Abstract
Relationship between the strength and the change of grain size and precipitation behavior of NbC by various heat treatments was examined with pure 0.2% carbon steels containing 0.02-0.09% Nb.
The results were summarized as follows:
1. Though NbC particles precipitated in Nb treated steels tend to coalesce with increase of austenitizing temperature, they remain in steel and prevent the grain coarsening of austenite because of its small solubility and dissolving rate in austenite.
2. The highest strength was obtained when the cooling rate from austenitizing temperature to room temperature was 25°C/min.
When Nb treated steel is continuously cooled after full solution treatment for long time at sufficiently high temperature such as 1250°C, NbC seems to precipitate not only at lower temperatures than A3 point but also at the temperature range of austenite, and the precipitation from austenite will cause less strengthening effect than that from ferrite.
3. Strengthening effect by Nb treatment may be explicable on the assumption that both effects of grain refinement and dispersion of fine particles of NbC are additive; that is, an increase of strength in Nb treated steels austenitized at lower temperatures than about 1050°C is due mainly to refining of ferrite grain size and that at higher temperatures is due mainly to fine precipitation of NbC.
