It has been known that one of the primary trouble on the production of middle carbon grade steel is longitudinal cracks on the cast surface, and occurrence frequency of these cracks is effectively reduced with a lower heat flux in a mold. In order to decrease the heat flux in the mold, a mold powder with high crystallization temperature has been ordinarily used. To make clear the mechanism of lowering heat flux in the mold using the mold powder, laboratory experiments were conducted. Thermal resistance was measured using a dipping test of copper mold, which had several thermocouples on the surface. Interfacial thermal resistance was about 2-3 times of thermal resistance of film. To clarify the change of the surface texture by crystallization, the surface of powder plate was observed with a laser microscope. The surface started to wave, when the glass changed to crystal above 830K. Therefore, it is supposed that the wave on the surface of the powder film caused an increase in the interfacial thermal resistance.
