The figure shows the matrix structure of lzqt600-3 ductile iron profile at different parts. In figure (a), the microstructure is fine and the size of graphite ball is small. From the edge to the center, the graphite ball becomes larger and the pearlite content increases obviously. Compared with GB / T 9441-2009 “metallographic examination of nodular cast iron”, the pearlite content can be determined.

Due to the great subjective error of artificial evaluation, in order to describe the pearlite content in lzqt600-3 ductile iron profile more accurately and quantitatively, the specific data are obtained by using image J statistics. The results are shown in Fig. 3-6. From the edge to the center, the content of pearlite increased from 64.36% to 65.90% and finally increased to 70.84%. The corresponding grade names were zhu65, zhu65 and zhu75. The grade of pearlite calculated by statistics is consistent with the result of artificial evaluation, and conforms to the classification chart of pearlite content in GB / T 9441-2009, but the calculation result can more quantitatively reflect the pearlite content in the tissue, and the artificial observation and evaluation can only obtain the approximate range of one grade, and can not get the accurate value. Through the calculation results, it can be accurately concluded that the pearlite content at the edge of lzqt600-3 ductile iron profile is the lowest, and the pearlite content in the core is the most, and the pearlite content is increasing from the edge to the core.

The reason for this phenomenon is that austenite can be transformed into pearlite in sufficient time in the horizontal continuous casting production process. The condensation rate of hot metal at the edge of the profile is the largest and that at the core is the smallest. Therefore, the transformation time at the edge is short and the austenite is too late to transform, so the content of pearlite is the highest Less. However, the austenite is fully transformed after a long time of transformation in the center, so the pearlite content in this part is the most. The other reason is that the cooling rate at the edge is greater than 1 / 2R and the center, and the molten iron has a shorter time as a liquid. The solute atoms such as carbon and silicon can not be fully diffused in the liquid iron because of the high content and uniform distribution. However, the solidification time of the liquid iron at 1 / 2R and the core is long, and the carbon atoms have enough time to diffuse to the graphite ball and make it grow up. Therefore, the ferrite near the graphite ball will appear carbon poor phenomenon, but the austenite far away from the graphite ball has high carbon content, which is beneficial to obtain pearlite during eutectoid transformation.