Analysis and prevention of microcracking phenomenon occurring during strip casting of an AISI 304 stainless steel
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INTRODUCTION
AUSTENITIC stainless steels, having excellent mechanical properties and corrosion resistance, have been widely used for tableware, building materials, and materials for petrochemical plants. Recently, intensive studies on a strip-casting process of these steels have been made to produce thin strips without hot rolling.[1,2,3] Strip casting offers many advantages, such as refinement of the solidified structure, reduction of microsegregation, and expansion of the solubility limit, because solidification occurs much faster than in conventional continuous casting. Strip casting also lowers the production cost by omitting hot-rolling processes and is an environmentally conscious process. Several companies have already finished constructing plant facilities for commercialization, and new products are currently being developed.[4,5] However, it is required that the quality of the surface and the bulk of final cast strips meets the quality level of hot-rolled strips produced by continuous casting, because the rolling ratio available after strip casting is limited. Critical quality problems arising from strip casting include surface cracks, tortoise-shell–shaped depressions, and microcracks formed along the depressions.[6–9] The thickness of solidification shells becomes uneven due to inhomogeneous solidification and thermal contraction, and transformation strains are concentrated in thin regions of the solidification shells,[10] thereby leading to the initiation of MANJIN HA, JUTAE CHOI, and SEONGIN JEONG, Research Engineers, HEEKYUNG MOON, Senior Research Engineer, and TAEWOOK KANG, General Manager, are with the Strip Casting Project Team, Research Institute of Industrial Science and Technology, Pohang 790-600, Korea, SUNGHAK LEE, Professor, Center for Advanced Aerospace Materials, Pohang University of Science and Technology, Pohang 790-784, Korea, is jointly appointed with the Materials Science and Engineering Department, Pohang University of Science and Technology. Contact e-mail: [email protected] Manuscript submitted July 19, 2001. METALLURGICAL AND MATERIALS TRANSACTIONS A
surface cracks in cast strips. These cracks can be prevented when homogeneous solidification of strips is induced by (1) intentionally providing homogeneous roughness on the casting-roll surface,[6,11–13] (2) using a N2 gas atmosphere, which has good wettability with the roll surface and good solubility in the melt,[6] (3) preventing the inclusion of oxides into the melt,[14] and (4) preventing the turbulence of the melt.[14] However, even these methods cannot eliminate microcracks formed along depressions. Microcracks, mostly hundreds of micrometers long, are not easily visible, but may grow to be long cracks, which act as defects to deteriorate the deformability and corrosion resistance of cast strips. These microcracks are presumed to be closely related to the presence of a gas gap and the inhomogeneous solidification behavior which frequently occurs as solidification shells are in direct contact with casting rolls. Very few stud
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