Journal of Materials Processing Technology 209 (2009) 4476–4483 Contents lists available at ScienceDirect Journal of Materials Processing Technology journal homepage: www.elsevier.com/locate/jmatprotec Surface wrinkle defect of carbon steel in the hot bar rolling process Hyuck-Cheol Kwon a , Ho-Won Lee b , Hak-Young Kim c , Yong-Taek Im b,∗ , Hae-Doo Park a , Duk-Lak Lee d a Rolling Technology and Process Control Group, POSCO Technical Research Laboratories, POSCO, 1, Goedongdong, Namgu, Pohang, Gyeongbuk 790-785, Republic of Korea b National Research Laboratory for Computer Aided Materials Processing, Department of Mechanical Engineering, Korea Advanced Institute of Science and Technology, 373-1, Gusongdong, Yusonggu, Daejeon 305-701, Republic of Korea c Advanced Technology Department, Hyundai Mobis Technical Research Institute, 80-10, Mabukdong, Giheunggu, Yongin, Gyeonggi 449-912, Republic of Korea d Wire Rod Research Group, POSCO Technical Research Laboratories, POSCO, 1, Goedongdong, Namgu, Pohang, Gyeongbuk 790-785, Republic of Korea article info Article history: Accepted 6 October 2008 Keywords: Surface wrinkle defect Hot bar rolling Specific deformation energy Instability abstract It is well known that surface defect is a common problem encountered in the multi-stage hot bar rolling process of carbon steel. In this study, the phenomenon was investigated by simulating the process by the finite element technique to identify the location where the surface defect might occur and checking the surface qualities obtained from the compression tests at various temperatures and strain rates to clarify the important parameter governing the possible surface defect formation. Also, the surface temperature was measured by employing pyrometer to support the experimental observation. The levels of temper- ature and specific deformation energy obtained from finite element simulations depending on the roll groove geometry were compared with the experimental observation to better understand the formation of the surface defect in the hot rolled bar. Based on this study, the surface defect might be formed by dissipating the excessive deformation energy accumulated by generating the new surface at the lower level of temperature where recrystallization cannot occur. According to this work, the comparison of the specific deformation energy level for determining the instability of the hot working process might be interesting for further investigation. © 2008 Elsevier B.V. All rights reserved. 1. Introduction Workability for hot deformation depends on both the material characteristics such as grain size, billet geometry, and distribu- tion of secondary phase and the processing characteristics such as strain, strain rate, stress, and temperature. For cold deformation, the processing characteristics are more important than the material characteristics to govern the workability. In steel company, surface defect in the multi-stage hot bar rolling is one of critical problems to be solved for quality assurance of the rolled product. The surface defect, which is frequently encountered in the hot bar rolling of steel, can easily develop into a fatal man- ufacturing defect during the secondary cold forging process of bar stocks as shown in Fig. 1. Thus, it is necessary to minimize such defect formations on the surface of hot rolled bars. Although many researchers had paid attention to understand and solve the prob- lems, the phenomenon is not quite well understood yet because of many deformation stages of roughing, intermediate roughing, inter- ∗ Corresponding author. Tel.: +82 42 869 3237; fax: +82 42 869 3210. E-mail address: ytim@kaist.ac.kr (Y.-T. Im). mediate finishing, and finishing mill to produce hot rolled bars as shown in Fig. 2. Schey (1980) described that cracking of the billet in rolling, whether in the form of edge or surface cracking, or damage to the billet center, invariably resulted in increased crap and production costs. In this work, it was concluded that through thickness inho- mogeneity resulted into surface or center cracks, whereas lateral inhomogeneity into edge cracking. From the comparison of bend test specimens with edge cracking by the help of metallographic observations, Fitzsimons and Kuhn (1984) pointed out that edge cracks were observed in the hot bar rolling at high strain rates. Crowther and Mintz (1986) had investigated the change of duc- tility at various temperatures depending on carbon contents of the material. Barlow et al. (1984) also discussed main rolling defects observed both during rolling and on finished rolled bars. They related defect formations to ingot casting such that if the molten metal was not poured carefully into the ingot at the proper temperature or not allowed to cool in a controlled environment, it might result into a longitudinal crack or seam along the bar. Hassani and Yue (1993) claimed that surface cracks were oxidized in the air in ingot cast- ing, resulting in defects that lowered the surface quality of the final product. For rolling in a blooming mill, Mil’man et al. (1979) estab- 0924-0136/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.jmatprotec.2008.10.032