Based on thermodynamics and kinetics, a new mathematical model was developed to calculate the CCT diagrams and the transformation kinetics in low carbon niobium steels, in which the effect of deformation on the degree...Based on thermodynamics and kinetics, a new mathematical model was developed to calculate the CCT diagrams and the transformation kinetics in low carbon niobium steels, in which the effect of deformation on the degree of supercooling was taken into account. The undercooling caused by deformation is the major reason for the increase of the starting transition temperature during continuous cooling. The critical cooling rate of bainite formation is within 2--5 ℃s for the studied niobium steels and deformation is suitable for the occurrence of pearlite. The ferrite volume fraction increases with the increase of the austenite boundary area, and decreases with the increase of the cooling rate. The calculated CCT diagrams and the volume fraction of each phase are in good agreement with the measurements.展开更多
Using Gleeble-1500 system, the influence of holding time on bainite transformation in deformed niobium microalloyed steel during continuous cooling was analyzed, and the carbides in upper bainite were also systematica...Using Gleeble-1500 system, the influence of holding time on bainite transformation in deformed niobium microalloyed steel during continuous cooling was analyzed, and the carbides in upper bainite were also systematically researched. The results show that the occurrence of the static recrystallization decreases the amount of bainite with an increase in the holding time and the emergence of retained austenite (RA) with the longer holding time. Two types of carbides were observed in upper bainite with regard to their precipitation sites. They either existed between the bainite ferrite laths or co-existed with RA. The formation mechanism of two kinds of carbides was analyzed by combining TEM micrographs with the model.展开更多
Cracking in a laser weld of niobium to stainless steel occurred due to the formation of brittle,continuously distributed Nb-Fe intermetallic compounds.A crack-free joint,which had a tensile strength of 147 MPa,was obt...Cracking in a laser weld of niobium to stainless steel occurred due to the formation of brittle,continuously distributed Nb-Fe intermetallic compounds.A crack-free joint,which had a tensile strength of 147 MPa,was obtained by using the W/Cu composite filler metals.To determine the reasons for cracking in the Nb/SS joint and the function of the W/Cu composite filler metals on the improvement of the cracking resistance of the Nb/W/Cu/SS joint,the microstructures of the joints were studied by optical microscopy,scanning electron microscopy and X-ray diffraction.The cracking susceptibilities of the joints were evaluated with microhardness test on the cross section of the Nb/W/Cu/SS joint.The results showed that the Nb/W/Cu/SS joint was characterized by various solid solution.The formation of solid solution reduced the cracking susceptibility of the joint.展开更多
In order to enhance the corrosion resistance of 304 stainless steel, niobium was electrodeposited on its surface in air- and water-stable ionic liquids. The electrochemical behaviors of bare and niobium-coated 304 sta...In order to enhance the corrosion resistance of 304 stainless steel, niobium was electrodeposited on its surface in air- and water-stable ionic liquids. The electrochemical behaviors of bare and niobium-coated 304 stainless steel were evaluated by electrochemical tests in a simulated proton exchange membrane fuel cell (PEMFC) environment. The results showed that niobium could be electrodeposited on the surface of 304 stainless steel from ionic liquids, and a smooth and strong chemical inert compound film was obtained on the surface of 304 stainless steel, which was mainly composed of NbO and Nb2Os. The thin composite film acted as a barrier and remarkably improved the corrosion resistance of 304 stainless steel in the PEMFC environment.展开更多
AISI H13 hot work tool steel is widely used for hot forging, hot-extrusion and die-casting because of its high temperature strength, impact toughness, heat checking resistance and wear resistance, etc. The thermally i...AISI H13 hot work tool steel is widely used for hot forging, hot-extrusion and die-casting because of its high temperature strength, impact toughness, heat checking resistance and wear resistance, etc. The thermally induced surface damage, i. e., thermal fatigne,is believed to be controlled by the magnitude of the imposed cyclic strain. The thermal fatigue on the surface of hot working die, which is responsible to the initiation of the cracks, is reported to result in more than 80 % of the failure of dies.展开更多
