Microstructural design via ultrafast heating to improve mechanical properties of a low carbon steel

Abstract

The current regulations carried out by the public institutions are focused on reducing the carbon emissions from any sector of society, including the automotive industry. In order to reduce the emissions generated by vehicles, car manufactures are looking for new solutions to lighten car weight. One way to achieve this goal is by increasing the mechanical properties of Advanced high-strength steels (AHSS), widely used in structural components, reducing the total car weight. Steel industry is searching for new processing routes to satisfy customer demands, reducing, at the same time, the greenhouse emissions during manufacturing. Among the new thermal treatments where research is focused on, the Ultrafast heating (UFH) is receiving significant attention. This process is based on using higher heating rates (>> 100 ºC/s) instead of conventional ones (<10 º C/s), followed by a short soaking time at maximum temperature and subsequent quenching, thus reducing the duration of entire treatment to a few seconds. The resultant steel has a hierarchic multiphase microstructure, formed by ferrite, martensite and retained austenite with the desirable combination of mechanical properties required for the structural components. Despite the efforts made by the scientific community to understand the influence of high heating rates on microstructure and properties, the effect of other processing parameters on the microstructural architecture and properties has not been explored. There are no studies about the mechanical properties of the individual microconstituents and their effect on overall mechanical performance of these steels. Therefore, the main goal of this work is to gain fundamental understanding of the effect of soaking time and peak temperature on the microstructure and properties of UFH treated steels at macro- and micro-scales. This knowledge will allow to develop a concept for microstructural design via UFH treatment to achieve enhanced combination of mechanical and functional properties in steels. This work demonstrates that an optimal combination of mechanical strength and tensile ductility can be reached in the UFH treated steels via microstructural design, since volume fraction and size of individual microconstituents strongly depend on peak temperature and soaking time during UFH process.