Low-temperature ductile iron, preparation method, and QT400-18L volute casting

Abstract

The invention belongs to the technical field of ductile iron and discloses low-temperature ductile iron, a preparation method and a QT400-18L volute casting. The low-temperature ductile iron is obtained by spheroidizing and inoculating the ductile iron with a pretreatment agent A and a special-effect inoculant B; the pretreatment agent A comprises: Si: 45-50%, Ca: 1.0-1.5%, Ba: 9-10%, La: 0.8-0.9%, Al: 0.8-1.0%, Ti≤0.005%, residual Fe, and a particle size of 2-5 mm; the special-effect inoculant B comprises: Si: 58-62%, Ca: 0.8-1.0%, B i: 1.9-2.0%, Al: 1.0-1.2%, Ti≤0.005%, balance Fe, particle size: 3-8 mm; the weight percentage composition of ductile iron is: C: 3.72-3.78%, Si: 2.40-2.50%, Mn: 0.09-0.12%, P≤0.015%, S: 0.005-0.010%, Ti≤0.008%, Sb: 0.0035-0.005%, V: 0.05-0.08%, La: 0.008-0.010%, Mg: 0.040-0.050%, balance Fe. The low-temperature ductile iron of the present invention effectively obtains ductile iron with high and low-temperature tensile strength, high elongation after fracture and high and low-temperature impact energy absorption by comprehensively adjusting the Si content, Mn content and V content in combination with the addition of pretreatment agent A and special-effect inoculant B in spheroidizing inoculation treatment under specific chemical composition.

Description

Technical Field

[0001] The present invention belongs to the technical field of ductile iron, and more specifically, relates to low-temperature ductile iron, a preparation method and a QT400-18L volute casting. Background Art

[0002] Ductile iron, a special type of cast iron, precipitates carbon as spherical graphite during its solidification process. This characteristic significantly differentiates its metallographic structure from gray cast iron, eliminating the cutting effect of the sharp graphite in gray cast iron on the metal matrix, thereby significantly reducing stress concentration. This results in a strength utilization rate of 70-90% in the metal matrix, fully demonstrating its superior mechanical properties.

[0003] With the continued growth in ductile iron production and its performance improving, it has gradually gained a foothold in the field of metal structural materials, even partially replacing forged and cast steel, becoming a metal material with great development potential. Therefore, in-depth research on the metallographic structure and mechanical properties of ductile iron is undoubtedly the key to fully utilizing its excellent properties and promoting its widespread application.

[0004] Ordinary ductile iron is prone to brittle fracture below -40°C, which restricts its application in engineering machinery, magnetic levitation track beams, wind turbine castings and other fields in extremely cold regions. Based on this, Western developed countries took the lead in formulating the EN-GJS series of low-temperature ductile iron standards (such as -40°C impact absorption energy ≥12J).

[0005] Low-temperature ductile iron has a wide range of applications. In mechanical engineering, it is often used to manufacture components requiring high strength and wear resistance, such as gears, cycloidal pinwheels, and diesel engine cylinder blocks. In automotive manufacturing, low-temperature ductile iron is often used to manufacture key components such as engine cylinder blocks and crankcases. In aerospace, it is also widely used to manufacture key components such as rotors, turbine blades, and combustion chambers in aircraft engines.

[0006] Traditional methods to improve low-temperature impact absorption energy are: (1) through graphitization annealing heat treatment; (2) adding nickel or other metal elements.

[0007] For example, prior art 1: Chinese patent application 202411325746.X discloses a low-temperature ductile iron, its preparation method, and its application. The low-temperature ductile iron has a CEL of 4.25-4.3 and a chemical composition by mass of 3.4% to 4.0% C, 1.8% to 2.3% Si, ≤0.2% Mn, ≤0.02% S, ≤0.03% P, 1.2% to 1.6% Ni, and the balance Fe. The preparation method includes the steps of selecting raw materials, batching, smelting, spheroidizing, inoculating, and pouring. In the as-cast state, the low-temperature ductile iron of the present invention exhibits a tensile strength σb ≥ 450 MPa, an elongation ≥ 10%, and an average impact energy absorption AKV ≥ 10 J for V-notched specimens at room temperature and as low as -40°C.

