Abstract
Linepipe steels complexly consisted of low-temperature transformation microstructures of bainitic ferrite, granular bainite, and acicular ferrite (AF) as well as polygonal ferrite (PF) which individually affect the Bauschinger effect occurring during the pipe-forming. In this study, microscopic analyses of electron back-scattered diffraction (EBSD) coupled with tension-compression and interrupted bending tests were performed for verification of the Bauschinger effect of AF and PF working as major microstructures in single-phase- and two-phase-rolled API X80 steels, respectively. With respect to microstructural effects on Bauschinger effect, the reduction in mobile dislocation density during the flattening was smaller in the AF than in the PF. However, the dislocation pile-up at low-angle substructures and high-angle grain boundaries was more frequently observed, thereby leading to the higher back stress and Bauschinger effect in the AF. Boundary kernel average misorientation (KAM) profile played a critical role in determining the Bauschinger effect because they were closely related with the back stress. Thus, the Bauschinger effect was higher in the single-phase-rolled steel than in the two-phase-rolled steel. The present ex-situ interrupted bending methods coupled with EBSD analyses are outstanding ones for the detailed explanation of Bauschinger effect and provide an important idea for the yield strength designs of linepipe steels.
Highlights
Linepipe steels used for long-distance transport of crude oil or natural gas require the higher strength and toughness to improve the transport efficiency and structural endurance, respectively
The present study provides a good way to examine effects of basic constituent microstructures such as acicular ferrite (AF) and polygonal ferrite (PF) on Bauschinger effect existed in the yield strength measurement before and after the pipe-forming
Two linepipe steels were fabricated by varying finish-rolling temperature, and effects of microstructural features of AF and PF on Bauschinger effect were investigated with respect to the density of mobile dislocation and grain boundary by electron back-scatter diffraction (EBSD) analyses coupled with ex-situ three-point bending tests
Summary
Linepipe steels used for long-distance transport of crude oil or natural gas require the higher strength and toughness to improve the transport efficiency and structural endurance, respectively. Linepipe steels are made in a coil or plate form according to sheet thickness requirements, and are used in seamless, UOE, electric resistance welded, and spiral pipe forms[1,2,3] These various pipe-forming procedures result in the generation of tensile and compressive strains in outer and inner sides of pipes, respectively[4,5,6,7], and the flattening followed by tensile test induces repeated tensile and compressive strains. Detailed mechanisms on how these microstructures affect the Bauschinger effect during pipe-forming and flattening procedures are hardly studied in linepipe steels containing AF and PF as basic constituent microstructures Their roles and characteristics in Bauschinger effect are important, but which is more unfavorable for the Bauschinger effect still remains to be addressed because of difficulties in the detailed elucidation of repeated reversible deformation mechanisms within very small areas of AF or PF. The resultant microstructural analysis data of AF or PF were utilized for understanding microstructural characteristics and for reducing Bauschinger effect
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