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1.
ZHOU Xiaoguang LIU Zhenyu YUAN Xiangqian WU Di WANG Guodong LIU Xianghua 《钢铁研究学报(英文版)》2008,15(3)
Considering the effect of strain and chemical composition on precipitation behavior, new models for the start and end time of Nb(C,N) precipitation in austenite under the conditions of different temperatures and strains have been investigated for Nb microalloyed steel. The value of n in the precipitation kinetic equation has been determined by using the available experimental data in literature, which indicated that n is a constant and independent of temperature. The values of the start and end time of the predicted precipitation are compared with the experimental values. Calculated results are in good agreement with the experimental results. Also, the evolution of austenite grains before ferrite transformation is simulated by taking the effect of precipitation into consideration. The measured austenite grain size is in good agreement with predicted one prior to ferrite transformation. 相似文献
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Precipitation kinetics of Nb(C,N) in microalloyed steels is crucial for the achievement of favoured steel properties. Therefore, numerous experimental studies have been performed in the past and various theoretical models have been developed to describe Nb(C,N) precipitation. However, the experimental data is sometimes contradictory and even the thermodynamic data for NbC solubility in austenite have a large scatter. In this paper, experimental results on the Nb(C,N) and NbV(C,N) precipitation kinetics in deformed and undeformed austenite are reviewed. Based on these data and with the precipitation kinetics module of the software package MatCalc, computer simulations are performed. The predicted interfacial energy of precipitates is adjusted to match the observed kinetics. A comparison between experimental information and simulation, i.e. time ‐ temperature ‐ precipitation (TTP) diagrams, is drawn and discussed. The results of the computer simulations using modified interfacial energies are in good agreement with the experiments. 相似文献
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The isothermal decomposition of austenite has been examined in a set of 0.1 C, 1.4 Mn steels containing small amounts of Ti,
V, or Nb. The volume fraction of ferrite was measured as a function of transformation temperature and holding time, after
hot rolling. Precipitation of carbonitrides, in both the austenite and the ferrite, was examined by electron microscopy of
extraction replicas. The decomposition is slowest in the Nb-alloyed steel, in which the start of transformation is delayed
and ferrite growth rates are much lower than in the other steels. In the V-alloyed steels, ferrite growth rates are lower
than in the plain carbon or Ti alloyed steels. These results are discussed in terms of the effects of carbonitride precipitation
in the austenite during high temperature deformation and in the ferrite during transformation. The roles of V and Nb in solution
are also considered. 相似文献
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Hot deformation processing was designed to study the effects of niobium (Nb) on DIFT. A prestrain of 051 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. 相似文献
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针对碳质量分数为0.47%中碳高铁车轮钢,研究了铌微合金化对前驱体为铁素体-珠光体的组织发生奥氏体逆相变的影响。结果表明,铁素体-珠光体钢的逆相变是一个由碳原子扩散控制的过程,奥氏体优先在珠光体内的铁素体与渗碳体(α/Fe3C)片层界面处形核,并且沿平行于珠光体片层方向的长大速率比垂直于珠光体片层方向更快。含铌车轮钢细化的珠光体组织可以提高奥氏体的形核率,有利于细化奥氏体晶粒。随着再加热温度的提高,含铌车轮钢的奥氏体混晶温度(960 ℃)比不含铌的钢高80 ℃,因此通过铌微合金化可扩大再加热奥氏体化温度窗口。结合Thermal-Calc热力学计算和透射电镜分析,铌在中碳钢中主要以析出物的形式存在,析出钉扎作用是其细化奥氏体晶粒、推迟混晶现象出现的主要机制。 相似文献
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为了研究铌对高强抗震钢筋生产过程中组织转变的影响,通过热模拟试验对比研究了无铌碳素钢筋及铌微合金化钢筋(铌质量分数为0.03%)形变奥氏体在不同冷却速率下的组织和相变规律,获得动态CCT曲线。研究结果表明,添加0.03%铌使试验钢奥氏体连续冷却转变有明显变化。从连续冷却曲线(CCT)可看出,添加铌后,发生先共析铁素体、珠光体相变的冷却速度范围减小,铁素体、珠光体转变温度降低;贝氏体相变的冷却速度区间整体右移。添加铌能细化组织,各冷却速度下含铌钢的硬度均大于无铌钢。利用TEM对不同冷却速度下含铌钢中析出相进行观察,发现Nb(C,N)弥散分布于钢中,随着冷却速度的增加,析出的Nb(C,N)逐渐减少,析出相尺寸呈先减小后增大的规律,2 ℃/s冷却速度冷却得到的析出相尺寸细小且数量较多。 相似文献
9.
