Vacuum induction smelting method for high-purity stainless steel

Abstract

A vacuum induction smelting method for high-purity stainless steel, the method comprising: adding a high-purity return material at the last stage of smelting, the addition amount of the high-purity return material being not less than the weight of molten steel contained under the condition that the height of a crucible for smelting is 100 mm, the high-purity return material being high-purity stainless steel cut blocks or cleaned and briquetted turnings smelted by a duplex process of vacuum induction smelting plus vacuum arc remelting, and the O content of the return material being ≤10ppm. The method reduces the metal material loss caused by incapability of melting metal rings into molten pools under conventional conditions, such that the stainless steel smelting yield in vacuum induction smelting stages reaches 98% or above. The method reuses high-purity return materials obtained after VIM+VAR, and avoids the formation of metal rings, thus avoiding the molten steel loss of 2-4%, raising the yield of molten steel in vacuum induction smelting stages, and effectively reducing the production cost of high-purity stainless steel without lowering the purity of the stainless steel.

Description

A vacuum induction smelting method for high-purity stainless steel

[0001] This disclosure claims priority to a Chinese patent application filed with the Patent Office of China on July 13, 2023, with application number 202310859025.6 and invention name “A Vacuum Induction Smelting Method for High-Purity Stainless Steel,” the entire contents of which are incorporated herein by reference. Technical Field

[0002] The invention belongs to the technical field of vacuum induction melting, and in particular relates to a vacuum induction smelting method for high-purity stainless steel. Background Art

[0003] In conventional vacuum induction smelting operations, to improve molten steel purity and reduce residual inclusions, the gaseous nature of carbon deoxidation products is typically exploited, intensifying the carbon deoxidation reaction under high-temperature, high-vacuum conditions to deoxidize the molten steel. However, high-temperature, high-vacuum carbon deoxidation can cause the molten steel to boil, and splashing molten steel adheres to the inner wall of the crucible, forming a metal ring approximately 100 mm high above the liquid surface (see Figure 1). This metal ring, located above the molten metal surface, cannot be completely melted by shaking the crucible, resulting in a 2-4% reduction in overall metal yield and hindering production cost control. Furthermore, with increasing smelting cycles, the metal ring increases in size (especially thickness), reducing the effective inner diameter of the crucible and making it more susceptible to material jamming during charging. To mitigate this risk, the metal ring must be removed from the crucible when it reaches a certain thickness. There are two common methods for removing the metal ring: first, after smelting, the vacuum furnace is cooled to room temperature and manually removed under vacuum. However, this method requires a long cooling cycle and is labor-intensive, hindering continuous smelting production. A second approach involves melting the metal rings by washing them. This method requires additional molten steel to ensure the level of the molten steel is just high enough to submerge and melt the metal rings. However, washing the furnace requires additional metal material, which is time-consuming and energy-intensive, hindering production cost control.

[0004] On the other hand, for ultra-pure stainless steel for special applications, the production process requires a dual vacuum induction melting + vacuum consumable remelting (VIM + VAR) process to improve product purity. Stainless steel produced by the dual vacuum process requires multiple peelings, and the high rate of head and tail removal of the consumable remelted ingots results in a very low overall yield of high-purity stainless steel, with each furnace generating approximately 30-40% return material. Because the return material is produced through the dual vacuum process, its purity is very high. Disposing of this return material as ordinary waste would result in significant waste.

[0005] Summary of the Invention

[0006] To address the problems existing in the prior art, the present invention provides a vacuum induction smelting method for high-purity stainless steel. This method adds high-purity return material to molten steel being smelted in a vacuum induction furnace and heats the metal rings adhering to the crucible. This method achieves efficient utilization of the high-purity return material and eliminates the problem of metal rings adhering to the crucible in the vacuum induction furnace, without compromising the purity of the molten steel. This significantly reduces the production cost of high-purity stainless steel.

[0007] In order to achieve the above object, the technical solution adopted by the present invention is as follows:

[0008] The present invention provides a vacuum induction smelting method for high-purity stainless steel, which comprises adding high-purity return material at the end of smelting, wherein the amount of the high-purity return material added is not less than the weight of molten steel contained in a smelting crucible at a height of 100 mm; the high-purity return material is high-purity stainless steel cuttings or cleaned and pressed turnings smelted by a dual process of vacuum induction smelting + vacuum consumable remelting, and the return material O is ≤10ppm.

[0009] Preferably, before adding the high-purity return material, the molten steel in the crucible has completed high-temperature refining and has reached the purity requirement.

