Process for increasing spheroidal graphite in malleable cast iron and cast iron piece

Abstract

The application provides a processing technology for increasing spheroidal graphite in malleable cast iron and a cast iron piece, and the processing technology comprises the following steps: adding raw materials into an electric furnace to smelt original molten iron, pouring the original molten iron into a ladle, pre-adding a modifier into the ladle, and the mass ratio of the modifier is 0.8-1.8%; pouring the molten iron into a sand mold to obtain a gray-white porosity green body; performing a tamping and annealing process on the gray-white porosity green body; the modifier is a rare earth magnesium silicon alloy, and after the modification of the original molten iron, the components of the molten iron include the following components in mass parts: iron 92-95%, carbon 2.8-3.8%, silicon 1.8-3.8%, manganese <0.6%, sulfur <0.2%, phosphorus <0.1%, magnesium <0.055%, and chromium <0.1%; and the cast iron piece is prepared by using the above process. The application has the advantages that the strength and toughness of the green body are increased, the green body is prevented from being broken during tamping, the annealing process temperature is low and the annealing process time is short, the production energy consumption is reduced, the production cost is saved, the proportion of the cavity in the sand mold is increased, the molten iron utilization rate is increased, the re-furnace loss is reduced, and the tamping difficulty is not increased.

Description

TECHNICAL FIELD

[0001] The present application belongs to the technical field of malleable cast iron, and particularly relates to a processing technology for increasing spheroidal graphite in malleable cast iron and a cast iron piece. BACKGROUND

[0002] Common gray cast iron can be divided into ordinary gray cast iron, malleable cast iron, spheroidal graphite cast iron and vermicular graphite cast iron according to the form of graphite. The graphite in ordinary gray cast iron is in the form of flake, and the content of carbon and silicon is generally controlled in the range of 2.5%-4.0% for carbon and 1.0%-3.0% for silicon. The ordinary gray cast iron has good casting performance and heat conduction performance, and is widely used.

[0003] The graphite in spheroidal graphite cast iron is in the form of sphere, and is obtained by spheroidizing the molten gray cast iron. The comprehensive mechanical property of the spheroidal graphite cast iron is close to that of steel, and the tensile strength and fatigue strength are higher. The spheroidal graphite cast iron has the most stringent composition requirement, and the carbon content is usually changed in the range of 4.5%-4.7% to facilitate spheroidization of the graphite. The tensile strength of the spheroidal graphite cast iron is far higher than that of the gray cast iron, and is comparable to that of steel. The spheroidal graphite cast iron is mainly used for important parts such as crankshafts, connecting rods and camshafts, and has relatively high production cost. The graphite in vermicular graphite cast iron is in the form of worm, and the strength of the vermicular graphite cast iron is close to that of the spheroidal graphite cast iron, and the vermicular graphite cast iron has high wear resistance. The vermicular graphite cast iron is mainly used for producing castings such as cylinder heads, cylinder liners, steel ingot molds and hydraulic valves.

[0004] The graphite in malleable cast iron is in the form of flocculus, and the malleable cast iron has the strongest plasticity and impact toughness among the gray cast irons. The malleable cast iron is mainly used for manufacturing parts with complex shape and subjected to impact and vibration load, such as pipe joints and low-pressure valves. About 90% of the malleable cast iron is black core malleable cast iron. Compared with other types of gray cast iron, the black core malleable cast iron has general requirements for tensile strength and wear resistance. The processing process of the black core malleable cast iron mainly includes the following steps: the original molten iron is first made into white hot green body, and then the white hot green body is subjected to graphitization annealing (heat treatment) to obtain the black core malleable cast iron.

[0005] The carbon in the white hot green body exists in the form of cementite, and only a trace of graphite is present. The fracture of the white hot green body is bright white, and the white hot green body has high hardness, poor plasticity and toughness, and great brittleness.

[0006] The malleable cast iron must be subjected to the processing process of the white hot green body. The volume of the parts to be produced is small, the shape structure of the parts is complex, and the demand for the parts is large. Therefore, the casting mold for batch production of the parts in industry usually has a plurality of cavities (6-92 cavities) arranged on the mold. The cavities are connected through a gating channel and a pouring riser (i.e. a gating and pouring system) to improve the yield and reduce the production cost. The molten iron is rapidly cooled into the green body of the parts in the mold, and then the green body is demolded from the mold. The green body of the parts is manually knocked out from the gating and pouring system after the process of knocking, inspection and transportation. The green body of the parts that passes the inspection is subjected to multi-stage annealing to obtain the black core malleable cast iron product with high plasticity and impact toughness. The remaining parts such as the gating channel and the pouring riser are reused as the return material.

