Deformation-induced ferrite transformation （DIFT） has been proved to be an effective approach to refine ferrite grains. This paper shows that the ferrite grains can further be refined through combination of DIFT and...Deformation-induced ferrite transformation （DIFT） has been proved to be an effective approach to refine ferrite grains. This paper shows that the ferrite grains can further be refined through combination of DIFT and V or V-N microalloying. Vanadium dissolved in γ matrix restrains DIFT. During deformation, vanadium carbonitrides rapidly precipitate due to strain-induced precipitation, which causes decrease in vanadium dissolved in matrix and indirectly accelerates DIFT. Under heavy deformation, deformation induced ferrite （DIF） grains in V microalloyed steel were finer than those in V free steel. The more V added to steel, the finer DIF grains obtained. Moreover, the addition of N to V microalloyed steels can remarkably accelerate precipitation of V, and then promote DIFT. However, DIF grains in V-N microalloyed steel easily coarsen.展开更多
A series of tests of deformation-induced ferrite transformation （DIP-T） in a low carbon steel were carried out by the Gleeble-3500 hot simulation machine at a temperature range of Ae3-Ar3. The overall stress-strain ...A series of tests of deformation-induced ferrite transformation （DIP-T） in a low carbon steel were carried out by the Gleeble-3500 hot simulation machine at a temperature range of Ae3-Ar3. The overall stress-strain curves during DIFT can be divided into three typical types： ＂double-humped＂,＂ single-humped＂ and ＂transitional＂. The peaks exhibited in the curve are involved with deformation-induced transformation which happened in grains or at the grain boundaries. According to the stress-time curve and strain-time curve, strain capacity dramatically postponed the strain-induced transformation, which leads to the start of the transformation right ahead of the finish of deformation and the majority of the ferrite transformation process mainly happened after the deformation. Deformation-induced transformation is a metadynamic transformation process with dynamic nucleation.展开更多
On the basis of transformation kinetics and thermodynamics, the austenite-ferrite transformation start temperature during deformation was predicted for several grades of low-carbon steels under different processing co...On the basis of transformation kinetics and thermodynamics, the austenite-ferrite transformation start temperature during deformation was predicted for several grades of low-carbon steels under different processing conditions. Results indicate that Ar3d temperature mostly depended on alloying composition and processing parameters. Ar3d increased as strain rate or strain increased for the same steel grade. In view of enhancement of deformation on transformation, the basic kinetics model was established to simulate deformation induced transformation behavior, using which the influence of the deformation stored energy and effective deformation ledge on the nucleation and growth can be considered. The simulated results are in good agreement with experiment results.展开更多
Hot deformation processing was designed to study the effects of niobium （Nb） on DIFT. A prestrain of 0.51 at 880 ℃ for different isothermal time was used for adjusting the deformed austenite constitution and Nb exi...Hot deformation processing was designed to study the effects of niobium （Nb） on DIFT. A prestrain of 0.51 at 880 ℃ for different isothermal time was used for adjusting the deformed austenite constitution and Nb existing state, followed by a secondary heavy deformation at 780 ℃ for inducing the ferrite transformation. The volume fraction and grain size of deformation induced ferrite （DIF） obtained at different isothermal time between double hits were investigated. It was found that Nb dissolved in austenite is adverse to DIFT; however, the precipitation of Nb is beneficial to DIFT. As Nb plays the role in the conventional TMCP, Nb retards the recrystallization of deformed austenite and enhances the deformation stored energy in the multipass deformation, and in result, Nb promotes DIFT.展开更多
In this work,DIFT technology and Q&P process were combined in order to introduce ultrafine-grained ferrite into the matrix of martensite and retained austenite to develop a new kind of advanced high strength steel...In this work,DIFT technology and Q&P process were combined in order to introduce ultrafine-grained ferrite into the matrix of martensite and retained austenite to develop a new kind of advanced high strength steel,and two kinds of steels were investigated by this novel combined process.The newly designed process resulted in a sophisticated microstructure of a large amount of ferrite(about 5 m in diameter),martensite and a considerable amount of retained austenite for TRIP 780 steel.The ultimate tensile strength can reach about 1200 MPa with elongation above 16% for TRIP 780,that is much higher than the one solely treated by Q&P process.Tensile tests showed that both steels with the novel combined process achieved a good combination of strength and ductility,indicating that the new process is promising for the new generation of advanced high strength steels.展开更多
