Inoculating agent and application thereof in preparation of machine tool castings

Abstract

The invention discloses a nucleating agent and application thereof in preparation of machine tool castings, and relates to the technical field of casting of gray cast iron. The nucleating agent provided by the invention comprises the following components in percentage by mass: 5-7% of nitrogen, 2-4.5% of magnesium, 1-3% of rare earth, 1-3% of barium, 1-2% of calcium, 0-1.5% of aluminum, 20-50% of manganese, 30-65% of silicon and the balance of iron and inevitable impurities. The inoculant is prepared by smelting, cooling and crushing raw materials into particles. When the inoculant provided by the invention is used for preparing machine tool cast iron, the strength of the cast iron can be ensured and the hardness uniformity can be improved while high carbon equivalent is maintained on the premise of controlling the cost; the casting stress is obviously reduced, and the energy absorption and shock absorption effects are better; the hardness is moderate, the good cutting performance is achieved, and the machining precision is guaranteed; deformation of machine tool castings is reduced, and precision retentivity is improved.

Description

Technical Field

[0001] The invention relates to the technical field of gray cast iron casting, in particular to an inoculant for preparing machine tool castings. Background Art

[0002] At present, in the casting production process, the carbon equivalent of heavy and ultra-heavy machine tool castings must be maintained at around 3.7-3.8% to ensure that most of the A-type graphite and more than 95% of the pearlite matrix structure can be maintained, and sufficient strength ≥250Mpa can be guaranteed. While maintaining high strength at a low carbon equivalent, the stress is also large, and there are many casting shrinkage defects. The uneven hardness makes its processing performance poor, and it is difficult to ensure the quality of machine tool castings and their precision stability.

[0003] Since the carbon equivalent is increased during the casting process, the smelting process requires the addition of large amounts of precious alloys such as copper, tin, and antimony. This not only significantly increases casting costs but also increases stress. It also causes the A-type graphite to thin and even form a large amount of E-type graphite, reducing the amount of A-type graphite. This prevents machine tool castings from obtaining high-quality A-type graphite and its quantity. Ultimately, the machine tool castings suffer from poor vibration damping and poor precision retention, ultimately forming a vicious cycle. Therefore, designing an inoculant that can be used in the manufacture of high-carbon equivalent, high-strength, low-stress machine tool castings is an effective solution to these problems. Summary of the Invention

[0004] The present invention provides an inoculant for preparing heavy-duty and ultra-heavy-duty machine tool castings, and its preparation and application methods. By using this inoculant, the strength of gray cast iron (such as HT250) can be guaranteed without increasing costs on a large scale; the inoculant can increase the liquidus temperature of the casting iron liquid, strengthen the austenite precipitation range and refine the primary austenite dendrites, shorten the graphite growth time in the early eutectic stage and precipitate a large amount of graphite for a long time in the late eutectic stage, ultimately making the graphite thinner, shorter, and blunt, and making the machine tool castings obtain more than 95% of excellent curved, blunt, thin and short A-type graphite, with better energy absorption and shock absorption effects; the inoculant can change the sub- The eutectic solidification process of eutectic gray cast iron optimizes the graphite and microstructure differences under different cooling rates, homogenizes them, refines graphite, and improves the uniformity of graphite and microstructure, significantly reducing the internal stress of the casting, with the stress value below 50Mpa; the inoculant can reduce the eutectoid transformation temperature and the time it takes for the eutectoid transformation temperature to rise, refine the pearlite layer spacing, and stably obtain a hardness of 170-190HBW, with good cutting performance, thereby ensuring good processing accuracy, ultimately reducing the deformation of machine tool castings and improving precision retention. The above-mentioned effects of the present invention are specifically achieved through the following technologies.

[0005] A gestating agent, characterized in that its composition, by mass fraction, includes 5-7% nitrogen, 2-4.5% magnesium, 1-3% rare earth, 1-3% barium, 1-2% calcium, 0-1.5% aluminum, 20-50% manganese, 30-65% silicon, and the remainder is iron and unavoidable impurities.

