1. Spheroidizing treatment

Graphite distortion and fragment graphite will be produced in nodular cast iron with thick and large section due to its spheroidization recession. The data show that ω The content of (mg residue) was increased to 0.05%~0.07%, and the spheroidization result was not ideal, ω On the contrary, the shrinkage porosity and slag inclusion of nodular cast iron increased significantly with the increase of (mg residual) content. Therefore, the spheroidization decline of thick and large cross-section nodular cast iron in the final solidification cannot be explained by the escape decline of Mg. The main reason is related to the s content in nodular cast iron. The higher the s content, the faster the spheroidization decline, and vice versa, the slower. Therefore, controlling the s content at a low level is an important measure to prevent spheroidization decline, while maintaining a certain ω (mg residual) content, after spheroidization ω (S) The content shall be controlled within 0.006%~0.01%.

Type	RE	Mg	Si	Dosage
Yttrium based heavy rare earth nodulizer	1.5-2.5	6-7	45	1.0-1.2

For the ductile iron punch disc, the punching method is used for spheroidization. The nodulizing agent is yttrium based heavy rare earth nodulizing agent. See Table 1 for main elements and addition amount.

2. Inoculation treatment

The multiple inoculation of thick and large cross-section nodular cast iron is different from that of small and medium-sized nodular cast iron in three aspects: first, the inoculation amount is large during tapping inoculation; Second, the ladle is 20~60 T, which is difficult to stir after inoculation; Third, it is difficult to inoculate with the flow due to the amount of molten iron up to dozens of tons during pouring.

Inoculation is an effective measure to prevent graphite distortion and fragment graphite of thick and large section ductile iron. Two problems should be paid attention to during inoculation: first, the content of s in the raw molten iron should not be too low, which should be greater than 0.006%, and the content of O in the molten iron should not be too low, which should be greater than 0.001%, otherwise the inoculation effect will be affected; Second, the total content of Si should not be too high, because the Si content in the spheroidizing agent is generally about 45%, and the Si content in the inoculant is more than 70%.

Type	Si	Ca	Al	Ba	RE	Dosage
High efficiency inoculant containing Ba	70-75	1.5-2.0	1.0-2.0	1.0-2.0	—	0.5-0.6
Long acting follow-up inoculant	55-65	—	—	—	1.0-2.0	0.1-0.2

Adopt multiple inoculation treatment, select high-efficiency throwing inoculant containing Ba and long-term follow-up inoculant. See Table 2 for the main elements and addition amount.