Simulation and Evaluation of Phase Transformations and Mechanical Response in the Hot Stamping Process

• Main Authors: Oldenburg, Mats, Akerstrom, Paul, Bergman, Greger, Salomonsson, Per
• Format: Conference Proceeding
• Language: English
• Subjects: AUSTENITE; BORON; COMPUTERIZED SIMULATION; CRYSTAL STRUCTURE; DEFORMATION; FERRITE; FINITE ELEMENT METHOD; HARDNESS; L CODES; MARTENSITE; MATERIALS SCIENCE; METALS; MICROSTRUCTURE; PEARLITE; PHASE TRANSFORMATIONS; PLASTICITY; SHEETS; SOLIDS; STEELS; THERMOMECHANICAL TREATMENTS
• ISSN: 0094-243X; 1551-7616
• DOI: 10.1063/1.2740970

When producing thin ultra high strength steel components with the hot stamping process it is essential that the final component achieves desirable material properties. This applies in particular to passive automotive safety components. Often the desirable microstructure consists of a mix of martensite and bainite. Therefore, it is of great importance to accurately predict the final microstructure of the component early in the product development process. In this work a model to predict the austenite decomposition into ferrite, pearlite, bainite and martensite during arbitrary cooling paths for thin sheet boron steel is used. The decomposition model is based on Kirkaldy's rate equations and later modifications by Li et al. The modified model accounts for the effect from the added boron. The model is implemented as part of a material subroutine in the Finite Element Program LS-DYNA 970. Both the simulated volume fractions of micro-constituents and hardness profiles show good agreement with the corresponding experimental observations. The phase proportions 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 material model is used in combination with a thermal shell formulation with quadratic temperature interpolation in the thickness direction to simulate the complete process of simultaneous forming and quenching of sheet metal components. The implemented model is used in coupled thermo-mechanical analysis of the hot stamping process and 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.