Method for improving elongation of qt500-7 castings

Abstract

The application discloses a method for improving elongation of QT500-7 castings, and belongs to the technical field of nodular cast iron casting and heat treatment. Step one: configuring furnace charge according to the component requirement of QT500-7 nodular cast iron castings, adding pig iron, low-manganese scrap steel, return-to-furnace material and carbon additive into a medium-frequency induction furnace for smelting to obtain original molten iron, and controlling the content of sulfur, phosphorus, manganese, titanium and anti-spheroidizing elements in the original molten iron in the smelting process; by controlling the mass of low-manganese scrap steel and return-to-furnace material, the amount of manganese, titanium, phosphorus, sulfur and anti-spheroidizing elements is reduced, the adverse effect of pearlite promoting elements and graphite distortion elements on the elongation of the castings is reduced, and a component basis is provided for obtaining high spheroidization rate, high graphite ball number and high ferrite proportion. The application adopts a higher Si content of 2.45-2.85 wt% for silicon solid solution strengthening, so that the ferrite matrix itself has higher strength, thereby reducing the requirement of high pearlite proportion for reaching the tensile strength of QT500-7.

Description

Technical Field

[0001] This invention belongs to the field of ductile iron casting and heat treatment technology, specifically relating to a method for improving the elongation of QT500--7 castings. Background Technology

[0002] QT500-7 ductile iron combines high strength, good plasticity, and excellent casting properties, making it widely used in automotive chassis brackets, construction machinery supports, pump and valve housings, gearbox housings, agricultural machinery connectors, and load-bearing support castings. Compared to ordinary gray cast iron, the graphite in QT500-7 exists in spherical form, which has a smaller cutting effect on the matrix, thus enabling a certain elongation while ensuring tensile strength.

[0003] However, in actual production, the elongation of QT500-7 castings fluctuates significantly. Especially for castings with uneven wall thickness, complex structures, or those produced in batches, the tensile strength often meets requirements, but the elongation after fracture is low. The main reasons for this problem include the following: On the one hand, inadequate control of elements such as manganese, titanium, chromium, and copper in the furnace charge can promote pearlite formation or interfere with graphite spheroidization. Manganese and copper can increase the tendency for pearlite formation, while elements such as titanium, lead, bismuth, and antimony can easily cause graphite distortion or spheroidization degradation, ultimately leading to a decrease in graphite spheroidization rate, a reduction in the number of graphite spheroids, an increase in the proportion of pearlite in the matrix, and a decrease in the elongation of the casting.

[0004] On the other hand, in conventional QT500-7 production, to ensure a tensile strength of over 500MPa, there is often a tendency to retain more pearlite or add pearlite-promoting elements such as copper and tin. While this approach is beneficial for improving strength, it significantly impairs elongation, making it difficult for castings to achieve stable high toughness.

[0005] On the other hand, ordinary single-stage inoculation or single-bundle inoculation cannot guarantee the nucleation capacity throughout the entire casting process. Especially when the waiting time after spheroidization is long, the temperature drop of the molten iron is large, or the thin-walled parts solidify quickly, inoculation decline, insufficient number of graphite spheroids, and local white iron or carbide tendencies are likely to occur, thereby reducing the elongation.

[0006] Furthermore, as-cast QT500-7 castings often exhibit pearlite segregation, localized carbides, residual stress, and uneven microstructure in regions with varying wall thicknesses. Without proper heat treatment, relying solely on as-cast microstructure control often fails to achieve a stable balance between strength, hardness, and elongation. If the heat treatment temperature is too low or the holding time is insufficient, pearlite and carbides cannot decompose sufficiently; conversely, if the heat treatment temperature is too high or the holding time is too long, it may lead to grain coarsening, graphite nodule growth, or a decrease in strength.

[0007] Therefore, there is an urgent need to provide a method suitable for industrial mass production to improve the elongation of QT500--7 castings. Under the premise of ensuring that the tensile strength is not less than 500MPa, the method can improve the proportion of matrix ferrite, reduce harmful pearlite and carbides, and improve the morphology of graphite spheroids by synergistic control of composition, spheroidization, inoculation, cooling and heat treatment, thereby obtaining QT500--7 castings with stable high elongation. Summary of the Invention

[0008] The purpose of this invention is to provide a method for improving the elongation of QT500-7 castings, so as to solve the problems of large fluctuations in elongation, unstable graphite spheroidization rate, high pearlite ratio, and large differences in local carbides and hardness in the mass production of existing QT500-7 castings.

[0009] To achieve the above objectives, the present invention provides the following technical solution: a method for improving the elongation of QT500-7 castings, comprising the following steps: Step 1: Prepare the furnace charge according to the composition requirements of QT500--7 ductile iron castings. Add pig iron, low-manganese scrap steel, recycled material and carbon raiser to the medium frequency induction furnace for smelting to obtain the original molten iron. During the smelting process, control the content of sulfur, phosphorus, manganese, titanium and anti-spheroidizing elements in the original molten iron.

