Modeling of Bainite Transformation During Partitioning Process and Atomic‐Scale Characterization of Bainite

Abstract

This paper presents a detailed investigation of the interaction between bainite transformation and carbon partitioning during isothermal holding of intercritical heating, quenching, and partitioning (I&Q&P) steel. The kinetics of isothermal bainite transformation is modeled based on the assumption of displacive mechanism. A systematic analysis of the autocatalytic parameters extracted from the fitting curves reveals that autocatalysis plays only a minor role in bainite transformation due to quite small size of parent austenite grains transforming into bainite, independent from each other. Atom probe tomography (APT) is used to investigate the localized composition changes at the interface between bainitic ferrite and retained austenite. The results indicate that carbon enrichment of austenite occurs and Mn atoms segregate at phase grain boundary. Furthermore, in terms of the thermodynamic analysis, bainitic ferrite forms under diffusionless growth followed by tempering, and in present work, the growth of bainitic ferrite can be divided into two parts: bainite transformation quickly (BTQ) and bainite transformation slowly (BTS).

Fundamentals and application of solid-state phase transformations for advanced high strength steels containing metastable retained austenite

“…However, it is unavoidable that austenite containing an insufficient percentage of carbon content would decompose into bainite during partitioning. The formation of bainitic ferrite can divide the unstable blocky austenite into film-like austenite [258][259][260][261], and also contribute to carbon enrichment in its surrounding austenite. Bainite formation in Q&P steels was also found to benefit the ductility and toughness [112,113,192,258].…”

“…In addition to experimental investigations, bainite formation during the Q&P process was also analysed based on diffusionless [168,261,264,265] or diffusional theories [266]. HajyAkbary et al [168] studied the influence of carbon partitioning from martensite into austenite on bainite formation in an Fe-0.3C-3.5 Mn-1.6Si steel.…”

Effect of Cold Rolling Reduction Rate on the Microstructure and Properties of Q&P Steel with a Ferrite-Pearlite Initial Structure

“…The Ms point temperature also decreases with the increase of CRRR which is related to the austenite grain size in the intercritical two-phase region [ 20 , 30 ]. At 420 °C, a significant change in the slope of the expansion curve is observed, indicating the occurrence of bainite transformation in addition to carbon partitioning from martensite to austenite [ 31 ]. The bainite phase transition becomes more prominent as the CRRR increases, as evidenced by the increased amount of expansion.…”