Effect of nitrogen on formation of martensite-austenite constituent in low carbon steels

The effect of dissolved nitrogen on phase transformations and microstructural evolution in low-carbon high manganese silicon steel wire-rod was examined for varying nitrogen levels of 30, 70, and 100 ppm, while keeping the base composition constant. It was found that nitrogen suppresses the pearlite reaction start temperature by up to 30°C. A strong correlation was discovered between the nitrogen level and the formation of martensite-austenite microconstituent (MA phase). The possible mechanisms by which nitrogen causes the formation of MA phase are discussed. The main conclusion drawn from this study is that levels of nitrogen between 70 and 110 ppm can have a significant effect on microstructure, such that up to approximately 10vol% of MA phase is formed in these steels. Microstructural changes in titanium-microalloyed steels were also investigated - particularly changes in the development of the MA and pearlite phases. The pearlite content was up to double that found in a Ti-free alloy. Microalloying with titanium was found to be effective in completely eliminating MA phase.