钢锭内部的金属流动与宏观偏析

钢锭内部的宏观偏析取决于凝固过程中凝固界面处的溶质分配及固－液两相区中的溶质流动。溶质分配和流体流动受钢锭合金成分、冷却条件及凝固形式等影响。根据钢锭中宏观偏析形成的最新理论，从枝晶间流动、凝固组织、成分、冷却条件等方面，详细论述了钢锭中宏观偏析的控制。     

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。     

VAR熔炼GH2130合金点状缺陷的分析及改进措施

采用元素热力学平衡分配计算的方法,对GH2130合金VAR(真空自耗重熔)出现的宏观偏析及其析出相的形成特点进行了探讨。结果表明,合金中"点偏区域"主要是富Ti、C的一次碳化物和富Ti、Cr、W的Laves相等。这主要是由于钢锭重熔过程中VAR熔速过高所致。降低熔速、减小熔池深度后,有效避免了这种宏观偏析的产生。     

VAR熔炼GH2130合金点状缺陷的分析

采用元素热力学平衡分配计算的方法,对GH2130合金VAR(真空自耗重熔)出现的宏观偏析及其析出相的形成特点进行了探讨。结果表明,合金中"点偏区域"主要是富Ti、C的一次碳化物和富Ti、Cr、W的Laves相等,这主要是由于钢锭重熔过程中VAR熔速过高所致。降低熔速、减小熔池深度后,有效避免了这种宏观偏析的产生。     

钛合金VAR铸锭中的偏析

钛合金铸锭通常都是将海绵钛与合金添加料配合,压制电极块,焊接一次电极,然后二次真空电弧熔炼(简称VAR)而成.该工艺方法本身就存在来自合金元素添加方法引起的偏析和凝固偏析两大弊病.前者一是由于合金元素在电极块中分布不均匀,熔炼中来不及消除就凝固了;二是合金元素中有高熔点和高密度元素,在两次VAR中不能完全熔化和混合均匀.后者是由于电磁力、浮力和重力等因素引起的合金元素集聚而产生的宏观偏析与微观偏析.     

垂直半连续铸造Cu-15Ni-8Sn合金组织及性能研究

采用垂直半连续铸造法制备Cu-15Ni-8Sn合金,并对合金的凝固组织、元素偏析以及热处理后的组织和性能进行了分析。采用普通熔铸法制备Cu-15Ni-8Sn合金的铸态组织主要由贫Sn的α-Cu(Ni, Sn)固溶体、富Sn的γ相以及片层状的（α+γ）组成,并且Sn元素主要偏聚在晶界上。在垂直半连续铸造的过程中同时施加机械振动和电磁场具有明显的晶粒细化效果,同时有效减轻了Sn元素的宏观反偏析和微观晶界偏析,富Sn相比较均匀地分布在晶粒内部和晶界上。Cu-15Ni-8Sn合金经过850℃固溶、 90%轧制和400℃时效1 h后可获得最佳的综合性能,此时合金的硬度为HV 401,导电率为8.4%IACS,抗拉强度为1233 MPa,屈服强度为1185 MPa,伸长率为4.5%。     

过热度对GCr15SiMn轴承钢钢锭宏观偏析的影响

针对GCr15SiMn钢锭易出现宏观偏析凝固缺陷的问题,研究了过热度对GCr15SiMn钢锭宏观偏析的影响规律,使用真空感应炉冶炼1 kg的GCr15SiMn钢锭,通过酸侵试验与OPA技术分别测定了钢锭的凝固组织与宏观偏析,并结合ProCAST软件分析了钢液流动的规律。结果表明,高过热度(70 ℃)时,中心下部出现一定程度的负偏析,中心上部形成了较严重的正偏析同时伴随疏松;中过热度(50 ℃)时,疏松范围较小,碳元素分布较均匀;低过热度(20 ℃)与极低过热度(-20 ℃)时,疏松范围扩大,凝固初期是严重负偏析,凝固末期是严重正偏析。过热度影响偏析的机理为,高过热度时,凝固过程热对流较强,溶质上浮,钢锭上部的正偏析严重;当过热度过低时,初期凝固大量形核并保留在钢锭底部,在底部形成严重的负偏析。     

超深淬硬层冷轧工作辊用钢预处理试验

在冷轧工作辊生产的过程中,轧辊钢锭在结晶凝固时具有固有的枝状偏析,锻造时又会形成碳化物的带状分布,这就不可避免地带来碳及合金元素的微区偏析和不均匀性。这些天然的弊端必须在随后的预处理过程中加以克服。同时,轧辊最终热处理的质     

