Effect of Cool Deformation on Mechanical Properties of a High-Strength Pipeline Steel

Effect of Cool Deformation on Mechanical Properties of a High-Strength Pipeline Steel

A.M. ELWAZRI, R. VARANO, F. SICILIANO, D. BAI, andS. YUE

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 gen-
erally 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 °,C and held for 20 minutes t
ensure significant dissolution of the Nb precipitates. Some specimens were aged at 400 °,C for time
ranging from 10 minutes to 10 hours followed by air cooling. Others were subjected to deformation
at 400 °,C (cool deformation) prior to aging. It was found that cool deformation improves the mechan
ical properties (both yield and tensile strengths are significantly higher than those of the aged only
specimens). A field emission gun(R)Cscanning electron microscope (FE-SEM) was applied successfull
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.

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