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Rapid solidification of steel droplets with different carbon contents in drop tube

Female, born in 1973, Ph.D., Associate Professor. Her research interests mainly focus on the rapid solidification and the segregation of alloying elements in alloys.E-mail: huatsing2006@yahoo.com.cnReceived: 2011-04-15; Accepted: 2011-10-06

*Li Na

*Li Na1, Sha Minghong1, Xu Qian2, Zhang Shuang1 and Li Shengli1

(1. School of Materials and Metallurgy, University of Science and Technology Liaoning, Anshan 114051, China; 2. Tianjin Iron and Steel

Group Co., Ltd, Tianjin 300301, China)

Rapid solidification is a significant research subject in the field of materials science and condensed physics [1-4] and

plays a major role in materials engineering and crystal growth [5 which can remarkably increase the solid solution of alloying elements, produce fine microstructures and reduce or eliminate the segregation of alloying elements. However, the segregation of P and C was also found in rapid solidified strip-casting steel strips [6].

Container-less processing is an important method to realize the undercooling and rapid solidification of materials. During container-less processing, the contact between the melt and container wall can be avoided and heterogeneous nucleation can be suppressed to some extent; hence high undercooling and rapid solidification can be achieved. A drop tube is a special technique for investigating rapid solidification through combining high undercooling and rapid cooling [7,8]. In this study, the steel droplets with the volume of about 2 mm × 2 mm × 2 mm (TM) and 5 mm × 5 mm × 5 mm (FM) were solidified in silicone oil after re-melting and falling in a short drop tube. The C contents of the droplet samples were set to different in order to observe and analyze the effects of C content and the size of droplets on the microstructures during rapid solidification.

Abstract: Rapid solidification is regarded as being an effective method to refine the microstructure and reduce or eliminate the segregation of alloying elements. In this study the microstructures of rapid solidified carbon steel droplets (cooled in silicone oil) with different C contents by drop tube processing were observed. The volumes of droplets were set to be 2 mm × 2 mm × 2 mm (TM) and 5 mm × 5 mm × 5 mm (FM). For most samples, the microstructures are nearly the same from the surface to the center region. The microstructures of the FM samples with higher C content are much finer than those of the TM samples, which is the opposite of the situation with the lower C content samples. The distribution of C along the diameter of each sample was detected. The segregation of C was observed in TM samples with higher C contents while not in FM samples. This is regarded as relating to recalescence and the diffusion of C atoms during the solidification process.

Key words: rapid solidification; carbon; droplet; drop tubeCLC numbers: TG142.31/249.9 Document code: A Article ID: 1672-6421(2012)01-020-04

1 Experimental proceduresCarbon steels with different C contents were prepared in a 2-kg high-frequency vacuum induction furnace, and the compositions are listed in Table 1. Small samples with the size of 2 mm × 2 mm × 2 mm (TM) and 5 mm × 5 mm × 5 mm (FM)were taken from the bulk. All the sides of the small samples were ground and then cleaned with alcohol.

Table 1: Chemical composition of steel droplets (mass%)Sample C Mn Si S P 1 0.035 0.179 0.036 0.004 0.089 2 0.141 0.178 0.033 0.004 0.091

Fig. 1: Schematic diagram of vacuum drop tube

The dry samples were re-melted in a suspension-type vacuum furnace and the melted droplets were then solidified in silicone oil, both the vacuum furnace and the silicone oil were placed in the vacuum drop tube. The schematic diagram of the experimental device is shown in Fig. 1.

Vacuum chamberSuspension furnaceInduction coil

Silicone oilVessel

Furnacecontrolling system

Movable bed

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because of the vacuum condition in the drop tube; and as the volume of the droplets was rather large, it was thought that the solidification process did not happen until the liquid drops met the silicone oil. It can be seen from Figs. 2 and 3 that the microstructures in Fig. 2 are a little finer than those of Fig. 3. It may indicate that the solidification speed is a little higher for the TM drop samples. By comparing Fig. 2 (a) with Fig. 2 (b), and Fig. 3 (a) with Fig. 3 (b), it was found that there is no obvious difference between the microstructure near the surface and that at the center. It indicates that the solidification speed is approximately the same from the surface to the center for both TM and FM samples.

The drop heights were set to be about 0.2 m. The microstructures were observed using an optical microscope, and the distribution of C along the diameter of each sample was detected by electron probe micro-analysis (EPMA-1610).

