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| In situ observation of solidification under transient and average cooling rate of 20CrMnTi continuous casting |
| SHANG Ting-rui1,2, WANG Wei-ling1,2, KANG Ji-bai1,2, ZHU Miao-yong1,2, LUO Sen1,2 |
1. Key Laboratory for Ecological Metallurgy of Multimetallic Mineral, Ministry of Education, Northeastern University, Shenyang 110819, Liaoning, China;
2. School of Metallurgy, Northeastern University, Shenyang 110819, Liaoning, China |
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Abstract Wide hardenability band and large-size TiN precipitates seriously endanger the product quality of 20CrMnTi gear steel. The key basis for its control is to master the behavior mechanism of solidification structure evolution during continuous casting. The traditional high temperature laser confocal scanning microscopy(HT-CSLM) in situ observation and research usually sets the cooling rate of mushy zone as a fixed value,which can not effectively reflect the change of cooling rate during continuous casting solidification. Therefore,20CrMnTi 160 mm×160 mm billet was the research object. Through the two-dimensional slice solidification heat transfer calculation,the thermal history,transient and average cooling rate of the paste area 20,40 and 60 mm below the inner arc surface were determined,and then the heating and cooling scheme of HT-CSLM experiment was designed. Then,HT-CSLM experiments under the conditions of transient and average cooling rate in the mushy zone at these locations were carried out to reveal the solidification process of 20CrMnTi under different cooling conditions δ grain growth kinetics and peritectic phase transformation mechanism. Finally,the effect of cooling conditions on solidification structure size was investigated by electron probe microanalysis(EPMA). The results show that due to the compensation of solidification latent heat,the cooling rate at the position of 20,40 and 60 mm under the inner arc is small in the initial solidification stage,and gradually increases in the middle and later stages of solidification,and becomes more significant with the deeper into the billet. The average cooling rates at these locations are 102.81,44.63 and 34.93 ℃/min,respectively. δ grain precipitates first from the liquid steel,and its average growth rate increases with the increase of cooling rate. Under the condition of transient cooling rate,instantaneous growth rate increases with the progress of solidification,but decreases slightly under the condition of average cooling rate. This is because under the condition of transient cooling rate,the cooling rate of mushy zone changes from slow to fast,which continuously compensates the latent heat of solidification,at the same time,the number of initial nucleation is small,the growth space is large,and the influence of solute undercooling is relatively weak. When the melt temperature decreases to the critical temperature of peritectic phase transformation,δ grain transition to γ grain rapidly,that is,massive transformation occurs,leading to a rapid increase in the solid fraction,accompanied by some γ grain polymerization rapidly. On the whole,the critical temperature of peritectic phase transition decreases with the increase of cooling rate,but it is also affected by the initial content of solute. After that,the remaining liquid phase transitions to the γ phase until complete solidification. The grain radius decreases with the increase of cooling rate,and it is smaller under the condition of average cooling rate than that under the condition of transient cooling rate,which depends on the number of nucleation in the initial solidification stage.
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Received: 14 February 2022
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2022, Vol. 57 
