Effects of annealing time on microstructure of cold-roiled niobium-titanium bearing micro-alloyed steel strips were investigated by optical microscopy, scanning electron microscopy, electron back-scatter diffraction ...Effects of annealing time on microstructure of cold-roiled niobium-titanium bearing micro-alloyed steel strips were investigated by optical microscopy, scanning electron microscopy, electron back-scatter diffraction （EBSD） and transmission electron microscopy. The complete recrystallization annealing temperature of 670 ℃ and complete annealing time of 9 min were determined using Vickers-hardness testing and EBSD analysis. The ferrite mi-crostructure with spheric cementite particles and nano-scale precipitates of Nb（C,N） in matrix was obtained. The ki-netics of the ferrite grain growth is lowered due to ferrite grain boundaries pinned by the cementite particles, so the ferrite grain size of 5.5 μm remains unchanged among the annealing time ranging from 9 to 30 min. In addition, the strength of tested steel also keeps unchanged with the increase of annealing time. The higher yield strength of ap-proximately 420 MPa can be obtained by grain refinement and precipitation hardening and the higher elongation of approximately 40% and work-hardening exponent of approximately 0.2 can be gained due to grain refinement and presence of cementite particles, indicating that the balance of strength, ductility and forming property is realized.展开更多
The dynamic recrystallization （DRX） behavior of Nb-Ti microalloyed steels was investigated by isothermal single compression tests in the temperature range of 900-1 150 ℃ at constant strain rates of 0.1-5 s^-1. DRX ...The dynamic recrystallization （DRX） behavior of Nb-Ti microalloyed steels was investigated by isothermal single compression tests in the temperature range of 900-1 150 ℃ at constant strain rates of 0.1-5 s^-1. DRX was retarded effectively at low temperature due to the onset of dynamic precipitation of Nb and Ti carbonitrides, resulting in higher values of the peak strain. An expression was developed for the activation energy of deformation as a function of the contents of Nb and Ti in solution as well as other alloying elements. A new value of corrective factor was determined and applied to quantify the retardation produced by increase in the amount of Nb and Ti dissolved at the reheating temperature. The ratio of critical strain to peak strain decreases with increasing equivalent Nb content. In addition, the effects of Ti content and deformation conditions on DRX kinetics and steady state grain size were determined. Finally, the kinetics of dynamic precipitation was determined and effect of dynamic precipitation on the onset of DRX was clarified based on the comparison between precipitate pinning force and recrystallization driving force.展开更多
The nanometer-size carbides formed in ferrite matrix of Nb-Ti microalloyed steel at different finishing cooling temperatures and holding time have been investigated. The characteristics of nanometer-size carbides in f...The nanometer-size carbides formed in ferrite matrix of Nb-Ti microalloyed steel at different finishing cooling temperatures and holding time have been investigated. The characteristics of nanometer-size carbides in ferrite were observed by transmission electron microscopy, and mechanical properties of ferrite were detected by a nano-hardness tester. The results showed that interphase precipitation and diffusion precipitation were observed at different finishing cooling temperatures, and the interphase precipitation was planar and curved. Sheet spacing of inter-phase precipitation increased with the increase of finishing cooling temperature and changed a little when holding for 50--1000 s. Interphase precipitation shows higher nano-hardness at 640℃ compared with diffusion precipitation at 600℃, and the contribution of interphase precipitation to the mechanical properties of ferrite was larger than that of diffusion precipitation.展开更多
Nb-Tihot-rolled TRIP-assisted steel with high plasticity and appropriate volume percentage of retained austenite based on fine ferrite grain have been developed in the experiment. The test results showed that niobium ...Nb-Tihot-rolled TRIP-assisted steel with high plasticity and appropriate volume percentage of retained austenite based on fine ferrite grain have been developed in the experiment. The test results showed that niobium tend to exist in solution state in matrix with less precipitation, and niobium-titanium could be precipitated in form of （Nb, Ti）C or （Nb, Ti） （C, N）, which play an important role in increasing yield strength （from 495 MPa to 610 MPa）. Besides, the retained austenite had a positive effect on improving the plasticity by transformation into martensite during tensile deformation.展开更多
