Solidification and crystallization properties of CaO-SiO_2-Na_2O based mold fluxes

Crystallization properties play an important role in keeping a smooth running of continuous casting process and high surface quality of cast strands. To reduce fluorine pollution in slag, a new type of CaO-SiO2-Na2O （CSN） based mold flux was studied. The solidification and crystallization properties, including crystallization temperature, crystallization ratio and solidification mineragraphy, were measured, which were compared with the CaO-SiO2-CaF2 （GF） mold flux. The results show that the crystallization performance is equal to the high fluoride mold powder and CSN can be used for peritectic steel grades sensitive to longitudinal cracking in continuous casting.

Effects of Rare Earth Oxide on Viscosity of Mold Fluxes for Continuous Casting

The effects of RE (rare earth) oxide on viscosity of mold fluxes were investigated with a rotary viscometer. The results show that: (1) The viscosity of mold fluxes is remarkably increased by RE oxide addition, especially when the mass fraction of RE oxide is more than 10%. (2) By addition of RE oxide, precipitation of the insoluble particles with high melting point from the molten slag with the decreasing of the temperature leads to the increase of viscosity. Viscosity curve shows that RE oxide is soluble in some extent in mold fluxes. When RE oxide is in a state of supersaturation, the existence of insoluble particles also makes the viscosity of mold fluxes increase. (3) Not only the viscosity of mold fluxes can be reduced, but also the capacity to dissolve and absorb RE oxide can be increased by Li_2O, B_2O_3 and BaO. However, the contents of Li_2O, B_2O_3, and BaO should be controlled to suitable levels. (4) The solidification temperature of mold fluxes can be increased by the addition of RE oxide, which is unfavorable to heat transfer and lubrication of mold fluxes between steel shell and mold.

Crystallization behavior of F-free mold fluxes

The time-temperature-transformation （TTT） diagrams of F-free mold fluxes was constructed using single hot thermocouple technique （SHTT） and confocal scanning laser microscopy （CSLM） to study the crystallization behavior of F-free mold fluxes. The tendency of crystallization is found to increase whereas the incubation time decreases with increasing basicity. Zirconia addition enhances the crystallization tendency due to its limited solubility in the slag melt and the solid particles acting as nucleation sites. Pseudo-wollastonite is found to precipitate in the slag with low basicity （CS-1 and CS-2）, kilchoanite and larnite are formed with further increasing basicity （CS-3）, and larnite is finally formed as the basicity beyond unit （CS-4）. The crystal morphology changes with varying compositions and isothermal temperatures. The measured growth rate is found to be linear with time under isothermal conditions and decreases with increasing isothermal temperature.

Effect of Carbon Properties on Melting Behavior of Mold Fluxes for Continuous Casting of Steels

During continuous casting of steel, the properties of mold fluxes strongly affect the casting performance, steel quality and environment of casting operation. The high temperature microscopy technique was used to investigate the melting behaviour of mold fluxes, and drip test method was used to determine their melting rate. The results showed that free carbon is a dominant factor in governing the melting behaviour of fluxes, and the melting rate is increased with increasing carbon reactivity and decreasing carbon content.

FACTORS ON VISCOSITY STABILITY OF MOLD FLUXES

Viscosity stability indexes of mold flux at high temperature and low temperature havebeen introduced and the effects of flux compositions on viscosity stability indexeshave been studied. Two mold fluxes have been developed by analyzing the effects offlux viscosity stability on the process and the condition of continuous casting slab ofmedium carbon steel. The results show that the fluxes are suitable for the process.

The optimization of mold fluxes used for casting the peritectic steel of heavy slabs at Baosteel

In order to solve the problem of the high surface longitudinal crack ratio of heavy peritectic steel slabs produced by the No. 3 continuous caster at Baosteel,the physical properties of the original mold flux and the optimized mold flux were compared in a comprehensive way by using analytical measures, such as a slag film heat-flow simulator, a thermowire molten flux crystallization tester and an X-ray diffractometer in the laboratory. The results reveal that one of the major reasons for the cracks is the poor heat transfer ability of the original mold flux. However, the optimized mold flux with a high basicity features a high crystallizing rate,low crystallization temperature and low heat-flow density. Therefore, the optimized mold flux is more suitable for casting peritectic steel by the heavy slab continuous caster. The test results show that the slabs produced by using the optimized mold flux had no surface longitudinal crack in four test casts, while the surface longitudinal crack ratio of the slabs produced by using the original mold flux was 5%. The optimized mold flux can effectively prevent slab surface longitudinal cracks from occurring.

