This paper reports the effect of nano-precipitation strengthening of ferrite on the tensile behavior of ferrite-martensite dual phase (DP) steels. Samples of ferrite-martensite DP steel containing a dispersion of nano-sized vanadium carbides (VCs) in the ferrite phase were produced by interphase precipitation and quenching of a V-added low carbon steel, and the mechanical properties are compared with those of conventional ferrite-martensite DP samples without VC particles. Both the yield stress and the ultimate tensile strength are significantly increased by nano-VC precipitates. For ferrite volume fractions of 20-50% a dispersion of VCs results in only a small change in the elongation, whereas for ferrite volume fractions of above 50% both uniform and post-uniform elongations are decreased by a VC dispersion. It is suggested that dispersion of nano-precipitates in ferrite is an effective approach to simultaneously improve the strength and the strength-ductility balance of DP steels. Digital image correlation (DIC) analysis demonstrates that the ferrite phase is more deformed than the martensite phase in both VC-free and VC-dispersed DP samples, but that such strain partitioning is less pronounced in the VC dispersion-hardened samples. It is found that the stress-strain relationship of DP samples can reasonably be explained based on a law of mixtures using partitioned strain and stress values as estimated from the DIC analysis.KEY WORDS: dual phase (DP) steel; interphase precipitation; vanadium carbides (VC); mechanical property; digital image correlation (DIC); strain and stress partitioning.

In this study, the microstructure, tensile strength, elongation, and reduction of area of near-¢ Ti alloys (Ti-17) were investigated after being subjected to solution and aging treatments. Ti-17 was forged at temperatures between 700 and 850°C followed by air cooling. Then, the forged Ti-17 was subjected to solution treatment at 800°C for 4 h followed by water quenching and aging treatment at 620°C for 8 h followed by air cooling. Tensile tests were performed at room temperature, 450°C, and 600°C. The change in microstructure at different forging temperatures was exhibited by only the volume fraction and morphology of the grain boundary (GB) ¡ phase. That is, a granular GB ¡ phase was formed in the samples forged at 700 and 750°C. Moreover, a film-like GB ¡ phase was formed in the samples forged at 800 and 850°C. The tensile strength was the same for all the tested samples, indicating that the microstructure has little effect on the tensile strength. The elongation and reduction of area increased with decreasing volume fraction in the GB ¡ phase. It is considered that the film-like morphology slightly improves ductility.

The current understanding of the microstructural features and mechanical properties of micro-alloyed low carbon steels strengthened by interphase precipitation of nano-sized alloy carbides are critically reviewed in this paper. The experimental results obtained via advanced quantitative characterization have revealed that interphase precipitation is promoted at the ferrite/austenite interface with a relatively lower degree of coherency caused by the deviation from the exact Kurdjumov–Sachs orientation relationship. Its dispersion becomes refined by enlarging the driving force for its precipitation, as adjusted by changing the transformation condition and chemical composition. The occurrence of interphase precipitation can significantly increase the strength of steels due to its large precipitation strengthening, and maintain good ductility as a result of enhanced work-hardening and dynamic recovery in different stages of tensile deformation. Finally, the application of interphase precipitation to ferrite/martensite dual-phase steels, together with our outlook on the challenging points in future research, are briefly explained.

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