Microstructure And Properties Optimization Of High Strength And Toughness Non-magnetic Amorphous Composite Structural Steel

Steel is the leading structural material nowadays.With the development of supercon-ducting and high magnetic field technology,the demand for new type non-magnetic high performance structural steel with magnetic permeability less than 1.5 has increased dramatically.It is required that steel in high magnetic field is not easy to be magnetized and produce eddy current,so as to avoid energy loss and component failure due to disturbing magnetic field distribution.Amorphous steel has super high strength,but its brittleness and strain softening at room temperature seriously restrict its application as a high performance structural material when loading.Exploring a new type of non-magnetic high performance amorphous steel system with both high strength and plastic toughness has important industrial application prospects.In this paper,Fe-Mn-Si alloys with certain amorphous forming ability and shape memory effect were selected,and the continuous tissue gradient amorphous composites with 2 mm diameter were prepared by vacuum magnetic suspension melting water-cooled copper crucible-negative pressure copper mould suction casting method through component control and microalloying.Specific research contents are as follows:the alloy composition of the non-magnetic amorphous composite structural steel is optimized by changing the content of C element.On this basis,the structure and properties of the alloy are optimized by adding rare earth element Ce.Finally,the non-magnetic amorphous composite structural steel with good comprehensive mechanical properties is prepared by pretreatment and impurity removal with industrial raw materials.XRD,OM,SEM,TEM,VSM,ICP-AES and Universal Capability Testing Machine were used to analyze the structure,composition,mechanical and magnetic properties of the alloy.The results show that the structure of Fe-15Mn-5Si-10Cr-xC?x=0.2-1.0,wt.%?amor-phous alloy composites is composed of amorphous and crystalline phases,and the as-cast structure from the edge to the center presents a gradient change from amorphous phase to crystalline phase.The alloy system has good compressive mechanical properties at room temperature.The martensite phase transformation from austenite to martensite occurs during compressive stress loading,and the TRIP effect strengthens and toughens the alloy.When x=0.2-0.5,the precipitated crystalline phases of the alloy are?-Fe undercooled austenite phase(CFe_15.1)and?-Fe ferrite phase?Fe-Cr?.When x=0.6-1.0,the precipitated crystalline phases of the alloy are single?-Fe undercooled austenite phase(CFe_15.1).The yield strength and fracture strength of the alloy x=0.6 are 857 MPa and 2366 MPa respectively.The alloy has22.8%plastic strain.The comprehensive mechanical properties of the alloy 0.9C are the best.The yield strength is 1300 MPa,the fracture strength is 2378 MPa and the plastic strain is17.2%.The macro paramagnetic amorphous composite structural steel was tested by VSM.It is found that the relative permeability?_r is lower than 1.04963,and the value of?_r decreases with the increase of C content.On this basis,the Fe-15Mn-5Si-10Cr-0.2C-1.0Ce alloy was obtained by adding rare earth element Ce instead of the main element Fe in Fe-15Mn-5Si-10Cr-0.2C alloy.The solidified structure of the alloy is still a composite structure of amorphous matrix+crystalline phase,and its crystalline phase is?-Fe undercooled austenite phase+?-Fe ferrite phase.The addition of rare earth element Ce increases the mixing entropy from 1.04R to 1.08R,decreases the mixing enthalpy from-14.37 to-14.79,and the fracture strength and plastic strain of the alloy reach 3146 MPa and 30.1%.Then,Ti₃₉Ni₄₀Cu₂₀Ce amorphous composites were prepared by studying the substitution of Ce for Ti.The crystalline phases of Ti39Ni40Cu20Ce amorphous composites were B2-Ti?Ni,Cu?austenite phase and thermotropic B19'-Ti?Ni,Cu?martensite phase,and the mechanical properties and work hardening behavior were compared with those of Fe-based amorphous composites.The addition of rare earth element Ce increases the mixing entropy from 1.06R to 1.10R,decreases the mixing enthalpy from-23.72 to-24.12,and the yield strength,fracture strength and plastic strain of the alloy reach 1091 MPa,2511 MPa and 13.8%respectively.Ti-based amorphous composites have stronger amorphous forming ability and higher yield strength,but their fracture strength and plastic strain capacity are lower than those of Fe-based amorphous composites.Then,from the thermodynamic and dynamic aspects,the pre-melted slag system and its components which meet the requirements of industrial raw materials for impurity removal by pretreatment were designed,and the preparation of alloys from industrial raw materials was explored to further optimize the structure and reduce the cost.The specific composition of CaO:SiO₂:Fe₂O₃=3:1:6 is calculated by FactSage software simulation.which satisfies the melting conditions of the magnetic suspension water-cooled copper crucible melting equipment.The basicity is 3.The liquidus temperature is 1322?.The desulfurization and dephosphorization reaction products CaS and 3CaO·P₂O₅ enter the liquid slag floating on the surface of liquid metal.After cooling,the slag is removed from the alloy and the slag is easy to separate.Four alloys were obtained by pre-treatment with pre-melted slag and rare earth mixed pre-melted slag from Fe-15Mn-5Si-10Cr-0.6C-0.06P-0.12S and Fe-15Mn-5Si-10Cr-0.9C-0.06P-0.12S,respectively.The four kinds of alloys are all composed of amorphous phase and single?-Fe austenite phase.The yield strength of Fe-15Mn-5Si-10Cr-0.6C-0.06P-0.12S alloy and Fe-15Mn-5Si-10Cr-0.9C-0.06P-0.12S alloy obtained by pretreatment of pre-melted slag are 942 MPa and 1143 MPa respectively,the fracture strength are 2672 MPa and 2380 MPa respectively,and the plastic strain are 25.1%and 20.3%,respectively.Compared to x=0.6 alloy?yield strength 857 MPa,fracture strength 2366 MPa,plastic strain22.8%?and x=0.9 alloy?yield strength 1300 MPa,fracture strength 2378 MPa,plastic strain17.2%?prepared with high purity elements,their comprehensive mechanical properties are similar or even better.The fracture strength and plastic strain of Fe-15Mn-5Si-10Cr-0.6C0.06P-0.12S alloy and Fe-15Mn-5Si-10Cr-0.9C-0.06P-0.12S alloy obtained by pretreatment with rare earth mixed pre-melting slag are higher than those of the two alloys obtained by pretreatment with pre-melting slag.The fracture strength of Fe-15Mn-5Si-10Cr-0.6C-0.06P-0.12S alloy is up to 3589MPa and the plastic strain is up to 30.8%.The strength and plasticity of the alloy are improved at the same time.After pretreatment,the dephosphorization and desulfurization rates of the four alloys are above 75%and 80%,respectively.Fe-15Mn-5Si-10Cr-0.6C-0.06P-0.12S alloy and Fe-15Mn-5Si-10Cr-0.9C-0.06P-0.12S alloy obtained by pretreatment of rare earth mixed pre-melting slag have more remarkable effects on dephosphorization and desulfurization,the desulfurization rates are 91.7%,91.4%,78.8%and81.7%respectively.