2 Results and discussion2.1 Microstructural characterization Figures 2 and 3 show the microstructures near the surface and at the center of TM sample 1 and FM sample 1 solidified in the drop tube. The microstructure is mainly fine ferrite. The convection heat transfer outside the melt was ignored

Fig. 3: Microstructures of steel droplet FM sample 1: (a) at the center, (b) near the surface

For sample 2 with higher carbon content, as shown in Figs. 4 and 5, the microstructures of FM sample 2 are rather finer than those of the FM sample 1 and of the TM samples 1 and 2. This is opposite to the observed results for sample 1; and the microstructures are uniform from the center to the surface. It was analyzed that, recalescence is an important phenomenon that could not be ignored during the rapid solidification process. Recalescence comes from the release of the latent heat of crystallization, which is in direct proportion to the volume of the melt. So the effect of recalescence on the FM samples is considerably greater (more than 15 times) than that on the TM samples. When the carbon content is increased in the steel, the

heat transfer capability and the latent heat of crystallization are decreased gradually [9]. So the solidification speed is higher in sample 2 than that in sample 1, which leads to the finer grains. Moreover, the C content of sample 2 approaches the eutectoid steel, which may make the microstructure further refined during the cooling process after solidification.

For sample 2 with higher C content, the microstructure of TM samples at the center (Fig. 4a) is quite different from that near the surface (Fig. 4b). There are more pearlite appearing near the surface. This means there is higher carbon content near the surface than at the center, where the microstructure presents more ferrite. The surface temperature of the droplet

Fig. 2: Microstructures of steel droplet TM sample 1: (a) at the center, (b) near the surface

(b)(a)

(b)(a)

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sample declines to γ phase zone earlier than the center during the solidification process, and the solidification speed near the surface reduces due to the release of latent heat of crystallization [10], even though it is not re-melted. The surface is kept a relative longer time in γ phase zone than that in the center region. The carbon atoms may diffuse to the γ phase zone as the solid solution of C in the γ phase is much higher than that in the α phase. This may lead to the segregation of carbon near the surface region. For the TM sample 2, the carbon content of most local regions is far away from the eutectoid steel, and there are more ferrite at the center and

more pearlite near the surface, so the microstructure is not so fine and uniform.

3.2 Distribution of alloying elementsThe distribution of C was detected along the diameter of each droplet sample. For sample 1, the distribution of C is nearly uniform throughout the whole sample, including both TM and FM droplet samples, as shown in Fig. 6. For the TM sample 1, the relatively higher C content was observed both near the surface and at the center as shown in Fig. 6(a). The fluctuation of C distribution indicates that the small volume samples are

Fig. 5: Microstructures of steel droplet FM sample 2: (a) at the center, (b) near the surface

Fig. 6: Distribution of C throughout the diameter of droplet: (a) TM sample 1 and (b) FM sample 1

(a) (b)

Fig. 4: Microstructures of steel droplet TM sample 2: (a) at the center, (b) near the surface

(a) (b)

(a) (b)

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affected greatly by the cooling and surrounding conditions, including the release of latent heat of crystallization. For the FM sample 1, the distribution of C is more uniform, as shown in Fig. 6(b); this may relate to the greater amount of latent heat of crystallization.

With higher C content, the segregation of C was observed in TM sample 2, as shown in Fig. 7(a). The C content near the surface is higher than that at the center. The distribution of C

4 Conclusions

A vacuum drop tube with a height of 0.2 m was used to obtain rapidly solidified droplet steel samples with different volumes and different C contents. The microstructures were observed and the distribution of C was detected. The conclusions are as follows:

(1) The solidification speed is approximately the same from the surface to the center of each sample.

(2) When the C content is rather low, the microstructures of the TM droplet samples are a little finer than those of the FM samples. When the C content increases to approach that of eutectoid steel, the microstructures of the FM samples are much finer than those of the TM samples. This may be related to the lower latent heat of crystallization when the carbon content increases; and the cooling rate also increases. Meanwhile, the C content of sample 2 approaches the eutectoid steel and the microstructures are refined during the cooling process after solidification.

(3) In TM samples with high C, more C is distributed near the surface than at the center, and more pearlites appear near the surface. The segregation of C is thought to relate to the solid solubility of alloying elements in different phases and the diffusibility of C during the solidification and recalescence processes.

The study was financially supported by the National Natural Science Foundation of China, Project No. 51074210.

Fig. 7: Distribution of C throughout the diameter of droplet: (a) TM sample 2 and (b) FM sample 2

(a) (b)

is corresponding to the microstructure of the TM sample 2, as shown in Fig. 4, where there are more pearlite near the surface and there are more ferrite near the center.

In the FM sample 2, the uniform C distribution was observed as well, as shown in Fig. 7(b), which also corresponds to the microstructures, as shown in Fig. 5. This may suggest that the intensity of cooling is equivalent to the latent heat of crystallization.

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