[0010] Step 2: Heat the original molten iron to 1480-1530℃ for superheated refining. After slag removal, adjust the carbon equivalent so that the molten iron after spheroidization and inoculation contains, by mass percentage: C 3.55-3.75%, Si 2.45-2.85%, Mn 0.12-0.28%, P≤0.035%, S≤0.012%, Mg 0.035-0.055%, RE 0.006-0.018%, Cu≤0.12%, Cr≤0.04%, Ti≤0.030%, with the balance being Fe and unavoidable impurities.

[0011] Step 3: The molten iron obtained in Step 2 is taken out of the furnace and placed in a spheroidizing ladle for spheroidizing treatment. The spheroidizing treatment uses a low rare earth magnesium silicon iron spheroidizing agent, and an inoculant and iron filings are covered on top of the spheroidizing agent to keep the residual magnesium and residual rare earth in the molten iron within a range suitable for forming fine and round graphite spheres.

[0012] Step 4: Perform graded composite inoculation on the spheroidized molten iron. The graded composite inoculation includes long-term inoculation in the ladle, supplementary inoculation in the transfer ladle, and instantaneous inoculation in the flow, so that the molten iron maintains a stable graphite nucleation ability before casting.

[0013] Step 5: After inoculation, the molten iron is filtered through a ceramic filter and then poured into a sand mold or a coated sand mold. The pouring temperature, pouring time and unpacking temperature are controlled to obtain the QT500--7 casting blank.

[0014] Step 6: The QT500--7 casting blank is subjected to a stabilized graphitization heat treatment, which includes high-temperature homogenization holding, slow cooling ferritization holding, and stress relief cooling, to obtain a QT500--7 casting with improved elongation.

[0015] The resulting QT500--7 casting has a matrix structure that is mainly composed of ferrite and contains a small amount of pearlite. The graphite spheroidization rate is not less than 85%, the tensile strength is not less than 500MPa, and the elongation after fracture is not less than 8.5%, preferably not less than 10.0%, and more preferably not less than 12.5%.

[0016] Further, in step one, the mass ratio of pig iron, low-manganese scrap steel, and recycled material in the furnace charge is 35-55:25-45:10-25; the low-manganese scrap steel has a Mn content of no more than 0.20wt%, a Ti content of no more than 0.020wt%, a P content of no more than 0.025wt%, and a S content of no more than 0.020wt%; the recycled material is ductile iron recycled material of the same or similar grade, which is shot-peened or shot-blasted before being added to reduce the amount of sand inclusions, oxide scale, and oil contamination introduced.

[0017] Furthermore, in steps one and two, the sulfur content of the raw molten iron before spheroidizing is controlled at 0.006-0.018 wt%, the phosphorus content is controlled at no more than 0.035 wt%, and the manganese content is controlled at 0.10-0.25 wt%. When the sulfur content of the raw molten iron is higher than 0.018 wt%, sodium carbonate, calcium carbide, magnesium-based desulfurizer or a combination thereof are used for pre-desulfurization outside the furnace, and the desulfurization slag is removed before spheroidizing.

[0018] Furthermore, in step two, the carbon equivalent (CE) of the molten iron after spheroidization inoculation is controlled to be 4.35-4.65, where CE = C + 1 / 3 Si; the Si content is controlled to be 2.45-2.85 wt%, which is used to improve the strength of the ferrite matrix through silicon solid solution strengthening, so that the volume fraction of pearlite in the final casting matrix is ​​not higher than 22%, providing a microstructure basis for obtaining high elongation.

[0019] Further, in step three, the low rare earth magnesium silicon iron spheroidizing agent is a FeSiMgRE spheroidizing agent, containing 5.0-6.5% Mg, 0.5-1.5% RE, and 42-48% Si by mass percentage, and its addition amount is 1.0-1.5% of the mass of the molten iron; the spheroidizing treatment temperature is 1430-1480℃, the spheroidizing reaction time is 60-120s, and after the spheroidizing reaction is completed, the surface slag is removed, and the residual Mg content in the treated molten iron is 0.035-0.055wt% and the residual RE content is 0.006-0.018wt%.

[0020] Further, in step four, the long-term inoculation within the ladle uses a ferrosilicon inoculant containing Ba, Ca, and Al, with an addition amount of 0.25-0.45% of the molten iron mass; the supplementary inoculation in the transfer ladle uses a 75% ferrosilicon inoculant or a Si-Ca inoculant, with an addition amount of 0.08-0.20% of the molten iron mass; the instantaneous inoculation in the flow uses a ferrosilicon inoculant containing Zr, Ca, and Ba, with an addition amount of 0.04-0.12% of the molten iron mass; after the graded composite inoculation, the time from the last inoculation to the completion of casting is controlled to be 3-8 minutes.

