Composition, microstructure, hardness, and wear properties of high-speed steel rolls
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I.
INTRODUCTION
ROLLING conditions are gradually becoming complicated and demanding in hot-rolling plants, because of the increasing volume of thin plates and high-strength steel plates. These conditions require rolls with high hardness and excellent wear properties such as a low friction coefficient and low surface roughness. Accordingly, high-speed steel (HSS) rolls, characterized by enhanced thermal fatigue properties and excellent resistance to wear and abrasion, are replacing conventional high-chromium (Hi-Cr) iron rolls.[1,2] The wear process of rolls is a very complex one and is integrally affected by various factors such as abrasive wear, oxidation wear, cracking by thermal fatigue and heat impact, fatigue wear by fall-off of carbides, and sticking of rolled material onto the roll.[3,4] Wear involves microscopic and dynamic processes occurring in the interface between the roll and the rolled material and is almost impossible to directly observe. Interpretation is also difficult because kinetic, thermal, chemical, and material factors are intricately in interaction. Since the roll surface is repeatedly under stress and heat during hot rolling and in friction with the rolled material, the surface layer is often damaged. This poses serious problems in roll performance and considerably deteriorates the surface quality of rolled products because of the surface roughness of the roll. In Hi-Cr rolls, a network of carbides formed along solidification cell boundaries work favorably for improving wear resistance. However, because the carbide networks often initiate thermal fatigue cracks and function as crack JOON WOOK PARK, Research Engineer, and HUO CHOON LEE, Senior Research Engineer, are with the Roll R&D Team, Roll Manufacturing Division, Kangwon Industries, Ltd., Pohang, 790-370 Korea. SUNGHAK LEE, Professor, is with the Center for Advanced Aerospace Materials, Pohang University of Science and Technology, Pohang, 790-784 Korea. Manuscript submitted February 25, 1998. METALLURGICAL AND MATERIALS TRANSACTIONS A
propagation sites, they seriously harm thermal fatigue properties.[5,6,7] These shortcomings are microstructurally alleviated in HSS rolls, where very hard carbides improve hardness and wear resistance while the tempered martensitic matrix, containing fine carbides, is responsible for fracture toughness.[8] Under this concept, HSS rolls contain strong carbide-forming elements such as V, Mo, and W, which help form very hard carbides that significantly improve hardness and wear resistance.[9,10,11] The properties of HSS rolls are determined by various microstructural factors such as (1) the type, morphology, volume fraction, and distribution of carbides; (2) the characteristics of the martensitic matrix; and (3) the solidification cell structure, composed of carbides. Carbides directly influence the wear resistance of rolls and the surface quality of rolled plates, since carbides are very hard. The matrix is related to the overall hardness, strength, and fracture toughness of the rolls and plays a sustain
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