Nodular cast iron and preparation method thereof

Abstract

The invention provides nodular cast iron and a preparation method thereof. The nodular cast iron comprises the following chemical components in percentage by mass: 3.45-3.50% of C; 0.035% to 0.050% of Mg; 1.50% to 1.55% of Si; mn: lt; 0.17%, 0.17%; ni: lt; 0.15% by weight; cu: lt; 0.12%, 0.12%; p: lt; 0.03% by weight; cr: lt; 0.02% of the total weight; sb: lt; 0.01% of the total weight; sn: lt; 0.01% of the total weight; and the balance Fe and impurities. The spheroidizing agent A and the spheroidizing agent B with specific components are added in a composite mode, the primary inoculation process and the stream inoculation process are combined, finally, the Si content is accurately controlled to be 2.95%-3.05%, and therefore the nodular cast iron with the mark being EN-GJS-450-18 is successfully prepared. Through stepped silicon increasing and inoculation treatment, synchronous optimization of the strength and toughness of the nodular cast iron is achieved, and the comprehensive mechanical property is excellent and stable.

Description

Technical Field

[0001] This invention relates to the field of ductile iron production and manufacturing technology, specifically to a ductile iron and its preparation method. Background Technology

[0002] Ductile cast iron (DCI) is a high-performance cast iron material produced by adding a spheroidizing agent to molten iron and undergoing inoculation treatment, causing graphite to precipitate in the form of tiny spherical particles. Compared to traditional gray cast iron, the graphite is distributed in a spherical rather than flake shape, improving the material's toughness, tensile strength, and ductility, while retaining the inherent high wear resistance, good fluidity, and low-cost casting characteristics of cast iron. In the wind power generation field, this material is widely used in critical load-bearing components, such as gearbox housings, hubs, and main shaft support seats, serving as an important medium connecting the mechanical transmission system and the main structural body.

[0003] Chinese Patent (CN106011607B) discloses a silicon-solution-strengthened ferritic ductile iron and its preparation process. By weight percentage, the ductile iron contains 3.05-3.30% C, 3.80-4.00% Si, 0.15-0.25% Mn, 0.035-0.050% Mg, ≤0.035% P, ≤0.015% S, 0.0030-0.0040% Ce, 0.0045-0.0055% Sb, with the remainder being iron. The preparation process uses high-quality pig iron and scrap steel. The molten iron is melted in an electric furnace while controlling the carbon, silicon, and manganese content. A pretreatment agent, spheroidizing agents A and B, and an inoculant are added to the spheroidizing ladle for molten iron treatment. Antimony is added to adjust the cerium content, resulting in good spheroidizing effect. The prepared silicon-solution-strengthened ferritic ductile iron exhibits superior tensile strength, yield strength, and elongation. However, this can lead to increased material brittleness, a significant tendency to shrink, and the risk of loss in toughness and casting performance. Introducing high silicon content in the initial melting stage not only exacerbates the generation of impurities during melting, but also inhibits ferrite formation due to premature solid solution strengthening of silicon, resulting in uneven matrix structure. After the spheroidization reaction, magnesium rapidly oxidizes and degrades, causing graphite distortion, ultimately leading to fluctuations in mechanical properties, especially insufficient toughness. Summary of the Invention

[0004] The purpose of this invention is to overcome the shortcomings of the prior art and provide a ductile iron and its preparation method.

[0005] To achieve the above objectives, the technical solution provided by the present invention is as follows: In the first aspect, a type of ductile iron comprises, by mass percentage: C, 3.45-3.50%; Mg, 0.035-0.050%; Si, 1.50-1.55%; Mn, <0.17%; Ni, <0.15%; Cu, <0.12%; P, <0.03%; Cr, <0.02%; Sb, <0.01%; Sn, <0.01%; with the balance being Fe and unavoidable impurities; In this process, the Si content reaches 2.95~3.05% by mass percentage after adding spheroidizing agent A and spheroidizing agent B and completing one inoculation and in-flow inoculation, thus producing ductile iron with the grade EN-GJS-450-18.

[0006] As a preferred technical solution, the spheroidizing agent A comprises, by mass percentage: Mg, 40-50%; Si, 40-45%; Ca, 1.0-2.0%; Al, ≤1.0%; RE, 0.5-1.5%; with the balance being Fe.

[0007] As a preferred technical solution, the spheroidizing agent B comprises, by mass percentage: Ba, 8-12%; Si, 45-50%; Ca, 1.0-2.5%; Al, ≤1.5%; with the balance being Fe.

[0008] Secondly, the above-mentioned method for preparing ductile iron includes the following steps: S1. By weight, select 20-65 parts pig iron, 15-35 parts scrap steel, and 10-30 parts recycled material, pack and melt them, adjust the chemical composition to meet the requirements, and heat the electric furnace to 1430-1480℃ to melt them into molten iron. S2. Based on the quality of the molten iron, add 0.6% of spheroidizing agent A and 0.4% of spheroidizing agent B to the spheroidizing ladle and compact it. Evenly cover the spheroidizing ladle with 0.20% (by weight of the molten iron) of ferrosilicon powder and 0.2-0.6% of iron filings, and compact it. The silicon content of the ferrosilicon powder should be 72-78 wt%. S3. Wait for the spheroidizing reaction to proceed for 60-180 seconds, then add 0.6% (by weight of molten iron) of inoculant A into the spheroidizing ladle for a first inoculation. After the first inoculation, remove the slag. S4. During the casting process, inoculant B, with a mass of 0.15% of the molten iron mass, is added to the mold for inoculation.

