A medium silicon nodular iron formulation and a preparation process thereof

Abstract

This invention discloses a medium-silicon ductile iron formulation and preparation process, specifically relating to the field of ductile iron material technology. The chemical composition of the medium-silicon ductile iron includes: 3.05%–3.15% carbon, 4.80%–5.20% silicon, 0.40%–0.50% chromium, 0.035%–0.045% magnesium, 0.015%–0.025% yttrium, 0.005%–0.015% lanthanum, 0.003%–0.010% cerium, 0.005%–0.015% bismuth, 0.003%–0.010% zirconium, with the balance being iron and impurities. The preparation process includes furnace charge preparation, molten iron smelting and alloying, gradient composite spheroidizing treatment, multi-stage time-sharing inoculation treatment, casting control, cooling intervention, and three-stage stepped heat treatment. This invention utilizes high-silicon solid solution strengthening and the synergistic effect of multiple trace elements, combined with gradient spheroidization, ternary inoculation and subcritical slow cooling processes, to enable medium-silicon ductile iron to possess both high strength, high plasticity and excellent acid corrosion resistance, making it suitable for the manufacturing of corrosion-resistant components in chemical, environmental protection, and vacuum equipment industries.

Description

Technical Field

[0001] This invention relates to the field of ductile iron materials technology, and more specifically, to a medium-silicon ductile iron formulation and preparation process. Background Technology

[0002] Due to its good mechanical properties and low cost, ductile iron is widely used in machinery manufacturing, chemical equipment, vacuum pump housings and other fields. Under certain corrosive conditions, the material is required to have high strength, good plasticity and excellent acid corrosion resistance.

[0003] Increasing the silicon content can form a dense SiO2 protective film on the surface of ductile iron, thereby improving its oxidation and corrosion resistance. There are reports of silicon solid solution strengthened ferritic ductile iron in the prior art. For example, CN118703873A discloses a method for preparing high-strength and high-toughness ferritic ductile iron with a silicon content of 2.0% to 3.3%. The silicon solid solution strengthening improves the strength of the material, but does not involve the improvement of corrosion resistance.

[0004] In addition, although conventional high-silicon ductile iron has good heat resistance, it suffers from severe spheroidization decay, low graphite spheroid count, and incomplete pearlite decomposition, resulting in an elongation rate that is usually less than 3%, making it difficult to meet the application scenarios that require both high plasticity and corrosion resistance.

[0005] To improve corrosion resistance, existing technologies also include acid-resistant ductile iron with added precious metal elements such as nickel and molybdenum. For example, CN1840722A discloses an alloy ductile iron material resistant to sulfuric acid corrosion. However, its raw material cost is high, and the matching of mechanical properties and plasticity is still not ideal.

[0006] Regarding inoculation treatment, CN115679188A discloses a ductile iron with high elongation and tensile strength and its preparation method, which uses a composite inoculator for inoculation treatment, but does not solve the key problems of spheroidization decay and carbon diffusion obstruction under high silicon content.

[0007] Therefore, how to achieve high sphericity, high ferrite content, high elongation and excellent acid corrosion resistance at a high silicon content is a technical problem that urgently needs to be solved in this field. Summary of the Invention

[0008] In order to overcome the above-mentioned defects of the prior art, the embodiments of the present invention provide a medium-silicon ductile iron formulation and preparation process to solve the problems mentioned in the background art.

[0009] To achieve the above objectives, the present invention provides the following technical solution: On one hand, the present invention provides a medium-silicon ductile iron formulation, wherein the chemical composition of the medium-silicon ductile iron, by mass percentage, comprises: carbon 3.05%–3.15%, silicon 4.80%–5.20%, manganese ≤0.25%, phosphorus ≤0.045%, sulfur ≤0.013%, chromium 0.40%–0.50%, magnesium 0.035%–0.045%, yttrium 0.015%–0.025%, lanthanum 0.005%–0.015%, cerium 0.003%–0.010%, bismuth 0.005%–0.015%, and zirconium 0.003%–0.010%. The balance consists of iron and unavoidable impurities; The impurity elements are controlled as follows: titanium ≤ 0.015%, lead ≤ 0.001%, and antimony + arsenic ≤ 0.005%.

