a-N curves of fatigue crack growth of U20Mn bainite rail after different heat treatment process were studied(The temperature is cooled from 900 to 20℃,the cooling rate was 0.5℃/s,1℃/s and salt bath isothermal respe...a-N curves of fatigue crack growth of U20Mn bainite rail after different heat treatment process were studied(The temperature is cooled from 900 to 20℃,the cooling rate was 0.5℃/s,1℃/s and salt bath isothermal respectively),the Paris formula of fatigue crack growth was fitted linearly,and the material parameters C and n were measured.The results show that the sample with cooling rate of 0.5℃/s has the fastest crack growth rate,and the sample with salt bath isothermal has the slowest crack growth rate.The coarse M/A islands with irregular shape in bainite structure with cooling rate of 0.5℃/s has poor resistance to fatigue crack propagation,which is not conducive to improving the fatigue performance.However,the sample with salt bath isothermal has longer fatigue life.Due to the combination of bainitie lamellar and retained austenite distributed between them,the salt bath isothermal sample can effectively improve the strength and toughness of bainite steel.The sample with cooling rate of 0.5℃/s is mainly composed of granular bainitie structure,and the fatigue crack growth trajectory is generally gentle without large angle deflection,the sample with salt bath isothermal is mainly composed of bainite lamellar structure,and the fatigue crack growth trajectory is not straight,with a large number of Z-shaped deflection.The fatigue cracks are prone to produce branch cracks at the stress concentration of propagation deflection,and the branch crack consumes the energy of the main fatigue cracks,thus reducing the fatigue crack growth rate and improving fatigue life.展开更多
The phase volume fraction has an important role in the match of the strength and plasticity of dual phase steel.The different bainite contents(18–53 vol.%)in polygonal ferrite and bainite(PF+B)dual phase steel were o...The phase volume fraction has an important role in the match of the strength and plasticity of dual phase steel.The different bainite contents(18–53 vol.%)in polygonal ferrite and bainite(PF+B)dual phase steel were obtained by controlling the relaxation finish temperature during the rolling process.The effect of bainite volume fraction on the tensile deformability was systematically investigated via experiments and crystal plasticity finite element model(CPFEM)simulation.The experimental results showed that the steel showed optimal strain hardenability and strength–plasticity matching when the bainite reached 35%.The 3D-CPFEM models with the same grain size and texture characters were established to clarify the influence of stress/strain distribution on PF+B dual phase steel with different bainite contents.The simulation results indicated that an appropriate increase in the bainite content(18%–35%)did not affect the interphase strain difference,but increased the stress distribution in both phases,as a result of enhancing the coordinated deformability of two phases and improving the strength–plasticity matching.When the bainite content increased to 53%,the stress/strain difference between the two phases was greatly increased,and plastic damage between the two phases was caused by the reduction of the coordinated deformability.展开更多
Bainite transformation has yet to be utilized and even thoroughly studied in medium Mn steels.Here,we investigate the isothermal bainite transformation in a 10Mn steel at 450°C experimentally and theoretically,fo...Bainite transformation has yet to be utilized and even thoroughly studied in medium Mn steels.Here,we investigate the isothermal bainite transformation in a 10Mn steel at 450°C experimentally and theoretically,focusing on the effect of dislocations introduced by warm deformation.We show that the bainite transformation in the studied medium Mn steel exhibits extremely sluggish kinetics(on a time scale of days),concurrent with the pearlite formation.The introduced dislocations can significantly accelerate bainite transformation kinetics while also facilitating the pearlite reaction.This is likely the first report on the simultaneous occurrence of these two solid-state reactions in medium Mn steels.With respect to the roles of dislocations in the acceleration of bainite transformation observed in this work,we propose a new‘carbon depletion mechanism’,in which dislocations-stimulated pearlite formation makes a twofold contribution:facilitating the formation of bainitic ferrite sub-units to further enhance the autocatalytic effect and preventing the carbon enrichment in the remaining austenite.On this basis,a physical model is developed to quantitatively understand the bainite transformation kinetics considering the effect of concurrent pearlite formation,revealing good agreements between model descriptions and experiment results.Our findings,herein,offer fundamental insights into the bainite transformation in medium Mn steels and uncover a previously unidentified role played by introduced dislocations in influencing the kinetics of bainite formation,which may guide its future application in manipulating microstructure for the development of advanced high-strength steels.展开更多
In this study, a high-carbon nano-bainitic GCr15Si1Mo bearing steel was investigated. Specifically, the effects of content and size of undissolved carbides on the microstructure and transformation kinetics of nano-bai...In this study, a high-carbon nano-bainitic GCr15Si1Mo bearing steel was investigated. Specifically, the effects of content and size of undissolved carbides on the microstructure and transformation kinetics of nano-bainite were analyzed. The results demonstrated that after prolonged austempering at low temperatures, the mixed microstructure composed of nano-bainite (NB), undissolved carbides (UC), and retained austenite (RA) was obtained in GCr15SiMo steel. When the experimental steel was austenitized at 900 ℃, the undissolved carbides gradually dissolved until reaching a stable state with increasing holding time. Furthermore, at the same austempering temperature, despite different volume fractions of undissolved carbides in the substrate, the volume fractions of nano-bainite in the final microstructures remained essentially the same. Moreover, the higher the content of undissolved carbides in steel, the faster the transformation rate of nano-bainite and the shorter the total transformation time.展开更多