Investigation of bubbles escape behavior from low basicity mold flux for high-Mn high-Al steels using 3D X-ray microscope

During the continuous casting process of high-Mn high-Al steels,various types of gases such as Ar need to escape through the top of the mold.In which,the behavior of bubbles traversing the liquid slag serves as a restrictive link,closely associated with viscosity and the thickness of liquid slag.In contrast to two-dimensional surface observation,three-dimensional(3D)analysis method can offer a more intuitive,accurate,and comprehensive information.Therefore,this study employs a 3D X-ray microscope(3D-XRM)to obtained spatial distribution and 3D morphological characteristics of residual bubbles in mold flux under different basicity of liquid slag,different temperatures,and different holding times.The results indicate that as basicity of slag increases from 0.52 to 1.03,temperature increases from 1423 to 1573 K,the viscosity of slag decreases,the floating rate of bubbles increases.In addition,when holding time increases from 10 to 30 s,the bubbles floating distance increases,and the volume fraction and average equivalent sphere diameter of the bubbles solidified in the mold flux gradually decreases.In one word,increasing the basicity,temperature,and holding time leading to an increase in the removal rate of bubbles especially for the large.These findings of bubbles escape behavior provide valuable insights into optimizing low basicity mold flux for high-Mn high-Al steels.

Viscosity and Viscosity Estimate Model of Fluoride-Free and Titanium-Bearing Mold Fluxes

The relationship between the binary basicity （CaO/SiO2）,TiO2,Na2O,Li2O,MgO,MnO,B2O3 and viscosity for fluoride-free and titanium-bearing mold fluxes were systematically researched. The rotating cylinder method was employed in the experiment to measure the viscosity of the slag. The results indicate that Li2O, B2O3 and Na2O play major roles in decreasing viscosity, especially Li2O, which is the most effective flux, while MgO and MnO exert little influence on viscosity. Meanwhile, it can be concluded that with increasing TiO2 content, the viscosity of fluoride-free and titanium-bearing mold fluxes increases at first but then falls when the amount of TiO2 is greater than 6.0%. Based on large amounts of experimental statistics of the viscosity of fluoride-free and titanium-bearing mold fluxes, an available model in literature for predicting the mold-slag viscosity was modified. This modified model can be used to predict the viscosity of fluoride-free and titanium-bearing mold fluxes. In fact, the predicted values approximate the observed values with a ±10.6% average deviation. Compared with the classical models,the average deviation is higher and it was found that the modified model can be used to estimate the viscosity of fluoride-free and titanium-bearing mold fluxes.

CCT and TTT Diagrams to Characterize Crystallization Behavior of Mold Fluxes

The isothermal and non-isothermal experiments were performed to construct the continuous cooling transformation （CCT） and temperature time transformation （TTT） diagrams of four industrial mold fluxes through visual observations in an experimental apparatus based on the single hot thermocouple technique （SHTT）. The results of the CCT diagrams indicate that ① the crystallization temperature of mold fluxes lowers as the cooling rate increases, ② the mold fluxes have larger critical cooling rate, higher crystallization temperature, and less onset time of crystallization when the basicity increases or the viscosity decreases, ③ the influences of the melting points of the mold fluxes on their crystallization tendency are not significant. Isothermal tests show that the onset time of crystallization decreases at first, and then increases, and finally represents a ＂C＂ shape with increasing isothermal temperature. The TTT diagrams of four industrial mold fluxes were divided into two separate ＂C＂ shape regions. The crystal phase of C20A selected was analyzed by X-ray diffraction, which is cuspidine （Ca4 Si2 O7F2 ） over I 100 ℃ and calcium silicon oxide fluoride （Ca2SiO2F2） below 1 100℃. When compared with the TTT diagram, the CCT diagram can provide a more realistic estimate of the critical cooling rate of the mold fluxes. Thus, both the CCT and TTT diagrams can unambiguously describe the crystallization phenomena of the mold fluxes.

