Austenite grain refinement and superplasticity in niobium microalloyed steel.

概要

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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.
The Iron and Steel Institute of Japanの論文