Niobium was electrodeposited on 316 stainless steel bipolar plates of a fuel cell in water and air-stable choline chloride based ionic liquids. The electrochemical corruption property of bipolar plates in simulated PE...Niobium was electrodeposited on 316 stainless steel bipolar plates of a fuel cell in water and air-stable choline chloride based ionic liquids. The electrochemical corruption property of bipolar plates in simulated PEMFC environment was investigated. It was showed that the plating film was distributed on the surface of 316 stainless steel like isolated islands with height less than 50 nm. The XPS, XRD results showed that a smooth and strong chemical inert film of Nb O and Nb2O5 was formed on the surface of 316 stainless steel. In simulated cathodic condition, the corrosion potential of Nb coated stainless steel was improved by 244 m V, whilst in an anodic condition, it was improved by 105 m V. The current densities for the coated 316 stainless steel were decreased to 2.479 4 μA·cm-2 from 14.810 μA·cm-2 at-0.1 V and to 0.576 μA·cm-2 from 13.417 μA/·cm-2 at 0.6 V, respectively. It was implied that the niobium coating effectively decreased the corrosion rate. The results of the electrochemical tests indicated that the corrosion resistance of stainless steel was greatly improved after coated with niobium.展开更多
An experinientally quantitative inveetisation has been made on the precipitationand coarsening behaviors of niobium carbide in a high strength low alloy(HSLA) steel. The results suggest that the static coarsening proc...An experinientally quantitative inveetisation has been made on the precipitationand coarsening behaviors of niobium carbide in a high strength low alloy(HSLA) steel. The results suggest that the static coarsening processes duringisothermal reaction in (α+γ) two phase region and during isothermal stay aftercertain amount of deformation of austenite are principally controlled by thediffusion of niobium atoms along dislocation pipes, and the volume fraction ofthe precipitates exerts sisnificant influence on the coarsening processes.Whereas the dynamic coarsening process during hot deformation of austeniteappoars to be dominated by the carbide/matrix interphase reaction.展开更多
为研究Nb含量对焊接热影响区微观组织和性能的影响,采用熔化极气体保护焊(gas metal arc welding,GMAW)和手工焊条电弧焊(shielded metal arc welding,SMAW)对0.055%Nb和0.075%Nb含量的X80钢管进行环焊.采用夏比冲击试验和金相分析方法...为研究Nb含量对焊接热影响区微观组织和性能的影响,采用熔化极气体保护焊(gas metal arc welding,GMAW)和手工焊条电弧焊(shielded metal arc welding,SMAW)对0.055%Nb和0.075%Nb含量的X80钢管进行环焊.采用夏比冲击试验和金相分析方法,研究热影响区的微观组织差异和夏比冲击韧性.并借助扫描电镜和超高温激光共聚焦显微镜分析不同Nb含量X80管体的微观组织对热影响区性能的影响.结果表明,在0℃和-20℃时,0.075%Nb和0.055%Nb的X80钢管GMAW环焊接头热影响区均具有较高的冲击韧性,其平均冲击吸收能量均高于150 J.其中0.055%Nb略高于0.075%Nb的GMAW环焊接头热影响区夏比冲击吸收能量;焊接热输入较低时,0.055%Nb低于0.075%Nb的X80环焊接头粗晶区的韧脆转变温度,具有更好的低温韧性.焊接热输入较高时,0.075%Nb的X80环焊接头粗晶区具有更高的上平台冲击吸收能量,且上平台温度和韧脆转变温度也更低,其低温韧性也更优异;还发现了X80环焊接头热影响区的冲击韧性不仅与热输入量和热影响区马氏体-奥氏体组织(M-A)的形状、大小、分布有关,而且还受管体中Nb含量、原始的强度与韧性、微观组织状态的遗传影响.展开更多
The characteristics and generation mechanism of(Ti,Nb,V)(C,N) precipitates larger than 2 μm in Nb-containing H13 bar steel were studied. The results show that two types of(Ti,Nb,V)(C,N) phases exist—a Ti-V-r...The characteristics and generation mechanism of(Ti,Nb,V)(C,N) precipitates larger than 2 μm in Nb-containing H13 bar steel were studied. The results show that two types of(Ti,Nb,V)(C,N) phases exist—a Ti-V-rich one and an Nb-rich one—in the form of single or complex precipitates. The sizes of the single Ti-V-rich(Ti,Nb,V)(C,N) precipitates are mostly within 5 to 10 μm, whereas the sizes of the single Nb-rich precipitates are mostly 2–5 μm. The complex precipitates are larger and contain an inner Ti-V-rich layer and an outer Nb-rich layer. The compositional distribution of(Ti,Nb,V)(C,N) is concentrated. The average composition of the single Ti-V-rich phase is(Ti_(0.511)V_(0.356)Nb_(0.133))(CxNy), whereas that for the single Nb-rich phase is(Ti_(0.061)V_(0.263)Nb_(0.676))(C_xN_y). The calculation results based on the Scheil–Gulliver model in the Thermo-Calc software combining with the thermal stability experiments show that the large phases precipitate during the solidification process. With the development of solidification, the Ti-V-rich phase precipitates first and becomes homogeneous during the subsequent temperature reduction and heat treatment processes. The Nb-rich phase appears later.展开更多
