Corrosion is an essential issue limiting the application of high-strength low-carbon steel in seawater environment. The impact of retained austenite on its corrosion behavior with immersion experiments and related cor...Corrosion is an essential issue limiting the application of high-strength low-carbon steel in seawater environment. The impact of retained austenite on its corrosion behavior with immersion experiments and related corrosion sensor technology was explored. A model that clarifies the micro-galvanic effect and the heat-induced changes to the shape and composition of retained austenite was used to discuss the findings. The results indicated that retained austenite was generated following an intercritical process and demonstrated approximately 48 mV higher Volta potential than the matrix. The retained austenite content first increased and then decreased with increasing intercritical temperatures, while reaching the maximum value of 8.5% at 660℃. With the increase in retained austenite content, the corrosion rate was increased by up to 32.8% compared to “quenching + tempering” (QT) specimen. The interfaces between the retained austenite and matrix were the priority nucleation sites for corrosion. Moreover, the retained austenite reduced the corrosion resistance of the steel by increasing the micro-galvanic effect and reducing rust layer compactness.展开更多
Hydrogen-induced cracking (HIC) is one of the most complex material problems that hydrogen can diffuse into and interact with microstructure, degrading their mechanical properties. Microstructural modification is an e...Hydrogen-induced cracking (HIC) is one of the most complex material problems that hydrogen can diffuse into and interact with microstructure, degrading their mechanical properties. Microstructural modification is an effective way to enhance the resistance to HIC. The present study focused on the relationship between the retained austenite (RA) and HIC behavior in NiCrMoV/Nb multi-alloying ultra-strength steel. Results demonstrated that the maximum volume fraction of RA of 9.31% was obtained for QL30T specimen. After the deep cryogenic pretreatment, the volume fraction of RA reduced to 8.8%. RA could reduce the effective diffusion coefficient, while deep cryogenic pretreatment increased the susceptibility of the steel to HIC by a maxim of 14.8%. This was mainly due to the transformation of retained austenite into martensite, degrading the mechanical properties under hydrogen-charged condition. In addition, the deep cryogenic pretreatment had a significant effect on the crack initiation and propagation, with the intergranular (IG) fracture becoming the dominant fracture mode where an increase in the number of secondary cracks in the section. The interfaces of RA and matrix, as well as the grain boundaries, were the preferred sites for cracks initiation.展开更多
The equation which reflects the relationship between the retained austenite and strain has been proposed and clear TRIP can be obtained while the S value (An index of retained austenite stability) is less than 6.5 for...The equation which reflects the relationship between the retained austenite and strain has been proposed and clear TRIP can be obtained while the S value (An index of retained austenite stability) is less than 6.5 for Silicon-Manganese TRIP steel展开更多
The systematic chemical compositions including common C, Si, Mn, Al, and micro-alloying elements of Ti and Nb were designed for high volume fraction of retained austenite as much as possible. The thermo-cycle experime...The systematic chemical compositions including common C, Si, Mn, Al, and micro-alloying elements of Ti and Nb were designed for high volume fraction of retained austenite as much as possible. The thermo-cycle experiments were conducted by using Gleeble 2000 thermo-dynamic test machine for finding the appropriate composition. The experimental results showed that chemical composition had a significant effect on retained austenite, and the appropriate compositions were determined for commercial production of TRIP steels.展开更多
Both microstrueture and mechanical properties of low alloy steels treated by quenching and partitioning （Q＆P） process were examined. The mixed microstructure of martensite and large-fractioned retained austenite （...Both microstrueture and mechanical properties of low alloy steels treated by quenching and partitioning （Q＆P） process were examined. The mixed microstructure of martensite and large-fractioned retained austenite （about 27.3%） was characterized and analyzed, excellent combinations of total elongation of 19% and tensile strength of 1 835 MPa were obtained, and three-stage work hardening behavior was demonstrated during tensile test. The en hanced mechanical properties and work hardening behavior were explained based on the transformation induced plas ticity effect of large fractioned austenite.展开更多
Effects of silicon （Si） content on the stability of retained austenite and temper embrittlement of ultrahigh strength steels were investigated using X-ray diffraction （XRD）,transmission electron microscopy （TEM）...Effects of silicon （Si） content on the stability of retained austenite and temper embrittlement of ultrahigh strength steels were investigated using X-ray diffraction （XRD）,transmission electron microscopy （TEM）,and other experimental methods.The results show that Si can suppress temper embrittlement,improve temper resistance,and hinder the decomposition of retained austenite.Reversed austenite appears gradually with the increase of Si content during tempering.Si has a significant effect on enhancing carbon （C） partitioning and improving the stability of retained austenite.Si and C atoms are mutually exclusive in lath bainite,while they attract each other in austenite.ε-carbides are found in 1.8wt% Si steel tempered at 250℃,and they get coarsened obviously when tempered at 400℃,leading to temper embrittlement.Not ε-carbides but acicular or lath carbides lead to temper embrittlement in 0.4wt% Si steel,which can be inferred as cementites and composite compounds.Temper embrittlement is closely related to the decomposition of retained austenite and the formation of reversed austenite.展开更多
