Fe-C-Mn-Al系TRIP钢两相区奥氏体转变模拟

为了研究Fe-C-Mn-A1系TRIP钢两相区奥氏体化过程中合金元素在奥氏体和铁素体中的分布,利用热膨胀仪、金相显微镜、电子探针等仪器,在对TRIP钢两相区奥氏体化过程进行热力学与动力学分析的基础上,建立了两相区奥氏体化过程的扩散模型,采用显式有限体积法对800℃与840℃的奥氏体化过程进行了数值求解.模拟结果表明:奥氏体转变初期受C元素在奥氏体中的扩散控制达到亚平衡,奥氏体转变速率较快;此时A1元素在奥氏体与铁素体界面处的浓度差较显著,Mn元素在奥氏体与铁素体界面处的浓度差不显著.奥氏体转变后期受Mn元素在铁素体内的扩散控制,转变速率较慢;此时A1元素在铁素体内已大量富集,Mn元素在奥氏体与铁索体界面处有较显著的浓度差.     

Formation of Austenite During Intercritical Annealing of Dual-Phase Steels

The formation of austenite during intercritical annealing at temperatures between 740 and 900 °C was studied in a series of 1.5 pct manganese steels containing 0.06 to 0.20 pct carbon and with a ferrite-pearlite starting microstructure, typical of most dual-phase steels. Austenite formation was separated into three stages: (1) very rapid growth of austenite into pearlite until pearlite dissolution is complete; (2) slower growth of austenite into ferrite at a rate that is controlled by carbon diffusion in austenite at high temperatures (~85O °C), and by manganese diffusion in ferrite (or along grain boundaries) at low temperatures (~750 °C); and (3) very slow final equilibration of ferrite and austenite at a rate that is controlled by manganese diffusion in austenite. Diffusion models for the various steps were analyzed and compared with experimental results.     

Modeling of ferrite growth in nodular cast iron

In nodular cast iron, ferrite forms around the graphite nodules and growth proceeds until pearlite nucleates and consumes the remaining austenite. In order to simulate the structure, it is therefore necessary to have accurate models for the ferrite growth. Some investigators have proposed that the growth is completely governed by carbon diffusion through the ferrite shell. In the present work, it is shown that the ferrite growth in nodular cast iron can be divided into three different stages where the growth initially is governed by carbon diffusion in the austenite until the graphite nodule is entirely enveloped by a ferrite shell. During the second stage, it is proposed that the growth is controlled by the incorporation rate of carbon atoms on the graphite nodule. During the later stages of the transformation, the diffusion distance has increased considerably, and therefore, the diffusion of carbon through the ferrite shell will determine the growth rate.     

Monte carlo-method simulation of the deformation-induced ferrite transformation in the Fe-C system

A Monte Carlo (MC) technique has been used to model deformation-induced ferrite transformation (DIFT) in an Fe-C binary system on a mesoscale. The effects of strain rate, strain, and recrystallization of the matrix on DIFT are investigated. Increasing the strain rate slightly retards the onset of DIFT. The volume fraction of ferrite increases gradually as the strain increases before the volume fraction of ferrite reaches its saturation value. After the volume fraction of ferrite becomes saturated, it oscillates around its saturation value. The recrystallization of austenite slightly retards the onset of the DIFT. Although the recrystallization of austenite reduces the equilibrium volume fraction of ferrite significantly, it cannot completely suppress DIFT. The stress concentration has been shown to induce the nucleation of ferrite near the grain boundaries and phase boundaries. The significance of the reverse transformation has been investigated. We found that there is a temporal oscillation of the volume fraction of ferrite and the stored energy after they arrive at their saturation values. We conclude that this oscillation and the effect of the strain rate on DIFT are both brought about by the reverse transformation from induced ferrite to undeformed austenite. The diffusion behavior of carbon atoms in the systems is different for different strain rates. The simulation shows that the dynamic recovery of austenite cannot occur in the system during deformation under the present conditions. The results of the simulation show that, other than the oscillation of the equilibrium volume fraction of ferrite and the unusual diffusion behavior of carbon atoms, the simulation agrees well with the corresponding experimental results. The temporal oscillation of the volume fraction of ferrite and stored energy and the unusual diffusion behavior are two new phenomena that have not been reported by other researchers.     

