How to smelt grey cast iron in medium frequency induction furnace?

In modern cast iron production, the cupola is being shut down gradually due to environmental protection problems, and most casting enterprises use medium frequency induction melting furnace to melt cast iron. Compared with cupola, the intermediate frequency furnace smelting process is relatively simple. The chemical composition and temperature of molten iron are easy to be controlled. The working environment and labor intensity of smelting in front of furnace are greatly improved. The production cost can be about the same as that of the cupola by using the trough smelting at night. For castings of the same chemical composition and cast in the same mold, the strength and hardness of gray iron melted in the intermediate frequency furnace are higher than those in the cupola furnace. Medium frequency molten iron has higher overheating temperature and poor fluidity than cupola molten iron, and has the following bad characteristics: the number of crystal nucleus of molten iron is small, the tendency of subcooling, white mouth and shrinkage is great, the thick wall of casting is easy to produce shrinkage cavity and porosity, the thin wall is easy to produce white mouth and hard edge and other casting defects. In the subeutectic gray cast iron, the amount of type A graphite is easily reduced, while the amount of D, E graphite, and its associated ferrite increases, and the amount of pearlite is small. All of these, together with some improper factors in daily production, are reflected in the fluctuation of casting quality, which affects the normal production of cast iron.

For medium frequency induction melting furnace smelting new problems in the gray cast iron, we overcame the electric furnace melting process, technical data, less practice, exploration is difficult, and many other difficulties, gradually grope and summary has accumulated some experience in production technology and experience, expect to rise level is difficult to operate and transition of small and medium foundry enterprises offer little help in pain.

1.The selection of raw materials and charge ratio

Burden fit and unfit quality directly affect the quality of molten iron, medium frequency furnace melting degree of grey iron for the burden of cleaning and drying the demand is higher, the charge is not clean, or melting control, containing harmful elements can result in molten iron oxide and purity is low, serious deterioration of molten iron metallurgy quality, organization, and influence of cast iron matrix graphite morphology, cause poor inoculation, white and large shrinkage tendency and a lot of stomata. Therefore, it is necessary to strengthen the management of raw and auxiliary materials, and strictly prohibit the use of heavy rust, a greasy burden. At the same time, in order to improve the purity of molten iron and to stabilize the chemical composition of molten iron, carbon steel scrap should be selected as the burden, and make it account for more than 50% of the burden ratio; For the return charge, casting riser of the same material should be selected, and the adhesive molding sand and paint should be cleaned before use. The usage amount should be about 40%. Scrap iron should also be the same material casting machine added scrap iron;For pig iron, because of the impurities and trace elements as well as the microstructure defects have inherited, should choose source stable, clean little embroidery, low harmful elements, it is best to Z18 above grade of casting pig-iron, such internal quality is good and stable production of pig iron casting, not to transform a source of iron, easily otherwise unqualified factors are for the use of the furnace charge and may cause quality problems will be available, and the addition of pig iron should join as well at the beginning of melting, ratio of 15%, to help improve the cast iron graphite morphology;Carburizer should choose commercial graphite carburizer or high temperature graphitized carburizer, and in the melting as early as possible, so that the carburizer and molten iron direct contact, and have enough time to melt absorption;Ferroalloy and inoculant should have qualified chemical composition and suitable particle size. The content of C, Si, Mn, and other elements should be calculated in advance according to the burden ratio and material composition, and the insufficient part should be adjusted with carburizer and ferroalloy. If C content is low, pig iron can be added to carburize. If C content is high, scrap steel can be added to reduce carbon.

