Microstructures and properties of three Nb micro-alloyed steels were studied through hot rolling experiment. The result indicates that the ferrite grain size （dF ） decreases with increasing Nb content （Nb）, and th...Microstructures and properties of three Nb micro-alloyed steels were studied through hot rolling experiment. The result indicates that the ferrite grain size （dF ） decreases with increasing Nb content （Nb）, and the bainite fraction （fB） increases with increasing Nb content （Nb）. The effect of ferrite grain size （dF） on yield strength （δy） is related to Nb content （Nb）, and the effect of bainite fraction （fB） on yield strength （δy） is unrelated to Nb content （Nb）. Modelling of yield strength （δy） for Nb micro-alloyed steels with high accuracy has been built up with Nb content （Nb） and bainite fraction （fB） taken into account as new parameters, and formulas for ferrite grain size （dF ） and bainite fraction （fB） vs Nb content （Nb） have also been established under the experiment conditions. The research results could provide instructions for industrial productions.展开更多
Dry sliding friction and wear test of Nb containing low carbon microalloyed steel was carried out at room temperature,and the effect of Nb on the wear behavior of the steel,as welll as the mechanism was studied.Scanni...Dry sliding friction and wear test of Nb containing low carbon microalloyed steel was carried out at room temperature,and the effect of Nb on the wear behavior of the steel,as welll as the mechanism was studied.Scanning electron microscopy(SEM) and energy dispersive spectrometry(EDS) were employed to analyze the morphology and composition of the worn surface,and the structure evolution of the plastic deformation layer.The carbide content and type in the steel were analyzed by the electrolytic extraction device and X-ray diffraction(XRD).The experimental results demonstrate that the addition of 0.2% Nb can refine the grain and generate Nb C to improve the wear resistance of the steel.By enhancing the load and speed of wear experiment,the wear mechanism of the test steel with 0.2% Nb changes from slight oxidation wear to severe adhesion wear and oxidation wear.Compared with the load,the increase in the rotation speed exerts a greater influence on the wear of the test steel.展开更多
This work investigated the effect of Cr and Si on the mechanical properties and oxidation resistance of press hardened steel.Results indicated that the microstructure of the Cr-Si micro-alloyed press hardened steel co...This work investigated the effect of Cr and Si on the mechanical properties and oxidation resistance of press hardened steel.Results indicated that the microstructure of the Cr-Si micro-alloyed press hardened steel consisted of lath martensite,M_(23)C_(6)carbides,and retained austenite.The retained austenite and carbides are responsible for the increase in elongation of the micro-alloyed steel.In addition,after oxidation at 930℃for 5 min,the thickness of the oxide scales on the Cr-Si micro-alloyed press hardened steel is less than 5μm,much thinner than 45.50μm-thick oxide scales on 22MnB5.The oxide scales of the Cr-Si micro-alloyed steel are composed of Fe_(2)O_(3),Fe_(3)O_(4),mixed spinel oxide(FeCr_(2)O_(4)and Fe_(2)SiO_(4)),and amorphous SiO_(2).Adding Cr and Si significantly reduces the thickness of the oxide scales and prevents the generation of the FeO phase.Due to the increase of spinel FeCr_(2)O_(4)and Fe_(2)SiO_(4)phase in the inner oxide scale and the amorphous SiO_(2)close to the substrate,the oxidation resistance of the Cr-Si micro-alloyed press hardened steel is improved.展开更多
The effect of molybdenum on the continuous cooling transformation behavior of the micro-alloyed low carbon steel containing niobium and titanium was investigated by a Gleeble 3800 thermo-mechanical simulator. The phas...The effect of molybdenum on the continuous cooling transformation behavior of the micro-alloyed low carbon steel containing niobium and titanium was investigated by a Gleeble 3800 thermo-mechanical simulator. The phase transformation temperature of the steel at various cooling rates was detected. The microstmcture was observed by optical microscope （OM） and scanning electronic microscope （ SEM）, and its Vickers hardness was tested. Based on these, its dynamic continuous cooling transformation （CCT） diagrams were determined. The results show that the transformation temperature from deformed austenite to acicular ferrite （AF） is decreased when Mo is added, and the formation of pro- eutectoid ferrite （F） and pearlite （P） is either inhabited or postponed. Mo can also enlarge the range of the cooling rate in forming AF, and refine the microstructure effectively.展开更多
