CN116037868A - Increase the processing technology of spherical graphite in malleable cast iron and cast iron parts - Google Patents

Abstract

The invention provides a processing technology for adding spheroidal graphite in malleable cast iron and an iron casting, wherein the processing technology comprises the following steps: adding raw materials into an electric furnace to smelt to obtain base molten iron, pouring the base molten iron into a ladle, and adding an alterant in the ladle in advance, wherein the weight ratio of the alterant is 0.8-1.8%; pouring molten iron into a sand mold to obtain a green blank of a white gray; tamping and annealing the gray-white green body; the modifier is rare earth magnesium silicon alloy, and each material after the original molten iron is modified comprises the following components in parts by mass: 92-95% of iron, 2.8-3.8% of carbon, 1.8-3.8% of silicon, less than 0.6% of manganese, less than 0.2% of sulfur, less than 0.1% of phosphorus, less than 0.055% of magnesium and less than 0.1% of chromium; the iron casting is manufactured by adopting the process. The invention has the advantages that: the strength and toughness of the green body are improved, the green body is prevented from being broken when stamping is performed, the annealing process temperature is low, and the time is short; the production energy consumption can be reduced, and the production cost can be saved; the proportion of the cavity in the sand mould is increased, the utilization rate of molten iron is increased, the return loss is reduced, and meanwhile, the stamping difficulty is not increased.

Description

Increase the processing technology of spherical graphite in malleable cast iron and cast iron parts

technical field

The invention belongs to the technical field of malleable cast iron, and in particular relates to a processing technology for increasing spherical graphite in malleable cast iron and iron castings.

Background technique

Common gray cast iron can be divided into: ordinary gray cast iron, malleable cast iron, nodular cast iron, vermicular graphite cast iron, etc. according to the shape of graphite. Graphite in ordinary gray cast iron is in the form of flakes. The content of carbon and silicon is generally controlled at 2.5%-4.0% of carbon and 1.0%-3.0% of silicon. It has good casting performance and thermal conductivity, the largest total output, and a wide range of uses.

Graphite in ductile iron is spherical, which is obtained by spheroidizing gray cast iron molten iron. The comprehensive mechanical properties are close to steel, and the tensile strength and fatigue resistance are higher. The composition requirements are the most stringent. The carbon content of ductile iron is relatively high. High, usually within the range of 4.5%-4.7%, in order to facilitate graphite spheroidization. The tensile strength of ductile iron is much higher than that of gray cast iron, comparable to that of steel, and its production cost is relatively high. It is mainly used for important parts such as crankshafts, connecting rods, and camshafts. Graphite in vermicular graphite cast iron is worm-like, its strength is close to that of nodular cast iron, and it has high wear resistance. It is mainly used in the production of castings such as cylinder heads, cylinder liners, steel ingot molds and hydraulic valves.

Graphite in malleable cast iron is flocculent, which belongs to the cast iron with the strongest plasticity and impact toughness in gray cast iron. It is mainly used to manufacture parts with complex shapes and shock and vibration loads, such as: pipe joints, low-pressure valves, etc.; 90% of malleable cast iron Both left and right are black-core malleable cast iron. Compared with other types of gray cast iron, black-core malleable cast iron has general requirements for tensile strength and wear resistance. The processing process is mainly to make white green body from raw molten iron, and then undergo graphitization annealing. (heat treatment) to prepare.

Almost all the carbon in the white green body exists in the form of cementite, only a very small amount of graphite structure, the fracture is bright white, high hardness, poor plasticity and toughness, and very brittle.

Malleable cast iron must go through the processing of white blanks, because the parts to be produced are small in size, complex in shape and structure, and have a large demand. Therefore, casting molds for mass production of parts in industry usually have multiple cavities (6 ~92), each cavity is connected through the sprue and the pouring riser (that is, the pouring riser system) to increase production and reduce production costs; after the molten iron is rapidly cooled in the mold to form a green part, it is removed from the mold The mold still needs to go through the process of stamping, inspection, and transportation. The green parts are removed from the pouring system by manual stamping. The finished product is black-heart malleable cast iron with strong impact toughness; the rest of the sprue, pouring riser, etc. are reused as recycled materials.

Therefore, the key intermediate product of green parts must have high brittleness to be easy to stamp, so it must be a white green body; but at the same time, if the parts of the white green body are too brittle, it is easy to occur during the stamping process. Brittle fracture produces various explicit or hidden cracks, resulting in a high green body scrap rate; especially the existence of invisible cracks is mostly difficult to detect, making the processed finished products prone to breakage during use, affecting the sales and use of finished parts.

