The effect of tempering and auto-tempering on the microstructure–property relationship of two ultra-high strength press hardening steels (PHS1500 and PHS2000) was studied. Both steels were austenitized, oil quenched, and subsequently tempered at four different temperatures ranging from 180 °C to 300 °C. For auto-tempering, the steels underwent austenitization and quenching using a press equipped with planar tools and were subsequently ejected at varying cooling durations. The tensile properties, hardness, microstructure, and dislocation densities after heat treatment were characterized. The results showed that the effect of tempering temperature on tensile properties and microstructure features was more pronounced than the effect of tempering time for both steels. Tensile strength and hardness decreased slightly with increasing tempering temperature up to 200 °C. Above that temperature, there was a further decrease in tensile strength and hardness, which is suggested to be due to the formation and coarsening of carbides in the highly dislocated martensitic matrix. In contrast to the tensile strength and hardness, the yield strength increased with increasing tempering temperatures, which is most probably due to internal stress relaxation. Total elongation was increased with increasing tempering temperatures, except for the samples tempered at 250 °C and 300 °C. These samples experienced a reduction in elongation at fracture, which was more pronounced after tempering at 300 °C than at 250 °C. This was most likely attributed to the so-called tempered martensite embrittlement effect. Calculation of dislocation densities before and after tempering treatments confirmed dislocation annihilation and recovery of martensitic microstructure. 

During production of components using press hardening, the steel will at one point be heated to an austenitic state. Grain growth can occur during this austenitization if the time and temperature are sufficient, where the microstructure becomes increasingly coarse. The final austenite grain size can affect both the phase transformations during quenching and the final mechanical properties of a fully martensitic microstructure. In this work, austenite grain growth was modeled using measurements of the mean grain diameters from isothermal experiments, while the model was validated using non-isothermal experiments. The temperature and time ranges used in the isothermal experiments were 900–960 C and 1-1200 seconds, respectively. Bending tests according to VDA 238-100 were performed, using samples previously austenitized at 900, 930, and 960 C and then rapidly quenched. The isothermal grain growth up to 930 C could be modeled using the average grain size, while at 960 C the microstructure displayed a more complex growth behavior. The grain growth during the non-isothermal validation experiments could be predicted with the exception of one thermal cycle. No effect of the austenitization temperature on the bending performance was observed when short austenitization times were utilized, and only a minor effect was observed for a longer austenitization time.