Yielding and Strain Aging Behaviors of an Fe-0.07C-1.6Mn Dual-phase Steel

Abstract

Yielding and strain aging behaviors of an Fe-0.070-1.7Mn dual phase steel were investigated by tensile testing after various cooling schedules from intercritical annealing.
Mechanical properties did not vary much with cooling rates between 20°C/sec and 100°C/sec when cooled from annealing at 720°C. At slower cooling rates than 20°C/sec discontinuous yielding was observed. The discontinuous yielding seemed to be caused by diffusion of interstitial solute to mobile dislocations below the austenite-martensite transformation temperature. Existence of pearlite did not affect the yielding behavior of the dual-phase steel.
Aging experiment between 60°C and 230°C with 2% prestrain showed that two stages of strain aging appeared with prolonged aging even at a constant temperature. The activation energies for the first and the second stages aging were estimated as 17 000cal/mol and 28 000cal/mol, respectively. The first stage was related to carbon diffusion to mobile dislocations and the second stage was explained to be due to precipitation growth. Manganese alloying did not affect the diffusion rate of carbon atoms at the aging process.