2.The influence of chemical composition

Carbon and silicon strongly promote graphitization. Higher C and Si will lead to graphite coarsening, increased ferrite content, decreased pearlite volume, and decreased strength and hardness of cast iron. The strength of the cast iron matrix increases with the increase of pearlite volume. Therefore, in high-strength grey iron, the contents of C and Si should be appropriately reduced within a certain range, which is conducive to refining graphite, promoting the formation of pearlite, and improving the mechanical properties while ensuring the gray opening. The CE and Si/C ratios of carbon equivalent significantly affect the microstructure and properties of gray iron. It is beneficial to select the appropriate CE and Si/C ratios to improve the microstructure and properties of cast iron.CE is the most important factor affecting the inner quality of gray iron castings. CE raising can greatly improve the casting performance of cast iron, reduce whiteness, shrinkage hole, shrinkage porosity, and leakage defects, and reduce the rejection rate, which is particularly important for thin-walled cast iron. However, if CE is too high, the amount of graphite precipitation increases and ferrite tendency is obvious, which will reduce the tensile strength and hardness of the casting. Due to slow cooling speed, the thick wall of the casting is prone to produce coarse grain and loose structure defects. If the CE is too low, the thin wall of the casting is prone to form a local hard zone, resulting in poor machining performance. Due to low CE, eutectic leite and D – and E-type supercooled graphite are easy to appear in gray iron structure, which results in reduced casting performance, increased sectional sensitivity, increased internal stress, and increased hardness. The strength and cutting performance of cast iron can be improved by increasing the Si/C ratio appropriately. Under the same conditions, the mechanical properties and microstructure of cast iron are greatly different with different Si/C ratios. When CE is constant, the Si/C value increases from 0.6 to 0.8, and the strength and hardness of gray iron peaks. When Si/C value is constant, the strength and hardness of gray iron decrease with the increase of CE. The appropriate Si/C ratio should be selected and controlled while the CE is strictly controlled at the production site. The CE and C content of grey iron smelted by intermediate frequency furnace should be about 0.3% higher than that of the cupola, and the Si/C ratio should be controlled around 0.6~0.7, so as to maintain appropriate hardness and high tensile strength of cast iron.

Manganese and sulfur are elements that stabilize pearlite and hinder graphitization. Manganese can promote and refine pearlite. The increase of manganese content can improve the strength and hardness of cast iron as well as the pearlite content in the structure. Manganese and sulfur also form compounds with high melting points that serve as heteronuclear and refine grains, so manganese is used more often in high-grade gray iron. However, the high manganese content also affects the nucleation of molten iron when it crystallizes and reduces the number of eutectic groups, leading to the thickening of graphite and the formation of supercooled graphite, which will also reduce the strength of cast iron. Sulfur is a limiting element in gray iron, and an appropriate amount of sulfur plays an active and beneficial role in the nucleation and growth of graphite, which can improve the breeding effect and machining performance of gray iron. In order to ensure the breeding effect of grey iron smelted by intermediate frequency furnace, w(S) is generally required to be ≥0.06%, and the content of S is properly increased, which can improve the morphology of graphite, refine eutectic group, shorten the length, bend the shape and blunt the end of flake graphite, weaken the fracture and destruction effect of graphite on the matrix, and thus improve the performance of cast iron. So sulfur is not as low as possible in gray iron. And phosphorus in gray iron is a harmful element, easy to form low melting point phosphorus eutectic in the grain boundary, resulting in cold cracking of cast iron. Therefore, the lower the phosphorus in gray iron is generally the better. For cast iron with density requirements, the phosphorus content should be lower than 0.06%.

In actual production, the chemical composition design should be optimized according to the grade, wall thickness, structural complexity and other factors of gray iron castings, and the fluctuation range of each element should be strictly controlled, which is very important to ensure the quality and performance of gray iron castings.

3.Process, quality control, and improvement of grey iron smelting in intermediate frequency melting furnace

3.1 Control of carburizing rate and use of carburizing agent

For intermediate frequency smelting furnace smelting gray iron, many people think that as long as the furnace before the control of the chemical composition and temperature of molten iron, you can melt high-quality molten iron, but the fact is not so simple. The most important task of smelting gray iron in an intermediate frequency furnace is to control the core function of carburizer, and the core technology is carburizing iron water. The higher the carburizing rate is, the better the metallurgical properties of molten iron are. The carbon increment rate mentioned here is the carbon added in the form of the carburizer in the molten iron, not the carbon brought in the charge. The production practice shows that the ratio of pig iron in the charge ratio is high and the tendency of white mouth is great. The proportion of carburizer increases and the tendency of white mouth decreases. This requires that more cheap scrap steel and return charges should be used in the ingredients, and less or no new pig iron should be used. This kind of waste steel carburizing process exists A large number of dispersed heterogeneous crystal nucleus, which reduces the supercooling degree of the molten iron and promotes the formation of graphite structure dominated by A-type stone ink. At the same time, the decrease of pig iron dosage also reduces the bad genetic effect of pig iron thick graphite, and the performance of gray iron also increases with the increase of scrap steel dosage. In the actual production, it has been found that in the case of scrap steel consumption of about 30%, the same use of scrap steel, return charge, new pig iron as the burden, in the same chemical composition is basically the same, if the furnace smelting of gray iron is lower than cupola smelting performance, strengthening the inoculation effect is not obvious, this is the scrap less, low carbon rate. This shows the importance of increasing carbon to ensure the smelting quality of grey iron and improve the microstructure and properties of cast iron.

