凝固过程显微偏析预测数值模拟研究的发展现状

显微偏析现象对于合金是非常重要的,因为它在很大程度上影响着合金的性能和后续加工,对它的数值模拟研究已受到许多研究的关注,本作回顾并介绍了显微偏析数值模拟的发展现状,指出了现有模型的不足和以后的发展方向。     

Analysis of Solute Distribution during the Solidification of Low Alloyed Steels

A numerical model that estimates the interdendritic segregation during the solidification of multicomponent low alloyed steels is presented. Some special features related to the solidification of these steels, like peritectic reaction, dendrite arm coarsening and inclusion precipitation, are taken into account by the model. Model results were extensively compared with experimental data published in the literature. In general, a good agreement between calculated and measured results was verified. Furthermore, the model was used to investigate the effect of steel composition on cracking tendency. Results showed that an increment of phosphorus and carbon content extends the solidification time in the vulnerable period, which may increase the risk of internal defects. Additionally, a peak contraction is observed for steels with carbon content around 0.10 %, which is in agreement with the lower mould heat transfer and higher surface defect index observed for these steels in plant data.     

Strain‐Rate Sensitivity in Hot Forming of Steels – Influence of Microstructure,Temperature and Chemical Composition

A methodology to determine the strain‐rate sensitivity index was developed, based on rolling of a set of samples with the same draught but different speed at defined temperatures. It was shown that initial grain size has nearly negligible influence on the investigated variable, in contrast to phase composition whose influence is very considerable. Combined influence of strain rate and temperature on deformation resistance of various types of steel was studied. For a selected group of steels a universal equation was set up, which described, with a good accuracy, impact of reciprocal temperature and chemical composition (expressed simply by nickel equivalent) on strain‐rate sensitivity in hot state.     

Morphology of the Cu‐rich Phase in Cu‐Pb Hypermonotectic Alloys under an Intense Magnetic Field

The solidification structure of a Cu‐80wt%Pb hypermonotectic alloy was investigated under four different experimental conditions. The results show that the spherical Cu‐rich phase in the matrix has three kinds of morphology, that is the larger “net‐shell type” and the smaller “egg‐type” or “eye‐type”. The cooling rate of the samples has a great effect on both macrostructure and microstructure. As the cooling rate becomes slow, the Cu‐rich phase changes from fine spheres to larger floating spheres and finally to coarse dendrites, and the thickness of the Cu‐shell and the net of Cu‐Pb phase in the Cu‐rich spheres become coarser. A magnetic field of 12 T has a remarkable effect: it restrains the gravity segregation of the Cu‐Pb alloy by preventing the floating of the larger Cu‐rich droplets and the sedimentation of the Pb‐matrix. Thus, the Cu‐Pb monotectic alloy exhibits a macrostructure and microstructure which are similar to those obtained at relatively fast cooling rates. The 12T magnetic field is supposed to restrain the collection of solute Cu extracted from the liquid Pb phase and the diffusion of solute Cu to the Cu‐shell in the centre of larger Cu‐rich droplets.     

高碳中锰耐磨钢凝固过程溶质微观偏析模型

通过定向凝固、电子探针（EPMA）、Factsage等多种实验与理论计算相结合的手段对中锰耐磨钢凝固过程溶质元素微观偏析行为进行了系统研究. 研究表明在定向凝固试验中当拉速为50 μm·s⁻¹时,中锰钢的二次枝晶间距平均值为59.77 μm;中锰钢凝固过程组织转变为L→ L + γ → γ属于奥氏体凝固模式,无包晶反应的发生,也无铁素体相及其他相的出现;中锰钢定向凝固过程中Mn、Cr在枝晶间的含量明显高于枝晶内,表明Mn、Cr元素发生了明显的正偏析行为;通过对各特征参数求解,构建了中锰钢溶质元素微观偏析模型,发现中锰钢定向凝固过程中Mn元素偏析指数与Brody–Flemings模型符合较好,而Cr元素偏析指数与Clyne–Kurz模型分布较为一致.     

