COMBINED ROLL OF BRAKE DOWN MILL IN PIPE FORMATION

PROBLEM TO BE SOLVED: To prevent the defect of buckling phenomenon, bending, etc., at the center part of a formation by constituting a cross sectional profile with a cross sectional curving surface connected together with plural curves. SOLUTION: An upper roll 1 is constituted of splitting rolls as symmetrical shape in the right and the left sides. A lower roll 2 is formed of a signal roll composed of the cross sectional profile having the fixed curvature of arc at the center part. The cross sectional profile of the upper roll 1 is specified by the curve having the position in the uniform gap with the lower roll 2. Successively, the curve is divided into plural parts and plural partial curves which do not mutually adjoin, are selected and the cross sectional profile constituted of the cross sections connected together with plural curves having the same curve as the curve obtd. by connecting end parts with the other end parts of the partial curves parallely moved in the order of the near one only by parallely moving the partial curve at the nearest position to this selected partial curve, is constituted. By this method, the load at the finish pass forming in the following process can be restrained to the min. limit.