A method for alloying metallic bismuth in molten steel of a vacuum induction furnace

Abstract

The present invention relates to the technical field of vacuum induction melting, and particularly to an alloying method for metallic bismuth in molten steel of a vacuum induction furnace. The characteristics include ramming, wrapping, sequential addition, and melting. The specific steps are as follows: 1) Ramming, the ramming position of the crucible is at the lower 1 / 4 of the middle part of the crucible; 2) Wrapping, divide bismuth into several parts and wrap them completely with iron sheets respectively; 3) Sequential addition, after putting bismuth into a certain grid of the bin, put lump-shaped industrial pure iron into the next grid behind it, and the bin is fed in order from front to back; 4) Melting, after adding all the other alloys except bismuth, turn off the power to cool down. When a film forms on the surface of the molten steel, add the bismuth blocks wrapped with iron sheets, and immediately add industrial pure iron again to quickly cool and solidify the molten steel above the molten pool, then supply power for heating; when the steel shell above the molten pool melts, stir, and then quickly pour into ingots. The advantages of the present invention are: bismuth is added to the steel in a wrapped form, reducing the volatilization caused by high-temperature gasification, and the recovery rate reaches more than 85%.

Description

Technical Field

[0001] The present invention relates to the technical field of vacuum induction melting, and particularly relates to an alloying method for adding metallic bismuth to molten steel in a vacuum induction furnace. Background Art

[0002] Bismuth is a low-melting-point element with a density greater than that of steel. Its density is 9.8 g / cm3, its melting point is 271.3 °C, and its boiling point is about 1560 ± 5 °C. In production practice, since lead is a toxic and harmful element, bismuth is often used to replace lead and added to free-cutting steel to improve the machinability of the steel. However, due to its high density, low melting point, low boiling point, and the property that its vapor pressure in steel at 1600 °C is much higher than the vapor pressure of iron, the phenomenon of very low recovery rate and uneven element distribution occurs after bismuth is added to molten steel. Especially in the process of vacuum smelting, volatilization loss is more likely to occur.

[0003] Vacuum smelting is a metallurgical process that uses a vacuum induction furnace to complete the melting and casting of materials such as ordinary steel, special steel, and superalloys under vacuum or protective atmosphere conditions. It can better control the alloy composition and has electromagnetic stirring and a powerful high-vacuum degassing ability. When using a vacuum furnace to smelt steel grades containing bismuth, since a vacuum induction furnace generally does not carry out slag making and is in a vacuum-sealed environment, the conventional method of adding bismuth alloy by feeding a bismuth wire into the tundish cannot be used. After adding bismuth alloy, the recovery rate is very low, the fluctuation range of bismuth content is large, and it is also impossible to smelt steel grades with a high bismuth content. The present invention provides a method for adding metallic bismuth during the smelting of molten steel in a vacuum induction furnace, with a recovery rate that can be stably maintained above 85%, and the distribution of bismuth in the steel is uniform, which can meet the smelting requirements for steel grades containing bismuth in scientific research work.

[0004] Currently, the commonly used methods for adding bismuth to molten steel are feeding a bismuth wire into the continuous casting tundish or mold, or directly adding bismuth alloy or metallic bismuth. The Chinese invention patent with the publication number CN 110205445 discloses an alloying method for adding metallic bismuth to a ladle. The molten steel is smelted by a converter and tapped into the ladle and undergoes RH refining treatment; after the RH refining of the molten steel is completed, argon is immediately blown from the bottom; when it is observed that the molten steel surface is churning, the Bi metal including iron sheet is immediately added to the designated area of the ladle by a charging trolley. When there are multiple bismuth-containing containers, continuous addition is carried out until the end; after the addition of bismuth is completed, argon is blown from the bottom for another 2 - 3 minutes; and the bismuth-containing container should be submerged by the molten steel within no more than 5 seconds to achieve the purpose of bismuth alloying. In this method, the distribution of bismuth among the head slab, middle slab, and tail slab is uniform, that is, the fluctuation of bismuth content is within ±2%, and the recovery rate of bismuth is stable at 40%. This method can make the distribution of bismuth uniform, but the recovery rate is too low.

