The Determining Role of Finish Cooling Temperature on the Microstructural Evolution and Precipitation Behavior in an Nb-
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decades, high-strength low-alloy (HSLA) steels have been widely used in buildings, bridges, and ships because of their potential to obtain high strength–toughness combination.[1] Superior mechanical properties were obtained through optimization of alloy design in conjunction with thermo-mechanical processing (TMCP).[2–5] The improvement in mechanical properties was a consequence of grain refinement together with microstructural control and precipitation strengthening. The carbide-forming elements include titanium and niobium that facilitate grain refinement and contribute to dispersion hardening through carbide precipitation in the matrix. The relative contribution of the microalloying elements is determined by the solubility of carbides in the microstructure. Titanium is beneficial because it combines with nitrogen at relatively high temperature, preventing grain growth, while niobium can effectively retard recovery and recrystallization during hot rolling leading to grain refinement. Moreover, these two elements can precipitate as carbides from the supersaturated ferrite solid solution or precipitate as interphase precipitates during austenite-to-ferrite transformation increasing strength. Vanadium is less effective in grain refinement but contributes to strength via precipitation hardening because of higher solubility in austenite as compared to niobium, leading to precipitation at lower temperature.[6] TMCP involving ultrafast cooling (UFC) technology developed by our laboratory is being currently applied to industrial production,[7–9] with the aim to reduce the consumption of alloying elements and make the steel making process economically viable,[10–12] which can greatly suppress the degree of precipitation in austenite and increase supersaturation in ferrite or bainite,[13] and the precipitation hardening can be enhanced. Thus, it is of practical significance to understand the effect of cooling parameters on the microstructural evolution, precipitation behavior, and mechanical properties in microalloyed steels, such as cooling rate and finish cooling temperature in UFC. The effect of cooling rate was recently studied.[14–17] However, the effect of the finish cooling temperature in UFC has not been explored. In the present work, we focus on the effect of finish cooling temperature on microstructural evolution and precipitation behavior in Nb-V-Ti microalloyed steel, with particular focus on the nucleation site, chemical composition, size distribution, and crystallography. Furthermore, the contribution of the precipitation strengthening to yield strength was calculated using empirical equation. The chemical composition of steel studied (in weight percent) was 0.09 C, 1.05 Mn, 0.25 Si, 0.03 V, 0.025 Nb, VOLUME 47A, MAY 2016—1929
0.011 Ti, 0.02 Al, 0.0037 N, and balance Fe. The alloy was prepared by vacuum melting and cast into ingots of thickness ~100 mm. The ingots were homogenized at 1473 K (1200 C) for 1 h and then hot-rolled into a steel plate of 12 mm thickness via seven passes on the F450 mm trial rolling mill. The dyn
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