Surface oxide formation for differentiated new steel grades

This thesis presents an in-depth investigation into the effects of silicon and boron composition on surface oxide formation in advanced high-strength steels (AHSS) model alloys, particularly in relation to their applications within the automotive industry. The addition of small amounts of boron is known to enhance the mechanical strength of AHSS grades, making it particularly relevant for automotive applications. However, this modification also alters the oxidation behaviour during the processing of these steels, which serves as the primary motivation for this research. The study addresses the need for materials that not only show better mechanical performance but also keep good manufacturability. It further explores how variations in the composition of silicon and boron influence other physical properties of the steel, such as hardness and microstructure. By integrating theoretical models with empirical data, this research aims to provide a thorough understanding of the impact of these alloying elements on AHSS. The DICTRA model focused on the formation of wustite (FeO) and silica (SiO₂) at elevated temperatures and varying oxygen activities. At 700°C, the concentrations of wustite and silica are similar; however, as the temperature increases to 1000°C, wustite levels rise significantly, showing its higher stability and rapid formation at elevated temperatures. The study explores the effects of oxygen activity, revealing that at very low levels, iron and silicon do not oxidize, while silicon begins to oxidize at slightly higher levels, and iron requires significantly more oxygen. Kinetic limitations hinder the formation of all thermodynamically stable oxides, with higher oxygen activity leading to increased iron oxide formation, showcasing its sensitivity to oxygen availability. Utilizing MICRESS simulation software, the research delves into oxidation dynamics, showing that boron can inhibit oxide formation for certain alloy components, thereby influencing alloy design strategies. Silicon is shown to oxidize, forming protective SiO₂ layers, although insufficient levels may result in partial oxide coverage, increasing the risk of corrosion. The analysis of grain boundaries reveals that they facilitate faster diffusion of elements, impacting oxidation behaviour. Experimental findings include scanning electron microscopy (SEM) images of oxide surfaces after 15 minutes at 700°C, which display various stages of oxide development with increasing silicon content. Silicon levels below 1 wt% yield coarse, irregular oxides, while concentrations above this produce uniform oxide formations, with a silicon range of 1 to 1.7 wt% necessary for achieving a stable silica layer. The impact of boron is also examined, showing that it reduces silicon oxide formation and alters oxide morphology, with no visible boron oxide detected.