Nodular cast iron for high-performance machine tool casting and preparation method of nodular cast iron

Abstract

The invention belongs to the field of casting production of castings, and particularly relates to nodular cast iron for high-performance machine tool castings and a preparation method thereof. The nodular cast iron comprises the following chemical components in percentage by mass: 3.10%-3.20% of C, 3.50%-3.60% of Si, 0.2%-0.3% of Mn, 1t of P and the balance of Fe and inevitable impurities. 0.03%, S: lt; the alloy comprises the following components in percentage by weight: 0.015% of Fe, 0.038%-0.052% of Mg, 0.30%-0.40% of Cu, 0.01%-0.02% of Sn, 0.01%-0.02% of Sb, 0.01%-0.02% of rare earth elements, 4.20%-4.40% of CE and the balance of Fe and inevitable impurity elements. The nodular cast iron disclosed by the invention has high strength, high rigidity and good toughness, and meets performance requirements on high-end machine tool castings. The preparation method is simple in process, convenient to operate and suitable for industrial production.

Description

Technical Field

[0001] The present invention belongs to the field of casting production of castings, and particularly relates to a ductile iron for high-performance machine tool castings and a preparation method thereof. Background Art

[0002] Ductile iron is widely used in fields such as machine tool castings due to its good castability and comprehensive mechanical properties. Traditional ductile iron usually increases the pearlite content by adding alloying elements to improve strength, but the elongation decreases as the strength increases. Under the same tensile strength, silicon solution-strengthened ductile iron has better plasticity, as well as good wear resistance and machining performance, and can meet the growing demand for key components of high-end machine tools. At present, the product quality of China's machine tool industry has been significantly improved, but there are still deficiencies in terms of stability and dimensional reliability during the processing compared with the international advanced level. The main problem lies in the uneven structure and poor comprehensive performance of key components such as machine tool guide rails and worktables, which cannot meet the usage requirements. Therefore, developing high-performance ductile iron materials for machine tool castings is of great significance and value for the development of China's machine tool manufacturing technology.

[0003] Chinese Patent with application number 202211720499.4 discloses an as-cast high-strength ductile iron composed of the following components by weight percentage: Mn 0.67 - 0.81 wt%, Cu 0.23 - 0.32 wt%, Ni 0.05 - 0.09 wt%, C 3.6 - 3.9 wt%, Si 2.3 - 2.7 wt%, P 0 - 0.03 wt%, S 0 - 0.02 wt%, Cr 0.01 - 0.63 wt%, Mg 0.04 - 0.08 wt%, RE 0.01 - 0.02 wt%, and the balance is Fe and inevitable impurities.

[0004] Chinese Patent with application number 202211286374.5 discloses a QT600 ductile iron and a preparation method thereof, including the following steps: Step S1, batching: By mass fraction, the raw material ratio is as follows: pig iron 20% - 30%, scrap steel 20% - 30%, return material 40% - 60%, electrolytic copper plate 0.6% - 0.7%, nickel plate 0.12% - 0.17%, electrolytic chromium 0.22% - 0.27%; Step S2, melting: First add scrap steel, pig iron and return material, and after all the scrap steel, pig iron and return material are melted, add nickel plate, electrolytic chromium and electrolytic copper plate, and the melt tapping temperature is controlled at 1470 - 1500 °C; Step S3, spheroidizing: Place the spheroidizing agent and inoculant in the spheroidizing chamber of the spheroidizing ladle, cover with a covering agent, and tap the molten iron for spheroidizing and in-ladle inoculation; Step S4, pouring: The pouring temperature is controlled at 1370 - 1400 °C, and the in-stream inoculant is added with the molten iron during pouring to obtain the finished QT600 ductile iron.

[0005] Although the mechanical properties of ductile iron are improved to a certain extent, its cost is relatively high. For the above reasons, in the face of the increasing quality requirements of high-end machine tools and to ensure the accuracy, stability and machining quality of machine tools, it is urgent to develop a high-performance and low-cost ductile iron material for machine tools. Summary of the Invention

[0006] To solve the above problems, the present invention provides a ductile iron for high-performance machine tool castings and a preparation method thereof. The ductile iron of the present invention has high strength, high stiffness and good toughness, meeting the performance requirements for high-end machine tool castings.

[0007] The technical solutions adopted by the present invention to achieve the above object are as follows:

[0008] A ductile iron for high-performance machine tool castings, in terms of mass percentage, contains the following chemical components: C: 3.10%-3.20%, Si: 3.50%-3.60%, Mn: 0.2%-0.3%, P: <0.03%, S: <0.015%, Mg: 0.038-0.052%, Cu: 0.30%-0.40%, Sn: 0.01%-0.02%, Sb: 0.01%-0.02%, rare earth elements: 0.01%-0.02%, CE (carbon equivalent): 4.20%-4.40%, and the balance is Fe and inevitable impurity elements.

[0009] In the ductile iron of the present invention, trace alloying elements of tin and antimony with relatively low comprehensive cost are used to strengthen the matrix, reducing the addition of other precious alloying elements and lowering the production cost. Both tin and antimony have the effect of refining grains, and they can make the structure of ductile iron more uniform and fine. The refined grains can improve the strength and toughness of the material, and are beneficial to improving the surface quality and dimensional accuracy during the machining process. Tin and antimony can also increase the spheroidization rate of the graphite structure and the content of pearlite structure in ductile iron, reduce the pearlite lamellar spacing, and effectively prevent the graphite from deforming, thereby improving the comprehensive mechanical properties of the casting. Tin and antimony cooperate with copper, and through grain boundary strengthening, jointly improve the mechanical properties of ductile iron. Rare earth elements have the functions of deoxidizing and desulfurizing in ductile iron, which can purify the molten iron. Rare earth elements also have a certain anti-decay performance, can improve the graphite morphology, inhibit chill, refine graphite balls, improve the roundness of graphite balls, and further improve the casting performance of ductile iron, reducing casting defects. At the same time, rare earth elements can reduce the supercooling tendency of antimony, reduce shrinkage porosity, and can form intermetallic compounds and composite precipitation phases with tin and antimony, such as LaSn, La 3 Sn 5 、Mg 3 Sb 2, MgSbS, etc., reduce the brittle phases formed by the segregation of tin and antimony at grain boundaries, and can also serve as an effective matrix for graphite nucleation to promote graphite nucleation, improving the microstructure and properties of castings.

