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An opposite combined vertical linear electromagnetic stirring (CV-LEMS) was proposed, which is applied in the final solidification zone of bloom continuous casting. The melt flow, heat transfer, and solidification under CV-LEMS were investigated by establishing a three-dimensional numerical simulation model and a pilot continuous casting simulation experiment and compared with the conventional rotary electromagnetic stirring (REMS). The results show that a longitudinally symmetric linear magnetic field is formed in the liquid core of the bloom by applying CV-LEMS, which induces a strong longitudinal circulation flow both on the inner arc side and the outer arc side in the liquid core of the bloom. The height of the melt longitudinal effective mixing range under CV-LEMS reaches 0.9 m, which is greater than that of the REMS and makes up for the deficiency of REMS sensitivity to the position of the final solidification zone. CV-LEMS strongly promotes the mixing of upper melt with high temperature and the lower part melt with low temperature in the liquid core, improves the uniformity of melt temperature distribution and significantly increases the melt temperature near the solidification front, and the width of the liquid core increases by 4.2 mm at maximum. This shows that the appliction of CV-LEMS is more helpful to strengthen the feeding effect of the upper melt to the solidification shrinkage of the lower melt than the conventional REMS and inhibits the formation of porosity, shrinkage cavity and crack defects in the center of the bloom.

The formation and propagation of the popular off-corner subsurface cracks in bloom continuous casting were investigated through thermo-mechanical analysis using three coupled thermo-mechanical models. A two-dimensional thermo-elasto-visco-plastic finite element model was developed to predict the mould gap evolution, temperature profiles and deformation behavior of the solidified shell in the mould region. Then, a three-dimensional model was adopted to calculate the shell growth, temperature history and the development of stresses and strains of the shell in the following secondary cooling zones. Finally, another three-dimensional model was used to analyze the stress distributions in the straightening region. The results showed that the off-corner cracks in the shell originated from the mould owing to the tensile strain developed in the crack sensitive regions of the solidification front, and they could be driven deeper by the possible severe surface temperature rebound and the extensive tensile stress in the secondary cooling zone, especially upon the straightening operation of the bloom casting. It is revealed that more homogenous shell temperature and thickness can be obtained through optimization of mould corner radius, casting speed and secondary cooling scheme, which help to decrease stress and strain concentration and therefore prevent the initiation of the cracks.

The dynamic soft reduction technology for 360 mm×450 mm bloom casting and its application results at Panzhihua Iron and Steel Co （PISCO） were presented. In order to analyze the influence of the soft reduction on internal defects of bloom, such as center porosity and central segregation, experiment for contrasting process with or without soft reduction for bloom was conducted. The operation shows that after applying the soft reduction of alloy steel 45CrMnMo bloom, the ratio of center porosity whose defect level is less than or equal to 1.0 increased from 66.67% to 85.71%, ratio of central segregation whose defect level is below 1.0 increased from 94.44% to 100%, ratio of central shrinkage cavity which is free increased from 88.89% to 96.42%, and central segregation index of carbon decreased from mean value of 1.15 to 1.05.

There are significant effects of process parameters on internal qualities of bloom, and these process parameters are as follows. position and reduction amount, reduction distribution, reduction rate, and so on. Developing a control model is the key to apply soft reduction technology successfully. As the research object, 360 mm ×450 mm bloom caster in PISCO （Panzhihua Iron and Steel Co. ） has been studied, and the research method for control model of dynamic soft reduction has been proposed. On the basis of solidification and heat transfer model, the position of soft reduction and reduction distribution of each frame are determined according to the bloom temperature distribution and solid fraction in bloom center calculated. Production practice shows that the ratio of center porosity which is less than or equal to 1.0, increased to 97.27%, ratio of central segregation which is less than or equal to 0.5, increased to 80.91%, and ratio of central carbon segregation index which is more than or equal to 1.10, decreased to 4% with the applying model of dynamic soft reduction.

