Double annealed steel sheet products with improved hole expansion ratios

Abstract

Steel sheet products are disclosed having controlled compositions that, in combination with controlled double annealing processes, produce desirable microstructures and favorable mechanical properties including high strengths and hole expansion ratios. The steel compositions may include carbon, manganese, chromium and molybdenum in controlled amounts, along with other alloying additions such as aluminum, silicon, niobium, boron and titanium.

1 . A method of producing a rolled steel sheet product comprising from 0.02 to 0.3 weight percent C, from 1.5 to 2.7 weight percent Mn, from 0.01 to 1.5 weight percent Mo, and up to 0.5 weight percent Cr, the method comprising:
 subjecting the steel sheet product to a first step annealing process at a temperature of from 800 to 980° C.; and   subjecting the steel sheet product to a second annealing process comprising soaking the sheet product at a temperature of from 750 to 900° C., followed by holding the sheet product a temperature of from 350 to 550° C., wherein the steel sheet product comprises ferrite, bainite, martensite, or a combination thereof, and has a hole expansion ratio of at least 30 percent.   
     
     
         2 . The method of  claim 1 , wherein the first annealing process is performed at a temperature above 820° C. 
     
     
         3 . The method of  claim 1 , wherein the first annealing process is performed at a temperature of from 830 to 940° C. 
     
     
         4 . The method of  claim 1 , wherein the soaking step of the second annealing process is performed at a temperature of from 780 to 880° C., and the holding step of the second annealing process is performed at a temperature of from 370 to 520° C. 
     
     
         5 . The method of  claim 1 , wherein the soaking step of the second annealing process is performed at a temperature of from 790 to 860° C., and the holding step of the second annealing process is performed at a temperature of from 370 to 520° C. 
     
     
         6 . The method of  claim 1 , wherein the steel sheet product is cooled to a temperature below 300° C. between the first annealing process and the second annealing process. 
     
     
         7 . The method of  claim 1 , wherein the Cr is at least 0.01 weight percent. 
     
     
         8 . The method of  claim 1 , wherein the C is from 0.09 to 0.17 weight percent, the Mn is from 2.0 to 2.5 weight percent, the Cr is from 0.15 to 0.3 weight percent, and the Mo is from 0.08 to 0.25 weight percent. 
     
     
         9 . The method of  claim 1 , wherein the sheet product comprises less than 0.4 weight percent Si and less than 0.4 weight percent Al. 
     
     
         10 . The method of  claim 9 , wherein the total combined amount of Si and Al is less than 0.5 weight percent. 
     
     
         11 . The method of  claim 1 , wherein the first annealing process is performed on a continuous annealing line, and the second annealing process is performed on a continuous annealing line. 
     
     
         12 . The method of  claim 11 , wherein the same continuous annealing line is used for both the first annealing process and the second annealing process. 
     
     
         13 . The method of  claim 11 , wherein separate continuous annealing lines are used for the first annealing process and the second annealing process. 
     
     
         14 . The method of  claim 1 , wherein the first annealing process is performed on a continuous annealing line, and the second annealing process is performed on a continuous galvanizing line. 
     
     
         15 . The method of  claim 1 , wherein the first annealing process and the second annealing process are performed in a single pass on a galvanizing line. 
     
     
         16 . The method of  claim 1 , further comprising electrolytically coating the rolled steel sheet product with a zinc-based coating. 
     
     
         17 . The method of  claim 1 , wherein the rolled steel sheet product is hot rolled prior to the first annealing process. 
     
     
         18 . The method of  claim 1 , wherein the rolled steel sheet product is cold rolled prior to the first annealing process. 
     
     
         19 . The method of  claim 1 , wherein the steel sheet product comprises less than 5 volume percent austenite. 
     
     
         20 . The method of  claim 1 , wherein the steel sheet product comprises less than 2 volume percent austenite. 
     
     
         21 . The method of  claim 1 , wherein the steel sheet product is substantially free of austenite. 
     
     
         22 . The method of  claim 1 , wherein the steel sheet product comprises at least 50 volume percent bainite. 
     
