Microstructure control for superplasticity of an ultra-high carbon steel

Relation between the (alpha+theta) 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 a matrix. By 90% warm rolling of a pearlite structure, an (alpha+theta) microstructure with fine and equiaxed a grains of 0.41 mu m in diameter and spheroidized theta particles of 0.2 mu m in diameter is obtained. Kikuchi pattern analysis has revealed that a matrix exhibits a recovered structure in which a large fraction of a grain boundaries are subgrain boundaries (low angle boundaries) not suitable for grain boundary sliding. When the 90% warm rolled specimens were austenitized in (gamma+theta) two phase region, quenched and tempered at the temperature below Al, an (alpha+theta) microduplex structure with the a and theta grain sizes equivalent to those in the as warm rolled specimen is formed. It was found that the fraction of high angle a 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 (alpha+theta) microduplex structure with high angle a 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.