Abstract
This work investigated the microstructural effect on stretch flangeability of ferrite–martensite dual-phase (DP) steels. Three types of DP steels with various martensitic structures were prepared for the research: fibrous martensite in water-quenched (WQ) sample, chained martensite in air-quenched (AQ) sample, and coarse martensite in step-quenched (SQ) sample. The WQ specimen exhibited the highest mechanical strength and hole expansion ratio compared to the AQ and SQ samples despite their similar fraction of martensite. Such a result was explained in view of uniform distribution of fine martensite and high density of geometrically necessary dislocations in the WQ specimen. Meanwhile, most cracks initiated at either rolling or transverse direction during the stretch flangeability test regardless of the martensitic morphology. It was attributed to the highest average normal anisotropy in the direction of 45° to rolling direction.
Highlights
Dual-phase (DP) steels composed of hard martensitic particles and soft ferritic matrix have received great attention for automotive applications due to their good combination of strength and elongation [1]
The volume fraction of martensite was comparable among the specimens, the cooling rate exerted a significant influence on the other microstructural characteristics
The specimen exhibited an inhomogeneous distribution of ferrite and martensite in rolling direction–normal direction (RD-ND) and transverse direction–normal direction (TD-ND) planes (Figure 1c)
Summary
Dual-phase (DP) steels composed of hard martensitic particles and soft ferritic matrix have received great attention for automotive applications due to their good combination of strength and elongation [1]. The mechanical properties of ferrite–martensite DP steels strongly depend on the volume fraction and morphology (e.g., size, shape, distribution, continuity) of martensitic constituent. Chang and Preban [3] described tensile properties of DP steel in terms of a ferritic grain size and martensitic volume fraction. Molaei and Ekrami [5] observed the higher fatigue strength of DP steels with fibrous martensite (i.e., the structure composed of parallel and narrow laths [6]) in comparison to the alloy with networked martensite (i.e., the structure connected through a phase interface [6]) due to the tortuous crack propagation in the former microstructure
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