Properties of cold-rolled high-strength steel sheets

For high-strength steel sheets, a new concept has become necessary,viz, the relation between strength and formability. When the relation between tensile strength and elongation is appraised for ranking in terms of the strengthening mechanism, it is found that the substitutional solid-solution hardening type is excellent and the precipitation hardening type is inferior. In batch annealing, the phosphorus-added aluminum-killed steel sheets are representative of the former type having excellent formability as indicated by a highr values despite their tensile strength of 450 N/mm². The titanium-added aluminum-killed steel sheets, which are representative of the latter type, have a tensile strength of 600 N/mm² and a relatively highr value. Continuous annealing of the highstrength steel sheets of the same chemical composition causes higher yield stresses and combinations of yield stress and elongation. Alternatively with continuous annealing the same level of strength can be achieved with smaller additions of alloying elements than with batch annealing. An additional advantage of continuous annealing is the uniformity of properties along the length of the coil. The rapid cooling possible after continuous annealing allows production of high strength steel sheets having excellent mechanical properties that are unobtainable in the batched annealed steels. For example, steel sheets of 0.4 pet Si and 1.4 pet Mn after continuous annealing, haven values and Erichsen values as higher than conventional low-carbon rimmed or capped steel sheets even while they have a tensile strengths of 550 N/mm².