[0005] The Chinese invention patent with the publication number CN108359768 discloses a method for bismuth alloying in a ladle. In this method, a wire feeder is used to feed metallic bismuth into the molten steel in the form of cored wire. The diameter of the bismuth cored wire is 8 - 14 mm, the thickness of the steel outer skin is 0.5 - 1.5 mm, and the diameter of the bismuth element particles is 0 - 3 mm. The bottom blowing stirring of the ladle is turned on, the feeding temperature is between 1600 - 1780 °C, the feeding rate is 80 - 110 m / min, and stirring continues for 3 - 8 min after feeding. The present invention avoids the problems of large volatilization, sedimentation, and oxidation losses caused by the low melting and boiling points and active properties of metallic bismuth. The recovery rate reaches over 50%, effectively reducing the splash risk and a large amount of flue gas pollution caused by the violent reaction when metallic bismuth is directly added. However, when using a vacuum induction furnace for smelting, the operation of feeding the bismuth wire is relatively complex and difficult to control, and the recovery rate is also relatively low.

[0006] The Chinese invention patent with the publication number CN112746215 discloses a smelting method for a steel containing low - melting - point and high - density elements with uniform composition; in this method, smelting and refining are carried out before bismuth alloying. During the refining process, when the temperature of the molten steel for smelting is 1550 °C - 1650 °C, alloying is carried out by adding alloys in at least 3 batches to obtain a molten steel containing alloy elements; the molten steel containing alloy elements is cast to obtain an ingot. This method improves the recovery rate of elements by controlling the temperature of the molten steel for smelting to ensure that the alloy is added to the molten steel for smelting near the boiling point, and alloying is carried out by adding alloys in at least 3 batches, effectively reducing the degree of reaction splashing. However, the recovery rate of bismuth in this method is relatively low, and it is relatively difficult to smelt steel grades with a relatively high bismuth content.

[0007] The Chinese invention patent with the publication number CN103388050 discloses a method for adding low - melting - point metal bismuth to free - cutting molten steel. Its main feature is that a sufficiently thick slag layer is added on the surface of the molten steel before bismuth addition. Metallic bismuth powder and iron powder (or alloy powder meeting the requirements of the smelted steel composition) are mixed in a certain proportion and used as the bismuth source, and are added to the molten steel deep enough under the slag layer in the form of wire feeding. Argon is always blown in for soft stirring during and after the addition of the bismuth source. The wire - feeding method of this method is relatively complex, not suitable for vacuum induction furnace smelting, and the processing cost of the cored wire is relatively high. Summary of the Invention

[0008] The purpose of the present invention is to provide a method for alloying metallic bismuth in molten steel melted by a vacuum induction furnace, overcoming the deficiencies of the prior art, increasing the recovery rate of bismuth to over 86%, ensuring uniform distribution of bismuth in the steel, being able to smelt steel grades with a high bismuth content, and meeting the usage requirements of bismuth - containing steel grades in production and scientific research work.

[0009] To achieve the above - mentioned purpose, the present invention is realized through the following technical solutions:

[0010] An alloying method for melting metallic bismuth in molten steel of a vacuum induction furnace, characterized by comprising ramming, wrapping, sequential addition and melting, and the specific steps are as follows:

[0011] 1) Ramming: Ram the crucible used in the vacuum induction furnace to ensure that it does not crack and leak steel during the melting of molten steel. The ramming position of the crucible is at the lower 1 / 4 of the middle part of the crucible, aligned with the middle of the induction coil;

