Influence of starting microstructure on the microstructure evolution during annealing and the yield strength in Nb-Ti microalloyed cold-rolled steel

Abstract

The microstructure evolution of cold-rolled microalloyed steel during annealing at different soaking temperatures was investigated, with emphasis on the influence of the starting microstructure prior to annealing. The starting microstructure of cold-rolled martensite exhibited the fastest recrystallization kinetics, followed by the cold-rolled ferrite-pearlite initial microstructure, while the recrystallization was nearly suppressed in the cold-rolled ferrite-cementite microstructure. The different recrystallization kinetics appear to be influenced by the competing effects of the driving pressure for recrystallization and the solute drag of microalloying elements, which leads to dissimilar post-annealing microstructures even with comparable fractions of new ferrite originating from the reversely transformed austenite. The yield strength following annealing was found to be affected by the starting microstructure. In addition to the dislocation strengthening associated with recrystallization retardation, contributions from other strengthening mechanisms, such as grain refinement and precipitation, varied depending on the starting microstructure. Consequently, the strengthening effects are subject to interplay between the starting microstructure and microalloying elements during annealing, which requires quantitative evaluation for a comprehensive understanding of changes in yield strength.