The dynamic recrystallization （DRX） behavior of Nb-Ti microalloyed steels was investigated by isothermal single compression tests in the temperature range of 900-1 150 ℃ at constant strain rates of 0.1-5 s^-1. DRX ...The dynamic recrystallization （DRX） behavior of Nb-Ti microalloyed steels was investigated by isothermal single compression tests in the temperature range of 900-1 150 ℃ at constant strain rates of 0.1-5 s^-1. DRX was retarded effectively at low temperature due to the onset of dynamic precipitation of Nb and Ti carbonitrides, resulting in higher values of the peak strain. An expression was developed for the activation energy of deformation as a function of the contents of Nb and Ti in solution as well as other alloying elements. A new value of corrective factor was determined and applied to quantify the retardation produced by increase in the amount of Nb and Ti dissolved at the reheating temperature. The ratio of critical strain to peak strain decreases with increasing equivalent Nb content. In addition, the effects of Ti content and deformation conditions on DRX kinetics and steady state grain size were determined. Finally, the kinetics of dynamic precipitation was determined and effect of dynamic precipitation on the onset of DRX was clarified based on the comparison between precipitate pinning force and recrystallization driving force.展开更多
Based on their hot ductilities, Nb, V and Ti microalloying steels can be classified into two groups. The first group includes steels with lower carbon content (≤0.10%). Ductilities of steels of this group recover and...Based on their hot ductilities, Nb, V and Ti microalloying steels can be classified into two groups. The first group includes steels with lower carbon content (≤0.10%). Ductilities of steels of this group recover and rise with decreasing temperature when temperature lowers to below 825℃. Another group includes steels which contain more carbon (>0.12%) or contain more Nb and V. The low ductility temperature Region Ⅲ for steels of mis group extends to temperature as low as 725℃. The occurrence of the transverse corner cracks of the Nb, V and Ti microalloying steel CC slabs has be considerably decreased by stabilizing casting speed, increasing mold steel level automatic control ratio,enhancing caster segment radial alignment and adopting proper secondary cooling patterns to make slab corner temperature at straightening out off the low ductility temperature region.展开更多
The CCT curves of supercooled austenite were investigated by using THERMECMASTOR-Z simulation machine, test samples refined from 50 kg vacuum induction furnace and two-stage controlled roiling in laboratorial conditio...The CCT curves of supercooled austenite were investigated by using THERMECMASTOR-Z simulation machine, test samples refined from 50 kg vacuum induction furnace and two-stage controlled roiling in laboratorial condition. Test results show that influence of cerium （Ce） addition in low sulfur Nb-Ti-Bearing steel is as follows： Ce can increase the transformation point tara and tAr1, enlarge the interval of transformation （tAr3-tAr1） . Ce moves CCT and rightwards , and reduces hardenability. Ce increases bainitic transformation temperature and enlarges curves upwards interval of bainite formation. With higher cooling rates, Ce changes the martensite from coarse strip to fine strip. It decreases Ms, enhances the trend of lath martensitic structure and limits the formation of lamellar crystal.展开更多
Influence of different cooling rates on the microstructure and the precipitation behavior of Nb-Ti microalloyed steel was investigated by CSLM, OM, SEM and EDS. The results show that the precipitation process of carbo...Influence of different cooling rates on the microstructure and the precipitation behavior of Nb-Ti microalloyed steel was investigated by CSLM, OM, SEM and EDS. The results show that the precipitation process of carbonitrides can be in-situ observed by CSLM, and with the increase of the cooling rate, the distribution of precipitates changes from along the austenitic grain boundaries to within the grains. With the increase of the cooling rate, the proeutectoid ferritic film becomes smaller and smaller and then disappears, and the original austenitic grains become finer and finer. In order to obtain non-film like proeutectoid ferrites or non-chain like precipitates along the austenitic grain boundaries and finer austenitic grains,the cooling rate should be at least 5℃/s.展开更多
The effect of rare earth on the microstructures, mechanical properties and inclu sions in low sulphur Nb-Ti-bearing steel were investigated. It is shown that t h e transverse yield point, the traverse tensile strength...The effect of rare earth on the microstructures, mechanical properties and inclu sions in low sulphur Nb-Ti-bearing steel were investigated. It is shown that t h e transverse yield point, the traverse tensile strength and elongation of testin g steels decrease initially and then rise with increasing content of rare earth. The impact energy values of the testing steels exhibit a contrary trend. Proper amount of rare earth in the steels can improve the anisotropy of impact toughne ss above -20 ℃ and it does not affect the type of microstructures which ar e st ill composed of ferrites and pearlites, but the pearlite amount increases. On one hand, rare earth cleans the molten steel and reduces the amount of inclusions; on the other hand, rare earth makes the inclusions spheroidizd, refi ned and dispersed, and thus improves the distribution of inclusions.展开更多