At the same time, the more complex the shape of the part, the greater the difference in thickness at different positions of the part. In the process of molten iron pouring, the cooling rate at different positions in the part mold will inevitably be inconsistent, which will increase the degree of supercooling of the white green body and excessive internal stress. , prone to shrinkage and porosity, and also lays hidden dangers for the generation of hidden cracks.

Before 1998, most of the equipment for producing malleable cast iron was a cupola. In the cupola, there was only free oxygen in a limited area near the tuyere. With the rise of the furnace gas and the reaction of coke, the free oxygen was quickly exhausted, and the Generally speaking, the atmosphere in the furnace is non-oxidizing, so it is easy to obtain molten iron with excellent properties when smelted in a cupola furnace. The molten iron has high fluidity and low shrinkage. However, after 1998, due to environmental protection issues, the cupola was gradually phased out, and the equipment used to produce malleable cast iron was changed to an electric furnace. Electric furnace smelting molten iron is prone to oxidation of molten iron and oxygen absorption during the smelting process, resulting in poor quality of molten iron, poor fluidity, and more likely to cause shrinkage. ; Therefore, it is usually necessary to add a large amount of metal bismuth to increase the whitening tendency of the green body. Due to the high price of bismuth, the production cost also increases.

Therefore, under the background of the existing electric furnace as the molten iron production equipment, the important issues in the processing of malleable cast iron are to control the brittleness of the white green body and prevent the shrinkage of the molten iron, so as to solve the hidden danger of the breakage of the white green body. At the same time, it is necessary to control the production cost.

Patent application CN113403526A discloses a white molten iron, white green body, cooked body and its production method and application. It adopts the technology of strictly controlling the silicon content in the original molten iron, and then adding ferrosilicon alloy in the outer bag of the molten iron furnace, and then adding Silicon can increase the non-spontaneous core of graphitization in the original molten iron. Under the condition of ensuring that the overall content of C and Si is within a reasonable range, the fine graphite in the molten iron can be expanded to prevent the shrinkage of the molten iron and improve the quality of the white green body. The strength, toughness and plasticity; this method reduces the addition of scarce and expensive bismuth iron, reduces the pollution of bismuth iron to the environment, and saves production costs.

However, during the implementation of the above method, the weight percentages of carbon and silicon in the original molten iron need to be strictly controlled, and the element content in the original molten iron needs to be strictly detected for each smelting, which increases the difficulty of implementation; Intensity has not yet completely eliminated the hidden danger of cracking of the blank green body. In the case of manual tamping of parts on the green body of parts by mistake, certain invisible cracks will still occur and some defective products will be produced.

Contents of the invention

Aiming at the problems in the prior art that the molten iron smelted in an electric furnace is prone to shrinkage, the white green body is brittle and has hidden dangers of rupture, resulting in a high rate of defective products and high production costs, the present invention provides a method for increasing spherical graphite in malleable cast iron. Processing technology and iron castings.

The technical scheme provided by the present invention is as follows: a processing technology for increasing spherical graphite in malleable cast iron, comprising the following steps:

S1, adding raw materials into the electric furnace for smelting to obtain raw molten iron;

S2. Pour the smelted raw molten iron into a ladle, add a modifier in advance in the ladle, and the modifier accounts for 0.8-1.8% by mass of the molten iron in the ladle;

S3, pouring the molten iron in the ladle into the sand mold to obtain a gray-white green body;

S4. Tamping the gray-white green body, and ramming the part green body from the pouring system;

S5, annealing the green part;

The intermediate modifier in step S2 is a rare earth magnesium-silicon alloy, and the various substances in the molten iron after the deterioration of the original molten iron include the following components in parts by mass: 92-95% iron, 2.8-3.8% carbon, 1.8-3.2% silicon, manganese < 0.6%, sulfur<0.2%, phosphorus<0.1%, magnesium<0.055%, chromium<0.1%.

Furthermore, the silicon content in alpine regions is 1.8% to 2.7%.

Preferably, the amount of rare earth magnesium silicon alloy added in step S2 is 1.0-1.4%.

Further, in step S1, the smelting time of the raw molten iron is 55 min-80 min, and the smelting temperature is 1500-1600°C.