The low carbon Nb-Ti mieroalloyed tested steel was prepared by the process of vacuum induction furnace smelting, forging and hot rolling. The new steel aims to meet the demand of high strength, high toughness and high...The low carbon Nb-Ti mieroalloyed tested steel was prepared by the process of vacuum induction furnace smelting, forging and hot rolling. The new steel aims to meet the demand of high strength, high toughness and high plasticity for building facilities. The effects of quenching process on microstructure and mechanical properties of tested steel were investigated. The results showed that prior austenite grain size, phase type and precipitation behavior of （ Nb, Ti） （ C, N） play important roles in mechanical properties of the steel. Through modified appropriately, the model of austenite grain growth during heating and holding is d^5.7778 = 5. 6478^5.7778 ＋ 7.04 × 10^22t^1.6136 exp（- 427. 15 ×10^3 /（RT））. The grain growth activation energy is Qg = 427. 15 kJ. During quenching, the microscopic structures are mainly martensite and lath bainite which contains lots of lath substructure and dislocations. The content of phases, fine and coarsening （ Nb, Ti ） （ C, N ） precipitated changes during different quenching temperatures and holding time. Finally compared with the hardness value, the best quenching process can be obtained that heating temperature and holding time are 900 ℃ and 50 mins, respectively.展开更多
The mechanical peratures and cooling rates properties and microstructure of as-cast Nb-Ti microalloyed steel at different tern- are investigated in this paper. The III brittle zone （700--900~C） is revealed. The redu...The mechanical peratures and cooling rates properties and microstructure of as-cast Nb-Ti microalloyed steel at different tern- are investigated in this paper. The III brittle zone （700--900~C） is revealed. The reduction of the hot ductility is due to the film-like proeutectoid ferrite or the Nb（C, N） precipitates along the austenitic grain boundaries. In the temperature range of 850 1000~C, with the increase of the cooling rate, the hot ductility decreases. However, in the range of 650--850 ~C, the appearance of large volume fractions of ferrite on austenite grain boundaries minimizes the effect of cooling rate on hot ductility. When the cooling rate is 10 ℃/s, austenite transforms more quickly to ferrite and at a lower temperature a larger amount of ferrite nucleates and precipitates in the grain, which leads to a sharper improvement in the hot ductility at 650 ℃.展开更多
The hot rolling experiment investigates into the relationship between the microstructures and the mechanical properties of Nb-Ti microalloyed steels with various Ti contents. The results indicate that the effect of th...The hot rolling experiment investigates into the relationship between the microstructures and the mechanical properties of Nb-Ti microalloyed steels with various Ti contents. The results indicate that the effect of the bainite fraction of Nb-Ti microalloyed steels on the yield strength of the steels is not related to Ti content, while the slope of the Hall- Petch relationship decreases with the increase of Ti content. Accordingly, the Misra model for the yield strength of Nb-Ti microalloyed steels is modified, and the factors which cause the change in the slope of Hall-Petch relationship are discussed.展开更多
Nb/Ti multilayers with different modulation periods were prepared by magnetron sputtering deposition. Microstructure and mechanical properties were investigated by XRD, SEM and Nanoindentation. It turns out that hardn...Nb/Ti multilayers with different modulation periods were prepared by magnetron sputtering deposition. Microstructure and mechanical properties were investigated by XRD, SEM and Nanoindentation. It turns out that hardness of samples increases with decreasing modulation wavelength （A） and then dropped at small A. Through scrutinizing other results in literature, we found that the coherent stress rather than modulus mismatch played more important role for hardness enhancement with de- creasing A of the bcc-hcp multilayers. Annealing of samples in low （or high） vacuum at 400 ~C for 30 min led to large （or medium） enhancement of hardness, while the modulated structure was still maintained. It was found that annealing in low vac- uum resulted in oxygen penetration into the multilayers as revealed by auger electron spectroscopy, but hardly changed metallic sheet resistivity. XRD results suggested that some niobium oxides were formed in Nb layers and oxygen distributed intersti- tially in Ti layers. The large hardness enhancement after annealing is mainly due to the strengthening effect from dispersive distribution of nano-scale niobium oxides and interstitial oxygen in the multilayers. In addition, interfaces between adjacent layers were more distinct after annealing which indicated good thermal stability of laminated structure.展开更多