[0009] As a preferred technical solution, the inoculant A comprises, by mass percentage: Si, 70-75%; Ba, 1.5-2.5%; Ca, 0.8-1.5%; Bi, 0.5-1.2%; Al, ≤1.2%; with the balance being Fe.

[0010] As a preferred technical solution, the inoculant B comprises, by mass percentage: Si, 72-78%; Sr, 0.8-1.5%; Zr, 0.3-0.8%; Al, ≤1.0%; with the balance being Fe.

[0011] As a preferred technical solution, in step S2, the spheroidizing ladle is a dam-type spheroidizing ladle, with the dam height being 1 / 3 to 1 / 2 of the height of the ladle's inner cavity. Before pouring molten iron, the spheroidizing ladle needs to be preheated to 600-750°C, and a covering agent comprising 0.1-0.3% of the total mass of the molten iron is added to the bottom of the ladle. The covering agent includes perlite or cryolite powder.

[0012] As a preferred technical solution, in step S4, after in-flow inoculation is completed, the pouring temperature of the molten iron is controlled at 1320-1360℃. After pouring, the casting is opened from the mold when it cools to 750-800℃ and immediately transferred to an annealing furnace for heat treatment. The heat treatment process is as follows: the temperature is raised to 900-920℃ at a rate of 80-100℃ / hour and held for 2-4 hours; then it is cooled in the furnace to 700-720℃ and held for 4-6 hours; finally, it is cooled in the furnace to below 300℃ and then air-cooled.

[0013] As a preferred technical solution, in step S4, after in-flow inoculation is completed, the pouring temperature of the molten iron is controlled at 1320-1360℃. After pouring, the casting is air-cooled in the mold to below 400℃ before being unloaded. Compared to heat-treated castings, the residual stress release is relatively lower by 20%, and the cooling process can be selected according to actual needs such as the construction period.

[0014] The advantages and beneficial effects of this invention lie in the precise control of the final silicon content within a narrow range of 2.95% to 3.05%, and the adoption of a stepped inoculation process combining the composite addition of spheroidizing agents A and B, and integrating primary inoculation with in-flow inoculation, thereby achieving simultaneous optimization of the strength and toughness of ductile iron. Spheroidizing agent A provides sufficient magnesium and rare earth elements to ensure graphite spheroidization, while barium in spheroidizing agent B prolongs the inoculation time and slows down degradation. Bismuth in inoculator A further promotes graphite spheroidization and refines the graphite morphology, while strontium and zirconium in in-flow inoculator B effectively enhance the inoculation's resistance to degradation and inhibit carbide precipitation. Ultimately, this results in ductile iron that achieves high strength while maintaining excellent plasticity and toughness, consistently meeting the performance requirements of EN-GJS-450-18. The production process is stable, reliable, and easy to control. Attached Figure Description

[0015] Figure 1 This is one of the scanning electron microscope schematic diagrams of the ductile iron of the present invention; Figure 2 This is the second scanning electron microscope schematic diagram of the ductile iron of the present invention; Figure 3This is the third scanning electron microscope schematic diagram of the ductile iron of the present invention; Figure 4 This is the fourth scanning electron microscope schematic diagram of the ductile iron of this invention. Detailed Implementation

[0016] To make the above-mentioned objectives, features, and advantages of this application more apparent and understandable, the specific embodiments of this application will be described in detail below with reference to the accompanying drawings. It should be understood that the specific embodiments described herein are for illustrative purposes only and are not intended to limit the scope of this application. Furthermore, it should be noted that, for ease of description, only the parts relevant to this application are shown in the accompanying drawings, not the entire structure. Based on the embodiments in this application, all other embodiments obtained by those skilled in the art without inventive effort are within the scope of protection of this application.

[0017] The terms “comprising” and “having”, and any variations thereof, used in this application are intended to cover non-exclusive inclusion. For example, a process, method, system, product, or apparatus that includes a series of steps or units is not limited to the steps or units listed, but may optionally include steps or units not listed, or may optionally include other steps or units inherent to such process, method, product, or apparatus.

[0018] In this document, the term "embodiment" means that a particular feature, structure, or characteristic described in connection with an embodiment may be included in at least one embodiment of this application. The appearance of this phrase in various places throughout the specification does not necessarily refer to the same embodiment, nor is it a separate or alternative embodiment mutually exclusive with other embodiments. It will be explicitly or implicitly understood by those skilled in the art that the embodiments described herein can be combined with other embodiments.

