钢锭铸造过程中宏观偏析的研究进展

宏观偏析现象普遍存在于铸造合金凝固过程中,直接影响材料的使用性能.近几十年来,研究工作者们对铸造过程中合金元素的宏观偏析进行了预测和控制研究,其目的是使钢锭中溶质元素的分布尽可能均匀.阐述了钢锭宏观偏析分布形式、形成机理和影响因素,详细地总结了宏观偏析数学模型的发展,介绍了钢锭宏观偏析的控制措施.最后,对铸锭宏观偏析数...     

关于热顶铸造6082合金圆铸锭表层成分偏析的研究

通过大量生产实践及严谨的实验,发现大直径规格6082圆铸锭表层存在宏观偏析层。Si、Mg、Mn主要合金元素都存在成分偏析,Si、Mg偏析规律及偏析程度高度一致。偏析层的形成是热顶铸造这种生产方式造成的,其形成机理是:铸锭在凝固结晶过程中,表层经历了急剧冷却凝固-晶界重熔-溶质在晶界网络流动-再凝固过程。晶界重熔引起溶质流动,溶质流动导致表层不同区域出现成分偏析,最终形成"勺子"形状的表层偏析层。     

铝合金热顶铸造圆铸锭的偏析层控制方法

针对铸锭特有的偏析层冶金缺陷造成后续加工制品化学成分不均匀的问题,简单阐述了偏析层在高、低倍检测下的组织形貌,并从热顶铸造法成型机理方面分析了偏析层的形成过程,同时通过实验验证了铸造速度、铸造温度、铸造水流量和工装位置等因素对热顶铸造法生产圆铸锭控制偏析层厚度的影响。     

超声处理对2219大规格铝锭微观组织与宏观偏析的影响

在半连续铸造过程中施加超声,成功制备了φ1250 mm 2219铝合金铸锭.利用光学显微镜、扫描电镜、能谱仪及直读光谱仪等仪器对铸锭的组织与成分分布进行检测与分析,探究超声对铸锭组织与偏析的内在作用机制.研究结果表明:超声振动引起的空化和声流效应能明显均匀组织结构,细化晶粒,尤其是心部晶粒细化率达到39.6%.超声促进铸锭晶间第二相呈枝丫状断续分布,晶内析出物点状弥散分布.同时,超声有效减小近表面负偏析,降低边部与心部之间的溶质浓度差异,弱化整个横截面的浓度波动,从而改善宏观偏析.      

Al─Fe合金熔炼与铸造的工艺探讨

本文主要从化学成份，宏观组织，铸锭偏析以及铸造过程中的工艺参数、质量控制等方面，叙述了含铁量较高的Ａｌ─Ｆｅ合金在熔炼与铸造过程中的工艺性能以及工艺控制     

低镍不锈钢光谱标准样品的研制

根据GB/15000《标准样品工作导则》、YB/T082-1996《冶金产品分析用标准样品技术规范 》和相关ISO导则研制了包含C、Si、Mn、P、S、Ni、Cr、Cu、Mo、V、Co、Sn、As、W、Nb、N 共16个元素的马氏体沉淀硬化型不锈钢S17400(17-4PH),双相不锈钢S32205、S32750和200系奥氏体不锈钢S20100(201)等4种典型低镍不锈钢光谱分析用标准样品。采用500 kg中频感应炉冶炼,冶炼过程利用感应磁力搅拌,301铝丝和硅-钙粉脱氧,小截面(145 mm)锭型金属模上注法浇铸,从而保证了钢液成分均匀、良好的脱氧效果和元素的较高回收率,避免了成分偏析和夹杂物的形成。铸锭经锻造和热轧后进行偏析和金相组织检验,结果表明被测元素没有偏析且组织结构能满足制作光谱标样的要求。用F方差法检验均匀性,各元素F值均小于临界值F0,表明被检元素的均匀性良好。由8家有资质的实验室采用多种不同原理,准确、可靠的分析方法协作定值分析,对分析结果进行统计和处理,得到了被定值元素的认定值和扩展不确定度。     

