Processing of ultrafine ferrite steels

Recently, the grain refinement of metallic materials has been investigated intensively. There are two basic methods, i.e., severe plastic deformation and thermomechanical processing, to refine grain sizes of bulk steels for the ultrafine or submicron range. By severe plastic deformation submicron-grained microstructure (less than 1 μm) in steels can be obtained. For example, the microstructure with ultrafine grains of 0.3 μm in a commercial 0.15 wt.%C–1.1 wt.%Mn–0.25 wt.%Si steel was obtained after ECAP at an accumulative strain of about 4, and the microstructure with ultrafine grains of 0.4 μm in a Ti-added interstitial free (IF) steel was obtained after ARB at an accumulative strain of about 5.6. As a result of thermomechanical processing for the grain refinement, the ultrafine ferrite (UFF) grains (about 1 μm) of steels could be obtained through hot rolling in the intercritical region, warm rolling in the ferrite region, or strip rolling under the condition of strain-induced transformation from austenite to ferrite. A composite microstructure, consisting of UFF grains in the surface layers penetrated to about one-quarter to one-third of thickness and coarser ferrite (about 5–10 μm) and pearlite in the core of the strip, is introduced by strain-induced transformation. Finally, the relationship of strength and ductility is discussed. The strength of ultrafine-grained steels increases with decreasing ferrite grain sizes, following the Hall–Petch relationship. On the other hand, ductility decreases with decreasing ferrite grain sizes. The composite microstructure of UFF surface and coarser core in steels has an advantage to keep both high strength and high ductility.