The influence of microalloying additions on the mechanical properties of a low-carbon cast steel containing combinations of V, Nb, and Ti in the as-cast condition was evaluated. Tensile and hardness test results indic...The influence of microalloying additions on the mechanical properties of a low-carbon cast steel containing combinations of V, Nb, and Ti in the as-cast condition was evaluated. Tensile and hardness test results indicated that good combinations of strength and ductility could be achieved by V and Nb additions. While the yield strength and UTS (ultimate tensile strength) increased up to the range of 378-435 MPa and 579- 590 MPa, respectively in the microalloyed heats, their total elongation ranged from 18% to 23%. The presence of Ti, however, led to some reduction in the strength. Microstructural studies including scanning electron microscopy (SEM) and optical microscopy revealed that coarse TiN particles were responsible for this behavior. The Charpy impact values of all compositions indicated that microalloying additions significantly decreased the impact energy and led to the dominance of cleavage facets on the fracture surfaces. It seems that the increase in the hardness of coarse ferrite grains due to the precipitation hardening is the main reason for brittle fracture.展开更多
The dynamic recrystallization (DRX) behavior of high-grade X80/X100 pipeline steels with different Nb contents was investi- gated through single pass compression experiment using a Gleeble 1500 thermomechanical simu...The dynamic recrystallization (DRX) behavior of high-grade X80/X100 pipeline steels with different Nb contents was investi- gated through single pass compression experiment using a Gleeble 1500 thermomechanical simulator. By the regression of stress-sWain data obtained in the experiment, the deformation activation energy of DRX was identified, and the critical strain was calculated with the Po- liak-Jonas (P-J) method. Based on the analysis, the occurrence condition and kinetics of DRX were determined. The results show that as the Nb content increases from 0.08wt% to 0.095wt%, the activation energy of recrystallization raises from 365 to 395 kJ/mol. The critical swain of DRX can be determined more accurately by the P-J method, and the ratios of critical swain to peak sWain of XS0 and X100 pipeline steels are 0.51 and 0.49, respectively, which are similar to the results achieved by other researchers and calculated with empirical formulae.展开更多
The increasing speed of trains necessitates the development of brake-disc materials that meet more stringent requirements.Therefore,Nb and V have been added to Cr–Mo–V steel to improve its thermal fatigue performanc...The increasing speed of trains necessitates the development of brake-disc materials that meet more stringent requirements.Therefore,Nb and V have been added to Cr–Mo–V steel to improve its thermal fatigue performance when used in brake discs.In this paper,the influences of Nb and V on the static continuous cooling transformation(CCT)behaviors of undercooled austenite were studied.The microstructures,hardness,and dislocation densities at different cooling rates and with the addition of different alloying elements were also investigated.The results show that the transformation products of ferrite,granular bainite,lower bainite,and martensite form under different cooling conditions.With increasing Nb and V contents,the CCT curves are shifted to the left,ferrite and bainite transformations are promoted,and the critical cooling rate of total martensite formation is increased.The added V mainly forms V-rich M_8C_7 precipitates and reduces the dissolved C content;therefore,the A_(c1),A_(c3),and M_s-point temperatures increase.Moreover,the stability of retained austenite is also reduced;its content therefore decreases.Compared with V,the effect of added Nb is weaker because of its smaller content.However,the addition of Nb improves the hardness at lower cooling rates because of the precipitation of fine Nb C particles and refining of the microstructure.展开更多
