Research Status of Bubble Removal Technology for Inclusions (Part II)

This article introduces the current research status of bubble removal technology for inclusion removal, mainly including ladle argon blowing technology, ladle long nozzle argon blowing technology, reaction induced micro-heterogeneous phase technology, tundish gas curtain retaining wall technology, pressurization and decompression method, ultrasonic cavitation method, nitrogen addition and nitrogen precipitation method and micro hydrogen bubble method.This article introduces the current research status of bubble removal technology, which mainly includes ladle argon blowing technology, ladle long nozzle argon blowing technology, reaction induced micro-heterogeneous phase technology, tundish gas curtain retaining wall technology, pressurization and decompression method, ultrasonic cavitation method, nitrogen addition and nitrogen precipitation method and micro hydrogen bubble method.

Argon blowing technology in ladle. Argon blowing in ladle is one of the important refining methods. It can not only make the composition and temperature of molten steel uniform, but also purify molten steel by allowing bubbles to adhere to inclusions and the bubble tail to carry inclusions to float. The structure of the air-permeable bricks used for argon blowing in ladle has a direct impact on the bubble size. The pore size is generally 2mm~4mm. The diameter of the bubbles generated within the commonly used argon blowing flow range is about 10mm~20mm. The bubbles generated by bottom argon blowing will expand rapidly during the floating process in molten steel. Therefore, the probability of bubbles capturing small particle inclusions is very small, and the removal effect of smaller inclusions is not ideal.Argon blowing technology in ladle. Argon blowing in ladle is one of the important refining methods. It can not only make the composition and temperature of molten steel uniform, but also purify molten steel by allowing bubbles to adhere to inclusions and the bubble tail to carry inclusions to float. The structure of the air-permeable bricks used for argon blowing in ladle has a direct impact on the bubble size. The pore size is generally 2mm~4mm. The diameter of the bubbles generated within the commonly used argon blowing flow range is about 10mm~20mm. The bubbles generated by bottom argon blowing will expand rapidly during the floating process in molten steel. Therefore, the probability of bubbles capturing small particle inclusions is very small, and the removal effect of smaller inclusions is not ideal.

Ladle argon blowing technology has the advantages of simple equipment, low investment and simple operation, and has been applied by major steel mills; however, its poor effect on removing microscopic inclusions is also an unavoidable "shortcoming".

Ladle argon blowing technology has the advantages of simple equipment, low investment and simple operation, and has been applied by major steel mills; however, its poor effect on removing microscopic inclusions is also an unavoidable "shortcoming".

Argon blowing technology for ladle shroud. During continuous casting, a large amount of argon is blown into the ladle protective sleeve below the joint. The turbulent molten steel in the sleeve breaks the gas into dispersed tiny bubbles. The formed bubbles enter the tundish with the turbulent molten steel, float and grow, and constantly collide and adhere to inclusions, and finally carry the inclusions to float and be removed. Compared with the traditional argon blowing at the connection between the shroud and the ladle, the argon blowing technology for the ladle shroud has a large amount of argon blowing, which can form a large number of dispersed tiny bubbles in the shroud and the injection area of the tundish, and has a good effect in removing inclusions.Argon blowing technology for ladle shroud. During continuous casting, a large amount of argon is blown into the ladle protective sleeve below the joint. The turbulent molten steel in the sleeve breaks the gas into dispersed tiny bubbles. The formed bubbles enter the tundish with the turbulent molten steel, float and grow, and constantly collide and adhere to inclusions, and finally carry the inclusions to float and be removed. Compared with the traditional argon blowing at the connection between the shroud and the ladle, the argon blowing technology for the ladle shroud has a large amount of argon blowing, which can form a large number of dispersed tiny bubbles in the shroud and the injection area of the tundish, and has a good effect in removing inclusions.

The argon blowing technology for the long shroud of the ladle requires blowing a large amount of argon into the molten steel, which is easy to form a "naked hole" in the tundish, causing secondary oxidation of the molten steel. With the improvement of tundish sealing technology, especially the adoption of sealed tundish, the argon blowing technology for the long shroud is expected to be well applied.