The presence of retained austenite gives rise to deterioration of the wear resistance and fracture strength of Cr-Mo steels in many cases. Thus, the effects of heat treatments including direct quenching, martempering,...The presence of retained austenite gives rise to deterioration of the wear resistance and fracture strength of Cr-Mo steels in many cases. Thus, the effects of heat treatments including direct quenching, martempering, and austempering on the retained austenite existing in the microstrueture of these steels were investigated. Specimens were austenized at 950 ℃ followed by direct quenching using compressed and still air. The specimens were also isothermally quenched in salt bath at 200 and 300 ℃ for 2, 8, 30, and 120min. Microstructures of the specimens were studied using optical microscope （traditional black and white etching as well as color etching）, scanning electron microscope （SEM）, microhardness tester, and X-ray diffraction （XRD）. The results showed that the lowest amount of retained austenite in the microstructure was obtained in the specimens quenched isothermally at 300 ℃ for 120 min.展开更多
We present a study concerning Fe-0. 176C-1.31Si-1.58Mn-0.26Al-0.3Cr （wt%） steel subjected to a quenching and partitioning （Q＆P） process. The results of scanning electron microscopy, transmission electron microsco...We present a study concerning Fe-0. 176C-1.31Si-1.58Mn-0.26Al-0.3Cr （wt%） steel subjected to a quenching and partitioning （Q＆P） process. The results of scanning electron microscopy, transmission electron microscopy, X-ray diffraction, and tensile tests demon- strate that the microstructures primarily consist of lath martensite, retained austenite, lower bainite （LB）, and a small amount of tempered martensite; moreover, few twin austenite grains were observed. In the microstrucmre, three types of retained austenite with different sizes and morphologies were observed： blocky retained austenite （-300 nm in width）, film-like retained austenite （80-120 nm in width）, and ul- tra-fine film-like retained austenite （30-40 nm in width）. Because of the effect of the retained austenite/martensite/LB triplex microstructure, the specimens prepared using different quenching temperatures exhibit high ultimate tensile strength and yield strength. Furthermore, the strength effect of LB can partially counteract the decreasing strength effect of martensite. The formation of LB substantially reduces the amount of retained austenite. Analyses of the retained austenite and the amount of blocky retained austenite indicated that the carbon content is critical to the total elongation of Q＆P steel.展开更多
Thermal decomposition of retained austenite in TRIP steel was investigated by means of differential scanning calorimetry（DSC）.The decomposition curve was abnormal,and the decomposition temperature and the activation...Thermal decomposition of retained austenite in TRIP steel was investigated by means of differential scanning calorimetry（DSC）.The decomposition curve was abnormal,and the decomposition temperature and the activation energy were measured by the Kissinger method,which were all higher than those in quenched steel.The thermal decomposition data of samples soaked in liquid nitrogen after TRIP treatment were all similar to those without additional low temperature treatment.It indicated that there is a high thermal stability in retained austenite of the TRIP steel at low temperature,which was also proved by XRD analysis.展开更多
The effects of finishing rolling temperature and coiling temperature on retained austen-ite were studied for hot-rolled transformation induced plasticity (TRIP) steels with different carbon content. The experimental r...The effects of finishing rolling temperature and coiling temperature on retained austen-ite were studied for hot-rolled transformation induced plasticity (TRIP) steels with different carbon content. The experimental results showed that an appropriate volume fraction of retained austenite from 6% to 11% could be obtained according to the different carbon content less than 0.20% by controlled finishing rolling and coiling for the hot-rolled TRIP steels. It can be concluded that carbon content has a significant effect on the fraction of retained austenite and coiling processing plays stronger role on retaining austenite than fishing rolling processing.展开更多
Tensile and bending properties are two critical attributes of press hardened steels(PHSs)for automotive body structural components.However,the research on these properties of PHSs with retained austenite(RA),which is ...Tensile and bending properties are two critical attributes of press hardened steels(PHSs)for automotive body structural components.However,the research on these properties of PHSs with retained austenite(RA),which is introduced to improve the mechanical properties,has not been well reported.In this study,the effect of RA on the quasi-static uniaxial tensile and three-point bending behaviors has been systematically investigated for a newly developed Cr and Si alloyed PHS.RA with various degrees of mechanical stabilities was obtained by tuning the auto-tempering of martensite through the control of the die contact pressure during the press hardening process.Mechanically stable RA provides the optimal combination of tensile and bending performance,while the PHS with RA of poor mechanical stability has a deteriorated bending toughness as manifested by a reduction of bending angle from approximately 61.6°to56.8°.However,itstensileproperties,incontrast,arethebestintermsof theproductofulti-mate tensile strength and total elongation(15.9 GPa%).This is mainly attributed to the unstable RA on the outermost surface of the bending sample.The unstable RA can easily transform into brittle martensite under local plane strain and strain gradient conditions in the bending test compared with uniaxial stress in the tensile test,promoting crack initiation and propagation.Furthermore,the effect of martensitic auto-tempering or contact pressure on the quantity and stability of RA are also discussed.This study would provide a useful reference to guide hot stamping process optimization for the newly developed Cr and Si-containing press-hardened steel.展开更多
