Patent ID: 7959747

Claim:
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 r3 -60)° C. and about 980° C. (about 1796° F.), where 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% weight, silicon in a range from about 0.05% by weight to about 2% by weight, chromium and nickel in combination from about 0.2% by weight to about 2% by weight where the chromium if present is in a range from about 0.1% by weight to about 2% by weight and nickel if present is in an amount up to about 1% by weight, aluminum in a range from about 0.01% by weight to about 0.10% by weight and nitrogen less than about 0.02% by weight, where the ratio of Al/N is more than about 2, 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.); (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 c3 temperature for longer than about 60 minutes; (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.