Heat transfer phenomenon in steel continuous casting molds is critical, because it determines surface quality of the product and productivity and safety of the process. Excessive, insufficient or uneven rate of heat extraction from the newly solidified shell lead to several problems, and hence precise control is required. This demands to study the thermal resistance between the cast product and the mold from angles encompassing different aspects of the film present in between – mold powder slag – and the use of different investigation techniques.
For studying heat transfer in the steel continuous casting mold, researchers have basically made use of methods that can be classified into three categories: sandwiching, where a mold slag film is held between hot and cold solid walls; dipping of a cold finger into molten slag, where slag is frozen over the finger; and infrared irradiation, where a slag films is exposed to a radiation energy source. These investigations have disclosed that there are many factors influencing heat transfer through the slag film and that general agreement has not been reached about the key characteristics determining heat transfer.
Group of researchers led by Prof. A. Humberto Castillejos E. from Mexico experimentally investigated the total effective thermal resistance between a hot Inconel surface and a cold copper surface separated by an initially glassy slag disk, made from powders for casting low and medium carbon steels in compact strip production, CSP, using the sandwiching method. Furthermore, they estimated the effective thermal resistance in the actual process combining a heat transfer model with plant measurements and additionally investigated the development of the gap between a slag film and a hot metallic surface using high temperature confocal laser scanning microscopy, CLSM. The new paper describing the research was published in the peer-reviewed journal Metallurgical and Materials Transactions B.
Heat flux measurements reveal that the interfacial thermal resistance of mold powders for casting low and medium carbon steels are fairly similar and suggest that the difference in thermal characteristics that they show during devitrification arises from distinctness in the characteristics of the films. This is confirmed by a posteriori microscopic inspection of the films treated in the sandwiching apparatus and by real time observation of films treated in CLSM. High temperature observations reveals for the first that during devitrification the gap between a slag film and a metallic surface, simulating the mold, does not broaden but narrows, contrary to what is widely claimed. The difference in the packing of crystalline grains in devitrified slags, for casting low and medium carbon steels, gives raise to considerable differences in porosity, greater in slags for medium carbon steels.
The total effective thermal resistances measured in the laboratory for a slag used for casting medium carbon steels was found to be 33% larger than that of a slag for low carbon steels. This difference agrees closely with the total thermal resistance estimated from plant data obtained in CSP molds operating with the same powders. Additionally, comparison of thermal resistances estimated from laboratory and plant data hints that the slag film thickness in the CSP process must be around 0.1-0.25 mm.
The findings of this study for the first time show observation of the motion of a devitrifying slag surface relative to a stationary parallel surface, thus shedding light on the phenomena occurring at the interface.
J. Manuel Gonzalez de la C., Tania M Flores F., A. Humberto Castillejos E., Study of Shell-Mold Thermal Resistance: Laboratory Measurements, Estimation from Compact Strip Production Plant Data, and Observation of Simulated Flux-Mold Interface, Metallurgical and Materials Transactions B, 47 (2016) 2509-2523.
Laboratory of Process Metallurgy, Department of Metallurgical Engineering, Centro de Investigación y de Estudios Avanzados, CINVESTAV – Unidad Saltillo, Ramos Arizpe, Mexico.Go To Metallurgical and Materials Transactions B