C. Capdevila F. G. Caballero C. García de Andrés 《Metallurgical and Materials Transactions A》2001,32(7):1591-1597
The present article is concerned with the theoretical and experimental study of the growth kinetics of idiomorphic ferrite
in a medium-carbon vanadium-titanium microalloyed steel. A theoretical model is presented to calculate the evolution of isothermal
austenite-to-idiomorphic ferrite transformation with time for a given temperature. Moreover, the nature, size, and distribution
of the inclusions that are responsible for the intragranular nucleation of idiomorphic ferrite have been characterized by
scanning electron microscopy (SEM). Finally, the influence of austenite grain size in the isothermal decomposition of austenite
in idiomorphic ferrite has been thoroughly analyzed. An excellent agreement (higher than 90 pct in R
2) has been obtained between the experimental and predicted values of the volume fraction of idiomorphic ferrite. 相似文献
10.
C. Capdevila F. G. Caballero C. García de Andrés 《Metallurgical and Materials Transactions A》2001,32(3):661-669
The present article is concerned with the theoretical and experimental study of the growth kinetics of allotriomorphic ferrite
in medium carbon vanadium-titanium microalloyed steel. A theoretical model is presented in this work to calculate the evolution
of austenite-to-allotriomorphic ferrite transformation with time at a very wide temperature range. At temperatures above eutectoid
temperature, where allotriomorphic ferrite is the only austenite transformation product, the soft-impingement effect should be taken into account in the modeling. In that case, the Gilmour et al. analysis reliably predicts the progress of austenite-to-allotriomorphic ferrite transformation in this steel. By contrast,
since pearlite acts as a carbon sink, the carbon enrichment of austenite due to the previous ferrite formation is avoided,
and carbon concentration in austenite far from the α/γ interface remains the same as the overall carbon content of the steel. Hence, the soft-impingement effect should be neglected,
and allotriomorphic ferrite is considered to grow under a parabolic law. Therefore, assumption of a semi-infinite extent austenite
with constant boundary conditions is suitable for the kinetics of the isothermal decomposition of austenite. An excellent
agreement (higher than 93 pct in R
2) has been obtained between the experimental and predicted values of the volume fraction of ferrite in all of the ranges of
temperature studied. 相似文献
11.
C. Capdevila F. G. Caballero C. García de Andrés 《Metallurgical and Materials Transactions A》2001,32(13):661-669
The present article is concerened with the theoretical and experimental study of the growth kinetics of allotriomorphic ferrite
in medium carbon vanadium-titanium microalloyed steel. A theoretical model is presented in this work to calculate the evolution
of austente-to-allotriomorphic ferrite transformation with time at a very wide temperature range. At temperatures above eutectoid
temperature, where allotriomorphic ferrite is the only austenite transormation product, thesoft-impingement effect should be taken into account in the modeling. In that case, the Gilmouret al., analysis reliably predicts the progress of austenite-to-allotriomorphic ferrite transformation in this steel. By contrast,
since pearlite acts as a carbon sink, the carbon enrichment of austenite due to the previous ferrite formation is avoided,
and carbon concentration in austenite far from the α/λ interface remains the same as the overal carbon content of the steel.
Hence, the soft-impingement effect should be neglected, and allotriomorphic ferrite is considered to grow under a parabolic
law. Therefore, assumption of a semi-infinite extent austenite with constant boundary conditions is suitable for the kinetics
of the isothermal decomposition of austenite. An excellent agreement (higher than 93 pct inR
2) has been obtained between the experimental and predicted values of the volume fraction of ferrite in all of the ranges of
temperature studied.