[0006] Furthermore, in the inoculant, the total mass of magnesium and rare earth is not less than 5% of the total mass of the inoculant.

[0007] Furthermore, in the inoculant, the mass ratio of magnesium to rare earth is 3:2.

[0008] Furthermore, the particle size of the inoculant is 0.1-30 mm

[0009] The present invention also provides a method for preparing the inoculant, which is prepared by smelting raw materials, cooling them, and crushing them into particles.

[0010] The present invention also provides an application method of any one of the above-mentioned inoculants, which is used to inoculate the molten iron of the machine tool cast iron to produce a machine tool casting; the composition of the machine tool casting includes, by mass fraction, 3.0-4.0% carbon, 1.5-3.5% silicon, 0.4-1.3% manganese, ≤0.12% sulfur, ≤0.1% phosphorus, ≤0.25% chromium, ≤0.4% copper, ≤0.4% tin, ≤0.1%, ≤0.02% magnesium, ≤0.02% nitrogen, and the balance is Fe and unavoidable impurities.

[0011] Furthermore, in the above application method, the amount of the inoculant added is 0.1-0.5% of the weight of the molten iron.

[0012] Furthermore, the amount of the inoculant added is increased or decreased according to the nitrogen content in the molten iron. Generally, the nitrogen content in the final molten iron after adding the inoculant does not exceed 0.02%.

[0013] Furthermore, the inoculant is added in the bag or added along with the flow.

[0014] Furthermore, when the inoculant is added in the ladle, the particle size is selected to be 5-15 mm, and the added amount is 0.1-0.5% of the weight of the molten iron; when the inoculant is added with the flow, the particle size is selected to be 0.2-0.8 mm, and the added amount is 0.05-0.25% of the weight of the molten iron.

[0015] Compared with the prior art, the present invention is beneficial in that: the present invention provides an inoculant for the production of machine tool castings (gray cast iron). By using this inoculant, while ensuring high carbon equivalent and casting strength (especially tensile strength), the stress of the casting can be significantly reduced (the tip graphite passivation reduces stress), the energy absorption and shock absorption effect is better, the cutting performance is good, and the processing accuracy is guaranteed; ultimately, the deformation of the machine tool casting is reduced and the accuracy retention is improved. BRIEF DESCRIPTION OF THE DRAWINGS

[0016] Figure 1 and Figure 2 Metallographic graphite photographs of gray cast iron prepared after the inoculants prepared in Example 1 and Example 2 were added to the bag, respectively, at 100 times magnification (left picture) and 400 times magnification (right picture).

[0017] Figure 3 These are metallographic graphite photographs of the gray cast iron prepared after the inoculant prepared in Example 2 was added into the stream, and are photographs magnified 100 times (left) and 400 times (right).

[0018] Figure 4 These are metallographic graphite photographs of gray cast iron prepared after adding ordinary silicon-barium-calcium inoculant into the ladle, magnified 100 times (left) and 400 times (right). DETAILED DESCRIPTION

[0019] 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.

[0020] The present invention provides an inoculant for manufacturing machine tool castings (gray cast iron), which comprises 5-7 wt% nitrogen, 2-4.5 wt% magnesium, 1-3 wt% rare earth, 1-3 wt% barium, 1-2 wt% calcium, 0-1 wt% aluminum, 20-50 wt% manganese, 30-65 wt% silicon, and the remainder being iron and unavoidable impurities.

[0021] In some embodiments of the present invention, the total mass of magnesium and rare earth is not less than 5% of the total mass of the inoculant.

[0022] In some embodiments of the present invention, the particle size of the inoculant is 0.2-15 mm.

[0023] The inoculant provided by the present invention is prepared by smelting raw materials containing nitrogen, magnesium, rare earth, barium, calcium, aluminum, manganese and silicon elements, cooling them and crushing them into particles.

