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Damage and phase transformation during the continuous casting

Damage during continuous casting of steel may appear in principle as surface defects and/or defects inside the product. The transformation of delta ferrite into austenite during the primary solidification in the mould can also contribute to a massive impairment of the product quality. The research activities in connection with the formation of surface cracks and internal defects as well as the phase transformation can be described as follows:

  • Physical and numerical modelling of hot tearing in the continuous casting process and the role of hot tears in the further processing of steels:
    The improvement of the exact knowledge on the mechanism of hot tearing, considering not only open hot tears but also segregated hot tears, is the major goal within this research area. Important points to consider are the role of microstructure, the definition of critical limits and the grain boundary cohesion in conjunction with the phenomena of coalescence. Further goals are as follows: (i) the improvement of existing hot tearing criteria by the consideration of interdendritic precipitates and dissolved gases, (ii) the enhancement of hot tearing models to high-alloyed steels and Ni-base-alloys; and (iii) the establishment of a deeper understanding of the behaviour of hot tears in the subsequent processing steps and for the final product quality.

  • Physical and numerical modelling of phase changes during and immediately after solidification of “critical” steel grades:
    The objective within this research area is to improve the understanding of the peritectic phase transition. The behaviour of these phenomena during solidification of steels will be studied. An important step represents the transfer of the results to the continuous casting process to enhance the product quality. A strong focus of these investigations will be put on the effect of high alloying element contents (e.g. Al) on the peritectic phase transition, which have not been considered very often up to now. The experimental investigations will be performed by means of the SSCC test (Submerged Split Chill Contraction) and the dilatometer method. Phase changes will be characterised by combining in-situ observations with thermodynamic modelling based on commercial software.

  • In-situ - Material characterization near the austenite/ferrite transformation temperature  under continuous casting conditions:
    Within this field of research the formation of surface cracks in the so-called second ductility trough is investigated using a newly developed experimental apparatus. The IMC (In-situ material characterization) test allows the straining of a specimen – which directly solidified from the melt – on the basis of various experimental parameters. In conjunction with a comprehensive analysis of the metallographic samples the understanding of the formation of cracks near the austenite/ferrite transformation temperature should be enlarged. A major advantage over previously used experimental apparatus is the possibility to determine the critical strain of surface cracking, which can be directly adopted in strand mechanics models.