Steel makers can now predict how heat treatment weakens metal
Researchers have developed a mathematical model that predicts how grain size changes when boron steel is heated during manufacturing—a critical factor affecting the final strength and durability of stamped metal parts. The breakthrough could help automakers and manufacturers reduce defects and optimize production cycles in press hardening processes worth billions annually.
Originaltitel: The Influence of Austenitization Conditions on Grain Growth and the Bending Performance of Boron Steel
Pressväxling av borstal är känslig för austenitiseringsförhållanden — högre temperatur och längre tid orsakar kornväxt som kan försämra mekaniska egenskaper. Erik Lundholm och kollegor vid Luleå tekniska universitet och Gestamp Hardtech har utvecklat en modell för kornväxt under austenitisering baserat på isotermiska försök mellan 900–960 °C under 1–1200 sekunder. Modellen förutsåg kornväxten vid upp till 930 °C med god noggrannhet, medan 960 °C visade mer komplex tillväxtkinetik. Böjprovning enligt VDA 238-100 avslöjade att kort austenitiseringstid inte påverkade böjförmågan märkbart — endast vid längre tider observerades mindre effekter. Resultaten ligger närmare praktisk processreglering än rena materialstudier. För leverantörer av pressväxlingsåtgärder möjliggör kornväxtmodellen optimering av värmecykler utan att offra mekaniska egenskaper, vilket kan reducera energiförbrukning och kvalitetskontrollkostnader.
<p>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.</p>