Further, the components in the rare earth magnesium silicon alloy include, by mass parts: 30-32% magnesium, 40-44% silicon, 2.5-4.0% calcium, and 2.5-4.0% rare earth.

Further, the particle size of the rare earth magnesium silicon alloy is 10-30mm.

Further, the annealing process in step S5 is as follows: firstly, the temperature is raised from room temperature to 720-750° C. at a rate of 100-140° C./h, kept for 2 hours, and then air-cooled out of the kiln.

Further, in step S3, the sand mold includes: a cavity and a pouring system; the volume ratio of the pouring system is 20% to 38%.

Further, the number of cavities in the sand mold ranges from 6 to 92 according to different types of casting products.

The present invention also provides an iron casting, which is manufactured by adopting the above-mentioned processing technology of increasing spherical graphite in malleable cast iron.

The advantages of the present invention are: the addition of the rare earth magnesium-silicon alloy prevents the shrinkage of molten iron, increases the strength and toughness of the green body, prevents the green body from breaking during stamping, and can accelerate the speed of graphitization during subsequent graphitization annealing. The annealing temperature is low and the time is short, and malleable cast iron products meeting the performance can be obtained; the production process of the present invention can reduce production energy consumption, save production costs, and produce malleable cast iron with a small portion of spherical graphite added; cooperate with the improvement of the sand mold pouring system , making the sprue thinner and the riser smaller, increasing the proportion of the cavity in the sand mold, increasing the utilization rate of molten iron, reducing the generation of returned materials, reducing the loss of returning furnace, and ensuring that the difficulty of stamping parts does not increase.

Description of drawings

Fig. 1 is the off-white green body of the present invention;

Fig. 2 is the white green body of traditional malleable cast iron;

Fig. 3 is the metallographic structure diagram of the sample that embodiment 1 obtains without polishing;

Fig. 4 is the metallographic structure figure after the sample that embodiment 1 obtains is processed through nital;

Fig. 5 is the metallographic structure diagram of the sample obtained in Comparative Example 1 without polishing;

Fig. 6 is a metallographic structure diagram of the sample obtained in Comparative Example 1 after being treated with nitric acid ethanol.

Detailed ways

In order to make the purpose, technical solutions and advantages of the embodiments of the present invention more clear, the technical solutions in the embodiments of the present invention will be clearly and completely described below in conjunction with the accompanying drawings in the embodiments of the present invention. Based on the embodiments of the present invention All other embodiments obtained by persons of ordinary skill in the art without creative efforts fall within the protection scope of the present invention.

The processing technology of adding spherical graphite in the malleable cast iron of the present invention includes two directions, on the one hand, it increases the content of fine graphite in the green body, prevents the shrinkage of molten iron, increases the strength and toughness of the green body, and prevents the green body from breaking when stamping. And the speed of graphitization can be accelerated during subsequent graphitization annealing (heat treatment). Combined with special annealing process, production energy consumption can be reduced, production cost can be saved, and malleable cast iron with a small portion of spherical graphite can be produced; on the other hand, it is necessary to cooperate with the The improvement of the pouring system of the sand mold makes the sprue thinner and the riser smaller, that is to say, the proportion of the cavity in the sand mold increases and the utilization rate of molten iron increases, which reduces the generation of returned materials and reduces the loss of returning to the furnace. The difficulty of the piece does not increase. The two aspects must cooperate with each other, which can greatly reduce the production cost while ensuring the quality of malleable cast iron products.

For the first aspect, the preparation method of malleable cast iron provided by the present invention, the steps are as follows:

S1. Add raw materials into the electric furnace for smelting to obtain raw molten iron;

S2. Pour the smelted raw molten iron into the molten iron ladle, and add the molten iron modifier in advance in the molten iron ladle. The molten iron modifier is a rare earth magnesium-silicon alloy, and the mass ratio of the modifier to the molten iron in the ladle is 0.8-1.8%; preferably 1.0 ~1.4%.

S3, pouring the molten iron in the ladle into the sand mold to obtain a gray-white green body;

S4. Tamping the gray-white green body, and ramming the part green body from the pouring system;

S5, annealing the green body of the part.

In the above step S1, the sources of molten iron raw materials are: steel scrap, pig iron, recycled materials, etc., the smelting time is 55min-80min, and the smelting temperature is 1500-1600°C.

