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

Abstract

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 1 000°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 1 000°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 1 000°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.KEY WORDS: steels; interface structure; scanning transmission electron microscopy; phase transformation; severe plastic deformation. Experimental ProcedureFor microstructural studies of the SPCC/SUS420J2 interface, a three-layered sheet was designed using commercialgrade roll steel SPCC and martensitic stainless steel SUS420J2; the SUS420J2 layer is sandwiched by the two SPCC layers. Compositions of these steels are summarized in Table 1. The three-layered sheet was submitted to a cold rolling process with reduction of ϳ60 % in thickness, and consequently the SPCC and SUS420J2 layers become 0.16 mm and 0.33 mm in final thickness, respectively (see Fig. 1 in Ref. 11 for details of specimen dimensions). Figure 1 shows the as-rolled SPCC/SUS420J2 microstructure by scanning electron microscopy (SEM), revealing that some thick oxide blocks (0.5-1 mm in thickness) are sparsely distributed along the as-rolled interface (see white arrows in Fig. 1) at which obvious voids are not observed at this magnification. The oxide blocks are presumably inherited from the original surface oxide layers (the sheet surfaces have been scratch-brushed before the cold-rolling 9) ), and, in fact, similar block oxides were also observed for the previous as-rolled SCM415/SUS304 interface treated by the same manner. 11,12) In the present work, we investigate microstructure evolutions at the apparently well-bonded interface region in Fig. 1 (namely, we do not deal with any microstructural change of the block oxides during the subsequent annealing).The rolled samples were annealed at 1 000°C for 0-10 min followed by quenching into water ("0 min" annealing means that the specimen was immediately quenched when it reached the annealing temperature 1 000°C). Note that water quenching is es...

Complex Oxide Layer at a Nickel/Steel Interface Bonded under a Moderate Vacuum Condition

“…The dissimilar steels are firmly bonded even without protective atmosphere, because surface oxide layers on the original steel sheets are mostly crushed into small pieces due to a severe plastic deformation during cold-rolling (reduction in thickness ~60%), and recrystallized grains grow and stride over the original cold-rolled interfaces during the annealing. 11,12) During systematic attempts of various roll-bonding procedures of the laminated steels, Nambu et al have found that even the annealing below a recrystallization temperature is able to achieve a firm bonding when combined with a unique surface activated bonding (SAB) under a moderate vacuum condition (~10 -3 Pa). The SAB is a solid-state bonding method via pre-surface polishing/activation by Arion sputtering to obtain clean surfaces, followed by in-situ cold rolling with a small reduction in thickness (~1-3%); the both processes are performed in a pre-evacuated system.…”