Relation between Precipitation Behavior of Mo, V and Nb and Yield Strength in Normalized and Tempered Steel

Abstract

The changes in the precipitation behavior of alloying elements and the yield strength due to normalizing and subsequent tempering of 0.15% C-1.4% Mn steels containing small amounts of Mo, V, Nb and Ni have been studied. The results are as follows: (1) The addition of any two or all of Mo, V and Nb to steel exhibits a synergistic effect on the increase in yield strength of normalized-tempered steel at room and moderate temperatures. (2) The amounts of Mo, V and Nb as precipitates increase by the coexistence of these elements in steel, compared to the simple sum of the amounts of precipitates in steels having any one element. (3) Fine precipitates of Mo₂C and more frequently those of NbC and V₄C₃ were detected through electron microscopic analyses. Peculiarly shaped precipitate likely composed of two kinds of precipitates was also observed in steels containing Mo, V and Nb. (4) The increase in the cooling rate from normalizing temperature and the addition of Ni contribute to the increase in the amounts of Mo and V precipitated by tempering. This effect is attributed to the increase in the percentage of bainite structure after normalizing which has many dislocations and can prepare precipitation sites during tempering.