Effect of Initial Microstructure and Parameters of Annealing of 4 and 6.7 Mn steels on the Evolution of Microstructure and Mechanical Properties

摘要

There are growing efforts aiming to increase significantly the balance of strength and ductility of Advanced High Strength Steels (AHSS) to improve safety performance and fuel consumption of mo dern automobiles. Recently, the unique combination of mechanical properties has been reported in 4～10% Mn containing steels after long term (batch) annealing in the inter-critical temperature range. This is mostly attribu ted to Transformation Induced Plasticity (TRIP) related to the significant amount of reta ined austenite that was stabilized by high Mn content due to Mn partitioning between ferrite and austenite. The previous investigation by a uthors has shown tha t superior combination of strength and ductility in such steels can be obtained after short tim e annealing and the significant amount of austenite was retained regardless of cooling rate without any isothermal holding in the bainite region. The stability of austenite was appeared to be a result of a combination of fine grain size, and high Mn / carbon content in the austenite at specific intercritical temperature. Therefore the control of gr ain size and Mn/C partitioning during annealing is a key process to achieve superior properties in the steels. The special focus of this paper was to investigate effect of initial microstructure of hot rolled band prior to cold rolling on the evolution of microstructure and mechanical properties after short time annealing.