High strength galvanized and galvannealed steel sheets and manufacturing method

Abstract

An ultra-high strength steel has improved processing windows for galvanizing and galvannealing though a novel alloying concept. In addition to the alloying concept, the steel includes ferrite and overaged martensite microstructures that are strengthened, and a volume fraction of the magnetic phases is controlled. The steel can be processed to sheet form and galvanized and galvannealed.

I/we claim: 
     
         1 . A steel comprising, by weight percent, 0.15-0.26% C, 2.10-3.60% Mn, 0.05-0.85% Si, 0.001-0.85% Al, 0.01-0.90% Cr, 0.01-0.50% Mo, 0.01-0.10% Ti, 0.01-0.04 Nb, 0.01-0.30% V, 0.0001-0.005% B, less than 0.01% N, less than 0.01% S, less than 0.05% P, and the balance Fe and impurities. 
     
     
         2 . The steel of  claim 1  wherein the steel is formed as a sheet. 
     
     
         3 . The steel of  claim 1 , wherein the steel is an ultra-high strength steel. 
     
     
         4 . The steel of  claim 1 , wherein an ultimate tensile strength is greater than or equal to about 1480 MPa and less than or equal to about 1750 MPa. 
     
     
         5 . The steel of  claim 1 , wherein a yield strength is greater than or equal to about 1050 MPa and less than or equal to about 1050 MPa. 
     
     
         6 . The steel of  claim 1 , wherein a total elongation is above or equal to about 7%. 
     
     
         7 . The steel of  claim 1 , wherein the steel microstructure comprises, based on area percent, about 10-25% ferrite, about 75-90% martensite, and about 3-10% retained austenite. 
     
     
         8 . The steel of  claim 7 , wherein the ferrite is preferably 13-19%. 
     
     
         9 . The steel of  claim 7 , wherein the martensite is preferably 75-80%. 
     
     
         10 . The steel of  claim 7 , wherein the retained austenite is preferably 5-8%. 
     
     
         11 . The steel of  claim 1 , wherein the steel microstructure comprises, based on area percent, about 20-65% tempered martensite, about 35-80% fresh martensite, and about 3-10% retained austenite. 
     
     
         12 . The steel of  claim 11 , wherein tempered martensite is preferably 35-50%. 
     
     
         13 . The steel of  claim 11 , wherein the fresh martensite is preferably 50-65%. 
     
     
         14 . The steel of  claim 11 , wherein the retained austenite is preferably 5-8%. 
     
     
         15 . The steel of  claim 1 , wherein an overall microstructure of the steel comprises ferrite and overaged martensite, wherein the ferrite and the overaged martensite are strengthened by one or more of (a) solid-solution strengthening, (b) precipitation of vanadium carbides and/or titanium-niobium carbides, (c) grain refinement, and (d) strain strengthening or work hardening. 
     
     
         16 . The steel of  claim 1 , wherein the steel is configured for use in a galvanizing process and a galvannealing process. 
     
     
         17 . The steel of  claim 1 , having 0.0001-0.003% B. 
     
     
         18 . A method of manufacturing steel comprising:
 (a) melting an ingot having a composition of 0.15-0.26% C, 2.10-3.60% Mn, 0.05-0.85% Si, 0.001-0.85% Al, 0.01-0.90% Cr, 0.01-0.50% Mo, 0.01-0.10% Ti, 0.01-0.04 Nb, 0.01-0.30% V, 0.0001-0.005% B, less than 0.01% N, less than 0.01% S, less than 0.05% P, and the balance Fe and impurities;   (b) hot rolling the ingot down to a hot band having a gauge of about 3.0-4.0 mm, wherein the hot rolling is at about 1260 C, with a finish temperature is about 850-900 C, and a coiling temperature of about 600 C;   (c) annealing the hot band; and   (d) cold rolling the annealed hot band to reduce the gauge to form a sheet with a gauge of about 1.2-2.0 mm.   
     
     
         19 . The method of  claim 18 , wherein the steel is fully annealed in a temperature range of about 840-860 C and above an Ac3 temperature, then quenched to a temperature range of about 250-375 C, then galvanized and galvannealed, and then temper rolled. 
     
     
         20 . The method of  claim 18 , wherein the steel is intercritically annealed in a temperature range of about 750-830 C and between an Ac1 and an Ac3 temperatures, then cooled below a galvanizing temperature, then galvanized and galvannealed, and then temper rolled. 
     
     
         21 . The method of  claim 18 , wherein the steel is intercritically annealed in a temperature range of about 750-830 C and between an Ac1 and an Ac3 temperatures, then quenched to a temperature below a galvanizing temperature, then galvanized and galvannealed, and then temper rolled. 
     
     
         22 . The method of  claim 18 , having 0.0001-0.003% B. 
     
     
         23 . The method of  claim 18 , wherein cold rolling the annealed hot band to reduce the gauge forms a sheet with a gauge of about 1.2-1.6 mm.