Abstract

Ultrafast heating (UFH) has recently attracted significant attention of steel research community due to its advantages over conventional heat treatment of advanced high-strength steel (AHSS) including very short processing time, reduced energy consumption and low carbon footprint. The UFH processing allows to generate in low alloyed steels a fine grained microstructure showing high ultimate tensile strength (UTS) and ductility, which strongly depend on the UFH processing parameters. Despite there is a body of research on the effect of ultrahigh heating rates on the microstructure and properties of steels, the influence of other processing parameters (such as soaking time at peak temperature) on the microstructure and properties of steels has never been studied. Another topic remaining unexplored is the properties of individual phases formed during UFH processing. Therefore, the main objective of this work was to analyse the effect of soaking time on the microstructure evolved in a low carbon steel, properties of individual microconstituents and overall properties, and to gain understanding of heating rate – microstructure – properties relationship on micro- and macro-scales.
The low carbon steel was heat treated with well controlled conventional and ultrafast heating rates and varying soaking time at peak temperature in a thermo-mechanical simulator Gleeble 3800. The microstructure of the material was studied, and its mechanical properties were determined by tensile testing, as well as by nanohanrdness measurements on individual microconstituents determined a priori by EBSD characterization. Formation of complex hierarchic microstructure in ferritic phase consisting of recovered and recrystallized grains is observed after UFH processing. The recrystallized grains are prone to show pop-ins on the load – depth curves during nanoindentation testing, while recovered grains tend to have a continuous character. The effect of heating rate and soaking time on nanohadrness of individual microconstituents and overall mechanical properties is discussed.