Role of Retained Austenite in Corrosion Resistance of Si-Mn Steel

Abstract

Si-Mn steels are used for many industrial applications because of their high strength. One of the most important characteristics of the microstructure of Si-Mn steels is the retained austenite embedded in the matrix of martensite or bainite. The existence of the retained austenite promotes TRIP (Transformation-Induced Plasticity) effect, and therefore, provides Si-Mn steels with their excellent strength. While the retained austenite has the beneficial effect on the mechanical properties, corrosion mechanisms of the steels with the retained austenite have been unclear. In reality, the actual application of high-strength steels almost always involves outdoor corrosion environments which can be characterized as predominantly near-neutral pH environments with chloride ions. Even if the steels are successfully protected from corrosion by coating and/or painting, pitting corrosion readily occurs at the cut edges and in coating defect areas due to the presence of chloride ions. In this research, the objective is to clarify the effect of the retained austenite on the corrosion resistance of the Si-Mn steel in near-neutral pH environments.The specimens were Si-Mn steel (0.39%C, 1.51%S, 2.01%Mn, 0.010%P, 0.002%S, 0.04%Al, 0.004%N). The metallographic inspection was performed to ascertain the characteristics of the microstructure of the specimen by using EPMA (Electron Probe Micro Analyzer). It was confirmed that the microstructure consisted of the retained austenite and the matrix of martensite. To assess the corrosion behavior of the specimen, immersion tests were performed in the boric-borate buffer solutions with NaCl (pH 7.0). It was confirmed that the pits were initiated only in the matrix and not in the retained austenite.To clarify the reason for the high corrosion resistance of retained austenite, the corrosion behavior of two types of specimens were compared: Full martensitic specimen (0.44%C, 0.20%S, 0.85%Mn, 0.008%P, 0.002%S, 0.035%Al, 0.003%N) and full austenitic specimen (0.45%C, 0.32%S, 29.40%Mn, 0.014%P, 0.002%S, 0.025%Al, 0.005%N). In the case of the austenitic specimen, higher amount of Mn was added to form full austenitic structure even at room temperature. The austenitic and martensitic specimens simulated the retained austenite and matrix phases in the Si-Mn steel. The depassivation pH values of the austenitic and martensitic specimens were measured in boric-borate buffer solutions. The pH of the solution was decreased stepwise with 0.35 M H3BO3, and the open circuit potentials were measured. It was confirmed that the depassivation pH of the austenitic specimen was lower than that of the martensitic specimen. In other words, the depassivation occurred more readily on the martensite than the austenite. AES (Auger Electron Spectroscopy) depth analysis was performed to investigate the thickness of the surface oxide films on the austenitic and martensitic specimens. It was clarified that the oxide film on the austenite was about 1 nm thicker than that on the martensite, suggesting that the protective ability of the oxide film formed on the austenite is higher than that on the martensite. According to the above results, the high corrosion resistance of the retained austenite in Si-Mn steels appeared to be related with the nature of the oxide film formed on the austenite.