Balance solubility products and enthalpy of for- mation for NbC0.75, NbC0.85, NbC0.88, NbC and NbN in oriented silicon steels were calculated and compared quali- tatively. Meanwhile, the mixing enthalpies of these fiv...Balance solubility products and enthalpy of for- mation for NbC0.75, NbC0.85, NbC0.88, NbC and NbN in oriented silicon steels were calculated and compared quali- tatively. Meanwhile, the mixing enthalpies of these five Nb compounds were calculated based on Miedema Model. The results show that the solubility products of Nb compounds in ferrite and austenite meet the following relationship, NbC0.75 〉 NbC0.85 〉 NbC0.88 〉 NbC 〉 NbN and NbN has the minimum enthalpy of formation. It indicates that NbN easily precipitate out, but it is more difficult for NbC0.75.展开更多
Based on thermodynamics and kinetics, precipitation behavior of microalloyed steels was analyzed. Deformation greatly promotes isothermal carbonitride precipitation and makes C-curve shift leftwards. The position and ...Based on thermodynamics and kinetics, precipitation behavior of microalloyed steels was analyzed. Deformation greatly promotes isothermal carbonitride precipitation and makes C-curve shift leftwards. The position and shape of C-curve also depend on the content of Nb and N. C-curve shifts leftwards a little when N content increases and the nose temperature is raised with increasing Nb content. Deformation shortened precipitation start time during continuous cooling, raised precipitation start temperature, accelerated precipitation kinetics of carbonitrides. With decreasing the finishing temperature and coiling temperature, the precipitates volume fraction increases and strength increment is raised during hot rolling. The simulated results are in agreement with experiment results.展开更多
The mechanism of the interfacial reaction of Al_2O_3/medium Mn steel containing Nb was studied by means of the observation on the interfacial reaction phenomenon of Al_2O_3/medium Mn steel,and the analyses on the inte...The mechanism of the interfacial reaction of Al_2O_3/medium Mn steel containing Nb was studied by means of the observation on the interfacial reaction phenomenon of Al_2O_3/medium Mn steel,and the analyses on the interracial phases.The results show that when T≥1550℃,the interfacial reac- tion of Al_2O_3/medium Mn steel containing Nb happened.In the medium Mn steel matrix,Nb exists in the form of NbC.NbC are the nucleating base of CO gas bubbles.展开更多
Austenite grain size is an important influence factor for ductility of steel at high temperatures during continuous casting. Thermodynamic and kinetics calculations were performed to analyze the characteristics of Ti(...Austenite grain size is an important influence factor for ductility of steel at high temperatures during continuous casting. Thermodynamic and kinetics calculations were performed to analyze the characteristics of Ti(C,N) precipitates formed during the continuous casting of micro-alloyed steel. Based on Andersen-Grong equation, a coupling model of second phase precipitation and austenite grain growth has been established, and the influence of second precipitates on austenite grain growth under different cooling rates is discussed. Calculations show that the final sizes of austenite grains are 2.155, 1.244, 0.965, 0.847 and 0.686 mm, respectively, under the cooling rate of 1, 3, 5, 7, and 10 ℃·s^(-1), when ignoring the pinning effect of precipitation on austenite growth. Whereas, if taking the pinning effect into consideration, the grain growth remains stable from 1,350 ℃, the calculated final sizes of austenite grains are 1.46, 1.02, 0.80, 0.67 and 0.57 mm, respectively. The sizes of final Ti(C,N) precipitates are 137, 79, 61, 51 and 43 nm, respectively, with the increase of cooling rate from 1 to 10 ℃·s^(-1). Model validation shows that the austenite size under different cooling rates coincided with the calculation results. Finally, the corresponding measures to strengthen cooling intensity at elevated temperature are proposed to improve the ductility and transverse crack of slab.展开更多
