Effect of cold rolling and annealing parameters on the microstructure and mechanical properties of dual phase steels

Poyraz, Okan
This study consists of change in the microstructure and mechanical properties of the 0.14C-1.98Mn steel with different cold rolling ratios, annealing temperature and annealing time. The starting sheet metal is 2.8mm thick and have a microstructure of ferrite-pearlite in as-received condition. Samples are cold rolled such that either 30%, 45% or 60% cold rolling is achieved. In order to see the change in microstructure below and in the intercritical temperatures, samples are annealed for 5 minutes or 10 minutes in the range 680ºC - 800ºC, then quenched in water. Finally, the as-received samples are annealed at the same temperatures range for 5 minutes in order to be able to compare the microstructural and mechanical property differences between cold rolled and ferritic-pearlitic samples. Between 730-760ºC, ferrite fraction and grain sizes of 45% samples is found to be highest at intermediate intercritical annealing temperatures, and as a result the lowest tensile strength is obtained. At higher temperatures, the effect of cold rolling ratio become negligible. The positive effect of increasing the annealing temperature and the annealing time on the amount of austenite formation is observed. Also, the main nucleation sites for austenite is determined as grain boundaries that are close to carbon sources.