[0010] Preferably, the rare earth elements include La: 28 - 32% by mass percentage.

[0011] The preparation method of the ductile iron for high-performance machine tool castings described above includes the following steps:

[0012] (1) Dry and process the raw materials for standby. The raw materials include: scrap steel, pig iron, return scrap, silicon carbide, pure copper, pure tin, pure antimony, lanthanum metal, rare earth ferrosilicon;

[0013] (2) Melting: Melting the dried scrap steel, pig iron, and return scrap to obtain molten iron, then adding a carbon increasing agent and silicon carbide to the molten iron for continuous melting. Before tapping, add a slagging agent to the molten iron for slag making and slag skimming treatment, and then add pure copper, pure tin, pure antimony, lanthanum metal, and rare earth ferrosilicon for melting and mixing, and then tap to obtain a cast iron melt. The melting temperature and tapping temperature are both 1510 - 1540 °C. The dosages of the carbon increasing agent and silicon carbide are 1 - 1.5% and 0.8 - 1.2% of the mass of the molten iron respectively, and the dosage of the slagging agent is 0.1 - 0.5% of the mass of the molten iron;

[0014] (3) Spheroidizing and inoculation process: After tapping, adopt the pressure-packed pouring method for spheroidizing treatment, and then use the first inoculant for the first inoculation; the dosage of the spheroidizing agent in the spheroidizing treatment is 1.0 - 1.2% of the mass of the cast iron melt, and the dosage of the first inoculant is 0.3 - 0.4% of the mass of the cast iron melt;

[0015] (4) Casting: Cast the cast iron melt processed in step (3) at a temperature of 1360 - 1390 °C to obtain; during casting, use the second inoculant for in-stream inoculation. The dosage of the second inoculant is 0.05 - 0.1% of the mass of the cast iron melt. During in-stream inoculation, mix the second inoculant with tungsten carbide and then add it to the cast iron melt.

[0016] Preferably, the slagging agent in step (2) includes by weight: 25 - 35 parts of NaCl, 40 - 50 parts of KCl, 12 - 17 parts of fluorite, and 7 - 12 parts of sodium carbonate.

[0017] Preferably, the spheroidizing agent in step (3) is a lanthanum-based magnesium spheroidizing agent, including the following components by weight percentage: 45 - 50% Si, 5 - 6% Mg, 0.4 - 0.7% La, and the balance is Fe.

[0018] Preferably, the first inoculant in step (3) is a ferrosilicon inoculant.

[0019] Preferably, the second inoculant in step (4) is a sulfur-oxygen inoculant, and the sulfur-oxygen inoculant comprises the following components by weight percentage: silicon (Si): 60-67%, calcium (Ca): 0.2-1.0%, barium (Ba): 0.3-0.5%, lanthanum (La): 1.5-2.0%, aluminum (Al): 0.5-1.5%, sulfur (S): 0.3-0.9%, oxygen (O): 0.3-0.9%, cerium: 0.5-1.2%, and the balance is iron.

[0020] Preferably, the sulfur-oxygen inoculant is spherical particles with a size of 0.2-0.8 mm.

[0021] Preferably: the amount of tungsten carbide used in step (4) is 0.03-0.05% of the mass of the cast iron melt, and the tungsten carbide is particles with a particle size of 800-1000 mesh.

[0022] The second inoculant used in the present invention is a sulfur-oxygen inoculant, which can strengthen the inoculation effect, improve the metallographic structure, refine graphite, and enhance the mechanical properties. Among them, silicon has a strong promoting effect on graphitization during the ductile iron casting process, can also reduce the melting point of the cast iron melt, and improve the fluidity of the cast iron melt. When casting complex-shaped castings, good fluidity can ensure that the cast iron melt smoothly fills all parts of the mold, avoiding casting defects such as misruns and cold shuts, thereby improving the forming quality of the castings. Calcium can promote the roundness of graphite balls, making the graphite balls more regular. In ductile iron, round graphite balls can better disperse stress, reduce stress concentration, and thus improve the mechanical properties of the castings, such as strength and toughness. Barium can improve the anti-decay ability of ductile iron. During a long solidification process or under complex casting conditions, the spheroidizing effect may decline. The presence of barium can make the inoculation effect last longer, ensuring that the graphite balls maintain good morphology and properties throughout the solidification process. Barium can also promote the refinement of graphite balls, and the refined graphite balls can increase the strength and toughness of ductile iron. Lanthanum and cerium, as rare earth elements, can change the growth morphology of graphite, making the graphite balls smaller, rounder, and more evenly distributed. Lanthanum can also improve the corrosion resistance and oxidation resistance of ductile iron. For machine tool castings, they may come into contact with various corrosive media or be in a high-temperature oxidation environment during operation. The presence of lanthanum can effectively delay the corrosion and oxidation processes of the castings and extend the service life of the castings. Aluminum can promote graphitization. During the solidification process of ductile iron, aluminum can change the crystallization conditions of the iron-carbon alloy, which is beneficial to the nucleation and growth of graphite balls. An appropriate amount of aluminum can make the graphite balls smaller and rounder, thereby improving the mechanical properties of ductile iron. Sulfur and oxygen are important components for nodule nucleation. During the ductile iron casting process, sulfur and oxygen can combine with other elements to form sulfides and oxides, which can serve as the nucleation cores of graphite balls and increase the number of graphite balls. More graphite balls can better disperse stress and improve the toughness and strength of ductile iron. At the same time, sulfur and oxygen in the sulfur-oxygen inoculant can supplement the deficiency of sulfur and oxygen in the cast iron melt after spheroidizing treatment, which is beneficial to maintaining the normal growth and distribution of graphite balls during the solidification process of ductile iron, thereby ensuring the quality and performance of the castings.

