Measurements, simulations, and analyses of instantaneous heat fluxes from solidifying steels to the surfaces of twin roll casters and of aluminum to plasma-coated metal substrates

Abstract

Using implanted thermocouples, and an inverse heat-transfer technique, heat fluxes and associated heat-transfer coefficients during the solidification of steel in a pilot scale 0.6-m-diameter twin roll caster, whose copper contact surfaces had been treated with a propriety coating, were measured. It was found that heat fluxes during initial contact of liquid steel with the rolls were low, rising to maximum values of about 5 to 6 MW per square meter halfway down the sump of liquid steel, but then diminishing toward zero as the strip approached the roll nip. These results corresponded to roll speeds of some 7 m/min, strip thicknesses of 7 mm, and a roll separating force of 20 kN. For higher speeds and thinner strip, a secondary peak in the heat flux was observed. Associated microstructures revealed acicular ferrite, large prior austenite grains, and secondary dendrite arm spacings in keeping with measurements.In parallel experiments simulating a single belt horizontal caster, heat fluxes from strips of various aluminum alloys to coated and uncoated steel and copper substrates were measured. Under these conditions, peak heat fluxes were recorded during the period of initial contact, and depending on the coating characteristics, these reduced to a lower plateau before declining, or continuously decreased toward zero, corresponding to complete solidification of the strip. A theoretical analysis of the maximum heat-transfer rates that can be expected given perfect thermal contact of metal with the rolls, and its moderations by gas films, and substrate coatings illustrates the dominant role of the gas film and the need for dynamic heat flux measurements for quantitative modeling of fluid flows and solidification phenomena in thin strip casting operations. A model for air gap formation is proposed, based on viscous laminar flows within the gas films. Predicted thicknesses are in reasonable accord with those deduced on the basis of heat flux measurements.

Effect of Process Parameters, Casting Thickness, and Alloys on the Interfacial Heat-Transfer Coefficient in the High-Pressure Die-Casting Process

“…The IHTC was usually determined by using an inverse approach, which aims at minimizing the difference between the measured and simulated temperatures. For the sand and gravity die-casting process, extensive work has been performed for the determination of the IHTC and many factors were found to have an influence on the value of the IHTC, including the die coatings (material and thickness), [4][5][6][7][8][9][10][11][12][13] the die material and preheating, [1,[14][15][16][17] the casting size, [15,18] the application of pressure, [19][20][21][22][23] the orientation of the casting with respect to gravity, [1,15,24] the alloy composition, [14][15][16]25,26] and the surface roughness of the die. [7,14,[27][28][29][30] The heat transfer in HPDC process is different from most of the conventional casting processes.…”

Progress in Strip Casting Technologies for Steel; Technical Developments

“…Heat fluxes were determined from temperature-time data measured by thermocouples embedded in the rolls, using the Inverse Heat Conduction Problem (IHCP) method. 47) At slow casting speed (~4 m/min), there was only a single peak in the heat flux vs. contact time plot, whereas at higher casting speeds (6-7.5 m/min), two peaks could be identified, both of which were significantly greater than the peak from the slower casting speed. This was explained by the mechanism proposed by Guthrie, Isac, Kim and Tavares, which took into account various phenomena such as thermal resistance from an air-film entrapped at the melt/mould interface, the presence of a thin insulating coating on the rolls causing poor initial melt/mould contact, thermal expansion of casting rolls, increasing metallostatic pressure further down the sump, diminishing air-film thickness due to increasing contact pressure, and the effect of rolling and solid shell shrinkage 47) (See Fig.…”

“…49) Numerical modeling has been heavily used, coupled with experiments, to understand the correlation between different operating parameters and the shape and stability of the meniscus. 47,50) Figure 18 shows the result from one such study carried out for the MMPC-HSBC delivery system, where the effects of a back-wall gap of 0.9 mm, and various casting speeds, on meniscus shape was investigated.…”

Numerical Simulation of Thermal Field of Work Roll during Top Side-pouring Twin-roll Casting of Steel

“…With inverse heat-transfer analysis for twin-roll caster, Guthrie et al 7) confirmed that the heat transfer in axial and circumferential directions of the roll could be ignored compared to the radial heat transfer. Consequently, a two-dimensional model was established and the periodic boundary conditions were set for the roll to investigate its thermal field and the corresponding influencing factors.…”