In this work, we investigate the sensitivity of the microstructure and mechanical properties of an ultrafast heat treated low carbon-steel to the peak temperature. In all studied cases, the steel was heated within the intercritical temperature range (i.e. between the A_C1 and A_C3 temperatures). Both the peak temperature and soaking time were varied, and their effect on the size, the fraction of individual microstructural constituents and their tensile mechanical response were investigated. It is shown that the increasing peak temperature and soaking time promote austenite formation and recrystallization processes in the ferritic matrix. The highest nanohardness is shown by martensitic grains, while recovered ferrite demonstrated slightly higher nanohardness compared to recrystallized ferrite. The applied heat treatment parameters have a strong effect on the nanohardness of martensite, whereas the nanohardness of ferrite microconstituents is not sensitive to variations of peak temperature and soaking time. The non-recrystallized ferrite is harder than its recrystallized counterpart due to the higher dislocation density of the former. Increasing peak temperatures promote strengthening in the material at the expense of its ductility mainly due to increased martensite fraction. The steel demonstrates enhanced strain hardening ability independently of the peak temperature. Analysis of the experimental results showed that the industrial processing window of ±10 ^°C may lead to some heterogeneity of the local microstructure in the ultrafast heat treated sheets. However, the latter should not have any negative effect on the overall mechanical behavior of the ultrafast heat treated steel sheets on the macro-scale.

The main objective of this study is to understand the effect of the soaking time during the ultrafast heat treatment of a low carbon steel on its complex multi-phase microstructure, tensile mechanical behavior and properties of individual microconstituents. Tensile tests were performed to determine the macro-mechanical properties. Nanoindentation testing was carried out on individual microconstituents (martensite, recrystallized ferrite and non-recrystallized ferrite) identified a priori via EBSD analysis to measure their properties. It is shown that ultrafast heating combined with short soaking times results in improved macro-mechanical properties due to finer grain size and higher fraction of non-recrystallized ferrite, that has a higher nanohardness than recrystallized ferrite. Prolonged soaking times eliminate the advantages of the ultrafast heat treatment. This occurs because, even though a long soaking time promotes a higher volume fraction of martensite than a short one, it also induces substantial grain growth and complete recrystallization of the ferritic matrix. On the micro-scale, the ferritic grains show two different types of mechanical response. The recrystallized ferritic grains are prone to show pop-in events on the nanoindentation curves that are associated to dislocation nucleation events as a consequence of their low dislocation density, while non-recrystallized ferritic grains demonstrate a continuous response. The relationship between microstructure and mechanical properties on the macro- and micro-scales is discussed with respect to the microstructure, which in turn strongly depends on the applied heating rate and soaking time. A general recipe for microstructural design to improve the tensile mechanical behavior of low carbon steels implementing controlled heating and soaking conditions is outlined.

The effect of ultrafast heating on the microstructure and properties of a low-carbon steel is studied at the microscale. Ultrafast heating results in the formation of a complex multiphase microstructure containing mainly martensite and retained austenite grains embedded into a ferritic matrix. The ferritic matrix exhibits a microstructure consisting of recovered and recrystallized grains. The recrystallized grains are softer and display pop-in events during nanoindentation, while the harder recovered grains show uniform deformation behavior.