An efficient inoculant for cast iron containing rare earth elements

Abstract

The present invention relates to the technical field of cast iron alloys, in particular to an efficient cast iron inoculant containing rare earth elements. The composition of the inoculant (by weight percentage): scandium 0.5 - 2.0%, ytterbium 0.3 - 1.5%, lutetium 0.1 - 0.8%, terbium 0.2 - 1.0%; silicon 30 - 45%, calcium 0.5 - 2.5%, molybdenum 0.2 - 1.0%, strontium 0.1 - 0.8%, zirconium 0.1 - 0.5%; aluminum 0.5 - 2.0%, and the balance is iron. By applying the synergistic effect of four rare earth elements, scandium - ytterbium - lutetium - terbium, at a certain ratio, the technical obstacles of high activity, easy oxidation and high cost of rare earth elements are overcome, and the comprehensive performance of cast iron is significantly improved.

Description

Technical Field

[0001] The present invention relates to the technical field of cast iron alloys, in particular to a high-efficiency cast iron inoculant containing rare earth elements. Background Art

[0002] Cast iron materials, as indispensable basic materials in the modern industrial system, are widely used in fields such as automotive parts, heavy machinery, energy equipment, and aerospace. With the progress of science and technology and the continuous upgrading of industrial demands, the requirements for the performance of cast iron materials are increasing day by day, especially the service performance under harsh environments such as high temperature, high pressure, and high corrosion. Inoculation treatment, as a key process for controlling the microstructure of cast iron and optimizing its performance, has always been a research hotspot in the casting field. The core of inoculation treatment is to add specific elements to the molten iron to provide heterogeneous nucleation cores during solidification, promote graphitization, refine the microstructure, and thus improve the comprehensive performance of cast iron.

[0003] Traditional cast iron inoculants mainly include silicon-based inoculants (such as ferrosilicon, calcium silicide, etc.) and light rare earth-based inoculants (mainly containing cerium, lanthanum, etc.). These inoculants have been widely used in industrial production, but at the same time, they have obvious limitations: the inoculation effect is prone to decline, especially when the molten iron is overheated or the holding time is extended; the inoculation effect is sensitive to the composition and state of the molten iron, and it is difficult to achieve universality; the inoculation effect on special grades of cast iron is limited; some inoculants will introduce inclusions during use, which instead affects the purity of cast iron. More importantly, with the development of cast iron products towards high strength, toughness, and lightweight, traditional inoculants are no longer able to meet the increasingly demanding performance requirements.

[0004] In recent years, due to their unique electronic structures and physicochemical properties, rare earth elements have shown great application potential in the field of materials science. However, current industrial applications mainly focus on light rare earth elements (such as cerium, lanthanum), while scandium-ytterbium-lutetium-terbium rare earth elements have not been fully utilized due to their scarcity, high cost, and large processing difficulty. With the progress of rare earth extraction and separation technologies, some scandium-ytterbium-lutetium-terbium rare earth elements that were previously overlooked are showing unique application values.

[0005] There is no report in the prior art on systematically using scandium-ytterbium-lutetium-terbium rare earth elements to construct an efficient inoculation system, nor is there an in-depth study on the synergistic action mechanism of these rare earth elements. Moreover, there is a lack of effective solutions to solve problems such as the high activity and easy oxidation of scandium-ytterbium-lutetium-terbium rare earth elements during application. Summary of the Invention

[0006] The object of the present invention is to provide an efficient inoculant for cast iron containing rare earth elements. By systematically utilizing the unique physical and chemical properties of scandium-ytterbium-lutetium-terbium rare earth elements, a multi-element synergistic inoculation system is constructed to significantly improve the tissue quality and comprehensive performance of cast iron, especially the high-temperature performance and service stability, while solving the technical obstacles in the application of scandium-ytterbium-lutetium-terbium rare earth elements.

[0007] The scandium-ytterbium-lutetium-terbium rare earth elements selected in the present invention include scandium (Sc), ytterbium (Yb), lutetium (Lu) and terbium (Tb).

[0008] As a transition metal element, scandium has the characteristics of small atomic radius, high ionization energy and strong oxygen affinity. Its high-melting-point compounds can serve as graphite nucleation substrates and help to stabilize the inoculation effect; ytterbium, as a heavy rare earth element, has variable +2 / +3 valence states and can affect the redox environment in molten iron under different temperature conditions; lutetium, as the heaviest rare earth element, may affect the uniformity of the casting structure through its compounds; terbium mainly affects the graphite nucleation environment through its chemical properties. These elements can act synergistically in the inoculation of cast iron and are expected to improve the structure and properties of cast iron.