[0007] Therefore, the key intermediate product of the green part must require higher brittleness to facilitate the tamping of the part, and thus must be a white hot green part. However, if the brittleness of the white hot green part is too large, the white hot green part is prone to brittle fracture during the tamping process, resulting in various apparent or hidden cracks, and thus a high green part rejection rate. In particular, the existence of hidden cracks is difficult to check, and thus the finished product is prone to breakage during use, affecting the sales and use of the finished part.

[0008] Meanwhile, the more complex the shape of the part, the greater the thickness difference at different positions of the part, which inevitably leads to inconsistent cooling speeds at different positions in the mold during the pouring of the molten iron, thereby increasing the supercooling degree of the white hot green part and the excessive internal stress, and thus easily causing shrinkage and hidden cracks.

[0009] Before 1998, the equipment for producing malleable cast iron was mainly a cupola furnace, and free oxygen only existed in a limited area near the tuyere in the cupola furnace. With the rising of the furnace gas and the reaction of the coke, the free oxygen is quickly consumed, and overall, the atmosphere in the furnace is non-oxidizing. Therefore, by using the cupola furnace for smelting, excellent molten iron with high fluidity and small shrinkage can be obtained, and the white hot green part obtained can also prevent shrinkage. However, since 1998, due to environmental protection issues, cupola furnaces have gradually been eliminated, and electric furnaces have been used for producing malleable cast iron. The smelting of molten iron in electric furnaces is prone to oxidation and oxygen absorption during the smelting process, resulting in poor molten iron quality and poor fluidity, which is more likely to cause shrinkage. Therefore, a large amount of bismuth is usually added to increase the white hot tendency of the green part, which increases the production cost due to the high price of bismuth.

[0010] Therefore, under the background of the existing electric furnace as the molten iron production equipment, the important problem in malleable cast iron processing is to control the brittleness of the white hot green part and prevent the shrinkage of the molten iron to solve the hidden problem of the breakage of the white hot green part, while also needing to control the production cost.

[0011] Patent application CN113403526A discloses a white hot molten iron, a white hot green part, a finished part, and a method and application thereof. The application strictly controls the silicon content in the original molten iron, and then adds silicon-iron alloy in the outer package of the molten iron furnace. The added silicon can increase the non-spontaneous core of graphitization in the original molten iron. Under the condition of ensuring the overall content of C and Si within a reasonable range, the fine graphite in the molten iron expands, preventing the shrinkage of the molten iron and improving the strength, toughness, and plasticity of the white hot green part. This method reduces the addition of scarce and expensive bismuth iron, reduces the environmental pollution caused by bismuth iron, and saves the production cost.

[0012] However, the implementation of the above method needs to strictly control the weight percentage of carbon and silicon in the original molten iron, and the element content in the original molten iron needs to be strictly detected each time of smelting, which increases the implementation difficulty; and the focus of the method is to improve the strength of the white birth blank, and the hidden danger of the white birth blank has not been completely eliminated, and under the condition that the artificial ramming part is mistakenly rammed on the part blank, certain invisible cracks will still be generated, and part of the defective products will be generated. SUMMARY

[0013] In view of the problems of the shrinkage of the molten iron smelted by the electric furnace, the brittleness of the white birth blank, the hidden danger of the white birth blank, the high defective product rate and the high production cost in the prior art, the application provides a processing technology for increasing the spheroidal graphite in forgeable cast iron and a cast iron part.

[0014] The technical scheme provided by the application is as follows: a processing technology for increasing the spheroidal graphite in forgeable cast iron, comprising the following steps:

[0015] S1, adding raw materials into an electric furnace to obtain original molten iron;

[0016] S2, pouring the smelted original molten iron into a molten iron ladle, and pre-adding a modifier into the molten iron ladle, the mass percentage of the modifier in the molten iron in the ladle being 0.8-1.8%;

[0017] S3, pouring the molten iron in the molten iron ladle into a sand mold to obtain a gray white birth blank;

[0018] S4, ramming the gray white birth blank, and ramming the part blank from the pouring and feeding system;

[0019] S5, annealing the part blank;

[0020] In the step S2, the modifier is a rare earth magnesium silicon alloy, and the smelted molten iron comprises the following components in mass parts: iron 92-95%, carbon 2.8-3.8%, silicon 1.8-3.2%, manganese <0.6%, sulfur <0.2%, phosphorus <0.1%, magnesium <0.055%, and chromium <0.1%.

