Solidification Mechanism And Key Control Technology Of Macro Segregation In Continuously Cast High-carbon Steel Blooms

Serious macro segregation occurs more easily in continuous cast high-carbon steel blooms due to high carbon content,wider mushy zone and longer solidification time.Segregation is an important factor in causing internal quality and performance nonuniformity for following large section rolling products.Macro segregation forms during the solidification process,and is closely related to solidification characteristic and solidification structure.Cooperative technology of complex electromagnetic stirring(M-EMS and F-EMS)and mechanical soft reduction(MSR)has been widely applied in bloom continuous casting for high carbon steel and has proven to be one effective way to reduce macro segregation.However difficulty of control technology exists in practical production.In this paper,systematic research on microsegregation of solute elements in the mushy zone,evolvement rule of solidification structure,solidification progress and key control technology of macro segregation in continuously cast high-carbon alloy blooms could been carried out.An important theoretical and practical basis has been provided for improving macro segregation in continuous cast high-carbon steel bylooms to the greatest extentCombined with the operating condition,microsegregation model of solute elements in the mushy zone during continuously cast high-carbon steel blooms is established in consideration of ?/? phase transformation.Accurate solidus and liquidus temperature and the relationship between temperature and solid fraction during the solidification process can been acquired.The results show that ? phase precipitates out from liquid steel directly without ? phase during the solidification process of high-carbon steel.Interdendritic solute segregation ratio of S,P and C is bigger at the final solidification stage than that of Mn,Si and Cr.C content has a great effect on zero strength temperature(ZST),liquid impenetrable temperature(LIT)and zero ductility temperature(ZDT),while the content of S,P,Mn and cooling rate only have a bigger effect on ZDT.A combined celluar automaton-finite element(CA-FE)model has been developed to investigate the effect of operating parameters such as casting speed,superheat and secondrary cooling ratio on solidification structure.The simulated results show that electromagnetic stirring and superheat have greater effect on the equiaxed ratio than casting speed and secondrary cooling ratio.Therefore,in order to improve center carbon macro segregation of high carbon steel bloom by increasing the equiaxed ratio,control of superheat and optimization of electromagnetic stirring should be first condidered in the practical production.High-temperature thermodynamics performance of high carbon steel with different carbon content has been obtained by differential scanning method and high temperature thermal simulation,and applied in developed solidification heat transfer model.The results show that from the mold meniscus to the secondary cooling zone export,solidified shell thickness in columnar zone and the square root of solidification time showes a linear relationship,which fits the solidification square root law.However,from the secondary cooling zone outlet to solidification end,the relationship between solidified shell thickness in equiaxed zone and the solidification time is non-linear.Moreover,casting speed has more obvious effect on solidification end and center solid fraction than secondary cooling rate and superheat.In order to control macro segregation,electromagnetic torque instrument in mold electromagnetic stirring has been developed in accordance with electromagnetic torque theory.Technical parameters of mold electromagnetic stirring are optimized by combination with gauss meter and industrial experiment.The results show that maximum magnetic flux density and maximum electromagnetic torque occur with different frequency,and larger section corresponds with smaller frequency.Optimal stirring frequency on BU steel for 310mm×360mm section is 1.8Hz in theory.After the optimal stirring frequency is applied,central equiaxed grain ratio increases by 30%and central carbon segregation index decreases from 1.1 to 1.05 and homogenization becomes better.Main technological parameters are systematically optimized by high temperature performance of high carbon steel,numerical simulation and industial trials.The effect of complex electromagnetic stirring and MSR processes on macro segregation of high-carbon steel blooms are analyzed by contrast.The results are as follows.On controlling of macro segregation of BU?B6 and B3 steel for 310mm×360mm bloom in F-EMS and MSR integrated process,the reasonable center solid fraction of bloom at the F-EMS implemented position is less than 0.1.The reasonable center solid fraction in optimum reduction zone of BU and B3 steel is 0.3?0.75.Higher casting speed should be in priority adjusted to meet the demand of reduction zone and corresponding casting speed is 0.52m/min and 0.5m/min,separate ly.Total reduction amount is 17 mm,reduction amount of single withdrawal straightening roller is below 2mm when the center solid fraction is between 0.3 and 0.4 at the beginning of reduction,and larger reduction amount is adopted when the center solid fraction is between 0.5 and 0.7.Centerline carbon segregation ratio of BU and B3 steel along the bloom longitudinal section is 0.97?1.04 and 0.97?1.08,respectively.V-shaped segregation and shrinkage cavity have almost disappeared and internal cracks could be avoided effectively.On controlling of macro segregation of GCrl5 steel for 220mmx260mm bloom in combination with M-EMS and F-EMS process,the reasonable center solid fraction at the F-EMS implemented position is 0.1?0.2.Lower casting speed should be adjusted to meet the demand of F-EMS implemented position,and corresponding casting speed is 0.75m/min.Centerline carbon segregation ratio along the bloom longitudinal section is 0.93?1.12,and it is not obvious to improve macro segregation.With regard to BU steel for 220mmx260mm bloom in combination with cooperative technology of complex electromagnetic stirring and MSR,the reasonable center solid fraction at the F-EMS implemented position is below 0.1,and corresponding casting speed is 0.9m/min.Centerline carbon segregation ratio along the bloom longitudinal section is 0.95?1.02,and macro segregation is obviously improved.