A method for preparing high-strength, high-toughness, low-temperature ductile iron

Abstract

The present invention discloses a preparation method of high-strength and high-toughness low-temperature ductile iron, and belongs to the technical field of metal material manufacturing. The mass percentage of each element component in the ductile iron of the present invention is: C 3.6-3.9%, Si1.8-2.1%, Mn 0.01-0.03%, Mg0.02-0.04%, Ni0.3%, Cu0.15-0.2%, P≤0.01%, S<0.01%, Ca0.0015-0.0025%, Ti0.002-0.003%, and the balance is Fe and inevitable impurities. The present invention optimizes the element composition, adds and regulates the Ni and Cu element contents at the same time, and then obtains a ferrite ductile iron with a high spheroidization rate by an efficient high and low temperature dual-stage gradient heat treatment method, has higher strength and excellent toughness and impact plasticity, excellent comprehensive mechanical properties, and a simple and efficient preparation method, which is suitable for large-scale application and promotion.

Description

Technical Field

[0001] The invention belongs to the technical field of metal material manufacturing, and particularly relates to a method for preparing high-strength, high-toughness, low-temperature ductile iron. Background Art

[0002] Ductile iron is a kind of cast iron with spherical graphite obtained by spheroidizing and inoculating molten iron. Its yield strength ratio (σ 0.2 / σ b ) is approximately 0.7-0.8, about twice that of steel. Its structure can be considered a combination of steel and spheroidal graphite, combining the advantages of both steel and cast iron. Through heat treatment, the properties of ductile iron can be modified over a wide range, achieving higher strength, toughness, wear resistance, and excellent low-temperature properties. Therefore, ductile iron, with its unique comprehensive mechanical properties and processability, is used in key components in the equipment manufacturing industry.

[0003] With the rapid development of industries such as wind power generation, automobiles, and high-speed trains in my country and even globally, the demand for high-toughness ductile iron castings has grown rapidly, and at the same time, higher requirements have been placed on low-temperature impact toughness. Especially in high-latitude areas, wind turbine hubs, engine compartment seats and other castings are required to operate at -40°C for 20 to 30 years without breaking down; key components such as automobile chassis suspension frames, high-speed train bogie axle boxes, gear boxes, etc. are required to maintain a balance between tensile strength and low-temperature impact energy under extremely cold conditions of -40°C to -60°C. Ductile iron castings for some wind power equipment require the average impact energy of three V-notch specimens at -40°C to be ≥12J / cm 2 The key ductile iron components for high-speed railways such as the Beijing-Harbin and Harbin-Dalian lines and those exported to cold regions such as Russia have been required to have an impact energy of QT400-18AL of ≥12J / cm at -60°C. 2 .

[0004] However, the comprehensive performance of current ductile iron still needs to be further improved. Existing technologies often optimize the performance of ductile iron by adding or reducing the proportions of metal or non-metallic elements. For example, Chinese patent CN103060671A discloses a "ferritic ductile iron and its preparation method." This invention utilizes high-quality raw materials, adjusts the carbon, silicon, and manganese contents in molten iron, and then deoxidizes, purifies, pre-treats, and spheroidizes the molten iron to produce high-strength and high-toughness ferritic ductile iron.

[0005] While adjusting the elemental composition does improve some properties, the arbitrary addition of various metallic elements is extremely detrimental to the low-temperature toughness of ductile iron, making it impossible to achieve both room-temperature strength and low-temperature toughness. Therefore, optimizing the comprehensive low-temperature mechanical properties of ductile iron is a technical problem that needs to be solved urgently. Summary of the Invention

[0006] The present invention addresses the problems existing in the prior art. By controlling the element composition and the step-by-step heat treatment process, the prepared ductile iron has fine grains and a good degree of spheroidization, effectively improving the strength and low-temperature impact performance of the finished product, and having excellent comprehensive mechanical properties.

