PURPOSE: To improve the resistance to cracking and exfoliation of a mold coating material for a chill and to improve the coating performance thereof by using a mixture prepd. by compounding a specific ratio of Al powder with refractory powder as aggregate.
CONSTITUTION: The powder mixture prepd. by compounding 5W20wt% Al powder with refractory powder is incorporated as the aggregate into the mold coating material for chilling a caste steel casting. The coated film formed on the chill surface by using such mold coating material is converted to the composite film composed of Al and refractory particles and to form alumina as the Al powder is melted by the heat of a molten metal. The coated film is thereby reinforced and the generation of cracking and exfoliation by the difference in the coefft. of thermal expansion from the chill is obviated; in addition, the welding and seizure of the chill and cast steel product are prevented. Since the Al powder has a deoxidiation effect, the generation of blowholes occurring in red rust is prevented even if the red rust exists on the chill surface.