The solidification sequence in an 18-8 stainless steel,investigated by directional solidification

The directional solidification technique was applied in order to investigate the complicated solidification sequence in a commercial austenitic stainless steel which was known to yield a primary precipitation of § ferrite when cast into a 5 tons ingot. Three stages of solidification were found. The first precipitation of § ferrite was interrupted by precipitation of austenite and at the end of the solidification there was a transition back to precipitation of § ferrite. The competition between the first two stages is affected by the cooling rate and the nitrogen content. The precipitation of austenite from the melt results in the usual coring whereas ô ferrite forms with a very homogeneous composition, presumably due to rapid diffusion in this phase. On cooling austenite forms from the § ferrite and this reaction also results in coring, presumably due to rapid diffusion in § ferrite.     

Modeling diffusional growth during austenite decomposition to ferrite in polycrystalline FeC alloys

A grain-boundary-nucleated, diffusional growth model of austenite decomposition to proeutectoid ferrite is developed for polycrystalline iron-carbon alloys. The diffusion equation is solved under restricted diffusion conditions using the quasi-static method and employing local thermodynamic equilibrium at the disordered austenite:ferrite interface. Decomposition kinetics for a model polycrystalline material consisting of a log-normal distribution of spherical grains are calculated numerically. Effects of temperature, overall carbon concentration, volume change, austenite grain size and carbon buildup in the centers of the austenite grains are included in the treatment. A scaling factor is deduced that enables the effect of austenite grain size on transformation kinetics to be characterized provided kinetic information is available for one grain size. Experiments carried out on a laboratory steel verified the applicability of the scaling factor, Also, partial I-T and C-T diagrams can be computed from the model and sample calculations are presented for an iron + 0.036 wt% carbon steel.     

中锰钢两相区退火奥氏体逆转变的数值分析

根据中锰钢热轧组织结构确立两相区奥氏体化的几何模型和初始条件,利用DICTRA动力学分析软件对中锰钢马氏体基体奥氏体化过程进行计算分析.在奥氏体化初期的形核过程中,马氏体中过饱和的碳锰元素从铁素体迅速转移到奥氏体并在相界面奥氏体一侧聚集.后续的相变过程中,碳在奥氏体中快速均化,但锰在相界面奥氏体一侧的聚集加剧.相变初期奥氏体界面推移速度比中后期高出若干个数量级,但随时间推移迅速衰减.相变初期相界面推移是碳扩散主导,相变后期界面推移受到锰在奥氏体中扩散速度制约.温度升高可显著提高相界面推移速度.达到相同数量奥氏体的情况下,低温长时退火有利于锰从铁素体向奥氏体转移并提高其在奥氏体中的富集度,从而提高奥氏体的稳定性.      

Decomposition of hypo-eutectoid Fe-C austenites; a numerical diffusion model

A numerical model is presented which describes the growth rate of ferrite during the decomposition of austenite in Fe-C alloys. The growth rate is modelled assuming carbon diffusion in austenite as the rate determining mechanism. The effect of the definition of the diffusion coefficient of carbon in austenite on the growth rate is shown. The model is used to examine the effects of initial carbon concentration, the local carbon concentration in austenite at the interface and the initial austenite grain size on the growth rate. Good agreement between theoretical results and both new and existing experimental data was observed.     

Growth of intragranular ferrite in Fe-Ni-P alloys

Analytical electron microscopy (AEM) techniques were used to study the growth of intragranular ferrite in Fe-Ni-P alloys. The spatial resolution of the AEM was exploited to gather microchemical information regarding elemental redistribution at ferrite/austenite interfaces in order to determine the growth mechanism for intragranular ferrite. In this alloy system, the growth kinetics are dictated by the bulk diffusion of Ni in austenite. Full equilibrium occurs during intragranular ferrite growth with full partitioning of Ni and P between austenite and ferrite, and chemical equilibrium occurs at the α/γ interface in both phases. A numerical model to simulate ferrite growth was developed based on equilibrium growth considerations. The Ni concentrations and precipitate sizes predicted by the model agree well with those measured by AEM techniques in the experimental alloys. The computer model has been extended to predict the thermal histories of iron meteorites and their parent asteroidal bodies.     