[0023] After the first inoculation, the nucleating particles in the cast iron melt may be partially consumed. For the second in-stream inoculation, sulfur and oxygen in the sulfur-oxygen inoculant can serve as important components for nodular graphite nucleation and provide nucleation sites again. These sulfur and oxygen can combine with other elements in the cast iron melt to form sulfides and oxides, increasing the number of graphite nodule nucleation. For example, during the solidification of ductile iron, more nucleation sites enable graphite nodules to grow at more positions, increasing the number of graphite nodules per unit volume, thus better dispersing stress and improving the toughness and strength of ductile iron. Rare earth elements such as lanthanum (La) and cerium (Ce) in the sulfur-oxygen inoculant for the second in-stream inoculation can play a modifying role. These elements can make the graphite nodules finer, more rounded, and more evenly distributed. Finer and more rounded graphite nodules can effectively reduce stress concentration and enhance the ability of ductile iron to resist external forces. At the same time, elements such as calcium (Ca) and barium (Ba) also help to maintain and improve the roundness of graphite nodules and improve the quality of graphite nodules. During the casting process, with the passage of time and the change of the temperature of the cast iron melt, the inoculation effect may decline. Barium (Ba) in the sulfur-oxygen inoculant for the second in-stream inoculation can improve the anti-decay ability of ductile iron. It can make the inoculation effect last longer, ensuring that graphite nodules can maintain good morphology and performance throughout the solidification process. This can effectively avoid problems such as poor spheroidization and poor graphite nodule morphology caused by the decline of the inoculation effect and ensure the stability of the casting quality. Barium (Ba), tin (Sn), and antimony (Sb) may promote grain refinement during the second in-stream inoculation. The refined grain structure can further improve the strength and toughness of ductile iron and is beneficial to obtaining better surface quality and dimensional accuracy during processing, which is very beneficial for the processing of high-performance machine tool castings, etc. Silicon (Si) can also reduce the melting point of the cast iron melt and improve the fluidity of the cast iron melt during the second in-stream inoculation. Good fluidity is particularly important for casting complex-shaped castings. It can ensure that the cast iron melt can smoothly fill all parts of the mold, avoiding casting defects such as incomplete pouring and cold shut, thus improving the forming quality of the casting. At the same time, the synergistic effect of other elements may also have a positive impact on the fluidity of the cast iron melt, making the casting process smoother.

[0024] During the second in-stream inoculation, tungsten carbide is fully mixed with the in-stream inoculant and added into the molten iron along with the flow. Tungsten carbide is evenly distributed in the molten iron. During the solidification process of the molten iron, tungsten carbide acts as a crystal nucleus, which is beneficial to the refinement of the microstructure. Since tungsten carbide has a high melting point, it hinders the growth of grains during the solution treatment in the heat treatment process, which is beneficial to the refinement of grains after heat treatment. The refined graphite spheres and grain structure can effectively reduce stress concentration and improve the strength and toughness of ductile iron. In addition, tungsten carbide particles have extremely high hardness. After it is added to ductile iron, dispersed hard phases are formed on the surface and inside of the casting. During the use of machine tool castings, such as in components like guide rails and sliders, these hard phases can effectively resist friction and wear, significantly improve the wear resistance of ductile iron, and extend the service life of the casting. Silicon (Si) in the sulfur-oxygen inoculant can lower the melting point of the cast iron melt and improve the fluidity of the cast iron melt. This good fluidity helps tungsten carbide to be evenly distributed in the casting and avoid the formation of new defects due to particle agglomeration.

[0025] The present invention has the following beneficial effects:

[0026] For the ductile iron for high-performance machine tool castings provided by the present invention, carbon and silicon synergistically promote the graphitization process, prompting carbon to precipitate in the form of graphite. Manganese mainly plays a role in stabilizing pearlite and also has certain deoxidation and desulfurization capabilities. In ductile iron, an appropriate amount of manganese can improve the strength and hardness of the matrix. It can combine with sulfur to form manganese sulfide (MnS), reducing the harmful effects of sulfur on spheroidization. Copper, tin, and antimony improve the strength and toughness of the matrix through solid solution strengthening and grain boundary strengthening. Manganese stabilizes pearlite and jointly adjusts the matrix structure with elements such as copper, tin, and antimony. Silicon dissolves in ferrite to produce solid solution strengthening. These elements cooperate with each other to achieve a good balance among the strength, hardness, and toughness of the matrix of ductile iron. The ductile iron of the present invention has high strength, high stiffness, and good toughness, meeting the performance requirements for high-end machine tool castings. The microstructure and properties of the ductile iron are as follows: the graphite size grade is 7-8, the spheroidization rate ≥ 90%, the spheroidization grade is 1-2, the pearlite content ≥ 30%, the tensile strength ≥ 600 MPa, the yield strength ≥ 450 MPa, the Brinell hardness ≥ 200 HB, the elastic modulus ≥ 160 GPa, the elongation rate ≥ 10%, and the impact absorption work at room temperature ≥ 20 J. Description of the Drawings

[0027] Figure 1 Among them, (a) is the graphite microstructure morphology diagram of the ductile iron obtained in Comparative Example 1, and (b) is the graphite microstructure morphology diagram of the ductile iron obtained in Example 3;

[0028] Figure 2 Among them, (a) is the matrix microstructure morphology diagram of the ductile iron obtained in Comparative Example 1, and (b) is the matrix microstructure morphology diagram of the ductile iron obtained in Example 3. Detailed implementation manners

[0029] Next, in combination with the embodiments of the present application, the technical solutions in the embodiments of the present application will be clearly and completely described. Obviously, the described embodiments are only a part of the embodiments of the present application, rather than all the embodiments. Based on the embodiments in the present application, all other embodiments obtained by those of ordinary skill in the art without making creative efforts belong to the scope of protection of the present application.

