Enhancement of Ductility in High Strength Steels

摘要

The conditions under which high strength and high ductility can be attained were studied in metastable austenitic steels. The stability of the austenite was shown to be the most important variable. It affected the tensile properties through the isothermal formation of martensite induced by elastic stresses and/or plastic strain. The results from 14 different alloys showed that low austenite stability resulted in high work hardening rates, high tensile strengths and low elongation values. Low austenite stability also caused the yield strength to decrease due to the stress induced formation of martensite. When the austenite was very stable, plastic deformation was initiated by slip and its work hardening rate was too low to prevent necking at small strains. A specific amount of strain induced martensite led to an 'optimum' work hardening rate, resulting in maximum elongation. The effects of varying C and Mn contents, different test and rolling temperatures, rolling time and various reductions in thickness on the austenite stability were investigated. The amount of plastic deformation at a temperature at which no phase transformation can be induced, i.e. above the M sub d temperature, had a negligible effect on the yield strength when the austenite was very unstable, its effect increased, however, as the austenite stability increased. The best combination of strength and ductility was attained when the austenite was stable enough for slip to initiate yielding but still sufficiently unstable so that martensite could form during straining. (Author)