A Study of the Influence of Thermomechanical Controlled Processing on the Microstructure of Bainite in High Strength Pla
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ROALLOYED steels for construction, shipbuilding, or linepipe applications require not only high XIAOJUN LIANG, Postdoc Associate, is with the Department of Mechanical Engineering and Materials Science, Basic Metals Processing Research Institute, University of Pittsburgh, Pittsburgh, PA 15261. ANTHONY J. DE ARDO, Professor, is with the Department of Mechanical Engineering and Materials Science, Basic Metals Processing Research Institute, University of Pittsburgh, and also Distinguished Professor, with the Centre for Advanced Steel Research, University of Oulu, Oulu, Finland. Contact e-mail: [email protected] Manuscript submitted January 16, 2014. Article published online July 17, 2014 METALLURGICAL AND MATERIALS TRANSACTIONS A
strength in steel plates, but also good toughness, weldability, and resistance to HIC and other hydrogen-related fractures.[1] The principal alloy and process design concepts have been known since before 1970 in ferrite–pearlite steels of lower strength levels near YS of 420 MPa.[2] These concepts include (1) reduction in the carbon contents and (2) combining the appropriate rolling parameters with proper microalloying to achieve a well-conditioned austenite. Achieving well-conditioned austenite involves rough rolling at high temperatures to reduce the austenite grain size and finish rolling at lower temperatures to change the austenite grain shape (controlled rolling), together with appropriate microalloying to raise the austenite recrystallization-stop VOLUME 45A, OCTOBER 2014—5173
temperature and increase the austenite near-planar interface area per unit volume (Sv); and (3) applying appropriate interrupted controlled cooling for the given CCT diagram to achieve the desired final microstructure.[2] However, in order to achieve a YS in excess of 690 MPa in heavier plates (6 to 25 mm), a matrix phase of bainitic ferrite is required, which forms together with a certain amount of unintended but unavoidable higher carbon second phase, micro-constituent or feature. This higher carbon second feature, which can be martensite, retained austenite, or cementite, depending on circumstances, is mainly martensite in low carbon, IDQ steels. This second feature will be referred to as ‘‘MA’’ in the remainder of the paper. In this current paper, recent studies of high strength bainite in a 0.06 pctC steel will be presented. However, as indicated above, the term bainite is used to describe a wide range of specific and complex microstructures comprised of mixtures of the bainitic ferrite matrix phase and a high carbon second micro-constituent. Bainitic ferrite plus MA is often referred to as carbidefree bainite, since the excess carbon is tied up as high carbon martensite islands rather than as Fe3C.[3] Unlike pearlite, where the ferrite and cementite form cooperatively, i.e., at the same temperature, and at the same advancing transformation front, the bainitic ferrite plus MA in bainite forms separately at different temperatures or isothermal holding time.[4] Therefore, the microstructure represented by the generi
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