1. Selection of raw materials
For the production of ferrite ductile iron, it is very necessary to select high-purity raw materials. The content of Si, Mn, S, and P in the raw materials should be low (Si<1.0%, Mn<0.3%, S<0.03%, P<0.03%), and the content of some alloying elements such as Cu, Cr, and Mo should be strictly controlled. Because many trace elements are most sensitive to spheroidization decay, such as tungsten, antimony, tin, titanium, and vanadium. Titanium has a great influence on spheroidization and should be controlled, but high titanium is a characteristic of pig iron in my country, which is mainly related to the metallurgical process of pig iron.
2. Desulfurization
The sulfur content of the raw iron liquid determines the amount of spheroidizing agent added. The higher the sulfur content in the raw iron liquid, the more spheroidizing agent is added, otherwise it is impossible to obtain castings with good spheroidization. The S content in the raw iron liquid before spheroidization is controlled below 0.02%. When the sulfur content of the raw iron liquid before spheroidization is high, desulfurization must be carried out.
· Mo alloy treatment:
Mo alloying treatment adopts eddy current process, and the addition amount is controlled at 0.5~1.0%, which is adjusted according to the final Mo content. In order to ensure the effective absorption of Mo, the particle size of the alloy should be strictly required.
· Spheroidizing agent and spheroidizing treatment
When producing thick and large-section ductile iron parts, in order to improve the anti-degradation ability, a certain proportion of heavy rare earth is added to the spheroidizing agent, which can not only ensure the content of Mg for spheroidization, but also increase the heavy rare earth elements with high anti-degradation ability, such as yttrium. According to the tests and production practices of many domestic factories, it is very ideal to use a composite spheroidizing agent of Re-Mg and yttrium-based heavy rare earth as a spheroidizing agent for the production of thick and large-section ductile iron parts. The use of this spheroidizing agent has also achieved good results in the actual production and application of our foundry enterprises. According to relevant data, the spheroidizing ability of yttrium is second only to magnesium, but its anti-degradation ability is much stronger than magnesium, and it does not resulfurize. Yttrium can be added in excess, and when high carbon is well inoculated, cementite will not appear. In addition, yttrium and phosphorus can form high iron melting point inclusions, which reduce and disperse the phosphorus eutectic, thereby further improving the elongation of ductile iron. During the spheroidizing treatment, in order to improve the absorption rate of magnesium, control the reaction speed and improve the spheroidizing effect, a unique spheroidizing process is used. The control of the spheroidizing treatment is mainly to control the reaction speed, and the spheroidizing reaction time is controlled to be about 1 minute.
For this purpose, a composite spheroidizing agent of medium and low Mg, Re spheroidizing agent and yttrium-based heavy rare earth is used, and the amount of spheroidizing agent added is determined according to the residual Mg amount.
Prevention of spheroidization decline: The reason for spheroidization decline is related to the reduction of Mg and RE elements escaping from the molten iron on the one hand, and the continuous decline of the inoculation effect on the other hand. In order to prevent spheroidization decline, the following measures are taken: A. The molten iron should maintain sufficient spheroidizing element content; C. Reduce the sulfur content of the original molten iron and prevent the molten iron from oxidation; C. Shorten the residence time of the molten iron after spheroidization treatment; D. After the molten iron is spheroidized and slag is removed, in order to prevent the escape of Mg and RE elements, the surface of the molten iron can be covered with a covering agent to isolate the air to reduce the escape of elements.
3. Inoculant and inoculation treatment
Spheroidization treatment is the basis of ductile iron production, and inoculation treatment is the key to ductile iron production. The inoculation effect determines the diameter, number and roundness of the graphite balls. In order to ensure the inoculation effect, the inoculation treatment adopts multi-stage inoculation treatment. The closer the inoculation treatment is to casting, the better the inoculation effect. It takes a certain amount of time from inoculation to casting. The longer the time, the more serious the inoculation decline. In order to prevent or reduce the decline of inoculation, the following measures are adopted:
A. Use long-term inoculants (silicon-based inoculants containing a certain amount of barium, strontium, zirconium or manganese);
B. Use multi-stage inoculation treatment (floating silicon inoculation, in-package inoculation, inverted package inoculation, flow inoculation, etc.);
C. Try to shorten the time from inoculation to pouring.
The amount of inoculant added is controlled at 0.6~1.4%. Too little inoculant addition will directly cause poor inoculation effect, and too much inoculation will lead to inclusions in castings.
4. Casting process control
The pouring should adopt the principle of fast pouring and steady injection. In order to improve the uniformity of instantaneous inoculation and prevent slag from entering the cavity, the total capacity of the nozzle basin should be equivalent to the gross weight of the casting. When pouring, put the inoculant in the nozzle basin, and inject all the molten iron into the nozzle at once to fully mix the molten iron and the inoculant, scrape off the surface slag, and raise the nozzle to block the pouring.
5. Control principles of casting process
1) Reasonable casting process is a crucial factor,
2) Control the solidification time through casting process. The principle is to place cold iron at the thick section to adjust the temperature field and accelerate the solidification of molten iron. (Some factories in the same industry use forced cooling process, that is, to use cold iron and then add water cooling or air cooling and other forced measures to strengthen the solidification of castings and reduce the solidification time, the effect is very good. But there are certain risks and high technical requirements. In addition, in order to obtain the ferrite matrix, the unpacking temperature must be controlled below 600.)
6. Result analysis
According to the above process method, in production practice, it is completely possible to obtain customer standard requirements in the cast state, so that the matrix is completely ferrite and the spheroidization level is 1-2.
7. Conclusion
By analyzing and controlling the various factors affecting cast iron ferrite ductile iron, various performance indicators fully meet the requirements; high elongation is obtained under the premise of obtaining high tensile strength and yield strength; reasonable control of spheroidization inoculation process, full ferrite structure is obtained in the cast state, avoiding subsequent heat treatment.
Based on the above-mentioned control technology of ferritic ductile iron, we have been fully verified in the production of wind power castings and injection molding parts, and achieved good results. The production technology of thick and large-section ductile iron has been reported both at home and abroad. Although the methods are different, the principles followed are the same. That is:
- Strictly control the raw materials, reasonably select the chemical composition, and reduce the content of elements that interfere with spheroidization.
- Adopt effective anti-degradation measures.
- Select a reasonable casting process.
- Reasonably select the spheroidizing inoculation treatment method
