Patent ID: 6066212
Filing Date: 2000-05-23
Classification: C21D,C22C

Abstract:
A method for preparing a dual phase steel plate having a microstructure comprising about 10 vol % to about 40 vol % of a first phase of essentially ferrite and about 60 vol % to about 90 vol % of a second phase of predominantly fine-grained lath martensite, fine-grained lower bainite, or mixtures thereof, said method comprising the steps of:(a) heating a steel slab to a reheating temperature sufficiently high to (i) substantially homogenize said steel slab, (ii) dissolve substantially all carbides and carbonitrides of niobium and vanadium in said steel slab, and (iii) establish fine initial austenite grains in said steel slab;(b) reducing said steel slab to form steel plate in one or more hot rolling passes in a first temperature range in which austenite recrystallizes;(c) further reducing said steel plate in one or more hot rolling passes in a second temperature range below about the T.sub.nr temperature and above about the Ar.sub.3 transformation temperature;(d) further reducing said steel plate in one or more hot rolling passes in a third temperature range between about the Ar.sub.3 transformation temperature and about the Ar.sub.1 transformation temperature;(e) quenching said steel plate at a cooling rate of about 10.degree. C. per second to about 40.degree. C. per second (18.degree. F./sec-72.degree. F./sec) to a Quench Stop Temperature below about the M.sub.s transformation temperature plus 200.degree. C. (360.degree. F.); and(f) stopping said quenching, so as to facilitate transformation of said microstructure of said steel plate to about 10 vol % to about 40 vol % of a first phase of ferrite and about 60 vol % to about 90 vol % of a second phase of predominantly fine-grained lath martensite, fine-grained lower bainite, or mixtures thereof.