[0021] Furthermore, in step five, the pore density of the ceramic filter is 10-20 ppi; the pouring temperature is 1350-1410℃, and the pouring time per box is 8-45s; the casting is opened after cooling in the sand mold to 350-550℃, and heat-insulating risers or chills are set for casting parts with a wall thickness greater than 40mm to adjust the cooling rate, so as to reduce the solidification temperature difference of different wall thickness areas of the casting.

[0022] Further, in step six, the stabilized graphitization heat treatment specifically involves: heating the QT500--7 casting blank to 880-920℃ at a heating rate of 60-120℃ / h and holding it at that temperature for 1.5-3.0h; then furnace cooling to 720-760℃ at a rate of 20-50℃ / h and holding it at that temperature for 2.0-5.0h; and then furnace cooling to 580-650℃ at a rate of 10-40℃ / h before air cooling. This process eliminates possible carbide and pearlite segregation in the as-cast structure, transforming the matrix structure into a mixed matrix dominated by ferrite and containing a small amount of pearlite, thereby obtaining the QT500--7 casting with improved elongation.

[0023] Furthermore, the obtained QT500--7 casting has a graphite spheroidization rate of 85-95%, a graphite spheroid number of 120-260 per mm², a graphite size of 6-8, a ferrite volume fraction of 75-90%, a pearlite volume fraction of 8-22%, a carbide volume fraction of no more than 1.0%, and a hardness difference of no more than 25 HBW between different test locations on the casting body.

[0024] Furthermore, the QT500--7 casting has a tensile strength of 500-580MPa, a yield strength of not less than 320MPa, an elongation after fracture of not less than 8.5%, preferably not less than 10.0%, more preferably not less than 12.5%, and a Brinell hardness of 160-210HBW; the QT500--7 casting is a ductile iron part such as an automobile chassis bracket, engineering machinery support, pump and valve housing, reducer housing, agricultural machinery connector, or load-bearing bracket.

[0025] Compared with the prior art, the present invention provides a method for improving the elongation of QT500--7 castings, which has the following beneficial effects: by controlling the quality of low-manganese scrap steel and recycled materials, the amount of manganese, titanium, phosphorus, sulfur and anti-spheroidizing elements introduced is reduced, the adverse effects of pearlite promoting elements and graphite distortion elements on the elongation of castings are reduced, and a compositional basis is provided for obtaining high spheroidization rate, high graphite spheroid number and high ferrite ratio.

[0026] This invention employs a relatively high Si content of 2.45-2.85 wt% for silicon solid solution strengthening, which enables the ferrite matrix itself to possess high strength. This reduces the need to rely on a high pearlite ratio to achieve a tensile strength of QT500-7, and can improve the elongation after fracture without the tensile strength being less than 500 MPa.

[0027] This invention uses a low-rare-earth magnesium-silicon-iron spheroidizing agent and controls the content of residual Mg and residual RE to avoid insufficient spheroidization due to excessively low residual magnesium, and also to avoid increased inclusions, graphite distortion or increased brittleness due to excessively high residual magnesium or rare earth content.

[0028] This invention constructs a graded composite inoculation system by combining long-term inoculation within the package, supplementary inoculation by sub-package, and instantaneous inoculation with the flow. This system can reduce inoculation degradation, improve graphite nucleation ability, increase the number of graphite spheres, refine the size of graphite spheres, and improve the sphericity of graphite.

[0029] This invention reduces the solidification temperature difference and microstructure variation at different locations of the casting by controlling the pouring temperature, pouring time, unpacking temperature, and cooling rate at different wall thicknesses, thereby reducing the impact of local white iron, carbides, and shrinkage defects on elongation.

[0030] This invention uses a stabilized graphitization heat treatment to decompose or reduce the pearlite segregation and carbides that may exist in the as-cast microstructure, transforming the matrix into a mixed matrix mainly composed of ferrite and containing a small amount of pearlite. This significantly improves the elongation while maintaining the QT500-7 strength grade.

[0031] The QT500-7 casting obtained by this invention can stably achieve an elongation at break of 8.5% or more, and under the preferred process, it can achieve 10.0% or more, and under the even better process, it can achieve 12.5% ​​or more. Moreover, the tensile strength, yield strength and hardness are all within the acceptable range for engineering applications. Attached Figure Description

[0032] Figure 1 The image shows a SEM comparison of the graphite spheroidization morphology in Example 6 (left) and Comparative Example 5 (right). Figure 2 This is a comparison chart of the number of graphite spheres and the number of inoculated and degenerated gold in Example 6 and Comparative Example 4 of the present invention; Figure 3This is a comparison diagram of the corroded matrix microstructure of gold in Example 6 and Comparative Example 6 of the present invention; Figure 4 This is a comparison diagram of Mn and Ti segregation between Example 6 and Comparative Example 1 of the present invention. Detailed Implementation

[0033] 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 some embodiments of the present invention, and not all 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.

