Precipitation of NbC in a model austenitic steel

A model Fe–30 wt% Ni, 0.1 C, 1.61 Mn, 0.1 Nb microalloyed steel, that simulates conventional microalloyed C–Mn steels, but does not transform from the austenite phase on cooling, is reported. Plane strain compression testing was undertaken at 950°C at a constant true strain rate of 10 s−1. Samples were deformed in a two stage process. An initial true strain of 0.25–0.45 was followed by unloading, a hold of 1–1000 s and a final deformation to a total true strain of 0.5–0.9. A single deformation was undertaken under identical conditions, but to the total true strain of the double deformation tests. Electron spectroscopic imaging (ESI) in the TEM was used to determine precipitate size and distribution. A 1 s hold time between equal strains of ϵ=0.25 was sufficient for appreciable strain induced precipitation, although 40% static recrystallisation occurred during the hold time. Precipitation occurred entirely on dislocations, present principally as microband walls but also as a rudimentary cell structure within the microbands. No evidence was found for NbC precipitation in the matrix, which therefore remains supersaturated with Nb. NbC particle diameter was in the range 2.5–15 nm, with a density of 3.8×1021 particles/m3 for a 100 s delay period between two strains of ϵ=0.45 at 950°C. Both the size and number density are consistent with those observed in conventional microalloyed C–Mn steels. The behaviour of the model microalloyed Fe–30 Ni steel is discussed in relation to the data on conventional microalloyed steels.