Effects of Annealing Treatment Prior to Cold Rolling on the Edge Cracking Phenomenon of Ferritic Lightweight Steel SEOK SU SOHN, BYEONG-JOO LEE, JAI-HYUN KWAK, and SUNGHAK LEE Effects of annealing treatment from 923 K to 1023 K (650 C to 750 C) prior to cold rolling on the edge cracking phenomenon of a ferritic lightweight steel were investigated. The edge cracking was severely found in the hot-rolled and 923 K (650 C)-annealed steels after cold rolling, whereas it hardly occurred in the 1023 K (750 C)-annealed steel. As the annealing temperature increased, lamellar j-carbides were dissolved and coarsened, and most of the j-carbides continuously formed along boundaries between ferrite and j-carbide bands disappeared. Microstructural observation of the deformed region of tensile specimens revealed that the removal of band boundary j-carbides reduced the difference in tensile elongation along the longitudinal direction (LD) and transverse direction (TD), which consequently led to the reduction in edge cracking. The 1023 K (750 C)-annealed steel showed fine ferrite grain size, weak texture, and decomposed band structure after subsequent cold rolling and intercritical annealing, because j-carbides actively worked as nucleation sites of ferrite and austenite. The present annealing treatment prior to cold rolling, which was originally adopted to prevent edge cracking, also beneficially modified the final microstructure of lightweight steel. DOI: 10.1007/s11661-014-2332-z The Minerals, Metals & Materials Society and ASM International 2014
I.
INTRODUCTION
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 microstructures and mechanical properties. In the present study, the annealing treatment at 923 K to 1023 K (650 C to 750 C) was conducted prior to cold rolling as a possible method to prevent cracking from occurring during the cold rolling of a ferritic lightweight steel, and the microstructural modification including j-carbides was examined in detail. In order to determine the fractions of the equilibrium phases, i.e., ferrite, austenite, and j-carbide, existing at high temperatures, thermodynamic calculations of the Fe-Mn-AlC four-component system were used.[23] In addition to the thermodynamic approach, the tensile deformation behavior of hot-rolled and annealed steel sheets was analyzed, METALLURGICAL AND MATERIALS TRANSACTIONS A
because the deformation behavior of the cold-rolled sheets could be estimated from the tensile test. Finally, the optimal annealing temperature to prevent cracking and to improve mechanical properties was suggested.
microscope (model: Leica DM4000, Wetzlar, Germany) and a scanning electron microscope (SEM, model: JSM6330F, JEOL*) operated at an acceleration voltage of *JEOL is a trademark for Japan Electron Optics Ltd., Tokyo.
II.
EXPERIMENTAL
A. Lightweight Steels A lightweight steel was fabricated by a vacuum induction melting method, and its chemical compositions are Fe-0.3C-3.5Mn-5.8Al-(
