Nitrogen-Enhanced Ferritic Microalloyed Steels
In many ferritic steels, “free” (uncombined) nitrogen is regarded as a harmful impurity, responsible for aging and embrittlement. In making “clean” steel, nitrogen is expected to be below 20 ppm. In “enhanced-nitrogen” ferritic steels, the amount of nitrogen may be either caused by a high residual content (e.g. up to 110 ppm in electric-arc-furnace (EAF) steels) or intentionally added to levels close to the solubility limit. In the presence of nitrogen binding elements, such as aluminium, titanium, and vanadium, nitrogen is removed from solid solution by forming metal nitrides. The degree of their dispersion affects the microstructure and thus steel properties. The following metallurgical phenomena may be involved the grain coarsening temperature, grain refinement, intragranular ferrite nucleation, optimized precipitation strengthening, and other factors. An enhanced-nitrogen content particularly benefits vanadium steels, contributing to property improvements and cost reduction.
Key words: Aluminum, vanadium, or titanium nitrides; precipitation; grain refinement; intragranular nucleation; weight reduction
The International High-Nitrogen Steel Conference (HS 2006), Jiuzhaigou Valley, China, 29-31 August 2006