The argon blowing technology for the long shroud of the ladle requires blowing a large amount of argon into the molten steel, which is easy to form a "naked hole" in the tundish, causing secondary oxidation of the molten steel. With the improvement of tundish sealing technology, especially the adoption of sealed tundish, the argon blowing technology for the long shroud is expected to be well applied.

Reaction-induced micro-heterogeneous phase technology. Reaction-induced micro-heterogeneous phase removal of fine inclusions in steel is achieved by adding fine sodium carbonate to the molten steel to generate tiny bubbles in the molten steel to make the inclusions float up and be removed. Some researchers have further studied this method and designed a composite sphere with this function. This tiny sphere is added to the molten steel and decomposes at high temperature to produce bubbles and slag droplets. The generated slag droplets collide, aggregate and grow with inclusions such as Al2O3, accelerating their floating removal. The composite sphere has been subjected to industrial test research in the RH refining furnace of Anshan Iron and Steel. After the molten steel is treated with this technology, the number of oxide inclusions in the ingot is significantly reduced, the size is reduced, and the total oxygen in the steel can be as low as 6×10-6.Reaction-induced micro-heterogeneous phase technology. Reaction-induced micro-heterogeneous phase removal of fine inclusions in steel is achieved by adding fine sodium carbonate to the molten steel to generate tiny bubbles in the molten steel to make the inclusions float up and be removed. Some researchers have further studied this method and designed a composite sphere with this function. This tiny sphere is added to the molten steel and decomposes at high temperature to produce bubbles and slag droplets. The generated slag droplets collide, aggregate and grow with inclusions such as Al2O3, accelerating their floating removal. The composite sphere has been subjected to industrial test research in the RH refining furnace of Anshan Iron and Steel. After the molten steel is treated with this technology, the number of oxide inclusions in the ingot is significantly reduced, the size is reduced, and the total oxygen in the steel can be as low as 6×10-6.

This technology has not yet been widely promoted and applied in steel enterprises, and the theoretical research on this technology is still incomplete. For example, the size of the bubbles generated, the distribution of bubbles in the molten steel, and the temperature drop of the molten steel have not been studied in depth.

This technology has not yet been widely promoted and applied in steel enterprises, and the theoretical research on this technology is still incomplete. For example, the size of the bubbles generated, the distribution of bubbles in the molten steel, and the temperature drop of the molten steel have not been studied in depth.

Tundish gas curtain retaining wall technology. Tundish gas curtain retaining wall technology, also known as tundish bottom argon blowing technology, works by blowing bubbles into the molten steel through porous bricks embedded in the bottom of the tundish. These bubbles collide with and adhere to inclusion particles in the molten steel, increasing the upward vertical movement of the inclusions and thus purifying the molten steel. At the same time, tundish argon blowing can change the flow state of the molten steel, promote mixing, and benefit uniform temperature and composition.

4 中间包气幕挡墙技术。中间包气幕挡墙技术即中间包底部吹氩技术，其原理是通过埋设于中间包底部的透气砖向钢液中吹入的气泡，与流经此处钢液中的夹杂物颗粒相互碰撞聚合吸附，增加了夹杂物的垂直向上运动，从而达到净化钢液的目的。同时，中间包吹氩可以改变钢液的流动状态，促进钢液的混合，有利于温度及成分的均匀。

Tundish air curtain retaining wall technology. Tundish air curtain retaining wall technology is the argon blowing technology at the bottom of the tundish. Its principle is that the bubbles blown into the molten steel through the air-permeable bricks buried at the bottom of the tundish collide with the inclusion particles in the molten steel flowing through this place, aggregate and adsorb, increase the vertical upward movement of the inclusions, and thus achieve the purpose of purifying the molten steel. At the same time, argon blowing in the tundish can change the flow state of the molten steel, promote the mixing of the molten steel, and is conducive to the uniformity of temperature and composition.

Although some progress has been made in the theoretical research of argon blowing in the tundish, some companies have reported that the effect is not very stable and is not widely used in practice. The main problems are: the generated bubbles are large, and the effect of capturing and removing inclusions is not obvious; the amount of gas injection is limited to prevent slag entrainment and secondary oxidation of molten steel; the cost of air-permeable bricks is slightly higher, and the installation is inconvenient.

