step 1
Material selection
For the production of ferrite ductile iron, it is necessary to select high-purity raw materials. The content of Si, Mn, S and P in raw materials is less (Si<1.0%, Mn<0.3% S<0.03%, P<0.03%). ), some alloying elements such as Cu, Cr, Mo should be strictly controlled. Because many trace elements are most sensitive to spheroidization decline, such as tungsten, tantalum, tin, titanium, vanadium and so on. The influence of titanium on spheroidization should be controlled, but the high titanium is the characteristic of pig iron in China, which is mainly related to the metallurgical process of pig iron.
Step 2
Desulfurization
The sulfur content of the original iron solution determines the amount of the spheroidizing agent added. The higher the sulfur content in the original molten iron, the more the spheroidizing agent is added, otherwise the spheroidized casting cannot be obtained. The S content in the original molten iron before the spheroidizing treatment is controlled to be 0.02% or less. When the sulphur content of the original molten iron is high before the spheroidizing treatment, it is necessary to carry out desulfurization treatment.
Step 3
Mo alloy treatment
Mo alloying treatment, using eddy current process, the amount of addition is controlled at 0.5 to 1.0%, specifically adjusted according to the final Mo content. In order to ensure effective absorption of Mo, the particle size of the alloy should be strictly required.
Step 4
Spheroidizing agent and spheroidizing treatment
In the production of thick section ductile iron parts, in order to improve the anti-recession ability, a certain proportion of heavy rare earth is added to the spheroidizing agent, which can ensure the spheroidizing Mg content and also increase the high anti-recession ability. Heavy rare earth elements, such as yttrium. According to the test and production practice of many factories in China, it is very desirable to use the composite spheroidizing agent of Re-Mg and yttrium-based rare earth as the spheroidizing agent for the production of thick-section ductile iron parts. The actual production is carried out using this spheroidizing agent. The application process also achieved good results. According to the relevant data, the spheroidizing ability of bismuth is second only to that of magnesium, but its anti-recession ability is much stronger than that of magnesium, and it does not return sulfur. It can be added in excess. When high carbon is well bred, no cementite will appear. In addition, bismuth and phosphorus can form high melting point inclusions, which reduce and diffuse the phosphorus eutectic, thereby further increasing the ductile iron elongation. In the spheroidization treatment, in order to increase the absorption rate of magnesium, control the reaction rate and improve the spheroidization effect, a unique spheroidization process is employed. The control of the spheroidization treatment is mainly controlled at the reaction rate, and the spheroidization reaction time is controlled to about 2 minutes.
For this purpose, a composite spheroidizing agent of medium-low Mg, Re spheroidizing agent and cerium-based heavy rare earth is used, and the amount of the spheroidizing agent added is determined according to the amount of residual Mg.
Spheroidization decline prevention: The reason for the spheroidal decline is related to the decrease of the escape of Mg and RE elements in the molten iron, and on the other hand, the gradual decline of the gestational effect. In order to prevent the spheroidization from declining, the following measures are taken:
A. There should be enough spheroidized element content in the molten iron;
B. Reduce the sulfur content of the original molten iron and prevent the oxidation of the molten iron;
C. Shortening the residence time of the molten iron after spheroidizing treatment;
D. After the spheroidizing treatment of the molten iron and slag slag, in order to prevent the escape of Mg and RE elements, the surface of the molten iron may be covered with a covering agent to isolate the air to reduce the escape of the elements.
Step 5
Inoculant and inoculation
Spheroidizing treatment is the basis of ductile iron production. Inoculation treatment is the key to the production of ductile iron. The effect of inoculation determines the diameter of graphite sphere, the number of graphite spheres and the roundness of graphite sphere. In order to ensure the inoculation effect, the inoculation treatment adopts multi-stage breeding. deal with. The closer the inoculation treatment is to the pouring, the better the inoculation effect. It takes a certain amount of time from gestation to pouring, and the longer the time, the more serious the decline is. In order to prevent or reduce the decline of pregnancy, the following measures are taken:
A. Use a long-acting inoculant (a silicon-based inoculant containing a certain amount of lanthanum, cerium, zirconium or manganese);
B. Multi-stage inoculation treatment (inoculation, inoculation, inoculation, etc.);
C. Try to shorten the time of incubation.
The amount of inoculant added is controlled at 0.6-1.4%, and the amount of inoculant added is too small, which directly causes poor breeding effect, and the amount of inoculum is too large, resulting in inclusions in castings.
Step 6
Pouring process control
Pouring should be based on the principle of fast pouring and smooth injection. In order to improve the uniformity of instantaneous gestation and prevent the slag from entering the cavity, the total capacity of the sump should be equivalent to the gross weight of the casting. When pouring, the inoculant is placed in the sump, and the molten iron is completely injected into the nozzle at one time to make the molten iron and the inoculant. Mix thoroughly, remove the surface scum, and propose a water plugging.