[0010] Preferably, the metallographic structure of the medium silicon ductile iron satisfies the following conditions: spheroidization rate ≥ 90%, graphite spheroid number ≥ 140 / mm², ferrite volume fraction ≥ 70%, and pearlite volume fraction ≤ 30%.

[0011] On the other hand, the present invention also provides a preparation process for preparing the above-mentioned medium-silicon ductile iron, comprising the following steps: S1. Prepare furnace charge, control the amount of pig iron added to ≤25%, and detect and control the content of anti-spheroidizing elements within the specified range; S2. Iron smelting and alloying: 0.15% silicon carbide is added during the middle stage of melting, and ferrochrome alloy is added according to the calculated amount to make the chromium content reach 0.40% to 0.50%. After heating to 1520±10℃, the power is turned off and the mixture is allowed to stand for 3 minutes. After standing, 0.05% to 0.10% of nano-silicon carbide particles with a particle size ≤100 nm are added. S3. Gradient composite spheroidizing treatment: The spheroidizing agent is filled into the spheroidizing pit in a gradient manner, with light rare earth elements at the bottom and heavy rare earth elements at the top. The spheroidizing agent is 1.0% yttrium-based heavy rare earth spheroidizing agent and 0.6% rare earth silicon-magnesium spheroidizing agent. The yttrium-based heavy rare earth spheroidizing agent contains yttrium, lanthanum, and cerium. The amount added is such that the final casting contains 0.015% to 0.025% yttrium, 0.005% to 0.015% lanthanum, and 0.003% to 0.010% cerium. S4. Multi-stage time-sharing gestation treatment, including: First inoculation: During the pouring process after spheroidization, 0.6% of barium silicon inoculant, 0.3% of zirconium silicon inoculant, and 0.2% of bismuth silicon inoculant are added to the bottom of the ladle. The amount of zirconium silicon inoculant added is such that the zirconium content in the final casting is 0.003% to 0.010%, and the amount of bismuth silicon inoculant added is such that the bismuth content in the final casting is 0.005% to 0.015%. Second inoculation: 0.1% silicon-bismuth inoculant is added during the casting process; Third incubation: For thick castings, spray a mixture of 0.05% nano-silica sol and 200-mesh ferrosilicon powder into the mold cavity; The interval between each pregnancy should be ≤2 minutes; S5. Casting control: the casting temperature is 1380~1390℃, and the total time from the start of the spheroidizing reaction to the completion of casting is ≤12min; S6. Cooling intervention: The casting is unpacked at 800±20℃. Thick and large parts are sprayed with water mist for 10 seconds and then covered with hot sand for slow cooling. The cooling rate is controlled to be ≤30℃ / h in the 700~500℃ range, and subcritical slow cooling is carried out at a rate of 15~25℃ / h in the 500~350℃ range. S7, three-stage stepped controlled cooling heat treatment, including: Stage A: Increase the temperature to 920±10℃ at a rate of ≤100℃ / h and hold for 2 hours; Phase B: Cooling from 920℃ to 750℃ at a rate of 30±5℃ / h; Stage C: Cooling from 750℃ to 720℃ at a rate of 7.5±1℃ / h; Stage D: Cooling from 720℃ to 350℃ at a rate of 40±5℃ / h; Stage E: Remove from the furnace at 350℃ and air cool to room temperature.

[0012] Preferably, the amount of nano-silicon carbide particles added in step S2 is 0.08%.

[0013] Preferably, in step S3, the yttrium content in the yttrium-based heavy rare earth spheroidizing agent is ≥50%, and the magnesium content in the rare earth silicon magnesium spheroidizing agent is 5%–8% and the rare earth content is 2%–5%.

[0014] Preferably, in step S4, the barium content in the barium silicon inoculant is 4%–6%, the zirconium content in the zirconium silicon inoculant is 3%–5%, and the bismuth content in the bismuth silicon inoculant is 2%–4%, with each inoculant having a particle size of 10–25 mm, and the particle size of the bismuth silicon inoculant in the second inoculation is 0.2–0.8 mm.

[0015] Preferably, the subcritical slow cooling rate in the 500-350°C range in step S6 is 20°C / h.

