Effect of cool deformation on mechanical properties of a high-strength pipeline steel

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SMALL additions of Nb to high-strength, low-alloy (HSLA) steels can result in significant improvements in strength, toughness levels, and weldability as compared to conventional HSLA steels[1–4] through the combination of C and N to form a dispersion of Nb(C, N) precipitates. The Nb(C, N) precipitates that form within the austenite matrix act to pin the grain boundaries and prevent recrystallization and excessive grain growth, thereby reducing the overall size of the ferrite grain.[5,6] The other strengthening component is by precipitation in the ferrite grains. The amount of Nb in solid solution at room temperature depends on the kinetics of Nb(C, N) in austenite and ferrite. Niobium carbonitride kinetics in ferrite has been studied rather infrequently[7] compared to the number of investigations lavished on precipitation in austenite. Investigating Nb carbonitrides in ferrite is comparatively difficult because of the low volume fraction of precipitates (104 to 103) and their fine size.[8] Niobium has a limited solubility in austenite and ferrite.[9,10] Consequently, during the thermomechanical processing of Nb microalloyed steels, the precipitation of Nb(C, N) particles in austenite inhibits softening and grain growth, providing microstructural refinement.[11] AddiA.M. ELWAZRI and R. VARANO, Research Associates, and S. YUE, Professor, are with the Department of Metals and Materials Engineering, McGill University, Montreal, PQ, Canada H3A 2B2. Contact e-mail: [email protected] D. BAI, Senior Research Engineer, is with ISPCO Company, Muscatine, IA 52761. F. SICILIANO, Market Development Engineer, is with CBMM Companhia Brasileira, and the Department of Metallurgy and Minerals, RHC Reference Metals Company, Inc. Manuscript submitted March 4, 2005. METALLURGICAL AND MATERIALS TRANSACTIONS A

tionally, during isothermal transformation, there is a strong tendency for Nb to precipitate in the ferrite, which has been found to increase the strength of low C steels through precipitation strengthening. These precipitates can be very fine, below 100 nm in size. Thus, proper characterization of such fine microstructural features requires highresolution imaging techniques. Normally, this would involve characterization using a transmission electron microscope (TEM). Nowadays, the field emission gun–scanning electron microscope (FE-SEM) is being used increasingly for the analysis of microstructural features in the nanometer size range. The FE-SEM conventionally uses bulk specimens. Therefore, there is a problem of the interaction volume lowering the resolution of the microscope. In other words, features below 100 nm can be difficult to characterize with respect to X-ray microanalysis using energy-dispersive spectroscopy (EDS). This limitation in the FE-SEM can be alleviated by using thin specimens that would normally be used in the TEM, such as carbon extraction replicas. In this work, small amounts of plastic deformation at very low working temperatures (e.g., 400 °C) in the single-phase ferrite region (i.e