MICROSTRUCTURE EVOLUTION AND MECHANICAL RESPONSE IN THE HOT STAMPING PROCESS
In the manufacturing of ultra high strength boron steel components with the hot stamping process, it is of great importance that the final product will have the desired material properties. This is especially true for safety related automotive components. Often the preferred microstructure is a mix of martensite and bainite. In this work a model is developed and implemented in order to predict the austenite decomposition into ferrite, pearlite, bainite and martensite during arbitrary cooling paths. The model is based on Kirkaldy’s rate equations and later modifications by Li et al. After modification, the model accounts for the effect from the added boron and the effect of straining at high temperatures. The implementation is as part of a material subroutine in the finite element program LS-Dyna. The achieved volume fractions of microconstituents and hardness profiles in the analyses show good agreement with the corresponding experimental observations. The phase content affect both the thermal and the mechanical properties during the process of continuous cooling and deformation of the material. A thermo-elastic-plastic constitutive model including effects from changes in the microstructure as well as transformation plasticity is implemented in the LS-Dyna code. The model is used together with a thermal shell formulation with quadratic temperature interpolation in the thickness direction. The developed methods are used to simulate the complete process of simultaneous forming and quenching of sheet metal components. The implemented models are used in coupled thermo-mechanical analysis of the hot stamping process and are evaluated by comparing the results from hot stamping experiments. The results from simulations such as local thickness variations, hardness distribution and spring-back in the component show good agreement with experimental results. However, it is shown that the simulation of the final cooling stage relies on a correct modelling of contact properties and heat transfer.
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MICROSTRUCTURE EVOLUTION AND MECHANICAL RESPONSE IN THE HOT STAMPING PROCESS
In the manufacturing of ultra high strength boron steel components with the hot stamping process, it is of great importance that the final product will have the desired material properties. This is especially true for safety related automotive components. Often the preferred microstructure is a mix of martensite and bainite. In this work a model is developed and implemented in order to predict the austenite decomposition into ferrite, pearlite, bainite and martensite during arbitrary cooling paths. The model is based on Kirkaldy’s rate equations and later modifications by Li et al. After modification, the model accounts for the effect from the added boron and the effect of straining at high temperatures. The implementation is as part of a material subroutine in the finite element program LS-Dyna. The achieved volume fractions of microconstituents and hardness profiles in the analyses show good agreement with the corresponding experimental observations. The phase content affect both the thermal and the mechanical properties during the process of continuous cooling and deformation of the material. A thermo-elastic-plastic constitutive model including effects from changes in the microstructure as well as transformation plasticity is implemented in the LS-Dyna code. The model is used together with a thermal shell formulation with quadratic temperature interpolation in the thickness direction. The developed methods are used to simulate the complete process of simultaneous forming and quenching of sheet metal components. The implemented models are used in coupled thermo-mechanical analysis of the hot stamping process and are evaluated by comparing the results from hot stamping experiments. The results from simulations such as local thickness variations, hardness distribution and spring-back in the component show good agreement with experimental results. However, it is shown that the simulation of the final cooling stage relies on a correct modelling of contact properties and heat transfer.
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