Effects of continuous casting mould fluxes on reducing longitudinal and star cracks on a slab surface

In the continuous casting production of medium carbon steel (whose mass fraction of carbon is 0.09 % to 0.16 %) and high strength low alloy steel (whose mass fraction of Mn is 0.90 % to 1.40 %), flecks occurring the most often are usually longitudinal and star cracks. In additional to the employment of a galvanized plate mould, two kinds of fluxes with special properties were studied and used to harmonize the conflicts between the function of heat transfer and lubrication. An industrial application revealed that the crystallizing temperature (Tc) and crystal ratio (R) of fluxes have a crucible effect on impeding the occurrence of above defects on a slab surface. In the case of slab section (180 to 240) mm(1000 to 1400) mm and casting speed Vc of (0.7 to 1.2) m/min, the optimized parameters of fluxes are Tc of (1170 to 1190) ℃, R of 80 % to 95 % for medium carbon steel, and Tc of (1100 to 1150) ℃, plus R of 40 % to 60 % for high strength low alloy steel.

Effects of Transition Metal Oxides on Thermal Conductivity of Mould Fluxes

The thermal conductivity of the mould fluxes containing transition metal oxides was measured by hotline method at different temperatures. The relationship between the thermal conductivity of mold fluxes and the contents of transition metal oxides was discussed. The synthetic slags were composed of 30.0% - 35.4%CaO, 34.7% - 38.6% SiO2, 6% Al2O3, 9% Na2O, 14.4% CaF2, 0-4% Cr2O3 and 0-8% MnO in mass percent. The results indicated that Cr2O3 and MnO had a negative effect on thermal conductivity of mold fluxes. The thermal conductivity of mold fluxes was about 0.25-0.55 W/（m · K） when the temperature reached 1 300 ℃, and it increased sharply to about 1.32-1.99 W/（m · K） when the temperature reduced from 1300 to 1000 ℃. The thermal conductivity of mold fluxes containing Cr2 O3 and MnO was 10 %- 25% lower than those of original fluxes. The decrease in thermal conductivity was attributed to the change of molecular structure of mold fluxes. In addition, the poor integrity and regulation of polycrystal structure, complexity of crystal structure, and effects of impurities in the boundary and lat- tice distortion leaded to the reduction in the thermal conductivity. Na2 CrO4, Mn2 SiO4 and other minor phases were also found in the samples containing Cr2O3 and MnO, respectively.

Development of Test Method for Measuring Sintering Temperature of Mould Fluxes

Excessive sintering of mould fluxes can readily cause defects and sticker breakouts in continuously cast strands.Studying the sintering property is important to minimize problems related to sintering arising from the use of mould fluxes in continuous casting.An effective method of measuring the apparent sintering temperature has been developed in this study.The method is based on monitoring the formation of cavities caused by melting of samples.For monitoring,the differential pressure of an inert gas flow was measured through a set volume of sample（mould flux A）held in a furnace tube.The apparent sintering temperature was defined in this test to determine sintering process.The sintering properties of fluxes with various contents of carbon black were examined along with identification of mineralogical phases and the nature of the sinter for samples of mould flux A held for one hour at different temperatures.The experimental results indicated that the apparent sintering temperature（AST）was a useful parameter to assess the threat of problems related to sinter.

Physical Properties and Regulating Mechanism of Fluoride-Free and Harmless B_2O_3-Containing Mould Flux

The flux agents in common mould fluxes were fluoride and sodium oxide, which would do great harm to environments. B2O3 was selected as flux. The physical properties of B2O3-containing mould fluxes were studied. The corresponding physical properties of 37. 91% CAO-43.09% SiO2-5% Al2O3-5% MgO-2% Li20-7% B2O3 mould fluxes were as follows： the melting point was 909℃, the flowing temperature was 1 160℃, the viscosity and surface tension at 1300 ℃ were 0. 4 Pa · s and 0.32 N/m respectively, which could meet the demands for certain kinds of steels for mould fluxes in continuous casting.

Viscosity estimation for slags containing calcium fluoride

Based on recently published experimental data, the Riboud model was modified for viscosity estimation of the slags containing calcium fluoride. The estimated values were in good agreement with measured data. Reasonable estimation can be achieved using the modified Riboud model for mould fluxes and ESR (eletro slag remelting) slags. Especially for ESR slags, the modified Riboud model can provide much more precise values than the original Riboud model.