     
         23 . The method of  claim 22 , wherein the bainite comprises from 60 to 95 volume percent. 
     
     
         24 . The method of  claim 22 , wherein the steel sheet product further comprises from 1 to 30 volume percent martensite, from 1 to 30 volume percent ferrite, or a combination thereof. 
     
     
         25 . The method of  claim 1 , wherein the steel sheet product comprises at least 50 volume percent ferrite. 
     
     
         26 . The method of  claim 25 , wherein the ferrite comprises from 60 to 95 volume percent. 
     
     
         27 . The method of  claim 25 , wherein the steel sheet product further comprises from 1 to 30 volume percent fresh martensite. 
     
     
         28 . The method of  claim 1 , wherein the steel sheet product comprises at least 50 volume percent fresh martensite. 
     
     
         29 . The method of  claim 28 , wherein the fresh martensite comprises from 60 to 95 volume percent. 
     
     
         30 . The method of  claim 28 , wherein the steel sheet product further comprises from 1 to 45 volume percent bainite. 
     
     
         31 . A rolled steel sheet product made by the method of  claim 1 . 
     
     
         32 . A rolled double annealed steel sheet product comprising from 0.02 to 0.3 weight percent C, from 1.5 to 2.7 weight percent Mn, from 0.01 to 1.5 weight percent Mo, and up to 0.5 weight percent Cr, wherein the steel sheet product comprises ferrite, bainite, martensite, or a combination thereof, and has a hole expansion ratio of at least 30 percent. 
     
     
         33 . The rolled double annealed steel sheet product of  claim 32 , wherein the Cr is at least 0.01 weight percent. 
     
     
         34 . The rolled double annealed steel sheet product of  claim 32 , wherein the C is from 0.09 to 0.17 weight percent, the Mn is from 2.0 to 2.5 weight percent, the Cr is from 0.15 to 0.3 weight percent, and the Mo is from 0.08 to 0.25 weight percent. 
     
     
         35 . The rolled double annealed steel sheet product of  claim 32 , wherein the sheet product comprises less than 0.4 weight percent Si and less than 0.4 weight percent Al. 
     
     
         36 . The rolled double annealed steel sheet product of  claim 35 , wherein the total combined amount of Si and Al is less than 0.5 weight percent. 
     
     
         37 . The rolled double annealed steel sheet product of  claim 32 , wherein the steel sheet product comprises less than 5 volume percent austenite. 
     
     
         38 . The rolled double annealed steel sheet product of  claim 32 , wherein the steel sheet product comprises less than 2 volume percent austenite. 
     
     
         39 . The rolled double annealed steel sheet product of  claim 32 , wherein the steel sheet product is substantially free of austenite. 
     
     
         40 . The rolled double annealed steel sheet product of  claim 32 , wherein the steel sheet product comprises at least 50 volume percent bainite. 
     
     
         41 . The rolled double annealed steel sheet product of  claim 40 , wherein the bainite comprises from 60 to 95 volume percent. 
     
     
         42 . The rolled double annealed steel sheet product of  claim 40 , wherein the steel sheet product further comprises from 1 to 30 volume percent martensite, from 1 to 30 volume percent ferrite, or a combination thereof. 
     
     
         43 . The rolled double annealed steel sheet product of  claim 32 , wherein the steel sheet product comprises at least 50 volume percent ferrite. 
     
     
         44 . The rolled double annealed steel sheet product of  claim 43 , wherein the ferrite comprises from 60 to 95 volume percent. 
     
     
         45 . The rolled double annealed steel sheet product of  claim 43 , wherein the steel sheet product further comprises from 1 to 30 volume percent fresh martensite. 
     
     
         46 . The rolled double annealed steel sheet product of  claim 32 , wherein the steel sheet product comprises at least 50 volume percent fresh martensite. 
     
     
         47 . The rolled double annealed steel sheet product of  claim 46 , wherein the fresh martensite comprises from 60 to 95 volume percent. 
     
     
         48 . The rolled double annealed steel sheet product of  claim 46 , wherein the steel sheet product further comprises from 1 to 45 volume percent bainite.