Patent ID: 7608155
Filing Date: 2009-10-27
Classification: C22C,C23C,Y10T

Abstract:
1. A method of making a galvanized steel sheet, comprising: (I) at a temperature in a range between about (A (II) cooling the hot band at a mean rate of at least about 3° C./s (about 5.4° F./s) to a temperature not higher than about 800° C. (about 1472° F.) obtaining a steel sheet comprising a dual phase microstructure comprising a martensite phase at least 3% by volume embedded in a ferrite matrix phase; (III) coiling the cooled band to form a coil; (IV) galvanizing the steel sheet by heating to a temperature higher than about 600° C. (1112° F.), holding the temperature in a soaking zone of a galvanizing line while using a line speed or process speed faster than about 30 m/min, cooling the steel sheet to a temperature close to the temperature in the galvanizing bath in a range between about 400° C. (about 752° F.) and about 550° C. (about 1022° F.), passing the steel sheet through the galvanizing bath to coat the steel sheet with a zinc coating or a zinc alloy coating, and cooling the galvanized steel sheet; and (V) obtaining a galvanized steel sheet comprising (a) a dual phase microstructure comprising a martensite phase and a ferrite phase, (b) said composition, and (c) one or more of a property chosen from (i) a weldability superior to that of known galvanized steel sheet having a dual phase microstructure of a martensite phase and a ferrite phase, (ii) an impact energy ≧ about 1200 g-m, measured on a V-notch Charpy specimen of about 1.5 mm thickness, or (iii) a yield strength/tensile strength ratio ≦ about 70%.