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The knowledge of the martensite start (Ms) temperature of steels is sometimes important during parts and structures fabrication, and it can not be always properly estimated using conventional empirical methods. The additions in newly developed steels of alloying elements not considered in the empirical relationships, or with compositions out of the bounds used to formulate the equations, are common problems to be solved by experimental trial and error. If the trial process was minimised, cost and time might be saved. This work outlines the use of an artificial neural network to model the calculation of Ms temperature in engineering steels from their chemical composition. Moreover, a physical interpretation of the results is presented.

It has been broadly reported that determination of the martensite start temperature in steels, M s , requires a complete description of their chemical composition. Recently, several neural networks models considering both chemical composition and austenite grain size (AGS) have been developed. Such models predict a moderate dependence of M s with AGS. The present work examines the validity of existing neural network models, but focusing on fine AGS (below 5 m).

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Cementite is responsible of the limited application of conventional bainitic steels, however it has been proof that cementite precipitation during bainite formation can be suppressed by the judicious use of silicon in medium carbon steels. In this work, thermodynamic and kinetic models were used to design steels with an optimum bainitic microstructure consisting of a mixture of bainitic ferrite, carbon-enriched retained austenite and some martensite. Using these models, a set of seven carbide free bainitic steels with a 0.3 wt% carbon content were proposed for manufacturing. The work presented here is concerned with the microstructural and mechanical characterisation of the steels manufactured. Except for the steel with the highest content of alloying elements, all the grades present the same microstructure composed of carbidefree upper bainite and retained austenite after hot rolling and a two-steps cooling. Theirs tensile strengths range from 1 600 to 1 950 MPa while keeping a uniform elongation equal to 4 % and a total elongation over 10 %. Regarding toughness at room temperature, they match quenched and tempered martensitic steels.

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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The main aim of this work is to study the mechanisms that control the austenitisation process in steels with different initial microstructures. The compiled knowledge in literature regarding the isothermal formation of austenite from different initial microstructures (pure and mixed microstructures), has been used in this work to develop a model for non-isothermal austenite formation in steels with initial microstructure consisting of ferrite and/or pearlite. The microstructural parameters that affect the nucleation and growth kinetics of austenite, and the influence of the heating rate have been considered in the modelling. Moreover, since dilatometric analysis is a technique very often employed to study phase transformations in steels, a second model to describe the dilatometric behaviour of the steel and calculate the relative change in length which occurs during the austenite formation has been developed. Both kinetics and dilatometric models have been validated. Experimental kinetic transformation, critical temperatures as well as the magnitude of the overall contraction due to austenite formation are in good agreement with calculations.