[0010] Preferably, a feeding barrel is used to add high-purity return material. When the return material is added, the molten steel scalds the bottom of the feeding barrel of the stainless steel material, and the return material block enters the molten pool smoothly through the scalded gap.

[0011] Preferably, the feeding barrel is a split structure, including a feeding barrel side wall 1 and a stainless steel basin 4 with an edge. There is a limiting flange 2 on the inner side of the bottom of the feeding barrel side wall 1. When the stainless steel basin 2 is placed inside the feeding barrel, it reaches the bottom of the feeding barrel and is stuck by the limiting flange 2.

[0012] Preferably, the side wall 1 of the feeding barrel is a straight tube made of curled stainless steel plate.

[0013] Preferably, the stainless steel basin 4 is a thin-walled stainless steel basin.

[0014] Preferably, the molten steel in the crucible is not frozen or cooled before the return material is added.

[0015] As a preference, the returned material is cleaned and then the temperature is adjusted for steelmaking.

[0016] Preferably, the method further comprises the steps of melting the new material and refining it at high temperature.

[0017] The present invention uses an innovative feeding container to steadily add a certain amount of high-purity return material at the end of smelting, thereby raising the liquid level of the molten pool and completely melting the metal rings that splash and adhere to the crucible wall during the high-temperature refining period, reducing the metal material loss caused by the inability of the metal rings to melt into the molten pool under conventional conditions, and achieving a stainless steel smelting yield of more than 98% in the vacuum induction smelting stage.

[0018] The present invention reuses the high-purity return material after VIM+VAR, avoids the formation of metal rings, circumvents the loss of 2-4% of molten steel, improves the yield of molten steel in the vacuum induction smelting stage, and effectively reduces the production cost of high-purity stainless steel without reducing the purity of stainless steel.

[0019] The method of the present invention is applicable to vacuum induction furnaces of various capacities such as 0.5t, 1t, and 3t, and to the smelting of various types of stainless steel. BRIEF DESCRIPTION OF THE DRAWINGS

[0020] The accompanying drawings are used to provide a further understanding of the present invention and constitute a part of the specification. Together with the embodiments of the present invention, they are used to explain the present invention and do not constitute a limitation of the present invention. In the accompanying drawings:

[0021] Figure 1 shows a metal ring (partial) stuck to the crucible wall.

[0022] FIG2 is a schematic diagram of the structure of the feeding barrel used in the conventional method.

[0023] FIG3 is a schematic diagram of the structure of the feeding barrel used in the method of the present invention.

[0024] FIG4 is a schematic diagram of adding materials using the feeding barrel of the present invention.

[0025] Among them, 1 is the side wall of the feeding barrel, 2 is a thin hemp rope, 3 is a loose-leaf leaf, 4 is a stainless steel basin, 5 is a limiting flange, 6 is a material block, 7 is a metal ring, 8 is the steel liquid surface, 9 is a crucible, and 10 is the steel liquid. DETAILED DESCRIPTION

[0026] The following examples are provided to facilitate a better understanding of the present invention, but are not intended to limit the present invention. The experimental methods in the following examples are all conventional methods unless otherwise specified.

[0027] The present invention provides a vacuum induction smelting method for high-purity stainless steel, comprising adding high-purity return material at the end of smelting, wherein the amount of the high-purity return material added is not less than the weight of the molten steel held by a 100mm high crucible. The high-purity return material is high-purity stainless steel blocks or cleaned and briquetted turnings smelted by the VIM+VAR process, and the oxygen content of the return material is ≤10ppm.

[0028] Adding low-purity metal raw materials at the end of smelting will cause contamination of the molten steel. Moreover, since the smelting has reached the end stage, the re-refining and purification operation will take too long and is not practical. In order to avoid the adverse effects of the final addition on the molten steel, high-purity raw materials are required.

[0029] During conventional vacuum induction smelting, metal rings adhered to the crucible's inner wall due to molten steel splashing typically remain approximately 100mm above the molten steel surface. If the amount of return material added later is less than the weight of the molten steel held in the crucible at a 100mm height, the added return material will not completely melt the formed metal rings. Only when the amount of return material exceeds the weight of the molten steel held in the crucible at a 100mm height can the metal rings be melted, minimizing molten steel loss and improving yield.

[0030] In some embodiments, before adding the high-purity return material, the molten steel in the crucible has completed high-temperature refining and meets the purity requirement (0≤50ppm).

