Preparation method of high-carbon-equivalent high-strength low-stress machine tool gray iron casting

Abstract

The invention discloses a preparation method of a high-carbon-equivalent, high-strength and low-stress machine tool gray cast iron piece, and belongs to the technical field of gray cast iron casting. According to the method, the specific inoculant is adopted, the gray cast iron molten iron is subjected to inoculation treatment for at least two times, scrap steel, pig iron and recirculated iron are used as main materials, the carbon equivalent is increased to 3.9% or above and is close to the eutectic point 4.23%, meanwhile, a small amount of alloy is used as an auxiliary material, the number of A-type graphite of the prepared gray cast iron reaches 95% or above, the pearlite content reaches 95% or above, and the structure uniformity is good. According to the method, the gray iron casting can obtain excellent A-type graphite and the quantity of the A-type graphite while the cost is not increased; and meanwhile, the gray iron casting has the properties of high and uniform hardness, high tensile strength, low stress and the like, the tensile strength value of the gray iron casting is larger than or equal to 250 Mpa, and the stress value is smaller than or equal to 50 Mpa.

Description

Technical Field

[0001] The invention relates to the technical field of gray cast iron casting, and in particular to a method for preparing a high-carbon equivalent, high-strength, and low-stress machine tool gray cast iron part. Background Art

[0002] Gray cast iron is a type of cast iron in which carbon exists in the form of flake graphite. Its fracture is gray, and it exhibits excellent wear resistance, casting, and cutting properties. It is widely used in applications requiring energy absorption and vibration reduction, such as gearboxes, blowers, and machine tool manufacturing. In current casting production, the carbon equivalent of heavy and ultra-heavy machine tool castings is maintained at around 3.7% to 3.8%. The graphite is A+E-type graphite, and the microstructure consists of a pearlite matrix of at least 85%. The strength of these castings is ≥250 MPa. However, this high strength also results in high stresses (over 100 MPa). High stresses increase the tendency of castings to crack, impair the accuracy of machine tool castings, and increase the formation of defects such as shrinkage and porosity. These defects reduce the tensile strength of the castings to below 250 MPa. Furthermore, the formation of undercooled E-type graphite, uneven hardness, and carbides during solidification significantly degrades the machining performance of machine tool castings, further complicating the accuracy and stability of machine tool castings.

[0003] To ensure a strength of 250 MPa or higher for gray iron castings, the common practice is to reduce the carbon equivalent (CE 3.7%-3.8%) without alloying, or maintain a high carbon equivalent (CE 3.8%-3.9%) with the addition of certain amounts of precious alloys such as Cr, Mo, Cu, Ni, Sn, and V. Lowering the carbon equivalent can easily lead to the formation of white cast iron, which deteriorates machining performance. High carbon equivalent combined with high alloying can achieve higher strength for gray iron, but this can lead to increased stress and significantly increase smelting costs. It can also cause the A-type graphite to become thinner, and even form a large amount of E-type graphite, reducing the amount of A-type graphite. This ultimately results in poor vibration damping and poor precision retention in machine tool castings, creating a vicious cycle. Summary of the Invention

[0004] In response to the deficiencies of the above-mentioned prior art, the present invention achieves the goal of increasing the amount of type A graphite in gray cast iron to over 95% through a specific inoculant and inoculation process, while at the same time passivating the tip of the type A graphite (reducing the cracking of the matrix) and achieving over 95% pearlite, thereby replacing the addition of a large amount of precious alloys to control costs. The present invention increases the carbon equivalent to over 3.9%, close to the eutectic point of 4.23%. By supplementing a small amount of alloy, the gray cast iron parts can obtain excellent type A graphite and its quantity, without significantly increasing costs. The gray cast iron parts can also have low stress and high strength, and can also solve defects such as shrinkage and shrinkage cavities in the castings. At the same time, the stress of the castings themselves is ultra-low, and the casting process can be simplified or the annealing process can be omitted. The gray cast iron parts of the present invention can have good strength, excellent type A graphite, uniform hardness, and excellent machinability at a high carbon equivalent of over 3.9%, thereby ensuring the stability and maintenance of machining accuracy.

[0005] To achieve the above purpose, the specific technical solutions of the present invention are as follows:

[0006] A method for preparing high-carbon equivalent, high-strength, low-stress gray cast iron parts for machine tools comprises performing at least two inoculation treatments on gray cast iron liquid; the inoculation agents used in the inoculation treatments include at least one low-magnesium rare earth inoculant and at least one nitrogen-containing inoculant.

