Correlation of microstructure and thermal fatigue property of three work rolls
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I.
INTRODUCTION
THE hot-rolling process is one of the areas that the steel industry is actively exploring in an effort to devise a new and advanced technique for quality improvement of products. Rolls with improved mechanical properties such as wear resistance, high strength, fracture toughness, and thermal fatigue have become required, because rolling conditions have become severe as demands increase for rolled plates with a more uniform and lower thickness, smoother surface, and higher strength.[1,2] Developing such rolls is mainly for the purpose of quality improvement of the rolled products and for roll durability. Thus, understanding thermal fatigue behavior is essential, since surface roughening and surface cracks are closely related to thermal fatigue. The thermal fatigue of rolls is associated with the sudden increase and decrease of temperature as they come into contact with rolled plates at high temperature followed by water cooling. However, the surface region that experiences the thermal fatigue, less than 1.0 mm from the surface, is in a complete constraint by the roll interior that is not affected by the heat.[3,4,5] Thus, the roll surface is consistently roughened because of the continuous expansion and contraction caused by contact with the hot-rolled plate and subsequent water cooling.[6,7,8] In order to determine the
roll-working conditions, the thermal stress variation at the complete constraint state should be understood, as well as the role of each microstructural factor on the thermal fatigue properties. In other words, to understand the thermal fatigue phenomenon and to figure out the factors that cause thermal fatigue during hot rolling, a systematic understanding through the investigation of the microstructural evolution is essential. The characterization of thermal fatigue behavior has been based on the analysis of surface roughening and prediction of the thermal fatigue property by performing simulation tests using a wear tester.[9] However, the quantitative analysis of the thermal fatigue behavior in relation to the microstructural evolution has hardly been reported. In this study, the microstructure and thermal fatigue property of the three work rolls in commercial use, i.e., a nickel-grain cast-iron roll (Ni-grain roll), a high-chromium cast-iron roll (Hi-Cr roll), and a recently developed highspeed steel roll (HSS roll), were investigated to obtain the fundamental data for the establishment of hot-rolling conditions and the extension of the roll durability. In addition, the thermal fatigue mechanism and the microstructural factors influencing thermal fatigue were analyzed and compared using the thermal fatigue results. II.
SUNGHAK LEE, Professor, is with the Center for Advanced Aerospace Materials, Pohang University of Science and Technology, Pohang, 790784 Korea. DO HYUNG KIM, formerly Research Assistant, Center for Advanced Aerospace Materials, Pohang University of Science and Technology, is Research Associate, Multi-Layer and Thin-Film Division, Samsung Electro-Mechanics Co., L
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