The microstructure–mechanical property relationship of a Cu-bearing low-carbon high-strength low-alloy steel,subjected to a novel multistage heat treatment including quenching(Q),lamellarization(L)and tempering(T),is...The microstructure–mechanical property relationship of a Cu-bearing low-carbon high-strength low-alloy steel,subjected to a novel multistage heat treatment including quenching(Q),lamellarization(L)and tempering(T),is presented.Yield strength of 989.5 MPa and average toughness at-80℃of 41 J were obtained in this steel after quenching and tempering(QT)heat treatments.Specimen QLT gained a little lower yield strength(982.5 MPa),but greatly enhanced average toughness at-80℃(137 J).To further clarify the strengthening and toughening mechanisms in specimen QLT,parameters of microstructural characteristic and crack propagation process were compared and analyzed for specimens Q,QL,QT and QLT.The microstructure of tempered martensite/bainite(M/B)in specimen QT changed to refined tempered M/B matrix mixed with minor IF(inter-critical ferrite)in specimen QLT.Cu-rich precipitates existed in tempered M/B for both specimens QT and QLT,as well as in IF.Compared with QT,adding a lamellarization step before tempering made the effective grains of specimen QLT refined and also led to coarser Cu-rich precipitates in tempered M/B matrix.The weaker strengthening effect of coarser Cu-rich precipitates should be a key reason for the slightly lower yield strength in specimen QLT than in specimen QT.No austenite was found in all specimens Q,QL,QT and QLT.Specimen QLT showed purely ductile fracture mode at-80℃due to the refined effective grains.The greatly improved toughness is mainly attributed to the enhanced energy of crack propagation.The combination of refined microstructure,softened matrix and deformation of minor'soft'IF during crack propagation led to the most superior toughness of specimen QLT among all specimens.展开更多
Through the thermo-mechanical control process （TMCP）, a high Nb low Mo fire resistant steel with the yield strength （YS） of 521 MPa at room temperature （RT） and 360 MPa at elevated temperature （ET） of 600 ℃ w...Through the thermo-mechanical control process （TMCP）, a high Nb low Mo fire resistant steel with the yield strength （YS） of 521 MPa at room temperature （RT） and 360 MPa at elevated temperature （ET） of 600 ℃ was developed based on MX （M=Nb, V, Mo; X=C,N） precipitation strengthening. A series of tensile and con- stant load tests were conducted to study the mechanical properties at ET. The dynamic continuous cooling transfor- mation （CCT） as well as precipitation behavior of microalloy carbonitride was investigated by means of thermal sim- ulator and electron microscopy approaches. Results showed that the failure temperature of tested steel was deter- mined as 653 ℃, and the granular bainite was obtained when the cooling rate was higher than 10 ℃/s. In the rolled state, a certain amount of M/A islands was observed. During heating from RT to ET, M/A islands disappeared, and cementites and high dense compound precipitates （Nb, Mo, V）C with size of less than 10 nm precipitated in ferrite at ET （600 ℃）, which resulted in precipitation strengthening at ET.展开更多
The corrosion and passive behaviour of duplex stainless steel 2205 at six cooling rates （1, 5, 10, 15, 20 ℃ s^-1 and water quenched） in a simulated marine-environment solution was investigated using electrochemical...The corrosion and passive behaviour of duplex stainless steel 2205 at six cooling rates （1, 5, 10, 15, 20 ℃ s^-1 and water quenched） in a simulated marine-environment solution was investigated using electrochemical measurements of potentiostatic critical pitting temperature, potentiodynamic polarisation curves, electrochemical impedance spectroscopy and Mott-Schottky curves. The microstructural evolution and pitting morphologies of the specimens were visualised using an optical microscope and scanning electron microscope. The electrochemical responses of the passive film show that passivity of the steel was enhanced as the cooling rate increased; however, the threshold cooling rate was 20 ℃ s^-1, beyond which pitting corrosion resistance remained stable. Based on the analyses of microstructural evolution and pit morphologies, the proportion of the ferrite phase increased with the cooling rate and the ratio of austenite and ferrite was close to 1：1. The pitting size decreased as the cooling rate increased, and most metastable pits on specimens were located in the ferrite phase and on the ferrite-austenite interface. Thus, pitting resistance of steel is governed by the phase that provides the lowest pitting resistance equivalent number. The optimised pitting corrosion resistance for steel 2205 was achieved when it was greater than or equal to 20℃ s^-1.展开更多
The relationship between micro structure and corrosion behavior of high-grade pipeline steel under low-temperature conditions was comparatively investigated by the potentiodynamic polarization and electrochemical impe...The relationship between micro structure and corrosion behavior of high-grade pipeline steel under low-temperature conditions was comparatively investigated by the potentiodynamic polarization and electrochemical impedance spec-troscopy test combining with optical micrographs,scanning electron microscopy,transmission electron microscopy and electron back-scattered diffraction.The results showed that,compared with room and high temperature,low temperature could influence the corrosion behavior of high-grade pipeline steel,which means that the corrosion potential and current density decreased and the corrosion resistance increased significantly.Moreover,double layer structures of the interface became thicker and more compact.Under low-temperature environment,acicular ferrite had the martensite/austenite constituents with less amount and smaller size,a higher density of low angle grain boundaries and smaller effective grain sizes compared with granular bainite,which demonstrated higher corrosion resistance.展开更多