[0007] In order to achieve the above technical objectives, the technical solution adopted by the present invention is:

[0008] A method for preparing high-strength, high-toughness, low-temperature ductile iron comprises the following steps:

[0009] (1) Raw material preparation and smelting: Scrap steel and pig iron are added to a medium frequency induction furnace for smelting. During the smelting process, the temperature of the molten iron is maintained at 1400-1500°C. After all the molten iron is melted, the content of each element in the molten iron is detected and adjusted to meet the requirements of the formula to obtain molten iron with qualified chemical composition. The molten iron is then placed and slag is removed to obtain molten iron;

[0010] (2) Spheroidizing and inoculating treatment: add 1.3%-1.5% of the total weight of molten iron to the spheroidizing bag and 0.5% of the total weight of molten iron to the spheroidizing bag.

[0011] -0.6% inoculant, three-quarters of the total weight of the molten iron is poured into the molten iron, after the spheroidization treatment and the first inoculation treatment are completed, another quarter of the total weight of the molten iron is poured into the molten iron, and 0.2%-0.4% of the inoculant of the total weight of the molten iron is poured into the molten iron along with the flow, the second inoculation treatment is completed, and the casting is obtained;

[0012] (3) Step heat treatment:

[0013] a. During the high temperature stage, heat the casting to 910-920℃ at a rate of 20-30℃ / min, keep it warm for 3-5h, and then cool it to

[0014] 590-600℃ and then air-cooled to room temperature;

[0015] b. The low temperature stage is then followed by heating the casting to 760-770°C at a rate of 5-10°C / min and holding for 3-5h, then cooling to 730-740°C and holding for 3-5h, cooling to 550-600°C at a rate of 2-4°C / min, and then air cooling to room temperature to obtain the ductile iron.

[0016] Furthermore, the mass percentages of the elemental components in the ductile iron are: C 3.6-3.9%, Si 1.8-2.1%, Mn 0.01-0.03%, Mg 0.02-0.04%, Ni 0.3%, Cu 0.15-0.2%, P ≤ 0.01%, S < 0.01%, Ca 0.0015-0.0025%, Ti 0.002-0.003%, and the balance is Fe and unavoidable impurities.

[0017] Furthermore, the Ni content and the Cu content satisfy Ni / Cu=1.5-2.

[0018] Furthermore, the inoculant in step (2) is composed of 45%-50% silicon, 2%-3% calcium, and the remainder iron in terms of mass percentage.

[0019] Furthermore, in step (2), the spheroidizing agent and the inoculant are added in a flow-by-flow manner.

[0020] Furthermore, the spheroidizing agent in step (2) is Mg.

[0021] All raw materials of the present invention are commercially available.

[0022] The present invention comprehensively improves the mechanical properties of ductile iron through two paths. The high strength and high toughness mentioned in the present invention refers to excellent comprehensive mechanical properties.

[0023] First, optimize the elemental composition. Control the amounts of the key elements C and Si. A high C content increases the number and size of graphite nodules in the cast iron, even causing graphite flotation. This reduces plastic deformation and deformation energy, manifesting as a decrease in the upper limit of impact absorption energy. A too low carbon content shifts the ductile-brittle transition temperature range toward higher temperatures, similarly reducing impact absorption energy. Taking all factors into consideration, the carbon content in ductile iron is set at 3.6-3.9%, which provides better molten iron fluidity and is less prone to shrinkage cavities and porosity in castings.

[0024] Silicon is a strong graphitization promoter, strengthening the ferrite matrix and improving the strength and toughness of ductile iron. However, as silicon content increases, the ductile-brittle transition temperature rises. Reducing silicon by 0.1% can lower the brittle transition temperature by approximately 5°C. To achieve superior low-temperature toughness, the silicon content of ductile iron must be minimized. However, this reduction in silicon reduces the tensile strength and yield strength of ferritic ductile iron. Taking all these factors into consideration, the silicon content of ductile iron is set at 1.8-2.1%.

[0025] In ductile iron, Mn promotes the formation of carbides and pearlite in the matrix, while also causing a severe tendency for segregation. This negatively impacts toughness, particularly the impact energy absorption capacity of low-temperature ductile iron. Therefore, the Mn content must be limited, preferably to below 0.05%. In the present invention, the Mn content is controlled within a range of 0.01-0.03%.

