Cross-Sectional Shapes of Corner Zones of Hexagonal Steel Pipes Formed by Extroll-Forming Mill with Expanding Inner Idler Rolls

Abstract

To improve the cross-sectional shapes (outer curvature, wall thickness change) of the corner zones of hexagonal steel pipes (168.0, 170.0 and 173.0 mm wide, STK400) formed by an extroll-forming mill, an expanding-type forming tool with six idler rolls is introduced into the groove of No. 1 grooved rolls to impose peripheral bending on the portions of the round pipes (outer diameter D₀ = 190.7 mm, wall thickness t₀ = 4.6 mm) that correspond to the corner zones of the finished products. Characteristics of the cross-sectional shapes of the corner zones of the hexagonal pipes are investigated with respect to the nose radius of the inner roll and the degree of corner bending by the inner rolls, and compared with the results obtained by conventional extroll-forming. The results show that a hexagonal steel pipe with a small outer radius at each corner zone can be manufactured using a pair of inner idler rolls with a small nose radius.