Ways of improving the quality of straight-seam welded pipe

Abstract

In [1], we examined the physical principles governing the effect of one-sided undulations of skelp on the quality of straight-seam electric-welded pipes. It is known that the quality of these pipes is affected most by tendency of the skelp to undergo corrugation during the forming of the pipe. In a number of cases, the corrugation can dismpt the lorming operation (in particular, it can lead to "biting" of the corrugations in closed passes), result in projection of the edges during their welding, and produce incomplete fusion. Other conditions being equal, the probability of the formation of corrugations on the edges of skelp increases under the lollowing conditions: a decrease in the thickness of the pipe wall, i.e., an increase in the ratio of the diameter of the pipe to the wall thickness: Dp/S; the use of skelp with crescent-shaped or wavy edges; the tbrmation of skelp from steel with a low elastic modulus; wide spacing of the stands of the mill; the use of passes not designed with allowance for the need to ensure longitudinal stability of the semifinished product, etc.Other conditions being equal, the longitudinal stability of the semifinished product (its tendency to undergo corrugation) depends mainly on the character of the change in the curvature of its longitudinal neutral axis (along the Z axis of the mill).This dependence can be represented in the form of a curve that connects the centers of gravity of the cross sections of the semifinished product [2]. The character of the curvature of the longitudinal neutral axis of the semifinished product depends in turn on the character of the change in the longitudinal bending moments of different signs that act on the product, as well as on deformation of its cross sections. It has been proposed that the main condition for the absence of corrugations on the semifinished product during continuous forming can be expressed by the following inequality: An increase in the distance between the stands increases the values of Mcz and Mtz. The increase in those values not only increases the above difference, but it also causes the section of the semifinished product in which the critical moment Mcr z has been reached to separate from the tool. Such separation also increases the probability of corrugation. Mcz -Mtz <-