稀土处理钢的偏析和夹杂物偏聚

为了研究钢中稀土夹杂物的分布和元素偏析,解剖了三支钢锭和一些钢坯。结果表明,稀土处理后,钢中 MnS 消失而形成稀土的硫化物和硫氧化物。这些稀土夹杂物与金属晶体一起沉降到钢锭底部形成夹杂物的宏观偏聚区,从而导致锭中稀土、硫及氧的偏析并使硫呈现反常分布。稀土夹杂物分布在铸态晶粒内部因而使枝晶间的组织发生变化,改善了铸态材料的性能。加入稀土金属的方法不同,锭中夹杂物的偏聚和钢中元素偏析状态亦有差别,一种锭模内加入法获得良好的结果。     

GH742合金钢锭的组织及偏析的消除

研究了GH742合金钢锭的显微组织、初熔点、元素偏析及其消除办法.研究表明,钢锭原始铸态组织基体γ中有大量γ'相,枝晶间存在少量γ/γ'共晶相、针状金属间化合物(Ni3Nb)、白色块状镧化物和部分骨架状碳化物.初熔点在1130～1160℃之间.钢锭经1135℃×24h均匀化处理可消除枝晶偏析,使组织均匀.     

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.     

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。     

Investigation of Macrosegregation Formation in Aluminium DC Casting for Different Alloy Systems

Direct chill (DC) casting of aluminum involves alloys employing different solute elements. In this article, a qualitative analysis and comparison of macrosegregation formation is presented for three different alloy systems: Al-Mg, Al-Zn and Al-Cu. For this purpose, a multiphase, multiscale solidification model based on a volume-averaging method accounting for shrinkage-induced flow, thermal-solutal convection and grain motion is used and applied to an industrial-scale DC-cast ingot. The primary difference between these alloys is the thermal-solutal convection with Al-Mg having a competing thermal and solutal convection, whereas the other two systems have a cooperating thermal and solutal convection. In the study, the combined effect of the macrosegregation mechanisms is analyzed for each alloy to assess the role of the alloy system on the final macrosegregation.

     

Thermosolutal convection and macrosegregation during solidification of hypereutectic and hypoeutectic binary alloys in statically cast trapezoidal ingots

Thermosolutal convection patterns and evolution of macrosegregation during solidification of hypereutectic and hypoeutectic NH₄CL-H₂O binary systems in trapezoidal side-chilled ingots with negative and positive slopes have been numerically investigated. The results have been compared with the base case of solidification in a rectangular ingot. During solidification of NH₄CL-70 pct H₂O hypereutectic alloy, channels and “A segregates” develop early in the solidification process. When the slope is positive, channels penetrate to a larger distance inside the ingot. Whereas, for negative slope, they are shifted outward toward the chilled wall and are vertically oriented. During solidification of NH₄CL10 pct-H₂O hypoeutectic alloy, circulation cells which emerge in the narrow melt at later stages of the process are shown to be responsible for the development of V-shaped segregates in the final casting. The final degree of macrosegregation is higher for both positive and negative slopes of the ingot chilled wall compared to the rectangular ingot. deceased.     

Modeling of Species Transport and Macrosegregation in Heavy Steel Ingots

In the current study, two significant phenomena involved in heavy steel ingot casting, i.e., species transport and macrosegregation, were numerically simulated. First, a ladle–tundish–mold species transport model describing the entire multiple pouring process of heavy steel ingots was proposed. Carbon distribution and variation in both the tundish and the mold of a 292-ton steel ingot were predicted. Results indicate high carbon concentration in the bottom of the mold while low concentration carbon at the top of mold after the pouring process. Such concentration distribution helps in reducing both negative segregation in the bottom of the solidified ingot and positive segregation at the top. Second, a two-phase multiscale macrosegregation model was used to simulate the solidification process of industrial steel ingots. This model takes into account heat transfer, fluid flow, solute transport, and equiaxed grain motion on a system scale, as well as grain nucleation and growth on a microscopic scale. The model was first used to analyze a three-dimensional industry-scale steel ingot as a benchmark. Then, it was applied to study macrosegregation formation in a 53-ton steel ingot. Macrosegregation predicted by the numerical model was presented and compared with experimental measurements. Typical macrosegregation patterns in heavy steel ingots are found to be well reproduced with the two-phase model.     

