Development of High Strength Non-Magnetic Steels for Concrete Applications

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In this investigation, a process to refine austenite grains to below 1mum was established in an Fe-15% Mn alloy by the use of reversion of deformation induced bcc martensite. It was also confirmed that yield stress of this alloy can be increased to 0.8GPa by the ultra grain refining (before grain refining : about 0.2GPa), and that this alloy has sufficient ductility due to TRIP phenomenon. However, even a small amount of strain causes the formation of ferro magnetic bcc martensite in this alloy, so that this alloy is not suitable for the application to a field where complete non-magnetism is required. Thus, in a series of Fe-Mn binary alloys, the relationship between Mn content and microstructures, and grain size dependence of hcp (epsilon) martensitic transformation were discussed in connection with the mechanical properties of these alloys. The results obtained are as follows :(1) In alloys with 20% Mn or more, ferro magnetic bcc martensite was not formed until the fracture in tensile testing. In Fe-20-27% Mn alloys, epsilon martensite with non-magnetic nature is induced during deformation and this contributes to a great increase in tensile strength.(2) When these alloys have a large grain size (around 100mum), however, they undergo brittle fractures in a early stage of deformation along platelet epsilon martensite or austenite grain boundary. It is also cleared that grain refining to 10mum or below is indispensable for the suppression of such brittle fractures.(3) Grain refining of austenite and the use of deformation induced epsilon martensite enabled the production of non-magnetic high-strengthened steels with the tensile strength of above 1.1GPa and moderate ductility.