[0008] Existing technology 1 can produce low-temperature ductile iron with a tensile strength σb ≥ 450 MPa. Its specification states: "Nickel is a graphitizing element, and nickel-strengthened ferrite can improve the strength and impact toughness of ductile iron." This shows that existing technology 1 is a good technology that enables low-temperature ductile iron to be used in the production of pads for railway track transportation in cold areas by adding nickel elements; however, due to the excessively high nickel content, the elongation of ductile iron is seriously affected. When it is applied to the production of volute castings in other fields such as fans, compressors, centrifugal pumps, etc., there will be a problem, that is, the elongation and average impact absorption energy of the low-temperature ductile iron are not high enough, which makes it difficult to meet the requirements of volute castings. Summary of the Invention

[0009] The main purpose of the present invention is to provide a low-temperature ductile iron that has excellent mechanical properties even at low temperatures. It not only has high tensile strength, but also has good elongation after fracture and impact absorption energy of V-notch specimens at -40°C and -60°C, which can fully meet product requirements.

[0010] Another object of the present invention is to provide a method for preparing low-temperature ductile iron. The low-temperature ductile iron prepared by this method can effectively ensure the quality of the ductile iron and has relatively excellent mechanical properties.

[0011] Another object of the present invention is to provide a QT400-18L volute casting having high tensile strength, elongation after fracture and impact absorption energy of -40°C and -60°C V-notch specimens, which can fully meet the needs.

[0012] To achieve the above object, the present invention provides a low-temperature ductile iron, which is obtained by spheroidizing and inoculating with a pretreatment agent A and a special-effect inoculant B; the pretreatment agent A comprises: Si: 45-50%, Ca: 1.0-1.5%, Ba: 9-10%, La: 0.8-0.9%, Al: 0.8-1.0%, Ti≤0.005%, residual Fe, and a particle size of 2-5 mm; the special-effect inoculant B comprises: Si: 58-62%, Ca: 0.8-1.0%, Bi: 1.9-2.0%, Al: 1.0-1.2%, Ti≤0.005%, residual Fe, and a particle size of 3-8 mm;

[0013] The ductile iron has the following weight percentage composition: C: 3.72-3.78%, Si: 2.40-2.50%, Mn: 0.09-0.12%, P≤0.015%, S: 0.005-0.010%, Ti≤0.008%, Sb: 0.0035-0.005%, V: 0.05-0.08%, La: 0.008-0.010%, Mg: 0.040-0.050%, and the remainder is Fe.

[0014] The existing technology generally adopts graphitization annealing heat treatment or the addition of elements such as nickel to improve the impact absorption energy of V-notch specimens of low-temperature ductile iron; however, during graphitization annealing heat treatment, especially for large castings (heavy weight or large dimensions), large-scale annealing is required, which is affected by the temperature control stability of the equipment, resulting in uneven amount and distribution of ferrite after annealing, thereby limiting the mechanical properties of ductile iron; the addition of nickel makes it difficult to balance the elongation and average impact absorption energy of low-temperature ductile iron; and the above two existing technologies significantly increase costs.

[0015] The present invention limits the composition of ductile iron and combines the selection of pretreatment agent A and special-effect inoculant B during spheroidizing inoculation treatment to effectively improve the elongation after fracture and impact absorption energy of low-temperature ductile iron, and the obtained low-temperature ductile iron has good tensile strength.

[0016] The present invention also discloses a method for preparing low-temperature ductile iron, comprising the following steps:

[0017] Step 1: Based on the total mass of high-purity pig iron and high-purity scrap steel being 100%, 30-40% high-purity pig iron, 60-70% high-purity scrap steel, 2.26-2.82% low-sulfur and low-nitrogen recarburizer, 1.86-2.23% ferrosilicon, and 0.07-0.11% ferrovanadium are added as raw materials into an electric furnace to be smelted into molten iron, and the outlet water temperature is controlled at 1460-1470°C;

[0018] Step 2: placing a spheroidizing agent, a pretreatment agent A, and a special-effect inoculant B in a ladle, with the pretreatment agent A and the special-effect inoculant B on both sides of the ladle, and then covering the ladle with antimony particles and crushed steel particles, and pouring the molten iron into the ladle toward the side of the pretreatment agent A for spheroidizing and inoculating treatment; the amount of the pretreatment agent A added is 0.4% by weight of the molten iron, and the amount of the special-effect inoculant B added is 0.3% by weight of the molten iron;

[0019] Step 3: pouring molten iron, adding special effect inoculant C for inoculation during pouring.

