Effect of nitrogen on formation of martensite-austenite constituent in lowcarbon steels

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