21Cr 린 듀플렉스 스테인리스강의 기계적 특성 및 내식성에 관한 연구

Abstract

The effects of Mo addition and an aging heat treatment on microstructural evolution and transformation-induced plasticity (TRIP) and localized corrosion behavior in cold-rolled lean duplex stainless steel (DSS) samples were investigated. 21Cr lean DSS with 0–2% Mo was cold-rolled into 1-mm-thick plates and solution-treated at 1050 ℃ for 5 min. During the subsequent aging treatment at 700 ℃, redistribution of alloying elements, such as chromium (Cr), nickel (Ni) and Nitrogen (N), occurred in the α and γ phases, which resulted in local segregation and secondary phase precipitate in the α-phase interior and/or the α/γ phase boundary; this precipitate was confirmed as Cr2N. With increasing Mo content, a smaller thermal α΄ phase were formed after solutionization. Decrease in the stability of the γ phase, allowing deformation-induced martensite transformation (DIMT) to occur more easily. Due to the instability of the γ phase, the fraction of the thermal α΄ phase increased with aging. steels with a higher Mo content exhibited a more stable γ phase, which suppressed the formation of deformation-induced martensite (DIM). The effect of DIM (caused by TRIP) on the strength and elongation of the steels according to the fraction of the thermal α΄ phase was evaluated. The pitting potential (Epit) and critical pitting temperature (CPT) values decreased drastically, even after 5 min of aging time, compared with the as-solutionized steel; the values decreased with aging time. Pitting corrosion resistance decreases rapidly with increasing aging time and increases with the addition of Mo. The results of CPT test and potentiodynamic test with the samples for various aging time showed the consistency. Intermetallic precipitates of Cr2N containing high chromium content formed at the α/γ phase boundaries after aging treatment caused to the localized depletion of Cr in their vicinity. the longer aging time, the more Cr2N precipitates formed at α/γ phase boundary rather than ferrite matrix which should result to the easier pit initiation at the α/γ phase boundary.