Method for preparing high purity molten steel for powder metallurgy in tundish

Abstract

The present application belongs to the technical field of steel metallurgy and powder metallurgy raw material preparation, and discloses a preparation method of tundish high-purity molten steel for powder metallurgy. The tundish is sequentially provided with a pre-agitation zone, a filtration zone and a final purification zone along the direction of molten steel flow. A main electromagnetic stirring coil is installed at the bottom of the pre-agitation zone, an open-pore retaining wall is arranged in the filtration zone, an auxiliary electromagnetic stirring coil and a pouring zone are arranged at the bottom of the final purification zone, and an array-type bottom argon-blowing gas permeable brick is arranged in the pouring zone. The problems of incomplete removal of inclusions and poor molten steel uniformity in the traditional tundish are solved. By using the method, the elimination rate of non-metallic inclusions with a particle size greater than 20 microns is greater than 97%, the molten steel temperature fluctuation is less than or equal to ±2℃, and the composition uniformity error is less than or equal to 0.01%. The method provides stable high-purity molten steel raw material for powder metallurgy atomization powder production, is simple and easy to implement, and is suitable for existing conventional production lines.

Description

Technical Field

[0001] This invention belongs to the field of iron and steel metallurgy and powder metallurgy raw material preparation technology, and particularly relates to a method for preparing high-purity molten steel in an intermediate ladle for powder metallurgy. Background Technology

[0002] In the powder metallurgy atomization process, the purity of molten steel directly determines the quality of the metal powder. Non-metallic inclusions with a particle size >20μm remaining in the molten steel lead to poor powder flowability, numerous forming defects, and significantly reduce the density, strength, and fatigue life of sintered products, failing to meet the requirements of high-end powder metallurgy. The tundish is the final refining process before the molten steel enters the atomization device. Existing conventional tundishes mostly employ a single baffle or dam for flow control, which can only remove large particle inclusions >50μm and cannot completely eliminate harmful inclusions >20μm. Some technologies use a single electromagnetic stirring or a single bottom-blowing argon process, which suffers from poor synergy between the two, low inclusion removal efficiency, and complex process control, making it difficult to meet the stable purity requirements of molten steel in powder metallurgy. Summary of the Invention

[0003] To address the aforementioned technical problems, this invention discloses a method for preparing high-purity molten steel for powder metallurgy tundishes. Through the synergistic effect of two-stage electromagnetic stirring and bottom-blowing argon gas, the method aims to completely eliminate inclusions with a particle size >20μm in the molten steel, providing high-purity and highly homogeneous molten steel raw materials for powder metallurgy atomization. The specific technical solution is as follows: This invention provides a method for preparing high-purity molten steel for powder metallurgy using an intermediate ladle, specifically comprising the following steps: (1) Tundish optimization: The tundish is set up with a pre-stirring zone, a filtration zone and a final purification zone in sequence along the steel flow direction; the main electromagnetic stirring coil is installed at the bottom of the pre-stirring zone, the filter zone is set with an open baffle wall, the final purification zone is set with an auxiliary electromagnetic stirring coil and a casting zone at the bottom, and the casting zone is equipped with an array of bottom-blown argon permeable bricks and connected to an argon gas pipeline. (2) Pre-refining treatment: After the molten steel is initially refined in a 15T medium frequency furnace, it enters the LF furnace for refining. After the composition and temperature are adjusted, it undergoes VD vacuum treatment. After vacuum treatment, it is soft-blown with argon for ≥25 minutes. After VD vacuum treatment, it is calmed and vertically cast into the tundish. (3) Pre-stirring stage: After the molten steel is injected into the pre-stirring zone, the main electromagnetic stirring coil is turned on to achieve uniformity of the molten steel composition and temperature, and at the same time, large particles of inclusions are initially separated by centrifugal force. (4) Final purification stage: After the molten steel enters the final purification zone through the perforated baffle wall of the filtration zone, the auxiliary electromagnetic stirring coil is turned on, and it works in conjunction with the bottom-blown argon gas in the casting zone to form a spiral upward flow field, which strengthens the collision and adsorption of inclusions and bubbles.

[0004] A further improvement of the present invention is that the distance between the main electromagnetic stirring coil of the pre-stirring zone and the surface of the molten steel is 150~200mm, and the magnetic field covers the entire effective depth of the pre-stirring zone, with no stirring blind zone.

[0005] A further improvement of the present invention is that the array of permeable bricks in the final purification zone is distributed in a 2×2 pattern, with a diameter of 120~150mm for a single permeable brick and a spacing of 250~300mm between adjacent permeable bricks. The permeable bricks are made of corundum porous refractory material.

