Low-temperature impact resistant austempered ductile cast iron and method for producing the same

Abstract

The application relates to the technical field of metal casting, in particular to a low-temperature impact-resistant isothermal quenching nodular cast iron and a preparation method thereof. The low-temperature impact-resistant isothermal quenching nodular cast iron comprises the following components in percentage by weight: C: 3.5-3.8%, Si: 2.3-2.5%, Cu: 0.5-1.0%, Ni: 0.5-1.5%, Mn: <=0.3%, P: <=0.04%, S: <=0.02%, Mg: 0.03-0.06%, RE: 0.01-0.03%, and the balance of Fe; wherein RE is a rare earth element. The application utilizes the synergistic complementary effect of silicon and nickel and combines with an accurately controlled isothermal quenching heat treatment process to obtain a complex phase structure composed of upper bainite and carbon-rich austenite, so that the material has high strength and excellent low-temperature impact resistance, solves the problems of insufficient strength of the existing low-temperature nodular cast iron, difficulty in bearing high stress and alternating load, simultaneously realizes lightweight design of the structure, and meets the harsh requirements of high-cold regions on the comprehensive mechanical properties of dynamic structural parts.

Description

Technical Field

[0001] This invention relates to the field of metal casting technology, and in particular to a low-temperature impact-resistant isothermal quenched ductile iron and its preparation method. Background Technology

[0002] Isothermally hardened ductile iron (ADI) is widely used in machinery manufacturing, transportation, and other fields due to its high strength, high wear resistance, and good toughness. However, critical structural components used in cold or frigid regions often require excellent low-temperature impact resistance while withstanding high stresses.

[0003] With the rapid development of equipment manufacturing in high-altitude and cold regions and the increasing demands for lightweight product design, the strength properties of traditional low-temperature ductile iron can no longer meet the requirements of higher strength, wear resistance, and dynamic structural components subjected to alternating loads. Specifically, to ensure low-temperature impact toughness, existing low-temperature ductile iron typically controls the silicon (Si) content at a low level (e.g., 1.8%~2.3%) to avoid increased cold brittleness caused by silicon. However, reducing silicon content weakens its role in promoting graphitization, inhibiting carbide precipitation, and improving graphite morphology, thus limiting further improvements in material strength and toughness. Furthermore, existing processes impose extremely stringent controls on casting composition and heat treatment, resulting in high production costs.

[0004] Existing Chinese patent CN 108754302 B discloses a method for preparing carbide-containing isothermal quenched ductile iron, specifically a heat treatment method to improve toughness and wear resistance, belonging to the field of wear-resistant materials technology. This patent produces ductile iron with the following chemical composition (wt%): C: 3.3-3.8, Si: 1.3-1.7, Mn: 0.4-0.7, Cr: 1.0-1.5, P≤0.040; S≤0.007, balance Fe. The isothermal quenched ductile iron exhibits an impact toughness improvement of over 120% without reducing hardness and wear resistance, but its impact toughness at low temperatures is insufficient.

[0005] Therefore, developing an isothermal quenched ductile iron with high strength, good elongation, and resistance to low-temperature impact, and its preparation method, is of great engineering significance and practical value for meeting the application requirements of dynamic structural components under harsh working conditions such as high stress, wear, or alternating loads, and for achieving product lightweighting, cost reduction, and efficiency improvement. Summary of the Invention

[0006] In view of this, the present invention proposes a low-temperature impact-resistant isothermal quenched ductile iron and its preparation method, in order to solve the problem that the strength and low-temperature toughness of traditional low-temperature ductile iron in the prior art are difficult to balance, and cannot meet the requirements of dynamic structural parts under high stress and alternating loads. The low-temperature impact-resistant isothermal quenched ductile iron has good comprehensive mechanical properties and low-temperature impact resistance, can adapt to high-stress working conditions in cold regions, and significantly improves the lightweight effect and service life of products.