[0020] Preferably, the amount of the spheroidizing agent added in step 2 is 1% of the weight of the molten iron, and the composition is: Mg: 5.5-6.0%, Si: 43-48%, Ca: 1.9-2.1%, La: 1.0-1.5%, Al: 0.5-0.6%, Ti≤0.005%, and the remainder is Fe, and the particle size is 5-20 mm.

[0021] Preferably, the particle size of the antimony particles and crushed steel particles in step 2 is 0.7-2 mm, the amount of antimony particles added is 0.0040-0.0056% of the weight of the molten iron, and the amount of crushed steel particles added is 1.5-2.0% of the weight of the molten iron.

[0022] Preferably, the addition amount of the special-effect inoculant C is 0.2% of the weight of the molten iron, and the composition is: Si: 70-72%, Ca: 1.5-1.7%, S+O: 0.8-0.9%, Al: 0.6-0.7%, Ti≤0.005%, and the remaining Fe, and the particle size is 0.2-0.7 mm.

[0023] Preferably, the molten iron obtained in step 1 has the following composition in weight percentage: C: 3.72-3.78%, Si: 1.47-1.65%, Mn: 0.09-0.12%, P≤0.015%, S: 0.010-0.015%, Ti≤0.008%, V: 0.05-0.08%, and the remainder is Fe.

[0024] The present invention also discloses a QT400-18L volute casting, which is obtained by casting the above-mentioned low-temperature ductile iron.

[0025] Beneficial effects

[0026] Compared with the prior art, the present invention has at least the following advantages:

[0027] (1) The present invention discloses a low-temperature ductile iron, which, under a specific chemical composition, improves the low-temperature tensile strength by adjusting the Si content and improves the low-temperature impact absorption energy by adjusting the V content;

[0028] (2) The low-temperature ductile iron of the present invention effectively obtains ductile iron with high and low temperature tensile strength, high elongation after fracture and high and low temperature impact absorption energy by comprehensively adjusting the Si content, Mn content and V content in combination with the addition of pretreatment agent A and special effect inoculant B in spheroidizing inoculation treatment under specific chemical composition;

[0029] (3) The present invention discloses a method for preparing low-temperature ductile iron, wherein during the spheroidizing inoculation process, molten iron is controlled to be poured toward the side of pretreatment agent A into a molten iron ladle for spheroidizing inoculation, thereby controlling the reaction sequence of pretreatment agent A and special-effect inoculant B and avoiding the simultaneous reaction of pretreatment agent A and special-effect inoculant B with molten iron, effectively exerting the synergistic cooperation of the trace alloy content in the pretreatment agent and the special-effect inoculant, and preparing ductile iron with high and low temperature tensile strength, high elongation after fracture and high and low temperature impact absorption energy. BRIEF DESCRIPTION OF THE DRAWINGS

[0030] The present invention will be further described below with reference to the accompanying drawings and embodiments;

[0031] Figure 1 This is a schematic diagram of the placement of pretreatment agent, special effect inoculant, spheroidizing agent, etc. in the molten iron ladle of the present invention;

[0032] Figure 2 The spheroidization rate of the test piece cast in Example 2 (100 times that before corrosion);

[0033] Figure 3 This is the matrix structure of the test block cast in Example 2 (100 times after corrosion);

[0034] Figure 4 It is a three-dimensional view of the volute casting of the QT400-18L invention.

[0035] Figure 1 In the figure, 1 is the dam of the ladle; 2 is the pretreatment agent A; 3 is the spheroidizing agent; 4 is the special effect inoculant B; 5 is the mixture of antimony particles and crushed steel particles. DETAILED DESCRIPTION

[0036] The present invention will be further described below in conjunction with the embodiments, but this does not constitute any limitation to the present invention. Any limited modifications made within the scope of the claims of the present invention are still within the scope of the claims of the present invention.

[0037] In order to explain the technical content of the present invention in detail, further description will be given below in conjunction with the embodiments.