A modified tempering treatment has been designed in order to avoid the direct transformation of retained austenite(Ar)during tempering of a low-alloy Cr-Mo-V steel.Instead of the direct transformation of Ar into ferri...A modified tempering treatment has been designed in order to avoid the direct transformation of retained austenite(Ar)during tempering of a low-alloy Cr-Mo-V steel.Instead of the direct transformation of Ar into ferrite and M23C6 carbides during conventional tempering at 700℃,transformation into aggregate of ferrite and cementite has been forced by a pre-tempering at 455℃ before conventional tempering.Experiments have been performed on specimens quenched with cooling rates 1.5,3 and 12℃/s,providing different types of Ar within the as-quenched microstructures.The results show that the tempering modification does not improve the Charpy impact toughness at the highest quenching rate of 12℃/s,where the specimens cannot incur cleavage cracking induced from fine and discontinuous M23C6 carbides along lath interfaces.For the lowest quenching rate 1.5℃/s,the Charpy impact toughness can be improved,and the failure is dominated by carbide aggregates,which originate from the decomposed products of blocky Ar.This is because the tempering modification effectively suppresses the formation of coarse M23C6 carbides at interfaces between the carbide aggregate and bainitic matrix,thereby resulting in a relatively homogeneous distribution of M23C6 carbides inside carbide aggregates.Therefore,the tempering modification is recommended for large-scale forgings,in which relatively high quenching rates are difficult to achieve.展开更多
The partitioning of nitrogen atoms and its effect on the retained austenite content(RAC)during quenching and partitioning(Q&P)process were investigated by dilatometry,X-ray diffraction,and field emission transmiss...The partitioning of nitrogen atoms and its effect on the retained austenite content(RAC)during quenching and partitioning(Q&P)process were investigated by dilatometry,X-ray diffraction,and field emission transmission electron microscopy with energy-dispersive spectrometer mapping in a 00Cr13Mn8N steel.Nitrogen partitioning by diffusion of N atoms from martensite to austenite occurred at 400℃after quenching.N atoms are enriched in austenite after partitioning,and the stability of these N-rich austenite is improved and retained at room temperature during subsequent cooling.The different quenching temperatures(QTs)result in different phase fractions after partitioning.With the increase in QT,RAC first increases and then decreases,and the maximum RAC is 28.5 vol.%after quenching at 80℃.A mathematical model was developed to rapidly and accurately characterize the phase fraction in Q&P process based on the relative length change of the samples partitioned after quenching at different QTs.展开更多
Through the comparison of microstructure for polygonal ferrite （PF） matrix transformation induced plasticity （TRIP） seamless steel tube at different positions before and after tensile rupture, the transformation b...Through the comparison of microstructure for polygonal ferrite （PF） matrix transformation induced plasticity （TRIP） seamless steel tube at different positions before and after tensile rupture, the transformation behavior of retained austenite （RA） was studied. The results showed that there were no yield points in tensile process and the splendid elongation and tensile strength were contributed by the uniform ferrite/bainite grains and the transformation of RA. The stability of RA was to some extent in inverse proportion with the ability of transformation induced plas ticity. The coarse retained austenite located in ferrite and ferrite/bainite laths were all transformed into martensite during the tensile process.展开更多
Microstructures and mechanical properties of Ti-V micro-alloyed TRIP（ transformation-induced plasticity） steel with different compositions were investigated by tensile test,scanning electron microscopy（ SEM）,trans...Microstructures and mechanical properties of Ti-V micro-alloyed TRIP（ transformation-induced plasticity） steel with different compositions were investigated by tensile test,scanning electron microscopy（ SEM）,transmission electron microscopy（ TEM）,X-ray diffraction（ XRD） and thermodynamic calculation（ TC）. The results indicated that the steel exhibited high ultimate tensile strength（ 1 079MPa）,sufficient ductility（ 28%） and the highest product of strength and ductility（ 30 212 MPa·%） heat treated after intercritical annealing at 800℃ for 3 min and bainitic annealing at 430 ℃ for 5 min. In addition,the change of volume fraction of retained austenite（ VF-RA） versus tensile strain was measured using in-situ analysis by X-ray stress apparatus and micro-electronic universal testing machine. It was concluded that a-value could be used to evaluate the stability of retained austenite（ S-RA） in the investigated Ti-V micro-alloyed TRIP steel. The smaller a-value indicated the higher stability of retained austenite（ S-RA） and the higher mechanical properties of Ti-V micro-alloyed TRIP steel.展开更多
