Formation of hot-top segregation in steel ingot and effect of steel compositions
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I.
INTRODUCTION
THE positive segregation region near the top of the killed steel ingots is called hot-top segregation, LIJ in which alloying elements and impurities, especially carbon, are concentrated. Therefore, the top of the ingot must be discarded. Because this problem is related to the saving of materials and energy consumption, it has been the main attraction for the researchers in this field. Wu t21 regarded this kind of segregation as a result of solute redistribution during solidification, but he did not give any details about the formation process. Olsson et al. 131studied two ingots, each weighing 1.7 tons, and attempted to calculate hot-top segregation in them with the models suggested by Hultgren, I*l Takahashi et al. ,[5] Jonsson,[6] and Fredriksson and Nilsson, tT~ but they found that none of the models was successful. In the present paper, the composition variation in the liquid zone during solidification of the H20-NH4C1 model ingot is studied. Based upon the experimental results, the formation mechanism of hot-top segregation and methods to prevent it are suggested. Theoretical analysis and experimental verification of the effect of alloy composition on the formation of hot-top segregation in steel ingot are given as well.
II.
STUDIES ON H20-NH4CI MODEL INGOTS
A. Experimental Procedure The mold for the H20-NH4C1 model ingot is shown in Figure 1. Its front and back walls are made of plexiglass, and both lateral sides and the bottom directly contact the cooling chambers, through which the coolant, a mixture of ethyl alcohol and condensed nitrogen, is circulated. Fifteen pairs of chromel-silicon thermocouples are used to measure the temperature distribution in the mold during solidification. Syringe needles are stuck into the mold to inject dyes to show the convection and take the liquid samples for composition analysis.
JIE WANQI, Lecturer, and ZHOU YAOHE, Professor, are with the Department of Materials Science and Engineering, Northwestern Polytechnical University, Xi'an, People's Republic of China. Manuscript submitted September 1, 1988. METALLURGICAL TRANSACTIONS B
The phase diagram of the H20-NH4CI system is shown in Figure 2.[81 H20-30 wt pct NH4C1 solution was used in the experiments. Because the density of the saturated liquid decreases with the decrease of the liquidus temperature (i.e., dpL/dTL > 0) for both H20-NH4C1 solution and most kinds of steel, they have a similar model of convection. During experiments, the prepared solution was poured into the preheated mold with about 20 ~ superheat. When the superheat decreased to 10 ~ the coolant started circulating. At a certain time during solidification, the liquid samples were taken from different levels in the liquid zone along the centerline of the ingot. The NH4C1 contents of the samples were determined by chemical analysis. B. Experiment Results and Discussion 1. Convection The convection model and its development during solidification traced by the track of the dye are shown in Figure 3. At the beginning of solidification, liqui
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