Study On The Control Of Carbon Macro-Segregation For Large Round Bloom During Continuous Casting

Recently,with the large-section continuous casting round blooms used in the heavy forging industry more and more widely,the quality requirements for controlling macro-segregation of the blooms are more stringent.The focus of quality is not only on controlling the central segregation,but also on minimizing the solute fluctuation on the whole cross-section of the bloom to improve the uniformity of product properties.However,so far it is very difficult for the large blooms with a long solidification time to fully control the uniform distribution of solute elements in the strand.Therefore,in terms of the super-large round blooms of ?380mm~?800mm diameter produced by an arc continuous caster,the control of macro-segregation has been selected as the present investigation topic.The carbon distribution on the cross-section of round bloom has been inspected by Optical Emission Spectrometer(OES),C/S analyzer using the drilling sampling method and Original in-situ Position Analyzer for Metal.Combined with the solidification macrostructure,the trend characteristics of macro-segregation distribution of the blooms have been revealed and the causes have been theoretically analyzed.And also systemic control strategy of carbon macro-segregation has been put forward,which includes a new idea of using the strand electromagnetic stirring(S-EMS)to control the positive segregation in the columnar to equiaxed crystal transition(CET)zone.Based on the measurement of stirrer magnetic field,solidifying shell thickness and surface temperature,coupled mathematical simulation of fluid flow,heat transfer and solidification has been done under the action of a combined electromagnetic stirring.And combined with the analysis of the relationship between white band and macrostructure,the optimization aim of electromagnetic stirring allocation and parameters has been given.Then,with a series of industrial tests,the effects of process parameters on carbon macro-segregation have been verified,such as superheat,electromagnetic stirring(including M-EMS,S-EMS,F-EMS)and secondary cooling.Based on the proposed carbon segregation control strategy,the casting parameters have been optimized and and the expected metallurgical effect has been confirmed by large-scale industrial production.Finally,a capping delayed running out process has been developed to solve the quality problems of trail bloom,such as low ratio of equiaxed crystal zone,high ratio of white band,serious central porosity,cavity and crack.And industrial tests have also been carried out to confirm the effect.Based on above investigations,the flowing conclusions can be drawn:(1)The main distribution trend characteristics of macro-segregation of large round bloom can be describes as follows.In the sub-surface,the carbon content decreases rapidly and then rises slowly.There exists a negative segregation valley.In the CET zone,the carbon content begins to decline after a rapid increase,and there exists a positive segregation peak.In the central equiaxed zone,the carbon content fluctuates greatly,and the positive and negative segregation coexists with a fluctuated peak or valley.In addition,the macro-segregation distribution of the strand in a arc continuous caster is asymmetrical,and the carbon segregation in the inner arc side is more serious than that of outer arc side.(2)Sub-surface negative segregation of the bloom is caused by the redistribution of solute elements and the flow of molten steel in the mold.The higher the intensity of mold electromagnetic stirring is,the lower the carbon content is and the higher the degree of negative segregation is in the sub-surface.The formation of positive segregation in CET region is due to the retention of solute elements enriched at the front of columnar crystals by selective crystallization and CET transformation.The strand electromagnetic stirring before CET can reduce the positive segregation near the CET zone so that the overall carbon uniformity is obviously improved.The reason why the carbon content in the central equiaxed crystal zone fluctuates greatly is that the liquid core at the end of solidification is a mixture of crystal rain with low carbon content and the melt enriched with solute elements.The carbon distribution in the center area is more uniform by increasing the intensity of electromagnetic stirring at the final solidification.(3)The formation of the white band on the large round bloom is closely related to the macro-structure at the stirring positions.It is easier to form white bands in columnar crystal zone than in equiaxed crystal zone with the same stirring intensity.When the electromagnetic stirring acts on the columnar crystal zone,the stirring cannot work fully,or the white bands will appear.So,the effect of electromagnetic stirring on solidification macrostructure is limited.(4)The capping delayed running out process delays the strand capping by adding some heating agent on the top surface and switching off the spraying water in order,which leads more sufficient contraction compensation of liquid core.So,the macrostructure unqualified ratio of trail bloom decreases from 6.25% to 1.96% though the tail cropping length is 0.5m reduced,and the liquid steel yield is 0.55% increased.(5)A carbon macro-segregation control strategy has been presented for the uniform distribution of carbon content over the whole cross-section,which includes low superheat pouring,weak mold electromagnetic stirring,increased secondary cooling intensity appropriately,strand electromagnetic stirring before the CET zone and strong final electromagnetic stirring at the appropriate position of the solidified end.The process parameters of continuous casting have been optimized according to this present study.The maximum carbon difference including central point has decreased by about 25% on average for the typical steel grades and bloom sections,and by about 30% except the central point.Furthermore,the qualified ratio of macrostructure has increased by more than 2 percentage points.The innovations of my work are:(1)Concerning the large round bloom with diameter from 380 mm to 800 mm,systematical solution about the carbon macrosegregation has been put forward,the key idea of which is controlling the positive segregation in CET zone by strand electromagnetic stirring.As the successful use in industrial production,the maximum carbon diffenence including central point has decreased by about 25% on average,and by about 30% except the central point.Furthermore,the qualified ratio of macrostructure has increased by more than 2 percentage points.(2)Based on the analysis of process requirements for forming white band,it is found that white bands are easier to form in columnar crystal zone than in equiaxed crystal zone with the same stirring intensity.(3)A new running out process of delayed capping is developed,the key idea of which is adding some heating agent on the top surface and switching off the spraying water in order to make more sufficient contraction compensation of liquid core.With the process,the macrostructure unqualified ratio of trail bloom decreases from 6.25% to 1.96% though the tail cropping length is 0.5m reduced,and the liquid steel yield is 0.55% increased.