The effect of the amount of isothermal martensite and bainite on the microstructure and properties in a medium-carbon quenching and partitioning(Q&P)steel was investigated by designing the different Q&P treatm...The effect of the amount of isothermal martensite and bainite on the microstructure and properties in a medium-carbon quenching and partitioning(Q&P)steel was investigated by designing the different Q&P treatment parameters.The results show that the amount of isothermal martensite increased gradually with the increase in quenching time.The increase in isothermal martensite amount improved the product of strength and elongation(PSE)of Q&P steels.In addition,the increase in carbides amount and the recovery in prior martensite with longer partitioning time led to an increase in PSE first and then,a decrease.It implies that a higher PSE could be obtained by the selection of a suitable partitioning time.Furthermore,the effect of bainite transformation during partitioning on PSE was investigated by designing the different partitioning temperatures,including 300,400(below bainite starting temperature,B_(s))and 480℃(above B_(s)).The results show that compared with the samples partitioned at temperature above B_(s),the bainite transformation was only detected when the samples were partitioned at temperature below B_(s).The bainite transformation amount increased with the decreasing partitioning temperature,leading to the inhibition of carbides precipitation and more stable RA and thus,resulting in the highest PSE.展开更多
The effect of bainite morphology on mechanical properties of the mixed bainite-martensite microstructure in D6AC low alloy ultra-high strength steel has been studied in the present work. For this purpose, samples aust...The effect of bainite morphology on mechanical properties of the mixed bainite-martensite microstructure in D6AC low alloy ultra-high strength steel has been studied in the present work. For this purpose, samples austenitized at 910℃ for 40 min were quenched in three different ways. Some of the samples were directly oil-quenched, some others were quenched in salt bath at 330 ℃ and the remaining samples were quenched in salt bath at 425 ℃ for various holding times. All samples were tempered at 200 ℃ for 2 h. Microstructures were examined by optical microscopy （OM） and scanning electron microscopy （SEM）. Fracture surfaces also were studied by SEM. Results showed that the mixed microstructure containing martensite and 28 vol.% of the lower bainite exhibited higher yield and tensile strengths than the fully martensitic microstructure. This could be mainly attributed to the partitioning of the prior austenite grains by the lower bainite and enhancing the strength of lower bainite in the mixed microstructure by plastic constraint. Charpy V-notch （CVN） impact energy and ductility were improved by increasing the volume fraction of the lower bainite. This is not the case about the mixed microstructure containing the upper bainite and martensite. As a result, the tensile and CVN impact properties of mixed upper bainite-martensite microstructure are lower than those of the fully martensitic microstructure. Finally, fractography studies showed cleavage fracture at the surface of CVN impact specimens with martensitic and upper bainitic microstructures confirming the tendency to brittle behavior.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 effects of deformation temperature and strain on bainite transformation of low carbon steel and boron- containing steel were investigated under continuous cooling conditions by means of dilatometric measurement an...The effects of deformation temperature and strain on bainite transformation of low carbon steel and boron- containing steel were investigated under continuous cooling conditions by means of dilatometric measurement and mi- crostructure observation. The results show that with decreasing the deformation temperature from 1000 to 800 ℃ and increasing strains, bainite start temperature for boron-containing low carbon steel increases, whereas it decreases for low carbon steel under the same condition. The bainite microstructures are easily obtained for boron-containing steel deformed at different temperatures and different strains compared with plain low carbon steel. With increasing the continuous cooling rate, the bainite start temperature under deformed condition is about 20 ℃ higher than that under undeformed condition for boron-containing steel, but it changes slightly when the cooling rate is 20 ℃/s or more.展开更多
The quantitative analysis of substructure in the martensite/bainite mixed structure, which is obtained from low-carbon NiCrMoV steels under different cooling conditions, was made by means of optical microscope （OM）,...The quantitative analysis of substructure in the martensite/bainite mixed structure, which is obtained from low-carbon NiCrMoV steels under different cooling conditions, was made by means of optical microscope （OM）, scanning electron microscope （SEM）, electron backscatter diffraction （EBSD）, and transmission electron microscope （TEM）, in order to research the effect on toughness. The test results indicate that the toughness of the steel is en- hanced with the decrease in the packet and block size under the condition of the same prior austenite grain size mixed with different ratios of martensite and bainite while the lath width is about 0.38μm. The calculation shows that both the packet and block boundaries have the same hindering effect on crack extension. Furthermore, the effect of the block width on impact energy is much larger than that of the packet. Therefore, the block can be used as microstruc- tural substructure to affect the toughness in low-carbon martensite steels, suggesting that the block size is ＂the effective grain size＂ for controlling toughness.展开更多