Properties of High Basicity Mold Fluxes for Peritectic Steel Slab Casting

In high speed continuous casting of peritectic steel slabs, mold fluxes with high basicity are required for less surface defect product. However, the basicity of remaining liquid slag film tends to decrease in casting process because of the crystallization of 3CaO ·2SiO2 · CaF2. Thus, a way is put forward to improve mold fluxesr properties by raising the original basicity. In order to confirm the possibility of this method, the effect of rising original basicity on the properties of mold fluxes is discussed. Properties of high fluorine based mold fluxes with different basicities and contents of CaF2 , Na2 O, and MgO were measured, respectively. Then, properties of higher basicity mold fluxes were discussed and compared with traditional ones. The results show that increasing the basicity index can improve the melting and flow property of mold fluxes. With the increasing basicity, crystallization rate of mold fluxes increases obviously and crystallization temperature tends to decrease when the basicity exceeds 1.35. The method presen- ted before is proved as a potential way to resolve the contradiction between horizontal heat transfer controlling and solidified shell lubricating for peritectic steel slab casting. But further study on improving the flow property of liquid slag is needed. This work can be used to guide mold fluxes design for high speed continuous casting of peritectic steel slabs.

Analysis of Crystallization Behavior of Mold Fluxes Containing TiO_2 Using Single Hot Thermocouple Technique

The crystallization behavior of mold fluxes containing 0-8 mass% TiO2 was investigated using the single hot therrnocouple technique （SHTT） and X ray diffraction （XRD） to study the possible effects on the coordination of heat transfer control and strand lubrication for casting crack sensitive peritectic steels. Time-temperature-transforma tion （TTT） and continuous-cooling transformation （CCT） curves were plotted using the data obtained from SHTT to characterize the crystallization of the mold fluxes. The results showed that crystallization of the mold fluxes during isothermal and non-isothermal processes was suppressed with TiO2 addition. From the TTT curves, it could be seen that the incubation and growth time of crystallization increased significantly with TiO2 addition. The CCT curves showed that the crystallization temperature initially decreased, and then suddenly increased with increasing the TiO2 content. XRD analysis suggested the presence of cuspidine in the mold fluxes with lower TiO2 content （〈4 mass%） , while both perovskite and cuspidine were detected in the mold fluxes when the TiO2 content was increased to 8 mass%. In addition, the growth mechanisms of the crystals changed during the isothermal crystallization process from interface controlled growth to diffusion-controlled growth with increasing the TiO2 content.

Crystallization Behavior in Fluoride-Free Mold Fluxes Containing TiO_2/ZrO_2

The time-temperature-transformation （TTT） diagrams of fluoride-free mold fluxes containing TiO2/ZrO2 were constructed through the confocal scanning laser microscope （CSLM） technique. It was found that the crystallization temperature of F-free mold fluxes containing TiO2 or ZrO2 increases,while incubation time decreases with increasing basicity. The crystallization tendency increases with the zirconia addition in slag melt,as it may be thought that the addition solubility is limited in molten slag and the solid particles act as heterogeneous nucleation sites. Three types of crystal morphologies were observed,corresponding to different crystallization mechanisms. CaSiO3 and Ca3Si2O7 precipitate in the slag with low basicity,and Ca2SiO4 was formed with increasing basicity. The addition of TiO2 promotes the precipitation of CaTiO3. The logarithm of crystal growth rate increased with increasing isothermal temperature,suggesting that the crystal/melt interface reaction is the controlling step in the present experiment.

