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碳氮化物在超纯铁素体不锈钢铸坯中的析出规律

采用光学显微镜和扫描电镜研究了超纯铁素体不锈钢铸坯在不同加热温度和保温时间下Nb、Ti碳氮化物的析出规律.结果表明,当加热温度相同时,随着保温时间延长,连铸凝固过程中析出的初始大尺寸(2～7μm)铌钛析出物数量逐渐减少,即初始铌钛析出物逐步回溶至钢中;随加热温度从1000℃升高至1100℃,初始铌钛析出物呈长大趋势,从...     

Nb(C,N)溶解对高铌奥氏体晶粒长大行为的影响

为了定量研究铌对高铌钢加热过程奥氏体晶粒长大的影响,采用化学溶解过滤分离及电感耦合等离子光谱测定不同加热温度两种试验钢固溶铌质量分数,并对比研究了奥氏体晶粒长大行为。结果表明,在低温条件下,低铌钢固溶铌质量分数高于高铌钢;随加热温度升高,高铌钢固溶铌质量分数快速增加,但即使在1 300 ℃时,铌也不能完成固溶,少量铌存在于(Ti,Nb)(N,C)析出相中;奥氏体晶粒快速长大的温度与固溶铌质量分数快速增加的温度有关。随铌质量分数由0.082%增加到0.120%,奥氏体晶粒快速长大的临界温度由1 050升高到1 150 ℃。高铌钢在1 150～1 250 ℃加热温度范围内,奥氏体晶粒尺寸小于100 μm。     

Nb-H13钢中一次碳化物的析出机理和控制

为了得出铌微合金化H13钢中合适的铌元素添加量,以H13钢中的一次碳化物为研究对象,通过实验室冶炼不同铌元素质量分数的H13钢,系统地研究了Nb-H13钢中一次碳化物的析出机理和合适的Nb添加量。结果表明,当H13钢中铌的质量分数小于0.03%时,Nb-H13钢中的一次碳化物主要为富钒相;当H13钢中铌的质量分数达到0.05%时,Nb-H13钢中的一次碳化物主要为富钒相和富铌相。铌微合金化H13钢中铌元素的添加量应小于0.03%。铌质量分数的高低会显著影响FCC相中碳化物的析出顺序,随着铌质量分数的增加,碳化物的析出顺序由首先析出富钒相转变为首先析出富铌相,且富铌相的热稳定性要远高于富钒相。理论计算结果与试验观察结果基本一致,工业试验结果同样论证了本研究的结论。     

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。     

轧制工艺对Nb微合金H型钢性能的影响

采用透射电镜、电解法对铌（Nb）微合金H型钢析出相进行分析,利用显微镜研究加热温度对奥氏体晶粒度的影响,以及通过轧制试验研究未再结晶区压下率和终轧温度对试验钢性能的影响,制定出微合金H型钢控轧工艺。该工艺应用于微合金H型钢工业生产,其实际晶粒度11级,屈服强度为400~420 MPa,-20℃横向夏比冲击功大于45 J,产品性能满足日标H型钢要求。     

微钛固氮降低含铌钢皮下裂纹敏感性的机理

 降低含铌低合金钢铸坯的裂纹敏感性,是实现热装热送工艺的必要条件,采用超高温激光共聚焦显微镜(HT-CLSM)、透射电镜(TEM)等手段研究了钛含量不同的含铌低合金钢(Q390、Q390GJD)高温铸坯的晶粒度及析出物特征,旨在揭示微钛固氮降低含铌钢皮下裂纹敏感性的机理。热力学计算与TEM检测结果表明,增加钢中钛质量分数(由0.010%上升到0.023%)提高了氮化钛粒子的析出温度(大于1 400 ℃),高温析出细小弥散的氮化钛粒子可钉扎奥氏体晶界,细化高温铸坯的晶粒度(由4级变成6.5级),晶粒尺寸降低了约44%,使高温铸坯的裂纹敏感性明显降低;氮化钛粒子优先析出固氮降低了铌碳氮化物、氮化铝的开始析出温度,并作为异质形核核心,抑制了铌、铝析出物在晶界析出概率,降低了析出物脆化晶界的危害。通过微钛固氮调控氮化物的析出温度、析出位置及细化晶粒的作用,有效降低了含铌钢第三脆性温度槽的宽度和深度,同时,高温抗拉强度提高了21.3%~27.5%,铸坯皮下裂纹发生率降低了80%以上。为了避免析出物的晶界链状析出导致含铌钢铸坯热装轧制裂纹,应将其钛质量分数控制在0.015%~0.020%的合理范围。     

