There are 180t top and bottom combined blowing converter in a steel plant. The lining is built by simple upper repair method, and the working layer is made of magnesia carbon refractory brick. The converter has been put into operation from February 2009 to March 2010, with a furnace life of 8288 times. In the process of converter shutdown and overhaul, it is observed that the lining damage is unbalanced and the corrosion rate of local refractory is faster.
According to the comprehensive analysis results, it can be inferred that the slag intrudes into the brick from the gap between the periclase grains and the pores, and the solid solution reaction between FeO and MgO in the slag results in the disintegration of the periclase grains and the final melting out of the brick into the slag. The main minerals in the reaction layer are dicalcium ferrite, dicalcium silicate and periclase solid solution with FeO. The minerals in the protoplast layer are mainly large particles of periclase and flake graphite.
A lot of researches have been made on the damage mechanism of MgO-C brick for converter. Generally speaking, the corrosion is caused by two factors: one is the oxidation of graphite by oxygen and iron oxide in slag; the other is the erosion of magnesia by slag.
No obvious decarburization area was found in this investigation, but pores left by carbon oxidation could be observed. The reaction to oxidize graphite is as follows:
FeO and SiO2 in slag, O2 and CO2 produced during converter blowing and MgO in MgO-C brick are all the causes of graphite oxidation. After the graphite is oxidized to form CO gas, pores are formed in the original brick, which leads to the loose structure of MgO-C brick, and destroys the carbon bonding network structure in the brick, reducing its strength and corrosion resistance. The molten slag can invade and wet the periclase particles. The oxide of iron in the slag reacts with the periclase in solid solution, resulting in the volume expansion and melting point reduction, resulting in the disintegration of the periclase grains and the reduction of the high temperature strength of the MgO-C brick working face. Under the combined action of molten steel, slag, dicalcium ferrite, dicalcium silicate and other low melting point compounds, the refractory was damaged and melted out. The damage process of MgO-C brick can be expressed as the cycle process of decarburization → slag infiltration → solid solution → melting out → decarburization. With the reaction layer continuously pushing towards the inside of the brick, MgO-C brick is continuously eroded and damaged until the furnace is shut down for overhaul.
The damage reason of MgO-C brick near slag line in the furnace is analyzed. The conclusion is as follows:
1) the residual thickness of MgO-C brick at the joint of furnace body and furnace cap, front and rear large surface, slag line, trunnion and other parts on the furnace lining is relatively thin and seriously corroded, which is the weak part of the furnace lining.
2) the reaction layer minerals of the residual brick are mainly dicalcium ferrite, dicalcium silicate and the solid solution of MgO and FeO. The isolated periclase grains melted by the slag are observed. The slag intrudes into the brick through the gaps and pores of the periclase grains.
3) no obvious decarburization area is found in this investigation. It is speculated that the reaction speed of slag brick interface is faster, and the brick body is melted out by slag immediately after decarburization.
4) the damage causes of MgO-C brick can be expressed as follows: graphite is oxidized → slag invades the brick → periclase reacts with slag → refractory is melted out → MgO-C brick is damaged.