Although some progress has been made in theoretical research on argon blowing in the tundish, some companies have reported that the effect of use is not very stable and is not widely used in practice. The main problems currently exist are: the size of the generated bubbles is large, and the effect of capturing and removing inclusions is not obvious; the amount of gas blowing is limited because it is necessary to prevent the slag from rolling up in the tundish and the secondary oxidation of the molten steel; the cost of the air-permeable bricks is slightly high, and it is inconvenient to bury them, etc.

5 Pressure increase and decrease method. In the early 1990s, Japan's NKK company proposed the Pressure Elevating and Reducing Method (PERM) to remove inclusions from steel. The principle is mainly divided into three steps: first, pressurizing to dissolve N2 in the molten steel to reach supersaturation; second, rapidly reducing the pressure, allowing bubbles to nucleate and grow heterogeneously on the surface of inclusions; third, the bubbles carry the inclusions to float up and eventually separate from the molten steel.

In the early 1990s, Japan's NKK company proposed the Pressure Elevating and Reducing Method (PERM) to remove inclusions from steel. The principle is mainly divided into three steps: first, pressurizing the steel to dissolve N2 in the molten steel to reach supersaturation; second, rapidly reducing the pressure, allowing bubbles to nucleate and grow heterogeneously on the surface of inclusions; third, the bubbles carry the inclusions to float up and eventually separate from the molten steel.

The pressure increase and pressure reduction method is significantly effective in removing inclusions from steel. However, since this method requires high-pressure treatment of molten steel, the operation is difficult, and it has not been industrialized yet.

The pressure increase and pressure reduction method is effective in removing inclusions from steel. However, since this method requires high pressure treatment of molten steel, it is difficult to operate and has not yet been industrialized.

6 Ultrasonic cavitation method. Ultrasonic waves are mechanical waves. During the propagation in liquid media, they produce periodic stress and sound pressure changes. When propagating in molten steel, they activate tiny bubble nuclei in the molten steel, causing a series of processes including oscillation, growth, contraction, and even collapse. This process of microbubbles from oscillation growth to collapse is called ultrasonic cavitation.

Ultrasonic cavitation method. Ultrasonic waves are mechanical waves that produce periodic stress and sound pressure changes during the propagation of liquid media. When propagating in molten steel, they activate the tiny bubble nuclei in the molten steel, causing them to undergo a series of processes including oscillation, growth, contraction, and even collapse. This process of microbubbles from oscillation growth to collapse is called ultrasonic cavitation.

The cavitation bubbles generated by ultrasonic waves have a small diameter of only tens of microns. During the floating process, cavitation bubbles have more opportunities to collide with tiny inclusions and adhere to each other to form clusters, thereby effectively removing tiny inclusions in the molten steel. However, due to the difficulty in introducing ultrasonic waves into molten steel and the difficulty in finding waveguide materials that can be used at high temperatures, research on the removal of inclusions by ultrasonic cavitation bubbles is still concentrated in the water model and laboratory experiment stage, and has not been applied on a large scale in industrial applications.

The cavitation bubbles generated by ultrasonic waves have a small diameter of only tens of microns. During the floating process, cavitation bubbles have more opportunities to collide with tiny inclusions and adhere to each other to form clusters, thereby effectively removing tiny inclusions in the molten steel. However, due to the difficulty in introducing ultrasonic waves into molten steel and the difficulty in finding waveguide materials that can be used at high temperatures, research on the removal of inclusions by ultrasonic cavitation bubbles is still concentrated in the water model and laboratory experiment stage, and has not been applied on a large scale in industrial applications.

7 Nitrogen enrichment and nitrogen precipitation method. Its technical principle is to fill N2 into the molten steel in the early stage to significantly increase the nitrogen content in the molten steel; in the later stage, the pressure is quickly reduced through vacuum treatment, so that the supersaturated gas in the steel generates a large number of dispersed tiny bubbles with inclusions as the core; finally, the bubbles carry the inclusions to float up, and continuously capture the tiny inclusions during the floating process, so as to achieve the purpose of removing microscopic inclusions.