[0021] Further, the silicon content in the high-cold region is 1.8%-2.7%.

[0022] Preferably, the rare earth magnesium silicon alloy is added in an amount of 1.0-1.4% in the step S2.

[0023] Further, in the step S1, the smelting time of the original molten iron is 55-80 min, and the smelting temperature is 1500-1600℃.

[0024] Further, the components in the rare earth magnesium silicon alloy comprise the following components in mass parts: magnesium 30-32%, silicon 40-44%, calcium 2.5-4.0%, and rare earth 2.5-4.0%.

[0025] Further, the particle size of the rare earth magnesium silicon alloy is 10-30 mm.

[0026] Further, the annealing process in step S5 is as follows: first, increase the temperature from room temperature to 720-750 DEG C at a speed of 100-140 DEG C / h, keep for 2 hours, and then air cool out of the kiln.

[0027] Further, the sand mold in step S3 includes a cavity and a pouring and feeding system, and the volume ratio of the pouring and feeding system is 20%-38%.

[0028] Further, the number of the cavities in the sand mold is 6-92 according to the type of the cast product.

[0029] The application also provides a cast iron piece prepared by the above process for increasing the spheroidal graphite in forgeable cast iron.

[0030] The application has the advantages that: the addition of the rare earth magnesium silicon alloy prevents the shrinkage of the molten iron, increases the strength and toughness of the green body, prevents the green body from being broken during the tamping, and accelerates the graphitization speed during the subsequent graphitization annealing, so that the annealing temperature is low, the annealing time is short, and the forgeable cast iron product with the required performance can be obtained; the production process of the application can reduce the production energy consumption and production cost, and generate the forgeable cast iron with a small amount of spheroidal graphite; in combination with the improvement of the pouring and feeding system of the sand mold, the pouring channel is made thin, the riser is made small, the proportion of the cavities in the sand mold is increased, the utilization rate of the molten iron is increased, the generation of the return material is reduced, the return loss is reduced, and the tamping difficulty is not increased. BRIEF DESCRIPTION OF DRAWINGS

[0031] Figure 1 It is the grey-white brittle green body of the application;

[0032] Figure 2 It is the white brittle green body of the traditional forgeable cast iron;

[0033] Figure 3 It is the metallographic structure diagram of the sample obtained in Example 1 without polishing;

[0034] Figure 4 It is the metallographic structure diagram of the sample obtained in Example 1 after nitric acid ethanol treatment;

[0035] Figure 5 It is the metallographic structure diagram of the sample obtained in Comparative Example 1 without polishing;

[0036] Figure 6 It is the metallographic structure diagram of the sample obtained in Comparative Example 1 after nitric acid ethanol treatment. DETAILED DESCRIPTION

[0037] In order to make the purposes, technical solutions and advantages of the embodiments of the present application clearer, the technical solutions in the embodiments of the present application will be described clearly and completely below with reference to the drawings in the embodiments of the present application. All other embodiments obtained by those of ordinary skill in the art without creative labor based on the embodiments in the present application belong to the protection scope of the present application.

[0038] The process for increasing spheroidal graphite in the malleable cast iron of the present application includes two aspects: on the one hand, the content of fine graphite in the green body is increased to prevent shrinkage of the molten iron, increase the strength and toughness of the green body, prevent the green body from breaking during the tamping, and accelerate the graphitization speed during the subsequent graphitization annealing (heat treatment), thereby reducing the production energy consumption and saving the production cost, and generating the malleable cast iron with increased small portion of spheroidal graphite; on the other hand, the sand mold pouring system needs to be improved to make the pouring channel thin and the riser small, i.e. the proportion of the cavity in the sand mold is increased, the utilization rate of the molten iron is increased, the generation of the return material is reduced, the return loss is reduced, and the tamping difficulty is not increased. The two aspects must be matched with each other to greatly reduce the production cost while ensuring the quality of the malleable cast iron product.

