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

One of the requirements to successfully produce high-strength pipeline steel is the optimization of precipitation strengthening. Some high-strength pipeline grades rely on increased levels of niobium (Nb); in these grades, it is important to ensure that all of the Nb is effectively employed. It is generally accepted that the choice of coiling temperature (i.e., the temperature of steel strip just before coiling) is critical in maximizing Nb carbonitrides in ferrite. Additional control of this precipitation may be attained by deformation at these coiling temperatures, an approach termed "cool deformation." In this work, steel specimens were heated to a temperature of 1200 degrees C and held for 20 minutes to ensure significant dissolution of the Nb precipitates. Some specimens were aged at 400 degrees C for times ranging from 10 minutes to 10 hours followed by air cooling. Others were subjected to deformation at 400 degrees C (cool deformation) prior to aging. It was found that cool deformation improves the mechanical properties (both yield and tensile strengths are significantly higher than those of the aged only specimens). A field emission gun-scanning electron microscope (FE-SEM) was applied successfully in the characterization of Nb precipitates using carbon extraction replicas. The FE-SEM observation of the replicas confirmed the presence of Nb precipitates in ferrite. The FE-SEM could analyze small particles (below 50 nm) embedded in the steel, but the analysis had to be carried out at low voltages to maximize spatial resolution, resulting in a poor signal. However, it is possible to analyze carbon extraction replicas in the FE-SEM using high voltages, because the interaction volume effect is no longer a problem.