The effects of chemical composition and cooling rate after solidication on the grain coarsening temperature,T_(GC),of the V-Ti-N microalloyed steels have been investigated.It is shown that the T_(GC) may be obviously ...The effects of chemical composition and cooling rate after solidication on the grain coarsening temperature,T_(GC),of the V-Ti-N microalloyed steels have been investigated.It is shown that the T_(GC) may be obviously raised by adding even a little Ti to the base steel so as to pre- cipitate a great deal of fine Ti-bearing particles of about 10 nm.The T_(GC) does not increase with the cooling rate,as it is over a certain critical value.The T_(GC) is insensitive to any varia- tion of N content at simulated cooling condition of 150 mm continuous cast slab.The T_(GC) may be dropped down about 100℃ by adding 0.33 wt-%Mo to the steels.The sensitivi- ty of T_(GC) to cooling condition relates to the Ti and V contents.展开更多
The low carbon Nb-Ti mieroalloyed tested steel was prepared by the process of vacuum induction furnace smelting, forging and hot rolling. The new steel aims to meet the demand of high strength, high toughness and high...The low carbon Nb-Ti mieroalloyed tested steel was prepared by the process of vacuum induction furnace smelting, forging and hot rolling. The new steel aims to meet the demand of high strength, high toughness and high plasticity for building facilities. The effects of quenching process on microstructure and mechanical properties of tested steel were investigated. The results showed that prior austenite grain size, phase type and precipitation behavior of （ Nb, Ti） （ C, N） play important roles in mechanical properties of the steel. Through modified appropriately, the model of austenite grain growth during heating and holding is d^5.7778 = 5. 6478^5.7778 ＋ 7.04 × 10^22t^1.6136 exp（- 427. 15 ×10^3 /（RT））. The grain growth activation energy is Qg = 427. 15 kJ. During quenching, the microscopic structures are mainly martensite and lath bainite which contains lots of lath substructure and dislocations. The content of phases, fine and coarsening （ Nb, Ti ） （ C, N ） precipitated changes during different quenching temperatures and holding time. Finally compared with the hardness value, the best quenching process can be obtained that heating temperature and holding time are 900 ℃ and 50 mins, respectively.展开更多
The hot rolling experiment investigates into the relationship between the microstructures and the mechanical properties of Nb-Ti microalloyed steels with various Ti contents. The results indicate that the effect of th...The hot rolling experiment investigates into the relationship between the microstructures and the mechanical properties of Nb-Ti microalloyed steels with various Ti contents. The results indicate that the effect of the bainite fraction of Nb-Ti microalloyed steels on the yield strength of the steels is not related to Ti content, while the slope of the Hall- Petch relationship decreases with the increase of Ti content. Accordingly, the Misra model for the yield strength of Nb-Ti microalloyed steels is modified, and the factors which cause the change in the slope of Hall-Petch relationship are discussed.展开更多
基金the National Natural Science Foundation of China(No.50474086,50334010)the Program for New Century Excellent Talents in Universities(No.NCET-04-0278)of the Ministry of Education
文摘The dynamic recrystallization （DRX） behavior of Nb-Ti microalloyed steels was investigated by isothermal single compression tests in the temperature range of 900-1 150 ℃ at constant strain rates of 0.1-5 s^-1. DRX was retarded effectively at low temperature due to the onset of dynamic precipitation of Nb and Ti carbonitrides, resulting in higher values of the peak strain. An expression was developed for the activation energy of deformation as a function of the contents of Nb and Ti in solution as well as other alloying elements. A new value of corrective factor was determined and applied to quantify the retardation produced by increase in the amount of Nb and Ti dissolved at the reheating temperature. The ratio of critical strain to peak strain decreases with increasing equivalent Nb content. In addition, the effects of Ti content and deformation conditions on DRX kinetics and steady state grain size were determined. Finally, the kinetics of dynamic precipitation was determined and effect of dynamic precipitation on the onset of DRX was clarified based on the comparison between precipitate pinning force and recrystallization driving force.
文摘Based on their hot ductilities, Nb, V and Ti microalloying steels can be classified into two groups. The first group includes steels with lower carbon content (≤0.10%). Ductilities of steels of this group recover and rise with decreasing temperature when temperature lowers to below 825℃. Another group includes steels which contain more carbon (>0.12%) or contain more Nb and V. The low ductility temperature Region Ⅲ for steels of mis group extends to temperature as low as 725℃. The occurrence of the transverse corner cracks of the Nb, V and Ti microalloying steel CC slabs has be considerably decreased by stabilizing casting speed, increasing mold steel level automatic control ratio,enhancing caster segment radial alignment and adopting proper secondary cooling patterns to make slab corner temperature at straightening out off the low ductility temperature region.
