Abstract
Flattened plates often show the lower or higher yield strength than initial leveled plates because tensile and compressive strains are repeatedly experienced at outer and inner walls during the pipe-forming and flattening, but reasons for the yield-strength variation after the pipe-forming are not sufficiently verified yet. In this study, ten line-pipe steels were fabricated by controlling alloying elements and finish rolling temperatures (FRTs), and the yield strength of pipe-flattened steel plates was predicted by using cyclic simulation tests, based on competing contributions of Bauschinger effect (BE) and strain hardening (SH) effect quantified from yield drop (YD) and yield rise (YR) parameters, respectively. High-FRT-treated steels (H steels) showed the lower BE and the higher SH than low-FRT-treated steels (L steels), thereby resulting in the smaller yield-strength reduction. This lower BE in the H steels was caused by the lower total boundary density, while the higher SH was caused by the higher fraction of granular bainite. According to the SH analyses between the YR parameters obtained from cyclic simulation tests and the yield ratios obtained from ordinary tensile tests, the decrease in yield-strength reduction with decreasing yield ratio was not attributed to the increase in ordinary tensile SH but to the increase in YR parameter.
Highlights
Flattened plates often show the lower or higher yield strength than initial leveled plates because tensile and compressive strains are repeatedly experienced at outer and inner walls during the pipe-forming and flattening, but reasons for the yield-strength variation after the pipe-forming are not sufficiently verified yet
Line-pipe steels generally consist of polygonal ferrite (PF) together with low-temperature-transformed microstructures of granular bainite (GB), bainitic ferrite (BF), and acicular ferrite (AF), and their volume fractions vary with alloying elements and manufacturing thermo-mechanical controlled process (TMCP) parameters[6,7,8,9,10]
Ten line-pipe steels were fabricated by controlling alloying elements and finish rolling temperatures (FRTs), and the yield strength of their pipes was predicted, based on competing contributions of Bauschinger effect (BE) and strain hardening (SH) quantified from cyclic simulation tests
Summary
Flattened plates often show the lower or higher yield strength than initial leveled plates because tensile and compressive strains are repeatedly experienced at outer and inner walls during the pipe-forming and flattening, but reasons for the yield-strength variation after the pipe-forming are not sufficiently verified yet. Ten line-pipe steels were fabricated by controlling alloying elements and finish rolling temperatures (FRTs), and the yield strength of pipe-flattened steel plates was predicted by using cyclic simulation tests, based on competing contributions of Bauschinger effect (BE) and strain hardening (SH) effect quantified from yield drop (YD) and yield rise (YR) parameters, respectively. Line-pipes used for transportation of natural gas or crude oil have been manufactured by various processes of hot rolling, coiling, leveling, and pipe making, and their final yield strength is measured after pipes are flattened During these processes, tensile and compressive strains are repeatedly experienced at outer and inner walls, respectively[1,2,3]. Effect of the decrease in yield ratio, which provides a useful parameter raising the SH obtained from ordinary tensile tests[13,14], was discussed
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