Patent ID: 8366844

Claim:
A method of making a hot rolled dual phase steel sheet, comprising: (I) hot rolling a steel slab into a hot band at a hot rolling 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 0.2% 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, molybdenum less than 0.2% by weight, copper less than about 0.4% 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 after completion of hot rolling at a mean rate of at least about 5° C./s (about 9° F./s) to a coiling temperature not higher than about 750° C. (about 1382° F.); and (III) coiling the hot band to form a coil at a temperature more than the martensite formation temperature obtaining a steel sheet comprising (a) a dual phase microstructure comprising a martensite phase of 11-28% by volume and a ferrite phase, (b) said composition, and (c) properties comprising a tensile strength of at least about 590 megapascals and a hole expansion ratio more than about 70%.