Kim Dae Woong, Sohn Seok Su, Kim Wan-Keun, Kim Ki-Seok, Lee Sunghak
Center for Advanced Aerospace Materials, Pohang University of Science and Technology, Pohang, 790-784, Korea.
Max-Planck-Institut für Eisenforschung, Max-Planck-Straße 1, 40237, Düsseldorf, Germany.
Sci Rep. 2018 Oct 22;8(1):15598. doi: 10.1038/s41598-018-34046-x.
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.
管线钢复杂地由贝氏体铁素体、粒状贝氏体和针状铁素体（AF）的低温转变微观组织以及多边形铁素体（PF）组成，这些微观组织各自影响管材成型过程中出现的包申格效应。在本研究中，分别对单相和两相轧制的API X80钢中作为主要微观组织的AF和PF进行了电子背散射衍射（EBSD）微观分析，并结合拉伸-压缩和间断弯曲试验，以验证包申格效应。关于微观组织对包申格效应的影响，AF在平整过程中可动位错密度的降低比PF小。然而，在低角度亚结构和高角度晶界处的位错堆积更频繁地被观察到，从而导致AF中更高的背应力和包申格效应。边界核平均取向差（KAM）分布在确定包申格效应中起关键作用，因为它们与背应力密切相关。因此，单相轧制钢中的包申格效应高于两相轧制钢。目前结合EBSD分析的非原位间断弯曲方法是详细解释包申格效应的优秀方法，并为管线钢的屈服强度设计提供了重要思路。
