Do refractory bricks with different properties for steelmaking have an effect on the performance of steel?
Refractory brick is a solid material that can resist the effects of high temperatures and is widely used in the metallurgical industry. According to their chemical properties and composition, refractory bricks can generally be divided into the following categories: acidic refractory bricks (such as quartz and silica bricks); semi-acidic refractory bricks (such as semi-silica bricks); neutral refractory bricks (such as chrome bricks, clay bricks, and high-alumina bricks); and alkaline refractory bricks (such as magnesia bricks, chrome-magnesia bricks, magnesia-alumina bricks, dolomite bricks, and other magnesia, dolomite, and magnesia-based refractory clays).
Refractory Bricks for Steelmaking
Alkaline refractory bricks are widely used in forging equipment due to their excellent properties, such as high refractoriness, good thermal stability, and strong slag resistance.
During the steelmaking process in converters and electric furnaces, the molten steel causes mechanical erosion of the furnace lining refractory bricks. At the same time, elements from the refractory bricks can dissolve into the steel, resulting in chemical reactions between the refractory materials and the molten steel.
This interaction-mechanical erosion and chemical reaction-on one hand, leads to the damage and corrosion of the furnace lining refractory bricks; on the other hand, it affects the quality of the steel.
In the steel forging process, the interaction between the furnace lining refractory bricks and the molten steel in the furnace pool impacts the quality of the steel in the following main aspects:
01. Refractory Brick Spalling Leading to Non-metallic Inclusions in Steel
During the steelmaking process, the furnace lining refractory bricks undergo corrosion. Structural changes in the brick's decarburization layer and reaction layer can cause loosening. Under the mechanical scouring of molten steel, slag, furnace gas, as well as added molten iron and scrap, the refractory bricks may spall and break off, becoming mixed into the molten steel. This results in non-metallic inclusions.
Non-metallic inclusions have very different properties from the steel itself. From a mechanical perspective, they act as stress concentration points, significantly impacting the strength, stiffness, durability, and other mechanical properties of the steel. Therefore, non-metallic inclusion is one of the major defects affecting steel quality.
02. Reaction Between Refractory Brick Elements and Molten Steel Elements
Some elements in the composition of refractory bricks can dissolve directly into the molten steel, increasing the content of non-metallic elements such as oxygen and carbon in the molten pool. Under certain conditions, these non-metallic elements in the steel may react with each other to form non-metallic inclusions. This also affects both the quality and the composition of the steel.
03. The Effect of Carbon Increase from Refractory Bricks on Steel Quality
Currently, carbon composite refractory bricks, developed in the 1980s, are widely used domestically and internationally in converters, electric furnaces, continuous casting, furnace refining, and steel ladles. These alkaline refractory bricks typically contain between 3% and 30% carbon.
During steelmaking, the oxidation of carbon is a primary reaction in the forging process. Reducing the carbon content in molten steel to meet the steel grade specifications is one of the main goals of steelmaking.
Decarburization of refractory bricks can lead to an increase in the carbon content of the steel, altering its composition. This effect is especially significant when producing pure steel and ultra-pure steel, where decarburization from refractory bricks greatly impacts steel quality.
The mechanism of refractory brick decarburization is as follows: during forging, a liquid-phase isolation layer forms between the steel and the refractory brick. Elements from the refractory brick diffuse through a solid-phase product layer into the steel. Simultaneously, some elements and oxides in the molten steel, mainly iron oxide (FeO) in the slag, react with the refractory brick layer at the decarburization interface, resulting in decarburization reactions that affect the composition of the molten steel. The main chemical reactions are:
(FeO)+C (solid)=[CO]+Fe (slag)
O2+2C(solid)=2[CO](furnace gas)
Additionally, during forging, oxygen is blown into converters and electric furnaces to decarburize the steel. Oxygen ([O₂]) oxidizes atomic iron ([Fe]) in the molten pool to form iron oxide ([FeO]), which dissolves in the molten steel. Carbon ([C]) then reacts with [FeO], oxidizing carbon to generate CO bubbles. These bubbles rise and escape through the slag layer.
Carbon oxidation also affects the content of other elements such as oxygen in the steel, thereby influencing steel quality. However, the rising CO bubbles vigorously stir the molten metal pool, promoting uniformity in composition and temperature, and enhancing the conditions for chemical reactions in the steel. This stirring effect has certain benefits for steelmaking.
03. The Desulfurization Effect of Refractory Materials and Its Contribution to Improving Molten Steel Quality
Sulfur in steel mostly exists in the form of sulfides. For the vast majority of steels, sulfur ([S]) is considered a harmful element. It primarily affects the performance of steel by causing hot brittleness, reducing mechanical properties, and impairing weldability. Therefore, reducing and controlling the sulfur content in steel is highly beneficial for improving steel quality.
Experimental studies have shown that at high temperatures, refractory bricks develop a certain amount of internal liquid phase. Between the molten steel and the refractory bricks, there is generally a liquid phase layer, mainly composed of silicate melt. This layer's composition and structure closely resemble that of slag, thus enabling similar oxidation recovery, desulfurization, phosphorus removal, and adsorption effects on impurities in the molten steel.
The principle of desulfurization by refractory materials in the molten steel pool is as follows: sulfur ions penetrate the liquid-phase isolation layer of the refractory and disperse into the refractory materials, where they react with [CaO] particles in the refractory. On the surface of these [CaO] particles, a layer of calcium sulfide (CaS) is formed. The chemical reaction is:
(CaO)+[S]=(CaS)+[O]
This reaction indicates that CaO has a strong desulfurization effect. Therefore, refractory bricks rich in MgO-CaO exhibit strong desulfurization capabilities.