マルテンサイトとベイナイト二相混合組織をもつ0.2%C-Ni-Cr-Mo鋼の引張特性について

抄録

A study has been made of room-temperature tensile properties of a 0.2%C-Ni-Cr-Mo steel having mixed structure of martensite and bainite.
When B-III and B-II type bainite, which precipitated acicularly so as to partition prior austenite grains, associated with lath martensite, a detrimental effect was found on the strength and ductility regardless of volume fraction of the bainite and tempering conditions, as opposed to a desirable effect in the case of mixed structure of lenticular martensite and lower bainite precipitated acicularly.
From the analyses of true stress-strain diagrams and microfractographs, the decrease in the ductility results from the fact that the bainite being elongated pararell to tensile axis fractures in brittle manner, it being attributed to higher stress concentration in the vicinity of twophase interface as a result of unequal strain in the two-phases durring plastic deformation.
From the above results, it was assumed that the strength and ductility of steels having mixed structure of martensite and bainite were significantly affected not only by the shape and distribution of the bainite, but also by the morphology of co-existent martensite.