In the automotive industry, structural components are often produced via press hardening, enabling rapid production and the use of ultra-high-strength steels. In this process, steels are heated to an austenitic state and are then formed and quenched in rapid succession. The initial steel that enters the press-hardening production line varies, where the microstructure is a result of previous production steps. This work was performed to investigate the possible effects of the initial microstructure on the final mechanical properties for rapidly quenched samples. Although the initial microstructure is transformed during austenitization, the steel can still be affected by its prior history. Steels with three different initial microstructures were evaluated, with only minor variations in chemical composition and thicknesses. The Lankford coefficients and the failure strains were dependent on the orientation of the samples. However, for a given orientation, there were only minor variations between the different steels with respect to anisotropy, strength, and ductility. The anisotropy could be correlated with the microstructure through the calculation of Taylor factors based on measurements using electron backscatter diffraction. The minor influence from the initial steel microstructure on the final mechanical properties indicates robustness suitable for mass production.

The continuous development of the press hardening technology has led to stronger, lighter and more environmentally friendly components. Utilising the varying properties of boron steel at different temperatures enables great design freedom, while also attaining high strength in the final component. This is achieved by heating the initial material to an austenitic state, where it has good formability, followed by forming and quenching using pressing tools. However, in order to simulate this thermo-mechanical process the microstructure evolution must be understood. Research has been performed using various initial material states, evaluating possible effects on the final mechanical properties. Studies have also been performed to evaluate the grain growth during austenitisation. The influence of the initial material and the evolution of the austenite morphology during austenitisation has previously been less researched compared to other parts of the process.

In this work, samples from commercially available materials have been heat treated to create test specimens, which subsequently have been used for mechanical testing and microstructure analysis. Digital image correlation was used to determine local fracture strains and anisotropic properties during plastic deformation. Samples were also heat treated using varying process parameters in order to study the grain growth during austenitisation. It was found that if hot rolled, cold rolled and soft annealed cold rolled samples were compared after hardening, their mechanical properties only exhibited minor variations. However, all samples displayed anisotropic properties during plastic deformation. There is therefore some microstructural trace from the production which is unaffected by soft annealing, austenitisation and subsequent quenching. The grain growth observed during the austenitisation was consistent within a temperature range not exceeding 930 ◦C. Using data retrieved from isothermal experiments a model could be fitted which described the growth using the temperature and current grain size. At 960 ◦C the microstructure was irregular, with large single grains and considerable variations in the average grain size within the same sample. The bending performance was not affected in a major way by the austenitisation temperature.

The lack of variation of the mechanical properties due to the initial microstructure or parent austenite grain size is a testament to the robustness of the process. It should be noted however, that all samples were rapidly quenched. If the microstructure is formed through diffusion dependent phase transformations, the final mechanical properties could be more sensitive to process parameters. Further research is needed to fully understand the microstructural evolution and thus the mechanical properties where a more general thermal cycle can be used.