Ferrite Grain Size Distributions in Ultra-Fine-Grained High-Strength Low-Alloy Steel After Controlled Thermomechanical D
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induced ferrite transformation at the surface layers of low-carbon steel strip (~2-lm thickness), using a laboratory rolling mill. Morimoto et al.[16] reported the industrial production of fine-grained steel at Nakayama Steel (Osaka, Japan) with a grain size of 2 to 5 lm in 2-mm-thick plain-C steel strip by applying asymmetric rolling. Previous studies indicated that the presence of microalloying elements (especially, Nb) in steel can promote DSIT by retarding both dynamic recrystallization (DRX) of deformed austenite, as well as the static c fi a transformation, and therefore, shifts the Tdef required to form UFF grains, to higher values (greater than 1073 K [800 C]).[17–20] An additional benefit of shifting the deformation temperature to higher values is the further softening of the steel, which makes it easy to roll. Grain boundary (GB) pinning from fine-strain-induced Nb precipitates can also retard the grain growth, subsequent to DSIT.[17–20] Pre-existing microalloy precipitates (undissolved during soaking) are inactive in retarding c-recrystallization and preventing the a-grain growth.[1] Therefore, to achieve fine-scale (strain-induced) microalloy precipitation during deformation, complete dissolution of pre-existing microalloy precipitates is required at the soaking stage. For that reason, high soaking temperature is preferred in industrial plate/ strip rolling.[1] Although previous studies looked into the effect of prior c-grain sizes, the grain structures are always represented in terms of average c-grain sizes (e.g., 70 to 80 lm for coarse grain structures and 10 to 20 lm for fine grain structures[17,18,21,22]). In reality, mixed grain structures, having bimodal austenite grain size distribution can develop after industrial soaking of microalloyed grades, where both coarse- and fine-cgrain sizes coexist.[4] The effect of such ‘‘wide’’ c-grain size distribution on DSIT was not considered before. In a similar way, the potential of DSIT and the extent of a-grain coarsening during cooling were evaluated through the average (a) grain sizes and the grain aspect ratios.[7–30] A limited number of studies characterized the entire a-grain size distributions and related those with the processing parameters.[18,21,23,24] The present study throws some light on this aspect. The possibility of breaking down a single, heavy deformation pass into multiple, lighter deformation passes was also explored to make the UFF structures industrially feasible.
II.
EXPERIMENTAL DETAILS
Low-carbon Nb-Ti-V microalloyed steel was used for experimental study. The chemical composition of the steel is given in Table I. The steel was provided in the form of an as-cast slab of ~200-mm thickness. A steel block (20 mm 9 60 mm 9
Table I. C 0.09
200 mm) was machined from the quarter-thickness location of the as-cast slab and is reheated to 1473 K (1200 C) for 1 hour inside a furnace, before ice-water quenching. Plane-strain compression testing samples (rectangular blocks of 10 mm 9 15 mm 9 20 mm) were machined from the steel block and are tested in
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