In situ measurements of austenite grain growth were made at various temperatures in the range of 1273-1473 K with subsequent isothermal holding time of 3600 s for the Nb-Ti-bearing and Nb-Ti-free high carb on steel by...In situ measurements of austenite grain growth were made at various temperatures in the range of 1273-1473 K with subsequent isothermal holding time of 3600 s for the Nb-Ti-bearing and Nb-Ti-free high carb on steel by using a confocal laser scanning microscope.The solid solute behavior of Nb-Ti carbides during austenitizing process was analyzed.The experimental results indicate that the austenite grains of both steels grow up gradually with increasing the heating temperature and holding time;the size and growth rate of austenite grain of Nb-Ti-bearing high carbon steel are much lower than those of Nb-Ti-free high carbon steel.A large amount of(Nb,Ti)(C,N)nanoparticles are observed in Nb-Tibearing steel,which retain the strong pinning effect on austenite grain boundary.The kinetics model of austenite grain growth of Nb-Ti-bearing steel during isothermal heat treatment is obtained and the predicted values calculated by using the model meet the experimental values very well.展开更多
The influence of Ti and Nb on the microstructure,mechanical properties,and second-phase precipitation of 430 ferritic stainless steel was investigated.In addition to optical microscopy,transmission electron microscopy...The influence of Ti and Nb on the microstructure,mechanical properties,and second-phase precipitation of 430 ferritic stainless steel was investigated.In addition to optical microscopy,transmission electron microscopy and X-ray diffraction analyses,tensile tests,and carbonitride extraction experiments were conducted to investigate the microscopic mechanisms.The results showed that the primary precipitates in SUS 430 ferritic stainless steel were Cr_（23）C_6,Mn_（23）C_6,and Cr_7C_3,and the primary strengthening mechanism was precipitation strengthening.When Ti was added separately,the main precipitates were TiC and TiN.However,coarse TiC adversely affected the mechanical properties of steel.When double-stabilized with Ti and Nb,coarse TiC was replaced by fine NbC.The type of precipitation was altered,and precipitation and solid solution strengthening occurred.Therefore,the tensile strength and plastic strain ratio（r-value） improved to 433.60 MPa and 1.37,respectively.展开更多
Hot ductility of the Nb- and Ti-containing line-pipe steel CC slab specimens were measured under the sirain rate of 1 x 10-3/s. Three types of precipitates were found in the fractured specimens. One was the block-shap...Hot ductility of the Nb- and Ti-containing line-pipe steel CC slab specimens were measured under the sirain rate of 1 x 10-3/s. Three types of precipitates were found in the fractured specimens. One was the block-shaped coarse TiN particles precipitated at high temperature. Another type was the fine dynamic precipitation products precipitated at 950～900℃ which caused remarkable ductility reduction of the steel. The third type was the co-existed precipitates formed by fine Nb precipitates nucleating and growing on TiN paricles. Compared with Nb-containing steel which contains no Ti, there was no ductility drop for Nb- and Ti-containing steel at temperature between 850℃ and Ar3 and, the γ→α transformation inside the grain matrixes proceeded faster, which both improved the ductility of the steel in the low ductility temperature Region Ⅲ.展开更多
The Nb-Ti microalloyed high strength IF steel sheet was used to study the effect of annealing temperature on the microstructures,mechanical properties and textures.The experimental results show that experimental steel...The Nb-Ti microalloyed high strength IF steel sheet was used to study the effect of annealing temperature on the microstructures,mechanical properties and textures.The experimental results show that experimental steel is incomplete recrystallization at 750℃ annealing,but complete recrystallization from 780℃ to 870℃ under experimental conditions.When the annealing temperature was increased,the yield strength and tensile strength would gradually reduce,the plastic strain ratio and yield point elongation would gradually increase.The yield strength,tensile strength,elongation,the plastic strain ratio and the strain hardening exponent were approximate 300MPa,410MPa,36.5%,1.7 and 0.22 respectively under annealing temperature 810℃ to 840℃.When the annealing temperature was increased,the α-textures and γ-textures were gradually weakened,and the α-textures have a trend to {111} texture.Therefore,the suggestion of the optimal recrystallization annealing temperature is about 810℃ to 840℃ in industrial production.展开更多
