Dissolving of Nb and Ti carbonitride precipitates in microalloyed steels

The dissolving behaviour of Nb and Ti carbonitride precipitates inmicroalloyed steels during isothermal holding at 1300℃ was investigated by Transmission electronmicroscopy (TEM) and energy dispersion x-ray spectrum (EDX). It was found that all precipitates inNb-Ti microalloyed steel are (Nb, Ti)(C, N). With holding time increasing, the atomic ratio of Nb/Tiin precipitates decrease gradually. These precipitates still existe even after holding for 48 h at1300℃ while Nb(C, N) precipitates dissolve away in Nb microalloyed steel only after 4 h at the sametemperature. These results show that formation and thermostability of precipitates are considerablyinfluenced by interaction between Nb and Ti.

Behavior of precipitation in bainitic steel during relaxation processing of RPC technique

Thermal simulation test, TEM (Transmission Electron Microscope) and nanobeamEDS (Energy Dispersed x-ray Spectrum) techniques were used to investigate the precipitation behaviorof Nb, Ti, Mo etc. in HSLA (High Strength Low Alloy) steel. The strain induced precipitationoccurred during the isothermal relaxation stage after deformed in the non-recrystallizationtemperature region. After 30% predeformation at 850 and 900℃, there are two kinds of particles,(Ti,Nb)(C,N) and a few Nb(C,N), to precipitate during holding. The content of Nb in particles riseswith the relaxation time increasing. During the final holding stage, some Nb and Ti atoms in thelattice sites of the precipitates would be replaced by Mo atoms, and the Mo content in theprecipitates increases with the relaxation time. The results were compared with the refinementeffect of microstructures caused by relaxation-precipitation controlling transformation (RPC)processing.

Precipitation behavior of Ti in high strength steels

Precipitation behavior of Ti in high strength steels was investigated by means of the equilibrium solid solubility theory. The contributions of Ti content to yield strength were calculated. The calculated results were verified by the hot rolling experiment for C–Mn steel and C–Mn–Ti micro alloyed steel, respectively. The research results show that the precipitates are mainly Ti N at the higher temperature. With the decreasing temperature, the proportion of Ti C in precipitates increases gradually. When the temperature drops to 800 °C, Ti C will become predominant for the precipitation of Ti. When Ti content is less than 0.014%(mass fraction), Ti has little influence on the yield strength. When Ti content is in the range of 0.014%–0.03%(mass fraction), the yield strength of Ti micro alloyed steel is greatly increased, which leads to instability of the mechanical properties of the steel. Therefore, the design of Ti content in high strength steels should avoid this Ti content range. When Ti content is higher than 0.03%, the yield strength increases stably. In this experiment, when added Ti content was controlled in the range of 0.03%–0.05%, the contribution to the yield strength of Ti micro alloyed steel can reach about 92.44 MPa.

Influence of Ti(C,N)precipitates on austenite growth of micro-alloyed steel during continuous casting

Austenite grain size is an important influence factor for ductility of steel at high temperatures during continuous casting. Thermodynamic and kinetics calculations were performed to analyze the characteristics of Ti(C,N) precipitates formed during the continuous casting of micro-alloyed steel. Based on Andersen-Grong equation, a coupling model of second phase precipitation and austenite grain growth has been established, and the influence of second precipitates on austenite grain growth under different cooling rates is discussed. Calculations show that the final sizes of austenite grains are 2.155, 1.244, 0.965, 0.847 and 0.686 mm, respectively, under the cooling rate of 1, 3, 5, 7, and 10 ℃·s^(-1), when ignoring the pinning effect of precipitation on austenite growth. Whereas, if taking the pinning effect into consideration, the grain growth remains stable from 1,350 ℃, the calculated final sizes of austenite grains are 1.46, 1.02, 0.80, 0.67 and 0.57 mm, respectively. The sizes of final Ti(C,N) precipitates are 137, 79, 61, 51 and 43 nm, respectively, with the increase of cooling rate from 1 to 10 ℃·s^(-1). Model validation shows that the austenite size under different cooling rates coincided with the calculation results. Finally, the corresponding measures to strengthen cooling intensity at elevated temperature are proposed to improve the ductility and transverse crack of slab.

