Patent ID: 7288158
Filing Date: 2007-10-30
Classification: C21D

Abstract:
1. A process for producing a steel product comprised of high strength, low alloy steel containing a hardness-promoting vanadium nitride microalloy and having a yield strength of at least about 100 ksi, the process comprising: (a) casting molten steel to form a solid, as-cast product having a thickness, the as-cast product comprising austenite; (b) transferring the as-cast product to a first rolling apparatus, wherein a temperature of the as-cast product as it enters the first rolling apparatus is greater than a recrystallization stop temperature of the austenite and above about 1020° C.; (c) conducting a first reduction step in the first rolling apparatus to reduce the thickness of the as-cast product by a first amount, thereby producing a first thickness-reduced product, wherein a temperature of the as-cast product entering the first rolling apparatus and a temperature of the first thickness-reduced product exiting the first rolling apparatus are above the recrystallization stop temperature and above about 1020° C.; (d) heating the first thickness-reduced product with a heating apparatus so as to maintain the first thickness-reduced product at a temperature above the recrystallization stop temperature and above about 1020° C., and holding the first thickness-reduced product at said temperature, while heating it with said heating apparatus, for a time sufficient to permit substantially complete recrystallization of the austenite and thereby reduce a grain size of the austenite; (e) transferring the first thickness-reduced product to a second rolling apparatus comprised of a plurality of rolling stands; (f) conducting a second reduction step in the second rolling apparatus to reduce the thickness of the first thickness-reduced product by a second amount, thereby producing a second thickness-reduced product, wherein a temperature of the first thickness-reduced product entering the second rolling apparatus is above the recrystallization stop temperature and above about 1020° C., and a temperature of the second thickness-reduced product exiting the second rolling apparatus is above a phase transformation temperature at which austenite is transformed to ferrite; (g) cooling the second thickness-reduced product to below the phase transformation temperature, thereby producing a cooled product; and (h) conducting a third reduction step in a third rolling apparatus to reduce the thickness of the cooled product by a third amount, thereby producing the steel product having a yield strength of at least about 100 ksi.