Development of High Strength Non-Magnetic Steels for Concrete Applications

在本研究中，利用变形诱导的bcc马氏体的逆转，在Fe-15% Mn合金中建立了将奥氏体晶粒细化到1mum以下的工艺。通过超晶粒细化，该合金的屈服应力可提高到0.8GPa（晶粒细化前约为0.2GPa），并且由于TRIP现象，该合金具有足够的延展性。然而，即使是少量的应变也会导致该合金中铁磁性bcc马氏体的形成，因此该合金不适合应用于要求完全无磁性的领域。因此，在一系列Fe-Mn二元合金中，结合合金的力学性能，讨论了Mn含量与合金组织的关系以及hcp （epsilon）马氏体相变的晶粒尺寸依赖性。结果表明：(1)在Mn含量为20%及以上的合金中，铁磁性bcc马氏体在拉伸试验中断裂后才形成。在Fe-20-27% Mn合金中，变形过程中产生非磁性的epsilon马氏体，使合金的抗拉强度大大提高。(2)当合金晶粒尺寸较大（约100ma）时，合金在变形初期沿片状马氏体或奥氏体晶界发生脆性断裂。同时，晶粒细化至10mum或以下是抑制脆性断裂的必要条件。(3)奥氏体的晶粒细化和形变诱导的epsilon马氏体的利用使生产出抗拉强度在1.1GPa以上、塑性适中的非磁性高强钢。

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.