[0030] The raw materials used in the following embodiments are all ordinary commercially available products. Among them, the scrap steel is high-quality carbon steel, purchased from Fengqiu County Xizhao Business Co., Ltd., the pig iron is Q10 pig iron, purchased from Anyang Iron and Steel Group Yongtong Ductile Iron Pipe Co., Ltd., the composition of the return material: C: 3.10%-3.80%, Si: 2.30%-3.60%, Mn: 0.1%-0.5%, P: <0.06%, S: <0.02%, Mg: 0.030-0.060%, and the balance is Fe and inevitable impurity elements; silicon carbide, in which SiC≥90%, F.C≤3%, purchased from Tianjin Yuye International Trade Co., Ltd.; pure copper, copper content 99.9%, nickel content 0.001%, zinc content 0.001%; pure tin, content 99.9%; pure antimony, antimony content 99.9%, impurity content 0.1%; lanthanum metal, purity 99.5%; rare earth ferrosilicon, effective component content 99.99%, purchased from Boyu Metal Co., Ltd.; carburizer, fixed carbon≥98.5%, ash+volatile matter≤1.5%, purchased from Zhuoyue New Materials Co., Ltd.

[0031] Example 1

[0032] A ductile iron for high-performance machine tool castings, in terms of mass percentage, contains the following chemical components: C: 3.175%, Si: 3.536%, Mn: 0.273%, P: <0.028%, S: 0.012%, Mg: 0.047%, Cu: 0.312%, Sn: 0.01%, Sb: 0.02%, rare earth elements: 0.015%, CE: 4.30%, and the balance is Fe and inevitable impurity elements. Among them, in the rare earth elements, by mass percentage, La: 30%, and other rare earth elements such as Ce, Gd, etc. are 70%.

[0033] The preparation method of the above-mentioned ductile iron for high-performance machine tool castings includes the following steps:

[0034] (1) Dry and process the raw materials for later use. The raw materials include: scrap steel, pig iron, return material, silicon carbide, pure copper, pure tin, pure antimony, lanthanum metal, rare earth ferrosilicon;

[0035] (2) Melting: The dried scrap steel, pig iron, and return materials are placed in an intermediate frequency induction electric furnace for melting to obtain molten iron. Then, a carbon increasing agent and silicon carbide are added to the molten iron for continuous melting. Before tapping, a slag forming agent is sprinkled on the surface of the molten iron for slag forming and slag skimming treatment. Then, pure copper, pure tin, pure antimony, lanthanum metal, and rare earth ferrosilicon are added in sequence, melted and mixed evenly, and then tapped to obtain a cast iron melt. The melting temperature and tapping temperature are both 1510 °C. The dosages of the carbon increasing agent and silicon carbide are 1.0% and 0.8% of the mass of the molten iron respectively, and the dosage of the slag forming agent is 0.4% of the mass of the molten iron. The slag forming agent includes, by weight parts: 30 parts of NaCl, 45 parts of KCl, 15 parts of fluorite, and 10 parts of sodium carbonate;

[0036] (3) Spheroidizing and inoculation process: After tapping, the spheroidizing treatment is carried out by the pressure pack injection method, and then the first inoculant is used for the first inoculation. In the spheroidizing treatment, the spheroidizing agent is a lanthanum-based magnesium spheroidizing agent with a particle size of 3 - 25 mm, and the dosage is 1.0% of the mass of the cast iron melt. The spheroidizing agent includes the following components by weight percentage: 47% Si, 5.6% Mg, 0.6% La, and the balance is Fe. The first inoculant is a 75FeSi inoculant with a particle size of 5 - 10 mm, and the dosage is 0.3% of the mass of the cast iron melt;

[0037] (4) Casting: The cast iron melt treated in step (3) is cast into a furan resin sand mold at a temperature of 1360 °C to obtain the product. During casting, a second inoculant is used for in-stream inoculation. The second inoculant is a sulfur-oxygen inoculant. During in-stream inoculation, the sulfur-oxygen inoculant is mixed with tungsten carbide and then added to the cast iron melt. The dosage of the sulfur-oxygen inoculant is 0.05% of the mass of the cast iron melt. The sulfur-oxygen inoculant is 0.2 - 0.8 mm spherical particles. The sulfur-oxygen inoculant includes the following components by weight percentage: silicon (Si): 65%, calcium (Ca): 0.8%, barium (Ba): 0.4%, lanthanum (La): 1.8%, aluminum (Al): 1.0%, sulfur (S): 0.6%, oxygen (O): 0.4%, cerium: 0.9%, and the balance is iron. The dosage of tungsten carbide is 0.05% of the mass of the cast iron melt, and the tungsten carbide is particles with a particle size of 800 - 1000 mesh.

[0038] Example 2

[0039] A ductile iron for high-performance machine tool castings, by mass percentage, contains the following chemical components: C: 3.135%, Si: 3.568%, Mn: 0.256%, P: 0.026%, S: 0.009%, Mg: 0.038%, Cu: 0.351%, Sn: 0.015%, Sb: 0.015%, rare earth elements: 0.01%, CE: 4.20%, and the balance is Fe and inevitable impurity elements. Among them, in the rare earth elements, by mass percentage, La: 28%, and other rare earth elements such as Ce, Gd, etc. are 72%.