We obtained a good combination of strength and ductility in a 0.4C-2.0Mn-1.7Si-0.4Cr（wt%） steel,namely,;.7 GPa of ultimate tensile strength and;6% of elongation,by conducting a Q-P-T（quenching-partitioning- temper...We obtained a good combination of strength and ductility in a 0.4C-2.0Mn-1.7Si-0.4Cr（wt%） steel,namely,;.7 GPa of ultimate tensile strength and;6% of elongation,by conducting a Q-P-T（quenching-partitioning- tempering） process incorporating the formation of carbide-free bainite. The tempering behavior of this steel was discussed by using experimental finding(scanning electron microscopy,X-ray diffraction（XRD）,transmission electron microscopy and dilatometer) and CCE（constrained carbon equilibrium） modeling. The XRD results combined with CCE calculation prove that carbon partitioning from martensite to austenite occurs during tempering. Consequently,the thermodynamic stability of retained austenite is enhanced. This idea can be utilized to design novel Q-P-T processes in future.展开更多
The roles of pre-formed martensite(PM)in below-Ms bainite formation,microstructure,crystallography,strain partitioning and mechanical properties of a low-carbon bainitic steel were investigated using electron-backscat...The roles of pre-formed martensite(PM)in below-Ms bainite formation,microstructure,crystallography,strain partitioning and mechanical properties of a low-carbon bainitic steel were investigated using electron-backscattered diffraction,transmission electron microscopy,micro digital image correlation technique and mechanical tests.It is demonstrated that the pre-formation of martensite eliminates the incubation time for bainite transformation at various austempering temperatures below Ms,indicative of its acceleration effect at the early stage of transformation.This effect is mainly attributed to the surfaces or tips of the PM acting as the nuclei of subsequently-formed bainite,with initial bainite tending to form around the PM.However,the finishing time for below-Ms bainite transformation,especially at even lower temperatures,is retarded,owing to the dividing effect of PM on parent austenite grains,the decreasing effect of lowered isothermal temperature on the diffusion rate of carbon atoms and the strengthening effect of lowered isothermal temperature on supercooled austenite.PM and its adjacent bainitic laths have nearly the same crystallographic orientation and belong to the same block.The pre-formation of martensite largely refines the bainitic blocks/laths and retained austenite.The specimens with PM show relatively uniform strain partitioning among various phases,contrasting with the specimens without PM,for which strains are highly concentrated in the bainite region nearby fresh martensite/austenite(M/A)blocks or between adjacent M/A blocks.The impact absorption energies of the specimens with PM,when austempered at 30-60℃ below Ms,are more than twice higher than those of the specimens without PM,at no expense of tensile properties.展开更多
Quenching partitioning tempering （QP-T） process were applied to a quenched carbide-free bainite/mar- tensite （CFB/M） multiphase steel 40Mn2Si2Cr and its effect on the mechanical properties was studied. The results...Quenching partitioning tempering （QP-T） process were applied to a quenched carbide-free bainite/mar- tensite （CFB/M） multiphase steel 40Mn2Si2Cr and its effect on the mechanical properties was studied. The results showed that the partitioning time of Q P T treatment can he increased greatly to 90 min or even longer. The tensile strength, elongation and product of strength and elongation （PSE） of the experimental steel can achieve 1650 MPa, about 20% and over 33 GPa ·%, respectively. The significant increase of partitioning time and excellent mechanical properties could be related to the CFB microstrueture introduced in the quenching process during the Q-P-T treat- ment.展开更多
The growth rates of bainite plates in an Fe-C-Mn-Si superbainitie steel were investigated by in situ observation. The lengthening rates of ferrite bainite during both cooling and isothermal holding processes were obse...The growth rates of bainite plates in an Fe-C-Mn-Si superbainitie steel were investigated by in situ observation. The lengthening rates of ferrite bainite during both cooling and isothermal holding processes were observed and the growth rates of bainite plates nucleating at grain boundaries, within grains and on preformed bainite were measured. It is indicated that the lengthening rates of bainite plates during the cooling and isothermal processes were different, and that the growth rates of bainite plates nucleating at different types of sites also demon- strated diversity. The bainite plates initiating at [vain boundaries during cooling grew the fastest, while the plates nucleating on preformed bainite did the slowest. However, the growth rate of the bainite plates nucleating at grain boundaries during isothermal transformation de- creased the most, whereas the bainite plates initiating within grains grew the fastest. In addition, the growth rate of ferrite bainite in the study supported the diffusion transformation mechanism of bainite from the viewooint of ~rowth rate.展开更多
A dilatometer was used to study the kinetics of bainite-to-austenite transformation in low carbon microalloyed steel with the initial microstructure of bainite during the continuous reheating process. The bainite-to-a...A dilatometer was used to study the kinetics of bainite-to-austenite transformation in low carbon microalloyed steel with the initial microstructure of bainite during the continuous reheating process. The bainite-to-austenite trans- formation was observed to take place in two steps at low heating rate. The first step is the dissolution of bainite, and the second one is the remaining bainite-to-austenite transformation controlled by a dissolution process. The calculation result of the kinetics of austenite formation shows that the two steps occur by diffusion at low heating rate. However, at high heating rate the bainite-to-austenite transformation occurs in a single step, and the process is mainly dominated by shear. The growth rate of austenite reaches the maximum at about 835℃ at different heating rates and the growth rate of austenite as a function of temperature increases with the increase in heating rate.展开更多