[0019] This invention provides a high-performance ductile iron and its preparation method, particularly suitable for key structural components in wind power generation equipment, such as gearboxes, hubs, and main shaft seats. The chemical composition of this ductile iron is precisely designed, with the silicon content ultimately controlled within a narrow range of 2.95% to 3.0%. Through a composite spheroidizing agent and a stepped inoculation process, a synergistic improvement in strength and toughness is achieved, ultimately meeting the performance requirements of EN-GJS-450-18 grade. Controlling the silicon content at a low level during the initial smelting helps reduce the generation of impurities during the smelting process and improves the fluidity of the molten iron. The post-siliconization process is completed through the compounding of spheroidizing agents A and B and the graded inoculation of inoculants A and B. This allows silicon to be introduced in a highly dispersed manner during the key stage after the spheroidization reaction. This not only improves the absorption and utilization rate of silicon and promotes the formation of ferrite and solid solution strengthening, but also avoids problems such as increased brittleness and shrinkage tendency of castings caused by excessively high silicon content in the early stage. As a result, high-strength and high-toughness EN-GJS-450-18 grade ductile iron can be stably obtained.

[0020] In the initial composition of ductile iron, the carbon content is controlled at 3.45%–3.50% to ensure sufficient graphitization; the magnesium content is 0.035%–0.050% to ensure sufficient core elements for graphitization; the silicon content is initially low (1.50%–1.55%), and is gradually increased to 2.95%–3.05% later through spheroidizing agents and inoculants. This avoids brittleness caused by premature silicon increase and also enhances ferrite content through solid solution strengthening. Other elements such as Mn, Ni, Cu, P, Cr, Sb, and Sn are all controlled at low levels to reduce their adverse effects on toughness and casting performance.

[0021] Nodulating agent A is mainly composed of Mg, Si, Ca, Al, and rare earth elements (RE), with Mg content of 40%–50% and RE content of 0.5%–1.5%. It has strong nodulating ability and anti-interference properties, making it particularly suitable for high-purity molten iron. Nodulating agent B is mainly composed of Ba, Si, and Ca, with Ba content of 8%–12%. It can prolong the inoculation time, slow down fading, and improve nodulation stability. The combined use of the two ensures both nodulation rate and improves process tolerance.

[0022] Inoculant A contains Bi, which can further promote graphite spheroidization and refine graphite morphology; inoculant B contains Sr and Zr, which have strong anti-fading ability and inhibit carbide precipitation. The combination of primary inoculation and in-flow inoculation forms a temporal functional relay, covering the entire process from spheroidization to casting, and improving the overall performance of ductile iron.

[0023] The process employs a dam-type spheroidizing ladle, with the dam height being 1 / 3 to 1 / 2 of the inner cavity height, which helps improve the spheroidizing reaction efficiency and magnesium absorption rate. The spheroidizing ladle is preheated to 600–750℃, and perlite or cryolite powder is pre-added as a covering agent to effectively prevent magnesium oxidation and burn-off. The pouring temperature is controlled at 1320–1360℃, and the unpacking temperature is 750–800℃. Combined with a stepped heat treatment process, the matrix structure is further optimized, internal stress is eliminated, and the toughness and stability of the material are improved.

[0024] Compared to existing technologies, which typically employ a one-time high silicon content addition method that achieves strengthening but also carries the risk of reduced toughness and casting performance, this invention combines low-silicon molten iron smelting with subsequent silicon addition. This mitigates the material brittleness caused by the increase in silicon content. The compounding and staged addition of multiple additives creates a sequential functional relay, covering the entire process from spheroidization to inoculation and casting. This successfully achieves a comprehensive improvement in product performance, production stability, and cost-effectiveness.

[0025] During the spheroidization stage, spheroidizing agent A rapidly releases magnesium in the early stages of the spheroidization reaction, promoting the initiation of graphite spheroidization. Spheroidizing agent B, on the other hand, continuously releases barium in the middle and later stages of the reaction, prolonging the inoculation time, slowing down magnesium decay, and improving spheroidization stability.

[0026] During the inoculation stage, Bi further promotes graphite spheroidization, refines graphite particles, and increases the spheroidization rate. Inoculant A mainly acts on the molten iron after spheroidization, stabilizing the formation of spheroidal graphite and reducing graphite distortion caused by spheroidization fading. Inoculant B is added before casting and dispersed in the molten iron through the in-flow inoculation process. Sr has a long-lasting inoculation effect, significantly extending the duration of the inoculation effect and inhibiting carbide precipitation, while Zr enhances the inoculant's resistance to fading, preventing the inoculation effect from weakening during casting.

[0027] In the preparation of this ductile iron, the silicon content is precisely increased to a narrow range of 2.95-3.05%. During the initial smelting, the silicon content is intentionally controlled at a low level. Subsequently, by adding spheroidizing agents A and B in combination, and employing both single-stage and in-flow inoculation processes, silicon is gradually introduced after the spheroidization reaction. This staged silicon-increasing method provides a foundation for controlling the silicon content.