[0007] Low-magnesium rare earth inoculant contains magnesium and rare earth. Magnesium and rare earth can remove O and S in the molten iron and form oxides. While purifying the molten iron, they can promote the formation of more type A graphite, making the type A graphite short and small. At the same time, the amount of type A graphite and the content of pearlite can reach more than 95%; the nitrogen in the nitrogen-containing inoculant can effectively passivate the tip of the type A graphite within a certain range; the combined use of the two inoculants can form more fine type A graphite. Since austenite and graphite in gray cast iron are symbiotic, the obtained austenite will also become short and small, eventually forming a large amount of grain refinement, which significantly reduces the splitting effect on the matrix. The present invention uses a specific inoculant to perform at least two inoculation treatments on gray cast iron liquid, thereby obtaining high-quality refined iron liquid with high nitrogen (greater than 100 ppm) and low oxygen (less than 40 ppm), so as to enable gray iron castings to have good type A graphite, with the amount of type A graphite reaching more than 95%, and the pearlite content reaching more than 95%; the carbon equivalent is increased to more than 3.9%, close to the eutectic point of 4.23%, and a small amount of alloy is added. Without significantly increasing costs, the gray iron castings are made to have low stress, high strength, and good cutting performance, thereby ensuring the stability and maintenance of processing precision.

[0008] The present invention uses a specific inoculant and inoculation process to increase the amount of type A graphite in gray cast iron to over 95%, while also passivating the tip of the type A graphite (to reduce cutting of the matrix) and achieving over 95% pearlite, thereby controlling costs by replacing the addition of a large amount of precious alloys. The present invention increases the carbon equivalent to over 3.9%, close to the eutectic point of 4.23%. By adding a small amount of alloy, the gray cast iron parts can obtain excellent type A graphite and its quantity, thereby achieving both low stress and high strength without significantly increasing costs. It can also solve defects such as shrinkage and shrinkage cavities in the castings. At the same time, the stress of the castings themselves is extremely low, and the casting process can be simplified or the annealing process can be omitted, thereby reducing production costs.

[0009] Preferably, the low-magnesium rare earth inoculant or nitrogen-containing inoculant is added into the bag, and correspondingly, the nitrogen-containing inoculant or low-magnesium rare earth inoculant is added with the flow; or, the low-magnesium rare earth inoculant and nitrogen-containing inoculant are added into the bag or added with the flow at the same time.

[0010] Preferably, the low-magnesium rare earth inoculant and / or nitrogen-containing inoculant is used in combination with a silicon-barium inoculant; the amount of the silicon-barium inoculant used is 0.15% to 0.25% of the total mass of the molten iron.

[0011] Preferably, the amount of the low-magnesium rare earth inoculant is 0.05% to 0.3% of the total mass of the molten iron.

[0012] Preferably, the content of Mg+RE (rare earth elements) in the low-magnesium rare earth inoculant is ≥5%, the content of Si is 61.43%, the content of Ca is 3.5%, and the rest is iron; the content ratio of Mg to RE in the low-magnesium rare earth inoculant is (2~4): (1~3).

[0013] Preferably, the nitrogen-containing inoculant is a nitride alloy inoculant.

[0014] Preferably, in the nitride alloy inoculant, the N content is 6.7% to 7.5%, the alloy content is 80% to 85%, and the balance is Fe.

[0015] Further preferably, the nitride alloy is a manganese nitride alloy.

[0016] The preparation method of the gray iron casting comprises the following steps:

[0017] (1) Smelting: Scrap steel, pig iron and recycled iron are used as main materials, and coal-based carburizer, ferrosilicon, ferromanganese and silicon carbide are used as auxiliary materials for smelting. The smelting overheating temperature is ≥1500℃;

[0018] (2) Pouring and inoculation: Before being taken out of the furnace, at least one inoculant is added to the ladle for at least one inoculation treatment; pouring is carried out when the molten iron temperature is 1320~1350℃, and a secondary inoculant is added during pouring for inoculation treatment.

[0019] Preferably, the amount of the main material is: scrap steel ≥ 40%, pig iron ≤ 20%, and the rest is recycled iron, the total amount being 100%.

[0020] Preferably, the amount of the coal-based recarburizer used is 1.2% to 2.5% of the total mass of the molten iron.

