Modeling of a thermomechanical process chain for sheet steels

Abstract

The purpose of this work is the development, identification and validation of a model for the simulation of a thermomechanical multistage production process chain for sheet steels. The process of interest consists of forming (in particular deep-drawing) followed by cutting and heat treatment. For the forming and cutting stages, the complete model is based in particular on a model for thermoelastic, viscoplastic (i.e., rate-dependent) material behavior in sheet steels accounting for isotropic and anisotropic (i.e., kinematic and cross) hardening. This is combined with a model for thermally induced phase transformations in order to model heat treatment. The particular material modeled here is the sheet steel LH800®. This steel has an initially ferritic microstructure which is maintained during forming and cutting. Heating of the workpiece after forming and cutting during heat treatment phase results in transformation of ferrite to austenite. Subsequent air-cooling back to room temperature is accompanied by a second transformation from austenite to martensite. Model predictions for the workpiece behavior during forming and cutting show quite good agreement with corresponding experimental results. In contrast, small discrepancies between the model predictions and experimental results for the change in workpiece geometry during cooling imply that the phase transformation from austenite to martensite in LH800® is not purely volumetric in nature as assumed in the model. Rather, it results in change in the deviatoric state of stress in the material and a corresponding change in shape of the workpiece.

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