The components in the rare earth magnesium silicon alloy in step S2 include by mass parts: 30-32% of magnesium (Mg), 40-44% of silicon (Si), 2.5-4.0% of calcium (Ca), 2.5-4% of rare earth (Re) 4.0%, and the balance is Cr, MgO, Fe and other elements, which are not particularly limited because of their small content. Rare earth magnesium silicon alloy particle size is 10-30mm. Rare-earth magnesium-silicon alloy is added to the original molten iron to cause a metamorphic reaction, and the various substances in the molten iron obtained by the modification include the following components in parts by mass: iron (Fe) content 92-95%, carbon (C) content 2.8-3.8%, silicon ( Si) content of 1.8-3.2% (need to be controlled at 1.8%-2.7% in alpine regions), manganese (Mn) content of less than 0.6%, sulfur (S) content of less than 0.2%, phosphorus (P) content of less than 0.1%, magnesium (Mg ) content is less than 0.055%, chromium (Cr) content is less than 0.1%.

In step S2, with the addition of the rare earth magnesium silicon alloy, Ca and Mg in it can react with the S in the original molten iron, consume the sulfur content in the molten iron, reduce the S that hinders the graphitization process, and the addition of Mg can also make The eutectic degree of molten iron increases slightly, which increases the fluidity of molten iron, facilitates pouring, and prevents the shrinkage of the green body; the addition of Si increases the non-spontaneous core of graphitization, and promotes the graphitization of C in the original molten iron; rare earths can adjust Si. The effect of promoting graphitization, adjusting the spheroidization rate and roundness of graphite, etc. However, the amount of rare earth magnesium silicon alloy should not be too high. If it is greater than 1.8%, the absolute amount of interference elements MgO, MgS, Re ₂ O ₃ , Re ₂ S ₃ produced by the reaction will increase, and the spheroidization tendency of graphite will be destroyed instead. , causing the green body to become brittle.

The addition of rare earth magnesium-silicon alloy can improve the strength and toughness of the green body of malleable cast iron as a whole, and at the same time make the green body have a certain amount of fine graphite, which is a kind of gray-white structure including cementite, pearlite, ferrite and graphite matrix Mouth raw body, as shown in Figure 1. The ordinary malleable cast iron white green body shown in Figure 2 is used as a reference. It can be seen that this kind of green body is a special green body between gray and white, which can not only meet the demand of being able to be rammed from the pouring system, but also increase the strength and toughness to prevent the ramming process. The green body breaks.

The annealing process in step S5 is as follows: first, the temperature is raised from room temperature to 720-750° C. at a rate of 100-140° C./h, kept for 2 hours, and then air-cooled out of the kiln.

In the present invention, the limit of C content in molten iron is higher than that of common malleable cast iron, and the range of Si content is wider (the C content in common malleable cast iron is 2.4～2.8%, and the silicon content is 1.2～1.8%). The green body of the intermediate product parts produced by the invention is not completely white, but a kind of grayish white, which requires a certain amount of carbon and silicon to increase the strength of the green body and ensure the shape of the graphite; at the same time, the use of malleable cast iron The requirements for plasticity and impact toughness are slightly higher, but the requirements for wear resistance are general, so there is no need to specifically limit the lower range of C and Si; but the content of C and Si should not exceed C 3.8%, Si 3.8% (alpine regions Do not exceed Si 2.7%, that is, the lower the temperature is in the region or season, the upper limit of the silicon content will decrease accordingly), otherwise the degree of expansion of graphite in molten iron is too high, which will also cause the matrix to be loose, resulting in graphite in the green body being more flakes In the form of existing, the gray mouth becomes gray mouth, the brittleness increases, the strength deteriorates, and it is easy to break.

The gray-white green body of the present invention has slightly lower requirements for annealing temperature (ordinary malleable cast iron is generally continuous two-stage or multi-stage annealing, and the first stage of multi-stage annealing is commonly used when the room temperature rises to 950-970 ° C and then heats it for 6- 8 hours, in the second stage, the cooling rate is 70-80 °C/h to 730 °C, and then the cooling rate is 20 °C/h to 570 °C, and finally air-cooled out of the kiln), this is due to the relative amount of graphite in the green body Slightly more, a lower annealing temperature can ensure that the graphite form between flocculent graphite and spherical graphite is formed in the obtained casting structure; it meets the finished product standard GB/T 9440-2010 of black-core malleable cast iron, that is, the tensile strength is greater than 300N/mm ² (MPa), the elongation rate is greater than 6.0%, and the flattening rate is greater than 8%. If the traditional malleable cast iron annealing process is adopted, although the casting performance will be higher and more nodular graphite will be generated in the casting structure, it is undoubtedly a kind of excess performance for malleable cast iron. The most important thing is this high temperature and long time annealing The process consumes a lot of electric energy, which is the reason for the high production cost. The present invention can achieve qualified malleable cast iron properties through this kind of annealing process with slightly lower temperature and shorter time, reduce the annealing temperature by about 230°C, save annealing time by 15-20 hours, greatly reduce production energy consumption, and save the cost of malleable cast iron. cost.

