Effects of heating temperature and atmosphere on element distribution and microstructure in high-Mn/Al austenitic low-density steel

The elemental distribution and microstructure near the surface of high-Mn/Al austenitic low-density steel were investigated after isothermal holding at temperatures of 900–1200°C in different atmospheres, including air, N₂, and N₂ + CO₂. No ferrite was formed near the surface of the experimental steel during isothermal holding at 900 and 1000°C in air, while ferrite was formed near the steel surface at holding temperatures of 1100 and 1200°C. The ferrite fraction was larger at 1200°C because more C and Mn diffused to the surface, exuded from the steel, and then reacted with N and O to form oxidation products. The thickness of the compound scale increased owing to the higher diffusion rate at higher temperatures. In addition, after isothermal holding at 1100°C in N₂, the Al content near the surface slightly decreased, while the C and Mn contents did not change. Therefore, no ferrite was formed near the surface. However, the near-surface C and Al contents decreased after holding at 1100°C in the N₂ + CO₂ mixed atmosphere, resulting in the formation of a small amount of ferrite. The compound scale was thickest in N₂, followed by the N₂ + CO₂ mixed atmosphere, and thinnest in air. Overall, the element loss and ferrite fraction were largest after holding in air at the same temperature. The differences in element loss and ferrite fraction between in N₂ and N₂ + CO₂ atmospheres were small, but the compound scale formed in N₂ was significantly thicker. According to these results, N₂ + CO₂ is the ideal heating atmosphere for the industrial production of high-Mn/Al austenitic low-density steel.