[0007] The technical solution of this invention is achieved as follows: This invention provides a low-temperature impact-resistant isothermal quenched ductile iron, which, by weight percentage, comprises the following components: C: 3.5~3.8%, Si: 2.3~2.5%, Cu: 0.5~1.0%, Ni: 0.5~1.5%, Mn: ≤0.3%, P: ≤0.04%, S: ≤0.02%, Mg: 0.03~0.06%, RE: 0.01~0.03%, balance Fe; wherein RE is rare earth element.

[0008] Silicon is a key element in promoting graphitization and inhibiting carbide precipitation, while also improving graphite morphology (making graphite spheres rounder and smaller, increasing the number of graphite spheres). However, silicon exhibits cold brittleness, which raises the ductile-brittle transition temperature of materials.

[0009] Nickel is a key element for improving low-temperature toughness, lowering the ductile-brittle transition temperature of materials and improving low-temperature impact resistance. Simultaneously, nickel also enhances hardenability. This invention utilizes the complementary properties of nickel and silicon in cold brittleness, allowing for an increase in silicon content without significantly compromising low-temperature toughness, thus achieving a synergistic improvement in both high strength and low-temperature impact resistance.

[0010] Copper can improve the hardenability of materials, stabilize austenite, promote the uniformity of bainite transformation during isothermal quenching, and help obtain an appropriate amount of carbon-rich austenite.

[0011] Based on the above technical solutions, preferably, the microstructure of the low-temperature impact-resistant isothermal quenched ductile iron is upper bainite + carbon-rich austenite, wherein the volume fraction of the carbon-rich austenite is 10%~20%.

[0012] Upper bainite consists of ferrite laths and carbon-rich austenite distributed between the laths, exhibiting both high strength and good plasticity. When the material is subjected to external forces, the carbon-rich austenite (retained austenite) undergoes transformation-induced plasticity (TRIP effect), absorbing impact energy and significantly improving low-temperature toughness. If the volume fraction of carbon-rich austenite is too low, the toughening effect is not significant; conversely, if it is too high, the strength will decrease.

[0013] Based on the above technical solutions, preferably, the low-temperature impact-resistant isothermal quenched ductile iron has a tensile strength ≥900 MPa, an elongation ≥8%, and an impact energy ≥6 J at -30℃.

[0014] This invention also provides a method for preparing low-temperature impact-resistant isothermal hardened ductile iron, the method comprising: S1. Using pig iron, scrap steel and recycled materials as raw materials according to mass percentage, molten iron is prepared by melting in a medium frequency induction furnace; S2. Heat the molten iron to 1480~1500℃ and perform spheroidizing and inoculation treatments to obtain ductile iron raw molten iron. S3. The molten ductile iron is poured into a casting at 1350~1390℃; S4. The casting is heated to 890~910℃ for austenitization and heat preservation, and then placed in a nitrate bath at 365~380℃ for isothermal quenching for 60~120 minutes. After that, it is taken out and air-cooled to obtain low-temperature impact resistant isothermal quenched ductile iron.

[0015] If the spheroidizing temperature is too low, the spheroidizing agent will not melt completely, resulting in poor spheroidizing effect; if the spheroidizing temperature is too high, magnesium will be severely burned off, and inclusions are likely to occur. This invention controls the temperature within this range, which can ensure the spheroidizing effect while reducing magnesium burn-off. If the casting temperature is too high, defects such as shrinkage cavities and sand adhesion are likely to occur; if the casting temperature is too low, the fluidity will be poor and the filling will be incomplete.

[0016] An austenitizing temperature range of 890~910℃ ensures that the casting matrix is ​​completely transformed into austenite while avoiding coarse grains. If the temperature is too low, austenitization will be insufficient; if the temperature is too high, grains will grow, reducing mechanical properties.

[0017] The isothermal quenching temperature range of 365~380℃ falls within the upper bainite transformation region. The choice of isothermal temperature directly affects the final microstructure: a lower temperature easily forms lower bainite, resulting in high strength but insufficient toughness; a higher temperature results in coarse bainite and decreased strength. If the isothermal time is too short, the bainite transformation is incomplete, leading to excessive retained austenite and insufficient strength; if the time is too long, production efficiency decreases, and the carbon-rich austenite may decompose. After isothermal quenching, air cooling can avoid rapid cooling that could lead to secondary phase transformations (such as martensitic transformation), thus stably preserving the desired bainite + carbon-rich austenite microstructure.

