Kinetics of Strain Aging in Cold Rolled Low Strength Multiphase Steel

The kinetics of strain aging in cold rolled multiphase steel processed for a yield strength of 250 MPa and a tensile strength of 450 MPa (250/450 MPa grade of mechanical strength) was studied by means of aging experiments in the temperature interval from 50 to 185°C and time intervals ranging from 1 to 4915 min followed by tensile tests.The aging kinetics law was determined in terms of changes in the bake hardening value with the aging time and temperature for specimens with a tensile prestrain of 0.5%. The steel studied showed two strain aging stages, the first one between 50°C and 155°C (for times shorter than 9 min) and the second one between 125°C (for times longer than 211 min) and 185°C. The changes in the bake hardening value suggest, for the first stage, a process controlled by the locking of the dislocations in the ferrite due to the formation of clusters and/or transition carbides, such as the e-carbide, with an activation energy close to 70 kJ/mol and following a kinetic law with a time exponent of 0.4. In the second stage, the phenomenon is controlled by tempering of martensite, particularly the precipitation of transition carbides, e-carbide and/or η-carbide. The corresponding activation energy is approximately 130 kJ/mol and the kinetics of this stage can be described with a time exponent of 0.5.