A method for internal strengthening of articles formed from deep drawing quality low carbon steel strip or sheet containing about 0.02 to about 0.3% titanium in solution, up to about 0.3% columbium in solution, columbium when present being in an amount of at least 0.025% in solution, with the sum total of titanium and columbium in solution not exceeding about 0.3%, comprising the steps of heating the articles at a temperature of about 570.degree. to about 580.degree. C in a nitriding fluid for a time sufficient to form a total nitrogen concentration adjacent the surfaces of the articles at least substantially equal to the amount theoretically required to combine completely with the nitride-forming elements in the steel; and diffusion annealing the articles in a non-nitriding fluid at a temperature of about 595.degree. to about 815.degree. C for a time sufficient to complete through-thickness strengthening by reaction of the nitrogen with the nitride-forming elements. A preferred nitriding fluid is a fused cyanate salt bath.