连铸板坯中心线偏析形成机制与轻压下技术

针对连铸板坯中心线偏析,利用原位分析技术对中心线区域进行扫描。发现连铸板坯中心线区域的宏观偏析由正偏析带和负偏析带共同组成,其中碳、磷、硫这3种元素在中心线正偏析带周围有明显的负偏析带。对这种其周围存在负偏析带的板坯中心线偏析提出了相应的形成机制。最后根据形成机制近似推断了轻压下技术中压下速率与临界固相率这2个重要参数...     

A Simple Model of the Mold Boundary Condition in Direct-Chill (DC) Casting of Aluminum Alloys

An accurate thermofluids model of aluminum direct-chill (DC) casting must solve the heat-transfer equations in the ingot with realistic external boundary conditions. These boundary conditions are typically separated into two zones: primary cooling, which occurs inside the water-cooled mold, and secondary cooling, where a film of water contacts the ingot surface directly. Here, a simple model for the primary cooling boundary condition of the steady-state DC casting process was developed. First, the water-cooled mold was modeled using a commercial computational fluid dynamics (CFD) package, and its effective heat-transfer coefficient was determined. To predict the air-gap formation between the ingot and mold and to predict its effect on the primary cooling, a simple density-based shrinkage model of the solidifying shell was developed and compared with a more complex three-dimensional (3-D) thermoelastic model. DC casting simulations using these two models were performed for AA3003 and AA4045 aluminum alloys at two different casting speeds. A series of experiments was also performed using a laboratory-scale rectangular DC caster to measure the thermal history and sump shape of the DC cast ingots. Comparisons between the simulations and experimental results suggested that both models provide good agreement for the liquid sump profiles and the temperature distributions within the ingot. The density-based shrinkage model, however, is significantly easier to implement in a CFD code and is more computationally efficient.     

弹簧钢连铸坯中心偏析的研究

为了探讨真空精炼和过热度对弹簧钢连铸坯中心偏析的影响,采用两种生产工艺:转炉→LF→VD→连铸和转炉→LF→连铸,并取不同的过热度(30℃和22℃),生产60Si2 Mn弹簧钢连铸坯。在矩形坯上取样分析碳和硫的偏析情况,并进行对比。其结果是:经过真空精炼,连铸坯的碳偏析指数从1.72降低到1.28;硫偏析指数从2.68降低到1.68。     

Three-Dimensional CFD Simulation Coupled with Thermal Contraction in Direct-Chill Casting of A390 Aluminum Alloy Hollow Billet

Metallurgical and Materials Transactions B - A three-dimensional CFD model coupled with melt flow, heat transfer, and thermal contraction was developed to simulate the direct-chill (DC) casting...     

A356.2铸造铝合金生产中Si偏析的影响

论述了铸造铝合金生产中不同时间和状态下采样对Si偏析的影响及Si偏析对产品的影响,并应用于指导生产实际。     

Influence of Melt Feeding Scheme and Casting Parameters During Direct-Chill Casting on Microstructure of an AA7050 Billet

Direct-chill (DC) casting billets of an AA7050 alloy produced with different melt feeding schemes and casting speeds were examined in order to reveal the effect of these factors on the evolution of microstructure. Experimental results show that grain size is strongly influenced by the casting speed. In addition, the distribution of grain sizes across the billet diameter is mostly determined by melt feeding scheme. Grains tend to coarsen towards the center of a billet cast with the semi-horizontal melt feeding, while upon vertical melt feeding the minimum grain size was observed in the center of the billet. Computer simulations were preformed to reveal sump profiles and flow patterns during casting under different melt feeding schemes and casting speeds. The results show that solidification front and velocity distribution of the melt in the liquid and slurry zones are very different under different melt feeding scheme. The final grain structure and the grain size distribution in a DC casting billet is a result of a combination of fragmentation effects in the slurry zone and the cooling rate in the solidification range.     

Heat-Transfer Measurements in the Primary Cooling Phase of the Direct-Chill Casting Process

Thermal modeling of the direct-chill casting process requires accurate knowledge of (1) the different boundary conditions in the primary mold and secondary direct water-spray cooling regimes and (2) their variability with respect to process parameters. In this study, heat transfer in the primary cooling zone was investigated by using temperature measurements made with subsurface thermocouples in the mold as input to an inverse heat conduction algorithm. Laboratory-scale experiments were performed to investigate the primary cooling of AA3003 and AA4045 aluminum alloy ingots cast at speeds ranging between 1.58 and 2.10 mm/s. The average heat flux values were calculated for the steady-state phase of the casting process, and an effective heat-transfer coefficient for the global primary cooling process was derived that included convection at the mold surfaces and conduction through the mold wall. Effective heat-transfer coefficients were evaluated at different points along the mold height and compared with values from a previously derived computational fluid dynamics model of the direct-chill casting process that were based on predictions of the air gap thickness between the mold and ingot. The current experimental results closely matched the values previously predicted by the air gap models. The effective heat-transfer coefficient for primary cooling was also found to increase slightly with the casting speed and was higher near the mold top (up to 824 W/m²·K) where the molten aluminum first comes in contact with the mold than near the bottom (as low as 242 W/m²·K) where an air gap forms between the ingot and mold because of thermal contraction of the ingot. These results are consistent with previous studies.     

