Isothermal Phase Transformations Below the Martensite Start Temperature in a Low-Carbon Steel

Abstract

Advanced High Strength Steels (AHSS) have been used extensively for the last three decades in the automotive industry as they exhibit an enhanced combination of strength and ductility which has successfully allowed the weight reduction of structural components. This breakthrough has been highly beneficial for the environment, as lighter vehicles have reduced the CO2 emissions during use. In the last decade, the development of AHSS has been focused on the design of complex microstructures containing high strength phases, such as bainite and martensite, as well as a softer phase providing ductility and strain hardening, such as austenite. However, the thermomechanical processing of these multiphase steels requires long, complex, and energy-intensive thermal treatments with a high environmental footprint. New alternative processing routes are being developed for producing these multiphase steels sustainably, without compromise on strength and ductility, thus achieving reduced CO2 emissions throughout the lifecycle of steel. In this framework, a new thermal treatment consisting of a rapid cooling below the martensite start temperature (Ms) followed by an isothermal treatment at the same quenching temperature is proposed as a promising environmentally sustainable alternative for the production of such multiphase steels. This Ph.D. thesis investigates, from a scientific point of view, the phase transformations and the interactions between the phases formed during the above-described novel isothermal treatment below Ms in a low-carbon high-silicon steel. The thermal treatment is applied in different combinations of quenching temperature and isothermal holding time in order to stimulate the formation of diverse phase fraction mixtures. The research also elucidates the effects of the formation of each of the phases on the microstructure-property relationships of these multiphase steels.