Three furnace pretreatment
Desulfurization
Desulfurization of molten iron reduces the amount of spheroidizing agent and the amount of sulfide slag in the casting. In the developed countries, almost all of the ductile iron production adopts the desulfurization process, which requires S≤0.01% after desulfurization. Under the condition of China, the target position is S≤0.02%, and Xichai and Changchai can reach S≤0.015%.
The bottom of the bag is treated by Na2CO3 flushing method. The method is simple, but the desulfurization rate is low, the effect is unstable, and the dust pollutes the environment during treatment, which is irritating to the throat. In the scale iron ore workshop, the desulfurization agent should be placed on the liquid surface, and the desulfurization should be carried out by mechanical stirring method, shaking method or pneumatic stirring method. Among them, the pneumatic method is relatively simple, and the power consumption is small, which is the most popular. The pneumatic desulfurization device can be arranged upstream of the forehearth for continuous desulfurization, or can be desulfurized in a single package in front of the furnace. The pneumatic source is mostly N2.
The desulfurizing agent is divided into two major categories: CaC2 system and CaO. The desulfurization effect of CaC2 is good, and the sulfur can be reduced from 0.04 to 0.06% to 0.01% or less. The scum is easy to remove in the form of granules, but its price is high, the melting point is high, and it is restricted to transportation and storage, so it is limited to near supply. CaO is more commonly used. Regardless of the desulfurizer, it should be resistant to moisture and explosion. The active CaO is formed by a film forming process and a composite desulfurizing agent prepared by adding an additive, which is free from moisture, has high desulfurization efficiency, and is desulphurized by pneumatic method, and the desulfurization rate is 60 to 90%. For plants with a small output, the single-pack flushing method should use a low-melting composite desulfurizer, and the desulfurization rate is 30-50%. At this time, the desulfurization slag is molten.
After pneumatic desulfurization, the molten iron enters the induction furnace to raise the temperature.
2. Deslagging
The slag removal process can be carried out in a package or in an induction furnace. The slag removing agent (slag slag agent) is sprinkled on the liquid surface to form a molten layer to cover and slag. The slag removing agent is heated and puffed, and many small holes are formed in the molten layer to provide good heat preservation.
The slag removal agent is divided into three grades: low, medium and high. The products that are simply crushed and sieved by the original ore are low-grade and are produced in all provinces. It is not optimistic about the foundry. The mid-range slag remover has a choice of ore, which increases the washing process and can meet the basic requirements of castings and is accepted by most factories. Imported high-grade slag remover, select high-quality mine source, the processing procedure is different from the general. The product is spread on the liquid surface and spreads quickly, covering the entire liquid surface. The slag removal ability is excellent, the dosage is small, the lining and the ladle are not adhered, and the molten slag layer can be provoked by the stick, and the slag is convenient and thorough. Over the years, it has begun to receive the attention of the domestic industry.
3. Spheroidization
The spheroidization treatment is still dominated by the in-package method. The choice of spheroidizing agent depends on the furnace, the temperature of the tapping iron, the desulfurization or not, and the type of ductile iron. In general, the cupola iron is selected from Mg7~9%, RE3~7% spheroidizing agent, and the induction furnace is made of low Mg (5~6%) and low RE (1.5~2.5%) spheroidizing agent. Some plants still use Mg8RE3 spheroidizing agent. According to the JB/T9228-1999 standard, the deviation of the composition range of Mg and RE is specified as ±1%, and some of the standards have been reduced to ±0.5%, and Ca, Al, etc. are also clearly defined. In the quality of the spheroidizing agent, attention should be paid to the MgO content, the degree of segregation of the components, and the concentration of the particle size. Any spheroidizing agent with uneven particle size, many powders and dark color should not be used.
In principle, as long as the chemical composition and incubation of the molten iron are mastered, the as-cast spheroidizing agent can be used to produce as-cast ductile iron. However, some factories use spheroidizing agents for as-cast ductile iron. Generally, Ba and Bi are contained in the as-cast ferrite spheroidizing agent, Ba and Sb are contained in the as-cast pearlite spheroidizing agent, but Sb spheroidizing is not recommended for important crank castings of QT700-2 or higher. In this case, the substrate should be controlled by adjusting the contents of Cu, Mo, and the like. It must be pointed out that no matter whether or not an as-cast spheroidizing agent is used, gestation is always an important part that cannot be ignored.
Heavy rare earth magnesium-containing spheroidizing agent is used for thick-walled ductile iron parts. In order to prevent spheroidization degradation and graphite distortion, the amount of Si should be lowered, and the method of increasing the number of graphite balls and refining the graphite sphere diameter should be adopted.
The cover bag treatment method greatly reduces the smoke pollution during the treatment, and is a simple process for saving the spheroidizing agent and ensuring the quality of the spheroidization, and is worthy of promotion. Inspired by the steel wire feeding technology, the feeding spheroidization has been successfully tested in enterprises such as the emerging casting pipe company. Feeding spheroidization is a process in which the alloy core wire is continuously fed to the bottom of the ladle by a wire feeder for spheroidizing treatment. The process can accurately control the amount of residual magnesium, and has the characteristics of high and low three, namely high spheroidization quality, less slag amount, less temperature drop and less pollution. The core wire sheath is a 0.3 mm thick cold-rolled steel sheet, and the core material is generally composed of Mg, 25 to 30%, and has a composite component of Ca, Ba, (RE) or the like. The cost of feeding is 20-40% lower than that of the flushing method. It is very attractive for large-scale production of cast pipe plants and automobile foundries. At present, a number of units in China have been able to supply a complete set of wire feeders and core-spun wires, including core-splitting, creeping, inoculation, desulfurization, carbon addition and alloying.
