Niobium in ferritic and martensitic stainless steels

A review has been presented of the effects of niobium in ferritic and martensitic stainless steels, introducing some new stainless steels containing niobium. An addition of niobium to the steel has a beneficial effect on improving the corrosion resistance of ferritic stainless steel, since niobium reacts with carbon to form stable carbides, consequently, resulting in keeping effective content of chromium in the matrix. Niobium added 22% chromium-1.2% molybdenum ferritic stainless steel shows remarkable environmental corrosion resistance in comparison with Type 304 and 316. An addition of niobium is one of the most effective ways to give a ferritic stainless steel a good deep drawability. An amount of niobium, which is necessary to improve this property, is related to a content of carbon and nitrogen. Ferritic stainless steels containing an appropriate amount of niobium exhibit good mean r-value. Soluted niobium and molybdenum are effective in improving the high temperature strength. An amount of soluted niobium depends on the annealing temperature, and niobium precipitates rapidly when the steel is annealed at the lower temperature. Therefore, this behavior of niobium affects complicatedly on mechanism of the influences of this alloying element on the high temperature strength. Niobium added martensitic stainless steel shows very good tempering resistance, since niobium is beneficial to inhibit forming chromium carbides. Consequently, martensitic stainless steel is able to keep effective content of chromium in the matrix, which causes for the steel to improve corrosion resistance, as same as ferritic stainless steel.