By measuring the expansion curves of a Nb bearing steel at different cooling rates by using Gleeble-3800 thermomechanical simulator, combining with metallographic analysis, different phase zones were determined. Also,...By measuring the expansion curves of a Nb bearing steel at different cooling rates by using Gleeble-3800 thermomechanical simulator, combining with metallographic analysis, different phase zones were determined. Also, precipitation behavior of Nb at different phase zones was investigated under ultra fast cooling conditions. The experimental results showed that adopting a proper deformation temperature, the ultra fast cooling process can restrain the precipitation of Nb at austenite phase zone. More quantities and smaller size precipitates of Nb can be found at the ferrite or bainite phase zone by controlling the ultra fast cooling ending temperature. With the increase of holding time at austenite, ferrite and bainite phase zones respectively, the volume fraction of precipitation, density, and average size of precipitates will increase obviously. With the decrease of early ultra fast cooling ending temperature, the density of Nb precipitates first increase（at ferrite phase zone） and then decrease（at bainite phase zone）, the volume fraction of Nb precipitation decreases and precipitates can be refined. The optimal early ultra fast cooling ending temperature is located at ferrite phase zone. The combination of high rolling temperature with early ultra fast cooling technology opens the way for new cooling schedules and makes the production of high strength steels easier and cheaper by making full use of Nb precipitation strengthening effect.展开更多
Utilizing Gleeble-1500 thermomechanical simulator, the influences of hot deformation parameters on continuous cooling bainite transformation in Nb-microalloyed low carbon steel were investigated. The results indicate ...Utilizing Gleeble-1500 thermomechanical simulator, the influences of hot deformation parameters on continuous cooling bainite transformation in Nb-microalloyed low carbon steel were investigated. The results indicate that bainite starting temperature decreases with raising cooling rate and increases with increasing deformation temperature. Deformation has an accelerative effect on the bainite transformation when the specimens are deformed at 950 ℃. When the deformation temperature increases, the effect of deformation on bainite starting temperature is weakened. The amount of bainite is influenced by strain, cooling rate, and deformation temperature. When the specimens are deformed below 900 ℃, equiaxed ferrites are promoted and the bainite transformation is suppressed.展开更多
The oxidation behaviors of three austenitic cast steels with different morphologies of primary carbides at 950℃ in air were investigated using scanning electron microscopy,energy dispersive spectroscopy,and focused i...The oxidation behaviors of three austenitic cast steels with different morphologies of primary carbides at 950℃ in air were investigated using scanning electron microscopy,energy dispersive spectroscopy,and focused ion beam/transmission electron microscopy.Their oxidation kinetics followed a logarithmic law,and the oxidation rate can be significantly decreased as long as a continuous silica layer formed at the scale/substrate interface.When the local Si concentration was inadequate,internal oxidation occurred beneath the oxide scale.The spallation of oxides during cooling can be inhibited with the formation of internal oxidation,owing to the reduced mismatch stress between the oxide scale and the substrate.The“Chinese-script”primary Nb(C,N)was superior to the dispersed primary Nb(C,N)in suppressing the oxidation penetration in the interdendritic region by supplying a high density of quick-diffusion Cr channels.In addition,the innermost and outermost oxidation layers were enriched with Cr,whereas the Cr evaporation in the outermost layer was significant when the water vapor concentration in the environment was high enough.These findings further the understanding regarding the oxidation behavior of austenitic cast steels and will promote the alloy development for exhaust components.展开更多
