Macrosegregation In Ingots Research Articles

Simulation and experimental validation of the effect of superheat on macrosegregation in large-size steel ingots

A 3D model was employed to study the effect of melt initial superheat on the macrosegregation formation using FE modeling and experimentation methods. The casting process of three ingots with the initial melt superheats of 75 °C, 65 °C, and 55 °C were simulated. The three cases represented three variables encountered in industry during casting of large size ingots. For the above three studied cases, all other casting conditions were kept the same. Results showed that the variation of initial melt superheat gave rise to changes in temperature pattern, liquid flow field, solidification speed, and thermomechanical contraction. Under the combined actions of all these changes, lower superheat tended to alleviate the segregation intensity in the upper part of the ingot body, in the hot-top, and in the solute-rich bands between the ingot centerline and periphery. The beneficial effect of lower superheat on alleviation of segregation severity was confirmed by experimental chemical measurement results. The results were analyzed in terms of heat and mass transfer theories and allow for a better understanding of the underlying mechanisms responsible for the occurrence of macrosegregation in ingot casting process. The findings should be helpful for the casting process design of a given ingot of high value-added steels or other alloys.

Experimental and Numerical Study of Macrosegregation in a 160 Steel Ingot

Large steel ingots are the important material for the equipment manufacturing industry. It is still difficult to predict and control the macrosegregation in ingot. In this paper, the cooling curves at the surface of ingot and temperature variation of the mold were measured. The carbon distribution was measured through the local region dissection of ingot. Then, based on the definite the heat transfer coefficient at the interface of mold/ingot, a two-phase model with consideration of the motion of equiaxed grains is applied for the prediction of macrosegregation in 160-t steel ingot formed during the solidification. The results indicate that the heat transfer coefficient at the interface of mold/ingot decreases sharply after starting solidification and then varies slowly. Negative segregation at the bottom of ingot forms due to the interaction of solidification interface and equiaxed grains deposition during solidification. The positive segregation appears in the riser with thanks to the solidification shrinkage and the floating enriched solute. Finally, the results of the predicted and the measured are in good agreement.

A Numerical Study of the Effect of Multiple Pouring on Macrosegregation in a 438-Ton Steel Ingot

In present paper, a ladle–tundish–mold CFD model and a macrosegregation model were utilized to investigate the effects of the multiple pouring (MP) process on the macrosegregation in a 438-ton steel ingot. Firstly, the model was partially proved as compared to the measured carbon distributions along the transverse sections in the riser of ingot. Then, the comparison between the single pouring (SP) and MP process has been carried out to study their influences on the macrosegregation in ingot. Besides, the predicted macrosegregation results in MP process which introduced the improved riser fixed with an insulating sleeve were compared with that in traditional MP process. The traditional MP process leads to certain favorable initial carbon distribution in the mold, which has some favorable influence on suppressing the positive segregation in ingot. The holding time of the low carbon in the riser is the main factor to suppress the positive segregation in ingot. Improved insulating sleeve can prolong the holding time of the low carbon in the riser and release the positive segregation in the riser of ingot. Improvement of the insulating effect of the riser is an efficient method to control macrosegregation in large steel ingot.

Numerical study of macrosegregation in a large steel ingot with multiple pouring process

Multiple pouring (MP) process with a higher concentration in the beginning and a lower concentration at the last is generally utilized to reduce macrosegregation in the large steel ingot production. However, it is insufficient to demonstrate the practical efficiency of MP process from the point view of theory. In this paper, the whole filling and solidification processes were investigated to analyzethe formation of macrosegregation in a large steel ingot. Firstly, a filling transportation model considering the entire MP process was presented. The carbon concentration variation through the outlet of the tundishat different times was predicted and provided as the boundary conditions for the moldfilling and the solidification processes simulation. Secondly, the macrosegregation model was integrated into the filling transportation model to investigate the influence of the MP on the macrosegregation in the ingot through the comparison of the MP and the single pouring process (SP). Finally, the top section of a 438-t steel ingot was cut off and the carbon concentration was measured to validate the numerical results. The results indicated that the MP process has a certain favorableeffect on reducing the positive segregation in the top region of the ingot.

Numerical Simulation of Macrosegregation During Vertical Solidification

A continuum formulation based mathematical model has been employed to analyze the evolution of thermosolutal convection, isotherms, liquid fraction profiles and macrosegregation profiles during vertical solidification of lead-rich binary lead–tin alloys. The initial conditions, boundary conditions and microstructural parameters of this model have been taken from controlled experiments performed on lead–tin system. Numerical simulations were carried out for three chosen experiments on Pb-35 wt pct. Sn alloy with different cooling rates. The convection was very weak in case of vertical solidification. It was also observed that the nature of the flow field did not show significant effect on thermal field. However, there was substantial effect of nature of flow on macrosegregation. It was found that the choice of permeability model, especially near the liquidus, played a significant role in the evolution of macrosegregation during vertical solidification. Through the use of appropriate permeability model, the predicted profile of average macrosegregation in ingots along the vertical (i.e., axial) direction showed reasonable agreement with experimental data over a wide range of cooling rates.

