A smelting method for electroslag remelting ultra-clean steel

Abstract

The invention discloses a method for smelting electroslag remelted ultra-clean steel, which uses a drawing ingot mold for electroslag melting; during the smelting process, a high-purity gas that does not participate in the reaction is blown into the slag pool at a position evenly distributed in a circle at the lower middle part of the slag pool at a rate of 0.1-0.5 L / min. In the method of the present invention, the slag pool will accelerate the convective circulation under the agitation of the high-purity gas bubbles, which will effectively increase the scouring effect of the slag on the electrode end face, forcing the metal droplets not to grow naturally and only break away from the electrode at a smaller size; at the same time, during the falling process of the droplets, they will be continuously impacted and broken by the slag flow and high-purity gas bubbles, further reducing the droplet size and increasing the specific surface area of the droplets, which will significantly increase the slag-metal contact interface and accelerate the reaction process. The invention can significantly reduce the metal droplet size and significantly reduce the droplet falling speed, thereby increasing the slag-metal reaction time and improving the inclusion removal efficiency. For the ultra-clean steel smelted by this method, the inclusion and sulfur content are more than 50% lower than those of traditional electroslag remelting.

Description

Technical Field

[0001] The present invention relates to an electroslag remelting method, in particular to a smelting method for electroslag remelting ultra-clean steel. Background Art

[0002] Electroslag metallurgy is one of the major breakthroughs in special metallurgy technologies in the 20th century. Since 1940 when an American, Hopkins, obtained an electroslag patent, electroslag metallurgy has developed vigorously. Currently, electroslag steel is widely used in industrial fields such as aviation, aerospace, and electric power. However, for some high-end materials with high requirements for harmful impurity components, such as some superalloys, corrosion-resistant alloys, and ultra-clean steel, a triple process of vacuum induction melting + electroslag remelting + vacuum consumable is currently required for production.

[0003] Producing ultra-clean steel using the triple process not only has a high cost, but also a long production cycle and great process difficulty, with many disadvantages. Therefore, metallurgists in various countries have tried to improve the electroslag smelting process and strive to obtain a more efficient method for removing inclusions and sulfur.

[0004] It is not difficult to know that the removal of sulfur and inclusions in electroslag mainly focuses on the two stages of metal droplet formation and falling. During the smelting process, the consumable electrode melts layer by layer along the surface at the end in contact with the slag, forming a thin film and droplets, and undergoing slag-metal reactions to remove a part of the impurities; when the droplets fall, the metal passing through the slag layer will also react with the slag to remove inclusions. Obviously, in these two processes, the contact area and the action time are the most critical factors affecting the inclusion removal efficiency.

[0005] As is well known, the slag-metal contact area has an exponential relationship with the droplet size. The smaller the droplet size, the larger the specific surface area and the contact area, and vice versa. A metal droplet of a certain mass, when broken into two droplets of the same mass, the specific surface area increases by 2.17 times. Therefore, controlling the formation of smaller metal droplets during the electroslag process is very beneficial for removing impurities.

[0006] On the other hand, the slag-metal action time is inversely proportional to the droplet falling time. When the metal droplet leaves the electrode and starts to fall, it will be jointly affected by the buoyancy of the slag, the viscous force (equivalent to the frictional force), and gravity. Among them, the slag buoyancy and the viscous force are resistances. The greater they are, the more beneficial it is to slow down the falling speed and extend the residence time, and the more beneficial it is to increase the degree of slag-metal reaction. Therefore, from the perspective of removing inclusions, measures should be taken to control the increase of slag buoyancy and viscous force during the remelting process.

[0007] In terms of reducing the droplet size and increasing the buoyancy and viscosity of the slag, metallurgists have conducted a lot of research. Generally speaking, the more effective work mainly focuses on two aspects: optimizing process parameters and improving the composition of the slag system. However, these efforts have both advantages and disadvantages. For example, increasing the current will indeed reduce the droplet size and improve the slag washing effect, but at the same time, it will also increase the melting rate, resulting in metal solidification segregation and uneven structure. In addition, increasing the density and viscosity of the slag will increase the buoyancy and viscosity, improving the inclusion removal effect, but at the same time, it will also thicken the slag skin, making it difficult to separate the steel slag and leading to a decline in the surface quality of the ingot.

