The aspects of two pipeline steels with different technologies were investigated by using transmission electron microscopy （TEM） and electron back-scattered diffraction （EBSD）. The microstructure presents a typica...The aspects of two pipeline steels with different technologies were investigated by using transmission electron microscopy （TEM） and electron back-scattered diffraction （EBSD）. The microstructure presents a typical acicular ferrite characteristic with fine particles of martensite/austenite （M/A） constituent, which distributes in grains and at grain boundaries. The bulk textures of the pipeline steel plate are {112}〈110〉 and 〈111〉 fibers, respectively, and the {112}〈110〉 component is the favorable texture benefiting for drop weight tear test. Moreover, low angle boundaries and low coincidence site lattice boundaries are inactive and more resistant to fracture than high energy random boundaries.展开更多
The formation mechanism of acicular ferrite and its microstructural characteristics in 430 ferrite stainless steel with TiC additions were studied by theory and experiment.Using an"edge?to?edge matching"mode...The formation mechanism of acicular ferrite and its microstructural characteristics in 430 ferrite stainless steel with TiC additions were studied by theory and experiment.Using an"edge?to?edge matching"model,a 5.25 mismatch between TiC(FCC structure)and ferritic stainless steel(BCC structure)was identified,which met the mismatch requirement for the heterogeneous nucleation of 430 ferritic stainless steel.TiC was found to be an effective nucleation site for the formation of acicular ferrite in a smelting experiment,as analyzed by metallographic examination,Image-Pro Plus 6.0 analysis software,and SEM–EDS.Furthermore,small inclusions in the size of 2–4?m increased the probability of acicular ferrite nucleation,and the secondary acicular ferrite would grow sympathetically from the initial acicular ferrite to produce multi-dimensional acicular ferrites.Moreover,the addition of Ti C can increase the average microstrain and dislocation density of 430 ferrite stainless steel,as calculated by Williamson-Hall(WH)method,which could play some role in strengthening the dislocation.展开更多
An ultra-low carbon acicular ferrite steel heavy plate was obtained with an advanced thermo-mechanical control process-relaxed precipitation controlled transformation （TMCP-RPC） at Xiangtan Steel, Valin Group. The h...An ultra-low carbon acicular ferrite steel heavy plate was obtained with an advanced thermo-mechanical control process-relaxed precipitation controlled transformation （TMCP-RPC） at Xiangtan Steel, Valin Group. The heavy plate has a tensile strength of approximately 600 MPa with a lower yield ratio. The impact toughness of the heavy plate achieves 280 J at ？40°C. The fine-grained mixed microstructures of the heavy plate mainly consist of acicular ferrite, granular bainite, and polygonal ferrite. The high strength and excellent toughness of the heavy plate are attributed to the formation of acicular ferrite microstructure. The prevention of blocks of martensite/retained austenite （M/A） and the higher cleanness are also responsible for the superior toughness.展开更多
The morphology, structure, and chemical composition of precipitates in the final microstructure of Nb-V-Ti microalloyed X70 acicular ferrite pipeline steel were investigated using transmission electron microscopy （TE...The morphology, structure, and chemical composition of precipitates in the final microstructure of Nb-V-Ti microalloyed X70 acicular ferrite pipeline steel were investigated using transmission electron microscopy （TEM） and energy dispersive X-ray spectroscopy （EDS）. Precipitates observed by TEM can be classified into two groups. The large precipitates are complex compounds that comprise square-shaped TiN precipitate as core with fine Nb-containing precipitate nucleated on pre-existing TiN precipitate as caps on one or more faces at high temperature. In contrast, the fine and spherical Nb carbides and/or carbonitrides precipitate heterogeneously on dislocations and sub-boundaries at low temperature. From the analysis in terms of thermodynamics, EDS and chemical cornposition of the steel, NbC precipitation is considered to be the predominant precipitation behavior in the tested steel under the processing conditions of this research.展开更多
Nucleation of acicular ferrite and its influence factors in non quenched-and-tempered steel was studied by using TEM and thermodynamic calculation. The results show that the complex particles with a center made of Ti ...Nucleation of acicular ferrite and its influence factors in non quenched-and-tempered steel was studied by using TEM and thermodynamic calculation. The results show that the complex particles with a center made of Ti oxide, Al2O3, and silicate and an outside made of a small quantity of mixture of TiN and MnS are able to act as ferrite nucleation nuclei. The acicular ferrite percentage changes little with Ti. When the oxygen content was 80 ppm, the volume percentage of acicular ferrite decreased due to an increase in allotriomorphic ferrite. The larger the cooling rate and the shorter the incubation time, the finer the titanium oxide and the higher the nucleation ratio of acicular ferrite.展开更多