[0021] Preferably, the amount of ferrosilicon used is 0.8% to 1.2% of the total mass of the molten iron.

[0022] Preferably, the amount of ferromanganese used is 0.5% to 1.0% of the total mass of the molten iron.

[0023] Preferably, the amount of silicon carbide used is less than 0.5% of the total mass of the molten iron.

[0024] Preferably, the coal-based recarburizer is a 93% coal-based recarburizer, and the nitrogen content of the coal-based recarburizer is 2000-8000 ppm.

[0025] Preferably, the ferrosilicon is 75% ferrosilicon.

[0026] Preferably, the ferromanganese is 65% ferromanganese.

[0027] The present invention utilizes a low-magnesium rare earth inoculant and a nitrogen-containing inoculant to inoculate gray iron molten iron at least twice, promoting the formation of more A-type graphite nuclei, with the number reaching over 95%. This results in high graphite curvature, reduced graphite tip passivation, and no E-type graphite. Furthermore, the pearlite content reaches over 95%, resulting in good structural uniformity, little or no carbides, and casting stress values ​​below 50 MPa. Furthermore, the nitrogen-containing inoculant can be used to adjust the nitrogen content of the gray iron castings. During the smelting process, the composition of the base iron must be monitored and its nitrogen content must be controlled to ensure that, after two or more inoculation treatments, the resulting gray iron castings have a nitrogen content of ≤0.02%. The amount of nitrogen-containing inoculant can be flexibly adjusted based on the nitrogen content of the base iron. If the nitrogen content of the base iron is too high, the amount of nitrogen-containing inoculant can be reduced; if the nitrogen content of the base iron is too low, the amount of nitrogen-containing inoculant can be appropriately increased.

[0028] The present invention also provides a high-carbon equivalent, high-strength, low-stress machine tool gray iron casting, whose chemical composition by mass percentage is: carbon 3.2%~4.0%, silicon 1.0%~2.4%, manganese 0.4%~1.3%, sulfur ≤0.12%, phosphorus ≤0.1%, chromium ≤0.4%, copper ≤0.4%, magnesium ≤0.02%, nitrogen ≤0.02%, and the balance is Fe and unavoidable impurity elements, and the carbon equivalent is ≥3.9%.

[0029] Compared with the prior art, the present invention is beneficial in that:

[0030] (1) The method provided by the present invention uses scrap steel ≥40%, pig iron ≤20%, and the rest recycled iron as the main materials, and increases the carbon equivalent to above 3.9%; uses a specific inoculant to perform at least two inoculation treatments on the gray cast iron liquid to obtain high-quality refined iron liquid with high nitrogen (greater than 100 ppm) and low oxygen (less than 40 ppm), so that the prepared gray cast iron parts are all A-type graphite, with large graphite curvature, passivation of the graphite tip, reduced splitting effect, no E-type graphite, and good structural uniformity.

[0031] (2) The gray iron castings prepared by the present invention have the advantages of high and uniform hardness, high tensile strength, and low stress. Among them, the tensile strength value is ≥250 MPa and the stress value is ≤50 MPa. Therefore, the deformation of the casting is small in the as-cast state, and the casting structure is uniform. It has good cutting performance and can achieve high-precision processing functions as well as good precision retention and stability.

[0032] (3) The gray iron castings prepared by the present invention have extremely low stress and little deformation in the as-cast state, which can save unnecessary annealing processes and reduce production costs. BRIEF DESCRIPTION OF THE DRAWINGS

[0033] Figure 1 This is the metallographic image of the high carbon equivalent, high strength, low stress machine tool gray iron casting body before corrosion of Example 1 (left image 100 times, right image 400 times);

[0034] Figure 2 This is the metallographic image of the high carbon equivalent, high strength, low stress machine tool gray iron casting body after corrosion in Example 1 (left image 100 times, right image 400 times);

[0035] Figure 3 This is the metallographic image of the high carbon equivalent, high strength, low stress machine tool gray iron casting body before corrosion of Example 2 (left image 100 times, right image 400 times);

[0036] Figure 4 This is the metallographic image of the high carbon equivalent, high strength, low stress machine tool gray iron casting body after corrosion in Example 2 (left image 100 times, right image 400 times);

[0037] Figure 5This is the metallographic image of the high carbon equivalent, high strength, low stress machine tool gray iron casting body before corrosion of Example 3 (left image 100 times, right image 400 times);

