The Effects of Finish Rolling Temperature and Cooling Interrupt Conditions on Precipitation in Microalloyed Steels Using
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TRADITIONAL strengthening mechanisms for high-strength microalloyed steels used in pipelines include grain refinement, dislocation strengthening, and precipitation strengthening. Precipitates that are produced in high-strength microalloyed steels can include Ti(C,N), Nb(C,N)[1–3] and other phases depending on the exact composition of the steel. In addition to composition, the processing parameters associated with thermomechanical controlled processing (TMCP) of these steels, including casting, the reheating process, rough rolling conditions, finish rolling conditions, and cooling conditions, all can affect the size and number of precipitates that form. However, to achieve any significant amount of strengthening via precipitation, precipitate diameters should be on the order of 5 nm[1] in diameter. Thus, an understanding of the relationship between the number density (i.e., distribution) of fine nano-size precipitates that form and their relation to processing is essential for the production of higherstrength microalloyed pipeline steels. Precipitation events during thermomechanical processing can be grouped according to which stage in the TMCP process the precipitates form.[6] In a very simplistic categorization, large (>0.5 lm), primarily TiN, precipitates form during the solidification process J.B. WISKEL, F.S.O., D.G. IVEY, Professor, and H. HENEIN, Professor, are with the Department of Chemical and Materials Engineering, University of Alberta, Edmonton, AB, Canada. Contact e-mail: [email protected] Manuscript submitted October 4, 2007. Article published online December 8, 2007. 116—VOLUME 39B, FEBRUARY 2008
and subsequent cooling to the reheat furnace temperature, medium- size Nb,Ti(C,N) precipitates in the size range from 50 to 500 nm form during the hot rolling schedule and very fine precipitates (
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