Viscous properties of new mould flux based on aluminate system with CeO_2 for continuous casting of RE alloyed heat resistant steel

The conventional mould fluxes can not be applied to the continuous casting of RE alloyed heat resistant steel, because severe slag-metal interface reactions occur generally in the mold. To restrain the interface reaction and improve conditions for continuous casting, a new mould flux based on aluminate system was devised. The viscous properties were investigated. Scanning electron microscopy and X-ray diffraction were applied to detect and characterize the crystalline phases in the continuous cooling process. The results showed that appropriate addition of CeOcould avoid the precipitation of CaO and decrease the viscosity of the mould flux. Increasing the mass ratio of CaO /AlO, especially to a value exceeding 1, could worsen the stability of the mould flux. With a content of less than 14 wt.%, LiO could reduce the viscosity and breaking temperature, but its effect could be weakened for the promoted precipitation of LiAlO. To obtain a mould flux with stable viscous properties, such as viscosity and breaking temperature, appropriate contents of CeOand LiO should be controlled to around 10 wt.% and 14 wt.%, while the mass ratio of CaO /AlOshould not be more than 1.

Influence of La_2O_3 on crystallization behavior of free-fluoride mould flux and heat transfer of slag films

In order to research the influence of La2O3 on crystallization behavior of free-fluoride mould flux and the heat transfer of slag film,free-fluoride mould flux with various La2O3 content were investigated by using self-made mould simulator,comprehensive thermal analyzer and SEM-EDS.With the increase of La2O3 content from 0% to 20%,the crystallization ratios of mould flux films were improved from 2% to 90% and the thicknesses of films were also enhanced over two times.Moreover,crystallization temperature was greatly raised and its increasing extent reached 100 oC.It could also decrease the melting temperature of casting powder about 50 oC.However,the undissolved La2O3 particles appeared in slag film if the ratio of La2O3 in free-fluoride mould flux was much too high.

Structural behavior of F-in mould flux melt of CaO-SiO2-Al2 O3-Na2O-CaF2 system

The influence of fluorine on the structure of CaO Si（）e-Al2Oa Na2O-CaF2 continuous-casting-type slag was measured by Raman spectroscopy, and the degree of polymerization of mould flux and the structural behavior of F- in the melt were investigated by classifying and quantifying the structural species of F ions. The results exhibit that the main structural units of Si O tetrahedra are Q1, Q1 and Q2, and the actual measured number of non bridging oxygen ions in the [SiO4 ]-tetrahedra （denoted by NBO/T） in creases from 2. 73 to 3.44 with increasing the molar ratio of F to （F＋O） （denoted by XF/X（F-O ） from 0.06 to 0. 19. It means that the degree of polymerization o1 meh structure decreases with an increase in XE/X（F＋O）. In addition, most of F- ions were distributed in Si-O tetrahedra and Al-O tetrahedra. With increasing XF/X（F＋O）, the complex structural units Al-Otetrahedra are gradually replaced hy discrete structural units A1F4 because of the breakage of Al-O bonds in Al-Otetrahedra by F ions, and the SiO （bridging oxygen） bonds of SiO tetrahedra are broken to form [SiOnF1-n]-tetrahedra by F ions coordinating with Si4 ＋ .

Viscosity and Structure Changes of CaO-SiO_2-Al_2O_3-CaF_2 Melts with Substituting Al_2O_3 for SiO_2

During the smelting process of the high Al steels,the reaction between SiOin molten slag and dissolved aluminum in liquid steel always takes place.This aluminathermic reduction reaction will lead to the substitution of 1mol SiOfor 2/3mol AlO.Therefore,the investigations about the influence of the ratio of AlOto SiOon viscosity and structure changes of mould flux during this process are very necessary.The viscosity variation of CaO-SiO-AlO-CaFv2 melts was studied by changing compositions considering the aluminathermic reduction reaction.It was found that viscosity increased monotonously with gradually increasing the substitution extent.According to the Raman analysis,the substitution of AlOfor SiOleads to the decrease of non-bridging oxygen but the increase of bridging oxygen.Therefore,degree of polymerization and viscosity increase as the substitution extent increases.By comparing the measured viscosities with the model calculated values,it was found that both the recently developed Zhang′s model and Roboud model could describe the viscosity variation behavior of CaO-SiO-AlO-CaFmelts very well.