Effect of Starting Microstructure through Severe Plastic Deformation and Ultra-Rapid Annealing of Low-Carbon Steel

摘要

In this research, low-carbon steel sheets with two different microstructures containing carbides (as-received steel) and pearlites (austenitized steel) are subjected to severe plastic deformation (SPD), and ultra-rapid annealing (URA) is applied with the heating rate of 200 degrees Cs-1 up to the intercritical temperature near Ac-1. Microstructure of SPDed as-received steel after URA is consisted of fully refined ferrite grains surrounded by fine pearlites; but for SPDed austenitized steel, grain refinement is not observed, and microstructure is consisted of martensite and high fraction of coarse non-recrystallized ferrite. Presence of carbides for SPDed as-received steel leads to strong interaction between ferrite recrystallization and ferrite transformation to austenite, which causes to refined final microstructure after URA. However, initial pearlites in SPDed austenitized sample prevent the occurrence of strong interaction during URA. For austenitized sample, URA of SPDed steel increases the strength and ductility about 28 and 12%, respectively. Also, for as-received sample, it increases the strength about 32% but decreases the ductility about 11%.