[0026] P, S, and Ti are harmful elements. Ti and C easily form TiC, which melts at 3180°C and exists as isolated blocks. When C and N are present in molten iron, Ti and C are more likely to form compounds like TiC and TiN. The formation of acicular Mo-Ti-V intergranular complex carbides can weaken mechanical properties.

[0027] S is an anti-spheroidizing element. Sulfur has a strong bonding ability with spheroidizing elements. Most of it exists in cast iron in the form of ferrous sulfide (FeS) and other sulfide inclusions (MnS, CeS), and is distributed on the grain boundaries, which greatly reduces the strength value and impact energy of the material.

[0028] Phosphorus is a harmful element. It significantly increases the ductile-brittle transition temperature and significantly reduces the upper impact energy limit of ductile iron. Reducing phosphorus by 0.01% can lower the brittle transition temperature by approximately 4°C. Therefore, reducing phosphorus content is a key factor in producing low-temperature, high-toughness ductile iron.

[0029] Importantly, the present invention adds Ni and Cu in appropriate amounts and controls the ratio of the two to be Ni / Cu=1.5-2. Ni plays a major role in solid solution strengthening in the matrix, strengthening ferrite, thereby improving the mechanical properties of ductile iron, and will not cause embrittlement at the eutectic cell boundaries due to segregation. At the same time, Ni belongs to a face-centered cubic crystal structure with more slip planes, which is very easy to cause slip deformation in more directions, and can improve the plasticity and toughness of the material. However, too much Ni will continuously increase the degree of lattice distortion, resulting in a decrease in plasticity and toughness. Cu can promote graphitization and refine graphite, making the graphite more rounded. In addition, Cu can also eliminate free cementite and promote the solidification of the stable system, thereby optimizing the eutectic transformation process. However, Cu is also an element that interferes with spheroidization. If the ratio is too high, flake graphite is easily generated and the spheroidization rate is difficult to ensure. If the ratio is too low, the refinement and homogenization effects are difficult to ensure. The graphite spheres formed in the ratio range of Ni / Cu=1.5-2 are round and small in shape, and have excellent mechanical properties.

[0030] Secondly, the present invention uses high and low temperature two-stage heat treatment to treat the casting, heating it to 910-920°C at a rate of 20-30°C / min. Rapid high-temperature graphitization annealing can eliminate cementite and fully diffuse carbon atoms, thereby eliminating the segregation of components and the unevenness of the structure, eliminating the internal stress of the casting, and obtaining high-toughness ferrite ductile iron castings. In the subsequent low-temperature treatment stage, slow heating and multi-stage heat preservation methods can decompose pearlite so that the ferrite content in the structure can reach more than 98%, and a matrix structure of graphite spheres + ferrite is obtained. Ductile iron subjected to rapid high temperature + two-stage low-temperature heat treatment has a uniform structure, a high spheroidization rate, more rounded graphite spheres, finer grains, and excellent comprehensive mechanical properties.

[0031] In summary, the beneficial effects of the present invention are:

[0032] The present invention optimizes the elemental composition, controls the contents of major elements such as C and Si, reduces the content of harmful elements, and simultaneously adds and regulates the contents of Ni and Cu elements. Subsequently, an efficient high-low temperature dual-stage gradient heat treatment is performed to obtain ferritic ductile iron with a high spheroidization rate, high strength, excellent toughness and impact plasticity, and excellent comprehensive mechanical properties. The preparation method is simple and efficient, and is suitable for large-scale application and promotion. BRIEF DESCRIPTION OF THE DRAWINGS

[0033] Figure 1 2 are metallographic organization diagrams of the examples of the present invention and the comparative example samples. DETAILED DESCRIPTION

[0034] The technical solution of the present invention will be further described below with reference to specific embodiments, but is not limited thereto.