[0038] In the following examples and comparative examples, the composition of the spheroidizing agent is: Mg: 5.7%, Si: 44%, Ca: 2%, La: 1.0%, Al: 0.53%, Ti: 0.005%, and the remainder is Fe, and the particle size is 5-20 mm;

[0039] The composition of pretreatment agent A is: Si: 46%, Ca: 1.3%, Ba: 9%, La: 0.8%, Al: 1.0%, Ti: 0.003%, and the remaining Fe. The particle size is 2-5 mm.

[0040] The composition of special inoculant B is: Si: 61%, Ca: 0.8%, Bi: 2.0%, Al: 1.2%, Ti: 0.004%, and the remaining Fe. The particle size is 3-8 mm.

[0041] The composition of special inoculant C is: Si: 72%, Ca: 1.6%, S+O: 0.8%, Al: 0.7%, Ti: 0.005%, residual Fe, particle size: 0.2-0.7mm;

[0042] The composition of pretreatment agent D is: Si: 47%, Ca: 1.5%, Ba: 10%, Al: 0.8%, and the remaining Fe, with a particle size of 2-5 mm;

[0043] The composition of special inoculant E is: Si: 60%, Ca: 1.1%, Al: 1.2%, and the remaining Fe, with a particle size of 3-8 mm.

[0044] The composition of high-purity pig iron is: C: 4.2%, Si: 0.5%, Mn: 0.05%, P: 0.012%, S: 0.010%, Ti: 0.005%, and the remaining Fe + other trace elements;

[0045] The composition of high-purity scrap steel is: C: 0.05%, Si: 0.11%, Mn: 0.12%, P: 0.010%, S: 0.009%, Ti: 0.006%, and the remaining Fe + other trace elements;

[0046] The composition of low sulfur and low nitrogen recarburizer is: C: 99.8%, S: 0.003%, N: 150ppm, residual ash, absorption rate: 90%;

[0047] The composition of ferrosilicon is: Si: 70%, Al: 0.12%, and the rest is Fe;

[0048] The main composition of antimony particles is: Sb: 99.9%, absorption rate: 90%;

[0049] The main composition of ferrovanadium is: V: 81%, absorption rate: 90%.

[0050] The absorption rate of Si element in the above raw materials (except high-purity pig iron and high-purity scrap steel) is 90%.

[0051] In the following examples and comparative examples, the particle sizes of the antimony particles and crushed steel particles are both 0.7-2 mm.

[0052] In the following examples and comparative examples, % refers to weight percentage unless otherwise specified.

[0053] In the following examples and comparative examples, the molten iron ladle used is Figure 1 A dam 1 is provided in the middle to separate the pretreatment agent A (or D) and the special effect inoculant B (or E).

[0054] Example 1

[0055] Step 1: With the total mass of high-purity pig iron and high-purity scrap steel being 100%, 34% high-purity pig iron, 66% high-purity scrap steel, 2.58% low-sulfur and low-nitrogen carburizer, 2.08% ferrosilicon, and 0.08% ferrovanadium are added as raw materials into an electric furnace to be smelted into molten iron, and the outlet water temperature is controlled at 1465°C;

[0056] Sampling was performed to test and adjust the composition of molten iron to the following weight percentages: C: 3.78%, Si: 1.55%, Mn: 0.10%, P: 0.013%, S: 0.012%, Ti: 0.008%, V: 0.058%, and the remainder was Fe;

[0057] Step 2: 1% of the weight of the molten iron in the form of a nodulizer, 0.4% of the weight of the molten iron in the form of a pretreatment agent A, and 0.3% of the weight of the molten iron in the form of a special inoculant B are placed in a ladle, with the pretreatment agent A and the special inoculant B on either side of the ladle. The ladle is then covered with 0.0040% of the weight of the molten iron in the form of antimony particles and 1.7% of the weight of the molten iron in the form of crushed steel particles. The molten iron is then poured into the ladle toward the pretreatment agent A to perform a spheroidizing and inoculating treatment.

[0058] Step 3: pouring molten iron, adding 0.2% of the weight of the molten iron special inoculant C for inoculation during pouring;

[0059] The final ductile iron sample analysis showed the following composition: C: 3.78%, Si: 2.40%, Mn: 0.10%, P: 0.013%, S: 0.008%, Ti: 0.008%, Sb: 0.0035%, V: 0.058%, La: 0.009%, Mg: 0.046%, and the remainder Fe.

