Abstract
A new high strength steel with dual matrix structure and exceptionally high toughness plus ductility have been produced by intermediate quenching of 0.22_wt% C microalloyed steel. The treatment consisted of initial austenitization and rapid quenching of the steel samples to achieve a fully martensitic state followed by annealing in the intercritical (α+γ) region of 730℃-810℃ for the period of 30, 60 and 90_minutes. These samples were subsequently quenched to obtain dual phase microstructure containing varying proportions of ferrite and martensite constituents. The mechanical properties of the samples were measured according to ASTM standard and their microstructures were analyzed by optical microscopy. The experimental results show that martensitic dual phase (MDP) steel samples developed within the intercritical temperature range of 770–790℃ revealed finer martensite and precipitate-free ferrite microstructure. The tensile and impact properties of the developed HMDP steels increased with intercritical annealing (ICA) temperatures, with an optimum properties obtained at 790℃ mainly due to finer microstructure of the constituent phases and absence of carbide precipitate that permit ease of dislocation flow. A further increase in the intercritical annealing temperature beyond 790℃ led to general decrease in the mechanical properties. This is attributed to the formation of coarse structure in this region. The results further show that with increasing intercritical treatment time from 30 to 90 minutes, the general mechanical properties of the MDP steels were found to increase except at the higher temperature of 810℃ which showed decreasing values. In general, the tensile strengths and ductility as well as the impact properties of the developed dual phase steel samples are greatly improved with the intercritical heat treatment investigated.
Highlights
The interest for high strength Dual-phase (DP) steel sheets having excellent ductility has increased significantly in recent years in various industries and in the automotive industry in particular [1]
4) Finer distribution of ferrite and martensite phases were obtained for intermediate quench (IQ)-treated martensitic dual phase (MDP) steel samples treated within the intercritical annealing treatments temperature range of 7707900C
Samples that were intercritically annealed at 810oC exhibited coarse structure and the MDP steels heat-treated at lower intercritical annealing temperatures (730oC-750oC) were found to contain coarse ferrite with carbide precipitates; the effect of which lowered the properties of the material
Summary
The interest for high strength Dual-phase (DP) steel sheets having excellent ductility has increased significantly in recent years in various industries and in the automotive industry in particular [1]. The tensile strength of the composite can be approximated by simple rule-of-mixture [3,4]: δDP = VFδF + VMδM, where δ is tensile strength and V is the volume fraction of the phase and the subscripts DP, F and M indicate the composite dual phase structure and the ferrite and martensite phases respectively Both the phase fractions and the properties of martensite can be adjusted by controlling the steel chemistry and intercritical annealing (ICA) temperature in the ferrite plus austenite region just prior to rapid quenching [4]. It has been shown that the ductility and fracture toughness can satisfactorily be optimized by developing microstructures with fine distribution of ferrite and martensite in DP steel either by introducing new alloy design or adopting an appropriate heat treatment procedure [3, 6, 7]. This paper present the effects of the intercritical thermal treatment parameters (temperature and time) on the microstructures and mechanical properties of 0.22 wt% C microalloyed steel
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