Effect of Degree of Corner-Bending Imposed on Round Welded Stee1 Pipe in Expanding Type Pre-Forming Process on Cross-Sectional Shape of Square Pipe Formed by Extroll-Forming

摘要

In order to improve the cross-sectional shape (outer curvature, inner curvature, wall thickness change at corner zones, flatness of sides) of a square steel pipe (150mm wide) formed using an extroll-forming mill, an expanding type pre-forming process is introduced to impose peripheral bending under tension on the portions of a round pipe (190.7mm in outer diameter, 4.6mm in wall thickness) that correspond to the corner zones of the finished product. Characteristics of the cross-sectional shapes of the formed square pipes are investigated with respect to the degree of corner-bending of the pre-formed pipes and compared with the results obtained by conventional extroll-forming. The outer and inner curvatures of the corner zone increase and the depth of the hollow along the cross-sectional profile of the side decreases with increase in the degree of corner-bending in the pre-forming. The increase in wall thickness of the corner zone which is caused by the forming rolls is greatly affected by the degree of corner-bending in the pre-forming. This wall thickness change at the corner zone can be reduced by imposing an appropriate degree of corner-bending on the round pipe in the pre-forming process.