Texture Inheritance on Phase Transition in Low-Carbon, Low-Alloy Pipe Steel after Thermomechanical Controlled Processing

Orientation microscopy (electron back scatter diffraction, EBSD) is used to investigate the structural and textural states of low-carbon, low-alloy pipe steel (resembling 06Г2MБ steel) after thermomechanical controlled processing (TMCP): heating to 1000°C with subsequent quenching in water; isothermal quenching with holding at 300°C; and slow cooling in the furnace. The heat treatment is associated with double phase recrystallization: α → γ → a_ht, where a_ht is martensite, bainite, or ferrite. The texture obtained after TMCP is mainly formed by two strong scattered orientations from {112}〈110〉and two weaker scattered orientations close to {110}〈223〉. Despite the double phase recrystallization, the main crystallographic orientations of the bainite after TMCP and after isothermal quenching are the same. That indicates structural and textural inheritance in the material. The structures obtained after other thermal treatments of the structure (both martensite and ferrite) also include complex multicomponent textures, which are nevertheless distinct. Some of the main textural components of martensite and ferrite are the same as bainitic components. All the structures after heat treatment have a similar spectrum of large-angle boundaries, with strongly expressed boundaries of the coincidence site lattices (CSL): Σ3, Σ11, Σ25b, Σ33c, and Σ41c. The orientations forming the texture of all the structures obtained are related to the main orientation of the deformed austenite grains formed on hot rolling in TMCP, in accordance with orientation relations intermediate between the Kurdjumov–Sachs and Nishiyama–Wasserman types. In all cases, the orientation relationship of the textural components of the initial material and the structure obtained by heat treatment may be explained in terms of the onset of phase transformations (both shear and diffusional transition) at crystallographically determined boundaries (including special boundaries) similar to the CSL boundaries Σ3 and Σ11.