Acta Materialia 50 (2002) 735–747 www.elsevier.com/locate/actamat Precipitation of NbC in a model austenitic steel W.M. Rainforth a,* , M.P. Black a , R.L. Higginson a , E.J. Palmiere a , C.M. Sellars a , I. Prabst b , P. Warbichler b , F. Hofer b a IMMPETUS (Institute for Microstructural and Mechanical Process Engineering: The University of Sheffield), Department of Engineering Materials, Sir Robert Hadfield Building, Mappin Street, Sheffield S1 3JD, UK b Forschungsinstitut fu ¨r Electronmikroskopie, Technische Universita ¨t Graz, Steyrergasse 17, 8010 Graz, Austria Received 27 October 2000; received in revised form 16 October 2001; accepted 17 October 2001 Abstract 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 = 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 3.8×10 21 particles/m 3 for a 100 s delay period between two strains of = 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 conven- tional microalloyed steels.  2002 Acta Materialia Inc. Published by Elsevier Science Ltd. All rights reserved. Keywords: Microalloyed steel; Strain induced precipitation; Electron spectroscopic imaging 1. Introduction Microalloyed (or high strength low alloy, HSLA) steels have received considerable interest over many years and continue to gain wider indus- * Corresponding author. Tel.: +44-114-222-2000; fax: +44- 114-222-5943. E-mail address: m.rainforth@sheffield.ac.uk (W.M. Rainforth). 1359-6454/02/$22.00  2002 Acta Materialia Inc. Published by Elsevier Science Ltd. All rights reserved. PII:S1359-6454(01)00389-5 trial applications. The addition of Nb to a steel is considered to have three primary effects [1–4]: (i) as an inhibitor of austenite grain coarsening during reheating, (ii) suppression of austenite recrystallis- ation prior to the α/γ transformation through the strain induced precipitation of NbC and (iii) pre- cipitation hardening from the NbC in the low tem- perature transformation product. The strongest contribution to strength is refinement of the final microstructure (e.g. ferrite grain size in pearlite