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The influence of bainite morphology on the impact toughness behaviour of continuously cooled cementite-free low carbon bainitic steels has been examined. In these steels, bainitic microstructures formed mainly by lath-like upper bainite, consisting of thin and long parallel ferrite laths, were shown to exhibit higher impact toughness values than those with a granular bainite, consisting of equiaxed ferrite structure and discrete island of marteniste/austenite (M/A) constituent. Results suggest that the mechanism of brittle fracture of cementite-free bainitic steels involves nucleation of microcracks in M/A islands but is controlled by the bainite packet size.

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In order to improve the ductility of carbide free bainitic microstructures, consisting of a bainitic ferrite matrix and a mixture of austenite and martensite, the TRIP effect i.e. the strain induced transformation of retained austenite to martensite, should be controlled. In this sense, the effect of the chemical composition on the mechanical stability of the retained austenite and the morphology, size, and distribution of this phase has been studied to determine the role that plays on the ductility behaviour of advanced bainitic steels. Results suggest that apart of the retained austenite, the morphology of the bainitic matrix is an important factor controlling ductility. Bainitic microstructures formed by coiling with coarse and blocky bainite morphology have shown higher uniform deformation values than those obtained by air cooling with the typical thin bainite platelets.KEY WORDS: ductility; TRIP effect; bainite; steels. mechanical stability to martensitic transformation.Further improvement can be achieved by TRIP effect i.e. strain induced transformation of retained austenite to martensite. In order to take full advantage of this effect, the mechanical stability of austenite, i.e. its capability to transform to martensite under strain, must be controlled. In this sense, the role of retained austenite on the mechanical properties of ultra high strength bainitic steels has been analysed in this work. The effect of the retained austenite chemical composition, morphology, size, and distribution on its mechanical stability of this phase has been studied. Moreover, the influence of the amount and size of martensite, and the morphology of bainitic ferrite matrix on the ductility behaviour of advanced bainitic sheet steels has been also examined. Materials and Experimental ProcedureChemical composition of these novel bainitic steels was designed to have the same bainitic transformation region in the TTT diagram and the same T o curve 6) as those of Ni 2 bainitic steel designed in previous work 9) and taken here as reference ( Table 1). The reference steel proved to have impressive mechanical properties when fully transformed to bainite. 10) Designed alloys were manufactured by ARCELOR RESEARCH (France) as 180ϫ80ϫ12 mm 3 plates. Their actual composition is shown in Table 1. All laboratory heats were elaborated in a 60 kg vacuum induction furnace under inert atmosphere (Ar, N 2 ). The generator power was 80 kW. Pure (Ͼ99.9 %) electrolytic iron and addition of the alloying elements one after each other were used. Carbon deoxidation was performed and an analysis of C, S, N, O was made on line during elaboration for the final adjustment of composition. During elaboration, the temperature was controlled by a thermocouple.Hot rolling simulations were performed on ARCELOR pilot plant according to the general scheme presented on Fig. 1. The desired bainitic microstructure was obtained in all the steels performing air cooling after accelerating cooling, and in CENIM 6 and 7 performing coiling after accelerating cooling. Spec...

The coarsening of the three-dimensional microstructure resulting from phase separation during ageing at 748 K of a Fe-based PM 2000 TM oxide dispersion strengthened (ODS) steel has been investigated by atom probe tomography and hardness measurements. Phase separation resulted in the formation of isolated particles of the chromium-enriched ␣ phase. The aluminum and titanium were found to preferential partition to the iron-rich ␣ phase. The partitioning of aluminum is consistent with theoretical calculations. The change in the scale of the chromium-enriched ␣ phase was found to fit a power law with a time exponent of 0.32 in accordance with that predicted by the classical Lifshitz, Slyozov and Wagner (LSW) theory. The solute concentrations of the coexisting ␣ and ␣ phases were estimated from concentration frequency distributions with the Langer-Bar-on-Miller (LBM) method and proximity histograms. The hardness was linearly related to the chromium content of the ␣ phase.

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