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

Abstract

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.KEY WORDS: continuous casting; mold powder; slag; flux; lubrication; oscillation marks; mold oscillation; stroke; frequency; modification ratio; negative strip time; positive strip time; casting speed; meniscus; models; plant experiments.ISIJ International, Vol. 46 (2006), No. 11, pp. 1635No. 11, pp. -1644 slag as shown in Fig. 1(a). The solid slag layers are glassy, crystalline, or mixtures of both 14) depending on the slag composition and local cooling rate history. [15][16][17] Several previous models of heat transfer and lubrication assume that the gap has a uniform thickness. [18][19][20] However, the real strand surface contains periodic transverse depressions called oscillation marks (OM) in Fig. 1(b). The volume of these depressions generally consumes a significant amount of slag, which greatly affects lubrication and leads to nonuniform heat transfer in the mold. 21) Thus, quantifying the oscillation mark shape is a necessary prerequisite for the prediction of lubrication and heat transfer in the mold.In this study, mold powder consumption was measured during casting trials conducted at POSCO Gwangyang Works for several different oscillation conditions and casting speeds. The measured consumptions were divided into two components: 1) the slag entrapped in the oscillation marks and 2) the remaining flux, which provides lubrication to prevent sticking, as shown in Fig. 1(b). This "lubrication consumption" is a more realistic concept to represent lubrication than total mold powder consumption.17) The oscillation mark shape is calculated by combining measured OM depths with a realistic profile based on fundamentals. 22) Correlations are found for OM depth, corresponding OM consumption, l...

Explicit Modelling of Slag Infiltration and Shell Formation during Mould Oscillation in Continuous Casting

“…This fact explains why slag consumption has been found to increase with increasing positive strip time in industrial practice. 32) The results also indicate that positive slag infiltration occurs throughout the oscillation cycle but decreases in the second half of the positive strip due to the decrease in liquid film thickness as the solid film recovers. No cases of negative consumption rates were identified in the cycle for the casting parameters studied here.…”

“…12 It was also found that the liquid slag thickness increased slightly when descending the mould, which is in agreement with the predictions of Okazawa et al 37) The most remarkable result is the replication of the effect of casting speed on consumption, since it is well known from plant measurements that increases in v c lead to decreases in consumption. However, with exception of the numerical models based in plant data fitting by Meng 16) and Shin,32) or the physical model by Kajitani, 31) this relationship has not been reproduced even qualitatively by previous models.…”

“…It is estimated that Q s for a width of 1.88 m (used in present study) would be 10% higher than that for average slabwidths (ϳ1.4 m). 15) (iii) Values due to Shin et al 32) would be expected to be low since they refer to a slag viscosity of 3.2 dPs (cf. 1.115 dPs in present study).…”

Transient Thermo-fluid and Solidification Behaviors in Continuous Casting Mold: Evolution Phenomena

“…Previous works 24,26,36) have shed light on the effect of casting speed on fluid flow, heat transfer, and solidification in the mold, but the continuous and associated phenomena are not described, especially at the initial stage of casting process. Figure 12 shows the variations of meniscus level and liquid pool depth measured at a location 40 mm from the mold during the casting process.…”

“…The predicted flux layer thicknesses were matched with experimental measurements, and a large recirculation zone in the liquid slag pool was observed by these model calculations. Shin et al 26) presented a semi-empirical model, which divided total powder consumption into two components: the lubrication consumption and the flux carried within the volume of oscillation marks, to predict mold lubrication phenomena based on the measurements from extensive plant trials on ultra-low carbon steels. This work was validated by the known trends of oscillation mark depth and slag consumption with various operation parameters, such as mold powder characteristics, casting speed and mold oscillation conditions.…”