Acicular ferrite microstructure was achieved for an ultralow carbon pipeline steel through the improved thermome chanical control process (TMCP), which was based on the transformation process of deformed austenite of ...Acicular ferrite microstructure was achieved for an ultralow carbon pipeline steel through the improved thermome chanical control process (TMCP), which was based on the transformation process of deformed austenite of steel. Compared with commercial pipeline steels, the experimental ultralow carbon pipeline steel possessed the satisfied strength and toughness behaviors under the current improved TMCP, although it contained only approximately 0.025% C, vvhich should mainly be attributed to the microstructural characteristics of acicular ferrite.展开更多
The effect of acicular ferrite （AF） on banded structures in low-carbon microalloyed steel with Mn segregation during both iso- thermal transformation and continuous cooling processes was studied by dilatometry and m...The effect of acicular ferrite （AF） on banded structures in low-carbon microalloyed steel with Mn segregation during both iso- thermal transformation and continuous cooling processes was studied by dilatometry and microscopic observation. With respect to the iso- thermal transformation process, the specimen isothermed at 550℃ consisted of AF in Mn-poor bands and martensite in Mn-rich bands, whereas the specimen isothermed at 450℃ exhibited two different morphologies of AF that appeared as bands. At a continuous cooling rate in the range of 4 to 50℃/s, a mixture of AF and martensite formed in both segregated bands, and the volume fraction of martensite in Mn-rich bands was always higher than that in Mn-poor bands. An increased cooling rate resulted in a decrease in the difference of martensite volume fraction between Mn-rich and Mn-poor bands and thereby leaded to less distinct microstrucmral banding. The results show that Mn segregation and cooling rate strongly affect the formation of AF-containing banded structures. The formation mechanism of microstructural banding was also discussed.展开更多
The volume fraction and morphology of acicular ferrite evolution in a high strength high toughness weld metal were studied and the mechanical properties of weld metal under heat input of 21 kJ/cm with and without fast...The volume fraction and morphology of acicular ferrite evolution in a high strength high toughness weld metal were studied and the mechanical properties of weld metal under heat input of 21 kJ/cm with and without fast cooling were tested. The results show the weld metal can obtain a large proportion of acicular ferrite during a wide range of cooling rate and the sizes of acicular ferrite in length and thickness decrease with cooling rate increasing. The weld metal exhibited high tensile strength （895 MPa and 870 MPa） and good low temperature toughness （average AKv-30℃ 104 J and 79. 2 J）. The higher tensile strength and better low temperature toughness of the weld metal under fast cooling are due to the more refined grain of acicular ferrite.展开更多
Acicular Ferrite(AF) is beneficial in welds because it provides a tough and strong microstructure.Therefore,much works have been done in the study of the effect of alloying elements and welding conditions aimed at obt...Acicular Ferrite(AF) is beneficial in welds because it provides a tough and strong microstructure.Therefore,much works have been done in the study of the effect of alloying elements and welding conditions aimed at obtaining more AF in the weld metal.However,the knowledge about the nature of AF is limited due to the lack of kinetics researches.In this paper,the kinetic curves of AF transformation in an X65 weld metal were established by means of welding thermal simulation test.The results showed that AF transformation is a typical diffusion process of nucleation and growth with incubation period.AF transformation in weld metal obeys the relationship described by JMA Equation.The effect of welding heat input was also studied.展开更多
Theacicular austeniteformation process duringintercritical annealing atslow heating speedof0 5 ℃/ min isinvestigated using TEMin the sample of 30 CrMnSiA. The resultsshow thatinthe preliminarystage, austenitesnucle...Theacicular austeniteformation process duringintercritical annealing atslow heating speedof0 5 ℃/ min isinvestigated using TEMin the sample of 30 CrMnSiA. The resultsshow thatinthe preliminarystage, austenitesnucleateintheform ofshortneedle atshort rod like cementites, which hasthesame orientation and precipitates along the boundary of original martensitelathes. Asthetemperatureincreasingslowly, the austenites grow up and connect witheach other alongtheboundaryoftheoriginallathesso astoform acicular austenite. At last,the fully grown austenite connectsin the direction verticalto the boundary of lathes, leavingthe remnant ferrites appear in the shape of rhombus, the original austenite grainshaperecovers and the feature of thethick lath remains. The superledgeinterface has been found and someaustenitegrowsintoferritein theshapeoflath.展开更多