铌的析出行为对形变诱导相变的影响

在Gleeble热模拟试验机上,研究含铌微合金钢中铌的析出物在形变诱导相变过程中的作用机理,结果表明:当加热温度为1000℃时,部分铌的碳化物未溶解于奥氏体中,而加热温度为1100℃时,铌的碳化物能够全部溶解到奥氏体中,含铌微合金钢加热温度范围约在1000～1100℃之间。铌的未溶解碳化物和在冷却过程中析出的碳化物都能作为铁素体形核位置,从而促进γ→α相变的发生。     

溶质元素对X80管线钢碳氮化物析出行为的影响

为了明晰X80管线钢凝固过程中析出物粒子析出规律。采用Thermo-calc软件对X80管线钢凝固过程中奥氏体、铁素体相转变温度,析出物类型,析出物开始析出温度与析出相最大质量分数进行热力学分析。计算结果表明:X80管线钢凝固过程析出相主要包括MnS相、AlN相、铁素体相及奥氏体相、富钛相碳氮析出相、富铌相碳氮析出相与碳化钒析出相。富钛相碳氮析出相析出温度为1 390℃,质量分数为1.2×10~(-4),析出相随C元素和Nb元素的含量增加而受到抑制。N元素和Ti元素含量增加会促进富钛相碳氮析出相生成,V元素质量分数增量对富钛相碳氮化物无明显规律。富铌相碳氮析出相在1 110℃析出,质量分数为1.3×10~(-3),随C元素和Nb元素的含量增加,促进富铌相碳氮析出相析出。增加N元素与Ti元素含量抑制富铌相碳氮析出相析出,V元素对富铌相碳氮化物无明显影响规律。碳化钒析出相析出温度为596℃,析出相质量分数为4.59×10~(-5),增加C元素和V元素含量利于碳化钒相析出,Nb元素含量增加对碳化钒相起到一定抑制作用。     

25Cr2Mo1VA钢奥氏体晶粒长大规律探讨

 为了研究不同加热温度和保温时间下25Cr2Mo1VA钢奥氏体晶粒长大规律,利用金相显微镜、TEM、EDS以及截距法分析了不同状态下奥氏体晶粒大小、形貌,析出相类型及其大小分布等。结果表明,当加热温度在900 ℃以下,试验钢的奥氏体晶粒长大缓慢且细小;随着温度加热至950 ℃,奥氏体晶粒出现了“混晶”现象,再加热超过1 000 ℃后,奥氏体晶粒异常长大。随保温时间的延长,奥氏体晶粒也会长大,但保温时间对奥氏体晶粒尺寸的影响没有加热温度显著。试验钢中出现近似球形状的析出相,即为富钒的非计量化合物M₈C₇析出相。根据Anelli改进模型和回归分析,建立了试验钢奥氏体晶粒长大的热力学模型,并结合析出相粒子对奥氏体晶粒钉扎作用,探讨了25Cr2Mo1VA钢的奥氏体晶粒长大规律及成因。     

Nb在高铝铁素体钢中的固溶析出行为

采用电解相分析方法, 结合X射线衍射分析和电感耦合等离子体原子发射光谱仪(ICP)、扫描电镜(SEM)、透射电镜(TEM)等对高铝铁素体基体中的析出相颗粒粉末和电解液进行定性定量分析. 试验结果表明, 试验钢中固态析出相主要为NbC以及少量的Al₂O₃和AlN夹杂. 通过扫描电镜观察不同再加热温度下NbC分布状态, 发现随着固溶温度的升高, 铸态组织中存在的NbC析出逐渐回溶, 数量随之减少且发生明显的粗化行为. 当温度升高到1100℃, 大部分NbC已经回溶到高温铁素体基体中. 在利用Thermo-Calc热力学计算软件分析Nb及其碳化物的热力学性质基础上, 计算得到Al与Nb的相互作用系数, 表明Al能够降低Nb在铁素体基体中的活度, 提高其在基体中的固溶度, 进一步得到了NbC在高铝铁素体钢中的固溶度积公式, 发展了高温铁素体中的Nb微合金化理论, 为进一步的应用提供了理论基础.      

