Deformation and fracture mechanisms in fine- and ultrafine-grained ferrite/martensite dual-phase steels and the effect of aging

 Abstract

Three ferrite/martensite dual-phase steels varying in the ferrite grain size (12.4, 2.4 and 1.2 lm) but with the same martensite content(-30 vol.%) were produced by large-strain warm deformation at different deformation temperatures, followed by intercritical annealing.Their mechanical properties were compared, and the response of the ultrafine-grained steel (1.2 lm) to aging at 170 -C was investigated.The deformation and fracture mechanisms were studied based on microstructure observations using scanning electron microscopy and electron backscatter diffraction. Grain refinement leads to an increase in both yield strength and tensile strength, whereas uniform elon-gation 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 martens-ite plasticity and better interface cohesion. Grain refinement further promotes ductile fracture mechanisms, which is a result of theimproved 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. 2010 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

Keywords: Ultrafine grains; Dual-phase steel; Aging; Deformation mechanisms; Fracture mechanisms.Introduction Grain refinement of metals is essential as it is the only strengthening mechanism that simultaneously enhances the toughness of a material. In recent years, a variety of methods have been developed to produce ultrafine-grained(UFG) materials with a ferrite grain size of around 1 lm[1,2]. These methods can be divided into advanced thermo-mechanical processing (ATMP) routes, which aim at improving conventional processing routes in commercial large-scale rolling mills, and severe plastic deformation (SPD) techniques, which are essentially confined to labora-tory-scale sample dimensions. The ATMP methods cover deformation-induced ferrite transformation (DIFT) [3],large-strain warm deformation [4], intercritical hot rolling[5], multi-directional rolling [6] and cold-rolling plus annealing of martensitic steel [7]. The most important SPD techniques are equal-channel angular pressing (ECAP) [8], accumulative roll bonding [9] and high-pressure torsion.

Conclusions

Three low-carbon dual-phase steels with nearly constant martensite fraction around 30 vol.% martensite and differ-ent ferrite grain sizes (1.2, 2.4 and 12.4 lm) were produced by applying hot deformation and large-strain warm defor-mation at different deformation temperatures, followed by intercritical annealing. Their deformation and fracture mechanisms were studied based on tensile test data and microstructure observations. The BH response was investi-gated for the UFG steel.

Click here to download full paper.