定向凝固Fe-Mn-C-Al系TWIP钢枝晶生长行为的研究

采用定向凝固技术结合光学显微镜对Fe-Mn-C-Al系TWIP钢的枝晶生长行为进行了研究,通过建立不同的数学模型对生长速率为25μm/s、50μm/s、100μm/s定向凝固TWIP钢试样中的一次和二次枝晶臂间距进行了预测,并与实验结果进行了比较,结果发现,在实验速度范围内,实验用钢的凝固组织都为枝晶组织,并且随着抽拉速度的增加,枝晶被明显细化。3种一次枝晶臂间距模型的计算值与实验值的综合误差率分别是22.45%、35.64%、78.49%,其中Hunt模型预测效果最好。3种二次枝晶臂间距模型的计算值与实验值的综合误差率分别是8.27%、19.70%、41.94%,其中Masana Imagumbai模型可以准确的预测SDAS值。     

PMO对42CrMoA钢?300 mm圆坯枝晶组织的影响

枝晶形貌是影响铸坯元素分布、缩孔缩松及性能的重要因素，细化枝晶能够有效提高铸坯质量。研究了PMO处理对42CrMoA钢?300 mm连铸圆坯枝晶形貌的影响。研究发现，PMO处理坯的柱状枝晶生长方向发生偏转，枝晶臂间距明显减小，其中一次枝晶臂间距减小率为10%～35%,二次枝晶臂间距减小率为25%～35%。分析认为，PMO造成凝固前沿热流方向及溶质场变化，导致枝晶重新竞争生长，因此一次枝晶生长方向发生改变，且一次枝晶臂间距减小。另外，PMO在铸坯凝固前沿形成感生电流，固液相电导率差异造成枝晶尖端电流聚集，进而导致曲率半径增大，降低了细枝晶的溶解速率，抑制了二次枝晶的熟化过程，因此二次枝晶臂间距减小。研究结果为今后利用PMO改善铸坯枝晶组织提供了指导。     

双辊连铸304不锈钢2.0 mm薄带中铁素体的形态与分布

采用化学侵蚀、彩色金相显示和电子探针(EPMA)分析研究了304奥氏体不锈钢(%:0.08C、1.37Mn、0.62Si、17.77Cr、9.09Ni)2.0 mm连铸薄带内残留铁素体的形态和分布。结果表明,在薄带表层铁素体呈棒状,其间距≤20μm;在薄带柱状树枝晶区,铁素体位于一次枝晶臂和二次枝晶臂中心,铁素体二次枝晶间距≤10μm;在中心等轴晶区,残留铁素体形貌和分布与等轴晶区半固态形成机理有关。先凝固的固相颗粒内部铁素体为弯曲树枝状和网状形貌,固相颗粒间隙铁素体呈岛状形貌。     

连铸板坯二次枝晶臂间距对中心碳偏析的影响

针对新余钢铁公司生产的1 900 mm×250 mm板坯,通过测量不同拉速、二冷比水量、过热度等工况条件下铸坯的二次枝晶臂间距,确定不同连铸工艺参数条件对二次枝晶臂间距的影响,结果表明二次枝晶臂间距越大,渗透率越高,中心碳偏析越严重.     

凝固组织对GCr15轴承钢220 mm×260 mm连铸坯中心偏析的影响

铸坯高中心等轴晶率及小的二次枝晶臂间距有利于降低高碳钢M+E-EMS连铸坯中心偏析。通过建立GCr15钢220 mm×260 mm连铸坯耦合有限元-元胞自动机模型(CAFE)及二次枝晶臂间距(SDAS)模型,研究结晶器电磁搅拌、过热度和拉速对中心等轴晶率及二次枝晶臂间距的影响。结果表明,相比于拉速,过热度和结晶器电磁搅拌对其影响明显。随着过热度降低及结晶器电磁搅拌强度增加,铸坯中心等轴晶率增加而二次枝晶臂间距减小,而拉速对凝固终点和中心固相率影响大。工业试验结果表明,采用结晶器与凝固末端电磁搅拌,相比于过热度35℃和拉速0.75 m/min,控制过热度小于25℃且拉速调整为0.8 m/min时,轴承钢GCr15铸坯中心等轴晶率由原27%增加至38%且二次枝晶臂间距细化,中心碳偏析指数由原1.06～1.39降至0.93～1.13。     

