Aggregation and engulfment behavior of AlN inclusions in Fe-Mn-Al-C low density steel
The aggregation and growth of AlN inclusions in Fe-Mn-Al-C low density steel have a serious effect on the quality of the steel.In situ observation of the evolution behavior of AlN inclusions in Fe-Mn-Al-C low density steel during liquid and solidification processes was conducted using a high-temperature confocal laser scanning micro-scope combined with a field emission scanning electron microscope.It was found that AlN inclusions aggregate in liq-uid steel,forming large-sized multi particle aggregation type AlN inclusions;During the subsequent cooling and solidification process,aggregated AlN inclusions approach the liquid-solid interface and are pushed forward by the front edge of the liquid-solid interface;as the liquid content decreases,the aggregated AlN inclusions are eventually engulfed by the solid phase.In order to clarify the aggregation mechanism,the viscous resistance and cavity bridge force between AlN inclusions with a radius of 1-10 μm in molten steel were calculated through force analysis,as most of the observed single particle AlN had a radius of 1-10 μm.It was found that the cavity bridge force is much higher than the viscous resistance,and the cavity bridge force is the main force causing the aggregation of AlN inclu-sions.Based on the theory of cavity bridge force,the total free energy variation of cavity bridge formation was fur-ther analyzed.Taking two single particle AlN inclusions with a radius of 8 μm as an example,the total free energy variation of cavity bridge formation was calculated.It was found that the critical distance between two AlN inclu-sions to form a stable cavity bridge dc is 1.5 μm;when the distance between two AlN inclusions d＞dc,the total free energy change during the aggregation process of AlN inclusions is higher than 0,and a stable cavity bridge will not be formed;when d＜dc,the total free energy change during the aggregation process is less than 0,and stable cavity bridges can be formed between AlN inclusions,leading to aggregation.The experimental observation and the calculation of the force balance of inclusions at the liquid-solid interface show that the van der Waals force plays a key role in the movement of AlN inclusions during solidification.AlN inclusions are first pushed by the front edge of the liquid-solid interface,and as the temperature and liquid volume decrease,AlN inclusions are ultimately captured and engulfed by the solid phase.
Fe-Mn-Al-C low density steelAlN inclusionaggregationcavity bridgefree energy changecritical distance
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