[0012] 2) Wrapping: Take the particle size of metallic bismuth to be 2 - 15 mm, divide the bismuth into several portions, and wrap each portion completely with iron sheets. The wrapping thickness of the iron sheets is 1 - 5 mm, and the wrapping thickness of each iron sheet can be different;

[0013] 3) Sequential addition: Put the bismuth wrapped with iron sheets into the feed bin above the vacuum induction furnace. There are multiple feeding areas in the feed bin. After putting the bismuth into a certain grid, put crushed industrial pure iron into the next grid behind it, and the feed bin feeds materials in the front - to - back order;

[0014] 4) Melting: Charge, melt, refine, and alloy according to the normal vacuum furnace smelting process. After all alloys except bismuth are added, turn off the power and cool down to 10 - 25 °C above the liquidus. At this time, a film begins to form on the surface of the molten steel, the pressure in the furnace is 10000 - 90000 Pa, add the bismuth blocks wrapped with iron sheets. After the bismuth blocks fall from a high place in the feeding hopper, they directly enter different depths inside the molten pool. Immediately add crushed industrial pure iron blocks after adding the bismuth blocks, causing the molten steel above the molten pool to cool and solidify rapidly. Wait for 1 - 2 minutes, then supply power at a low power; after the solidified steel shell above the molten pool melts, apply electromagnetic stirring, and then quickly cast into ingots.

[0015] The upper end of the crucible is flush with the upper edge of the induction coil or slightly higher than the upper edge by within 5 cm.

[0016] In step 2), the single - layer thickness of the iron sheet is 2 mm - 7 mm.

[0017] In step 2), the material of the iron sheet is industrial pure iron or IF steel.

[0018] In step 3), the relationship between the addition amount m of the crushed industrial pure iron and the crucible size is: m > r 2 ×3.14×0.01×ρ, where: m is the mass, in kg; r is the inner radius of the crucible, in m; ρ is the density of molten steel, in kg / m 3 .

[0019] In step 4), the power of the low - power power supply is 30% - 60% of the effective power of the equipment power supply.

[0020] In step 4), the power of the electromagnetic stirring is 80% - 100% of the maximum effective power of the equipment.

[0021] In step 4), the pressure in the furnace is 70,000 Pa - 80,000 Pa.

[0022] In the technical solution of the present invention, due to the relatively large density of bismuth, after being wrapped, it is relatively heavy by itself. After falling from a high place in the feeding hopper, it directly enters the molten pool, resulting in a decrease in the temperature of the molten steel. After adding bismuth, a large amount of industrial pure iron chunks are immediately added, causing the molten steel above the molten pool to cool and solidify rapidly. Since the iron sheet wrapped on the surface of bismuth has not completely melted, bismuth does not participate in alloying. After waiting for 1 - 2 minutes, power is supplied at a low power. At this time, because the position where the crucible is knotted is relatively high, when the power is supplied for heating and the temperature rises slowly, the temperature rises relatively fast in the lower part of the molten pool, and the temperature rise at the solidified part above the molten pool is very slow. Therefore, the bismuth block undergoes alloying in a closed environment. And because the bismuth block is small in size and relatively dispersed in the molten steel, the alloying occurs sequentially, so bismuth undergoes alloying slowly, and the vaporized part of bismuth will eventually melt in the molten steel.

[0023] Compared with the prior art, the beneficial effects of the present invention are:

[0024] 1) Bismuth is added to the steel in a wrapped form, reducing the volatilization caused by high-temperature gasification, so that its recovery rate can reach more than 85%, and can reach up to 95% at most, and there is no splashing and flue gas pollution;

[0025] 2) The composition control of bismuth in the molten steel is relatively stable, and steel grades with high bismuth content can be stably smelted;

[0026] 3) The bismuth composition of the steel ingot smelted by the vacuum induction furnace is evenly distributed, and steel grades with high bismuth content can be smelted, which can meet the usage requirements of bismuth-containing steel grades in production and scientific research work. Specific Embodiments

[0027] The technical solution of the present invention will be clearly and completely described below in conjunction with specific embodiments. Obviously, the described embodiments are some, but not all, of the embodiments of the present invention.