C. CAPDEVILA, Research Associate, formerly with the Department of Physical Metallurgy, Centro Nacional de Investigaciones
Metalurgicas (CENIM), Consejo Superior de Investigaciones Cientificas (CSIC), 28040 Madrid, Spain 相似文献
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Thermo‐mechanical simulation tests were performed on V–Ti–N microalloyed steel under three hot working conditions by using Gleeble‐3800 thermo‐mechanical simulator to study the effects of hot deformation and post‐deformation holding process on the continuous cooling transformation behaviors of overcooled austenite. The continuous cooling transformation diagrams (CCT diagrams) were determined by thermal dilation method and metallographic method. The effects of the hot deformation, post‐deformation holding, and cooling rate on the microstructure evolution were analyzed. The results show that deformation promotes ferrite and pearlite transformation. In addition, deformation leads to an increase in bainite start temperature, which becomes more markedly with the increase in cooling rate. The post‐deformation holding process is much favorable to promote carbonitride precipitation of the microalloying elements, which contributes to ferrite nucleation and smaller austenite grains. As a result, an increase in ferrite quantity and a decrease in ferrite grain size can be observed. And further more, the post‐deformation holding process reduces the effect of hot deformation on the bainite start temperature. 相似文献
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Yves J. M. Bréchet Christopher R. Hutchinson Hatem S. Zurob Chad W. Sinclair 《国际钢铁研究》2007,78(3):210-215
A promising new method for steel design is based on controlling alloy chemistry and thermomechanical processing parameters to tailor microstructural evolution though an explicit understanding of the physical mechanisms governing microstructural change. Additions of Nb have been shown to have a large effect on microstructural processes in steels and this contribution summarizes recent work on elucidating the effect of Nb on the processes of recrystallization in ferrite and the kinetics of the austenite to ferrite phase transformation. In particular, emphasis is placed on distinguishing the effects of Nb in solution and Nb present as Nb‐containing precipitates. Nb in solution is shown to have a very strong effect on the recrystallization in ferrite and this can be quantified and understood in terms of the well‐known solute‐drag effect. The effect of NbC particles on the kinetics of the austenite to ferrite phase transformation is, however, less clear. Theoretical considerations would lead us to expect interphase boundary carbide precipitation to influence the transformation rate but novel decarburization experiments suggest this is not the case. This illustrates that although we are making progress on our understanding of the physical mechanisms governing change in Nb containing steels, there remains a number of important issues requiring further work. 相似文献
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During the continuous casting of low‐carbon Nb–Ti microalloyed steel, control of the slab surface microstructure and the behavior of the second‐phase precipitation are significantly influenced by the cooling rate. Through confocal laser scanning microscopy, the effect of the cooling rate on the behavior of ferrite precipitation both at the grain boundary and within the austenite was observed in situ and analyzed. The relationship between the cooling rate and precipitation of the microalloying elements on the slab surface microstructure was further analyzed by transmission electron microscopy. The results showed that the effect of microalloying element precipitation on proeutectoid ferrite phase transformation is mainly manifested in two aspects: (i) the carbonitrides of microalloying elements act as inoculant particles to promote nucleation of the proeutectoid ferrite and (ii) the carbon near the grain boundary is depleted when the microalloying elements precipitate into carbonitrides, inducing a decrease in the local carbon concentration and promoting ferrite precipitation. 相似文献
16.