The effect of multi-step tempering on retained austenite content and mechanical properties of low alloy steel used in the forged cold back-up roll was investigated.Microstructural evolutions were characterized by opti...The effect of multi-step tempering on retained austenite content and mechanical properties of low alloy steel used in the forged cold back-up roll was investigated.Microstructural evolutions were characterized by optical microscope,X-ray diffraction,scanning electron microscope and Feritscope,while the mechanical properties were determined by hardness and tensile tests.The results revealed that the content of retained austenite decreased by about 2% after multi-step tempering.However,the content of retained austenite increased from 3.6% to 5.1% by increasing multi-step tempering temperature.The hardness and tensile strength increased as the austenitization temperature changed from 800 to 920 ℃,while above 920 ℃,hardness and tensile strength decreased.In addition,the maximum values of hardness,ultimate and yield strength were obtained via triple tempering at 520 ℃,while beyond 520 ℃,the hardness,ultimate and yield strength decreased sharply.展开更多
The thermal stability of retained austenite（RA）and the mechanical properties of the quenched and intercritical annealed 0.1C-5Mn steel with the starting ultrafine lamellar duplex structure of ferrite and retained au...The thermal stability of retained austenite（RA）and the mechanical properties of the quenched and intercritical annealed 0.1C-5Mn steel with the starting ultrafine lamellar duplex structure of ferrite and retained austenite during tempering within the range from 200 to 500°C were studied by X-ray diffraction（XRD）,transmission electron microscopy（TEM）and tensile testing.The results showed that there was a slight decrease in the RA volume fraction with increasing tempering temperature up to 400°C.This caused a slight increase in the ultimate tensile strength（UTS）and a slight decrease in the total elongation（TE）;thus,the product of UTS to TE（UTS×TE）as high as 31GPa·% was obtained and remained nearly unchanged.However,aportion of the RA began to decompose when tempered at 500°C and thus caused a^35% decrease of the RA fraction and a^16%decrease of the value of UTS×TE.It is concluded that the ultrafine lamellar duplex structure is rather stable and the excellent combination of strength and ductility could be retained with tempering temperature up to 400°C.Thus,thermal processes such as galvanization are feasible for the tested steel provided that their temperatures are not higher than 400°C.展开更多
Enhancing the ductility and toughness of advanced high-strength steels is essential for the wide range of promising applications.The retained austenite(RA)is a key phase due to the austenite-to-martensite transformati...Enhancing the ductility and toughness of advanced high-strength steels is essential for the wide range of promising applications.The retained austenite(RA)is a key phase due to the austenite-to-martensite transformation and its transformation-induced plasticity effect.It is commonly accepted that slow RA-to-martensite transformation is beneficial to ductility;therefore,the RA fraction and stability should be carefully controlled.The RA stability is related to its morphology,size,carbon content,neighboring phase and orientation.Importantly,these factors are cross-influenced.It is noteworthy that the influence of RA on ductility and fracture toughness is not consistent because of their difference in stress state.There is no clear relationship between fracture toughness and tensile properties.Thus,it is important to understand the role of RA in toughness.The toughness is enhanced during the RA-to-martensite transformation,while the fracture toughness is decreased due to the formation of fresh and brittle martensite.As a result,the findings regarding to the effect of RA on fracture toughness are conflicting.Further investigations should be conducted in order to fully understand the effects of RA on ductility and fracture toughness,which can optimize the combination of ductility and toughness in AHSSs.展开更多
In this study,the impact of retained austenite on corrosion initiation,propagation,and resistance of ultrafine bainitic steels in marine environments based on the one-step and two-step bainitic isothermal transformati...In this study,the impact of retained austenite on corrosion initiation,propagation,and resistance of ultrafine bainitic steels in marine environments based on the one-step and two-step bainitic isothermal transformation was investigated using a combination of theoretical calculations and experimental methods.According to the results,the microstructure of ultrafine bainitic steels was composed of parallel arranged bainite ferrite(BF)and retained austenite(RA).The fraction of the block RA was significantly reduced,whereas the amount of the film RA increased through the two-step bainitic transformation.In the initial stage of corrosion,micro-galvanic effects occurred between the multiphases due to the difference in nobility,leading to the selective dissolution of BF sheaves,which is adjacent to the block RA in the one-step bainitic steel.However,uniformly distributed film RA in the two-step bainitic steel acted as a barrier against corrosion propagation.The electrochemical measurements and neutral salt spray tests revealed a relatively lower corrosion rate for the two-step bainitic steel,indicating a higher corrosion resistance than the one-step bainitic steel.The corrosion products layer mainly consisted ofα-FeOOH,γ-FeOOH,and Fe_(3)O_(4),which were stable and favorable for the formation of a protective rust layer.展开更多
