A smooth tap hole opening operation with suitable and constant drilling speed has been required to keep a stable blast furnace operation. In practice, however, the operation is occasionally disturbed with sudden interruption, forcing operators the biggest efforts and time for further progress. Furthermore, production rate of iron should be decreased because of a higher slag level in a blast furnace, which raises during drilling interruption. For figuring out the mechanism and/or the causes for such incidents, the investigation was made on tap hole mix structure, combining laboratory experiment and simulative study utilizing the actual drilling operation data attained at a certain customer’s blast furnace. Laboratory experiments on various types of tap hole mix revealed that drilling speed and drill wear are highly influenced by tap hole mix’s Young’s modulus. A numerical model simulating the actual drilling operations, in which the laboratory experiment results are adopted, suggests the followings ; １）Drilling speed in actual operation is predictable with tap hole mix’s Young’s modulus. ２）Young’s modulus of plugged tap hole mix is almost constant, except for approximately 10％ of farthest end near metal/slag phase, and apparent drilling speed slow down is attributable to the drill wear. ３）The position where drilling operation encounters the sudden interruption is thought to consist of the complex structures with tap hole mix and metal and/or slag, whose apparent Young’s modulus should be, at least, five time of well sintered tap hole mix The results indicate the necessities for further investigation on the complex structure formation mechanism from the operational and the refractory material viewpoints.

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