Heat transfer through mould flux with titanium oxide additions

Abstract

Mould powders are synthetic slags that contain mixtures of silica (SiO2), lime (CaO), sodium oxide (Na2O), fluorspar (CaF2), and carbon (C). When heated to elevated temperatures these powders liquefy and float on the liquid steel in the mould. Mould oscillation helps the liquid flux to penetrate the tiny gap between the mould and the newly formed solid steel shell. In this position the liquid flux partially solidifies against the water cooled mould, while a small portion of the flux remains liquid next to the steel shell to provide lubrication between the moving parts. Effective horizontal heat transfer in the mould is critical for solidifying the liquid steel inthe mould. This process is largely influenced by the thickness and the nature of the flux layer that infiltrates the mould/shell gap. When casting titanium stabilised stainless steels the alloying element reacts with the molten flux, ultimately changing the behaviour of the flux. During the casting process, titanium from the liquid steel reacts with the molten flux producing solids at high temperatures known as perovskite (CaTiO3). Research has shown that perovskite reduces the lubrication capabilities of casting fluxes leading to detrimental effects on product quality while posing a serious threat of machine damage (breakout). The focus of this study is to investigate the effect of titanium pickup on the solidification nature of mould flux and the consequences on horizontal heat transfer. To achieve this, an experimental setup was constructed to simulate the behaviour of mould flux during continuous casting. Analyses of the test flux indicated that the liquid flux closest to the cold side (mould) instantly froze to produce a glassy solid structure. Closer to the hot side (steel shell), solid particles such as perovskite, cuspidine (Ca4Si2O7F2), olivine (Ca,Mg,Mn)2SiO4 and nepheline (Na2O.Al2O3.(SiO2)2) could be identified. Similar solid particles were also found in a slag rim sample taken during the industrial casting of 321- titanium stabilised stainless steel using SPH-KA1 mould powder. Further investigations of the crystalline flux layers showed the entrapment of many tiny gas bubbles during solidification. This porous structure acted as a thermal heat barrier limiting horizontal heat transfer. Experimental testing on 3.0 and 6.0mm flux thickness revealed that the overall thermal conductivity of mould flux decreased as the flux porosity increased. Larger amounts of gas entrapment (in the solid flux structure) resulted in higher thermal resistances which ultimately reduced the heat transfer capabilities of the flux. A second heat barrier, which has a far more dominating effect on the overall heat transfer, is created on mould surface during flux solidification. This thermal contact resistance is also found to be the result of entrapped gas bubbles. Experimental results concluded that the effect of titanium pickup on heat transfer is primarily overshadowed by the larger effect of the thermal contact resistance that is formed during mould flux solidification. The contact resistance in combination with gas entrapment in the solid crystalline structure is considered to be the key factors preventing horizontal heat transfer during continuous casting.