The Effect of Mould Flux Properties on Thermo-mechanical Behaviour during Billet Continuous Casting

Abstract

During continuous casting mould powder forms a pool of liquid flux which infiltrates into the solidifying shell/mould gap and forms a flux film containing liquid and/or solid layers. Subsequent crystallisation of the film results in the formation of an air gap at the mould wall. An air gap can also be formed by the shrinkage of the solidified shell. In practise, no air gap is formed by shell shrinkage in the upper mould since molten flux will flow immediately into and fill any gap formed. However, in the lower mould, if the shell temperature falls below the break temperature of the flux there is no liquid to fill any gap formed by shell shrinkage and hence an air gap can form. A model was developed which determines the effect of the various layers (including the air gap) on the horizontal heat transfer between shell and mould. A fully coupled, heat transfer/stress analysis was used to calculate: (i) the thickness of the various layers of the flux film; (ii) horizontal heat flux; and (iii) the resultant stress in the solidified shell.The model predicted that: (a) the hoop stress increased as the interfacial thermal resistance decreases (i.e. the heat flux increases); (b) the maximum hoop stress at the exit decreased with increasing flux film thickness; (c) the ferro-static pressure tended to hinder the formation of an air gap at mid-face positions whereas in the corners the thicker shell tended to counteract the ferro-static pressure resulting in air gap formation; (d) the thickness of the liquid film at various positions in the mould was computed for fluxes with optimum break temperatures to determine if they provided liquid lubrication throughout the mould; and (e) the maximum stress occurred in the corner when casting with mould fluxes but at the mid-face when using oil.KEY WORDS: continuous casting; mould flux; heat transfer; finite element; stress analysis; billet; oil casting. 95© 2007 ISIJ metal. The molten flux forms a liquid pool on the top of the mould, infiltrating the mould/shell gap and partially solidifies near the mould. The flux film formed initially is glassy in nature but in time, crystallisation occurs. 17,18) Since the density of the crystalline phase is greater than that of the glass phase, crystallisation results in shrinkage of the solid flux layer and the formation of an air gap at the copper mould/flux interface. Thus the crystal/glass distribution has a significant effect on the horizontal heat transfer.A second type of air gap can also be formed by shell shrinkage. However, in the upper part of the mould liquid flux will flow into and fill any gap formed. In the lower part of the mould if the shell temperature is lower than the break temperature of the flux there will be no liquid to fill the gap (due to shell shrinkage or inadequate mould taper). Consequently an air gap can be formed under these circumstances. In this paper the air gap formed by crystallisation of the flux film is referred to as an interfacial contact resistance, whereas the air gap formed by shell shrinkage is referre...

Mathematical Modeling of Thermal Resistances of Mold Flux and Air Gap in Continuous Casting Mold Based on an Inverse Problem

“…At the exit of the mold, the thickness of the air gap is approximately 0.5-0.6 mm, and the thermal resistance is about 7.5 × 10 − 4 − 9 × 10 − 4 m 2 ·K/W. These results are in accordance with other published research 11) and verify the reliability of the model. As the heat transfer coefficients of the air gap and solid slag are relatively small, they severely hinder heat transfer lubrication between the mold and slab; therefore, it is of great significance to predict the non-uniform distribution of the air gap and slag films exactly.…”

Heat-Transfer Phenomena Across Mold Flux by Using the Inferred Emitter Technique

“…[2] Thus, the mold fluxes strongly affect the final surface quality of the slab and involve with the formation of casting defects, such as longitudinal cracking, star cracking, oscillation marks, mold wear, and breakouts. [3] During continuous casting, the liquid mold flux infiltrates into the shell/mold gap and solidifies on the water-cooled copper mold forming complex amorphous and crystalline structures due to the large temperature gradient distribution between the steel shell and copper mold. [4] The mold flux is usually composed of two to three layers, depending upon the cooling rate and its chemical compositions, i.e., a liquid layer next to the steel shell, a solid glassy layer against the mold wall, and a crystallized layer in between, or sometimes just a liquid and a solid crystalline layer.…”

Study of Shell-Mold Thermal Resistance: Laboratory Measurements, Estimation from Compact Strip Production Plant Data, and Observation of Simulated Flux-Mold Interface

“…[7][8][9] Normally, it is stated that the gap arises because the crystalline phase has a higher density than the glass, and therefore crystallization results in shrinkage of the solid flux layer. [10,11] In an investigation carried out to determine the surface roughness of solidified mold fluxes, it was found that this was in the range of 10 to 30 lm, when the crystalline phase precipitated; [12] this surface roughness was claimed to agree with the calculated thickness of the air gap assumed to form between the mold and the solidified slag layer. Furthermore, it was disclosed that the surface roughness of mold powder slags for casting medium-carbon steels is larger than that of mold slags used for low-carbon steels.…”