[0016] Preferably, the cooling rate of stage C in step S7 is 7.5℃ / h, and the cooling rate of stage D is 40℃ / h.

[0017] An application of the above-described medium-silicon ductile iron formulation in the preparation of corrosion-resistant fluid transport equipment components.

[0018] The technical effects and advantages of this invention are as follows: 1. This invention achieves high tensile strength and significantly improves elongation after fracture while maintaining a suitable hardness range through the synergistic effect of high silicon solid solution strengthening, gradient composite spheroidization, ternary time-sequential inoculation, and three-stage stepped controlled cooling heat treatment. The ferrite volume fraction and spheroidization rate in the material reach a high level, and the number of graphite spheroids is sufficient, thus achieving a balance between high strength, high plasticity, and good toughness, overcoming the defects of low elongation and high brittleness of traditional high silicon ductile iron. 2. This invention adds an appropriate amount of chromium to medium-silicon ductile iron, which synergistically forms a dense composite oxide film with high silicon content, significantly enhancing the material's corrosion resistance in acidic media. The corrosion rate in sulfuric acid solution is much lower than that of conventional ductile iron, and is significantly reduced compared to the comparative scheme without chromium and without subcritical slow cooling. It can meet the long-term use requirements under strong acid conditions and is suitable for chemical, environmental protection and other fields with high requirements for corrosion resistance. 3. This invention effectively suppresses the problems of spheroidization decay and carbon diffusion obstruction caused by high silicon by controlling the amount of pig iron added, precisely regulating the content of anti-spheroidizing elements, and adopting gradient spheroidization and multi-stage inoculation treatment. The preparation process parameters are clear, the operating window is wide, and the raw material sources are extensive. It has good industrial applicability and repeatability, and is suitable for promotion and application in the field of corrosion-resistant fluid transportation equipment component manufacturing. Attached Figure Description

[0019] Figure 1 This is a flowchart illustrating the overall steps of the present invention.

[0020] Figure 2 The image shows the metallographic structure analysis results of silicon ductile iron in Example 1 of this invention. Detailed Implementation

[0021] The technical solutions of the present invention will be clearly and completely described below with reference to the accompanying drawings in the embodiments of the present invention.

[0022] Example 1 This embodiment provides a medium-silicon ductile iron formulation, the chemical composition of which, by mass percentage, is as follows: Carbon 3.072%, Silicon 5.014%, Manganese 0.247%, Phosphorus 0.0345%, Sulfur 0.011%, Chromium 0.452%, Magnesium 0.039%, Yttrium 0.018%, Lanthanum 0.009%, Cerium 0.006%, Bismuth 0.008%, Zirconium 0.005%; The balance consists of iron and unavoidable impurities; Impurity elements are controlled as follows: titanium ≤ 0.015%, lead ≤ 0.001%, antimony + arsenic ≤ 0.005%.

