining unit, the vacuum degree is controlled at 39Pa, the vacuum treatment time is 23min, the circulating argon flow rate is 1380L / min, the immersion tube insertion depth is 190mm, and the temperature of the molten steel drops by 22℃ during the vacuum treatment. S6, Static Stirring Treatment: After RH vacuum treatment, static stirring is performed using argon gas soft blowing at a flow rate of 45 L / min for 23 min. The molten steel surface is kept covered during the stirring process. After stirring, the molten steel is allowed to stand for 9 min, and samples are taken to check for inclusions; the number is 6 inclusions / mm. 2 The requirement is less than 10 pieces / mm. 2 To meet the requirements of obtaining high-purity molten steel.

[0024] To further verify the technical effect of the present invention, a comparative example is also provided: a conventional high-purity steelmaking process is used, the oxygen content of the converter steel is 600 ppm, aluminum is not used for sufficient deoxidation, the slag basicity of refining is 2.8, no calcium treatment is performed, only RH vacuum treatment and static stirring are performed, and other process parameters are the same as in Example 1. The final number of inclusions in the molten steel is 18 / mm. 2 It cannot meet the requirements for high-purity molten steel.

[0025] By comparing Examples 1-3 with the comparative examples, it can be seen that the present invention, through precise control of the oxygen content in the converter tapping steel, aluminum deoxidation, high-basicity slag refining, calcium treatment, RH vacuum treatment, and static stirring process, can effectively reduce the number of inclusions in molten steel and stably obtain an inclusion count of less than 10 per mm. 2 The high-purity molten steel produced is significantly superior to that produced by conventional processes.

[0026] In addition to the embodiments described above, the present invention may have other implementations. All technical solutions formed by equivalent substitution or equivalent transformation fall within the protection scope claimed by the present invention.

Claims

1. A method for preparing high-purity molten steel, characterized in that: (1) Converter tapping control: After the converter blowing is completed, the oxygen content of the tapped steel is controlled to be ≤500ppm. Slag blocking operation is adopted during the tapping process. The tapping temperature is controlled at 1620~1680℃ and the tapping speed is controlled at 3~5t / min. (2) Fully deoxidizing aluminum: During the tapping process, metallic aluminum is added to the molten steel for full deoxidation. The amount of metallic aluminum added is 1.2~1.8 kg / ton of steel. After deoxidation, the acid-soluble aluminum content in the molten steel is controlled at 0.020%~0.040%. (3) Refining to produce high-basicity slag: The deoxidized molten steel is transferred to the refining furnace for refining and slag production. The raw materials for slag production are lime, fluorite and bauxite. The basicity of the refining slag CaO / SiO2 is controlled at 3.5~5.0, the slag amount is controlled at 5~8 kg / ton of steel, the refining temperature is controlled at 1580~1620℃, the refining time is 30~45 min, and the FeO+MnO content in the slag is controlled at ≤0.9%. (4) Calcium treatment control: During the refining process, calcium wire is added to the molten steel for calcium treatment. The calcium content in the molten steel is controlled to be stable at 5~10ppm after calcium treatment. After calcium treatment, weak argon blowing and stirring are performed for 5~10min. (5) RH vacuum treatment: The molten steel after calcium treatment is transferred to the RH vacuum refining unit for vacuum treatment. The vacuum degree is controlled at ≤40Pa and the vacuum treatment time is 15~25min. Argon is used for circulation gas and the flow rate is controlled at 1300~1450L / min. (6) Static stirring treatment: After the RH vacuum treatment is completed, the molten steel is subjected to static stirring treatment. The static stirring is carried out by argon soft blowing, the argon flow rate is controlled at 30~50L / min, the static stirring time is 15~25min, the molten steel surface is kept covered during the static stirring process, and the molten steel is left to stand for 5~10min after the static stirring is completed to obtain high purity molten steel.

2. The method for preparing high-purity molten steel according to claim 1, characterized in that: In step (1), the molten steel needs to be sampled and tested before tapping from the converter to ensure that the oxygen content of the tapped steel is ≤500ppm. If the oxygen content exceeds the standard, supplementary blowing is required, and the supplementary blowing time is controlled within 1~2 minutes.

3. The method for preparing high-purity molten steel according to claim 1 or 2, characterized in that: In step (2), the aluminum is aluminum granules with a purity of ≥99.5% and a particle size of 5~10mm, which are added uniformly with the steel flow.

4. The method for preparing high-purity molten steel according to claim 1 or 2, characterized in that: In step (3), the amount of lime added is 8~12 kg / ton of steel, the amount of fluorite added is 1~2 kg / ton of steel, and the amount of bauxite added is 2~3 kg / ton of steel.

5. The method for preparing high-purity molten steel according to claim 1 or 2, characterized in that: In step (3), weak stirring is used during the refining and slag-making process, and the argon flow rate is controlled at 80~120L / min.

6. The method for preparing high-purity molten steel according to claim 1 or 2, characterized in that: In step (4), the calcium wire used is silicon-calcium wire with a calcium content ≥30%, the wire feeding speed is 120~150m / min, and the wire feeding amount is adjusted according to the composition of the molten steel.

7. The method for preparing high-purity molten steel according to claim 1 or 2, characterized in that: In step (5), during RH vacuum treatment, the insertion depth of the immersion tube is 150~200mm, and the temperature drop of the molten steel during the vacuum treatment process is controlled to not exceed 30℃.

8. The method for preparing high-purity molten steel according to claim 1 or 2, characterized in that: In step (6), after static stirring, the molten steel is inspected for inclusions. The Aspex index is used to count the number of inclusions to ensure that the number of inclusions in the molten steel is less than 10 per mm. 2 .