Abstract
A multi-element and multi-phase internal oxidation model that couples thermodynamics with kinetics is developed to predict the internal oxidation behaviour of Fe–Mn–Cr steels as a function of annealing time and oxygen partial pressure. To validate the simulation results, selected Fe–Mn–Cr steels were annealed at 950 °C for 1–16 h in a gas mixture of Ar with 5 vol% H2 and dew points of − 30, − 10 and 10 °C. The measured kinetics of internal oxidation as well as the concentration depth profiles of internal oxides in the annealed Fe–Mn–Cr steels are in agreement with the predictions. Internal MnO and MnCr2O4 are formed during annealing, and both two oxides have a relatively low solubility product. Local thermodynamic equilibrium is established in the internal oxidation zone of Fe–Mn–Cr steels during annealing and the internal oxidation kinetics are solely controlled by diffusion of oxygen. The internal oxidation of Fe–Mn–Cr steels follows the parabolic rate law. The parabolic rate constant increases with annealing dew point, but decreases with the concentration of the alloying elements.
Highlights
Advanced high-strength steels (AHSS) are extensively used for automotive applications to reduce the weight of car bodies and thereby reducing fuel consumption and CO2 emissions; see, for example, [1]
1.6Cr steel annealed at 950 °C for 16 h in the gas mixture with dew points ranging from - 45 to 10 °C
The transition from external to internal oxidation of the Fe–1.9Mn–1.6Cr steel occurs at an annealing dew point of - 30 °C, with oxides formed both at steel surface and in the alloy matrix
Summary
Advanced high-strength steels (AHSS) are extensively used for automotive applications to reduce the weight of car bodies and thereby reducing fuel consumption and CO2 emissions; see, for example, [1]. A zinc coating is applied onto the steel surface usually by hot-dip galvanizing [2] in order to protect the AHSS against corrosion. Before entering the zinc bath, the steel strip passes through continuous annealing furnaces. Oxidation of the alloying elements (e.g. Mn, Cr, Si, Al) in AHSS is thermodynamically favourable during annealing process prior to galvanizing. The presence of these alloying element oxides at the steel surface reduces the zinc wettability [3] and impairs the quality of the galvanized steel product [4]. Understanding the nature of oxidation of AHSS and predicting the oxidation behaviour of AHSS are essential to the steel industry in terms of designing new steel grades and optimizing annealing parameters
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