Textural heredity at phase transformations in low-carbon low-alloy pipe steel after controlled thermomechanical processing

Abstract

The orientation microscopy (EBSD) was used for studying of the structural and textural states of the low-carbon low-alloy pipe steel close to 06G2MB after the thermomechanical controlled processing (TMCP) and subsequent thermal treatments – heating up to 1000 °C followed by: 1) water quenching; 2) isothermal quenching with holding at 300 °C; 3) slow cooling in a furnace. All heat treatments included double phase recrystallization: α → γ → αN (where αN is martensite, ferrite or bainite, respectively). The texture obtained after TMCP, was formed mainly by two strong orientations {112}<110> and two weaker orientations close to {110}<223>. It was shown that despite dual phase recrystallization the main crystallographic orientations of bainite after TMCP and after isothermal quenching are consistent with each other. This indicates some mechanism of structure and texture heredity within the material. The structures obtained through other thermal treatments, martensite and ferrite, were also characterized by complex multi-textures. Part of the basic textural components of martensite and ferrite were the same as in case of bainite. All structures after various thermal treatments have common spectrum of high angle boundaries with the most pronounced boundaries of the coincidence site lattice (CSL): Σ3, Σ11, Σ25b, Σ33с, Σ41c. It has been demonstrated that the orientations inside textures of all obtained structures are associated with the major orientations of the strained austenite grains formed as a result of hot rolling during TMCP and the orientation relationships (ORs) that are intermediate between ORs of Kurdyumov-Sachs and Nishiyama-Wasserman. In all cases, the fact of correspondence between orientation bonds of textures in initial and all resulting states is explained through a selective initiation of phase transformations (both shear and diffusion) on crystallographic (including special) boundaries close to the CSL boundaries Σ3 and Σ11. © 2017, National University of Science and Technology MISIS. All rights reserved.