Abstract：Magnesia-carbon (MgO-C) refractories are composed of magnesia clinker, flake graphite, antioxidants, and resin. Due to the components, they exhibit high corrosion resistance, spalling resistance, and slag infiltration resistance. MgO-C refractories have been partially applied to converters since the late 1970s and are now used in all converter parts. By changing the components according to the damage type of each part of the converter, the refractory greatly contributes to improving the life of the converter and reducing the cost of refractory. Furthermore, higher durability has been achieved through the development of new evaluation methods, reduction of graphite, and suppression of the MgO-C reaction.
Keyword：Magnesia-Carbon brick
Magnesia-carbon (MgO-C) bricks were developed for use on the hot spots of electric arc furnaces and began to be applied to converters in the late 1970s. Since they were found to have higher durability than dolomite-carbon bricks that were mainly used for converters then,1) they have significantly increased in use applications and quantities as converter refractories. Today, MgO-C bricks are widely applied as steelmaking refractories, not only for converters but also for vacuum degassers and ladles, among other equipment. MgO-C bricks are composed of magnesia clinker, flake graphite, antioxidants, resin components, etc., and have high resistance to corrosion, spalling, and slag penetration thanks to the characteristics of their constituents. Furthermore, their strength and physical properties, corrosion resistance, and spalling resistance can be significantly changed by changing the particle size composition of magnesia clinker and the addition contents of flake graphite and antioxidants. In actual operation, the blend composition of MgO-C brick raw materials is finely tuned to suit the wear pattern of specific converter lining areas. The zoned lining method is employed to use different refractories in different converter lining zones. As a result, the converter life is extended and the converter refractory cost is reduced.
Raw Material Composition
Typical constituent raw materials of MgO-C bricks are outlined below.
1.1 Magnesia clinker
Sintered magnesia and electrofused magnesia are mainly used for MgO-C bricks. Table 1 shows typical quality examples of sintered magnesia and electrofused magnesia.2) The purity, crystal grain size, crystal grain boundary flux CaO/SiO2 ratio, and other properties of magnesia clinker affect the corrosion resistance of MgO-C bricks.3) It is necessary to select appropriate aggregate types to suit specific converter lining areas.
1.2 Graphite
Natural flake graphite is commonly used for MgO-C bricks. Graphite in MgO-C bricks is not easily wetted by slag and suppresses slag infiltration. It also inhibits spalling with its high thermal conductivity and low coefficient of thermal expansion. In addition, CaO and SiO2 contained in the ash of graphite migrate to the boundaries between graphite and magnesia clinker at high temperature and form low-melting point compounds. The low-melting point compounds are considered to form liquid phases, reducing the hot modulus of rupture of MgO-C bricks and facilitating the dissolution of the magnesia clinker into the slag. High-purity graphite is often used as a component of MgO-C bricks for the converter lining areas where high corrosion resistance is required.
1.3 Resins
Phenolic resin is generally used as binder for magnesia clinker, graphite, and other raw materials in MgO-C bricks. The phenolic resin
• Has high affinity for and is kneaded well with graphite and magnesia clinker.
• Is high in fixed carbon and forms strong carbon bonds.
• Is less harmful to environmental health than tar pitch.
Phenolic resin comes in two types: the thermosetting resol type and thermoplastic novolac type. The type to be used is determined by considering the manufacturing process, manufacturing equipment, and other conditions
1.4 Other additives
Carbon components contained in MgO-C bricks are oxidized by oxygen and carbon dioxide in the atmosphere or by iron oxide in the slag. Antioxidants such as metals are added mainly to suppress this oxidation.
Manufacturing Process
First, the raw materials are graded into coarse, medium, and fine sizes and are classified as necessary. Next, they are mixed and kneaded with a binder in predetermined blend proportions by particle size. The kneaded mixture is press formed into bricks. Uniaxial forming with an oil press or a friction press is generally adopted as the pressing method. The magnesia clinker and graphite in MgO-C bricks exhibit orientability depending on the forming direction of the press. The strength and thermal conductivity of MgOC bricks exhibit anisotropy. It is therefore important to consider the forming direction of bricks when laying the bricks. A cold isostatic press (CIP) with small anisotropy is also used for the manufacture of large refractory products such as bottom blowing tuyere bricks and tap hole bricks. Formed bricks are dried to remove moisture and other volatile components, processed and coated as required, visually inspected for cracks, chips, and other defects, and shipped after removing defective bricks.
Wear Mechanisms of MgO-C Bricks
The basic wear mechanisms of MgO-C bricks are roughly classified into the following five.
4.1 Corrosion
The dissolution and elution phenomena of magnesia clinker by slag can be divided into the following two:
• Dissolution and elution of the magnesia clinker by intrusion of the SiO2 and CaO components into the periclase grain boundaries in the magnesia clinker
• Dissolution of the periclase by diffusion of the FeO component into periclase crystals (melting point reduction by formation of MgO-FeO complete solid solution) The above phenomena proceed at the same time. In any case, the dissolution and elution phenomena of the magnesia clinker into the slag greatly affect the wear mechanism of MgO-C bricks. This is supported by the fact that high-purity raw materials and electrofused magnesia with few grain boundaries are applied to badly damaged areas, that the MgO content of the slag during blowing is intention ally increased, and that the wear rate of bricks is reduced by coating with the slag whose MgO content is adjusted.