Microalloying element Nb in low carbon steels produced by compact strip production (CSP) process plays an important role in inhibiting recrystallization, decreasing the transformation temperature and grain refinemen...Microalloying element Nb in low carbon steels produced by compact strip production (CSP) process plays an important role in inhibiting recrystallization, decreasing the transformation temperature and grain refinement.With decreasing the rolling temperature, dislocations can be pinned by carbonitrides and the strength is increased. Based on the two sublattice model, with metal atom sublattice and interstitial atom sublattice,a thermodynamic model for carbonitride was established to calculate the equilibrium between matrix and carbonitride. In the steel produced by CSP, the calculation results showed that the starting temperature of precipitation of Ti and Nb are 1340℃ and 1040℃, respectively. In the range of 890-950℃, Nb rapidly precipitated. And the maximum of the atomic fraction of Nb in carbonitride was about 0.68. The morphologies and energy spectrum of the precipitates showed that (NbTi) (CN) precipitated near the dislocations. The experiment results show that Nb rapidly precipitated when the temperature was lower than 970℃, and the atomic fraction of Nb in carbonitride was about 60%-80%. The calculation results are in agreement with the experiment data. Therefore the thermodynamic model can be a useful assistant tool in the research on the precipitates in the low carbon steels produced by CSP.展开更多
Four Nb-Ti microalloyed steels were refined and rolled to study the composition optimization of Nb-Ti microalloyed steels. The effects of Nb and Ti on the microstructures, precipitates and properties of Nb-Ti microall...Four Nb-Ti microalloyed steels were refined and rolled to study the composition optimization of Nb-Ti microalloyed steels. The effects of Nb and Ti on the microstructures, precipitates and properties of Nb-Ti microalloyed steel were investigated. The results showed that an increase in Ti content resulted in the appearance of many fine precipitates leading to a strong precipitation strengthening effect. Hence, the yield strength increased. Besides, the increased strength by the combined increase of Nb and Ti was similar to that observed for the increase in Ti content alone. This increase in strength was attributed widely to the increase in the Ti content alone rather than Nb. Moreover, the increase in Nb content beyond 0.036 wt% exerted no significant effect on the strength of Ti-Nb microalloyed steels, in which more Ti could be added to further improve the strength of steels.展开更多
The effect of Ti and Nb on the recrystallization and the recrystallized grain size of the hot-rolled plates and cold-rolled sheets of ultra low carbon and nitrogen (ULCN) Cr18 ferritic stainless steel (FSS) is inv...The effect of Ti and Nb on the recrystallization and the recrystallized grain size of the hot-rolled plates and cold-rolled sheets of ultra low carbon and nitrogen (ULCN) Cr18 ferritic stainless steel (FSS) is investigated. The results show that the recrystallization of hot-rolled plates and cold-rolled sheets with 0.29% Nb is delayed,and the recrystallized grains are significantly refined.展开更多
基金Item Sponsored by National Natural Science Foundation of China (50504007 ,50474086 ,50334010)Doctor Startup Foundation of Liaoning Province of China (20041009)
文摘Based on thermodynamics and kinetics, a new mathematical model was developed to calculate the CCT diagrams and the transformation kinetics in low carbon niobium steels, in which the effect of deformation on the degree of supercooling was taken into account. The undercooling caused by deformation is the major reason for the increase of the starting transition temperature during continuous cooling. The critical cooling rate of bainite formation is within 2--5 ℃s for the studied niobium steels and deformation is suitable for the occurrence of pearlite. The ferrite volume fraction increases with the increase of the austenite boundary area, and decreases with the increase of the cooling rate. The calculated CCT diagrams and the volume fraction of each phase are in good agreement with the measurements.
基金Item Sponsored by High Technology Development Program of China (863) (2001AA332020)National Natural Science Foundation of China (50271015)
文摘Using Gleeble-1500 system, the influence of holding time on bainite transformation in deformed niobium microalloyed steel during continuous cooling was analyzed, and the carbides in upper bainite were also systematically researched. The results show that the occurrence of the static recrystallization decreases the amount of bainite with an increase in the holding time and the emergence of retained austenite (RA) with the longer holding time. Two types of carbides were observed in upper bainite with regard to their precipitation sites. They either existed between the bainite ferrite laths or co-existed with RA. The formation mechanism of two kinds of carbides was analyzed by combining TEM micrographs with the model.
