（α＋γ）二相ステンレス鋼における超塑性変形中の均一微細組織の生成とキャビティの生成挙動

Abstract

Superplastic deformation behaviors in an (α+γ) duplex stainless steel have been studied with particular emphasis on microstructural change and cavity formation during tensile deformation at 950°C.The main results obtained are as follows;(1) Homogeneous (α+γ) microduplex structure with very fine α and γ grains can be obtained during tensile testing at 950°C at a strain rate of 1.0×10-2/S in the specimens which were solution-treated in the α single phase region and subsequent heavily cold-rolled. The initial cold rolling after solution treatment can play an important role for the formation of homogeneous (α+γ) microduplex structure, that is, the heavy cold rolling promotes the recrystallization of α matrix during hot deformation, resulting in the homogeneous (α+γ) microduplex structure.(2) Superplasticity is considerably enhanced by the prior heavy cold rolling, that is, the maximum elongation at a strain rate of 1.0×10-2S-1 in 90% cold-rolled specimens was 930%. The improvement of superplasticity arises from grain refinement of α and γ during hot deformation.(3) Two types of cavities were observed during hot deformation, that is, the one initiated in the early stage of deformation at the inclusions which were elongated by the prior heavy cold rolling, and the other initiated at grain boundaries and interfaces after the intermediate stage of deformation, suggesting that these cavities especially the former type of large ones may have deleterious effect on the strength and ductility during room temperature service.