[0038] Figure 6 This is the metallographic image of the low carbon equivalent gray iron casting body before corrosion of Comparative Example 1 (left image 100 times, right image 400 times);

[0039] Figure 7 This is the metallographic image of the low carbon equivalent gray iron casting body after corrosion of Comparative Example 1 (left image 100 times, right image 400 times);

[0040] Figure 8 Metallographic images of the high carbon equivalent, high strength, low stress furnace test bar of Example 1 before corrosion (left image 100 times, right image 400 times);

[0041] Figure 9 This is the metallographic image of the low-equivalent furnace test bar of Comparative Example 1 before corrosion (left image 100 times, right image 400 times). DETAILED DESCRIPTION

[0042] The technical solutions of the present invention are described clearly and completely below. Obviously, the embodiments described are only some of the embodiments of the present invention, not all of them. All other embodiments obtained by persons of ordinary skill in the art based on the embodiments of the present invention without creative effort are within the scope of protection of the present invention.

[0043] The invention provides a method for preparing high-carbon equivalent, high-strength and low-stress gray cast iron parts for machine tools, wherein the gray cast iron liquid is inoculated at least twice; the inoculants used in the inoculation treatment include at least one low-magnesium rare earth inoculant and at least one nitrogen-containing inoculant.

[0044] In some examples, the low-magnesium rare earth inoculant or the nitrogen-containing inoculant is added in the bag, and correspondingly, the nitrogen-containing inoculant or the low-magnesium rare earth inoculant is added with the flow; or, the low-magnesium rare earth inoculant and the nitrogen-containing inoculant are added in the bag or added with the flow at the same time.

[0045] In some examples, the low-magnesium rare earth inoculant and the nitrogen-containing inoculant are used in combination with a silicon-barium inoculant; the amount of the silicon-barium inoculant used is 0.15% to 0.25% of the total mass of the molten iron.

[0046] In some examples, the low-magnesium rare earth inoculant is used in an amount of 0.05% to 0.3% of the total mass of the molten iron.

[0047] In some examples, the low-magnesium rare earth inoculant has a Mg+RE (rare earth element) content of ≥5%, a Si content of 61.43%, a Ca content of 3.5%, and the remainder being iron; and a content ratio of Mg to RE in the low-magnesium rare earth inoculant is (2-4): (1-3).

[0048] In the following specific embodiments, the nitrogen-containing inoculant is a nitride alloy inoculant; the nitride alloy inoculant is a manganese nitride alloy inoculant; in the manganese nitride alloy inoculant, the nitrogen content is 7.1%, the alloy content is 83%, and the balance is Fe.

[0049] In the following specific embodiments, the method for preparing the gray iron casting comprises the following steps:

[0050] (1) Smelting: Scrap steel, pig iron and recycled iron are used as main materials, and coal-based carburizer, ferrosilicon, ferromanganese and silicon carbide are used as auxiliary materials for smelting. The smelting overheating temperature is ≥1500℃;

[0051] (2) Pouring and inoculation: Before being taken out of the furnace, at least one inoculant is added to the ladle for at least one inoculation treatment; pouring is carried out when the molten iron temperature is 1320~1350℃, and a secondary inoculant is added during pouring for inoculation treatment.

[0052] In some examples, the amount of the main material is: scrap steel ≥ 40%, pig iron ≤ 20%, and the rest is recycled iron, with the total amount being 100%.

[0053] In some examples, the amount of the coal-based recarburizer used is 1.2% to 2.5% of the total mass of the molten iron.

[0054] In some examples, the amount of ferrosilicon used is 0.8% to 1.2% of the total mass of the molten iron.

[0055] In some examples, the amount of ferromanganese used is 0.5% to 1.0% of the total mass of the molten iron.

[0056] In some examples, the amount of silicon carbide used is less than 0.5% of the total mass of the molten iron.

[0057] In the following specific examples, the coal-based recarburizer is a 93% coal-based recarburizer, and the nitrogen content of the coal-based recarburizer is 5000 ppm; the ferrosilicon is 75% ferrosilicon; and the ferromanganese is 65% ferromanganese.

[0058] Example 1

[0059] A high carbon equivalent, high strength, low stress machine tool gray iron casting, the preparation method of which is as follows:

[0060] (1) Smelting: 40% scrap steel and 60% recycled iron were used as the main materials, and 1.5% coal-based recarburizer, 1% ferrosilicon, 0.8% ferromanganese, and 0.3% silicon carbide were used as auxiliary materials. The overheating temperature was 1503 °C and the overheating time was 5 min.