[0008] To sum up, there is no particularly effective technical means to solve the problem of electroslag remelting for smelting ultra-clean steel at present. Summary of the Invention

[0009] The technical problem to be solved by the present invention is to provide a smelting method for electroslag remelting ultra-clean steel that can reduce the size of metal droplets and lower the falling speed of droplets.

[0010] To solve the above technical problem, the technical solution adopted by the present invention is: using a drawing type mold for electroslag melting; during the smelting process, blowing 0.1 - 0.5 L / min of non-reactive high-purity gas into the slag pool at a position evenly distributed in a circle at the lower middle part of the slag pool.

[0011] The high-purity gas of the present invention is blown horizontally into the slag pool.

[0012] The high-purity gas of the present invention is symmetrically blown into both sides at the lower middle part of the slag pool.

[0013] The high-purity gas of the present invention is high-purity helium gas.

[0014] The temperature of the high-purity gas of the present invention is 200 - 300 °C, and the pressure is 0.06 - 0.08 MPa.

[0015] The process idea of the present invention is: First, the slag pool will accelerate the convective circulation under the agitation of high-purity gas bubbles, which will effectively increase the scouring effect of the slag on the electrode end face, forcing the metal droplets not to grow naturally and only break away from the electrode at a smaller size. At the same time, during the falling process of the droplets, they will be continuously impacted and broken by the slag flow and high-purity gas bubbles, further reducing the droplet size and increasing the specific surface area of the droplets, which will significantly increase the slag-metal contact interface and accelerate the reaction process.

[0016] Secondly, due to the reduced mass, the broken droplets can undergo convective motion with the slag pool, greatly extending the dripping path and increasing the residence time, enabling the slag-metal reaction to proceed fully.

[0017] From the above analysis, it can be known that the droplet size and the action time are exactly the two most critical parameters for removing inclusions and impurities such as sulfur in electroslag metallurgy. Therefore, the present invention can effectively improve the inclusion removal efficiency.

[0018] In addition, the selection of helium as the blowing medium is also related to its physical and chemical properties. Helium is an inert gas that does not react with metals. As the blowing medium, it will not cause compositional changes. At the same time, its density is smaller than that of argon. When blown into the slag layer, it will expand rapidly and float up quickly, and will not remain in the steel. Moreover, helium is dehydrated, heated, and pressure-stabilized before being filled, so it will not increase H and will not reduce the slag temperature and increase the operation difficulty, and it has good process adaptability. Its usage amount is also small, only 0.1 - 0.5 L / min, which is economical and practical.

[0019] The beneficial effects of adopting the above technical solution are as follows: The present invention breaks through the traditional methods and uses "external force" to interfere with the electroslag remelting process, which can greatly reduce the size of metal droplets and significantly reduce the droplet falling speed, thereby increasing the slag-metal reaction time and improving the inclusion removal efficiency. After the electroslag remelting of the present invention, there is no need for secondary melting. The inclusions and sulfur content in the ultra-clean steel smelted are more than 50% lower than those of traditional electroslag remelting. Description of the Drawings

[0020] The present invention will be further described in detail below with reference to the drawings and specific embodiments.

[0021] Figure 1 It is a schematic structural diagram of the electroslag remelting device described in the present invention.

[0022] In the figure: 1 - consumable electrode; 2 - mold; 3 - blow hole; 4 - ingot; 5 - metal molten pool; 6 - slag pool; 7 - high-purity gas bubble. Specific Embodiments

[0023] The smelting method of the present electroslag remelting ultra-clean steel is applicable to electroslag steel, such as ultra-clean steels like GCr15, GH4169, W18Cr4V, NS333, etc. A drawing-out type mold is adopted, especially applicable to a drawing-out type mold with a diameter of Φ300 mm or more; the drawing-out type mold is provided with blow holes in the lower middle part of the slag pool, and there are at least two blow holes, which are evenly arranged on the same plane around the slag pool; preferably, there are two blow holes, which are symmetrically arranged on both sides of the slag pool; the drawing-out type mold needs to be improved, and the following structure of the drawing-out type mold can be adopted: Figure 1 As shown, 2 fine holes are symmetrically drilled through the middle part of the mold 2, and copper pipes are welded in parallel. The interfaces are polished smoothly. The inner diameter of the pipes is 0.12 - 0.16 mm, and the pipes are the blow holes 3; the blow holes 3 are horizontally arranged; the rest of the mold is the same as the structure of the conventional drawing-out type mold.