[0031] If the molten steel in the crucible has not completed the necessary refining and purification before the return material is added, adding the high-purity return material will not produce a pure alloy liquid. If high-temperature, high-vacuum deoxidation is performed again for further purification, it will still cause the molten steel to splash and form new metal rings. Therefore, the molten steel in the crucible must reach the required purity before adding the return material. Once the return material is clear, the temperature can be adjusted and the steel can be tapped.

[0032] In some embodiments, a feeding barrel is used to add high-purity return material. When the return material is added, the molten steel scalds the bottom of the feeding barrel of the stainless steel material, and the return material block smoothly enters the molten pool through the scalded gap.

[0033] The vacuum induction furnace consists of a charging chamber and a melting chamber. The charging chamber is located directly above the melting chamber and connected by a vacuum valve. A crucible is placed in the melting chamber, along with a charging bucket, with the crucible and bucket coaxial. Twenty minutes before the completion of high-temperature refining, the material blocks are placed in the charging bucket, and the charging chamber is then evacuated. When the vacuum level in the charging chamber matches that of the melting chamber, the vacuum valve is opened, connecting the charging and melting chambers. The charging bucket is then lowered from the charging chamber into the melting chamber for charging.

[0034] There are generally three methods for charging metal in conventional vacuum induction smelting. The first involves welding a metal hanger wire to the incoming metal lumps, which slowly lowers them from the charging chamber into the molten steel. This method allows for smooth, individual addition of lumps into the molten pool, but it is slow and requires opening the vacuum valve for each addition, increasing valve wear and reducing valve life. The second method involves using a charging bucket. The bottom of a conventional charging bucket has six to eight hinges, secured together with fine hemp rope (see Figure 2). During charging, the bucket is slowly lowered directly above the molten steel. When the bucket is approximately one meter above the molten steel surface, the hemp ropes heat up, breaking the hinges and releasing a large number of lumps from the bucket into the molten steel. This method is fast, but the large lumps have high gravitational potential energy, which can cause a large amount of molten steel to splash into the molten pool. The third method involves side loading. That is, the material blocks are placed in the feeding hopper above the crucible side, and the material blocks are added by shaking to cause them to fall into the molten steel. Under this condition, the feeding speed is faster, but the gravitational potential energy of the added metal blocks is still high, and a large number of them falling will still cause the molten steel to splash.

[0035] To circumvent the drawbacks of the three aforementioned charging methods, the molten steel can be chilled before the return material is added. Large quantities of metal chunks can then be added via a charging barrel or side hopper and dropped onto the solidified alloy surface. This approach avoids frequent opening of the vacuum valve, allows for the addition of a larger quantity of metal at once, and prevents molten steel from splashing. However, this operation requires chilling the high-temperature molten steel, typically cooling it from its original temperature of 1600-1650°C to 1450-1480°C. This requires a significant wait time and increases time costs. Furthermore, the addition of large quantities of metal chunks to the molten pool causes the molten steel to heat up slowly, wasting the heat energy of the initially high-temperature molten steel.

[0036] The method of the present invention eliminates the need to chill or cool the molten steel before adding high-purity return material, and allows for faster feeding. The key advantage is that the metal material is fed directly into the molten pool using a feeding bucket, minimizing its gravitational potential energy and preventing steel splashing. Furthermore, the molten steel temperature is already high before addition, shortening the melting time of the subsequent metal lumps and saving energy.

[0037] According to some embodiments of the present application, the feeding barrel is a split structure, including a feeding barrel side wall 1 and a stainless steel basin 4 with an edge. There is a limiting flange 2 on the inner side of the bottom of the feeding barrel side wall 1. When the stainless steel basin 4 is placed inside the feeding barrel, it reaches the bottom of the feeding barrel and is stuck by the limiting flange 2.

[0038] According to some embodiments of the present application, the side wall 1 of the feeding barrel is a straight tube made of curled stainless steel plate.

[0039] The present invention has no specific requirements for the shape of the stainless steel basin 4, as long as it has a rim and can hold the return material.

[0040] The present invention has no specific requirements for the material of the stainless steel 4; for example, food-grade stainless steel or common commercially available stainless steel can be used. A common commercially available thin-walled stainless steel basin is preferred, as it easily melts during addition of the material. Common commercially available stainless steel basins are also inexpensive, further reducing smelting costs. For ton-scale smelting processes, the addition of a small amount of common stainless steel has little effect on the purity of the steel ingot.

