Tempering characteristics of a vanadium containing dual phase steel
- PDF / 3,092,889 Bytes
- 8 Pages / 594 x 774 pts Page_size
- 3 Downloads / 190 Views
I.
INTRODUCTION
Yield Point Elongation
DUALphase
steels are characterized by a microstructure that consists of 75 to 85 vol pet fine grained ferrite with the balance being a uniformly distributed mixture of martensite, retained austenite, and lower bainite 1-7 in varying proportions. These steels are currently manufactured and used commercially. They are produced by continuously annealing certain low carbon Si-Mn 4 or microalloyed high strength, low alloy (HSLA) steels,l'2'5 or directly off the hot mill. 6 The continuous annealing generally consists of heating the steel for a short time at temperatures in the ferriteaustenite, (a + ~), or austenite, (3') regions of the Fe-C phase diagram and cooling to room temperature. The cooling rate may be in the range between air cooling and water quenching depending on steel chemistry, available facilities, and desired mechanical properties. The stress-strain behavior of dual phase steels 2 is characteristically different from that of ferrite-pearlite steels such as plain carbon steel or the HSLA steels (Figure 1). The ferrite-pearlite steels have yield point elongation, a high yield strength to ultimate tensile strength (YS/UTS) ratio, and their strength and ductility (uniform elongation) are inversely related (Figure 2). Dual phase steels have a continuous stress-strain curve with no yield point elongation. They work harden very rapidly at low strains, have a low YS, a high UTS, and hence a low YS/UTS ratio. They have better formability than the ferrite-pearlite steels of equivalent tensile strength, and their strength-ductility data fall on a separate curve (Figure 2) than that for ferritepearlite steels.
M.S. RASHID is Assistant Head, Metallurgy Department, General Motors Research Laboratories, General Motors Technical Center, Warren, MI 48090. B. V. N. RAO, formerly Senior Research Engineer, Analytical Chemistry Department, General Motors Research Laboratories, is presently with Exxon Research and Engineering, Materials Technology Division, Florham Park, NJ 07932. Manuscript submitted March 26, 1981. METALLURGICAL TRANSACTIONS A
9
GM 9 8 0 X
oopl , I~ /
SAE c~80X
11/"-v
s,,.s, i sl
ma400'~ / ~ ' I . ~ SAE 9"SOX
~
li
un 200 i~,,,,~ I il ii
Plain Carbon
~ '
I
0 I -,--- I----0.2
Total Elongation, e t I I L 10 20 30 Percent Strain
= !
I
40
Fig. 1 --Schematic stress-strain curves for plain carbon, HSLA, and dual phase steels. SAE 950X and 980X are Society of Automotive Engineers designations for HSLA steels of different strength levels. GM 980X is a General Motors developed dual phase steel. GM 980X is more ductile than SAE 980X although both steels have similar tensile strength.
The mechanical properties of dual phase steels can be altered by low temperature heating, or tempering which produces various microstructural changes in the steel. Tempering has been successfully used as a post-continuous annealing step to improve the strength-ductility relationship of certain microalloy-free dual phase steels. 4 Also, elevated temperatures might be experienc
Data Loading...