[0028] Mg provides the spheroidizing nucleus between spheroidizing agents A and B, while Ba delays its oxidation and burn-off, improving magnesium utilization. Rare earth elements RE work synergistically with Ba to improve the purity of molten iron and enhance its resistance to interference. Both spheroidizing agents contain Si, providing a foundation for subsequent solid solution strengthening. The Mg and RE elements released from the spheroidizing agents provide a favorable reaction environment for Bi in inoculant A, promoting its inoculation effect. Sr and Zr in inoculant B continue to maintain the inoculation effect even after the effects of the spheroidizing agents and inoculant A diminish.

[0029] Inoculant A, rich in magnesium and rare earth elements, is responsible for initiating the graphite spheroidization reaction, while barium inoculant B prolongs the inoculation time, slows down magnesium decay, and ensures spheroidization stability. Simultaneously, bismuth inoculant A further promotes graphite spheroidization and refinement, while strontium and zirconium inoculant B enhance resistance to decay and inhibit carbide precipitation. This multi-stage, multi-component synergistic effect allows silicon to be efficiently and uniformly dissolved in the molten iron, thus achieving precise control of silicon content in the final casting.

[0030] This invention, through the rational combination of additives and time control, ultimately forms a matrix structure with ferrite as the main component and pearlite as the auxiliary component. The stepwise introduction of Si promotes the formation of ferrite, while achieving solid solution strengthening, inhibiting pearlite transformation, stabilizing ferrite content, and exhibiting a good balance between strength and plasticity.

[0031] Specifically, this invention significantly improves the comprehensive mechanical properties of materials, consistently meeting and even surpassing the requirements of EN-GJS-450-18. The ductile iron produced by this invention has a tensile strength ≥450MPa and an elongation ≥16%. The production process exhibits excellent stability and reproducibility, reducing casting defects and scrap rates caused by inoculation degradation and poor spheroidization. Furthermore, this process is more adaptable to raw materials, helping to control production costs. It is particularly suitable for the mass production of critical components of wind turbine generators, such as gearboxes, hubs, and main shafts, which require extremely high strength and toughness.

[0032] Example 1 A method for preparing ductile iron includes the following steps: S1. By mass, select 20 parts pig iron, 35 parts scrap steel and 15 parts recycled material, pack them together and melt them. After adjusting the chemical composition, heat them to 1430℃ in an electric furnace to melt them into molten iron. S2. Based on the quality of the molten iron, add 0.6% of spheroidizing agent A and 0.4% of spheroidizing agent B to the dam-type spheroidizing ladle and compact it. The spheroidizing agent A, by mass percentage, comprises: Mg: 40%; Si: 45%; Ca: 1.0%; Al: 0.8%; RE: 0.5%; with the balance being Fe. The spheroidizing agent B, by mass percentage, comprises: Ba: 8%; Si: 50%; Ca: 1.0%; Al: 1.0%; with the balance being Fe. Then, evenly cover the spheroidizing agents with 0.20% of the molten iron mass of ferrosilicon powder (72% silicon content) and 0.2% of iron filings and compact them. S3. After 60 seconds of spheroidizing reaction, inoculant A, with a mass of 0.6% of the molten iron, is added to the molten iron in the spheroidizing ladle for a first inoculation. The inoculant A comprises, by mass percentage: Si: 70%; Ba: 1.5%; Ca: 0.8%; Bi: 0.5%; Al: 1.0%; with the balance being Fe. After 30 seconds of the first inoculation, slag is removed. S4. During the casting process, inoculant B, with a mass of 0.15% of the molten iron mass, is added to the mold along with the molten iron flow for in-flow inoculation. The inoculant B, by mass percentage, comprises: Si: 72%; Sr: 0.8%; Zr: 0.3%; Al: 0.8%; with the balance being Fe. The casting temperature is 1320℃. When the casting cools to 750℃ in the mold, it is opened and immediately transferred to an annealing furnace for heat treatment. The heat treatment process is as follows: the temperature is increased to 900℃ at a rate of 80℃ / hour and held for 2 hours; then cooled in the furnace to 700℃ and held for 4 hours; finally, it is cooled in the furnace to below 300℃ and then air-cooled.

[0033] The ductile iron prepared in this embodiment has a tensile strength of 452 MPa and an elongation of 18%.