Hot deformation experiments were carried out on Gleeble 1500 thermo-mechanical simulator. The bainite transformation after deformation was investigated by optical microstructure analysis. The results indicated that th...Hot deformation experiments were carried out on Gleeble 1500 thermo-mechanical simulator. The bainite transformation after deformation was investigated by optical microstructure analysis. The results indicated that the deformation accelerated the bainite transformation when the deformation was carried out at high temperature and no or little ferrite was precipitated before bainite transformation; when the deformation was carried out at low temperature, the deformation hindered the bainite transformation because a lot of ferrite precipitated before bainite transformation.展开更多
The ultra-high cycle fatigue behavior of a novel high strength steel with carbide-free bainite/martensite （CFB/M） complex microstructure was studied. The ultra-high cycle fatigue properties were measured by ultrason...The ultra-high cycle fatigue behavior of a novel high strength steel with carbide-free bainite/martensite （CFB/M） complex microstructure was studied. The ultra-high cycle fatigue properties were measured by ultrasonic fatigue testing equipment at a frequency of 20 kHz. It is found that there is no horizontal part in the S-N curve and fatigue fracture occurs when the life of specimens exceeds 10^7 cycles. In addition, the origination of fatigue cracks tends to transfer from the surface to interior of specimens as the fatigue cycle exceeds 10^7, and the fatigue crack originations of many specimens are not induced by inclusions, but by some kind of ＂soft structure＂. It is shown that the studied high strength steel performs good ultra-high cycle fatigue properties. The ultra-high fatigue mechanism was discussed and it is suggested that specific CFB/M complex microstructure of the studied steel contributes to its superior properties.展开更多
The crystallography of bainite, transformed isothermally at 450 ℃ in 0.2C-2.0Mn-1.5Si-0.6Cr steel, was investigated by electron backscatter diffraction （EBSD） analysis. The orientation relationship （OR） was found...The crystallography of bainite, transformed isothermally at 450 ℃ in 0.2C-2.0Mn-1.5Si-0.6Cr steel, was investigated by electron backscatter diffraction （EBSD） analysis. The orientation relationship （OR） was found to be closer to Nishiyama-Wassermann （N-W） than Kurdjumov-Sachs orientation relationship. Bainite microstructure consisted of parallel laths forming a morphological packet structure. Typically, there were three different lath orientations in a morphological packet. These orientations were dictated by a three specific N-W OR variants sharing the same {111} austenite plane. A packet of bainite laths with common {111} austenite plane was termed as crystallographic packet. Generally, the crystallographic packet size corresponded to the morphological packet size. Locally, crystallographic packets with only two dominant orientations were observed. This indicates strong local variant selection during isothermal bainite transformation. The relative orientation between the variants in crystallographic packets was found to be near 60°/〈110〉. This appears to explain the strong peak observed in the grain boundary misorientation distribution near 60°. Bainite also contained pronounced fraction of boundaries with their misorientation in the range of 2.5°-8° with quite widely dispersed rotation angles. Spatially these boundaries were found to locate inside the bainite laths, forming lath-like sub-grains.展开更多
The effect of bainite transformation and microstructure on the mechanical properties in 0.3%C bainitic steel was investigated via the heat treatment process of quenching at higher initial temperature and partitioning ...The effect of bainite transformation and microstructure on the mechanical properties in 0.3%C bainitic steel was investigated via the heat treatment process of quenching at higher initial temperature and partitioning below martensite-start temperature. The results show that bainite transformation takes place with the partitioning time increasing during partitioning below martensite-start temperature. The microstructure of samples treated by this two-step quenching and partitioning process consists of lath bainite, lath martensite and retained austenite. This kind of multiphase microstructure exhibits better strength of 1420 MPa, ductility of 21.8 % and the product of strength and elongation of 30.8 GPa%. Furthermore, the excellent impact toughness of 103 J is exhibited by partitioning at 280 ℃ for 3 h. In addition, the coalescence of bainite platelets was found in the sample treated by partitioning for 8 h, leading to the deterioration of toughness.展开更多
The quantitative effects of chromium content on continuous cooling transformation （CCT） diagrams of novel air-cooled bainite steels were analyzed using artificial neural network models. The results showed that the c...The quantitative effects of chromium content on continuous cooling transformation （CCT） diagrams of novel air-cooled bainite steels were analyzed using artificial neural network models. The results showed that the chromium may retard the high and medium-temperature martensite transformation.展开更多
The effects of aluminum addition on bainite transformation and properties of carbide-free bainitic steels containing 0.22 wt.% carbon were investigated by two different types of heat treatment processes: continuous co...The effects of aluminum addition on bainite transformation and properties of carbide-free bainitic steels containing 0.22 wt.% carbon were investigated by two different types of heat treatment processes: continuous cooling process (CCP) and isothermal transformation process (ITP). The results indicate that for the CCP treatment, Al addition significantly promoted the ferrite and bainite transformation;however, it did not significantly increase the product of tensile strength and total elongation (PSE). For the ITP treatment, Al addition significantly promoted the kinetics of bainite transformation, and thus, more bainite was formed with Al addition;however, it was found that Al addition resulted in a decrease in tensile strength and an increase in elongation of the tested bainitic steels. Moreover, the effects of Al addition on comprehensive property were profoundly dependent on austempering temperatures. When the austempering temperature was higher (430℃), PSE significantly increased with Al addition, whereas it decreased at the lower austempering temperature (400℃). Therefore, it can be concluded that the effects of Al on properties of bainitic steels were more significant at higher austempering temperatures.展开更多