[0034] Please see Figures 1-4 This invention provides a technical solution for improving the elongation of QT500-7 castings: In this invention, unless otherwise stated, all percentages are mass percentages. CE is the carbon equivalent, CE = C + 1 / 3Si. The volume fractions of ferrite, pearlite, and carbides are obtained through metallographic image analysis. The number of graphite spheres is measured in terms of the number of graphite spheres per square millimeter of field of view.

[0035] Main raw materials and equipment The pig iron used in the examples is low-phosphorus and low-sulfur cast iron; the low-manganese scrap steel is stamping scrap or low-manganese carbon steel scrap; the remelting material is ductile iron risers, scrap parts and test bar residues of the same or similar grade; the carbon raiser is a low-sulfur graphitized carbon raiser with a fixed carbon content of not less than 98 wt% and a sulfur content of not more than 0.05 wt%.

[0036] The spheroidizing agent is FeSiMgRE, and the inoculants include long-acting ferrosilicon inoculants containing Ba, Ca, and Al, 75% ferrosilicon inoculants, Si-Ca inoculants, and in-flow inoculants containing Zr, Ca, and Ba. The smelting equipment is a medium-frequency induction furnace, the spheroidizing ladle is a dam-type spheroidizing ladle, and the casting system is equipped with ceramic filters. The heat treatment equipment is a bogie-type resistance furnace or a gas-fired heat treatment furnace.

[0037] Test methods 1. Chemical composition testing: Samples were taken from the molten iron before each casting and analyzed using a spark discharge atomic emission spectrometer after white casting, in accordance with GB / T 24234-2009 "Determination of Multi-element Content in Cast Iron by Spark Discharge Atomic Emission Spectrometry (Conventional Method)"; publicly available standard information shows that this standard is currently in effect.

[0038] 2. Mechanical property testing: Tensile specimens are prepared from the casting block attached to the casting body or from a designated location on the casting body, and the tensile strength, yield strength and elongation after fracture are tested according to GB / T228.1-2021 "Metallic materials - Tensile testing - Part 1: Test method at room temperature"; publicly available standard information shows that this standard is currently in effect.

[0039] 3. Grade and performance evaluation of ductile iron castings: The performance of QT500--7 castings was evaluated according to GB / T 1348-2019 "Ductile Iron Castings"; publicly available standard information shows that this standard is currently in effect.

[0040] 4. Metallographic structure test: Samples were taken from typical thick and thin-walled parts, as well as critical stress parts of the casting, and the graphite spheroidization rate, graphite size, number of graphite spheroids, ferrite, pearlite and carbide volume fraction were tested according to GB / T9441-2021 "Metallographic Examination of Ductile Iron"; publicly available standard information shows that this standard is currently in effect.

[0041] 5. Brinell hardness test: The hardness of the casting body shall be tested in accordance with GB / T 231.1-2018 "Metallic materials - Brinell hardness test - Part 1: Test method". The load and indenter shall be selected according to the requirements for cast iron hardness test. According to publicly available standard information, this standard is currently in effect.

[0042] Example 1 A method for improving the elongation of QT500-7 castings includes the following steps: S1: Pig iron, low-manganese scrap steel, and recycled materials are mixed in a mass ratio of 35:45:20, and a low-sulfur graphitizing recarburizing agent is added. The mixture is then smelted in a medium-frequency induction furnace to obtain molten iron. The low-manganese scrap steel contains 0.18wt% Mn, 0.018wt% Ti, 0.020wt% P, and 0.018wt% S. The recycled materials are shot-blasted before being added.

[0043] S2: The original molten iron is heated to 1480℃ for superheat refining. After holding at this temperature for 8 minutes, the slag is removed, and the composition is adjusted so that the molten iron after spheroidization and inoculation contains, by mass percentage, 3.55% C, 2.45% Si, 0.12% Mn, 0.032% P, 0.012% S, 0.035% Mg, 0.006% RE, 0.05% Cu, 0.025% Cr, and 0.030% Ti, with the balance being Fe and unavoidable impurities. The CE value is 4.37.

[0044] S3: The molten iron is taken out of the furnace at 1430℃ and transferred to the spheroidizing ladle for spheroidizing treatment. The spheroidizing agent contains 5.0% Mg, 0.5% RE and 42% Si. The amount added is 1.0% of the mass of the molten iron. The spheroidizing reaction time is 60s. After the reaction is completed, the slag is removed.

[0045] S4: Perform graded composite inoculation on the spheroidized molten iron. The amount of long-acting inoculant added in the ladle is 0.25% of the mass of molten iron, the amount of supplementary inoculant added in the sub-ladle is 0.08% of the mass of molten iron, the amount of instantaneous inoculant added during flow is 0.04% of the mass of molten iron, and the time from the last inoculation to the completion of casting is 3 minutes.

[0046] S5: After inoculation, the molten iron is filtered through a 10ppi ceramic filter and then poured at a pouring temperature of 1350℃. The pouring time for a single mold is 8s. The casting is cooled to 350℃ in the sand mold and then opened to obtain the QT500--7 casting blank.