The effect of rare earth on the microstructures, mechanical properties and inclu sions in low sulphur Nb-Ti-bearing steel were investigated. It is shown that t h e transverse yield point, the traverse tensile strength...The effect of rare earth on the microstructures, mechanical properties and inclu sions in low sulphur Nb-Ti-bearing steel were investigated. It is shown that t h e transverse yield point, the traverse tensile strength and elongation of testin g steels decrease initially and then rise with increasing content of rare earth. The impact energy values of the testing steels exhibit a contrary trend. Proper amount of rare earth in the steels can improve the anisotropy of impact toughne ss above -20 ℃ and it does not affect the type of microstructures which ar e st ill composed of ferrites and pearlites, but the pearlite amount increases. On one hand, rare earth cleans the molten steel and reduces the amount of inclusions; on the other hand, rare earth makes the inclusions spheroidizd, refi ned and dispersed, and thus improves the distribution of inclusions.展开更多
文摘Microstructures and properties of three Nb micro-alloyed steels were studied through hot rolling experiment. The result indicates that the ferrite grain size （dF ） decreases with increasing Nb content （Nb）, and the bainite fraction （fB） increases with increasing Nb content （Nb）. The effect of ferrite grain size （dF） on yield strength （δy） is related to Nb content （Nb）, and the effect of bainite fraction （fB） on yield strength （δy） is unrelated to Nb content （Nb）. Modelling of yield strength （δy） for Nb micro-alloyed steels with high accuracy has been built up with Nb content （Nb） and bainite fraction （fB） taken into account as new parameters, and formulas for ferrite grain size （dF ） and bainite fraction （fB） vs Nb content （Nb） have also been established under the experiment conditions. The research results could provide instructions for industrial productions.
文摘Dry sliding friction and wear test of Nb containing low carbon microalloyed steel was carried out at room temperature,and the effect of Nb on the wear behavior of the steel,as welll as the mechanism was studied.Scanning electron microscopy(SEM) and energy dispersive spectrometry(EDS) were employed to analyze the morphology and composition of the worn surface,and the structure evolution of the plastic deformation layer.The carbide content and type in the steel were analyzed by the electrolytic extraction device and X-ray diffraction(XRD).The experimental results demonstrate that the addition of 0.2% Nb can refine the grain and generate Nb C to improve the wear resistance of the steel.By enhancing the load and speed of wear experiment,the wear mechanism of the test steel with 0.2% Nb changes from slight oxidation wear to severe adhesion wear and oxidation wear.Compared with the load,the increase in the rotation speed exerts a greater influence on the wear of the test steel.
基金The authors gratefully acknowledge the financial support from the National Natural Science Foundation of China(Nos.52274372 and 52201101)the National Key R&D Program of China(No.2021YFB3702404)the Fundamental Research Funds for the Central Universities(No.FRF-TP-22-013A1).
文摘This work investigated the effect of Cr and Si on the mechanical properties and oxidation resistance of press hardened steel.Results indicated that the microstructure of the Cr-Si micro-alloyed press hardened steel consisted of lath martensite,M_(23)C_(6)carbides,and retained austenite.The retained austenite and carbides are responsible for the increase in elongation of the micro-alloyed steel.In addition,after oxidation at 930℃for 5 min,the thickness of the oxide scales on the Cr-Si micro-alloyed press hardened steel is less than 5μm,much thinner than 45.50μm-thick oxide scales on 22MnB5.The oxide scales of the Cr-Si micro-alloyed steel are composed of Fe_(2)O_(3),Fe_(3)O_(4),mixed spinel oxide(FeCr_(2)O_(4)and Fe_(2)SiO_(4)),and amorphous SiO_(2).Adding Cr and Si significantly reduces the thickness of the oxide scales and prevents the generation of the FeO phase.Due to the increase of spinel FeCr_(2)O_(4)and Fe_(2)SiO_(4)phase in the inner oxide scale and the amorphous SiO_(2)close to the substrate,the oxidation resistance of the Cr-Si micro-alloyed press hardened steel is improved.