GCr15SiMn轴承钢中碳元素偏析与疏松的相关性

针对GCr15SiMn钢锭在凝固过程中容易出现宏观偏析与疏松等凝固缺陷的问题,为了制定更合理的模铸浇注工艺,通过真空感应炉冶炼1 kg的GCr15SiMn钢锭,从模铸工艺和钢锭宏观组织的角度研究了实验室与工业生产的相似性,采用OPA、SEM等检验方法研究了碳偏析与疏松的相关性。研究表明,碳元素的偏析最为严重,碳元素偏析与疏松相关。得出了碳偏析与疏松的相关性公式SC=0.92P－0.89,该公式所体现出的基本规律适用于工业生产的铸锭。根据SEM和OPA的统计结果,建立了疏松的当量直径与其定量表征值(表观致密度P)的对应关系。结果表明,随着疏松的当量直径增大,碳的偏析度逐渐增大。利用Scheil微观偏析模型从原理上进行了分析与讨论,得出碳的偏析度和钢液收缩量呈正相关关系。     

Solidification and microstructure of a high alloy stainless steel

The steel considered in the present work can be classified as a 6Mo austenitic stainless steel, which has a much higher corrosion resistance than many other commercially available stainless steels. However, because of macrosegregation during solidification of ingot casting, the high alloy content in the steel, especially Mo, N, C, enhances precipitation of intermetallic phases such asa phase. Such precipitation mainly occurs in the center of the material and has normally no significant effect on mechanical properties or surface corrosion resistance. The introduction of modern production methods such as continuous casting of slabs for stainless sheet production has reduced the macrosegregation tendency and related precipitation. However, the microsegregation in a small scale may be enhanced as the higher cooling rate prevents the diffusion in the solidified region. In the present work, the continuous casting solidification process was simulated in a laboratory gradient     

Computational Fluid Dynamics Modeling of Macrosegregation and Shrinkage in Large-Diameter Steel Roll Castings

Minimizing macrosegregation and shrinkage in large cast steel mill rolls challenges the limits of commercial foundry technology. Processing improvements have been achieved by balancing the total heat input of casting with the rate of heat extraction from the surface of the roll in the mold. A submerged entry nozzle (SEN) technique that injects a dilute alloy addition through a nozzle into the partially solidified net-shaped roll ingot can mitigate both centerline segregation and midradius channel segregate conditions. The objective of this study is to optimize the melt chemistry, solidification, and SEN conditions to minimize centerline and midradius segregation, and then to improve the quality of the transition region between the outer shell and the diluted interior region. To accomplish this objective, a multiphase, multicomponent computational fluid dynamics (CFD) code was developed for studying the macrosegregation and shrinkage under various casting conditions for a 65-ton, 1.6-m-diameter steel roll. The developed CFD framework consists of solving for the volume fraction of phases (air and steel mixture), temperature, flow, and solute balance in multicomponent alloy systems. Thermal boundary conditions were determined by measuring the temperature in the mold at several radial depths and height locations. The thermophysical properties including viscosity of steel alloy used in the simulations are functions of temperature. The steel mixture in the species-transfer model consists of the following elements: Fe, Mn, Si, S, P, C, Cr, Mo, and V. Density and liquidus temperature of the steel mixture are locally affected by the segregation of these elements. The model predictions were validated against macrosegregation measured from pieces cut from the 65-ton roll. The effect of key processing parameters such as melt composition and superheat of both the shell and the dilute interior alloy are addressed. The influence of mold type and thickness on macrosegregation and shrinkage also are discussed.     

一种合金焊丝模铸锭中心孔洞的成因和措施

高强度合金焊丝钢广泛用于工程机械行业,具有残余元素低、纯净度高的特点,氢、氧、氮气体元素含量均要求严格,国内该系列焊丝材料主要依赖进口。为了进一步开拓系列高端焊丝钢市场,开发了新型高强度合金焊丝钢。生产过程发现模铸钢锭轧制后探伤不合格,取样发现初轧坯横截面低倍中心出现孔洞缺陷,影响材料质量。通过低倍试验和EPMA分析结合材料成分采用Thermo-Calc软件计算固液相线,分析发现钢种固液线差值相对较小是导致钢锭中心孔洞的主要原因。采用优化钢液浇注过热度和模铸浇注时补缩控制,避免了钢锭中心孔洞缺陷的产生。