Mathematical Modelling of Carbonitride Precipitation during Hot Working in Nb Microalloyed Steels

Based on thermodynamics and kinetics, precipitation behavior of microalloyed steels was analyzed. Deformation greatly promotes isothermal carbonitride precipitation and makes C-curve shift leftwards. The position and shape of C-curve also depend on the content of Nb and N. C-curve shifts leftwards a little when N content increases and the nose temperature is raised with increasing Nb content. Deformation shortened precipitation start time during continuous cooling, raised precipitation start temperature, accelerated precipitation kinetics of carbonitrides. With decreasing the finishing temperature and coiling temperature, the precipitates volume fraction increases and strength increment is raised during hot rolling. The simulated results are in agreement with experiment results.

Precipitation Behavior of Nb in Steel under Ultra Fast Cooling Conditions

By measuring the expansion curves of a Nb bearing steel at different cooling rates by using Gleeble-3800 thermomechanical simulator, combining with metallographic analysis, different phase zones were determined. Also, precipitation behavior of Nb at different phase zones was investigated under ultra fast cooling conditions. The experimental results showed that adopting a proper deformation temperature, the ultra fast cooling process can restrain the precipitation of Nb at austenite phase zone. More quantities and smaller size precipitates of Nb can be found at the ferrite or bainite phase zone by controlling the ultra fast cooling ending temperature. With the increase of holding time at austenite, ferrite and bainite phase zones respectively, the volume fraction of precipitation, density, and average size of precipitates will increase obviously. With the decrease of early ultra fast cooling ending temperature, the density of Nb precipitates first increase（at ferrite phase zone） and then decrease（at bainite phase zone）, the volume fraction of Nb precipitation decreases and precipitates can be refined. The optimal early ultra fast cooling ending temperature is located at ferrite phase zone. The combination of high rolling temperature with early ultra fast cooling technology opens the way for new cooling schedules and makes the production of high strength steels easier and cheaper by making full use of Nb precipitation strengthening effect.

KINETICS OF STRAIN-INDUCED PRECIPITATION OF (Nb,V) CARBONITRIDE IN A MICROALLOYED STEEL

The kinetics of the isothermal precipitation of(Nb,V)CN in Nb-V alloys has been investi- gated by using the Formastor-press simulator and the extraction replica method.Under four deformation amounts(0,10,30,50%)and three temperatures(1100,1000,850℃), four types of kinetic curves were found.

Influence of cooling rates on the precipitation behavior and microstructure of Nb-Ti microalloyed steel

Influence of different cooling rates on the microstructure and the precipitation behavior of Nb-Ti microalloyed steel was investigated by CSLM, OM, SEM and EDS. The results show that the precipitation process of carbonitrides can be in-situ observed by CSLM, and with the increase of the cooling rate, the distribution of precipitates changes from along the austenitic grain boundaries to within the grains. With the increase of the cooling rate, the proeutectoid ferritic film becomes smaller and smaller and then disappears, and the original austenitic grains become finer and finer. In order to obtain non-film like proeutectoid ferrites or non-chain like precipitates along the austenitic grain boundaries and finer austenitic grains,the cooling rate should be at least 5℃/s.

Carbide and Nitride Precipitation in High Temperature TensiledSpecimens and Hot Ductility of Nb-and Ti-ContainingSteel CC Slabs

Hot ductility of the Nb- and Ti-containing line-pipe steel CC slab specimens were measured under the sirain rate of 1 x 10-3/s. Three types of precipitates were found in the fractured specimens. One was the block-shaped coarse TiN particles precipitated at high temperature. Another type was the fine dynamic precipitation products precipitated at 950～900℃ which caused remarkable ductility reduction of the steel. The third type was the co-existed precipitates formed by fine Nb precipitates nucleating and growing on TiN paricles. Compared with Nb-containing steel which contains no Ti, there was no ductility drop for Nb- and Ti-containing steel at temperature between 850℃ and Ar3 and, the γ→α transformation inside the grain matrixes proceeded faster, which both improved the ductility of the steel in the low ductility temperature Region Ⅲ.

