Using methods of single-hit hot compression and stress relaxation after deformation on a Gleeble 1500D thermomechanical simulator, the curves of flow stress and stress relaxation, the microstructure and the recrystall...Using methods of single-hit hot compression and stress relaxation after deformation on a Gleeble 1500D thermomechanical simulator, the curves of flow stress and stress relaxation, the microstructure and the recrystallization behavior of Nb-V-Ti high strength microalloyed low carbon pipeline steel were studied, and the influence of the thermomechanical treatment parameters on dynamic and static recrystallization of the steel was investigated. It was found that microalloying elements improved the deformation activation energy and produced a retardation of the recrystallization due to the solid solution and precipitation pinning. The deformation conditions such as deformation temperature, strain, and strain rate influenced the recrystallization kinetics and the microstructure respectively. Equations obtained can be used to valuate and predict the dynamic and static recrystallizations.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
The mechanical properties dependence on the microstructure was reviewed and analyzed,and the ultrafine grained duplex microstructure of BCC matrix and large fractioned austenite was given as one of the optimum structu...The mechanical properties dependence on the microstructure was reviewed and analyzed,and the ultrafine grained duplex microstructure of BCC matrix and large fractioned austenite was given as one of the optimum structures to develop the third generation steel with high strength and high ductility.The medium-Mn steels with different carbon contents processed by austenite reverted transformation(ART-annealing) were studied thoroughly to fabricate the ultrafine duplex steels with large fractioned austenite.The lamellar typed ultrafine structure,the granular typed ultrafine duplex structure and the corresponding mechanical properties of the medium-Mn steels processed by ART-annealing were demonstrated in this paper.It was revealed that the duplex structure with large fraction of austenite and ultrafine grain structure is capable of producing steels with excellent combination of strength and ductility,i.e.,Rm A about 30-50 GPa%,which is about two times of that of the conventional automobile steels and close to that of the TWIP steels.It was concluded that the ART-annealing of the medium-Mn steels would be at least one of the promising ways to fabricate the third generation automobile steels in the near future.展开更多
The austenite grain growth behavior in a simulated coarse-grained heat-affected zone during thermal cycling was investigated via in situ observation. Austenite grains nucleated at ferrite grain boundaries and then gre...The austenite grain growth behavior in a simulated coarse-grained heat-affected zone during thermal cycling was investigated via in situ observation. Austenite grains nucleated at ferrite grain boundaries and then grew in different directions through movement of grain boundaries into the ferrite phase. Subsequently, the adjacent austenite grains impinged against each other during the α→γtransformation. After the α→γ transformation, austenite grains coarsened via the coalescence of small grains and via boundary migration between grains. The growth process of austenite grains was a continuous process during heating, isothermal holding, and cooling in simulated thermal cycling. Abundant finely dispersed nanoscale TiN particles in a steel specimen containing 0.012wt% Ti effectively retarded the grain boundary migration, which resulted in refined austenite grains. When the Ti concentration in the steel was increased, the number of TiN particles de- creased and their size coarsened. The big particles were not effective in pinning the austenite grain boundary movement and resulted in coarse austenite grains.展开更多
The passive film formed on 2205 duplex stainless steel（DSS） in 0.5 M NaHCO3＋0.5 M NaCl aqueous solution was characterized by electrochemical measurements,including potentiodynamic anodic polarization and dynamic el...The passive film formed on 2205 duplex stainless steel（DSS） in 0.5 M NaHCO3＋0.5 M NaCl aqueous solution was characterized by electrochemical measurements,including potentiodynamic anodic polarization and dynamic electrochemical impedance spectroscopy（DEIS）.The results demonstrate that there is a great difference between the passive film evolutions of ferrite and austenite.The impedance values of ferrite are higher than those of austenite.The impedance peaks of ferritic and austenitic phases correspond to the potential of 0.15 and 0.25 V in the low potential range and correspond to 0.8 and 0.75 V in the high potential range.The evolutions of the capacitance of both phases are reverse compared to the evolutions of impedance.The thickness variations obtained from capacitance agree well with those of impedance analysis.The results can be used to explain why pitting corrosion occurs more easily in austenite phase than in ferrite phase.展开更多
The effect of compressive deformation of austenite on continuous cooling transformation microstructures for 22CrSH gear steel has been investigated using a Gleeble 1500 thermal simulator. The experimental results show...The effect of compressive deformation of austenite on continuous cooling transformation microstructures for 22CrSH gear steel has been investigated using a Gleeble 1500 thermal simulator. The experimental results show that the deformation of austenite promotes the formation of proeutectoid ferrite and pearlite, and leads to the increase of critical cooling rate of proeutectoid ferrite plus pearlite microstructure. The grain boundary allotriomorphic ferrite occupies the austenite grain surfaces when the prior deformation takes place or the cooling rate is decreased, which causes a transition from bainite to acicular ferrite. The deformation enhances the stability of transformation from austenite to acicular ferrite, which results in an increase of M/A constituent.展开更多