[0006] A further improvement of the present invention is that: the height of the retaining wall is 450~500mm, and two 100mm * 100mm square through holes are opened. The through holes are located at 2 / 3 of the height of the retaining wall, and a filter screen is embedded in the through holes.

[0007] A further improvement of the present invention is that the total residence time of the molten steel in the tundish is 60-120 minutes, and argon gas is used for sealing and protection during the entire casting process to maintain the superheat of the molten steel at 15-25°C, thereby avoiding secondary oxidation and slag entrapment.

[0008] A further improvement of the present invention is that: in step (2), when performing VD vacuum treatment, the vacuum degree is ≤100Pa and the treatment time is ≥25min.

[0009] A further improvement of the present invention is that, in step (3), the stirring current of the main electromagnetic stirring coil is set to 350~450A, the rotation speed of the molten steel is 25~35r / min, and the stirring time is ≥8min.

[0010] A further improvement of the present invention is that: in step (4), the stirring current of the auxiliary electromagnetic stirring coil is 300~380A and the stirring time is ≥10min.

[0011] A further improvement of the present invention is that the bottom blowing argon process parameters in step (4) are as follows: high-purity argon with a purity of ≥99.999% is used and introduced through the bottom permeable brick of the final purification zone. The argon flow rate is 0.05~0.12m³ / h·t steel, the gas supply pressure is 0.3~0.5MPa, and the bubble diameter is controlled at 50~100μm.

[0012] A further improvement of the present invention is that the molten steel is any one of powder metallurgy bearing steel, stainless steel, or high-strength steel, and the molten steel pouring temperature is 15~25°C above the liquidus line.

[0013] Compared with existing technologies, this invention achieves the following technical advantages: 1. Through two-stage synergistic refining, 97% of non-metallic inclusions with a particle size >20μm in the molten steel are removed, fully meeting the core requirements of powder metallurgy atomization powder production; 2. The process is simple and easy to implement, requiring no complex non-standard structures or precision control equipment. It can be implemented by modifying existing conventional tundishes. The electromagnetic stirring and bottom-blowing argon process parameters have a wide range, low on-site operation difficulty, and strong adaptability; 3. The molten steel has good homogenization effect. With the cooperation of two-stage electromagnetic stirring, the temperature fluctuation of the molten steel is ≤±2℃, and the composition uniformity error is ≤0.01%, which can effectively ensure the batch stability of atomized powder; 4. The production stability is strong. The whole-process protective casting and stable flow design can effectively avoid slag entrapment and secondary oxidation. The purity fluctuation of the molten steel during the ladle change process is small, enabling continuous and stable production, which is suitable for large-scale industrial applications. Attached Figure Description

[0014] The accompanying drawings, which are included to provide a further understanding of the invention and form part of this invention, illustrate exemplary embodiments of the invention and are used to explain the invention, but do not constitute an undue limitation of the invention. In the drawings: Figure 1 This is a schematic diagram of the overall structure of the intermediate package of the present invention; Figure 2 This is a schematic diagram of the process flow of the present invention; Among them, 1-pre-mixing zone, 2-filtration zone, 3-final purification zone, 4-main electromagnetic stirring coil, 5-perforated baffle wall, 6-auxiliary electromagnetic stirring coil, 7-pouring zone, 8-permeable brick, 9-argon gas pipeline, 10-filter screen. Detailed Implementation

[0015] The following will describe in detail the implementation of the present invention with reference to the accompanying drawings and embodiments, so as to fully understand how the present invention uses technical means to solve technical problems and achieve technical effects and to implement it accordingly.

[0016] This embodiment provides a method for preparing high-purity molten steel using a tundish for powder metallurgy. Firstly, it provides a tundish structure, such as... Figure 1As shown, the tundish is sequentially configured with a pre-stirring zone 1, a filtration zone 2, and a final purification zone 3 along the molten steel flow direction. A main electromagnetic stirring coil 4 is installed at the bottom of the pre-stirring zone 1, with a distance of 150-200 mm between the coil and the molten steel surface. The magnetic field covers the entire effective depth of the pre-stirring zone, eliminating any blind spots. An open baffle wall 5 with a height of 450-500 mm is installed within the filtration zone 2, containing two 100 mm * 100 mm square through holes located at 2 / 3 of the baffle wall height. Filter screens 10 are embedded within these through holes. An auxiliary electromagnetic stirring coil 6 and a casting zone 7 are located at the bottom of the final purification zone 3, with the auxiliary electromagnetic stirring coil 6 positioned around the periphery of the casting zone 7. The pouring area 7 is equipped with an array of bottom-blown argon permeable bricks 8. The array of permeable bricks is distributed in 2×2. The diameter of a single permeable brick is 120~150mm, and the spacing between adjacent permeable bricks is 250~300mm. The permeable bricks are made of corundum porous refractory material. The permeable bricks 8 are connected to argon gas pipes 9.