The microstructures and mechanical properties of deformation induced ferrite （DIF） in the low carbon steel Q235 under different deformation temperatures have been investigated systematically. Through deformation ind...The microstructures and mechanical properties of deformation induced ferrite （DIF） in the low carbon steel Q235 under different deformation temperatures have been investigated systematically. Through deformation induced ferrite transformation （DIFT）, ferrite grain can be refined to 3 μm and accounts for above 85% of the overall fraction. Yield strength of DIF （〉500 MPa） is increased by up to 100% compared with the conventional low carbon steel. Comparison of microstructure and mechanical properties in the Q235 steel with DIF and tempered DIF microstructure illustrates that the strengthening mechanism of DIF microstructure is the combination of grain boundary strengthening and carbon supersaturated strengthening. Electron back-scattered diffraction （EBSD） analysis and high magnification scanning electron microscopy （SEM） observation denote that high-angle grain boundary among ultrafine ferrite grain and the transformation product of retain austenite membrane along ferrite boundaries are responsible for the stability of ferrite grain size during tempering process. Transmission electron microscopy （TEM） analysis demonstrates that the transformation product of retained austenite membrane between ferrite grain boundaries is cementite.展开更多
A 0.06%C low carbon steel was deformed in torsion over the temperature range 877-917℃in a 2% H_2 - Ar gas atmosphere.Strains of 0.25 -5.0 were applied at strain rates ofε= 0.04 s^(-1) andε= 0.4 s^(-1) to study the ...A 0.06%C low carbon steel was deformed in torsion over the temperature range 877-917℃in a 2% H_2 - Ar gas atmosphere.Strains of 0.25 -5.0 were applied at strain rates ofε= 0.04 s^(-1) andε= 0.4 s^(-1) to study the formation of ferrite by dynamic transformation(DT) at temperatures above the A_(e3).The critical strain for ferrite formation by DT was aboutε= 0.2 and its volume fraction increased with strain and decreased with temperature above the A_(e3).Average ferrite grain sizes of 1.5μm to 5μm were produced,which decreased with strain rate.At the lower strain rate(ε= 0.04 s^(-1)) reverse transformation(RT) took place during deformation once an incubation time of about 40 s,was exceeded.An increase in strain rate fromε= 0.04 s^(-1) toε= 0.4 s^(-1) arrested RT during testing at all temperatures as the total test times did not exceed 13 s.The present work shows that DT is favored at higher strain rates by increasing the driving force(i.e.stored energy ) and by suppressing RT.展开更多
The ultra-fine-grained ferrite(UFGF) with the size of less than 1 μm is often difficult to be obtained for low-alloyed steel in practical production processing.In this study,considering the rod and wire production pr...The ultra-fine-grained ferrite(UFGF) with the size of less than 1 μm is often difficult to be obtained for low-alloyed steel in practical production processing.In this study,considering the rod and wire production process,a new method for preparing the UFGF with submicron scale is proposed by warm deformation of six passes with total strain of 2.6,followed by the cooling process in Gleeble-3500 thermo-mechanical simulator.The results show that the UFGF with an average size of 0.64 μm could be obtained via the phase transformation from austenite grains with an average size of 3.4 μm,which are achieved by the deformation-induced reversal austenization during the high strain rate warm deformation.The main driving force for the reversal transformation is the stress.And the interval between the passes also plays an important role in the reversal austenization.展开更多
文摘Deformation-induced ferrite transformation （DIFT） has been proved to be an effective approach to refine ferrite grains. This paper shows that the ferrite grains can further be refined through combination of DIFT and V or V-N microalloying. Vanadium dissolved in γ matrix restrains DIFT. During deformation, vanadium carbonitrides rapidly precipitate due to strain-induced precipitation, which causes decrease in vanadium dissolved in matrix and indirectly accelerates DIFT. Under heavy deformation, deformation induced ferrite （DIF） grains in V microalloyed steel were finer than those in V free steel. The more V added to steel, the finer DIF grains obtained. Moreover, the addition of N to V microalloyed steels can remarkably accelerate precipitation of V, and then promote DIFT. However, DIF grains in V-N microalloyed steel easily coarsen.
文摘A series of tests of deformation-induced ferrite transformation （DIP-T） in a low carbon steel were carried out by the Gleeble-3500 hot simulation machine at a temperature range of Ae3-Ar3. The overall stress-strain curves during DIFT can be divided into three typical types： ＂double-humped＂,＂ single-humped＂ and ＂transitional＂. The peaks exhibited in the curve are involved with deformation-induced transformation which happened in grains or at the grain boundaries. According to the stress-time curve and strain-time curve, strain capacity dramatically postponed the strain-induced transformation, which leads to the start of the transformation right ahead of the finish of deformation and the majority of the ferrite transformation process mainly happened after the deformation. Deformation-induced transformation is a metadynamic transformation process with dynamic nucleation.
基金This work was financially supported by the High Technology Development Program(No.2001AA339030)the National Nat ural Science Foundation of China(No.50100404).
文摘On the basis of transformation kinetics and thermodynamics, the austenite-ferrite transformation start temperature during deformation was predicted for several grades of low-carbon steels under different processing conditions. Results indicate that Ar3d temperature mostly depended on alloying composition and processing parameters. Ar3d increased as strain rate or strain increased for the same steel grade. In view of enhancement of deformation on transformation, the basic kinetics model was established to simulate deformation induced transformation behavior, using which the influence of the deformation stored energy and effective deformation ledge on the nucleation and growth can be considered. The simulated results are in good agreement with experiment results.
