On the Combination of Tensile Strength and Ductility of High Tensile Strength Cold Rolled Steel Sheets and the Metallurgical Factors
1974 Volume 60 Issue 2 Pages 284-295
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1974 Volume 60 Issue 2 Pages 284-295
Aiming at the use for the inner and the outer steel panels of the automobiles on which the safety of the rider should be considered as the first importance, various high tensile strength cold rolled steel sheets have been developed. A trial has been carried out to find what strengthening mechanism is the best for the combination of strength and ductility (n-value and the total elongation).
The following results were obtained:
1) The combination of the strength and ductility can be ranked as follows from the best to the worst; Substitutional solution hardening>Grain boundary hardening>Precipitation hardening This ranking well coincides with the work hardening behavior.
2) In the substitutional solution hardening type, a collinear distribution of dislocations is dominantly observed and the work hardening still increases at a large tensile strain.
3) In the grain boundary hardening type, more cell substructures and less structures of collinear distribution of dislocations are formed than in the substitutional solution hardening type.
4) In the precipitation hardening type, most of dislocations often have interactions with the uniformly distributed fine precipitates in the matrix and the strain increases forming dislocation loops at an early stage of tensile strain. Thus, work hardening is fairly large at the early stage of strain but soon saturates without forming clear cell substructions at the late stage of tensile strain.
5) The poor combination of strength and ductility of the precipitation hardened steels by the usual treatment is much improved up to a level of the grain boundary hardening type by taking a special treatment, i. e. 950°C×1hr pre-heating before cold rolling and causing the precipitating particlescoalesce and distribute heterogeneously. This improvement coincides with the result of the transimission electron micro - scopic obserbation that the microstructure after deformation resembles that of the grain boundary hardening type.
