1. Sand properties

In order to ensure the good surface quality of ductile iron castings, the performance of green clay sand is particularly important, and the control of molding sand performance is an important link in the production process control. Use a 3 t rotor sand mixer, add a certain proportion of recycled sand, new sand, bentonite and pulverized coal into the sand mixer in sequence, and after a certain time of sand mixing, mix the molding sand that meets the requirements. The specific sand mixing parameters are shown in Table 1.

Element	C	Si	Mn	P	S	Cu
Content	3.3~3.9	2.2~3.2	0.1~0.4	≤0.08	≤0.02	≤0.2

Feeding sequence: (recycled sand+new sand)+water+(pulverized coal+bentonite)+water replenishment. The first water replenishment is 80% of the historical water replenishment. The water replenishment is automatically adjusted according to the online detection results. When the total sand mixing time exceeds 200 s, the molding sand will be discharged and no longer used. The control range of molding sand properties is shown in Table 2.

Raw material	Recycled sand	Fresh sand	Pulverized coal	Bentonite
Dosage/kg	3 000	10	15	35

2. Smelting and spheroidizing process

Q10 pig iron, briquette scrap and QT450-10 pig iron are used as raw materials. The charging ratio is 50% pig iron+20% pig iron+30% scrap steel. The charging sequence is pig iron+scrap steel+carburetor+pig iron+scrap steel. In order to ensure the absorption rate of the carburetor, the carburetor is added to the electric furnace with the scrap steel. The static temperature is 1 535~1 555 ℃, the static time is 3~5 minutes, the outlet temperature is 1 495~1 525 ℃, and the sampling temperature is ≥ 1 490 ℃, The tapping mass is 696-704 kg.

After smelting adjustment, the chemical composition of molten iron after melting is controlled as shown in Table 3. The residence time in the furnace of molten iron with qualified composition shall not exceed 30 min, otherwise, the carbon content shall be re-detected and re-adjusted if necessary.

Project	Moisture/%	Compaction rate/%	Air permeability	Crushing index/%	Wet compression strength/MPa	Temperature/℃	AFS fineness	Effective bentonite content/%	Burn loss rate/%	Mud content/%
Parameter	3.2~3.4	38~42	100~120	≥75	0.17~0.21	20~50	60~65	7~9	3~6	9~13

The content of w (Mg) in the spheroidized wire is 29.5%~32.5%, the content of w (RE) is 2.0%~2.5%, the diameter of the core wire is 13.0~13.7 mm, and the particle size is 0.1~2.5 mm. During the casting process, attention must be paid to ensure the consistency of the casting quality, the casting temperature and the liquid iron turnover time of each ladle of molten iron. At the same time, in order to prevent the shrinkage defects in the ductile iron castings caused by the excessive residual magnesium content, and to ensure the spheroidization rate of the ductile iron castings, the parameters such as the spheroidization temperature, the spheroidization time, and the pouring temperature must be strictly controlled. In the process of wire feeding spheroidization reaction, the cover of the ladle must be tightly covered to avoid and reduce the splashing of molten iron outside the ladle, at the same time, it is conducive to the smoking and dust removal effect of environmental protection equipment in the workshop, and avoid environmental pollution. The process parameters of wire feeding spheroidization treatment are shown in Table 4.

Element	C	Si	Mn	P	S	Cu
Content	3.80~3.85	2.00~2.10	0.25~0.30	≤0.06	≤0.025	0.1~0.2

The inoculation treatment of nodular cast iron is divided into three stages: for the first stage, when the molten iron is discharged from the furnace, the silicon barium inoculant with particle size of 2.6~4.5 mm is added into the transfer bag, with the addition amount of 0.2%; For the second inoculation, after spheroidizing treatment, when the molten iron is transferred to the pouring basin, the silicon barium inoculant with particle size of 0.6~2.5 mm is added, and the addition amount is 0.3%; For the third inoculation, during pouring, the silicon barium inoculant with particle size of 0.2~0.5 mm is added by the method of flow inoculation, and the inoculation amount is 3 g/s. To ensure spheroidization and inoculation effect, the pouring time of the whole package shall not exceed 8 min.