Improving the formability of low carbon sheet steel by control of interstitial carbon content and temperature
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THE role of higher strength
steels in promoting weight reduction in automobiles through gage reduction in strength limited structures is obvious. It is perhaps less obvious that significant weight and gage reduction may also be accomplished in stiffness limited structures provided that the section modulus can be increased to compensate for the reduction in stiffness which accompanies downgaging. Increased section modulus may be achieved through the use of deeper sections, provided the formability of the material is adequate to permit the deeper section to be formed. Thus, downgaging and weight reduction in stiffness limited structures may require improvements in formability. The effect of interstitial carbon content on the room temperature mechanical properties of a low carbon steel was reported previously, l In addition, a few lower temperature results were also reported. The most striking feature of the low temperature results was the marked increase in strain rate sensitivity observed in samples of high (110 ppm*, 190 ppm*) interstitial * ppm by weight
carbon content at - 17 ~ compared to that observed at room temperature. The practical significance of this result lies in the association of high strain rate sensitivity with extended post-uniform elongation (and hence increased total elongation) in the tensile test? In its turn, increased elongation in the tensile test has been associated with enhanced formability. 3 It appeared, then, that it might be possible to promote enhanced formability in low carbon steels at reduced temperature by appropriate selection of temperature and interstitial carbon content. To investigate this possibility, a program was instituted to determine the tensile and forming behavior of low carbon steel with variable interstitial content at reduced temperature.
ROBIN STEVENSON is Senior Research Engineer, Physics Department, General Motors Research Laboratories, Warren, MI 48090. Manuscript submitted February 14, 1980.
PROCEDURE As in the previous study, I the appropriate range of interstitial carbon contents was obtained by "equilibrating" samples at between 200 and 700 ~ and quenching in an aqueous 5 pct N a O H solution to develop high metastable interstitial carbon contents at room temperature. For temperatures of 250 ~ or higher molten salt baths were employed, while an oil bath was used at 200 ~ In all cases, the time at temperature was calculated to yield a constant (50 ~m) diffusion distance (corresponding to \/~Dt, where D = Diffusion coefficient for carbon in c~ iron, t = time). Since the intercarbide spacing in this material was determined to be ~ 2 5 / z m , it was anticipated that this would result in a uniform interstitial carbon content which was a close approximation to the equilibrium value. A complete listing of heat treatment schedules is given in Table I. Interstitial carbon contents were then computed using the relationship: 7 wt pct C = 2 exp
(-9100/RT)
where R = gas constant, 1.98 cal d e g - 1 rnol- J, T = temperature (K) from which the specimen was quenched
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