[0034] Example 2 A method for preparing ductile iron includes the following steps: S1. By mass, select 62 parts pig iron, 15 parts scrap steel and 30 parts recycled material, pack them together and melt them. After adjusting the chemical composition, heat them to 1450℃ in an electric furnace to melt them into molten iron. S2. Based on the quality of the molten iron, add 0.6% of spheroidizing agent A and 0.4% of spheroidizing agent B to the dam-type spheroidizing ladle and compact it. The spheroidizing agent A, by mass percentage, comprises: Mg: 45%; Si: 42%; Ca: 1.5%; Al: 0.5%; RE: 1.0%; with the balance being Fe. The spheroidizing agent B, by mass percentage, comprises: Ba: 10%; Si: 48%; Ca: 1.8%; Al: 1.2%; with the balance being Fe. Then, evenly cover the spheroidizing agents with 0.20% of the molten iron mass of ferrosilicon powder (75% silicon content) and 0.4% of iron filings and compact them. S3. After the spheroidizing reaction for 120 seconds, inoculant A, with a mass of 0.6% of the molten iron, is added to the molten iron in the spheroidizing ladle for a first inoculation. The inoculant A, by mass percentage, comprises: Si: 72%; Ba: 2.0%; Ca: 1.2%; Bi: 0.8%; Al: 1.0%; with the balance being Fe. After the first inoculation for 45 seconds, slag is removed. S4. During the casting process, inoculant B, with a mass of 0.15% of the molten iron mass, is added to the mold along with the molten iron flow for in-flow inoculation. The inoculant B, by mass percentage, comprises: Si: 75%; Sr: 1.2%; Zr: 0.5%; Al: 0.8%; with the balance being Fe. The casting temperature is 1340℃. When the casting cools to 770℃ in the mold, it is opened and immediately transferred to an annealing furnace for heat treatment. The heat treatment process is as follows: the temperature is increased to 910℃ at a rate of 90℃ / hour and held for 3 hours; then it is cooled in the furnace to 710℃ and held for 5 hours; finally, it is cooled in the furnace to below 300℃ and then air-cooled.

[0035] The ductile iron prepared in this embodiment has a tensile strength of 455 MPa and an elongation of 17%.

[0036] Example 3 A method for preparing ductile iron includes the following steps: S1. By mass, select 45 parts pig iron, 15 parts scrap steel and 10 parts recycled material, pack them together and melt them. After adjusting the chemical composition, heat them to 1480℃ in an electric furnace to melt them into molten iron. S2. Based on the quality of the molten iron, add 0.6% of spheroidizing agent A and 0.4% of spheroidizing agent B to the dam-type spheroidizing ladle and compact it. The spheroidizing agent A, by mass percentage, comprises: Mg: 50%; Si: 40%; Ca: 2.0%; Al: 0.5%; RE: 1.5%; with the balance being Fe. The spheroidizing agent B, by mass percentage, comprises: Ba: 12%; Si: 45%; Ca: 2.5%; Al: 1.5%; with the balance being Fe. Then, evenly cover the spheroidizing agent with 0.20% of the molten iron mass of ferrosilicon powder (78% silicon content) and 0.6% of iron filings and compact it. S3. After the spheroidizing reaction for 180 seconds, inoculant A, with a mass of 0.6% of the molten iron, is added to the molten iron in the spheroidizing ladle for a first inoculation. The inoculant A comprises, by mass percentage: Si: 75%; Ba: 2.5%; Ca: 1.5%; Bi: 1.2%; Al: 1.2%; with the balance being Fe. After the first inoculation for 60 seconds, slag is removed. S4. During the casting process, inoculant B, at a mass of 0.17% of the molten iron mass, is added to the mold along with the molten iron flow for in-flow inoculation. The inoculant B, by mass percentage, comprises: Si: 78%; Sr: 1.5%; Zr: 0.8%; Al: 1.0%; with the balance being Fe. The casting temperature is 1360℃. When the casting cools to 800℃ in the mold, it is opened and immediately transferred to an annealing furnace for heat treatment. The heat treatment process is as follows: the temperature is increased to 920℃ at a rate of 100℃ / hour and held for 4 hours; then cooled in the furnace to 720℃ and held for 6 hours; finally, it is cooled in the furnace to below 300℃ and then air-cooled.

[0037] The ductile iron prepared in this embodiment has a tensile strength of 450 MPa and an elongation of 16%.

[0038] Example 4 A method for preparing ductile iron includes the following steps: S1. By mass, select 55 parts pig iron, 20 parts scrap steel and 25 parts recycled material, pack them together and melt them. After adjusting the chemical composition, heat them to 1440℃ in an electric furnace to melt them into molten iron. S2. Based on the quality of the molten iron, add 0.6% of spheroidizing agent A and 0.4% of spheroidizing agent B to the dam-type spheroidizing ladle and compact it. The spheroidizing agent A, by mass percentage, comprises: Mg: 42%; Si: 44%; Ca: 1.2%; Al: 0.6%; RE: 0.8%; with the balance being Fe. The spheroidizing agent B, by mass percentage, comprises: Ba: 9%; Si: 49%; Ca: 1.5%; Al: 1.3%; with the balance being Fe. Preheat the spheroidizing ladle to 600°C, and add perlite covering agent accounting for 0.1% of the total mass of the molten iron to the bottom of the ladle beforehand. Then, evenly cover the spheroidizing agent with 0.20% of the molten iron mass of ferrosilicon powder (silicon content 73%) and 0.3% of iron filings and compact it. S3. After 90 seconds of spheroidizing reaction, inoculant A, at a mass of 0.6% of the molten iron, is added to the molten iron in the spheroidizing ladle for a first inoculation. Inoculant A comprises, by mass percentage: Si: 71%; Ba: 1.8%; Ca: 1.0%; Bi: 0.7%; Al: 1.1%; with the balance being Fe. After 40 seconds of the first inoculation, slag is removed. S4. During the casting process, inoculant B, with a mass of 0.15% of the molten iron mass, is added to the mold along with the molten iron flow for in-flow inoculation. The inoculant B, by mass percentage, comprises: Si: 74%; Sr: 1.0%; Zr: 0.4%; Al: 0.9%; with the balance being Fe. The casting temperature is 1330℃. When the casting cools to 760℃ in the mold, it is opened and immediately transferred to an annealing furnace for heat treatment. The heat treatment process is as follows: the temperature is increased to 905℃ at a rate of 85℃ / hour and held for 2.5 hours; then cooled in the furnace to 705℃ and held for 4.5 hours; finally, it is cooled in the furnace to below 300℃ and then air-cooled.

