Dynamic strain-induced transformation of the low carbon steel Q（235） at 770℃ and 850℃ leads to fine ferrite grains. The microstructure characterization and mechanism of the fine ferrite grain were studied by scann...Dynamic strain-induced transformation of the low carbon steel Q（235） at 770℃ and 850℃ leads to fine ferrite grains. The microstructure characterization and mechanism of the fine ferrite grain were studied by scanning electron microscopy （SEM）, transmission electron microscopy （TEM） and electron backscattered diffraction （EBSD） technique. The results show that strain-induced microstructure is the mixed microstructure of ferrite and pearlite, with cementite randomly distributed on ferrite grain boundaries and the grains interiors. EBSD images of grain boundaries demonstrate that high angle grain boundaries （HAGBs） are dominant in both of the deformation induced microstructures occurring below and above A（e3） , with only a few low angle grain boundaries （LAGBs） existing in the grain interiors. It implies that the dynamic strain-induced transformation （DSIT） happens above and below A（e3） temperature and has the same phase transition mechanisms. The refinement of ferrite is the cooperative effect of DSIT and continuous dynamic recrystallization （CDRX） of ferrite. Besides, DSIT is deemed as an incomplete carbon diffusion phase transition through the analysis of microstructure and the previous simulated results. The strengths of the Q（235） steel with refined ferrite and pearlite structure get doubled than the initial state without treated by DSIT and the residual stress in the refined structure is partly responsible for the ductility loss.展开更多
The RAFM（reduced activation ferritic/martensitic）steels containing different tantalum contents（0wt.%,0.027wt.%,0.073wt.%）were designed and cast.Differential scanning calorimetry and optical microscopy were employe...The RAFM（reduced activation ferritic/martensitic）steels containing different tantalum contents（0wt.%,0.027wt.%,0.073wt.%）were designed and cast.Differential scanning calorimetry and optical microscopy were employed to explore the influence of tantalum content on the austenitic transformation of RAFM steels.The austenitic transformation kinetics was described by aphase-transformation model.The model,involving site saturation nucleation,diffusion-controlled growth and impingement correction,was established based on the classical Johnson-Mehl-Avrami-Kolmogorov model.The phase-transformation kinetics parameters,including D_0（pre-exponential factor for diffusion）and Q_d（activation energy for diffusion）,were calculated by fitting the experimental data and the kinetic model.The results indicated that the average grain size is decreased with the increase of tantalum.The values of A_（c1） and A_（c3） （onset and finish temperature of austenitic transformation,respectively）are increased by increasing the tantalum content.The increase of tantalum caused the decrease of D_0.However,Q_d is increased with the increase of tantalum.In addition,as a carbides forming element,tantalum would reduce the carbon diffusion coefficient and slow down the austenitic transformation rate.展开更多
基金support from the National Natural Science Foundation of China (NSFC) under Grantb No. 50871109
文摘Dynamic strain-induced transformation of the low carbon steel Q（235） at 770℃ and 850℃ leads to fine ferrite grains. The microstructure characterization and mechanism of the fine ferrite grain were studied by scanning electron microscopy （SEM）, transmission electron microscopy （TEM） and electron backscattered diffraction （EBSD） technique. The results show that strain-induced microstructure is the mixed microstructure of ferrite and pearlite, with cementite randomly distributed on ferrite grain boundaries and the grains interiors. EBSD images of grain boundaries demonstrate that high angle grain boundaries （HAGBs） are dominant in both of the deformation induced microstructures occurring below and above A（e3） , with only a few low angle grain boundaries （LAGBs） existing in the grain interiors. It implies that the dynamic strain-induced transformation （DSIT） happens above and below A（e3） temperature and has the same phase transition mechanisms. The refinement of ferrite is the cooperative effect of DSIT and continuous dynamic recrystallization （CDRX） of ferrite. Besides, DSIT is deemed as an incomplete carbon diffusion phase transition through the analysis of microstructure and the previous simulated results. The strengths of the Q（235） steel with refined ferrite and pearlite structure get doubled than the initial state without treated by DSIT and the residual stress in the refined structure is partly responsible for the ductility loss.
基金financially sponsored by the China National Funds for Distinguished Young Scientists (Granted No.51325401)the National Natural Science Foundation of China(Granted No.51501126)the National Magnetic Confinement Fusion Energy Research Program(Granted No.2015GB119001)
文摘The RAFM（reduced activation ferritic/martensitic）steels containing different tantalum contents（0wt.%,0.027wt.%,0.073wt.%）were designed and cast.Differential scanning calorimetry and optical microscopy were employed to explore the influence of tantalum content on the austenitic transformation of RAFM steels.The austenitic transformation kinetics was described by aphase-transformation model.The model,involving site saturation nucleation,diffusion-controlled growth and impingement correction,was established based on the classical Johnson-Mehl-Avrami-Kolmogorov model.The phase-transformation kinetics parameters,including D_0（pre-exponential factor for diffusion）and Q_d（activation energy for diffusion）,were calculated by fitting the experimental data and the kinetic model.The results indicated that the average grain size is decreased with the increase of tantalum.The values of A_（c1） and A_（c3） （onset and finish temperature of austenitic transformation,respectively）are increased by increasing the tantalum content.The increase of tantalum caused the decrease of D_0.However,Q_d is increased with the increase of tantalum.In addition,as a carbides forming element,tantalum would reduce the carbon diffusion coefficient and slow down the austenitic transformation rate.