轴承钢大方坯二次枝晶臂间距数值模拟

针对某钢厂GCr15钢连铸坯质量问题,利用实验室试验和Pro CAST模拟软件对连铸坯二次枝晶臂间距进行研究,分析过热度、拉速和二冷水量对二次枝晶臂间距的影响,并进一步探究碳偏析与二次枝晶臂间距的关系。结果表明,铸坯二次枝晶臂间距从铸坯表面到中心呈先增大后减小的趋势,与Pro CAST模拟结果基本一致。降低过热度和拉速、增大二冷水量均有助于减小二次枝晶臂间距;为了提高连铸坯质量,建议将过热度、拉速和二冷水量分别控制在20℃、0.45 m/min、0.32 L/kg左右。铸坯碳偏析最大值位于柱状晶向等轴晶转变区域(CET)。     

试样尺寸对一种单晶高温合金组织的影响

 通过制备两种不同尺寸单晶试样的凝固试验,研究了试样尺寸对DD3单晶高温合金组织的影响。结果表明,在相同的凝固工艺条件下,试样尺寸增大,合金的一次枝晶间距和二次枝晶间距变大,合金元素的偏析程度稍有增加,枝晶干和枝晶间的铸态γ′相尺寸稍有增大,显微疏松无明显改变,共晶的尺寸和含量增加;经相同的热处理工艺处理后,试样尺寸增大,合金枝晶干和枝晶间的γ′相尺寸稍有增大。     

铝合金铸件定向凝固过程的数值模拟

为了研究铝合金定向凝固组织的变化规律,采用有限元软件ProCAST对Al Si Cu合金定向凝固过程进行模拟,分析了不同浇注温度和抽拉速率对铸件定向凝固过程中的温度梯度、固液界面前沿、糊状区宽度、枝晶生长速率和二次枝晶臂间距的影响。结果表明,当浇注温度越高时,温度梯度越大,而固液界面前沿下凹越小,糊状区宽度也越窄,从而越有利于顺序凝固的发生;随着抽拉速率的增大,枝晶生长速率先增大后减小,当抽拉速率为200 μm/s时,最大生长速度达到0.093 mm/s,铸件凝固组织最佳;当抽拉速率大于300或小于200 μm/s时,都会导致枝晶生长速率缓慢,枝晶生长不平稳,二次枝晶臂粗大。对模拟得到较优的工艺参数进行试验验证,可以制备出具有较好力学性能的铸件。     

连铸参数对高碳钢小方坯二次枝晶间距的影响

通过研究高碳钢小方坯连铸参数对铸坯二次枝晶臂间距的影响发现:采用结晶器电磁搅拌、加大二冷比水量,二次枝晶臂间距减小;过热度升高、拉速增大,二次枝晶臂间距增大.无M-EMS的铸坯接近中心的区域,仍然是枝晶臂发达的柱状晶组织;采用结晶器电磁搅拌,促使铸坯中心区域形成粒状等轴晶组织;随搅拌电流增大,二次枝晶间距明显减小.随二次枝晶臂间距增大,渗透率增加,造成铸坯中心碳偏析加重.     

高强度二冷对高碳钢小方坯凝固组织和中心碳偏析的影响

试验了250A/5Hz电磁搅拌连铸150 mm×150 mm方坯时二冷比水量对0．64％-0．82％C钢铸坯冷却速度、凝固组织和中心碳偏析的影响。结果表明,当二冷比水量由0．83 L/kg增加至1．55 L/kg时,距铸坯边缘46 mm处的一次枝晶臂间距由380μm降至300μm,平均中心碳偏析指数由1．15降至1．10,同时等轴晶比例由45％降至40％,二次强冷工艺适合于小方坯连铸。     

Flow of interdendritic liquid and permeability in pb-20 Wt Pct Sn alloys

Directionally solidified Pb-20 wt pct Sn alloys of uniform microstructures were produced with various primary and secondary dendrite arm spacings. Permeabilities of these alloys were investigated with approximately 0.19 and 0.29 volume fraction liquid, for flow parallel to the direction of primary dendrite arms. The permeabilities of the samples with approximately 0.19 volume fraction liquid were also obtained for flow normal to the primary dendrite arms. It was found that for flow parallel to the primary dendrite arms, permeability varied with d ₁ ² and g _L ² (d₁ is the primary arm spacing and g_L is the volume fraction of liquid). There appears to be no relation between permeability for this parallel flow and the secondary dendrite arm spacing. For flow perpendicular to the primary dendrite arms, permeability is approximately 0.06 to 0.20 that for parallel flow, and in this case the permeability appears to be strongly dependent upon the secondary dendrite arm spacing.     