The properties of gray iron are determined by the matrix structure and the morphology, size, quantity, and distribution of graphite. In comparison, the matrix structure is easier to control, and it mainly depends on the chemical composition of the molten iron and the cooling rate. However, the graphite morphology is not easy to control. It requires the degree of graphitization of molten iron. The strange thing is that only the new carbon takes part in graphitization, and the original carbon in the furnace does not. Without the use of carburizer, although the chemical composition of molten iron is qualified, the temperature is appropriate, the breeding is reasonable, but the molten iron performance is poor: seemingly high temperature, the flow is not very good, shrinkage, the loose tendency is large, easy to breathe, easy to produce white mouth, section sensitivity is large, iron inclusions. These are caused by the low carburization rate and graphitization degree of molten iron.

Carbon in the form of the original molten iron mainly for small graphite and carbon atoms, considering from the perspective of refined graphite, not wishing to have too many in molten iron of the carbon atoms, it will reduce the number of graphite core, and carbon atoms in the cooling process are easier to form cementite, and fine small graphite can be as heterogeneous nucleation core directly. Refining graphite and increasing core are the keys to achieve high performance of cast iron. Increasing the amount of carburizer can increase the number of nucleating core, thus laying a solid foundation for refining graphite.

Therefore, in the actual production should emphasize the use of carburizing agent and carburizing effect: The absorption rate of carburizing agent is directly related to its C content, the higher the C content, the higher the absorption rate.(2) the size of the carburizer is the main factor affecting the molten iron, the practice has proved that the size of carburizer should be 1~4mm for good, there are micro powder and coarse carburizing effect is not good.(3) Silicon has a greater impact on the effect of carburizing, high ferric silicon water carburizing is poor, carburizing speed is slow, so ferric silicon should be added after the carburizing in place, to follow the principle of carburizing after the silicon. (4) sulfur can hinder the absorption of carbon, high ferric sulfide water than low ferric sulfide water increases the rate of carbon slows a lot. (5) Graphite carburizer can improve the nucleation ability of molten iron, and the absorption rate is more than 10% higher than that of non-graphite carburizer, so low nitrogen graphite carburizer should be selected. Carbonizing agent is recommended to use with the furnace loading method, that is, in the bottom of the furnace to add a certain amount of small pieces of furnace feed and scrap, and then add the carburizing agent according to the number of ingredients need to be all, and then press a layer of small pieces of waste steel and pig iron, after melting and charging.This method is simple and easy, with high production efficiency and absorption rate of up to 90%. If a large amount of carburizer is added, it can be added in two batches, first, add 60%~70% to the bottom scrap pad, the rest is added in the process of continuing to add scrap.Carburizer can also be added when the temperature of molten iron is 1400~1430 ℃. The goal is to increase the content of molten iron C to reach the upper limit of grade requirements.7 add carbon agent time can not be too late, in the smelting late add carbon agent has two adverse aspects: one, carbon agent is easy to burn, the carbon absorption rate is very low. Secondly, the addition of carburizer in the later stage requires additional melting and absorption time, which delays the adjustment of chemical composition and heating time, reduces production efficiency, increases power consumption, and may cause harm due to excessive heating.8 the stirring of iron can promote carbon, especially attached to the wall of the stone ink group, if not excessive temperature and a certain time of iron heat preservation, not easy to dissolve in iron, medium frequency furnace strong electromagnetic stirring to carbon.