[0028] In order to more clearly illustrate the specific embodiments of the present invention or the technical solutions in the prior art, the specific embodiments required for use in the description of the specific embodiments or the prior art will be briefly introduced below. Obviously, the specific embodiments described below are some embodiments of the present invention. For those of ordinary skill in the art, without creative efforts, other specific embodiments can be obtained based on these specific embodiments.

[0029] The components of the embodiments of the present invention that are usually described and shown in detail in the specific embodiments herein can be arranged and designed in countless different configurations. Therefore, the following detailed description of the embodiments of the present invention provided in the specific embodiments is not intended to limit the scope of the claimed invention, but merely represents selected embodiments of the present invention.

[0030] An alloying method for melting metallic bismuth in molten steel in a vacuum induction furnace of the present invention includes ramming, wrapping, sequential addition, and melting. The specific steps are as follows:

[0031] 1) Ramming: First, ram the crucible used in the vacuum induction furnace to ensure that it does not crack and leak molten steel during the melting of molten steel. The relative position of the crucible and the induction coil is different from the conventional one. In the conventional crucible placement position, the high-temperature stirring area of the molten steel should be in the middle of the crucible. However, in this invention patent, the ramming position of the crucible is moved upward. The ramming position of the crucible is at the 1 / 4 position in the lower-middle part of the crucible, aligned with the middle part of the induction coil, so that the high-temperature stirring area is around the 1 / 4 in the lower-middle part of the crucible;

[0032] 2) Wrapping: Metallic bismuth is selected or crushed into a particle size of 2 - 15 mm. Calculate the addition amount of bismuth according to the bismuth content in the steel grade. Bismuth is divided into several portions and each portion is completely wrapped with iron sheets. The wrapping thickness of the iron sheets is 1 - 5 mm, and the wrapping thickness of each iron sheet can be different;

[0033] 3) Sequential addition: Put the wrapped bismuth into the feed bin above the vacuum induction furnace. There are multiple grid feeding areas in the feed bin, which can realize the function of continuous feeding. After putting bismuth into a certain grid, put lump-shaped industrial pure iron into the next grid. The feed bin feeds in sequence according to the front-back order;

[0034] 4) Melting: Charge, melt, refine, and alloy according to the normal vacuum furnace smelting process. After adding all alloys except bismuth, turn off the power and cool down to 10 - 25 °C above the liquidus line. At this time, a film begins to form on the surface of the molten steel, and the furnace pressure is 10,000 - 90,000 Pa, with the optimal pressure value being 80,000 Pa. Then add the wrapped bismuth blocks. Due to the relatively large density of bismuth and the heavy weight after wrapping, when they fall from a high place in the charging hopper, they directly enter the molten pool at different depths, causing the temperature of the molten steel to drop. Immediately after adding bismuth, add a large amount of industrial pure iron scraps, causing the molten steel above the molten pool to cool and solidify rapidly. At this time, since the iron sheet wrapped on the surface of bismuth has not completely melted, bismuth has not been alloyed. After waiting for 1 - 2 minutes, power on at a low power. At this time, because the position of the crucible knotting is relatively high, when heating slowly with electricity, the lower part of the molten pool heats up faster, and the solidified part above the molten pool heats up very slowly. Therefore, bismuth blocks are alloyed in a closed environment. And because the bismuth blocks are small in size and relatively dispersed in the molten steel, the alloying process proceeds sequentially and is slow, and the vaporized part of bismuth will eventually also melt in the molten steel. After the solidified steel shell above the molten pool melts, increase the power for electromagnetic stirring, and then quickly cast into ingots.