[0024] Adopt the following process: (1) Select a drawing-out type mold, place an arc starter on the bottom water tank, and place slag materials around the arc starter; install the mold 2, consumable electrode 1, and protective cover, and introduce argon;

[0025] (2) Turn on the smelting power supply, ignite the arc, add the slag material. When the slag temperature rises to 1700 - 1800 °C, enter the normal smelting stage; the slag material is baked before being added, preferably baked in a heating furnace at 600 - 650 °C for 8 - 12 h; the slag material melts to form a slag pool 6, and the metal droplets melted from the consumable electrode 1 sink to the bottom of the slag pool 6 and gradually accumulate to form a metal melt pool 5; the metal melt pool 5 cools and solidifies to form an ingot 4.

[0026] (3) In the normal smelting stage, start blowing gas when the slag layer covers the gas injection holes 3, and start drawing the ingot when the gas injection holes are located in the middle and lower part of the slag layer until the smelting ends. The gas injection holes 3 horizontally inject a high-purity gas with a total flow rate of 0.1 - 0.5 L / min that does not participate in the reaction into the middle and lower part of the slag pool. The high-purity gas is preferably high-purity helium gas. The high-purity helium gas is dehydrated, heated, and pressure-stabilized before entering the crystallizer. When entering the crystallizer, the water content of the gas is ≤ 0.0001 vol%, the temperature is 200 - 300 °C, and the pressure is 0.06 - 0.08 MPa. During the blowing process of the gas injection holes 3, high-purity gas bubbles 7 are formed in the slag pool 6, and the molten droplets are continuously impacted and broken by the slag flow and the high-purity gas bubbles 7 during the falling process, thereby reducing the size of the molten droplets, increasing the specific surface area of the molten droplets, extending the dripping path, and further accelerating the reaction process and enabling the slag-metal reaction to proceed fully.

[0027] (4) After the smelting ends, stop blowing helium gas, and demold after 30 - 120 min.

[0028] Example 1: The following specific process is adopted for the method of smelting ultra-clean steel by electroslag remelting.

[0029] Electroslag smelt GCr15. Specifically include: The specifications of the ingot-drawing crystallizer: Φ300×700 mm; the specifications of the consumable electrode: Φ200×2600 mm; the steel grade composition: C 1.02%, Si 0.24%, Mn 0.26%, P 0.010%, S 0.008%, Cr 1.49%, and the balance is Fe and inevitable impurities; the slag composition (wt): 70% CaF2, 30% Al2O3; the slag amount: 28 kg. Two gas injection holes with an inner diameter of 0.12 mm are symmetrically arranged at the middle part of the crystallizer; the smelting process is as follows:

[0030] (1) Polish and weld the consumable electrode.

[0031] (2) Place the slag material in a heating furnace at 600 °C and bake for 8 h.

[0032] (3) Lay the slag-fixing arc starter, install the electrode, and introduce Ar into the crystallizer.

[0033] (4) Give an electric arc, add slag, and after melting the slag for 30 min, raise the temperature to 1750 °C.

[0034] (5) Enter the smelting stage. When the slag layer covers the gas injection holes, start injecting high-purity helium gas at 230 °C, 0.06 MPa, 0.3 L / min, and a water content of 0.00006 vol%.

[0035] (6) Start ingot withdrawal when the gas injection holes are located in the middle and lower part of the slag layer until the smelting ends.

[0036] (7) Stop injecting helium gas and demold after 45 minutes.