[0060] Example 2

[0061] Step 1: With the total mass of high-purity pig iron and high-purity scrap steel being 100%, 30% high-purity pig iron, 70% high-purity scrap steel, 2.75% low-sulfur and low-nitrogen recarburizer, 2.06% ferrosilicon, and 0.11% ferrovanadium are added as raw materials into an electric furnace to be smelted into molten iron, and the outlet water temperature is controlled at 1465°C;

[0062] Sampling was performed to test and adjust the composition of molten iron to the following weight percentages: C: 3.77%, Si: 1.60%, Mn: 0.099%, P: 0.010%, S: 0.011%, Ti: 0.005%, V: 0.08%, and the remainder was Fe;

[0063] Step 2: 1% of the weight of the molten iron in the form of a nodulizer, 0.4% of the weight of the molten iron in the form of a pretreatment agent A, and 0.3% of the weight of the molten iron in the form of a special inoculant B are placed in a ladle, with the pretreatment agent A and the special inoculant B on either side of the ladle. The ladle is then covered with 0.0048% of the weight of the molten iron in the form of antimony particles and 1.5% of the weight of the molten iron in the form of crushed steel particles. The molten iron is poured into the ladle toward the pretreatment agent A side for spheroidization and inoculation.

[0064] Step 3: pouring molten iron, adding 0.2% of the weight of the molten iron special inoculant C for inoculation during pouring;

[0065] The final ductile iron sample analysis composition is as follows: C: 3.76%, Si: 2.45%, Mn: 0.099%, P: 0.010%, S: 0.007%, Ti: 0.005%, Sb: 0.0043%, V: 0.08%, La: 0.010%, Mg: 0.043%, and the rest Fe.

[0066] Example 3

[0067] Step 1: With the total mass of high-purity pig iron and high-purity scrap steel being 100%, 40% high-purity pig iron, 60% high-purity scrap steel, 2.26% low-sulfur and low-nitrogen recarburizer, 2.18% ferrosilicon, and 0.1% ferrovanadium are added as raw materials into an electric furnace to be smelted into molten iron, and the outlet water temperature is controlled at 1465°C;

[0068] Sampling was performed to test and adjust the composition of molten iron to the following weight percentages: C: 3.73%, Si: 1.64%, Mn: 0.092%, P: 0.015%, S: 0.008%, Ti: 0.006%, V: 0.073%, and the remainder was Fe;

[0069] Step 2: 1% of the weight of the molten iron in the form of a nodulizer, 0.4% of the weight of the molten iron in the form of a pretreatment agent A, and 0.3% of the weight of the molten iron in the form of a special inoculant B are placed in a ladle, with the pretreatment agent A and the special inoculant B on either side of the ladle. The ladle is then covered with 0.0056% of the weight of the molten iron in the form of antimony particles and 2% of the weight of the molten iron in the form of crushed steel particles. The molten iron is then poured into the ladle toward the pretreatment agent A to perform a spheroidizing and inoculating treatment.

[0070] Step 3: pouring molten iron, adding 0.2% of the weight of the molten iron special inoculant C for inoculation during pouring;

[0071] The final ductile iron sample analysis composition is as follows: C: 3.72%, Si: 2.49%, Mn: 0.0092%, P: 0.015%, S: 0.007%, Ti: 0.006%, Sb: 0.0050%, V: 0.073%, La: 0.010%, Mg: 0.0448%, and the rest Fe.

[0072] Comparative Example 1

[0073] It is generally the same as Example 2, except that the pretreatment agent A is replaced by pretreatment agent D.

[0074] Comparative Example 2

[0075] It is generally the same as Example 2, except that the special effect inoculant B is replaced by special effect inoculant E.

[0076] Comparative Example 3

[0077] It is generally the same as Example 2, except that in step 2, the pretreatment agent A and the special effect inoculant B are on the same side of the ladle.

[0078] Comparative Example 4

[0079] The method is substantially the same as Example 2, except that step 2 is changed to: 1% of the weight of the molten iron by weight of a spheroidizing agent, 0.4% of the weight of the molten iron by weight of a pretreatment agent A, and 0.3% of the weight of the molten iron by weight of a special-effect inoculant B are placed in a ladle, with the pretreatment agent A and the special-effect inoculant B on both sides of the ladle, respectively, and then covered with 0.0048% of the weight of the molten iron by weight of antimony particles and 1.5% of crushed steel particles, and the molten iron is poured into the ladle toward the side of the special-effect inoculant B for spheroidization and inoculation treatment.