[0040] The preparation method of the above ductile iron for high-performance machine tool castings includes the following steps:

[0041] (1) Dry and process the raw materials for standby. The raw materials include: scrap steel, pig iron, return scrap, silicon carbide, pure copper, pure tin, pure antimony, lanthanum metal, rare earth ferrosilicon;

[0042] (2) Melting: Place the dried scrap steel, pig iron, and return scrap in an intermediate frequency induction furnace for melting to obtain molten iron. Then add a carbon increasing agent and silicon carbide to the molten iron and continue melting. Before tapping, sprinkle a slag forming agent on the surface of the molten iron for slag forming and slag skimming treatment. Then successively add pure copper, pure tin, pure antimony, lanthanum metal, and rare earth ferrosilicon, melt and mix them evenly, and then tap to obtain a cast iron melt. The melting temperature and tapping temperature are both 1520°C. The dosages of the carbon increasing agent and silicon carbide are 1.2% and 1.0% of the mass of the molten iron respectively, and the dosage of the slag forming agent is 0.2% of the mass of the molten iron. The slag forming agent includes, by weight parts: 25 parts of NaCl, 42 parts of KCl, 12 parts of fluorite, and 9 parts of sodium carbonate;

[0043] (3) Spheroidizing and inoculation process: After tapping, adopt the pressure package pouring method for spheroidizing treatment, and then use the first inoculant for the first inoculation. In the spheroidizing treatment, the spheroidizing agent is a lanthanum-based magnesium spheroidizing agent with a particle size of 3 - 25 mm and a dosage of 1.1% of the mass of the cast iron melt. The spheroidizing agent includes the following components by weight percentage: 45% Si, 5.2% Mg, 0.4% La, and the balance is Fe. The first inoculant is a 75FeSi inoculant with a particle size of 5 - 10 mm and a dosage of 0.35% of the mass of the cast iron melt;

[0044] (4) Casting: Pour the cast iron melt treated in step (3) into a furan resin sand mold at a temperature of 1360 °C to obtain the product. During casting, in-stream inoculation is carried out using a second inoculant, and the second inoculant is a sulfur-oxygen inoculant. When performing in-stream inoculation, the sulfur-oxygen inoculant is mixed with tungsten carbide and then added to the cast iron melt. The dosage of the sulfur-oxygen inoculant is 0.07% of the mass of the cast iron melt. The sulfur-oxygen inoculant is spherical particles with a size of 0.2 - 0.8 mm. The sulfur-oxygen inoculant comprises the following components by weight percentage: silicon (Si): 63%, calcium (Ca): 0.6%, barium (Ba): 0.3%, lanthanum (La): 2.0%, aluminum (Al): 0.5%, sulfur (S): 0.3%, oxygen (O): 0.9%, cerium: 0.8%, and the balance is iron; the dosage of tungsten carbide is 0.04% of the mass of the cast iron melt, and the tungsten carbide is particles with a particle size of 800 - 1000 mesh.

[0045] Example 3

[0046] A ductile iron for high-performance machine tool castings, by mass percentage, contains the following chemical components: C: 3.125%, Si: 3.549%, Mn: 0.27%, P: 0.027%, S: 0.012%, Mg: 0.044%, Cu: 0.347%, Sn: 0.02%, Sb: 0.01%, rare earth elements: 0.02%, CE: 4.40%, and the balance is Fe and inevitable impurity elements. Among them, in the rare earth elements, by mass percentage, La: 32%, and other rare earth elements such as Ce, Gd: 68%.

[0047] The preparation method of the above ductile iron for high-performance machine tool castings includes the following steps:

[0048] (1) Dry and process the raw materials for standby. The raw materials include: scrap steel, pig iron, return scrap, silicon carbide, pure copper, pure tin, pure antimony, lanthanum metal, rare earth ferrosilicon.

[0049] (2) Melting: Place the dried scrap steel, pig iron, and return scrap in an intermediate frequency induction furnace for melting to obtain molten iron. Then, add a carbon increasing agent and silicon carbide to the molten iron and continue melting. Before tapping, sprinkle a slag forming agent on the surface of the molten iron for slag forming and slag skimming treatment. Then, add pure copper, pure tin, pure antimony, lanthanum metal, and rare earth ferrosilicon in sequence, melt and mix them evenly, and then tap to obtain a cast iron melt. The melting temperature and tapping temperature are both 1540 °C. The dosages of the carbon increasing agent and silicon carbide are 1.5% and 1.2% of the mass of the molten iron respectively, and the dosage of the slag forming agent is 0.3% of the mass of the molten iron. The slag forming agent comprises by weight parts: 35 parts of NaCl, 45 parts of KCl, 17 parts of fluorite, and 7 parts of sodium carbonate.

[0050] (3) Spheroidizing and inoculation process: After tapping, the spheroidizing treatment is carried out by the method of pressing and flushing, and then the first inoculant is used for the first inoculation; in the spheroidizing treatment, the spheroidizing agent is a lanthanum-based magnesium spheroidizing agent with a particle size of 3-25 mm and a dosage of 1.2% of the mass of the cast iron melt; the spheroidizing agent includes the following components by weight percentage: 50% Si, 5.5% Mg, 0.7% La, and the balance is Fe. The first inoculant is a 75FeSi inoculant with a particle size of 5-10 mm and a dosage of 0.4% of the mass of the cast iron melt;

[0051] (4) Casting: The cast iron melt treated in step (3) is cast into a furan resin sand mold at a temperature of 1370 °C to obtain the product; during casting, the second inoculant is used for in-stream inoculation. The second inoculant is a sulfur-oxygen inoculant. During in-stream inoculation, the sulfur-oxygen inoculant is mixed with tungsten carbide and then added to the cast iron melt. The dosage of the sulfur-oxygen inoculant is 0.1% of the mass of the cast iron melt. The sulfur-oxygen inoculant is spherical particles with a size of 0.2-0.8 mm. The sulfur-oxygen inoculant includes the following components by weight percentage: silicon (Si): 60%, calcium (Ca): 0.2%, barium (Ba): 0.5%, lanthanum (La): 1.5%, aluminum (Al): 1.5%, sulfur (S): 0.9%, oxygen (O): 0.3%, cerium: 1.0%, and the balance is iron; the dosage of tungsten carbide is 0.03% of the mass of the cast iron melt, and the tungsten carbide is particles with a particle size of 800-1000 mesh.