Solution and Precipitation Behavior of Nb and Its Role in Medium Carbon Long Product Steels

Controlling of the solution and precipitation for microalloying additions of Nb,V,Ti and their combination is the basic building block of microalloyed steels.Among three microalloying elements,Nb has been regarded the most effective microalloying ingredient used for low carbon flat products due to marked strengthening effect of grain refinement and relatively weak precipitation strengthening effect by matching proper thermomechanical processing(TMP).However,Nb was not viewed as attractive as V for medium carbon long products in the past because of limited solubility during reheating process.What is more,hot working is usually carried out at high temperatures in the recrystallization regime,so it is difficult to exert conventional controlled rolling to obtain pancaked austenite,which has further affected the research and application of Nb in medium carbon long products.Because of these factors,studies of Nb in medium carbon long products were incomplete,and even some recognitions and conclusions are subjected to debate.In order to clarify the strengthening effects of Nb in medium carbon long products,the reinforcing bars have been chosen as experimental steels to clarify the role of Nb on microstructural changes along the whole hot working,cooling processes.In addition,qualitative and quantitative analyses of Nb’s state and distribution in reheating,rolling and cooling had been carried out to illustrate some singularities.

鲁米诺—H_2O_2—K_3Fe(CN)_6—Nb体系化学发光反应的研究

铌的化学发光测定尚未见文献报导。本文发现,铌对鲁米诺—H_2O_2—K_3Fe(CN)_6体系的化学发光反应有较强的抑制作用,且在一定的范围内,试液中铌的浓度与体系的发光强度成线性关系。在此基础上,建立了痕量铌的化学发光测定法。本方法检测下限为2.3×10^-克/毫升、对1×10^-克/毫升铌量的测定,相对标准偏差为0.45％。数种离子对本方法测定铌有干扰,可用纸色谱法将铌与其它杂质分离再进行测定。利用本方法对两种矿石标样中铌量测定的结果比较满意。

基于前期降雨指数的SCS-CN模型在三峡库区改进与应用

径流是土壤流失的主要载体,准确预测径流量对于三峡库区土壤侵蚀预报和水库安全管理至关重要。为优化径流曲线数模型(SCS-CN)中土壤前期含水量核算方法,提高径流模拟准确性,研究通过引入前期降雨指数来表征土壤含水量,并基于实测的土壤饱和含水量设定上限阈值,对SCS-CN模型进行了改进,利用忠县水土保持监测站2019-2022年的场次降雨径流监测数据评估了模型准确性,并对改进前后模型在不同降雨类型和土地利用条件的适用性进行评价。结果表明,在两种初始模型和两种改进模型中,基于MSCS-CN改进的模型(M4)对于径流深的模拟精度更高,纳什效率系数(NSE)较原始SCS-CN模型(M1)提高了134%,尤其是在前期土壤含水量达饱和状态的情况下NSE提升至0.74,引入最大前期降雨指数(APImax)作为前期降雨指数上限值能减弱以往模型对高径流事件预测值的高估。此外,改进模型M4对三峡库区主要的产流性降雨类型(高雨强)以及主要的土地利用(耕地、果园)适用性良好,有效提高了小雨量产流事件和高侵蚀风险土地利用类型的径流模拟精度(雨型Ⅱ的NSE为0.66,耕地、果园NSE分别为0.69和0.75)。总体而言,改进模型M4提高了径流模拟精度,更适合三峡库区的产流机制、土地利用类型,能为三峡库区径流预测及水土流失防治工作提供参考。

5Cr21Mn9Ni4Nb2WN耐热钢M(CN)和M_(23)(CN)_6的可逆析出反应

通过扫描电镜(SEM)、透射电镜(TEM)和选区电子衍射分析(EDA)、能量色散X-射线衍射分析(EDAX)等技术研究了5Cr21Mn9Ni4Nb2WN耐热钢锻态,1 200℃120 min水冷固溶,+1 030℃20~240 min水冷固溶状态,再经1 220℃20~240 min水冷固溶状态的晶内和晶界面上碳化物相、显微组织和力学性能。结果表明,5Cr21Mn9Ni4Nb2WN钢在1 030℃固溶时晶界晶内不但有粒状M_(23)(CN)_6析出,同时M(CN)块作为补充碳源发生了向M_(23)(CN)_6的退化反应,其室温冲击功急剧下降,但对钢的屈服强度影响不大;再经在1 220℃固溶处理时出现M_(23)(CN)_6向M(CN)的逆析出反应,该钢又显示出较高的室温冲击功。