[0009] Specifically, the present invention adopts the following technical solutions:

[0010] An efficient inoculant for cast iron containing rare earth elements, by weight percentage, comprises the following components:

[0011] Scandium-ytterbium-lutetium-terbium rare earth elements: scandium (Sc) 0.5 - 2.0%, ytterbium (Yb) 0.3 - 1.5%, lutetium (Lu) 0.1 - 0.8%, terbium (Tb) 0.2 - 1.0%;

[0012] Auxiliary elements: silicon (Si) 30 - 45%, calcium (Ca) 0.5 - 2.5%, molybdenum (Mo) 0.2 - 1.0%, strontium (Sr) 0.1 - 0.8%, zirconium (Zr) 0.1 - 0.5%;

[0013] Carrier element: aluminum (Al) 0.5 - 2.0%, the balance is iron (Fe).

[0014] The total content of the scandium-ytterbium-lutetium-terbium rare earth elements is 1.1 - 5.3%, preferably 1.5 - 3.5%.

[0015] The weight ratio of the four rare earth elements of scandium, ytterbium, lutetium and terbium is 3:2:1:1.5.

[0016] The beneficial effects of the present invention:

[0017] Compared with traditional ferrosilicon inoculants and conventional rare-earth inoculants, the inoculant of the present invention shows significant advantages in improving the mechanical properties of gray cast iron. The tensile strength is increased by about 50%, the hardness is increased by about 38%, and at the same time, the elongation at break and impact toughness are also significantly improved, achieving a synergistic improvement in strength and toughness.

[0018] The inoculant of the present invention has excellent high-temperature properties. The strength retention rate at 500 °C reaches 64%, which is much higher than 40 - 50% of traditional inoculants, greatly improving the serviceability of cast iron parts in high-temperature environments.

[0019] The inoculant of the present invention exhibits excellent anti-decay performance. After heat preservation for 60 minutes, the decay rate of the inoculation effect is only 7.6%, while that of traditional inoculants is 30 - 40%, significantly extending the effective time window of inoculation treatment.

[0020] The inoculant of the present invention has a significant effect on refining and homogenizing the structure of cast iron. Description of the Drawings

[0021] Figure 1 (a) is a comparison chart of the proportion of A-type graphite in the example group, Figure 1 (b) is a comparison chart of the graphite size in the example group, Figure 1 (c) is a comparison chart of the graphite quantity in the example group, Figure 1 (d) is a comparison chart of the proportion of A-type graphite in the comparative example group, Figure 1 (e) is a comparison chart of the graphite size in the comparative example group, Figure 1 (f) is a comparison chart of the graphite quantity in the comparative example group;

[0022] Figure 2 (a) is a graph showing the relationship between the proportion of A-type graphite and the addition amount, Figure 2 (b) is a graph showing the relationship between the graphite size and the addition amount, Figure 2 (c) is a graph showing the relationship between the graphite quantity and the addition amount, Figure 2 (d) is a graph showing the relationship between the graphite distribution uniformity and the addition amount;

[0023] Figure 3 (a) is a comparison chart of the pearlite and ferrite contents, Figure 3 (b) is a comparison chart of the grain size, Figure 3 (c) is a graph showing the relationship between the pearlite content and the mechanical properties (tensile strength and hardness), Figure 3 (d) is a graph showing the relationship between the grain size and the impact toughness;

[0024] Figure 4 (a) is a comparison chart of the tensile strength in the example group, Figure 4 (b) is a comparison chart of the tensile strength in the comparative example group, Figure 4 (c) is a graph of high-temperature strength characteristics,Figure 4 Figure (d) is a graph showing the relationship between hardness and impact toughness;

[0025] Figure 5 Figure (a) is a graph showing the high-temperature strength retention rate of the example group, Figure 5 Figure (b) is a graph showing the high-temperature strength retention rate of the comparative example group;

[0026] Figure 6 is a graph showing the improvement of the high-temperature strength retention rate relative to Comparative Example 1;

[0027] Figure 7 Figure (a) is a curve graph showing the relationship between the addition amount and the tensile strength, Figure 7 Figure (b) is a curve graph showing the relationship between the growth rate of the tensile strength and the addition amount;

[0028] Figure 8 Figure (a) is a curve graph showing the relationship between the tensile strength and the holding time, Figure 8 Figure (b) is a curve graph showing the relationship between the proportion of Type A graphite and the holding time, Figure 8 Figure (c) is a curve graph showing the relationship between the decay rate and the holding time (example), Figure 8 Figure (d) is a curve graph showing the relationship between the decay rate and the holding time (comparative example);

[0029] Figure 9 Figure (a) is a graph showing the relationship between the tensile strength and the overheating temperature, Figure 9 Figure (b) is a graph showing the relationship between the proportion of Type A graphite and the overheating temperature, Figure 9 Figure (c) is a graph showing the relationship between the strength retention rate and the overheating temperature;

[0030] Figure 10 Figure (a) is a curve graph showing the relationship between the crack density and the number of thermal cycles, Figure 10 Figure (b) is a curve graph showing the relationship between the maximum crack depth and the number of thermal cycles, Figure 10 Figure (c) is a curve graph showing the relationship between the grain size and the crack propagation rate, Figure 10 Figure (d) is a graph showing the improvement of the thermal fatigue resistance;