[0039] The ductile iron prepared in this embodiment has a tensile strength of 453 MPa and an elongation of 17.5%.

[0040] Example 5 A method for preparing ductile iron includes the following steps: S1. By weight, select 60 parts pig iron, 30 parts scrap steel, and 10 parts recycled material, pack them together, melt them, adjust the chemical composition, and then heat them to 1460℃ in an electric furnace to melt them into molten iron. S2. Based on the quality of the molten iron, add 0.6% of spheroidizing agent A and 0.4% of spheroidizing agent B to the dam-type spheroidizing ladle and compact it. The spheroidizing agent A, by mass percentage, comprises: Mg: 48%; Si: 41%; Ca: 1.8%; Al: 0.7%; RE: 1.2%; with the balance being Fe. The spheroidizing agent B, by mass percentage, comprises: Ba: 11%; Si: 46%; Ca: 2.2%; Al: 1.4%; with the balance being Fe. Preheat the spheroidizing ladle to 700°C. Add cryolite powder covering agent, accounting for 0.2% of the total mass of the molten iron, to the bottom of the ladle beforehand. Then, evenly cover the spheroidizing agent with 0.20% of the molten iron mass of ferrosilicon powder (76% silicon content) and 0.5% of iron filings and compact it. S3. After the spheroidizing reaction for 150 seconds, inoculant A, with a mass of 0.6% of the molten iron, is added to the molten iron in the spheroidizing ladle for a first inoculation. The inoculant A, by mass percentage, comprises: Si: 73%; Ba: 2.2%; Ca: 1.3%; Bi: 1.0%; Al: 1.0%; with the balance being Fe. After the first inoculation for 50 seconds, slag is removed. S4. During the casting process, inoculant B, with a mass of 0.15% of the molten iron mass, is added to the mold along with the molten iron flow for in-flow inoculation. The inoculant B, by mass percentage, comprises: Si: 76%; Sr: 1.3%; Zr: 0.6%; Al: 0.7%; with the balance being Fe. The casting temperature is 1350℃. When the casting cools to 780℃ in the mold, it is opened and immediately transferred to an annealing furnace for heat treatment. The heat treatment process is as follows: the temperature is increased to 915℃ at a rate of 95℃ / hour and held for 3.5 hours; then cooled in the furnace to 715℃ and held for 5.5 hours; finally, it is cooled in the furnace to below 300℃ and then air-cooled.

[0041] The ductile iron prepared in this embodiment has a tensile strength of 457 MPa and an elongation of 16.5%.

[0042] Example 6 A method for preparing ductile iron includes the following steps: S1. By weight, select 55 parts pig iron, 25 parts scrap steel and 15 parts recycled material, pack them together and melt them. After adjusting the chemical composition, heat them to 1470℃ in an electric furnace to melt them into molten iron. S2. Based on the quality of the molten iron, add 0.6% of spheroidizing agent A and 0.4% of spheroidizing agent B to the dam-type spheroidizing ladle and compact it. The spheroidizing agent A, by mass percentage, comprises: Mg: 43%; Si: 43%; Ca: 1.6%; Al: 0.9%; RE: 1.1%; with the balance being Fe. The spheroidizing agent B, by mass percentage, comprises: Ba: 10.5%; Si: 47%; Ca: 2.0%; Al: 1.1%; with the balance being Fe. Preheat the spheroidizing ladle to 750°C, and add perlite covering agent accounting for 0.3% of the total mass of the molten iron to the bottom of the ladle beforehand. Then, evenly cover the spheroidizing agent with 0.20% of the molten iron mass of ferrosilicon powder (silicon content 77%) and 0.4% of iron filings and compact it. S3. After the spheroidizing reaction for 100 seconds, inoculant A, with a mass of 0.6% of the molten iron, is added to the molten iron in the spheroidizing ladle for a first inoculation. The inoculant A comprises, by mass percentage: Si: 74%; Ba: 2.3%; Ca: 1.4%; Bi: 0.9%; Al: 0.9%; with the balance being Fe. After the first inoculation for 35 seconds, slag is removed. S4. During the casting process, inoculant B, at a mass of 0.15% of the molten iron mass, is added to the mold along with the molten iron flow for in-flow inoculation. The inoculant B, by mass percentage, comprises: Si: 77%; Sr: 1.4%; Zr: 0.7%; Al: 0.6%; with the balance being Fe. The casting temperature is 1345℃. When the casting cools to 790℃ in the mold, it is opened and immediately transferred to an annealing furnace for heat treatment. The heat treatment process is as follows: heating to 918℃ at a rate of 88℃ / hour and holding for 3.8 hours; then cooling in the furnace to 718℃ and holding for 5.2 hours; finally cooling in the furnace to below 300℃ and then air-cooling.

