Ultra-Rapid Intercritical Annealing to Improve Deep Drawability of Low-Carbon, Al-Killed Steels

Ultra-rapid annealing (URA) cycles were performed on a low-carbon, Al-killed steel coiled at a high temperature. The parameters (heating and cooling rates, and annealing temperature) of these cycles were varied to analyze their impact on the microstructure and the tensile properties of the steel. It was established that the final microstructure of the steel is governed by an interaction between the growth of the primary ferrite grains (resulting from the recrystallization) and the development of the austenite grains formed above AC1, and that the heating rate strongly influences this interaction. A microstructure favorable to deep drawability (leading to a mean plastic strain ratio of 1.8) was obtained by adjusting the parameters of the URA cycles. The current work showed that three conditions have to be fulfilled to produce this optimized microstructure: (1) Intercritical annealing should lead to the formation of about 20 pct of austenite, (2) the heating rate has to be rapid to minimize the carbon content in solution during recrystallization, and (3) the cooling rate must be relatively slow (≈50 K/s).