Part I. The microstructural evolution in Ti-Al-Nb O+Bcc orthorhombic alloys
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The simulation of macrosegregation in a 2.45-ton steel ingot with the three-phase mixed columnar-equiaxed model was presented previously. The results showed an overestimation of the intensity of bottom negative segregation. The reason is due to the assumed globular morphology for the equiaxed crystal. Therefore, in this paper a simple approach is suggested to treat the dendritic morphology of equiaxed crystals. Three aspects are improved: the drag force between the moving equiaxed crystals and the surrounding melt, the mechanism of the columnar-to-equiaxed transition, the packing limit of the equiaxed crystals. The modified model is used to calculate the macrosegregation of the same ingot. It is found that the modified model predicts less severe negative segregation in the bottom equiaxed zone than the previous globular equiaxed model does, i.e. it agrees better to the experiment. The model considering simplified-dendritic morphology improves the calculation accuracy.
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The varying regularity of microhardness of Cu-3.2Ni-0.75Si alloy was analysed during aging. Results show that the alloy can obtain maximum microhardness when aging at 500℃, while electric conductivity can reach the highest value when aging at 550℃. Anlysis show that, the strengthening effect of the alloy lean mainly upon the volume fraction and radius of second phase. Before volume fraction reach equilibrium, the precipitation of second phase has important influence on strenghening of the alloy , while after equilibrium, the main factor that leads to decrease of strenghening effect is the growth of second phase.
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复杂 Cu-Ni 基于合金被开发为高温度申请制作穿抵抗灌木丛。在发达合金铸块锭的 equiaxed 谷物的控制的担心焦点由离心的扔准备了。结果证明 equiaxed 谷物被 melt，冷却的率和模子的旋转速度的流的温度决定。与在倒温度的减少，在铸块锭的横向的节的 equiaxed 谷物的部分增加，圆柱的谷物的平均长度减少。当流的温度在 1,2500C 下面被限制时，完全的 equiaxed 谷物能是 obtained.Based 在上处理的最佳的离心的扔，分别地，有完全的 equiaxed 谷物的发达合金铸块锭的张力的力量在房间温度和 5000C 到达 MPa 和 435 MPa 到 810 它比普通商业 QAI10-4-4 合金的高是14%和110%。比 QAI10-4-4 合金的低的发达合金时间的 wear 率。
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The effect of various stirring parameters on the solidification structure of medium-carbon steel during the initial stage of solidification was studied in this paper.The results showed that with different stirring processing time,the macrostructure was refined and equiaxed crystal also increased,as well as columnar crystal reduced and the proportion of equiaxed grains enhanced with the increasing stirring time;by changing the depth inserted into liquid steel,it can be seen that the proportion of equiaxed crystal zone significantly increased,grain size was refined and distributed more evenly with the increase of insertion depth;By changing stirring rate,it was inferred that the slower the stirring rate,the larger the proportion of equiaxed grains.The mechanism of these phenomena were also analyzed.
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Liquid steel
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The equiaxed dendrite evolution during the solidification of a pure material was numerically simulated using the phase field model. The equiaxed dendritic growth in a 2-D square region of undercooled liquid nickel with four fold anisotropy was computed. The phase field model equations have been solved using the explicit finite difference method on an uniform mesh. The formation of various equiaxed dendritic patterns was shown by a series of simulation.
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Effects of electromagnetic stirring on the solidification structure of continuously cast SUS430 slabs were investigated to reduce the ridging of cold-rolled sheets by grain refinement of the solidification structure. The following results were obtained:(1) The area in which equiaxed crystals form increases with increasing stirring intensity and decreasing superheat (ΔT) of molten steel. The most suitable stirring mode is the alternate stirring. The effect of ΔT can be moderated by lowering the position of the stirrer, but the ratio of equiaxed crystals decreases in this case. Appropriate conditions for obtaining a ratio of equiaxed crystals over 60% were determined.(2) Ridging is reduced as the ratio of equiaxed crystals increases. Stable ridging obtained for 50% or more equiaxed crystals is the same as that of steel sheets rolled from ingots.
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Abstract Correct prediction of composition heterogeneities and grain structure across a steel ingot is critical in optimizing the industrial processing parameters for enhanced performance. The columnar to equiaxed transtion (CET) is a microstructural transition which is strictly controlled as it affects the mechanical properties of the final product along with the macrosegregation patterns. Larger equiaxed regions are preferred for most industrial applications. CET is significantly affected by the number density of equiaxed grains and by the nucleation undercooling. 8 kg 42CrMo4 alloy steel ingots (240 mm x 60 mm x 60 mm) were cast. The cast structure was characterized by ASCOMETAL. The experiments were simulated with a process-scale model of solidification that incorporates a multiscale description of the microstructure formation. The goal of the present study is to show the capabilities of such a process-scale solidification model to explain the observed structure distributions (extent of the columnar and equiaxed zones, equiaxed-to-columnar transition).
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