中碳CrMo钢中Ti、Nb液析形貌及生成机理分析

通过观察微合金Ti、Nb、V在42CrMo钢连铸坯中的液析形貌,以及液析经加热轧制后的变化,同时,采用热力学计算的方式推导液析生成机理。结果表明,在含w[Ti]0.012%、w[Nb]0.030%、w[V]0.030%、w[N]0.003 5%的42CrMo钢410 mm×530 mm连铸坯中,可观察到Ti、Nb以复合碳氮化物的形式析出,形成微米级液析,w[Ti]较高(70%~83%)的多呈多边形块状,w[Nb]较高(70%~85%)的多呈薄膜状或不规则块状,液析中基本不含V元素;Ti和Nb的碳氮化物液析可互溶并共同生长,Nb含量较高的液析以Ti含量较高的液析为核心长大为块状,部分液析以含硫夹杂物为核心形核并长大;连铸坯心部的碳氮化物液析较多,并且尺寸较大(最长达35μm),连铸坯表面的数量较少,尺寸较小(10μm以下);连铸坯经1 200℃加热并轧制后,液析可部分回溶,数量减少,尺寸减小,并且在轧制过程中破碎。

Nb对GH4169合金管材组织性能的影响

采用真空感应、电渣重熔、真空自耗、均匀化热处理、锻造、热挤压、冷轧、热处理等工艺得到Nb含量分别为4.90%和5.30%的两种GH4169合金管,在其他成分基本保持一致的情况下,研究了Nb含量对GH4169合金管材组织性能的影响。结果表明,在相同的热处理工艺下,随着Nb含量的增加,Nb含量越高,晶粒尺寸越细小;200℃和室温抗拉强度、屈服强度都随着Nb含量的增加而增加;且在GH4169合金管材中的析出相含量也增加。GH4169合金管在不同的固溶热处理和相同的时效热处理工艺下,得到GH4169合金管材的晶粒度尺寸在4.0~7.0级,晶粒尺寸越小,管材的抗拉强度和屈服强度越高;在相同的晶粒度下,Nb含量越高,管材的抗拉强度、屈服强度都随着Nb含量的增加而增加。

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。

含Nb微合金钢变形后弛豫及时效过程中的析出行为

采用热模拟手段、金相及硬度测量并结合透射电子显微镜观察 ,研究了简单成分的含Nb微合金钢在热变形弛豫后、5 2 5℃时效时硬度变化及析出过程。结果表明 :经弛豫淬火后的含Nb钢在时效过程中出现双硬化峰现象 ,相变前原有细小析出相在时效时会长大和粗化 ,形成第一个硬化峰 ;而相变后α相中过饱和的Nb在时效时会进一步析出 ,形成第二个硬化峰。

高钛低碳钢中Nb、Ti的析出行为

通过热力学计算和实验观察,对一种高钛含铌钢中Nb、Ti的析出行为进行研究。结果表明:实验钢中的铌的氮化物和碳化物在液相和凝固过程中不能生成;氮化钛在凝固过程中析出,碳化钛在凝固末期形成;在奥氏体相中,实验钢中的Nb、Ti碳氮化物析出的先后顺序为TiN>Nb(CN)>NbC>TiC;铸坯中的析出相主要为氮化钛,而成品板材的二相粒子均为Ti、Nb复合的C、N化物。

