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:
GUO Dongwei, GUO Kunhui, ZHANG Fuli, ZHANG Fei, CAO Jianghai, HOU Zibing. A New Method for CET Position Determination of Continuous Casting Billet Based on the Variation Characteristics of Secondary Dendrite Arm Spacing. Acta Metall Sin, 2022, 58(6): 827-836.
High-end steel products play an essential role in economic development and infrastructure projects. Nowadays, continuous casting is an important production process of high-end steel products due to higher efficiency and lower energy consumption. However, the quality and properties of end products, such as steel billets, bloom, and bar, will be affected by internal segregation defects, which are closely related to solidification structure characteristics. In the research on the solidification structure characteristics of billets, the columnar to equiaxed transition (CET) determination is of great significance to determine equiaxed crystal zones and the quality control of continuous casting billets. In this work, the secondary dendrite arm spacing (SDAS) of typical dendrites was measured and analyzed using the actual solidification structure of continuous casting billets and the mutation of SDAS during the solidification process from the billet surface to the center was found. Combined with the two-dimensional temperature field numerical model of the billet cross section, it can be seen that the CET will affect the heat transfer process in the billet and this will be reflected as the mutation of SDAS in typical dendrites. This work proposed a new method for the quantitative determination of CET in the continuous casting billets based on this mutation, and the starting position of the maximum SDAS increase rate is determined as the starting position of the CET. The CET positions calculated using the new method correspond to changes in the thermal gradient and growth rate in the billet, and are consistent with the positions of the actual solidification structure morphology transformations, which prove the effectiveness of this method.
Fig.1 Schematic of sampling method for cross section of the selected continuous casting billets
Billet
Superheat
Casting speed
Specific water
No.
oC
m·min-1
amount / (L·kg-1)
1
42
1.8
0.68
2
40
2.1
0.68
Table 1 Production parameters of selected SCM440 continuous casting billet
Fig.2 Schematic of measurement position of secondary dendrite arm spacing (SDAS) in cross section of selected continuous casting billets
Section
Length
Water amount
Boundary condition
Computational formula
m
m3·h-1
No.1
No.2
Mold
0.9
114
114
qm
Foot roller section
0.5
4.16
5.19
qf = hf(Tb - Tf)
hf = 153.6(w / 60)0.351
First section of secondary cooling zone
2.7
6.69
8.24
qk = hk(Tb - Tw)
hk = 160 + 8.35w0.851
Second section of secondary cooling zone
2.9
2.08
2.48
qk = hk(Tb - Tw)
hk = 200 + 10.44w0.851
Third section of secondary cooling zone
3.5
1.63
2.04
qk = hk(Tb - Tw)
hk = 200 + 10.44w0.851
Air cooling zone
5.4
-
-
qa = ɛσ(Tb4 - Ta4)
ε = 0.8
Table 2 Parameters and relevant computational formulas at different sections in the calculation of temperature field[18-20]
Temperature measuring point No.
Distance from the meniscus
m
Measured surface center temperature / oC
Calculated surface center temperature / oC
1
11.67
1026
1002.23
2
13.65
998
979.00
3
26.90
823
793.20
4
28.90
776
754.32
Table 3 Comparison of temperature measurement results and numerical simulation results of temperature field at the center of right surface of billet No.1
Fig.3 SDAS changes on the cross section of the billets No.1 (a, c, e) and No.2 (b, d, f) (a, b) near inner arc side (c, d) centerline (e, f) near outer arc side
Fig.4 Thermal gradient changes on the left centerline of billets No.1 (a) and No.2 (b) (Insets show the high magnified images)
Fig.5 Growth rate changes of solidification structure on the left centerline of billets No.1 (a) and No.2 (b) (CET—columnar to equiaxed transition)
Fig.6 Typical morphological changes of the solidified structure from the surface to the center of the billet
Fig.7 Schematic of billet CET determination based on the SDAS change (dSi —SDAS of typical dendrite)
Billet No.
Side
Measured CET position / mm
Average
Near inner arc
Centerline
Near outer arc
mm
1
Left
30.12
32.33
31.13
30.82
Right
29.25
31.32
30.74
2
Left
29.27
28.58
28.03
28.71
Right
28.98
28.99
28.40
Table 4 Measurement results of the CET position on the cross section of billets No.1 and No.2
Fig.8 Calculated CET positions of actual billets No.1 (a) and No.2 (b)
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