Controlled rolling of steel plate and strip.

Abstract

The major purpose of controlled rolling is to refine grain structure and, thereby, to enhance both the strength and toughness of steel in the as-hot-rol1ed condition. If a survey is made of the development of controlled rolling, it can be seen that controlled rolling consists of three stages: (a) deformation in the recrystallization region at high temperatures; (b) deformation in the non-recrystallization region within a low temperature range above Ar3; and (c) deformation in the austenite–ferrite region. It is stressed that the importance of deformation in the nonrecrystallization region is in dividing an austenite grain into several blocks by the introduction of deformation bands within it. Deformation in the austenite-ferrite region gives a mixed structure consisting of equiaxed grains and subgrains after transformation and, thereby, it increases further the strength and toughness. The fundamental difference between conventionally hot-rolled and controlled -rolled steels lies in the fact that the nucleation of ferrite occurs exclusively at austenite grain boundaries in the former, while it occurs in the grain interior as well as at grain boundaries in the latter, leading to a more refined grain structure. In controlled-rolled steel a crystallographic texture develops, which causes planar anisotropies in mechanical properties and embrittlement in the through -thickness direction. The latter is shown to be the main cause of the delamination which appeared in the fractured Charpy specimens. Fundamental aspects of controlled rolling, such as the recrystallization behaviour of austenite, the retardation mechanism of austenite recrystallization due to 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.