Center porosity and centerline segregation in continuously cast bloom can be minimized by the well-known method of dynamic soft reduction. Metallurgical results of soft reduction previously employed in continuous bloom casting for heavy rail steel in Panzhihua Iron and Steel Group were not fully achieved because of the improper soft reduction process. Therefore, experiments for optimizing the process parameters of soft reduction for bloom were carried out. The results show that the proportion of the center porosity, which is less than 1.0, increases from 28.41% to 99.81%, while the proportion of the centerline segregation class increases from 40.91% to 100%, and the proportion of the central cavity increases from 92.05% to 100%, whereas the center carbon segregation index decreases from 1.17 to 1.05. The internal quality and the mechanical performance of the rails produced from continuously cast blooms meet the requirement of high-speed tracks of 350 km/h.

In order to save equipment investment, the commercial continuous caster is widely used to produce diverse round bloom with a compatible final electromagnetic stirring (F-EMS) device, and thus, the eccentric final electromagnetic stirring is commonly observed during the continuous casting process. In this study, based on our previously proposed three-phase solidification model for continuous casting process, the effect of eccentric F-EMS on the magnetic field, solidification structure, and macrosegregation are fully investigated in the 42CrMoS4 steel continuously cast round bloom with diameter of 350, 450, 550, and 650 mm, respectively. The eccentric degrees of geometric, electromagnetic force, velocity, and solute concentration are introduced to clarify the effect of eccentric F-EMS on electromagnetic force, stirring velocity, and macrosegregation. The results indicate the eccentric round bloom solidification structure is mainly caused by the grain sedimentation during the solidification process, which is benefit for the equiaxed grains accumulation at the front of columnar grains and leads the columnar to equiaxed transition (CET) later the inner arc side than on the external arc side of round bloom. But the eccentric solidification structure of round bloom is almost not affected by the eccentric F-EMS, because the location of F-EMS is at 15.0 m from the meniscus, and the CET has occurred in the round bloom before the application of F-EMS. Moreover, the eccentric F-EMS has an influence on the electromagnetic field, stirring velocity, and solute concentration in the round bloom. Both the electromagnetic force and stirring velocity on the inner arc side are lower than those on the external arc side with eccentric F-EMS. The negative segregation difference near the center of the round bloom becomes more pronounced, and the center positive segregation also increases as the geometric eccentricity increases. When the geometric eccentric degrees are 15.4, 30.8, and 46.2 pct, the eccentric degrees of electromagnetic force are, respectively, 44.7, 54.1, and 64.1 pct, the eccentric degrees of maximum tangential velocity are, respectively, 20.0, 33.3, and 50.0 pct, the eccentric degrees of negative segregation induced by the F-MES are, respectively, 1.1, 2.1, and 3.1 pct, and the center segregation ratios are, respectively, 1.08, 1.09, and 1.10.

The CCM productivity and the quality of steel blanks largely depend on how well the mold meets all technological requirements, the main of which is to ensure the necessary uniformity and intensity of heat removal from the ingot shell without collapsing under the exposure to heat from the steel melt, ferrostatic pressure of the liquid phase, and thermal cracking. Mold tubes are the main replaceable tool of the molds of the modern billet and bloom casters. They represent high-tech products manufactured with high precision, and contain a special wear-resistant coating applied to their working surface.