基金supported by the National Natural Science Foundation of China(Grant No.51605205)the Natural Science Foundation of the Jiangsu Higher Education Institutions of China(Grant No.15KJB460009)
文摘Cracking in a laser weld of niobium to stainless steel occurred due to the formation of brittle,continuously distributed Nb-Fe intermetallic compounds.A crack-free joint,which had a tensile strength of 147 MPa,was obtained by using the W/Cu composite filler metals.To determine the reasons for cracking in the Nb/SS joint and the function of the W/Cu composite filler metals on the improvement of the cracking resistance of the Nb/W/Cu/SS joint,the microstructures of the joints were studied by optical microscopy,scanning electron microscopy and X-ray diffraction.The cracking susceptibilities of the joints were evaluated with microhardness test on the cross section of the Nb/W/Cu/SS joint.The results showed that the Nb/W/Cu/SS joint was characterized by various solid solution.The formation of solid solution reduced the cracking susceptibility of the joint.
基金supported by the National High-Tech Research and Development Program of China (No. 2009AA05Z120)the Young Teachers Program of Dalian Maritime University (No. DLMU-ZL-200720)
文摘In order to enhance the corrosion resistance of 304 stainless steel, niobium was electrodeposited on its surface in air- and water-stable ionic liquids. The electrochemical behaviors of bare and niobium-coated 304 stainless steel were evaluated by electrochemical tests in a simulated proton exchange membrane fuel cell (PEMFC) environment. The results showed that niobium could be electrodeposited on the surface of 304 stainless steel from ionic liquids, and a smooth and strong chemical inert compound film was obtained on the surface of 304 stainless steel, which was mainly composed of NbO and Nb2Os. The thin composite film acted as a barrier and remarkably improved the corrosion resistance of 304 stainless steel in the PEMFC environment.
文摘AISI H13 hot work tool steel is widely used for hot forging, hot-extrusion and die-casting because of its high temperature strength, impact toughness, heat checking resistance and wear resistance, etc. The thermally induced surface damage, i. e., thermal fatigne,is believed to be controlled by the magnitude of the imposed cyclic strain. The thermal fatigue on the surface of hot working die, which is responsible to the initiation of the cracks, is reported to result in more than 80 % of the failure of dies.
基金Funded by the National Natural Science Foundation of China(No.21276036)the Fundamental Research Funds for the Central Universities(No.3132014323)
文摘Niobium was electrodeposited on 316 stainless steel bipolar plates of a fuel cell in water and air-stable choline chloride based ionic liquids. The electrochemical corruption property of bipolar plates in simulated PEMFC environment was investigated. It was showed that the plating film was distributed on the surface of 316 stainless steel like isolated islands with height less than 50 nm. The XPS, XRD results showed that a smooth and strong chemical inert film of Nb O and Nb2O5 was formed on the surface of 316 stainless steel. In simulated cathodic condition, the corrosion potential of Nb coated stainless steel was improved by 244 m V, whilst in an anodic condition, it was improved by 105 m V. The current densities for the coated 316 stainless steel were decreased to 2.479 4 μA·cm-2 from 14.810 μA·cm-2 at-0.1 V and to 0.576 μA·cm-2 from 13.417 μA/·cm-2 at 0.6 V, respectively. It was implied that the niobium coating effectively decreased the corrosion rate. The results of the electrochemical tests indicated that the corrosion resistance of stainless steel was greatly improved after coated with niobium.
文摘An experinientally quantitative inveetisation has been made on the precipitationand coarsening behaviors of niobium carbide in a high strength low alloy(HSLA) steel. The results suggest that the static coarsening processes duringisothermal reaction in (α+γ) two phase region and during isothermal stay aftercertain amount of deformation of austenite are principally controlled by thediffusion of niobium atoms along dislocation pipes, and the volume fraction ofthe precipitates exerts sisnificant influence on the coarsening processes.Whereas the dynamic coarsening process during hot deformation of austeniteappoars to be dominated by the carbide/matrix interphase reaction.