[0018] Based on the above technical solutions, preferably, in step S1, the pig iron is high-purity pig iron, and the total amount of interfering elements Ti, Cr, and V in the high-purity pig iron is ≤0.1wt%; the scrap steel does not include alloy steel; the remelting material is ductile iron scrap, and the remelting material content in the raw materials is 10~30wt%.

[0019] High-purity pig iron is selected, and interfering elements (Ti, Cr, V, etc.) are strictly controlled because these elements consume magnesium, affect spheroidization, promote carbide formation, and reduce low-temperature toughness. Alloy steel is not used for scrap steel to avoid introducing unnecessary alloying elements and ensure precise control over the composition. Scrap ductile iron is used for remelting, which reduces production costs and achieves resource recycling.

[0020] Based on the above technical solutions, preferably, in step S2, the spheroidizing treatment adopts the injection method or the wire feeding method, and the spheroidizing agent adopts a low rare earth magnesium alloy, with an addition amount of 1% to 1.6% of the weight of the molten iron.

[0021] Before spheroidizing, molten iron should undergo desulfurization treatment using the wire feeding method. Currently, spheroidizing and desulfurization are carried out simultaneously.

[0022] Based on the above technical solutions, preferably, in step S2, the inoculation treatment adopts composite inoculation, including in-flow inoculation and instantaneous inoculation, the inoculant is silicon barium calcium inoculator, and the addition amount is 0.3%~0.8% of the weight of the molten iron. During the inoculation treatment, the number of as-cast graphite spheres is ≥150 / mm. 2 .

[0023] A barium-silicon-calcium inoculant was used, added in two stages. The in-flow inoculation stage was added after spheroidization treatment to promote graphite nucleation; the instantaneous inoculation stage was added during casting to prevent inoculation degradation. Through this composite inoculation, the number of as-cast graphite spheroids was ≥150 per mm². 2 This provides a uniform and fine graphite distribution for subsequent isothermal quenching, which is beneficial for improving mechanical properties.

[0024] Based on the above technical solutions, preferably, in step S3, a resin sand casting mold is used to complete rapid casting within 12 minutes.

[0025] After spheroidizing and inoculation treatments, the inoculation effect gradually diminishes over time, leading to a decrease in the number of graphite spheroids. Therefore, pouring must be completed within 12 minutes to maximize the preservation of the inoculation effect and ensure a uniform as-cast microstructure. Resin sand has good permeability, excellent collapsibility, and uniform cooling, which is beneficial for obtaining a uniform as-cast microstructure and avoids microstructure differences caused by uneven cooling.

[0026] Based on the above technical solutions, preferably, in step S4, the austenitizing holding time is calculated as 1.5~2.0 min / mm based on the casting wall thickness.

[0027] Calculating the holding time based on the casting wall thickness ensures that the core is also fully austenitized, avoiding uneven microstructure due to differences in wall thickness.

[0028] Based on the above technical solutions, preferably, in step S4, the isothermal quenching nitrate bath is a mixed salt bath of KNO3 and NaNO2, and the mass ratio of KNO3 to NaNO2 is 0.9~1.35:1.

[0029] Nitrate baths are characterized by low melting point, good fluidity, and uniform and stable temperature, making them suitable for isothermal quenching. Mixed salt baths within this ratio range have moderate melting points and a wide operating temperature window, ensuring uniform temperature of castings during the isothermal process and preventing localized overheating or undercooling.

[0030] The low-temperature impact-resistant isothermal quenched ductile iron and its preparation method of the present invention have the following advantages over the prior art: (1) By optimizing the chemical composition design, the present invention controls the silicon (Si) content within the middle range of 2.3% to 2.5%, utilizes silicon to promote graphitization and inhibit carbide precipitation, and adds 0.5% to 1.5% nickel (Ni) to improve hardenability and low-temperature toughness, which complements the cold brittleness of silicon. This breaks through the technical bias of traditional low-temperature ductile iron that reduces silicon content in pursuit of toughness, so that the material can obtain high strength while maintaining excellent low-temperature impact resistance.

