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 (α) to ultra grain refining of austenite (γ), was proposed for metastable austenitic stainless steels. To determine optimal steels for the treatment, the effect of chemical composition on the γ-α transformation behavior during cold rolling and the α-γ reversion behavior by successive annealing was investigated in Fe-Cr-Ni ternary alloys. An ultra fine y grain structure was obtained when steels satisfied the following three compositional conditions: (1) Metastable γ should be almost completely transformed to α’ during cold rolling at room temperature. The amount of α’ 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 α transforms to α’ during 90% cold rolling at room temperature. (2) Most of deformation induced α’ must revert to y 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 a’ induced by the 90% cold rolling reverts to y through 873 K-0.6 ks annealing. Retained α’ is less than 10 vol%. (3) The Ms temperature of the reversed y 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 γ is stable at room temperature. For the Fe-Cr-Ni ternary alloys which satisfy these three conditions, an ultra fine y grain structure might be obtained through the reversion from deformation induced α.’. For example, y grains of 0.5 μm were observed in a 1 5.5%Cr-10%Ni steel which was subjected to 90% cold rolling and subsequent 873 K-0.6 ks annealing.