Reduction in macrosegregation on 380 mm6490 mm bloom caster equipped combination MzF EMS by optimising casting speed H. Sun* 1,2 , L. Li 1 , X. Cheng 2 , W. Qiu 2 , Z. Liu 2 and L. Zeng 2 Based on the plant trials and the developed coupled mathematical model of electromagnetism, turbulent fluid flow, heat and solute transport, the effect of F-EMS position on the degree of bloom macrosegregation cast with combination stirring (MzF-EMS) has been evaluated and compared for medium carbon steel, 40Cr, and high carbon steel, GCr15. The optimum F-EMS position is mainly related to the solidification characteristic of the molten steel in the liquid pool that depends on the average natural convection velocity and solidification time during the final solidification stage. For a given steel grade and a given F-EMS position, the trend of centre porosity and shrinkage cavity variations is to first decrease and then increase with increasing casting speed. The optimal solidification ratio at the F-EMS centre is increased as the carbon content increases due to the increasing solidification time, where the optimal solidification ratios for GCr15 and 40Cr are 72 and 68% respectively. Keywords: Bloom continuous casting, Macrosegregation, F-EMS, Implemented position, Combination stirring Introduction Macrosegregation, caused by solute redistribution and macroscopic transportation of the solute rich liquid ahead of the solidification front, continues to be one of the major internal quality problems in the continuous casting (CC) process, especially for large section sizes or high carbon steel blooms. The resulting chemical anisotropy in cast products may lead to a significant variation of physical and mechanical properties for the final steel products due to the persistence of heavily segregated regions during the subsequent hot rolling or forging. To control the degree of macrosegregation during the CC process, technologies, such as low tem- perature casting, 1,2 electromagnetic stirring (EMS), 3–6 soft reduction 7,8 and intensive cooling 9,10 at the final solidification stage have been proposed. Among these means, the EMS technology has been widely applied and proven to be one effective way to reduce the macrosegregation. 11,12 It is well known that the EMS technology can be classified by the installation location of EMS in the casters: in the mould region (M-EMS), in the secondary cooling zone (S-EMS) and in the final solidification zone (F-EMS). At the present, M-EMS, which is beneficial to the superheat dissipation in the mould region, is commonly used for the high carbon steel blooms and billets. 3,13,14 To further decrease the macrosegregation of cast products, combination stirring technology, such as M-EMS plus S-EMS and/or F-EMS, has been employed in the steel mills. 11,15 For the S-EMS, its effect on the macrosegregation reduction can be obtained by optimis- ing the operation parameters of EMS including the current and frequency, but white band defect and the skin effect should be avoided. 16–18 In addition to the need to avoid these negative metallurgical effects, F-EMS is also very dependent on the solidification behaviour of the steel grade and the ratio between thicknesses of the shell and liquid pool in the stirring region. 4,9–11 However, few papers 19–21 have reported on the evolution of fluid flow, heat and solute transport behaviour during the final solidification stage in the CC process and its effect on the optimisation of stirring condition and F-EMS position. Moreover, the previous empirical results on the opti- mum F-EMS position investigated using the ingot tests 22,23 or plant trials 9,11 may be not appropriate for other casting cases. To reveal and evaluate the macroscopic transport phenomena in the final solidification region, in this work, a coupled mathematical model of electromagnet- ism, turbulent fluid flow, heat and solute transport using 3D plus 2D hybrid modelling method was employed, which has been developed and proven to be suitable for the CC processes. 24 Combined with the numerical 1 School of Mechanical & Automotive Engineering, South China University of Technology, Guangzhou 510640, China 2 Department of Steel-making, Baosteel Group Guangdong Shaoguan Iron & Steel Co., Ltd., Shaoguan 512123, China *Corresponding author, email sunmyseven@126.com ß 2014 Institute of Materials, Minerals and Mining Published by Maney on behalf of the Institute Received 26 August 2014; accepted 20 October 2014 DOI 10.1179/1743281214Y.0000000247 Ironmaking and Steelmaking 2014 VOL 000 NO 000 1