| Steels are characterized by high strength with good plasticity and have a series of advantages of abundant resources,low price and easy processing.And thus,steels are widely used in aerospace,chemical,petroleum,marine and other engineering machinery industries.In recent years,with the upgrading of the engineering machinery industry,the requirements of large engineering components on the mechanical properties of steels are getting higher.Precipitation strengthening is one of the most simple and effective methods to improve the strength and plasticity of steels.Now,searching for nano-reinforced phases that "match" the matrix is the most popular research direction.Recent studies have found that G-phase has a cube-on-cube orientation relationship with the martensitic matrix and is dispersed in martensitic steels in the form of nanoparticles,which has the potential as a nanosacle strengthening phase.However,there are few reports on the G-phase strengthened BCC structural steels,especially its precipitation behavior is still unclear.Therefore,this study designed and prepared a series of G-phase strengthened Fe-Ni-Si-based BCC structural steels through the combination of thermodynamic calculation and experimental research.The precipitation behavior of the nanoscale G-phase in BCC structural steel was characterized by using transmission electron microscopy and atom probe.On this basis,the mechanical properties of G-phase strengthened Fe-Ni-Si-based BCC structural steels were systematically studied by optimizing the alloy composition,heat treatment process and rolling process.The main results of this study are as follows:(1)In this study,the Calphad method was used to carry out alloy design on G-phase strengthened Fe-Ni-Si-based BCC structural steels to explore and find a series of Fe-7Ni-2Si-1X(X=Ti,Nb,Ta,Zr,Mn)alloys with the microstructural characteristics of "high-temperature austenite+G-phase",Fe-20Cr-3Ni-3Si-2X(X=Ti,Nb,Ta,Zr,Mn)alloys characterized by "ferritic matrix(BCC)+G-phase" and Fe-20Cr-3Ni-3Si-I Ti-2X(X=Cu,Al)alloys with "ferrite matrix(BCC)+G-phase+NiAl phase+Cu-rich phase”microstructure.The experimental results are in good agreement with the calculated results.(2)Based on the alloy design,a series of Fe-7Ni-2Si-1X(X=Ti,Nb,Ta.Zr,Mn)quaternary martensitic alloys were prepared.Systematic studies have been carried out on their microstructure,crystal structure and phase transition temperatures and so on.The results showed that the water-quenched samples of the Fe-7Ni-2Si-1X series alloys presented the body-centered cubic(b.c.c.)lath-martensite microstructure,and their austenitizing temperature are～800℃.At the same time,the G-phase precipitation behavior of.the Fe-7Ni-2Si-1X series alloys during aging at 500℃ was solved by using transformission electron microscopy and atom probe.Among them,the averaged radius of the Ni16Ti6Si7_G phase in Fe-7Ni-2Si-1Ti alloy is about 1.4±0.7nm and the number density is up to 1.9×1024m-3.The room temperature tensile strength and plasticity of the alloy are～1800MPa and 4%,respectively.(3)Based on the alloy design,a series of the Fe-20Cr-3Ni-3Si-2X(X=Ti,Nb,Ta,Zr,Mn)ferritic alloys were prepared.Combined with transmission electron microscopy and atom probe,the basic information of chemical composition,morphology,structure and coherence relationship of the nanoscale Ni16X6Si7_G-phase in Fe-20Cr-3Ni-3Si-2X series alloys were obtained for the first time.Among them,the averaged radius and the number density of G-phase in Fe-20Cr-3Ni-3Si-2Ti alloy are～1.64±0.45nm and 6.05×1023m-3,respectively.At the same time,the creep threshold of the alloy at 660℃is～140MPa,which is much higher than the creep threshold of 100MPa for the commercial P92 steel at 650℃.(4)Based on the alloy design,a series of the Fe-20Cr-3Ni-3Si-1Ti-2X(X=Cu,Al)ferritic alloys were prepared.The atomic probe technique was used to study the crystal structure,coherent relationship,precipitation sequences and synergistic behavior of G-phase,Cu-rich phase and NiAl phase in Fe-20Cr-3Ni-3Si-1Ti-2X series alloys.For the first time,a multi-layer core-shell nanostructure from core to outer layer of G/B2-NiAl/Cu-rich/B2-NiAl was found in Fe-20Cr-3Ni-3Si-1Ti-2Cu-2Al alloy.At the same time,the Fe-20Cr-3Ni-3Si-0.75Ti-1Cu-lAl alloy has a 1300 MPa tensile strength and a 5%tensile plasticity at room temperature. |
