Anisotropy of Dynamic Compressive Properties of Non-Heat-Treating Cold-Heading-Quality Steel Bars
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COLD-HEADING-QUALITY bars are mainly used for connecting parts such as bolts, nuts, and threads in many industrial areas such as automobiles, machineries, electronics, and constructions. These bars are mostly classified into heat-treated and non-heat-treating ones, and are fabricated by various processes of drawing, forging, and heat-treatment. Heat-treated bars are generally spheroidized before cold drawing or forging processes. Recently developed non-heat-treating bars are subjected to cold drawing or forging to achieve required mechanical properties without heat treatments, which can give economical merits of fabrication costs and productivity.[1–3] However, these non-heat-treating bars often show low toughness values in final product forms. In order to solve this problem, the formation of low-temperature transformation microstructures such as bainites or degenerated pearlites by controlling the Mn content and cooling rate is recommended.[1–6] The grain HYUNMIN KIM and MINJU KANG, Research Assistants, are with the Center for Advanced Aerospace Materials, Pohang University of Science and Technology, Pohang 790-784, Korea. CHUL MIN BAE, Group Leader, is with the Wire Rod Research Group, Technical Research Laboratories, POSCO, Pohang 790-785, Korea. HYOUNG SEOP KIM and SUNGHAK LEE, Professors, are with the Center for Advanced Aerospace Materials, Pohang University of Science and Technology, and also with the Materials Science and Engineering, Pohang University of Science and Technology. Contact e-mail: [email protected] Manuscript submitted February 9, 2013. Article published online January 10, 2014 1294—VOLUME 45A, MARCH 2014
refinement obtained from hot- or cold-working processes with high reduction ratios is also suggested to improve the toughness.[7–9] Another problem of non-heat-treating bars is anisotropic mechanical properties which are dependent on the alignment direction of pearlite bands inevitably formed during hot- or cold-working processes.[10–12] The anisotropy often has an advantage to control the required mechanical properties, but can be a fatal shortcoming of the shear cracking occasionally occurring during the cold heading. It is more sensitive under the dynamic loading than under the quasistatic loading.[12] When the orientation between the pearlite band alignment and loading directions increases from the 0 deg (longitudinal) direction to 90 deg (transverse) direction, the strength and ductility can be varied particularly under the dynamic compressive loadings such as cold heading.[13–15] In this case, dynamic compressive loads are generally concentrated along maximum shear stress planes, and unidirectionally aligned pearlite bands can work as shear crack-initiation sites for the brittle fracture.[16,17] Therefore, systematic studies on deformation and cracking mechanisms of cold-heading-quality bars in relation with microstructural modification under the dynamic and quasistatic compressive loading conditions are essentially needed. In addition, effects of the abrupt shear deformation cau
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