Ultrafine ferrite in low carbon steel

Abstract

In previous studies related to the austenite to ferrite transformation of controlled rolled steels it was shown that there is a limiting ferrite grain size of approximately 5 {micro}m regardless of the level of retained strain introduced into the austenite. This is at least partly due to ferrite coarsening during transformation. If, however, a 1 {micro}m grain size -- here called ultrafine ferrite (UFF) -- could be produced and retained then this would increase the yield strength by almost 350 MPa compared to a 5 {micro}m ferrite. Most recently, the current authors have reported the development of a new thermomechanical process which produces UFF in hot rolled steel strip. This grain refinement appears to be the result of a strain induced transformation reaction activated over a significant volume of the austenite. This requires high undercooling and high effective rolling strains. This paper describes how this combination of austenite conditioning, deformation conditions and temperature control can lead to the formation of significant volumes of UFF in low carbon steel.