Over the last 20 years, high-temperature Laser Scanning Confocal Microscopy (HT-LSCM) has been established as in-situ observation method for high temperature microstructure evolution in steels. The related phenomena, such as the coarsening of austenite grains or the formation of pro-eutectoide ferrite along the austenite grain boundaries are commonly seen as major causes for the development of the so-called second ductility trough and consequently the formation of surface defects in continuous casting.
The Montanuniversitaet Leoben and the Materials Center Leoben operate a HT-LSCM since 2010. The present work deals with the previous experiences in the observation of microstructure evolution under conditions close to the continuous casting process.
HT-LSCM provides the simulation of a wide variety of thermal cycles such as the cooling cycle of a strand surface. Samples come either from production or from laboratory (re)melting facilities. Austenite and δ-ferrite grain growth as well as phase transformations become visible.
The presentation will focus on two examples for current research activities:
-Isothermal and non-isothermal observation of austenite grain growth: HT-LSCM prooved as flexible and efficient tool to observe the growth of austenite grains at temperature between 850 and 1250 °C. The grains size is determined by digital image analysis from the recorded videos. The measurements are the basis for the adjustment of parameters for grain growth models as well as precipitation kinetics. The influence of certain alloying elements on the grain growth kinetics will be briefely discussed.
-The Montanuniversitaet Leoben also developed an in-situ bending experiment to simulate the formation of surface defects. HT-LSCM has become an essential tool to simulate the thermal cycle of the bending experiments and thus to observe the microstructure evolution such as the austenite to ferrite transformation. This turned out to be essential for the interpretation of the bending results. The derived phase transformation temperatures were cross-checked with dilatometer measurements and results from commercial software packages such as IDS or JMatPro.
Finally some results for recently developed surface cooling strategies will be presented: The results show the austenite to ferrite or bainite transformation during temporary undercooling of the strand surface and the formation of fine austenite grains during the subsequent reheating. This so-called “surface structure control by cooling” strategy was simulated for a microalloyed steel and the results will be briefly discussed.