[0035] Example 1

[0036] A method for preparing high-strength, high-toughness, low-temperature ductile iron comprises the following steps:

[0037] (1) Raw material preparation and smelting: Scrap steel and pig iron are added to a medium frequency induction furnace for smelting. During the smelting process, the temperature of the molten iron is maintained at 1400°C. After all the molten iron is melted, the content of each element in the molten iron is detected and adjusted to meet the requirements of the formula to obtain molten iron with qualified chemical composition. The molten iron is then placed and slag is removed to obtain molten iron;

[0038] (2) Spheroidizing and inoculating: 1.3% of the total weight of the molten iron is added to the spheroidizing ladle, and 0.5% of the total weight of the molten iron is added to the spheroidizing ladle. Three-quarters of the total weight of the molten iron is poured in. After the spheroidizing and first inoculation are completed, another quarter of the total weight of the molten iron is poured in. 0.2% of the total weight of the molten iron is poured in along with the flow of the inoculant to complete the second inoculation. The casting is then cast. (3) Stepwise heat treatment:

[0039] a. During the high temperature stage, heat the casting to 910℃ at a rate of 20℃ / min, keep it at that temperature for 3 hours, then cool it to 590℃ in the furnace and then air cool it.

[0040] to room temperature;

[0041] b. During the low temperature stage, heat the casting to 760℃ at a rate of 10℃ / min and keep it for 3 hours, then cool it down to 730℃ and keep it for 3 hours.

[0042] The temperature was lowered to 550° C. at a rate of 4° C. / min, and then air-cooled to room temperature to obtain the ductile iron.

[0043] The mass percentages of the element components in the ductile iron are: C 3.6%, Si 1.8%, Mn 0.01%, Mg 0.02%, Ni 0.3%, Cu 0.15%, P ≤ 0.01%, S < 0.01%, Ca 0.0015%, Ti 0.002%, and the balance is Fe and unavoidable impurities.

[0044] The Ni content and the Cu content satisfy Ni / Cu=2.

[0045] The composition of the inoculant in step (2) is as follows in terms of mass percentage: 47% silicon, 2% calcium, and the remainder iron.

[0046] In step (2), the spheroidizing agent and the inoculant are added in a flow-by-flow manner.

[0047] The spheroidizing agent in step (2) is Mg.

[0048] Example 2

[0049] A method for preparing high-strength, high-toughness, low-temperature ductile iron comprises the following steps:

[0050] (1) Raw material preparation and smelting: Scrap steel and pig iron are added to a medium frequency induction furnace for smelting. During the smelting process, the temperature of the molten iron is maintained at 1500°C. After all the molten iron is melted, the content of each element in the molten iron is detected and adjusted to meet the formula requirements to obtain molten iron with qualified chemical composition. The molten iron is then subjected to static slag treatment to obtain molten iron;

[0051] (2) Spheroidizing and inoculating: 1.3% of the total weight of the molten iron is added to the spheroidizing ladle, and 0.6% of the total weight of the molten iron is added to the spheroidizing ladle. Three-quarters of the total weight of the molten iron is poured in. After the spheroidizing and first inoculation are completed, another quarter of the total weight of the molten iron is poured in. 0.4% of the total weight of the molten iron is poured in along with the flow of the inoculant to complete the second inoculation. The casting is then cast. (3) Stepwise heat treatment:

[0052] a. During the high temperature stage, heat the casting to 910℃ at a rate of 25℃ / min, keep it at that temperature for 4 hours, then cool it to 590℃ in the furnace and then air cool it.

[0053] to room temperature;

[0054] b. During the low temperature stage, heat the casting to 760℃ at a rate of 7℃ / min and keep it for 4 hours, then cool it down to 730℃ and keep it for 4 hours.

[0055] The temperature was lowered to 550° C. at a rate of 3° C. / min, and then air-cooled to room temperature to obtain the ductile iron.

[0056] The mass percentages of the element components in the ductile iron are: C 3.6%, Si 2.0%, Mn 0.02%, Mg 0.04%, Ni 0.3%, Cu 0.18%, P ≤ 0.01%, S < 0.01%, Ca 0.0025%, Ti 0.003%, and the balance is Fe and unavoidable impurities.

[0057] The Ni content and the Cu content satisfy Ni / Cu=1.67.

[0058] The composition of the inoculant in step (2) is as follows in terms of mass percentage: 45% silicon, 3% calcium, and the balance iron.

[0059] In step (2), the spheroidizing agent and the inoculant are added in a flow-by-flow manner.

[0060] The spheroidizing agent in step (2) is Mg.