[0031] Figure 11 Figure (a) is a graph showing the mass loss under different wear conditions, Figure 11 Figure (b) is a graph showing the relative wear resistance under different conditions, Figure 11 Figure (c) is a graph showing the relationship between hardness and wear resistance, Figure 11 Figure (d) is a graph showing the relationship between the graphite size and the wear resistance;

[0032] Figure 12 Figure (a) is a curve graph showing the relationship between the proportion of Type A graphite of a single element and the addition amount, Figure 12 Figure (b) is a curve graph showing the synergistic effect of combined rare earth elements, Figure 12 Figure (c) is a column line combination graph showing the characteristics of nucleating particles, Figure 12(d) is a bar chart of physical and chemical properties;

[0033] Figure 13 is a comparison chart of nucleation efficiency coefficients of different rare earth combinations;

[0034] Figure 14 is a relationship chart between supercooling degree and rare earth combination of different rare earth combinations. Specific implementation manner

[0035] The composition of the inoculant is selected as follows.

[0036] Selection of active element system:

[0037] The active element system is composed of four scandium-ytterbium-lutetium-terbium rare earth elements, namely scandium (Sc), ytterbium (Yb), lutetium (Lu), and terbium (Tb), which is the core functional part of the inoculant. According to the results of a large number of preliminary experiments and theoretical calculations, the optimal proportion range of these four elements is determined, that is, Sc:Yb:Lu:Tb = 3:2:1:1.5.

[0038] The content of scandium (Sc) is 0.5 - 2.0%, which acts as the main nucleation promoter to form high-melting-point compounds as heterogeneous nucleation cores;

[0039] The content of ytterbium (Yb) is 0.3 - 1.5%, which acts as a valence state regulating element to dynamically regulate the oxidation-reduction environment of the molten iron;

[0040] The content of lutetium (Lu) is 0.1 - 0.8%, which acts as a grain boundary regulating element to improve the matrix strength and thermal fatigue resistance;

[0041] The content of terbium (Tb) is 0.2 - 1.0%, which acts as a magnetic regulation and multivalent state regulation element to promote uniform nucleation.

[0042] The total content of the four rare earth elements is controlled within 1.1 - 5.3%, preferably within the range of 1.5 - 3.5%. This range not only ensures sufficient activity but also takes into account economy. It should be noted that there are complex interactions among the four elements, and any change in the content of one element will affect the optimal content of other elements, forming a complex multi-variable optimization problem.

[0043] Selection of auxiliary element system:

[0044] In order to enhance the effect of the scandium-ytterbium-lutetium-terbium rare earth elements, and at the same time considering economy and process practicability, the present invention selects the following auxiliary element system:

[0045] The content of silicon (Si) is 30 - 45%, which acts as a traditional inoculating element to promote graphitization and regulate the matrix structure;

[0046] The calcium (Ca) content is 0.5 - 2.5%, which enhances the deoxidation and desulfurization effects and reduces the surface tension of the molten iron;

[0047] The molybdenum (Mo) content is 0.2 - 1.0%, which enhances the high-temperature stability of rare earth elements and prolongs the inoculation effect;

[0048] The strontium (Sr) content is 0.1 - 0.8%, which forms a synergistic deoxidation effect with scandium-ytterbium-lutetium-terbium rare earth elements;

[0049] The zirconium (Zr) content is 0.1 - 0.5%, which stabilizes scandium-ytterbium-lutetium-terbium compounds and prevents premature decomposition.

[0050] The addition of auxiliary elements can not only enhance the inoculation effect, but also reduce the dosage of rare earth elements and improve the economy. More importantly, there are various complex synergistic mechanisms between auxiliary elements and scandium-ytterbium-lutetium-terbium rare earth elements. For example, the Mo-Sc-O composite oxide has a higher nucleation efficiency than the single Sc2O3; the Zr-Lu composite compound has better high-temperature stability than the single Lu compound, etc.

[0051] Selection of carrier elements:

[0052] Considering the high activity and easy oxidation characteristics of scandium-ytterbium-lutetium-terbium rare earth elements, it is crucial to select a suitable carrier element. In this invention, ferrosilicon alloy (Fe-Si) is selected as the basic carrier, and a small amount of aluminum (Al) is added to form a protective oxide layer to prevent the excessive oxidation of rare earth elements during the preparation and use processes. The composition of the carrier element is:

[0053] Iron (Fe): the balance;

[0054] Aluminum (Al): 0.5 - 2.0%.

