Development of ultra fine grain structure by martensitic reversion in stainless steel

Abstract

Austenitic stainless steels have good corrosion resistance and good formability but they have also relative low yield strength. It is well known that the mechanical properties of austenitic stainless steels are very sensible to the chemical composition (which can induce hardening by both substitutional and interstitial solid solution) and to microstructural features (such as grain size and δ-ferrite content). Recently there have been commercial developments to exploit the effect of these variables in stainless steel taking advantage of changes in the chemical composition induced by nitrogen addition [1, 2]. Another effective way to increase yield strength without impairing good ductility is grain refining. Although this approach has induced the development of ultrafine grain carbon steels (e.g. [3]), no attempts have been still reported on this approach for austenitic stainless steels. In fact, austenitic stainless steels do not undergo phase transformation at typical annealing temperatures and then the only way to refine the grain is recrystallization after cold rolling. However, the strengthening by grain refining is limited, due to the high recrystallization temperature of this stainless steel grade. For instance, the recrystallization temperature of the AISI 301 steel is above 900 ◦C and the minimum grain size obtained is in the range 10–30 μm [4]. In austenitic stainless steels, plastic deformation of austenite creates the proper defect structure which acts as embryo for martensite deformation: the successive reversion of deformation-induced martensite (α′) enables a marked grain refining [5, 6]. In this letter the production of an ultra fine microstructure in an AISI 301 stainless steel by martensitic reversion is reported. The chemical composition of the steel used is shown in Table I. The procedure used to refine the grain is the following (see Fig. 1): • Metastable γ is almost entirely transformed to α′ by heavy cold rolling: in fact the retained γ cannot be refined during the subsequent annealing. • α′ reverts to recrystallized austenite γR during annealing at low temperature.