Influence of martensite content and morphology on tensile and impact properties of high-martensite dual-phase steels

Abstract

A series of dual-phase (DP) steels containing finely dispersed martensite with different volume fractions of martensite (V m ) were produced by intermediate quenching of a boron-and vanadiumcontaining microalloyed steel. The volume fraction of martensite was varied from 0.3 to 0.8 by changing the intercritical annealing temperature. The tensile and impact properties of these steels were studied and compared to those of step-quenched steels, which showed banded microstructures. The experimental results show that DP steels with finely dispersed microstructures have excellent mechanical properties, including high impact toughness values, with an optimum in properties obtained at ϳ0.55 V m . A further increase in V m was found to decrease the yield and tensile strengths as well as the impact properties. It was shown that models developed on the basis of a rule of mixtures are inadequate in capturing the tensile properties of DP steels with V m Ͼ 0.55. Jaoul-Crussard analyses of the work-hardening behavior of the high-martensite volume fraction DP steels show three distinct stages of plastic deformation.

Micromechanical modeling of dual phase steels

“…They relate this to the martensite phase becoming softer as its volume fraction increases. Other authors (Bag et al (1999)) show that softening of the aggregate occurs at v,,, ~ 0.55 and not V,II = 0.3 and this explains why they get three distinct regions ev en though they used VIII> 0.3. The difference is thought to be due to different alloying and morphology used in each investigation.…”

“…Increasing the volume fraction of the harder phase was found to increase the yield and ultimate strengths of the aggregate. Bag et al (1999) reported that the increase in strength with v'll only extends up to v'll ~ 55% after which a reduction in strength is observed. The same was observed by Byun and Kim (1993) but at a different value of VIII ' Shen, Lei and Liu (1986) have observed this as weIl without specifying the value of VIII at which this takes place.…”

“…When the Joaul-Crussard analysis is employed, each region above corresponds to a distinct mechanism in the elastic-plastic deformation process (Bag et al (1999) &Byun andKim (1993)). The Joaul-Crussard analysis is based on the following equation:…”

The effect of cooling rates on the microstructure and mechanical properties of thermo-mechanically processed Ti–Al–Mo–V–Cr–Fe alloys

“…Table 5 shows Vickers micro-hardness values for α and β phases from which 0.2% offset yield strength of the samples is calculated using the rule of mixtures 2 The work hardening behaviour is analysed using the modified Crussard -Jaoul (C-J) model [59][60][61][62][63], as the samples contain two phases similar to dual phase steel for which this model was developed. The modified C-J model uses the following Eq.…”

“…It is mentioned in [61][62]72] that three stages in the work hardening behaviour depend upon the amount and type of the constituents present in the microstructure. Stage I corresponds to the plastic deformation of β phase that work hardens due to the glide of dislocations and formation of pile ups in the vicinity of the hard α phase grains which serve as barriers to dislocation motion and initially do not deform plastically.…”