Yield-strength prediction of flattened steel pipes by competing Bauschinger effect and strain hardening during pipe-forming

Dae Woong Kim,Won-Doo Choi,Sunghak Lee,Wan-Keun Kim,Jin-Ho Bae,Hyoung Seop Kim

doi:10.1038/s41598-019-50328-4

Abstract

Since flattened steel sheets often show the unexpectedly lower or higher yield strength than leveled sheets, unceasing efforts have been made to accurately predict the yield strength in pipe-forming industries. In the present investigation, the yield strength of line-pipe or casing-pipe steels was predicted by competing Bauschinger effect and strain hardening occurred during the pipe-forming. Yield drop (YD) and yield rise (YR) parameters were newly defined from cyclic simulation analyses of outer and inner walls of pipes to express more reasonably the Bauschinger effect and strain hardening. The YD increased abruptly until the pre-strain of about 1%, and then saturated, while the YR increased linearly with increasing pre-strain. By combining the YD and YR, the variation in yield strength (Δσ) showed a down-and-up behavior as the Bauschinger effect and strain hardening were dominant at low and high pre-strains, respectively, and plausibly explained the relationship of Δσ and piping strain used in pipe-forming industries. According to the microstructural analyses related to the down-and-up Δσ behavior, the polygonal ferrite reduced the yield-strength reduction in the low pre-strain range, whereas the granular bainite or pearlite expanded it. This yield strength prediction coupled with microstructural analyses provide a good idea for designing and reliably predicting the yield strength of in various steel pipes.

Highlights

Line-pipes used widely for the long-distance transportation of crude oil and natural gas have been produced by hot-rolling, coiling, leveling, and pipe-forming procedures, and their mechanical properties are evaluated after the flattening of a pipe wall segment
The yield strength of flattened line-pipe or casing-pipe steel sheets was predicted by competing Bauschinger effect and strain hardening quantified from cyclic simulation tests
(1) Yield drop (YD) and yield rise (YR) parameters were newly defined from cyclic simulation analyses of outer and inner walls of steel pipes to express more reasonably the Bauschinger effect and strain hardening occurred during the pipe-forming

Summary

Introduction

Line-pipes used widely for the long-distance transportation of crude oil and natural gas have been produced by hot-rolling, coiling, leveling, and pipe-forming procedures, and their mechanical properties are evaluated after the flattening of a pipe wall segment. After the pipe-forming, pipes’ outer and inner layers in the walls experience repeatedly tensile and compressive strains[1,2,3,4], respectively Because of these different strain histories, the flattened segment of pipe walls often show the unexpectedly much lower or higher yield strength than that of the leveled sheets[5]. In ultra-high-strength API X100-grade steels or the higher ones, martensite-austenite constituent (MA) is utilized[8,9,10] These microstructures influence both Bauschinger effect and strain hardening, and their volume fractions appropriately control the yield-strength level. Line-pipe or casing-pipe steel sheets having different constituent phases were made by varying steel compositions and coiling temperatures, and the Bauschinger effect and strain hardening were quantified by cyclic simulation tests of their leveled sheets to predict yield strength of the pipes without conducting the pipe-forming, flattening, and tensile testing. This study is expected to contribute to yield-strength designs of the line-pipe or casing-pipe steel sheets in relation to microstructural characteristics and to the reduction of unnecessary spending of time

Methods

Results

Discussion

Conclusion