[0041] The feeding container of the present invention uses a bottomless feeding bucket with a rim, and is equipped with a commercially available thin-walled stainless steel basin with an edge as the bottom. First, the return material is placed in the feeding bucket, and the feeding bucket is steadily lowered just above the molten steel. When the bottom of the stainless steel basin contacts the surface of the molten steel, the bottom of the basin melts, and the material block enters the molten steel (see Figure 3). The advantage of this method is that the bottom of the stainless steel basin will not melt during the descent process, and can only melt when it contacts the surface of the molten steel, reducing the gravitational potential energy of the material block and avoiding the splashing of the molten steel. At the same time, the molten steel does not need to be frozen or cooled when the return material is added. Freezing and cooling are to reduce splashing when the material block falls into the molten steel. The use of the feeding bucket of the present invention avoids the splashing of the molten steel, and naturally eliminates the freezing and cooling processes.

[0042] The present invention has no specific requirements for the height of the stainless steel basin 4, as long as it can hold a sufficient amount of high-purity return material.

[0043] In some embodiments, the return material is cleaned and then the temperature is adjusted for steelmaking.

[0044] The purity of the returned material is relatively high and has reached the purity requirement of the finished product. After the returned material is added with chemical clarifier, it does not need to undergo deoxidation and denitrification treatment and can be temperature-controlled for steelmaking.

[0045] When the temperature is adjusted for tapping, the tapping temperature commonly used in the prior art can be used, such as overheating by 150-180° C. above the melting point of stainless steel.

[0046] In some embodiments, the method of the present invention further comprises the steps of melting the new material and high-temperature refining. The melting and high-temperature refining of the new material can be performed using methods in the prior art.

[0047] If a metal ring is already present, the method of the present invention can only melt it in one go if the amount of molten steel is increased. If the amount of molten steel is not increased (i.e., the molten steel level must be above the top edge of the metal ring), the metal ring will remain unremovable. For example, if a 1-ton furnace normally produces 1 ton of molten steel and forms a metal ring, the present invention would require at least 1.2 tons of molten steel to remove the metal ring in one go.

[0048] If the method of the present invention is used for smelting from the beginning, no metal ring will be formed no matter how many times the smelting is repeated.

[0049] The method of the present invention is applicable to vacuum induction furnaces of various capacities such as 0.5t, 1t, and 3t, and to the smelting of various types of stainless steel.

[0050] The present invention will be further described below with reference to the accompanying drawings and embodiments.

[0051] Example 1

[0052] From Figures 3 and 4,

[0053] 304 stainless steel is melted in a 1-ton vacuum induction furnace. The specific melting process is as follows:

[0054] (1) Charging the furnace: Place all 500 kg of new steelmaking material in the crucible 9, and prepare 500 kg of high-purity return material blocks 6 smelted by the VIM+VAR method and place them in the charging barrel. The O content of the return material is 6 ppm.

[0055] The feeding bucket comprises a feeding bucket side wall 1 and a stainless steel basin 4 with an edge. A limiting flange 2 is provided on the inner side of the bottom of the feeding bucket side wall 1. When the stainless steel basin 2 is placed inside the feeding bucket, it reaches the bottom of the feeding bucket and is clamped by the limiting flange 2.

[0056] The side wall 1 of the feeding barrel is a straight tube made of curled stainless steel plate, and the stainless steel basin 4 can be a common commercially available thin-walled stainless steel basin with a strip edge.

[0057] (2) Melting: Start supplying power when the vacuum degree is less than 1.0 Pa, keep warm at 300KW for 30 minutes, then increase the power to 500KW until the melt is clear (2 to 3 hours).

[0058] (3) High temperature refining: After the new material is purified, the temperature is raised to 1600℃, the vacuum degree is ≤1.0Pa, and the temperature is kept for 60 minutes to meet the purity requirements (O≤50ppm).

[0059] (4) Feeding period: The feeding bucket is steadily lowered from just above the liquid steel 10 to the liquid steel surface 8. The bottom of the stainless steel basin 4 is melted, and the return material block 6 all smoothly enters the liquid steel 10. Then the feeding bucket is quickly raised to the feeding chamber. At this time, the liquid steel surface is below the upper edge of the metal ring.

[0060] (5) Pouring: After the return material is cleared, adjust the temperature of the molten steel to 1600℃ and pour with electricity.

[0061] After the steel ingot was removed from the mold, it was weighed and the weight of the steel ingot was 991 kg, with a yield of 99.1%.

