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Effect of ladle-lining refractory on non-metallic inclusions in steel

Title: Effect of ladle-lining refractory on non-metallic inclusions in steel
Keyword: non-metallic inclusion; ladle-lining refractory; secondary refining; interface reaction; molten steel purification
Description:This paper summarizes the influence of current mainstream refractory materials for ladle lining on non-metallic inclusions in steel, and discusses the development direction of refractory materials for ladle lining in the future.

ladle-lining-refractory

Seconclary refining is an important turning point in the control of nonmetallic inclusions in steel during the steelmaking process. The ladle-lining refractory are always confronted with severe physiochemical reactions during the whole secondary refining process, which leads to the introduction of inclusions in the molten steel and resulting in less than expected refining results. The interface reaction between ladle-lining refractories and the molten steel and introduce the resulting influences on the formation of inclusions was focused, and it was found that the ladle lining refractory material will affect the morphology, composition and physical and chemical properties of the inclusions in the steel. It is possible not only to introduce inclusions into the steel, but also to absorb and remove them. It is proposed that the future refractory materials for ladle lining should be given more development direction for the purification of molten steel and other functional indicators.

 

  • Influence of refractory materials for typical active ladle lining on non-metallic inclusions

In the actual refining process, the refractory material used for the ladle lining is in real-time contact with the molten steel in a large area. With the action of external fields such as high temperature and stirring, the molten steel will penetrate into the refractory material and cause an interface reaction, which gradually leads to the corrosion of the refractory material and affects the composition of the molten steel. Based on the difference in the material of the refractory for the lining of the refining ladle and the difference in the refining process, there are differences in the degree of corrosion of the refractory for the lining of the ladle and the i

nfluence on the inclusions in the steel. Therefore, understanding the influence of refractories for ladle lining on inclusions in steel is not only of great significance for the control of inclusions in the metallurgical industry, but also provides guidance for the development of refractories for ladle linings in the future. Taking the mainstream MgO-based refractories and magnesia-aluminum refractories in service as the objects, the influence laws of their use as ladle linings on the inclusions in aluminum deoxidized steel and silicon-manganese deoxidized steel in the refining process are reviewed and summarized.

 

1.1 Influence of MgO-based refractories on non-metallic inclusions in steel

1.1.1 Influence on inclusions in aluminum deoxidized steel

Metal aluminum has the advantages of low price, strong oxygen affinity and fast deoxidation speed, and is a very popular deoxidizer. When using aluminum for deoxidation, the total oxygen content in the steel can be controlled at a very low level. Therefore, aluminum is generally used as a deoxidizer for steels that require a higher total oxygen content in the steel. Typical representative steels include bearing steel and pipeline steel, IF steel, etc. A lot of research has been carried out on the effect of MgO-based ladle lining refractories on inclusions in aluminum deoxidized steel.

When Harada A, Brabie V and Jansson S studied the reaction mechanism between MgO-C refractory and aluminum killed steel, they found that not only MgO and MgO·Al2O3 inclusions appeared in the steel without any magnesium addition, Moreover, a thin layer of magnesia-aluminum spinel is formed at the interface between the MgO-C refractory and the molten steel. MgO-C refractory material dissolves magnesium in steel by reducing its own MgO through carbon and reacts with dissolved aluminum in steel to form magnesium-aluminum spinel inclusions, see formula (1) and formula (2).

When aluminum deoxidation is used, there will be more dissolved aluminum in the steel. These dissolved aluminums also play a role in the dissolution of MgO-C refractories. When LIU C studied the reaction between MgO-C refractory and aluminum deoxidized steel, it was found that MgO in MgO-C refractory would undergo replacement reaction with dissolved aluminum in molten steel, resulting in dissolved magnesium and MgO·Al2O3 inclusions. The reaction is shown in formula (3).

The reaction with the reduction of its own MgO with carbon to dissolve magnesium in the steel will vary in the distribution of elements formed on the surface of the refractory. The element distribution at the interface of MgO-C refractory after reaction with molten steel is shown in Figure 1. It can be seen that a magnesium-aluminum spinel layer is formed at the reaction interface after the displacement reaction between the dissolved aluminum in the steel and the MgO in the refractory (Fig. 1(a)). However, after the MgO-C refractory dissolves magnesium into the steel by reducing MgO by its own carbon, it is found that there is no magnesia-aluminum spinel layer at the interface between the refractory and the molten steel (Fig. 1(b)). The chemical composition changes of inclusions in steel are shown in Fig. 2 [41]. Before the start of the test, the inclusions in the steel were mainly Al2O3 inclusions formed after aluminum deoxidation. With the prolongation of the reaction time, the mass fraction of MgO in the inclusions gradually increased. When the reaction time exceeds 3.5 min, the types of inclusions in the steel are mainly MgO·Al2O3.

