Study On Mechanical Property And Strengthening Mechanism Of Fe-0.2C-2Nb(wt%) Steel

In recent years,with the extensive development and application of a series of heat-resistant steels with excellent high-temperature properties,the design of heat-resistant alloys has gradually attracted the attention of researchers.Most of them are highly alloyed,such as containing alloying elements of Ni,Cr,Nb and W etc,such as Fe-20Ni-14Cr-2.5Al-（1-3）Nb（wt.%）heat-resistant steel developed by Professor Yamamoto in Oak Ridge National Laboratory,USA,as reported in Science journal.However,the more the alloying elements added,the more the complexity of the material system and harder to control the microstructures and properties.Furthermore,to sustain the development of global resources and environment,reducing the addition of alloy elements or improving the utilization of alloy elements without deteriorating the material properties,increasing the recyclability and improving the sustainability is very urgent to be considered for material design.Generally,niobium（Nb）is used as a microalloying element to enhance the mechanical properties of steel in the development and production of steel,but it is added in large quantities in the composition design of heat-resistant steels.The main purpose is to enhance the high temperature mechanical properties of heat-resistant steel by using intermetallic compound Fe₂Nb and carbide Nb C formed by Nb element with Fe or C elements in steel as precipitation strengthening phases.In the present study,a Fe-0.2C-2Nb（wt%）low carbon alloy steel was designed.The microstructure of the alloy was effectively controlled by plastic deformation and heat treatment.The microstructure evolution of the alloy was systematically characterized by optical microscope（OM）,scanning electron microscope（SEM）,Electron backscatter diffraction（EBSD）and transmission electron microscope（TEM）imaging techniques.The mechanical properties of the alloy at high temperature and room temperature were measured.The quantitative model between the microstructure parameters and mechanical properties was established to reveal the deformation mechanism and strengthening mechanism of the alloy,thus providing experimental and theoretical basis for the subsequent research and development of the alloy.The main results are summarized as follows:（1）The hot forging process of the sample was simulated by hot compression experiments with deformation temperature ranging from 800℃to 1150℃and strain rate ranging from 0.01s^-1-10 s^-1.The constitutive equations of the sample hot deformation was established by using Zener-Hollomon model:=2.15×10⁷sinh 0.0089^5.25exp（-197.23×10³/）（800℃-950℃）;=1.77×10⁷sinh 0.0099^4.96exp（-213.86×10³/）（1000℃-1150℃）.The optimal process parameters of hot forging were determined by hot processing maps of different phase areas as follows:920℃-950℃,0.01 s^-1-0.02 s^-1and 1000℃,0.01s^-1.（2）After forging,the average grain size of the sample was greatly refined to 8μm.The Nb C precipitates and particles in the matrix were substantially broken,dispersed and isolated in the grain interiors and on the grain boundaries,pinning the dislocations with small precipitates and blocking the grain boundaries’movement with large particles.（3）The yield strength,ultimate tensile strength,total elongation of the as-forged sample are 339 MPa,498 MPa and 29%,respectively.Comparing with the as-casted sample,the yield strength and ultimate tensile strength of the as-forged sample are increased by 10%,while the total elongation is increased by 222%.By quantitative calculation,the contribution of fine grain strengthening and precipitation strengthening are 88 MPa and 217 MPa,respectively.（4）The Nb C precipitates in the as-forged sample have obvious effect on fine grain strengthening and precipitation strengthening when deformed under 600℃.The yield strength,ultimate tensile strength and total elongation of the as-forged sample at 600℃are 297 MPa,430 MPa and 25%,respectively,which are close to those of 12Cr-ODS steel and 1.3（Cr-Cu-Ni）-0.3（Mo-Nb-Ti-V）（wt%）steel.By quantitative calculation,the contribution of fine grain strengthening and precipitation strengthening is 64 MPa and215 MPa,respectively.（5）To further improve the mechanical properties,the as-forged sample was hot rolled at 1100℃with 80%plastic deformation.The yield strength,ultimate tensile strength,total elongation of the as-forged sample are 552 MPa,715 MPa and 14%,respectively.The sample was then agedto obtain the peak aged state（200℃for 15min）,whose yield strength,the ultimate tensile strength,the total elongation at room temperature are 592 MPa,752 MPa and 14%,respectively.Comparing with the as-forged sample,the yield strength and ultimate tensile strength of the as-peak aged sample are increased by 75%and 51%,respectively.By quantitative calculation,the contribution of fine grain strengthening,precipitation strengthening and dislocation strengthening is 102 MPa,281 MPa and 150 MPa,respectively.（6）Finally,the as-forged sample was cold rolled with 80%deformation.The yield strength,ultimate tensile strength,total elongation of the as-forged sample are 955MPa,1058 MPa and 4%,respectively.After annealing at 600℃for 0.5 h,the sample has good strength and ductility matching:the yield strength of 713 MPa,the ultimate tensile strength of 844 MPa and the total elongation of 13%.Comparing with the as-forged sample,the yield strength and the ultimate tensile strength are increased by110%and 70%,respectively.By quantitative calculation,the contribution of fine grain strengthening,precipitation strengthening and dislocation strengthening is 357 MPa,215 MPa and 114 MPa,respectively.