[0039] For the first aspect, the present application provides a preparation method of the malleable cast iron, and the steps are as follows:

[0040] S1, adding raw materials into an electric furnace for smelting to obtain original molten iron;

[0041] S2, pouring the smelted original molten iron into a ladle, and pre-adding a molten iron modifier into the ladle, wherein the molten iron modifier is a rare earth magnesium silicon alloy, and the mass proportion of the modifier in the molten iron in the ladle is 0.8-1.8%, preferably 1.0-1.4%.

[0042] S3, pouring the molten iron in the ladle into a sand mold to obtain a gray-white green body;

[0043] S4, tamping the gray-white green body to tump the part green body from the pouring and riser system;

[0044] S5, annealing the part green body.

[0045] In the step S1, the raw materials of the molten iron include scrap steel, pig iron, return material, etc., the smelting time is 55-80 min, and the smelting temperature is 1500-1600℃.

[0046] The rare earth magnesium-silicon alloy in step S2 comprises, by mass, the following components: magnesium (Mg) 30-32%, silicon (Si) 40-44%, calcium (Ca) 2.5-4.0%, rare earth (Re) 2.5-4.0%, with the balance being elements such as Cr, MgO, and Fe, which are not specifically limited due to their small content. The particle size of the rare earth magnesium-silicon alloy is 10-30 mm. The rare earth magnesium-silicon alloy is added to the original molten iron to undergo a modification reaction. The resulting molten iron contains, by mass, the following components: iron (Fe) 92-95%, carbon (C) 2.8-3.8%, silicon (Si) 1.8-3.2% (requiring control at 1.8%-2.7% in cold regions), manganese (Mn) less than 0.6%, sulfur (S) less than 0.2%, phosphorus (P) less than 0.1%, magnesium (Mg) less than 0.055%, and chromium (Cr) less than 0.1%.

[0047] In step S2, with the addition of rare earth magnesium-silicon alloy, the Ca and Mg in it can react with the S in the original molten iron, consuming the sulfur content in the molten iron and reducing the S that hinders the graphitization process. The addition of Mg can also slightly increase the eutecticness of the molten iron, increasing its fluidity, facilitating casting, and preventing shrinkage porosity in the green billet. The addition of Si increases the non-spontaneous nuclei for graphitization, promoting the graphitization of C in the original molten iron. Rare earth can adjust the graphitization-promoting effect of Si, adjusting the spheroidization rate and roundness of graphite. However, the amount of rare earth magnesium-silicon alloy added should not be too high. If it is greater than 1.8%, it will lead to an increase in the absolute amount of interfering elements MgO, MgS, Re2O3, and Re2S3 produced by the reaction, which will destroy the spheroidization tendency of graphite and cause the green billet to become brittle.

[0048] The addition of rare earth magnesium-silicon alloys can improve the overall strength and toughness of malleable cast iron green billets, while also introducing a certain amount of fine graphite into the green billet. This results in a grayish-white green billet with a microstructure including cementite, pearlite, ferrite, and a graphite matrix. Figure 1 As shown. Figure 2 The ordinary malleable white cast iron green billet is used as a reference. It can be seen that this green billet is a special green billet between gray cast iron and white cast iron. It can meet the requirement of being tamped out of the gating system, while increasing the strength and toughness and preventing the green billet from cracking during the tamping process.

[0049] The annealing process in step S5 is as follows: first, the temperature is raised from room temperature to 720-750℃ at a rate of 100-140℃ / h, and then held at that temperature for 2 hours before being air-cooled out of the kiln.

[0050] The C content in the molten iron in the present application is higher than the general malleable cast iron, and the Si content range is wider (the C content in the general malleable cast iron is 2.4-2.8%, and the Si content is 1.2-1.8%), because the green body of the intermediate product part prepared in the present application is not completely white, but a kind of gray-white, and a certain amount of carbon and silicon is needed to increase the strength of the green body and ensure the shape of the graphite therein; meanwhile, the use of the malleable cast iron slightly requires higher plasticity and impact toughness, and the general requirement for wear resistance is not high, so the lower range of C and Si is not particularly limited; however, the C and Si contents should not exceed 3.8% and 3.8% (in high-cold regions, the upper limit of the Si content is 2.7%, that is, the lower the temperature of the region or season, the lower the upper limit of the Si content), otherwise the graphite in the molten iron will expand too much, which will cause the matrix to be loose, and the graphite in the green body will exist in the form of a sheet, the gray-white will become gray, the brittleness will increase, the strength will decrease, and the green body will be prone to breakage.

