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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 D 0 ¼ 190:7 mm, wall thickness t 0 ¼ 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.

A heavy gauge square steel pipe is manufactured by a hot roll sizing process, in view of difficulty in manufacturing such a pipe with sharp corners by cold roll forming. In this paper, the effect of pass schedule on a cross-sectional shape is discussed by referring to experimental measurements, and results calculated by the rigid-plastic finite element method. The experiment was carried out at the last step of the sizing process for seamless pipes. The corners of a product become sharper as the magnitude of total reduction increases. In the case of a two-roll type roller, the corner near the roll flange becomes sharper than the corner near the groove bottom. The hollow depth at the sides is small when the incremental reduction at each sizing stand is high at the early reshaping stage with a large bending curvature (ðD 0 =2Þ=R i , R i : bending radius, D 0 : initial external diameter of a circular seamless pipe) and low at the late reshaping stage with a small bending curvature. (Received August 7, 2003; Accepted September 19, 2003) Keywords: hot roll forming, tube forming, seamless pipe, heavy gauge square pipe, rigid-plastic finite element method PrefaceSquare steel pipes are often used as pillars for low-rise to high-rise steel constructions. They are mainly cold-formed square steel pipes, which are produced by a roll forming system or a press forming system. The roll forming system continuously reshapes ERW pipe into square steel pipes by tandem pass forming rolls. The press forming system forms a steel plate into a square cross-section or a pair of groove cross-sections and finishes the thus-produced intermediate products into a square steel pipe. Square steel pipes formed by these systems are required to have high dimensional precision. The requisites of square steel pipes used for pillar and beam joints (connections) are a cross-sectional shape with a large flat area and small corners for high processability, and excellent flatness on the sides.In production of square steel pipe by the roll forming system, the corners can be reduced to some extent by increasing the amount of forming (reduction) with pass forming rolls. As the reduction is increased and the corners are made smaller, the corner walls become obviously thicker, 1) work hardening progresses, and deformability (toughness) decreases. Excess reduction may crack corners and disable forming. Since thick materials make the corner walls even thicker, the cold forming of thick-wall square steel pipes with small corners is known to be difficult. The work hardening of corners poses not only a forming problem, but also a problem in earthquake resistance, because corners exhibit low plastic deformability.The thermal treatment of formed square steel pipes is a means of eliminating work hardening strain but involves many problems, including high cost and generation of lengthwise warpage.Bearing these problems in mind, the authors performed hot roll forming of circular pipes by heating to the recrystallization temperature or higher, and conducted an experi...

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