Microstructure of AHSS produced via UFH and thermal cycling

The development of new routes to produce steel is motivated by the global regulations that have emerged currently. These rules are focused on the reduction of polluting emissions that are generated as a result of the industrial manufacture of steel as well as during the internal combustion of vehicles. Therefore, the current trend when it comes to solving this industrial problem is to decrease the weight of the car structures. In this direction, the efforts are centred on the development of advanced high strength steels (AHSS), which are known to combine both good elongation and strength properties. Thus, recent research and technological advances in the also called the third generation of steels, are oriented towards the obtention of effective alloying methods and microstructural designs for new grades of AHSS that balance the good mechanical properties, low weight, and a cost-effective manufacturing technique. The microstructural design of new steel grades is commonly obtained via controlled transformations and chemical stabilization of austenite during heat treatments, nowadays performed in many prolongated stages. Nevertheless, there are emerging technologies involving thermal treatments that are known to be environmentally friendly and energy-saving processes. These technologies might allow increasing the capacity of the industrial continuous annealing lines by employing high heating rates during thermal treatment, which is the case of the ultra-fast heating methodology. Therefore, there is no theoretical or practical restriction for the combination of ultra-fast heating with another heating route for mechanical properties enhancement. Driven by the potential development of optimized and well-known heat treatments, the present research conducted in this master’s thesis focuses on the evaluation of the microstructure and mechanical properties of an AHSS subjected to annealing cycles and ultra-fast heating. The thermal treatments studied in this work were designed to gain insight into the grain refinement effect of both heat-treating methodologies. Hence, two thermal pathways were designed to be performed on an initial cold-rolled low-carbon steel, i.e., triple annealing cycles combined with ultra-fast annealing (TA&UFA), and a single ultra-fast annealing cycle (UFA). The microstructure, texture and hardness properties of the treated steel samples were evaluated. Those posterior analyses provided significant data. From these measurements results and conclusions are obtained, regarding the effectiveness of the studied thermal routes as grain refinement techniques