[0047] S6: Heat the QT500--7 casting blank to 880℃ at 60℃ / h and hold for 1.5h; then furnace cool to 720℃ at 20℃ / h and hold for 2.0h; then furnace cool to 580℃ at 10℃ / h and air cool to obtain QT500--7 casting with improved elongation.

[0048] Example 2 This embodiment is basically the same as Embodiment 1, except that: The mass ratio of pig iron, low-manganese scrap steel, and recycled material is 40:40:20; the raw molten iron is heated to 1495℃ for superheated refining; after spheroidization and inoculation, the molten iron contains C 3.60%, Si 2.55%, Mn 0.16%, P 0.030%, S 0.010%, Mg 0.040%, RE 0.008%, Cu 0.06%, Cr 0.026%, and Ti 0.026%, CE is 4.45; the amount of spheroidizing agent added is 1.1% of the mass of molten iron, the spheroidizing treatment temperature is 1445℃, and the spheroidizing reaction time is 70s; the amounts of long-acting inoculant in the ladle, the supplementary inoculant in the sub-ladle, and the instantaneous inoculant added in the flow are 0.30%, 0.10%, and 0.06%, respectively; the pouring temperature is 1370℃, the single-box pouring time is 15s, and the opening temperature is 400℃; the heat treatment is to heat to 890℃ and hold for 2.0h, furnace cool to 730℃ and hold for 2.5h, then furnace cool to 600℃ and air cool after exiting the furnace.

[0049] Example 3 A method for improving the elongation of QT500-7 castings includes the following steps: S1: Pig iron, low-manganese scrap steel and recycled material are prepared in a mass ratio of 45:35:20. The low-manganese scrap steel contains 0.16wt% Mn, 0.015wt% Ti, 0.020wt% P and 0.016wt% S.

[0050] S2: The original molten iron is heated to 1505℃ for superheated refining. After slag removal, the composition is adjusted so that the molten iron after spheroidization and inoculation contains, by mass percentage, 3.65% C, 2.65% Si, 0.20% Mn, 0.028% P, 0.009% S, 0.045% Mg, 0.012% RE, 0.08% Cu, 0.030% Cr, and 0.022% Ti, with the balance being Fe and unavoidable impurities, and CE being 4.53.

[0051] S3: The spheroidizing agent contains 5.8% Mg, 1.0% RE, and 45% Si. The amount added is 1.25% of the mass of the molten iron. The spheroidizing treatment temperature is 1455℃, the spheroidizing reaction time is 90s, and the slag is removed after the reaction is completed.

[0052] S4: The amount of long-acting inoculant added in the package is 0.35%, the amount of inoculant added by subcontracting is 0.14%, the amount of instantaneous inoculant added with the flow is 0.08%, and the time from the last inoculant addition to the completion of pouring is 5 minutes.

[0053] S5: After being filtered by a 15ppi ceramic filter, the casting is poured at a temperature of 1385℃, and the single-box pouring time is 25s. The casting is opened after cooling to 450℃ in the sand mold. Insulating risers are set for parts with a wall thickness greater than 40mm to regulate the cooling rate.

[0054] S6: Heat the casting blank to 900℃ at 90℃ / h and hold for 2.2h; then furnace cool to 740℃ at 35℃ / h and hold for 3.5h; then furnace cool to 620℃ at 25℃ / h and air cool to obtain QT500--7 casting.

[0055] Example 4 This embodiment is basically the same as embodiment 3, except that: The mass ratio of pig iron, low-manganese scrap steel, and recycled material is 50:30:20; the molten iron after spheroidizing and inoculation contains C 3.60%, Si 2.85%, Mn 0.24%, P 0.026%, S 0.008%, Mg 0.050%, RE 0.015%, Cu 0.10%, and Cr 0.035% Ti, 0.020% CE, 4.55%; superheated refining temperature 1515℃; spheroidizing treatment temperature 1470℃, spheroidizing agent addition 1.35%; long-acting inoculant in the ladle, supplementary inoculant in the sub-ladle and instantaneous inoculant added in the flow are 0.40%, 0.16% and 0.10% respectively; casting temperature 1395℃, single-box casting time 35s, opening temperature 500℃; heat treatment is to heat to 910℃ and hold for 2.6h, furnace cool to 750℃ and hold for 4.0h, then furnace cool to 635℃ and air cool after exiting the furnace.

[0056] Example 5 A method for improving the elongation of QT500-7 castings includes the following steps: S1: Pig iron, low-manganese scrap steel and recycled material are prepared in a mass ratio of 55:25:20. The low-manganese scrap steel contains 0.20wt% Mn, 0.020wt% Ti, 0.024wt% P and 0.020wt% S.

[0057] S2: The original molten iron is heated to 1530℃ for superheated refining. After slag removal, the composition is adjusted so that the molten iron after spheroidization and inoculation contains, by mass percentage, 3.75% C, 2.70% Si, 0.28% Mn, 0.030% P, 0.010% S, 0.055% Mg, 0.018% RE, 0.12% Cu, 0.040% Cr, and 0.030% Ti, with the balance being Fe and unavoidable impurities, and CE being 4.65. The casting temperature and heat treatment process are controlled to avoid graphite floating due to excessive carbon equivalent.