For the second aspect, due to the increased strength and toughness of the intermediate product part green body of the present invention, although it can still be pounded from the pouring system, compared with the traditional white malleable cast iron green body, the difficulty of pounding is still increased , so only adopting the first aspect of the process will increase the time of manual ramming and the fatigue of the ramming piece, which will also lead to a reduction in production efficiency; in addition, the content of C and Si in the molten iron obtained in the first aspect is higher than that of ordinary malleable cast iron. The addition of rare earth magnesium-silicon alloy also improves the fluidity of molten iron and prevents the shrinkage of the green body. Shrinkage, usually the diameter of the riser is 2.5 to 3.5 times the wall thickness of the part. In addition to the cavity in the sand mold, the remaining gating system volume generally accounts for 45 to 50% of the entire sand mold). There will also be a certain size surplus.

If the addition process of the molten iron component and the rare earth magnesium silicon alloy of the present invention is adopted, the volume of the whole gating system in the sand mold can be appropriately reduced to 20%-38%. This design improves the yield of sand molds, reduces the generation of recycled materials, and improves the green body production. At the same time, because the pouring system becomes thinner and smaller, it is convenient for manual tamping, and solves the gray hole of the present invention. The problem of increased difficulty in stamping blanks.

For the specific size reduction form of sand molds, those skilled in the art can adjust parameters such as runner size, riser diameter and height, and ingate length in the gating system within the scope of relevant standards according to different product models. Only the principle of reduction design is provided, and the specific design form is not limited.

So far, the combination of the two malleable cast iron processing methods can more effectively solve the problem of brittle fracture of the original white green body. When the product meets the performance standard, the processing cost is greatly reduced and the production efficiency is improved. According to calculations, the cost of producing one ton of malleable cast iron parts according to the present invention can save 100-250 yuan.

The present invention will be described in detail below through several groups of specific examples.

The type of rare earth magnesium silicon alloy used in all embodiments is 13mm, the content of magnesium is 30.1%, the content of silicon is 43.5%, the content of calcium is 3.6%, and the content of rare earth Re is 2.7%. Test the tensile strength, elongation and flattening rate of the finished product. Test instruments include: microcomputer multi-element analyzer, hydraulic testing machine, metallographic microscope.

Example 1

The processing technology of increasing spherical graphite in malleable cast iron includes the following steps:

S1. Add raw materials into the electric furnace for smelting, and smelt 3000kg of molten iron;

S2. Pour the smelted raw iron into a 500kg ladle, and add 9kg of rare earth magnesium silicon alloy to the ladle in advance, with a mass ratio of 1.8%;

S3. Pouring the molten iron in the ladle into a tee pipe sand mold with a wall thickness of 2.8-14 mm to obtain a gray-white tee pipe green body;

S4. Tamping the green body of the gray-white mouth of the tee pipe, and pounding the green body of the part from the pouring system;

S5, annealing the green body of the tee pipe part to obtain a finished tee part.

In step S1, the melting time is 55 minutes; the melting temperature is 1500°C.

In step S2, the content of each component in the test molten iron is: Fe content 93.2%, C content 2.95%, Si content 3.51%, Mn content 0.215%, S content 0.011%, P content 0.028%, Mg content 0.042%, Cr content 0.016%, Re content 0.02%.

In step S3, the volume ratio of the gating system of the tee pipe sand mold is 33%, that is, the product yield is 67%. The specification of the three-way pipe is 2.8-14mm, which means that the thinnest part of the inner wall of the three-way pipe is 2.8mm, and the thickest part is 14mm.

In step S5, the temperature is raised from room temperature to 720° C. at a rate of 100° C./h, kept for 2 hours, and then air-cooled out of the kiln. Test the tensile strength, elongation and flattening rate of the obtained tee pipe.