[0037] (8) Use this mold and smelt electrodes of the same composition and specification by the above process, but without injecting helium gas during the smelting process for a comparative experiment to produce comparative electroslag. The electroslag obtained in this example (hereinafter referred to as the electroslag of this method) and the comparative electroslag are tested. The detected droplet size and quality of the slag layer are shown in Table 1, and the sulfur, phosphorus, and non-metallic inclusion contents of the ingot are shown in Table 2.

[0038] Table 1: Droplet Size and Quantity of Remelted Metal in Electroslag

[0039]

[0040] It can be seen from Table 1 that the average particle diameter of the metal droplets in the electroslag of this method is 2.86 mm, while that of the comparative electroslag is 7.44 mm, a decrease of 61.5%. The specific surface area increases by 130% under the same mass; the residual amount of droplets in the slag of the electroslag of this method is 130.7 g, and that of the comparative electroslag is 80.9 g, indicating that the residence time of the electroslag of this method is 61.6% longer than that of the comparative electroslag.

[0041] Table 2: Non-Metallic Inclusion and Sulfur and Phosphorus Contents (wt) after Electroslag Melting

[0042]

[0043] It can be seen from Table 2 that the total amount of inclusions, the rating, and the S content of the electroslag of this method are all lower than those of the comparative electroslag; among them, the total amount of inclusions is 61.9% lower than that of the comparative electroslag, and the sulfur content is 70.0% lower than that of the comparative electroslag.

[0044] Example 2: The following specific process is adopted for the smelting method of this electroslag remelting ultra-clean steel.

[0045] Electroslag remelting of GH4169. Specifically, it includes: The specifications of the ingot-pulling crystallizer: Φ220×600mm; The specifications of the consumable electrode: Φ160×1800mm; The chemical composition of the steel grade: C 0.05%, P 0.008%, S 0.005%, Cr 19%, Mo 3%, Ni 52%, Fe 19%, Nb 5.2%, Al 0.5%, Ti 1.0%, and inevitable impurities; The slag composition (wt): 50% CaF2, 25% CaO, 20% Al2O3, 5% TiO2; The slag amount: 10kg. Two gas blowing holes with an inner diameter of 0.16mm are symmetrically arranged at the middle part of the crystallizer; The smelting process is as follows:

[0046] (1)The consumable electrode is polished until it is bright and then welded;

[0047] (2)The slag material is placed in a heating furnace at 600°C and baked for 8h;

[0048] (3)The solid slag arc starter is laid, the electrode is installed, and Ar is introduced into the crystallizer;

[0049] (4)Electric arc ignition is carried out, slag is added, and after melting the slag for 15min, the temperature is raised to 1700°C;

[0050] (5)Enter the melting stage. When the slag layer covers the gas blowing holes, high-purity helium gas at 300°C, 0.06MPa, 0.1L / min, and a water content of 0.00009vol% is blown in;

[0051] (6)When melting reaches the position where the gas blowing holes are in the middle and lower part of the slag layer, ingot pulling starts until the melting ends;

[0052] (7)Stop blowing helium gas, and demold after 30min.

[0053] (8)Using this crystallizer, electrodes with the same composition and specifications are smelted by the above process without blowing helium gas during the smelting process for a comparative experiment, and the comparative electroslag is produced. The electroslag obtained in this example (the electroslag of this method hereinafter) and the comparative electroslag are detected. The detected molten droplet size and quality of the slag layer are shown in Table 3, and the sulfur, phosphorus, and non-metallic inclusion contents of the ingot are shown in Table 4.

[0054] Table 3: Molten Droplet Size and Quantity of Electroslag Remelting Metal

[0055]

[0056] It can be seen from Table 3 that the average particle diameter of the metal molten droplets of the electroslag of this method is 1.49mm, while that of the comparative electroslag is 3.02mm, a decrease of 50.7%. The specific surface area increases by 103% under the same mass; The residual amount of molten droplets in the slag of the electroslag of this method is 81.4g, and that of the comparative electroslag is 50.5g, indicating that the residence time increases by 61.4% compared with the comparative electroslag.

[0057] Table 4: Non-metallic inclusions and sulfur and phosphorus contents after electroslag melting (wt)

[0058]

[0059] As can be seen from Table 4, the total amount of electroslag inclusions, the rating, and the S content of this method are all lower than those of the comparative electroslag. Among them, the total amount of inclusions is 53.7% lower than that of the comparative electroslag, and the sulfur content is 62.5% lower than that of the comparative electroslag.