烧结过程氮气分压对(Ti, Nb)(C,N)基金属陶瓷组织和性能的影响

以(Ti, Nb)(C,N)固溶体粉末为原料在不同氮气分压下制备(Ti, Nb)(C,N)基金属陶瓷,并制备了Ti(C,N)基金属陶瓷进行对比。采用带有能谱分析仪的扫描电子显微镜及维氏硬度计,并结合热力学计算,研究了在不同氮气分压下烧结对金属陶瓷组织结构和力学性能的影响。结果表明,相同烧结工艺下,与Ti(C,N)基金属陶瓷相比,(Ti, Nb)(C,N)基金属陶瓷晶粒更细小,且仅少数晶粒出现特殊的弱"芯-环"结构。真空烧结(Ti, Nb)(C,N)基金属陶瓷表面形成富粘结相Ni的梯度层。随着烧结过程氮气分压的增大,(Ti, Nb)(C,N)基金属陶瓷表层硬质相异常长大。(Ti, Nb)(C,N)基金属陶瓷梯度结构的形成是由不同氮气分压下材料表面和芯部的N活度存在差异,在热力学耦合作用下,Ti、Nb等元素扩散方向和速率不同所导致的。

(Ti,W,Mo,Nb,Ta)(C,N)纳米固溶体粉的合成及性能

采用高能球磨-碳热氮化还原法合成了(Ti,W,Mo,Nb,Ta)(C,N)纳米固溶体粉,利用X射线衍射仪、扫描电子显微镜和元素分析仪等分析了还原温度和还原时间对粉体物相组成、晶格常数及碳氮含量的影响。结果表明:金属元素在Ti(C,N)中的固溶按难易程度排序为钽、铌、钨、钼;随还原温度的升高和还原时间的延长,固溶体粉的晶格常数、碳含量及粒径均逐渐增大;在1 500℃碳热氮化还原1 h后,可合成单相且平均粒径为100 nm的(Ti,W,Mo,Nb,Ta)(C,N)固溶体粉,其碳、氮质量分数分别为9.1%和7.2%。

Ti-Nb微合金化对低碳高强度钢组织和性能的影响

0.153Ti-0.038Nb和0.080Ti-0.062Nb两种Ti-Nb微合金化低碳钢(/%:0.061～0.075C、0.22Si、1.76～1.77Mn、0.002～0.003S、0.006P、0.003Als、0.003 8～0.004 2N、0.004 0～0.004 5O)由50 kg真空感应炉冶炼,轧成10 mm板,终轧温度880℃,水冷至630℃。试验结果表明,两种Ti-Nb微合金化钢的析出物均为(Nb,Ti)(C,N)复合析出物;当Ti含量由0.080%增加至0.153%,同时Nb含量由0.062%降至0.038%时,钢屈服和抗拉强度分别从558 MPa和653 MPa提高至633 MPa和756 MPa,屈强比、伸长率和断面收缩率变化较小。表明,添加Ti代替部分Nb进行复合微合金化可提高钢材强度,降低生产成本。

Effects of Ti addition on low carbon hot strips produced by CSP process

Large quantity of fine Ti（C,N） particles, 15-30 nm in size, were observed in low carbon hot strips added to a small amount of Ti and produced by CSP process. The results showed that the precipitation of Ti（C,N） mostly took place during soaking and hot rolling, which is significantly different from that in the conventional production. These fine Ti carbonitride particles could be very effective on the austenite grain refinement by hindering grain growth of recrystallized austenite. Their precipitation behavior was discussed and compared with that of the steels produced in the conventional production.

Nb钢及Nb-Ti钢的应变诱导析出行为

利用应力弛豫法在热模拟机上分别测定了Nb钢及Nb-Ti微合金钢的应变诱导析出曲线,并采用萃取复型技术在电镜下直接观察了应变诱导析出过程。结果表明:析出过程分为明显的三个阶段,在高温回溶充分的前提下,微合金元素Ti的加入推迟了Nb(C,N)的析出速度。随着弛豫时间的延长,析出颗粒由圆形逐渐转化为不规则形状。

Ti-Nb复合微合金化高强度钢强化机理研究

以国内某钢厂的热轧高强度钢为研究对象,进行拉伸试验,利用OM和TEM分析了钢显微组织、第二相析出粒子形貌和尺寸等。结果表明,实验钢的屈服强度657 MPa、抗拉强度744 MPa、伸长率18.7%。微合金元素的加入使钢材晶粒明显细化,屈服强度的细晶强化增量为321.5 MPa,为提高材料强度的主要强化机制;研究析出的第二相粒子可知,基体中纳米级析出的(Nb,Ti)(C,N)粒子阻碍位错运动,是提高材料强度的主要机制之一,起到析出强化作用,屈服强度的析出强化量为78.58 MPa。