Solidification Properties of CaO-SiO_2-TiO_2 Based Mold Fluxes

In continuous casting of peritectic steel grades sensitive to longitudinal cracking, the solidification properties of mold fluxes play an important role in keeping smooth running of continuous casting process and achieving high surface quality of casting strands. To reduce fluorine pollution in molten slag, types of CaO-SiO2-TiO2 （CTS） based mold fluxes were investigated. The solidification and crystallization properties, including viscosity η at 1 573 K, break temperature Tbr, crystallization ratio Rc and solidification mineragraphy were measured, which were compared with those of CaO-SiO2-CaF2 based mold fluxes. The experiments show that there are unstable viscosity-high tem- perature properties and high Tbr in part of CTS slag system, which are bad for lubrication between liquid flux film and strand. And when temperature is below Tbr, the viscosities change slowly during cooling in some part of this slag system, which imply that liquid mold fluxes solidify slowly and it is easy to cause surface longitudinal cracks on strand. Major mineragraphy of the CaO-SiO2-TiO2 based mold fluxes are CaO · TiO2 or CaO · TiO2 and CaO · SiO2 · TiO2. TiN and Ti（C,N） can be formed in mold fluxes bearing high TiO2 during the continuous casting.

Crystallization Temperature and Crystallization Ratio of Mold Flux

The factors influencing the crystallization ratio of mold flux were researched by rapid cooling technolo gy, and the factors affecting crystallization temperature were studied by single thermocouple technique. The results showed that the crystallization ratio of mold flux increases with the basicity and the content of Na2O, CaF2, Li2O and NaF, and decreases with the increase of the content of Al2O3, MgO, BaO, MnO and B2O3. However, the crystallization temperature of mold flux rises with the basicity and the content of NaF, Na2O and CaF2, and reduces with the increase of the content of Al2O3, MgO, BaO, MnO and B2O3. But for Li2O, crystallization temperature decreases firstly to a minimum value at 2%, and then increases gradually with the increase of Li2O.

Research on mold flux for hypo-peritectic steel at high casting speed

Mold fluxes having adaptable properties were developed in the laboratory to solve quality defects, such as depressions and longitudinal cracks when casting hypo-peritectic steel at high casting speed. Firstly, the effect of components on the high basicity mold flux properties was first studied using the orthogonal method. In the scope of the studied content, Li2O has the largest effect on the melting temperature, and the least effect on the viscosity; CaF2 has the largest effect on the viscosity, and the least effect on the melting temperature; Na2O and CaO/SiO2 have no obvious influence on the melting temperature and viscosity. Secondly, two powders （Z1 and Z6） have reasonable viscosity-temperature curves, higher solidification temperatures, and porous structure after solidification, but the crystal property of Z6 is worse than that of Z1, and thus Z1 is more suitable for continuous casting hypo-peritectic steel at high casting speed. Thirdly, a higher basicity of powder, a less free enthalpy of crystal compound, and a lower baffle energy are good for a higher tendentiousness of crystal.

Dissolution of TiO_(2) and TiN inclusions in CaO–SiO_(2)–B_(2)O_(3)-based fluorine-free mold flux

Mold flux serves the crucial metallurgical function of absorbing inclusions, directly impacting the smoothness of the casting process as well as the cast slab quality. In this study, the dissolution behavior and mechanism of Ti O_(2)and Ti N inclusions in molten Ca O–Si O_(2)–B_(2)O_(3)-based fluorine-free mold flux were explored by in situ single hot thermocouple technology combined with X-ray photoelectron spectroscopy.The results showed that Ti O_(2) inclusions are effectively dissolved by the molten slag within 76 s, during which the original octahedral [Ti O_(6)]^(8-)structures are destroyed and convert to the networker tetrahedral [Ti O_(4)]^(4-)structures. However, the dissolution rate is much lower for Ti N inclusions than for Ti O_(2)inclusions. This can be attributed to the fact that the Ti N particles need to be oxidized and then dissolved in the molten slag to form tetrahedral [Ti O4]4-and octahedral [Ti O_(6)]^(8-)structures during the Ti N inclusion dissolution process, which is accompanied by the generation of a large amount of N_(2)gas. Moreover, Ca Ti O_(3)crystals tend to nucleate and grow on bubble surfaces with sufficient octahedral [Ti O_(6)]^(8-)structures and Ca^(2+)ions, eventually resulting in the molten slag being in a solid–liquid mixed state.