轻压下条件下管线钢铸坯凝固组织与中心偏析的关系

 为研究在动态轻压下条件下等轴晶和柱状晶组织与管线钢板坯中心偏析的关系,试验通过改变中包钢水过热度来获得铸坯中心不同凝固组织,并分析不同凝固组织内的中心偏析情况。试验铸坯分别采用热酸腐蚀、化学分析的方法,定性、定量分析过热度在11~32℃之间,铸坯厚度方向上元素含量分布和不同凝固组织中的偏析形貌。结果显示：过热度在14℃以下的坯样中心为等轴晶组织,过热度在20℃以上的坯样中心完全为柱状晶组织;在现有轻压下条件下,中心为等轴晶和过热度为32℃时中心柱状晶组织的坯样中心偏析度均较低,但因铸坯中心整个等轴晶区内会分布着偏析点,因而较之中心柱状晶组织,中心为等轴晶组织中点状偏析分布范围较宽;当中心为柱状晶组织时,适当提高过热度、优化压下量,可降低中心偏析度,提高中心偏析评级。     

Film Boiling and Water Film Ejection in the Secondary Cooling Zone of the Direct-Chill Casting Process

Accurate thermal modeling of the direct-chill casting process relies nowadays on increasingly complex boundary conditions for the secondary cooling zone. A two-dimensional axisymmetric finite-element model of the direct-chill casting process was developed to quantify the importance of secondary cooling at the surface compared with internal heat conduction within the billet. Boiling water heat transfer at the surface was found to dominate and be the governing factor only when stable film boiling or water film ejection take place; all other cases were dominated by internal heat conduction. The influence of various parameters (casting speed, cooling water flow rate, and thermophysical properties of the cast material) on the occurrence of water film ejection was analyzed. An exponential relationship was found between the cooling water flow rate and the minimum casting speed at which water film ejection takes place.     

连铸过热度和电磁搅拌对铁路车轴坯碳偏析的影响

通过分析连铸不同过热度和结晶器电磁搅拌方式对铁路车轴坯碳偏析的对比分析,确定了过热度的控制范围和结晶器电磁搅拌方式.     

Effects of Solid–Liquid Mixing on Microstructure of Semi-Solid A356 Aluminum Alloy

The desired starting material for semi-solid processing is the semi-solid slurry in which the solid phase is present as fine and globular particles. A modified solid–liquid mixing (SLM) is reported wherein semi-solid slurry can be produced by mixing a solid alloy block into a liquid alloy, and mechanical vibration is utilized to enhance the mixing. Effects such as liquid alloy temperature, mass ratio, and mixing intensity on the microstructure and the cooling curves during SLM were evaluated. 2D and 3D microstructure analysis of treated A356 aluminum alloy shows that microstructure can be refined significantly with a considerable morphology change in primary Al phase. It is critical that the temperature of mixture after mixing is lower than its liquidus temperature to obtain a valid SLM process. Specially, mixing intensity is identified as a primary factor for a favorable microstructure of semi-solid slurry.     

Time-Averaged Mixing Behavior of Circular Jet in Counterflow: Velocity and Concentration Measurements

The time-averaged mixing behavior of a circular jet issuing into a counterflow is investigated in the laboratory at a number of jet-to-current velocity ratios between 3 and 15. The velocity field is obtained from laser Doppler anemometry measurements and the mean concentration field of the jet is measured by laser-induced fluorescence. A similarity analysis is performed on the radial profiles of mean velocity at successive streamwise stations. This is achieved by expressing the mean velocities as jet excess velocities over the ambient counterflow. The agreement of data to the similarity profiles suggested by previous studies is discussed. A similarity analysis is also attempted for the radial distributions of mean concentration. Data are presented on the axial growth of the momentum width and the concentration width of the jet in a counterflow.     

轴承钢连铸坯偏析和致密度的分析

采用原位分析方法对轴承钢连铸大方坯不同部位的偏析和疏松进行了分析;将原位分析与低倍组织分析结合起来,完整地体现铸坯试样的成分分布和组织特征。轴承钢同一铸坯断面中部的C、Si、Mn元素偏析程度大于边部。轴承钢铸坯边缘部位致密度和均匀性都好于中心部位,即相应的柱状晶区致密度高于等轴晶区。     

铝熔铸工序提高废铝利用率及减少铝损耗的方法

在建筑铝型材生产的熔铸工序中，采用合理的原材料配比及有效的铝渣回收方法，可以提高铝资源的利用率，减少铝的损耗，降低生产成本。