熔炼工艺对TC10铸锭中Cu含量的影响

在真空自耗熔炼TC10铸锭时,Cu的化学成分均匀性很难控制,易产生偏析缺陷,影响铸锭的冶金质量和材料性能。针对真空自耗熔炼过程中Cu元素易偏析,不易控制的技术难点,研究了经二次VAR熔炼的Ф480mm及Ф560mm锭型中Cu的横向及纵向成分分布,对Cu元素的偏析规律及熔炼工艺对其分布的影响进行了进一步的探索。结果表明,选用不同的原料、锭型及熔炼电流,对Cu含量在铸锭横向、纵向分布均匀性有影响。     

用电解铝液生产7050铝合金圆铸锭的质量缺陷及其预防措施

基于薄壁管挤压用7050铝合金圆铸锭的化学成分和质量要求,简述了Φ178 mm7050铝合金圆铸锭热顶铸造工艺,分析了用电解铝液生产Φ178 mm7050铝合金圆铸锭热顶铸造过程中出现偏析层超标、晶粒粗大、羽毛状晶等铸造缺陷和漏铝现象的原因并提出了相应的改进措施,实现了生产高品质Φ178 mm7050铝合金薄壁管挤压用圆铸锭的目的。     

Ti2AlNb合金锭真空自耗过程宏观偏析的数值模拟

Ti₂AlNb合金锭的真空电弧重熔(VAR)是一种超高温且不透明冶金过程,很难对这一过程中的熔体流动行为和宏观偏析的形成过程进行试验研究。发展了基于欧拉多相流的电磁场、温度场、流场、溶质场的多场强耦合数学模型,研究了真空自耗过程中的多物理场相互作用机制,对Ti₂AlNb合金锭中成分偏析形成过程及分布规律进行了预测。模拟结果表明,电磁力主要分布于熔池表面,自感电磁力推动金属液由中心向下流动而加深熔池;搅拌电磁力的离心效应则大幅提升熔池的温度场均匀度,促使熔池内金属液中的溶质混合均匀。尽管铸锭外围和中心分别形成了大范围的正、负偏析区,但区域内的成分较为均匀。在搅拌和沉降的作用下,金属熔池中的等轴晶极大地缩短了铸锭中的柱状晶区。该模型的模拟结果在熔池深度与宏观偏析分布方面与试验结果吻合良好,可进一步应用于预测和研究工业级大型铸锭中的成分偏析。     

激光诱导击穿光谱原位分析仪和电子探针在CrMo耐磨铸钢元素偏析分析中的联合应用

CrMo耐磨铸钢是重要的耐磨钢铁材料,凝固过程中的溶质元素偏析是影响CrMo耐磨铸钢组织和性能的重要因素,了解凝固过程中的溶质元素偏析对于CrMo耐磨铸钢的工业化生产具有重要的借鉴意义。宏观偏析和微观偏析是衡量材料偏析程度的两个指标,准确的测量其偏析状况是研究溶质元素偏析的基础。实验以CrMo耐磨铸钢为研究对象,采用激光诱导击穿光谱原位分析仪(LIBSOPA)和电子探针(EPMA)分析钢锭不同部位的宏观偏析和凝固组织中的微观偏析,结果发现,Cr、V和Mn元素在CrMo耐磨铸钢铸锭中宏观偏析程度较小,偏析比接近1,而Mo元素宏观偏析程度较大,其最大宏观偏析比超过1.20;Cr、Mo、V和Mn元素在CrMo钢凝固组织中均存微观偏析,且随着冷却速度的增加,Cr、Mo、V和Mn微观偏析程度也随之增加,其最大微观偏析比分别为1.39、2.63、3.47和1.83。LIBSOPA与EPMA在CrMo耐磨铸钢元素偏析分析中的联合应用,对全面了解CrMo钢铸锭元素偏析,优化铸造以及后续的热加工工艺具有重要借鉴意义。     

光谱分析用标准物质/样品均匀性初检的符合性评价

通常,有色光谱分析用标准物质/样品均匀性初检是依据YS/T 409-1998有色金属产品分析用标准样品技术规范进行,本文对铝-锌-铟系合金标准物质研制过程中的炉前检测数据进行实例分析,发现此技术规范的不足在于未进行块内测量数据平均值差异显著性检验,因此提出采用t检验法进行块内测量数据平均值显著性检验,这一方法较技术规范更为严密,它不仅考虑块内测量数据的精度,而且还考虑了块内测量数据平均值之间的差异,为最终均匀性复检提供了强有力的技术保证。     

