Effects of Annealing Treatment Prior to Cold Rolling on the Edge Cracking Phenomenon of Ferritic Lightweight Steel
- PDF / 6,950,727 Bytes
- 13 Pages / 593.972 x 792 pts Page_size
- 98 Downloads / 203 Views
UCTION
AUTOMOTIVE steels generally require excellent strength to sustain automotive structures.[1–5] Recently, a considerable amount of Al was added to automotive steels to achieve a lightweight effect as well as excellent strength and ductility.[6,7] The addition of 5 to 6 wt pct of Al leads to 8 to 10 pct weight reduction in comparison with conventional automotive steels such as transformation-induced plasticity steels or twinning-induced plasticity steels. Aluminum, a ferrite stabilizer, helps to form a dual-phase structure of ferrite and austenite at high temperatures and promotes precipitation during cooling.[8,9] When the steels contain hardenability elements such as C, precipitates such as j- or j¢-carbides (composition: (Fe,Mn)3-Al-C) are mainly formed on grain boundaries or coherently within the matrix, respectively. Amounts of j- or j¢-carbides are varied with alloying contents of Mn and Al as well as C and aging temperature or time.[10–14] In lightweight steels, j-carbide was used for increasing strengths by controlling their size, fraction, and distribution,[6,13,15] but it is generally harmful to ductility because of the steels’ hardness.[16,17] However, the SEOK SU SOHN, Research Assistant, is with the Center for Advanced Aerospace Materials, Pohang University of Science and Technology, Pohang 790-784, Korea. BYEONG-JOO LEE and SUNGHAK LEE, Professors, are with the Center for Advanced Aerospace Materials, Pohang University of Science and Technology, also with the Materials Science and Engineering, Pohang University of Science and Technology, Pohang 790-784, Korea. Contact e-mail: [email protected] JAI-HYUN KWAK, Senior Principal Researcher, is with the Sheet Products & Process Research Group, Technical Research Laboratories, POSCO, Kwangyang 545-090, Korea. Manuscript submitted December 10, 2013. Article published online May 14, 2014 3844—VOLUME 45A, AUGUST 2014
formation processes of j-carbides are not fully understood yet, and j-carbides often cause cracking by forming band structures during hot or cold rolling.[17,18] It was reported that this cracking was caused by inhomogeneous microstructures due to the segregation of C and Mn at high temperatures, rolling anisotropy due to the formation of dendrite-type microstructures, and formation of oxides or nitrides due to exposure at high temperatures.[19–22] The lightweight steels containing about 10 wt pct Mn and Al are newly developed, but very few studies on the cracking phenomenon of rolled steel sheets have been conducted. Furthermore, methods for systematically preventing cracking, such as annealing treatment prior to cold rolling, and detailed deformation and fracture mechanisms related to annealing have hardly been studied. Therefore, for the successful development of lightweight steels, the formation of j-carbides, which can vary with annealing conditions as well as alloying elements, is essentially verified. From these understandings, it is possible to fabricate lightweight steels without any cracks and to simultaneously improve their microstr
Data Loading...
