Fundamental Study And Application Of Mold Fluxes For Special And High-Alloy Steels

Mold fluxes play significant role during continuous casting to achieve stable casting and good casting slab quality. Special and high-alloy steels high-Al non-magnetic steel 20Mn23AlV, low-expansion alloy Fe-36Ni and heat-resistant alloy Incoloy 800 were produced by ingot casting traditionally. More casting and casting slab quality issues related to mold fluxes apperared after trial production by vertical continuous casting. However, study on mold fluxes for these special and high-alloy steels is especially less, there are almost no references can be refered to. Based on the above background, mold fluxes for special and high-alloy steels high-Al non-magnetic 20Mn23AlV, low-expansion alloy Fe-36Ni and heat-resistant alloy Incoloy 800, as well as the property change rules of liquid mold fluxes and flux films for stainless steel 304,3Crl3 and 42Cr9Si2 billets were studied and evaluated systematically from laboratory and industrial perspectives.(I) Study on mold fluxes for high-Al steel shows:(1) Comparative analysis of chemical compositions, physico-chemical properties and crystallized mineralogical phases of liquid slag and slag rim showed that the main compositions and properties changed obviously within 15min since starting cast, and then tended to be stable after 15min. The increasing viscosity and precipitation of high melting-point phase gehlenite were important causes of the formation of slag rim.(2) Ion and molecule coexistence theory was employed to study the reaction thermodynamics of slag and molten steel, as well as the effect of Al2O3/SiO、 CaO/Al2O3、B2O3 on mass action concentrations of high-melting temperature components and low-melting temperature components, which can provide reasonable explaination and guidance for the design of low-SiO2 CaO-Al2O3-based mold flux and varying effect of compositions on melting temperature, viscosity characteristics.(3) In order to improve the CaO-SiO2-based mold flux for high-Al non-magnetic steel, the replacement of CaO with BaO in CaO-SiO2 based flux was carried out, comparative studies on the effect of Al2O3/SiO2 and BaO/CaO on melting temperature, viscosity, viscous flow activation energy, heat transfer, crystallization characteristics showed BaO/CaO counteracted the effect of increased Al2O3/SiO2 on the above properties to varying degrees.(4) In order to determine the main compositions of CaO-Al2O3-based mold flux for high-Al non-magnetic steel, effect of CaO/Al2O3 on melting temperature, viscosity, heat transfer and crystallization characteristics. The results indicated CaO-Al2O3-based mold flux with medium CaO/Al2O3 range 1.1～1.6 showed appreciated comprehensive properties.(5) Study on F-free CaO-Al2O3-based mold flux is extremely limited. In order to develop F-free or low-Free CaO-Al2O3-based mold flux, systematic and comparative studies on effect of CaF2 and B2O3 on melting temperature, viscosity, viscous flow activation energy, heat transfer and crystallization characteristics suggested CaF2 and B2O3 played similar role in affecting the above properties but which was different from the effect of CaF2 on those properties of CaO-SiO2-based mold flux. From the perspectives of property characteristics, CaF2 in CaO-Al2O3-based mold flux can be replaced by B2O3 to develop F-free or low-Free CaO-Al2O3-based mold flux.(6) A model with high accuracy was also developed in this thesis based on Weymann-Frenkel equation and optical basicity which can reliably predict the viscosity of mold flux with high Al2O3 content.(II) Study on mold fluxes for low-expansion alloy Fe-36Ni shows:Cracking of low-expansion alloy Fe-36Ni was related to unmatched heat transfer of mold flux. The compositions of improved mold flux were determined by studying their effects on heat transfer crystallization characteristics. The improved mold fluxes were tried and evaluated and showed flux film had the low crystalline fraction of 21.4%, heat transfer capability increased remarkably. During field trial of the improved mold fluxes, comprehensive properties of the molten fluxes were steady and met the needs of continuous casting stability and quality of casting slabs. The improved mold fluxes exhibited a better heat transfer capability. Border solidification structures of casting slabs were refined obviously, hot cracking was eliminated effectively, and good industrial production performance was achieved.(III) Study on mold fluxes for heat-resistant steel Incoloy 800 shows:Formation of surface cracks of Incoloy was because the solidification characteristic didn’t match the heat transfer of mold flux. The mold flux was modified by studying the effect of main components on primary viscosity and heat transfer properties. Compared to the original mold flux, the newly-designed flux has lower characteristic melting temperatures, an increased value for the ηVc and a greater heat transfer ability. Good liquid flux and flux film performance were achieved throughout the period of casting with negligible changes in composition and properties. The flux film had a moderate thickness of 1.1mm and a low crystalline fraction of 30%. In both the flux film and samples of the solidified flux, the dominant crystalline phase was cuspidine rather than the undesirable, high melting point compounds gehlenite or perovskite. The new mold flux with reduced characteristic melting temperatures, good lubrication properties and low crystalline fraction promoted heat transfer which enhanced thickening and strengthening of the initial shell, all of which contributed to improved slab quality.(IV) Study on mold fluxes for stainless steel 304,3Crl3 and 42Cr9Si2 billets shows:(1) Mold fluxes for stainless steel billets 304,3Crl3 and 42Cr9Si2 have minor chemical changes within 1% and basicity changes less than 0.05 during continuous casting. The break temperature and viscosity changes were less than 25℃ and 0.2dPa·s, respectively. All the changes tended to be stable after 5～10min since casting. The original mold fluxes for martensitic stainless steel billets 3Crl3 and 42Cr9Si2 have high melting temperature, and also has a big change up to 50℃ during casting, which may be the cause for formation of slag rims.(2) Flux film for 304 billet was rougher than that for 3Crl3 billet. The thicknesses of flux film for 304 and 3Crl3 were 1.6mm and 0.74mm but they have similar structure. Flux film for 304 has higher crystallization fraction 80% than 50% of flux film for 3Crl3 but they have same crystallization phase cuspidine and nephline. Flux film for 304 has stronger ability to control heat transfer across flux film than flux film for 3Crl3. The performances of flux films for 304 and 3Crl3 stainless steel billets agreed with the solidification characteristics to some extent.