In-situ observation on austenite grain growth of a Nb microalloyed high-carbon steel

It was reported in previous studies that the growth of austenite was inhibited by the pinning effect of Nb containing precipitates and the solute dragging effect of solute Nb. The effect of Nb on austenite grain growth of high carbon steel was investigated by laser scanning confocal microscope (LSCM). Microstructure evolution during heating process of the tested steel was observed by in situ observation. The results show that even without the pinning effect of Nb containing precipitates (at high temperatures), Nb can hinder the growth of austenite grains due to the solute dragging effect of Nb. Two models were used to fit the austenite grain growth process, and the Beck growth models of Nb microalloyed high carbon steels at different heating temperatures were established. The austenite grain growth kinetics model considering the influence of heating temperature and holding time can accurately predict the austenite grain growth process of Nb microalloyed high carbon steels.     

Nb微合金化高碳钢奥氏体晶粒长大原位观察

摘要：以往研究表明Nb析出相钉扎和固溶Nb溶质拖曳作用共同阻碍奥氏体晶粒长大。采用高温共聚焦显微镜研究了Nb对一种高碳含Nb钢奥氏体晶粒长大的影响,对含Nb钢加热过程组织演变进行原位观察。结果表明,Nb在没有钉扎作用下（即高温条件下）仍能起到阻碍奥氏体晶粒长大的作用,该阻碍效果主要是固溶Nb的溶质拖曳作用引起的。采用2种模型对奥氏体晶粒长大行为进行拟合,给出了不同加热温度下Nb微合金化高碳钢的Beck长大方程,同时考虑到加热温度和保温时间的共同影响,根据原位观察结果得到实验钢的奥氏体晶粒长大动力学模型,该模型能够较准确地预测Nb微合金化高碳钢奥氏体晶粒长大行为。     

Microalloyed Steels for Heat Treating Applications at Higher Process Temperatures

The potential is considered for use of microalloyed bar steels,in conjunction with thermomechanical processing,to enhance the properties of steels heat treated at higher process temperatures than have been used historically.Two examples are highlighted:microalloyed spring steels with enhanced resistance to tempering and Nb-modified gear steels for high temperature vacuum carburizing,e.g.on the order of 1050℃ versus 930℃ for a typical gas carburizing operation.In the spring steel example,the Nb+V steel results in significantly finer prior austenite grain sizes than the other steels considered,enhanced fatigue performance,and improved toughness.In the Nb-modified carburizing steel,Nb additions up to 0.1 wt pct to a Ti-modified 8620 steel,in conjunction with thermomechanical processing to control initial precipitate distributions prior to carburizing,are shown to lead to materials with improved resistance to abnormal austenitic grain growth at the higher process temperatures.Alloy content and heating rate to the carburizing temperature were shown to be important variables and suppression of abnormal grain growth was correlated with the development of a critical distribution of fine NbC precipitates,stable at the austenitizing temperature leading to improved fatigue performance in steels with fine and uniform grain structures.Opportunities for extending the results of this study to alloy design and controlled rolling in bar mills are assessed.     

Effect of Nb Solute and NbC Precipitates on Dynamic or Static Recrystallization in Nb Steels

 Nb is often considered to be a powerful alloying element for controlling the recrystallization process in microalloyed high strength steels. However, Nb can be presented either as solute in solution, where it is thought to exhibit a strong solute drag effect, or as NbC precipitates, which are thought to be effective at pinning grain boundaries. Therefore, it is very important to quantitatively measure Nb in solution or in NbC precipitates. A quantitative analysis method of Nb in solution and in precipitates was proposed. The test procedure involved chemical dissolution, filtration and inductively coupled plasma atomic emission spectroscopic (ICP-AES) analysis. The amount of Nb in solution in Nb-microallyed steels under different treatment conditions was evaluated. The results show that the niobium and carbon contents in steels have a great effect on niobium dissolution kinetics. The solute Nb is more effective to retard dynamic recrystallization, while the NbC precipitates are more effective to inhibit static recrystallization. The results may help to comprehend effect of Nb in steels, and provide some guides in the design of new high strength Nb-bearing steels.     

Microstructure and Properties of Ti and Ti+Nb Ultra-Low Carbon Bake Hardening Steels

 Hot rolling, cold rolling and continuous annealing processes of Ti bearing and Ti+Nb stabilized ultra low carbon bake hardening(ULC-BH) steels were experimentally studied. The microstructure and texture evolution, as well as the morphology, size and distribution of the second phase precipitates during the hot rolling, cold rolling and continuous annealing were also analyzed. The results showed that the size of NbC precipitates in Ti+Nb stabilized ULC-BH steel was smaller than that of TiC precipitates in Ti bearing ULC-BH steel, this caused the average grain size of Ti+Nb stabilized ULC-BH steel to be finer than that of Ti bearing ULC-BH steel, for the yield strength, the former was higher than the latter, but for the r value which reflecting the deep drawing performance, the former was lower than the latter.     