文摘The effect of molybdenum on the continuous cooling transformation behavior of the micro-alloyed low carbon steel containing niobium and titanium was investigated by a Gleeble 3800 thermo-mechanical simulator. The phase transformation temperature of the steel at various cooling rates was detected. The microstmcture was observed by optical microscope （OM） and scanning electronic microscope （ SEM）, and its Vickers hardness was tested. Based on these, its dynamic continuous cooling transformation （CCT） diagrams were determined. The results show that the transformation temperature from deformed austenite to acicular ferrite （AF） is decreased when Mo is added, and the formation of pro- eutectoid ferrite （F） and pearlite （P） is either inhabited or postponed. Mo can also enlarge the range of the cooling rate in forming AF, and refine the microstructure effectively.
基金financially supported by the National Natural Science Foundation of China (Nos. 51274083 and 51074062)
文摘Balance solubility products and enthalpy of for- mation for NbC0.75, NbC0.85, NbC0.88, NbC and NbN in oriented silicon steels were calculated and compared quali- tatively. Meanwhile, the mixing enthalpies of these five Nb compounds were calculated based on Miedema Model. The results show that the solubility products of Nb compounds in ferrite and austenite meet the following relationship, NbC0.75 〉 NbC0.85 〉 NbC0.88 〉 NbC 〉 NbN and NbN has the minimum enthalpy of formation. It indicates that NbN easily precipitate out, but it is more difficult for NbC0.75.
基金This work was financially supported by the National Natural Science Foundation of China(Nos.50334010,50504007 and 50474086)the Ph.D.Startup Foundation of liaoning Province(2003hp1009).
文摘Based on thermodynamics and kinetics, precipitation behavior of microalloyed steels was analyzed. Deformation greatly promotes isothermal carbonitride precipitation and makes C-curve shift leftwards. The position and shape of C-curve also depend on the content of Nb and N. C-curve shifts leftwards a little when N content increases and the nose temperature is raised with increasing Nb content. Deformation shortened precipitation start time during continuous cooling, raised precipitation start temperature, accelerated precipitation kinetics of carbonitrides. With decreasing the finishing temperature and coiling temperature, the precipitates volume fraction increases and strength increment is raised during hot rolling. The simulated results are in agreement with experiment results.
文摘The mechanism of the interfacial reaction of Al_2O_3/medium Mn steel containing Nb was studied by means of the observation on the interfacial reaction phenomenon of Al_2O_3/medium Mn steel,and the analyses on the interracial phases.The results show that when T≥1550℃,the interfacial reac- tion of Al_2O_3/medium Mn steel containing Nb happened.In the medium Mn steel matrix,Nb exists in the form of NbC.NbC are the nucleating base of CO gas bubbles.
基金supported by the National Natural Science Foundation of China(No.51504172)and(No.51474163)China Postdoctoral Science Foundation(No.2015M572212)
文摘Austenite grain size is an important influence factor for ductility of steel at high temperatures during continuous casting. Thermodynamic and kinetics calculations were performed to analyze the characteristics of Ti(C,N) precipitates formed during the continuous casting of micro-alloyed steel. Based on Andersen-Grong equation, a coupling model of second phase precipitation and austenite grain growth has been established, and the influence of second precipitates on austenite grain growth under different cooling rates is discussed. Calculations show that the final sizes of austenite grains are 2.155, 1.244, 0.965, 0.847 and 0.686 mm, respectively, under the cooling rate of 1, 3, 5, 7, and 10 ℃·s^(-1), when ignoring the pinning effect of precipitation on austenite growth. Whereas, if taking the pinning effect into consideration, the grain growth remains stable from 1,350 ℃, the calculated final sizes of austenite grains are 1.46, 1.02, 0.80, 0.67 and 0.57 mm, respectively. The sizes of final Ti(C,N) precipitates are 137, 79, 61, 51 and 43 nm, respectively, with the increase of cooling rate from 1 to 10 ℃·s^(-1). Model validation shows that the austenite size under different cooling rates coincided with the calculation results. Finally, the corresponding measures to strengthen cooling intensity at elevated temperature are proposed to improve the ductility and transverse crack of slab.
