Nb-Ti微合金钢中第二相析出的热力学计算

分析了Nb-Ti微合金钢在液相、凝固过程、奥氏体以及铁素体相中氮化物、碳化物和碳氮化物等第二相析出的热力学条件,计算了钛和铌的碳化物和氮化物在不同温度下的溶度积,由此得出微合金钢在不同相阶段的析出规律。计算结果表明,在该微合金钢中,氮化物、碳化物和碳氮化物不可能在液相中析出;TiC0.03N0.97和TiN在凝固过程中具备析出的热力学条件;在奥氏体相中,随着温度的降低,氮化物、碳化物及碳氮化物的析出顺序为:NbC0.75N0.25、NbC、TiC、NbN。     

Nb-Ti微合金高强度钢1.5mm冷轧板退火组织和第二相析出行为

用透射电镜实验研究了(%):0.08C-1.0Mn-Nb+Ti<0.10微合金高强度钢经53%冷变形1.5 mm板650℃和680℃退火的组织和第二相析出行为。结果表明,试验钢中的第二相为(Ti,Nb)(C,N)复合析出相,第二相粒子尺寸一般为20~30 nm,随退火温度提高,第二相粒子的数量增加。由于退火过程第二相析出强化和第二相粒子抑制晶粒长大,使钢中晶粒细小,该钢650℃退火组织具有较高强度(屈服强度≥480 MPa)。     

Ti微合金钢热变形后连续冷却相变及第二相析出行为

采用热力模拟试验技术、金相显微镜(OM)、扫描电镜(SEM)及透射电镜(TEM),并结合宏观硬度测试,研究了钛微合金钢形变奥氏体的连续冷却相变和组织演变规律,建立了试验钢连续冷却转变曲线,探讨了不同冷速对第二相析出的影响。结果表明,不同冷速下,试验钢获得了不同的微观组织,随冷速增加,第二相析出量增多,尺寸更细小,但冷速过高,析出被抑制。     

一种Nb-Ti微合金钢微合金碳氮化物析出行为的研究

利用热模拟和TEM技术研究了Nb—Ti微合金钢中微合金碳氮化物的析出行为,研究结果表明,高温奥氏体区析出的微合金碳氮化物数量随变形量的增大而增加,尺寸随着变形温度的升高稍有增大。铁素体区析出的微合金碳氮化物尺寸比在形变奥氏体中析出的更为细小,数量随着保温时间的增加而增多,但尺寸变化不大;当温度较低的时候,微合金碳氮化物主要在位错线等晶内缺陷处析出。     

Ti-Nb微合金钢及焊接热影响区中的第二相粒子

利用萃取复型和焊接热模拟技术，对0．14C-1．34Mn-0．017Ti-0．023Nb微合金钢及模拟焊接粗晶热影响区中第二相粒子的形态、数量及物相进行了研究。结果表明，母材中第二相粒子形状不规则，Nb的相对含量(Nb／(Nb Ti))在25％～82％之间的粒子平均直径为14．2am，尺寸较大的粒子(～50nm)含Ti较高，呈方形，尺寸较小的粒子中Nb含量较高，呈球形。焊接热循环后，第二相粒子数量显著减小，平均尺寸增大，呈方形。800℃至500℃冷却时间t8/5从16s增加至60s时，粒子中Nb的相对含量为20％～50％，粒子平均尺寸由31．4nm增大至37．2nm，粒子数量由1．95/μm^2减少至1．20／μm^2。但t8/5从60s增至120s时，因冷速慢，析出温度低，粒子平均尺寸减小至26．3nm，粒子数量增加至3．56／μm^2，又重新出现一些含Nb量较高的球形粒子。     

EAF-CSP流程V-N微合金钢中第二相的析出

目前,在EAF-CSP流程中通过V-N微合金化技术可成功生产屈服强度为550 MPa的高强度低合金钢.然而,基于热力学平衡考虑,对铸坯中V(C,N)的析出分析存在争议.针对珠钢CSP流程生产的V-N微合金钢,采用物理化学相分析方法、透射电镜的复型分析以及扫描电镜观察了流程中不同阶段试样的V(C,N)析出情况.并通过Thermo-Calc 理论计算,初步探讨了EAF-CSP流程V-N微合金钢中Ti以及Al的析出对V(C,N)析出的影响.     

