The work hardening of dual-phase steels at small plastic strains

Abstract

The work hardening at small plastic strains in dual-phase steels has been studied as a function of the microstructure and the volume fraction of the hard phase (phase II) in a series of fully characterized steels with between 0 and 100 vol.% of phase II. Three series of steels were examined: banded, homogenized, and homogenized and tempered at 200°C. In all the steels the ferrite was free of fine precipitate particles and the martensite contained 0.37 wt.% C and negligible amounts of retained austenite. The elastic limits of the steels (the stress at a strain of less than 5 × 10 −5 and the stress-strain curves at small plastic strains were measured using balanced electrical resistance strain gauges of high sensitivity. The internal stresses were assessed from measurements of the Bauschinger strains as a function of the volume fraction of phase II and, in the untempered homogenized steel with 26 vol.% of phase II, as a function of the plastic prestrain. The cyclic hardening characteristics of the steels were determined by continuing the cyclic tests at a constant strain amplitude for 10 cycles or more. In all the dual-phase steels the elastic limit is equal to or less than the elastic limit of annealed ferrite, an effect which is attributed to the presence of local internal elastic stresses in the ferrite resulting from the strains produced by the martensitic transformation. The rapid work hardening in the dual-phase steels with a ferrite matrix is due to the heterogeneous deformation of the ferrite at plastic strains of less than about 3.5 × 10 −2, a heterogeneity which is due to the large variations in dislocation density within the ferrite. Heterogeneous deformation of the ferrite is a factor also in the initial deformation of steels with a martensitic matrix but, in this case, the high local elastic stresses are more important. The long-range elastic stresses in the ferrite of steels with a ferrite matrix are small at zero plastic strain, increase with increasing plastic strain and level off to a constant value at large strains. The contribution of these stresses to the hardening at small plastic deformations (less than 5 × 10 −3) is not significant but at larger deformations it can account for a measurable fraction of the hardening. The origins of the short- and long-range elastic stresses are discussed in some detail. There is no long-range internal stress in the martensite as is shown by the absence of permanent softening in the Bauschinger test. The cyclic hardening and softening of the dual-phase steels and of the constituent phases are described and discussed, as are the effects on all the properties considered here produced by low temperature tempering.