强磁场下保温时间对Fe-0.76%C钢先共析铁素体组织形貌的影响

 借助于光学显微镜研究了磁场（12 T）对Fe 0.76%C合金在807 ℃奥氏体化保温不同时间(10 min、30 min、60 min)后以2 ℃/min的冷速冷却后,先共析铁素体显微组织的形貌变化。结果表明：在相同奥氏体化保温时间下,经强磁场热处理样品的先共析铁素体的面积分数和晶粒数量明显高于无磁场热处理样品。这可归结为强磁场降低了先共析铁素体形核所需的驱动力。随着奥氏体化保温时间的延长先共析铁素体晶粒沿着强磁场方向伸长的趋势明显变弱。这主要是由于奥氏体晶粒随着奥氏体化保温时间的延长逐渐增大,导致铁素体晶核之间的距离增大,从而造成奥氏体中的Fe原子向先共析铁素体晶粒扩散的距离增大所致。     

Partitioning of carbon from supersaturated ferrite plates

The time required for the diffusion of carbon out of suturated plates of ferrite, into the residual austenite, is investiagted using a finite difference model. The results are compare with an earlier approximate analytical solution. It is found that in all cases investigated, the analytical solution underestimates the diffusion time, the discrepany increasing at lower temperatures, or when the concentration of substitutional solutes which stabilise austenit is reduced. This is attributed to the fact that the analytical method fails to take account of the coupling of the diffusion fluxes that arise in both the austenite and the ferrite. The results are discussed in the context of displacive transformations in steels.     

Alloying Element Partition and Growth Kinetics of Proeutectoid Ferrite in Hot-Deformed Fe-0.1C-3Mn-1.5Si Austenite

Alloying element partition and growth kinetics of proeutectoid ferrite in deformed austenite were studied in an Fe-0.1C-3Mn-1.5Si alloy. Very small ferrite particles, less than several microns in size, were formed within the austenite matrix, presumably at twin boundaries as well as at austenite grain boundaries. Scanning transmission electron microscopy–energy-dispersive X-ray (STEM-EDX) analysis revealed that Mn was depleted and Si was enriched in the particles formed at temperatures higher than 943 K (670 °C). These were compared with the calculation of local equilibrium in quaternary alloys, in which the difference in diffusivity between two substitutional alloying elements was assumed to be small compared to the difference from the carbon diffusivity in austenite. Although the growth kinetics were considerably faster than calculated under volume diffusion control, a fine dispersion of ferrite particles was readily obtained in the partition regime due to sluggish growth engendered by diffusion of Mn and Si.     

Calculation of Austenite Formation Kinetics of Copper-Bearing Steel During Continuous Heating

The kinetics of austenite formation in a new type of copper-bearing steel with initial microstructure composed of ferrite and bainite was investigated by using dilatometric analysis and measurement during continuous heating. The formation of austenite was observed to occur in two stages. The first stage is the dissolution of ferrite and most bainite, followed by the second stage of dissolution of bainite and formation of austenite. The first stage takes place through diffusion and the second stage through shear. The critical temperature of austenite formation during continuous heating increases with increasing heating rates, which therefore exerts a greater influence on the As1 temperature of the austenite formation. Kinetics calculation shows that the process is mainly controlled by diffusion when the heating rate is over 1 ℃/s.     

铌微合金化对高铁车轮钢奥氏体形核和长大的影响

 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。     

Modeling of Austenite Decomposition in Low Si-Mn TRIP Steel During Cooling

Transformation behavior in low carbon Si Mn TRIP steel was investigated by means of microstructural ob servation and computer modelling. A transformation model in which transformation is controlled by carbon diffusion was suggested, which well described the volume fractional change of ferrite, pearlite, and bainite during continuous cooling. The influence of Si content and austenite grain size was thoroughly investigated. The calculated results indicated that Si retards pearlite transformation, accelerates polygonal ferrite transformation, refines the austenite grain, and increases the ferrite transformation rate.     

低碳微合金钢晶界铁素体三维形态及形成机理

为了更好地研究低碳微合金钢晶界铁素体的三维形态及形成机理,利用配有电子背散射衍射(EBSD)的聚焦离子束场发射扫描电子显微镜构建了低碳微合金钢不同析出位置的晶界铁素体三维形貌,通过高温激光共聚焦显微镜原位观察技术计算了晶界铁素体在试验条件下的生长速率,最后利用Matlab建立了试样钢成分下不同温度下溶质元素的扩散模型,...     

