Carbide free bainite is an advanced high strength steel of the 3rd generation exhibiting relatively low alloying concentrations while simultaneously providing excellent mechanical properties. However, there is no consistent process-microstructure or microstructure-property model available. In the present work, a process-microstructure-property model chain is proposed focusing on the influence of carbon for a (0.11 – 0.6)C1.5Si0.9Mn1.2Cr0.67Mo0.1Al0.1V carbide free bainitic steel during isothermal bainite formation.
Dilatometry, optical microscopy and hardness test are conducted for 6 different carbon contents and at least seven different isothermal holding temperatures per chemical composition. Subsequently, specific material conditions are selected for further characterization of the microstructure using EDX, XRD, HEXRD and EBSD as well as mechanical properties (i.e. yield strength and uniform elongation).
This data is used for the adaption of the following models. Phase fractions and carbon contents of retained austenite and bainitic ferrite as well as starting temperature of bainite and martensite are modeled thermodynamically. With these quantities as input, the kinetic model is made predictive by introducing a carbon dependency in the model parameters. For the grain size of retained austenite and bainitic ferrite a fully data driven approach has been used. Finally, constrained symbolic regression is applied to derive a model for the yield strength and flow curve based on Taylors formula, the Hall-Petch relationship and the Kocks-Mecking-model.
The carbon dependence modelled in the present study is a first step towards a consistent process-microstructure-property modelling approach for carbide free bainitic steels and will be expanded for additional alloying elements in further research.