Analysis and Prevention of Dent Defects Formed during Strip Casting of Twin-Induced Plasticity Steels
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THE rapid solidification of strip casting, compared to conventional continuous casting, can be exploited for reducing microsegregation, extending solute solubility limit, and refining the solidification structure. It is also an environmentally conscious production process that can possibly reduce manufacturing costs, labor, and energy since the hot-rolling step can be omitted. Thus, studies on strip casting have been intensively undertaken to capitalize on these advantages. Production facilities to commercialize STS 304 stainless steels and other steels have been completed, and commissioning of the steel-strip production is in progress.[1–7] Since strip casting can omit the hot-rolling process, it is also especially suited for the production of steels that are problematic during hot rolling and can open the opportunity to develop new advanced steels. Twin-induced plasticity (TWIP) steels are promising steels with a high potential for automotive applications, since they have dramatically improved elongation, high strength, and high formability, according to the twin transformation behavior during the forming processes.[8–10] However, they face many problems arising MANJIN HA and WAN-SOO KIM, Principal Researchers, and HEE-KYUNG MOON, Senior Principal Researcher, are with the POSTRIP R&E Project Department, Technical Research Laboratories, POSCO, Pohang, 790-785 Korea. BYEONG-JOO LEE, Professor, is with the Center for Advanced Aerospace Materials, Pohang University of Science and Technology. SUNGHAK LEE is Professor, Center for Advanced Aerospace Materials, and the Department of Materials Science and Engineering, Pohang University of Science and Technology, Pohang, 790-784 Korea. Contact e-mail: [email protected] Manuscript submitted May 20, 2007. Article published online March 26, 2008 METALLURGICAL AND MATERIALS TRANSACTIONS A
from hot rolling, such as decreased production yield by grain-boundary oxidization during reheating, generation of microscabs on the surface by selective oxidization due to excessive containment of Al and Mn in the steel,[11,12] and crack initiation during hot rolling of slabs. Therefore, these problems are expected to be solved by applying the strip-casting process to TWIP steels, which eliminates the hot-rolling step. One of the defects found on the surface of cast strips of TWIP steels are gas pockets (Figure 1). These are also called ‘‘dent’’ defects and are 0.1- to 0.5-mm deep and 0.3 to 1.5 mm in diameter. These defects are ‘‘dented’’ on the cast-strip surface, and surface cracks can be initiated along these dents. Dent defects are present only on the surface of cast strips, whereas pinhole defects are not present beneath the surface. Since dent defects can deteriorate mechanical properties and cause surface defects of the steel strips, their prevention or reduction is an important consideration. These dent defects observed in the stripcasting process have not been reported previously. The shape of the dent in Figure 1 suggests that these defects apparently form due to gases presen
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