The unidirectional solidification of Al-4 Wt pct Cu ingots in microgravity

Three Al-4 wt pct Cu alloy ingots, 10 mm in diameter and 25-mm long, were unidirectionally solidified in microgravity during the flight of a sounding rocket, with solidification rates of about 1.6 × 10⁻⁴ m/s and temperature gradients of about 2600 K/m. The apparatus was comprised of three muffle furnaces, which melted the ingots prior to the launch of the rocket. Unidirectional solidification of the ingots was accomplished by chill plates attached to the furnaces, which were withdrawn from the ingots during the microgravity portion of the flight, bringing the chill plates into contact with the bases of the capsules containing the ingots. Solidification was complete in less than 4 minutes. For comparison, several ground-based ingots were solidified in unit gravity under similar conditions. Metallographic analysis of the solidified ingots showed that the macrostructures of the unit-gravity and microgravity ingots were similar, all exhibiting columnar grains. However, the microstructures were significantly different, with the microgravity ingots exhibiting primary dendrite spacings about 40 pct larger than the unit-gravity ingots and secondary dendrite arm spacings about 85 pct larger. The larger dendrite spacings for the ingots solidified in microgravity are explained by lower dendrite growth velocities. The absence of convective mixing in the microgravity ingots slightly increased temperature gradients in the liquid portion of the alloy during solidification, which resulted in decreased growth velocities.     

The unidirectional solidification of Al-4 wt pct Cu ingots in microgravity

Three Al-4 wt pct Cu alloy ingots, 10 mm in diameter and 25-mm long, were unidirectionally solidified in microgravity during the flight of a sounding rocket, with solidification rates of about 1.6×10⁻⁴ m/s and temperature gradients of about 2600 K/m. The apparatus was comprised of three muffle furnaces, which melted the ingots prior to the launch of the rocket. Unidirectional solidification of the ingots was accomplished by chill plates attached to the furnaces, which were withdrawn from the ingots during the microgravity portion of the flight, bringing the chill plates into contact with the bases of the capsules containing the ingots. Solidification was complete in less than 4 minutes. For comparison, several ground-based ingots were solidified in unit gravity under similar conditions. Metallographic analysis of the solidified ingots showed that the macrostructures of the unit-gravity and microgravity ingots were similar, all exhibiting columnar grains. However, the microstructures were significantly different, with the microgravity ingots exhibiting primary dendrite spacings about 40 pct larger than the unit-gravity ingots and secondary dendrite arm spacings about 85 pct larger. The larger dendrite spacings for the ingots solidified in microgravity are explained by lower dendrite growth velocities. The absence of convective mixing in the microgravity ingots slightly increased temperature gradients in the liquid portion of the alloy during solidification, which resulted in decreased growth velocities. K.N. TANDON, formerly Associate Professor, Materials Engineering Laboratory, Department of Mechanical and Industrial Engineering, University of Manitoba     

Solidification of hypereutectic Al-38 Wt Pct Cu alloy in microgravity and in unit gravity

Solidification in microgravity aboard the space shuttle Endeavour resulted in a dramatic change in the morphology of the primary Al₂Cu phase compared to ground-based solidification in unit gravity. An Al-38 wt pct Cu ingot directionally solidified at a rate of 0.015 mm/s with a temperature gradient of 1.69 K/mm exhibited large, well-formed dendrites of primary Al₂Cu phase. Ingots solidified under similar conditions in unit gravity contained primary Al₂Cu phase with smooth, faceted surfaces. The primary Al₂Cu phase spacing in the microgravity ingot was much greater than that in the unit gravity ingot, 670 μm compared to 171 μm. It is suggested that thermosolutal mixing in the unit gravity ingot reduces the buildup of an Al-rich layer at the solid/liquid interface, which increases the stability of the interface resulting in smooth, faceted particles of Al₂Cu phase. It is also suggested that the large difference in primary phase spacings is due mostly to the difference in morphology rather than changes in parameters that might influence dendrite ripening mechanisms. The presence or absence of gravity had no effect on the interlamellar spacing of the inter-Al₂Cu phase eutectic. The ingot solidified in microgravity exhibited almost no longitudinal macrosegregation, in agreement with the theory of inverse segregation in the absence of thermosolutal convection. The ingot solidified in unit gravity exhibited considerable longitudinal macrosegregation, with the chilled end having about 6 wt pct more Cu than the average composition. It is not clear whether the segregation results from thermosolutal convection during solidification or from sedimentation during melting.     

