Grain Growth of Deformation Induced Ferrite in V Microalloyed Low Carbon Steel During Controlled Cooling Process

The effect of vanadium on the DIFT (Deformation Induced Ferrite Transformation) microstructure coarsening in low carbon steel during the continuous cooling processes and isothermal processes at different temperatures were investigated using thermo-simulator.The results showed that the steel containing a small amount of vanadium had the similar velocity of grain growth with the vanadium free steel during the continuous cooling process,but a lower velocity of grain growth than that of vanadium free steel during isothermal processes at high temperatures.On the other hand,the vanadium remarkably inhibited grain growth in the steel containing a high amount of vanadium during both the continuous cooling and isothermal processes.Vanadium dissolved in matrix is indicated as an important factor on restraining grain growth through estimating the driving force of normal grain growth and the resistance of precipitation particles of vanadium on grain growth.The influencing mechanism of vanadium dissolved in matrix on the grain growth during the controlled cooling process is discussed.     

钒对高氮钢形变诱导相变的影响

通过实验室模拟试验,研究了高氮钒钢的金相组织和形变诱导铁素体体积分数,分析了钒对高氮钒钢变形诱导相变的影响.试验结果表明,钒的加入促进了形变诱导铁素体相变,细化了铁素体晶粒,提高了形变诱导铁素体体积分数.     

Effect of Dissolved and Precipitated Niobium in Microalloyed Steel on Deformation Induced Ferrite Transformation （DIFT）

 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.     

工艺参数对耐蚀钢12CuCrNiV组织与性能的影响

采用热力模拟试验机、光学显微镜、显微硬度计研究了耐蚀钢12CuCrNiV在不同冷却速率下的连续冷却组织转变规律,并绘制其CCT曲线,同时研究了形变温度和冷却速度对耐蚀钢热变形后的组织和硬度的影响规律。结果表明:连续冷却转变情况下,耐腐蚀钢在冷速小于15℃/s时,有铁素体转变;冷速小于1℃/s时,有珠光体转变;冷速在0.5~20℃/s之间时,有贝氏体转变。控制冷速在5~15℃/s可得到铁素体和贝氏体复相组织。随变形温度的降低,试验钢形变过程中形变诱导铁素体相变现象显著,且伴随有M/A岛生成;随冷却速度的增高,形变诱导相变现象减弱,M/A岛数量减少。与连续冷却试验相比较,形变诱导析出现象明显,其硬度增量为40~50HV,形变可使试验钢的析出向更高冷速移动。     

Microstructural stability of ultrafine grained low-carbon steel containing vanadium fabricated by intense plastic straining

Two grades of low-carbon steel, one containing vanadium and the other without vanadium, were subjected to equal channel angular pressing (ECAP) at 623 K up to an effective strain of ∼4. After equal channel angular pressing, a static annealing treatment for 1 hour was undertaken on both pressed steels in the temperature range of 693 to 873 K. By comparing the microstructural evolution during annealing and the tensile properties of the two steels, the effect of the addition of vanadium on the thermal stability of ultrafine-grained (UFG) low-carbon steel fabricated by intense plastic straining was examined. For the steel without vanadium, coarse recrystallized ferrite grains appeared at annealing temperatures above 753 K, and a resultant degradation of the strength was observed. For the steel containing vanadium, submicrometer-order ferrite grain size and ultrahigh strength were preserved up to 813 K. The enhanced thermal and mechanical stabilities of the steel containing vanadium were attributed to its peculiar microstructure, which consisted of ill-defined pearlite colonies and ultrafine ferrite grains with uniformly distributed nanometer-sized cementite particles. This microstructure resulted from the combined effects of (a) the preservation of high dislocation density providing an effective diffusion path, due to the effect of vanadium on increasing the recrystallization temperature of the steel; and (b) precipitation of fine cementite particles at ferrite grain boundaries through the enhanced diffusion of carbon atoms (which were dissolved from pearlitic cementite by severe plastic straining) along ferrite grain boundaries and dislocation cores.     