通过金相、扫描电镜,相变临界点和过冷奥氏体等温转变的分析,研究了Nb对共析钢(质量分数:C 075%,C 0.78%)珠光体相变微观组织和等温转变动力学的影响。结果表明:微量Nb(质量分数为0.04%、0064%)的加入使钢中先共析铁素体的量较未含铌钢有所增加,这很可能是Nb的加入使得共析碳含量明显升高的结果,同时使珠光体相变产物的形貌发生明显变化,珠光体中渗碳体的展弦比显著降低。此外Nb提高了先共析铁素体和珠光体的开始转变温度,即Nb的加入在一定程度上降低了过冷奥氏体的热力学稳定性;相变动力学表明加入Nb 0.04%可使珠光体相变的鼻子点温度升高约50℃,且最快开始相变时间比不含铌钢推迟一个数量级以上,即Nb的加入推迟了鼻子点和较低温度下的珠光体相变,提高了钢的淬透性。 相似文献
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通过Gleeble热模拟实验研究了含0.038%Nb(质量分数)的热轧TRIP钢在高温奥氏体区的热加工工艺,借助光学显微镜、扫描电镜和透射电镜分析了组织演变和Nb的析出行为,并利用电感耦合等离子体发射光谱仪定量分析了Nb的固溶/析出程度.在1250℃奥氏体化5 min后添加Nb有70%固溶于奥氏体.在1000℃以上的奥氏体再结晶区变形过程中Nb的析出量仅占总固溶量的3%,不能有效抑制静态再结晶,奥氏体晶粒得到明显细化.在900℃的奥氏体未再结晶区变形前析出Nb量已达到总固溶量的9%,再结晶被抑制而获得拉长状奥氏体.奥氏体未再结晶区变形可促进铁素体转变并细化铁素体晶粒.再结晶奥氏体或形变奥氏体状态下冷却至650℃时分别有占总添加量的48%和40%的Nb仍以固溶态存在. 相似文献
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In this article, a detailed study was conducted to evaluate the microstructural evolution and mechanical properties of microalloyed
steels processed by thermomechanical schedules incorporating cool deformation. Cool deformation was incorporated into a full
scale simulation of hot rolling, and the effect of prior austenite conditioning on the cool deformability of microalloyed
steels was investigated. As well, the effect of varying cooling rate, from the end of the finishing stage to the cool deformation
temperature, 673 K (400 °C), on mechanical properties and microstructural evolution was studied. Transmission electron microscopy
(TEM) analysis, in particular for Nb containing steels, was also conducted for the precipitation evaluation. Results show
that cool deformation greatly improves the strength of microalloyed steels. Of the several mechanisms identified, such as
work hardening, precipitation, grain refinement, and strain-induced transformation (SIT) of retained austenite, SIT was proposed,
for the first time in microalloyed steels, to be a significant factor for strengthening due to the deformation in ferrite.
Results also show that the effect of precipitation in ferrite for the Nb bearing steels is greatly overshadowed by SIT at
room temperature. 相似文献
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A new theory and kinetic modeling of strain-induced precipitation of Nb(CN) in microalloyed austenite 总被引:1,自引:0,他引:1
W. J. Liu 《Metallurgical and Materials Transactions A》1995,26(7):1641-1657
A new theory of strain-induced precipitation of Nb(CN) in microalloyed austenite has been developed by considering the effect
of nonequilibrium segregation of Nb on the nucleation and growth of the precipitates. With the aid of the theory, the discrepancy
existing between the conventional theory for strain-induced precipitation and experimental data can be reconciled. A kinetic
model has been developed based on the new theory. The model is capable of predicting both the Nb(CN) precipitation kinetics
and the resulting microstructure in microalloyed austenite. Excellent agreement has been obtained between the model predictions
and the experimental observations reported by different investigators. 相似文献
20.
M J Crooks A J Garratt-Reed J B Vander Sande W S Owen 《Metallurgical and Materials Transactions A》1982,13(8):1347-1353
The effect of vanadium on the isothermal austenite-ferrite transformation, between 725 °C and 775 °C, of a hot-deformed microalloyed
steel has been studied by examination of the microstructure and measurement of the volume fraction of ferrite in specimens
quenched from the reaction temperature. The accompanying precipitation was studied by transmission electron microscopy of
thin foils and carbon extraction replicas and by electron energy-loss spectroscopy. Very early in the transformation a continuous
band of fine-grained ferrite forms at austenite grain boundaries. After some time some of these grains coarsen to form large
equiaxed ferrite grains. It is found that vanadium has no effect on the time to the start of coarsening but thereafter accelerates
the rate of formation of ferrite. Interphase precipitation of VN occurs throughout the transformation in the vanadium steels
and this is thought to influence the rate at which the ferrite coarsens at the lower temperatures (750 ° and 725 °C) in the
range studied. 相似文献