[0061] (2) Pouring and inoculation: Before leaving the furnace, add 0.2% nitrogen-containing inoculant and 0.2% low-magnesium rare earth inoculant into the ladle. The Mg content of the low-magnesium rare earth inoculant used is 3% and the RE content is 3%. Pouring is carried out at a molten iron temperature of 1330 °C. At the same time of pouring, 0.2% silicon-barium inoculant is added for inoculation.

[0062] The gray iron casting in this embodiment is a 7-ton small beam, with an average wall thickness of 70 mm and a guide rail thickness of 120 mm. The chemical composition is shown in Table 1, and the mechanical properties are shown in Table 2.

[0063] Example 2

[0064] A high carbon equivalent, high strength, low stress machine tool gray iron casting, the preparation method of which is as follows:

[0065] (1) Smelting: 40% scrap steel and 60% recycled iron were used as the main materials, and 1.5% coal-based recarburizer, 1% ferrosilicon, 0.8% ferromanganese, and 0.3% silicon carbide were used as auxiliary materials. The overheating temperature was 1503 °C and the overheating time was 5 min.

[0066] (2) Pouring and inoculation: Before leaving the furnace, add 0.2% low-magnesium rare earth inoculant into the ladle, wherein the Mg content of the low-magnesium rare earth inoculant used is 3% and the RE content is 3%; pouring is carried out at the molten iron temperature of 1330 ℃, and 0.2% nitrogen-containing inoculant is added into the ladle for inoculation.

[0067] The gray iron casting in this embodiment is a 7-ton small beam, with an average wall thickness of 70 mm and a guide rail thickness of 120 mm. The chemical composition is shown in Table 1, and the mechanical properties are shown in Table 2.

[0068] Example 3

[0069] A high-carbon equivalent, high-strength, low-stress gray iron casting for machine tools. The preparation method of the gray iron casting in this embodiment is basically the same as that in Example 1, except that the Mg content of the low-magnesium rare earth inoculant used is 4% and the RE content is 2%.

[0070] The gray iron casting in this embodiment is a 16.4-ton column (gross weight), the guide rail thickness is 200 mm, the chemical composition is shown in Table 1, and the mechanical properties are shown in Table 2.

[0071] Comparative Example 1

[0072] The preparation method of the gray iron castings in this comparative example is basically the same as that in Example 1, except that a low carbon equivalent of about 3.7% and a normal silicon-barium inoculant are used for inoculation, 0.3% silicon-barium inoculant is added to the ladle, and 0.2% silicon-barium inoculant is added along the flow.

[0073] The gray iron casting in this comparative example is a 7-ton small beam (the same type of casting as in Examples 1 and 2). The average wall thickness of the casting is 70 mm, and the thickness of the guide rail is 120 mm. The chemical composition is shown in Table 1, and the mechanical properties are shown in Table 2.

[0074] Comparative Example 2

[0075] The preparation method of the gray iron castings in this comparative example is basically the same as that in Example 1, except that a high carbon equivalent is used and an alloy with 0.3% ferrochrome, 0.5% copper and 0.05% tin is added, and the casting and inoculation are carried out according to step (2): before being taken out of the furnace, 0.3% silicon-barium inoculant is added to the ladle; the casting is carried out when the molten iron temperature is 1330°C, and 0.2% silicon-barium inoculant is added during the casting to carry out in-stream inoculation.

[0076] The iron casting in this comparative example is a 16.4-ton column (gross weight), with a casting rib of 30 mm, an average wall thickness of 80 mm, and a guide rail thickness of 200 mm. The chemical composition is shown in Table 1, and the mechanical properties are shown in Table 2.

[0077] Table 1: Chemical composition of gray iron castings of Examples 1 to 3 and Comparative Examples 1 to 2 (%)

[0078]

[0079] Example 1, Example 2, and Example 3 all use high carbon equivalent (carbon equivalent exceeding 3.9%) and are treated using different inoculation methods.

[0080] Comparative Example 1 is a low-carbon equivalent gray iron casting in the prior art. The inoculant used for ladle inoculation and stream inoculation is a normal silicon-barium inoculant. Except for the low content of C, the contents of other elements are within the range of the present invention.

