Fluorine‐Free Mould Powders for Slab Casting: Crystallization Control in the CaO–SiO₂–TiO₂–Na₂O–Al₂O₃ System

Abstract

The main problem related to the development of fluorine‐free mould powders for slab casting is effectively controlling the heat transfer between the steel shell and mould. In commercial mould powders crystallization of cuspidine (Ca4Si2O7F2) from mould slag has a great effect on heat‐transfer control. In industrial process the crystallization rate for a fluorine‐free mould slag should be similar to the crystallization rate of cuspidine. To evaluate the crystallization rate for slags time‐temperature‐transformation (TTT) diagrams can be constructed using the Single Hot Thermocouple Technique by in situ observation. In the present work, fundamental information related to crystallization control in the CaO–SiO2–TiO2–Na2O–Al2O3 system was obtained. It was observed that the addition of Na2O in CaO–SiO2–TiO2 slags dramatically shortens the crystals' incubation times in TTT diagrams to the range of seconds. It is possible to control the crystallization kinetics in CaO–SiO2–TiO2 slags by changing the Na2O content. Some observations for the crystals' morphology are reported.

Effect of TiO₂ Addition on Crystallization Characteristics of CaO-Al₂O₃-based Mould Fluxes for High Al Steel Casting

“…Crystallization of mould fluxes could be investigated by employing many techniques, such as differential thermal analysis (DTA) 17) or differential scanning calorimeter (DSC), 18) Single/Double Hot thermocouple technique (SHTT/DHTT) 19,20) and laser confocal scanning microscopy (LCSM). 21) In the present work, crystallization behaviors of CaO-Al2O3-based mould fluxes with varying TiO2 content were investigated by the differential thermal analysis (DTA).…”

ZrO2 ADDITION IN MOULD POWDERS: EFFECT ON HORIZONTAL HEAT TRANSFER RATE AND SEN EROSION RATE

“…10% lower. That is an important result, considering that reduction of heat transfer rate helps alleviate problems associate with longitudinal cracking that arises from the mismatch in the thermal contraction coefficients of δ-ferrite and austenite [7]. …”

“…specific values of basicity and break temperature, promoting the crystallisation of cuspidine (3CaO.2SiO 2 .CaF 2 ) [5]. During continuous casting process, the presence of crystals in mould slags has a decisive effect on both the lubrication and the heat transfer rate between the mould and the steel strand [6,7]. The crystallisation of cuspidine from mould slag is thought to be the most effective way of exerting horizontal heat transfer control.…”

Phase Equilibria of Fluoride‐Free Boracic Mould Flux for Steel Continuous Casting

“…The major problem in the development of fluorine‐free mould flux is to find appropriate substitutes for fluorides. Feasible candidates are those oxides of TiO 2 , B 2 O 3 , and Na 2 O which are considered to be suitable additives for mould fluxes . However, no single oxide can take all functions which fluorides can do, which combine low viscosity, good lubrication, and also efficient heat transfer control .…”

“…Feasible candidates are those oxides of TiO 2 , B 2 O 3 , and Na 2 O which are considered to be suitable additives for mould fluxes . However, no single oxide can take all functions which fluorides can do, which combine low viscosity, good lubrication, and also efficient heat transfer control . This paper investigates the phase equilibria of the fluoride‐free fluxes, containing B 2 O 3 , Na 2 O, TiO 2 , MgO, and other oxides using thermochemical software FACTSage which has not yet been systematically carried out; it also includes phase analyses of selected fluxes using XRD and SEM/EDS analyses.…”