[0023] The preparation process in this embodiment includes the following steps: S1. Prepare the furnace charge, control the amount of pig iron added to ≤25%, and test the content of anti-spheroidizing elements in the furnace charge, such as titanium, lead, antimony, and arsenic, to ensure that they are within the specified range. S2. Iron smelting and alloying: 0.15% silicon carbide is added during the middle of the melting process, and ferrochrome alloy is added according to the calculated amount to make the final chromium content reach 0.452%. After heating to 1520±10℃, the power is turned off and the mixture is allowed to stand for 3 minutes. After standing, nano-silicon carbide particles with a particle size ≤100 nanometers are added at a rate of 0.08%. S3. Gradient composite spheroidizing treatment: The spheroidizing agent is filled into the spheroidizing pit in a gradient manner, with light rare earth elements at the bottom and heavy rare earth elements at the top. The spheroidizing agent consists of 1.0% yttrium-based heavy rare earth spheroidizing agent and 0.6% rare earth silicon-magnesium spheroidizing agent. The yttrium-based heavy rare earth spheroidizing agent contains yttrium, lanthanum, and cerium. The amount added ensures that the final casting contains 0.018% yttrium, 0.009% lanthanum, and 0.006% cerium. Among them, the yttrium content in the yttrium-based heavy rare earth spheroidizing agent is ≥50%, and the magnesium content in the rare earth silicon-magnesium spheroidizing agent is 5%–8% and the rare earth content is 2%–5%. S4. Multi-stage time-sharing inoculation treatment: First inoculation: After spheroidization, during the pouring process, add 0.6% barium silicon inoculant, 0.3% zirconium silicon inoculant, and 0.2% bismuth silicon inoculant to the bottom of the ladle. The barium content in the barium silicon inoculant is 4%–6%, the zirconium content in the zirconium silicon inoculant is 3%–5%, and the bismuth content in the bismuth silicon inoculant is 2%–4%. The particle size of each inoculant is 10–25 mm. The amount of zirconium silicon inoculant added ensures that the zirconium content in the final casting is 0.005%, and the amount of bismuth silicon inoculant added ensures that the bismuth content in the final casting is 0.008%. Second inoculation: During the pouring process, add 0.1% bismuth silicon inoculant with a particle size of 0.2–0.8 mm. Third inoculation: For thick castings, spray a mixture of 0.05% nano-silica sol and 200-mesh ferrosilicon powder into the mold cavity. The interval between each inoculation is ≤2 min. S5. Casting control: the casting temperature is 1380~1390℃, and the total time from the start of the spheroidizing reaction to the completion of casting is ≤12min; S6. Cooling intervention: The casting is unpacked at 800±20℃. Thick and large parts are sprayed with water mist for 10 seconds and then covered with hot sand for slow cooling. The cooling rate is controlled to be ≤30℃ / h in the 700~500℃ range, and subcritical slow cooling is carried out at a rate of 20℃ / h in the 500~350℃ range. S7. Three-stage stepped controlled cooling heat treatment: Stage A: Heat to 920±10℃ at a rate of ≤100℃ / h and hold for 2h; Stage B: Cool from 920℃ to 750℃ at a rate of 30±5℃ / h; Stage C: Cool from 750℃ to 720℃ at a rate of 7.5℃ / h; Stage D: Cool from 720℃ to 350℃ at a rate of 40℃ / h; Stage E: Remove from the furnace at 350℃ and air cool to room temperature.

[0024] Example 2 The difference between this embodiment and Embodiment 1 lies in the chemical composition and some process parameters: In terms of chemical composition: carbon 3.10%, silicon 4.95%, manganese 0.24%, phosphorus 0.035%, sulfur 0.010%, chromium 0.41%, magnesium 0.038%, yttrium 0.016%, lanthanum 0.008%, cerium 0.005%, bismuth 0.007%, zirconium 0.004%, with the balance being iron and unavoidable impurities, and impurity control being the same as in Example 1.

[0025] In terms of preparation process: the amount of nano-silicon carbide particles added is 0.06%, the subcritical slow cooling rate is 18℃ / h, the cooling rate of stage C is 7.0℃ / h, and the cooling rate of stage D is 38℃ / h.

[0026] The remaining steps and parameters are the same as in Example 1.

[0027] Example 3 The difference between this embodiment and Embodiment 1 lies in the chemical composition and some process parameters: In terms of chemical composition: carbon 3.06%, silicon 5.18%, manganese 0.25%, phosphorus 0.033%, sulfur 0.012%, chromium 0.49%, magnesium 0.041%, yttrium 0.022%, lanthanum 0.012%, cerium 0.008%, bismuth 0.011%, zirconium 0.007%, with the balance being iron and unavoidable impurities, and impurity control being the same as in Example 1; In terms of preparation process: the amount of nano-silicon carbide particles added is 0.10%, the subcritical slow cooling rate is 22℃ / h, the cooling rate of stage C is 8.0℃ / h, and the cooling rate of stage D is 42℃ / h.

[0028] The remaining steps and parameters are the same as in Example 1.

[0029] Comparative Example 1 It adopts conventional ductile iron formula and process, that is, it does not add elements such as chromium, yttrium, lanthanum, cerium, bismuth, zirconium, etc., and does not perform gradient composite spheroidization, ternary time-sharing inoculation, subcritical slow cooling and three-stage step heat treatment.

[0030] Its typical chemical composition by mass fraction is: carbon 3.6%, silicon 2.5%, manganese 0.3%, phosphorus 0.05%, sulfur 0.02%, magnesium 0.04%, with the balance being iron.

