In-Situ Observation of Crystallization and Growth in High-Temperature Melts Using the Confocal Laser Microscope
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CONTINUOUS casting technology has been one of the main drivers that has substantially driven production costs down and allowed steel to become a dominant structural material of choice for manufacturing industries. Beyond the primary and secondary steelmaking processes, when the steel chemistry and cleanliness is fulfilled, the steel melt must be cooled and the superheat removed through a water-cooled copper mold to produce solid semi-finished products. The main objective of continuous casting, and in particular the mold, is to remove this excess heat uniformly; to control the amount of heat removed (which depends on the steel grade being cast); to ensure that a thick enough solidified steel shell is uniformly formed with sufficient physical strength to withstand the thermal stresses, strains, friction, and ferro-static pressure imposed on the solidifying shell.[1] Without optimal control of the heat removal, a non-uniform shell can cause localized stresses concentrations, subsequent crack formation such as transverse cracks, longitudinal face cracks, depressions, and in the worst case break-outs. The propensity toward cracks and surface problems is exacerbated in crack-sensitive grades in which sulfur, phosphorous, and other alloying elements segregate during cooling between the growing dendrites, resulting in localized concentrations
IL SOHN, Associate Professor, is with Materials Science and Engineering, Yonsei University, Seoul 120-749, Republic of Korea. Contact e-mail: [email protected] RIAN DIPPENAAR, Professor, is with Engineering of Information Sciences, University of Wollongong, Wollongong, NSW 2522, Australia. Contact e-mail: rian@ uow.edu.au. Manuscript submitted November 29, 2015. METALLURGICAL AND MATERIALS TRANSACTIONS B
and subsequent formation of liquid films in the inter-dendritic regions.[2,3] It is therefore imperative that fundamental understanding be developed of the solidification behavior as well as the control of heat removal and their influence on the microstructural evolution of the steel within the mold with a view to optimizing continuous casting operations. In light of this importance to understand the solidification behavior of steel, several techniques have been developed to determine the solidification and phase transformation temperatures by controlling the cooling rate. Due to the high temperatures involved in the solidification of steel, researchers used low-temperature transparent and translucent materials by which high-temperature solidification could be modeled, such as the studies initiated by Jackson and Hunt.[4] In order to study phase transitions at high temperatures, differential scanning calorimetry (DSC) and differential thermal analysis (DTA) have been utilized and reasonable agreement was found between DSC and DTA measurements and theoretical predictions at near-equilibrium conditions as well as during undercooling.[5–7] However, due to the limited ability to detect small enthalpy changes and the limited range of cooling rates achievable in DSC and DTA measurements, the
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