Influence of different cooling rates on the microstructure and the precipitation behavior of Nb-Ti microalloyed steel was investigated by CSLM, OM, SEM and EDS. The results show that the precipitation process of carbo...Influence of different cooling rates on the microstructure and the precipitation behavior of Nb-Ti microalloyed steel was investigated by CSLM, OM, SEM and EDS. The results show that the precipitation process of carbonitrides can be in-situ observed by CSLM, and with the increase of the cooling rate, the distribution of precipitates changes from along the austenitic grain boundaries to within the grains. With the increase of the cooling rate, the proeutectoid ferritic film becomes smaller and smaller and then disappears, and the original austenitic grains become finer and finer. In order to obtain non-film like proeutectoid ferrites or non-chain like precipitates along the austenitic grain boundaries and finer austenitic grains,the cooling rate should be at least 5℃/s.展开更多
基金Sponsored by Fundamental Research Funds for Central Universities of China(N110607003)
文摘Effects of annealing time on microstructure of cold-roiled niobium-titanium bearing micro-alloyed steel strips were investigated by optical microscopy, scanning electron microscopy, electron back-scatter diffraction （EBSD） and transmission electron microscopy. The complete recrystallization annealing temperature of 670 ℃ and complete annealing time of 9 min were determined using Vickers-hardness testing and EBSD analysis. The ferrite mi-crostructure with spheric cementite particles and nano-scale precipitates of Nb（C,N） in matrix was obtained. The ki-netics of the ferrite grain growth is lowered due to ferrite grain boundaries pinned by the cementite particles, so the ferrite grain size of 5.5 μm remains unchanged among the annealing time ranging from 9 to 30 min. In addition, the strength of tested steel also keeps unchanged with the increase of annealing time. The higher yield strength of ap-proximately 420 MPa can be obtained by grain refinement and precipitation hardening and the higher elongation of approximately 40% and work-hardening exponent of approximately 0.2 can be gained due to grain refinement and presence of cementite particles, indicating that the balance of strength, ductility and forming property is realized.
基金the National Natural Science Foundation of China(No.50474086,50334010)the Program for New Century Excellent Talents in Universities(No.NCET-04-0278)of the Ministry of Education
文摘The dynamic recrystallization （DRX） behavior of Nb-Ti microalloyed steels was investigated by isothermal single compression tests in the temperature range of 900-1 150 ℃ at constant strain rates of 0.1-5 s^-1. DRX was retarded effectively at low temperature due to the onset of dynamic precipitation of Nb and Ti carbonitrides, resulting in higher values of the peak strain. An expression was developed for the activation energy of deformation as a function of the contents of Nb and Ti in solution as well as other alloying elements. A new value of corrective factor was determined and applied to quantify the retardation produced by increase in the amount of Nb and Ti dissolved at the reheating temperature. The ratio of critical strain to peak strain decreases with increasing equivalent Nb content. In addition, the effects of Ti content and deformation conditions on DRX kinetics and steady state grain size were determined. Finally, the kinetics of dynamic precipitation was determined and effect of dynamic precipitation on the onset of DRX was clarified based on the comparison between precipitate pinning force and recrystallization driving force.
基金Item Sponsored by National Natural Science Foundation of China(51104046)Fundamental Research Funds for Central Universities of China(N120407001,N120807001)
文摘The nanometer-size carbides formed in ferrite matrix of Nb-Ti microalloyed steel at different finishing cooling temperatures and holding time have been investigated. The characteristics of nanometer-size carbides in ferrite were observed by transmission electron microscopy, and mechanical properties of ferrite were detected by a nano-hardness tester. The results showed that interphase precipitation and diffusion precipitation were observed at different finishing cooling temperatures, and the interphase precipitation was planar and curved. Sheet spacing of inter-phase precipitation increased with the increase of finishing cooling temperature and changed a little when holding for 50--1000 s. Interphase precipitation shows higher nano-hardness at 640℃ compared with diffusion precipitation at 600℃, and the contribution of interphase precipitation to the mechanical properties of ferrite was larger than that of diffusion precipitation.
