Effect of Superheat on Integral Morphology Characteristics of Solidification Structure and Permeability in Bearing Steel Billet
CAO Jianghai1,2, HOU Zibing1,2(), GUO Zhongao1,2, GUO Dongwei1,2, TANG Ping1,2
1.College of Materials Science and Engineering, Chongqing University, Chongqing 400044, China 2.Chongqing Key Laboratory of Vanadium-Titanium Metallurgy and New Materials, Chongqing University, Chongqing, 400044, China
Cite this article:
CAO Jianghai, HOU Zibing, GUO Zhongao, GUO Dongwei, TANG Ping. Effect of Superheat on Integral Morphology Characteristics of Solidification Structure and Permeability in Bearing Steel Billet. Acta Metall Sin, 2021, 57(5): 586-594.
As a typical bearing steel, GCr15 steel tends to solidify over a wide temperature range during casting because of its high carbon content. The size of the mushy zone is relatively large, causing macrosegregation and porosity defects in bearing steel. The morphology of the solidification structure plays an important role in governing macrosegregation severity. Solidification structures have conventionally been characterized by measuring the primary or secondary dendrite arm spacing in a dendritic network, but these measures do not adequately describe the branched appearance of secondary and tertiary arms. In this work, fractal dimension and specific surface area have been introduced, and the solidification structure integral morphology characteristics of different locations in the continuous casting billet of GCr15 bearing steel have been quantitatively investigated. Then, the permeability of the interdendritic channels were calculated based on fractal dimension and specific surface area. The size of the billets was 220 mm × 220 mm, and the sampling location was in the cross section of the billet. Two superheats (20 and 35oC) were considered for studying the integral characteristics of the solidification structure. First, fractal dimension can describe the self-similar complexity of the solidification structure, and specific surface area can describe dendritic coarsening. Second, it was determined that the fractal dimension was larger and the specific surface area was smaller at 35oC superheat compared with 20oC superheat. This indicates that the self-similar complexity of dendrites is larger, and the dendrite coarsening is more significant at high superheating. Finally, the permeability in the equiaxed grain zone calculated using fractal dimension and specific surface area is lower at 20oC superheat. The smaller the permeability, the greater the flow resistance of the liquid, which is more conducive to the control of the macrosegregation defects. In addition, to effectively restrain the formation of macrosegregation defects at high superheat, the cooling rate in the equiaxed grains zone should increase by adjusting the process parameters under isothermal solidification conditions.
Fund: United Funds between National Natural Science Foundation and Baowu Steel Group Corporation Limited of China(U1860101);Chongqing Fundamental Research and Cutting-Edge Technology Funds(cstc2017jcyjAX0019)
About author: HOU Zibing, associate professor, Tel: (023)65105202, E-mail: houzibing@cqu.edu.cn
Fig.1 Schematic of sampling locations in cross section of billet (unit: mm)
Fig.2 Macrostructures of samples 1#-20# (a-t) at different locations of billet under 20oC superheat
Fig.3 Macrostructures of samples 1#-20# (a-t) at different locations of billet under 35oC superheat
Fig.4 Calculation process of fractal dimension in sample 1# under 20oC superheat
Sample
20oC
35oC
D
R2
D
R2
1#
1.6106
0.9976
1.6746
0.9991
2#
1.6362
0.9976
1.6856
0.9990
3#
1.6656
0.9976
1.6911
0.9987
4#
1.7033
0.9974
1.7651
0.9990
5#
1.7200
0.9979
1.7680
0.9989
6#
1.7144
0.9979
1.7549
0.9988
7#
1.7568
0.9987
1.7820
0.9991
8#
1.7672
0.9990
1.7914
0.9992
9#
1.7578
0.9986
1.7729
0.9989
10#
1.7798
0.9987
1.8047
0.9992
11#
1.7745
0.9987
1.8140
0.9993
12#
1.7468
0.9985
1.8144
0.9993
13#
1.7372
0.9982
1.7666
0.9989
14#
1.7085
0.9974
1.7657
0.9990
15#
1.7310
0.9980
1.7775
0.9990
16#
1.7465
0.9982
1.7380
0.9982
17#
1.7428
0.9983
1.7223
0.9986
18#
1.6830
0.9981
1.7202
0.9988
19#
1.6715
0.9981
1.7175
0.9987
20#
1.6766
0.9984
1.7343
0.9990
Table 1 Fractal dimensions (D) and corresponding R2 at different locations under 20 and 35oC superheats
Fig.5 Fractal dimensions at different locations under 20 and 35oC superheats
Fig.6 Specific surface areas at different locations under 20 and 35oC superheats
Fig.7 Effects of superheat on fractal dimension (a) and specific surface area (b)
Fig.8 Relationship between fractal dimension and specific surface area
Fig.9 Permeability in equiaxed grains zone
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