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Deformation and fracture in fine- and ultrafine-grained ferrite-martensite dual-phase steels

For this project three ferrite/martensite dual-phase steels varying in the ferrite grain size (12.4, 2.4 and 1.2 um) but with the same martensite content (30 vol.%) were produced by large-strain warm deformation at different deformation temperatures, followed by intercritical annealing.

The mechanical properties of these dual phase steels were compared, and the response of the ultrafine-grained steel (1.2 um) to aging at 170 °C was investigated. The deformation and fracture mechanisms were studied based on microstructure observations using scanning electron microscopy (SEM) and electron backscatter diffraction (EBSD). Grain refinement leads to an increase in both yield strength and tensile strength, whereas uniform elongation and total elongation are less affected. This can be partly explained by the increase in the initial strain-hardening rate. Moreover, the stress/strain partitioning characteristics between ferrite and martensite change due to grain refinement, leading to enhanced martensite plasticity and better interface cohesion. Grain refinement further promotes ductile fracture mechanisms, which is a result of the improved fracture toughness of martensite. The aging treatment leads to a strong increase in yield strength and improves the uniform and total elongation. These effects are attributed to dislocation locking due to the formation of Cottrell atmospheres and relaxation of internal stresses, as well as to the reduction in the interstitial carbon content in ferrite and tempering effects in martensite.

Fig:  EBSD maps taken perpendicular to the fracture surface of the coarse grained (CG) specimen (a–c), the ultra fine grained (UFG) specimen (d–f) and the UFG-BH specimen (g–i). (a, d, g) IQ maps of areas within the area of UE. IQ maps taken close to the neck (b, e, h) and the respective IPF maps (c, f, i) show the microstructure evolution during straining. The tensile direction is horizontal and equals the rolling direction; the normal direction is vertical.

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