Influence of NbC-Precipitation on Hot Ductility in Microalloyed Steel - TEM Study and Thermokinetic Modeling

Abstract:

One of the main challenges during continuous casting of microalloyed steel is to avoid the formation of transverse surface cracks on the steel slabs. These cracks occur due to severe mechanical and thermal stresses in the strand during the straightening operation. The reason for this phenomenon is a ductility loss of austenite in a typical temperature range of 700°C - 1100°C. One of the main mechanisms reducing the ductility is the precipitation of carbides and nitrides. In this work, we correlate ductility loss and precipitation state accompanying two model cooling strategies in a microalloyed steel grade. Continuously cooled samples show a minimum of ductility at temperatures around 750°C. With increasing temperature, deformability recovers again to reach full ductility again at 950°C. In contrast, samples treated with a fast intermediate cooling and reheating show constant low ductility in this entire temperature range. A transmission electron microscopy (TEM) investigation shows nanometer-sized NbC precipitates in the low ductility states. In contrast, in the samples with high ductility, larger NbC precipitates with lower number densities are observed. The experimental results show a good accordance with corresponding precipitation kinetics simulations carried out with the MatCalc software package.

• Periodical: Materials Science Forum (Volume 879)
• Pages: 2107-2112
• DOI: https://doi.org/10.4028/www.scientific.net/MSF.879.2107
• Online since: November 2016
• Authors: Tomasz Wojcik*, Ernst Kozeschnik
• Keywords: Hot Ductility, Microalloyed Steel, Precipitation, Transmission Electron Microscopy (TEM)

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