Thermodynamic Prediction and Experimental Verification of Multiphase Composition of Scale Formed on Reheated Alloy Steel
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Thermodynamic Prediction and Experimental Verification of Multiphase Composition of Scale Formed on Reheated Alloy Steels RICHARD OSEI, SIMON LEKAKH, and RONALD O’MALLEY The structure, phase, and composition of scale formed on a continuously cast steel slab during reheating depend on intrinsic factors (steel chemistry, microstructure, and as-cast surface condition) and extrinsic parameters (temperature, time, composition, and velocity of combustion gas atmosphere). The scale that forms on a slab normally has several layers with differing compositions and phases and knowledge of this scale structure is important in subsequent descaling and hot rolling processing steps. Formation of multiphase scale structures on steel during high temperature oxidation in reheat furnace proceeds according to a local thermodynamic equilibrium, while thickness of layers depends on kinetic conditions (mostly by diffusion). In this study, the local thermodynamic equilibrium conditions through the scale layer were simulated using different oxygen/steel ratios, which mimicked the conditions for scale formation at the external, internal, and sublayer oxide region at metal/scale boundary. Experiments were performed in a simulated combustion atmosphere using typical industrial reheat time/temperature conditions. The phases that developed in layered scale structure were documented using SEM/EDX and Raman spectroscopy. The predicted scale compositions and phases were in good agreement with the experimental results for studies with Mn and Si-alloyed carbon steel, Cr-alloyed ferritic, and Cr, Ni-alloyed austenitic steels. https://doi.org/10.1007/s11663-020-02023-3 Ó The Author(s) 2020
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INTRODUCTION
SCALE formation in a typical steel processing plant begins during the continuous casting of a slab and continues through slab reheating and hot rolling of the steel. Scale formation during reheating is a complex phenomenon which has been described in numerous studies conducted over the past several decades.[1–6] The formation of oxide scale on steel at various processing stages is affected by a combination of intrinsic factors (steel grade, microstructure, segregation, grain boundary, surface roughness) and extrinsic factors (gas atmosphere, gas flow, furnace pressure, temperature, exposure time, furnace throughput, and mechanical and thermal stresses).[4–9] With a combination of these factors, the formed oxide scale might be easily removed by water jet descaling or it may be sticky and hard to remove from the steel surface, leaving a residual scale after descaling that interferes with subsequent processes RICHARD OSEI, SIMON LEKAKH, and RONALD O’MALLEY are with the Peaslee Steel Manufacturing Research Center, Department of Materials Science and Engineering, Missouri University of Science and Technology, Rolla, MO 65409. Contact e-mail: [email protected] Manuscript submitted June 30, 2020; accepted October 29, 2020.
METALLURGICAL AND MATERIALS TRANSACTIONS B
such as hot rolling.[10,11] To promote surface cleanliness of the steel, water jet descaling
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