3.2 Temperature control

The melting temperature of gray iron should not be too high, generally controlled below 1400℃. If the melting temperature is too high, the burning loss or reduction of the alloy will affect the composition adjustment at the later melting stage. After the furnace temperature reaches 1460℃, the sample is quickly tested, then the slag is scraped, and the remaining burden such as iron alloy is added. Slag temperature has a great influence on the quality of molten iron. It is closely related to the stable chemical composition and inoculation effect, and directly affects the control of the temperature. If the slag temperature is too high, it will aggravate the burning loss of molten iron graphite crystal nucleus and the reduction of silicon, which is on the high side (in the acidic lining), and produce the carbon expulsion effect, affecting the stable system crystallization. If the slag temperature is too low, the molten iron is exposed for a long time, and The burning loss of C and Si is serious, the composition shall be adjusted again, the smelting time shall be prolonged, the molten iron shall be overheated, the degree of supercooling shall be increased, and the composition is easy to get out of control and destroy the normal crystallization.

The control of discharging temperature must ensure the best temperature of inoculation treatment and pour. Generally, the discharging temperature should be controlled according to the actual situation at 1460 ~ 1500℃, the overheating temperature can be controlled at 1510 ~ 1530℃, and stand for 5 ~ 8min.In the range of 1500 ~ 1550℃, increasing the overheating temperature of molten iron and extending the standing time at high temperature will refine the graphite and matrix structure, improve the strength of cast iron, which is conducive to inoculation treatment and eliminate the adverse effects of porosity, inclusion defects and charge heredity on the structure and performance of cast iron. If the standing temperature is too low and the time is too short, the carburizer cannot be completely dissolved in the molten iron, which is not conducive to the impurity of molten iron floating up to be removed by slag. However, if the overheating temperature is too high or the standing time at high temperature is too long, the graphite shape will be deteriorated, the matrix will be roughened, the supercooling degree will be increased, and the white tendency will be increased. As a result, the existing heterogeneous core of molten iron will disappear, and the oxidation will be serious, which will reduce the performance of cast iron and affect the control of the temperature of discharging. If the oven temperature is too high, the white depth of the pouring triangulated block will be too large or there will be a hemp mouth in the center, despite the moderate C and Si content. If this is the case, the need to reduce the intermediate frequency power, to the furnace to add pig iron cooling carbon.

Casting temperature should not be high, or it will cause serious sand defects in the casting, and some are even difficult to clean and make the casting scrap. Besides, high casting temperature and a high degree of supercooling are not conducive to the formation of A-type graphite. If the casting temperature is too low, it is not conducive to degassing, and it will also cause the casting to be hard and cold insulation, unclear outline, and other problems. At a lower casting temperature, the liquid molten iron shrinks less, which helps to reduce the shrinkage cavity and obtain a dense casting. Castings of different wall thickness and different weight have a different ideal pouring temperature, which is generally controlled at 1450 ~ 1380℃ in daily production. For thick and large castings, it is necessary to ensure that “hot out, low temperature fast pouring”.In order to shorten the waiting time for the molten iron temperature to drop to the pouring temperature and prevent the incubation decline, the hot metal can be rapidly cooled by the method of inverted bag and standing, so as to prevent shrinkage and loosen and improve the production efficiency.

