Much recent work has been devoted to characterize the microstructure and mechanical properties bainitic nanostructured steels. The microstructure is developed by isothermal heat treatment at temperatures as low as 125-350ºC and adapted steel grades typically contain high carbon contents to achieve sufficient depletion of the B S-M S temperature range, and above 1.5 Si wt.% to suppress carbide formation during isothermal holding. On the latter, most of the published literature agrees on a limit of around 1.2-1.5 wt.% to suppress cementite in high carbon steels. For this reason perhaps, additions of Si significantly above this limit have not been investigated systematically in the context of nanostructured bainitic steels. The present work is concerned with the effect of up to ~3 Si wt.% in a steel grade otherwise adapted to low temperature bainitizing. Tensile properties as compared to similar grades, though with lower Si contents, exhibited unrivalled combinations of strength and ductility, with above 21% total elongation for a UTS above 2GPa. An attempt is made to explain the mechanical properties of this microstructure in terms of some of its most relevant and unique morphological and microstructural features.

The present study is concerned with the potential of high carbon, high silicon steel grades isothermally transformed to bainite at low temperature (< 300 °C). A first part gives an overview of design principles allowing very high strength and ductility to be achieved, while minimising transformation duration. Wear and fatigue properties are then investigated for over ten variants of such material, manufactured in the laboratory or industrially. The results are discussed against published data. Tensile strength above 2 GPa are routinely achieved, with, in one case, an exceptional and unprecedented total elongation of over 20%. Bainite plate thickness and retained austenite content are shown to be important factors in controlling the yield strength, though additional, non negligible parameters remain to be quantified. Rolling-sliding wear performances are found to be exceptional, with as little as 1% of the specific wear rate of conventional bainitised 100Cr6. It is suggested that this results from the decomposition of retained austenite in the worn layer, which considerably increases hardness and presumaby introduces compressive residual stresses. Fatigue performance were slightly improved over 100Cr6 for one of the two industrially produced material, but significantly lower otherwise. Factors controlling fatigue resistance require further investigations.

Specially designed steels with carbon contents from 0.6 to 1.0 wt.% were isothermally transformed at very low temperatures, between 220 and 270°C, in order to obtain a nano-structured bainitic microstructure. It is shown that the wear resistance in dry rolling-sliding of these nano-structured steels is significantly superior to that of bainitic steels transformed at higher temperatures with similar hardness values. In addition to the highly refined microstructure, the transformation under strain to martensite (TRIP effect), contributes to the plasticity of the nanoscaled steels, increasing surface hardness during testing, thus reducing the wear rate.

It is not the first time that a consortium of steel makers, end users and scientists ends up with unique approaches and developments in the physical metallurgy of steels. This paper reveals the scientific and technological developments of a consortium sharing a common intrigue and interest for a unique microstructure, nanostructured bainite. Also known as low temperature bainite, as its unique properties rely solely on the scale of the miscrostructure obtained by heat treatment at low temperature (150-350ºC). Careful design based on phase transformation theory, some well known metallurgy facts and the necessary industrial experience were the ingredients for a further step towards the industrialization of these microstructures.

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