The research and trial production of X80 pipeline steel in Baosteel is briefly introduced in this paper. It is shown that the X80 pipeline steel with high toughness using acicular ferrite microstructure is designed by...The research and trial production of X80 pipeline steel in Baosteel is briefly introduced in this paper. It is shown that the X80 pipeline steel with high toughness using acicular ferrite microstructure is designed by ultra-low carbon, micro-alloying and Mo-low-alloying in the compositions, and can be produced by combining with the clean steel-making technology and thermal-mechanical controlled processing. The pipes made from the trial producing coils meet the requirements for grade X80 line pipe of the API spec.5L, and have high Charpy V-notch impact energy and good dynamic tear-resistance ability at low temperature. The X80 pipeline steel can be applied to the high-pressure transporting gas pipeline project.展开更多
It is introduced in this paper that a high strength and high toughness X70 pipeline steel with an ultra low acicular ferrite microstructure has been researched and developed at Baosteel according to the requirements o...It is introduced in this paper that a high strength and high toughness X70 pipeline steel with an ultra low acicular ferrite microstructure has been researched and developed at Baosteel according to the requirements of the West East Gas Pipeline(WEGTP) project. The developed steel has higher strength, higher toughness, lower ductile brittle transition temperature and higher dynamic tear resistance than the conventional X70 with ferrite and pearlite microstructure. The excellent properties of the steel are benefited from ultra-low carbon acicular ferrite consisting of interaction of very fine precipitated particles and high-density dislocation. And the steel has been applied on the WEGTP in China.展开更多
High strength low alloy（HSLA） steels have been widely used in pipelines,power plant components,civil structures and so on,due to their outstanding mechanical properties as high strength and toughness,and excellent w...High strength low alloy（HSLA） steels have been widely used in pipelines,power plant components,civil structures and so on,due to their outstanding mechanical properties as high strength and toughness,and excellent weldability.Multi-phase microstructures containing acicular ferrite or acicular ferrite dominated phase have been proved to possess good comprehensive properties in HSLA steels.This paper mainly focuses on the formation mechanisms and control methods of acicular ferrite in HSLA steels.Effect of austenitizing conditions,continuous cooling rate,and isothermal quenching time and temperature on acicular ferrite transformation was reviewed.Furthermore,the modified process to control the formation of multi-phase microstructures containing acicular ferrite,as intercritical heat treatments,step quenching treatments and thermo-mechanical controlled processing,was summarized.The favorable combination of mechanical properties can be achieved by these modified treatments.展开更多
Inclusion characteristic and microstructure steel were evaluated with scanning electron of rare earth （RE） elements containing microscopy with energy dispersive spec- troscopy （SEM-EDS）, element-mapping, optical m...Inclusion characteristic and microstructure steel were evaluated with scanning electron of rare earth （RE） elements containing microscopy with energy dispersive spec- troscopy （SEM-EDS）, element-mapping, optical microscopy （OM）, and automated feature analysis （AFA） option equipped with ASPEX PSEM. Factsage was used to calculate the equilibrium inclusion composition. Based on the calculation, an inclu- sion evolution mechanism was proposed. Furthermore, line scanning analysis was used to elucidate the intra-granular acicular ferrite （IAF） nucleation mechanism. The re- sult showed that two different inclusions exist in sample steel： （Mn-A1-Si-Ti-La-Ce-O） ＋MnS complex inclusion and isolated MnS inclusion. Almost all nucleation sites for IAF are complex inclusions, while single MnS inclusion cannot induce IAF. A possible formation mechanism of complex inclusion is proposed based on calculated results using Factsage, which agrees well with experimental results. A Mn-depletion zone （MDZ） which exists adjacent to the （Mn-A1-Si-Ti-La-Ce-O） ＋MnS complex inclusion can account for the IAF formation. However, the low volume fraction （1.49× 10-7） of effective inclusion may result in only 10% （volume fraction） IAF.展开更多
The effects of Mg content, inclusion size, and austenite grain size on the intragranular acicular ferrite （IAF） nucleation in heat-affected zone of steel plate after high-heat-input welding of 400 kJ/cm were investi...The effects of Mg content, inclusion size, and austenite grain size on the intragranular acicular ferrite （IAF） nucleation in heat-affected zone of steel plate after high-heat-input welding of 400 kJ/cm were investigated by welding simulation and observation using a scanning electron microscope equipped with an energy dispersive spectrometer and an optical microscope. The IAFs are observed in steel with Mg addition, and the volume fraction of IAF is as high as 55.4% in the steel containing 0.0027 mass% Mg. The MgO-Al2O3-Ti2O3-MnS inclusions with size around 2 μm are effective nucleation sites for IAF, whereas Al2O3-MnS inclusions are impotent to nucleate the acicular ferrite. The prior-austenite grain （PAG） size distribution in low Mg steel is similar to that in steel without Mg addition. The austenite grain with size about 200 μm is favorable for the IAF formation. In the steel with high Mg content of 0.0099%, the growth of PAG is greatly inhibited, and PAG sizes are smaller than 100 μm. Therefore, the nucleation of IAF can hardly be observed.展开更多
