Ferrite recrystallization and austenite formation at the early stage of annealing in cold-rolled low-carbon steels

Abstract

Background: High strength low carbon steels used in the automobile industry are often annealed at the intercritical temperature, and recrystallization of ferrite and formation of austenite can occur at the same time during the intercritical annealing. However, detailed studies on the competition between recrystallization of ferrite and formation of austenite at the early stage of annealing have not been carried. Therefore, ferrite recrystallization and austenite formation in cold-rolled low carbon steels at an early stage of annealing have been observed. Methods: The chemical composition of the steel (in mass %) was 0.1C-2.0Mn. After hot and cold rolling, the specimens were heated from room temperature to 1003, 1013 and 1023 K at a rate of 30 K s-1, and then gas-quenched to room temperature at a rate of 50 K s-1. We observed the microstructure of the annealed specimens. Results: Fractions of recrystallized ferrite and austenite gradually increased with increasing annealing temperature, whereas the fraction of non-recrystallized ferrite gradually decreased with increasing annealing temperature. A large portion of austenite was formed at the grain boundaries of recrystallized ferrite and at the interface between recrystallized and non-recrystallized ferrite grains, whereas formation of austenite was scarcely confirmed at subgrain boundaries inside non-recrystallized ferrite grains. The grain size of the recrystallized ferrite and the subgrains formed in the non-recrystallized ferrite hardly changed during the annealing. Conclusions: Ferrite recrystallization was able to proceed during intercritical annealing when the austenite fraction was less than at least 10%. A large portion of austenite was formed at the grain boundaries of the recrystallized ferrite and the interface between the recrystallized ferrite and non-recrystallized ferrite at an early stage of annealing at the intercritical temperature.