[0062] Example 2

[0063] The difference between this embodiment and embodiment 1 is that 300 kg of new steelmaking material and 700 kg of high-purity return material blocks smelted by the VIM+VAR process are used. The rest is the same as embodiment 1.

[0064] Example 3

[0065] The difference between this embodiment and embodiment 1 is that 400 kg of new steelmaking material and 600 kg of high-purity return material blocks smelted by the VIM+VAR process are used. The rest is the same as embodiment 1.

[0066] Example 4

[0067] The difference between this embodiment and embodiment 1 is that the feeding barrel is an integrated structure. The rest is the same as embodiment 1.

[0068] Using such a feeding bucket, the yield and purity of the steel ingots finally prepared are almost the same as those in Example 1. However, compared with Example 1, the feeding bucket needs to be replaced each time the material is added, resulting in material waste and increased costs.

[0069] Comparative Example 1

[0070] 304 stainless steel is melted in a 1-ton vacuum induction furnace. The specific melting process is as follows:

[0071] (1) Charging the furnace: Place all 1000 kg of new steelmaking material from the same batch as in Example 1 into a crucible.

[0072] (2) Melting: Start supplying power when the vacuum degree is less than 1.0 Pa, keep warm at 300KW for 30 minutes, then increase the power to 500KW until the melt is clear (3 to 4 hours).

[0073] (3) High temperature refining: After the new material is purified, heat it to 1600℃, vacuum degree ≤1.0Pa, and keep it warm for 60 minutes.

[0074] (4) Pouring: Adjust the temperature of molten steel to 1600℃ and pour under electricity.

[0075] After the steel ingot is taken out of the mold, it is weighed and the weight of the steel ingot is 952 kg, with a yield of 95.2%. A metal ring is formed on the inner wall of the crucible. As the number of smelting increases, the volume of the metal ring increases.

[0076] Finally, it should be noted that the above descriptions are merely preferred embodiments of the present invention and are not intended to limit the present invention. Although the present invention has been described in detail with reference to the aforementioned embodiments, those skilled in the art will be able to modify the technical solutions described in the aforementioned embodiments or substitute equivalents for some of the technical features. Any modifications, equivalent substitutions, and improvements made within the spirit and principles of the present invention shall be included within the scope of protection of the present invention.

Claims

1. A vacuum induction smelting method for high purity stainless steel, characterized in that: The method comprises adding high-purity return material at the end of smelting, wherein the addition amount of the high-purity return material is not less than the weight of molten steel contained in a smelting crucible at a height of 100 mm; the high-purity return material is high-purity stainless steel cut blocks or cleaned and pressed turnings smelted by a vacuum induction smelting + vacuum consumable remelting dual process, and the return material O≤10ppm.

2. A vacuum induction smelting method for high purity stainless steel according to claim 1, characterized in that: Before adding high-purity return material, the molten steel in the crucible has completed high-temperature refining and reached the purity requirement.

3. The vacuum induction smelting method of high purity stainless steel according to claim 1, characterized in that: Use a feeding bucket to add high-purity return material. When the return material is added, the molten steel scalds the bottom of the feeding bucket of the stainless steel material, and the return material block smoothly enters the molten pool through the scalded gap.

4. A vacuum induction smelting method for high purity stainless steel according to claim 3, characterized in that: The feeding barrel is a split structure, comprising a feeding barrel side wall (1) and a stainless steel basin (4) with an edge. A limiting flange (2) is provided on the inner side of the bottom of the feeding barrel side wall (1). When the stainless steel basin (4) is placed inside the feeding barrel, it reaches the bottom of the feeding barrel and is clamped by the limiting flange (2).

5. A vacuum induction smelting method for high purity stainless steel according to claim 4, characterized in that: The side wall (1) of the feeding barrel is a straight tube formed by curling a stainless steel plate.

6. A vacuum induction smelting method for high purity stainless steel according to claim 4, characterized in that: The stainless steel basin (4) is a thin-walled stainless steel basin.

7. A vacuum induction smelting method for high purity stainless steel according to any one of claims 4 to 6, characterized in that: Before the return material is added, the molten steel in the crucible does not freeze or cool down.

8. The vacuum induction smelting method of high purity stainless steel according to claim 1, characterized in that: The returned material is cleaned and then the temperature is adjusted for steelmaking.

9. The vacuum induction smelting method of high purity stainless steel according to claim 1, characterized in that: The method further comprises the steps of melting and high-temperature refining the new steelmaking material.

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