Heat treatment process for producing cold rolled transformation induced plasticity-aided （TRIP-aided） steels with bainitic ferrite matrix was adopted. Characteristics of retained austenite （RA） in such TRIP steels...Heat treatment process for producing cold rolled transformation induced plasticity-aided （TRIP-aided） steels with bainitic ferrite matrix was adopted. Characteristics of retained austenite （RA） in such TRIP steels were investigated. SEM and OM determination results showed that the stable austenite retained at room temperature were mainly located between laths and some of them inside the coarse ferrite. The grains were uniformly distributed in heat treated steel matrix and the regularly dispersed RA represented to be triangular morphology. XRD analysis indicated that RA content in matrix was not less than 10%, and TEM testified that RA inside the matrix were formed at the prior austenite boundaries and represented to be single or twin crystals. The ductile fracture originated from the boundaries of martensite islands from RA and ferrite. The cracks propagated along grain boundaries and some passed through the large ferrite grains and induced transgranular fracture.展开更多
文摘Corrosion is an essential issue limiting the application of high-strength low-carbon steel in seawater environment. The impact of retained austenite on its corrosion behavior with immersion experiments and related corrosion sensor technology was explored. A model that clarifies the micro-galvanic effect and the heat-induced changes to the shape and composition of retained austenite was used to discuss the findings. The results indicated that retained austenite was generated following an intercritical process and demonstrated approximately 48 mV higher Volta potential than the matrix. The retained austenite content first increased and then decreased with increasing intercritical temperatures, while reaching the maximum value of 8.5% at 660℃. With the increase in retained austenite content, the corrosion rate was increased by up to 32.8% compared to “quenching + tempering” (QT) specimen. The interfaces between the retained austenite and matrix were the priority nucleation sites for corrosion. Moreover, the retained austenite reduced the corrosion resistance of the steel by increasing the micro-galvanic effect and reducing rust layer compactness.
文摘Hydrogen-induced cracking (HIC) is one of the most complex material problems that hydrogen can diffuse into and interact with microstructure, degrading their mechanical properties. Microstructural modification is an effective way to enhance the resistance to HIC. The present study focused on the relationship between the retained austenite (RA) and HIC behavior in NiCrMoV/Nb multi-alloying ultra-strength steel. Results demonstrated that the maximum volume fraction of RA of 9.31% was obtained for QL30T specimen. After the deep cryogenic pretreatment, the volume fraction of RA reduced to 8.8%. RA could reduce the effective diffusion coefficient, while deep cryogenic pretreatment increased the susceptibility of the steel to HIC by a maxim of 14.8%. This was mainly due to the transformation of retained austenite into martensite, degrading the mechanical properties under hydrogen-charged condition. In addition, the deep cryogenic pretreatment had a significant effect on the crack initiation and propagation, with the intergranular (IG) fracture becoming the dominant fracture mode where an increase in the number of secondary cracks in the section. The interfaces of RA and matrix, as well as the grain boundaries, were the preferred sites for cracks initiation.
文摘The equation which reflects the relationship between the retained austenite and strain has been proposed and clear TRIP can be obtained while the S value (An index of retained austenite stability) is less than 6.5 for Silicon-Manganese TRIP steel
文摘The systematic chemical compositions including common C, Si, Mn, Al, and micro-alloying elements of Ti and Nb were designed for high volume fraction of retained austenite as much as possible. The thermo-cycle experiments were conducted by using Gleeble 2000 thermo-dynamic test machine for finding the appropriate composition. The experimental results showed that chemical composition had a significant effect on retained austenite, and the appropriate compositions were determined for commercial production of TRIP steels.
基金Item Sponsored by Youth Science Funds of China(51101036)National Basic Research Program of China(2010CB630803)National Key Technology Support Program of China(2013BAE07B05)
文摘Both microstrueture and mechanical properties of low alloy steels treated by quenching and partitioning （Q＆P） process were examined. The mixed microstructure of martensite and large-fractioned retained austenite （about 27.3%） was characterized and analyzed, excellent combinations of total elongation of 19% and tensile strength of 1 835 MPa were obtained, and three-stage work hardening behavior was demonstrated during tensile test. The en hanced mechanical properties and work hardening behavior were explained based on the transformation induced plas ticity effect of large fractioned austenite.
基金supported by the Project of Scientific and Technical Supporting Program of China during the 11th Five-Year Plan(No.2006BAE03A06)
文摘Effects of silicon （Si） content on the stability of retained austenite and temper embrittlement of ultrahigh strength steels were investigated using X-ray diffraction （XRD）,transmission electron microscopy （TEM）,and other experimental methods.The results show that Si can suppress temper embrittlement,improve temper resistance,and hinder the decomposition of retained austenite.Reversed austenite appears gradually with the increase of Si content during tempering.Si has a significant effect on enhancing carbon （C） partitioning and improving the stability of retained austenite.Si and C atoms are mutually exclusive in lath bainite,while they attract each other in austenite.ε-carbides are found in 1.8wt% Si steel tempered at 250℃,and they get coarsened obviously when tempered at 400℃,leading to temper embrittlement.Not ε-carbides but acicular or lath carbides lead to temper embrittlement in 0.4wt% Si steel,which can be inferred as cementites and composite compounds.Temper embrittlement is closely related to the decomposition of retained austenite and the formation of reversed austenite.