[0051] The gray-white green body of the present application slightly requires lower annealing temperature (the general malleable cast iron is generally annealed in two or more stages, and the first stage of the multi-stage annealing is usually used, that is, the temperature is raised to 950-970°C and then kept for 6-8 hours, then the temperature is lowered to 730°C at a rate of 70-80°C / h, then the temperature is lowered to 570°C at a rate of 20°C / h, and finally the furnace is cooled), because the amount of graphite in the green body is relatively large, and a lower annealing temperature can ensure that the graphite morphology in the obtained castings is between the flocculent graphite and the spherical graphite; the finished product standard of the black core malleable cast iron GB / T 9440-2010 is met, that is, the tensile strength is greater than 300 N / mm 2 (MPa), the elongation is greater than 6.0%, and the flattening rate is greater than 8%. If the traditional malleable cast iron annealing process is used, although the performance of the obtained castings will be higher, and more spherical graphite will be generated in the castings, it is undoubtedly a performance surplus for the malleable cast iron, and most importantly, this high-temperature and long-time annealing process consumes a lot of electric energy, which is the reason for the high production cost. The present application can realize the qualified performance of the malleable cast iron through this slightly lower temperature and short time annealing process, reduce the annealing temperature by about 230°C, save the annealing time by 15-20h, greatly reduce the production energy consumption, and save the cost of the malleable cast iron.

[0052] For the second aspect, due to the increase of the strength and toughness of the intermediate product green part blank of the present application, although it can still be knocked down from the pouring system, the difficulty of knocking the part is increased compared with the conventional malleable cast iron white cast iron green part blank, therefore, only using the process of the first aspect, the time and fatigue of manual knocking of the part are increased, which in turn leads to the reduction of production efficiency; in addition, the C and Si contents of the molten iron obtained by the first aspect are relatively high compared with the ordinary malleable cast iron, the addition of the rare earth magnesium silicon alloy also improves the fluidity of the molten iron, and the shrinkage prevention performance of the green part is good, therefore, there is a certain size surplus in the original sand mold pouring system (the pouring and pouring system in the malleable cast iron is to prevent the shrinkage of the molten iron and to compensate for the shrinkage, and the diameter of the pouring head is usually 2.5-3.5 times the wall thickness of the part, and in addition to the cavity in the sand mold, the remaining pouring system accounts for 45-50% of the entire sand mold).

[0053] If the process of adding the molten iron composition and the rare earth magnesium silicon alloy of the present application is used, the overall pouring system volume in the sand mold can be appropriately reduced to 20-38%. This design improves the yield of the sand mold, reduces the generation of return materials, increases the yield of the green part, and at the same time, due to the thinning and small size of the pouring system, manual knocking of the part is facilitated, and the problem of increased difficulty of knocking the gray-white cast iron green part of the present application is solved.

[0054] For the specific size reduction form of the sand mold, those skilled in the art can adjust the parameters such as the pouring channel size, the diameter and height of the pouring head, and the length of the ingate in the pouring system according to different product models within the relevant standard range, and the present application only provides the principle of reduction design, and does not limit the specific design form.

[0055] At this point, the combination of the two aspects of malleable cast iron processing methods can more effectively solve the problem of brittle fracture of the original white cast iron green part, and at the same time, due to the reduction of the temperature and time of the annealing process and the reduction design of the sand mold, the processing cost of the malleable cast iron product is greatly reduced within the range of meeting the performance standards, and the production efficiency is improved. It is calculated that the cost of producing one ton of malleable cast iron parts according to the present application can be saved by 100-250 yuan.

[0056] The present application will be described in detail below through several specific examples.

[0057] The rare earth magnesium silicon alloy used in all examples is 13 mm in size, with a magnesium content of 30.1%, a silicon content of 43.5%, a calcium content of 3.6%, and a rare earth Re content of 2.7%. The tensile strength, elongation, and flattening rate of the finished product are tested. The testing instruments include a computer multi-element analyzer, a hydraulic testing machine, and a metallographic microscope.

[0058] Example 1

[0059] The processing process for increasing spheroidal graphite in malleable cast iron includes the following steps:

[0060] S1, adding raw materials into an electric furnace for smelting, smelting 3000 kg of molten iron;

[0061] S2, pouring the smelted molten iron into a 500 kg ladle, and adding 9 kg of rare earth magnesium silicon alloy into the ladle in advance, with a mass ratio of 1.8%;

[0062] S3, pouring the molten iron in the ladle into a 2.8-14 mm thick tee pipe sand mold to obtain a tee pipe grayish white green body;

[0063] S4, tamping the tee pipe grayish white green body, and tamping the green body from the pouring and feeding system;

[0064] S5, annealing the tee pipe green body to obtain a finished tee pipe part.