[0043] The ductile iron prepared in this embodiment has a tensile strength of 451 MPa and an elongation of 18.2%.

[0044] Example 7 A method for preparing ductile iron includes the following steps: S1. By mass, select 45 parts pig iron, 15 parts scrap steel and 20 parts recycled material, pack them together and melt them. After adjusting the chemical composition, heat them to 1435℃ in an electric furnace to melt them into molten iron. S2. Based on the quality of the molten iron, add 0.6% of spheroidizing agent A and 0.4% of spheroidizing agent B to the dam-type spheroidizing ladle and compact it. The spheroidizing agent A, by mass percentage, comprises: Mg: 47%; Si: 42.5%; Ca: 1.7%; Al: 0.6%; RE: 1.3%; with the balance being Fe. The spheroidizing agent B, by mass percentage, comprises: Ba: 9.5%; Si: 48.5%; Ca: 2.3%; Al: 1.0%; with the balance being Fe. Preheat the spheroidizing ladle to 650°C. Add cryolite powder covering agent accounting for 0.15% of the total mass of the molten iron to the bottom of the ladle beforehand. Then, evenly cover the spheroidizing agent with 0.20% of the molten iron mass of ferrosilicon powder (silicon content 74%) and 0.25% of iron filings and compact it. S3. After 70 seconds of spheroidizing reaction, inoculant A, at a mass of 0.6% of the molten iron, is added to the molten iron in the spheroidizing ladle for a first inoculation. Inoculant A comprises, by mass percentage: Si: 72.5%; Ba: 2.1%; Ca: 1.1%; Bi: 0.6%; Al: 1.2%; with the balance being Fe. After 55 seconds of the first inoculation, slag is removed. S4. During the casting process, inoculant B, with a mass of 0.15% of the molten iron mass, is added to the mold along with the molten iron flow for in-flow inoculation. The inoculant B, by mass percentage, comprises: Si: 73.5%; Sr: 0.9%; Zr: 0.45%; Al: 0.95%; with the balance being Fe. The casting temperature is 1325℃. When the casting cools to 755℃ in the mold, it is opened and immediately transferred to an annealing furnace for heat treatment. The heat treatment process is as follows: the temperature is increased to 902℃ at a rate of 82℃ / hour and held for 2.2 hours; then cooled in the furnace to 702℃ and held for 4.8 hours; finally, it is cooled in the furnace to below 300℃ and then air-cooled.

[0045] The ductile iron prepared in this embodiment has a tensile strength of 454 MPa and an elongation of 17.8%.

[0046] Example 8 A method for preparing ductile iron includes the following steps: S1. By weight, select 65 parts pig iron, 35 parts scrap steel, and 30 parts recycled material, pack them together, melt them, adjust the chemical composition, and then heat them to 1475℃ in an electric furnace to melt them into molten iron. S2. Based on the quality of the molten iron, add 0.6% of spheroidizing agent A and 0.4% of spheroidizing agent B to the dam-type spheroidizing ladle and compact it. The spheroidizing agent A, by mass percentage, comprises: Mg: 44%; Si: 44.5%; Ca: 1.9%; Al: 0.8%; RE: 0.9%; with the balance being Fe. The spheroidizing agent B, by mass percentage, comprises: Ba: 11.5%; Si: 46.5%; Ca: 2.4%; Al: 1.3%; with the balance being Fe. Preheat the spheroidizing ladle to 720°C. Add perlite covering agent at 0.25% of the total mass of the molten iron to the bottom of the ladle beforehand. Then, evenly cover the spheroidizing agent with 0.20% of the molten iron mass of ferrosilicon powder (silicon content 75.5%) and 0.55% of iron filings and compact them. S3. After the spheroidizing reaction for 130 seconds, inoculant A, with a mass of 0.6% of the molten iron, is added to the molten iron in the spheroidizing ladle for a first inoculation. The inoculant A, by mass percentage, comprises: Si: 74.5%; Ba: 2.4%; Ca: 1.35%; Bi: 1.1%; Al: 0.8%; with the balance being Fe. After the first inoculation for 42 seconds, slag is removed. S4. During the casting process, inoculant B, with a mass of 0.15% of the molten iron mass, is added to the mold along with the molten iron flow for in-flow inoculation. The inoculant B, by mass percentage, comprises: Si: 76.5%; Sr: 1.35%; Zr: 0.65%; Al: 0.75%; with the balance being Fe. The casting temperature is 1355℃. When the casting cools to 795℃ in the mold, it is opened and immediately transferred to an annealing furnace for heat treatment. The heat treatment process is as follows: the temperature is increased to 919℃ at a rate of 98℃ / hour and held for 3.2 hours; then cooled in the furnace to 717℃ and held for 5.8 hours; finally, it is cooled in the furnace to below 300℃ and then air-cooled.