[0080] Comparative Example 5

[0081] The method is substantially the same as Example 2, except that in step 1, the sample is taken for detection and the composition of the molten iron is adjusted to the following weight percentages: C: 3.77%, Si: 1.25%, Mn: 0.099%, P: 0.010%, S: 0.011%, Ti: 0.005%, V: 0.08%, and the remainder is Fe;

[0082] The final ductile iron sample analysis composition is as follows: C: 3.77%, Si: 2.10%, Mn: 0.0099%, P: 0.010%, S: 0.005%, Ti: 0.005%, Sb: 0.0043%, V: 0.08%, La: 0.009%, Mg: 0.042%, and the rest Fe.

[0083] Comparative Example 6

[0084] The method is substantially the same as Example 2, except that in step 1, the sample is taken for detection and the composition of the molten iron is adjusted to the following weight percentages: C: 3.77%, Si: 1.60%, Mn: 0.099%, P: 0.010%, S: 0.011%, Ti: 0.005%, V: 0.04%, and the remainder is Fe;

[0085] The final ductile iron sample analysis composition is as follows: C: 3.76%, Si: 2.45%, Mn: 0.099%, P: 0.010%, S: 0.007%, Ti: 0.005%, Sb: 0.0043%, V: 0.04%, La: 0.010%, Mg: 0.044%, and the rest Fe.

[0086] Comparative Example 7

[0087] The method is substantially the same as Example 2, except that in step 1, the sample is taken for detection and the composition of the molten iron is adjusted to the following weight percentages: C: 3.77%, Si: 1.60%, Mn: 0.099%, P: 0.010%, S: 0.011%, Ti: 0.005%, V: 0.09%, and the remainder is Fe;

[0088] The final ductile iron sample analysis composition is as follows: C: 3.76%, Si: 2.45%, Mn: 0.099%, P: 0.010%, S: 0.007%, Ti: 0.005%, Sb: 0.0043%, V: 0.09%, La: 0.010%, Mg: 0.045%, and the rest Fe.

[0089] Test results

[0090] 1. Ingredients

[0091] The contents of each element in the above examples and comparative examples were obtained by spectrometer analysis.

[0092] 2. Mechanical properties

[0093] The ductile iron obtained in the above examples and comparative examples was cast into test blocks with a size of 400 mm×400 mm×400 mm.

[0094] The center positions of the test blocks obtained in Examples 1-3 and Comparative Examples 1-6 were sampled and mechanical properties tested in accordance with the provisions of GB / T1348-2019 "Ductile Iron Castings". The impact absorption energy of the -40°C and -60°C V-notch specimens was the average of the results of the three impact specimens. The results are shown in Table 1.

[0095] Table 1 Test results of mechanical properties of ductile iron

[0096]

[0097] It can be seen from the results in Table 1 that:

[0098] It can be seen from the data of Examples 1-3 that the ductile iron obtained by the present invention has high tensile strength, high elongation after fracture and high average impact energy absorption at low temperature, and is significantly higher than the requirements of the performance indexes corresponding to Grade QT400-18L and casting wall thickness 60 < t ≤ 200 mm in Tables 1 and 2 of GB / T 1348-2019 "Ductile Iron Castings". Therefore, the obtained ductile iron has very excellent comprehensive mechanical properties;

[0099] It should be noted that according to the general knowledge in the field, it is considered that the best tensile strength, elongation after fracture and low-temperature impact energy absorption can be obtained when the silicon content of ductile iron is between 2.1-2.4%; however, the present invention finds that through the selection of the pretreatment agent and the special inoculant in the spheroidizing inoculation process, effectively exerting the synergistic cooperation of the trace alloy content in the pretreatment agent and the special inoculant, and then cooperating with the control of the silicon content, the mechanical properties of ductile iron can be effectively improved. While maintaining a high tensile strength, the elongation after fracture and the low-temperature impact energy absorption are both effectively improved.

