Patent ID: 7442268

Claim:
A batch annealing method of making a dual phase steel sheet, comprising: (I) at a temperature in a range between about (A r3 -60)° C. and about 980° C. (about 1796° F.), hot rolling a steel slab into a hot band, wherein the steel slab comprises a composition comprising: carbon in a range from about 0.01% by weight to about 0.2% by weight, manganese in a range from about 0.3% by weight to about 3% by weight, silicon in a range from about 0.05% by weight to about 2% by weight, chromium in a range from about 0.1% by weight to about 2% by weight, aluminum in a range from about 0.01% by weight to about 0.10% by weight, and calcium in a range from about 0.0005% by weight to about 0.01% by weight, with the balance of said composition comprising iron and incidental ingredients; (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 1 temperature for longer than about 60 minutes; (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.