Fe-12Mn강에서 망간에 의한 입계 취화와 공석 반응

Abstract

Mn has been considered as a cheaper alternative to Ni because Mn produces similar effects to Ni which is expensive as a major alloying element. Unfortunately, the Fe-Mn steels are brittle at low temperatures, exhibiting the fracture path along prior austenite grain boundaries. There has, however, been no decisive evidence for the reason of the embrittlement observed in the high Mn steels. From the present results on the grain boundary embrittlement of an Fe-12Mn binary steel which were obtained using simple tensile tests in liquid nitrogen, Auger electron spectroscopy and scanning or transmission electron microscopy, it was clarified that the intergranular embrittlement in the present Mn steel is due to the prior austenite grain boundary segregation of Mn which acts as a strong embrittler. The concave profile of intergranular granular fracture strength corresponded to the convex profile of segregated Mn at the grain boundaries. It was also shown that the eutectoid reaction producing α-Mn precipitates occurred surprisingly at a temperature (400oC) higher than the equilibrium eutectoid reaction temperature (247oC) in the present Mn steel. This is due to the hydrostatic pressure induced by the transformation of epsilon martensite to austenite during heating to the tempering temperature. It was proposed from the present results that the embrittlement in high Mn steels may be due to the grain boundary segregation of Mn.