文摘The presence of retained austenite gives rise to deterioration of the wear resistance and fracture strength of Cr-Mo steels in many cases. Thus, the effects of heat treatments including direct quenching, martempering, and austempering on the retained austenite existing in the microstrueture of these steels were investigated. Specimens were austenized at 950 ℃ followed by direct quenching using compressed and still air. The specimens were also isothermally quenched in salt bath at 200 and 300 ℃ for 2, 8, 30, and 120min. Microstructures of the specimens were studied using optical microscope （traditional black and white etching as well as color etching）, scanning electron microscope （SEM）, microhardness tester, and X-ray diffraction （XRD）. The results showed that the lowest amount of retained austenite in the microstructure was obtained in the specimens quenched isothermally at 300 ℃ for 120 min.
基金funded by the China Scholarship Council (No. 201406460053)
文摘We present a study concerning Fe-0. 176C-1.31Si-1.58Mn-0.26Al-0.3Cr （wt%） steel subjected to a quenching and partitioning （Q＆P） process. The results of scanning electron microscopy, transmission electron microscopy, X-ray diffraction, and tensile tests demon- strate that the microstructures primarily consist of lath martensite, retained austenite, lower bainite （LB）, and a small amount of tempered martensite; moreover, few twin austenite grains were observed. In the microstrucmre, three types of retained austenite with different sizes and morphologies were observed： blocky retained austenite （-300 nm in width）, film-like retained austenite （80-120 nm in width）, and ul- tra-fine film-like retained austenite （30-40 nm in width）. Because of the effect of the retained austenite/martensite/LB triplex microstructure, the specimens prepared using different quenching temperatures exhibit high ultimate tensile strength and yield strength. Furthermore, the strength effect of LB can partially counteract the decreasing strength effect of martensite. The formation of LB substantially reduces the amount of retained austenite. Analyses of the retained austenite and the amount of blocky retained austenite indicated that the carbon content is critical to the total elongation of Q＆P steel.
基金Item Sponsored by National Natural Science Foundation of China(50171038)Belgium-China Bilateral Project BIL04/13
文摘Thermal decomposition of retained austenite in TRIP steel was investigated by means of differential scanning calorimetry（DSC）.The decomposition curve was abnormal,and the decomposition temperature and the activation energy were measured by the Kissinger method,which were all higher than those in quenched steel.The thermal decomposition data of samples soaked in liquid nitrogen after TRIP treatment were all similar to those without additional low temperature treatment.It indicated that there is a high thermal stability in retained austenite of the TRIP steel at low temperature,which was also proved by XRD analysis.
文摘The effects of finishing rolling temperature and coiling temperature on retained austen-ite were studied for hot-rolled transformation induced plasticity (TRIP) steels with different carbon content. The experimental results showed that an appropriate volume fraction of retained austenite from 6% to 11% could be obtained according to the different carbon content less than 0.20% by controlled finishing rolling and coiling for the hot-rolled TRIP steels. It can be concluded that carbon content has a significant effect on the fraction of retained austenite and coiling processing plays stronger role on retaining austenite than fishing rolling processing.
基金The work was financially supported by the National Natural Science Foundation of China(Nos.52111530093,52011530032,and 52071066).
文摘Tensile and bending properties are two critical attributes of press hardened steels(PHSs)for automotive body structural components.However,the research on these properties of PHSs with retained austenite(RA),which is introduced to improve the mechanical properties,has not been well reported.In this study,the effect of RA on the quasi-static uniaxial tensile and three-point bending behaviors has been systematically investigated for a newly developed Cr and Si alloyed PHS.RA with various degrees of mechanical stabilities was obtained by tuning the auto-tempering of martensite through the control of the die contact pressure during the press hardening process.Mechanically stable RA provides the optimal combination of tensile and bending performance,while the PHS with RA of poor mechanical stability has a deteriorated bending toughness as manifested by a reduction of bending angle from approximately 61.6°to56.8°.However,itstensileproperties,incontrast,arethebestintermsof theproductofulti-mate tensile strength and total elongation(15.9 GPa%).This is mainly attributed to the unstable RA on the outermost surface of the bending sample.The unstable RA can easily transform into brittle martensite under local plane strain and strain gradient conditions in the bending test compared with uniaxial stress in the tensile test,promoting crack initiation and propagation.Furthermore,the effect of martensitic auto-tempering or contact pressure on the quantity and stability of RA are also discussed.This study would provide a useful reference to guide hot stamping process optimization for the newly developed Cr and Si-containing press-hardened steel.