基金Funded by Inner Mongolia Autonomous Region Science and Technology Program(No.2023YFHH0036)Natural Science Foundation of Inner Mongolia(No.2024LHMS05033)the Basic Scientific Research Fees for Colleges and Universities Directly under the Inner Mongolia(Nos.2023QNJS002,2023YXXS007,2024YXXS039)。
文摘a-N curves of fatigue crack growth of U20Mn bainite rail after different heat treatment process were studied(The temperature is cooled from 900 to 20℃,the cooling rate was 0.5℃/s,1℃/s and salt bath isothermal respectively),the Paris formula of fatigue crack growth was fitted linearly,and the material parameters C and n were measured.The results show that the sample with cooling rate of 0.5℃/s has the fastest crack growth rate,and the sample with salt bath isothermal has the slowest crack growth rate.The coarse M/A islands with irregular shape in bainite structure with cooling rate of 0.5℃/s has poor resistance to fatigue crack propagation,which is not conducive to improving the fatigue performance.However,the sample with salt bath isothermal has longer fatigue life.Due to the combination of bainitie lamellar and retained austenite distributed between them,the salt bath isothermal sample can effectively improve the strength and toughness of bainite steel.The sample with cooling rate of 0.5℃/s is mainly composed of granular bainitie structure,and the fatigue crack growth trajectory is generally gentle without large angle deflection,the sample with salt bath isothermal is mainly composed of bainite lamellar structure,and the fatigue crack growth trajectory is not straight,with a large number of Z-shaped deflection.The fatigue cracks are prone to produce branch cracks at the stress concentration of propagation deflection,and the branch crack consumes the energy of the main fatigue cracks,thus reducing the fatigue crack growth rate and improving fatigue life.
基金supported by the Project of Liaoning Marine Economic Development(Development of high strength pipeline steel for submarine oil and gas transmission)State Key Laboratory of Metal Material for Marine Equipment and Application Funding(No.SKLMEA-K202205).
文摘The phase volume fraction has an important role in the match of the strength and plasticity of dual phase steel.The different bainite contents(18–53 vol.%)in polygonal ferrite and bainite(PF+B)dual phase steel were obtained by controlling the relaxation finish temperature during the rolling process.The effect of bainite volume fraction on the tensile deformability was systematically investigated via experiments and crystal plasticity finite element model(CPFEM)simulation.The experimental results showed that the steel showed optimal strain hardenability and strength–plasticity matching when the bainite reached 35%.The 3D-CPFEM models with the same grain size and texture characters were established to clarify the influence of stress/strain distribution on PF+B dual phase steel with different bainite contents.The simulation results indicated that an appropriate increase in the bainite content(18%–35%)did not affect the interphase strain difference,but increased the stress distribution in both phases,as a result of enhancing the coordinated deformability of two phases and improving the strength–plasticity matching.When the bainite content increased to 53%,the stress/strain difference between the two phases was greatly increased,and plastic damage between the two phases was caused by the reduction of the coordinated deformability.
基金support from National Key Research and Development Program of China(No.2019YFA0209900)National Natural Science Foundation of China(No.52130102)+5 种基金Research Grants Council of Hong Kong(No.R7066–18)Guangzhou Municipal Science and Technology Project(No.202007020007)Guangdong Basic and Applied Basic Research Foundation of China(No.2020B1515130007)support from National Natural Science Foundation of China(No.52130110)support from National Natural Science Foundation of China(No.52271116)Hong Kong Scholars Program(No.XJ2019029).
文摘Bainite transformation has yet to be utilized and even thoroughly studied in medium Mn steels.Here,we investigate the isothermal bainite transformation in a 10Mn steel at 450°C experimentally and theoretically,focusing on the effect of dislocations introduced by warm deformation.We show that the bainite transformation in the studied medium Mn steel exhibits extremely sluggish kinetics(on a time scale of days),concurrent with the pearlite formation.The introduced dislocations can significantly accelerate bainite transformation kinetics while also facilitating the pearlite reaction.This is likely the first report on the simultaneous occurrence of these two solid-state reactions in medium Mn steels.With respect to the roles of dislocations in the acceleration of bainite transformation observed in this work,we propose a new‘carbon depletion mechanism’,in which dislocations-stimulated pearlite formation makes a twofold contribution:facilitating the formation of bainitic ferrite sub-units to further enhance the autocatalytic effect and preventing the carbon enrichment in the remaining austenite.On this basis,a physical model is developed to quantitatively understand the bainite transformation kinetics considering the effect of concurrent pearlite formation,revealing good agreements between model descriptions and experiment results.Our findings,herein,offer fundamental insights into the bainite transformation in medium Mn steels and uncover a previously unidentified role played by introduced dislocations in influencing the kinetics of bainite formation,which may guide its future application in manipulating microstructure for the development of advanced high-strength steels.