High temperature carburization is becoming more and more attractive because it can remarkably reduce processing time and increase productivity. However, the commonly used gear steels which are microalloyed by Al are n...High temperature carburization is becoming more and more attractive because it can remarkably reduce processing time and increase productivity. However, the commonly used gear steels which are microalloyed by Al are not suitable for high temperature carburization due to abnormal grain coarsening. The gear steel 20CrMnTiNb, which is microalloyed with 0. 048% Nb and 0. 038% Ti, has been compared with the gear steel 20CrMn in terms of microstructure in the case of hardened layer and in the core after carburizing at 1000 ℃ for 4 h and mechanical prop- erties after carburizing and pseudo-carburizing. The results indicate that the fine austenite grains exist in the carbu- rized case of 20CrMnTiNb steel, while there is abnormal coarsening and duplex grain structure in the case and core of steel 20CrMn. The average prior austenite grain sizes are 19.5 and 34.2 μm for the steels 20CrMnTiNb and 20CrMn, respectively. In addition, the mechanical properties of 20CrMnTiNb steel are superior to those of 20CrMn steel. In particular, the HV hardness of the former is higher than that of the latter by about 40--70 in the range of less than 0. 7 mm in depth. Therefore, the steel 20CrMnTiNb is suitable for high temperature carburization.展开更多
The mechanical properties of an austenite-based Fe-Mn-Al-C lightweight steel were improved by co-precipitation of nanoscale Cu-rich and κ-carbide particles.The Fe-28Mn-9Al-0.8C-(0,3)Cu (wt.%) strips were near-rapidly...The mechanical properties of an austenite-based Fe-Mn-Al-C lightweight steel were improved by co-precipitation of nanoscale Cu-rich and κ-carbide particles.The Fe-28Mn-9Al-0.8C-(0,3)Cu (wt.%) strips were near-rapidly solidified and annealed in the temperature range from 500 ℃ to 700 ℃.The microstructure evolution and mechanical properties of the steel under different annealing processes were studied.Microstructural analysis reveals that nanoscale κ-carbides and Cu-rich particles precipitate in the austenite and ferrite of the steel in this annealing temperature range.Co-precipitation of nanoscale Cu-rich particles and κ-carbides provides an obvious increment in the yield strength.At the annealing temperature of 600 ℃,both the yield strength and ultimate tensile strength of Fe-28Mn-9Al-0.8C-3Cu (wt.%) steel strip are the highest.The total elongation is 25%,which is obviously higher than that of Cu-free steel strips,for the addition of Cu reduces the large sized κ-carbides precipitated along austenite/ferrite interfaces.When the annealing temperature rises to 700 ℃,the strength and ductility of the two steel strips deteriorate due to the formation of massive intergranular κ-carbides precipitated along austenite/ferrite interfaces.It can be concluded that a proper co-precipitation of Cu-rich particles and κ-carbides would improve the properties of austenite-based Fe-Mn-Al-C steel.展开更多
In this study, the effects of heating temperature(850–1100°C) and holding time(30–150 min) on the grain growth behavior of austenite in medium-carbon alloy steel were investigated by conducting experiments. The...In this study, the effects of heating temperature(850–1100°C) and holding time(30–150 min) on the grain growth behavior of austenite in medium-carbon alloy steel were investigated by conducting experiments. The abnormal grain growth and mixed grain structure phenomenon are explained using an equilibrium precipitation phase diagram calculated by Thermo-Calc software package. The Al N particles were observed by field-emission scanning electron microscopy(FESEM), and the amount of AlN precipitations was detected by electron probe microanalysis(EPMA). Based on the research results, it was found that the average grain size of austenite in the test steel increased continuously with the increase of temperature and holding time. Furthermore, the abnormal growth of austenite occurred in the test steel at 950°C, and the heating temperature affected the austenite grain size more significantly. In addition, the decline in the amount of AlN second-phase particle in the test steel, which weakened the "pinning" effect on austenite grain boundaries, resulted in abnormal growth and the development of mixed austenite grain structures. The prediction model for describing the austenite grain growth of medium-carbon alloy steel during heating was established by regression analysis of the experimental data, and the model was verified to be highly accurate.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
基金Item Sponsored by National Key Technologies Research and Development Program of China (2006BAE03A15)
文摘Using methods of single-hit hot compression and stress relaxation after deformation on a Gleeble 1500D thermomechanical simulator, the curves of flow stress and stress relaxation, the microstructure and the recrystallization behavior of Nb-V-Ti high strength microalloyed low carbon pipeline steel were studied, and the influence of the thermomechanical treatment parameters on dynamic and static recrystallization of the steel was investigated. It was found that microalloying elements improved the deformation activation energy and produced a retardation of the recrystallization due to the solid solution and precipitation pinning. The deformation conditions such as deformation temperature, strain, and strain rate influenced the recrystallization kinetics and the microstructure respectively. Equations obtained can be used to valuate and predict the dynamic and static recrystallizations.