Nitrogen enrichment and nitrogen precipitation method. Its technical principle is to fill N2 into the molten steel in the early stage to significantly increase the nitrogen content in the molten steel; in the later stage, the pressure is quickly reduced through vacuum treatment, so that the supersaturated gas in the steel generates a large number of dispersed tiny bubbles with inclusions as the core; finally, the bubbles carry the inclusions to float up, and continuously capture the tiny inclusions during the floating process, so as to achieve the purpose of removing microscopic inclusions.

The nitrogen enrichment and nitrogen precipitation method is still in the laboratory research stage, has not been industrially verified, and is not applicable to the production of steel that is sensitive to nitrogen content.

Nitrogen enrichment and nitrogen precipitation method is still in the laboratory research stage, has not been industrially verified, and is not applicable to the production of steel that is sensitive to nitrogen content.

8 Tiny hydrogen bubble method. Considering the difficulty of controlling the nitrogen content in steel by the nitrogen enrichment and nitrogen precipitation method, researchers have developed a tiny hydrogen bubble method to remove inclusions from steel. The principle is to introduce coke oven gas or natural gas into the molten steel. The coke oven gas or natural gas interacts with the molten steel, and the hydrogen component dissolves in the molten steel, making the hydrogen content in the molten steel reach more than 8 ppm; after refining and deoxidation of the molten steel, the molten steel is subjected to vacuum treatment, and the dissolved hydrogen in the steel generates fine bubbles with inclusions as heterogeneous nucleation cores; the bubbles carry the inclusions to float up to the slag for removal; the bubbles also promote the inclusions to float up to the slag for removal by adhering to the inclusions during the floating process.

Tiny hydrogen bubble method. Considering the difficulty of controlling the nitrogen content in steel using the nitrogen enrichment and precipitation method, some researchers have developed a tiny hydrogen bubble method to remove inclusions in steel. The principle is to introduce coke oven gas or natural gas into the molten steel. The coke oven gas or natural gas interacts with the molten steel, and the hydrogen component dissolves in the molten steel, increasing the hydrogen content to more than 8ppm. After the molten steel is refined and deoxidized, it undergoes vacuum treatment. The dissolved hydrogen in the steel uses the inclusions as heterogeneous nucleation cores to generate tiny bubbles, which carry the inclusions to the slag for removal. During this process, the bubbles adhere to the inclusions, further promoting their removal.

This technology can be used for hydrogen charging treatment at argon blowing stations, CAS treatment stations, LF treatment stations, AOD treatment stations, RH treatment stations, and VD/VOD treatment stations. Instead of blowing argon into the molten steel, natural gas or coke oven gas is used. Vacuum treatment is then performed using RH, VD/VOD, etc. It is applicable to a wide range of equipment and requires almost no modifications to existing equipment. It is simple to operate and low cost. The generated hydrogen bubbles are small, effectively removing microscopic inclusions and nitrogen from the steel. Compared to the nitrogen enrichment and precipitation method, this technology effectively removes nitrogen from steel and remains suitable for steel grades sensitive to nitrogen content.

This technology can be used for hydrogen charging treatment at argon blowing stations, CAS treatment stations, LF treatment stations, AOD treatment stations, RH treatment stations and VD/VOD treatment stations. The original argon blowing into the molten steel is changed to blowing natural gas or coke oven gas, and then vacuum treatment is performed through RH treatment, VD/VOD treatment stations, etc. It has a wide range of applicable equipment and almost no need to modify existing equipment; it is simple to operate and low in cost; the generated hydrogen bubbles are small in volume and have a good effect on removing microscopic inclusions and nitrogen in steel. Compared with the nitrogen enrichment and nitrogen precipitation method, this technology has a good removal effect on nitrogen in steel and is still applicable to steels sensitive to nitrogen content.

Due to its advantages—good removal effect on microscopic inclusions and nitrogen in steel, wide equipment applicability, and simple operation—the micro hydrogen bubble method is expected to see large-scale industrial application in the future.

Due to the advantages of the micro hydrogen bubble method, such as good removal effect on microscopic inclusions and nitrogen in steel, wide application of equipment, and simple operation, it is expected to achieve large-scale industrial application in the future.