[0024] The present invention also provides a method for producing machine tool castings using the above-mentioned inoculant. Specifically, molten iron with qualified composition is first smelted according to a predetermined ratio and poured into a ladle. Subsequently, any of the following inoculation methods can be used for inoculation:

[0025] (1) Before pouring the molten iron into the ladle, the inoculant provided by the present invention is first added into the ladle;

[0026] (2) After pouring the molten iron into the ladle, remove the slag, then add the inoculant provided by the present invention to the surface of the molten iron, and then pour the ladle to make the molten iron uniform, which has a better effect;

[0027] (3) During pouring, the inoculant provided by the present invention is added along with the flow.

[0028] Optionally, the element composition of the machine tool casting includes, by mass fraction, carbon 3.0-4.0%, silicon 1.5-3.5%, manganese 0.4-1.3%, sulfur ≤0.12%, phosphorus ≤0.1%, chromium ≤0.25%, copper ≤0.4%, tin ≤0.1%, magnesium ≤0.02%, nitrogen ≤0.02%, and the balance is Fe and unavoidable impurities.

[0029] Preferably, the elemental composition of the machine tool casting includes, by mass fraction, carbon 3.2-3.4%, silicon 2.0-2.5%, manganese 0.6-1.0%, sulfur ≤0.12%, phosphorus ≤0.1%, chromium ≤0.25%, copper ≤0.4%, tin ≤0.06%, magnesium 0.001-0.005%, nitrogen 0.01-0.014%, and the balance being Fe and unavoidable impurities.

[0030] Example 1

[0031] The elemental composition of the inoculant provided in this embodiment includes, by mass fraction, 7% nitrogen, 4.5% magnesium, 1% rare earth, 1% barium, 2% calcium, 0.5% aluminum, 20% manganese, 50% silicon, and the remainder is iron and unavoidable impurities.

[0032] Example 2

[0033] The elemental composition of the inoculant provided in this embodiment includes, by mass fraction, 6% nitrogen, 3.5% magnesium, 1.5% rare earth, 3% barium, 1% calcium, 0.5% aluminum, 25% manganese, 45% silicon, and the remainder is iron and unavoidable impurities.

[0034] Example 3

[0035] The elemental composition of the inoculant provided in this embodiment includes, by mass fraction, 5% nitrogen, 3% magnesium, 2% rare earth, 3% barium, 2% calcium, 0.5% aluminum, 30% manganese, 40% silicon, and the remainder is iron and unavoidable impurities.

[0036] Application Example 1: Performance testing of gray iron castings prepared using different preparation methods

[0037] This application example uses the inoculant prepared in Examples 1-3 and adopts different preparation methods to prepare gray iron castings.

[0038] The first specific preparation method is:

[0039] (1) Melting: The main materials are scrap steel, pig iron and recycled iron with a total mass of not less than 40%, and coal-based recarburizer, ferrosilicon, ferromanganese and silicon carbide as auxiliary materials. The smelting overheating temperature is ≥1500℃.

[0040] (2) Pouring and inoculation: Before the molten iron is discharged from the furnace, the inoculant of Example 1 is added to the bottom of the ladle at a level of 0.25% of the total amount of the discharged molten iron. The molten iron is poured when the temperature is above 1330°C.

[0041] The second specific preparation method is:

[0042] (1) Melting: The main materials are scrap steel, pig iron and recycled iron with a total mass of not less than 40%, and coal-based recarburizer, ferrosilicon, ferromanganese and silicon carbide as auxiliary materials. The smelting overheating temperature is ≥1500℃.

[0043] (2) Pouring and inoculation: After the molten iron is discharged from the furnace, the slag is removed and the inoculant of Example 2 is added to the surface of the molten iron ladle at a concentration of 0.2% of the total amount of the molten iron. The molten iron is poured when the temperature is above 1330°C.

[0044] The third specific preparation method (control group) is:

[0045] (1) Melting: The main materials are scrap steel, pig iron and recycled iron with a total mass of not less than 40%, and coal-based recarburizer, ferrosilicon, ferromanganese and silicon carbide as auxiliary materials. The smelting overheating temperature is ≥1500℃.