[0035] Example 1

[0036] The alloying method of metallic bismuth in molten steel by the vacuum induction furnace of the present invention. Example 1 is applied to smelt a certain steel grade with a bismuth content of 0.25% in a 200 kg vacuum induction furnace. The inner diameter of the crucible is 275 mm. The specific steps are as follows:

[0037] 1) Knotting: Knot the crucible used in the vacuum induction furnace. The knotting height of the crucible exceeds the induction coil by 2 cm, so that the high-temperature stirring area is about 1 / 4 of the middle and lower part of the crucible.

[0038] 2) Wrapping: Divide 512 g of the added metallic bismuth into 9 parts with a particle size less than 15 mm, and wrap 3 pieces each with 2 layers, 3 layers, and 4 layers of industrial pure iron sheets with a thickness of 0.5 mm.

[0039] 3) Sequential addition: Put the bismuth wrapped with iron sheets into the bin, and put 10 kg of small pieces of industrial pure iron in the subsequent bin, and add them in sequence. The total weight of the smelted molten steel is designed to be 175 kg.

[0040] 4) Melting: Close the door of the vacuum induction furnace, evacuate the air, heat it after evacuation, conduct refining after melting, add other alloys, and ensure that the pressure in the furnace is 70,000 Pa with argon as the protective atmosphere. Turn off the heating power supply and cool down to 1540 °C. At this time, a thin film begins to form on the surface of the molten steel. Then add bismuth blocks. The bismuth blocks fall to the deep part of the molten steel. Immediately add small pieces of industrial pure iron. The upper surface of the molten steel quickly solidifies into a shell. Wait for 1 minute and then supply power to increase the temperature with a low power of 60 kW. The lower and middle parts of the crucible molten pool begin to heat up, while the temperature rise at the upper solidified shell is slow. The bismuth slowly undergoes alloying. After the solidified shell above the molten pool is completely melted, use the maximum power of 110 kW of the equipment for electromagnetic stirring to adjust the temperature and then quickly tap the steel.

[0041] Take a sample of the steel ingot smelted in Example 1 for analysis and testing. The recovery rate of bismuth is 88%.

[0042] Example 2

[0043] The alloying method of metallic bismuth in molten steel by the vacuum induction furnace of the present invention. Example 2 is applied to smelt a certain steel grade with 0.1% bismuth content in a 50 kg vacuum induction furnace. The inner diameter of the crucible is 180 mm. The specific steps are as follows:

[0044] 1) Lining: The height of the crucible lining exceeds the induction coil by 1 cm, so that the high-temperature stirring area is about 1 / 4 of the lower and middle parts of the crucible.

[0045] 2) Wrapping: Divide the 50 g of metallic bismuth that needs to be added, which is calculated, into 4 parts with a particle size less than 10 mm. Wrap two pieces each with 2 layers and 3 layers using industrial pure iron sheets with a thickness of 0.3 mm.

[0046] 3) Sequential addition: Put the bismuth wrapped with iron sheets into the feed bin, and put 4 kg of small pieces of industrial pure iron in the subsequent feed bin. The total weight of the smelted molten steel is designed to be 45 kg.

[0047] 4) Melting: Close the door of the vacuum induction furnace, evacuate the air, heat it after evacuation, conduct refining after melting, add other alloys, and ensure that the pressure in the furnace is 60,000 Pa with argon as the protective atmosphere. Turn off the heating power supply and cool down to 1535 °C. At this time, a thin film begins to form on the surface of the molten steel. Then add bismuth blocks. The bismuth blocks fall to the deep part of the molten steel. Immediately add small pieces of industrial pure iron. The upper surface of the molten steel quickly solidifies into a shell. Wait for 2 minutes and then supply power to increase the temperature with a low power of 30 kW. The lower and middle parts of the crucible molten pool begin to heat up, while the temperature rise at the upper solidified shell is slow. The bismuth slowly undergoes alloying. After the solidified shell above the molten pool is completely melted, use the maximum power of 70 kW of the equipment for stirring to adjust the temperature and then quickly tap the steel.