 

The behavior of MgO-C refractory to dissolve magnesium into steel is greatly affected by its own carbon. In order to strip the effect of carbon on the dissolution of magnesium from MgO into steel, CHI Y G and Deng Z conducted experiments to study the effect of pure MgO refractory on aluminum deoxidized steel. dissolution behavior in .

CHI Y G investigated the effect of MgO refractory on inclusions in aluminum deoxidized steel under different reaction times by immersing MgO refractory rods in molten aluminum deoxidized steel. The results of the interface reaction between the magnesium oxide rod and the molten steel are shown in Figure 3. It was found that a thin MgO·Al2O3 layer was formed at the interface between the MgO refractory rod and the molten steel. As shown in Figure 3(a). In addition, MgO-based refractories also changed the composition and morphology of Al2O3 inclusions in the steel. In the early stage of the reaction, MgO·Al2O3 phase appeared at the edges of Al2O3 inclusions. As shown in Figure 3(b). With the prolongation of the reaction time, the Al2O3 inclusions were all transformed into angular MgO·Al2O3 inclusions. As shown in Figure 3(c).

This shows that at the refining temperature, the dissolved aluminum in the steel will react with the MgO refractory rod to form the MgO·Al2O3 layer and dissolved magnesium (equation (3)). The dissolved magnesium will further react with the existing Al2O3 in the steel to form MgO·Al2O3 inclusions, see formula (4).

Deng Z compared the effect of pure MgO refractory on Al2O3 inclusions in steel under the conditions of oxygen activity of 0.000 1% and 0.0500%. The results show that the oxygen activity in steel is the key factor for MgO refractories to form dissolved magnesium and promote the transformation of Al2O3 inclusions into MgO·Al2O3 inclusions. When the oxygen activity is very low (≤0.000 1%), more dissolved magnesium will be formed in the refractory material, which promotes the transformation of Al2O3 inclusions into MgO·Al2O3 inclusions. As shown in Figure 4; when the oxygen activity is high enough (≥0.050 0%), the Al2O3 inclusions will not change.

In the actual refining process of aluminum deoxidized steel, both MgO-based refractories and MgO-containing refining slag will dissolve magnesium into the steel, which will affect the inclusions in the steel. LIU C compared the effects of MgO-containing refining slag and MgO-based refractories on the transformation of inclusions in aluminum-killed steel through experiments. The rate of supplying dissolved magnesium to the steel by MgO refractory, MgO-C refractory and MgO-containing refining slag was calculated by dynamic model, and the transfer rate was obtained as 2.18×10-4, 5.0×10-4, 6.3×10-4 m/s. The literature believes that although the magnesium supply rate of the MgO-containing refining slag is faster than that of the MgO-C and MgO refractories, if the interaction area with the molten steel is considered. The contact area of refractory material and molten steel is about 4 times that of refining slag and molten steel. Therefore, MgO-based refractories can promote the transformation of Al2O3 inclusions to MgO·Al2O3 inclusions in aluminum deoxidized steel more than MgO-containing refining slag in the same time.

To sum up, the ability of MgO-based refractories to dissolve magnesium in steel is stronger than that of refining slag, and it will promote the transformation of Al2O3 inclusions in steel into MgO·Al2O3 inclusions.

 

1.1.2 Influence on Inclusions in Silicon Manganese Deoxidized Steel

For special-purpose medium and high carbon long products, such as cord steel, cutting wire steel, spring steel, etc. In order to avoid wire breakage caused by Al2O3 inclusions in the later cold drawing process, silicon-manganese composite deoxidation is generally used in the production process to control the shape of the inclusions and make them have better plasticity. DENG Z investigated the independent effect of MgO refractory on inclusions in silico-manganese deoxidized steel. The test results show that with the increase of the reaction time, the melting points of the original SiO2-Al2O3-MnO and SiO2-Al2O3-CaO inclusions in the silicon-manganese deoxidized steel will decrease, and finally stabilize below 1 500 ℃, as shown in the figure 5 shown.

1.2 Influence of magnesium-aluminum refractories on non-metallic inclusions in steel

1.2.1 Influence on inclusions in aluminum deoxidized steel

Magnesium-aluminum refractories have long service life and stability, excellent comprehensive properties, and avoid the carbonization behavior of MgO-C bricks into steel, so they are widely used in the production of aluminum deoxidized steel. Deng Z studied the effect of magnesium-aluminum refractories on inclusions in aluminum-deoxidized low-alloy steels. The results show that the magnesia-aluminum refractory has little effect on it and will not promote the transformation of Al2O3 into spinel inclusions, because the MgO activity in the magnesia-aluminum refractory is very low (about 0.06). It is difficult to react with molten steel to generate dissolved magnesium to affect the inclusions in the steel.