[0017] This embodiment provides a method for preparing high-purity molten steel for powder metallurgy using an intermediate ladle, which specifically includes the following steps: (1) Pre-refining treatment: After the molten steel is initially refined in a 15T medium-frequency furnace, it enters an LF furnace for refining. After the composition and temperature are adjusted, it undergoes VD vacuum treatment with a vacuum degree of 80Pa and a treatment time of 30min. After vacuum treatment, soft argon blowing is performed for ≥25min. After VD vacuum treatment, it is killed and vertically cast into the tundish. In this embodiment, an 8t powder metallurgy tundish with electromagnetic stirring and bottom argon blowing is used as described above. (2) Pre-stirring stage: After the molten steel is injected into the pre-stirring zone of the tundish, the main electromagnetic stirring coil is turned on. The parameters are set as follows: current 400A, molten steel rotation speed 30r / min, stirring time 10min. This is used to initially stir the molten steel to achieve uniformity of molten steel composition and temperature, and at the same time, to initially separate large particle inclusions through centrifugal force. (4) Final purification stage: The molten steel flows into the final purification zone after passing through the perforated baffle wall of the filtration zone and the filter screen. After entering the final purification zone, the auxiliary electromagnetic stirring coil is turned on. The stirring current and the bottom-blown argon gas in the casting zone work together to form a spiral upward flow field. The parameters are set as follows: current 350A, stirring time 12min, bottom-blown argon gas: purity 99.999%, flow rate 0.08m³ / h·t steel, pressure 0.4MPa, bubble diameter 50~80μm; molten steel superheat 20℃, total residence time 12min. Argon gas is used to seal and protect the casting throughout the process to enhance the collision and adsorption of inclusions and bubbles. (5) Testing: The test results showed that the removal rate of non-metallic inclusions with a particle size >20μm was 97.6% and the removal rate of inclusions with a particle size of 10~20μm was 95.2%; the temperature fluctuation of the molten steel was ±1.5℃ and the composition uniformity error was 0.05%.

[0018] This embodiment also provides two comparative examples: Comparative Example 1: The conventional baffle tundish used the same steel grade and preceding refining process as Example 1. The tundish only had a conventional baffle dam, without electromagnetic stirring or bottom-blowing argon. Test results: The removal rate of inclusions with a particle size >20μm in the total oxygen content of the molten steel was only 58%, and the maximum inclusion particle size was 62μm, which could not meet the requirements for powder metallurgy applications.

[0019] Comparative Example 2: A single electromagnetic stirring tundish was used with the same steel grade and preceding refining process as in Example 1. The tundish was equipped with only a single electromagnetic stirrer and no bottom blowing argon. Test results: The removal rate of inclusions with a particle size >20μm was 76%, but a large number of inclusions with a particle size of 20~40μm remained.

[0020] This embodiment mainly optimizes the tundish, constructing a two-stage refining system of pre-stirring homogenization separation and final purification synergistic capture, as follows: 1. Pre-stirring homogenization separation: The main electromagnetic stirring drives the molten steel to rotate, which on the one hand quickly and uniformly homogenizes the composition and temperature of the molten steel, avoiding powder quality instability caused by fluctuations in the properties of the molten steel during the subsequent atomization process; on the other hand, centrifugal force is used to cause large particles of inclusions in the molten steel to migrate to the cavity wall and be adsorbed by the refractory material, completing the initial separation of inclusions. 2. Final purification synergistic capture: The auxiliary electromagnetic stirring and bottom-blown argon work together. The spiral upward flow field formed by the electromagnetic stirring can prolong the residence time of argon bubbles in the molten steel, increase the probability of collision between bubbles and inclusions, and prevent bubbles from coalescing and growing, ensuring the adsorption efficiency of bubbles; after adsorbing inclusions, the argon bubbles quickly float to the slag layer for removal, achieving deep purification of residual inclusions, with a final elimination rate of non-metallic inclusions with a particle size >20μm >97%. 3. Stable flow and quality control design: By combining baffles and dual-channel electromagnetic forces, the flow path of molten steel is optimized, the residence time of molten steel in the tundish is extended, dead zones are reduced, short-circuit flow of molten steel is avoided, and sufficient time is provided for inclusions to float. Argon gas is used to seal and protect the casting process throughout to prevent secondary oxidation of molten steel and ensure stable purity.