Non-Steady Equilibrium during Pro-Eutectoid Ferrite Formation in Fe-0.2C Alloy

Ｔｈｅｐｒｅｃｉｐｉｔａｔｉｏｎｏｆｐｒｏ ｅｕｔｅｃｔｏｉｄｆｅｒｒｉｔｅｆｒｏｍａｕｓｔｅｎｉｔｅｉｓｃｏｎｔｒｏｌｌｅｄｂｙｄｉｆｆｕｓｉｏｎｉｎＦｅ Ｃａｌｌｏｙｓ .Ｗｉｔｈｔｈｅａｓｓｕｍｐｔｉｏｎｔｈａｔｔｈｅｍｉｇｒａｔｉｎｇｉｎｔｅｒｆａｃｅｉｓａｌｗａｙｓｉｎｌｏｃａｌｅｑｕｉｌｉｂｒｉｕｍ ,ｅａｒｌｙｉｎ 194 9,ＺｅｎｅｒＣ[1] ｅｘｐｌａｉｎｅｄｔｈｅｄｉｆｆｕｓｉｏｎａｌｇｒｏｗｔｈｏｆｐｒｏ ｅｕｔｅｃ ｔｏｉｄｆｅｒｒｉｔｅｂｙｕｓｉｎｇｔｈｅＦｉｃｋ’ｓｌａｗ .Ｓｉｎｃｅ…     

A kinetic model of the γ → α + Gr eutectoid transformation in spheroidal graphite cast irons

The eutectoid transformation of austenite in spheroidal graphite cast iron can follow one of two paths: (a) transformation to a mixture of ferrite and graphite or (b) transformation to pearlite. The extents to which the two reactions occur determine the relative amounts of ferrite and pearlite in the microstructure and, hence, the properties of the iron. In this paper, the kinetics of the γ → α+ Gr reaction is studied, and a model is developed to predict the isothermal transformation rates. The transformation occurs at a rate determined by the rate of carbon diffusion. The diffusion of carbon through ferrite, as well as through austenite, has been considered. The model predicts that the volume fraction of austenite transformed isothermally increases with increasing number density of graphite spheroids. Predictions of the model are compared with data available in literature.     

上贝氏体铁素体的形核及长大

 贝氏体铁素体的长大速度与转变机制密切相关。应用QUANTA 400型环扫电镜,观测了20CrMo钢和35CrMo钢的贝氏体铁素体形核及长大情况。结果表明,上贝氏体铁素体在原奥氏体晶界形核,可沿着晶界生长,也可平行地向晶内长大。测得贝氏体铁素体片条沿晶界延伸的平均速度为14 998 nm/s,而向晶内长大线速度为17 763 nm/s。应用计算和理论分析方法研究了贝氏体片条的长大机制,认为在晶界形成的上贝氏体铁素体晶核与两侧的奥氏体不同时具有共格界面,因此不能以共格切变长大。按照体扩散和界面扩散进行理论计算,计算结果表明：铁素体长大速度比实测值小3~4个数量级,因此扩散 台阶机制不能成立。另外,提出了上贝氏体铁素体晶核长大的原子热激活跃迁机制。     

Modeling of the Recrystallization and Austenite Formation Overlapping in Cold-Rolled Dual-Phase Steels During Intercritical Treatments

Austenite formation kinetics of a DP1000 steel was investigated from a ferrite–pearlite microstructure (either fully recrystallized or cold-rolled) during typical industrial annealing cycles by means of dilatometry and optical microscopy after interrupted heat treatments. A marked acceleration of the kinetics was found when deformed ferrite grains were present in the microstructure just before austenite formation. After having described the austenite formation kinetics without recrystallization and the recrystallization kinetics of the steel without austenite formation by simple JMAK laws, a mixture law was used to analyze the kinetics of the cold-rolled steel for which austenite formation and recrystallization may occur simultaneously. In the case where the interaction between these two phenomena is strong, three main points were highlighted: (i) the heating rate greatly influences the austenite formation kinetics, as it affects the degree of recrystallization at the austenite start temperature; (ii) recrystallization inhibition above a critical austenite fraction accelerates the austenite formation kinetics; (iii) the austenite fractions obtained after a 1 hour holding deviate from the local equilibrium fractions given by Thermo-Calc, contrary to the case of the recrystallized steel. This latter result could be due to the fact that the dislocations of the deformed ferrite matrix could promote the diffusion of the alloying elements of the steel and accelerate austenite formation.