[0061] Example 3

[0062] A method for preparing high-strength, high-toughness, low-temperature ductile iron comprises the following steps:

[0063] (1) Raw material preparation and smelting: Scrap steel and pig iron are added to a medium frequency induction furnace for smelting. During the smelting process, the temperature of the molten iron is maintained at 1500°C. After all the molten iron is melted, the content of each element in the molten iron is detected and adjusted to meet the formula requirements to obtain molten iron with qualified chemical composition. The molten iron is then subjected to static slag treatment to obtain molten iron;

[0064] (2) Spheroidizing and inoculating: 1.5% of the total weight of the molten iron in the spheroidizing ladle and 0.6% of the total weight of the molten iron in the inoculating ladle are added, and three-quarters of the total weight of the molten iron is poured in. After the spheroidizing and first inoculation are completed, another quarter of the total weight of the molten iron is poured in, and 0.4% of the total weight of the molten iron inoculating ladle is poured in along with the flow to complete the second inoculation. The casting is then cast. (3) Stepwise heat treatment:

[0065] a. During the high temperature stage, heat the casting to 920℃ at a rate of 20℃ / min, keep it at that temperature for 5 hours, then cool it to 600℃ in the furnace and then air cool it.

[0066] to room temperature;

[0067] b. During the low temperature stage, heat the casting to 770℃ at a rate of 5℃ / min and keep it for 5 hours, then cool it down to 740℃ and keep it for 5 hours.

[0068] The temperature was lowered to 600° C. at a rate of 2° C. / min, and then air-cooled to room temperature to obtain the ductile iron.

[0069] The mass percentages of the element components in the ductile iron are: C 3.9%, Si 2.1%, Mn 0.01%, Mg 0.04%, Ni 0.3%, Cu 0.2%, P ≤ 0.01%, S < 0.01%, Ca 0.0015%, Ti 0.002%, and the balance is Fe and unavoidable impurities.

[0070] The Ni content and the Cu content satisfy Ni / Cu=1.5.

[0071] The composition of the inoculant in step (2) is as follows in terms of mass percentage: 50% silicon, 2% calcium, and the balance iron.

[0072] In step (2), the spheroidizing agent and the inoculant are added in a flow-by-flow manner.

[0073] The spheroidizing agent in step (2) is Mg.

[0074] Comparative Example 1

[0075] A method for preparing low-temperature ductile iron comprises the following steps:

[0076] (1) Raw material preparation and smelting: Scrap steel and pig iron are added to a medium frequency induction furnace for smelting. During the smelting process, the temperature of the molten iron is maintained at 1500°C. After all the molten iron is melted, the content of each element in the molten iron is detected and adjusted to meet the formula requirements to obtain molten iron with qualified chemical composition. The molten iron is then subjected to static slag treatment to obtain molten iron;

[0077] (2) Spheroidizing and inoculating: 1.5% of the total weight of the molten iron in the spheroidizing ladle and 0.6% of the total weight of the molten iron in the inoculating ladle are added, and three-quarters of the total weight of the molten iron is poured in. After the spheroidizing and first inoculation are completed, another quarter of the total weight of the molten iron is poured in, and 0.4% of the total weight of the molten iron inoculating ladle is poured in along with the flow to complete the second inoculation. The casting is then cast. (3) Stepwise heat treatment:

[0078] a. During the high temperature stage, heat the casting to 920℃ at a rate of 20℃ / min, keep it at that temperature for 5 hours, then cool it to 600℃ in the furnace and then air cool it.

[0079] to room temperature;

[0080] b. During the low temperature stage, heat the casting to 770℃ at a rate of 5℃ / min and keep it for 5 hours, then cool it down to 740℃ and keep it for 5 hours.

[0081] The temperature was lowered to 600° C. at a rate of 2° C. / min, and then air-cooled to room temperature to obtain the ductile iron.

[0082] The mass percentages of the element components in the ductile iron are: C 3.9%, Si 2.1%, Mn 0.01%, Mg 0.04%, Ni 0.2%, Cu 0.2%, P ≤ 0.01%, S < 0.01%, Ca 0.0015%, Ti 0.002%, and the balance is Fe and unavoidable impurities.

