Yielding mode transition and yield point elongation evolution in thermomechanically controlled processed low-carbon HSLA steels: Role of strain aging and dislocation density

In low-carbon high-strength low-alloy (HSLA) steels, discontinuous yielding is generally considered undesirable because it causes localized deformation, thereby compromising dimensional accuracy and structural reliability. Although discontinuous yielding has traditionally been interpreted based on the classical strain aging mechanism, the relationship between processing-induced microstructural evolution, strain aging, and the resulting yielding mode transition has not yet been systematically clarified. In the present study, a single-ingot thermomechanical controlled processing (TMCP) approach was employed to systematically control the microstructure and phase constitution. With increasing TMCP degree, the ferrite fraction increased while the grain size decreased, accompanied by a transition in yielding behavior from continuous to discontinuous yielding and a progressive increase in yield point elongation (YPE). This behavior is attributed to the reduction in dislocation density associated with the increase in ferrite fraction, which enhances solute pinning efficiency, delays the activation of mobile dislocations, and promotes strain aging. Consequently, the yielding mode transition and YPE evolution can be systematically interpreted through the interplay between dislocation density and solute pinning under different TMCP conditions. Furthermore, three distinct regimes of strain aging behavior – negligible, moderate, and strong – were identified, suggesting a threshold-like, stage-wise evolution of strain aging with increasing TMCP degree, thereby establishing a quantitative relationship between TMCP conditions and yielding behavior.