基金financially supported by the Shenyang National Laboratory for Materials Science,Institute of Metal Research,Chinese Academy of Sciences(Grant No.L2019F48)。
文摘A modified tempering treatment has been designed in order to avoid the direct transformation of retained austenite(Ar)during tempering of a low-alloy Cr-Mo-V steel.Instead of the direct transformation of Ar into ferrite and M23C6 carbides during conventional tempering at 700℃,transformation into aggregate of ferrite and cementite has been forced by a pre-tempering at 455℃ before conventional tempering.Experiments have been performed on specimens quenched with cooling rates 1.5,3 and 12℃/s,providing different types of Ar within the as-quenched microstructures.The results show that the tempering modification does not improve the Charpy impact toughness at the highest quenching rate of 12℃/s,where the specimens cannot incur cleavage cracking induced from fine and discontinuous M23C6 carbides along lath interfaces.For the lowest quenching rate 1.5℃/s,the Charpy impact toughness can be improved,and the failure is dominated by carbide aggregates,which originate from the decomposed products of blocky Ar.This is because the tempering modification effectively suppresses the formation of coarse M23C6 carbides at interfaces between the carbide aggregate and bainitic matrix,thereby resulting in a relatively homogeneous distribution of M23C6 carbides inside carbide aggregates.Therefore,the tempering modification is recommended for large-scale forgings,in which relatively high quenching rates are difficult to achieve.
基金supported by the Shanghai Engineering Research Center of Hot Manufacturing(No.18DZ2253400)Natural Science Foundation-Steel and Iron Foundation of Hebei Province(No.E2020203195).
文摘The partitioning of nitrogen atoms and its effect on the retained austenite content(RAC)during quenching and partitioning(Q&P)process were investigated by dilatometry,X-ray diffraction,and field emission transmission electron microscopy with energy-dispersive spectrometer mapping in a 00Cr13Mn8N steel.Nitrogen partitioning by diffusion of N atoms from martensite to austenite occurred at 400℃after quenching.N atoms are enriched in austenite after partitioning,and the stability of these N-rich austenite is improved and retained at room temperature during subsequent cooling.The different quenching temperatures(QTs)result in different phase fractions after partitioning.With the increase in QT,RAC first increases and then decreases,and the maximum RAC is 28.5 vol.%after quenching at 80℃.A mathematical model was developed to rapidly and accurately characterize the phase fraction in Q&P process based on the relative length change of the samples partitioned after quenching at different QTs.
基金Item Sponsored by Key Project of National Eleventh-Five Year Research Program of China(2006BAE03A08)Fundamental Research Funds for Central Universities of China(N100607002)
文摘Through the comparison of microstructure for polygonal ferrite （PF） matrix transformation induced plasticity （TRIP） seamless steel tube at different positions before and after tensile rupture, the transformation behavior of retained austenite （RA） was studied. The results showed that there were no yield points in tensile process and the splendid elongation and tensile strength were contributed by the uniform ferrite/bainite grains and the transformation of RA. The stability of RA was to some extent in inverse proportion with the ability of transformation induced plas ticity. The coarse retained austenite located in ferrite and ferrite/bainite laths were all transformed into martensite during the tensile process.
基金supported by the Shanghai Municipal Science and Technology Commission(Grant No.15DZ2260300,15DZ2260301)Shanghai Municipal Natural Science Foundation(17ZR1410400)
文摘Microstructures and mechanical properties of Ti-V micro-alloyed TRIP（ transformation-induced plasticity） steel with different compositions were investigated by tensile test,scanning electron microscopy（ SEM）,transmission electron microscopy（ TEM）,X-ray diffraction（ XRD） and thermodynamic calculation（ TC）. The results indicated that the steel exhibited high ultimate tensile strength（ 1 079MPa）,sufficient ductility（ 28%） and the highest product of strength and ductility（ 30 212 MPa·%） heat treated after intercritical annealing at 800℃ for 3 min and bainitic annealing at 430 ℃ for 5 min. In addition,the change of volume fraction of retained austenite（ VF-RA） versus tensile strain was measured using in-situ analysis by X-ray stress apparatus and micro-electronic universal testing machine. It was concluded that a-value could be used to evaluate the stability of retained austenite（ S-RA） in the investigated Ti-V micro-alloyed TRIP steel. The smaller a-value indicated the higher stability of retained austenite（ S-RA） and the higher mechanical properties of Ti-V micro-alloyed TRIP steel.
文摘The effect of multi-step tempering on retained austenite content and mechanical properties of low alloy steel used in the forged cold back-up roll was investigated.Microstructural evolutions were characterized by optical microscope,X-ray diffraction,scanning electron microscope and Feritscope,while the mechanical properties were determined by hardness and tensile tests.The results revealed that the content of retained austenite decreased by about 2% after multi-step tempering.However,the content of retained austenite increased from 3.6% to 5.1% by increasing multi-step tempering temperature.The hardness and tensile strength increased as the austenitization temperature changed from 800 to 920 ℃,while above 920 ℃,hardness and tensile strength decreased.In addition,the maximum values of hardness,ultimate and yield strength were obtained via triple tempering at 520 ℃,while beyond 520 ℃,the hardness,ultimate and yield strength decreased sharply.
