Toward Eliminating Discontinuous Yielding Behavior of the EA4T Steel

Cold-rolled EA4T steel was heat-treated by inter-critical holding at 755 °C for 90 s, 120 s, 180 s, and 240 s, respectively, and then quenching in water. The tensile testing results of the EA4T specimens show an evident transition from the discontinuous yielding to the continuous yielding of the steel specimens by prolonging the holding time. A novel relationship between the discontinuous yielding behavior of tensile-deformed steel specimens and the carbide size was proposed based on experimental results and Cottrell’s theory. The model may provide a new clue for avoiding discontinuous yielding and improving mechanical properties of metals with static strain aging behaviors.

Effects of DP steel microstructure on the disappearance of discontinuous yielding

Although in terms of quality is not as high as TRIP steel, DP steel has been developed as an option to meet the minimum standard industrial requirement for automotive. Steel development for automotive material leads to posses high strength without suffering its ductility and weld ability, in addition there is no appearance of discontinuous yielding that cause stretcher strain marking. The change of micro structure as a result of the intercritical annealing process not only increases the strength of Dual Phase Steel, but also eliminates the discontinuous yielding. The phenomenon of discontinuous yielding is no longer appear in Dual Phase Steel which heating time on intercritical annealing process more than 1 minute, the minimum time is equal to the disappearance of discontinuous yielding. The phenomenon of upper yield point and discontinuous yielding on carbon steel is strongly influenced by the magnitude of the stress acting on the Ferrite phase to move the dislocation of the pinned state by the interstitial atom. From the recent works gives the result that the disappearance of discontinuous yielding in Dual Phase Steel resulting from the intercritical annealing process makes Dual Phase Steel very suitable as an automotive material.