抄録
Experiments have been carried out to clarify the effect of casting conditions on the formation of large nonmetallic inclusions in the top poured killed steel. Tapping and pouring temperatures are closely related with the appearance of macroscopic inclusions. Pouring temperature is particularly important. Being poured at low temperatures, ingot contains many macroscopic inclusions, whose diameter is larger than hundred microns, at the bottom equiaxed zone. Being poured at high temperatures, ingot has few macroscopic inclusions. In the case of adequate refinings, the main sources of macroscopic inclusions are airoxidation products and eroded refractories during a pouring. It is unfavourable that sources of inclusions are formed at the last stage of casting practice when molten steel temperature falls. Without air-oxidation and erosion of refractories, few macroscopic inclusions are formed even in the ingot poured at low temperatures. This shows that dissolved oxygen alone cannot form macroscopic inclusions which cause ultrasonic defects. In the case of silicon killed ingots, inclusions are globular manganese silicate containing a little alumina, and are almost in equilibrium with molten steel in composition. In case of aluminium killed one, on the other hand, inclusions just inside the surface of a ingot are globular aluminium-manganese-silicate, which are not in equilibrium with steel and differ from the large high alumina inclusions at the bottom equiaxed zone.