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supported by the National Basic Research Program of China "973 program" (Grant No. 2010CB630803)
文摘The mechanical properties dependence on the microstructure was reviewed and analyzed,and the ultrafine grained duplex microstructure of BCC matrix and large fractioned austenite was given as one of the optimum structures to develop the third generation steel with high strength and high ductility.The medium-Mn steels with different carbon contents processed by austenite reverted transformation(ART-annealing) were studied thoroughly to fabricate the ultrafine duplex steels with large fractioned austenite.The lamellar typed ultrafine structure,the granular typed ultrafine duplex structure and the corresponding mechanical properties of the medium-Mn steels processed by ART-annealing were demonstrated in this paper.It was revealed that the duplex structure with large fraction of austenite and ultrafine grain structure is capable of producing steels with excellent combination of strength and ductility,i.e.,Rm A about 30-50 GPa%,which is about two times of that of the conventional automobile steels and close to that of the TWIP steels.It was concluded that the ART-annealing of the medium-Mn steels would be at least one of the promising ways to fabricate the third generation automobile steels in the near future.
基金financially supported by the Postdoctoral Science Foundation of China (No. 2014M550415)the National Natural Science Foundation of China (No. 50734004)
文摘The austenite grain growth behavior in a simulated coarse-grained heat-affected zone during thermal cycling was investigated via in situ observation. Austenite grains nucleated at ferrite grain boundaries and then grew in different directions through movement of grain boundaries into the ferrite phase. Subsequently, the adjacent austenite grains impinged against each other during the α→γtransformation. After the α→γ transformation, austenite grains coarsened via the coalescence of small grains and via boundary migration between grains. The growth process of austenite grains was a continuous process during heating, isothermal holding, and cooling in simulated thermal cycling. Abundant finely dispersed nanoscale TiN particles in a steel specimen containing 0.012wt% Ti effectively retarded the grain boundary migration, which resulted in refined austenite grains. When the Ti concentration in the steel was increased, the number of TiN particles de- creased and their size coarsened. The big particles were not effective in pinning the austenite grain boundary movement and resulted in coarse austenite grains.
基金supported by the National Natural Science Foundation of China(No.50871020)
文摘The passive film formed on 2205 duplex stainless steel（DSS） in 0.5 M NaHCO3＋0.5 M NaCl aqueous solution was characterized by electrochemical measurements,including potentiodynamic anodic polarization and dynamic electrochemical impedance spectroscopy（DEIS）.The results demonstrate that there is a great difference between the passive film evolutions of ferrite and austenite.The impedance values of ferrite are higher than those of austenite.The impedance peaks of ferritic and austenitic phases correspond to the potential of 0.15 and 0.25 V in the low potential range and correspond to 0.8 and 0.75 V in the high potential range.The evolutions of the capacitance of both phases are reverse compared to the evolutions of impedance.The thickness variations obtained from capacitance agree well with those of impedance analysis.The results can be used to explain why pitting corrosion occurs more easily in austenite phase than in ferrite phase.
