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

Abstract

The slag film that forms between the shell and mold in steel continuous casting is key in regulating the heat transfer between them. Generally, the mechanisms proposed are related to the phenomena associated with the formation of crystals in the solid layer of the film, such as the appearance of internal pores and surface roughness, which decrease phononic conduction through the layer and interfacial gap with the mold, respectively, and the emergence of crystals themselves, which reduce the transmissivity of infrared radiation across the layer. Due to the importance of the solid layer, this study investigates experimentally the effective thermal resistance, R T , between a hot Inconel surface and a cold Cu surface separated by an initially glassy slag disk, made from powders for casting low and medium-carbon steels, denoted as A and B, respectively. In the tests, an initially mirror-polished disk is sandwiched for 10,800 seconds while the Inconel temperature, away from the disk face, is maintained steady at a value, T c , between 973 K and 1423 K (700°C and 1150°C)-below the liquidus temperature of the slags. The disks have a thickness, d t , between~0.7 and 3.2 mm. Over the range of conditions studied, mold slag B shows R T 33 pct larger than slag A, and microscopic observation of disks hints that the greater resistance arises from the larger porosity developed in B. This finding is supported by high-temperature confocal laser scanning microscope observations of the evolution of the surface of slag parallelepipeds encased between Pt sheets, which reveal that during devitrification the film surface moves outward not inward, contrary with what is widely claimed. This behavior would favor contact of the slag with the mold for both kinds of powders. However, in the case of slag A, the crystalline grains growing at or near the surface pack closely together, leaving only few and small empty spaces. In slag B, crystalline grains pack loosely and many and large empty spaces arise in and below the surface. Estimation from plant data shows R T values smaller than measured ones, suggesting that the process film slag thickness must be considerably thinner than those of laboratory disks. However, the difference in thermal resistance of both powders, averaged over the mold length, is close to the dissimilarity found in laboratory.

Mold Simulator Study of Heat Transfer Phenomenon During the Initial Solidification in Continuous Casting Mold

“…[52][53][54][55] However, all above techniques for the measurement of thermal resistance cannot represent the real scenario of continuous casting mold where the mold/slag thermal resistance varies with the position below the meniscus. A pilot caster [2,56] had been used to investigate the heat transfer across the slag film, while the thermal resistance of slag film along the casting direction was difficult to be calculated due to the complex and transient heat transfer in the vicinity of the meniscus.…”

High-temperature investigation of mould slag crystallization by single and double hot thermocouple techniques

“…Nevertheless, this process is only valid for the middle area of the slag film. In contrast, recent studies [8][9][10][11] have focused on the devitrification of crystals from the glassy phase, which is formed upon contact with the water-cooled copper mould immediately after infiltration. With increasing casting time, partial reheating of this glassy area occurs and enables the crystal formation, which in turn influences the heat transfer throughout the whole slag film [8].…”