1. An increase of the silicon or carbon content substantially increases the notch sensitivity of heat treated silicon steel, which is associated with an increase of the ductile-brittle transition temperature. 2. Analysis of the increase in the ductile-brittle transition temperature confirms the conclusion that it is expedient to use steels with a high silicon content when the material is heat treated to a high strength (low or moderate tempering temperature). 3. Unlike chromium or manganese, under severe stress conditions silicon increases the strength to an equal degree regardless of the tempering temperature. Small silicon additions have the greatest effect at low carbon concentrations.