[0047] The ductile iron prepared in this embodiment has a tensile strength of 456 MPa and an elongation of 16.8%.

[0048] The above embodiments demonstrate that, by precisely controlling the composition and process parameters, the present invention can stably produce EN-GJS-450-18 grade ductile iron with a tensile strength ≥450 MPa and an elongation ≥16%, which has good prospects for industrial application.

[0049] The embodiments described above are merely illustrative of several implementations of the present invention, and while the descriptions are relatively specific and detailed, they should not be construed as limiting the scope of the invention patent. It should be noted that those skilled in the art can make various modifications and improvements without departing from the concept of the present invention, and these all fall within the protection scope of the present invention. Therefore, the protection scope of this invention patent should be determined by the appended claims.

Claims

1. A type of ductile iron, characterized in that, The composition by mass percentage is as follows: C, 3.45-3.50%; Mg, 0.035-0.050%; Si, 1.50-1.55%; Mn, <0.17%; Ni, <0.15%; Cu, <0.12%; P, <0.03%; Cr, <0.02%; Sb, <0.01%; Sn, <0.01%; with the balance being Fe and unavoidable impurities. Among them, the Si content reaches 2.95~3.05% by mass percentage after adding spheroidizing agent A and spheroidizing agent B and completing one inoculation and in-flow inoculation, and ductile iron with grade EN-GJS-450-18 is obtained. The spheroidizing agent A comprises, by mass percentage: Mg: 40-50%; Si: 40-45%; Ca: 1.0-2.0%; Al: ≤1.0%; RE: 0.5–1.5%; with the balance being Fe; The spheroidizing agent B comprises, by mass percentage: Ba: 8-12%; Si: 45-50%; Ca: 1.0-2.5%; Al: ≤1.5%; with the balance being Fe.

2. A method for preparing ductile iron, characterized in that, Includes the following steps: S1. By mass, select 20-65 parts pig iron, 15-35 parts scrap steel, and 10-30 parts recycled material, pack them together, and melt them. After adjusting the chemical composition, heat the mixture to 1430-1480℃ using an electric furnace induction heating system to melt it into molten iron. S2. Based on the quality of the molten iron, add 0.6% of spheroidizing agent A and 0.4% of spheroidizing agent B to the spheroidizing ladle and compact it; then evenly cover the spheroidizing agents with 0.20% of the molten iron mass of ferrosilicon powder and 0.2~0.6% of iron filings and compact it; the ferrosilicon powder contains 72-78% silicon by mass. S3. After the spheroidizing reaction for 60-180 seconds, add inoculant A (0.6% by mass of molten iron) to the molten iron in the spheroidizing ladle for a first inoculation; after the first inoculation for 30-60 seconds, remove the slag. S4. During the casting process, inoculant B, which accounts for 0.15% of the mass of the molten iron, is added to the mold along with the molten iron for in-flow inoculation. After the in-flow inoculation is completed, the casting temperature of the molten iron is controlled at 1320-1360℃.

3. The preparation method according to claim 2, characterized in that, The spheroidizing agent A comprises, by mass percentage: Mg: 40-50%; Si: 40-45%; Ca: 1.0-2.0%; Al: ≤1.0%; RE: 0.5–1.5%; with the balance being Fe.

4. The preparation method according to claim 2, characterized in that, The spheroidizing agent B comprises, by mass percentage: Ba: 8-12%; Si: 45-50%; Ca: 1.0-2.5%; Al: ≤1.5%; with the balance being Fe.

5. The preparation method according to claim 2, characterized in that, The inoculant A comprises, by mass percentage: Si: 70-75%; Ba: 1.5-2.5%; Ca: 0.8-1.5%; Bi: 0.5-1.2%; Al: ≤1.2%; with the balance being Fe.

6. The preparation method according to claim 2, characterized in that, The inoculant B comprises, by mass percentage: Si: 72-78%; Sr: 0.8-1.5%; Zr: 0.3-0.8%; Al: ≤1.0%; with the balance being Fe.

7. The preparation method according to claim 2, characterized in that, In step S2, the spheroidizing bag is a dam-type spheroidizing bag, and the height of the dam is 1 / 3 to 1 / 2 of the height of the inner cavity of the spheroidizing bag.

8. The preparation method according to claim 2 or 7, characterized in that, In step S2, before pouring molten iron, the ball ladle needs to be preheated to 600-750°C, and a covering agent of 0.1-0.3% of the total mass of molten iron is added to the bottom of the ladle. The covering agent is perlite or cryolite powder.

9. The preparation method according to claim 2, characterized in that, In step S4, after the casting is completed, the casting is air-cooled in the mold to below 400°C before being removed from the furnace.

10. The preparation method according to claim 2 or 9, characterized in that, In step S4, after pouring, the casting is opened when it cools to 750-800°C in the mold and is immediately transferred to an annealing furnace for heat treatment. The heat treatment process is as follows: the temperature is raised to 900-920°C at a rate of 80-100°C / hour and held for 2-4 hours; then it is cooled to 700-720°C in the furnace and held for 4-6 hours; finally, it is cooled to below 300°C in the furnace and then air-cooled.

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