[0058] S3: The spheroidizing agent contains 6.5% Mg, 1.5% RE, and 48% Si. The amount added is 1.5% of the mass of the molten iron. The spheroidizing treatment temperature is 1480℃, the spheroidizing reaction time is 120s, and the slag is removed after the reaction is completed.

[0059] S4: The amount of long-acting inoculant added in the package is 0.45%, the amount of inoculant added by subcontracting is 0.20%, the amount of instantaneous inoculant added with the flow is 0.12%, and the time from the last inoculant addition to the completion of pouring is 8 minutes.

[0060] S5: After filtration through a 20ppi ceramic filter, the casting is poured at a temperature of 1410℃, with a single-box pouring time of 45s. The casting is opened after cooling to 550℃ in the sand mold. Chips are used to regulate the cooling rate for parts with a wall thickness greater than 40mm.

[0061] S6: Heat the casting blank to 920℃ at 120℃ / h and hold for 3.0h; then furnace cool to 760℃ at 50℃ / h and hold for 5.0h; then furnace cool to 650℃ at 40℃ / h and air cool to obtain QT500--7 casting.

[0062] The elongation of Example 5 is slightly lower than that of Example 6, indicating that although the strength is higher under the high-end point parameters, it needs to be precisely matched with the pouring temperature, unpacking temperature and the stabilization graphitization heat treatment regime to obtain the best plasticity while suppressing graphite floating and maintaining strength.

[0063] Example 6 A method for improving the elongation of QT500-7 castings includes the following steps: S1: Pig iron, low-manganese scrap steel, and recycled materials are mixed in a mass ratio of 48:34:18, with the addition of a low-sulfur graphitizing carburizing agent, and smelted in a 3t medium-frequency induction furnace. The low-manganese scrap steel contains 0.14wt% Mn, 0.012wt% Ti, 0.018wt% P, and 0.015wt% S. The recycled materials are ductile iron risers of the same grade, which are shot-blasted before being added.

[0064] S2: The original molten iron is heated to 1510℃ for superheat refining. After holding at this temperature for 10 minutes, the slag is removed, and the composition is adjusted so that the molten iron after spheroidization and inoculation contains, by mass percentage, 3.66% C, 2.70% Si, 0.18% Mn, 0.024% P, 0.007% S, 0.046% Mg, 0.010% RE, 0.06% Cu, 0.025% Cr, and 0.018% Ti, with the balance being Fe and unavoidable impurities. The CE is 4.56.

[0065] S3: The molten iron is tapped from the furnace at 1460℃ into a dam-type spheroidizing ladle and spheroidized using a FeSiMgRE spheroidizing agent. The spheroidizing agent contains 5.8% Mg, 0.9% RE, and 45% Si, and the amount added is 1.25% of the molten iron mass. The spheroidizing agent is covered with 0.20% of the ladle inoculant and dry iron filings. The spheroidizing reaction time is 90 seconds. After the spheroidizing reaction is completed, the surface slag is removed.

[0066] S4: The spheroidized molten iron is subjected to graded composite inoculation. Long-term inoculation within the ladle uses a ferrosilicon inoculant containing Ba, Ca, and Al, added at 0.36% of the molten iron mass; supplementary inoculation in the transfer ladle uses a 75% ferrosilicon inoculant, added at 0.14% of the molten iron mass; instantaneous inoculation in the flow uses a ferrosilicon inoculant containing Zr, Ca, and Ba, added at 0.08% of the molten iron mass; the time from the final inoculation to the completion of casting is 5 minutes.

[0067] S5: After inoculation, the molten iron is filtered through a 15ppi ceramic filter and then poured into a coated sand mold at a pouring temperature of 1380℃. The pouring time for a single mold is 28s. The casting is opened after cooling to 460℃ in the sand mold. Insulating risers are set for thick and hot parts, and local insulation coatings are used at thin-walled joints to reduce cooling differences, thus obtaining the QT500--7 casting blank.

[0068] S6: The casting blank is heated to 900℃ at a heating rate of 90℃ / h and held for 2.5h; then furnace cooled to 740℃ at 30℃ / h and held for 4.0h; then furnace cooled to 620℃ at 20℃ / h and then air cooled to obtain a high elongation QT500--7 casting.

[0069] Comparative Example 1 differs only in that: in step S1, ordinary scrap steel is used instead of low-manganese and low-titanium scrap steel, thereby increasing the Mn content in the final spheroidized and inoculated molten iron to 0.42 wt% and the Ti content to 0.055 wt%. The remaining component control, spheroidization, inoculation, casting, and heat treatment steps are the same as in Example 6.

