Synopsis Mold powder technology for continuous casting of aluminum-killed steel was developed to produce the clean and crack free steel slabs. The following items were investigated: 1) Mass balance of A120~ in the molten powder pool. 2) A1203 absorption capacity o f the molten powder. 3) Uniformization of heat removal through the mold during continuous casting. 4) Change in powder viscosity with absorption of A1203. 5) Design of optimum powder viscosity by consideration of A1203 absorption during continuous casting. The results obtained were as follows: (1) The A1203 content of the molten powder pool increases with the elapse of casting time in accordance with the A1203 absorption capacity of the powder, but after some time, reaches a constant value that matches the A1203 absorption capacity of the powder. (2) The A1203 absorption rate of the molten powder increases as the basicity index B; of the powder increases. (3) The viscosity of the molten powder generally increases with increasing A1203 content. (4) To uniformize heat removal through the mold during casting, it is effective to control the parameter iV over the range of 1.0 to 3.5, where 2 is the viscosity of the molten powder in poise at 1300 °C and V is the casting speed of the machine in m/min. (5) A method for designing the optimum powder viscosity that allows for the absorption of A1203 during casting has been established by the combination of the results mentioned above. (6) Powders designed by the above method have been applied to continuous slab casters to prevent the longitudinal surface cracking of mediumcarbon aluminum-killed steel slabs and to realize the hot direct rolling of unconditioned slabs.

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