[0083] The Ni content and the Cu content satisfy Ni / Cu=1.

[0084] In this comparative example, except for changing the usage ratio of Ni and Cu to Ni / Cu=1, other raw materials and preparation methods are the same as those in Example 3.

[0085] Comparative Example 2

[0086] In this comparative example, except for changing the ratio of Ni to Cu, that is, Ni / Cu=3, other raw materials and preparation methods are the same as those in Example 3.

[0087] The mass percentages of the element components in the ductile iron are: C 3.9%, Si 2.1%, Mn 0.01%, Mg 0.04%, Ni 0.6%, Cu 0.2%, P ≤ 0.01%, S < 0.01%, Ca 0.0015%, Ti 0.002%, and the balance is Fe and unavoidable impurities.

[0088] Comparative Example 3

[0089] In this comparative example, except for changing the ratio of Ni to Cu to Ni / Cu=0.5, other raw materials and preparation methods are the same as those in Example 3.

[0090] The mass percentages of the element components in the ductile iron are: C 3.9%, Si 2.1%, Mn 0.01%, Mg 0.04%, Ni 0.3%, Cu 0.6%, P ≤ 0.01%, S < 0.01%, Ca 0.0015%, Ti 0.002%, and the balance is Fe and unavoidable impurities.

[0091] Comparative Example 4

[0092] In this comparative example, except for changing the ratio of Ni to Cu, that is, Ni / Cu=4, other raw materials and preparation methods are the same as those in Example 3.

[0093] The mass percentages of the element components in the ductile iron are: C 3.9%, Si 2.1%, Mn 0.01%, Mg 0.04%, Ni 0.3%, Cu 0.075%, P ≤ 0.01%, S < 0.01%, Ca 0.0015%, Ti 0.002%, and the balance is Fe and unavoidable impurities.

[0094] Comparative Example 5

[0095] A method for preparing low-temperature ductile iron comprises the following steps:

[0096] (1) Raw material preparation and smelting: Scrap steel and pig iron are added to a medium frequency induction furnace for smelting. During the smelting process, the temperature of the molten iron is maintained at 1500°C. After all the molten iron is melted, the content of each element in the molten iron is detected and adjusted to meet the formula requirements to obtain molten iron with qualified chemical composition. The molten iron is then subjected to static slag treatment to obtain molten iron;

[0097] (2) Spheroidizing and inoculating: 1.5% of the total weight of the molten iron in the spheroidizing ladle and 0.6% of the total weight of the molten iron in the inoculating ladle are added, and three-quarters of the total weight of the molten iron is poured in. After the spheroidizing and first inoculation are completed, another quarter of the total weight of the molten iron is poured in, and 0.4% of the total weight of the molten iron inoculating ladle is poured in along with the flow to complete the second inoculation. The casting is then cast. (3) Stepwise heat treatment:

[0098] a. During the high temperature stage, heat the casting to 920℃ at a rate of 20℃ / min, keep it at that temperature for 5 hours, then cool it to 600℃ in the furnace and then air cool it.

[0099] to room temperature;

[0100] b. The casting was heated to 770°C at a rate of 5°C / min and kept warm for 10h, then cooled to 600°C at a rate of 2°C / min, and then air-cooled to room temperature to obtain the ductile iron.

[0101] In this comparative example, except that only a single temperature insulation method was used during the heat treatment, the remaining raw materials and preparation methods were the same as those in Example 3.

[0102] Performance Testing

[0103] The ductile iron obtained by the embodiment and the comparative example was subjected to performance testing, and the testing method was as follows:

[0104] Hardness tests were conducted on specimens using a 310HBS-3000 Brinell hardness tester in accordance with the national standard GB / T 231.1-2018, "Metallic Materials - Brinell Hardness Test - Part 1: Test Method." The Brinell hardness value was calculated by averaging the data from five evenly distributed points on the specimen surface. Mechanical properties such as yield strength and tensile strength were determined by testing each specimen three times using a universal tensile tester, with the average value taken. Reference was made to GB / T 228.1-2010, "Metallic Materials - Tensile Test - Part 1: Room Temperature Test Method."

