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, aluminum 0.008% or less by weight, 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.

What is claimed is: 
     
         1 - 25 . (canceled) 
     
     
         26 . A method of preparing a coiled 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;   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, aluminum 0.008% or less by weight, and between about 0.01% and about 0.20% niobium, and having a microstructure with a majority 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;   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., with the niobium retained in solid solution during coiling.   
     
     
         27 . The method of preparing a coiled thin cast steel strip as claimed in  claim 26  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. after coiling. 
 
     
     
         28 . The method of preparing a coiled thin cast steel strip as claimed in  claim 27  wherein the precipitation hardening occurs at a temperature between 650° C. and 800° C. 
     
     
         29 . The method of preparing a coiled thin cast steel strip as claimed in  claim 27  wherein the precipitation hardening occurs at a temperature between 675° C. and 750° C. 
     
     
         30 . The method of preparing a coiled thin cast steel strip as claimed in  claim 26  wherein the steel strip has a total elongation between at least 6% and 18%. 
     
     
         31 . The method of preparing a coiled thin cast steel strip as claimed in  claim 26 , wherein the steel strip is cast at a thickness of no more than 3 mm. 
     
     
         32 . The method of preparing a coiled thin cast steel strip as claimed in  claim 31  wherein the thin strip is hot rolled to a thickness of between 1.1 and 1.5 mm. 
     
     
         33 . The method of preparing a coiled thin cast steel strip as claimed in  claim 26  wherein the steel strip is hot rolled to a thickness of between 0.5 mm to 2.5 mm prior to coiling. 
     
     
         34 . The method of preparing a coiled thin cast steel strip as claimed in  claim 26  wherein the steel strip is hot rolled to a thickness of between 0.5 mm to 2.0 mm prior to coiling. 
     
     
         35 . The method of preparing a coiled thin cast steel strip as claimed in  claim 26  wherein the steel strip is hot rolled to a thickness of between 1.1 mm to 1.5 mm prior to coiling. 
     
     
         36 . The method of preparing a coiled thin cast steel strip as claimed in  claim 26  wherein the steel strip has a total elongation of at least 10%. 
     
     
         37 . The method of preparing coiled thin cast steel strip as claimed in  claim 26 , wherein the niobium retained in solid solution is available to provide precipitation hardening on subsequent ageing. 
     
     
         38 . The method of preparing coiled thin cast steel strip as claimed in  claim 26 , wherein the steel strip has a yield strength of at least 55 ksi (380 MPa). 
     
     
         39 . The method of preparing a coiled thin cast steel strip as claimed in  claim 26 , wherein the steel strip has a yield strength in the range of 61-64 ksi (420-440 MPa) and tensile strengths of about 74 ksi (510 MPa). 
     
     
         40 . The method of preparing coiled thin cast steel strip as claimed in  claim 26 , wherein the steel strip was coiled at coiling temperatures of 590-620° C.