超细夹杂物析出研究

非金属夹杂物往往是钢材表面和内部缺陷的成因.但是,当夹杂物的分布适当和颗粒小到一定尺寸时,就能对钢的性能有利.本文研究了氧化物在凝固过程中的析出,采用定向凝固工艺,精确控制了凝固速率,研究了钛、锰、硅及硅-锰复合元素的加入以及冷却速率对夹杂物析出的影响规律.     

SWRH 92A帘线钢中异相形核TiN复合夹杂析出机制

摘要：高碳帘线钢中析出的非金属夹杂物如钛夹杂,对钢材疲劳强度影响巨大。系统分析过共析帘线钢中钛夹杂的析出行为对控制夹杂物形成及产品质量提升尤为重要。结合形核理论以及热力学耦合模型,对SWRH 92A帘线钢盘条存在的Al2O3 TiN复合夹杂物的形成机制进行了研究。结果表明：TiN夹杂在钢液凝固进程凝固分数约为0.866(温度为1457K)时开始析出;凝固前期Ti、N偏析比基本相当,然而当凝固接近结束时Ti的偏析比明显大于N,分别为6.059和2.367×10-6。TiN夹杂物形成过程中,其异相形核功小于均相形核功,异相形核半径大于均相形核半径,并且形核功与形核半径变化曲线在1457K时均达峰值。呈球形形态的Al2O3夹杂在TiN析出之前就已经析出,并且尺寸较小的Al2O3将被推动至凝固前沿被TiN捕获并以此为核心析出长大最终形成Al2O3 TiN复合夹杂。     

Ti-Mg复合脱氧和硫含量对钢中夹杂物特征及MnS析出行为的影响

采用扫描电镜/能谱仪表征了管线钢中夹杂物的形貌、尺寸、成分及数量,考察了不同Ti/Mg比的钢中夹杂物特征、硫含量及脱氧产物数量对MnS析出行为的影响,并进行了热力学计算.结果表明:Ti-Mg脱氧钢中夹杂物以MgO-Al₂O₃-Ti₂O₃、MgO-Ti₂O₃或MgO为核心,表面包裹或局部析出MnS,粒径小于1.3μm,数量为300~450 mm-2,形貌为圆形、多边形和方形;夹杂物中Ti/Mg原子数比为0.05~0.2时,夹杂物细小且近圆形;随硫含量减少,凝固过程中MnS析出倾向减小,MnS在夹杂物表面由包裹析出向局部析出转变,提高氧化物夹杂数量,有利于细小MnS的包裹或局部异质形核;Ti-Mg复合脱氧产物细小、弥散,可作为MnS异质形核核心,可同时降低MnS及氧化物的危害.      

含钛低碳洁净钢凝固过程氧化钛的析出行为

通过热力学研究,确定了含钛低碳钢中形成氧化钛的最佳钢液条件.在溶质元素偏析和扩散的基础上,建立了凝固过程氧化钛长大模型.计算结果表明,凝固过程对Ti 203的析出、长大有很大影响.在凝固过程中,析出的Ti203随凝固过程的进行很容易长大,并且随冷却速率的提高,Ti203的尺寸明显减小.相比较来说,凝固前液相中形成的较大的Ti 203在凝固过程中不容易长大,尺寸变化相对较小,并且受冷却速率的影响也小于凝固过程中析出的Ti203.     

Ti在Cr12MoV钢中的作用分析

通过计算Ti、V、Mo和Cr等元素对应的碳化物在不同温度范围内的溶度积,分析了Ti对Cr12MoV模具钢凝固和冷却过程中奥氏体和碳化物形成过程的影响。结果表明,在Cr12MoV钢中加入的Ti超过0.108 4%后才能析出TiC,析出的TiC有可能成为奥氏体形核的核心;凝固后的冷却过程中奥氏体中的TiC优先于VC析出,有可能作为VC异质形核的核心,使VC颗粒变得粗大,以致于淬火加热过程中VC颗粒很难均匀地溶解在奥氏体中,进而影响淬火回火过程中的二次硬化效果。     

7050合金淬火析出相的脱溶析出行为研究

采用事温水作为淬火介质进行Jominy末端淬火实验,采集了合金淬火过程试样的温度-时间变化曲线,测定了末端淬火试样距淬火端不同距离合金的淬火态电导率和硬度,结合透射电镜组织观察,研究了冷却速率对合金过饱和固溶体脱溶析出的影响,通过观察淬火析出相非均质形核,考察了7050合金过饱和固溶体淬火脱溶析出相的析出规律.结果表明,淬火冷却速率对7050合金的过饱和固溶体的脱溶析出行为有着显著影响.末端淬火实验过程中,合金过饱和固溶体大角晶界、小角晶界、晶内弥散相周围分别在距淬火端5,10和15 mm出现淬火析出相.随着距淬火端距离的增加,淬火态合金导电率呈上升趋势,合金淬火析出相尺寸变大,体积分数增加.合金淬火平衡η相的非均质形核核心的优先次序为:晶界、亚晶界、A国<,3>Zr等纳米级弥散相粒子.     