4.2 Oxidation
The carbon contained in the MgO-C bricks plays the role of suppressing the penetration of slag components into the bricks, but it also has the drawback of being oxidized. The carbon oxidation phenomena can be divided into the following three types:
• Liquid phase oxidation
• Gas phase oxidation
• Oxidation of carbon by MgO (MgO-C reaction)
Liquid-phase oxidation is mainly caused by iron oxides in the slag. The iron oxide concentration in the slag has a great influence on the wear rate of MgO-C bricks.10) As expressed by the reaction formula of FeO(s)+C(s)→Fe(s)+CO(g), this phenomenon gasifies the carbon comprising the matrix of the brick and induces the structural embrittlement of the brick. Figure 4 shows an example of liquid phase oxidation. Highly brilliant Fe precipitates are confirmed in the void layer below the working surface or immediately below the void layer. Gas phase oxidation is the phenomenon by which the carbon in the brick matrix burns. It is caused by oxygen and carbon dioxide in the atmosphere. In general converters, gas-phase oxidation is likely to become a problem in the converter cone that is not adequately protected with slag and is easily exposed to air. A common remedy is the preliminary addition of active metal powder or similar material to the brick mixture as described in Section 2. The MgO-C reaction is a phenomenon likened to the wear mechanism of MgO-C bricks and will be described in detail in the next section.
4.3 MgO-C reaction
The oxidation reaction of carbon in MgO (MgO-C reaction) is given by
MgO(s) + C(s) → Mg(g) + CO(g)
Whether this reaction proceeds to the right depends on the temperature, Mg partial pressure, and CO partial pressure. The reaction is controlled by the dissipation rate of Mg(g) and CO(g) from the
working surface of the lining. Figure 5 shows an Ellingham diagram at various magnesia and CO partial pressures.11) In the equilibrium state where each partial pressure is 1 atm, the above reaction starts at 1850°C. If either or both of Mg(g) and CO(g) fall below 1atm, the reaction proceeds from the left to the right. In a refractory that can be regarded as an open system, formed Mg(g) diffuses and the Mg partial pressure in the refractory decreases considerably. As a result, the above reaction occurs at a significantly low temperature and causes the structural embrittlement of the refractory.
4.4 Spalling
Spalling damage is classified into thermal spalling and mechanical spalling. Figure 6 shows the relationship between the graphite content and spalling resistance. Generally, the higher the content of graphite with high thermal conductivity, the smaller the temperature gradient in the thickness direction of the refractory lining becomes. That is, the thermal expansion difference in the refractory lining decreases and the spalling resistance improves. Converters are generally lined with MgO-C bricks with a graphite content of 15 to 20 mass%. Given the large effect of equipment availability, MgOC bricks with higher graphite contents are often used in intermittently operating electric arc furnaces, for example. Mechanical spalling is caused by the thermal stress produced when the refractory lining thermally expands under restraint conditions. Mechanical spalling is likely to occur in the converter lining after a relatively few heats. Generally, the converter lining continuously peels off in the circumferential direction. If mechanical spalling occurs, the stress concentrations in the converter lining are mitigated by measures such as providing expansion allowance joints, changing brick allocations, and adjusting the number of joints.
Characteristics of MgO-C Bricks and Their Ap- plication to Specific Converter Lining Areas
In the design of the converter refractory lining, the respective lining areas differ in the damage mechanism, frequency, and amount. Zoned lining is generally adopted for changing the thickness and quality of MgO-C bricks in different lining areas to make the overall damage balance as uniform as possible throughout the converter refractory lining. Table shows the main damage factors and especially required properties for the specific converter lining areas.
The converter throat and cone have damage problems, such as gas phase oxidation, physical impact during deslagging, and cracks due to the thermal expansion of the barrel. The dislodgment of bricks is prevented by such measures as adding SiC as an antioxidant, employing anchors driven into the steel shell to secure the bricks, and using metal cases for fusion bonding.
Damage of the tap hole sleeve is dominated by abrasion by molten steel flow and is considered to be accelerated by repeated heating and cooling during operation and by gas phase oxidation. The durability of the tap hole sleeve is being improved by adjusting the antioxidant addition and increasing the hot modulus of rupture.
Slag corrosion is dominant in the slag line, trunnion, and steel bath areas. Improvements have been made, such as densification by changing the particle size composition and binder type, suppression of structural deterioration due to cyclic thermal loading, and use of CaO-containing clinker with good slag coating properties.
The charging pad is subjected to mechanical impact when the hot metal is received from the hot metal ladle and when scrap is charged as a cold iron source. The MgO-C bricks for the charging pad area are increased in strength by decreasing the carbon content and increasing the metal addition content. Concerning spalling damage, another issue, reports are available about improving the spalling resistance by changing the binder and flake graphite types.
Factors causing damage to the bottom tuyeres include graphite oxidation, slag corrosion, and spalling. Another factor is mechanical damage due to the back attack of the bottom blown gas and due to the flow abrasion by the molten steel. The bottom tuyere area is constructed of MgO-C bricks that have a higher graphite content than that of the bricks used in the walls. These bottom tuyere MgO-C bricks also have additives made to prevent the oxidation of graphite and to improve their strength.
In recent years, Nippon Steel Corporation developed the MultiRefining Converter (MURC) process (Fig. 8) whereby dephosphorization and decarburization are continuously conducted in a single converter. As the new process became widely applied within the company, the penetration of low-basicity slag into refractories and the corrosion of the refractories by low-basicity slag exerted conspicuous effects on the MURC converter. The quality deterioration of iron ore, coke, and other steel raw materials increased the impurities ([Si], [P], [S]) in the hot metal. This situation in turn exacerbated the operating severity of converters, increased the corrosion rate of MgO-C bricks, and urged the need for increasing the durability of MgO-C bricks.