[0081] Comparative Example 2 is a common silicon-barium inoculant and adopts a large amount of alloy. The Mg content in the obtained gray iron casting is 0, and the contents of Cr, Cu, and Sn are relatively high.

[0082] Table 2: Properties of gray iron castings obtained in Examples 1 to 3 and Comparative Examples 1 to 2

[0083]

[0084] From the data in Table 2, it can be seen that the gray iron castings prepared in Examples 1 to 3 have uniform hardness, a small difference in hardness between thick and thin walls, and a uniform difference of about 10 HBW (generally, the hardness difference is 20 HBW or even more), and have good cutting performance; the guide rail stress is less than 50 MPa, the deformation is small, and there are no shrinkage defects. Figure 1 and Figure 2 The metallographic images of the gray iron casting body prepared in Example 1 before and after corrosion are shown respectively; Figure 3 and Figure 4 The metallographic images of the gray iron casting body prepared in Example 2 before and after corrosion are shown respectively. Figures 1 to 4 It can be seen that the gray iron castings prepared in Examples 1 and 2 are all A-type graphite, with large graphite curvature, passivated graphite tips, reduced cutting effect, no E-type graphite, and good structural uniformity.

[0085] From the data in Table 2, it can be seen that the gray iron castings prepared in Example 3 have a small hardness difference and are easy to process; the machine tool castings have low stress and small deformation after processing. Figure 5 This is the metallographic image of the gray iron casting body prepared in Example 3 before corrosion. Figure 5 It can be seen that the gray iron casting prepared in Example 3 is all A-type graphite, with large graphite curvature, passivated graphite tips, reduced cutting effect, no E-type graphite, and good organizational uniformity.

[0086] Comparative Example 1 is an ordinary low-carbon equivalent gray iron casting. From the data in Table 2, it can be seen that the hardness deviation of the gray casting of Comparative Example 1 is large, which is not conducive to cutting processing; and its guide rail stress is 199 MPa, which is close to the tensile strength value. The tensile stress is large, and the casting is prone to cracks, and may even be scrapped due to cracking. Figure 6 and Figure 7 The metallographic images of the gray iron casting body before and after corrosion prepared in Comparative Example 1 are shown respectively. Figure 6 、 7 As can be seen from the figure, the gray cast iron prepared in Comparative Example 1 exhibits uneven metallographic distribution, with less than 70% type A graphite and the presence of type E graphite (directional graphite). The gray cast iron in Comparative Example 2, inoculated with conventional silicon-barium inoculants for both in-laundering and stream inoculation, and supplemented with a large amount of alloying aid at a high carbon equivalent, results in a gray cast body with only 55% pearlite. The data in Table 2 demonstrates that the hardness of the gray cast iron in Comparative Example 2, both for the body and the guide rail, is low, and the high ferrite content makes it difficult to machine (tool sticking). Furthermore, the guide rail stress, at 152 MPa, makes the casting susceptible to cracking and deformation after machining, resulting in high casting costs.

[0087] From the data in Table 2, it can be seen that the tensile strength of the test bar of Example 1 is high and the hardness is uniform; while the tensile strength of the test bar of Comparative Example 1 is low and the hardness is uneven, with the hardness difference exceeding 20 HBW. Figure 8 This is the metallographic image of the test bar before corrosion in Example 1; Figure 9 This is the metallographic image of the test bar before corrosion in the furnace of comparative example 1. Figure 8 It can be seen that the test rod of Example 1 is all A-type graphite, with large graphite curvature, passivated tip graphite, reduced cutting effect, no E-type graphite, and good organizational uniformity; Figure 9 It can be seen that the type A graphite of the test rod of Comparative Example 1 is obviously very straight, without blunting (sharp tip, high stress), and not bent.

[0088] In summary, the present invention uses scrap steel ≥40%, pig iron ≤20%, and recycled iron as main materials, and adopts a specific inoculant to perform at least two inoculation treatments on gray cast iron liquid, thereby obtaining high-quality refined iron liquid with high nitrogen (greater than 100 ppm) and low oxygen (less than 40 ppm). The obtained gray iron castings have good all-A-type graphite, large graphite curvature, passivated graphite tips, reduced splitting effect, no E-type graphite, and a test bar tensile strength stably reaching HT250 or above, with good hardness structure uniformity and a hardness difference within 10 HBW. The present invention increases the carbon equivalent to above 3.9% or even 4.0%, approaching the eutectic point of 4.23%. By adding a small amount of alloying agents, gray iron castings can obtain high-quality all-A graphite and its quantity. This allows the gray iron castings to achieve both low stress and high strength without significantly increasing costs, thus addressing defects such as shrinkage and shrinkage cavities. Furthermore, the castings themselves exhibit ultra-low stress, simplifying post-casting processes or eliminating the primary or even secondary annealing required for machine tool castings. Annealing takes approximately 3-4 days and costs approximately 600 yuan per ton. The preparation methods of Examples 1 and 3 can save 4,200 yuan and 9,840 yuan over 3 days, respectively, significantly reducing casting production cycle and casting costs. For a 10,000-ton annual production of machine tool castings, this can even save 600,000 yuan.