[0065] In step S1, the smelting time is 55 min, and the smelting temperature is 1500℃.

[0066] In step S2, the content of each component in the molten iron is tested as follows: 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%, and Re content 0.02%.

[0067] In step S3, the volume ratio of the pouring and feeding system of the tee pipe sand mold is 33%, that is, the product yield is 67%. The specification of the tee pipe is 2.8-14 mm, which means that the thinnest part of the tee pipe is 2.8 mm, and the thickest part is 14 mm.

[0068] In step S5, the temperature is raised from room temperature to 720℃ at a rate of 100℃ / h, and then the furnace is cooled after 2h of heat preservation. The tensile strength, elongation, and flattening rate of the obtained tee pipe product are tested.

[0069] Example 2

[0070] The processing technology for increasing spheroidal graphite in forgeable cast iron is basically the same as that in Example 1, and a bend pipe with an inner diameter of 2.8-14 mm is processed.

[0071] In step S1, the smelting time is 60 min, and the smelting temperature is 1520℃.

[0072] In step S2, the amount of rare earth magnesium silicon alloy added is 1.6%; the content of each component in the molten iron is tested as follows: Fe content 93.1%, C content 3.08%, Si content 3.38%, Mn content 0.226%, S content 0.012%, P content 0.033%, Mg content 0.045%, Cr content 0.024%, and Re content 0.018%.

[0073] In step S3, the volume ratio of the pouring and gating system of the elbow pipe sand mold is 30%, i.e. the product yield is 70%.

[0074] In step S5, the temperature is increased from room temperature to 730℃ at a rate of 120℃ / h, and the pipe is taken out of the furnace after holding for 2h and air cooling; the tensile strength, elongation and flattening rate of the obtained elbow pipe product are tested.

[0075] Example 3

[0076] The processing technology of adding spheroidal graphite in malleable cast iron is basically the same as that in Example 1, and the pipe joint with an inner diameter of 2.8-14mm is processed.

[0077] In step S1, the melting time is 65min, and the melting temperature is 1540℃.

[0078] In step S2, the adding amount of the rare earth magnesium silicon alloy is 1.4%, and the content of each component in the molten iron is as follows: 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%, and Re content 0.037%.

[0079] In step S3, the volume ratio of the pouring and gating system of the pipe joint sand mold is 25%, i.e. the product yield is 75%.

[0080] In step S5, the temperature is increased from room temperature to 740℃ at a rate of 110℃ / h, and the pipe joint is taken out of the furnace after holding for 2h and air cooling; the tensile strength, elongation and flattening rate of the obtained pipe joint product are tested.

[0081] Example 4

[0082] The processing technology of adding spheroidal graphite in malleable cast iron is the same as that in Example 1, and the four-way pipe with an inner diameter of 2.8-14mm is processed.

[0083] In step S1, the melting time is 70min, and the melting temperature is 1560℃.

[0084] In step S2, the adding amount of the rare earth magnesium silicon alloy is 1.2%, and the content of each component in the molten iron is as follows: 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%, and Re content 0.019%.

[0085] In step S3, the volume ratio of the pouring and gating system of the four-way pipe sand mold is 28%, i.e. the product yield is 72%.

[0086] In step S5, the temperature is raised from room temperature to 750℃ at a rate of 120℃ / h, and after holding for 2h, the furnace is cooled to room temperature; the tensile strength, elongation, and flattening ratio of the obtained pipe fitting are tested.

[0087] Example 5

[0088] The processing technology for increasing spheroidal graphite in malleable cast iron is the same as that in Example 1, and pipe fittings with an inner diameter of 2.8-14mm are processed.

[0089] In step S1, the melting time is 70min, and the melting temperature is 1580℃.

[0090] In step S2, the addition amount of the rare earth magnesium silicon alloy is 1.0%. The content of each component in the molten iron is as follows: 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%, and Re content 0.02%.

[0091] In step S3, the volume proportion of the gating and feeding system of the pipe fitting sand mold is 37%, i.e., the product yield is 63%.

[0092] In step S5, the temperature is raised from room temperature to 740℃ at a rate of 110℃ / h, and after holding for 2h, the furnace is cooled to room temperature; the tensile strength, elongation, and flattening ratio of the obtained pipe fitting are tested.

