1970 Volume 56 Issue 12 Pages 1501-1510
A study was made on the strengthening phenomena and fracture behaviours in alminium or Ca-Si killed low carbon manganese steels with small amounts of Nb, V or Nb-V addittion. It was found that the addition of Nb, V, or in combination of these elements produced an additional strengthening due to the fine carbonitride precipitation above the strength level presumed from the refined grain size, and this was controlled by the various heat treatments. The yield strength increment in the Nb-V, -and Nb-V-steels can be divided into the two terms, i. e. grain refinement and friction stress increment, the contribution of these two depends on the kinds of the steels and heat treatment. In Al-killed steels the impact transition temperature was generally lower than that of corresponding Ca-Si killed steels. The impact transition characteristics seem to be classified into four groups by the yield strength against the strengthening mechanism, and a linear dependence of the transition temperature on the friction stress increment has been found. A good combination of strength and toughness was obtained in the steels strengthened by the balanced effect of both the grain refinement and the friction stress factor. Finally, the fracture appearance of impact specimens was examined by microfractography.