文摘Nb-Tihot-rolled TRIP-assisted steel with high plasticity and appropriate volume percentage of retained austenite based on fine ferrite grain have been developed in the experiment. The test results showed that niobium tend to exist in solution state in matrix with less precipitation, and niobium-titanium could be precipitated in form of （Nb, Ti）C or （Nb, Ti） （C, N）, which play an important role in increasing yield strength （from 495 MPa to 610 MPa）. Besides, the retained austenite had a positive effect on improving the plasticity by transformation into martensite during tensile deformation.
基金Sponsored by the Major State Basic Research Development Program of China(Grant No.2010CB630801)
文摘The low carbon Nb-Ti mieroalloyed tested steel was prepared by the process of vacuum induction furnace smelting, forging and hot rolling. The new steel aims to meet the demand of high strength, high toughness and high plasticity for building facilities. The effects of quenching process on microstructure and mechanical properties of tested steel were investigated. The results showed that prior austenite grain size, phase type and precipitation behavior of （ Nb, Ti） （ C, N） play important roles in mechanical properties of the steel. Through modified appropriately, the model of austenite grain growth during heating and holding is d^5.7778 = 5. 6478^5.7778 ＋ 7.04 × 10^22t^1.6136 exp（- 427. 15 ×10^3 /（RT））. The grain growth activation energy is Qg = 427. 15 kJ. During quenching, the microscopic structures are mainly martensite and lath bainite which contains lots of lath substructure and dislocations. The content of phases, fine and coarsening （ Nb, Ti ） （ C, N ） precipitated changes during different quenching temperatures and holding time. Finally compared with the hardness value, the best quenching process can be obtained that heating temperature and holding time are 900 ℃ and 50 mins, respectively.
文摘The mechanical peratures and cooling rates properties and microstructure of as-cast Nb-Ti microalloyed steel at different tern- are investigated in this paper. The III brittle zone （700--900~C） is revealed. The reduction of the hot ductility is due to the film-like proeutectoid ferrite or the Nb（C, N） precipitates along the austenitic grain boundaries. In the temperature range of 850 1000~C, with the increase of the cooling rate, the hot ductility decreases. However, in the range of 650--850 ~C, the appearance of large volume fractions of ferrite on austenite grain boundaries minimizes the effect of cooling rate on hot ductility. When the cooling rate is 10 ℃/s, austenite transforms more quickly to ferrite and at a lower temperature a larger amount of ferrite nucleates and precipitates in the grain, which leads to a sharper improvement in the hot ductility at 650 ℃.
文摘The hot rolling experiment investigates into the relationship between the microstructures and the mechanical properties of Nb-Ti microalloyed steels with various Ti contents. The results indicate that the effect of the bainite fraction of Nb-Ti microalloyed steels on the yield strength of the steels is not related to Ti content, while the slope of the Hall- Petch relationship decreases with the increase of Ti content. Accordingly, the Misra model for the yield strength of Nb-Ti microalloyed steels is modified, and the factors which cause the change in the slope of Hall-Petch relationship are discussed.
基金supported by National Natural Science Foundation (Grant No.50871057)National High Technical Research and Development Programme of China (Grant No.2009AA034001)
文摘Nb/Ti multilayers with different modulation periods were prepared by magnetron sputtering deposition. Microstructure and mechanical properties were investigated by XRD, SEM and Nanoindentation. It turns out that hardness of samples increases with decreasing modulation wavelength （A） and then dropped at small A. Through scrutinizing other results in literature, we found that the coherent stress rather than modulus mismatch played more important role for hardness enhancement with de- creasing A of the bcc-hcp multilayers. Annealing of samples in low （or high） vacuum at 400 ~C for 30 min led to large （or medium） enhancement of hardness, while the modulated structure was still maintained. It was found that annealing in low vac- uum resulted in oxygen penetration into the multilayers as revealed by auger electron spectroscopy, but hardly changed metallic sheet resistivity. XRD results suggested that some niobium oxides were formed in Nb layers and oxygen distributed intersti- tially in Ti layers. The large hardness enhancement after annealing is mainly due to the strengthening effect from dispersive distribution of nano-scale niobium oxides and interstitial oxygen in the multilayers. In addition, interfaces between adjacent layers were more distinct after annealing which indicated good thermal stability of laminated structure.