[0031] (2) The present invention adopts a precisely controlled isothermal quenching heat treatment process to obtain a multiphase structure of upper bainite + carbon-rich austenite with a volume fraction of 10%~20%. This structure has both high strength and good plasticity, which solves the problem that the existing low-temperature ductile iron has insufficient strength and cannot withstand high stress and alternating loads, and significantly expands the application scope of ductile iron in the field of dynamic structural parts in cold regions such as engineering machinery and rail transit.

[0032] (3) The isothermal quenched ductile iron prepared by this invention has a tensile strength ≥900MPa, an elongation ≥8%, and an impact energy ≥6J at -30℃. Compared with traditional low-temperature ductile iron, it has a higher specific strength, which can realize the lightweight design of the structure and reduce the wall thickness and material usage of the components. At the same time, 10%~30% of recycled materials are used reasonably in the preparation process, resulting in high resource utilization and stable and controllable process, which meets the stringent requirements of high-end equipment for comprehensive mechanical properties and lightweighting. Attached Figure Description

[0033] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0034] Figure 1This is a schematic diagram of the main process flow of the preparation method of low-temperature impact-resistant isothermal quenched ductile iron according to the present invention. Figure 2 The metallographic structure of the low-temperature impact-resistant isothermal quenched ductile iron of the present invention is shown in the figure. Detailed Implementation

[0035] The technical solutions of the present invention will be clearly and completely described below with reference to the embodiments of the present invention. Obviously, the described embodiments are only a part of the embodiments of the present invention, and not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative effort are within the scope of protection of the present invention.

[0036] Example 1 This embodiment provides a low-temperature impact-resistant isothermal quenched ductile iron, which comprises the following components by weight percentage: C: 3.65%, Si: 2.4%, Cu: 0.75%, Ni: 1.0%, Mn: 0.25%, P: 0.035%, S: 0.015%, Mg: 0.045%, RE: 0.02%, balance Fe; wherein RE is rare earth element.

[0037] The preparation method of low-temperature impact-resistant isothermal quenched ductile iron includes: adding pig iron, scrap steel, and recycled material (20wt%) into a medium-frequency induction furnace, heating until completely melted into molten iron, and adjusting the composition to the target range according to the above mass percentages. The furnace exit temperature is controlled at 1520℃, and the molten iron temperature is controlled at 1490℃. A low-rare-earth magnesium alloy spheroidizing agent Mg5Re1 (1.3% by weight of molten iron) is added by the pouring method for spheroidization treatment. After spheroidization treatment, a composite inoculation process is adopted: first, in-flow inoculation (adding 0.3% by weight of molten iron silicon-barium-calcium inoculant), and then instantaneous inoculation during casting (adding 0.2% by weight of molten iron silicon-barium-calcium inoculant). The total inoculant addition is 0.5% by weight of molten iron. After inoculation treatment, the number of as-cast graphite spheroids is measured to be 185 / mm². 2 The spheroidization rate was 92%, and the graphite spheres were of size grade 6. The spheroidized molten iron was poured into castings at 1370℃ using resin sand molds. The entire pouring process was completed within 10 minutes after the spheroidization treatment.

[0038] The casting is placed in a protective atmosphere furnace and heated to 900℃. It is held for 50 minutes (1.67 min / mm) according to the casting wall thickness (average 30mm). Then, the casting is quickly transferred to a nitrate bath at 365℃ (KNO3:NaNO2 mass ratio of 1.1:1) for isothermal quenching and held for 90 minutes. The casting is then removed and allowed to cool naturally to room temperature in the air. The surface of the casting is then cleaned with 80℃ hot water to remove any residual nitrates.

