Influence of cold work on mechanical properties of Fe–Ti–N alloys

Abstract

High-strength steels offer fuel savings in automotive components through weight reduction and when high strength is achieved in low-alloy steels, an economical material is produced, suitable for a wide range of applications. ‘Formable and strengthenable’ (FS) steels have been developed as an alternative to present high-strength low-alloy (HSLA) steels, and are produced in a manner different from the conventional alloys. The FS steel is formed in the soft, fully annealed condition, and the final strength is developed by a nitriding heat treatment. The present paper describes the microstructure and properties of an FS steel containing titanium as the nitride-forming element. Annealed and nitrided Fe–Ti alloys are extremely hard and brittle but cold working before nitriding greatly improves ductility. The replacement of boundaries on cold working by a fine subcell structure eliminates embrittlement, and added strength is obtained by the stabilization of the cell structure by Ti–N clusters or TiN particles. Cold worked and nitrided alloys have a microstructure which varies with depth from the nitrided surface into the component, producing a complex structure which prevents detailed analysis of the strengthening process. However, the present work demonstrates the value of FS steels as a useful addition to the range of HSLA steels.

MST/379