Mold flux entrainment mechanisms

This paper presents a comprehensive and critical review of mold flux entrainment mechanisms in continuous casting of steel.Entrainment introduces inclusions into the final product,and thus greatly hinders clean steel production.By understanding the mechanisms that cause entrainment,the operating conditions of casters can be tuned to reduce the number of defects.Nine different mechanisms have been proposed over the last three decades,including vortex formation around the submerged entry nozzle(SEN),argon bubble interactions with the slag layer,shear-layer instability at the slag-steel interface,excessive upward flow impingement upon the meniscus, top surface level fluctuations,meniscus freezing and hook formation,top surface "balding",top surface standing wave instability,and slag crawling down the SEN.The previous work done for each of these mechanisms is presented,including both quantitative and qualitative descriptions of their behavior.

Wetting behavior of CaO-Al_(2)O_(3)-based mold flux with various BaO and MgO contents on the steel substrate

The Interfacial phenomena in mold have a great impact on the smooth operation of continuous casting process and the quality of the casting product.In this paper,the wetting behavior of CaO-Al_(2)O_(3)-based mold flux with different BaO and MgO contents was studied.The results showed that the contact angle between molten flux and interstitial free(IF)steel substrate increased from 62.4°to 74.5°with the increase of BaO content from 3 wt%to 7 wt%,while it decreased from 62.4°to 51.3°with the increase of MgO content from 3 wt%to 7 wt%.The interfacial tension also increased from 1630.3 to 1740.8 mN/m when the BaO content increased,but it reduced from 1630.3 to 1539.7 mN/m with the addition of MgO.The changes of contact angle and interfacial tension were mainly due to the fact that the bridging oxygen(O^(0)) at the interface was broken into non-bridging oxygen(O^(-)) and free oxygen(O_(2-)) by MgO.However,more O^(-) and O_(2-) connected into O^(0) when BaO was added,since the charge compensation effect of BaO was so stronger that it offset the effect of providing O_(2-).

Influence of substituting B_(2)O_(3) with Li_(2)O on the viscosity,structure and crystalline phase of low-reactivity mold flux

The low-reactivity mold flux with low SiO_(2)content is considered suitable for the continuous casting of high-aluminum steel since it can significantly reduce the reaction between Al in steel and SiO_(2)in mold flux.However,the traditional low-reactivity mold flux still presents some problems such as high viscosity and strong crystallization tendency.In this study,the co-addition of Li_(2)O and B_(2)O_(3)in CaO–Al_(2)O_(3)–10wt%Si O_(2)based low-reactivity mold flux was proposed to improve properties of mold flux for high-aluminum steel,and the effect of Li_(2)O replacing B_(2)O_(3)on properties of mold flux was investigated.The viscosity of the mold flux with 2wt%Li_(2)O and 6wt%B_(2)O_(3)reached a minimum value of 0.07 Pa·s.The break temperature and melting point showed a similar trend with the viscosity.Besides,the melt structure and precipitation of the crystalline phase were studied using Raman and X-ray diffraction spectra to better understand the evolution of viscosity.It demonstrated that with increasing Li_(2)O content in the mold flux from 0 to 6 wt%,the degree of polymerization of aluminate and the aluminosilicate network structure increased because of increasing Li+released by Li_(2)O,indicating the added Li_(2)O was preferentially associated with Al^(3+)as a charge compensator.The precipitation of LiAlO_(2)crystalline phase gradually increased with the replacement of B_(2)O_(3)by Li_(2)O.Therefore,Li_(2)O content should be controlled below 2wt%to avoid LiAlO_(2)precipitation,which was harmful to the continuous casting of highaluminum steels.

Abstracts from International Journal of Minerals,Metallurgy and Materials

The time-temperature-transformation （TIT） diagrams of F-free mold fluxes were constructed using single hot thermocouple technique （SITtT） and confocal scanning laser microscopy （CSLM） to study the crystallization behavior of F-free mold fluxes. The tendency of crystallization is found to increase whereas the incubation time decreases with increasing basicity. Zirconia addition enhances the crystallization tendency due to its limited solubility in the slag melt and the solid particles acting as nucleation sites. Pseudo-wollastonite is found to precipitate in the slag with low basicity （CS-I and CS-2）, kilchoanite and larnite are formed with further increasing basicity （CS-3）, and lamite is finally formed as the basicity beyond unit （CS-4）. The crystal morphology changes with varying compositions and isothermal temperatures. The measured growth rate is found to be linear with time under isothermal conditions and decreases with increasing isothermal temperature.