Dendrite morphology of several steady state unidirectionally solidified iron base alloys

The experimental data are presented of the dendrite morphology of steady state unidirectionally solidified steels. The dendrite arm spacings were correlated with the equation A =KR ^mG^mwhereR is the growth rate and G the temperature gradient. The exponentsm andn for the primary arms are fairly close to the theoretical values (m = -0.25,n = -0.5). For the secondary arms they are about the same (-0.4). The primary arm spacings do not depend much on composition. The secondary arm spacings, however, decrease at fixed carbon content with increasing content of substitutional elements, and they were found to be smaller in a steel freezing as ferrite compared to steels freezing as austenite. All the authors were with Max-Planck Institut für Eisenforschung, 4 Düsseldorf 1, Max-Planck-Str. 1, Germany, at the time this investigation was carried out.     

Interdendritic fluid flow in a lead-tin alloy

Fluid flow through interdendritic channels of a partially remelted Pb-20 pct Sn alloy has been measured. The flow, resulting from gravity forces, was determined for columnar and equiaxed dendritic structures as a function of dendrite arm spacing. The initial interdendritic flow was found to be consistent with Darcy’s law with a tortuosity factor of 4.6. The initial permeability of the dendritic array was found to be a function of the square of the primary dendrite spacing, and a more complex function of secondary dendrite arm spacing. There was little difference in flow rates for columnar and equiaxed structures of similar size. After the initial flow period the flow rate was observed to increase above that defined by Darcy’s law for castings with small dendrite spacings. This was shown to be a result of coarsening of the liquid channels during flow. The observed coarsening process is considered in terms of diffusion controlled ripening.     

The effect of enhanced gravity levels on microstructural development in Pb-50 wt pct Sn alloys during controlled directional solidification

The Pb-50 wt pct Sn alloys were directionally solidified at 21.1 μm s⁻¹ through a temperature gradient of ∼3.5 K mm⁻¹. With the aid of a centrifuge, the solidifying dendritic interfaces were subjected to constant gravity levels, opposite to the growth direction, up to 15.3 times that of Earth’s. Microstructural examination revealed no significant change in the secondary dendrite arm spacing, the interdendritic eutectic spacing, or the primary dendrite trunk diameter as a function of increasing gravity level; the primary dendrite arm spacing, however, decreased significantly. The primary spacing decrease is argued to result from suppressing convection in the bulk liquid or by modification of the rejected solute layer as a result of enhanced buoyancy.     

Dendrite morphology of steady state unidirectionally solidified steel

Steady state directional freezing experiments have been performed with two steels containing 0.59 and 1.48 pct carbon. Primary and secondary arm spacings were directly measured. In addition, average primary arm spacings were computed from the number of arms present on the observed area using the model of a hexagonal arrangement. The latter method seems to be more objective and reproducible than the line counting method. Arm spacings λ were related by the empirical equation λ =c R ^mGⁿ to growth rateR and temperature gradientG. For primary arms, the exponentsm andn were different, whereas for secondary arms they were almost identical. Some consideration is given to dendrite spacings in ingot solidified steel, where under parabolic growth conditions thermal gradients and growth velocity are coupled by heat flow. Hence, a single variable may be used if the boundary condition for heat flow remains the same. Using the present results the laws describing dendrite spacings as a function of local solidification time are derived and compared with previous data available in the literature.     

Dendrite morphology of steady state unidirectionally solidified steel

Steady state directional freezing experiments have been performed with two steels containing 0.59 and 1.48 pct carbon. Primary and secondary arm spacings were directly measured. In addition, average primary arm spacings were computed from the number of arms present on the observed area using the model of a hexagonal arrangement. The latter method seems to be more objective and reproducible than the line counting method. Arm spacings λ were related by the empirical equation λ =c R ^mGⁿ to growth rateR and temperature gradientG. For primary arms, the exponentsm andn were different, whereas for secondary arms they were almost identical. Some consideration is given to dendrite spacings in ingot solidified steel, where under parabolic growth conditions thermal gradients and growth velocity are coupled by heat flow. Hence, a single variable may be used if the boundary condition for heat flow remains the same. Using the present results the laws describing dendrite spacings as a function of local solidification time are derived and compared with previous data available in the literature.     

连铸坯枝晶臂间距检测方法的优化与应用实践

介绍了连铸坯凝固组织中树枝晶臂间距检测方法的优化试验及其应用效果。通过对连铸坯凝固冷却过程的分析及批量数据的采集,确定了测定连铸坯树枝晶臂间距的代表性区域,建立了一种科学测量枝晶臂间距的方法。将该方法应用于连铸圆坯生产检验中,能快速准确地找出铸坯枝晶臂间距随连铸二冷强度变化的规律,采取调整连铸二冷强度的措施,来控制一、二次枝晶臂间距大小,达到了减轻或消除铸坯开裂缺陷、提高连铸质量的目的。