Development of microalloyed steels for high temperature carburizing

The potential for use of microalloy additions to suppress abnormal austenite grain growth and produce steels with enhanced bending fatigue resistance after high temperature vacuum carburizing was investigated in a series of Ti-modified SAE 8620 steels with w(niobium) additions up to 0.1%.Results are considered from a series of papers at the Advanced Steel Processing and Products Research Center on the effects of Nb content,heating rate, rolling history,and processing temperature on the evolution of austenite grain structures in carburizing steels. Emphasis is placed on understanding the effects of alloying and processing on each stage in the annealing process including the as received laboratory rolled conditions,during the onset of carburizing after annealing at different heating rates,and after annealing for various times at carburizing temperatures up to 1 100℃.Heating rate to the carburizing temperature was shown to be an influential variable and suppression of abnormal grain growth was dependent on the development of a critical distribution of fine NbC precipitates,stable at the austenitizing temperature.The importance to industrial carburizing practice of heating rate effects on precipitates and austenite grain size evolution are discussed and correlated to selected data on fatigue performance.     

含0.029%铌F40MnVS钢连铸坯微米级大颗粒NbC特征及生成机理研究

针对铌微合金化(加0.029%Nb)F40MnVS非调质钢(220 mm×260 mm)连铸坯中含铌相进行研究，结果表明：F40MnVS钢连铸坯中可见较多微米级大颗粒NbC相(/%:80～94Nb, 1.2～7.1V,1.6～17.1Ti)。大颗粒NbC分布在枝晶间，其形貌可呈块状、长条状或与钢基体的共晶形态，部分依附于硫化物存在。在连铸坯边部NbC多在5μm以下，形貌以点状居多；连铸坯中间及心部NbC可达数十微米，多为细长条。这些大颗粒NbC相在热轧材中仍然存在。依据Thermo-Calc计算结果，钢液凝固末期，当固相率达到0.961时，NbC可在元素富集的钢液中析出，其成分存在Ti可能是与先析出的TiN成分互溶的结果；通过将轧前铸坯加热温度提高至1200℃,可明显降低大颗粒NbC的尺寸。     

罩式退火铌微合金化汽车钢的强化机理

 通过比较相同冷轧与罩式退火工艺下Mn-Si系和铌微合金化2种汽车用低合金高强钢的显微组织与力学性能,研究微量铌在冷轧罩式退火低合金高强钢中的强化机理。利用OM、SEM、TEM和拉伸试验机分别对2种钢的显微组织与力学性能进行了表征。对比分析表明：相对热轧板来说,2种钢冷轧退火板的铁素体晶粒和第二相析出物的尺寸都有所长大,导致了强度降低。相对Mn-Si钢而言,铌微合金化钢热轧板和冷轧退火板中的铁素体晶粒和第二相析出物尺寸更细小,细小第二相析出物的数量也更多,在相同的伸长率水平下明显提高了强度。冷轧罩式退火板的强化机理分析表明,铌微合金化低合金高强钢的主要强化方式是细晶强化和NbC的沉淀强化;研究认为添加质量分数为0.025%的铌时细晶强化更强烈。     

Effect of Nb on Microstructure Evolution in Cold Rolled Dual-Phase Steels:Interaction of Recovery,Recrystallization and Phase Transformations

The effects of Nb addition,individually and in combination with Ti,were evaluated over a range of coiling temperatures.Coiling temperature influences the ratio of soluble and precipitated Nb in the hot rolled steel containing 0.08 % C and 2.2 % Mn.Nb bearing precipitates can co-precipitate on TiN and impact the microstructure and mechanical properties of the steel after annealing treatment.Microstructure characterization revealed that recovery and recrystallization processes preceded austenite formation.The effects of Nb on austenite formation in cold rolled steels during heating and isothermal holding and on austenite decomposition during subsequent cooling were investigated using dilatometry.The addition of Nb retarded ferrite recrystallization starting temperature,but had no significant effect on the starting temperature of austenite formation during heating.The Nb addition also accelerated austenite formation once the transformation started,and was beneficial for the formation of a finer and homogeneous microstructure.