Vacuum degassed, interstitial-free, low carbon steel and method for producing same

Claim 1

The steel claimed in claim 1, consisting essentially of from about 0.002 percent to about 0.010 percent carbon up to about 0.35 percent manganese, from greater than 0.025 percent to about 0.060 percent columbium, at least one of titanium and zirconium, titanium being from about 0.015 percent to about 0.061 percent, zirconium being from about 0.028 percent to about 0.12 percent, from about 0.002 percent to about 0.006 percent nitrogen, up to about 0.004 percent total oxygen, up to about 0.020 percent sulfur, up to about 0.010 percent phosphorus, from about 0.015 percent to about 0.020 percent total aluminum, up to about 0.015 percent silicon, and balance substantially iron except for incidental impurities.

Claim 6

A method of producing a non-aging, low carbon steel having substantially no yield point elongation in the hot rolled and cold rolled and annealed conditions, which comprises melting a charge of steel having a maximum carbon content of about 0.05 percent, vacuum degassing the steel to obtain a melt having a carbon content of about 0.020 percent maximum, a total oxygen content of about 0.010 percent maximum, a nitrogen content of about 0.008 percent maximum, up to about 0.6 percent manganese, up to about 0.035 percent sulfur, and balance substantially iron, deoxidizing by addition of a deoxidant chosen from the class consisting of aluminum, titanium and silicon, adding at least one of titanium and zirconium, titanium when used being added in an amount sufficient to obtain a titanium content in the hot rolled stage within the range of about 0.015 percent to about 0.12 percent, and zirconium when used being added in an amount sufficient to obtain a zirconium content in the hot rolled stage within the range of about 0.028 percent to about 0.18 percent, adding columbium in an amount sufficient to produce greater than 0.025 percent columbium in solid solution in the hot rolled stage as determined by sheet analysis at room temperature, casting and solidifying said steel, hot rolling to band thickness with a finishing temperature of at least about 816* C, and coiling, the titanium content being equal to or less than four times the weight percent carbon plus 3.43 times the weight percent nitrogen except titanium as titanium oxide, the zirconium content being equal to or less than 7.6 times the weight percent carbon plus 6.51 times the weight percent nitrogen except zirconium as zirconium sulfide and zirconium oxide, and not more than 0.004 percent carbon being combined with columbium.