[0070] Comparative Example 2 differs only in that the Si content in step S2 is controlled at 2.10 wt%, which is lower than the range defined in this invention. The remaining component control, spheroidization, inoculation, casting and heat treatment steps are the same as in Example 6.

[0071] Comparative Example 3 differs only in that the Si content in step S2 is controlled at 3.20 wt%, which is higher than the range defined in this invention. The remaining component control, spheroidization, inoculation, casting and heat treatment steps are the same as in Example 6.

[0072] Comparative Example 4 differs only in that: in step S4, subcontracting for supplementary inoculation and in-flow instantaneous inoculation are cancelled, and only in-package long-term inoculation is retained, and the amount of inoculating agent added in the package remains 0.36%. The remaining component control, spheroidizing, casting, and heat treatment steps are the same as in Example 6.

[0073] Comparative Example 5 differs only in that the amount of spheroidizing agent added in step S3 is reduced, so that the residual Mg content in the treated molten iron is 0.025 wt%, which is lower than the range defined in this invention. The remaining component control, inoculation, casting, and heat treatment steps are the same as in Example 6.

[0074] Comparative Example 6 differs only in that the stabilization graphitization heat treatment in step S6 is omitted, and the casting blank is directly tested as a finished product after unpacking and cleaning. The remaining composition control, spheroidization, inoculation, and casting steps are the same as in Example 6.

[0075] Performance testing Table 1. Metallographic structure and mechanical property test results of the examples and comparative examples.

[0076] As shown in Table 1, Example 1, using the lower end process within the main numerical range of the claims, yielded a QT500--7 casting with a tensile strength of 506 MPa, an elongation after fracture of 8.8%, a graphite spheroidization rate of 85%, a ferrite volume fraction of 75%, and a pearlite volume fraction of 22%. This demonstrates that the present invention can still obtain a QT500--7 casting with an elongation after fracture of not less than 8.5% under the lower end conditions.

[0077] Example 3 uses the intermediate region process within the main numerical range of the claims. The resulting QT500--7 casting has a tensile strength of 531 MPa, an elongation after fracture of 11.6%, a graphite spheroidization rate of 91%, and a graphite spheroid number of 190 / mm². This indicates that under the intermediate region process parameters, the present invention can obtain QT500--7 castings with relatively stable strength and elongation.

[0078] Example 5 uses the upper endpoint combination process within the main numerical range of the claims. The resulting QT500-7 casting has a tensile strength of 562 MPa and an elongation after fracture of 10.6%, indicating that the casting strength is significantly improved under conditions of higher carbon equivalent, higher residual Mg, and higher inoculation. Its elongation is slightly lower than that of Example 6, indicating that the upper endpoint process needs to be precisely matched with the pouring cooling and stabilization graphitization heat treatment regime in order to obtain better plasticity while avoiding graphite floating, reducing segregation, and maintaining strength.

[0079] Example 6 represents the preferred process, yielding a QT500-7 casting with a tensile strength of 538 MPa, a yield strength of 358 MPa, an elongation after fracture of 13.5%, a Brinell hardness of 182 HBW, a graphite spheroidization rate of 94%, a graphite spheroid number of 235 / mm², a ferrite volume fraction of 88%, a pearlite volume fraction of 10%, a carbide volume fraction of only 0.2%, and a bulk hardness difference of 12 HBW. These results demonstrate that Example 6, through the synergistic effect of low Mn and low Ti charge, high Si solid solution strengthening, low rare earth spheroidization, graded composite inoculation, and stabilized graphitization heat treatment, can obtain a QT500-7 casting with superior elongation.

[0080] The excellent elongation at break of 13.5% achieved in Example 6 is a result of the synergistic effect of "low impurity control + high silicon solid solution strengthening + graded composite inoculation + fully stabilized graphitization heat treatment": low manganese and low titanium charge reduces the interference of pearlite promoting elements and anti-spheroidizing elements, clearing obstacles for graphite spheroidization; appropriate high silicon content enables solid solution strengthening of the ferrite matrix, allowing the casting to meet the QT500-7 strength requirement without relying on a high pearlite ratio; graded composite inoculation provides continuous graphite nucleation capability, resulting in finer, rounder, and more numerous graphite spheres; stabilized graphitization heat treatment further eliminates pearlite segregation and carbides in the as-cast microstructure, making the matrix microstructure close to the ideal state of being mainly ferrite with a small amount of pearlite. Therefore, Example 6 exhibits superior performance in terms of strength, elongation, hardness uniformity, and microstructure stability.

[0081] Compared to Example 6, Comparative Example 1 only replaced the low-manganese, low-titanium scrap steel with ordinary scrap steel, increasing the Mn and Ti content. Test results showed that the graphite spheroidization rate of Comparative Example 1 decreased to 78%, the pearlite volume fraction increased to 34%, the carbide volume fraction increased to 2.0%, and the elongation after fracture was only 6.8%. This indicates that stringent control of manganese, titanium, and anti-spheroidizing elements plays a crucial role in improving the elongation of QT500-7 castings.

