Superhigh Strengthening and Net Shaping for Sintered Low Alloy Steels by Utilizing the Partial Constituent Gradient Microstructure in MIM Process

金属注射成形（MIM）技术是一种先进的粉末成形技术，它以低的加工能量和100%的材料利用率实现了形状复杂的净成形。本研究旨在阐明和优化MIM工艺制备的超高强度低合金烧结钢（Fe-Ni系）的力学性能与显微组织之间的关系。6%mass%NiMIM钢具有2000 MPa的抗拉强度、5%的延伸率和500 MPa的疲劳强度。当Ni粉粒度小于18μ m时，富Ni相仅由马氏体组织组成，而当Ni粉粒度大于18μm时，富Ni相中心形成残余奥氏体。因此，当Ni粉粒度为20μm左右时，即使是相同成分的Fe-6 Ni-0.5Mo-0.2Mn钢，也可获得上级力学性能。此外，代替Fe-Ni，研究Fe-Mn低合金钢以获得高性能，因为Ni对人体具有有害影响，例如Ni过敏。

Metal Injection molding (MIM) process is an advanced powder processing technique because of net shaping with shape complexity at low processing energy and 100% material utilization. This study has been performed to clarify and to optimize the relationship between the mechanical properties and the microstructures for the superhigh strengthening sintered low alloy steels (Fe-Ni system) by using MIM process. A 6%mass%Ni MIM steel showed excellent properties of 2000MPa tensile strength, 5% elongation, and 500MPa fatigue strength. The Ni rich phase consists of only the martensitic structure if the size of agglomerated Ni powders is below 18μm, whereas the retained austenite is formed in the center of Ni rich phases if the size of agglomerated Ni powders is above 18μm. If the Ni powders of approximately 20μm particle size is dispersed uniformly in the matrix, therefore, superior mechanical properties may be obtained by even the same composition of Fe-6Ni-0.5Mo-0.2Mn steels. Instead of Fe-Ni, moreover, the Fe-Mn low alloy steel was investigated to obtain high performance properties, because Ni has harmful influence upon the human body, for example, a Ni allergy.