Abstract

In the steelmaking process, the RH degasser is a major facility for improving steel cleanliness, by removing gases and non-metallic inclusions from the molten steel. To improve the productivity of the RH degasser, it is necessary to extend the life of the lower vessel and snorkels to reduce the time loss that occurs when they are exchanged. Injection refractories are widely used to repair the lower vessel and snorkels of the RH degasser. At Oita Steel Works, alumina-magnesia (Al2O3-MgO) castable refractories are used as the injection refractories. It is well known by lab investigations, that the performance of alumina-magnesia castables is influenced by the curing conditions, such as the curing temperature and time, but the optimum conditions for RH repair in the steel works have not been clearly defined. This study focused on the effect of the curing temperature and time on the lifetime of the injection refractories in the actual RH degasser.

In the actual RH degasser, the temperatures of the injection refractories in the upper, middle and lower part of the snorkel were measured using thermocouples, during the curing and drying processes. The temperature of the injected refractories at 10 hours after injection was 200℃ in the upper part of the snorkel, and 50℃ in the lower part of the snorkel. So the curing temperature of the lower part was less than 100℃, which indicated that the free water in the injection refractories did not completely dehydrate, whereby the injection refractories were exposed to high temperature steam during the curing time.

 Therefore it was necessary to clarify the influence of the high temperature steam on the properties of the alumina-magnesia castable refractories. Alumina-magnesia castable samples (40×40×160mm) were cast in steel molds after mixing, sealed with a steel lid, and cured at 100℃, and 200℃, similar to the curing condition in the actual RH degasser. For a comparison, other samples were cured without being sealed with a steel lid.

The bending strength of each sample was measured at 1, 3, 6, 10, 24 hours after casting. It appeared that the bending strength of the samples exposed to high temperature steam increased up to 3 hours, and then decreased thereafter. The strength reduction was considered due to the slaking of MgO, caused by the high temperature steam. On the other hand, without the sealing lid on the mold, the bending strength of the injection refractories increased continuously, even after 3 hours.

The test results made it clear that the high temperature steam caused the strength deterioration of the alumina-magnesia castable refractories, because of the slaking of MgO. To suppress this strength deterioration, the curing including dehydration should be completed within 3 hours. By applying this new, controlled curing condition in the field, the wear rate of the injection refractories in the actual RH degasser was reduced by 14%.