The research in the literature shows that the magnesium-aluminum refractory will undergo a displacement reaction with the dissolved manganese in the aluminum-killed medium-manganese steel (equation (5)), and a (Mn, Mg)O·Al2O3 layer will be formed at the refractory-steel liquid interface. , As the reaction time increases, the erosion layer becomes thicker and thicker, and the manganese element slowly diffuses from the edge of the refractory to the center. As shown in Figure 6, when the interfacial metamorphic layer is washed by molten steel, it will peel off from the refractory matrix and become the source of (Mn,Mg)O·Al2O3 inclusions in the molten steel. The typical (Mn,Mg)O· Al2O3 inclusions are shown in Figure 7.

In summary, although magnesia-aluminum refractories have little effect on inclusions in Al-deoxidized low-alloy steels. However, a relatively strong interfacial reaction occurs with the aluminum-killed medium-manganese steel, and the morphology and composition of the inclusions in the steel are affected.

 

1.2.2 Influence on Inclusions in Silicon Manganese Deoxidized Steel

Magnesium-aluminum refractories also have certain applications in silicon-manganese deoxidized steel due to their excellent high-temperature service performance. Therefore, some researchers have carried out the influence of magnesia-aluminum refractories on inclusions in silicon-manganese deoxidized steel.

The literature carried out the dynamic interaction test between the magnesium-aluminum refractory and the silicon-manganese alloy steel. The test results show that the magnesium-aluminum refractory will be corroded by the silicon-manganese alloy steel at high temperature and form a liquid phase metamorphic layer. Under the dynamic action of molten steel, the metamorphic layer will emulsify with molten steel, and the steel droplets will be dispersed and distributed in the liquid phase metamorphic layer, causing the molten steel to continue to react with the refractory material through the interface layer. Such cyclic action will eventually cause the refractory material to be continuously eroded by molten steel. When eroded by molten steel, the liquid metamorphic layer will enter the steel to form inclusions. The erosion mechanism is shown in Figure 8 .

It was found when studying the interaction of magnesia-aluminum refractories with 95Cr cutting wire steel. The magnesia-aluminum refractory has a certain removal effect on the inclusions in the steel, and can distribute the melting point of most of the inclusions in the cutting wire steel in the low melting point region of 1 300 ℃. As shown in Figure 9 .

In the aluminum deoxidized low-alloy steel, the magnesium-aluminum refractory material has a small MgO activity and will not dissolve magnesium into the steel to affect the inclusions in the steel. However, when used in aluminum-killed medium-manganese steel, under the influence of manganese in the steel, a liquid metamorphic layer is formed at the interface, and inclusions are formed in the steel by the dynamic action of the molten steel. When used in silicon-manganese deoxidized steel, a metamorphic layer will also be formed, and inclusions will be transported to the steel through the metamorphic layer, but the melting point of the inclusions in the steel will be lowered to a certain extent.

It can be seen by summarizing the influence of the refractory materials used in typical active mainstream ladle linings on the inclusions in Al-deoxidized and Si-Mn-deoxidized steels. The ladle lining refractory will not only be damaged and peeled off under the mechanical action of the molten steel to form foreign large inclusions in the steel, but also have an interface reaction with the molten steel. It affects the morphology, types and properties of existing inclusions in steel. Therefore, in actual production, the influence of refractory materials for ladle lining on molten steel should be paid attention to.

 

  • Influence of new ladle lining refractories on inclusions in steel

The current mainstream refractories for ladle lining meet the service cycle requirements of the current refining technology. However, there are still many problems in its own structure, high temperature service performance and harmless treatment of inclusions in steel. Therefore, the new type of refractory for ladle lining should also play a role in purifying molten steel while having stable high-temperature service performance.

With the deepening of the research on the interfacial reaction between refractories and molten steel, it was found that CaO-based refractories [20, 22, 56] have the effect of modification and even adsorption and removal of Al2O3, SiO2 inclusions and phosphorus and sulfur elements in steel. For example, Al2O3 inclusions that are unfavorable to the properties of steel, free CaO and Al2O3 at the refining temperature will form low-melting complexes Ca12Al14O33 (C12A7) and Ca3Al2O6 (C3A) [56], see equations (6) and (7), It is liquid at the refining temperature (1 600-1 700 ℃). Liquid inclusions are not only easier to aggregate and grow at the refining temperature, increasing the probability of floating removal, but also reducing the probability of nozzle blockage. However, the hydration of CaO limits the promotion of CaO-based refractories in industrial production.

