Correlation of Microstructure and Cracking Phenomenon Occurring during Hot Rolling of Lightweight Steel Plates
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INTRODUCTION
MUCH effort to reduce the weight of steel plates in automotive industries has been undertaken in order to increase fuel efficiency and decrease CO2 emissions. In addition to light weight needs, automotive steels require excellent strength to sustain automotive structures and to reduce the impact or shock in case of accidents. Recently, low-density elements such as Mn and Al have been added to lightweight steels, and their total amounts are generally larger than 15 wt pct. This addition leads to an approximately 10 pct weight reduction in automotive components, and often offers excellent properties such as strengths of over 780 MPa and elongation of over 30 pct.[1–6] Lightweight steels containing a large amount of Mn have posed problems such as increased manufacturing costs and deteriorated productivity, because the temperature of the steel melt can be lowered during the steelmaking process. Thus, efforts to reduce the Mn
SANG YONG SHIN, Researcher, SEUNG YOUB HAN, Student, and SUNGHAK LEE, Professor, Center for Advanced Aerospace Materials, and CHANG-HYO SEO and KAYOUNG CHOI, Students, and NACK J. KIM, Professor, Graduate Institute of Ferrous Technology, are with the Pohang University of Science and Technology (POSTECH), Pohang 790-784, Korea. Contact e-mail: [email protected] HACKCHEOL LEE, Researcher, Plate Research Group, Technical Research Laboratories, and JAI-HYUN KWAK and KWANG-GEUN CHIN, Researchers, Automotive Steels Research Group, Technical Research Laboratories, are with the Pohang Iron and Steel Company (POSCO), Pohang 790-785, Korea. Manuscript submitted May 21, 2009. Article published online October 30, 2009 138—VOLUME 41A, JANUARY 2010
content in the lightweight steels have been undertaken. Even when the Mn and Al addition is reduced, e.g., from 15 to 10 wt pct, the cracking occurring during hot rolling can be a serious problem.[7,8] It was reported that this cracking was caused by the following: (1) inhomogeneous microstructures due to the segregation of C and Mn at high temperatures, (2) rolling anisotropy as a result of the formation of dendrite-type microstructures, (3) formation of oxides or nitrides due to the exposure at high temperatures, and (4) void formation arising from oxides fallen off from the friction or impact between rolls and steel plates. Because the lightweight steels containing approximately 10 wt pct Mn and Al are newly developed, their detailed microstructures or deformation and fracture mechanisms are hardly known. Furthermore, very few studies have been conducted in order to systematically explain the cracking phenomenon of hot-rolled steel plates in terms of microstructural parameters, except for some studies related to textures and the segregation of precipitates.[7–10] In the present study, two hot-rolled lightweight steel plates were fabricated, and the cracking phenomenon occurring during hot rolling was clarified in relation to the microstructure. Microstructural analysis, tensile testing, and high-temperature compression testing were conducted to study the c
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