AbstractThe mechanisms responsible for strengthening a series of high-strength cold-rolled steels with tensile strengths up to 800 MN m−2have been investigated. The magnitude of precipitation strengthening in the annealed steels is shown to be in agreement with the Orowan-Ashby model for non-deforming particles. Strengthening depends only upon the volume fraction and diameter of the precipitates, and is not influenced by their chemical composition, nor by whether the precipitation-hardening elements are added singly or in combination. Manganese alone is a weak solid-solution-strengthening agent, but has a synergistic effect in combination with titanium or niobium, which is attributed to its depression of the austenite-ferrite transformation and precipitation temperatures. Vanadium is a much less efficient strengthening element than titanium or niobium in annealed steel, owing to the rapid coarsening rate of vanadium carbonitride precipitates and the considerable loss in strength on processing from hot-rolled coil to annealed sheet. Sulphur acts to reduce the strength of the annealed steels and tends to coarsen the grain structure. The physical basis of this effect is not known, but it is suggested that it may be associated with the partial solution of managanese sulphide during slab reheating and its subsequent re-precipitation during hot rolling. Phosphorus and nitrogen are the most efficient strengthening agents up to tensile strengths of 450 MN m−2, but stronger materials require a combination of strengthening modes, depending upon the application.MST/111