Heat transfer through mould flux with titanium oxide additions

Abstract

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 (SiO 2 ), lime (CaO), 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. -HEAT TRANSFER THROUGH MOULD FLUX WITH TITANIUM OXIDE ADDITIONS - J.A. BOTHMA -MASTER OF ENGINEERING (METALLURGY) -UNIVERSITY OF PRETORIA -2006iv 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. -HEAT TRANSFER THROUGH MOULD FLUX WITH TITANIUM OXIDE ADDITIONS - J.A. BOTHMA -MASTER OF ENGINEERING (METALLURGY) -UNIVERSITY OF PRETORIA -2006v ACKNOWLEDGEMENTS

Liquidus Temperature of Mould Fluxes in Casting of Ti-stablized Stainless Steel

“…FactSage package has been employed to study the crystallization product of mould fluxes in casting of Ti-stabilized stainless steel [9]. However, due to scarcity of experimental data in slag system bearing fluoride, the accuracy of data generated by thermodynamic software package is questionable.…”

“…viscosity, liquidus temperature and crystallization behavior. There are some reports [5][6][7][8][9][10] regarding the effect of TiO 2 concentration on the viscosity, solidification temperature and crystallization. However, these studies were performed to investigate the TiO 2 effect on mould fluxes at fix basicity (defined as CaO/SiO 2 ).…”

Effect of substituting SiO2with TiO2on viscosity and crystallisation of mould flux for casting titanium stabilised stainless steel

“…[2][3][4][5][6][7][8] For example, it was reported 5 that the content of TiO 2 in mould fluxes increases from 0 to near 5% in 20 min during casting of ferritic stainless steel type 409. Mould powder can inevitably pick up the TiO 2 inclusions floating up from liquid steel, [2][3][4][5][6]8 and the titanium in steel can be oxidised by SiO 2 in the mould powder to form TiO 2 which enters into mould flux, 2-8 as follows 7 Ti…”

“…5,16,17 The existence of TiO 2 in mould fluxes can lead to precipitation of crystals (CaTiO 3 or CaSiTiO 5 ), which could alter the heat transfer performance. [6][7][8]15,16 However, all these studies are very limited in clearly indicating the change trend of properties and performance of mould flux with titanium in steel State Key Laboratory of Advanced Metallurgy, School of Metallurgical and Ecological Engineering, University of Science and Technology Beijing, Beijing 100083, China *Corresponding author, email shuqifeng@gmail.com being oxidised by SiO 2 or with the absorption of TiO 2 in continuous casting process of titanium stabilised stainless steel.…”

<i>In-situ</i> Study of Crystallisation Behaviour of CaO–SiO₂–Na₂O–B₂O₃–TiO₂–Al₂O₃–MgO–Li₂O Fluorine-free Mould Fluxes with Different CaO/SiO₂ Ratios

“…[7][8][9][10] Heat transfer is also suppressed by the formation of high thermal resistance pores, cracks and air gaps in the wake of the shrinkage of crystalline layer. 5,[11][12][13][14] Currently, most of the commercial mould fluxes contain fluorides, like CaF 2 and NaF, to improve lubricity and control heat transfer rate through the formation of cuspidine (Ca 4 Si 2 O 7 F 2 ). [15][16][17][18] However, the high volatility of fluorides at high temperature brings about serious problems such as health hazards to operators, acid rain and corrosion of submerged entry nozzle and secondary cooling system.…”