[0060] Example 3: The following specific process is adopted for the smelting method of this electroslag remelting ultra-clean steel.

[0061] Electroslag smelting of W18Cr4V. Specifically, it includes: The specifications of the ingot-drawing type mold: Φ550×1000mm; the specifications of the consumable electrode: Φ400×3900mm; the steel grade composition: C 0.75%, Si 0.30%, Mn 0.28%, P 0.010%, S 0.009%, Cr 4%, W 18%, Mo 0.3%, V 1.2%, and inevitable impurities. The slag composition (wt): 40% CaF2, 30% CaO, 30% Al2O3; the slag amount: 95 kg. Three blow holes with an inner diameter of 0.14 mm are evenly distributed on the plane at the middle part of the mold; the smelting process is as follows:

[0062] (1) Polish the consumable electrode until it is bright and then weld it.

[0063] (2) Place the slag material in a heating furnace at 620°C and bake it for 12 h.

[0064] (3) Lay the solid slag arc starter, install the electrode, and introduce Ar into the mold.

[0065] (4) Start the power supply, add slag, and after melting the slag for 60 min, raise the temperature to 1800°C.

[0066] (5) Enter the melting stage. When the slag layer covers the blow holes, start blowing high-purity helium gas at 200°C, 0.08 MPa, 0.5 L / min, and a water content of 0.0001 vol%.

[0067] (6) Start ingot drawing when the blow hole position is in the middle and lower part of the slag layer until the melting is completed.

[0068] (7) Stop blowing helium gas and demold after 120 min.

[0069] (8) Use this mold to smelt electrodes with the same composition and specifications by adopting the above process without blowing helium gas for a comparative experiment, and produce a comparative electroslag. The obtained electroslag in this example (hereinafter referred to as the electroslag of this method) and the comparative electroslag are detected. The detected droplet size and quality of the slag layer are shown in Table 5, and the sulfur, phosphorus, and non-metallic inclusion contents of the ingot are shown in Table 6.

[0070] Table 5: Dimensions and Quantities of Metal Droplets in Electroslag Remelting

[0071]

[0072] It can be seen from Table 5 that the average particle size of the electroslag metal droplets of the present invention is 4.55 mm, while that of the comparative electroslag is 9.67 mm, a decrease of 52.9%. Under the same mass, the specific surface area increases by 112%. The residual amount of droplets in the slag of this method is 165.9 g, while that of the comparative electroslag is 97.8 g, indicating that the residence time increases by 69.6% compared with the comparative electroslag.

[0073] Table 6: Non-Metallic Inclusions and Sulfur and Phosphorus Contents (wt) after Electroslag Melting

[0074]

[0075] It can be seen from Table 6 that the total amount of inclusions, the rating, and the S content of the electroslag of the present invention are all lower than those of the comparative electroslag. Among them, the total amount of inclusions is 62.4% lower than that of the comparative electroslag, and the sulfur content is 73.6% lower than that of the comparative electroslag.

[0076] Example 4: The following specific process is adopted for the smelting method of this electroslag remelting ultra-clean steel.

[0077] Electroslag smelting of NS333. Specific process: Specification of the ingot-pulling crystallizer: Φ400×850 mm; Specification of the consumable electrode: Φ300×2800 mm; Alloy composition: C 0.04%, Si 0.4%, Mn 0.4%, P 0.008%, S 0.005%, Cr 15%, Mo 16%, Co 2.5%, W 4%, Fe 5%, V 0.2%, the balance is Ni and inevitable impurities; Slag composition (wt): 60% CaF2, 25% CaO, 10% Al2O3, 5% MgO; Slag amount: 50 kg. Four gas blowing holes with an inner diameter of 0.15 mm are evenly distributed on the plane at the middle part of the crystallizer; The smelting process is as follows:

[0078] (1) Polish and weld the consumable electrode until it is shiny;

[0079] (2) Place the slag material in a heating furnace at 650°C and bake for 10 h;