Microstructure and Precipitation Behavior in HAZ of V and Ti Microalloyed Steel

Three steels containing 0. 05%C-0. 1%V-0. 01%N (steel V-LN), 0. 05%C-0. 1%V-0. 02%N (steel V-HN), and 0. 05%C-0. 1%V-0. 02%N-0. 01%Ti (steel V-HN-Ti), which were all essentially vanadium microalloyed steels, were subjected to simulating the microstructure of a coarse grained heat affected zone (CGHAZ). The process involved reheating to 1 350 °C, rapid cooling to room temperature, and varying the welding heat input from 15 kj/cm to 54 kj/cm, including four cooling rates of t_8/5 equal to 7. 5 s, 20 s, 40 s, 100 s, and the relationship of heat input to tg/_s was calculated by Quiksim software. The microstructure and precipitation of vanadium and titanium carbon nitrides are studied. The results indicate that the microstructure consists of granular bainite and some side plate ferrite in the grain boundary when the steels are produced with the highest heat input. As the heat input decreased, numerous polygonal ferrites and grain boundary ferrites appeared, and the size apparently increased. When the steel contained high nitrogen, it was considerably easier to form martensite-austenite island, which was even worse for the toughness and other properties of the steel. For the limitation of cooling time, vanadium carbon nitrides could not precipitate sufficiently, but as titanium was added, the unmelted or precipitated TiN on cooling absorbed some fraction of nitrogen in the matrix and made more precipitate positions for the round V(C, N), and thus several useful round particles could be seen in titanium-contained steel, and most of them were around TiN. By this experiment, we can conclude that with the help of titanium, nitrogen-enhanced steel had a better prior austenite grain size, was considerably easier to precipitate, reduced free nitrogen in the matrix effectively, and provided a very effective mechanism for restriction grain growth in the HAZ.     

Effect of Thermo‐Mechanical Process on Phase Transformation Behaviors of V–Ti–N Microalloyed Steel

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.     

微合金化控轧控冷钢筋纵向金相组织研究

 对微合金化控轧控冷钢筋的纵向金相组织进行了研究,并分析了不同成分试验钢纵向“条带”组织的差异及形成原因。研究结果表明：偏析元素（P、Si、Mn等）在轧制过程中沿轧制方向呈条状分布,是20MnSi、20MnSiV钢产生带状组织的原因。铌及其碳氮化物的溶质拖曳和“钉扎”作用,使20MnSiNb钢的奥氏体未再结晶轧制温度提高到1050℃,在冷却过程中,先共析铁素体在形变奥氏体晶界和内部变形带均匀析出,随后沿形变奥氏体晶界（在先共析铁素体与奥氏体的界面上）生成珠光体带,最后在形变奥氏体晶粒内部形成贝氏体条。研究条件下优势形核点的排序为：形变奥氏体晶界和形变奥氏体晶内变形带、偏析元素和夹杂、再结晶奥氏体晶界。     

Effect of Austenite Deformation on Continuous Cooling Transformation Microstructures for 22CrSH Gear Steel

 The effect of compressive deformation of austenite on continuous cooling transformation microstructures for 22CrSH gear steel has been investigated using a Gleeble 1500 thermal simulator. The experimental results show that the deformation of austenite promotes the formation of proeutectoid ferrite and pearlite, and leads to the increase of critical cooling rate of proeutectoid ferrite plus pearlite microstructure. The grain boundary allotriomorphic ferrite occupies the austenite grain surfaces when the prior deformation takes place or the cooling rate is decreased, which causes a transition from bainite to acicular ferrite. The deformation enhances the stability of transformation from austenite to acicular ferrite, which results in an increase of M/A constituent.     

低碳钢中形变诱导铁素体相变过程中的应力—应变曲线研究

利用热模拟机Gleeble-3500在温度Ae3～Ar3之间对低碳钢进行了形变诱导铁素体相变的试验研究,发现常见的形变诱导铁素体相变过程中的应力—应变曲线可归纳为三种情况:"双峰"、"单峰"以及"过渡类型"。曲线上所表现出来的峰值是由形变诱导相变在晶界或者晶内发生而导致的软化效应所引起的,是加工硬化和相变软化的平衡点。对应变—时间曲线和应力—时间曲线的分析表明,形变对诱导相变过程中在晶内开始大量发生相变的时刻具有明显的推迟作用,形变诱导相变在形变接近结束之时才开始大量发生,导致大部分的相变过程发生在应变结束之后。研究发现,形变诱导铁素体相变具有亚动态相变的特征,是一个动态形核的亚动态相变过程。     

Effects of thermomechanical processing on the microstructure and mechanical properties of a Ti-V-N steel

Based on studies of austenite deformation behavior and continuous-cooling-transformation behavior of a Ti-V microalloyed steel by cam plastometer and quench-deformation dilatometer, respectively, plate rolling schedules were designed to produce (i) recrystallized austenite, (ii) unrecrystallized austenite, (iii) deformed ferrite + unrecrystallized austenite. The effects of austenite condition and cooling rate on the final microstructure and mechanical properties were investigated. To rationalize the variation in final ferrite grain size with different thermomechanical processing schedules, it is necessary to consider the kinetics of ferrite grain growth in addition to the density of ferrite nucleation sites. The benefit of dilatometer studies in determining the optimum deformation schedule and cooling rate for a given steel is domonstrated. A wide range of tensile and impact properties results from the different microstructures studied. Yield strength is increased by increasing the amount of deformed ferrite, bainite, or martensite, and by decreasing the ferrite grain size. Impact toughness is most strongly influenced by ferrite grain size and occurrence of rolling plane delaminations. B. Dogan, Formerly with CANMET, Ottawa, Canada,     

