Dual-phase Specimens Research Articles

Characterization of local deformation and fracture behavior in ferrite + martensite dual-phase steels having different grain sizes

Low carbon dual-phase (DP) steels composed of soft ferrite and hard martensite have been widely used in the automotive industry due to their good strength-ductility balance and large strain hardening ability. DP steels have a wide variation in mechanical properties depending on several microstructural features such as grain size, phase fraction and distribution. Among them, the grain refinement of DP steels is known to be an effective option for enhancing mechanical performance in strength and ductility (especially post-uniform elongation). However, the exact reason for the significant improvement of post-uniform elongation by grain refinement has not been fully understood. It is considered that the characterization of local deformation behavior and micro-void formation/growth behavior in connection with microstructures is an essential approach for understanding the enhanced post-uniform elongation realized in the fine-grained DP specimen. In the present study, we prepared two kinds of DP specimens with mean ferrite grain sizes of 14.9 μm (coarse-grained DP) and 7.1 μm (fine-grained DP), and carefully investigated local strain distribution of tensile specimen and micro-void formation/growth behavior using digital image correlation (DIC) analysis and SEM observations. The fine-grained DP specimen exhibited a gradual strain localization after necking and had sufficient strain capacity that could endure against fracture. The fine-grained DP structure had a great number of micro-voids in the necked region, but almost all the micro-voids maintained a very small size, which was contrasted with the case of coarse-grained DP specimen containing very large-sized micro-voids. Such a significant difference in micro-void size/number characters between two kinds of DP specimens would be one possible reason for exhibiting greatly different post-uniform elongation behavior.

Effect of martensite volume fraction on abnormal work hardening behavior of a low carbon low alloy ferrite–martensite dual-phase steel

Abstract This paper investigates the work hardening behavior of a low carbon low alloy steel under various ferrite-martensite dual-phase (DP) microstructures. For this purpose, several DP microstructures containing 23- 87% martensite volume fraction (MVF) were developed by application of inter-critical annealing heat treatment processes. The Hollomon analysis was employed in conjunction with mechanical testing and electron microscopy observation to describe the work hardening behavior of DP samples. The experimental results indicated that the DP specimens containing lower than 36% MVF were associated with a ‘single stage’ of work hardening behavior while those of higher than 36% of martensitic microconstituents were related to a ‘double stage’ of work hardening pattern. With increasing the MVF from 23 to 87%, the strain hardening exponent for the first stage of work hardening showed a peak value of 0.111 at MVF = 73%, while those for the second stage of strain hardening exponents were continuously increased from 0.162 to 0.197. This abnormal trend of strain hardening exponents with MVF was correlated with the variable hardening responses of ferrite and martensite microconstituents.

Reason for high strength and good ductility in dual phase steels composed of soft ferrite and hard martensite

Dual phase (DP) steels in which the microstructures are composed of a soft ferrite phase and a hard martensite phase are known to show good strain-hardening, high strength and large elongation, but reasons for their superior mechanical properties are still unclear. In the present study, two types of DP structures, having either networked martensite or isolated martensite were fabricated in a low-carbon steel by different heat treatment routes, and their tensile deformation behavior was analyzed using the digital image correlation (DIC) technique. It was revealed that the DP specimens having networked martensite microstructures showed a better strength-ductility balance than the DP specimens with isolated martensite structures. The microscopic DIC analysis of identical areas showed that the strain distribution within the DP microstructures was not uniform and the plastic strain was localized in soft ferrite grains. The strain localized regions tended to detour around hard martensite but eventually propagated across the martensite. It was found also from the DIC analysis that the degree of strain partitioning between ferrite and martensite in the networked DP structure was lower than that in the isolated DP structure. The deformation became more homogeneous when the hard phase (martensite) was connected to form a network structure, which could be one of the reasons for the better strength-ductility balance in the networked DP structure compared to that in the isolated DP structure.

Ductility of Advanced High-Strength Steel in the Presence of a Sheared Edge

The ductility of dual-phase (DP) 980 and transformation-induced plasticity (TRIP) assisted bainitic ferritic (TBF) 980 steels was studied in the presence of a sheared edge. Specimens were tested in uniaxial tension in a standard test frame as well as in situ in the scanning electron microscope (SEM). Incremental tensile straining was done in the SEM with images taken at each strain increment. Then digital image correlation (DIC) was used to compute the effective strain at the level of the individual phases in the microstructure. Shear banding across multiple phases was seen in strained TBF specimens, while the DP specimens exhibited more of a patchwork strain pattern, with high strains concentrated in ferrite and low strains observed in the martensite. Two-point statistics were applied to the strain data from the DIC work and the corresponding microstructure images to evaluate the effect of phase hardness on localization and fracture. It was observed that the DP 980 material had a greater tendency for localization around hard phases compared to the TBF 980. This at least partially explains the greater ductility of the TBF material, especially in specimens where a sheared edge was present.

Effect of prior austenite carbon partitioning on martensite hardening variation in a low alloy ferrite–martensite dual phase steel

Abstract The aim of this research work is to investigate in detail the carbon partitioning within prior austenite developed during austenite to ferrite phase transformation, and consequently its relation to the martensite hardening variation in a low alloy ferrite–martensite dual phase (DP) steel. For this purpose, a wide variety of ferrite–martensite DP samples with different volume fractions of ferrite and martensite have been developed using step quenching heat treatment processes at 600 °C for various holding times after being austenitized at 860 °C for 60 min. Both spot and line scan energy dispersive X-ray spectroscopy for carbon analyses have been used in conjunction with nanoindentation tests to follow the variation of carbon partitioning within prior austenite areas and consequently the associated martensite hardening response in the DP specimens. Experimental results showed that the martensite hardening behavior was quite variable in the ferrite–martensite DP samples and even within a specific martensite area within a specific DP microstructure. A higher level and also a more scattered nanohardness were observed for martensite in the DP samples treated at 600 °C for longer holding times. These results were rationalized due to the variation of carbon partitioning within the prior austenite area developed during isothermal holding in the ferrite–austenite DP region. Longer isothermal holding times were associated with more carbon redistribution within prior austenite as a consequence of more ferrite formation, which resulted in the formation of harder martensite with a more scattered hardening response. Furthermore, compared to the central locations of martensite area, those nearer to the ferrite–martensite interfaces contained higher carbon concentration and consequently higher hardening responses.