Patent ID: 7959747
Filing Date: 2011-06-14
Classification: C22C

Abstract:
1. A batch annealing method of making a dual phase steel sheet, comprising: (I) hot rolling a steel slab into a hot band and completing the hot rolling process at a termination temperature in a range between about (A (II) cooling the hot band at a mean rate of at least about 5° C./s (about 9° F./s) to a temperature not higher than about 750° C. (about 1382° F.); (III) coiling the band to form a coil at a temperature higher than the martensite formation temperature; (IV) cooling the coil to a temperature lower than the martensite formation temperature to form a dual phase microstructure comprising a martensite phase of less than 35% by volume and a ferrite phase of more than; 65% by volume; (V) cold rolling the coil to a desired steel sheet thickness, with a total reduction of at least about 35%; (VI) annealing the cold rolled steel sheet in a batch furnace at a temperature higher than about 500° C. (about 932° F.) and lower than about the A (VII) cooling the annealed steel sheet to a temperature lower than about 400° C. (about 752° F.), and (VIII) obtaining a steel sheet comprising (a) a dual phase microstructure comprising a martensite phase of no more than 35% by volume and embedded in a ferrite matrix phase, (b) said composition, and (c) properties comprising a tensile strength of at least about 400 megapascals and an n-value of at least about 0.175.