文摘为研究Nb含量对焊接热影响区微观组织和性能的影响,采用熔化极气体保护焊(gas metal arc welding,GMAW)和手工焊条电弧焊(shielded metal arc welding,SMAW)对0.055%Nb和0.075%Nb含量的X80钢管进行环焊.采用夏比冲击试验和金相分析方法,研究热影响区的微观组织差异和夏比冲击韧性.并借助扫描电镜和超高温激光共聚焦显微镜分析不同Nb含量X80管体的微观组织对热影响区性能的影响.结果表明,在0℃和-20℃时,0.075%Nb和0.055%Nb的X80钢管GMAW环焊接头热影响区均具有较高的冲击韧性,其平均冲击吸收能量均高于150 J.其中0.055%Nb略高于0.075%Nb的GMAW环焊接头热影响区夏比冲击吸收能量;焊接热输入较低时,0.055%Nb低于0.075%Nb的X80环焊接头粗晶区的韧脆转变温度,具有更好的低温韧性.焊接热输入较高时,0.075%Nb的X80环焊接头粗晶区具有更高的上平台冲击吸收能量,且上平台温度和韧脆转变温度也更低,其低温韧性也更优异;还发现了X80环焊接头热影响区的冲击韧性不仅与热输入量和热影响区马氏体-奥氏体组织(M-A)的形状、大小、分布有关,而且还受管体中Nb含量、原始的强度与韧性、微观组织状态的遗传影响.
文摘The characteristics and generation mechanism of(Ti,Nb,V)(C,N) precipitates larger than 2 μm in Nb-containing H13 bar steel were studied. The results show that two types of(Ti,Nb,V)(C,N) phases exist—a Ti-V-rich one and an Nb-rich one—in the form of single or complex precipitates. The sizes of the single Ti-V-rich(Ti,Nb,V)(C,N) precipitates are mostly within 5 to 10 μm, whereas the sizes of the single Nb-rich precipitates are mostly 2–5 μm. The complex precipitates are larger and contain an inner Ti-V-rich layer and an outer Nb-rich layer. The compositional distribution of(Ti,Nb,V)(C,N) is concentrated. The average composition of the single Ti-V-rich phase is(Ti_(0.511)V_(0.356)Nb_(0.133))(CxNy), whereas that for the single Nb-rich phase is(Ti_(0.061)V_(0.263)Nb_(0.676))(C_xN_y). The calculation results based on the Scheil–Gulliver model in the Thermo-Calc software combining with the thermal stability experiments show that the large phases precipitate during the solidification process. With the development of solidification, the Ti-V-rich phase precipitates first and becomes homogeneous during the subsequent temperature reduction and heat treatment processes. The Nb-rich phase appears later.
文摘The influence of microalloying additions on the mechanical properties of a low-carbon cast steel containing combinations of V, Nb, and Ti in the as-cast condition was evaluated. Tensile and hardness test results indicated that good combinations of strength and ductility could be achieved by V and Nb additions. While the yield strength and UTS (ultimate tensile strength) increased up to the range of 378-435 MPa and 579- 590 MPa, respectively in the microalloyed heats, their total elongation ranged from 18% to 23%. The presence of Ti, however, led to some reduction in the strength. Microstructural studies including scanning electron microscopy (SEM) and optical microscopy revealed that coarse TiN particles were responsible for this behavior. The Charpy impact values of all compositions indicated that microalloying additions significantly decreased the impact energy and led to the dominance of cleavage facets on the fracture surfaces. It seems that the increase in the hardness of coarse ferrite grains due to the precipitation hardening is the main reason for brittle fracture.
文摘The dynamic recrystallization (DRX) behavior of high-grade X80/X100 pipeline steels with different Nb contents was investi- gated through single pass compression experiment using a Gleeble 1500 thermomechanical simulator. By the regression of stress-sWain data obtained in the experiment, the deformation activation energy of DRX was identified, and the critical strain was calculated with the Po- liak-Jonas (P-J) method. Based on the analysis, the occurrence condition and kinetics of DRX were determined. The results show that as the Nb content increases from 0.08wt% to 0.095wt%, the activation energy of recrystallization raises from 365 to 395 kJ/mol. The critical swain of DRX can be determined more accurately by the P-J method, and the ratios of critical swain to peak sWain of XS0 and X100 pipeline steels are 0.51 and 0.49, respectively, which are similar to the results achieved by other researchers and calculated with empirical formulae.