3.3 Control of sulfur and nitrogen

There is no sulfur source and the S content of molten iron in intermediate frequency furnace is low, which has a great advantage in producing ductile iron. But for gray iron, low sulfur and high manganese will increase the casting stress, greatly increasing the probability of crack occurrence, and the appropriate amount of sulfur in molten iron can improve the inoculation effect. In the past cupola production of gray iron, because coke will sulfur on molten iron, do not worry about low sulfur. Intermediate frequency furnace to produce gray iron, but not add sulfur, but also because of the large use of scrap steel, the S content is lower (about 0.04%).In gray iron, W (S)≤ 0.06% will lead to poor graphite morphology, difficulty in breeding, shrinkage, and white mouth tendency. In the past production, it was found that most of the castings with cracks and white mouth defects were made of D and E type graphite. In order to obtain normal graphite morphology, the suitable S content, low sulfur and sulfide content, the number of crystal nucleus will be reduced, the graphite nucleation ability will be reduced, the white hole will be increased, the A-type graphite will be reduced, the D-type and E-type supercooled graphite and ferrite will be increased, and the grain size and strength will be reduced. Moreover, with the extension of the heat preservation time of high-temperature molten iron, the subcooling degree continues to increase. The higher the grade of gray iron, the more significant the influence of heat preservation temperature and time on the subcooling degree is. It is pointed out that the content of molten iron is low and the number of eutectic clusters is small. With the increase of S content, the number of eutectic clusters increases sharply. The more the number of eutectic clusters, the smaller the size is, the better the mechanical properties of cast iron are. Therefore, intermediate frequency furnace smelting grey iron tend to increase the S content to 0.06% ~ 0.1%, the beneficial effects of sulfur into full play, improve the inoculation effect, increase the number of nucleation of molten iron, castings microstructure is given priority to with A type of graphite, the matrix organization pearlite content, thus improve the strength and machinability of cast iron. The specific method is to add FeS to increase sulfur after the composition is adjusted in the later stage of smelting. Some coke is also used as a carbonizing agent to increase the content of S to more than 0.06% at the same time. However, S content should not be too high, because sulfur is a hindrance to the graphitization element, too high will increase the white mouth. Moreover, when S content is high, with the increase of Mn content, the MnS generated fully plays the role of heterogeneous nucleation, creating conditions for good breeding. However, when Mn content is greater than 1%, too many MnS are formed and concentrated in the grain boundary, which weakens the grain boundary and even produces slag inclusion, reducing the strength of cast iron. From the Angle of reducing MnS slag inclusion, S content should be controlled to be less than 0.1%, so as to allow a higher manganese content, which is beneficial to improve the performance of gray iron.

As a large number of scrap steel is used for smelting gray iron in an intermediate frequency furnace, and with the increase of scrap ratio, the amount of carburizer also increases. In addition, the carburizer has higher nitrogen content, so the N content of molten iron in an intermediate frequency furnace is relatively high. When N content in the molten iron is greater than 100×10-6, the casting is prone to turtle crack, shrinkage porosity, and cracked subcutaneous porosity. The most effective way to control the N content in hot metal is to keep the hot metal at a high temperature. With the extension of time, the N content will gradually decrease. However, high temperature hot metal for a long time will increase the degree of supercooling and white mouth tendency, so the daily production should use low N content of graphite carburizer.If necessary, 10% iron oxide powder may be added to the coating to eliminate the effects of high nitrogen. But as belong to limit the grey of nitrogen and sulfur elements, trace amounts of nitrogen in molten iron can make grey iron grain and eutectic group refinement, pearl dimension increased in the matrix, and the mechanical properties improved, to improve the grey iron graphite morphology, promote matrix was pearlite can play a positive role, nitrogen compounds can also serve as crystal nucleus, to create growth conditions for graphite nucleation. In actual production, N content should be generally controlled below 0.008%.

3.4 Enhanced inoculation treatment

Inoculation, add a lot of artificial crystal core, forcing the cast iron in eutectic solidification under controlled conditions, its purpose is to promote the graphitization, reduce chilling tendency and sensitivity of the area, control the graphite morphology, reduce the undercooling graphite and pig iron, ferrite, increasing number of the eutectic group, promote the formation of pearlite, thus improve the strength of the cast iron and machining performance. In the actual production, it is necessary to select the appropriate inoculant and inoculating method. For high temperature hot metal with CE between 3.9% ~ 4.1% and temperature around 1480℃, it is necessary to use an efficient inoculating agent to strengthen inoculating, so as to get gray iron castings with good casting properties and high mechanical properties, rather than increase inoculating amount. Different inoculants have different characteristics, so inoculants and inoculants should be selected according to the characteristics of inoculants and their own production conditions.After selecting and determining the treatment method which is most suitable for the characteristics of the enterprise, the technological process should be strictly controlled to ensure the stability of casting quality.

