Heat transfer through mould flux with titanium oxide additions
KEY WORDS: continuous casting, crystallisation, cuspidine, heat transfer, heat
barrier, mould flux, perovskite, slag, stainless steel, titanium.
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 in
the 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 321titanium
stabilised stainless steel using SPH-KA1 mould powder.
School:University of Pretoria/Universiteit van Pretoria
School Location:South Africa
Source Type:Master's Thesis
Keywords:heat titanium dioxide steel stainless continuous casting flux metallurgy slag crystallization high temperatures perovskite
Date of Publication: