Strain hardening of high-strength steels

Abstract

Strain hardening and the loss of ductility with increasing strength have been investigated for a series of cold-rolled high-strength steels. The strain hardening exponent n decreases with increasing strength S according to the relationship nSp=C, where p and C are constants for a given strengthening mechanism. The value of p (and hence the ductility) at a fixed strength level depends on the operative strengthening mode. Solid-solution hardening is the most ductile method of strengthening, followed, in order of decreasing ductility, by grain refining, precipitation hardening, partial annealing, and cold working. It is proposed that the influence of a strengthening mechanism is related to the strain at which a steady-state dislocation-cell structure is developed in the material. The lower this critical strain, the lower the ductility, since further strain must then be accommodated by microband formation. The critical strain is considered to be dependent on the rate of accumulation of geometrically necessary dislocations in the structure, which is a function of the material's geometric slip distance. Strengthening mechanisms relying on obstacles in the microstructure - e.g. precipitation hardening - will have relatively short geometric slip distances, and thus low ductilities. Solid-solution hardening is preferable, because it operates only by increasing the lattice friction resistance to dislocation motion, and so does not reduce the geometric slip distance. The cell structure in the cold-worked steels is expected to approach that of the steady state at the start of the tests, a feature which, will result in very low critical strains and ductilities.MST/92