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M. Schöbel, S. Großeiber, J. Jonke, G. Requena, M. Hofmann, R. Wimpory, S. Ilie:
"Residual Stresses in Continuously Cast Steel Slabs";
Poster: EUROMAT 2013, Sevilla; 09-08-2013 - 09-13-2013.

Steel slabs produced by continuous casting allow fast processing. The molten metal is poured into a channel through a nozzle and bent by rolling while solidification takes place. The partly solidified slab suffers a combination of internal stresses by thermal gradients, phase transformation and externally applied deformation. A complex superimposed stress situation is responsible for crack formation during cooling and further manufacturing. The current simulation models take into account thermal stress calculations, which deliver the starting stress condition to predict crack formation and failure. The experimental proof of this initial stress state in steel slabs is still missing.
Non-destructive neutron diffraction was applied in-situ on a large continuously cast steel slab, with an initial thermal gradient to simulate solid state cooling during the casting process. Neutrons were chosen as probe particles due to their high penetration depth in metals to enable diffraction on coarse grained cast structures under extreme conditions. In-situ strain scanning was performed to determine the temperature dependent strains as a function of depth in the slab during cooling. Additional stress mapping at room temperature in segments of a steel slab revealed the 3D macro stress state after casting.
The stresses originating from the austenite (f.c.c.) to ferrite (b.c.c.) phase transformation dominate the stress state, during and after cooling. The volume increase of ~1% by phase transformation produces high macro stress gradients which superimpose to the thermal stresses. The phase sensitive strain analysis of the in-situ experiment during cooling, while passing the transformation temperature, was correlated to the final 3D stress state measured in the as cast condition at room temperature. Compared to the thermal model, an overall inverse stress situation was observed, as a consequence of transformation gradients during cooling.