In-type spheroidization is to set up a reaction chamber in the runner, and the spheroidization reaction occurs when the molten iron flows. In-type spheroidized labor conditions, less oxidative loss, Mg absorption rate of up to 80%, overcome the spheroidization decline and gestation decline, improve the performance of ductile iron. In-type spheroidization requires strict requirements for the amount of sulfur in the molten iron, and the yield of the castings is low. It was used in China in the 1970s and is now not seen in production.
Inoculation
Inoculation is an important part of improving the quality of cast iron. After more than ten years of hard work, China's inoculants have basically achieved serialization and commercialization.
Nowadays, domestic inoculants are mainly silicon and carbon silicon.
The silicon inoculant has a low melting point and is suitable for various iron liquids with low carbon and low carbon, low sulfur and high sulfur, and is most widely used. The special ferrosilicon has strong inoculation effect and low dosage, which causes the fluctuation of the chemical composition of the molten iron to be small, and the quality of the cast iron is stable. FeSi-Ba-Ca, which has a large coverage, significantly increases the number of eutectic groups, inhibits D and E type graphite in gray iron, promotes the formation of type A graphite, and improves the spheroidization level. Ba, especially Ba and Ca coexist. When it comes to recession resistance. FeSi-Sr is an inoculant for thin-walled engine parts. It has excellent anti-white-mouth ability and does not significantly increase the number of eutectic groups. Therefore, it can eliminate the drawbacks caused by shrinkage. In the production of ductile iron, the effect of Sr is weakened due to the presence of Ce. The Zr-containing ferrosilicon has a similar effect to Ba-containing ferrosilicon and is weaker. However, Zr has degassing function and microalloying effect, which stabilizes the mechanical properties of cast iron. Zr ferrosilicon has a high melting point, so Zr-Mn often coexists to lower its melting point. High Ca inoculants, such as CaSi, have both desulfurization, deoxidation, anti-whitening and graphite nucleation. They were used in the 1950s and 1960s. Later, due to the improvement of coke and iron quality, CaSi has Rarely used alone, occasionally used in combination with FeSi or FeSiRE. The carbon (graphite) in the carbon silicon inoculant makes this inoculant very excellent in preventing white mouth. It is more suitable for producing gray iron parts with high corners and thin wall thickness from high temperature molten iron. Water-thin thin-walled parts are also available as an option when leak-proof requirements are required. In the production of ductile iron, carbon silicon inoculants should not be used.
The large dose of iron trough is replaced by various forms of postpartum breeding, which is the general trend of the development of inoculation methods. Post-pregnancy should pay attention to the size and uniformity of particle size of the inoculant, especially when the bag mouth is inoculated with the flow and the jet stream is inoculated, the particle size is too large, and the poor melting will produce defects such as hard spots and intergranular inclusions in the casting. When using high-efficiency inoculants, it is forbidden to breed excessively to prevent the occurrence of shrinkage, pores and graphite segregation.
There are many methods for gestation in the type. Most of the methods are used to place the block inoculant at the bottom of the sprue for over-current gestation. This method captures the nucleation peak of the gestation and can theoretically obtain the best gestation. effect.
5. Alloying
Common alloying elements for cast iron are Si, Mn, Cu, Cr, Mo, Ni, Sn, Sb, V, Ti, P, and the like. Among them, Si, Mn, Cr, Mo, and P are added in the form of iron alloy, Cu, Ni, Sb, and Sn are added in the form of metal, and V and Ti are added in the form of iron alloy or VTi pig iron. The timing of addition is determined by the melting point, oxidizability and density of the furnace after the furnace (the cupola), the furnace (induction furnace) or the furnace. In order to facilitate the melting of the high-melting-point ferroalloy, in addition to controlling the particle size, it is an ideal measure to use a heating alloying agent or a feeding method.
Practice has shown that a part of FeMn, FeCr and silicon inoculant are added together, which has a weighting effect on gestation. When possible, delaying the timing of the addition of low alloying elements is beneficial to performance.
Four filters
Metal filtration technology is widely used in industrialized countries. Practice has proved that the failure of key components is not entirely due to the mechanical properties of the test bars, but the soundness of the interior of the castings. Therefore, the pursuit of perfection of metallographic organization is not comprehensive. The factory that the author visited has not only paid attention to the final level of filtration. Generally, small pieces are made of fiber filter, and the flow rate is large or the important parts are honeycomb ceramic filter or ceramic filter. Compared with the filter, the filter has a stronger slag-resistance (the ceramic filter can remove fine inclusions of several micrometers to several tens of micrometers) and has obvious degassing effect. As a quest, a versatile filter that combines spheroidization – gestation – filtration is being tested.
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