The morphology and characteristics of granular bainite (GB) in pipeline steels at different continuous cooling rates were investigated by scanning electron microscopy, transmission electron microscopy and electron bac...The morphology and characteristics of granular bainite (GB) in pipeline steels at different continuous cooling rates were investigated by scanning electron microscopy, transmission electron microscopy and electron back-scattered diffraction (EBSD). The results show that the morphology of ferrite matrix in GB turned from the lath sheaf structure into the nearly equiaxed large grain with the cooling rate decreasing from high (60℃/s) to low (5-10℃/s). At the medium cooling rate (20-40℃/s), GB consisted of the irregular ferrite matrix, the granular martensite/austenite (M/A) constituents and abundant substructures inside. The formation of the irregular ferrite and substructure was attributed to the high-temperature recovery which occurred at relatively high-temperature stage before phase transformation. The granular morphology of M/A constitu-ents was formed from the carbon-rich triple junctions which were produced by the multidirectional substructure interfaces converged with each other. Particularly, some martensite in M/A constituents was misoriented from the adjacent ferrite by very small misorientation angle, which could be characterized by the mean band contrast function of EBSD qualitatively or semiquantitatively.展开更多
The effect of grain size on the mechanical properties of a high-manganese(Mn)austenitic steel was investigated via electron-backscattered diffraction,transmission electron microscope,X-ray diffraction,and tensile and ...The effect of grain size on the mechanical properties of a high-manganese(Mn)austenitic steel was investigated via electron-backscattered diffraction,transmission electron microscope,X-ray diffraction,and tensile and impact tests at 25°C and-196°C.The Hall–Petch strengthening coefficients for the yield strength of the high-Mn austenitic steels were 7.08 MPa mm 0.5 at 25°C,which increased to 14 MPa mm 0.5 at-196°C.The effect that the grain boundary strengthening had on improving the yield strength at-196°C was better than that at 25°C.The impact absorbed energies and the tensile elongations were enhanced with the increased grain size at 25°C,while they remained nearly unchanged at-196°C.The unchanged impact absorbed energies and the tensile elongations were primarily attributed to the emergence of the micro-twin at-196°C,which promoted the cleavage fracture in the steels with large-sized grains.Refining the grain size could improve the strength of the high-Mn austenitic steels without impairing their ductility and toughness at low temperature.展开更多
The secondary phases of the steels have significant effects on the microstructure and mechanical properties, making controlling these secondary phases important. The control of MnS inclusions and A1N precipitates in a...The secondary phases of the steels have significant effects on the microstructure and mechanical properties, making controlling these secondary phases important. The control of MnS inclusions and A1N precipitates in a N-alloyed high-Mn twin-induced plastic cryogenic steel via solution treatment was investigated with several different techniques including microstructural characterization, 298 K tensile testing, and 77 K impact testing. The solutionizing temperature （ST） increased from 1323 to 1573 K, where the elongated MnS inclusions and large-sized AlN precipitates became spheroidized and dissolved. The aspect ratio of the MnS inclusions decreased as the ST increased and the number density increased. The impact toughness of the steels showed anisotropy and low impact energy values, due to the elongated MnS inclusions and large-sized AIN precipitates. The anisotropy was eliminated by spheroidizing the MnS inclusions. The impact energy was improved by dissolving the large-sized AlN precipitates during the solution treatment. The austenite grain size increased when the dissolution of the AlN precipitate increased, but the effect of the grain size on the yield strength, toughness, and the strength--ductility balance was weak.展开更多
The microstructure,precipitates and properties of 25CrNiMoV(DZ2)steel for high-speed railway axles with different Nb contents were investigated by means of optical microscopy,scanning electron microscopy,electron back...The microstructure,precipitates and properties of 25CrNiMoV(DZ2)steel for high-speed railway axles with different Nb contents were investigated by means of optical microscopy,scanning electron microscopy,electron back-scattering diffraction,transmission electron microscopy and physicochemical phase analysis.The results show that the grain size of the original austenite of the test steels decreases from 20.5 to 14.2 and 10.8μm after adding 0.026 and 0.039 wt.%Nb to a 25CrNiMoV steel,respectively.Moreover,the block width of the tempered martensite in the test steels is refined from 1.91 to 1.72 and 1.60µm,respectively.MC-type precipitates in 25CrNiMoV steel are mainly VC,while(Nb,V)C gradually precipitates when Nb is microalloyed,and the amount of precipitates increases with increasing Nb content.Through strengthening mechanism analysis,it is found that grain refinement strengthening is the primary way to increase the strength.The improvement in the yield strength with increasing Nb content is attributed to a significant increase in precipitation strengthening,grain refinement strengthening and dislocation strengthening.展开更多
