The diameter distribution of graphite balls in three parts of lzqt600-3 ductile iron profile was analyzed by Excel and Image J software. The results are shown in the figure. According to figure (a), the diameter distribution of graphite ball at the edge of section of lzqt600-3 ductile iron profile is mainly concentrated in the range of 0-70 μ m, accounting for 98.85715% and the average diameter is 29.86 μ m; from figure (b), the diameter distribution of graphite ball at 1 / 2R section is between 0-70 μ m, accounting for 99.6988% and the average diameter is 26.78 μ m; from figure (c), the diameter distribution of graphite ball at the center of section is 0-80 The average diameter was 31.03 μ M. It can be concluded that the size of graphite ball from the edge to the center increases, but the average diameter of graphite ball at 1 / 2R is smaller than that at the edge and center, and the microstructure is the smallest. As a result, the cooling rate of the molten iron decreases gradually in the process of casting.
However, due to its own gravity, the molten iron at 1 / 2R is inclined to the upper part of the liquid core during solidification. The cooling rate is the fastest, the nucleation rate is high, and the microstructure is uniform and fine. Secondly, due to the randomness of the selected parts in this experiment, Zhao Zhen  divided the sample selection of horizontal continuous casting profile into the left and the bottom of the profile. The research shows that the average diameter of graphite ball at the upper edge, 1 / 2R and center of the profile section is the smallest. The experimental results are in accordance with the law.