Uniquely structured rutile TiO2 microspheres with exposed nano-acicular single crystals have been successfully synthesized via a facile hydrothermal method. After calcination at 450 ℃ for 2 h, the futile TiO2 microsp...Uniquely structured rutile TiO2 microspheres with exposed nano-acicular single crystals have been successfully synthesized via a facile hydrothermal method. After calcination at 450 ℃ for 2 h, the futile TiO2 microspheres with a high surface area of 132 m2/g have been utilized as a light harvesting enhancement material for dye-sensitized solar cells （DSSCs）. The resultant DSSCs exhibit an overall light conversion efficiency of 8.41% for TiO2 photoanodes made of futile TiO2 microspheres and anatase TiO, nanoparticles （mass ratio of 1：1）, significantly higher than that of pure anatase TiO2 nanoparticle photoanodes of similar thickness （6.74%）. Such a significant improvement in performance can be attributed to the enhanced light harvesting capability and synergetic electron transfer effect. This is because the photoanodes made of futile TiO2 microsphere possess high refractive index which improves the light utilisation efficiency, suitable microsphere core sizes （450-800 nm） to effectively scatter visible light, high surface area for dye loading, and synergetic electron transfer effects between nanoparticulate anatase and nano-acicular futile single crystals phases giving high electron collection efficiency.展开更多
The transformation behavior and microstructural characteristics of a low carbon high niobium-bearing microalloyed pipeline steel were investigated by deformation dilatometry and microstructure observation. The continu...The transformation behavior and microstructural characteristics of a low carbon high niobium-bearing microalloyed pipeline steel were investigated by deformation dilatometry and microstructure observation. The continuous cooling transformation curves of the test steel were constructed. The results showed that high niobium content and deformation enhanced the formation of acicular ferrite; the microstructures changed from polygonal ferrite, quasi-polygonal ferrite to acicular ferrite with increasing cooling rates from 0.5 to 50 ℃/s and was dominated by acicular ferrite in a broadened cooling rate higher than 5 ℃/s. The chaotic microstructure consisted of non-equiaxed ferrite and interwoven ferrite laths with high density dislocations and subunits. The results of isothermal holding treatment showed that acicular ferrite microstructure was formed at 550-600 ℃ and consisted of highly misoriented plate packets having internal low angle boundaries. With increasing the holding time or temperature,some low misorientation boundaries changed to high misorientation owing to the movement of dislocations and coarsening of grain.展开更多
To verify the formation behaviors and mechanisms of intra-granular acicular ferrite（ IAF） grains nucleated by Mg-Al-O in low carbon steel,the steels containing different Mg contents were refined in a vacuum inductio...To verify the formation behaviors and mechanisms of intra-granular acicular ferrite（ IAF） grains nucleated by Mg-Al-O in low carbon steel,the steels containing different Mg contents were refined in a vacuum induction furnace. The effect of Mg addition on the formation of IAF structure in Al-killed low carbon steel was investigated by optical microscope（ OM） and scanning electron microscope with energy dispersive X-ray spectroscope（ SEM-EDX）. It reveals that the IAFs are only detected in Mg-added steels,and the volume fraction of IAF increases with the Mg concentration from 8 × 10^（-6） to 26 × 10^（-6）. It shows that not only the MgO-Al_2O_3-MnS and MgO-Al_2O_3-P_2O_5 particles are the effective nucleation sites for IAF,but also the pure MgO·Al_2O_3 phase can promote the ferrite nucleation. A Mn-depletion zone（ MDZ） is characterized adjacent to the MgO-Al_2O_3-MnS,which is believed to be one of the possible mechanisms to explain the IAF nucleation. The MDZ around the MgO-Al_2O_3-MnS inclusion would be induced by the Mn S precipitation on the inclusion. It seems that the ability of Mg-containing inclusions to induce the nucleation of ferrite might be attributed to a new mechanism,i. e.,the Prich zone formed on a few Mg-Al-O inclusions might be another factor for promoting the IAF formation.展开更多
To investigate the effect of surface-modified nanoparticles(NPs)on the inclusion refinement and microstructure evolution,deoxidized experiment ingots with different amounts of modified NPs were manufactured under diff...To investigate the effect of surface-modified nanoparticles(NPs)on the inclusion refinement and microstructure evolution,deoxidized experiment ingots with different amounts of modified NPs were manufactured under different cooling conditions.Laser scanning confocal microscope(LSCM)was hereby used for in-situ observation of the phase transition and microstructural evolution during heat cycle process.The results revealed that the inclusion size was always smaller under water quenching than under air cooling,and the number of inclusions was greater under water quenching.After NP addition,the nucleant inclusions were identified as MgAl_(2)O_(4)spinel and irregular TiN inclusion from SEM-EDS measurement and equilibrium calculations using Factsage thermodynamic software.The higher cooling rate under water quenching resulted in less polygonal ferrite decrease and the formation of bainite in the steel.The LSCM experiments showed that ferrite side plates(FSP)always formed on the boundary prior to the formation of acicular ferrite(AF)on the intragranular inclusions,and the start transformation temperatures of FSP and AF phases both lowered after NP addition.The higher cooling rate and NP addition contributed to AF formation and increased the degree of interlocking of the AF phase.Finally,the relationship between the characteristics of inclusions and the kinetics of AF was investigated.展开更多
