Axial heat-flux profiles have been determined quantitatively from temperature measurements conducted on a slab mold under routine operating conditions. As in earlier studies, the heat flux was observed to have a maximum value at the meniscus and to decline with increasing distance down the mold. The mold heat flux increased with increasing casting speed and was greater with a mold powder having lower viscosity and melting point being applied as lubricant. The heat extraction was largest while casting 0.29 pet carbon steel and least for a 0.09 pet carbon grade; reducing the depth of the submerged entry nozzle increased the heat flux slightly in the upper region of the mold. Most significant was the higher heat flux observed at the meniscus of the outside-radius face, attributable to the locally greater copper plate thickness compared to that of the opposite broad face. All of the measurements can be explained straightforwardly by heat flow in the vicinity of the meniscus and the resulting behavior of the so-called slag rim adjacent to the mold wall. It is postulated that the difference in copper plate thickness between the two broad faces at the meniscus causes the slag rim to be smaller on the outside-radius face which gives rise to shallower oscillation marks, as observed, higher heat transfer, and a slightly thicker solid shell. The dissimilar behavior has implications for quality because the inside-radius shell, experiencing reduced heat extraction, cools and shrinks less than the outside-radius shell. Thus, for a given end-plate taper, the narrow face of the slab adjacent to the inside radius can push against the end plate, accelerating copper wear, and, owing to squeezing of the broad face, cause an off-corner depression and subsurface crack toward the mold exit. If this is correct, maintenance of the same copper plate thickness at the meniscus is fundamental to preventing such an occurrence. Moreover, adjustment of the heat extraction at the meniscus should be achievable by changing copper plate thickness, mold coating thickness/conductivity, cooling water velocity, cooling channel configuration, and mold flux composition for a given steel grade.

Mold behavior and its influence on quality in the continuous casting of steel slabs: Part II. Mold heat transfer, mold flux behavior, formation of oscillation marks, longitudinal off-corner depressions, and subsurface cracks

This paper reviews the current state of the art in the application of electromagnetic forces to control fluid flow to improve quality in continuous casting of steel slabs. Many product defects are controlled by flow-related phenomena in the mold region, such as slag entrapment due to excessive surface velocity and level fluctuations, meniscus hook defects due to insufficient transport of flow and superheat to the meniscus region, and particle entrapment into the solidification front, which depends on transverse flow across the dendritic interface. Fluid flow also affects heat transfer, solidification, and solute transport, which greatly affect grain structure and internal quality of final steel products. Various electromagnetic systems can affect flow, including static magnetic fields and traveling fields which actively accelerate, slow down, or stir the flow in the mold or strand regions. Optimal electromagnetic effects to control flow depends greatly on the caster geometry and other operating conditions. Previous works on how to operate electromagnetic systems to reduce defects are discussed based on results from plant experiments, validated computational models, and lab scale model experiments.

Electromagnetic Forces in Continuous Casting of Steel Slabs

A 3D numerical model coupling the electromagnetic field, fluid flow, heat transfer and solidification phenomena was developed to simulate the 160 mm × 160 mm billet continuous casting process of SWRT82B steel with dual electromagnetic stirrings of mold and the final solidification. The columnar region was treated as a porous zone and a generalized approach was used to calculate permeability, while for the equiaxed zone, a variable apparent viscosity model was applied to simulate the fluid flow at the initial growing stage of equiaxed dendrite. The model was validated by the measured data of magnetic induction intensity in the stirrer center and strand surface temperature. The results show that it is indispensable to consider the solidification phenomenon in the mold zone with EMS, as the fluid flow mechanism near the meniscus is quite different. With the current intensity of M-EMS and F-EMS increasing, the tangential velocity of molten steel rises and the central temperature decreases. As the F-EMS is applied, the nature convection mechanism is destroyed and the temperature is promoted to distribute evenly in the mushy zone. Moreover, the results show that the coherent solid fraction has a significant effect on the temperature distribution and the fluid flow in F-EMS zone.

Flow and Solidification in Billet Continuous Casting Machine with Dual Electromagnetic Stirrings of Mold and the Final Solidification

Refinement of as-cast structures is one of the most effective approaches to improve mechanical properties, formability, and surface quality of steel castings and ingots. In the past few decades, addition of rare earths (REs), lanthanum and cerium in particular, has been considered as a practical and effective method to refine the as-cast steels. However, previous reports contained inconsistent, sometime even contradictory, results. This review summaries the major published results on investigation of the roles of lanthanum or/and cerium in various steels, provides reviews on the similarity and difference of previous studies, and clarifies the inconsistent results. The proposed mechanisms of grain refinement by the addition of lanthanum or/and cerium are also reviewed. It is concluded that the grain refinement of steels by RE additions is attributed to either heterogeneous nucleation on the in-situ formed RE inclusions, a solute effect, or the combined effect of both. The models/theories for evaluation of heterogeneous nucleation potency and for solute effect on grain refinement of cast metals are also briefly summarized.

Roles of Lanthanum and Cerium in Grain Refinement of Steels during Solidification

Influence of electromagnetic stirrer position on fluid flow and solidification in continuous casting mold

Chemical and metallographic studies have been made of the “white band” (a zone of negative segregation) formed by the electromagnetic stirring of 127 mm (5 inch) square strand-cast steel billets. Three stirrer locations were studied. At all locations, the white band was found to consist of a zone of negative segregation corresponding to the position of the solid-liquid interface during stirring, followed by a zone of positive segregation related to the cessation of stirring. Metallographic studies showed the curvature of columnar dendrites within the white band and the presence of a transition zone of nondirectional growth between the white band and the equiaxed core. A mechanism for the formation of the segregated zones at the white band is presented, along with theoretical calculations. Agreement between the calculations and experimental results was good. The role of electromagnetic stirring in the formation of the white band and in the associated columnar-equiaxed transition is discussed.

G. D. Rogers

Papers

Structural Effects and Band Segregate Formation during the Electromagnetic Stirring of Strand-Cast Steel