[0105] The spheroidization rate, graphite nodule diameter and other indicators were statistically analyzed according to GB / T 9441-2009 Metallographic Examination of Ductile Iron. The spheroidization rate, graphite nodule diameter and grain area were calculated using Image-Pro-Plus image processing software.

[0106] Low-temperature impact test: The impact specimens were processed according to the provisions of the national standard GB / T229-2007 "Charpy Pendulum Impact Test Method for Metallic Materials", i.e., 10mm×10mm×55mm "V"-notch Charpy impact specimens. The impact test was performed using a NI300F impact testing machine. Before the test, the specimens were placed in anhydrous ethanol at -20°C to cool for 20 minutes, then quickly removed from the anhydrous ethanol and placed on the impact testing machine. The impact test was completed within 2 seconds. Three specimens were tested in each group, and the results were averaged.

[0107] Table 1 Performance test results

[0108]

[0109] From the experimental data of the present invention, we can see that Examples 1-3 of the present invention exhibit good comprehensive mechanical properties, especially excellent toughness at -60°C. However, Comparative Examples 1-4, which changed the Ni / Cu element doping ratio, and Comparative Example 5, which changed the heat treatment and insulation method, all showed varying degrees of weakening in various properties. This may be because under the parameters of the embodiments of the present invention, the graphite nodules of the ductile iron structure are more rounded, the grains are more refined, and the macroscopic performance is better mechanical properties. Therefore, the process parameters of the present invention are key technical means, forming a unified whole; the lack of any one of them will weaken the effect.

[0110] It should be noted that the above embodiments are only some of the preferred embodiments of the present invention, and not all of them. Obviously, based on the above embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative work should fall within the scope of protection of the present invention.

Claims

1. A method for preparing high-strength, high-toughness, low-temperature ductile iron, characterized in that: The mass percentages of the elements in the ductile iron are: C 3.9%, Si 2.1%, Mn 0.01%, Mg 0.04%, Ni 0.3%, Cu 0.2%, P≤0.01%, S<0.01%, Ca 0.0015%, Ti 0.002%, and the balance is Fe and unavoidable impurities. The preparation method of the ductile iron comprises the following preparation steps: (1) Raw material preparation and smelting: Scrap steel and pig iron are added to a medium frequency induction furnace for smelting. During the smelting process, the temperature of the molten iron is maintained at 1500°C. After all the molten iron is melted, the content of each element in the molten iron is detected and adjusted to meet the formula requirements to obtain molten iron with qualified chemical composition. The molten iron is then subjected to static slag treatment to obtain molten iron; (2) Spheroidizing and inoculating: 1.5% of the total weight of the molten iron by weight of a spheroidizing agent and 0.6% of the total weight of the molten iron by weight of an inoculant are added to a spheroidizing ladle, and three-quarters of the total weight of the molten iron is poured in. After the spheroidizing and first inoculation treatment is completed, another quarter of the total weight of the molten iron is poured in, and 0.4% of the total weight of the molten iron by weight of an inoculant is poured in along with the flow to complete the second inoculation treatment, and then poured to obtain a casting; (3) Step heat treatment: a. During the high temperature stage, the casting is heated up to 920℃ at a rate of 20℃ / min, kept at this temperature for 5 hours, then furnace cooled to 600℃ and air cooled to room temperature; b. The low temperature stage was then followed by heating the casting to 770°C at a rate of 5°C / min for 5h, then cooling to 740°C for 5h, cooling to 600°C at a rate of 2°C / min, and then air cooling to room temperature to obtain the ductile iron.

2. The method for preparing high-strength, high-toughness, low-temperature ductile iron according to claim 1, characterized in that: The spheroidizing agent in step (2) is Mg.

3. The method for preparing high-strength, high-toughness, low-temperature ductile iron according to claim 1, characterized in that: The composition of the inoculant in step (2) is as follows: 45%-50% silicon, 2%-3% calcium, and the balance iron, in terms of mass percentage.

4. The method for preparing high-strength, high-toughness, low-temperature ductile iron according to claim 1, characterized in that: In step (2), the spheroidizing agent and the inoculant are added in a flow-by-flow manner.

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