Production of Ultrahigh-Strength and High-Ductility TWIP Steel by Precipitation of β-Mn during Large Deformation during Warm-Rolling

Herein, twinning-induced plasticity (TWIP) steel having large deformation is rolled at different rolling temperatures to improve the tensile strength and retain a certain plastic deformation capacity. Based on X-ray diffraction and transmission electron microscope analysis, β-Mn is found as the precipitate at the grain boundary during the warm-rolling process (500–650 °C). To investigate the impact of β-Mn on the tensile properties, the microstructure of the TWIP steel rolled at the temperature value of 600 °C is observed by carrying out electron backscatter diffraction and scanning electron microscope measurements. The intergranular β-Mn phase can help the material to accumulate geometric necessary dislocation (GND) density, inhibit crack propagation, as well as improve the strength and plasticity of the material. Once TWIP steel is warm-rolled above the temperature value of 600 °C, and serrated flow appears in the tensile process, which is also conducive to improving the material properties.     

Effect of Cr and N on the Martensitic Transformation in Fe‐8%Mn Alloys

The influence of Cr and N on the transformation temperatures of a Fe‐8%Mn alloy has been investigated by means of equilibrium thermodynamics and dilatometry. The addition of Cr and N resulted in the presence of ferrite or α'‐martensite at room temperature, with the microstructure transforming to a single phase austenitic microstructure with increasing temperature. Only high amounts of Cr or N in excess of 0.2% prevented the transformation to a single phase austenitic microstructure. The addition of alloying elements resulted in a decrease of the martensite start temperature M_s. The effect on the austenite start temperature A_s was smaller. The effect of thermal cycling resulted in a stabilization of the transformation temperatures. More cycles were required to reach stable phase transformation temperatures when N was added to Fe‐Mn‐Cr alloys.     

相场法在金属凝固过程的应用现状

相场法是在计算材料学中发展最快的一种强大的计算方法,以基本的热力学和动力学为输入,可用于模拟和预测材料的介观尺度形貌和微观结构的演变。首先总结了相场模型的历史发展、物理基础、数学表达及其数值求解,其次分析了相场法在纯物质、二元合金、多组分系统以及定向凝固、增材制造等领域中的应用情况,最后对相场法进行了总结及展望,并指出相场法发展应趋向于超大尺度相场模拟技术,更高效算法的开发,相场模型与热力学、动力学数据库的结合,工业应用的探索以及与实验观察技术的进一步结合。     

强制对流影响下Fe−C合金定向凝固微观组织的相场法研究

定向凝固技术能够获得特定柱状晶结构,对于优化合金轴向力学性能具有非常显著的效果。本文采用耦合流场的相场模型模拟了定向凝固过程中枝晶的生长过程,研究了各向异性系数、界面能对定向凝固枝晶生长的影响以及强制对流作用下枝晶的生长行为。数值求解过程中,选用基于均匀网格的有限差分方法对控制方程进行离散,实现了格子中标记点算法（MAC）和相场离散计算方法的联合求解。处理微观速度场和压力场耦合时,采用MAC算法求解Navier-Stokes方程和压力Poisson方程,采用交错网格法处理复杂的自由界面。结果表明：随着各向异性系数的增大,枝晶尖端生长速度增大,曲率半径减小,枝晶根部溶质浓度逐渐降低;随着界面能的增大,枝晶尖端曲率半径增大,当界面能为最大（0.6 J·m⁻²）时,凝固呈现平界面的凝固方式向前推进;强迫对流对定向凝固枝晶生长方向影响较大,上游方向定向凝固枝晶粗大且生长速度更快,其现象随流速的增大而愈加明显。     

On Restricting Aspects in the Production of Nonmagnetic Cr‐Mn‐N‐alloyed Steels

Cr‐Mn steel grades with high nitrogen contents are becoming increasingly important in the field of austenitic stainless steels. Industrial production facilities allow to use two different strategies to reach a high nitrogen content. The first involves taking advantage of the pressurised‐electroslag remelting process, which is operated at elevated nitrogen partial pressure; the second consists of adding elements which increase the nitrogen solubility of the melt so that high nitrogen contents can be achieved at atmospheric pressure. This paper focuses on nitrogen solubility and austenite stability. These have been observed as important and in some cases restricting for the successful implementation and production of high alloyed Cr‐Mn austenitic steels. The precondition for a stable austenitic microstructure can be predicted with the help of equations using chromium and nickel equivalents. Different formulae were tested and their results compared to the microstructure of the alloys. The nitrogen solubility in the melt is particularly important for the steel grades cast under atmospheric conditions. It has been found feasible to produce steel grades up to 0.9 mass percent nitrogen at atmospheric pressure on an industrial scale. Several theoretical approaches for calculating the nitrogen solubility in the melt were tested for atmospheric conditions and compared to the chemical analyses of conventionally cast Cr‐Mn steel grades.