Joo Choi

Simulation of low carbon steel solidification and mold flux crystallization in continuous casting using a multi-mold simulator

An inverted water-cooled multi-mold continuous casting simulator was used to investigate initial solidification of low-carbon steels and crystallization of mold flux. Embedded mold thermocouples showed characteristic temperature profiles dependent on parameters including casting speed, oscillation frequency, and stroke. Higher maximum temperatures for thermocouples at higher casting speeds, higher frequencies, and lower stroke lengths were observed. The surface of the as-cast steel strips showed oscillation marks similar to those of industrially cast slabs and higher casting speeds resulted in shallower oscillation marks. The measured pitch agreed well with the theoretical pitch suggesting the multi-mold simulator to be a cost-effective alternative to pursue fundamental studies on initial solidification in the mold. Analysis of the mold flux taken between the copper mold and solidified steel shell showed highly dendritic uni-directional crystallization occurring within the flux film suggesting that the heat transfer direction is dominantly horizontal towards the water-cooled copper mold. In addition, the solidified flux located at the upper to lower part of the mold suggested morphological differences in the size and shape of the crystalline phases indicating that crystallization ratio can increase depending upon the retention in the mold and subsequently decrease radiative heat transfer as the flux traverses down the mold.

Characteristics of medium carbon steel solidification and mold flux crystallization using the multi-mold simulator

An oscillating multi-mold simulator with embedded thermocouples was used to study the initial solidification of medium carbon steels and crystallization characteristics of the mold flux. Casting speed variations in the simulator from 0.7 m/min to 1.4 m/min at fixed oscillation frequency and stroke resulted in higher copper mold temperatures. Frequency modifications from 2.5 Hz to 5.0 Hz and stroke changes from 8.1 mm to 5.4 mm at fixed casting speeds also resulted in higher copper mold temperatures. Surface profile analysis of as-cast steel strips showed characteristic oscillation marks comparable to the narrow faces of the industrial cast slabs. The apparent effect of casting variables on the temperature and surface profiles during the solidification of the medium carbon steels could be correlated to the variations in the negative strip time and subsequent changes in the extent of mold flux infiltration. Back scattered scanning electron microscope analysis of the full length of the retrieved flux film after casting showed cuspidine crystallization ratio that increased from the upper to lower portion of the flux film. This dynamic crystallization and growth of the cuspidine phase increases as the flux is sustained at high temperatures for longer periods. Additional experiments with industrial fluxes designed for soft cooling of medium carbon steel grades showed comparable infiltration thickness of the flux, but the crystallization characteristics were significantly different, which could have a significant impact on the heat transfer rate and mechanism through the flux film.

The effects of ferritic transformation on hot ductility of medium carbon steel

The hot ductilities of Nb-bearing and Nb-free medium carbon steel have been investigated by the hot tensile tests. A high carbon steel (0.4%) was also tested as a reference to determine the relationship between transformation temperature and hot ductility. C and Nb are very effective in delaying the ferrite transformation so that the ductility trough extended to a lower temperature compared to Nb-free medium carbon steel. Decreasing the strain rate promotes the formation of grain boundary ferrite films at higher temperatures and, on the low temperature side, the intergranular failure could still occur even when the amount of ferrite reached 40% or more. The short time temperature oscillation accelerated the precipitation of grain boundary films at higher temperatures, extending the ductility trough to higher temperatures. The samples tested on different machines (direct heating and induction heating) exhibited different hot ductility behavior.