[0039] Example 2 This embodiment provides a low-temperature impact-resistant isothermal quenched ductile iron, which comprises the following components by weight percentage: C: 3.60%, Si: 2.30%, Cu: 0.70%, Ni: 0.50%, Mn: 0.22%, P: 0.032%, S: 0.012%, Mg: 0.042%, RE: 0.018%, balance Fe; where RE is rare earth element.

[0040] The preparation method of low-temperature impact-resistant isothermal quenched ductile iron includes: adding pig iron, scrap steel, and recycled material (15wt%) into a medium-frequency induction furnace, heating until completely melted into molten iron, adjusting the composition to the target range according to the above mass percentages, controlling the tapping temperature to 1500℃, adjusting the molten iron temperature to 1485℃, and adding 1.2% (by weight) of low rare earth magnesium alloy spheroidizing agent Mg5Re1 by the pouring method for spheroidization treatment. After spheroidization treatment, a composite inoculation process is adopted: adding 0.25% (by weight) of silicon barium calcium inoculant for in-flow inoculation, adding 0.15% (by weight) of molten iron for instantaneous inoculation, and the total inoculant addition is 0.4% (by weight) of molten iron. After inoculation treatment, the number of as-cast graphite spheroids is 170 / mm. 2 The spheroidization rate was 91%, and the graphite spheres were grade 6 in size. The spheroidized molten iron was poured into a casting at 1360℃ using a resin sand mold, and the pouring was completed within 11 minutes after the spheroidization treatment. The casting was then placed in a protective atmosphere furnace and heated to 895℃, held for 42 minutes (1.68 min / mm) according to the casting wall thickness (average 25 mm). It was then rapidly transferred to a 370℃ nitrate bath (KNO3:NaNO2 mass ratio 1:1) for isothermal quenching, held for 80 minutes. After isothermal quenching, it was air-cooled and washed with 80℃ hot water.

[0041] Example 3 This embodiment provides a low-temperature impact-resistant isothermal quenched ductile iron, which comprises the following components by weight percentage: C: 3.70%, Si: 2.50%, Cu: 0.85%, Ni: 1.50%, Mn: 0.28%, P: 0.038%, S: 0.018%, Mg: 0.055%, RE: 0.025%, balance Fe; wherein RE is rare earth element.

[0042] The preparation method of low-temperature impact-resistant isothermal quenched ductile iron includes: adding pig iron, scrap steel, and recycled material (25wt%) into a medium-frequency induction furnace, heating until completely melted into molten iron, adjusting the composition to the target range according to the above mass percentages, controlling the tapping temperature to 1530℃, adjusting the molten iron temperature to 1500℃, and adding 1.5% (by weight) of low rare earth magnesium alloy spheroidizing agent Mg5Re1 for spheroidization treatment using a wire feeding method. After spheroidization treatment, a composite inoculation process is adopted: adding 0.4% (by weight) of silicon-barium-calcium inoculant for in-flow inoculation, and adding 0.3% (by weight) of silicon-barium-calcium inoculant for instantaneous inoculation, with a total inoculant addition of 0.7% (by weight) of molten iron. After inoculation treatment, the number of as-cast graphite spheroids is 200 / mm. 2 The spheroidization rate was 93%, and the graphite spheres were grade 6 in size. The spheroidized molten iron was poured into a casting at 1380℃ using a resin sand mold, and the pouring was completed within 9 minutes after the spheroidization treatment. The casting was then placed in a protective atmosphere furnace and heated to 905℃, held for 60 minutes (1.71 min / mm) according to the casting wall thickness (average 35 mm). It was then rapidly transferred to a 375℃ nitrate bath (KNO3:NaNO2 mass ratio 1.2:1) for isothermal quenching, and held for 100 minutes. After isothermal quenching, it was air-cooled and washed with 80℃ hot water.

[0043] Example 4 This embodiment provides a low-temperature impact-resistant isothermal quenched ductile iron, which comprises the following components by weight percentage: C: 3.55%, Si: 2.35%, Cu: 0.65%, Ni: 0.80%, Mn: 0.20%, P: 0.030%, S: 0.010%, Mg: 0.040%, RE: 0.015%, balance Fe; wherein RE is rare earth element.

