Selective Oxidation during the Austenitic Annealing of a CMnSi Steel

Abstract

High strength multiphase CMnSi steel is increasingly used in passenger cars. Si and Mn alloying levels are typically in the range of 1-2% in mass. While Si improves the mechanical properties, it considerably deteriorates the galvanisability of steel. Residual water vapour in the reducing gas atmosphere during the intercritical or austenitic annealing results in the selective oxidation of Si and Mn at the steel surface. Besides Mn and Si, C is oxidized as well at the steel surface, leading to the formation of CO gas and decarburisation of the steel surface. This decarburisation has a major influence on the phase composition in the steel surface region: it shifts the ferrite to austenite transformation to higher annealing temperatures, leading to differences in surface and bulk microstructure. The phase composition influences the solubility and diffusivity of all alloying elements near the surface. The evolution with temperature of the selective oxidation at the steel surface has been studied by interrupted annealing in a protective atmosphere containing residual water vapour. The influence of the annealing temperature on the selective oxidation of Mn and Si is characterized by XPS (X-ray Photo-electron Spectroscopy) analysis.

Selective Oxidation of a CMnSi Steel during Heating to 1000 °C: Reversible SiO2 Oxidation

“…This air exposure only caused additional carbon and oxygen adsorption and the oxidation of the metallic iron in contact with the air. [17] All other elements are already in their oxidized state when present at the surface. The air exposure therefore allows distinguishing between the iron in contact with the atmosphere and the iron underneath a protective layer.…”

“…Previous work on similar CMnSi alloys [8,17,18] already reported a decreased selective oxidation during austenitic annealing. Due to the limited heating power in continuous strip processing facilities, such as annealing and particularly galvanizing lines, selective oxidation in the ferritic and intercritical region is practically inevitable.…”

Insight into selective oxidation of dual‐phase steel after annealing in pure H2 atmosphere at different heating rates and temperatures

“…It has been reported that reaction rates generally increase with increasing temperature . According to the Wagner theory for parabolic kinetics, the rate of oxide growth is directly proportional to the external diffusion coefficient and temperature of the alloying elements.…”

“…It has been reported that reaction rates generally increase with increasing temperature. [15,20,21] According to the Wagner theory for parabolic kinetics, [17] the rate of oxide growth is directly proportional to the external diffusion coefficient and temperature of the alloying elements. It has been reported by Van de Putte et al [21] that there is almost no oxidation of the surface at low temperatures.…”

“…[15,20,21] According to the Wagner theory for parabolic kinetics, [17] the rate of oxide growth is directly proportional to the external diffusion coefficient and temperature of the alloying elements. It has been reported by Van de Putte et al [21] that there is almost no oxidation of the surface at low temperatures. While, after heating to 650°C, Mn starts to be oxidized, and as the temperature rises, the oxidation of Si becomes dominant.…”