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 degrees 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 degrees C in air, while ferrite was formed near the steel surface at holding temperatures of 1100 and 1200 degrees C. The ferrite fraction was larger at 1200 degrees 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 degrees 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 degrees 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.