[0052] Example 4

[0053] A ductile iron for high-performance machine tool castings, by mass percentage, contains the following chemical components: C: 3.10%, Si: 3.60%, Mn: 0.3%, P: 0.029%, S: 0.012%, Mg: 0.052%, Cu: 0.30%, Sn: 0.016%, Sb: 0.017%, rare earth elements: 0.012%, CE: 4.25%, and the balance is Fe and unavoidable impurity elements. Among them, in the rare earth elements, by mass percentage, La: 30%, and other rare earth elements such as Ce, Gd, etc. are 70%.

[0054] The preparation method of the above ductile iron for high-performance machine tool castings includes the following steps:

[0055] (1) Dry the raw materials for standby. The raw materials include: scrap steel, pig iron, return scrap, silicon carbide, pure copper, pure tin, pure antimony, lanthanum metal, rare earth ferrosilicon;

[0056] (2) Smelting: placing the dried scrap steel, pig iron and recycled materials in a medium frequency induction furnace for smelting to obtain molten iron, then adding a carburizer and silicon carbide to the molten iron and continuing to smelt, and before taking out of the furnace, sprinkling a slag-forming agent on the surface of the molten iron to perform slag-forming and slagging treatment, and then sequentially adding pure copper, pure tin, pure antimony, lanthanum metal and rare earth ferrosilicon to melt and mix, and then taking out of the furnace to obtain a cast iron melt, wherein the melting temperature and the furnace temperature are both 1530° C., the amounts of the carburizer and silicon carbide are 1.2% and 1.2% of the mass of the molten iron respectively, and the amount of the slag-forming agent is 0.1% of the mass of the molten iron; the slag-forming agent comprises, by weight, 28 parts of NaCl, 50 parts of KCl, 15 parts of fluorite and 12 parts of sodium carbonate;

[0057] (3) Spheroidizing and inoculating process: after being discharged from the furnace, the steel is spheroidized by a pressing and punching method, and then the first inoculation is performed by a first inoculant; the spheroidizing agent in the spheroidizing process is a lanthanide magnesium spheroidizing agent, the particle size of which is 3-25 mm, and the amount used is 1.1% of the mass of the cast iron melt; the spheroidizing agent comprises the following components by weight percentage: 45% Si, 6% Mg, 0.6% La, and the balance is Fe; the first inoculant is a 75FeSi inoculant, the particle size of which is 5-10 mm, and the amount used is 0.4% of the mass of the cast iron melt;

[0058] (4) Casting: Casting the molten cast iron treated in step (3) into a furan resin sand mold at a temperature of 1380° C. to obtain a molten cast iron; during casting, a second inoculant is used for inoculation, wherein the second inoculant is a sulfur-oxygen inoculant. During inoculation, the sulfur-oxygen inoculant is mixed with tungsten carbide and then added to the molten cast iron. The amount of the sulfur-oxygen inoculant is 0.08% of the mass of the molten cast iron. The sulfur-oxygen inoculant is a spherical particle of 0.2-0.8 mm. The sulfur-oxygen inoculant includes the following components by weight percentage: silicon (Si): 67%, calcium (Ca): 1.0%, barium (Ba): 0.3%, lanthanum (La): 1.7%, aluminum (Al): 1.2%, sulfur (S): 0.6%, oxygen (O): 0.5%, cerium: 1.2%, and the balance is iron; the amount of tungsten carbide is 0.035% of the mass of the cast iron melt, and the tungsten carbide is a particle size of 800-1000 mesh.

[0059] Example 5

[0060] A ductile iron for high-performance machine tool castings, comprising the following chemical components by mass percentage: C: 3.20%, Si: 3.60%, Mn: 0.2%, P: 0.027%, S: 0.014%, Mg: 0.043%, Cu: 0.40%, Sn: 0.012%, Sb: 0.02%, rare earth elements: 0.017%, CE: 4.36%, and the balance is Fe and unavoidable impurity elements. Among them, the rare earth elements are La: 30% by mass, and other rare earth elements such as Ce, Gd, etc. are 70%.

[0061] The preparation method of ductile iron for the above-mentioned high-performance machine tool castings comprises the following steps:

[0062] (1) Dry and process the raw materials for standby. The raw materials include: scrap steel, pig iron, return scrap, silicon carbide, pure copper, pure tin, pure antimony, lanthanum metal, rare earth ferrosilicon;

[0063] (2) Melting: Place the dried scrap steel, pig iron, and return scrap into an intermediate frequency induction furnace for melting to obtain molten iron. Then, add a carbon increasing agent and silicon carbide to the molten iron and continue melting. Before tapping, sprinkle a slag forming agent on the surface of the molten iron for slag forming and slag skimming treatment. Then, successively add pure copper, pure tin, pure antimony, lanthanum metal, and rare earth ferrosilicon, melt and mix them evenly, and then tap to obtain a cast iron melt. The melting temperature and tapping temperature are both 1540°C. The dosages of the carbon increasing agent and silicon carbide are 1.3% and 1.1% of the mass of the molten iron respectively, and the dosage of the slag forming agent is 0.5% of the mass of the molten iron. The slag forming agent includes, by weight parts: 32 parts of NaCl, 40 parts of KCl, 14 parts of fluorite, and 10 parts of sodium carbonate;