The combined effects of martensite-austenite（MA）constituent and pearlite colony on cleavage crack initiation in the simulated coarse-grained heat-affected zone（CGHAZ）of V-N-Ti microalloyed offshore platform steel ...The combined effects of martensite-austenite（MA）constituent and pearlite colony on cleavage crack initiation in the simulated coarse-grained heat-affected zone（CGHAZ）of V-N-Ti microalloyed offshore platform steel under different heat inputs were investigated.The results of welding simulation,instrumented impact test,and quantitative analysis indicated that the size of the MA constituent decreased with the increase in cooling time,and by contrast,the size of the pearlite colony increased.According to Griffith theory,the critical sizes of cleavage microcracks were calculated.With the increase of cooling time,the calculated microcrack size could be characterized by the size of the MA constituent first,and then fitted with the size of the pearlite colony.Moreover,the calculated microcrack size variation was opposite to the microcrack initiation energy.This phenomenon is probably due to the combined effects of the MA constituent and pearlite colony with increasing the cooling time of the specimen′s temperature from800 to 500 ℃.展开更多
A superior combination of yield strength(1001 MPa)and-20℃ impact toughness(166 J)was obtained in Nb-V-Timicroalloyed Ni-Cr-Mo-Cu steel treated by direct quenching and tempering route(DQT).The tested steels treated by...A superior combination of yield strength(1001 MPa)and-20℃ impact toughness(166 J)was obtained in Nb-V-Timicroalloyed Ni-Cr-Mo-Cu steel treated by direct quenching and tempering route(DQT).The tested steels treated by DQT route and re-austenitization and tempering route(QT)were compared with each other in terms of mechanical properties and microstructures characterized by optical microscopy,transmission electron microscopy,X-ray diffraction,electron back-scattered diffraction method and so on.Strength and Vickers hardness of the tested steel treated by the above two routes vary with isothermal aging temperature(400-600℃),shown as under-aged state,peak-aged state and overaged state.All DQT specimens show higher strength and Vickers hardness than QT specimens with the same aging condition.Furthermore,the largest difference of yield strength between DQT and QT specimens was shown in DQT600 and QT600 specimens.DQT600 or QT600 specimens refers to direct quenched(DQ)or quenched(Q)specimens isothermally aged at 600℃.The main disparities in quenched microstructure between DQ and Q specimens are mainly in morphology of prior austenite grains,dislocation density of martensite matrix and solution amount of Nb and Mo elements dissolving in martensite matrix,which play key roles in affecting microstructure and mechanical properties of DQT and QT specimens.Higher dislocation density of matrix and finer average diameter of both MC(M is any combination of Nb,Mo and V)and Cu-rich particles were shown in DQT600 specimens than in QT600 specimens.Strengthening from dislocations and nanosized MC and Cu-rich particles mainly leads to the largest difference of yield strength between DQT600 and QT600 specimens.In addition,strong dislocation strengthening and precipitation strengthening in DQT600 specimen also elevated its ductile-to-brittle-transition-temperature,compared with QT600 specimen.展开更多
The austenite grain refinement through control of the grain growth during reheating process after thermomechanical controlled process （TMCP） in a vanadium microalloyed steel was achieved. The formation of ultra-fine...The austenite grain refinement through control of the grain growth during reheating process after thermomechanical controlled process （TMCP） in a vanadium microalloyed steel was achieved. The formation of ultra-fine grained austenite was attributed to the high density of austenite nucleation at the ferrite/martensite structure and to the inhibition of austenite growth by （Ti~ V）C particles at the relatively low reheating temperature. Corresponding with the precipitation behavior of （Ti,V）C with temperature, the growth behavior of austenite in the vanadium mi- croalloyed steel could be divided into two regions. At lower reheating temperature, austenite grains grew slowly, and ultra-fine grained austenite smaller than 5 ~m was successfully obtained. By contrast, the austenite grains grew rap- idly at high temperature due to the dissolution of （Ti, V）C particles. According to the measured and predicted results of austenite growth kinetics, two models were developed to describe the growth behavior of austenite grains in two different temperature regions, and the apparent activation energy Qapp for grain growth was estimated to be about 115 and 195 kJ/mol, respectively.展开更多
Hot roll bonding was carried out between commercially pure titanium TA2 and high-strength low-alloy steel Q390 using pure Nb or Mo interlayer at 950 ~C with a total reduction ratio of 86.7%. Interfacial microstructure...Hot roll bonding was carried out between commercially pure titanium TA2 and high-strength low-alloy steel Q390 using pure Nb or Mo interlayer at 950 ~C with a total reduction ratio of 86.7%. Interfacial microstructure and bonding properties of titanium clad steel plates were investigated by electron microscopy and mechanical tests. The results showed β-Ti, TiC and Fe2Ti reaction phases were generated at Ti/steel interface for the clad plates with no interlayer. Inserting Nb or Mo interlayer can effectively suppress the formation of brittle phases, while the weak bonding joint transferred to Nb/steel or Mo/steel interface. And some micro-voids were found at the interface of Nb/steel and Mo/steel. The improved shear strength of clad plates with Nb interlayer might be attributed to the elimination of brittle phases at bonding interface. The small size and little quantities of the micro-voids at Nb/steel interface had a relatively weak effect on shear strength. However, the large number and big size of micro-voids were responsible for the degradation of shear strength for the clad plates with Mo interlayer.展开更多