Except the stream joins the nucleating agent, and with the flow of time, for the amount of control to prevent a pregnant recession, improve the effect of inoculation and pay attention to the following aspects: (1) because of the limitation of melting temperature and holding time, bulky of graphite in cast iron piece can’t completely cancer, not dissolve as bulky graphite shape will be passed on to cast iron, greatly to offset the effect of inoculation and so on should try to reduce the amount of iron in the process of production, to eliminate iron hereditary, improve the inoculation effect, improve the performance of gray iron. (2) should be selected containing calcium, aluminum, refractory heterogeneous nucleating core inoculants, and control inoculants have the appropriate size because the size of inoculants on the effect of inoculants is very large. The particle size is too fine, easy to be oxidized into slag, and lose its function; The particle size is too large, inoculant melting is not enough, not only can not give full play to the inoculant function but also will cause partial analysis, hardpoint, supercooled graphite defects. The particle size of the inoculation agent is generally controlled in 3 ~ 8mm (iron water content of less than 1 ton), and the inoculation amount is controlled in about 0.3% ~ 0.5% of the weight of molten iron. The shrinkage and slag inclining tendency of cast iron will be increased by too much inoculation. (3) Multiple inoculations can effectively prevent inoculation recession, improve the graphite distribution uniformity inside cast iron, reduce the overcooling tendency of iron, make A type of graphite occupy high, moderate length, and promote the number of non-spontaneous crystal nucleus increase, refine grain, strengthen the matrix, improve the strength and performance of cast iron. For example, the selection of silica-Barium long-term inoculant with A strong ability to promote graphitization in secondary pregnancy can improve the morphology and distribution of graphite in thin-walled castings, increase eutectic groups, promote the formation of A-type graphite, eliminate supercooled graphite, inhibit the production of free cementite, and slow down the inoculating decline.(4) The influence of molten iron temperature on the inoculation is in a certain range to increase the overheating temperature of molten iron, and keep the appropriate time, can make the remaining molten iron undissolved graphite completely dissolve into the molten iron, eliminate the influence of genetic factors, give full play to the role of inbreeding agent, improve the fertility of molten iron. The superheat temperature should be increased to 1520℃, and the inoculation temperature should be controlled at 1460 ~ 1420℃.

3.5 Adjustment and improvement of process technology

(1) Process operation sequence of grey iron smelting in induction melting furnace: small return charge and scrap steel + graphite carburizer + scrap steel and new cast iron + return furnace charge + ferroalloy + suitable inoculation. In order to improve the ill effects of molten iron at high temperature and long heat preservation, based on the medium frequency furnace temperature is easy to improve, and the advantage of smelting, fast, fast melting fast process operation method, try to shorten the melting time, melting speed, make the hot metal in the furnace factually, as soon as possible after adjusting chemical composition, temperature and speed up the casting speed, strive to finish casting around 5 min, the biggest limit shorten the molten iron in the furnace and the holding time.

(2) Slag inclusion has a great influence on casting quality. To a lesser extent, fine slag inclusion can split the matrix and reduce tensile strength. Serious slag inclusion defects can directly lead to castings being scrapped. After the melting of the charge with more slag in it, the slag attached to the wall of the furnace and the slag in the molten iron are stirred by the electric furnace

The buoyancy effect of mixing molten iron continuously rises, in the later smelting stage needs to frequently, the efficiency picks up the slag, especially when the high temperature stands the impurity rises, should pick up the slag in time, until the molten iron surface is clean, does not add the scum, this to remove the slag, eliminates the slag hole flaw, reduces the slag to the matrix to break up the function is very big.

(3) Because a large number of scrap steel and back furnace iron are used in the smelting of gray iron in induction melting furnace, on the one hand, it will promote the generation of cast iron dendrite ink and the increase of the tilt of the white mouth, the increase of hardness, and the deterioration of the processing performance. So than cupola molten iron should pay more attention to the inoculation, to promote the graphitization, refine the eutectic group, change the graphite morphology, reduce chilling tendency, make white or mottled into fine pearlite, D, E type graphite into A type of uniform distribution of graphite, improve the uniformity of the casting wall thickness difference organization, to improve the performance of cast iron. On the other hand, with the increase of scrap steel consumption, the content of molten iron S becomes lower, and when W (S)≤ 0.06%, it is easy to cause breeding difficulties. Generally, the inoculation treatment with FeSi75 is not obvious, so sulfur-increasing measures should be taken.

(4) Thin-walled castings have serious white mouth defects, machining difficulties, and a high rejection rate. Solve the outstanding problem first to stop using scrap steel, appropriate to improve the CE, and control process before the Si content of molten iron in more than 1.6%, S content is greater than 0.06%, increasing amount of pregnancy) to 0.5%, and make molten iron nucleation number increases, the graphite shape nuclear capabilities enhance, promote the formation of A-type graphite, D, E graphite was subdued, a pearl in matrix organization size increases, the degree of the supercooling and chilling tendency of cast iron is reduced, the strength and machinability of improvement. The key to improving the processing property of gray iron is to control the microstructure of gray iron properly. If necessary, 2% clean and rust-free pig iron can be added into the package before the iron is released to effectively increase the graphite particles and eliminate the whiteness.

