Mold-slag friction and fracture may cause heat-transfer variations in continuous casting, which leads to steel shell temperature and stress variations, resulting in surface cracks Analytical transient models of liquid slag flow and solid slag stress have been coupled with a finite-difference model of heat transfer in the mold, gap, and steel shell to predict transient shear stress, friction, slip, and fracture of the slag layers The models are validated by comparing with numerical models and plant measurements of mold friction Using reported slag-fracture strength and time-temperature-transformation (TTT) diagrams, the models are applied to study the effect of casting speed and mold-powder viscosity properties on slag-layer behavior between the oscillating mold wall and the solidifying steel shell The study finds that liquid-slag lubrication would produce negligible stresses A lower mold-slag consumption rate leads to high solid friction and results in solid-slag-layer fracture and movement below a critical value Crystalline slag tends to fracture near the meniscus and glassy slag tends to fracture near the mold exit A medium casting speed may be the safest to avoid slag fracture, due to its having the lowest critical lubrication consumption rate The high measured friction force in operating casters could be due to three sources: an intermittent moving solid slag layer, excessive mold taper, or mold misalignment

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Modeling transient slag-layer phenomena in the shell/mold gap in continuous casting of steel

The flow of melted mold powder into the interfacial gap between the strand and the mold wall is important for productivity and quality in continuous cast slabs. Some of the mold slag (flux) consumption provides true lubrication, while much of the rest is trapped in the oscillation marks on the slab surface. This work presents measurements of powder consumption from extensive careful plant trials on ultra-low carbon steels, and a new, simple, semi-empirical model to predict slag consumption. The model predicts “lubrication consumption” by deducting the slag carried in the oscillation marks from the measured total. The oscillation mark shape is estimated from a theoretical analysis of equilibrium meniscus shape, which is based on metallographic analysis of many hook and oscillation mark shapes. The model demonstrates that the fraction consumed in the oscillation marks decreases with increasing casting speed, because the oscillation mark depth depends more on casting speed than on mold oscillation conditions. The model is validated by successful prediction of known trends of oscillation mark depth and mold powder consumption with changing various operation parameters. The model provides new insight into mold lubrication phenomena, which is important for extending casting operation to higher speeds and new lubrication regimes.

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Measurement and Prediction of Lubrication, Powder Consumption, and Oscillation Mark Profiles in Ultra-low Carbon Steel Slabs

The behavior of the slag layer between the oscillating mold wall, the slag rim, the slag/liquid steel interface, and the solidifying steel shell, is of immense importance for the surface quality of continuous-cast steel. A computational model of the meniscus region has been developed, that includes transient heat transfer, multi-phase fluid flow, solidification of the slag, and movement of the mold during an oscillation cycle. First, the model is applied to a lab experiment done with a “mold simulator” to verify the transient temperature-field predictions. Next, the model is verified by matching with available literature and plant measurements of slag consumption. A reasonable agreement has been observed for both temperature and flow-field. The predictions show that transient temperature behavior depends on the location of the thermocouple during the oscillation relative to the meniscus. During an oscillation cycle, heat transfer variations in a laboratory frame of reference are more severe than experienced by the moving mold thermocouples, and the local heat transfer rate is increased greatly when steel overflows the meniscus. Finally, the model is applied to conduct a parametric study on the effect of casting speed, stroke, frequency, and modification ratio on slag consumption. Slag consumption per unit area increases with increase of stroke and modification ratio, and decreases with increase of casting speed while the relation with frequency is not straightforward. The match between model predictions and literature trends suggests that this methodology can be used for further investigations.

Transient Thermo-fluid Model of Meniscus Behavior and Slag Consumption in Steel Continuous Casting

Thin slab direct rolling (TSDR) of microalloyed steels belongs to a new generation of thermomechanical treatments. TSDR shows some metallurgical peculiarities that significantly differentiate this process from the traditional route. This paper reviews some of these singularities, considering in a more detail those processing aspects which need to be optimized in order to obtain final homogeneous microstructures that bring about the strength and toughness levels required for specific applications.

Thin Slab Direct Rolling of Microalloyed Steels

As potential substitutes for fluorides, the influences of basicity and B2O3 content on viscosity, melting and crystallization behaviors of low fluorine mold fluxes for casting medium carbon steels were investigated by using Brookfield viscometer and Single Hot Thermocouple Technique in this study. Results suggested that, the break temperature, crystallization temperature and critical cooling rate of low fluorine mold fluxes were increased with the increase of basicity; while they were decreased with the further addition of B2O3. Meantime, the viscosity and melting temperature range were first attenuated, and then increased greatly with the increase of basicity; however, they would tend to be reduced with the addition of B2O3 content. Also, it was found that the viscosity of mold flux is not only decided by the degree of polymerization of silicate structure, but also greatly affected by its crystallization behavior.

Effect of basicity and B 2 O 3 on viscosity, melting and crystallization behaviors of low fluorine mold fluxes for casting medium carbon steels

The mechanism of formation of longitudinal facial cracks during continuous casting of peritectic steels has been studied using a process modeling approach. First, the delta-to-gamma transformation was modeled, assuming carbon diffusion control. The problem of the moving boundary (delta/gamma interface) was solved by employing a one-dimensional finite-difference method. Second, a heat-transfer model for continuous casting of steel was developed and combined with the phase transformation model. Three heat-flux conditions (i.e., low, medium, and high) were obtained from literature data and assumed as the thermal boundary condition. The delta-to-gamma transformation rate was evaluated as a function of the heat-flux conditions. Finally, the results of the coupled model were adopted to calculate the stresses in the solid shell applying the commercial finite-element program, ABAQUS. The results showed that the transformation from delta to gamma is comparatively rapid due to the high diffusivity of carbon in this temperature range. The variation of the heat flux at the meniscus results in large changes of the transformation rate in the meniscus region. Based on the results of the stress calculations, it was concluded that, in order to generate a longitudinal crack on the solid shell surface, not only the tensile stress caused by rapid transformation (i.e., rapid cooling) but also the presence of hot spots is required. A threshold value of approximately 16 pct was obtained for the retardation of shell growth required to generate longitudinal cracks.

J. K. Brimacombe

Papers

Modeling the formation of longitudinal facial cracks during continuous casting of hypoperitectic steel