Effects Of Carbonization And Vanadium Reduction On Continuous Heating Transition And High Temperature Oxidation Characteristics Of Ultra-high Strength Spring Steel

After quenching and tempering,54SiCrV steel has a tensile strength of up to 2100MPa and is an alternative steel grade for ultra-high strength spring steels,which contain approximately 0.54%C and 0.17%V.In order to reduce costs,this paper takes the steel as the research object,using thermal expansion meter,thermogravimetric test system,nanoindendenter,high temperature microscope,SEM+EDS and XRD and other material performance test instruments and analytical test methods,to study carbon addition（carbon from about 0.54%to about 0.63%）,vanadium reduction（vanadium from about 0.17%to about 0.12%）to continuous heating of the steel austenite transformation process,isothermal process of austenite grain growth tendency,high temperature oxidation kinetics,oxidative weight gain,The effects of phase composition,decarburization layer characteristics and mechanical properties of iron oxide nanoinderation provide experimental and theoretical basis for evaluating the effectiveness of reducing the production cost of steel by carbonizing and vanadium reduction from the perspectives of continuous heating austenite transformation characteristics,austenite grain growth tendency of insulation process and high temperature oxidation characteristics.The main conclusions are as follows:（1）In the continuous heating speed range of 0.1～50°C/s,the initial temperature Ac1 and the final temperature Ac3 of the austenite transition of 54SiCrV steel increase with the increase of heating rate.Carbonization and vanadium reduction make the critical temperature of the austenite transformation of the steel,A_c1,to the high temperature region by about 3 to 7°C,and A_c3 to the high temperature region by about4 to 22°C.In the range of 800～1200°C,the austenitic grain size of 54SiCrV steel showed a parabolic increase trend with the increase of holding temperature,and it had the characteristics of normal growth.Carbonization and vanadium reduction make the steel appear abnormally long after the insulation temperature is increased to 1100°C,and the austenitic grain size grows exponentially with the increase of the holding temperature.（2）In the four-factor three-level orthogonal high-temperature oxidation experiment such as 2～7%oxygen volume concentration,1080～1120°C insulation temperature,75～95 min holding time and 5～8°C/min heating rate,carbon-increasing and vanadium reduction made the oxidation rate constant K_P of 54SiCrV steel during the insulation period increase,and the oxidation activation energy Q decreased,indicating that carbonization and vanadium reduction made the steel easy to be oxidized,and the oxidation weight per unit area increased by about 8 to 43%compared with before carbonization and vanadium reduction.The minimum oxidative weight gain process of 54SiCrV steel is 5%O₂,1080°C,85 min,6.5°C/min,and the minimum oxidation weight gain process of steel after carbonization and vanadium reduction is2%O₂,1080°C,75 min,5°C/min.（3）The iron oxide sheet formed on the surface of 54SiCrV steel before and after carbonization and vanadium reduction is layered,mainly composed of the outermost layer Fe₂O₃,the middle layer Fe₃O₄ and the innermost Layer Fe O+Fe₂Si O₄.Under the same oxidation process conditions,carbonization and vanadium reduction make the iron oxide sheet generated on the surface of 54SiCrV steel more loose.（4）From the nanoindentation test results of the innermost iron oxide sheet,it can be seen that the load-displacement curve of 54SiCrV steel before and after carbonization and vanadium reduction occurs at the loading end（pop-in）phenomenon and pop-out phenomenon occurs at the unloading stage.In the process range of 2～7%oxygen volume concentration,1080～1120°C holding temperature,75～95 min holding time and 5～8°C/min heating rate,the process of easy removal of iron oxide skin on the surface of 54SiCrV steel is oxygen volume concentration 7%O₂,holding temperature1080°C and holding time 95 min and heating speed 8°C/min.Within the scope of the above processes,the carbonization and vanadium reduction processes that make the iron oxide skin of the steel easy to remove are 7%O₂,5°C/min,1100°C and 85 min,respectively.（5）In the four-factor three-level orthogonal high-temperature oxidation experiments such as 2～7%oxygen volume concentration,1080～1120°C holding temperature,75～95 min holding time and 5～8°C/min heating speed,a partial decarburization layer with a thickness of about 776μm and a full decarburization layer with a thickness of about 194μm appeared on the surface of 54SiCrV steel.Carbonization and vanadium reduction reduce the thickness of the partial decarburization layer of the steel to about 632μm,but have less impact on the thickness of the full decarburization layer,and the thickness of the full decarburization layer is about 195μm.