1985 Volume 71 Issue 2 Pages 228-235
Austenite grain refinement of low carbon steels prepared with and without addition of minor amount of niobium was attained by a thermomechanical processing in order to obtain the materials with (α+γ) microduplex structure in the intercritical temperature field. Such materials showed superplastic behavior during high temperature tensile deformation. The largest elongation to failure value of 738% and high value of strain rate sensitivity exponent, m, (>0.6) were obtained by tensile deformation at 790°C and at strain rate of 5×10-3 min-1in the Nb-bearing steel. Moreover, elongation to failure value of about 300% was obtained by tensile deformation at around 800°C and at strain rate of 5×10-1 min-1 which may be applicable for practical forming process. The largest elongation to failure value and the highest m-value were, as a whole, obtained at around the temperature where the specimens are composed of nearly equal volume fraction of α and γ phase.
A minor addition of niobium presumably promoted superplasticity of low carbon steel through a suppressive effects of finely dispersed precipitates of niobium-carbide (or-carbonitride) on grain growth during high temperature deformation.