文摘The aspects of two pipeline steels with different technologies were investigated by using transmission electron microscopy （TEM） and electron back-scattered diffraction （EBSD）. The microstructure presents a typical acicular ferrite characteristic with fine particles of martensite/austenite （M/A） constituent, which distributes in grains and at grain boundaries. The bulk textures of the pipeline steel plate are {112}〈110〉 and 〈111〉 fibers, respectively, and the {112}〈110〉 component is the favorable texture benefiting for drop weight tear test. Moreover, low angle boundaries and low coincidence site lattice boundaries are inactive and more resistant to fracture than high energy random boundaries.
基金financially supported by the National Natural Science Foundation of China (No. 51674071)
文摘The formation mechanism of acicular ferrite and its microstructural characteristics in 430 ferrite stainless steel with TiC additions were studied by theory and experiment.Using an"edge?to?edge matching"model,a 5.25 mismatch between TiC(FCC structure)and ferritic stainless steel(BCC structure)was identified,which met the mismatch requirement for the heterogeneous nucleation of 430 ferritic stainless steel.TiC was found to be an effective nucleation site for the formation of acicular ferrite in a smelting experiment,as analyzed by metallographic examination,Image-Pro Plus 6.0 analysis software,and SEM–EDS.Furthermore,small inclusions in the size of 2–4?m increased the probability of acicular ferrite nucleation,and the secondary acicular ferrite would grow sympathetically from the initial acicular ferrite to produce multi-dimensional acicular ferrites.Moreover,the addition of Ti C can increase the average microstrain and dislocation density of 430 ferrite stainless steel,as calculated by Williamson-Hall(WH)method,which could play some role in strengthening the dislocation.
基金supported by the National Natural Science Foundation of China (Nos.50471107 and 50734004)
文摘An ultra-low carbon acicular ferrite steel heavy plate was obtained with an advanced thermo-mechanical control process-relaxed precipitation controlled transformation （TMCP-RPC） at Xiangtan Steel, Valin Group. The heavy plate has a tensile strength of approximately 600 MPa with a lower yield ratio. The impact toughness of the heavy plate achieves 280 J at ？40°C. The fine-grained mixed microstructures of the heavy plate mainly consist of acicular ferrite, granular bainite, and polygonal ferrite. The high strength and excellent toughness of the heavy plate are attributed to the formation of acicular ferrite microstructure. The prevention of blocks of martensite/retained austenite （M/A） and the higher cleanness are also responsible for the superior toughness.
文摘The morphology, structure, and chemical composition of precipitates in the final microstructure of Nb-V-Ti microalloyed X70 acicular ferrite pipeline steel were investigated using transmission electron microscopy （TEM） and energy dispersive X-ray spectroscopy （EDS）. Precipitates observed by TEM can be classified into two groups. The large precipitates are complex compounds that comprise square-shaped TiN precipitate as core with fine Nb-containing precipitate nucleated on pre-existing TiN precipitate as caps on one or more faces at high temperature. In contrast, the fine and spherical Nb carbides and/or carbonitrides precipitate heterogeneously on dislocations and sub-boundaries at low temperature. From the analysis in terms of thermodynamics, EDS and chemical cornposition of the steel, NbC precipitation is considered to be the predominant precipitation behavior in the tested steel under the processing conditions of this research.
基金This work was financially supported by the National Natural Science Foundation of China (No.50574010)the National Doctor-ate Fund of the Ministry of Education of China (No.20060008015)
文摘Nucleation of acicular ferrite and its influence factors in non quenched-and-tempered steel was studied by using TEM and thermodynamic calculation. The results show that the complex particles with a center made of Ti oxide, Al2O3, and silicate and an outside made of a small quantity of mixture of TiN and MnS are able to act as ferrite nucleation nuclei. The acicular ferrite percentage changes little with Ti. When the oxygen content was 80 ppm, the volume percentage of acicular ferrite decreased due to an increase in allotriomorphic ferrite. The larger the cooling rate and the shorter the incubation time, the finer the titanium oxide and the higher the nucleation ratio of acicular ferrite.