基金Item Sponsored by National Basic Research Programof China (G2000067208-4)
文摘The effect of compressive deformation of austenite on continuous cooling transformation microstructures for 22CrSH gear steel has been investigated using a Gleeble 1500 thermal simulator. The experimental results show that the deformation of austenite promotes the formation of proeutectoid ferrite and pearlite, and leads to the increase of critical cooling rate of proeutectoid ferrite plus pearlite microstructure. The grain boundary allotriomorphic ferrite occupies the austenite grain surfaces when the prior deformation takes place or the cooling rate is decreased, which causes a transition from bainite to acicular ferrite. The deformation enhances the stability of transformation from austenite to acicular ferrite, which results in an increase of M/A constituent.
基金Item Sponsored by National High Technology Research and Development Program(863)of China(2006AA03Z526)
文摘High temperature carburization is becoming more and more attractive because it can remarkably reduce processing time and increase productivity. However, the commonly used gear steels which are microalloyed by Al are not suitable for high temperature carburization due to abnormal grain coarsening. The gear steel 20CrMnTiNb, which is microalloyed with 0. 048% Nb and 0. 038% Ti, has been compared with the gear steel 20CrMn in terms of microstructure in the case of hardened layer and in the core after carburizing at 1000 ℃ for 4 h and mechanical prop- erties after carburizing and pseudo-carburizing. The results indicate that the fine austenite grains exist in the carbu- rized case of 20CrMnTiNb steel, while there is abnormal coarsening and duplex grain structure in the case and core of steel 20CrMn. The average prior austenite grain sizes are 19.5 and 34.2 μm for the steels 20CrMnTiNb and 20CrMn, respectively. In addition, the mechanical properties of 20CrMnTiNb steel are superior to those of 20CrMn steel. In particular, the HV hardness of the former is higher than that of the latter by about 40--70 in the range of less than 0. 7 mm in depth. Therefore, the steel 20CrMnTiNb is suitable for high temperature carburization.
基金financially supported by the National Natural Science Foundation of China(No.51974184)the National MCF Energy R&D Program of China(No.2018YFE0306102)。
文摘The mechanical properties of an austenite-based Fe-Mn-Al-C lightweight steel were improved by co-precipitation of nanoscale Cu-rich and κ-carbide particles.The Fe-28Mn-9Al-0.8C-(0,3)Cu (wt.%) strips were near-rapidly solidified and annealed in the temperature range from 500 ℃ to 700 ℃.The microstructure evolution and mechanical properties of the steel under different annealing processes were studied.Microstructural analysis reveals that nanoscale κ-carbides and Cu-rich particles precipitate in the austenite and ferrite of the steel in this annealing temperature range.Co-precipitation of nanoscale Cu-rich particles and κ-carbides provides an obvious increment in the yield strength.At the annealing temperature of 600 ℃,both the yield strength and ultimate tensile strength of Fe-28Mn-9Al-0.8C-3Cu (wt.%) steel strip are the highest.The total elongation is 25%,which is obviously higher than that of Cu-free steel strips,for the addition of Cu reduces the large sized κ-carbides precipitated along austenite/ferrite interfaces.When the annealing temperature rises to 700 ℃,the strength and ductility of the two steel strips deteriorate due to the formation of massive intergranular κ-carbides precipitated along austenite/ferrite interfaces.It can be concluded that a proper co-precipitation of Cu-rich particles and κ-carbides would improve the properties of austenite-based Fe-Mn-Al-C steel.
基金financially supported by the National Natural Science Foundation of China (No. 51774037)
文摘In this study, the effects of heating temperature(850–1100°C) and holding time(30–150 min) on the grain growth behavior of austenite in medium-carbon alloy steel were investigated by conducting experiments. The abnormal grain growth and mixed grain structure phenomenon are explained using an equilibrium precipitation phase diagram calculated by Thermo-Calc software package. The Al N particles were observed by field-emission scanning electron microscopy(FESEM), and the amount of AlN precipitations was detected by electron probe microanalysis(EPMA). Based on the research results, it was found that the average grain size of austenite in the test steel increased continuously with the increase of temperature and holding time. Furthermore, the abnormal growth of austenite occurred in the test steel at 950°C, and the heating temperature affected the austenite grain size more significantly. In addition, the decline in the amount of AlN second-phase particle in the test steel, which weakened the "pinning" effect on austenite grain boundaries, resulted in abnormal growth and the development of mixed austenite grain structures. The prediction model for describing the austenite grain growth of medium-carbon alloy steel during heating was established by regression analysis of the experimental data, and the model was verified to be highly accurate.
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.
