The major intent behind controlled rolling is usually to refine grain structure and, thereby, to further improve both strength and toughness of steel in the as-hot-rol1ed condition. In case a survey is made from the development of controlled rolling, it can be seen that controlled rolling contains three stages: (a) deformation in the recrystallization region at high temperatures; (b) deformation within the non-recrystallization region inside a low temperature range above Ar3; and (c) deformation within the austenite-ferrite region.
It is actually stressed that the significance of deformation from the nonrecrystallization region is dividing an austenite grain into several blocks by the roll-out of deformation bands inside it. Deformation inside the austenite-ferrite region provides a mixed structure comprising equiaxed grains and subgrains after transformation and, thereby, it improves further the strength and toughness.
The essential distinction between conventionally hot-rolled and controlled -rolled steels is based on the reality that the nucleation of ferrite occurs exclusively at austenite grain 34dexppky within the former, though it takes place in the grain interior as well as at grain boundaries from the latter, ultimately causing a much more refined grain structure. In Clad Plate a crystallographic texture develops, which then causes planar anisotropies in mechanical properties and embrittlement in the through -thickness direction.
The latter is demonstrated to be the main source of the delamination which appeared from the fractured Charpy specimens. Fundamental elements of controlled rolling, like 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.