Role of carbon and nitrogen content on microstructural homogeneity in thin slab direct rolled microalloyed steels

Abstract

This paper analyses the effect of carbon and nitrogen content on the austenite microstructural homogeneity before transformation in the thin slab direct rolling of Nb and Nb–V microalloyed steels. The study was made with the help of a microstructural hot working model adapted to the metallurgical peculiarities associated with thin slab direct rolling. The results show that an increase in carbon content from 0·04 to 0·09% in 0·05%Nb microalloyed steels requires a significant increase in the initial rolling temperature in order to avoid the presence of isolated as cast austenite grains prior to transformation. Similarly, an increase in nitrogen content from 30 to 120 ppm does not imply changes as drastic as in the case of carbon. In both situations the changes required in the rolling temperature can be explained by the interaction between post-dynamic softening mechanisms and strain induced precipitation kinetics. In this context, the incidence of different final gauge thicknesses on microstructural homogeneity is evaluated.