The author’s research group abandoned the previous idea based on mechanical mixing of natural raw materials, and systematically studied CA6 materials from the perspective of composition regulation and structure matching. As a ladle lining refractory material, CA6 has several advantages: high melting point (1 875 ℃) and good stability under high temperature reducing atmosphere. High resistance to strong alkali slag and low wettability with molten metal. Does not react chemically with molten steel. However, due to the principle of CA6 sub-regulation, the preparation of dense CA6 refractories (with a density of 3.10 g/cm3) was achieved for the first time. On this basis, two kinds of CaO-MgO-Al2O3 (CMA) ternary functional raw materials were synthesized through the coordination and assembly of M-based blocks (magnepenite-based blocks) and S-based blocks (spinel-based blocks), namely CaMg2Al16O27 (CM2A8) and Ca2Mg2Al28O46 (C2M2A14) (density 3.66 g/cm3 and 3.71 g/cm3, respectively). This series of materials has a stable structure and can solve the hydration problem of traditional CaO-containing refractories.

In the laboratory of the author’s research group, the CMA material (C2M2A14) and the corundum-magnesium-aluminum spinel material were subjected to slag erosion (basicity 3.2) comparative tests. The results show that the two materials are eroded by high basicity slag for 3 h at the same temperature (1 550 ℃), and the results show that the slag thickness of the two materials is significantly different. Among them, the C2M2A14 slag intrusion layer is only a thin 179 μm, while the thickness of the corundum-magnesium aluminum spinel material slag intrusion layer is as high as 406 μm. And the composition of the slag intrusion layer is also different. The C2M2A14 slag intrusion layer is composed of dense CA and CA2, and has no slag composition, which effectively inhibits the erosion of the slag. The corundum-magnesium-aluminum spinel material slag layer contains more slag components besides CA and CA2 components. With the extension of time, the slag will further erode the refractory matrix, as shown in Figure 10.

In addition, the effects of CMA series materials and corundum-magnesium aluminum spinel materials on inclusions in aluminum deoxidized steel were further compared with the addition of refining slag. The results show that when the test time is controlled within 40 min, the inclusions in the molten steel using CMA material are mainly MgO·Al2O3 with a size of less than 5 μm. In the molten steel using corundum-magnesium-aluminum spinel material, there are more MgO·Al2O3 inclusions wrapped by MnS, and their size also increases. This shows that within the actual refining time (generally no more than 40 min), the CMA material has strong slag resistance and exhibits low wettability with molten metal, and will not introduce CaO-containing inclusions into the steel. At the same time, it also has an adsorption effect on the sulfur element in the steel, avoiding the precipitation of sulfide in the later solidification process.

In order to explore the influence of CMA material slag on the inclusions in the steel after invading the metamorphic layer, the test time was extended. When the test time increased to 50 min, the thickness of the eroded layer of the CMA material was stable at about 150 μm, and low-melting CaO-MgO-Al2O3 composite inclusions appeared in the steel, and the size was mainly controlled within 1-4 μm. This shows that with the further erosion of the slag, a transitional protective layer with better high temperature performance will be formed between the CMA material and the slag, which hinders the further erosion of the slag. At the same time, the CMA material will release Ca12Al14O33 (C12A7) through the erosion reaction of the slag, and modify the MgO·Al2O3 inclusions in the steel into CaO-MgO-Al2O3 composite inclusions with small size and low melting point. Play a refining slag effect.

The successful development of CMA materials provides new ideas for the development of refractory materials in the future. The author’s research group has cooperated with refractory material companies to carry out industrial production and industrialized trials in some steel plants. In the near future, the new functional ladle lining refractories represented by CMA are expected to provide stronger support for advanced refining technology and jointly promote the localization of high-quality steel.

 

  • Conclusion

(1) MgO-based refractories will dissolve magnesium into aluminum deoxidized steel, which will promote the transformation of Al2O3 inclusions in the steel to MgO·Al2O3 inclusions. However, in the smelting process of silicon-manganese deoxidized steel, it is beneficial to the formation of low-melting-point inclusions in the steel.

(2) The magnesium-aluminum refractory has little effect on the inclusions in the aluminum deoxidized low-alloy steel, but it will have a relatively strong interface reaction with the aluminum-killed medium-manganese steel to form a metamorphic layer, and the inclusions are transported to the steel through the metamorphic layer. When used in silicon-manganese deoxidized steel, inclusions will also be transported to the steel in the same way, but to a certain extent, the melting point of the inclusions in the steel will be lowered.

(3) In the future, refractory materials for ladle lining should break the traditional research and development thinking, and should not be limited to prolonging the service life. More consideration should be given to giving them functionality under the premise of stable service performance and no pollution of molten steel. The inclusions and impurity elements are removed or harmless. The successful development of CMA material provides new ideas for the development of refractory materials for ladle lining in the future.

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