[0044] The preparation method of low-temperature impact-resistant isothermal quenched ductile iron includes: adding pig iron, scrap steel, and recycled material (18wt%) into a medium-frequency induction furnace, heating until completely melted into molten iron, adjusting the composition to the target range according to the above mass percentages, controlling the tapping temperature at 1510℃, adjusting the molten iron temperature to 1490℃, and adding 1.1% (by weight) of low rare earth magnesium alloy spheroidizing agent Mg5Re1 by the pouring method for spheroidization treatment. After spheroidization treatment, a composite inoculation process is adopted: adding 0.28% (by weight) of silicon barium calcium inoculant for in-flow inoculation, adding 0.22% (by weight) of molten iron for instantaneous inoculation, and the total inoculant addition is 0.5% (by weight) of molten iron. After inoculation treatment, the number of as-cast graphite spheroids is 180 / mm. 2The spheroidization rate was 90%, and the graphite sphere size was grade 7. The spheroidized molten iron was poured into a casting at 1355℃ using a resin sand mold, and the pouring was completed within 12 minutes after the spheroidization treatment. The casting was placed in a protective atmosphere furnace and heated to 900℃, held for 50 minutes (1.79 min / mm) according to the casting wall thickness (average 28 mm). Then, it was rapidly transferred to a 380℃ nitrate bath (KNO3:NaNO2 mass ratio 1.1:1) for isothermal quenching, held for 120 minutes. After isothermal quenching, it was air-cooled and washed with 80℃ hot water.

[0045] Example 5 This embodiment provides a low-temperature impact-resistant isothermal quenched ductile iron, which comprises the following components by weight percentage: C: 3.68%, Si: 2.45%, Cu: 0.80%, Ni: 1.20%, Mn: 0.24%, P: 0.033%, S: 0.014%, Mg: 0.048%, RE: 0.022%, balance Fe; where RE is rare earth element.

[0046] The preparation method of low-temperature impact-resistant isothermal quenched ductile iron includes: adding pig iron, scrap steel, and recycled material (22wt%) into a medium-frequency induction furnace, heating until completely melted into molten iron, adjusting the composition to the target range according to the above mass percentages, controlling the tapping temperature to 1525℃, adjusting the molten iron temperature to 1495℃, and adding 1.4% (by weight) of low rare earth magnesium alloy spheroidizing agent Mg5Re1 for spheroidization treatment using the wire feeding method. After spheroidization treatment, a composite inoculation process is adopted: adding 0.35% (by weight) of silicon barium calcium inoculant for in-flow inoculation, adding 0.25% (by weight) of molten iron for instantaneous inoculation, and the total inoculant addition is 0.6% (by weight) of molten iron. After inoculation treatment, the number of as-cast graphite spheroids is 190 / mm. 2 The spheroidization rate was 92%, and the graphite spheres were grade 6 in size. The spheroidized molten iron was poured into a casting at 1375℃ using a resin sand mold, and the pouring was completed within 10 minutes after the spheroidization treatment. The casting was then placed in a protective atmosphere furnace and heated to 898℃, held for 55 minutes (1.72 min / mm) according to the casting wall thickness (average 32 mm). It was then rapidly transferred to a 360℃ nitrate bath (KNO3:NaNO2 mass ratio 1:1) for isothermal quenching, held for 70 minutes. After isothermal quenching, it was air-cooled and washed with 80℃ hot water.

[0047] Comparative Example 1 This comparative example provides a conventional low-temperature ductile iron (QT400-18L type), which, by weight percentage, comprises the following components: C: 3.60%, Si: 2.00%, Cu: 0%, Ni: 0%, Mn: 0.20%, P: 0.030%, S: 0.015%, Mg: 0.045%, RE: 0.020%, balance Fe.

