Development of High Strength Non-Magnetic Steels for Concrete Applications

在这项研究中，一个过程细化奥氏体晶粒低于1 μ m的Fe-15%Mn合金中建立了使用的变形诱导体心立方马氏体的逆转。还证实了该合金的屈服应力可以通过超晶粒细化提高到0.8GPa（晶粒细化前：约0.2GPa），并且该合金由于TRIP现象而具有足够的延展性。然而，即使少量的应变也会导致在该合金中形成铁磁bcc马氏体，使得该合金不适合应用于需要完全无磁性的领域。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含量与显微组织的关系，以及hcp（ε）马氏体相变的晶粒尺寸依赖性。结果表明：（1）Mn含量在20%以上的合金，在拉伸试验中，直到断裂时才形成铁磁体心立方马氏体。Fe-20-27% Mn合金在变形过程中诱发了非磁性马氏体，这有助于抗拉强度的大幅度提高。(2)然而，当这些合金具有大的晶粒尺寸（约100 μ m）时，它们在变形的早期阶段沿着沿着片状马氏体或奥氏体晶界经历脆性断裂。还清楚的是，晶粒细化到10 μ m或更低对于抑制这种脆性断裂是必不可少的。(3)奥氏体晶粒细化和形变诱发马氏体的应用使无磁高强度钢的抗拉强度达到1.1GPa以上，塑性适中。

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.