Deformation characteristics of dual-phase steels
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I.
INTRODUCTION
THE deformation characteristics of dual-phase steels are known to be different from those of normal high-strength, low-alloy steels in that at equivalent tensile strength levels, dual-phase steels exhibit a much higher ductility. Araki et al' sought to characterize the combination of high strength and ductility by a single parameter: the product of tensile strength and ductility (TS • They suggested that the TS x EL value should be constant across the entire range of vol pct second phase. However, since their investigation showed that the TS x EL abruptly decreased when fro, the volume percent second phase, exceeded 0.2, they advanced the explanation that the abrupt drop in the TS x EL value was caused by "premature" void formation near the harder second phase. They suggested that at high fro, void formation in dual-phase steels may appear at strains much below the uniform elongation, thus causing premature failure. However, Lagneborg 2 is of the opinion that the behavior of the parameter of TS x EL vs fm may be a simple mathematical consequence of the approximately linear relationship between UTS and elongation, which would result in a parabolic decline of TS x EL with increasing martensite content. Davies 3 reported that whenf~ exceeds 0.5, martensite in the form of "stringers," i.e., banding, affects the tensile strength and uniform elongation by "precracking" of the Table 1.
steels. The present study determined the effect of the range of martensite volume fractions up to 0.65 on the tensile characteristics of dual-phase steels.
II.
EXPERIMENTAL PROCEDURE
Hot band material (Table 1) was processed to 2.8 mm thickness and heat treated in the laboratory at various intercritical annealing temperatures between 760 ~ (1400 ~ and 816 ~ (1500 ~ Tensile specimens with a 50 m m gage length were heat treated in a tube furnace with a 300 mm constant-temperature zone. A chromel-alumel thermocouple was flash welded to the center of the grip region of each specimen to record the complete thermal profile during heat treatment. After annealing at three minutes, the specimens were quickly transferred from the furnace and cooled to room temperature at rates ranging from 1.5 to 600 C per second. The cooling rate was measured between the annealing temperature and 500 ~ All tensile tests were conducted at a strain rate of 0.05 per minute to three pct strain and then at a strain rate of 0.5 per minute to failure. Yield strength was taken at 0.2 pct offset, and uniform elongation was designated at the point at which the load began to decrease, i.e., the onset of necking. Metallographic specimens were taken from the extended-grip Chemistry
Alloy
C
Mn
P
S
Si
Mo
V
Ti
A1
50 52 53 55
.11 .098 .ll .098
1.45 1.47 1.44 1.45
.020 .014 .022 .018
.004 .003 .005 .002
.45 .45 .57 .58
.16 .02 .18
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