Microstructure Control for Superplasticity of an Ultra-High Carbon Steel

Abstract

Relation between the (α+θ) microstructure and the superplastic behavior in an ultra-high carbon steel (Fe-1.4Cr-1.0C) has been studied. Special attention was paid to the substructure and grain boundary character in α matrix. By 90% warm rolling of a pearlite structure, an (α+θ) microstructure with fine and equiaxed α grains of 0.4μm in diameter and spheroidized θ particles of 0.2μm in diameter is obtained. Kikuchi pattern analysis has revealed that α matrix exhibits a recovered structure in which a large fraction of α grain boundaries are subgrain boundaries (low angle boundaries) not suitable for grain boundary sliding. When the 90% warm rolled specimens were austenitized in (γ+θ) two phase region, quenched and tempered at the temperature below Al, an (α+θ) microduplex structure with the α and θ grain sizes equivalent to those in the as warm rolled specimen is formed. It was found that the fraction of high angle α grain boundary significantly increases in comparison with the as warm rolled specimen. The austenitizing, quenching and tempering treatment without warm rolling of pearlite structure also produces an (α+θ) microduplex structure with high angle α grain boundaries. The specimen austenitized, quenched and tempered shows much larger superplastic elongation in the tensile tests at 973K than the as warm rolled specimens.