The influence of convection during solidification on fragmentation of the mushy zone of a model alloy

Experiments have been conducted to observe fragmentation events in a model alloy (succinonitrile and acetone) solidifying in the presence of forced convection in the superheated melt. Measurements of fragmentation rates have been made, and an attempt was made to relate the results to the controllable parameters of the system. A microscope-video system recorded the mushy zone-melt interface, and the fragmentation process and fragmentation rates could be determined from a frame-by-frame analysis of the video images. Experiments were conducted for varying cooling rates, overall temperature differences, melt flow rates, and for two different concentrations of acetone (1.3 and 6.1 wt pct). Significant dendritic fragmentation occurred for all runs. In addition, the influence of buoyancy forces is clearly evident from particle motion near the mushy zone-melt interface. Fragmentation rates appear to correlate well with the magnitude of particle velocities near the interface, with increasing fragmentation being associated with higher particle velocity magnitude (either in the same or the opposite direction to the mean flow) for the 1.3 wt pct acetone mixture. However, the correlation is quite different for the higher concentration. The relationship between these results and the possible mechanisms for fragmentation are discussed. Although it appears that either constitutional remelting or capillary pinching are likely of importance, hydrodynamic shear forces or some other mechanism as yet undiscovered cannot be completely discounted, although circumstantial evidence suggests that mechanical shearing is inconsistent with observations made both here and already in published literature. The results provide a step in the development of solidification models that incorporate fragmentation processes in the mushy zone as an important mechanism of grain refinement and a potential source of macrosegregation in ingots and large castings.

On the Solidification of Steel Ingots and Continuously Cast Steel Billets and Slabs

AbstractThe cast macrostructure in steel ingots and continuously cast steel billets is often divided into three zones, a chill zone at the surface, a columnar zone in the intermediate region, and large grained equiaxed zone in the centre. This paper discusses the formation of the three zones and attempts to theoretically describe their formation. The formation of the three crystal zones influences the macrosegregation in both ingots and continuously cast billets. The effect of the cast structure on macrosegregation is discussed. Centreline segregation in continuously cast billets and slabs is considered theoretically, and related to the macrosegregation in ingots. The different mechanisms leading to macro segregation in continuously cast billets; slabs, and static cast ingots are considered. Pipe formation in ingots is primarily dependent on solidification shrinkage. A simple theoretical model of pipe formation in ingots is used to delineate the most important parameters for minimizing pipe formation. The...

鋳塊におけるマクロ偏析のコンピューターシミュレーション

鋳塊におけるマクロ偏析のコンピューターシミュレーション

Theoretical Analysis and Model Study on the Structure of A-type Segregation

Concerning the formation of macrosegregation in ingots, it is well known that the channels are smooth on the side facing the outside of ingot and rough on the side facing the inside of ingot.In this work, these phenomena were reproduced by the model experiments using NH4Cl-H2O system and the progression of channel formation was precisely visualized. The experiment indicated that the channels were melted on the side facing the outside of ingot and solidified on the other side, and that the interdendritic liquid flowed into the channels through the wall facing the inside of ingot and a part of the liquid flowed out the channels through the other wall. The difference in the roughness between both walls is theoretically explained on the basis of transport phenomena in this paper.

鋳塊の正偏析および重力偏析について

Macrosegregation in ingots mainly result from interdendritic fluid flow which arises because of the difference in the density of the interdendritic liquid from that of the bulk liquid. In this paper, Al-5.5%, 15%, 25%Cu, Al-14%Mg, and Al-6%Mg-1.2%Cu alloys were holizontally, unidirectionally solidified to clarify the effect of the solute elements and the freezing condition on macrosegregation.Almost no macrosegregation occurred in Al-6%Mg-1.2%Cu alloy, in which the density of interdendritic liquid (ρLs) is nearly equal to that of bulk liquid (ρP0) during the solidification, while normal and gravity segregation and string ghosts appeared at low solidification rates (R) in Al-Cu and Al-Mg alloys in which ρLs differs from ρL0.The effective segregation coefficient (Ke), the ratio of the solute concentration in the solid to that in the bulk liquid decreased almost linearly with the increase in the reciprocal of R. The effect of R was larger in Al-Cu alloys than in Al-Mg alloys.Normal and gravity segregation was found to result mainly from the flow of solute rich interdendritic liquid into the bulk liquid, through the string ghosts in the liquid-solid region.

凝固遷移層モデルによるマクロ偏析の解析

凝固遷移層モデルによるマクロ偏析の解析

The Importance of Solute-Induced Interdendritic Fluid Flow in Macrosegregation

AbstractExperimental studies have been conducted on alloys of the tin-gallium system in order to evaluate the relative contributions of the inverse segregation, solute pushing, and solute induced fluid flow mechanisms to macrosegregation during directional solidification. Alloys that were calculated to either expand or contract during the solid–liquid transformation were solidified upwards and downwards. The patterns of solute redistribution indicate that the effect of soluteinduced fluid flow on macrosegregation is exceedingly small in this system. The implications of these observations with respect to current theories of macrosegregation in ingots are discussed.