凝固过程重轨钢中MnS粒子形核与长大动力学分析

利用经典形核理论和扩散控制长大模型计算分析了重轨钢中MnS粒子析出的动力学行为,计算结果表明,MnS粒子在重轨钢凝固过程以均匀形核和晶界形核为主,主要在凝固末期析出。在设定的重轨钢成分下,计算出MnS的有效形核温度为1 634K,即Mn、S实际浓度积等于平衡浓度积。降低S的质量分数小于5.0×10-5能够推迟MnS接近固相线析出,而对MnS的长大半径影响较小;提高冷却速率从0.14K/s到1.45K/s,连铸坯内柱状晶区中MnS的长大半径比中心等轴晶区的大1个数量级,但对MnS的析出时机无影响。S元素是MnS在凝固过程中粗化长大的控制性环节,在凝固过程冷却速率对MnS粒子长大半径起着决定性的作用。     

钢中第二相诱导铜元素析出的晶体学分析

对钢液凝固温度下钛的化合物、Al2O3、MnS等基底与形核相铜元素的二维点阵错配度进行了计算,并对其成为铜元素非均质形核核心的有效性进行了分析.结果表明:基底与形核相的错配度δ越小,越有利于非均质形核.Ti2O3、MnS和Al2O3与铜元素的错配度较小,具有良好的匹配关系,可以作为铜元素形核的质点并促进其异质形核.Ti2O3和MnS是钢中残余铜元素非均质形核的最有效核心;Al2O3为中等有效核心;TiC、TiN为无效核心.     

20CrMnTi齿轮钢中TiN夹杂物的特征与分布

为了研究含钛齿轮钢中TiN夹杂物的析出与控制,利用扫描电镜对20CrMnTi齿轮钢两个端淬试样中TiN夹杂的分布与形貌进行了观察,通过夹杂物自动分析系统统计了试样中TiN夹杂的尺寸、数量,建立了数学模型,对试样中TiN夹杂的析出与长大进行了计算。结果表明,较高钛含量的试样凝固过程中TiN析出更早,析出量也更大,由于析出了更多数量的TiN夹杂,从而使得析出TiN夹杂的最终尺寸更小,当TiN夹杂的数量密度由37.28增加为58.65个/mm2时,TiN平均尺寸由2.33减小为1.49 μm。通过在凝固过程中为TiN夹杂提供充足数量的氧化物颗粒作为析出核心,提高异质形核率,从而增大TiN的析出数量,就可以降低析出TiN的整体尺寸,这为含钛齿轮钢中TiN夹杂物的控制提供了一种新的思路。     

低合金高强度钢奥氏体相中碳氮化物的析出模型

采用规则溶液描述低合金高强度钢奥氏体相中碳氮化物的热力学性质,以经典形核理论为基础,建立了低合金高强度钢奥氏体相中碳氮化物在连续冷却过程中碳氮化物析出模型.计算结果表明:冷却速率对碳氮化铌的析出及长大有重要影响.在不同冷却条件下,碳氮化铌的析出量随反应时间呈先增加后减少的趋势;随着冷却速率的提高,达到析出量峰值的时间越短,析出量和析出粒子的平均直径均减小.     

Investigation on Precipitation Behavior and Morphology of TiN Particles in High-Ti Low-Carbon High-Strength Steel

The precipitation behavior of TiN particles under homogeneous and heterogeneous nucleation conditions in a low-carbon high-strength steel with 0.11 wt% Ti was investigated by thermodynamics and dynamics equations combining with the microstructural observations on the surface and at the core of the casting slabs. The effect of cooling rate on the morphology and dimension of TiN particles was also analyzed in the present study. In addition, the three-dimensional and the spatial tridimensional morphology of compound TiN–Al2O3 was innovatively characterized by a focused ion beam scanning electron microscope. It was found that TiN particles possessed inerratic facet morphology (such as squares and clusters), and the square morphology was predominant. The preferential plane for TiN precipitates in the tested steel was {100}. Moreover, TiN particles precipitated both in solid and liquid phases from the solid–liquid phase region during the solidification process, and preferential precipitation was observed in the solid phase. Furthermore, the size of TiN particles decreased with the increasing cooling rate during the casting process. Al2O3 particles provided nucleation sites for the heterogeneous nucleation of TiN particles.     

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。     

Modeling of Microporosity Size Distribution in Aluminum Alloy A356

Porosity is one of the most common defects to degrade the mechanical properties of aluminum alloys. Prediction of pore size, therefore, is critical to optimize the quality of castings. Moreover, to the design engineer, knowledge of the inherent pore population in a casting is essential to avoid potential fatigue failure of the component. In this work, the size distribution of the porosity was modeled based on the assumptions that the hydrogen pores are nucleated heterogeneously and that the nucleation site distribution is a Gaussian function of hydrogen supersaturation in the melt. The pore growth is simulated as a hydrogen-diffusion-controlled process, which is driven by the hydrogen concentration gradient at the pore liquid interface. Directionally solidified A356 (Al-7Si-0.3Mg) alloy castings were used to evaluate the predictive capability of the proposed model. The cast pore volume fraction and size distributions were measured using X-ray microtomography (XMT). Comparison of the experimental and simulation results showed that good agreement could be obtained in terms of both porosity fraction and size distribution. The model can effectively evaluate the effect of hydrogen content, heterogeneous pore nucleation population, cooling conditions, and degassing time on microporosity formation.     