Example 2

The processing technology for adding spherical graphite to the malleable cast iron is basically the same as that of Embodiment 1, and the elbow pipe with an inner diameter of 2.8-14 mm is processed.

In step S1, the melting time is 60 minutes; the melting temperature is 1520°C.

In step S2, the addition amount of the rare earth magnesium silicon alloy is 1.6%; the contents of each component in the test molten iron are: Fe content 93.1%, C content 3.08%, Si content 3.38%, Mn content 0.226%, S content 0.012%, P content of 0.033%, Mg content of 0.045%, Cr content of 0.024%, and Re content of 0.018%.

In step S3, the volume ratio of the gating system of the elbow pipe sand mold is 30%, that is, the product yield is 70%.

In step S5, the temperature was raised from room temperature to 730°C at a rate of 120°C/h, kept for 2 hours, and then air-cooled out of the kiln; the tensile strength, elongation, and flattening rate tests were performed on the finished elbow pipe.

Example 3

The processing technology for adding spherical graphite to the malleable cast iron is basically the same as that of Embodiment 1, and the pipe collar with an inner diameter of 2.8-14 mm is processed.

In step S1, the melting time is 65 minutes; the melting temperature is 1540°C.

In step S2, the addition amount of the rare earth magnesium silicon alloy is 1.4%; the contents of each component in the molten iron are: Fe content 93.0%, C content 3.48%, Si content 3.18%, Mn content 0.208%, S content 0.011%, P content 0.031%, Mg content 0.043%, Cr content 0.021%, Re content 0.037%.

In step S3, the volume ratio of the pouring system of the pipe hoop sand mold is 25%, that is, the product yield is 75%.

In step S5, the temperature was raised from room temperature to 740° C. at a rate of 110° C./h, held for 2 hours, and then air-cooled out of the kiln; the tensile strength, elongation, and flattening rate tests were performed on the finished pipe collar.

Example 4

The processing technology for adding spherical graphite to the malleable cast iron is the same as that in Embodiment 1, and the four-way pipe with an inner diameter of 2.8-14 mm is processed.

In step S1, the melting time is 70 minutes; the melting temperature is 1560°C.

In step S2, the addition amount of the rare earth magnesium silicon alloy is 1.2%, and the contents of each component in the molten iron are: Fe content 93.0%, C content 3.62%, Si content 3.04%, Mn content 0.198%, S content 0.013%, P content 0.033%, Mg content 0.042%, Cr content 0.021%, Re content 0.019%.

In step S3, the volume ratio of the gating system of the four-way pipe sand mold is 28%, that is, the product yield is 72%.

In step S5, the temperature was raised from room temperature to 750° C. at a rate of 120° C./h, held for 2 hours, and then air-cooled out of the kiln; the tensile strength, elongation, and flattening rate of the obtained four-way pipe were tested.

Example 5

The processing technology for adding spherical graphite to the malleable cast iron is the same as that in Embodiment 1, and the pipe collar with an inner diameter of 2.8-14 mm is processed.

In step S1, the melting time is 70 minutes; the melting temperature is 1580°C.

In step S2, the addition amount of the rare earth magnesium silicon alloy is 1.0%. The contents of each component in molten iron are: Fe content 92.9%, C content 3.71%, Si content 2.95%, Mn content 0.226%, S content 0.015%, P content 0.026%, Mg content 0.045%, Cr content 0.014%, Re content 0.02%.

In step S3, the volume ratio of the pouring system of the pipe hoop sand mold is 37%, that is, the product yield is 63%.

In step S5, the temperature was raised from room temperature to 740° C. at a rate of 110° C./h, held for 2 hours, and then air-cooled out of the kiln; the tensile strength, elongation, and flattening rate tests were performed on the finished pipe collar.

Example 6

The processing technology for adding spherical graphite to the malleable cast iron is the same as that in Embodiment 1, and the four-way pipe with an inner diameter of 2.8-14 mm is processed.

In step S1, the melting time is 70 minutes; the melting temperature is 1600°C.

In step S2, the addition amount of the rare earth magnesium silicon alloy is 0.8%. The contents of each component in molten iron are: Fe content 94.0%, C content 3.75%, Si content 1.84%, Mn content 0.286%, S content 0.032%, P content 0.034%, Mg content 0.043%, Cr content 0.028%, Re content 0.021%.