[0031] The preparation process consists of conventional spheroidization by injection, single inoculation, natural cooling, and ordinary normalizing treatment.

[0032] Comparative Example 2 Based on Example 1, the addition of chromium is omitted, and the subcritical slow cooling step is also cancelled. That is, in S6, only the cooling rate of 700-500°C is controlled, and the 500-350°C range of controlled cooling is not performed. The rest of the formula and process are the same as in Example 1.

[0033] The chemical composition of Comparative Example 2 does not contain chromium, the chromium content is ≤0.01%, and the remaining elements are similar to those of Example 1.

[0034] The mechanical properties, metallographic structure, and corrosion resistance of Examples 1-3 and Comparative Examples 1-2 were tested below, and the results are shown in the table below: Note: The corrosion rate test conditions in the table are: 10% sulfuric acid solution, immersion at room temperature for 24 hours.

[0035] The test data in the table above shows that: 1. Examples 1-3 all exhibit excellent comprehensive performance, with tensile strength reaching 705-728 MPa, elongation after fracture reaching 5.2%-5.4%, hardness reaching 188-196 HB, spheroidization rate ≥91%, ferrite volume fraction ≥78%, and corrosion rate of 0.052-0.062 mm / year. This indicates that within the composition range and process parameters defined by this invention, a good match of high strength, high plasticity, and high corrosion resistance can be stably achieved. 2. Compared with conventional ductile iron in Comparative Example 1, the tensile strength of Examples 1-3 is increased by about 22% to 25%, the elongation after fracture is increased by about 73% to 80%, and the corrosion rate is reduced by about 86% to 88%. This fully demonstrates that the present invention significantly improves the mechanical properties and corrosion resistance of the material through the synergistic effect of high silicon solid solution strengthening, composite rare earth spheroidization, ternary time-sequential inoculation, and three-stage step heat treatment. 3. Compared with Comparative Example 2, which lacked chromium and had no subcritical slow cooling, the tensile strength of Example 1 increased by about 16%, the elongation after fracture increased by about 54%, and the corrosion rate decreased by about 80%. This indicates that chromium and high silicon form a dense composite oxide film, which significantly enhances the acid corrosion resistance. At the same time, subcritical slow cooling promotes the full decomposition of residual carbides, increases the ferrite content and plasticity. The two work synergistically and are indispensable. In summary, the medium-silicon ductile iron formulation and preparation process provided by this invention achieves a corrosion rate ≤0.062 mm / year in 10% sulfuric acid solution, tensile strength ≥705 MPa, elongation after fracture ≥5.2%, and ferrite volume fraction ≥78%, realizing a good match of high strength, high plasticity, and high corrosion resistance, and has outstanding substantive characteristics and significant progress.

[0036] The medium-silicon ductile iron formulation and preparation process provided by this invention have a wide range of raw material sources, clear process parameters, and good repeatability. It has been verified by actual production and is suitable for promotion and application in the foundry industry, especially in fields with high requirements for corrosion resistance such as chemical, environmental protection, and vacuum equipment.

[0037] The above description is merely a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A medium-silicon ductile iron formulation, characterized in that: The chemical composition of the silicon ductile iron, by mass percentage, includes: carbon 3.05%–3.15%, silicon 4.80%–5.20%, manganese ≤0.25%, phosphorus ≤0.045%, sulfur ≤0.013%, chromium 0.40%–0.50%, magnesium 0.035%–0.045%, yttrium 0.015%–0.025%, lanthanum 0.005%–0.015%, cerium 0.003%–0.010%, bismuth 0.005%–0.015%, and zirconium 0.003%–0.010%. The balance consists of iron and unavoidable impurities; The impurity elements are controlled as follows: titanium ≤ 0.015%, lead ≤ 0.001%, and antimony + arsenic ≤ 0.005%.

2. The medium-silicon ductile iron formulation according to claim 1, characterized in that: The metallographic structure of the medium silicon ductile iron meets the following requirements: spheroidization rate ≥ 90%, graphite spheroids number ≥ 140 / mm², ferrite volume fraction ≥ 70%, and pearlite volume fraction ≤ 30%.