[0046] (2) Pouring and inoculation: Before the molten iron is discharged from the furnace, a common silicon-barium-calcium inoculant (mass ratio: silicon 65-75%, barium 4-6%, calcium 1-2%, aluminum <1.5%, the rest is iron and unavoidable impurities) accounting for 0.25% of the total amount of molten iron is added to the bottom of the ladle. The molten iron is poured when the temperature is above 1330℃.

[0047] The gray iron castings produced using the two specific methods described above are 7-ton small beams with an average wall thickness of 70 mm and a guide rail thickness of 120 mm. The chemical compositions of the two gray iron castings are shown in Table 1, and their mechanical properties are shown in Table 2.

[0048] Table 1

[0049]

[0050] As can be seen from the data in Table 1, the control group is a gray iron casting with a high carbon equivalent in the prior art. The inoculant used for the bottom inoculation in the control group is a common silicon-barium-calcium inoculant. The Mg content in the obtained gray iron casting is 0 (the content is relatively low), and the contents of Cr and Cu are relatively high. The contents of other elements are all within the range of the present invention.

[0051] Table 2

[0052]

[0053] Figure 1 and 2 The following are metallographic photographs of the first specific preparation method described above, using the inoculants of Examples 1 and 2 at a 0.3% addition level at the bottom of the ladle (the left image is magnified 100x, and the right image is magnified 400x). It can be seen that the inoculant thins and shortens the graphite, blunting its tip. Furthermore, the machine tool castings possess over 95% excellent curved, blunted, and short A-type graphite, ensuring casting strength (especially tensile strength) while significantly reducing casting stress (passivation of the graphite tip reduces stress). The over 95% A-type graphite exhibits enhanced energy absorption and vibration reduction, excellent cutting performance, and guaranteed machining accuracy. Ultimately, the machine tool castings exhibit reduced deformation and improved precision retention.

[0054] Figure 3 The metallographic images of the inoculant prepared using the second specific preparation method and Example 2 at a 0.2% addition rate are shown (the left image is magnified 100 times, and the right image is magnified 400 times). It can be seen that the graphite morphology is more curved, fine, and blunt. Furthermore, it can be seen that the inoculant makes the graphite thinner and shorter, and the tip is blunted. Furthermore, the machine tool castings obtain more than 95% of excellent curved, blunt, and fine A-type graphite, which can ensure the strength of the castings (especially tensile strength) while significantly reducing the stress of the castings (the blunting of the tip graphite reduces stress). More than 95% of the A-type graphite has better energy absorption and shock absorption effects, good cutting performance, and guaranteed processing accuracy. Ultimately, the deformation of the machine tool castings is reduced, and the accuracy retention is improved.

[0055] Figure 4Metallographic images of a conventional silicon-barium-calcium inoculant at a 0.25% addition level (left image, 100x magnification, right image, 400x magnification) are shown. The graphite is large, long, and straight, with minimal or no curvature. The graphite tips are slightly blunted, with over 30% E-type graphite present. The A-type graphite content is less than 70%, and it is difficult to achieve a percentage exceeding 90%. The resulting gray cast iron exhibits an uneven metallographic distribution. The thinning of the graphite tips increases stress in the casting (the graphite tip increases stress). The 30% E-type graphite not only degrades cutting performance but also reduces energy absorption and vibration damping, leading to uncertainties in precision. Ultimately, this increases deformation in machine tool castings and reduces precision retention.

[0056] By comparison Figure 1-4 It can be found that, regardless of whether the inoculant is added in the bag or added with the flow, the inoculant of the present invention can make the graphite thinner, shorter, and passivated, and enable the machine tool casting to obtain more than 95% of excellent curved, passivated, thin and short type A graphite, with little cutting effect, no E-type graphite, and good structural uniformity; while ensuring the strength of the casting (especially the tensile strength), the stress of the casting is significantly reduced (the passivation of the tip graphite reduces stress), more than 95% of the type A graphite has better energy absorption and shock absorption effect, has good cutting performance, and ensures processing accuracy; ultimately, the deformation of the machine tool casting is reduced, and the precision retention is improved.

