| As the application of steel materials expands,it has become a developing trend to strengthen the steel properties by adding alloying elements such as Al.However,the high content of Al is prone to react with traditional Ca O-Si O2 based mold flux during casting process of steel,resulting in deterioration of slag properties and a series of problems such as slag inclusions,cracks and even breakout accident.Therefore,this paper proposes a non-reactive Ca O-Al2O3-Si O2-B2O3-R2O（alkali metal oxides）mold flux system,based on the idea of adding network formers of Si O2 and B2O3,and the alkali metal oxides with the charge compensation effect.Firstly,the roles of Si O2 and B2O3 in Ca O-Al2O3 based mold flux were explored by Fourier transform infrared spectroscopy（FTIR）,Raman spectroscopy and X-ray photoelectron spectroscopy（XPS）.The results show that the main form of Al3+is the four-coordinated[Al O4]5-tetrahedral network structure in Ca O-Al2O3 based mold flux without other network formers.With the addition of small amounts of Si O2 and B2O3,the aluminate network structure in the melt is disrupted and the transformation of[Al O4]5-tetrahedral into network-modified[Al O6]9-octahedral is promoted.Many stable aluminosilicate and aluminoborate networks will be formed when sufficient Si O2 and B2O3 is added.And a massive of the[Al O6]9-octahedral transforms back into[Al O4]5-tetrahedral.Thus,the degree of polymerization of the melt structure is significantly increased.Secondly,the effects of alkaline earth metal oxides and alkali metal oxides on the melt structure and crystallization properties of Ca O-Al2O3-Si O2-B2O3 mold flux systems were investigated using FTIR and Raman spectroscopy,combined with differential scanning calorimetry（DSC）and single hot thermocouple technique（SHTT）.The results show that the additions will first release O2-to disrupt the existing aluminate structure and promote the nucleation of crystals.On the other hand,the released metal cations can promote the formation of stable[Al O4]5-tetrahedral aluminate and Si-O-Al aluminosilicate structure by the charge compensation effect,which also directly promote the precipitation of Ba-bearing phase of Ba3Al2O6 and Li-bearing phase of Li Al O2.Then,isothermal and non-isothermal crystallization behavior of Ca O-Al2O3-Si O2-B2O3-R2O mold flux were in-situ investiged by single/double thermocouple technique.The results show that at a high supercooling,isothermal temperature has no significant effect on the crystallization incubation time;but the nucleation rate and growth rate of the crystals are notably promoted with the decrease of isothermal temperature.And the crystal growth is obviously inhibited when the isothermal temperature is below 1250°C due to the dramatic increase in viscosity.Under non-isothermal crystallization conditions,different temperature gradients affect the distribution of slag film.The thickness of the glass layer in slag film increases and the proportion of crystal layer decreases prominently with the reduction of the temperature at the low temperature side.Finally,the nucleation mechanism and growth behavior of the crystals in Ca O-Al2O3-Si O2-B2O3-R2O mold flux were elucidated through high-temperature Raman spectroscopy and time-of-flight secondary ion mass spectrometry and SHTT.The results show that the polymerization degree of the melt structure firstly decreases during the crystallization incubation period,which is conducive to solute migration,collision and nucleation,and the structural units associate with the nucleation of Ca2（Al（Al Si）O7）will fluctuate and segregate.Then,once the nuclei are formed,the crystal growth will absorb the surrounding solutes and causes both the segregation of elements and atomic groups.And during the growth process,the crystals will contact with each other and squeeze to form gaps.Those results in this paper will provide a theoretical support for the regulation of the melt structure and crystallization behavior in Ca O-Al2O3 based mold flux by optimizing the components at the molecular structure level. |