[0055] Traditional inoculants mainly rely on a single type of nucleation core (such as silicate or oxide) to promote graphite nucleation, while the scandium-ytterbium-lutetium-terbium inoculant of this invention significantly improves the nucleation efficiency and uniformity by forming multiple types of nucleation cores:

[0056] (1) Sc-O-S composite nucleation core: A composite compound formed by scandium with oxygen and sulfur, whose lattice constant highly matches that of graphite (the mismatch degree < 3%), and has extremely high nucleation efficacy;

[0057] (2) Yb-Tb oxide nucleation core: A composite oxide formed by ytterbium and terbium, which has variable valence states and can dynamically adapt to the changes in the molten iron environment;

[0058] (3) Lu-Mo-Zr composite nucleation core: A composite phase formed by lutetium with molybdenum and zirconium, which has excellent thermal stability and can maintain its activity in high-temperature molten iron for a long time.

[0059] These multiple nucleation cores are evenly distributed in the molten iron, providing a large number of efficient heterogeneous nucleation sites, making the graphite nucleation more uniform, finer, and with a larger quantity.

[0060] In the present invention, the variable valence characteristics of scandium-ytterbium-lutetium-terbium rare earth elements (especially Yb and Tb) enable the inoculant to dynamically adjust its chemical state according to the changes in the molten iron environment:

[0061] (1) Temperature-responsive oxygen potential regulation: The valence of ytterbium changes at different temperatures. It is mainly in the +3 valence at high temperatures and has strong deoxidation ability. When the temperature decreases, part of it transforms into the +2 valence, releasing a small amount of oxygen to create a suitable micro-oxygen environment for graphite nucleation;

[0062] (2) Local oxidation-reduction potential gradient: The +3 / +4 valence transformation of terbium forms an oxidation-reduction potential gradient in the micro-region, affecting the migration and aggregation behavior of carbon atoms;

[0063] (3) Dynamic regulation of sulfur activity: The affinity between scandium-ytterbium-lutetium-terbium rare earth elements and sulfur changes with temperature, realizing the dynamic regulation of sulfur activity, which not only purifies the molten iron but also does not desulfurize excessively (excessive desulfurization is instead not conducive to graphite nucleation).

[0064] This dynamic regulation mechanism enables the inoculant to adapt to molten iron with different compositions and different process conditions, maintaining a stable and efficient inoculation effect.

[0065] Through systematic research on the synergistic action mechanism of four kinds of scandium-ytterbium-lutetium-terbium elements in cast iron inoculation, the present invention has developed an efficient cast iron inoculant. The technical solutions and their effects of the present invention are described in detail below through multiple examples and comparative examples.

[0066] The present invention adopts the following test methods:

[0067] To ensure the reliability and comparability of the results, the following unified test methods are adopted for all examples and comparative examples:

[0068] 1. Test molten iron: HT250 gray cast iron is used, and the basic composition (wt%) is: C 3.2 - 3.4, Si 1.8 - 2.0, Mn 0.5 - 0.7, P ≤ 0.08, S ≤ 0.06, and the rest is Fe.

[0069] 2. Melting process: Medium-frequency induction furnace is used for melting, the melting temperature is 1480 ± 20 °C, and the tapping temperature is 1420 ± 10 °C.

[0070] 3. Inoculation treatment: The inoculant is added to the molten iron at 1380 ± 10 °C, and the pouring temperature is 1350 ± 10 °C.

[0071] 4. Specimen preparation: Standard Y-shaped test blocks (GB / T 9441) and Φ30 mm cylindrical specimens are used.

[0072] 5. Microstructure Observation and Quantitative Analysis:

[0073] Graphite Morphology: Observed and analyzed according to the standard of GB / T 7216

[0074] Matrix Microstructure: Etched with 4% nitric acid alcohol solution and observed under a metallographic microscope

[0075] Quantitative Analysis: Using Image-Pro Plus image analysis software, 10 fields of view were randomly selected for each specimen for statistics

[0076] 6. Performance Testing:

[0077] Tensile Strength: Tested according to the standard of GB / T 228

[0078] Hardness: Brinell hardness was tested according to the standard of GB / T 231

[0079] Impact Toughness: Tested according to the standard of GB / T 229

[0080] High-Temperature Performance: Tensile strength was tested at 300 °C, 400 °C and 500 °C

[0081] Thermal Fatigue Performance: Tested under the temperature cycle condition of 250 ± 10 °C to 650 ± 10 °C

[0082] Wear Resistance: Tested using an MM-200 type wear testing machine according to the standard of GB / T 12444

[0083] Inoculant Decline Characteristics: The performance changes were tested after holding at 1380 ± 10 °C for 5 min, 15 min, 30 min and 60 min

[0084] Example 1: Base Formula Inoculant

[0085] By weight percentage, the components of the inoculant are: scandium (Sc) 1.2%, ytterbium (Yb) 0.8%, lutetium (Lu) 0.4%, terbium (Tb) 0.6%, silicon (Si) 38%, calcium (Ca) 1.5%, molybdenum (Mo) 0.5%, strontium (Sr) 0.3%, zirconium (Zr) 0.3%, aluminum (Al) 1.0%, and the balance is iron (Fe).

