Nitriding of niobium steel and product made thereby

Abstract

A nitrided steel product or 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 0.01% aluminum, niobium between 0.01 and about 0.20%, and between 0.01 and 0.075% nitrogen, and having a majority of the microstructure comprised of bainite and acicular ferrite, having more than 70% niobium in solid solution prior to nitriding and having after nitriding for one hour yield strength between 600 MPa and 800 MPa and tensile strength between 650 MPa and 900 MPa and no more than a 2% decrease in total elongation from the steel product prior to nitriding.

1 . A nitrided 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 0.01% aluminum, niobium between 0.01 and about 0.20%, and between 0.01 and 0.075% nitrogen, and having a majority of the microstructure comprised of bainite and acicular ferrite, having more than 70% niobium in solid solution prior to nitriding and after nitriding for between 30 and 90 minutes, having yield strength between 600 MPa and 800 MPa and tensile strength between 650 MPa and 900 MPa and no more than a 2% decrease in total elongation from the steel product prior to nitriding. 
     
     
         2 . The nitrided steel product as claimed in  claim 1  where the niobium is less than 0.1%. 
     
     
         3 . The nitrided steel product as claimed in  claim 1  where the nitrided steel product has yield strength of at least 40% greater than a similar steel composition without nitriding. 
     
     
         4 . The nitrided steel product as claimed in  claim 1  where the nitrided steel product has tensile strength of at least 30% greater than a similar steel composition without nitriding. 
     
     
         5 . The nitrided steel product as claimed in  claim 1  where the nitrogen is between 0.035 and 0.065%. 
     
     
         6 . The nitrided steel product as claimed in  claim 1  where the nitrogen is between 0.045 and 0.065%. 
     
     
         7 . The nitrided steel product as claimed in  claim 1  where the nitrided steel product has yield strength between 650 MPa and 750 MPa and tensile strength between 750 MPa and 850 MPa. 
     
     
         8 . The nitrided steel product as claimed in  claim 1  where the nitrided steel product has a total elongation of less than 25% than a similar steel composition without nitriding. 
     
     
         9 . The nitrided steel product as claimed in  claim 1  where the nitrided steel product has a total elongation of at least 1%. 
     
     
         10 . The nitrided steel product as claimed in  claim 1  where the steel product has a total elongation of at least 6%. 
     
     
         11 . The nitrided steel product as claimed in  claim 1  where the steel product has a total elongation of at least 10%. 
     
     
         12 . The nitrided steel product as claimed in  claim 1  where the steel product is a thin cast steel strip of less than 3 millimeters in thickness. 
     
     
         13 . The nitrided steel product as claimed in  claim 12  where the thin cast steel strip is hot rolled. 
     
     
         14 . The nitrided steel product as claimed in  claim 12  where the thin cast steel strip is cold rolled. 
     
     
         15 . A method of preparing nitrided 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 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 casting 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 at least 10° C. per second, coiling the cast strip and nitriding the steel strip for between 30 and 90 minutes 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 0.01% aluminum, between 0.01% and 0.20% niobium, and between 0.01 and 0.075% nitrogen, and having a majority of the microstructure comprised of bainite and acicular ferrite, having more than 70% niobium in solid solution prior to nitriding and after nitriding having yield strength between 600 MPa and 800 MPa and tensile strength between 650 MPa and 900 MPa and no more than a 2% decrease in total elongation from the steel product prior to nitriding.   
     
     
         16 . The method of preparing nitrided thin cast steel strip as claimed in  claim 15  where the steel strip as coiled may have fine oxide particles of silicon and iron distributed through the steel microstructure having an average particle size less than 50 nanometers. 
     
     
         17 . The method of preparing nitrided thin cast steel strip as claimed in  claim 15  further comprising the steps of:
 hot rolling the steel strip; and 
 coiling the hot rolled steel strip at a temperature between about 450 and 700° C. 
 
     
     
         18 . The method of preparing nitrided thin cast steel strip as claimed in  claim 15  further comprising the steps of:
 hot rolling the steel strip; and 
 coiling the hot rolled steel strip at a temperature less than 650° C. 
 
     
     
         19 . The method of preparing a nitrided thin cast steel strip as claimed in  claim 15  where the nitriding process is selected from the group consisting of salt bath nitriding, gas nitriding, and plasma nitriding. 
     
     
         20 . A nitrided 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 0.01% aluminum, niobium between 0.01 and about 0.20%, and between 0.01 and 0.075% nitrogen, and having a majority of the microstructure comprised of bainite and acicular ferrite, having more than 70% niobium in solid solution prior to nitriding and having after nitriding for between 30 and 90 minutes, an average Vickers hardness at the surface of the steel product at least 50% greater than an average Vickers hardness at the surface of the steel product prior to nitriding. 
     
     
         21 . A nitrided steel strip of less than 5 mm in thickness comprising, by weight, less than 0.25% carbon, between 0.20 and 2.0% manganese, between 0.05 and 0.50% silicon, less than 0.01% aluminum, niobium between 0.01 and about 0.20%, and between 0.01 and 0.075% nitrogen, and having a majority of the microstructure comprised of bainite and acicular ferrite, having more than 70% niobium in solid solution prior to nitriding and having after nitriding for between 30 and 90 minutes, a soft center portion of less than 250 Vickers hardness and a hard outer portion of greater than 250 Vickers hardness with the hard outer portion being at least 30% of the thickness of the strip. 
     
     
         22 . A nitrided steel strip of less than 2 mm in thickness comprising, by weight, less than 0.25% carbon, between 0.20 and 2.0% manganese, between 0.05 and 0.50% silicon, less than 0.01% aluminum, niobium between 0.01 and about 0.20%, and between 0.01 and 0.075% nitrogen, and having a majority of the microstructure comprised of bainite and acicular ferrite, having more than 70% niobium in solid solution prior to nitriding and having after nitriding for between 30 and 90 minutes, a soft center portion of less than 250 Vickers hardness and hard outer portion of greater than 250 Vickers hardness with the hard outer portion being at least 30% of the thickness of the strip. 
     
     
         23 . A nitrided 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 0.01% aluminum, niobium between 0.01 and about 0.20%, and between 0.01 and 0.075% nitrogen, and having a majority of the microstructure comprised of bainite and acicular ferrite, having more than 70% niobium in solid solution prior to nitriding and having after nitriding for between 30 and 90 minutes, a nitride cluster density of 2600×10 15  clusters/cm 3  at a depth of 25 microns below the steel product surface. 
     
     
         24 . The nitrided steel product as claimed in  claim 23 , where the nitride clusters include at least one of the nitrides selected from the consisting of niobium nitride, manganese nitride, chromium nitride, iron nitride, and combinations thereof.