高碳鋼連鑄方坯拉坯方向偏析C元素分布的時間序列波動特征

高碳鋼連鑄方坯拉坯方向偏析C元素分布的時間序列波動特征TIME-SERIES FLUCTUATION CHARACTERISTICS OF SEGREGATION CARBON ELEMENT DISTRIBUTION ALONG CASTING DIRECTION IN HIGH CARBON CONTINUOUS CASTING BILLET

引入時間序列自回歸移動平均模型法，從固有周期和阻尼率2個方面對高碳鋼連鑄方坯拉坯方向上偏析C元素含量分布的波動特征進行定量化分析。結(jié)果表明，固有周期和阻尼率可以定量描述C元素時間序列的固有波動特征（周期和極值）。強冷使柱狀晶區(qū)C元素時間序列的平均固有周期和平均阻尼率均變小，對應波動程度增加，則偏析程度增大；同時使等軸晶區(qū)C元素時間序列的平均固有周期和平均阻尼率均變大，對應波動程度降低，則偏析程度降低。柱狀晶區(qū)的周期和阻尼率分別主要受枝晶間距和溫度梯度影響，而等軸晶區(qū)的周期和阻尼率分別主要受凝固過程液相流動（V形偏析）和局部冷卻速率影響。本研究能為連鑄坯拉坯方向元素分布均勻性的精細控制以及類似質(zhì)量波動問題的定量分析提供新的理論參考。

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 research, 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 170mm×170mm 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.

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