[0086] Preparation Process: Iron, silicon, calcium, molybdenum, strontium, zirconium and aluminum were placed in an induction furnace for melting. After melting, it was held at 1550 ± 20 °C for 10 minutes to homogenize the alloy, and then scandium, lutetium, terbium and ytterbium were added in sequence. It was held at 1500 ± 20 °C for 5 minutes, cast into ingots, and crushed into 15 - 50 μm particles after cooling.

[0087] Inoculation treatment was carried out on HT250 gray cast iron, and the addition amount was 0.8%.

[0088] Example 2: High-strength formula inoculant

[0089] Composition: Scandium (Sc) 1.8%, Ytterbium (Yb) 1.2%, Lutetium (Lu) 0.6%, Terbium (Tb) 0.9%, Silicon (Si) 35%, Calcium (Ca) 2.0%, Molybdenum (Mo) 0.8%, Strontium (Sr) 0.5%, Zirconium (Zr) 0.3%, Aluminum (Al) 1.5%, the balance is Iron (Fe).

[0090] The preparation process is the same as that of Example 1, and the addition amount is 1.0%.

[0091] Example 3: High-toughness formula inoculant

[0092] Composition: Scandium (Sc) 0.8%, Ytterbium (Yb) 0.5%, Lutetium (Lu) 0.25%, Terbium (Tb) 0.4%, Silicon (Si) 42%, Calcium (Ca) 1.0%, Molybdenum (Mo) 0.3%, Strontium (Sr) 0.2%, Zirconium (Zr) 0.2%, Aluminum (Al) 0.8%, the balance is Iron (Fe).

[0093] The preparation process is the same as that of Example 1, and the addition amount is 0.8%.

[0094] Example 4: High-temperature formula inoculant

[0095] Composition: Scandium (Sc) 1.5%, Ytterbium (Yb) 0.6%, Lutetium (Lu) 0.7%, Terbium (Tb) 0.5%, Silicon (Si) 36%, Calcium (Ca) 1.2%, Molybdenum (Mo) 0.9%, Strontium (Sr) 0.3%, Zirconium (Zr) 0.4%, Aluminum (Al) 1.2%, the balance is Iron (Fe).

[0096] The preparation process is the same as that of Example 1, and the addition amount is 0.9%.

[0097] Example 5: Economical formula inoculant

[0098] Composition: Scandium (Sc) 0.6%, Ytterbium (Yb) 0.4%, Lutetium (Lu) 0.2%, Terbium (Tb) 0.3%, Silicon (Si) 43%, Calcium (Ca) 2.2%, Molybdenum (Mo) 0.3%, Strontium (Sr) 0.6%, Zirconium (Zr) 0.2%, Aluminum (Al) 0.6%, the balance is Iron (Fe).

[0099] The preparation process is the same as that of Example 1, and the addition amount is 0.6%.

[0100] Example 6: High-rare-earth formula inoculant

[0101] Composition: Scandium (Sc) 2.0%, Ytterbium (Yb) 1.3%, Lutetium (Lu) 0.7%, Terbium (Tb) 1.0%, Silicon (Si) 32%, Calcium (Ca) 2.5%, Molybdenum (Mo) 1.0%, Strontium (Sr) 0.8%, Zirconium (Zr) 0.5%, Aluminum (Al) 2.0%, the balance is Iron (Fe).

[0102] The preparation process is the same as that of Example 1, and the addition amount is 1.2%.

[0103] Example 7: Low-rare-earth formula inoculant

[0104] Composition: Scandium (Sc) 0.5%, Ytterbium (Yb) 0.3%, Lutetium (Lu) 0.1%, Terbium (Tb) 0.2%, Silicon (Si) 45%, Calcium (Ca) 0.5%, Molybdenum (Mo) 0.2%, Strontium (Sr) 0.1%, Zirconium (Zr) 0.1%, Aluminum (Al) 0.5%, the balance is Iron (Fe).

[0105] The preparation process is the same as that of Example 1, and the addition amount is 0.5%.

[0106] Example 8: High-scandium - high-terbium formula inoculant

[0107] Composition: Scandium (Sc) 1.9%, Ytterbium (Yb) 0.5%, Lutetium (Lu) 0.3%, Terbium (Tb) 0.8%, Silicon (Si) 40%, Calcium (Ca) 1.8%, Molybdenum (Mo) 0.6%, Strontium (Sr) 0.4%, Zirconium (Zr) 0.3%, Aluminum (Al) 1.2%, the balance is Iron (Fe).

[0108] The preparation process is the same as that of Example 1, and the addition amount is 0.9%.