Modeling for Formation of Proeutectoid Ferrite in Steel during Continuous Cooling

A novel model of the evolution of microstructure during continuous cooling with the formation of proeutectoid ferrite in steel was proposed from a Voronoi construction for the austenite grains, based on the Rappaz‘s integral nucleation model and the assumption that the ferrite nucleates at the edges of the original austenite grains and the successive growth of the ferrite grain is radial. The model can be used to calculate the fraction of ferrite as a function of time or temperature during continuous cooling, and to determine the microstructure of ferrite. The calculated results are in agreement with experimental results investigated in 0.38C-0.28Si-0.55Mn-0.92Cr-0.20Mo steel under continuous cooling using a Gleeble 1500 thermomechanical simulator.     

低碳钢在形变作用下产生的超平衡数量铁素体

试验研究了低碳钢变形后经历等温及随后快冷的组织转变,发现在Ae3附近及以下一定温度范围内大变形会使室温组织中出现超平衡数量的铁素体.形变后等温,出现超平衡数量铁素体向奥氏体的逆相变,该弛豫过程具有类似C曲线的动力学特征;而且经过等温后,伴随着铁素体数量的减少存在其晶粒尺寸增大的现象.随着变形温度的降低,超平衡数量铁素体在数量以及晶粒尺寸上的稳定性均提高.     

变形诱导铁素体相变现象与理论

晶粒超细化是钢铁材料的重要发展方向，而变形诱导铁素体相变是最接近于现行钢铁生产TMCP的晶粒超细化方法，应用前景良好。从实验证实、热力学、动力学、影响因素和相变机制5个方面系统地评述了变形诱导铁素体相变。对存在的学术分歧既进行了客观评述，又明确地展示了作者自己的观点。     

Investigation into the stress-strain curves of deformation-induced ferrite transformation in a low carbon steel

A series of tests of deformation-induced ferrite transformation ( DIFT) in a low carbon steel were carried out by the Gleeble-3500 hot simulation machine at a temperature range of Ae 3 -Ar 3 . 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.     

奥氏体形变对50CrV4钢相变行为的影响

利用Thermecmastor-Z型热模拟试验机,结合金相显微镜(OM)、扫描电镜(SEM)、维氏硬度计等,系统研究了奥氏体区变形对50CrV4钢连续冷却相变和等温相变规律的影响。建立了试验钢动态CCT曲线。研究结果表明,奥氏体变形能促进连续冷却转变过程中铁素体-珠光体、贝氏体转变,但亦可提高奥氏体的机械稳定性,进而抑制马氏体转变,Ms点由331.6℃(奥氏体未变形)降低至291℃(950℃下变形50%+890℃下变形50%,变形速率均为5s-1,变形后冷速为20℃/s)。当轧后冷速小于0.5℃/s时,试验钢中可获得铁素体+珠光体组织。此外,在研究不同变形量对试验钢等温相变规律影响时发现,650℃等温时,试验钢中发生铁素体-珠光体相变。随着变形量的增加(由30%增加至50%),其等温相变动力学加快(相变完成时间由197.6s减小至136.5s),铁素体体晶粒尺寸、珠光体片层间距减小,硬度增加。     

镁处理对船体钢组织细化作用的高温原位分析

 船体钢经镁处理后其焊接热影响区性能显著提升。为进一步弄清镁的作用机制,采用高温激光共聚焦显微镜原位观察传统船体钢和镁处理船体钢在升降温过程中显微组织的演变过程,并对不同保温时间下钢组织在连续冷却过程中的相变进行研究。通过对比发现,镁处理船体钢在升温过程中因微细夹杂物粒子的钉扎作用使得奥氏体晶粒尺寸并未发生粗化,并且在冷却过程中奥氏体晶粒内部出现大量夹杂物诱发的针状铁素体,使钢组织得到进一步细化,而夹杂物这种诱发晶内针状铁素体的能力则呈现出随保温时间延长而逐渐增强的规律。与此同时,夹杂物数量及分布情况的统计也为上述结果提供了有力佐证。     

冷却速度对0.45C钢铸坯夹杂物和组织的影响

借助控制冷却速度的高温激光共聚焦显微镜,结合光学显微镜和扫描电镜、能谱分析仪研究1150℃至500℃时,冷却速度100~10℃/min对0.45C钢铸坯中夹杂物的属性、诱发晶内铁素体形核行为的影响。结果表明,随冷却速度减小,高熔点夹杂物基本没有变化,低熔点夹杂物的数量和尺寸有增加的趋势,尤其是钒的碳氮化物数量增加明显;随冷却速度降低,析出的晶内铁素体的数量增加,平均粒径增大,铁素体比例先增加后减小。当30℃/min连续冷却时,钢中夹杂物尺寸小于10μm比例高达93%,铁素体平均粒径为9.2μm,铁素体面积比例达30.2%。