Hot roll bonding was carried out between commercially pure titanium TA2 and high-strength low-alloy steel Q390 using no interlayer and Fe interlayer at temperatures of 850,900,950 and 1000℃with a total reduction rati...Hot roll bonding was carried out between commercially pure titanium TA2 and high-strength low-alloy steel Q390 using no interlayer and Fe interlayer at temperatures of 850,900,950 and 1000℃with a total reduction ratio of 86.7%.Interfacial microstructure and bonding property of clad plates were investigated by electron microscopy and shear test.The results showed that with the increase in heating temperature,the shear strength of titanium clad steel plates decreases obviously.Compared with that of titanium clad steel plates with no interlayer,inserting Fe interlayer between titanium and steel can improve the shear strength at 850 and 900℃but deteriorate the shear strength at 950 and 1000℃.The reason for this consequence was that Fe interlayer prevented the reaction diffusion of C in titanium to form a thinner TiC layer at 850 and 900℃,while Fe interlayer promoted the reaction diffusion of Fe in titanium to generate thicker Fe2Ti and FeTi intermetallic compound layers at 950 and 1000℃.展开更多
The effect of dissolved niobium on the eutectoid transformation behavior in near-eutectoid high-carbon steels has been studied.Dissolved niobium is important in the eutectoid transformation behavior.It increases the e...The effect of dissolved niobium on the eutectoid transformation behavior in near-eutectoid high-carbon steels has been studied.Dissolved niobium is important in the eutectoid transformation behavior.It increases the eutectoid carbon content significantly（by^0.0477% per 0.00001% dissolved niobium）,increases the hardenability of steel markedly,yields finer,more uniform,polygonal proeutectoid ferrite,induces a transition in morphology of eutectoid cementite from lamellar to somewhat spheroidal,and increases the misorientation angle of pearlite colonies from being focused near 0°to near 60°.展开更多
A novel hot rolled steel LG600A with the tensile strength exceeding 700 MPa was developed for automatic teller machine application. Thelow-cost C-Mn steel was microalloyed with：0.08 mass%- 0.12 mass% Ti rather than n...A novel hot rolled steel LG600A with the tensile strength exceeding 700 MPa was developed for automatic teller machine application. Thelow-cost C-Mn steel was microalloyed with：0.08 mass%- 0.12 mass% Ti rather than noble alloying elements, such as Nb, V, Mo, and Cu, etc. The novel steel had a good surface quality and welding property. After the hot rolled steel coils were leveled, the steel plates, the length of which was even down to 1 500 mm, had an excellent flatness. The effects of hot rolling parameters on mechanical per formance, m icrostructure and recrystallization behavior were studied. The metallurgical concept for the steel production was also discussed. The result shows that decreasing the finish rolling temperature, increasing cooling rate in the first cooling stage and decreasing the cooling rate in the last cooling stage, together with coiling at a modestly high coiling temperature all resulted in the refined grains and TiC precipitates, thereby improving the strength and toughness of this new steel greatly.展开更多
The China Jinping Underground Laboratory（CJPL）, which has the lowest cosmic-ray muon flux and the lowest reactor neutrino flux of any laboratory, is ideal to carry out low-energy neutrino experiments. With two detec...The China Jinping Underground Laboratory（CJPL）, which has the lowest cosmic-ray muon flux and the lowest reactor neutrino flux of any laboratory, is ideal to carry out low-energy neutrino experiments. With two detectors and a total fiducial mass of 2000 tons for solar neutrino physics（equivalently, 3000 tons for geo-neutrino and supernova neutrino physics）, the Jinping neutrino experiment will have the potential to identify the neutrinos from the CNO fusion cycles of the Sun, to cover the transition phase for the solar neutrino oscillation from vacuum to matter mixing, and to measure the geo-neutrino flux, including the Th/U ratio. These goals can be fulfilled with mature existing techniques. Efforts on increasing the target mass with multi-modular neutrino detectors and on developing the slow liquid scintillator will increase the Jinping discovery potential in the study of solar neutrinos,geo-neutrinos, supernova neutrinos, and dark matter.展开更多
Grain size determination is essential in producing and testing iron and steel materials.Grain size determination of martensitic steels usually requires etching with picric acid to reveal the prior austenite grain boun...Grain size determination is essential in producing and testing iron and steel materials.Grain size determination of martensitic steels usually requires etching with picric acid to reveal the prior austenite grain boundaries.However,picric acid is toxic and explosive and belongs to hazardous chemicals,which makes it difficult for laboratories and testing institutions to obtain.A new experimental method was developed to use Nital etchant instead of picric acid.The deep learning method was used to recognize the prior austenite grain boundaries in the etched martensite microstructure,and the grain size could be determined according to the recognition result.Firstly,the polished martensite specimen was etched twice with Nital etchant and picric acid,respectively,and the same position was observed using an optical microscope.The images of the martensitic structure and its prior austenite grain boundary label were obtained,and a data set was constructed.Secondly,based on this data set,a convolutional neural network model with a semantic segmentation function was trained,and the accuracy rate of the test set was 87.53%.Finally,according to the recognition results of the model,the grain size rating can be automatically determined or provide a reference for experimenters,and the difference between the automatic determination results and the measured results is about 0.5 level.展开更多
基金National Key Research and Development Program of China(No.2017YFB0304501).