In the continuous casting of special steel blooms, low casting speeds result in slow renewal of the molten steel surface in the mold, adversely affecting mold flux melting and liquid slag layer supply, which may lead to surface cracks, slag entrapment, and breakout incidents. To optimize the flow and heat transfer behavior in the mold, a three-dimensional numerical model was developed based on the VOF multiphase flow model, k−ϵ RNG turbulence model, and DPM discrete phase model, employing the finite volume method with SIMPLEC algorithm for solution. The effects of casting speed, argon injection rate, and mold flux properties were systematically investigated. Simulation results demonstrate that when casting speed increases from 0.35 m·min−1 to 0.75 m·min−1, the jet penetration depth increases by 200 mm and meniscus velocity rises by 0.014 m·s−1. Increasing argon flow rate from 0.50 L·min−1 to 1.00 L·min−1 leads to 350 mm deeper bubble penetration, 10 mm reduction in jet penetration depth, 0.002 m·s−1 increase in meniscus velocity, and decreased meniscus temperature due to bubble cooling. When mold flux viscosity increases from 0.2 Pa·s to 0.6 Pa·s, the average liquid slag velocity decreases by 0.006 m·s−1 with a maximum temperature drop of 10 K. Increasing density from 2484 kg·m−3 to 2884 kg·m−3 results in 0.005 m·s−1 higher slag velocity and average 8 K temperature reduction. Comprehensive analysis indicates that optimal operational parameters are casting speed 0.35–0.45 m·min−1, argon flow ≤ 0.50 L·min−1, mold flux viscosity 0.2–0.4 Pa·s, and density 2484–2684 kg·m−3. These conditions ensure more stable flow and heat transfer characteristics, effectively reducing slab defects and improving casting process stability.

Three-dimensional characterization of defects in continuous casting blooms of a heavy rail steel with different electromagnetic stirring intensities at the solidification end, also named final electromagnetic stirring, was performed employing a laboratory-based X-ray computed tomography to evaluate the effect of final electromagnetic stirring on the internal quality of blooms. The three-dimensional distribution and morphology of defects including porosities and oxide inclusions in blooms were characterized. The amount, size and shape complexity of defects increased gradually from the loose-side surface to the center of blooms. The total volume fraction of defects was 5265 and 3942 ppm when the current of final electromagnetic stirring was 200 and 300 A, respectively. The sphericity of defects varied from 0.1 to 0.75, and the equivalent spherical diameter varied between 20 and 450 μm. Most defects with a sphericity > 0.6 were nearly spherical and were assumed to be oxide inclusions. The volume fraction of both porosities and inclusions was small in the chilled layer and the columnar crystal zone and then increased rapidly toward the equiaxed zone. Increasing the current of F-EMS from 200 to 300 A significantly decreased the volume fraction of porosities in the center of the bloom from 2906 to 1873 ppm. It could also decrease the volume fraction and average diameter of oxide inclusions in the bloom by reducing the number of large inclusions.

In this paper, a mold electromagnetic stirring (M-EMS) model was established to investigate the behavior of M-EMS for round bloom castings under different conditions, and an electromagnetic-flow-heat transfer-solidification coupling model was established to explore the problem of eccentric stirring for various formats of round blooms. The results show that the magnetic flux density decreased with the increase in the current frequency, but the electromagnetic torque increases first and then decreases with it, and the same structure of M-EMS for round blooms has the same optimal current frequency at any current intensity. The electromagnetic torque and electromagnetic force both increase as a quadratic function of the current intensity, and the electromagnetic torque, which drives the steel flow, can directly characterize the real M-EMS performance. The mold copper tube has a significant magnetic shielding effect on M-EMS, the stirring intensity decreases rapidly as the tube thickness increases, and the optimal stirring frequency decreases as well. In fact, the deviation between the stirrer center and the geometric center of the strand can result in the eccentric stirring phenomenon. When blooms with a section size of Φ350 mm are produced by Φ650 mm SMS-Concast casting machine, the upper region of the inner arc side and the lower region of the outer arc side are subject to a stronger washing effect, which makes the temperature of the inner and outer arcs show alternating differences. The jet flow from the five-port nozzle can suppress the difference in initial solidification symmetry between the inner and outer arcs of round blooms caused by eccentric stirring. This paper reveals the magnetic shielding effect of the mold copper tube and the magnetic field loss of the air between the stirrer and the inner and outer arcs of the copper, which lead to the stirring intensity and the eccentric stirring phenomenon.