An LS-DYNA material model for simulations of hot stamping processes of ultra high strength steels

This paper present a new material model (*MAT_244) in LS-DYNA capable of simulating phase transition during quenching and forming. Usually during forming the blank is initially heated to become fully austenitized and then continuously formed and cooled. When the temperature is decreasing the austenite decomposes into different product phases. The amount of each phase does not only depend on the mechanical history, but also on the cooling rate of the blank. A higher cooling rate increases the amount of the harder phases (bainite and martensite) whereas a slow process gives higher content of ferrite and pearlite. This thermo-elastoplastic model is based on the isotropic von-Mises yield criterion with an associated plastic flow rule. It includes both the decomposition of austenite into ferrite, pearlite, bainite and martensite, and transformation plasticity. The examples show that the model is well suited for hot stamping simulations and it should be possible to simulate different steel compositions at different cooling rates to obtain a good prediction of the hardening process and the properties of the final product.

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