The Effect of Simulated Thermomechanical Processing on the Transformation Behavior and Microstructure of a Low-Carbon Mo
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OVER the last several decades linepipe steels have undergone extensive development, driven due to the constantly increasing demand for a more cost-effective, higher-performance pipeline design, which has also been reflected in the related research publications, e.g., References 1 through 28. Critical to the design of these steels is a low carbon equivalent for good field weldability. To compensate for the loss in strength, additions of microalloying elements such as molybdenum, niobium, and titanium are used. These additions contribute to an increase in strength both directly, through microstructural refinement, solid solution strengthening, and precipitation hardening, as well as indirectly, through enhanced hardenability and associated modification of the resultant transformation microstructures. Optimum product microstructures, with a desired balance of mechanical properties at a given steel composition, are being achieved through suitably designed thermomechanical processing schedules,[29] which commonly involve controlled rolling followed by controlled P. CIZEK, Senior Researcher, and P.D. HODGSON, Professor, are with the Institute for Frontier Materials, Deakin University, Waurn Ponds, VIC 3216, Australia. Contact e-mail: pavel.cizek@ deakin.edu.au B.P. WYNNE, Professor, is with the Department of Materials Science and Engineering, The University of Sheffield, Mappin Street, Sheffield S1 3JD, U.K. C.H.J. DAVIES, Professor, is with the Department of Mechanical and Aerospace Engineering, Monash University, Clayton, VIC 3800, Australia. Manuscript submitted June 24, 2014. Article published online October 8, 2014 METALLURGICAL AND MATERIALS TRANSACTIONS A
accelerated cooling. The controlled rolling step usually includes heavy deformation of the austenite, carried out in the non-recrystallization temperature region, which brings about significant refinement of the final transformation microstructures. The accelerated cooling step aims to suppress the formation of polygonal ferrite and, instead, encourage non-equilibrium, non-polygonal ferrite microstructures to be formed. The non-equilibrium ferrite microstructures do not contain cementite and possess some unique morphological features. A specific terminology[3] has been adopted in the present work in an attempt to describe all possible ferrite morphologies formed by the decomposition of austenite in the present steel. Apart from martensite (M), this terminology recognizes the following forms of ferrite: (i) polygonal ferrite (PF), the equilibrium microstructural constituent characterized by roughly equiaxed grains with smooth boundaries, mostly containing a low dislocation density and no substructure; (ii) quasipolygonal ferrite (QF), characterized by grains with irregular shape aligned in arbitrary directions, containing dislocation substructure, and occasional martensiteaustenite (M/A) micro-constituent; (iii) granular bainite (GB), which consists of sheaves of elongated ferrite crystals with low misorientations and a high dislocation density, containing roughly equiaxed isla
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