文摘In situ measurements of austenite grain growth were made at various temperatures in the range of 1273-1473 K with subsequent isothermal holding time of 3600 s for the Nb-Ti-bearing and Nb-Ti-free high carb on steel by using a confocal laser scanning microscope.The solid solute behavior of Nb-Ti carbides during austenitizing process was analyzed.The experimental results indicate that the austenite grains of both steels grow up gradually with increasing the heating temperature and holding time;the size and growth rate of austenite grain of Nb-Ti-bearing high carbon steel are much lower than those of Nb-Ti-free high carbon steel.A large amount of(Nb,Ti)(C,N)nanoparticles are observed in Nb-Tibearing steel,which retain the strong pinning effect on austenite grain boundary.The kinetics model of austenite grain growth of Nb-Ti-bearing steel during isothermal heat treatment is obtained and the predicted values calculated by using the model meet the experimental values very well.
文摘The influence of Ti and Nb on the microstructure,mechanical properties,and second-phase precipitation of 430 ferritic stainless steel was investigated.In addition to optical microscopy,transmission electron microscopy and X-ray diffraction analyses,tensile tests,and carbonitride extraction experiments were conducted to investigate the microscopic mechanisms.The results showed that the primary precipitates in SUS 430 ferritic stainless steel were Cr_（23）C_6,Mn_（23）C_6,and Cr_7C_3,and the primary strengthening mechanism was precipitation strengthening.When Ti was added separately,the main precipitates were TiC and TiN.However,coarse TiC adversely affected the mechanical properties of steel.When double-stabilized with Ti and Nb,coarse TiC was replaced by fine NbC.The type of precipitation was altered,and precipitation and solid solution strengthening occurred.Therefore,the tensile strength and plastic strain ratio（r-value） improved to 433.60 MPa and 1.37,respectively.
文摘Hot ductility of the Nb- and Ti-containing line-pipe steel CC slab specimens were measured under the sirain rate of 1 x 10-3/s. Three types of precipitates were found in the fractured specimens. One was the block-shaped coarse TiN particles precipitated at high temperature. Another type was the fine dynamic precipitation products precipitated at 950～900℃ which caused remarkable ductility reduction of the steel. The third type was the co-existed precipitates formed by fine Nb precipitates nucleating and growing on TiN paricles. Compared with Nb-containing steel which contains no Ti, there was no ductility drop for Nb- and Ti-containing steel at temperature between 850℃ and Ar3 and, the γ→α transformation inside the grain matrixes proceeded faster, which both improved the ductility of the steel in the low ductility temperature Region Ⅲ.
文摘The Nb-Ti microalloyed high strength IF steel sheet was used to study the effect of annealing temperature on the microstructures,mechanical properties and textures.The experimental results show that experimental steel is incomplete recrystallization at 750℃ annealing,but complete recrystallization from 780℃ to 870℃ under experimental conditions.When the annealing temperature was increased,the yield strength and tensile strength would gradually reduce,the plastic strain ratio and yield point elongation would gradually increase.The yield strength,tensile strength,elongation,the plastic strain ratio and the strain hardening exponent were approximate 300MPa,410MPa,36.5%,1.7 and 0.22 respectively under annealing temperature 810℃ to 840℃.When the annealing temperature was increased,the α-textures and γ-textures were gradually weakened,and the α-textures have a trend to {111} texture.Therefore,the suggestion of the optimal recrystallization annealing temperature is about 810℃ to 840℃ in industrial production.
文摘Influence of different cooling rates on the microstructure and the precipitation behavior of Nb-Ti microalloyed steel was investigated by CSLM, OM, SEM and EDS. The results show that the precipitation process of carbonitrides can be in-situ observed by CSLM, and with the increase of the cooling rate, the distribution of precipitates changes from along the austenitic grain boundaries to within the grains. With the increase of the cooling rate, the proeutectoid ferritic film becomes smaller and smaller and then disappears, and the original austenitic grains become finer and finer. In order to obtain non-film like proeutectoid ferrites or non-chain like precipitates along the austenitic grain boundaries and finer austenitic grains,the cooling rate should be at least 5℃/s.
