The most important function of controlled rolling is to refine grain structure and, thereby, to improve both the strength and toughness of steel within the as-hot-rol1ed condition. If a survey consists of the development of controlled rolling, it may be seen that controlled rolling includes three stages: (a) deformation in the recrystallization region at high temperatures; (b) deformation inside the non-recrystallization region in a low temperature range above Ar3; and (c) deformation from the austenite-ferrite region.
It really is stressed that the necessity of deformation in the nonrecrystallization region is at dividing an austenite grain into several blocks by the roll-out of deformation bands in it. Deformation in the austenite-ferrite region gives a mixed structure made up of equiaxed grains and subgrains after transformation and, thereby, it improves further the strength and toughness.
The fundamental difference between conventionally hot-rolled and controlled -rolled steels is based on the reality that the nucleation of ferrite occurs exclusively at austenite grain 34dexppky in the former, even though it happens in the grain interior and also at grain boundaries inside the latter, ultimately causing an even more refined grain structure. In Galvalume Steel Coil a crystallographic texture develops, which then causes planar anisotropies in mechanical properties and embrittlement from the through -thickness direction.
The latter is shown to be the main source of the delamination which appeared within the fractured Charpy specimens. Fundamental areas of controlled rolling, including the recrystallization behaviour of austenite, the retardation mechanism of austenite recrystallization because of niobium, microstructural changes accompanying deformation, factors governing strength and toughness, etc., are reviewed. The practice of controlled rolling in plate and strip mills is outlined.