Patent ID: 7442268
Filing Date: 2008-10-28
Classification: C22C

Abstract:
1. A batch annealing method of making a dual phase steel sheet, comprising: (I) at a 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.) obtaining a steel sheet comprising a dual phase microstructure comprising a martensite phase no more than about 35% by volume embedded in a ferrite matrix phase; (III) coiling the cooled band to form a coil; (IV) cold rolling the coil to a desired steel sheet thickness, with a total reduction of at least about 35%; (V) 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 Ac (VI) cooling the annealed steel sheet to a temperature lower than about 400° C. (about 752° F.) obtaining a steel sheet comprising (a) a dual phase microstructure comprising a martensite phase and a ferrite phase, (b) said composition, and (c) properties comprising a tensile strength of at least about 400 MPa and an n-value of at least about 0.175.