Microstructural Features Controlling Mechanical Properties in Nb-Mo Microalloyed Steels. Part I: Yield Strength
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se to the demanding requirements of various structural applications (gas and oil transportation pipes, offshore structures, and naval applications) and in order to achieve a good balance between strength and toughness properties, new generations of microalloyed steels are being developed.[1,2] In order to meet mechanical property requirements, it is vital to control the microalloying additions and the optimization of the processing parameters. The tensile properties, and more specifically the yield strength, are controlled by combining different strengthening mechanisms such as microstructural refinement, solid solution hardening, precipitation strengthening, and dislocation hardening related to the modification of the final microstructures, from the conventional polygonal ferrite to non-polygonal phases or bainitic structures. In the current context, Nb-Mo microalloyed steels are one of the most promising combinations for simultaneously achieving high strength and low ductile–brittle transition temperatures. Nb is used for its ability to induce strain accumulation in the austenite prior to transformation, leading to an additional microstructural refinement[3–5] while the NEREA ISASTI and DENIS JORGE-BADIOLA, Researchers, and PELLO URANGA, Researcher and Associate Professor, are with the CEIT and TECNUN (University of Navarra), Paseo de Manuel Lardizabal 15, 20018, Donostia-San Sebastia´n, Basque Country, Spain. Contact e-mail: [email protected] MITRA L. TAHERI, Hoeganaes Assistant Professor, is with the Department of Materials Science, Drexel University, Lebow 344, 3141 Chestnut Street, Philadelphia, PA 19104. Manuscript submitted February 12, 2014. Article published online July 12, 2014 4960—VOLUME 45A, OCTOBER 2014
addition of Mo enhances the formation of more complex non-polygonal microstructures.[6] In order to understand the relationship between microstructural parameters and mechanical properties, an exhaustive microstructural characterization has to be carried out. Optical microscopy presents limitations in terms of the characterization and quantification of the microstructure, when the presence of non-polygonal phases is important. Therefore, the electron-backscattered diffraction technique (EBSD) technique is becoming a more conventional characterization tool. The quantification of mean unit crystallographic sizes and the nature of the boundaries and low misorientation defects are being obtained through EBSD and these data contribute to our understanding of the different mechanisms acting in the mechanical properties. Following the assumptions used recently in several studies dealing with the quantification of complex microstructures,[7,8] boundaries with a misorientation angle (h) smaller than 2 deg are assumed to contribute to dislocation strengthening, whereas boundaries h > 2 deg are considered effective for contributing to grain size strengthening. Moreover, the boundaries located between the 2 deg < h < 15 deg interval and h > 15 deg are treated as low angle boundaries and high angle boundaries, respectively. Low a
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