PURPOSE: To prevent the development of a linear flaw on the surface of a steel tube by making a hoop steel after bending almost the triangular steel tube constituted with three large arcs and three small arcs, in the mechanism for forming the steel tube from the hoop steel.
CONSTITUTION: In the mechanism for forming the steel tube by gradually bending the hoop steel with forming rolls, the hoop steel after bending is formed to almost the triangular steel tube constituted with the three large arcs (d), (e), (f) and the three small arcs (a), (b), (c). By this method, the surface of the roll is not wholly brought into contact with each surface of the hoop steel rolled flower and the development of the linear flaw between the edge part and the surface of the steel tube caused by the difference with the peripheral velocity of the roll surface can be eliminated. Further, as this hoop steel is formed with almost the triangle having the three small arcs and the three large arcs, the change of a brake-down roll is made to be unnecessary.
HAMAMOTO NOBUYUKI
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