文摘The CCT curves of supercooled austenite were investigated by using THERMECMASTOR-Z simulation machine, test samples refined from 50 kg vacuum induction furnace and two-stage controlled roiling in laboratorial condition. Test results show that influence of cerium （Ce） addition in low sulfur Nb-Ti-Bearing steel is as follows： Ce can increase the transformation point tara and tAr1, enlarge the interval of transformation （tAr3-tAr1） . Ce moves CCT and rightwards , and reduces hardenability. Ce increases bainitic transformation temperature and enlarges curves upwards interval of bainite formation. With higher cooling rates, Ce changes the martensite from coarse strip to fine strip. It decreases Ms, enhances the trend of lath martensitic structure and limits the formation of lamellar crystal.
文摘Influence of different cooling rates on the microstructure and the precipitation behavior of Nb-Ti microalloyed steel was investigated by CSLM, OM, SEM and EDS. The results show that the precipitation process of carbonitrides can be in-situ observed by CSLM, and with the increase of the cooling rate, the distribution of precipitates changes from along the austenitic grain boundaries to within the grains. With the increase of the cooling rate, the proeutectoid ferritic film becomes smaller and smaller and then disappears, and the original austenitic grains become finer and finer. In order to obtain non-film like proeutectoid ferrites or non-chain like precipitates along the austenitic grain boundaries and finer austenitic grains,the cooling rate should be at least 5℃/s.
文摘The effect of rare earth on the microstructures, mechanical properties and inclu sions in low sulphur Nb-Ti-bearing steel were investigated. It is shown that t h e transverse yield point, the traverse tensile strength and elongation of testin g steels decrease initially and then rise with increasing content of rare earth. The impact energy values of the testing steels exhibit a contrary trend. Proper amount of rare earth in the steels can improve the anisotropy of impact toughne ss above -20 ℃ and it does not affect the type of microstructures which ar e st ill composed of ferrites and pearlites, but the pearlite amount increases. On one hand, rare earth cleans the molten steel and reduces the amount of inclusions; on the other hand, rare earth makes the inclusions spheroidizd, refi ned and dispersed, and thus improves the distribution of inclusions.
文摘The effects of chemical composition and cooling rate after solidication on the grain coarsening temperature,T_(GC),of the V-Ti-N microalloyed steels have been investigated.It is shown that the T_(GC) may be obviously raised by adding even a little Ti to the base steel so as to pre- cipitate a great deal of fine Ti-bearing particles of about 10 nm.The T_(GC) does not increase with the cooling rate,as it is over a certain critical value.The T_(GC) is insensitive to any varia- tion of N content at simulated cooling condition of 150 mm continuous cast slab.The T_(GC) may be dropped down about 100℃ by adding 0.33 wt-%Mo to the steels.The sensitivi- ty of T_(GC) to cooling condition relates to the Ti and V contents.
基金Sponsored by the Major State Basic Research Development Program of China(Grant No.2010CB630801)
文摘The low carbon Nb-Ti mieroalloyed tested steel was prepared by the process of vacuum induction furnace smelting, forging and hot rolling. The new steel aims to meet the demand of high strength, high toughness and high plasticity for building facilities. The effects of quenching process on microstructure and mechanical properties of tested steel were investigated. The results showed that prior austenite grain size, phase type and precipitation behavior of （ Nb, Ti） （ C, N） play important roles in mechanical properties of the steel. Through modified appropriately, the model of austenite grain growth during heating and holding is d^5.7778 = 5. 6478^5.7778 ＋ 7.04 × 10^22t^1.6136 exp（- 427. 15 ×10^3 /（RT））. The grain growth activation energy is Qg = 427. 15 kJ. During quenching, the microscopic structures are mainly martensite and lath bainite which contains lots of lath substructure and dislocations. The content of phases, fine and coarsening （ Nb, Ti ） （ C, N ） precipitated changes during different quenching temperatures and holding time. Finally compared with the hardness value, the best quenching process can be obtained that heating temperature and holding time are 900 ℃ and 50 mins, respectively.
文摘The hot rolling experiment investigates into the relationship between the microstructures and the mechanical properties of Nb-Ti microalloyed steels with various Ti contents. The results indicate that the effect of the bainite fraction of Nb-Ti microalloyed steels on the yield strength of the steels is not related to Ti content, while the slope of the Hall- Petch relationship decreases with the increase of Ti content. Accordingly, the Misra model for the yield strength of Nb-Ti microalloyed steels is modified, and the factors which cause the change in the slope of Hall-Petch relationship are discussed.