[0100] It can be seen from the data comparison between Example 2 and Comparative Example 1 that the composition of the pretreatment agent will seriously affect the tensile strength of ductile iron, and it also has a certain influence on the elongation after fracture and the low-temperature impact energy absorption of ductile iron; it is speculated that the reason is that in the pretreatment reaction process of spheroidizing inoculation, the La element in the pretreatment agent of Example 2 will affect the quality of the alloy reaction, thereby affecting the graphite morphology, matrix structure, etc.; however, Comparative Example 1 does not have the La element, so the alloy quality is not well improved during the reaction of the molten iron with the pretreatment agent, and therefore its mechanical properties are significantly weaker than those of Example 2.

[0101] It can be seen from the data comparison between Example 2 and Comparative Example 2 that the selection of the special inoculant in the spheroidizing inoculation treatment will seriously affect the elongation and the low-temperature average impact energy absorption of ductile iron. Comparative Example 2 of the present invention uses a special inoculant without Bi element, which may seriously affect the ferrite distribution morphology during the spheroidizing inoculation reaction process, so that the ductile iron conforms to the general knowledge in the field that further increase in the silicon content of ductile iron will lead to a significant decrease in the elongation after fracture and the low-temperature impact energy absorption after exceeding 2.4%.

[0102] According to the data comparison of Example 2, Comparative Example 3 and Comparative Example 4, it can be seen that in the ductile iron production process of the present invention, the reaction order of the spheroidizing inoculation treatment molten iron with the pretreatment agent A and the special effect inoculant B will have serious consequences on the mechanical properties of the ductile iron; the present invention can effectively improve the comprehensive mechanical properties of the ductile iron by controlling the selection and reaction order of the pretreatment agent and the special effect inoculant during the spheroidizing inoculation reaction, and then matching the component composition of the ductile iron, especially the elongation after fracture and the low-temperature impact absorption energy of the ductile iron are significantly improved.

[0103] According to the data comparison of Example 2 and Comparative Example 5, if the silicon content of ductile iron is too low, the tensile strength and elongation after fracture of ductile iron will be greatly reduced, and the low-temperature impact absorption energy will also be significantly reduced;

[0104] According to the data comparison of Example 2 and Comparative Examples 6 and 7, if the V content of ductile iron is too low, although it will not have a significant impact on the tensile strength of ductile iron, it will seriously affect the low-temperature impact absorption energy of ductile iron and have a certain impact on the elongation after fracture; if the V content of ductile iron is too high, the tensile strength, elongation after fracture and low-temperature impact absorption energy of ductile iron will all decrease significantly, resulting in poor mechanical properties.

[0105] It is explained that the improvement of the mechanical properties of ductile iron by the present invention requires the synergistic combination of three points: (1) the need to control the composition of ductile iron; (2) the need to control the composition of the pretreatment agent and the special effect inoculant during the spheroidizing inoculation process of ductile iron; (3) during the spheroidizing inoculation process, the reaction sequence of molten iron with the pretreatment agent and the special effect inoculant needs to be strictly controlled; only when the three are synchronized and coordinated can ductile iron with relatively excellent properties such as tensile strength, elongation and low-temperature impact absorption energy be obtained.

[0106] 3. Metallographic inspection

[0107] The test block cast in Example 2 was subjected to corrosion treatment, and the center point was taken for metallographic microscope examination and analysis of its metallographic structure before and after corrosion. The results are as follows: Figure 2 and Figure 3 As shown;

[0108] Figure 2 This is the metallographic structure of the test piece before corrosion, and it can be seen that the spheroidization rate is 95%;

[0109] Figure 3 This is the metallographic structure diagram of the test block after corrosion. It can be analyzed that the pearlite content is 0% and the ferrite content is 100%.

[0110] Application Examples

[0111] The ductile iron of Example 2 was cast into a furan resin sand casting mold to obtain Figure 4 The volute casting is of grade QT400-18L, with a casting weight of 16,300 kg, casting dimensions: 4,250 mm × 3,820 mm × 1,530 mm, and a main wall thickness of 72 mm.

[0112] Casting composition: C: 3.75%, Si: 2.46%, Mn: 0.09%, P: 0.011%, S: 0.006%, Ti: 0.005%, Sb: 0.0044%, V: 0.079%, La: 0.009%, Mg: 0.049%, and the remainder is Fe.

[0113] The mechanical properties of the casting test piece (type: D type) were tested and the results were as follows: tensile strength Rm: 485MPa, yield strength R P0.2 :318MPa, elongation after fracture A:28%, hardness:161HBW. Impact absorbed energy of V-notch specimen at -40℃:25J, 23.5J, 22J, average:23.5J; impact absorbed energy of V-notch specimen at -60℃:20J, 19J, 20.5J, average:20J.