基金The present work was financially supported by a China National‘973’Project No.G1998061511the Natural Science Fund of Hebei Province No.501205.
文摘Acicular ferrite microstructure was achieved for an ultralow carbon pipeline steel through the improved thermome chanical control process (TMCP), which was based on the transformation process of deformed austenite of steel. Compared with commercial pipeline steels, the experimental ultralow carbon pipeline steel possessed the satisfied strength and toughness behaviors under the current improved TMCP, although it contained only approximately 0.025% C, vvhich should mainly be attributed to the microstructural characteristics of acicular ferrite.
基金financially supported by the China National Funds for Distinguished Young Scientists (No. 51325401)the International Thermonuclear Experimental Reactor (ITER) Program Special Project (No. 2014GB125006)+1 种基金the Major State Basic Research Development Program of China (No. 2014CB046805)the Natural Science Foundation of Tianjin City (No. 14JCZDJC38700)
文摘The effect of acicular ferrite （AF） on banded structures in low-carbon microalloyed steel with Mn segregation during both iso- thermal transformation and continuous cooling processes was studied by dilatometry and microscopic observation. With respect to the iso- thermal transformation process, the specimen isothermed at 550℃ consisted of AF in Mn-poor bands and martensite in Mn-rich bands, whereas the specimen isothermed at 450℃ exhibited two different morphologies of AF that appeared as bands. At a continuous cooling rate in the range of 4 to 50℃/s, a mixture of AF and martensite formed in both segregated bands, and the volume fraction of martensite in Mn-rich bands was always higher than that in Mn-poor bands. An increased cooling rate resulted in a decrease in the difference of martensite volume fraction between Mn-rich and Mn-poor bands and thereby leaded to less distinct microstrucmral banding. The results show that Mn segregation and cooling rate strongly affect the formation of AF-containing banded structures. The formation mechanism of microstructural banding was also discussed.
基金support from NSFC(National Natural Science Foundation of China,under Grant No. 50734004)
文摘The volume fraction and morphology of acicular ferrite evolution in a high strength high toughness weld metal were studied and the mechanical properties of weld metal under heat input of 21 kJ/cm with and without fast cooling were tested. The results show the weld metal can obtain a large proportion of acicular ferrite during a wide range of cooling rate and the sizes of acicular ferrite in length and thickness decrease with cooling rate increasing. The weld metal exhibited high tensile strength （895 MPa and 870 MPa） and good low temperature toughness （average AKv-30℃ 104 J and 79. 2 J）. The higher tensile strength and better low temperature toughness of the weld metal under fast cooling are due to the more refined grain of acicular ferrite.
文摘Acicular Ferrite(AF) is beneficial in welds because it provides a tough and strong microstructure.Therefore,much works have been done in the study of the effect of alloying elements and welding conditions aimed at obtaining more AF in the weld metal.However,the knowledge about the nature of AF is limited due to the lack of kinetics researches.In this paper,the kinetic curves of AF transformation in an X65 weld metal were established by means of welding thermal simulation test.The results showed that AF transformation is a typical diffusion process of nucleation and growth with incubation period.AF transformation in weld metal obeys the relationship described by JMA Equation.The effect of welding heat input was also studied.
文摘Theacicular austeniteformation process duringintercritical annealing atslow heating speedof0 5 ℃/ min isinvestigated using TEMin the sample of 30 CrMnSiA. The resultsshow thatinthe preliminarystage, austenitesnucleateintheform ofshortneedle atshort rod like cementites, which hasthesame orientation and precipitates along the boundary of original martensitelathes. Asthetemperatureincreasingslowly, the austenites grow up and connect witheach other alongtheboundaryoftheoriginallathesso astoform acicular austenite. At last,the fully grown austenite connectsin the direction verticalto the boundary of lathes, leavingthe remnant ferrites appear in the shape of rhombus, the original austenite grainshaperecovers and the feature of thethick lath remains. The superledgeinterface has been found and someaustenitegrowsintoferritein theshapeoflath.