[0093] Example 6

[0094] The processing technology for increasing spheroidal graphite in malleable cast iron is the same as that in Example 1, and pipe fittings with an inner diameter of 2.8-14mm are processed.

[0095] In step S1, the melting time is 70min, and the melting temperature is 1600℃.

[0096] In step S2, the addition amount of the rare earth magnesium silicon alloy is 0.8%. The content of each component in the molten iron is as follows: 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%, and Re content 0.021%.

[0097] In step S3, the volume proportion of the gating and feeding system of the pipe fitting sand mold is 37%, i.e., the product yield is 63%.

[0098] In step S5, the temperature is raised from room temperature to 750℃ at a rate of 140℃ / h, and after holding for 2h, the furnace is cooled to room temperature; the tensile strength, elongation, and flattening ratio of the obtained pipe fitting are tested.

[0099] Comparative Example 1

[0100] The processing technology for producing tee pipe with wall thickness of 2.8-14mm by traditional forgeable cast iron without adding rare earth magnesium silicon alloy includes the following steps:

[0101] S1, adding raw materials into an electric furnace for smelting, smelting 3000kg of molten iron;

[0102] S2, pouring the smelted molten iron into a 500kg ladle, and pouring the molten iron in the ladle into a tee pipe sand mold with a wall thickness of 8mm to obtain a tee pipe white cast blank;

[0103] S3, tamping the tee pipe gray-white cast blank, and tamping the part blank off the pouring and running system;

[0104] S4, annealing the tee pipe part blank to obtain a finished tee part.

[0105] In step S1, the smelting time is 55min, the smelting temperature is 1500℃, and the content of each component in the molten iron is tested as follows: Fe content 94.9%, C content 2.96%, Si content 1.68%, Mn content 0.286%, S content 0.032%, Cr content 0.022%, and P content 0.034%.

[0106] In step S3, the volume ratio of the pouring and running system of the tee pipe sand mold is 45%, i.e., the product yield is 55%.

[0107] In step S5, multi-stage annealing is adopted, the first stage is to increase the temperature from room temperature to 950℃ at a rate of 120℃, and then to keep the temperature for 8 hours, the second stage is to decrease the temperature to 730℃ at a rate of 80℃ / h, then to decrease the temperature to 570℃ at a rate of 20℃ / h, and finally to air cool out of the furnace; the tensile strength, elongation and flattening rate of the obtained tee pipe finished product are tested.

[0108] Comparative Example 2

[0109] The processing technology for increasing spheroidal graphite in forgeable cast iron by increasing the content of rare earth magnesium silicon alloy includes the following steps:

[0110] S1, adding raw materials into an electric furnace for smelting, smelting 3000kg of molten iron;

[0111] S2, pouring the smelted molten iron into a 500kg ladle, and adding 10.5kg of rare earth magnesium silicon alloy into the ladle in advance, with a mass ratio of 2.1%;

[0112] S3, pouring the molten iron in the ladle into a tee pipe sand mold with a wall thickness of 2.8-14mm to obtain a tee pipe gray-white cast blank;

[0113] S4, the grayish white pipe blank is tamped, and the part blank is tamped off from the pouring and pouring system;

[0114] S5, the three-way pipe blank is annealed to obtain the finished three-way pipe.

[0115] In step S1, the melting time is 55 min; the melting temperature is 1500℃.

[0116] 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%.

[0117] In step S3, the volume ratio of the pouring and pouring system of the three-way pipe sand mold is 33%, that is, the product yield is 67%.

[0118] In step S5, the temperature is raised from room temperature to 720℃ at a rate of 100℃ / h, and then air-cooled out of the kiln after 2h of holding. The tensile strength, elongation, and flattening rate of the obtained three-way pipe product are tested.

[0119] The performance test results of each embodiment and the comparative example are as follows:

[0120] Table 1 Performance test of malleable cast iron of each embodiment and comparative example

[0121] Performance indicators Tensile strength (N / mm 2 )]]> Elongation (%) Flattening (%) 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

[0122] The results show that the addition amount of rare earth magnesium silicon alloy in examples 1-6 in the range of 0.8-1.8% will make the performance of the malleable cast iron casting obtained have a certain degree of improvement compared with ordinary malleable cast iron, fully meet and exceed the industry standard of malleable cast iron, and the important thing is that the grayish white pipe blank is not easy to be tamped, the annealing time is short, and the yield is high; but with the addition amount of rare earth magnesium silicon alloy exceeding 1.4%, the overall performance improvement benefit shows a downward trend.

