| Freckles, channel-like macrosegregation defects, are commonly observed in electroslag remelting (ESR) steel ingots, and appear as long black liner trails composed by chain of equiaxed dendrite structure in castings. Freckle channels rich in solute elements and dramatically reduce the mechanical property of castings, and they cannot be removed by any subsequent forging or heat treatment. Now, freckles have been the critical difficulty that restrict the production and development of large castings and ingots. In china, since the lack of knowledge related to freckle defects, electroslag remlting plants cannot take effective methods to limit the formation of freckles in ingots.Most of existing macrosegregation theories not suitable to explain the freckle formation during the electroslag remelting processes because of the large difference in solidification conditions of casting processes. Moreover, studies on freckles in the electroslag remelting ingots are especially less, almost no references can be refered to, there are still many issues kept unclear, especially the freckle formation mechanism and tendency evaluation of electro-slag remelting (ESR) solidification processes. This paper systematically investigated the freckles formation in ESR large High-strength, low alloy (HSLA) steels ingot by combined the research methods of production plant trials, thermodynamic calculation, kinetic models for microsegregation, and multicomponent continuum macrosegregation model, and properties analysis of ESR slag system.(?)Study on the freckle formation in ESR ingot shows:(1) Freckle formation in an ESR ingot is associated with a high melt rate. There is a critical melt rate, below which freckle formation does not occur. In this study, freckles were formed when the melt rate exceeded 9kg/min. While the effect of melt rate on freckle formation is predominant, the effect of process perturbations is not a necessary condition for freckle formation.(2) Freckles were found only within the mid-radius region and appeared as parallel straight lines with basically the same orientation. And there is. only one freckle channel within a given radial region (namely no parallel freckle channels along the radial direction), which confirmed that the evolution of freckle channel is conducted by the mechanism of competition and merging.(3) Microstructural analysis shows that the dendritic structure in the mid-radius region is the coarsest, and the dendrite arm spacing increases as the height of the ingot increases. The chain of equiaxed dendrite structure inside the freckle channel is quite different from the developed columnar dendrite structure of the surrounding matrix. Compared with the matrix, the freckle region is enriched with light elements, such as C. Cr, Mn, and Si. The macrosegregation degree of denser element Ni is not obvious.(4) The proposed mechanism of freckle formation involves positive density gradients and anisotropic permeability within the mushy zone.These conditions promote the upward flow of solute-rich fluid along a low resistance path (inclined upward) which is at an angle to the vertical direction.(?) Study on the tendency of freckle formation in ESR ingot shows:The freckles in the 20SiMn2MoV ingot have a composition corresponding to the interdendritic liquid with a liquid fraction of 0.24 to 0.41. This result is compared with the liquid fraction range where the calculated maximum Ra appears, and good agreement is found between the experiment and the calculation. Therefore, the freckles in ESR ingots initiate in the lower part of the mushy zone. The effect of the liquid composition variation on the density and viscosity of the liquid can be explicitly considered by combining thermodynamic calculation with thermophysical property calculation, and this approach shows good reliability for evaluating the tendency for forming freckles in industrial-scale ESR ingots. For 30CrMnSiNi2A, the estimated the threshold Rayleigh number Ra* was about -0.12 according to the present calculated results.(?) Study on the freckle formation in HSLA steel by multicomponent macrosegregation simulation shows:Freckle channels form in the upper-right corner (far away cooling interface) of solidification equipment, and the orientation of the solute-segregated channels is incline-upward (deviate from cooling interface), which result from the preference of segregated liquid tends to choose a path of least hydrodynamic resistance. The large liquid density difference result from interdendritic high solute concentration is the key factor that promote the freckle formation in HSLA steel casting. The freckle channel is actually formed from the progressively connection of many small liquid pools with concentrated solutes in the initial stage of solidification. The flow field and concentration distribution predicted by model shows that, the solute-enriched liquid inside the channels flows along the channel, however, in the neighboring regions with higher solid fraction, the flow almost perpendicular to the channels.(IV) Study on the properties of ESR slag system shows:(1) The melting temperature of slags with a CaO/AlO3 ratio of 1.0 reached a lower value at 50 wt% CaF2. For slags with 50 wt% CaF2. the melting temperature decreases as the CaO/Al2O3 ratio increases.(2) Increase in CaF2 content increases the thermal conductivity of the liquid slag. In contrast, increasing the CaO/Al2O3 ratio has the opposite effect. However the influence of CaF2 on these slag properties is very much greater than that of the CaO/Al2O3 ratio. And viscosity measurements showed that the break temperature of slags increases as the CaO/Al2O3 ratio decreases.(3) Based on the measurement of properties, a slag B2 containing 50%CaF2 and with a CaO/Al2O3 ratio of 1.5 with relatively low melting temperature and break temperature and high thermal conductivty was selected for further evaluation on ESR production facilities.(V) The study on the optimizing of ESR progress parameters shows:Reducing melt rate of electrode is the most direct and effective method for the prevention of freckle formation. For limiting the formation of freckle defects, the ESR HSLA 720-mm-diameter steel ingot melt rate should be limited between 7kg/min-7.5kg/min. Although freckle formation in the ingot can be effective limited by a low melt rate, but for ESR process using a conventional slag it will result in slag skin thickening and generates severe surface defects, futher make the ingot had to be scraped. Plant trials with ingot B confirmed that the B2 slag system formulation was effective for thinning the slag skin, improving the lubrication, eliminating surface defects. It can be concluded that reducing melt rate, intensifying the cooling of mold, and using slag system with well high temeperature viscous flow performance are effective methods for limiting freckle formation in HSLA steel ingots. |
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