To eliminate uneven thickness produced in base stock, forming and bead grinding, to allow on-line tube making of a steel tube having a satisfactory dimensional precision comparable to a conventional drawn tube, and to obtain a technology of liberalizing a schedule for resistance welded tube such as reduction in the number of times of changing rolls.
In the routine process for a resistance welded tube making machine, drawing is performed on the outer diameter of a tube by imparting a reduction rate of area of 10% or more using a caliber roll, and successively, rolling is carried out for the tube thickness by means of an internal tool. Thus, it is possible to increase the reduction rate of area and also to suppress the incidental degradation of thickness precision; as a result, a resistance welded tube is obtained with an extremely high dimensional precision, along with implementation of coping with a small lot incident to a base tube intensive operation.
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