High strength deep drawing steel developed by reaction with ammonia

A method of providing a sheet of deep drawing quality special killed, fully stabilized type steel of uniform enhanced strength and good formability and weldability, comprising:

• a) providing an essentially unalloyed interstitial free carbon steel melt having a carbon content from about 0.001 to about 0.01 weight percent;
• b) adding to the steel melt a strengthening element selected from the group consisting of titanium, niobium and vanadium and mixtures thereof in total amount from about 0.01 to about 0.3 free atomic percent available strengthening element uncombined with other elements ;
• c) casting and rolling the steel into a sheet according to a practice selected from the group consisting of (A) hot rolling and (B) hot rolling followed by cold rolling, wherein, when practice (A) is selected, the steel slab is hot rolled to a bar at a starting temperature between 2350.degree. F. and 1750.degree. F., followed by finish rolling, with a ferrite structure, starting toward the high end of a temperature range of about 1200.degree.-1675.degree. F. and finishing toward the low end of this temperature range, and coiling below about 1250.degree. F., and wherein, when practice (B) is selected, hot rolling is carried out by a practice selected from the group consisting of (1) rolling the steel slab, with an austenite structure, in the temperature range of about 2350.degree. F. to 1500.degree. F., followed by coiling (2) rolling the steel slab, with a ferrite structure, in the temperature range from a starting temperature of about 1675.degree. F. and finishing and coiling at a temperature above 1375.degree. F., with coiling temperature not less than about 1350.degree. F., and the hot rolling is followed by cold rolling of the thus hot-rolled sheet to a reduction in thickness of at least about 60%;
• d) coiling the rolled sheet;
• e) annealing the rolled sheet at a temperature in the range from about 1275.degree. F. to about 1350.degree. F. to optimize formation of a (111) grain structure of the steel;
• f) treating the annealed sheet in an open coil annealing furnace in an isothermal step at a nitriding temperature from about 800.degree. F. to about 1250.degree. F. with a nitriding gas delivered to the annealing furnace and consisting of ammonia and a inert buffer gas in such a ratio that exhaust gas composition at an exit edge of the open coil is about 1 vol. % to about 11 vol. % ammonia to all other gases present in the exhaust mixture, and for a time from about 1/2 hour to about 12 hours depending on the sheet thickness and the desired depth of strengthening, to nitride the steel through at least a portion of the sheet thickness;
• g) recirculating the nitriding gas through the furnace at a rate and in a manner to provide fully developed laminar gas flow across the width of the sheet, and substantially equal gas flow rates in coil interwrap spaces from inner to outer wraps;
• h) controlling the strengthening of the steel sheet as a function of steel composition, the nitriding gas composition, nitriding time, nitriding temperature, thickness of the steel sheet and depth of strengthening desired to provide a steel sheet having an 0.2% off-set yield strength after temper rolling of at least about 45 ksi and an r value in excess of about 1.8 for the cold rolled sheet.