Microstructure Evolution at Severely-deformed Ferrite/Martensite Interfaces in a Layer-integrated Steel

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The microstructure evolution at interfaces of a layer-integrated steel sheet constructed by ferritic (SPCC) and martensitic stainless (SUS420J2) steel layers, which were bonded through a cold-rolling and subsequently annealed at 1000°C, has been investigated using scanning transmission electron microscopy combined with energy dispersive X-ray spectroscopy. We find that microstructures around the SPCC/SUS420J2 interfaces are significantly reconstructed during a short-time annealing at 1000°C followed by quenching into water, and the resultant ferrite/martensite interface is found to be extended into the SPCC side from the original cold-rolled interface. Occurrence of such interface migration can be reasonably explained as due to a martensitic transformation across the composition-gradient interface that is caused by element diffusions during annealing at 1000°C. These microstructural characteristics are discussed by comparing with our recent observations of austenite (SUS304)/martensite (SCM415) interface microstructures (Hayashi et al., ISIJ Int., 49 (2009), 1406), which are formed via the same rolling/annealing procedures, aiming to provide a common aspect on how the strong bonding between the hetero-interfaces can be achieved in the layer-integrated steels.