Nb-Ti微合金钢中的奥氏体晶粒长大行为研究

Nb、Ti是管线钢中常用的合金元素。主要通过热处理和喷碳处理等手段研究了合金元素Nb、Ti的含量及加热制度对再加热奥氏体晶粒长大的影响。试验结果表明:试验钢在再加热过程中,奥氏体晶粒尺寸随加热温度升高而增大;在常规含铌钢中,为获得较小的加热态奥氏体晶粒,钛的质量分数应控制在一定范围内(0.010%~0.015%),钛含量过高或过低都对晶粒细化有不利影响。此外,在钛含量相同的情况下,高铌钢奥氏体晶粒长大明显,高铌钢的最佳钛含量范围也与常规含铌钢的最佳钛含量不同。     

加热工艺对Nb-Ti微合金钢奥氏体晶粒长大的影响

 为了解加热制度对Nb Ti微合金钢的奥氏体晶粒长大和析出行为的影响,采用OM、TEM和EDS分析技术,研究了Nb Ti微合金钢在不同加热温度和保温时间的奥氏体晶粒长大行为,以及微合金元素碳氮化物析出行为。结果表明,随加热温度升高,奥氏体晶粒尺寸逐渐长大,当加热温度超过1 200 ℃时奥氏体晶粒尺寸快速长大。随保温时间延长,奥氏体晶粒尺寸逐渐长大,当保温时间超过2.0 h时奥氏体晶粒尺寸快速长大。EDS分析显示Nb Ti钢中的析出物为(Nb,Ti)(C,N)复合相,随着加热温度升高和保温时间延长,析出相体积分数减少,尺寸增大,从而减弱对奥氏体晶粒的细化作用;Nb Ti微合金试验钢合适的加热温度范围为1 150～1 200 ℃,保温时间低于2.0 h。     

Ti微合金钢及其焊接粗晶区中的第二相粒子分析

利用萃取复型及金属薄膜电子显微技术对Ｔｉ微合金钢及其热影响区中的第二相粒子进行了分析,结果表明Ｔｉ微合金钢及其热影响区中的主要粒子为ＴｉＮ,此外还有少量ＡｌＮ等。经热循环后,粒子的尺寸明显增大、数量减少。与Ｔｉ、Ｎ含量比较高的钢相比,Ｔｉ／Ｎ较低的钢中ＴｉＮ粒子数量较多,尺寸较小,且热循环过程中ＴｉＮ粒子的溶解及粗化程度较小,对原始奥氏体晶粒长大的阻碍作用较大。     

取向电工钢加工过程中第二相粒子的析出行为

利用场发射扫描电镜观察了以MnS为主要抑制剂的普通取向电工钢加工过程中第二相粒子的分布状态,统计了粒子面密度、平均尺寸以及相应的尺寸分布.结果显示,热轧加工造成了大量第二相粒子弥散、细小地析出,同时基体仍保持过饱和状态.冷轧变形会造成第二相粒子的回溶行为,而基体的过饱和状态会减弱回溶现象.中间退火与脱碳退火过程中会同时存在新粒子的形核及已析出粒子的粗化两个过程,而在最终二次再结晶升温阶段则以第二相粒子明显粗化为主.     

铌-钛微合金化高强钢连续冷却的相变规律

利用Gleeble 3500热模拟试验机研究了铌-钛微合金化试验钢在变形与未变形条件下的连续冷却相变规律。研究结果表明:试验钢在2~50℃/s的较大冷速范围内均可获得贝氏体组织,且随着冷却速度的增加,组织中粒状贝氏体的量下降,板条贝氏体的量增加;同时变形促进相变,有利于奥氏体中新相的形成。用热膨胀法建立了试验钢静态与动态条件下的连续冷却转变(CCT)曲线。     

V含量对V-Mo-N微合金钢贝氏体区析出行为的影响

通过热模拟和透射电镜(TEM)等研究方法对三种不同V含量的V-Mo-N微合金钢在贝氏体铁素体基体中析出粒子分布、形貌和尺寸进行了观察和分析。结果表明:在V-Mo-N微合金钢中,当Mo的添加量较低时,析出粒子为V(C,N),Mo元素全部固溶于基体中起到固溶强化作用,并不会形成Mo的碳氮化物析出。随着V含量的提高,组织中的贝氏体板条逐渐变细并向粒状贝氏体转变。V含量在0.05%~0.16%时,试验钢析出量随V含量增加而增加,同时,钢的硬度也增加。     

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.     

