# Effect of Strain Hardening on Increase in Collapse Pressure during the Manufacture of ERW Pipe
## 1 Introduction  
Electric resistance welded (ERW) pipes have excellent productivity and economic efficiency because they are manufactured in a single continuous process from beginning to end. Thus, demand has increased for ERW pipes as the material of choice for deep-water offshore pipelines [1; 2; 3]. Due to extremely high pressure in deep-water applications, the collapse pressure governs the design of offshore pipelines. Since ERW pipes are manufactured via a cold-forming process, variation in the material properties of the ERW pipes due to plastic deformation is an important factor for evaluating the collapse pressure. The nature of the continuous manufacture process of ERW pipes, however, creates difficulties in tracking the plastic deformation [4]. The manufacture process of an ERW pipe generally involves the sequences of leveling, breakdown, finpass, welding, and sizing. In the leveling process, the coil supplied from the steel mill is uncoiled, and the uncoiled steel plate simultaneously passes through a breakdown roller to form both sides of the steel plate. In the finpass process, the curvature of the plate becomes constant while the cross-section is formed into a circular shape. The welding surfaces are brought into contact with the fin of the roller to increase the quality of the welding surfaces before the welding. During this process, there is compression in the circumferential direction of the pipe. During welding, both surfaces are welded bya high frequency current and are bounded by a physical external pressure as they pass through the squeezing roller. Finally, the pipe passes through sizing rollers to improve the out-of-roundness of the cross-section by compressing the steel pipe circumferentially to match the final target diameter. During manufacture, plastic deformation is governed by the Bauschinger effect and strain hardening occurs with repeated cyclic loading [3; 5; 6; 7]. Since this plastic deformation changes the yield strength of the steel plate, evaluation of the post-manufacture yield strength is essential to accurately determine the quality of the pipe. Recent studies on changes in the yield strength during manufacture have confirmed that changing the stress-strain curve by manipulating the compression during the sizing process increases the longitudinal tensile yield strength [6; 7; 8]. On the other hand, there are several limitations in evaluating yield strength in the circumferential direction. While a specimen can be extracted directly from the pipe to evaluate yield strength in the longitudinal direction, flattening is required in order to evaluate the yield strength in the circumferential direction. However, it is questionable whether the actual stress-strain curve in the circumferential direction of the pipe can be obtained because plastic deformation during the flattening process could have caused changes in the material properties [9; 10]. Moreover, the flattening process is not standardized, which places limitations on compensating for the plastic deformation caused by the flattening when attempting to trace the steps that determine the yield strength of the pipe [10; 11]. Repeated cyclic loading during cold forming creates differences between the tensile and compressive yield strengths in the circumferential direction. Therefore, the stress-strain curves in establishing the tensile and compressive stresses should be individually evaluated. Due to these limitations, it could be effective to derive a compressive stress-strain curve in the circumferential direction using finite element analysis. Establishing the collapse pressure is essential for designing offshore pipelines, because this determines the buckling strength against external pressure, which is significantly influenced by the circumferential compressive stress-strain curve of the pipe [4]. A hardening of the stress-strain curve could increase the collapse pressure and affect the post-buckling behavior, but the effect of hardening depends on the slenderness of the pipe, which is the diameter-to-thickness ratio [12].