Nb-W-Mo-Zr-C高温合金中第二相的形成演化与力学性能

采用扫描电镜(SEM)、透射电镜(TEM)、Olympus光学显微镜和真空高温拉伸机,研究了名义成分为Nb-5%W-2%Mo-1%Zr-0.1%C(质量分数)的铌合金制备过程中第二相的形成演化及其高温力学性能。结果表明:烧结条中存在(Nb,Zr)C相,电子束熔炼铸锭中为Nb2C,铸锭退火后析出ZrO2,成品板材中存在大量片状Nb2C和弥散分布的细小ZrO2。采用两种加热方法测定了板材1650℃的力学性能并讨论了不同高温拉伸机的测定值不同的原因。

Nb的析出对变形诱导铁素体相变的影响

通过Gleeble2000热模拟实验机,研究了X65管线钢中Nb在变形奥氏体中的析出状态对变形诱导铁素体相变(DIFT)的影响。试验结果表明,在奥氏体临界区变形时,第一道次变形后,随变形后等温时间延长,诱导铁素体量变化不大。等温时间达120s时,变形奥氏体仍未发生再结晶。在道次间随时间延长,Nb的析出量增加,第二道次变形后诱导的铁素体也显著增加。微合金元素Nb通过碳氮化物的析出作用促进变形诱导铁素体相变。

Nb含量和热输入量对X80管线钢焊接粗晶区的影响

以工业含铌X80管线钢为对象,研究了不同Nb含量条件下,焊接热影响区粗晶区中的原奥氏体晶粒和析出的状态;通过对焊接热影响区中粗晶区的热模拟实验,研究了不同热输入下,高Nb管线钢焊接粗晶区的晶粒粗化、显微组织演变、及大角晶界分布等情况。结果表明,高Nb钢的粗晶区范围更窄,粗晶区内的晶粒尺寸更小;经过双道焊的热循环后,高Nb钢粗晶区的析出尺寸更小,没有对韧性有害的大尺寸析出。此外,原奥氏体晶粒的粗化,以及显微组织中大角晶界密度的下降,明显降低了高Nb管线钢焊接粗晶区的韧性。

低碳微合金钢中Nb、V、Ti碳氮化物的回溶研究

采用透射电镜和X射线光谱技术,研究了低碳微合金钢中Nb、V、Ti的碳氮化物在不同温度保温1 h后的回溶行为。结果表明,低碳微合金钢中存在尺寸明显不同的两类析出,其中颗粒尺寸较大的在80 nm以上;这种颗粒的心部是(Nb,V,Ti)(C,N)相,而颗粒边缘为(Nb,Ti)(C,N)相;颗粒尺寸较小的在20 nm以下,其类型为(Nb,Ti)(C,N)。两类析出物中Nb与Ti的原子比均随回溶温度的升高而减小。

含Nb钢应变诱导析出过程的动力学模拟

以热力学和动力学为基础,对Fe Nb C N系统在奥氏体中的析出行为进行模拟,预测了奥氏体在等温和冷却过程中的析出动力学,分析了变形工艺和冷却制度对碳氮化物析出体积分数、析出开始和结束时间等动力学参数的影响·结果表明,在900℃左右的鼻子区,析出动力学过程明显加速;变形大大促进了碳氮化物的析出,缩短了析出的开始时间;随着冷却速率的增大,连续冷却析出的开始温度和结束温度均有所降低,并且析出变得越来越不充分·用该方法进行的计算机模拟结果和实验结果吻合良好·

Nb含量对NM550耐磨钢组织和力学性能的影响

通过光学显微镜、扫描电子显微镜、透射电子显微镜以及力学分析等方法研究了Nb对高强耐磨钢组织和力学性能的影响,并讨论了含Nb耐磨钢的强韧化机理。结果表明,在200℃回火时,实验用钢展示了较好的力学性能;随着回火温度的提高,实验钢的力学性能逐渐下降。加入少量的Nb元素,一方面在加热过程中抑制了晶粒的长大,另一方面在轧制过程中形变诱导析出NbC析出抑制了晶粒的回复和再结晶,保留了较大的位错密度,在细晶强化、析出强化和位错强化的共同作用下,含Nb钢展示了更为优越的力学性能,并且Nb元素能够抑制渗碳体的形成,防止由于回火脆性造成韧性的大幅下降。