文摘The microstructure–mechanical property relationship of a Cu-bearing low-carbon high-strength low-alloy steel,subjected to a novel multistage heat treatment including quenching(Q),lamellarization(L)and tempering(T),is presented.Yield strength of 989.5 MPa and average toughness at-80℃of 41 J were obtained in this steel after quenching and tempering(QT)heat treatments.Specimen QLT gained a little lower yield strength(982.5 MPa),but greatly enhanced average toughness at-80℃(137 J).To further clarify the strengthening and toughening mechanisms in specimen QLT,parameters of microstructural characteristic and crack propagation process were compared and analyzed for specimens Q,QL,QT and QLT.The microstructure of tempered martensite/bainite(M/B)in specimen QT changed to refined tempered M/B matrix mixed with minor IF(inter-critical ferrite)in specimen QLT.Cu-rich precipitates existed in tempered M/B for both specimens QT and QLT,as well as in IF.Compared with QT,adding a lamellarization step before tempering made the effective grains of specimen QLT refined and also led to coarser Cu-rich precipitates in tempered M/B matrix.The weaker strengthening effect of coarser Cu-rich precipitates should be a key reason for the slightly lower yield strength in specimen QLT than in specimen QT.No austenite was found in all specimens Q,QL,QT and QLT.Specimen QLT showed purely ductile fracture mode at-80℃due to the refined effective grains.The greatly improved toughness is mainly attributed to the enhanced energy of crack propagation.The combination of refined microstructure,softened matrix and deformation of minor'soft'IF during crack propagation led to the most superior toughness of specimen QLT among all specimens.
基金Item Sponsored by National Basic Research Program of China(2010CB630805)
文摘Through the thermo-mechanical control process （TMCP）, a high Nb low Mo fire resistant steel with the yield strength （YS） of 521 MPa at room temperature （RT） and 360 MPa at elevated temperature （ET） of 600 ℃ was developed based on MX （M=Nb, V, Mo; X=C,N） precipitation strengthening. A series of tensile and con- stant load tests were conducted to study the mechanical properties at ET. The dynamic continuous cooling transfor- mation （CCT） as well as precipitation behavior of microalloy carbonitride was investigated by means of thermal sim- ulator and electron microscopy approaches. Results showed that the failure temperature of tested steel was deter- mined as 653 ℃, and the granular bainite was obtained when the cooling rate was higher than 10 ℃/s. In the rolled state, a certain amount of M/A islands was observed. During heating from RT to ET, M/A islands disappeared, and cementites and high dense compound precipitates （Nb, Mo, V）C with size of less than 10 nm precipitated in ferrite at ET （600 ℃）, which resulted in precipitation strengthening at ET.
文摘The corrosion and passive behaviour of duplex stainless steel 2205 at six cooling rates （1, 5, 10, 15, 20 ℃ s^-1 and water quenched） in a simulated marine-environment solution was investigated using electrochemical measurements of potentiostatic critical pitting temperature, potentiodynamic polarisation curves, electrochemical impedance spectroscopy and Mott-Schottky curves. The microstructural evolution and pitting morphologies of the specimens were visualised using an optical microscope and scanning electron microscope. The electrochemical responses of the passive film show that passivity of the steel was enhanced as the cooling rate increased; however, the threshold cooling rate was 20 ℃ s^-1, beyond which pitting corrosion resistance remained stable. Based on the analyses of microstructural evolution and pit morphologies, the proportion of the ferrite phase increased with the cooling rate and the ratio of austenite and ferrite was close to 1：1. The pitting size decreased as the cooling rate increased, and most metastable pits on specimens were located in the ferrite phase and on the ferrite-austenite interface. Thus, pitting resistance of steel is governed by the phase that provides the lowest pitting resistance equivalent number. The optimised pitting corrosion resistance for steel 2205 was achieved when it was greater than or equal to 20℃ s^-1.