[0109] Example 9: High-ytterbium - high-lutetium formula inoculant

[0110] Composition: Scandium (Sc) 0.7%, Ytterbium (Yb) 1.4%, Lutetium (Lu) 0.7%, Terbium (Tb) 0.3%, Silicon (Si) 37%, Calcium (Ca) 1.5%, Molybdenum (Mo) 0.7%, Strontium (Sr) 0.5%, Zirconium (Zr) 0.2%, Aluminum (Al) 0.9%, the balance is Iron (Fe).

[0111] The preparation process is the same as that of Example 1, and the addition amount is 0.8%.

[0112] Example 10: Balanced ratio formula inoculant

[0113] Composition: Scandium (Sc) 1.0%, Ytterbium (Yb) 0.7%, Lutetium (Lu) 0.3%, Terbium (Tb) 0.5%, Silicon (Si) 40%, Calcium (Ca) 1.5%, Molybdenum (Mo) 0.5%, Strontium (Sr) 0.4%, Zirconium (Zr) 0.3%, Aluminum (Al) 1.0%, the balance is Iron (Fe).

[0114] The preparation process is the same as that of Example 1, and the addition amount is 0.8%.

[0115] Comparative Example 1: Traditional FeSi inoculant

[0116] Composition: Silicon (Si) 75%, Aluminum (Al) 1.2%, Calcium (Ca) 1.0%, and the balance is Iron (Fe).

[0117] The addition amount is 0.8%.

[0118] Comparative Example 2: Traditional rare earth inoculant

[0119] Composition: Silicon (Si) 45%, Cerium (Ce) 1.8%, Lanthanum (La) 0.9%, Calcium (Ca) 2.0%, Aluminum (Al) 1.0%, and the balance is Iron (Fe).

[0120] The addition amount is 0.8%.

[0121] Comparative Example 3: Single scandium inoculant

[0122] Composition: Scandium (Sc) 1.2%, Silicon (Si) 40%, Calcium (Ca) 1.5%, Aluminum (Al) 1.0%, and the balance is Iron (Fe).

[0123] The addition amount is 0.8%.

[0124] Comparative Example 4: Single ytterbium inoculant

[0125] Composition: Ytterbium (Yb) 0.8%, Silicon (Si) 40%, Calcium (Ca) 1.5%, Aluminum (Al) 1.0%, and the balance is Iron (Fe).

[0126] The addition amount is 0.8%.

[0127] Comparative Example 5: Single lutetium inoculant

[0128] Composition: Lutetium (Lu) 0.4%, Silicon (Si) 40%, Calcium (Ca) 1.5%, Aluminum (Al) 1.0%, and the balance is Iron (Fe).

[0129] The addition amount is 0.8%.

[0130] Comparative Example 6: Single terbium inoculant

[0131] Composition: Terbium (Tb) 0.6%, Silicon (Si) 40%, Calcium (Ca) 1.5%, Aluminum (Al) 1.0%, and the balance is Iron (Fe).

[0132] The addition amount is 0.8%.

[0133] Comparative Example 7: Scandium-ytterbium combined inoculant

[0134] Composition: Scandium (Sc) 1.2%, Ytterbium (Yb) 0.8%, Silicon (Si) 40%, Calcium (Ca) 1.5%, Aluminum (Al) 1.0%, the balance being Iron (Fe).

[0135] The addition amount is 0.8%.

[0136] Comparative Example 8: Lutetium-Terbium combined inoculant

[0137] Composition: Lutetium (Lu) 0.4%, Terbium (Tb) 0.6%, Silicon (Si) 40%, Calcium (Ca) 1.5%, Aluminum (Al) 1.0%, the balance being Iron (Fe).

[0138] The addition amount is 0.8%.

[0139] Comparative Example 9: Scandium-Lutetium-Terbium combined inoculant (without ytterbium)

[0140] Composition: Scandium (Sc) 1.2%, Lutetium (Lu) 0.4%, Terbium (Tb) 0.6%, Silicon (Si) 40%, Calcium (Ca) 1.5%, Aluminum (Al) 1.0%, the balance being Iron (Fe).

[0141] The addition amount is 0.8%.

[0142] Comparative Example 10: Four rare earths in equal proportion inoculant

[0143] Composition: Scandium (Sc) 0.75%, Ytterbium (Yb) 0.75%, Lutetium (Lu) 0.75%, Terbium (Tb) 0.75%, Silicon (Si) 40%, Calcium (Ca) 1.5%, Aluminum (Al) 1.0%, the balance being Iron (Fe).

[0144] The addition amount is 0.8%.

[0145] The specific experimental effects are as follows:

[0146] As Figure 1 shown, there are significant differences between the inoculants of the present invention (Examples 1-10) and the comparative examples (Comparative Examples 1-10) in terms of the proportion of A-type graphite, graphite size and graphite quantity. The proportion of A-type graphite in the example group is generally higher than 90%, while that of the traditional 75Si-Fe inoculant (Comparative Example 1) is only 68.5%, and that of the conventional rare earth inoculant (Comparative Example 2) is 82.3%. In terms of graphite size, the average in the example group is 35-40 μm, much smaller than 72.6 μm of Comparative Example 1. In terms of graphite quantity, the example group reaches 350-385 pieces / mm², about 2.5 times that of Comparative Example 1.