[0089] The above specific embodiments describe the implementation of the present invention in detail, but the present invention is not limited to the specific details of the above embodiments. Within the scope of the claims and technical concept of the present invention, various simple modifications and changes can be made to the technical solution of the present invention, and these simple modifications all fall within the scope of protection of the present invention.

Claims

1. A method for preparing high carbon equivalent, high strength and low stress machine tool gray iron castings, characterized in that: Gray cast iron liquid is inoculated at least twice; the inoculants used in the inoculation treatment include at least one low-magnesium rare earth inoculant and at least one nitrogen-containing inoculant.

2. The method for preparing a high carbon equivalent, high strength, low stress machine tool gray iron casting according to claim 1, characterized in that: The low-magnesium rare earth inoculant or nitrogen-containing inoculant is added into the bag, and correspondingly, the nitrogen-containing inoculant or low-magnesium rare earth inoculant is added with the flow; or, the low-magnesium rare earth inoculant and nitrogen-containing inoculant are added into the bag or added with the flow at the same time.

3. The method for preparing a high carbon equivalent, high strength, low stress machine tool gray iron casting according to claim 1, characterized in that: The low-magnesium rare earth inoculant and / or nitrogen-containing inoculant is used in combination with a silicon-barium inoculant; the amount of the silicon-barium inoculant used is 0.15% to 0.25% of the total mass of the molten iron.

4. The method for preparing a high carbon equivalent, high strength, low stress machine tool gray iron casting according to claim 1, characterized in that: The dosage of the low-magnesium rare earth inoculant is 0.05% to 0.3% of the total mass of the molten iron.

5. The method for preparing a high carbon equivalent, high strength, low stress machine tool gray iron casting according to claim 1, characterized in that: The content of Mg+RE in the low-magnesium rare earth inoculant is ≥5%; the content ratio of Mg to RE is (2-4): (1-3).

6. The method for preparing a high carbon equivalent, high strength, low stress machine tool gray iron casting according to claim 1, characterized in that: The nitrogen-containing inoculant is a nitride alloy inoculant.

7. The method for preparing a high carbon equivalent, high strength, low stress machine tool gray iron casting according to claim 6, characterized in that: In the nitride alloy inoculant, the nitrogen content is 6.7% to 7.5%, the alloy content is 80% to 85%, and the balance is Fe.

8. The method for preparing a high carbon equivalent, high strength, low stress machine tool gray iron casting according to claim 1, characterized in that: The following steps are involved: (1) Smelting: Scrap steel, pig iron and recycled iron are used as main materials, and coal-based carburizer, ferrosilicon, ferromanganese and silicon carbide are used as auxiliary materials for smelting. The smelting overheating temperature is ≥1500℃; (2) Pouring and inoculation: Before being taken out of the furnace, at least one inoculant is added to the ladle for at least one inoculation treatment; pouring is carried out when the molten iron temperature is 1320~1350℃, and a secondary inoculant is added during pouring for inoculation treatment.

9. The method for preparing a high carbon equivalent, high strength, low stress machine tool gray iron casting according to claim 8, characterized in that: The dosage of the main materials is: scrap steel ≥ 40%, pig iron ≤ 20%, and the rest is recycled iron, with the total amount being 100%.

10. A high carbon equivalent, high strength, low stress machine tool gray iron casting, characterized in that: Its chemical composition by mass percentage is: carbon 3.2%~4.0%, silicon 1.0%~2.4%, manganese 0.4%~1.3%, sulfur ≤0.12%, phosphorus ≤0.1%, chromium ≤0.4%, copper ≤0.4%, magnesium ≤0.02%, nitrogen ≤0.02%, the remainder is Fe and unavoidable impurity elements, and the carbon equivalent is ≥3.9%.

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