Lining refractories suitable for steel making furnaces

With the development of the application of induction furnaces in the steel casting industry, since the 1980s, the application of aluminum-magnesium spinel lining materials has gradually received attention. At present, the crucible-type induction furnaces used in the steel casting industry in various industrial countries are basically all adopt spinel type furnace lining.

In recent years, some steel casting enterprises in my country have adopted spinel-type furnace lining materials. Among them, some manufacturers have purchased materials pre-configured by foreign manufacturers. Although the use effect is very good, the production cost has increased a lot, and its characteristics are not good. I understand very well. Here, I mainly want to introduce some characteristics of spinel-type refractory lining, which can be used as a reference for colleagues in the industry to prepare lining materials and further improve lining materials. At the same time, in view of the fact that silica sand furnace lining, magnesia furnace lining and alumina furnace lining are still used in my country’s steel casting industry, a brief introduction to the characteristics of these materials is also made for reference.

(1) Silica sand furnace lining

The furnace lining built with silica sand as the basic refractory material is also called acid furnace lining. Silica sand has many advantages: First, it is rich in resources and low in price; Moreover, the crucible made of silica ramming mass as the basic refractory material still has good strength at high temperatures close to its melting point, and has good resistance to sudden cooling and sudden heating; In particular, it should be mentioned that during the sintering process of the furnace lining, the expansion of the quartz phase change of the silica sand can compensate for the volume shrinkage during the sintering process, thereby increasing the density of the sintered layer and reducing the porosity in the sintered layer of the furnace lining. Therefore, in the foundry industry of various countries, the furnace lining material based on silica ramming mass is widely used in crucible induction furnaces for melting various cast irons.

However, the low refractoriness of SiO2 basically cannot adapt to the steelmaking temperature. Moreover, SiO2 has strong chemical activity at high temperature, and can interact with various basic oxides and even neutral oxides in the steelmaking process. For example, FeO is easy to form fayalite with a melting point of 1205°C after contacting with silica sand ( Fe2SiO4), fayalite can further interact with SiO2 or FeO to form a eutectic component with a melting point of 1130 °C. In addition, SiO2 may also be reduced by some more active elements in molten steel. Therefore, the silica sand furnace lining is used in steelmaking, which can neither guarantee the metallurgical quality of the steel nor the life of the furnace lining. Since the late 1980s, foundries in various industrial countries that use induction furnaces to produce steel castings have not used silica sand linings. As far as I know, there are still some enterprises in our country that use silica sand furnace lining to smelt cast steel. This situation needs to be improved urgently.

(2) Magnesia furnace lining

The commonly used furnace lining material is metallurgical magnesite with an MgO content of more than 86%, which is made of magnesite calcined at high temperature. If the metallurgical magnesia is remelted in an electric arc furnace, the content of impurities such as SiO2 and Fe2O3 can be reduced, and fused magnesia with higher purity (MgO content above 96%) can be obtained. Fused magnesia is mostly used to make the lining of vacuum induction furnace.

Metallurgical magnesia has high refractoriness and is a conventional lining material for basic electric arc steelmaking furnaces. Although it has a high melting point, is not easy to sinter, and has a large expansion coefficient, due to the thick lining of Bonding materials and knotting with wet methods can completely make up for this deficiency.

If metallurgical magnesia is used as the lining material of the induction furnace, due to the restriction of the thickness of the lining, it is not suitable to use wet knotting. The negative effects of these shortcomings are very obvious, so. The crucible made of magnesia material is prone to cracks, especially in intermittently operated furnaces.

(3) Alumina furnace lining

Both alumina and zircon sand are neutral refractory materials, among which alumina is the most widely used, and zircon sand is rarely used as furnace lining material.

Using alumina alone as the lining material has a strong ability to resist cracking and prevent acid slag erosion, but it is not suitable for making alkaline slag. Moreover, due to its high refractoriness and poor sintering performance, the lining life is not very high.

(4) Spinel-type furnace lining

Spinel minerals have the characteristics of isomorphism, there are many varieties, and the composition is relatively complex. Its molecular formula can be written as M2+O·M3+2O3, where: M2+ represents some divalent metal atoms, such as Mg, Fe, Zn, Mn, etc.; M3+ Represents some trivalent metal atoms, such as Mg, Fe, Zn, etc. Therefore, it can also be written as (Mg, Fe, Zn, Mn) O • (Al, Cr, Fe) 2 O3.

Among the divalent metal atoms contained in spinel minerals, Mg2+ and Fe2+ can replace each other in any proportion; Among the trivalent metal atoms contained in them, Al3+ is the majority, but Cr3+ can replace Al3+ in any proportion, and Fe3+ can only be in a certain proportion. Replace Al3+ or Cr3+ within the limits. The common spinels are as follows:

Magnesium aluminum spinel MgO·Al2O3

Ferrite spinel FeO·Al2O3

Chromite (iron chromium spinel) FeO Cr2O3

Magnetite (iron spinel) FeO•Fe2O3

Magnesium iron spinel (Mg,Fe)O•(Al,Fe)2O3

Spinel ZnO·Al2O3

Magnesium chromium spinel MgO Cr2O3

Zinc iron spinel ZnO•Fe2O3

Manganese chromium spinel FeO Cr2O3

Spinel MnO·Al2O3

At present, in various industrial countries, magnesium-aluminum spinel (MgO·Al2O3) is mainly used as the lining material of induction furnaces for steelmaking, usually referred to as ‘spinel’ for short. In pure magnesium aluminum spinel, the MgO content is only 28.2%, but it is still an alkaline refractory material.

Magnesium-aluminum spinel materials have high refractoriness, small thermal expansion coefficient, good thermal stability at high temperature, and strong resistance to alkaline slag erosion. In particular, it should be mentioned that MgO and Al2O3 have a volume expansion of 7.9% in the process of sintering to form spinel, which can make up for the volume shrinkage in the sintering process and reduce the porosity of the sintered layer. This is an important advantage of silica sand furnace lining is consistent.