[0147] Figure 1The display example groups of (a)-(c) have a higher proportion of type A graphite, smaller graphite size, and larger number of graphite, indicating that the inoculant of the present invention significantly optimizes the graphite morphology of cast iron.

[0148] Figure 2 The influence of the inoculant addition amount of Example 1 on the graphite morphology is shown. The four curve graphs clearly show that as the addition amount increases from 0.3% to 1.2%, as Figure 2 shown in (a), the proportion of type A graphite increases from 78.5% to 99.2%, as Figure 2 shown in (b), the graphite size decreases from 52.3 μm to 30.8 μm, as Figure 2 shown in (c), the number of graphite increases from 215.6 pieces / mm² to 425.3 pieces / mm², as Figure 2 shown in (d), the uniformity of graphite distribution increases from 82.5% to 97.5%.

[0149] It is particularly noteworthy that the curves show obvious non-linear characteristics. After the addition amount reaches 0.8%, the improvement degree of each index tends to be gentle, indicating that the range of 0.8-1.0% is the most economical and effective addition amount range.

[0150] Figure 3 The influence of different inoculants on the matrix structure and its relationship with mechanical properties are shown. As Figure 3 shown in (a)-(b), the pearlite content in the example group is generally higher than 90%, the ferrite content is lower than 10%, and the average grain size is 18-22 μm, which is significantly better than the control group.

[0151] Figure 3 (c)-(d) The scatter plots reveal the positive correlation between the pearlite content and the tensile strength and hardness, and the negative correlation between the grain size and the impact toughness. The pearlite content of Example 1 reaches 96.5%, and the grain size is only 18.3 μm, so the best combination of mechanical properties is obtained. As Figure 3 shown in (d), the impact toughness reaches the best value when the grain size is about 20 μm, which highly coincides with the grain size range generated by the inoculant of the present invention.

[0152] Figure 4 The room temperature and high temperature mechanical properties of different inoculants are comprehensively shown. Figure 4 (a)-(b) Compare the tensile strength of each example with the control group. The strength of the example group is generally 320-390 MPa, while that of the control group 1 is only 235.6 MPa. The high-strength formulations (Example 2 and Example 6) are particularly prominent, reaching 386.5 MPa and 390.3 MPa.

[0153] Figure 4(c)-(d) compared the tensile strengths of different samples at room temperature, 300 °C, 400 °C and 500 °C. At a high temperature of 500 °C, the strength of the example group remained at 225 - 240 MPa, while that of Comparative Example 1 was only 135.2 MPa. The scatter plot in the lower right shows the correlation between hardness and impact toughness, indicating that the inoculant of the present invention has successfully achieved a synergistic improvement in strength and toughness.

[0154] Figure 5 and Figure 6 The high-temperature strength retention rate was analyzed emphatically. Figure 5 The strength retention rates of different samples at a high temperature of 500 °C were compared. The retention rate of the example group reached 61.6 - 65.5%, which was significantly higher than that of the comparative example group at 57 - 59%.

[0155] Figure 6 More intuitively showing the percentage increase compared with Comparative Example 1, Example 1 and Example 4 increased by 55.8% and 59.7% respectively, showing the significant advantages of the present invention in high-temperature applications. This is of great significance for cast iron parts that need to serve in high-temperature environments.

[0156] Figure 7 The non-linear relationship between the addition amount and the tensile strength was shown. Figure 7 (a) shows that the strength changes in an "S" shape with the increase of the addition amount: it increases rapidly in the range of 0.1 - 0.8%, levels off in the range of 0.8 - 1.2%, and slightly decreases after exceeding 1.2%. Figure 7 (b) shows the change of the strength growth rate. The growth rate is the highest (18.9%) when the addition amount is 0.3%, and then gradually decreases. Even a negative growth (-1.2%) occurs when the addition amount is 1.5%.

[0157] This non-linear relationship proves the existence of an optimal addition amount window (0.8 - 1.2%), and also reveals the principle that excessive addition is disadvantageous, providing important guidance for industrial applications.

[0158] Figure 8 The decay characteristics of different inoculants were shown. Figure 8 (a)-(b) compared the effects of holding time on the tensile strength and the proportion of A-type graphite. After holding for 60 minutes, the tensile strength of Example 1 still remained at 325.6 MPa (decay rate 7.6%), while that of Comparative Example 1 decreased to 160.2 MPa (decay rate 38.6%).

[0159] Figure 8(c)-(d) made a detailed comparison of the decay rate changes. The example group (especially Example 4) showed excellent anti-decay performance, and the decay rate was only 6.8 - 9.2% after 60 minutes, far lower than 38.6% of Comparative Example 1 and 22.5% of Comparative Example 2. This long-term stable inoculation characteristic is extremely important for improving the quality stability of castings.