基金supported by the National Key Research and Development Program of China(Grant No.2017YFB0304900)the National Natural Science Foundation of China(Grant No.51911530129).
文摘The relationship between micro structure and corrosion behavior of high-grade pipeline steel under low-temperature conditions was comparatively investigated by the potentiodynamic polarization and electrochemical impedance spec-troscopy test combining with optical micrographs,scanning electron microscopy,transmission electron microscopy and electron back-scattered diffraction.The results showed that,compared with room and high temperature,low temperature could influence the corrosion behavior of high-grade pipeline steel,which means that the corrosion potential and current density decreased and the corrosion resistance increased significantly.Moreover,double layer structures of the interface became thicker and more compact.Under low-temperature environment,acicular ferrite had the martensite/austenite constituents with less amount and smaller size,a higher density of low angle grain boundaries and smaller effective grain sizes compared with granular bainite,which demonstrated higher corrosion resistance.
文摘The morphology and characteristics of granular bainite (GB) in pipeline steels at different continuous cooling rates were investigated by scanning electron microscopy, transmission electron microscopy and electron back-scattered diffraction (EBSD). The results show that the morphology of ferrite matrix in GB turned from the lath sheaf structure into the nearly equiaxed large grain with the cooling rate decreasing from high (60℃/s) to low (5-10℃/s). At the medium cooling rate (20-40℃/s), GB consisted of the irregular ferrite matrix, the granular martensite/austenite (M/A) constituents and abundant substructures inside. The formation of the irregular ferrite and substructure was attributed to the high-temperature recovery which occurred at relatively high-temperature stage before phase transformation. The granular morphology of M/A constitu-ents was formed from the carbon-rich triple junctions which were produced by the multidirectional substructure interfaces converged with each other. Particularly, some martensite in M/A constituents was misoriented from the adjacent ferrite by very small misorientation angle, which could be characterized by the mean band contrast function of EBSD qualitatively or semiquantitatively.
基金supported financially by the National Key Research and Development Program of China (No.2017YFB0305100)
文摘The effect of grain size on the mechanical properties of a high-manganese(Mn)austenitic steel was investigated via electron-backscattered diffraction,transmission electron microscope,X-ray diffraction,and tensile and impact tests at 25°C and-196°C.The Hall–Petch strengthening coefficients for the yield strength of the high-Mn austenitic steels were 7.08 MPa mm 0.5 at 25°C,which increased to 14 MPa mm 0.5 at-196°C.The effect that the grain boundary strengthening had on improving the yield strength at-196°C was better than that at 25°C.The impact absorbed energies and the tensile elongations were enhanced with the increased grain size at 25°C,while they remained nearly unchanged at-196°C.The unchanged impact absorbed energies and the tensile elongations were primarily attributed to the emergence of the micro-twin at-196°C,which promoted the cleavage fracture in the steels with large-sized grains.Refining the grain size could improve the strength of the high-Mn austenitic steels without impairing their ductility and toughness at low temperature.
基金the financial support of the National Key Research and Development Program of China(Grant No.2017YFB0305101)
文摘The secondary phases of the steels have significant effects on the microstructure and mechanical properties, making controlling these secondary phases important. The control of MnS inclusions and A1N precipitates in a N-alloyed high-Mn twin-induced plastic cryogenic steel via solution treatment was investigated with several different techniques including microstructural characterization, 298 K tensile testing, and 77 K impact testing. The solutionizing temperature （ST） increased from 1323 to 1573 K, where the elongated MnS inclusions and large-sized AlN precipitates became spheroidized and dissolved. The aspect ratio of the MnS inclusions decreased as the ST increased and the number density increased. The impact toughness of the steels showed anisotropy and low impact energy values, due to the elongated MnS inclusions and large-sized AIN precipitates. The anisotropy was eliminated by spheroidizing the MnS inclusions. The impact energy was improved by dissolving the large-sized AlN precipitates during the solution treatment. The austenite grain size increased when the dissolution of the AlN precipitate increased, but the effect of the grain size on the yield strength, toughness, and the strength--ductility balance was weak.
基金supported by National Key R&D Program of China(No.2017YFB0304600).
文摘The microstructure,precipitates and properties of 25CrNiMoV(DZ2)steel for high-speed railway axles with different Nb contents were investigated by means of optical microscopy,scanning electron microscopy,electron back-scattering diffraction,transmission electron microscopy and physicochemical phase analysis.The results show that the grain size of the original austenite of the test steels decreases from 20.5 to 14.2 and 10.8μm after adding 0.026 and 0.039 wt.%Nb to a 25CrNiMoV steel,respectively.Moreover,the block width of the tempered martensite in the test steels is refined from 1.91 to 1.72 and 1.60µm,respectively.MC-type precipitates in 25CrNiMoV steel are mainly VC,while(Nb,V)C gradually precipitates when Nb is microalloyed,and the amount of precipitates increases with increasing Nb content.Through strengthening mechanism analysis,it is found that grain refinement strengthening is the primary way to increase the strength.The improvement in the yield strength with increasing Nb content is attributed to a significant increase in precipitation strengthening,grain refinement strengthening and dislocation strengthening.