基金support from the National Natural Science Foundation of China(Nos.52001105 and 52122410)the Hebei Natural Science Foundation(Nos.E2022402107,E2023203259 and E2020402101)+1 种基金the Science and Technology Project of Hebei Education Department(No.BJ2021012)the Central Guidance for Local Science and Technology Development Funding Project(No.236Z1021G).
文摘In this study, a high-carbon nano-bainitic GCr15Si1Mo bearing steel was investigated. Specifically, the effects of content and size of undissolved carbides on the microstructure and transformation kinetics of nano-bainite were analyzed. The results demonstrated that after prolonged austempering at low temperatures, the mixed microstructure composed of nano-bainite (NB), undissolved carbides (UC), and retained austenite (RA) was obtained in GCr15SiMo steel. When the experimental steel was austenitized at 900 ℃, the undissolved carbides gradually dissolved until reaching a stable state with increasing holding time. Furthermore, at the same austempering temperature, despite different volume fractions of undissolved carbides in the substrate, the volume fractions of nano-bainite in the final microstructures remained essentially the same. Moreover, the higher the content of undissolved carbides in steel, the faster the transformation rate of nano-bainite and the shorter the total transformation time.
基金The authors gratefully acknowledge the financial supports from the National Natural Science Foundation of China(No.52104381)the China Postdoctoral Science Foundation(No.2023M732721)also the help on microstructure analysis from Dr.Zhen Wang at the Analytical and Testing Center of Wuhan University of Science and Technology.
文摘The effect of the amount of isothermal martensite and bainite on the microstructure and properties in a medium-carbon quenching and partitioning(Q&P)steel was investigated by designing the different Q&P treatment parameters.The results show that the amount of isothermal martensite increased gradually with the increase in quenching time.The increase in isothermal martensite amount improved the product of strength and elongation(PSE)of Q&P steels.In addition,the increase in carbides amount and the recovery in prior martensite with longer partitioning time led to an increase in PSE first and then,a decrease.It implies that a higher PSE could be obtained by the selection of a suitable partitioning time.Furthermore,the effect of bainite transformation during partitioning on PSE was investigated by designing the different partitioning temperatures,including 300,400(below bainite starting temperature,B_(s))and 480℃(above B_(s)).The results show that compared with the samples partitioned at temperature above B_(s),the bainite transformation was only detected when the samples were partitioned at temperature below B_(s).The bainite transformation amount increased with the decreasing partitioning temperature,leading to the inhibition of carbides precipitation and more stable RA and thus,resulting in the highest PSE.
文摘The effect of bainite morphology on mechanical properties of the mixed bainite-martensite microstructure in D6AC low alloy ultra-high strength steel has been studied in the present work. For this purpose, samples austenitized at 910℃ for 40 min were quenched in three different ways. Some of the samples were directly oil-quenched, some others were quenched in salt bath at 330 ℃ and the remaining samples were quenched in salt bath at 425 ℃ for various holding times. All samples were tempered at 200 ℃ for 2 h. Microstructures were examined by optical microscopy （OM） and scanning electron microscopy （SEM）. Fracture surfaces also were studied by SEM. Results showed that the mixed microstructure containing martensite and 28 vol.% of the lower bainite exhibited higher yield and tensile strengths than the fully martensitic microstructure. This could be mainly attributed to the partitioning of the prior austenite grains by the lower bainite and enhancing the strength of lower bainite in the mixed microstructure by plastic constraint. Charpy V-notch （CVN） impact energy and ductility were improved by increasing the volume fraction of the lower bainite. This is not the case about the mixed microstructure containing the upper bainite and martensite. As a result, the tensile and CVN impact properties of mixed upper bainite-martensite microstructure are lower than those of the fully martensitic microstructure. Finally, fractography studies showed cleavage fracture at the surface of CVN impact specimens with martensitic and upper bainitic microstructures confirming the tendency to brittle behavior.
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Sponsored by National High-tech Research and Development Program(863Program) of China(2006AA03Z529)
文摘The effects of deformation temperature and strain on bainite transformation of low carbon steel and boron- containing steel were investigated under continuous cooling conditions by means of dilatometric measurement and mi- crostructure observation. The results show that with decreasing the deformation temperature from 1000 to 800 ℃ and increasing strains, bainite start temperature for boron-containing low carbon steel increases, whereas it decreases for low carbon steel under the same condition. The bainite microstructures are easily obtained for boron-containing steel deformed at different temperatures and different strains compared with plain low carbon steel. With increasing the continuous cooling rate, the bainite start temperature under deformed condition is about 20 ℃ higher than that under undeformed condition for boron-containing steel, but it changes slightly when the cooling rate is 20 ℃/s or more.
文摘The quantitative analysis of substructure in the martensite/bainite mixed structure, which is obtained from low-carbon NiCrMoV steels under different cooling conditions, was made by means of optical microscope （OM）, scanning electron microscope （SEM）, electron backscatter diffraction （EBSD）, and transmission electron microscope （TEM）, in order to research the effect on toughness. The test results indicate that the toughness of the steel is en- hanced with the decrease in the packet and block size under the condition of the same prior austenite grain size mixed with different ratios of martensite and bainite while the lath width is about 0.38μm. The calculation shows that both the packet and block boundaries have the same hindering effect on crack extension. Furthermore, the effect of the block width on impact energy is much larger than that of the packet. Therefore, the block can be used as microstruc- tural substructure to affect the toughness in low-carbon martensite steels, suggesting that the block size is ＂the effective grain size＂ for controlling toughness.
