ICME approach to develop 3rd Generation of Advanced High Strength Steel
3rd Generation Advanced High Strength Steel (AHSS) alloys have been identified as the steel of future for lightweight vehicles because of the capability to increase passenger safety and fuel efficiency while decreasing the carbon footprint of the vehicle. A heat treatment process, Quenching and Partitioning (Q&P), has been developed by the Colorado School of Mines; Q&P yields steel properties as described by American Iron and Steel Institute for 3rd Generation AHSS. The steel can be successfully produced in the lab but not on the production line, as the process requires multiple thermal cycles currently not available on most production lines. Observations showed that the mechanical properties of Q&P steel produced in the lab depend significantly on the quenching temperature, partitioning temperature and partitioning time. The key lies in martensite decarburization and austenite retention, where austenite stabilization is a problem which can be solved by introduction of alloying elements. The challenge now is to identify alloying elements, heat treatments and manufacturing processes that will allow mass production of Q&P steels with acceptable uncertainties in the mechanical properties. To overcome these challenges, we first need to have a good understanding of the process-structure-property relationship. Developing Q&P steels is a multiscale problem and Integrated Computational Mechanical Engineering (ICME) approach can be used to find the mechanisms driving the effect of alloying on the mobility of martensite interface and stability of austenite, in an effort to find the alloys and the designing process. Figure 1 shows a multiscale ICME approach to developing 3rd generation AHSS.
Use Density function theory (DFT) to identify modified embedded-atom method (MEAM) potentials which will be the bridge to nanoscale. Elastic Moduli obtained from the DFT calculations will be the bridge to internal state variable (ISV) continuum model.
Molecular dynamics (MD) simulations can be used to perform temperature and stress loading on grain boundary and phase boundary to calculate mobility and mechanisms that lead to grain boundary and phase boundary migrations. Important Grain Boundaries and Phase Boundaries can be found from experimental transmission electron microscopy (TEM) and electron backscatter diffraction (EBSD) results and reconstructed through MD simulations. This simulation will form a bridge with the continuum model by providing boundary energy. The simulations will also provide intrinsic defective structure associated with the grain boundary which will be a helpful parameter for dislocation dynamics and phase field modeling at the mesoscale. Micro-3D can be used to calculate defects/strengthening/hardening, providing dislocation motion for the macroscale model and hardening rules for the next scale.
The effect of intrinsic defective structure of grain boundary and phase boundary on mobility, grain growth and phase transformation can be integrated in the phase field model. The simulations can show the effect of different alloying elements on the microstructure and texture of the Q&P steel.
Microstructure-based finite element approach can be used to test the formability of the Q&P steel. The results obtained through simulations can be validated with the forming limit curved obtained by experimental data.
At the macroscale, multiphase ISV theory can be developed by collecting all the data from the downscaling bridge with some uncertainties.