[0114] The volute casting was subjected to pressure resistance testing according to standard ISO 11484, and the result showed that the pressure resistance reached 35MPa; ultrasonic flaw detection was performed on all positions of the casting and no defects were found.

[0115] The embodiments presented herein are merely embodiments selected from a combination of all possible embodiments. The appended claims should not be limited by the embodiments describing the present invention. Some numerical ranges used in the claims include subranges therein, and variations in these ranges should also be covered by the appended claims.

Claims

1. A low-temperature ductile iron, characterized in that: The ductile iron is obtained by spheroidizing and inoculating with a pretreatment agent A and a special-effect inoculant B; the composition of the pretreatment agent A is as follows: Si: 45-50%, Ca: 1.0-1.5%, Ba: 9-10%, La: 0.8-0.9%, Al: 0.8-1.0%, Ti≤0.005%, residual Fe, and a particle size of 2-5 mm; the composition of the special-effect inoculant B is as follows: Si: 58-62%, Ca: 0.8-1.0%, Bi: 1.9-2.0%, Al: 1.0-1.2%, Ti≤0.005%, residual Fe, and a particle size of 3-8 mm; The ductile iron has a weight percentage composition of: C: 3.72-3.78%, Si: 2.40-2.50%, Mn: 0.09-0.12%, P≤0.015%, S: 0.005-0.010%, Ti≤0.008%, Sb: 0.0035-0.005%, V: 0.05-0.08%, La: 0.008-0.010%, Mg: 0.040-0.050%, and the remainder is Fe; The preparation method of the low-temperature ductile iron comprises the following steps: Step 1: Based on the total mass of high-purity pig iron and high-purity scrap steel being 100%, 30-40% high-purity pig iron, 60-70% high-purity scrap steel, 2.26-2.82% low-sulfur and low-nitrogen recarburizer, 1.86-2.23% ferrosilicon, and 0.07-0.11% ferrovanadium are added as raw materials into an electric furnace to be smelted into molten iron, and the outlet water temperature is controlled at 1460-1470°C; Step 2: placing a spheroidizing agent, a pretreatment agent A, and a special-effect inoculant B in a ladle, with the pretreatment agent A and the special-effect inoculant B on both sides of the ladle, and then covering the ladle with antimony particles and crushed steel particles, and pouring the molten iron into the ladle toward the side of the pretreatment agent A for spheroidizing and inoculating treatment; the amount of the pretreatment agent A added is 0.4% of the weight of the molten iron, and the amount of the special-effect inoculant B added is 0.3% of the weight of the molten iron; Step 3: pouring molten iron, adding special effect inoculant C for inoculation during pouring; The addition amount of the special-effect inoculant C is 0.2% of the weight of the molten iron, and the composition is: Si: 70-72%, Ca: 1.5-1.7%, S+O: 0.8-0.9%, Al: 0.6-0.7%, Ti≤0.005%, and the remaining Fe, and the particle size is 0.2-0.7 mm.

2. The low-temperature ductile iron according to claim 1, characterized in that The amount of the spheroidizing agent added in step 2 is 1% of the weight of the molten iron, and the composition is: Mg: 5.5-6.0%, Si: 43-48%, Ca: 1.9-2.1%, La: 1.0-1.5%, Al: 0.5-0.6%, Ti≤0.005%, and the remainder is Fe, and the particle size is 5-20 mm.

3. The low-temperature ductile iron according to claim 1, characterized in that: In step 2, the particle size of the antimony particles and the crushed steel particles are both 0.7-2 mm, the amount of antimony particles added is 0.0040-0.0056% of the weight of the molten iron, and the amount of crushed steel particles added is 1.5-2.0% of the weight of the molten iron.

4. The low-temperature ductile iron according to claim 1, characterized in that The composition of the molten iron obtained in step 1 is as follows by weight: C: 3.72-3.78%, Si: 1.47-1.65%, Mn: 0.09-0.12%, P≤0.015%, S: 0.010-0.015%, Ti≤0.008%, V: 0.05-0.08%, and the remainder is Fe.

5. A QT400-18L volute casting, characterized in that: The low-temperature ductile iron according to claim 1 is used for casting.

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