Thin cast strip product with microalloy additions, and method for making the same

Abstract

A steel product or thin steel cast strip comprised of, by weight, less than 0.25% carbon, between 0.20 and 2.0% manganese, between 0.05 and 0.50% silicon, less than 0.06% aluminum, and at least one element selected from the group consisting of titanium between about 0.01% and about 0.20%, niobium between about 0.01% and about 0.20%, molybdenum between about 0.05% and about 0.50%, and vanadium between about 0.01% and about 0.20%, and having a microstructure comprised of a majority bainite, and further comprising fine oxide particles of silicon and iron distributed through the steel microstructure having an average precipitate size less than 50 nanometers. The steel product or thin cast steel strip may have a yield strength of at least 55 ksi (380 MPa) or a tensile strength of at least 500 MPa, or both. The steel product or thin cast steel strip may have a total elongation of at least 6% or 10%. The thin cast steel strip may have thickness less than 3.0 mm, or less than 2.5 mm, or less than 2.0 mm.

1 . A steel product comprising, by weight, less than 0.25% carbon, between 0.20 and 2.0% manganese, between 0.05 and 0.50% silicon, less than 0.06% aluminum, and at least one element selected from the group consisting of titanium between about 0.01% and about 0.20%, niobium between about 0.01% and about 0.20%, molybdenum between about 0.05% and about 0.50%, and vanadium between about 0.01% and about 0.20% and having a majority of the microstructure comprised of bainite and comprising fine oxide particles of silicon and iron distributed through the steel microstructure having an average precipitate size less than 50 nanometers.  
     
     
         2 . The steel product as claimed in  claim 1  wherein the steel product has a yield strength of at least 55 ksi (380 MPa).  
     
     
         3 . The steel product as claimed in  claim 1  wherein the steel product has a tensile strength of at least 72 ksi (500 MPa).  
     
     
         4 . The steel product as claimed in  claim 1  wherein the steel product has a total elongation of at least 6%.  
     
     
         5 . The steel product as claimed in  claim 1  wherein the steel product has a total elongation of at least 10%.  
     
     
         6 . A thin cast steel strip comprising, by weight, less than 0.25% carbon, between 0.20 and 2.0% manganese, between 0.05 and 0.50% silicon, less than 0.06% aluminum, and at least one element selected from the group consisting of titanium between about 0.01% and about 0.20%, niobium between about 0.01% and about 0.20%, molybdenum between about 0.05% and about 0.50%, and vanadium between about 0.01% and about 0.20% and having a majority of the microstructure comprised of bainite.  
     
     
         7 . The thin cast strip as claimed in  claim 6  comprising in addition having fine oxide particles of silicon and iron distributed through the steel microstructure having an average precipitate size less than 50 nanometers.  
     
     
         8 . The thin cast steel strip as claimed in  claim 6  wherein the steel product has a yield strength of at least 55 ksi (380 MPa).  
     
     
         9 . The thin cast steel strip as claimed in  claim 6  wherein the steel product has a tensile strength of at least 72 ksi (500 MPa).  
     
     
         10 . The thin cast steel strip as claimed in  claim 6  wherein the thin cast steel strip has a thickness of less than 3.0 mm.  
     
     
         11 . The thin cast steel strip as claimed in  claim 6  wherein the thin cast steel strip has a thickness of less than 2.5 mm.  
     
     
         12 . The thin cast steel strip as claimed in  claim 6  wherein the thin cast steel strip has a thickness of less than 2.0 mm.  
     
     
         13 . The thin cast steel strip as claimed in  claim 6  wherein the thin cast steel strip has a thickness in the range from about 0.5 mm to about 2 mm.  
     
     
         14 . The thin cast steel strip as claimed in  claim 6  wherein the steel product has a total elongation of at least 6%.  
     
     
         15 . The thin cast steel strip as claimed in  claim 6  wherein the steel product has a total elongation of at least 10%.  
     
     
         16 . A method of preparing a thin cast steel strip comprising the steps of: 
 assembling internally a cooled roll caster having laterally positioned casting rolls forming a nip between them, and forming a casting pool of molten low carbon steel supported on the casting rolls above the nip and confined adjacent the ends of the casting rolls by side dams,    counter rotating the casting rolls to solidify metal shells on the casting rolls as the rolls move through the casting pool; and    forming from the metal shells downwardly through the nip between the casting rolls a steel strip; and    cooling the steel strip at a rate of 10° C. per second and above to provide a composition comprising by weight, less than 0.25% carbon, between 0.20 and 2.0% manganese, between 0.05 and 0.50% silicon, less than 0.06% aluminum, and between about 0.01% and about 0.20% niobium, and having a microstructure with a majority comprised of bainite.    
     
     
         17 . The method of preparing a thin cast steel strip as claimed in  claim 16  wherein steel strip as coiled may have fine oxide particles of silicon and iron distributed through the steel microstructure having an average precipitate size less than 50 nanometers.  
     
     
         18 . The method of preparing a thin cast steel strip as claimed in  claim 16  further comprise the steps of: 
 hot rolling the molten low carbon steel strip; and    coiling the hot rolled low carbon steel strip at a temperature in the range from about 500-700° C.    
     
     
         19 . The method of preparing a thin cast steel strip as claimed in  claim 18  further comprise the steps of: 
 precipitation hardening the low carbon steel strip to increase the tensile strength at a temperature of at least 550° C.    
     
     
         20 . The method of preparing a thin cast steel strip as claimed in  claim 18  further comprise the steps of: 
 precipitation hardening may occur at a temperature between 650° C. and 800° C.    
     
     
         21 . The method of preparing a thin cast steel strip as claimed in  claim 18  further comprise the steps of: 
 precipitation hardening may occur at a temperature between 675° C. and 750° C.