Microstructural evolution of the bonding zone in TRIP-TWIP laminate produced by accumulative roll bonding

Abstract

For the first time a four-layered laminate made of X5CrMnNi-16-6-6 (TRIP steel) and X5CrMnNi16-6-9 (TWIP steel) was manufactured by accumulative roll bonding (ARB). An intermediate annealing between rolling passes allowed to achieve good bonding between the laminate layers. The microstructure of the laminate at each step of production was analysed with the main focus on the bonding interface and deformation lenses by means of scanning electron microscopy, including chemical and orientation mapping techniques, and microhardness measurements. Additionally, a special measuring routine was developed to reveal spatial distribution of the grain size along the cross section. Finally, tensile specimens were cut out from the laminate and tested under quasi-static loading with subsequent fracture surface analysis. Repeated rolling and annealing of the steel layers resulted in the alteration of the grain size and microhardness, whereas significant texture appeared only after tensile loading. The grain size in the layer of TWIP steel after first roll bonding and annealing was found to drop down near the surface, whereas no such effect was found in TRIP steel. The microstructure of a deformation lens was found to fully change from severely deformed brittle steel matrix with excessive oxygen content after rolling to a ductile mixture of sub-micron globular oxides and austenite grains after annealing. These ductile deformation lenses ensure excellent bonding of layers and microcrack blunting. This allowed to achieve a remarkable combination of mechanical properties of the four-layered TRIP-TWIP laminate with a yield strength up to 800 MPa and an elongation to failure up to 45%, which resulted in the work of deformation up to 41 GPa·%.