Processing and stability of ultrafine grained structures in some microalloy steels

The dynamic strain-induced transformation (DSIT) of ferrite from austenite in intense deformation at temperatures close to A,3 were applied to one C-Mn steel and several Nb and Nb-Ti microalloy steels to obtain an ultrafine ferrite grain size. As another route the static recrystallisation of severely cold-rolled martensite (SRM) was utilized. It was found that in the DSIT route a fine prior austenite grain size was crucial to form ferrite with the grain size of 1-3 mu m with a considerable fraction of a secondary phase, carbide aggregate/pearlite or martensite. Grain sizes achieved were somewhat finer in steels with a higher microalloying content. In the SRM route, the ferrite grain size of 1-1.5 mu m was obtained by using the cold rolling of 80-90% reduction. Thermal stability of the ultrafine-grained structures, especially those from the DSIT route, was found to be excellent. In electron or laser beam welding of 1-2.5 mm sheets neither any coarse-grained zone existed in the heat-affected zone, nor did form any softened zone.