The Influence of Austenitization Conditions on Grain Growth and the Bending Performance of Boron Steel

Abstract 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 $$^\circ $$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mmultiscripts> <mml:mrow/> <mml:mrow/> <mml:mo>∘</mml:mo> </mml:mmultiscripts> </mml:math> C and 1-1200 seconds, respectively. Bending tests according to VDA 238-100 were performed, using samples previously austenitized at 900, 930, and 960 $$^\circ $$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mmultiscripts> <mml:mrow/> <mml:mrow/> <mml:mo>∘</mml:mo> </mml:mmultiscripts> </mml:math> C and then rapidly quenched. The isothermal grain growth up to 930 $$^\circ $$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mmultiscripts> <mml:mrow/> <mml:mrow/> <mml:mo>∘</mml:mo> </mml:mmultiscripts> </mml:math> C could be modeled using the average grain size, while at 960 $$^\circ $$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mmultiscripts> <mml:mrow/> <mml:mrow/> <mml:mo>∘</mml:mo> </mml:mmultiscripts> </mml:math> 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.