基金Funded by the National Natural Science Foundation of China(51004037)the Fundamental Research Funds for the Central Unversities(N140704002)
文摘By measuring the expansion curves of a Nb bearing steel at different cooling rates by using Gleeble-3800 thermomechanical simulator, combining with metallographic analysis, different phase zones were determined. Also, precipitation behavior of Nb at different phase zones was investigated under ultra fast cooling conditions. The experimental results showed that adopting a proper deformation temperature, the ultra fast cooling process can restrain the precipitation of Nb at austenite phase zone. More quantities and smaller size precipitates of Nb can be found at the ferrite or bainite phase zone by controlling the ultra fast cooling ending temperature. With the increase of holding time at austenite, ferrite and bainite phase zones respectively, the volume fraction of precipitation, density, and average size of precipitates will increase obviously. With the decrease of early ultra fast cooling ending temperature, the density of Nb precipitates first increase（at ferrite phase zone） and then decrease（at bainite phase zone）, the volume fraction of Nb precipitation decreases and precipitates can be refined. The optimal early ultra fast cooling ending temperature is located at ferrite phase zone. The combination of high rolling temperature with early ultra fast cooling technology opens the way for new cooling schedules and makes the production of high strength steels easier and cheaper by making full use of Nb precipitation strengthening effect.
基金Item Sponsored by High Technology Development Programof China (863) (2001AA332020) and National Natural ScienceFoundation of China (50271015)
文摘Utilizing Gleeble-1500 thermomechanical simulator, the influences of hot deformation parameters on continuous cooling bainite transformation in Nb-microalloyed low carbon steel were investigated. The results indicate that bainite starting temperature decreases with raising cooling rate and increases with increasing deformation temperature. Deformation has an accelerative effect on the bainite transformation when the specimens are deformed at 950 ℃. When the deformation temperature increases, the effect of deformation on bainite starting temperature is weakened. The amount of bainite is influenced by strain, cooling rate, and deformation temperature. When the specimens are deformed below 900 ℃, equiaxed ferrites are promoted and the bainite transformation is suppressed.
基金finacially supported by the University Research Program between Ford Motor Company and the University of Science and Technology Beijing,and the Fundamental Research Funds for the Central Universities(Nos.FRF-IC-16-005 and FRF-IC-19-009)。
文摘The oxidation behaviors of three austenitic cast steels with different morphologies of primary carbides at 950℃ in air were investigated using scanning electron microscopy,energy dispersive spectroscopy,and focused ion beam/transmission electron microscopy.Their oxidation kinetics followed a logarithmic law,and the oxidation rate can be significantly decreased as long as a continuous silica layer formed at the scale/substrate interface.When the local Si concentration was inadequate,internal oxidation occurred beneath the oxide scale.The spallation of oxides during cooling can be inhibited with the formation of internal oxidation,owing to the reduced mismatch stress between the oxide scale and the substrate.The“Chinese-script”primary Nb(C,N)was superior to the dispersed primary Nb(C,N)in suppressing the oxidation penetration in the interdendritic region by supplying a high density of quick-diffusion Cr channels.In addition,the innermost and outermost oxidation layers were enriched with Cr,whereas the Cr evaporation in the outermost layer was significant when the water vapor concentration in the environment was high enough.These findings further the understanding regarding the oxidation behavior of austenitic cast steels and will promote the alloy development for exhaust components.
文摘The effect of rare earth on the microstructures, mechanical properties and inclu sions in low sulphur Nb-Ti-bearing steel were investigated. It is shown that t h e transverse yield point, the traverse tensile strength and elongation of testin g steels decrease initially and then rise with increasing content of rare earth. The impact energy values of the testing steels exhibit a contrary trend. Proper amount of rare earth in the steels can improve the anisotropy of impact toughne ss above -20 ℃ and it does not affect the type of microstructures which ar e st ill composed of ferrites and pearlites, but the pearlite amount increases. On one hand, rare earth cleans the molten steel and reduces the amount of inclusions; on the other hand, rare earth makes the inclusions spheroidizd, refi ned and dispersed, and thus improves the distribution of inclusions.
