Isothermal Transformation Behaviour of Extremely Low Carbon 3%Mn-1%Cr Steels

Abstract

Isothermal decomposition processes of austenite in ultra-low carbon 3%Mn-1%Cr steels have been investigated mainly by means of surface relief and microstructural observations and the following results have been obtained: (1) Bainitic reaction occurs even in the very low carbon steels where carbide particles do not precipitate. (2) Widmanstatten ferrite sub-units grow often from grain boundary ferrite allotriomorphs, while bainite nucleates specifically at an austenite grain boundary. These structures grow into austenite in the shape of laths, the longitudinal growth of them, however, being thought to be displacive. (3) The coalescence of them and the further diffusional growth lead to the formation of Widmanstatten ferrite and bainite laths, respectively. (4) The differences in the formation mechanism between Widmanstatten ferrite and bainitic ferrite exist in the diffusional nucleation events, i.e., the former occurs probably in the orthoequilibrium condition while the latter is in the paraequilibrium condition. © 1994, The Iron and Steel Institute of Japan. All rights reserved.