[0064] (3) Spheroidizing and inoculation process: After tapping, adopt the pressing and pouring-in method for spheroidizing treatment, and then use the first inoculant for the first inoculation. In the spheroidizing treatment, the spheroidizing agent is a lanthanum-based magnesium spheroidizing agent with a particle size of 3 - 25 mm and a dosage of 1.0% of the mass of the cast iron melt. The spheroidizing agent includes the following components by weight percentage: 48% Si, 5% Mg, 0.5% La, and the balance is Fe. The first inoculant is a 75FeSi inoculant with a particle size of 5 - 10 mm and a dosage of 0.38% of the mass of the cast iron melt;

[0065] (4) Casting: Pour the cast iron melt treated in step (3) into a furan resin sand mold at a temperature of 1390°C to obtain the product. During casting, use the second inoculant for in-stream inoculation. The second inoculant is a sulfur-oxygen inoculant. During in-stream inoculation, mix the sulfur-oxygen inoculant with tungsten carbide and then add it to the cast iron melt. The dosage of the sulfur-oxygen inoculant is 0.06% of the mass of the cast iron melt. The sulfur-oxygen inoculant is spherical particles with a size of 0.2 - 0.8 mm. The sulfur-oxygen inoculant includes the following components by weight percentage: silicon (Si): 62%, calcium (Ca): 0.7%, barium (Ba): 0.5%, lanthanum (La): 1.9%, aluminum (Al): 0.8%, sulfur (S): 0.7%, oxygen (O): 0.4%, cerium: 0.5%, and the balance is iron. The dosage of tungsten carbide is 0.045% of the mass of the cast iron melt, and the tungsten carbide is particles with a particle size of 800 - 1000 mesh.

[0066] Comparative Example 1

[0067] A ductile iron for high-performance machine tool castings, by mass percentage, contains the following chemical components: C: 3.641%, Si: 2.502%, Mn: 0.469%, P: 0.026%, S: 0.014%, Mg: 0.048%, Cu: 0.544%, Sn: 0, Sb: 0, rare earth elements: 0, CE: 0, and the balance is Fe and unavoidable impurity elements.

[0068] In its preparation method, the raw materials only use scrap steel, pig iron, return materials, carburizer, silicon carbide and pure copper, do not add pure tin, pure antimony, lanthanum metal, rare earth ferrosilicon, and no in-stream inoculation is carried out during the casting process, and tungsten carbide is not added. The rest is the same as in Example 1.

[0069] Comparative Example 2

[0070] A ductile iron for high-performance machine tool castings, in which, Sn: 0.009%, Sb: 0.009%, and the rest is the same as in Example 1.

[0071] Comparative Example 3

[0072] A ductile iron for high-performance machine tool castings, in which, Sn: 0.021%, Sb: 0.021%, and the rest is the same as in Example 1.

[0073] Comparative Example 4

[0074] A ductile iron for high-performance machine tool castings, in its preparation method, the second in-stream inoculation uses the same inoculant 75FeSi as the first inoculation, and tungsten carbide is not added. The rest is the same as in Example 1.

[0075] Performance Test

[0076] Refer to GB / T228.1-2021 and GB / T9441-2021 to conduct performance tests on Examples 1-5 and Comparative Examples 1-4, and the results are shown in Table 1.

[0077] Table 1. Mechanical Property Test

[0078]

[0079]

[0080] As can be seen from Table 1, the mechanical properties of the ductile iron specimens in Examples 1-5 are good, reaching the performance of QT600-10 ductile iron, and can meet the performance requirements of high strength, high stiffness and good plasticity and toughness of high-end machine tools. From Examples 1-5 and Comparative Examples 1-3, it can be seen that the ductile iron obtained by the scheme of the present invention has better mechanical properties than Comparative Examples 1-3. It can be seen that the total composite addition amount of Sn and Sb is 0.02-0.04%, too low will result in the tensile strength not meeting the requirements, and too high will not guarantee the plasticity and toughness. In this application, the tin content is controlled at 0.01%-0.02%, and the antimony content is controlled at 0.01%-0.02%, and the mechanical properties are the best. The mechanical properties of each specimen in Examples 1-5 are better than those in Comparative Example 4. It can be seen that by using the second in-stream inoculation of the present invention and adding the sulfur-oxygen inoculant and tungsten carbide in-stream together, the strength and toughness of the ductile iron are further improved.

[0081] As Figure 1 shown, it is the graphite microstructure morphology diagram of the ductile iron obtained in Comparative Example 1 and Example 3. From Figure 1 this, it can be seen that the graphite ball sizes of the two groups of ductile iron specimens are not much different, the distribution is uniform, the roundness of the graphite balls is relatively high, the size grade of the graphite balls is 7-8, and the spheroidization grade is 1-2. The graphite spheroidization rate in Comparative Example 1 is 90%, while the graphite spheroidization rate in Example 3 is 95%. The results show that within a reasonable test composition range, adding trace amounts of tin, antimony and rare earth elements can improve the graphite spheroidization rate. Tin and antimony elements can not only reduce the solubility of carbon in the molten iron, that is, increase the activity of carbon, thereby promoting the precipitation of carbon in the form of graphite, but also form substances with a good coherent relationship with the graphite crystal and not easily separated from the molten iron as an effective matrix for heterogeneous nucleation of graphite, promoting the formation of graphite nuclei, which is conducive to obtaining a graphite structure with high roundness, a large number, small size and uniform distribution.

