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Acta Metall Sin  2018, Vol. 54 Issue (6): 851-858    DOI: 10.11900/0412.1961.2017.00379
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Time-Series Fluctuation Characteristics of Segregation Carbon Element Distribution Along Casting Direction in High Carbon Continuous Casting Billet
Zibing HOU(), Rui XU, Yi CHANG, Jianghai CAO, Guanghua WEN, Ping TANG
College of Materials Science and Engineering, Chongqing University, Chongqing 400044, China
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High carbon steel is prone to produce macroscopic/semi-macroscopic segregation, and C segregation along casting direction will have a serious impact on the quality of billet. Based on qualitative analysis (macroscopic quality rating and elemental macro content analysis) and relatively simple quantitative analysis (segregation index, mean square error), the existing technologies have judged the degree of segregation of billet in different levels, but with requirement of stricter quality stability standard, especially for the typical raw material of high-level rod-wire steel (e.g. 70 high carbon steel), it is very necessary to introduce a new method to measure the fluctuation characteristics more effectively. In this work, the ARMA (auto regressive moving average) model in the time series is firstly used to study the segregation degree of high carbon steel billet at different positions from the aspects of inherent period and damping rate in terms of the fluctuation characteristics (period and extremum) of C element along the casting direction. The size of the billet is 170 mm×170 mm and the sampling location is in the center longitudinal plane of the billet. The experiment is conducted by considering the effect of cooling strength (conventional cooling and strong cooling) on the fluctuation characteristics. Firstly, it is shown that the inherent period and the damping rate can quantitatively describe the inherent characteristics (periodic and extreme characteristics) of the C element along the casting direction. Secondly, strong cooling makes the average inherent period and the average damping rate of C element time series in the columnar grain region smaller, thus increasing the fluctuation degree and the segregation degree. However, strong cooling makes the average inherent period and the average damping rate of the C element time series in the equiaxed grain region larger, thus decreasing the fluctuation degree and the segregation degree. Finally, the period and the damping rate of the columnar grain region are respectively affected by the dendritic spacing and the temperature gradient, and the period and the damping rate of the equiaxed grain zone are respectively affected by the liquid flow in the solidification process (V-shaped segregation) and local cooling rate. By this research, a new theoretical basis may be supplied for delicacy control of element segregation and related quality fluctuation phenomenon.

Key words:  segregation      high carbon steel      continuous casting      fluctuation characteristics      casting direction      time series     
Received:  08 September 2017     
ZTFLH:  TG113.12  
Fund: Supported by National Natural Science Foundation of China (No.51504047) and Fundamental Research Funds for the Central Universities (No.CDJPY14130001)

Cite this article: 

Zibing HOU, Rui XU, Yi CHANG, Jianghai CAO, Guanghua WEN, Ping TANG. Time-Series Fluctuation Characteristics of Segregation Carbon Element Distribution Along Casting Direction in High Carbon Continuous Casting Billet. Acta Metall Sin, 2018, 54(6): 851-858.

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Fig.1  Schematic of sampling location in the center longitudinal plane of the billet (20 and 30 mm in the columnar grain region; 40 and 50 mm in the mixed grain region; 60 mm in the equiaxed grain region)
Fig.2  Fluctuation characteristics in distribution of C element along casting direction (60 mm location in sample 2)
Fig.3  Relationship between the inherent period T and damping rate ξ with the fluctuation characteristics

(a) T=1 s, ξ=1 (b) T=0.5 s, ξ=0.5

(c) T=0.75 s, ξ=0.75

Fig.4  Time series of C elements of sample 1 (a~c) and sample 2 (d~f) at 20 and 30 mm (a, d), 40 and 50 mm (b, e) and 60 mm (c, f)
Fig.5  Average inherent period of C element time series
Fig.6  Average damping rate of C element time series
Fig.7  Change of solidification structure in columnar grain region under conventional cooling and strong cooling (Small boxes connected to primary dendrite are secondary dendrites,d1 and d1 represent primary dendrite arm spacing under conventional cooling and strong cooling, respectively, dashed lines represent measuring line of C element along casting direction in columnar grain zone)
Fig.8  Macrostructures of center longitudinal plane including V-shape segregation for sample 1 (a) and sample 2 (b)
Distance to left border / mm Sample 1 Sample 2
30 172.46 160.14
40 209.55 200.92
60 225.04 221.81
Table 1  Average secondary dendritic spacing d2 of the different grain regions of sample 1 and sample 2
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