基金Item Sponsored by Vanitec-CISRI Vanadium Technology Center
文摘The combined effects of martensite-austenite（MA）constituent and pearlite colony on cleavage crack initiation in the simulated coarse-grained heat-affected zone（CGHAZ）of V-N-Ti microalloyed offshore platform steel under different heat inputs were investigated.The results of welding simulation,instrumented impact test,and quantitative analysis indicated that the size of the MA constituent decreased with the increase in cooling time,and by contrast,the size of the pearlite colony increased.According to Griffith theory,the critical sizes of cleavage microcracks were calculated.With the increase of cooling time,the calculated microcrack size could be characterized by the size of the MA constituent first,and then fitted with the size of the pearlite colony.Moreover,the calculated microcrack size variation was opposite to the microcrack initiation energy.This phenomenon is probably due to the combined effects of the MA constituent and pearlite colony with increasing the cooling time of the specimen′s temperature from800 to 500 ℃.
基金supported by the National Key R&D Program of China(Nos.2017YFB0701802.2017YFB0703002 and 2017YFB0304501)the National Natural Science Foundation of China(No.51701044).
文摘A superior combination of yield strength(1001 MPa)and-20℃ impact toughness(166 J)was obtained in Nb-V-Timicroalloyed Ni-Cr-Mo-Cu steel treated by direct quenching and tempering route(DQT).The tested steels treated by DQT route and re-austenitization and tempering route(QT)were compared with each other in terms of mechanical properties and microstructures characterized by optical microscopy,transmission electron microscopy,X-ray diffraction,electron back-scattered diffraction method and so on.Strength and Vickers hardness of the tested steel treated by the above two routes vary with isothermal aging temperature(400-600℃),shown as under-aged state,peak-aged state and overaged state.All DQT specimens show higher strength and Vickers hardness than QT specimens with the same aging condition.Furthermore,the largest difference of yield strength between DQT and QT specimens was shown in DQT600 and QT600 specimens.DQT600 or QT600 specimens refers to direct quenched(DQ)or quenched(Q)specimens isothermally aged at 600℃.The main disparities in quenched microstructure between DQ and Q specimens are mainly in morphology of prior austenite grains,dislocation density of martensite matrix and solution amount of Nb and Mo elements dissolving in martensite matrix,which play key roles in affecting microstructure and mechanical properties of DQT and QT specimens.Higher dislocation density of matrix and finer average diameter of both MC(M is any combination of Nb,Mo and V)and Cu-rich particles were shown in DQT600 specimens than in QT600 specimens.Strengthening from dislocations and nanosized MC and Cu-rich particles mainly leads to the largest difference of yield strength between DQT600 and QT600 specimens.In addition,strong dislocation strengthening and precipitation strengthening in DQT600 specimen also elevated its ductile-to-brittle-transition-temperature,compared with QT600 specimen.
基金Item Sponsored by National Basic Research Program of China(2010CB630805)National Natural Science Foundation of China(51201036)China Iron and Steel Research Institute Group(12060840A)
文摘The austenite grain refinement through control of the grain growth during reheating process after thermomechanical controlled process （TMCP） in a vanadium microalloyed steel was achieved. The formation of ultra-fine grained austenite was attributed to the high density of austenite nucleation at the ferrite/martensite structure and to the inhibition of austenite growth by （Ti~ V）C particles at the relatively low reheating temperature. Corresponding with the precipitation behavior of （Ti,V）C with temperature, the growth behavior of austenite in the vanadium mi- croalloyed steel could be divided into two regions. At lower reheating temperature, austenite grains grew slowly, and ultra-fine grained austenite smaller than 5 ~m was successfully obtained. By contrast, the austenite grains grew rap- idly at high temperature due to the dissolution of （Ti, V）C particles. According to the measured and predicted results of austenite growth kinetics, two models were developed to describe the growth behavior of austenite grains in two different temperature regions, and the apparent activation energy Qapp for grain growth was estimated to be about 115 and 195 kJ/mol, respectively.
文摘Hot roll bonding was carried out between commercially pure titanium TA2 and high-strength low-alloy steel Q390 using pure Nb or Mo interlayer at 950 ~C with a total reduction ratio of 86.7%. Interfacial microstructure and bonding properties of titanium clad steel plates were investigated by electron microscopy and mechanical tests. The results showed β-Ti, TiC and Fe2Ti reaction phases were generated at Ti/steel interface for the clad plates with no interlayer. Inserting Nb or Mo interlayer can effectively suppress the formation of brittle phases, while the weak bonding joint transferred to Nb/steel or Mo/steel interface. And some micro-voids were found at the interface of Nb/steel and Mo/steel. The improved shear strength of clad plates with Nb interlayer might be attributed to the elimination of brittle phases at bonding interface. The small size and little quantities of the micro-voids at Nb/steel interface had a relatively weak effect on shear strength. However, the large number and big size of micro-voids were responsible for the degradation of shear strength for the clad plates with Mo interlayer.