[0123] The white pipe blank obtained by the traditional malleable cast iron processing method in comparative example 1 has the problems of large brittleness, easy to produce obvious or invisible cracks during tamping, low yield, and high processing cost. The test results of comparative example 2 show that once the addition amount of rare earth magnesium silicon alloy exceeds 1.8%, the absolute amount of interference elements MgO, MgS, Re2O3, and Re2S3 generated by the reaction will increase, which will destroy the spheroidization tendency of graphite, and the gray tendency is serious, which is easy to form flaky graphite, resulting in the brittleness of the blank, and the performance of the processed casting is not up to standard.

[0124] Figure 3 The metallographic structure of the sample obtained from Example 1 without polishing can be seen that most of the graphite matrix is in the transition state from flocculent graphite to spherulitic graphite, and part of the spherulitic graphite is present, and the content of the spherulitic graphite is about 30%; Figure 4 The metallographic structure of the sample of the example after polishing with 4% nitric acid ethanol can be seen that most of the C exists in the form of ferrite, and does not contain cementite.

[0125] Figure 5 The metallographic structure of the sample obtained from Comparative Example 1 without polishing can be seen that most of the graphite matrix is in the form of primary flocculent graphite, and no spherulitic graphite is present; Figure 6 The metallographic structure of the sample of the comparative example after polishing with 4% nitric acid ethanol can be seen that most of the C exists in the form of a mixture of ferrite and pearlite, the content of the pearlite is about 20%, and no cementite is present.

[0126] The above is only a preferred embodiment of the present application, and does not limit the present application in any form. Any simple modification or equivalent change made on the basis of the technical essence of the present application to the above embodiment falls within the protection scope of the present application.

Claims

1. A process for increasing spheroidal graphite in malleable cast iron, characterized by, It comprises the following steps: S1, adding raw materials into an electric furnace to smelt and obtain raw molten iron; S2, pouring the smelted raw molten iron into a ladle, and adding a modifier into the ladle in advance, the mass ratio of the modifier in the molten iron in the ladle being 0.8-1.8%; S3, pouring the molten iron in the ladle into a sand mold to obtain gray-white green body; S4, tamping the gray-white green body to separate the green body from the pouring and pouring system; S5, annealing the green body; In step S2, the modifier is a rare earth magnesium silicon alloy, and the molten iron after modification comprises the following components by mass fraction: iron 92-95%, carbon 2.8-3.8%, silicon 1.8-3.2%, manganese <0.6%, sulfur <0.2%, phosphorus <0.1%, magnesium <0.055%, and chromium <0.1%; In step S5, the annealing process is as follows: first, increasing the temperature from room temperature to 720-750℃ at a rate of 100-140℃ / h, and then air cooling after 2 hours of heat preservation; In step S3, the sand mold comprises a cavity and a pouring and pouring system. The volume ratio of the pouring and pouring system is 20-38%.

2. The process for increasing spheroidal graphite in malleable cast iron according to claim 1, characterized by, The silicon content in the high-cold region is 1.8-2.7%.

3. The process for increasing spheroidal graphite in malleable cast iron according to claim 1, characterized by, In step S2, the preferred amount of the rare earth magnesium silicon alloy is 1.0-1.4%.

4. The process for increasing spheroidal graphite in malleable cast iron according to claim 1, characterized by, In step S1, the smelting time of the raw molten iron is 55-80 minutes, and the smelting temperature is 1500-1600℃.

5. The process for increasing spheroidal graphite in malleable cast iron according to claim 1, characterized by, The rare earth magnesium silicon alloy comprises the following components by mass fraction: magnesium 30-32%, silicon 40-44%, calcium 2.5-4.0%, and rare earth 2.5-4.0%.

6. The process for increasing spheroidal graphite in malleable cast iron according to claim 5, characterized by, The particle size of the rare earth magnesium silicon alloy is 10-30mm.

7. The process for increasing spheroidal graphite in malleable cast iron according to claim 1, characterized by, The number of cavities in the sand mold is 6-92, which varies according to the type of the cast product.

8. A cast iron piece, characterized by, The processing technology for increasing the spheroidal graphite in forgeable cast iron is prepared by using the processing technology according to any one of claims 1-7.

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