Microstructure Evolution of Ferrite and Martensite Phases during Heat Treatment of the Ultralow‐Carbon 11.3Cr–1.7Mn–0.8Ni Steel

Significant phase transformations occur in the ultralow‐carbon 11.3Cr–1.7Mn–0.8Ni steel during heat treatment processes, producing distinct microstructures. Herein, optical microscopy, electron backscattered diffraction, electron probe microanalysis, and transmission electron microscopy techniques are employed to investigate the structural transformation of the steel after heat treatment at temperatures ranging from 730 to 1000 °C. The results reveal that the steel's microstructure gradually transforms from predominantly ferrite phase with minor martensite to mainly martensite with a small amount of ferrite structure, ultimately reaching almost complete martensite structure with a uniform distribution of low levels of retained δ‐ferrite strip at all tested temperatures. The band contrast curve exhibits unimodal characteristics in a single‐phase structure and bimodal characteristics in a biphase structure; the rise in the average kernel average misorientation value corresponds to higher degrees of misorientation and the formation more martensite; a grain orientation spread threshold of 4° could be utilized for qualitative determination regarding whether complete austenitic transformation occurs or not. Furthermore, the segregation of Cr, Mn, and Ni elements in hot‐rolled samples constitutes an intrinsic factor leading to microstructural alterations. Finally, hot‐rolled martensite exhibits a cellular structure, whereas heat‐treated martensite displays a lath bundle structure; both possess a body‐centered‐cubic crystal structure.