文摘The research and trial production of X80 pipeline steel in Baosteel is briefly introduced in this paper. It is shown that the X80 pipeline steel with high toughness using acicular ferrite microstructure is designed by ultra-low carbon, micro-alloying and Mo-low-alloying in the compositions, and can be produced by combining with the clean steel-making technology and thermal-mechanical controlled processing. The pipes made from the trial producing coils meet the requirements for grade X80 line pipe of the API spec.5L, and have high Charpy V-notch impact energy and good dynamic tear-resistance ability at low temperature. The X80 pipeline steel can be applied to the high-pressure transporting gas pipeline project.
文摘It is introduced in this paper that a high strength and high toughness X70 pipeline steel with an ultra low acicular ferrite microstructure has been researched and developed at Baosteel according to the requirements of the West East Gas Pipeline(WEGTP) project. The developed steel has higher strength, higher toughness, lower ductile brittle transition temperature and higher dynamic tear resistance than the conventional X70 with ferrite and pearlite microstructure. The excellent properties of the steel are benefited from ultra-low carbon acicular ferrite consisting of interaction of very fine precipitated particles and high-density dislocation. And the steel has been applied on the WEGTP in China.
基金the China National Funds for Distinguished Young Scientists (Grant No.51325401)the National Magnetic Confinement Fusion Energy Research Project (Grant No.2015GB119001)the National Natural Science Foundation of China (Grant Nos.51501126,51474156 and U1660201) for grant and financial support
文摘High strength low alloy（HSLA） steels have been widely used in pipelines,power plant components,civil structures and so on,due to their outstanding mechanical properties as high strength and toughness,and excellent weldability.Multi-phase microstructures containing acicular ferrite or acicular ferrite dominated phase have been proved to possess good comprehensive properties in HSLA steels.This paper mainly focuses on the formation mechanisms and control methods of acicular ferrite in HSLA steels.Effect of austenitizing conditions,continuous cooling rate,and isothermal quenching time and temperature on acicular ferrite transformation was reviewed.Furthermore,the modified process to control the formation of multi-phase microstructures containing acicular ferrite,as intercritical heat treatments,step quenching treatments and thermo-mechanical controlled processing,was summarized.The favorable combination of mechanical properties can be achieved by these modified treatments.
基金supported by the National Natural Science Foundation of China(No.2010CB30806)
文摘Inclusion characteristic and microstructure steel were evaluated with scanning electron of rare earth （RE） elements containing microscopy with energy dispersive spec- troscopy （SEM-EDS）, element-mapping, optical microscopy （OM）, and automated feature analysis （AFA） option equipped with ASPEX PSEM. Factsage was used to calculate the equilibrium inclusion composition. Based on the calculation, an inclu- sion evolution mechanism was proposed. Furthermore, line scanning analysis was used to elucidate the intra-granular acicular ferrite （IAF） nucleation mechanism. The re- sult showed that two different inclusions exist in sample steel： （Mn-A1-Si-Ti-La-Ce-O） ＋MnS complex inclusion and isolated MnS inclusion. Almost all nucleation sites for IAF are complex inclusions, while single MnS inclusion cannot induce IAF. A possible formation mechanism of complex inclusion is proposed based on calculated results using Factsage, which agrees well with experimental results. A Mn-depletion zone （MDZ） which exists adjacent to the （Mn-A1-Si-Ti-La-Ce-O） ＋MnS complex inclusion can account for the IAF formation. However, the low volume fraction （1.49× 10-7） of effective inclusion may result in only 10% （volume fraction） IAF.
文摘The effects of Mg content, inclusion size, and austenite grain size on the intragranular acicular ferrite （IAF） nucleation in heat-affected zone of steel plate after high-heat-input welding of 400 kJ/cm were investigated by welding simulation and observation using a scanning electron microscope equipped with an energy dispersive spectrometer and an optical microscope. The IAFs are observed in steel with Mg addition, and the volume fraction of IAF is as high as 55.4% in the steel containing 0.0027 mass% Mg. The MgO-Al2O3-Ti2O3-MnS inclusions with size around 2 μm are effective nucleation sites for IAF, whereas Al2O3-MnS inclusions are impotent to nucleate the acicular ferrite. The prior-austenite grain （PAG） size distribution in low Mg steel is similar to that in steel without Mg addition. The austenite grain with size about 200 μm is favorable for the IAF formation. In the steel with high Mg content of 0.0099%, the growth of PAG is greatly inhibited, and PAG sizes are smaller than 100 μm. Therefore, the nucleation of IAF can hardly be observed.