Ti,Nb和W复合强化超纯铁素体不锈钢的高温析出行为

为提高汽车尾气排放系统中高温端排气歧管等所用铁素体不锈钢的综合性能,采用真空熔炼制备Ti,Nb和W复合强化的超纯铁素体不锈钢（不含Ni）,并利用硬度和力学性能测试、场发射扫描电子显微镜和透射电子显微镜分析研究固溶态材料在550,600℃和700℃的时效硬化规律、拉伸性能以及时效析出行为。600℃时效40h之后材料力学性能达到最佳值,在晶粒内部和晶界区域弥散分布着两种形态的纳米析出相,一种是呈不规则颗粒状的（Ti,Nb）C,另一种是呈长条状的Laves相Fe_2（Nb,W）;这两种析出相与铁素体基体均存在固定的晶体学取向关系,即[011]Fe∥[011]（Ti,Nb）C和（0 11）Fe∥（200）（Ti,Nb）C（晶面偏差约3°）以及[011]Fe∥[0001]Fe2（Nb,W）和（200）Fe∥（01 10）Fe_2（Nb,W）（晶面偏差4°-5°）。随时效温度提高,析出相的尺寸增大,且析出相由（Ti,Nb）C逐渐转变为Fe2（Nb,W）。

Nb-V-Ti-N微合金化结构钢中碳氮化物析出的热力学分析与实验研究

为了更好地研究Nb-V-Ti-N微合金化结构钢中碳氮化物(MX)的析出行为,通过Thermo-Calc热力学软件并辅以TCFE8数据库分析了钢中析出碳氮化物的种类、数量、温度及成分,并利用场发射扫描电镜、透射电镜和能谱仪分析了碳氮化物的形貌、成分、尺寸及分布。结果表明,钢中MX析出相分别为面心立方结构的富Ti高温析出相、富Nb中温析出相和富V低温析出相;随Nb质量分数由0.050%增至0.100%,富Nb相析出温度由1210℃升高到1270℃,析出量由6.877×10^(-4)增加到1.265×10^(-3),而富Ti和富V相析出温度分别由1451℃和1080℃降低到1434℃和1020℃,析出量分别由3.663×10^(-4)和5.562×10^(-3)减少到2.331×10^(-4)和5.281×10^(-3)。实验研究得到的非平衡条件下MX析出特征与热力学计算结果基本一致:高温奥氏体区析出的亚微米颗粒能够钉扎晶界,诱导晶内铁素体形成,提高细晶强化效果;而中温两相区和低温铁素体区析出的纳米颗粒分布在基体、位错和晶界上,能有效阻碍位错运动,提高析出强化效果,二者共同作用改善了结构钢的强韧性。

Nb(N,C)的析出行为对含铌16MnR钢板性能的影响

通过两组不同的试样，分析含铌16MnR钢板中Nb(N，C)的析出行为对钢板性能的影响，并提出了解决途径。

含Nb微合金钢应变诱导析出的研究现状

简述了微合金钢中微量元素 Nb 的主要作用,回顾了应变诱导析出的研究方法及优缺点,阐明了变形奥氏体应变诱导析出相的种类,晶体结构,析出相的形成顺序、形貌和分布,分析了影响析出动力学的主要因素并简要介绍了应变诱导析出动力学模型的最新发展。

相变诱发塑性钢中Nb-V析出计算模型

在Gleeble 3500热模拟试验机上采用应力松弛的方法研究了Nb-V-C-N系TRIP钢在奥氏体区中的应变诱导析出行为。应力松弛结果表明:在奥氏体区中的应变诱导析出最快温度范围为915～925℃,其最快析出开始时间约6.5 s。同时根据实际测定的PTT曲线和模型计算的结果,推导出Nb-V-C-N系TRIP(ε=0.20,ε=0.1 s-1)的形变储能大约为2860 J·mol-1。奥氏体中的沉淀相形貌只有球形或近似球形一种,析出物经能谱分析为(Nb,V)(C,N),其粒子平均尺寸为12.91 nm。

含Nb多元微合金钢的应变诱导析出行为

利用松弛法,在Gleeble-1500热模拟机上测定了多元微合金钢的应力弛豫曲线,并采用萃取复型技术跟踪了析出物的长大过程。实验结果表明:应力弛豫曲线分为三个阶段,分别对应析出的形核、长大和粗化;在850℃变形后等温弛豫60～200s时,应变诱导析出颗粒的尺寸仍然小于10nm,析出相为复杂的(Nb,Ti,Mo)(C,N)。