3. A preparation process for preparing the medium-silicon ductile iron according to any one of claims 1-2, characterized in that: Includes the following steps: S1. Prepare furnace charge, control the amount of pig iron added to ≤25%, and detect and control the content of anti-spheroidizing elements within the specified range; S2. Iron smelting and alloying: 0.15% silicon carbide is added during the middle of the melting process, and ferrochrome alloy is added according to the calculated amount to make the chromium content reach 0.40% to 0.50%. After heating to 1520±10℃, the power is turned off and the mixture is allowed to stand for 3 minutes. After standing, 0.05% to 0.10% of nano-silicon carbide particles with a particle size ≤100nm are added. S3. Gradient composite spheroidizing treatment: The spheroidizing agent is filled into the spheroidizing pit in a gradient manner, with light rare earth elements at the bottom and heavy rare earth elements at the top. The spheroidizing agent is 1.0% yttrium-based heavy rare earth spheroidizing agent and 0.6% rare earth silicon-magnesium spheroidizing agent. The yttrium-based heavy rare earth spheroidizing agent contains yttrium, lanthanum, and cerium. The amount added is such that the final casting contains 0.015% to 0.025% yttrium, 0.005% to 0.015% lanthanum, and 0.003% to 0.010% cerium. S4. Multi-stage time-sharing gestation treatment, including: First inoculation: During the pouring process after spheroidization, 0.6% of barium silicon inoculant, 0.3% of zirconium silicon inoculant, and 0.2% of bismuth silicon inoculant are added to the bottom of the ladle. The amount of zirconium silicon inoculant added is such that the zirconium content in the final casting is 0.003% to 0.010%, and the amount of bismuth silicon inoculant added is such that the bismuth content in the final casting is 0.005% to 0.015%. Second inoculation: 0.1% silicon-bismuth inoculant is added during the casting process; Third incubation: For thick castings, spray a mixture of 0.05% nano-silica sol and 200-mesh ferrosilicon powder into the mold cavity; The interval between each pregnancy should be ≤2 minutes; S5. Casting control: the casting temperature is 1380~1390℃, and the total time from the start of the spheroidizing reaction to the completion of casting is ≤12min; S6. Cooling intervention: The casting is unpacked at 800±20℃. Thick and large parts are sprayed with water mist for 10 seconds and then covered with hot sand for slow cooling. The cooling rate is controlled to be ≤30℃ / h in the 700~500℃ range, and subcritical slow cooling is carried out at a rate of 15~25℃ / h in the 500~350℃ range. S7, three-stage stepped controlled cooling heat treatment, including: Stage A: Increase the temperature to 920±10℃ at a rate of ≤100℃ / h and hold for 2 hours; Phase B: Cooling from 920℃ to 750℃ at a rate of 30±5℃ / h; Stage C: Cooling from 750℃ to 720℃ at a rate of 7.5±1℃ / h; Stage D: Cooling from 720℃ to 350℃ at a rate of 40±5℃ / h; Stage E: Remove from the furnace at 350℃ and air cool to room temperature.

4. The preparation process according to claim 3, characterized in that: In step S2, the amount of nano-silicon carbide particles added is 0.08%.

5. The preparation process according to claim 3, characterized in that: In step S3, the yttrium content in the yttrium-based heavy rare earth spheroidizing agent is ≥50%, and the magnesium content in the rare earth silicon magnesium spheroidizing agent is 5%–8% and the rare earth content is 2%–5%.

6. The preparation process according to claim 3, characterized in that: In step S4, the barium content in the barium silicon inoculant is 4%–6%, the zirconium content in the zirconium silicon inoculant is 3%–5%, and the bismuth content in the bismuth silicon inoculant is 2%–4%. The particle size of each inoculant is 10–25 mm. In the second inoculation, the particle size of the bismuth silicon inoculant is 0.2–0.8 mm.

7. The preparation process according to claim 3, characterized in that: In step S6, the subcritical slow cooling rate in the 500-350℃ range is 20℃ / h.

8. The preparation process according to claim 3, characterized in that: In step S7, the cooling rate of stage C is 7.5℃ / h, and the cooling rate of stage D is 40℃ / h.

9. The application of the medium-silicon ductile iron formulation according to any one of claims 1-2 in the preparation of corrosion-resistant fluid transport equipment components.

Images