Inhibition of Abnormal Grain Growth during Isothermal Holding after Heavy Deformation in Nb Steel

Abstract

Equal channel angular pressing (ECAP) was conducted on the two grades of low carbon steel, with or without vanadium, in order to produce an ultarfine grained structure. As a result, the ferrite grains were refined from 30 mm to 0.20.3 mm. The strength of the ECAPed steels increased remarkably, over twice of the strength of the steels before ECAP. A series of static annealing experiments showed that the increment of ECAP strain and the dilute addition of microalloying element such as vanadium were very effective on enhancing thermal stability of the ultrafine gained low carbon steels produced by ECAP in terms of microstructure and tensile properties. This enhanced thermal stability resulted from; (a) presence of excessive carbon content in the ferrite matrix by carbon dissolution from pearlitic cementite during ECAP; (b) preservation of high dislocation density due to addition of vanadium, providing the effective diffusion path for dissolved carbon atoms; (c) precipitation of excessive carbon as the form of nano-sized cementite particles during subsequent annealing and its effect on suppressing grain growth.