OPTIMAL CHEMICAL-COMPOSITION IN FE-CR-NI ALLOYS FOR ULTRA GRAIN REFINING BY REVERSION FROM DEFORMATION INDUCED MARTENSITE

A thermomechanical treatment, which applies reversion from deformation induced martensite (alpha') to ultra grain refining of austenite (gamma), was proposed for metastable austenitic stainless steels. To determine optimal steels for the treatment, the effect of chemical composition on the gamma-alpha' transformation behavior during cold rolling and the alpha'-gamma reversion behavior by successive annealing was investigated in Fe-Cr-Ni ternary alloys. An ultra fine gamma grain structure was obtained when steels satisfied the following three compositional conditions: (1) Metastable gamma should be almost completely transformed to alpha' during cold rolling at room temperature. The amount of alpha' induced by 90% cold rolling can be estimated by the Ni equivalent (Ni + 0.35Cr). For steels with the Ni equivalent of less than 16.0 mass%, over 90 vol% of gamma transforms to alpha' during 90% cold rolling at room temperature. (2) Most of deformation induced alpha' must revert to gamma again at relatively low temperatures where grain growth is difficult to occur. When the Cr equivalent (Cr-1.2Ni) is less than 4.0 mass%, most alpha' induced by the 90% cold rolling reverts to gamma through 873 K-0.6 ks annealing. Retained alpha' is less than 10 vol%. (3) The Ms temperature of the reversed gamma obtained through 873 K-0.6 ks annealing should be below room temperature. When the Ni equivalent (Ni + 0.65Cr) of steels is more than 19.7 mass%, the reversed gamma is stable at room temperature. For the Fe-Cr-Ni ternary alloys which satisfy these three conditions, an ultra fine gamma grain structure might be obtained through the reversion from deformation induced alpha'. For example, gamma grains of 0.5-mu-m were observed in a 15.5%Cr-10%Ni steel which was subjected to 90% cold rolling and subsequent 873 K-0.6 ks annealing.