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Martensite is a key constituent in advanced high strength steels and plays an important role in providing the high strength. While the strength of martensite has been extensively studied in the past, its low elastic limit and extremely high strain hardening rate remain a puzzle for the steel community. Composite models proposed recently can successfully reproduce these features as result of gradual yielding of microstructural constituents with either variations in intrinsic yield strengths or transformation induced residual stresses. Although these composite models can explain certain observations associated with the deformation of as-quenched martensite, neither can self-consistently describe all the key characteristics in the tension-compression behaviour of asquenched martensite. Attempts to extend these composite models to tempered martensite have been limited.In this contribution, we conduct a systematic experimental study on the strain hardening of as-quenched and tempered martensite with mechanical testing (e.g. monotonic tension and tension-compression) and interrupted X-ray diffraction. It is shown that the high strain hardening rate, large Bauschinger effect and diffraction line narrowing found in as-quenched martensite during straining can be sustained in tempered martensite tempered up to 400°C. These phenomena can be understood by considering martensite as a multi-constituent composite having both variations in intrinsic yield strengths and relaxation of transformation induced residual stresses during straining.

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Carbon and manganese combined effect on the mechanical behavior of martensite was characterized and analyzed using literature and new experimental data of various carbon-manganese steels. A synergy effect of carbon and manganese on the martenstite strength and strain hardening was detected and was then taken into account in a specific way in the simplified model, based on a Continuous Composite Approach. Model was adjusted with only one fitting parameter and the obtained results are in good agreement with experimental stress-strain curves.

Double annealing of low carbon medium Mn steel was studied. The second intercritical annealing was done at 650°C within a range of holding time: 3min to 30h. Tensile properties of the steel were measured as a function of holding time and the relation between microstructure and mechanical behavior was analyzed. Furthermore, a model, based on the mixture law combined with the considerations of equivalent increment of work in each microstructural constituent during mechanical loading, was proposed. The individual mechanical behavior of each considered microstructural constituent was described with the approaches existing in the literature. The complete model shows a very good agreement with the experimental stress-strain curves and predicts well the optimum strength-ductility balance after 2h holding.