基金sponsored by the High-level Scientific Research Foundation for the Introduction of Talent of Beijing Jiaotong University(M14RC00010)the Fundamental Research Funds for the Central Universities(No.2014JBM108)
文摘The thermal stability of retained austenite（RA）and the mechanical properties of the quenched and intercritical annealed 0.1C-5Mn steel with the starting ultrafine lamellar duplex structure of ferrite and retained austenite during tempering within the range from 200 to 500°C were studied by X-ray diffraction（XRD）,transmission electron microscopy（TEM）and tensile testing.The results showed that there was a slight decrease in the RA volume fraction with increasing tempering temperature up to 400°C.This caused a slight increase in the ultimate tensile strength（UTS）and a slight decrease in the total elongation（TE）;thus,the product of UTS to TE（UTS×TE）as high as 31GPa·% was obtained and remained nearly unchanged.However,aportion of the RA began to decompose when tempered at 500°C and thus caused a^35% decrease of the RA fraction and a^16%decrease of the value of UTS×TE.It is concluded that the ultrafine lamellar duplex structure is rather stable and the excellent combination of strength and ductility could be retained with tempering temperature up to 400°C.Thus,thermal processes such as galvanization are feasible for the tested steel provided that their temperatures are not higher than 400°C.
基金supported by the National Natural Science Foundation of China(Nos.52271004 and 51901021).
文摘Enhancing the ductility and toughness of advanced high-strength steels is essential for the wide range of promising applications.The retained austenite(RA)is a key phase due to the austenite-to-martensite transformation and its transformation-induced plasticity effect.It is commonly accepted that slow RA-to-martensite transformation is beneficial to ductility;therefore,the RA fraction and stability should be carefully controlled.The RA stability is related to its morphology,size,carbon content,neighboring phase and orientation.Importantly,these factors are cross-influenced.It is noteworthy that the influence of RA on ductility and fracture toughness is not consistent because of their difference in stress state.There is no clear relationship between fracture toughness and tensile properties.Thus,it is important to understand the role of RA in toughness.The toughness is enhanced during the RA-to-martensite transformation,while the fracture toughness is decreased due to the formation of fresh and brittle martensite.As a result,the findings regarding to the effect of RA on fracture toughness are conflicting.Further investigations should be conducted in order to fully understand the effects of RA on ductility and fracture toughness,which can optimize the combination of ductility and toughness in AHSSs.
基金supported by the Wuhan Knowledge Innovation Program(20220108101020316)the Hubei Province Key Laboratory of Systems Science in Metallurgical Process(Y202104)the National Nature Science Foundation of China(52071238 and U20A20279).
文摘In this study,the impact of retained austenite on corrosion initiation,propagation,and resistance of ultrafine bainitic steels in marine environments based on the one-step and two-step bainitic isothermal transformation was investigated using a combination of theoretical calculations and experimental methods.According to the results,the microstructure of ultrafine bainitic steels was composed of parallel arranged bainite ferrite(BF)and retained austenite(RA).The fraction of the block RA was significantly reduced,whereas the amount of the film RA increased through the two-step bainitic transformation.In the initial stage of corrosion,micro-galvanic effects occurred between the multiphases due to the difference in nobility,leading to the selective dissolution of BF sheaves,which is adjacent to the block RA in the one-step bainitic steel.However,uniformly distributed film RA in the two-step bainitic steel acted as a barrier against corrosion propagation.The electrochemical measurements and neutral salt spray tests revealed a relatively lower corrosion rate for the two-step bainitic steel,indicating a higher corrosion resistance than the one-step bainitic steel.The corrosion products layer mainly consisted ofα-FeOOH,γ-FeOOH,and Fe_(3)O_(4),which were stable and favorable for the formation of a protective rust layer.
基金Funded by the Chinese Key Technology P&D Program of the Eleventh Five-year Plan(2006BAE03A08)the Fundamental Research Funds for the Central Universities of China(N100607002)
文摘Heat treatment process for producing cold rolled transformation induced plasticity-aided （TRIP-aided） steels with bainitic ferrite matrix was adopted. Characteristics of retained austenite （RA） in such TRIP steels were investigated. SEM and OM determination results showed that the stable austenite retained at room temperature were mainly located between laths and some of them inside the coarse ferrite. The grains were uniformly distributed in heat treated steel matrix and the regularly dispersed RA represented to be triangular morphology. XRD analysis indicated that RA content in matrix was not less than 10%, and TEM testified that RA inside the matrix were formed at the prior austenite boundaries and represented to be single or twin crystals. The ductile fracture originated from the boundaries of martensite islands from RA and ferrite. The cracks propagated along grain boundaries and some passed through the large ferrite grains and induced transgranular fracture.
