Atom Probe Tomography Study of Multi-microalloyed Carbide and Carbo-Nitride Precipitates and the Precipitation Sequence
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INTRODUCTION
MICROALLOYED steels use a combination of grain size refinement, microstructure control, and precipitation strengthening, via microalloying with elements such as Ti, Nb, V, and Mo, to obtain an optimal combination of strength and toughness.[1–8] The combination of these microalloying elements with interstitial solutes, such as C and N, leads to the formation of carbides, nitrides, and complex carbo-nitrides.[7,9–13] In general, Ti, Nb, and V facilitate grain refinement through precipitation in the austenite and contribute to dispersion hardening through carbide precipitation in ferrite. Nb can retard recovery and recrystallization during hot rolling resulting in ferrite grain refinement, and Ti combines with nitrogen to form TiN precipitates at high temperature in austenite which prevent grain growth.[1] In contrast, V contributes more to MONICA KAPOOR, formerly Postdoctoral Fellow with the Department of Metallurgical and Materials Engineering, The University of Alabama, Tuscaloosa, AL, is now ORISE Postdoctoral Fellow with National Energy Technology Laboratory, Albany, OR 97321. GREGORY B THOMPSON, Professor, is with the Department of Metallurgical and Materials Engineering, The University of Alabama. Contact e-mail: [email protected] RONALD O’MALLEY, formerly Plant Metallurgist with Nucor Steel Decatur, LLC, 4301 Iverson Blvd., Trinity, AL 35673, is now F. Kenneth Iverson Chair Professor and Director with PSMRC, Materials Science and Engineering, Missouri S&T, 284 McNutt Hall, 1400 N. Bishop Ave., Rolla, MO 65409-0340. Manuscript submitted March 30, 2015. Article published online March 2, 2016 1984—VOLUME 47A, MAY 2016
precipitation strengthening because it has a low solubility in ferrite and precipitates readily at lower temperatures.[1,2] The relative contribution of microalloying elements is determined by the solubility of their carbide/nitride/carbo-nitride phases in the microstructure. In the innovative mini-mill process used to make thin slabs, lower temperatures and short reheating times are employed.[12] This limits the amount of elements that can be added and held in solution to be precipitated for microstructure control. So, multiple strengthening mechanisms, using various elements, need to be employed to achieve strength levels that are comparable to those that can be achieved with single-element microalloy systems and long high-temperature slab soaking times. Combining microalloy systems can result in synergistic behaviors resulting in steels that exhibit very high strength levels, low rolling loads, and also good impact toughness. However, interaction of multiple elements can effect precipitation behavior of each element and not necessarily result in additive strengthening mechanisms, thereby effecting the mechanical properties of the steel.[9,11–13] For instance, addition of Ti to V-N microalloyed steels can decrease the yield strength as a result of preferential formation of TiN precipitates at higher temperatures in austenite.[14] The V-Ti-N precipitates forming at a higher temperature r
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