基金financially supported by the National Natural Science Foundation of China (No.51301012)
文摘We obtained a good combination of strength and ductility in a 0.4C-2.0Mn-1.7Si-0.4Cr（wt%） steel,namely,;.7 GPa of ultimate tensile strength and;6% of elongation,by conducting a Q-P-T（quenching-partitioning- tempering） process incorporating the formation of carbide-free bainite. The tempering behavior of this steel was discussed by using experimental finding(scanning electron microscopy,X-ray diffraction（XRD）,transmission electron microscopy and dilatometer) and CCE（constrained carbon equilibrium） modeling. The XRD results combined with CCE calculation prove that carbon partitioning from martensite to austenite occurs during tempering. Consequently,the thermodynamic stability of retained austenite is enhanced. This idea can be utilized to design novel Q-P-T processes in future.
基金financially supported by the National Natural Science Foundation of China(No.U1760116)the Natural Science Foundation of HeBei Province(No.E2019203478)。
文摘The roles of pre-formed martensite(PM)in below-Ms bainite formation,microstructure,crystallography,strain partitioning and mechanical properties of a low-carbon bainitic steel were investigated using electron-backscattered diffraction,transmission electron microscopy,micro digital image correlation technique and mechanical tests.It is demonstrated that the pre-formation of martensite eliminates the incubation time for bainite transformation at various austempering temperatures below Ms,indicative of its acceleration effect at the early stage of transformation.This effect is mainly attributed to the surfaces or tips of the PM acting as the nuclei of subsequently-formed bainite,with initial bainite tending to form around the PM.However,the finishing time for below-Ms bainite transformation,especially at even lower temperatures,is retarded,owing to the dividing effect of PM on parent austenite grains,the decreasing effect of lowered isothermal temperature on the diffusion rate of carbon atoms and the strengthening effect of lowered isothermal temperature on supercooled austenite.PM and its adjacent bainitic laths have nearly the same crystallographic orientation and belong to the same block.The pre-formation of martensite largely refines the bainitic blocks/laths and retained austenite.The specimens with PM show relatively uniform strain partitioning among various phases,contrasting with the specimens without PM,for which strains are highly concentrated in the bainite region nearby fresh martensite/austenite(M/A)blocks or between adjacent M/A blocks.The impact absorption energies of the specimens with PM,when austempered at 30-60℃ below Ms,are more than twice higher than those of the specimens without PM,at no expense of tensile properties.
基金Item Sponsored by National Natural Science Foundation of China(51271013)Key Project of Chinese Ministry of Railways(2012G011-C)Central College Basic Scientific Research Business Expenses Special Funds of China(2011JBZ021)
文摘Quenching partitioning tempering （QP-T） process were applied to a quenched carbide-free bainite/mar- tensite （CFB/M） multiphase steel 40Mn2Si2Cr and its effect on the mechanical properties was studied. The results showed that the partitioning time of Q P T treatment can he increased greatly to 90 min or even longer. The tensile strength, elongation and product of strength and elongation （PSE） of the experimental steel can achieve 1650 MPa, about 20% and over 33 GPa ·%, respectively. The significant increase of partitioning time and excellent mechanical properties could be related to the CFB microstrueture introduced in the quenching process during the Q-P-T treat- ment.
基金financially supported by the National Natural Science Foundation of China(No.51274154)the National High-Tech Research and Development Program of China(No.2012AA03A504)+1 种基金the State Key Laboratory of Development and Application Technology of Automotive Steels(Baosteel Group)the Key Project of Hubei Education Committee,China(No.20121101)
文摘The growth rates of bainite plates in an Fe-C-Mn-Si superbainitie steel were investigated by in situ observation. The lengthening rates of ferrite bainite during both cooling and isothermal holding processes were observed and the growth rates of bainite plates nucleating at grain boundaries, within grains and on preformed bainite were measured. It is indicated that the lengthening rates of bainite plates during the cooling and isothermal processes were different, and that the growth rates of bainite plates nucleating at different types of sites also demon- strated diversity. The bainite plates initiating at [vain boundaries during cooling grew the fastest, while the plates nucleating on preformed bainite did the slowest. However, the growth rate of the bainite plates nucleating at grain boundaries during isothermal transformation de- creased the most, whereas the bainite plates initiating within grains grew the fastest. In addition, the growth rate of ferrite bainite in the study supported the diffusion transformation mechanism of bainite from the viewooint of ~rowth rate.
文摘A dilatometer was used to study the kinetics of bainite-to-austenite transformation in low carbon microalloyed steel with the initial microstructure of bainite during the continuous reheating process. The bainite-to-austenite trans- formation was observed to take place in two steps at low heating rate. The first step is the dissolution of bainite, and the second one is the remaining bainite-to-austenite transformation controlled by a dissolution process. The calculation result of the kinetics of austenite formation shows that the two steps occur by diffusion at low heating rate. However, at high heating rate the bainite-to-austenite transformation occurs in a single step, and the process is mainly dominated by shear. The growth rate of austenite reaches the maximum at about 835℃ at different heating rates and the growth rate of austenite as a function of temperature increases with the increase in heating rate.
