Role of Heavy Deformation in Thermomechanical Processing on the Formation of Ultrafine-Grained Structure in Steels

The formation of ultrafine-grained structure in steels by various thermomechanical processings is reviewed from a metallurgical point of view. In the recent new type TMCP, ultrafine ferrite grains with a grain size of about 1μm are obtained when the austenite is heavily deformed at lower temperatures. In this case, dynamic phenomena such as dynamic recrystallization become prominent in the process. In the aging after heavy cold rolling of supersaturated matrix phase in two-phase alloys, the competition between the recovery or recrystallization of matrix phase and the precipitation of second phase occurs, resulting in various types of two-phase structures including microduplex structure. Microduplex structure is also obtained by annealing after heavy cold rolling of coarse two-phase structure in duplex stainless steel and high carbon steel. Recently, various severe plastic deformation processings, in which very large plastic strain over 4 is applied to the materials, have been developed to produce ultrafine grained materials with nanocrystalline and/or submicrocrystalline structures.