[0057] From the data in Table 2, it can be seen that the gray iron castings prepared by adding the inoculant prepared in Example 1 in a ladle manner have uniform hardness, high tensile strength, 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, only 38.2 MPa, the deformation is small, and there are no shrinkage defects.

[0058] The inoculant prepared in Example 2 was added in-stream, and the gray iron castings prepared had a small hardness difference and were easy to process; the stress of the machine tool castings was small, the maximum stress of the guide rail was only 33.7 MPa, and the deformation after processing was also small.

[0059] The control group's gray iron castings were inoculated with a standard silicon-barium-calcium inoculant. The resulting gray cast guide rails contained only 55% pearlite, resulting in slow cooling and low hardness in the center, and rapid cooling and high hardness at the ends. This resulted in low tensile strength, a large hardness difference between thick and thin walls, and extremely uneven hardness distribution. The maximum stress was 115 MPa, resulting in high compressive stress. Consequently, the machine tool caused significant casting deformation, poor precision retention, poor processability, and increased casting costs.

[0060] In summary, the present invention utilizes scrap steel (≥40%), pig iron (≤20%), and recycled iron as primary materials, and utilizes the inoculant of the present invention to inoculate molten gray iron, resulting in high-quality refined molten iron with high nitrogen (greater than 100 ppm) and low oxygen (less than 40 ppm). This results in gray iron castings with excellent all-A-type graphite, high graphite curvature, passive graphite tips, reduced splitting, no E-type graphite, and excellent structural uniformity. The present invention increases the carbon equivalent to over 3.9%, approaching the eutectic point of 4.23%. By adding a small amount of alloying agents, the gray iron castings achieve high-quality A-type graphite and sufficient quantity. This achieves both low stress and high strength without significantly increasing costs, addressing defects such as shrinkage and porosity in the castings. Furthermore, the ultra-low stress inherent in the castings simplifies the casting process or eliminates the annealing step, further reducing casting costs.

[0061] 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. An inoculant, characterized in that Its composition by mass fraction includes nitrogen 5-7%, magnesium 2-4.5%, rare earth 1-3%, barium 1-3%, calcium 1-2%, aluminum 0-1.5%, manganese 20-50%, silicon 30-65%, and the rest is iron and inevitable impurities.

2. The inoculant according to claim 1, characterized in that The total mass of magnesium and rare earth is not less than 5% of the total mass of the inoculant.

3. The inoculant according to claim 2, characterized in that The mass ratio of magnesium to rare earth is 3:

2.

4. The inoculant according to claim 1, characterized in that The particle size of the inoculant is 0.1-30 mm.

5. The method for preparing the inoculant according to any one of claims 1 to 4, characterized in that: The raw materials containing nitrogen, magnesium, rare earth, barium, calcium, aluminum, manganese and silicon are smelted, cooled and crushed into particles.

6. Use of the inoculant according to any one of claims 1 to 4, characterized in that: Used to inoculate the molten iron of the machine tool cast iron to produce a machine tool casting; the composition of the machine tool casting includes, by mass fraction, 3.0-4.0% carbon, 1.5-3.5% silicon, 0.4-1.3% manganese, ≤0.12% sulfur, ≤0.1% phosphorus, ≤0.25% chromium, ≤0.4% copper, ≤0.1% tin, ≤0.02% magnesium, ≤0.02% nitrogen, and the balance is Fe and unavoidable impurities.

7. The use according to claim 6, characterized in that The amount of the inoculant added is 0.1-0.5% of the weight of the molten iron.

8. The use according to claim 7, characterized in that The amount of the inoculant added is based on the nitrogen content in the final molten iron after the inoculant is added not exceeding 0.02%.

9. The use according to claim 6, characterized in that The inoculant is added in the bag or added along with the flow.

10. The use according to claim 6, characterized in that When the inoculant is added in the ladle, the particle size is selected to be 5-15 mm, and the added amount is 0.1-0.5% of the weight of the molten iron; when the inoculant is added with the stream, the particle size is selected to be 0.2-0.8 mm, and the added amount is 0.05-0.25% of the weight of the molten iron.

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