文摘Hot deformation experiments were carried out on Gleeble 1500 thermo-mechanical simulator. The bainite transformation after deformation was investigated by optical microstructure analysis. The results indicated that the deformation accelerated the bainite transformation when the deformation was carried out at high temperature and no or little ferrite was precipitated before bainite transformation; when the deformation was carried out at low temperature, the deformation hindered the bainite transformation because a lot of ferrite precipitated before bainite transformation.
基金supported by the National Key Fundamental Research and Development Program of China (No.2004CB619105)
文摘The ultra-high cycle fatigue behavior of a novel high strength steel with carbide-free bainite/martensite （CFB/M） complex microstructure was studied. The ultra-high cycle fatigue properties were measured by ultrasonic fatigue testing equipment at a frequency of 20 kHz. It is found that there is no horizontal part in the S-N curve and fatigue fracture occurs when the life of specimens exceeds 10^7 cycles. In addition, the origination of fatigue cracks tends to transfer from the surface to interior of specimens as the fatigue cycle exceeds 10^7, and the fatigue crack originations of many specimens are not induced by inclusions, but by some kind of ＂soft structure＂. It is shown that the studied high strength steel performs good ultra-high cycle fatigue properties. The ultra-high fatigue mechanism was discussed and it is suggested that specific CFB/M complex microstructure of the studied steel contributes to its superior properties.
基金Funding from the Finnish Funding Agency for Technology and Innovation(Tekes) is gratefully acknowledged(QaMiS project,DNo 1691/31/07No.40197/07)
文摘The crystallography of bainite, transformed isothermally at 450 ℃ in 0.2C-2.0Mn-1.5Si-0.6Cr steel, was investigated by electron backscatter diffraction （EBSD） analysis. The orientation relationship （OR） was found to be closer to Nishiyama-Wassermann （N-W） than Kurdjumov-Sachs orientation relationship. Bainite microstructure consisted of parallel laths forming a morphological packet structure. Typically, there were three different lath orientations in a morphological packet. These orientations were dictated by a three specific N-W OR variants sharing the same {111} austenite plane. A packet of bainite laths with common {111} austenite plane was termed as crystallographic packet. Generally, the crystallographic packet size corresponded to the morphological packet size. Locally, crystallographic packets with only two dominant orientations were observed. This indicates strong local variant selection during isothermal bainite transformation. The relative orientation between the variants in crystallographic packets was found to be near 60°/〈110〉. This appears to explain the strong peak observed in the grain boundary misorientation distribution near 60°. Bainite also contained pronounced fraction of boundaries with their misorientation in the range of 2.5°-8° with quite widely dispersed rotation angles. Spatially these boundaries were found to locate inside the bainite laths, forming lath-like sub-grains.
文摘The effect of bainite transformation and microstructure on the mechanical properties in 0.3%C bainitic steel was investigated via the heat treatment process of quenching at higher initial temperature and partitioning below martensite-start temperature. The results show that bainite transformation takes place with the partitioning time increasing during partitioning below martensite-start temperature. The microstructure of samples treated by this two-step quenching and partitioning process consists of lath bainite, lath martensite and retained austenite. This kind of multiphase microstructure exhibits better strength of 1420 MPa, ductility of 21.8 % and the product of strength and elongation of 30.8 GPa%. Furthermore, the excellent impact toughness of 103 J is exhibited by partitioning at 280 ℃ for 3 h. In addition, the coalescence of bainite platelets was found in the sample treated by partitioning for 8 h, leading to the deterioration of toughness.
文摘The quantitative effects of chromium content on continuous cooling transformation （CCT） diagrams of novel air-cooled bainite steels were analyzed using artificial neural network models. The results showed that the chromium may retard the high and medium-temperature martensite transformation.
基金The authors gratefully acknowledge the financial supports from National Natural Science Foundation of China (Nos. 51274154 and 51704217)Major Projects of Technological Innovation in Hubei (No. 2017AAA116)the Special Fund of Wuhan University of Science and Technology for Master Students' Short-Term Studying Abroad, and the State Scholarship Fund of China Scholarship Council.
文摘The effects of aluminum addition on bainite transformation and properties of carbide-free bainitic steels containing 0.22 wt.% carbon were investigated by two different types of heat treatment processes: continuous cooling process (CCP) and isothermal transformation process (ITP). The results indicate that for the CCP treatment, Al addition significantly promoted the ferrite and bainite transformation;however, it did not significantly increase the product of tensile strength and total elongation (PSE). For the ITP treatment, Al addition significantly promoted the kinetics of bainite transformation, and thus, more bainite was formed with Al addition;however, it was found that Al addition resulted in a decrease in tensile strength and an increase in elongation of the tested bainitic steels. Moreover, the effects of Al addition on comprehensive property were profoundly dependent on austempering temperatures. When the austempering temperature was higher (430℃), PSE significantly increased with Al addition, whereas it decreased at the lower austempering temperature (400℃). Therefore, it can be concluded that the effects of Al on properties of bainitic steels were more significant at higher austempering temperatures.