[0048] The traditional method for preparing low-temperature ductile iron includes: adding pig iron, scrap steel, and recycled material (15 wt%) to a medium-frequency induction furnace, heating until completely melted into molten iron, adjusting the composition to the target range according to the above mass percentages, controlling the tapping temperature at 1500℃, adjusting the molten iron temperature to 1480℃, and then adding 1.3% (by weight) of rare earth magnesium alloy spheroidizing agent (Mg8Re3) of the molten iron using a pouring method for spheroidization treatment. After spheroidization treatment, adding 0.4% (by weight) of 75% ferrosilicon inoculant of the molten iron using a single pouring method for inoculation treatment. After inoculation treatment, the number of as-cast graphite nodules is 120 per mm. 2 The spheroidization rate is 85%, and the graphite spheroids are 5-6 in size. The spheroidized molten iron is poured into castings at 1380℃ using resin sand molds. The entire pouring process is completed within 12 minutes after spheroidization. The castings are then allowed to cool naturally to room temperature without isothermal quenching (some castings undergo conventional annealing: heating to 720℃, holding for 2 hours, furnace cooling to 600℃, and then air cooling). The final product is ferritic matrix ductile iron (ferrite volume fraction ≥90%).

[0049] Comparative Example 2 This comparative example provides a low-temperature impact-resistant isothermal quenched ductile iron, which, by weight percentage, comprises the following components: C: 3.65%, Si: 2.70%, Cu: 0.75%, Ni: 1.00%, Mn: 0.25%, P: 0.035%, S: 0.015%, Mg: 0.045%, RE: 0.020%, balance Fe.

[0050] The preparation method of the low-temperature impact-resistant isothermal quenched ductile iron is the same as that in Example 1.

[0051] Comparative Example 3 This comparative example provides a low-temperature impact-resistant isothermal quenched ductile iron, which, by weight percentage, comprises the following components: C: 3.65%, Si: 2.40%, Cu: 0.75%, Ni: 0.30%, Mn: 0.25%, P: 0.035%, S: 0.015%, Mg: 0.045%, RE: 0.020%, balance Fe.

[0052] The preparation method of the low-temperature impact-resistant isothermal quenched ductile iron is the same as that in Example 1.

[0053] Comparative Example 4 This comparative example provides a low-temperature impact-resistant isothermal quenched ductile iron, whose composition is the same as that of Example 1 by weight percentage.

[0054] The preparation method of the low-temperature impact resistant isothermal quenched ductile iron is basically the same as that in Example 1, except that the casting is then quickly transferred to a nitrate bath at 350°C (KNO3:NaNO2 mass ratio is 1.1:1) for isothermal quenching.

[0055] Comparative Example 5 This comparative example provides a low-temperature impact-resistant isothermal quenched ductile iron, whose composition is the same as that of Example 1 by weight percentage.

[0056] The preparation method of the low-temperature impact resistant isothermal quenched ductile iron is basically the same as that in Example 1, except that the casting is then quickly transferred to a 400°C nitrate bath (KNO3:NaNO2 mass ratio of 1.1:1) for isothermal quenching.

[0057] The test results of the mechanical properties of ductile iron obtained from each embodiment and comparative example are shown in the table below: ; As shown in the table above, the low-temperature impact-resistant isothermal quenched ductile iron prepared by the technical solution of this invention exhibits significant advantages in strength, plasticity, and low-temperature impact performance through optimized chemical composition and precise isothermal quenching process. Specifically, the tensile strength of Examples 1 to 5 is ≥900MPa, the elongation is ≥8%, and the impact energy at -30℃ is ≥6J, demonstrating excellent comprehensive mechanical properties and fully meeting the stringent requirements for high strength and high toughness of dynamic structural components in cold regions. Among them, Example 1 has the best comprehensive performance, Examples 2 to 4 maintain good performance after adjusting the composition or process parameters, and Example 5 still achieves an excellent level of strength of 1053MPa and impact energy of 6.1J near the lower limit of the isothermal temperature.

[0058] In contrast, Comparative Examples 1 to 4 all showed varying degrees of deficiencies in their performance indicators. Comparative Example 1 (traditional low-temperature ductile iron) had a high low-temperature impact energy, but its tensile strength was only 580 MPa, which could not meet the requirements of high-stress conditions. The low-temperature impact energy of Comparative Example 2 (too high Si content) and Comparative Example 3 (too low Ni content) were only 3.5 J and 4.2 J, respectively, which were lower than the requirement of 6 J. Comparative Example 4 (too low isothermal temperature) had high strength but poor plasticity, with an impact energy of only 3.0 J and an elongation of only 5%.