4.A view on improving the quality and performance of grey iron castings

It is known to all insiders that the performance and finish of domestic and imported castings with basically the same chemical composition and metallographic analysis are quite different. The imported castings with the same carbon equivalent are 1 ~ 2 grades higher than domestic castings. The cutting performance of imported castings whose hardness is higher than that of domestic castings is better than that of domestic castings. These phenomena are caused by the high purity and carbon equivalent of imported castings, fewer inclusions and free carbides, and good uniformity of microstructure.

The inherent quality, appearance quality of cast iron, and whether there will be casting defects are closely related to the various factors of molten iron. High-quality molten iron is the most basic and important prerequisite for obtaining high-quality castings. The quality of molten iron is determined by the temperature, chemical composition, and purity of molten iron. It is very easy to obtain molten iron with high temperature and precise chemical composition higher than 1500℃ by smelting gray iron in an intermediate frequency furnace. Each element in molten iron has a certain influence and effect on solidification crystallization, microstructure, and properties of cast iron. The superheated temperature of molten iron directly affects the composition and purity of molten iron, and the improvement in a certain range can make the graphite refined, the matrix structure dense, the tensile strength increased, the casting property improved, and the impurities in molten iron more easily to float up and be removed by the slag. Only the purity of molten iron, so far still stays in the high-temperature smelting, slag, filter screen on these levels. In fact, experts in the industry understand that it is difficult to obtain high clean molten iron through these measures, can only make the situation improve, and for the deep purification of molten iron, the occurrence mechanism analysis, and prevention of casting defects but little research, rarely countermeasures. All kinds of harmful gases and nonmetallic inclusions in molten iron remain in the casting after solidification, causing various casting defects and affecting the performance of castings; The hard particles formed by non-metallic inclusions lead to the difficulty of casting machining. And the impurity harmful elements in molten iron directly affect the structure and performance of the casting. It is these factors that make the comprehensive quality of domestic castings lower than that of imported castings for a long period. Therefore, we should improve the quality of molten iron metallurgy, efforts to obtain low content of harmful elements and gas, high clean molten iron, for the purpose of small debris, based on the grey iron medium frequency furnace melting process, we will further improve the modern molten iron purification technology and technological process, to ensure that must be of high purity iron for casting, thus to ensure the high quality and high performance of castings.

5.conclusion

(1) If the medium frequency induction melting furnace smelting gray iron, scrap steel must have a certain ratio, generally should account for more than 50% of the charge. Low nitrogen graphite carburizing agent should be selected and a high carburizing rate should be guaranteed in order to obtain high-quality molten iron with good graphitization degree, white mouth, and small shrinkage tendency. At the same time, a large number of scrap steel and furnace iron, less or no new pig iron, to eliminate the genetic impact of thick graphite. The production cost can be greatly reduced by using the price difference between pig iron and scrap steel and smelting at the trough of electricity price at night.

(2) If the content of S in the molten iron is generally low, sulfur measures should be taken to increase the content of S in molten iron to 0.06%~0.1%, increase nuclear capacity, increase the number of crystal nuclei and pearlite content, improve graphite morphology, refine graphite, promote the formation of A-type graphite, improve the breeding effect and cutting performance, and improve the strength.

(3) through the adoption of scrap steel carburization process + increasing CE and Si/C than + fast the operation method of fuse, strengthening production technology such as inoculation, control of hot metal overheating temperature at 1510 ~ 1530 ℃, the temperature at 1480 ~ 1500 ℃ oven, to reduce casting defects, increase performance, strong ash to improve the quality of molten iron and casting quality, reduce scrap rate.

(4) The quality of molten iron is an important factor affecting the quality of cast iron. Without high-quality molten iron, it is impossible to have high-quality castings. In order to ensure the high quality and high performance of grey iron castings, it is necessary to improve the purity of molten iron on the basis of the current smelting process of grey iron in intermediate frequency furnace and further perfect the modern purifying technology and technological process of molten iron.

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