[0082] As Figure 2 shown, it is the matrix microstructure morphology diagram of the ductile iron obtained in Comparative Example 1 and Example 3. From Figure 2It can be seen that the matrix structures of the ductile iron specimens are all ferrite and pearlite. Compared with Comparative Example 1, in Example 3, the lamellar spacing is reduced and the structure is more refined. Tin elements will be enriched on the graphite spheres and phase interfaces, which can hinder the diffusion of carbon atoms in the cementite structure to the graphite structure, thus playing a role in promoting and stabilizing pearlite. Antimony has a lower affinity with carbon and is usually enriched around the graphite structure, which can hinder the diffusion of carbon atoms during the decomposition of austenite and promote the transformation tendency of austenite to pearlite in the eutectoid reaction, thus promoting the formation of pearlite. In addition, the addition of rare earth elements reduces the supercooling tendency, improves the fluidity of the molten iron, makes the structure more uniform, and thus improves the strength and machinability of ductile iron. Tungsten carbide acts as a crystal nucleus, which is beneficial to the refinement of the structure; tungsten carbide has a high melting point and has an obstructive effect on the growth of grains during the solution treatment in the heat treatment process, which is beneficial to the refinement of grains after heat treatment; tungsten carbide has a high hardness and belongs to hard particles, which is beneficial to improving the wear resistance of machine tools.

[0083] It should be noted that in this article, relational terms such as first and second are only used to distinguish one entity or operation from another entity or operation, and do not necessarily require or imply any actual relationship or order between these entities or operations. Moreover, the term "comprising", "including" or any other variant thereof is intended to cover non-exclusive inclusion, so that a process, method, article or device comprising a series of elements not only includes those elements, but also includes other elements not expressly listed, or further includes elements inherent to such process, method, article or device.

[0084] Although the embodiments of the present application have been shown and described, it will be understood by those of ordinary skill in the art that various changes, modifications, substitutions and variations can be made to these embodiments without departing from the principles and spirit of the present application, and the scope of the present application is defined by the appended claims and their equivalents.

Claims

1. A ductile iron for high-performance machine tool castings, characterized in that: In terms of mass percentage, it contains the following chemical components: C: 3.10%-3.20%, Si: 3.50%-3.60%, Mn: 0.2%-0.3%, P: <0.03%, S: <0.015%, Mg: 0.038-0.052%, Cu: 0.30%-0.40%, Sn: 0.01%-0.02%, Sb: 0.01%-0.02%, rare earth elements: 0.01%-0.02%, CE: 4.20%-4.40%, and the balance is Fe and unavoidable impurity elements.

2. The ductile iron for high performance machine tool castings according to claim 1, characterized in that: The rare earth elements include La in a mass percentage of 28-32%.

3. The method for preparing ductile iron for high-performance machine tool castings according to any one of claims 1 to 2, characterized in that: The following steps are involved: (1) drying raw materials for standby use, wherein the raw materials include scrap steel, pig iron, recycled materials, silicon carbide, pure copper, pure tin, pure antimony, lanthanum metal, and rare earth ferrosilicon; (2) Smelting: Smelting the dried scrap steel, pig iron and recycled materials to obtain molten iron, then adding a carburizer and silicon carbide to the molten iron and continuing to smelt, adding a slag-forming agent to the molten iron before it is discharged from the furnace, slagging and slagging treatment, then adding pure copper, pure tin, pure antimony, lanthanum metal and rare earth ferrosilicon to melt and mix, and then taking out of the furnace to obtain cast iron melt, wherein the melting temperature and the discharge temperature are both 1510-1540° C., the amount of the carburizer is 1-1.5% of the mass of the molten iron, and the amount of the slag-forming agent is 0.1-0.5% of the mass of the molten iron; (3) Spheroidizing and inoculating process: after being discharged from the furnace, the spheroidizing treatment is carried out by the pressing and punching method, and then the first inoculation is carried out by the first inoculant; the amount of the spheroidizing agent in the spheroidizing treatment is 1.0-1.2% of the mass of the molten cast iron, and the amount of the first inoculant is 0.3-0.4% of the mass of the molten cast iron; (4) Casting: Casting the molten cast iron treated in step (3) at a temperature of 1360-1390° C. to obtain a molten cast iron; during casting, a second inoculant is used for inoculation, and the amount of the second inoculant is 0.05-0.1% of the mass of the molten cast iron. During inoculation, the second inoculant is mixed with tungsten carbide and then added to the molten cast iron.

4. The method for preparing ductile iron for high-performance machine tool castings according to claim 3, characterized in that: The slag-forming agent in step (2) comprises, by weight: 25-35 parts of NaCl, 40-50 parts of KCl, 12-17 parts of fluorite and 7-12 parts of sodium carbonate.

5. The method for preparing ductile iron for high-performance machine tool castings according to claim 3, characterized in that: The spheroidizing agent in step (3) is a lanthanide magnesium spheroidizing agent, which comprises the following components by weight percentage: 45-50% Si, 5-6% Mg, 0.4-0.7% La, and the balance is Fe.

6. The method for preparing ductile iron for high-performance machine tool castings according to claim 3, characterized in that: In step (3), the first inoculant is a ferrosilicon inoculant.

7. The method for preparing ductile iron for high-performance machine tool castings according to claim 3, characterized in that: In step (4), the second inoculant is a sulfur-oxygen inoculant, which comprises the following components by weight percentage: Si: 60-67%, Ca: 0.2-1.0%, Ba: 0.3-0.5%, La: 1.5-2.0%, Al: 0.5-1.5%, S: 0.3-0.9%, O: 0.3-0.9%, Ce: 0.5-1.2%, and the balance is iron.

8. The method for preparing ductile iron for high-performance machine tool castings according to claim 7, characterized in that: The sulfur-oxygen inoculant is in the form of spherical particles with a diameter of 0.2-0.8 mm.

9. The method for preparing ductile iron for high performance machine tool castings according to claim 3, characterized in that: In step (4), the amount of tungsten carbide used is 0.03-0.05% of the mass of the cast iron melt, and the tungsten carbide is in the form of particles with a particle size of 800-1000 meshes.

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