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addition of microalloying additives, such as Nb, Ti, and V, to steels combined with thermomechanical controlled processing (TMCP) can enhance the strength and ductility of steel. The improvement of mechanical properties results mainly from the refinement of ferrite grain size, precipitation, solid-solution strengthening, and dislocation strengthening.[1–3] The first three can be expressed by Eq. [1]. Dislocation strengthening is not included in Eq. [1] because cold deformation is not used commonly in hot-rolled or normalized microalloyed steels.[3]

JUNFANG LU, Engineer, is with the Enbridge Pipelines Inc., Edmonton, AB T5J 3N7, Canada. OLADIPO OMOTOSO, Research Engineer, is with the Suncor Energy Inc., Fort McMurray, AB T9H 3E3, Canada. J. BARRY WISKEL, Research Associate, and DOUGLAS G. IVEY and HANI HENEIN, Professors, are with the Department of Chemical and Materials Engineering, University of Alberta, Edmonton, AB T6G 2V4, Canada. Contact e-mail: [email protected] Manuscript submitted June 16, 2011. Article published online March 29, 2012 METALLURGICAL AND MATERIALS TRANSACTIONS A

1=2

rys ðMPaÞ ¼ ri þ ky d þ

X

1=2

ki Ci

10:8vf þ X

!   X ln 6:125  104 ½1

where rys is the yield stress, ri is the friction stress of iron, ky is the strengthening coefficient for grain size, d is the grain diameter in microns, X is the size of precipitates in microns, vf is the volume fraction of a given precipitate size (X), ki is the strengthening coefficient for solute strengthening of solute i, and Ci is the concentration of solute i. The TMCP schedule is illustrated in Figure 1, and it includes the following steps[1]: (a) Reheating the steel to a selected temperature to dissolve many of the microalloyed precipitates (with the notable exception of TiN, which may only partially dissolve) in the steel. (b) Rough rolling at temperatures above the no-recrystallization temperature (Tnr) to break down the austenite grains through multiple recrystallization cycles. (c) Finish rolling at temperatures below Tnr but above the austenite to ferrite transformation temperature (Ar3), to produce heavily pancaked austenite. VOLUME 43A, SEPTEMBER 2012—3043

Fig. 1—Schematic of a thermomechanical-controlled processing (TMCP) schedule.

(d) Accelerated cooling from a temperature above Ar3 down to a temperature at which the desired stable microstructure can be obtained. (e) Slow cooling of the coil, during which fine precipitates are formed. Precipitation occurs during TMCP. It is a complex process because it takes place within the steel matrix and the matrix itself is evolving continuously from the austenite to ferrite. Many factors affect the precipitation of nanoprecipitates, such as steel chemistry, processing histories, strain, etc. Different sized precipitates have been identified in microalloyed steels, such as large TiN precipitates that are several microns in size and Nb-rich precipitates less than 5 nm in size. Intermediate-size Ti/Nb precipitates have been observed as well.[4] The size difference among these precipitates is caused