Roll processing technology has recently made great advancements to meet the demands for enhanced surface properties of rolled products and diversified consumer needs. Continuous efforts also have been made to improve the quality of work rolls, specifically in wear resistance, strength, fracture toughness, and thermal fatigue properties. [1,2,3] The prime objectives in the development of advanced work rolls are to improve the surface quality of rolled products and to extend the life of rolls themselves. To achieve these objectives, it is essential to understand fracture properties that directly affect the surface roughness, spalling, failure, etc.Fracture toughness, which is used to evaluate the structural stability of materials, is one of the most important material properties to be considered when designing structures and selecting structural materials. To improve fracture toughness, systematic studies on its correlation with the microstructural characteristics are highly required; specifically, clarification on the microfracture mechanism based on the microstructural control of each material concerned should proceed. [4] For example, fracture toughness in ductile materials is strongly affected by the presence of secondphase particles that induce the dimple formation by void initiation and propagation. When the second-phase particles are coarse and very brittle, they often induce cleavage fracture and deteriorate the material properties. Thus, the improvement of fracture toughness can be achieved by clarifying each microstructural factor involved in each microfracture mechanism. Work rolls can be fractured unexpectedly by spalling or thermal fatigue caused by the growth of internal cracks or by the roughening of the roll surface. This results from the repeated formation of thermal-fatigue cracks due to the abrupt temperature rise arising from the contact with high-temperature rolled plates and the ensuing drop in temperature by cooling water, and from the repeated impact load and the increased rolling load caused by abrupt intrusion of rolled plates. [5][6][7][8] Accordingly, analyses of the microstructural factors determining fracture properties are critical in establishing the overall rolling conditions, such as the roll surface roughness and the roll life.The present study aims at providing the basic data for the establishment of using conditions of work rolls by investigating the microstructure, hardness, and fracture tough-GWIHWAN BYUN, Section Manager, is with the Hot Rolling ness of the five currently commercialized work rolls and by clarifying their respective fracture mechanisms. They are high speed steel roll (HSS roll), high chromium cast iron roll (Hi-Cr roll), Ni-grain cast iron roll (Ni-grain roll), Adamite roll, and ductile cast iron roll (DCI roll). The microfracture behavior was observed by using an in situ loading stage installed inside a scanning electron microscope (SEM) chamber, while simultaneously measuring apparent fracture toughness. These experiments enable evaluation and interpretation...
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