[0080] (3) Lay the solid slag arc-starter, install the electrode, and introduce helium gas into the crystallizer;

[0081] (4) Start the power supply, add slag, and after melting the slag for 45 min, raise the temperature to 1750°C;

[0082] (5) Enter the smelting stage. When the slag layer covers the gas blowing holes, start blowing high-purity helium gas at 250°C, 0.07 MPa, 0.35 L / min, and a water content of 0.00007 vol%;

[0083] (6)Start ingot withdrawal when the blowing hole position reaches the middle - lower part of the slag layer until the smelting ends;

[0084] (7)Stop blowing helium gas and demold after 80 min.

[0085] (8)Using this mold, smelt electrodes with the same composition and specifications by the above process without blowing helium gas for a comparative experiment to produce comparative electroslag. Detect the obtained electroslag in this example (hereinafter referred to as the electroslag of this method) and the comparative electroslag. The detected molten droplet size and mass in the slag layer are shown in Table 7, and the sulfur, phosphorus and non - metallic inclusion contents in the ingot are shown in Table 8.

[0086] Table 7: Molten Droplet Size and Quantity of Remelted Electroslag

[0087]

[0088] It can be seen from Table 7 that the average particle diameter of the molten droplets of the electroslag of this method is 3.74 mm, while that of the comparative electroslag is 8.16 mm, a decrease of 54.2%. The specific surface area increases by 118% under the same mass. The residual amount of molten droplets in the slag of the electroslag of this method is 143.8 g, and that of the comparative electroslag is 88.5 g, indicating that the residence time increases by 62.5% compared with the comparative electroslag.

[0089] Table 8: Contents of Non - metallic Inclusions, Sulfur and Phosphorus after Electroslag Melting (wt)

[0090]

[0091] It can be seen from Table 8 that the total amount of inclusions, the rating and the S content of the electroslag of this method are all lower than those of the comparative electroslag. Among them, the total amount of inclusions is 63.0% lower than that of the comparative electroslag, and the sulfur content is 66.7% lower than that of the comparative electroslag.

[0092] It can be concluded from the above 4 examples that the larger the cross - section, the greater the advantage of this method. For molds with a diameter of Φ300 mm and above, the sulfur and inclusion contents are both reduced by more than 60% compared with traditional electroslag. Therefore, this method has broad application value.

Claims

1. A method for smelting electroslag remelted ultra-clean steel, characterized in that: Electroslag melting is carried out by using a drawing mold; the drawing mold is provided with air blowing holes in the middle and lower parts of the slag pool, and there are at least two air blowing holes, which are evenly arranged on the same plane around the slag pool, and the air blowing holes are horizontally arranged; during the smelting process, the air blowing holes horizontally blow high-purity gas that does not participate in the reaction into the middle and lower parts of the slag pool, and the total flow rate of the high-purity gas is 0.1-0.5 L / min, and the pressure of the high-purity gas is 0.06-0.08 MPa; high-purity gas bubbles are formed in the slag pool during the air blowing process, and the molten droplets are continuously impacted and broken by the slag flow and the high-purity gas bubbles during the falling process; the high-purity gas is horizontally blown into the slag pool.

2. The smelting method of an electroslag remelting ultra-clean steel according to claim 1, characterized in that: The inner diameter of the conduit is 0.12-0.16 mm, and the conduit is the air blowing hole.

3. The smelting method of electroslag remelting ultra-clean steel according to claim 1, characterized in that: The high-purity gas is symmetrically blown into both sides of the middle and lower parts of the slag pool.

4. The smelting method of electroslag remelting ultra-clean steel according to claim 1, characterized in that: The high-purity gas is high-purity helium gas.

5. The smelting method of electroslag remelting ultra-clean steel according to claim 1, characterized in that: The temperature of the high-purity gas is 200-300 °C.

6. A method for smelting electroslag remelted ultra-clean steel according to any one of claims 1-5, characterized in that: In the normal smelting stage, start blowing air when the slag layer covers the air blowing holes (3), and start drawing the ingot when the air blowing holes are located in the middle and lower parts of the slag layer until the smelting is completed.

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