CRACRACKS

Hot tearing phenomena seriously affect steel cast products (ingots and continuously cast). Industrial concerns are considerable, in terms of quality (extra-process like scarfing, or rejected parts) and productivity (the crack-sensitivity of "automotive" Ultra High Strength steels restricts continuous casting speeds). However, a prediction of associated internal and surface defects is still out of reach, preventing industry from a rational optimisation of processes, basically for two reasons: • In ingot casting, defects occur during filling, in a phase where no simulation software is able to provide a reliable stress state. • Physical phenomena are still poorly understood, and the different criteria proposed so far prove insufficient. Consequently, four major objectives are defined: 1. Progress in basic understanding of physical mechanisms, with help of small scale (microstructure) numerical simulation, in order to better define the nature of macroscopic criteria. 2. Set up a methodology for the identification of the constitutive parameters of selected grades, together with parameters of fracture criteria, using a large palette of instrumented tests. 3. Extend the capacity of numerical simulation, by modelling fluid-structure interaction in solidification conditions, in order to provide relevant thermomechanical information during ingot filling or within the thin solidified shells in continuous casting moulds. 4. Develop an improvement methodology for industrial processes, based on such enriched numerical simulation. The methodology is the following: A. The material characterization of three selected steel grades will provide data to both micro and macroscale numerical simulations. It will especially encompass high temperature and semi-solid state rheological identification, by means of resistance (Joule) heating tensile tests. B. Coupled micro-macro numerical simulation, applied to the analysis of different instrumented hot tearing tests (constrained solidification, with additional tensile effort), will give access to the nature and the parameters of macroscopic criteria. C. Those criteria will be implemented in THERCAST simulation package, which will be equipped with a new solver permitting the coupled solution of fluid flow (ingot mould filling, nozzle jet in continuous casting) and solid mechanics (stress-strain in thin solidified regions). D. In the framework of Experiment Design Methodology, the application of this new solver to industrial-scale tests will result in a validation of concepts at industrial scale and in a demonstration of process improvement. The consortium is well balanced: 3 steel suppliers, 1 technical centre, 1 software company and 2 research laboratories. All of them are major actors in each of the industrial, technical and scientific domain. The consortium is compact and robust; it is in a position to propose major breakthroughs in three domains: • Multi-physic numerical simulation: fluid-structure interaction (liquid-solid) in solidifiication conditions • Rheological characterisation of steels at high temperature and in the semi-solid state • Multiscale numerical modelling of solidification phenomena