In step S3, the volume ratio of the gating system of the four-way pipe sand mold is 35%, that is, the product yield is 65%.

In step S5, the temperature was raised from room temperature to 750° C. at a rate of 140° C./h, held for 2 hours, and then air-cooled out of the kiln; the tensile strength, elongation, and flattening rate of the obtained four-way pipe were tested.

Comparative example 1

The processing technology of traditional malleable cast iron without adding rare earth magnesium silicon alloy to produce tee pipe with wall thickness of 2.8-14mm is as follows:

S1. Add raw materials into the electric furnace for smelting, and smelt 3000kg of molten iron;

S2. Pour the smelted raw molten iron into a 500kg ladle; pour the molten iron in the ladle into a tee pipe sand mold with a wall thickness of 8 mm to obtain a green body of a tee pipe with a white mouth;

S3. Tamping the green body of the gray-white mouth of the tee pipe, and pounding the green body of the part from the pouring system;

S4. Annealing the green body of the tee pipe part to obtain a finished tee part.

In step S1, the smelting time is 55 minutes; the smelting temperature is 1500°C; the contents of each component in the test molten iron are: Fe content 94.9%, C content 2.96%, Si content 1.68%, Mn content 0.286%, S content 0.032%, Cr content Content 0.022%, P content 0.034%.

In step S3, the volume ratio of the gating system of the tee pipe sand mold is 45%, that is, the product yield is 55%.

In step S5, multi-stage annealing is adopted. The first stage is to raise the temperature from room temperature to 950°C at a heating rate of 120°C and then keep it warm for 8 hours. The cooling rate of h is reduced to 570°C, and finally air-cooled out of the kiln and then air-cooled out of the kiln; the tensile strength, elongation and flattening rate of the obtained tee pipe are tested.

Comparative example 2

The processing technology of increasing spherical graphite in the malleable cast iron with increased content of rare earth magnesium silicon alloy comprises the following steps:

S1. Add raw materials into the electric furnace for smelting, and smelt 3000kg of molten iron;

S2. Pour the smelted raw iron into a 500kg ladle, and add 10.5kg of rare earth magnesium-silicon alloy to the ladle in advance, with a mass ratio of 2.1%;

S3. Pouring the molten iron in the ladle into a tee pipe sand mold with a wall thickness of 2.8-14 mm to obtain a gray-white tee pipe green body;

S4. Tamping the green body of the gray-white mouth of the tee pipe, and pounding the green body of the part from the pouring system;

S5, annealing the green body of the tee pipe part to obtain a finished tee part.

In step S1, the melting time is 55 minutes; the melting temperature is 1500°C.

In step S2, the content of each component in the molten iron is: Fe content 92.6%, C content 3.68%, Si content 3.32%, Mn content 0.206%, S content 0.011%, P content 0.032%, Mg content 0.068%, Cr content 0.024% %, Re content 0.031%.

In step S3, the volume ratio of the gating system of the tee pipe sand mold is 33%, that is, the product yield is 67%.

In step S5, the temperature is raised from room temperature to 720° C. at a rate of 100° C./h, kept for 2 hours, and then air-cooled out of the kiln. Test the tensile strength, elongation and flattening rate of the obtained tee pipe.

Obtain the performance test result of each embodiment and comparative example as follows:

The malleable iron performance test of each embodiment and comparative example of table 1

Performance <![CDATA[Tensile Strength (N/mm²)]]> Elongation (%) Flattening rate (%) Example 1 338 8.5 9.5 Example 2 341 9.8 10.6 Example 3 366 10.4 11.9 Example 4 361 10.9 11.6 Example 5 358 10.3 10.3 Example 6 337 8.8 9.1 Comparative example 1 305 6.2 6.9 Comparative example 2 216 1.8 1.6

The results show that the addition of the rare earth magnesium silicon alloy in Examples 1 to 6 in the range of 0.8 to 1.8% will make the properties of the processed malleable iron castings improved to a certain extent compared with ordinary malleable cast irons, fully satisfying and higher than the requirements of malleable cast irons. According to the industry standard, the important thing is that the gray-white green body is not easy to be smashed, the annealing time is short, and the yield is high; but as the addition of rare earth magnesium-silicon alloy exceeds 1.4%, the overall performance improvement benefit shows a downward trend.