文摘Uniquely structured rutile TiO2 microspheres with exposed nano-acicular single crystals have been successfully synthesized via a facile hydrothermal method. After calcination at 450 ℃ for 2 h, the futile TiO2 microspheres with a high surface area of 132 m2/g have been utilized as a light harvesting enhancement material for dye-sensitized solar cells （DSSCs）. The resultant DSSCs exhibit an overall light conversion efficiency of 8.41% for TiO2 photoanodes made of futile TiO2 microspheres and anatase TiO, nanoparticles （mass ratio of 1：1）, significantly higher than that of pure anatase TiO2 nanoparticle photoanodes of similar thickness （6.74%）. Such a significant improvement in performance can be attributed to the enhanced light harvesting capability and synergetic electron transfer effect. This is because the photoanodes made of futile TiO2 microsphere possess high refractive index which improves the light utilisation efficiency, suitable microsphere core sizes （450-800 nm） to effectively scatter visible light, high surface area for dye loading, and synergetic electron transfer effects between nanoparticulate anatase and nano-acicular futile single crystals phases giving high electron collection efficiency.
基金Item Sponsored by National Key Technologies Research and Development Program of China (2006BAE03A15)
文摘The transformation behavior and microstructural characteristics of a low carbon high niobium-bearing microalloyed pipeline steel were investigated by deformation dilatometry and microstructure observation. The continuous cooling transformation curves of the test steel were constructed. The results showed that high niobium content and deformation enhanced the formation of acicular ferrite; the microstructures changed from polygonal ferrite, quasi-polygonal ferrite to acicular ferrite with increasing cooling rates from 0.5 to 50 ℃/s and was dominated by acicular ferrite in a broadened cooling rate higher than 5 ℃/s. The chaotic microstructure consisted of non-equiaxed ferrite and interwoven ferrite laths with high density dislocations and subunits. The results of isothermal holding treatment showed that acicular ferrite microstructure was formed at 550-600 ℃ and consisted of highly misoriented plate packets having internal low angle boundaries. With increasing the holding time or temperature,some low misorientation boundaries changed to high misorientation owing to the movement of dislocations and coarsening of grain.
基金Item Sponsored by National Natural Science Foundation of China(51374059,51374060)Scientific Research Fund of Liaoning Provincial Education Department of China(2012221013)
文摘To verify the formation behaviors and mechanisms of intra-granular acicular ferrite（ IAF） grains nucleated by Mg-Al-O in low carbon steel,the steels containing different Mg contents were refined in a vacuum induction furnace. The effect of Mg addition on the formation of IAF structure in Al-killed low carbon steel was investigated by optical microscope（ OM） and scanning electron microscope with energy dispersive X-ray spectroscope（ SEM-EDX）. It reveals that the IAFs are only detected in Mg-added steels,and the volume fraction of IAF increases with the Mg concentration from 8 × 10^（-6） to 26 × 10^（-6）. It shows that not only the MgO-Al_2O_3-MnS and MgO-Al_2O_3-P_2O_5 particles are the effective nucleation sites for IAF,but also the pure MgO·Al_2O_3 phase can promote the ferrite nucleation. A Mn-depletion zone（ MDZ） is characterized adjacent to the MgO-Al_2O_3-MnS,which is believed to be one of the possible mechanisms to explain the IAF nucleation. The MDZ around the MgO-Al_2O_3-MnS inclusion would be induced by the Mn S precipitation on the inclusion. It seems that the ability of Mg-containing inclusions to induce the nucleation of ferrite might be attributed to a new mechanism,i. e.,the Prich zone formed on a few Mg-Al-O inclusions might be another factor for promoting the IAF formation.
基金The Chinese Scholarship Council(CSC)financially supported by the National Natural Science Foundation of China(Nos.51874024 and 51734003)the Fundamental Research Funds for the Central Universities(No.FRF-TP-18–009C1)
文摘To investigate the effect of surface-modified nanoparticles(NPs)on the inclusion refinement and microstructure evolution,deoxidized experiment ingots with different amounts of modified NPs were manufactured under different cooling conditions.Laser scanning confocal microscope(LSCM)was hereby used for in-situ observation of the phase transition and microstructural evolution during heat cycle process.The results revealed that the inclusion size was always smaller under water quenching than under air cooling,and the number of inclusions was greater under water quenching.After NP addition,the nucleant inclusions were identified as MgAl_(2)O_(4)spinel and irregular TiN inclusion from SEM-EDS measurement and equilibrium calculations using Factsage thermodynamic software.The higher cooling rate under water quenching resulted in less polygonal ferrite decrease and the formation of bainite in the steel.The LSCM experiments showed that ferrite side plates(FSP)always formed on the boundary prior to the formation of acicular ferrite(AF)on the intragranular inclusions,and the start transformation temperatures of FSP and AF phases both lowered after NP addition.The higher cooling rate and NP addition contributed to AF formation and increased the degree of interlocking of the AF phase.Finally,the relationship between the characteristics of inclusions and the kinetics of AF was investigated.
