Microstructure control for superplasticity of an ultra-high carbon steel

Relation between the (a+0) 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 (α+θ) microstructure with fine and equiaxed a grains of 0.4um in diameter and spheroidized 0 particles of 0.2um 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 (y+9) two phase region, quenched and tempered at the temperature below Ai, an (a+0) microduplex structure with the a and 0 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 .(a+9) 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.