Effect of Carbon Content on Cracking Phenomenon Occurring during Cold Rolling of Three Light-Weight Steel Plates
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MANY efforts to reduce the weight of steel plates in automotive industries have been conducted in order to increase fuel efficiency and to decrease CO2 emissions.[1–8] In addition to light weight needs, automotive steels require excellent strength to sustain automotive structures and to reduce the impact or shock in cases of accidents. Thus, highly deformable steel plates such as transformation induced plasticity (TRIP) steels and twinning induced plasticity (TWIP) steels have been actively developed.[1–5,8–12] Recently, a large amount of Mn and Al has been added to automotive steels to achieve the light-weight effect as well as excellent strength and ductility, and their total amounts are generally larger than 15 wt pct. This addition leads to about 10 wt pct reduction in comparison with TRIP or TWIP steels, and often offers excellent properties such as strengths of over 780 MPa and elongations of over 30 pct.[9–15] SEUNG YOUB HAN, Research Assistant, SANG YONG SHIN, Research Professor, and SUNGHAK LEE, Professor, are with the Center for Advanced Aerospace Materials and the Materials Science and Engineering Department, Pohang University of Science and Technology, Pohang 790-784, Korea. Contact e-mail: shlee@ postech.ac.kr NACK J. KIM, Professor, is with the Graduate Institute of Ferrous Technology and the Center for Advanced Aerospace Materials, Pohang University of Science and Technology. JAI-HYUN KWAK, Senior Principal, Sheet Products & Process Research Group, and KWANG-GEUN CHIN, Steel Research Project Team, TWIP Steel Research Project Team, are with the Technical Research Laboratories, POSCO, Kwangyang, 545-090, Korea. Manuscript submitted April 9, 2010. Article published online October 19, 2010 138—VOLUME 42A, JANUARY 2011
Alloying elements used for light-weight steels mainly include Mn, Al, and C because the decreased specific gravity due to substitutional or interstitial atoms works to reduce the weight.[16–19] Mn raises the volume fraction of austenite at high temperatures as an austenite stabilizer, but the increased Mn content often leads to the formation of a large amount of ferrite during cooling.[20] It also poses problems such as increased manufacturing costs and deteriorated productivity, because the temperature of the steel melt can be lowered during the steel-making process. Thus, efforts to reduce the Mn content in the light-weight steels have been made. Aluminum, a ferrite stabilizer, helps form a dual-phase structure of ferrite and austenite at high temperatures and promotes the precipitation during cooling.[16–18] When the steels contain hardenability elements such as C, precipitates such as carbides or nitrides are well formed, and the amount of precipitates varies with contents of Mn and Al as well as C.[16–18] According to these precipitates, the steels are often exposed to the cracking occurring during cold rolling. The light-weight steels containing about 10 wt pct of Mn and Al are newly developed, but their detailed microstructures or deformation and fracture mechanisms are hardly kno
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