[0021] The foregoing description illustrates and describes several preferred embodiments of the present invention. However, as previously stated, it should be understood that the present invention is not limited to the forms disclosed herein and should not be construed as excluding other embodiments. It can be used in various other combinations, modifications, and environments, and can be altered within the scope of the inventive concept described herein through the foregoing teachings or techniques or knowledge in related fields. Any modifications and variations made by those skilled in the art that do not depart from the spirit and scope of the present invention should be within the protection scope of the appended claims.

Claims

1. A method for preparing high-purity molten steel for powder metallurgy tundishes, characterized in that: Specifically, the steps include the following: (1) Tundish optimization: The tundish is set up with a pre-stirring zone, a filtration zone and a final purification zone in sequence along the steel flow direction; the main electromagnetic stirring coil is installed at the bottom of the pre-stirring zone, the filter zone is set with an open baffle wall, the final purification zone is set with an auxiliary electromagnetic stirring coil and a casting zone at the bottom, and the casting zone is equipped with an array of bottom-blown argon permeable bricks and connected to an argon gas pipeline. (2) Pre-refining treatment: After the molten steel is initially refined in a 15T medium frequency furnace, it enters the LF furnace for refining. After the composition and temperature are adjusted, it undergoes VD vacuum treatment. After vacuum treatment, it is soft-blown with argon for ≥25 minutes. After VD vacuum treatment, it is calmed and vertically cast into the tundish. (3) Pre-stirring stage: After the molten steel is injected into the pre-stirring zone, the main electromagnetic stirring coil is turned on to achieve uniformity of the molten steel composition and temperature, and at the same time, large particles of inclusions are initially separated by centrifugal force. (4) Final purification stage: After the molten steel enters the final purification zone through the perforated baffle wall of the filtration zone, the auxiliary electromagnetic stirring coil is turned on, and it works in conjunction with the bottom-blown argon gas in the casting zone to form a spiral upward flow field, which strengthens the collision and adsorption of inclusions and bubbles.

2. The method for preparing high-purity molten steel for powder metallurgy using an intermediate ladle according to claim 1, characterized in that: The distance between the main electromagnetic stirring coil in the pre-stirring zone and the surface of the molten steel is 150~200mm, and the magnetic field covers the entire effective depth of the pre-stirring zone with no stirring blind spots.

3. The method for preparing high-purity molten steel for powder metallurgy using an intermediate ladle according to claim 1, characterized in that: The array of permeable bricks in the final purification zone is distributed in a 2×2 pattern. The diameter of a single permeable brick is 120~150mm, and the spacing between adjacent permeable bricks is 250~300mm. The permeable bricks are made of corundum porous refractory material.

4. The method for preparing high-purity molten steel for powder metallurgy using an intermediate ladle according to claim 1, characterized in that: The retaining wall has a height of 450~500mm and two 100mm*100mm square through holes. The through holes are located at 2 / 3 of the height of the retaining wall, and filter screens are embedded in the through holes.

5. The method for preparing high-purity molten steel for powder metallurgy using an intermediate ladle according to claim 1, characterized in that: The total residence time of molten steel in the tundish is 60-120 minutes. Argon gas is used for sealing and protection during the entire casting process to maintain the superheat of the molten steel at 15-25°C and avoid secondary oxidation and slag entrapment.

6. The method for preparing high-purity molten steel for powder metallurgy using an intermediate ladle according to claim 1, characterized in that: In step (2), when performing VD vacuum treatment, the vacuum degree is ≤100Pa and the treatment time is ≥25min.

7. The method for preparing high-purity molten steel for powder metallurgy using an intermediate ladle according to claim 1, characterized in that: In step (3), the stirring current of the main electromagnetic stirring coil is set to 350~450A, the rotation speed of the molten steel is 25~35r / min, and the stirring time is ≥8min.

8. The method for preparing high-purity molten steel for powder metallurgy using an intermediate ladle according to claim 1, characterized in that: In step (4), the stirring current of the auxiliary electromagnetic stirring coil is 300~380A, and the stirring time is ≥10min.

9. The method for preparing high-purity molten steel for powder metallurgy using an intermediate ladle according to claim 1, characterized in that: The bottom-blowing argon process parameters in step (4) are as follows: high-purity argon with a purity of ≥99.999% is used and introduced through the permeable brick at the bottom of the final purification zone. The argon flow rate is 0.05~0.12m³ / h·t steel, the gas supply pressure is 0.3~0.5MPa, and the bubble diameter is controlled at 50~100μm.

10. The method for preparing high-purity molten steel for powder metallurgy using an intermediate ladle according to claim 1, characterized in that: The molten steel is any one of powder metallurgy bearing steel, stainless steel, or high-strength steel, and the pouring temperature of the molten steel is 15~25℃ above the liquidus.

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