Development of Low-Cost CrNiMoWMnV Ultrahigh-Strength Steel with High Impact Toughness for Advanced Engineering Applications

Steel is produced using two main production routes: the ore-based route, which emits about 2.1 tonnes of CO2 per tonne of crude steel, and the scrap-based route, which emits 0.6 tonnes of CO2 per tonne of crude steel. So, increasing the share of crude steel production using the scrap-based production route can be considered one of the possibilities to reduce CO2 emissions and the problem of global warming and climate change. On the other hand, increasing demands for low-cost ultrahigh-strength steels (UHSSs) in commercial applications have raised interest in finding alternatives to the expensive steels currently used.

This thesis studies the possibilities for using low-cost air induction melting followed by refining using electroslag remelting (ESR) for the production of ultrahigh-strength, tough steel from steel scrap. It provides extensive results and analyses concerning the effect of ESR on the chemical composition, cleanliness, microstructure, and mechanical properties of novel UHSSs that can be considered as platforms for future alternative production routes. The effect of cooling rate and chemical composition on phase transformation temperatures, microstructures, and mechanical properties are covered. Furthermore, the evolution of precipitation and microstructure during the thermomechanical processing is described. Various heat treatment cycles have been studied with the aim of obtaining a good property combination.

It is shown that good combinations of strength and toughness can be obtained through the use of the following process route: low-cost air induction melting, ESR, thermomechanical hot-working and cooling followed by reheating, quenching and tempering at 200 °C. Charpy-V impact toughness can be improved by applying double austenitization and quenching prior to tempering.