[0059] The above description is only 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 type of low-temperature impact-resistant isothermal quenched ductile iron, characterized in that: By weight percentage, it includes the following components: C: 3.5~3.8%, Si: 2.3~2.5%, Cu: 0.5~1.0%, Ni: 0.5~1.5%, Mn: ≤0.3%, P: ≤0.04%, S: ≤0.02%, Mg: 0.03~0.06%, RE: 0.01~0.03%, balance Fe; wherein RE is rare earth element.

2. The low-temperature impact-resistant isothermal quenched ductile iron as described in claim 1, characterized in that: The microstructure of the low-temperature impact-resistant isothermal quenched ductile iron is upper bainite + carbon-rich austenite, wherein the volume fraction of the carbon-rich austenite is 10%~20%.

3. The low-temperature impact-resistant isothermal quenched ductile iron as described in claim 1, characterized in that: The low-temperature impact-resistant isothermal quenched ductile iron has a tensile strength ≥900 MPa, an elongation ≥8%, and an impact energy ≥6 J at -30℃.

4. A method for preparing low-temperature impact-resistant isothermal quenched ductile iron, characterized in that: The preparation method is used to prepare the low-temperature impact-resistant isothermal quenched ductile iron as described in claim 1, and the preparation method includes: S1. Using pig iron, scrap steel and recycled materials as raw materials according to mass percentage, molten iron is prepared by melting in a medium frequency induction furnace; S2. Heat the molten iron to 1480~1500℃ and perform spheroidizing and inoculation treatments to obtain ductile iron raw molten iron. S3. The molten ductile iron is poured into a casting at 1350~1390℃; S4. The casting is heated to 890~910℃ for austenitization and heat preservation, and then placed in a nitrate bath at 365~380℃ for isothermal quenching for 60~120 minutes. After that, it is taken out and air-cooled to obtain low-temperature impact resistant isothermal quenched ductile iron.

5. The method for preparing low-temperature impact-resistant isothermal quenched ductile iron as described in claim 4, characterized in that: In step S1, the pig iron is high-purity pig iron, and the total amount of interfering elements Ti, Cr, and V in the high-purity pig iron is ≤0.1wt%; the scrap steel does not include alloy steel; the remelting material is ductile iron scrap, and the remelting material content in the raw materials is 10~30wt%.

6. The method for preparing low-temperature impact-resistant isothermal quenched ductile iron as described in claim 4, characterized in that: In step S2, the spheroidizing treatment is carried out by either the injection method or the wire feeding method, and the spheroidizing agent is a low rare earth magnesium alloy, with an addition amount of 1% to 1.6% of the weight of the molten iron.

7. The method for preparing low-temperature impact-resistant isothermal quenched ductile iron as described in claim 4, characterized in that: In step S2, the inoculation treatment employs composite inoculation, including in-flow inoculation and instantaneous inoculation. The inoculant is a silicon-barium-calcium inoculant, added at a rate of 0.3% to 0.8% of the weight of the molten iron. During the inoculation treatment, the number of as-cast graphite spheres is ≥150 per mm. 2 .

8. The method for preparing low-temperature impact-resistant isothermal quenched ductile iron as described in claim 4, characterized in that: In step S3, a resin sand casting mold is used to complete rapid pouring within 12 minutes.

9. The method for preparing low-temperature impact-resistant isothermal quenched ductile iron as described in claim 4, characterized in that: In step S4, the austenitizing holding time is calculated to be 1.5~2.0 min / mm based on the casting wall thickness.

10. The method for preparing low-temperature impact-resistant isothermal quenched ductile iron as described in claim 4, characterized in that: In step S4, the isothermal quenching nitrate bath is a mixed salt bath of KNO3 and NaNO2, and the mass ratio of KNO3 to NaNO2 is 0.9~1.35:1.

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