Effects of preforming methods on cross-sectional shapes of extroll-formed square steel pipes

摘要

For improvement of the cross-sectional shape (outer curvature, inner curvature, wall thickness change at corner zones, and flatness of sides) of square steel pipes (150.0 mm wide) formed by use of an extroll-forming mill, two expansion-type preforming processes are used individually to impose peripheral bending on the portions of round pipes (190.7 mm in outer diameter, 4.6 mm wall thickness) that correspond to the corner zones of the finished product with four inner idler rolls or a pair of four inner and outer idler rolls (outer rolls: grooved rolls). Characteristics of the cross-sectional shapes of the formed square pipes are investigated with respect to the degree of corner bending in the preforming and compared with the results obtained by conventional extroll-forming. The results show that the cross-sectional shape of the finished product can be markedly improved by imposition of an appropriate degree of corner bending on the round pipe by use of a pair of inner and outer preforming rolls.