Effect of Cooling Rates on the Second‐Phase Precipitation and Proeutectoid Phase Transformation of a Nb–Ti Microalloyed Steel Slab

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.     

钼、铬对低碳铌钛微合金钢连续冷却转变行为的影响

 为研究合金元素钼、铬对低碳铌钛微合金钢连续冷却转变行为的影响,采用Gleeble-3800热模拟试验机和热膨胀试验方法,测定了钼、铬含量不同的3种低碳铌钛成分微合金钢在不同冷却速度下的相变点,采用光学显微镜及扫描电子显微镜观察了其转变产物的微观组织,同时结合维氏硬度测试,绘制了动态CCT曲线。结果表明,钼和铬均降低奥氏体向针状铁素体转变的相变温度,并且在冷速大于1℃/s时,钼比铬的作用效果更加明显。钼、铬均能抑制先共析铁素体和珠光体的转变,扩大针状铁素体形成冷速范围,并能够显著细化组织。     

加热温度对钛铌复合微合金化高强钢析出和性能的影响

在实验室冶炼了一种低碳高强度微合金钢,进行了轧制和回溶试验,对轧制试样进行了拉伸试验﹑检验了金相组织,并用TEM对比分析了不同加热温度下试样的析出物形貌、成分、尺寸。结果表明,1 280℃加热保温后轧制产品的性能优于1 180℃加热轧制后产品的性能;两种加热温度下轧制试样的组织都由铁素体和珠光体组成,晶粒尺寸基本相同;轧制试样的析出物均为(Ti,Nb)(C,N)复合析出,但1 280℃加热温度的试样析出更加细小、数量更多。析出强化理论计算表明,加热温度高的试样析出强化要比加热温度低的试样高131 MPa。回溶试验表明,铸坯在1 280℃保温1.5 h后,析出物基本回溶,而1 180℃保温1.5 h后,析出物不能充分溶解,证明了加热温度对钛铌微合金化高强钢析出强化有较大的影响。     

Nb-Ti微合金钢的静态再结晶行为

 通过双道次压缩试验研究了Nb-Ti微合金钢的静态再结晶行为,确定了应变诱导沉淀析出前Nb-Ti微合金钢的静态再结晶激活能,并建立了静态再结晶动力学模型。采用面积法及积分-能量法计算的软化率,很好地反映了微合金钢的静态再结晶行为及应变诱导沉淀析出行为。应变诱导沉淀析出的鼻尖温度在900～925℃之间,静态再结晶的临界温度（SRCT）高于950℃。     

Kinetics of Precipitation Behavior of Second Phase Particles in Ferritic Ti-Mo Microalloyed Steel

 Two types of stress relaxation tests were carried out to investigate the incubation time for incipient precipitation of Ti(C,N) in deformed austenite and (Ti,Mo)C in ferrite of ferritic Ti-Mo microalloyed steel. The size distribution, amount and chemical composition of precipitates were obtained by using physicochemical phase analysis, and calculated according to thermodynamics and kinetics. The experimental results demonstrated that the incubation time was reduced with increasing Ti content, and prolonged with the addition of Mo. After 30% deformation at 850 ℃, the nucleation of strain-induced Ti(C,N) was a relatively slow process. On the other hand, the temperature where the nucleation rate of (Ti,Mo)C in ferrite was the highest descended first and then ascended with increasing Ti content, and so did the temperature where the incubation time was the shortest. The key point is that the temperature of steel containing about 0. 09% Ti is the lowest. The mass fraction of MC-type particles with size smaller than 10 nm in steel containing 0. 09% Ti and 0. 2% Mo reached 73. 7%. The size distributions of precipitates in steel containing 0. 09% Ti were relatively concentrated compared with that in steel containing 0. 07% Ti.