Effect of Heat-Treatment Schedule on the Microstructure and Mechanical Properties of Cold-Rolled Dual-Phase Steels

doi:10.1007/s40195-015-0305-x

Abstract

Abstract:

Low-carbon (0.08 wt% C) steel has been subjected to three different heat treatments to obtain dual-phase steels with different microstructures. An understanding of structure-property was established through tensile tests, in conjunction with scanning electron microscope and transmission electron microscope. The results show that the steel after intermediate quenching (IQ) consisting of fine and fibrous martensite exhibited the intermediate strength, highest elongation and the best comprehensive performance of mechanical properties, whereas the steel subjected to intercritical annealing (IA) produced a network martensite along ferrite grain boundaries, having the lowest strength and intermediate elongation. Besides, step quenching (SQ) resulted in a coarse and blocky ferrite-martensite microstructure showing the worst mechanical properties of the three different heat-treatment conditions. The strain-hardening behavior was studied through the modified Crussard-Jaoul model, indicating two stages of strain-hardening behavior for all three samples. The highest magnitude of strain-hardening ability was obtained by IQ annealing routes. The analysis of the fractured surface revealed that ferrite/martensite interfaces are the most susceptible for microvoid nucleation. However, martensite microcracks were also observed in SQ sample, and the microvoids are nucleated within the ferrite grain in IA sample as well. The variations in strength, elongation, strain-hardening behavior and fracture mechanism of the steel with different heat-treatment schedules were further discussed in relation to the microstructural features.

Key words: Dual-phase steel, Heat-treatment schedule, Strain-hardening behavior, Fracture mechanism

Yong-Gang Deng, Hong-Shuang Di, Jie-Cen Zhang. Effect of Heat-Treatment Schedule on the Microstructure and Mechanical Properties of Cold-Rolled Dual-Phase Steels[J]. Acta Metallurgica Sinica (English Letters), 2015, 28(9): 1141-1148.

Figures/Tables 10

Fig. 1 Microstructure of the as-received cold-rolled steel: a OM, b SEM.

Fig. 2 Schematics of the three kinds of heat-treatment schedules studied: a IQ, b IA, c SQ. WQ means water quenching.

Fig. 3 SEM images of DP microstructure obtained by IQ a, IA b, SQ c.

Table 1 Mechanical properties obtained from the tensile tests and microstructural characteristics obtained from SEM micrographs

Specimen	YS (MPa)	UTS (MPa)	TEL (%)	UE (%)	UTS × UE (MPa%)	Yield ratio	d _f (μm)	V _m (%)
IQ	399.6	754.9	19.9	12.3	9285	0.53	7.3	45.7
IA	382.7	700.4	16.4	10.1	7074	0.55	10.2	36.2
SQ	488.3	838.7	13.8	7.5	6290	0.58	9.4	55.3

Fig. 4 TEM images of the annealed samples: a IQ, b IA, c SQ (F ferrite, M martensite).

Fig. 5 Engineering a, true stress-strain b curves of the investigated steels.

Fig. 6 Modified C-J plots of ln (dσ/dε) versus ln σ for ferrite-martensite DP microstructures developed by varying heat-treatment paths: a IQ, b IA, c SQ.

Fig. 7 SEM images on the fracture surfaces of IQ a, IA b, SQ c.

Fig. 8 Optical micrographs of the surfaces perpendicular to the fracture surface near the fracture surface: a IQ, b IA, c SQ.

Fig. 9 SEM images of subsurfaces close to the fracture surface: a IQ, b IA, c, d SQ samples. The arrows indicate the sites of crack and/or void nucleation.

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