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T. Wojcik, E. Kozeschnik, M. Lückl:
"Precipitation-Induced Hot Ductility Loss in Microalloyed Steel - A TEM-Study";
Talk: EUROMAT 2015, Warszawa; 2015-09-20 - 2015-09-24.

One of the main challenges during continuous casting of microalloyed steel is to avoid the formation of transverse surface cracks during the straightening operation of the slab. These cracks occur due to severe mechanical and thermal stresses and a simultaneous loss of ductility of austenite in the temperature range of 600°C - 1100°C. One mechanism aiding to this phenomenon is the precipitation of carbides and nitrides. In the present work, we characterize these particles with transmission electron microscopy (TEM) and correlate their occurrence to the measured ductility. In the experimental setup, two different cooling strategies are investigated: one with linear continuous cooling and one with a fast intermediate cooling segment with subsequent reheating. The ductility of the continuously cooled samples shows a minimum at around 750°C. Ductility recovers with increasing temperatures up to 950°C. In contrast, intermediate cooling leads a constantly low ductility in this temperature range. In the analysis of the Nb-microalloyed steel grades, we observe nanometer-sized NbC precipitates, which form long chains in the low ductile states. In the samples with high ductility, large NbC particles with sizes over 100 nm are detected. In comparison, the same experimental setup applied to Al-killed plain steel shows similar ductility profiles for both cooling strategies. In this case, only coarse AlN particles with sizes from 50 to 100 nm are observed in TEM in the samples with low ductility, whereas no precipitates are detected in the ductile states. The experimental results show a good accordance with thermo-kinetic precipitation simulations on these steel grades, which are finally presented and discussed.