準安定オーステナイトステンレス鋼における加工誘起マルテンサイト変態の組成および結晶粒度依存性

Abstract

Composition and grain size dependencies of Md30, the parameter proposed by Angel for indicating austenite stability during deformation in metastable austenitic Fe-Ni-Cr stainless steels, were examined.The measurement of Md30 by changing composition, C, N, Si, Mn, Ni, Cr, and others, give the following empirical equation: Md30 (C) =551-462 (C%+N%) -9.2Si%-8.1Mn%-13.7Cr%-29.0 (Ni%+Cu%) -18.5Mo% -68.0Nb%. A great coefficient for Ni in this equation is quite different from that in the Angel's equation. With decreasing grain size of austenite, the amount of strain-induced martensite decreases, and Md30 is lowered. Consequently, MdGS30 which is Md30 modified by the grain size effect, can be expressed as MdGS30=Md30 -1.42 (ν-8.0); where ν is ASTM grain size number.The proof stress drops during deformation at lower temperatures because of the inducement of martensite at a small strain. This phenomenon easily occurs in the specimen with finer grain size. On the stress-strain curve the serrated region can be easily found in the deformation of the specimen with finer grain size. The amplitude of serration becomes too small to be observed in such cases as the specimen with coarser grain size, lower deformation temperature and stabilization of austenitic phase, even though a lot of martensite is induced.