TSCR流程Ti微合金化超高强钢的第二相析出行为研究

利用Gleeble-1500D模拟机进行了Ti微合金钢的热模拟试验,研究了其在薄板坯连铸连轧(TSCR)流程下Ti(C,N)沉淀析出行为。结果表明,Ti微合金钢在TSCR流程中存在两类含Ti析出物:正方形的或矩形的TiN和球形或不规则形状TiC。TiN析出物主要在连铸凝固前钢液中和铸坯均匀化加热时析出,在连轧时起到抑制动态再结晶晶粒生长的作用;而TiC析出物主要是在连轧和卷取时析出,对Ti微合金钢起沉淀强化作用。为了提高Ti微合金钢沉淀强化效果,应在TSCR流程中增加TiC的体积分数和减小TiC的尺寸。

Effect of Ti(C,N)Precipitation on Texture Evolution and Fish-Scale Resistance of Ultra-Low Carbon Ti-Bearing Enamel Steel

The Ti（C,N）precipitation and texture evolution in ultra-low carbon Ti-bearing enamel steel were studied to discover their correlation.Two different cooling paths of simulative coiling were adopted to gain different morphologies of Ti（C,N）precipitate in hot strips.It is found that the Ti（C,N）precipitate in hot strip using Path 2（put into asbestos box and air cooled）are finer and much more in quantity than that in hot strip using Path 1（held at 730℃ for 30min and furnace cooled）.The morphology of Ti（C,N）precipitate in hot strip has great effect on recrystallization texture in the subsequent cold-rolled annealed sheet.γ-fiber texture of strong recrystallization is gained in coldrolled and annealed sheet came from Path 1.On the contrary,in the one came from Path 2,theγ-fiber texture is weak.This may be due to the pinning force of numerous fine Ti（C,N）precipitate which retard the growth of preferential nucleated{111}orientated grains.Annealed steel sheets in the test came from both paths have excellent fishscale resistance.

Characterization of( Nb,Ti,Mo) C Precipitates in an Ultrahigh Strength Martensitic Steel

A study on ultrahigh strength steel plate subjected to novel thermo-mechanical control process was presented. The mechanical properties examination showed that the investigated steel exhibited excellent combination of ultra-high strength（ 2 200 MPa） and toughness（ 26 J）. The microstructure of the experimental steel was observed by scanning electron microscope and transmission electron microscope. Desired martensitic lath with width of about 180- 250 nm was obtained. Nanostructured carbide precipitates with sizes of 20-50 nm,which contained Nb,Ti and Mo,were observed in the lath martensitic microstructure,and confirmed to be MC-type carbides with B1 structure by means of selected area electron diffraction.The compositional characteristics revealed by energy dispersive X-ray spectrometer mapping implied that the carbide forming elements Nb,Ti and Mo distributed in the precipitates evenly. Three-dimensional atom probe tomography reconstruction further indicated that Mo incorporated into the precipitates without enrichment in the carbide-matrix interface and probably substituted for Nb and Ti to form the（ Nb,Ti,Mo） C carbides.

铌微合金化H13钢中异相形核(Ti,Nb,V)(C,N)相形成机理研究

AISI H13热作模具钢中碳氮化物的析出会对钢的强度、硬度、韧性及耐疲劳等性能产生不利影响。对电渣重熔含铌H13钢锻造棒材中尺寸大于1μm的异相形核析出相的特征进行分析,同时利用热力学软件Thermo-Calc对其形成过程进行合理的解释。棒材中存在的析出相包含3层结构,中心为氧化物核心,主要为氧化铝或镁铝尖晶石,中间层为富Ti-V的碳氮化物相,最外层为富Nb碳氮化物相。2类碳氮化物相成分分布较为集中,富Ti-V相的平均成分为(Ti_(0.477)V_(0.336)Nb_(0.187))C_xN_y,富Nb的平均成分为(Ti_(0.128)V_(0.255)Nb_(0.617))C_xN_y。钢中观察到的析出相的尺寸甚至可以达到10μm以上。Thermo-Calc计算表明,核心镁铝尖晶石及氧化铝在凝固前即已生成,凝固过程中,当固相率达到0.823时,富Ti相依附于氧化物首先析出,随后富V相、富Nb相依次析出。在随后的热处理中,富V相溶解于富Ti相中,形成棒材中观察到的3层(Ti,Nb,V)(C,N)相。