[0160] Figure 9 Demonstrated the performance retention characteristics under overheating conditions. Under the extreme condition of 150 °C overheating of the molten iron, the tensile strength of Example 1 still remained at 305.6 MPa (retention rate 86.6%), and the proportion of Type A graphite remained at 82.6%, while the retention rates of Comparative Example 1 and Comparative Example 2 were only 63.9% and 72.1% respectively.

[0161] Figure 9 The gentle slope of the (c) curve indicates that the inoculant of the present invention has excellent adaptability to overheating conditions, which is of great significance for the adaptability to temperature control fluctuations in actual production.

[0162] Figure 10 Fully demonstrated the thermal fatigue resistance performance. Figure 10 (a)-(b) showed that after 1000 thermal cycles, the surface crack density and the maximum crack depth of Example 1 were 15.6 cracks / cm² and 385.5 μm respectively, only 37% and 45% of Comparative Example 1.

[0163] Figure 10 (c) revealed the relationship between the grain size and the crack propagation rate, indicating that the crack propagation rate decreased significantly in the grain size range of 15 - 30 μm, which is exactly the grain size range (15 - 20 μm) that can be obtained by the inoculant of the present invention. Figure 10 (d) comparison showed that Example 4 showed the best performance in thermal fatigue resistance, with a 58.8% improvement compared to Comparative Example 1.

[0164] Figure 11 Demonstrated the wear resistance and its relationship with the microstructure. Figure 11 (a)-(b) compared the mass loss and relative wear resistance under different conditions. The relative wear resistance of Example 1 under dry grinding, oil lubrication and water lubrication conditions reached 210%, 214% and 220% respectively, far superior to the traditional inoculant.

[0165] Figure 11 (c)-(d) revealed the relationship between hardness, pearlite content, graphite size and wear resistance. The scatter plot showed that the higher the hardness, the higher the pearlite content, and the smaller the graphite size, the better the wear resistance. Example 2 showed the best performance, with a relative wear resistance of 240%, corresponding to its highest pearlite content (98.2%) and hardness (285.6 HB).

[0166] Figure 12 Demonstrates the synergistic mechanism of scandium, ytterbium, lutetium, and terbium elements. Figure 12 (a) Shows the effect of a single rare earth element on the proportion of A-type graphite, which varies non-linearly with the increase in the addition amount. Figure 12 (b) Compares the optimal proportion combination, the equal-proportion combination, and the theoretical linear superposition effect. The optimal proportion combination is significantly higher than the linear superposition value, proving the existence of a strong synergistic effect.

[0167] Figure 12 (c) Compares the effects of different combinations on the characteristics of nucleation sites. The quaternary optimal proportion combination forms the largest number of nucleation sites with the smallest size. Figure 12 (d) Shows that the oxygen activity coefficient and sulfur activity coefficient of the quaternary optimal proportion combination are the lowest, indicating the best synergistic purification effect.

[0168] Figure 13 Demonstrates the nucleation efficiency coefficients of different rare earth combinations. As the complexity of the combination increases, the efficiency coefficient increases significantly, from 0.7 - 1.0 for a single element to 3.5 for the quaternary optimal proportion, indicating the existence of a significant synergistic effect.

[0169] Figure 14 Analyzes the effect of different rare earth combinations on the supercooling degree. The supercooling degree of the quaternary optimal proportion combination is the lowest (12.5°C), far lower than the theoretical linear superposition value (18.6°C) and the single element value (22 - 26°C), indicating that the composite system can significantly improve the nucleation conditions and promote the formation of A-type graphite.

Claims

1. An efficient inoculant for cast iron containing rare earth elements, characterized in that, By weight percentage, it includes the following components: Active elements: scandium (Sc) 0.5 - 2.0%, ytterbium (Yb) 0.3 - 1.5%, lutetium (Lu) 0.1 - 0.8%, terbium (Tb) 0.2 - 1.0%; Auxiliary elements: silicon (Si) 30 - 45%, calcium (Ca) 0.5 - 2.5%, molybdenum (Mo) 0.2 - 1.0%, strontium (Sr) 0.1 - 0.8%, zirconium (Zr) 0.1 - 0.5%; Carrier element: aluminum (Al) 0.5 - 2.0%, and the balance is iron (Fe).

2. The high-efficiency cast iron inoculant containing rare earth elements according to claim 1, characterized in that, The weight ratio of the four rare earth elements of scandium, ytterbium, lutetium and terbium is 3:2:1:1.

5.

3. The high-efficiency cast iron inoculant containing rare earth elements according to any one of claims 1-2, characterized in that, The inoculant is used for inoculation treatment of cast iron, and the addition amount is 0.3 - 1.2%.

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