基金financially supported by the National Nature Science Foundation of China (Nos.51674020 and 51571019)the National Key Research and Development Program of China (No.2016YFB0300102-5)
文摘The increasing speed of trains necessitates the development of brake-disc materials that meet more stringent requirements.Therefore,Nb and V have been added to Cr–Mo–V steel to improve its thermal fatigue performance when used in brake discs.In this paper,the influences of Nb and V on the static continuous cooling transformation(CCT)behaviors of undercooled austenite were studied.The microstructures,hardness,and dislocation densities at different cooling rates and with the addition of different alloying elements were also investigated.The results show that the transformation products of ferrite,granular bainite,lower bainite,and martensite form under different cooling conditions.With increasing Nb and V contents,the CCT curves are shifted to the left,ferrite and bainite transformations are promoted,and the critical cooling rate of total martensite formation is increased.The added V mainly forms V-rich M_8C_7 precipitates and reduces the dissolved C content;therefore,the A_(c1),A_(c3),and M_s-point temperatures increase.Moreover,the stability of retained austenite is also reduced;its content therefore decreases.Compared with V,the effect of added Nb is weaker because of its smaller content.However,the addition of Nb improves the hardness at lower cooling rates because of the precipitation of fine Nb C particles and refining of the microstructure.
基金This work was supported by the National Natural Science Foundation of China under grant Nos. 50334010 and 50271009.
文摘Microalloying element Nb in low carbon steels produced by compact strip production (CSP) process plays an important role in inhibiting recrystallization, decreasing the transformation temperature and grain refinement.With decreasing the rolling temperature, dislocations can be pinned by carbonitrides and the strength is increased. Based on the two sublattice model, with metal atom sublattice and interstitial atom sublattice,a thermodynamic model for carbonitride was established to calculate the equilibrium between matrix and carbonitride. In the steel produced by CSP, the calculation results showed that the starting temperature of precipitation of Ti and Nb are 1340℃ and 1040℃, respectively. In the range of 890-950℃, Nb rapidly precipitated. And the maximum of the atomic fraction of Nb in carbonitride was about 0.68. The morphologies and energy spectrum of the precipitates showed that (NbTi) (CN) precipitated near the dislocations. The experiment results show that Nb rapidly precipitated when the temperature was lower than 970℃, and the atomic fraction of Nb in carbonitride was about 60%-80%. The calculation results are in agreement with the experiment data. Therefore the thermodynamic model can be a useful assistant tool in the research on the precipitates in the low carbon steels produced by CSP.
基金Funded by the Major Projects of Technology Innovation of Hubei Province(No.2017AAA116)the National Natural Science Foundation of China(NSFC)(Nos.51274154,51704217)Hebei Joint Research Fund for Iron and Steel(No.E2018318013)
文摘Four Nb-Ti microalloyed steels were refined and rolled to study the composition optimization of Nb-Ti microalloyed steels. The effects of Nb and Ti on the microstructures, precipitates and properties of Nb-Ti microalloyed steel were investigated. The results showed that an increase in Ti content resulted in the appearance of many fine precipitates leading to a strong precipitation strengthening effect. Hence, the yield strength increased. Besides, the increased strength by the combined increase of Nb and Ti was similar to that observed for the increase in Ti content alone. This increase in strength was attributed widely to the increase in the Ti content alone rather than Nb. Moreover, the increase in Nb content beyond 0.036 wt% exerted no significant effect on the strength of Ti-Nb microalloyed steels, in which more Ti could be added to further improve the strength of steels.
文摘The effect of Ti and Nb on the recrystallization and the recrystallized grain size of the hot-rolled plates and cold-rolled sheets of ultra low carbon and nitrogen (ULCN) Cr18 ferritic stainless steel (FSS) is investigated. The results show that the recrystallization of hot-rolled plates and cold-rolled sheets with 0.29% Nb is delayed,and the recrystallized grains are significantly refined.