Comparative example 1. The white green body obtained by the traditional malleable cast iron processing method. Although the properties of the malleable cast iron obtained by high temperature and long-term annealing meet the industry standards, there are some problems such as high brittleness of the white green body, and obvious or invisible cracks are easy to occur when the workpiece is rammed. The problem of low yield and high processing cost. The test results of Comparative Example 2 show that once the addition of the rare earth magnesium silicon alloy exceeds 1.8%, the absolute amount of the interfering elements MgO, MgS, Re ₂ O ₃ , Re ₂ S ₃ produced by the reaction will increase, which will destroy the graphite. Spheroidization tendency and serious ash tendency, easy to form flake graphite, resulting in brittle green body, and the performance of processed castings is not up to standard.

Fig. 3 is the metallographic structure diagram of the unpolished sample obtained in Example 1. It can be seen that most of the graphite matrix is in the transition state from group flocculent graphite to spherical graphite, and there are some spherical graphites, and the content of spherical graphite is about 30%. ; The sample in Figure 4 is a metallographic structure diagram after polishing with 4% nital, and it can be seen that most of C exists in the form of ferrite without cementite.

Fig. 5 is the metallographic structure diagram of the sample obtained in comparative example 1 without polishing, and it can be seen that most of the graphite matrix is in the form of first-order flocculent graphite, without spherical graphite; Fig. 6 is the sample of comparative example after 4% It can be seen from the metallographic structure diagram after the nitric acid ethanol polishing treatment that most of the C is in the form of a blend of ferrite and pearlite, the pearlite content is about 20%, and no cementite is contained.

The above is only a preferred embodiment of the present invention, and does not limit the present invention in any form. Any simple modification or equivalent change made to the above embodiments according to the technology of the present invention falls within the scope of the present invention. within the scope of protection.

Claims (10)

1. a processing technology that increases spherical graphite in malleable cast iron, is characterized in that, comprises the steps: S1, adding raw materials into the electric furnace for smelting to obtain raw molten iron; S2. Pour the smelted raw molten iron into a ladle, add a modifier in advance in the ladle, and the modifier accounts for 0.8-1.8% by mass of the molten iron in the ladle; S3, pouring the molten iron in the ladle into the sand mold to obtain a gray-white green body; S4. Tamping the gray-white green body, and ramming the part green body from the pouring system; S5, annealing the green part; The intermediate modifier in step S2 is a rare earth magnesium-silicon alloy, and the various substances in the molten iron after the deterioration of the original molten iron include the following components in parts by mass: 92-95% iron, 2.8-3.8% carbon, 1.8-3.2% silicon, manganese < 0.6%, sulfur<0.2%, phosphorus<0.1%, magnesium<0.055%, chromium<0.1%. 2. The process for increasing spherical graphite in malleable cast iron according to claim 1, characterized in that the silicon content in alpine regions is 1.8% to 2.7%. 3. The processing technology for increasing spherical graphite in malleable cast iron according to claim 1, characterized in that the amount of rare earth magnesium silicon alloy added in step S2 is preferably 1.0-1.4%. 4. The processing technology for increasing spherical graphite in malleable cast iron according to claim 1, characterized in that, in step S1, the melting time of the original molten iron is 55 min-80 min, and the melting temperature is 1500-1600°C. 5. The processing technology for increasing spherical graphite in malleable cast iron according to claim 1, characterized in that, each component in the rare earth magnesium silicon alloy comprises: 30-32% magnesium, 40-44% silicon, Calcium 2.5-4.0%, rare earth 2.5-4.0%. 6. The process for increasing spherical graphite in malleable cast iron according to claim 5, characterized in that the particle size of the rare earth magnesium silicon alloy is 10-30mm. 7. The processing technology for increasing spherical graphite in malleable cast iron according to claim 1, characterized in that the annealing process in step S5 is as follows: first rise from room temperature to 720-750°C at a rate of 100-140°C/h, and keep warm for 2 Leave the kiln for air cooling after an hour. 8. The processing technology for increasing spherical graphite in malleable cast iron according to any one of claims 1 to 7, characterized in that, in step S3, the sand mold comprises: a cavity and a pouring system; the volume ratio of the pouring system is 20 %~38%. 9. The processing technology for increasing spherical graphite in malleable cast iron according to claim 7, characterized in that, the number of cavities in the sand mold is 6-92 according to the type of cast product. 10. An iron casting, characterized in that it is produced by the process of increasing spherical graphite in malleable cast iron according to any one of claims 1 to 9.

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