基金Item Sponsored by National Key Technologies Research and Development Program of China(G1998061502)
文摘Hot deformation processing was designed to study the effects of niobium （Nb） on DIFT. A prestrain of 0.51 at 880 ℃ for different isothermal time was used for adjusting the deformed austenite constitution and Nb existing state, followed by a secondary heavy deformation at 780 ℃ for inducing the ferrite transformation. The volume fraction and grain size of deformation induced ferrite （DIF） obtained at different isothermal time between double hits were investigated. It was found that Nb dissolved in austenite is adverse to DIFT; however, the precipitation of Nb is beneficial to DIFT. As Nb plays the role in the conventional TMCP, Nb retards the recrystallization of deformed austenite and enhances the deformation stored energy in the multipass deformation, and in result, Nb promotes DIFT.
基金supported by the National Engineering Research Center of Advanced Steel Technology (NERCAST)the National Basic Research Program of China "973 Program" (Grant No. 2010CB630803)the National Natural Science Foundation of China (Grant No. 51174251)
文摘In this work,DIFT technology and Q&P process were combined in order to introduce ultrafine-grained ferrite into the matrix of martensite and retained austenite to develop a new kind of advanced high strength steel,and two kinds of steels were investigated by this novel combined process.The newly designed process resulted in a sophisticated microstructure of a large amount of ferrite(about 5 m in diameter),martensite and a considerable amount of retained austenite for TRIP 780 steel.The ultimate tensile strength can reach about 1200 MPa with elongation above 16% for TRIP 780,that is much higher than the one solely treated by Q&P process.Tensile tests showed that both steels with the novel combined process achieved a good combination of strength and ductility,indicating that the new process is promising for the new generation of advanced high strength steels.
基金supported by the National Natural Science Foundation of China (NSFC) under Grant No. 50871109
文摘The microstructures and mechanical properties of deformation induced ferrite （DIF） in the low carbon steel Q235 under different deformation temperatures have been investigated systematically. Through deformation induced ferrite transformation （DIFT）, ferrite grain can be refined to 3 μm and accounts for above 85% of the overall fraction. Yield strength of DIF （〉500 MPa） is increased by up to 100% compared with the conventional low carbon steel. Comparison of microstructure and mechanical properties in the Q235 steel with DIF and tempered DIF microstructure illustrates that the strengthening mechanism of DIF microstructure is the combination of grain boundary strengthening and carbon supersaturated strengthening. Electron back-scattered diffraction （EBSD） analysis and high magnification scanning electron microscopy （SEM） observation denote that high-angle grain boundary among ultrafine ferrite grain and the transformation product of retain austenite membrane along ferrite boundaries are responsible for the stability of ferrite grain size during tempering process. Transmission electron microscopy （TEM） analysis demonstrates that the transformation product of retained austenite membrane between ferrite grain boundaries is cementite.
文摘A 0.06%C low carbon steel was deformed in torsion over the temperature range 877-917℃in a 2% H_2 - Ar gas atmosphere.Strains of 0.25 -5.0 were applied at strain rates ofε= 0.04 s^(-1) andε= 0.4 s^(-1) to study the formation of ferrite by dynamic transformation(DT) at temperatures above the A_(e3).The critical strain for ferrite formation by DT was aboutε= 0.2 and its volume fraction increased with strain and decreased with temperature above the A_(e3).Average ferrite grain sizes of 1.5μm to 5μm were produced,which decreased with strain rate.At the lower strain rate(ε= 0.04 s^(-1)) reverse transformation(RT) took place during deformation once an incubation time of about 40 s,was exceeded.An increase in strain rate fromε= 0.04 s^(-1) toε= 0.4 s^(-1) arrested RT during testing at all temperatures as the total test times did not exceed 13 s.The present work shows that DT is favored at higher strain rates by increasing the driving force(i.e.stored energy ) and by suppressing RT.
基金financially supported by the National Natural Science Foundation of China (Grant Nos. 51574107, 51501056, 51975593)the Natural Science Foundation of Hebei Province (Grant Nos. E2015209243, E2017209048)+1 种基金the Liaoning Provincial Natural Science Foundation of China (Grant No. 2019-KF-25-01)the Research Funds from Department of Education of Hebei Province (Grant Nos. QN2019051, ZD 2019064).
文摘The ultra-fine-grained ferrite(UFGF) with the size of less than 1 μm is often difficult to be obtained for low-alloyed steel in practical production processing.In this study,considering the rod and wire production process,a new method for preparing the UFGF with submicron scale is proposed by warm deformation of six passes with total strain of 2.6,followed by the cooling process in Gleeble-3500 thermo-mechanical simulator.The results show that the UFGF with an average size of 0.64 μm could be obtained via the phase transformation from austenite grains with an average size of 3.4 μm,which are achieved by the deformation-induced reversal austenization during the high strain rate warm deformation.The main driving force for the reversal transformation is the stress.And the interval between the passes also plays an important role in the reversal austenization.
