Effects of morphology and strength of martensite on cyclic deformation behaviour in dual-phase steels

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Abstract
Tension–compression cyclic deformation behaviour in dual-phase steels has been studied. Three different ferrite (α)–martensite (α′) microstructures, i.e. isolated α′-colonies dispersed in α-matrix (I), continuous α′ (C), and laminated α–α′ (L), were prepared by appropriate heat treatments, keeping the α′ volume fraction at ~0·3. The work hardening and the Bauschinger effect are found to be greater in microstructure C or L than in I when they are compared at an arbitrary forward (tension) prestrain level. An increase in the hardness of α′ enhances the Bauschinger effect and then narrows the stress–strain hysteresis loop. The stress evolved as a result of the Bauschinger stress (defined as the difference between forward prestress and backward (compression) 0·1% proof stress) is found to be almost independent of microstructure and hardness when it is compared at an arbitrarily fixed prestress level. The slip lines are very fine and relatively straight in microstructure C, but wavy in microstructure I. These findings are discussed from the standpoints of the accumulation of the average internal stress resulting from inhomogeneous plastic flow between two constituent phases and the plastic relaxation. MST/382