文摘Hot roll bonding was carried out between commercially pure titanium TA2 and high-strength low-alloy steel Q390 using no interlayer and Fe interlayer at temperatures of 850,900,950 and 1000℃with a total reduction ratio of 86.7%.Interfacial microstructure and bonding property of clad plates were investigated by electron microscopy and shear test.The results showed that with the increase in heating temperature,the shear strength of titanium clad steel plates decreases obviously.Compared with that of titanium clad steel plates with no interlayer,inserting Fe interlayer between titanium and steel can improve the shear strength at 850 and 900℃but deteriorate the shear strength at 950 and 1000℃.The reason for this consequence was that Fe interlayer prevented the reaction diffusion of C in titanium to form a thinner TiC layer at 850 and 900℃,while Fe interlayer promoted the reaction diffusion of Fe in titanium to generate thicker Fe2Ti and FeTi intermetallic compound layers at 950 and 1000℃.
文摘The effect of dissolved niobium on the eutectoid transformation behavior in near-eutectoid high-carbon steels has been studied.Dissolved niobium is important in the eutectoid transformation behavior.It increases the eutectoid carbon content significantly（by^0.0477% per 0.00001% dissolved niobium）,increases the hardenability of steel markedly,yields finer,more uniform,polygonal proeutectoid ferrite,induces a transition in morphology of eutectoid cementite from lamellar to somewhat spheroidal,and increases the misorientation angle of pearlite colonies from being focused near 0°to near 60°.
文摘A novel hot rolled steel LG600A with the tensile strength exceeding 700 MPa was developed for automatic teller machine application. Thelow-cost C-Mn steel was microalloyed with：0.08 mass%- 0.12 mass% Ti rather than noble alloying elements, such as Nb, V, Mo, and Cu, etc. The novel steel had a good surface quality and welding property. After the hot rolled steel coils were leveled, the steel plates, the length of which was even down to 1 500 mm, had an excellent flatness. The effects of hot rolling parameters on mechanical per formance, m icrostructure and recrystallization behavior were studied. The metallurgical concept for the steel production was also discussed. The result shows that decreasing the finish rolling temperature, increasing cooling rate in the first cooling stage and decreasing the cooling rate in the last cooling stage, together with coiling at a modestly high coiling temperature all resulted in the refined grains and TiC precipitates, thereby improving the strength and toughness of this new steel greatly.
基金Supported by the National Natural Science Foundation of China(11235006,11475093,11135009,11375065,11505301,and11620101004)the Tsinghua University Initiative Scientific Research Program(20121088035,20131089288,and 20151080432)+3 种基金the Key Laboratory of Particle&Radiation Imaging(Tsinghua University)the CAS Center for Excellence in Particle Physics(CCEPP)U.S.National Science Foundation Grant PHY-1404311(Beacom)U.S.Department of Energy under contract DE-AC02-98CH10886(Yeh)
文摘The China Jinping Underground Laboratory（CJPL）, which has the lowest cosmic-ray muon flux and the lowest reactor neutrino flux of any laboratory, is ideal to carry out low-energy neutrino experiments. With two detectors and a total fiducial mass of 2000 tons for solar neutrino physics（equivalently, 3000 tons for geo-neutrino and supernova neutrino physics）, the Jinping neutrino experiment will have the potential to identify the neutrinos from the CNO fusion cycles of the Sun, to cover the transition phase for the solar neutrino oscillation from vacuum to matter mixing, and to measure the geo-neutrino flux, including the Th/U ratio. These goals can be fulfilled with mature existing techniques. Efforts on increasing the target mass with multi-modular neutrino detectors and on developing the slow liquid scintillator will increase the Jinping discovery potential in the study of solar neutrinos,geo-neutrinos, supernova neutrinos, and dark matter.
文摘Grain size determination is essential in producing and testing iron and steel materials.Grain size determination of martensitic steels usually requires etching with picric acid to reveal the prior austenite grain boundaries.However,picric acid is toxic and explosive and belongs to hazardous chemicals,which makes it difficult for laboratories and testing institutions to obtain.A new experimental method was developed to use Nital etchant instead of picric acid.The deep learning method was used to recognize the prior austenite grain boundaries in the etched martensite microstructure,and the grain size could be determined according to the recognition result.Firstly,the polished martensite specimen was etched twice with Nital etchant and picric acid,respectively,and the same position was observed using an optical microscope.The images of the martensitic structure and its prior austenite grain boundary label were obtained,and a data set was constructed.Secondly,based on this data set,a convolutional neural network model with a semantic segmentation function was trained,and the accuracy rate of the test set was 87.53%.Finally,according to the recognition results of the model,the grain size rating can be automatically determined or provide a reference for experimenters,and the difference between the automatic determination results and the measured results is about 0.5 level.