冷却速率对薄带连铸低碳钢微观组织的影响

摘要：为了探究冷却速率对薄带连铸低碳钢微观组织的影响，采用相变仪设备对铸带重新加热并进行不同冷却速率冷却，采用光学显微镜、场发射电子探针和电子背散射衍射等手段对微观组织和针状铁素体形核所利用的夹杂物进行了分析。结果表明，铸带经1200℃保温3min后，原奥氏体晶粒尺寸约为150～650μm，可以满足晶内针状铁素体形核对原奥氏体晶粒尺寸的需要。在2～5℃/s的冷却速率范围内，试样中得到了大量的针状铁素体组织，冷却速率为2℃/s的试样中大角度晶界所占比例约为60%；当冷却速率大于20℃/s，针状铁素体的形成受到抑制。铸带中针状铁素体形核所利用的夹杂物是Ti-Al-Si-Mn-O+MnS复合夹杂物。     

Effect of titanium on the microstructure and hardness uniformity of non-quenched and tempered prehardened steel for large-section plastic mould

Two non-quenched and tempered (NQT) prehardened plastic mould steels, differing by titanium content, were produced to have good hardness uniformity in large sections (485?×?970 and 465?×?1325?mm²). The effect of titanium on continuous cooling transformation curves, hardness uniformity and microstructure (grain size, phase constituents) were studied. It was found titanium had refined the grain size of NQT steel by the pinning effect of Ti (C, N) at grain boundaries, and titanium also effectively postponed the nucleation of proeutectoid ferrite in intercritical region. Using finite element method, the cooling rate from core to surface along longitude direction of the steels was predicted. The results indicated that the NQT steel with titanium had better process adaptability than that of NQT steel without titanium. Compared with production of NQT steel without titanium, production for NQT steel with titanium can largely save the production cost by shortening the production cycle and reducing the resources.     

CeO_2对SiC_p/Al-Si复合材料中析出Si影响机理的研究

采用粉末冶金法制备含CeO_2的SiC_p/Al-Si复合材料,研究CeO_2的添加量对复合材料显微组织的影响,探讨CeO_2在烧结过程中对Si析出的作用机理。结果表明,添加CeO_2可以有效细化析出Si颗粒的平均尺寸,并提高析出Si颗粒的数量。CeO_2的最佳添加质量分数为0.6%,过高或过低的CeO_2含量会降低细化效果。CeO_2作为析出Si相的非均匀形核基底,提高了析出Si相的形核率,从而细化析出Si颗粒尺寸,提高了析出Si颗粒的数量。     

3D Quantitative Characterization of Rapidly Solidified Al-36 Wt Pct Ni

The rapid solidification of a peritectic alloy is studied. Various 2D and 3D characterization techniques were effectively utilized to investigate the effect of cooling rate on both the phase fractions and the shrinkage porosity. Particles of Al-36 wt pct Ni were produced using a drop tube impulse system. Neutron diffraction and Rietveld analysis were used to quantify the phases formed during solidification. The microstructure of the produced particles was analyzed using SEM and X-ray microtomography. It was found that increasing cooling rate resulted in decreasing the Al₃Ni₂ to Al₃Ni ratio. Also, quantitative analysis of the microtomography images revealed that the volume percent of porosity increased with increasing particle size. The distribution of porosity was found to be significantly different in small and large particles. It was concluded that the extensive growth of Al₃Ni₂ at lower cooling rates followed by the peritectic reaction made the feeding of the shrinkages more difficult, and as a result, the volume percent of porosity increased. Other findings showed that high cooling rate during solidification would result in the formation of a quasicrystalline phase, known as D-phase, and suppression of the primary Al₃Ni₂. Also, investigation of the 3D structure of the solidified particles revealed that large particles of Al-36 wt pct Ni contain multiple nucleation sites, while smaller particles contain only one single nucleation site.     

晶内铁素体及其组织控制技术研究概况

介绍了晶内铁素体的组织特点以及晶内铁素体组织对钢材力学性能的影响,得出晶内铁素体能显著提高钢的冲击韧性。详细阐述了晶内铁素体的形核机理,分析表明目前晶内铁素体形核机理仍不够完善,尚未形成统一的机制。同时介绍了钛氧化物、MnS、稀土氧化物等促进晶内铁素体形核的夹杂物,指出含Ti复合夹杂物是理想的晶内铁素体形核核心。最后分析了夹杂物尺寸、冷却速度对晶内铁素体形核的影响,并简述了一些晶内铁素体组织控制技术,结果表明Ti-B 处理、Ti-Mg处理效果优于单独的Ti处理。