PURPOSE:To easily increase the coefft. of friction of a steel material by chemically treating the smooth surface of mill scale produced on the steel material to form a specified rough surface. CONSTITUTION:The smooth surface of mill scale produced on a steel material during the manufacture of the steel material is chemically treated with a mill scale removing soln. impregnated into a nonwoven fabric or the like to form a rough surface having about 25-50s roughness and to stabilize the coefft. of friction at about 0.55-0.60. A mixed soln. contg. dil. hydrochloric acid, a thickener, a surfactant, an etching agent and a hydrogen brittleness controller may be used as the chemical treating soln. The coefft. of friction of the surface of the steel material to be joined can be increased to the standard value or above.
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