Physical Simulation of the Hot Direct Rolling of Steel

Thin slab casting and direct rolling technologies became important for hot strip production to increase the productivity, to lower the investment costs and to increase the energy efficiency. The main objective of our investigation is to gain a better understanding of the microstructural evolution starting from solidification through to the final hot rolled sheet. Small ingots were cast and direct rolled and reheated and rolled, using a thermomechanical testing machine. The differences in the recrystallization kinetics during both production routes were measured using double hit hot compression tests. In addition, multipass deformation trials were performed to reveal differences in the resulting microstructure and the grain size homogeneity. It was found, that the coarse initial microstructure before rolling become negligible after three rolling passes. This means that despite the simple and economical direct rolling similar mechanical strip properties can be obtained in comparison to conventional strip production routes.     

A mathematical model for surface segregation in aluminum direct chill casting

A two-dimensional mathematical model for the development of macrosegregation at and close to the ingot surface during direct chill (DC) casting of aluminum rolling sheet ingots is presented. The model accounts for macrosegregation caused by exudation of interdendritic melt and macrosegregation associated with solidification shrinkage. Equations for the conservation of energy, solute, momentum, and mass during the stationary phase of the process are solved numerically by a finite-element method. The solution domain corresponds to a vertical cross section at the middle of the longest side of the slab. The main simplifications in the modeling concept are to assume that the solid in the mushy zone moves with the casting speed, and that the alloy is binary and solidifies according to the lever rule. The thickness and solute concentration of the surface layer and the macrosegregation close to the surface are calculated, and modeling results are compared to measurements on full-scale castings.     

Effect of thermal conditions and alloying constituents (Ni,Cr) on macrosegregation in continuously cast high-carbon (0.8 Pct), low-alloy steel

The aim of this study was to investigate the influences of heat transfer, thermal gradient, solidification rate, and the addition of up to 1 pct Ni and 1 pct Cr on the solidification and macrosegregation of high-carbon (0.8 pct), low-alloy steel. Sixteen 13.6-kg laboratory ingots were horizontally and unidirectionally cast in a static moll, fitted on one face with a water-cooled copper chill to simulate a continuous casting mold. Thermocouples, placed in the chill and the mold, were used to calculate heat flux, thermal gradient, and solidification rate. The ingots were examined with respect to macro- and microstructures, distribution of phases, dendrite arm spacing, and solute element distribution. The extent of macrosegregation of carbon and sulfur was determined by wet chemical analysis of drillings, and a TANDEM VAN DE GRAAFF accelerator was used for A1, Si, P, V, S, C, Mn, Ni, and Cu. The extent of macrosegregation of these elements was correlated with heat transfer and thermodynamic distribution coefficient data.     

Experimental investigation of thermomechanical effects during direct chill and electromagnetic casting of aluminum alloys

The deformation and the temperature field within direct chill (DC) and electromagnetic (EM) cast aluminum ingots have been measuredin situ using a simple experimental setup. The deformation of the cross section of the cold ingots has also been characterized as a function of the casting speed, alloy composition, and inoculation condition. The pull-in of the lateral rolling faces has been found to occur in two sequences for DC cast ingots, whereas that associated with electromagnetic casting (EMC) was continuous. The pull-in was maximum at the center of these faces (about 7 to 9 pct) and strongly depended upon the casting speed. Near the short sides of the ingots, the deformation was only about 2 pct and was nearly independent of the casting parameters and alloy composition. Based upon these measurements, it was concluded that the pull-in of the rolling faces was mainly due to the bending of the ingots induced by the thermal stresses. This conclusion was further supported by a simple two-dimensional thermoelastic model.     

Jet Mixing in Direct-Chill Casting of Aluminum: Crater Effects and its Consequence on Centerline Segregation

Recent reports have demonstrated the possibility of mitigating macrosegregation during the Direct-Chill casting of rolling slab ingots using an impinging jet. Herein, an analytical model is presented to predict the shape of the crater formed due to the impact of the jet on the slurry region. The model takes into account alloy composition, physical dimension, and casting speed on the distribution of forces and crater shape. The calculated shape of the crater profile is used to explain the centerline depletion in the impingement region previously reported.