College of Materials Science and Engineering, Chongqing University, Chongqing 400044, China
Cite this article:
Zibing HOU,Jianghai CAO,Yi CHANG,Wei WANG,Han CHEN. Morphology Characteristics of Carbon Segregation in Die Steel Billet by ESR Based on Fractal Dimension. Acta Metall Sin, 2017, 53(7): 769-777.
Macro/semi-macro carbon segregation plays a key role for improving the steel product quality. Based on macrostructure qualitative rating comparison and element macro content analysis, the segregation extent has been controlled at different levels by the existing technologies, but there is an obvious shortcoming on segregation morphology description. Nowadays, delicacy control is demanded for higher quality requirement, especially for the production of high-quality H13 die steel by electro-slag remelting (ESR) technique. In this work, as to segregation point morphology, fractal dimension is introduced, and segregation characteristics of different locations in the ESR billet are quantitatively investigated in terms of area, number and outline morphology. The size of the billet is 160 mm×160 mm, and the sampling location in the central plane of billet. Two melting rates (350 and 400 kg/h) are considered for studying essential characteristics of segregation. Firstly, it is shown that the whole segregation extent in the billet is mostly influenced by the large segregation point (e.g., the area is larger than 0.1 mm2). The segregation ratio will be increased when increasing the number or area of the large segregation point. Secondly, it is found that fractal is a very important characteristic of the segregation point morphology in the billet. Moreover, fractal dimension can be used as a criterion for measuring the dispersion degree of the segregation. The dispersion degree will be increased when increasing the corresponding fractal dimension, and the large segregation point will be disintegrated by the small segregation point. Finally, the fractal dimensions in the columnar-equiaxed transition area and the solidifying end equiaxed area are less than the value of other locations. In addition, more researches are needed for accurately obtaining the influence factors of fractal dimensions of segregation point in the future.
Fund: Supported by National Natural Science Foundation of China (No.51504047) and Fundamental Research Funds for the Central Universities (No.CDJPY14130001)
Fig.1 Schematic of sampling location in the central plane of billet
Fig.2 Examples of I, II and III three kinds of segregation points (Area of I>0.1 mm2, 0.01 mm2<area of II<0.1 mm2,area of III<0.01 mm2)
Fig.3 Macrostructures of samples 1#~12# (a~l) at different locations of billet under melting rate 350 kg/h
Fig.4 Macrostructures of samples 1#~12# (a~l) at different locations of billet under melting rate 400 kg/h
Fig.5 Segregation ratios at different locations under 350 and 400 kg/h
Fig.6 Area ratios of I, II and III three kinds of segregation points under 350 kg/h (a) and 400 kg/h (b)
Fig.7 Amount ratios of I, II and III three kinds of segregation points under 350 kg/h (a) and 400 kg/h (b)
Fig.8 Changes of segregation ratio with area ratio (a) and amount ratio (b) of I segregation point
Fig.9 Relationship between lnP and lnA for fractal dimension of 1# sample under 350 kg/h (P—perimeter, A—area)
Sample No.
350 kgh-1
400 kgh-1
D
R2
D
R2
1
1.799
0.9779
1.822
0.9109
2
1.760
0.9807
1.779
0.9697
3
1.760
0.9818
1.769
0.9688
4
1.755
0.9820
1.786
0.9731
5
1.760
0.9761
1.785
0.9729
6
1.747
0.9724
1.790
0.9704
7
1.757
0.9738
1.775
0.9726
8
1.755
0.9783
1.796
0.9718
9
1.762
0.9823
1.804
0.9701
10
1.742
0.9841
1.788
0.9719
11
1.735
0.9824
1.781
0.9651
12
1.767
0.9787
1.793
0.9680
Table 1 Fractal dimensions D and corresponding fitting coefficients R2 at different locations under 350 and 400 kg/h
Fig.10 Fractal dimensions at different locations under 350 and 400 kg/h
(a) different locations (b) average value
Fig.11 Changes of segregation ratio (a) and average area of segregation point (b) with fractal dimension
Fig.12 Changes of area ratios of I (a), II (b) and III (c) three kinds of segregation points with fractal dimension
Fig.13 Changes of amount ratios of I (a), II (b) and III (c) three kinds of segregation points with fractal dimension
Fig.14 Changes of whole amount of segregation point at different locations with fractal dimension
Fig.15 Local solidification time at different locations in the billet
Fig.16 Changes of fractal dimension with local soli-dification time
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