Austenite Grain Refinement and Superplasticity in Niobium Microalloyed Steel

Abstract

Auslenite grain refinement of low carbon steels prepared with or without the addition of a very small amount of niobium was achieved by thermomechanical processing in order to obtain materials with an (α+γ) microduplex structure in the intercritical temperature range. Such materials showed superplastic behavior during high temperature tensile deformation. Large values of elongation to failure, the largest being 738% and high values of strain rate sensitivity exponent, m, of >0.6 were obtained by tensile deformation at 790°C and a strain rate of 5×10^-3min^-1 in the Nb-microalloyed steel. Moreover, an elongation to failure of about 300% was obtained by tensile deformation at around 800°C and a strain rate of 5×10^-1min^-1, a set of work condition which may be adaptable to commercial forming processes. The largest elongation to failure value and the highest m-value were, as a whole, obtained at around the temperature where the specimens were composed of nearly equal volume fractions of α and γ phases.
Microalloying cf niobium promoted superplasticily of low carbon steel presumably through the suppressive effects of finely dispersed precipitates of niobium carbide (or carbonitride) on grain growth during high temperature deformation.