The Influence of Precipitation Mode and Dislocation Substructure on the Properties of Vanadium-treated Steels.

Abstract

A study was conducted to examine the influence on the Charpy impact properties of various sizes and distributions of precipitates in microalloyed steels and also of the combined effects of precipitation and dislocation strengthening. Most of the work was done using vanadium steels but two niobium containing steels and a C-Mn steel were also included for comparison. Isothermal and continuous cooling treatments were used to obtain a range of sizes and distributions of precipitate particles while three different rolling schedules were used to obtain variations in the amount and type of dislocation substructure.It was found that continuous transformation gave greater precipitation strengthening than isothermal transformation and was also less detrimental to impact properties. The embrittlement vector was not constant but varied over the range 0.3-0.8°C/N/mm2 and increased as the level of precipitation strengthening increased and as the transformation temperature decreased. The combined effects of precipitation and dislocation strengthening led to much greater strengths in the vanadium steels than in the niobium steels and also tended to be less embrittling. The optimum microstructure in the vanadium steels consisted of fine ferrite grains with a well recovered substructure containing row precipitates formed at a relatively high transformation temperature.