[0048] Take a sample of the steel ingot smelted in Example 2 for analysis and testing. The recovery rate of bismuth is 91%.

[0049] Although embodiments of the present invention have been shown and described, those of ordinary skill in the art will appreciate that various changes, modifications, substitutions, and variations can be made to these embodiments without departing from the principles and spirit of the present invention. The scope of the present invention is defined by the appended claims and their equivalents.

Claims

1. A method for alloying molten steel in a vacuum induction furnace with metallic bismuth, characterized in that, It includes knotting, wrapping, sequential addition and melting, and the specific steps are as follows: 1) Knotting: Knot the crucible used in the vacuum induction furnace to ensure that it does not crack and leak molten steel during the melting of molten steel. The knotting position of the crucible is at the lower-middle 1 / 4 position of the crucible, aligned with the middle of the induction coil; 2) Wrapping: Take the particle size of metallic bismuth to be 2 - 15 mm, divide the bismuth into several portions, and wrap each portion completely with iron sheets. The wrapping thickness of the iron sheets is 1 - 5 mm, and the wrapping thickness of each iron sheet can be different; 3) Sequential addition: Put the bismuth wrapped with iron sheets into the hopper above the vacuum induction furnace. There are multiple grid feeding areas in the hopper. After putting the bismuth into a certain grid, put broken lump industrial pure iron into the next grid. The hopper feeds materials in sequence; 4) Melting: Charge, melt, refine, and alloy according to the normal vacuum furnace smelting process. After adding all alloys except bismuth, turn off the power and cool down to 10 - 25 °C above the liquidus. At this time, a film begins to form on the surface of the molten steel, and the furnace pressure is between - 10000 Pa and - 90000 Pa. Add the bismuth blocks wrapped with iron sheets in portions, adding one portion or several portions each time. After the bismuth blocks fall from a high place in the feeding hopper, they directly enter different depths inside the molten pool. Immediately add broken pieces of industrial pure iron after adding the bismuth blocks, causing the molten steel above the molten pool to cool and solidify rapidly. Wait for 1 - 2 minutes and then supply power at a low power; after the solidified steel shell above the molten pool melts, apply electromagnetic stirring, and then quickly cast into ingots.

2. The alloying method for alloying metallic bismuth in molten steel of a vacuum induction furnace according to claim 1, characterized in that, The upper end of the crucible is flush with the upper edge of the induction coil or slightly higher than the upper edge within 5 cm.

3. The alloying method for metallic bismuth in molten steel of a vacuum induction furnace according to claim 1, characterized in that, In step 2), the single-layer thickness of the iron sheet is 0.3 mm or 0.5 mm.

4. The alloying method for bismuth metal in molten steel of a vacuum induction furnace according to claim 1, characterized in that, In step 2), the material of the iron sheet is industrial pure iron or IF steel.

5. A method for alloying metallic bismuth in molten steel of a vacuum induction furnace according to claim 1, characterized in that, In step 3), the relationship between the addition amount m of the commercially pure iron fragments and the crucible size is: m > r 2 × 3.14 × 0.01 × ρ, where: m is the mass, in kg; r is the inner radius of the crucible, in m; ρ is the density of the molten steel, in kg / m 3 .

6. The alloying method for melting metallic bismuth in molten steel by a vacuum induction furnace according to claim 1, characterized in that, In step 4), the power of the low-power power supply is 30% - 60% of the effective power of the equipment power supply.

7. The alloying method of metallic bismuth in molten steel of a vacuum induction furnace according to claim 1, characterized in that, In step 4), the power of the electromagnetic stirring is 80% - 100% of the maximum effective power of the equipment.

8. The alloying method for melting metal bismuth in molten steel of a vacuum induction furnace according to claim 1, characterized in that, In step 4), the furnace pressure is 70000 Pa - 80000 Pa.