JIS SK5及JIS S60C鋼之變韌鐵組織與其它組織的性質比較

Abstract

This study investigates the microstructures and mechanical properties of various structures in JIS SK5 and JIS S60C steels. Strength falls and ductility raises as carbon content in steels increases for all structures. At equivalent hardness, the toughness and ductility of bainite are superior to those of tempered martensite. Tempered martensite embrittlement (TME) occurred in the ~310℃ tempered specimens, whose hardness is equal to lower bainite one. It is caused by grain boundary segregation of phosphorus and grain boundary precipitation of carbide during tempering. The fracture mode of TME is dominated by intergranular failure. The fracture surfaces of both lower and upper bainite structures exhibit transgranular cleavage. The size of the cleavage facet of bainite was demonstrated to be correlated with the width of the bainite sheaf. The results of EBSD analysis indicates not only that the sheaf boundary is a high-angle boundary, but also that the cleavage crack travels along the {001}α, {112}α and {123}α ferrite planes, whose surface energy are low. According to TEM diffraction analysis, the orientation relationship of cementite / bainitic ferrite satisfies the Bagaryatskii relation, and the habit plane of cementite precipitated in the bainite sheaf locates on || . The orientation relationship of austenite / bainitic ferrite satisfies between K-S and N-W relation. Two sets of parallelogram cross-section of binitic ferrite were found to locate on ~ and respectively. As isothermal holding time increases, one set of parallel plane inclines to via a diffusional interface migration and bainite subunits gradually coalesce.