EXPANDING DEVICE OF STEEL TUBE AND MANUFACTURE OF STEEL TUBE

PROBLEM TO BE SOLVED: To improve the circularity of a steel tube by providing grooves of prescribed number which are slightly wider than the width of the inner weld seam of the steel tube in the surface of one of a plurality of dies so that the interval of the grooves is prescribed times the width of the inner weld seam. SOLUTION: A plurality of dies 2 are arranged in a cylindrical manner, and the displacement is given in the direction orthogonal to the axis of the dies 2 to expand a steel tube 6 from the inner surface. Two or more grooves 4a, 4b are provided in the surface of one die 2b of a plurality of dies 2. The width of the grooves 4a, 4b is slightly wider than the width of the inner weld seam of the steel tube 6. The interval between the groove 4a and the groove 4b is >=1.5 times the width of the inner weld seam. The groove 4a agrees with the inner surface weld seam of the steel tube 6 to achieve the first expansion. A part 6b at which the inner surface of the steel tube 6 corresponds to the interval of the die 2 is straight. The groove 4b agrees with the inner weld line to achieve the second expansion. The part 6b is brought into contact with the surface of the die 2 and the straight part is curved.