[0082] Compared to Example 6, Comparative Example 2 only reduced the Si content to 2.10 wt%. Test results showed that Comparative Example 2 had a higher ferrite content and an elongation after fracture of 13.2%, but its tensile strength was only 488 MPa, failing to meet the strength requirement of QT500-7. This result indicates that if the Si content is insufficient, the solid solution strengthening of the ferrite matrix is ​​inadequate. Although the plasticity is good, it is difficult to guarantee the strength required for QT500-7 under low pearlite conditions.

[0083] Compared to Example 6, Comparative Example 3 only increased the Si content to 3.20 wt%. Test results showed that the tensile strength of Comparative Example 3 increased to 568 MPa, but the elongation after fracture decreased to 7.5%, and the Brinell hardness increased to 218 HBW. These results indicate that excessive Si content leads to excessive solid solution strengthening of the matrix and a decrease in plasticity. Therefore, limiting the Si content to 2.45-2.85 wt% in this invention is reasonable.

[0084] Compared to Example 6, Comparative Example 4 only omitted subcontracted supplementary inoculation and in-flow instantaneous inoculation. Test results showed that in Comparative Example 4, the number of graphite spheroids decreased to 96 / mm², the graphite spheroidization rate decreased to 83%, the pearlite volume fraction increased to 27%, and the elongation after fracture was only 7.4%. These results indicate that graded composite inoculation can effectively reduce inoculation degradation, improve graphite nucleation ability, and thus increase elongation.

[0085] Compared with Example 6, Comparative Example 5 only reduced the amount of spheroidizing agent, reducing the residual Mg content to 0.025 wt%. Test results showed that the graphite spheroidization rate of Comparative Example 5 decreased to 75%, and the elongation after fracture decreased to 6.9%, indicating that controlling the residual Mg within the range of 0.035-0.055 wt% plays a crucial role in ensuring the graphite spheroidization morphology and casting plasticity.

[0086] Compared to Example 6, Comparative Example 6 only omitted the stabilization graphitization heat treatment. Test results showed that the ferrite volume fraction in Comparative Example 6 was only 48%, the pearlite volume fraction increased to 48%, the carbide volume fraction increased to 2.3%, the elongation after fracture decreased to 5.8%, and the bulk hardness difference reached 42 HBW. These results indicate that the stabilization graphitization heat treatment can eliminate or reduce pearlite segregation and carbide in the as-cast microstructure, transforming the matrix into a mixed matrix dominated by ferrite and containing a small amount of pearlite. This is an important step in improving the elongation of QT500-7 castings.

[0087] In summary, Examples 1, 3, and 5 respectively cover the lower end, middle region, and upper end of the main numerical range in the claims, proving that the present invention is feasible within the scope of the claims. Example 6 demonstrates that the present invention, under the preferred process, can obtain QT500-7 castings with a high elongation of not less than 12.5% ​​after fracture. Comparative Examples 1-6 are all single-factor variable comparisons, which respectively demonstrate that the control of low manganese and low titanium furnace charge, the range of high silicon solid solution strengthening, graded composite inoculation, residual magnesium control, and stabilized graphitization heat treatment make substantial contributions to improving elongation and stabilizing microstructure properties.

Claims

1. A method for improving the elongation of QT500-7 castings, characterized in that, Includes the following steps: Step 1: Prepare the furnace charge according to the composition requirements of QT500--7 ductile iron castings. Add pig iron, low manganese scrap steel, recycled material and carbon raiser to the medium frequency induction furnace for smelting to obtain the original molten iron. Control the content of sulfur, phosphorus, manganese, titanium and anti-spheroidizing elements in the original molten iron during the smelting process. Step 2: Heat the original molten iron to 1480-1530℃ for superheat refining. After slag removal, adjust the carbon equivalent so that the molten iron after spheroidizing and inoculation contains, by mass percentage: C 3.55-3.75%, Si 2.45-2.85%, Mn 0.12-0.28%, P≤0.035%, S≤0.012%, Mg 0.035-0.055%, RE 0.006-0.018%, Cu≤0.12%, Cr≤0.04%, Ti≤0.030%, with the balance being Fe and unavoidable impurities. Step 3: The molten iron obtained in Step 2 is taken out of the furnace and sent to the spheroidizing ladle for spheroidizing treatment. The spheroidizing treatment uses a low rare earth magnesium silicon iron spheroidizing agent, and an inoculant and iron filings are covered on top of the spheroidizing agent to keep the residual magnesium and residual rare earth in the molten iron within a range suitable for forming fine and round graphite spheres. Step 4: Perform graded composite inoculation on the spheroidized molten iron. The graded composite inoculation includes long-term inoculation in the ladle, supplementary inoculation in the ladle, and instantaneous inoculation in the flow, so that the molten iron maintains a stable graphite nucleation ability before casting. Step 5: After inoculation, the molten iron is filtered through a ceramic filter and then poured into a sand mold or a coated sand mold. The pouring temperature, pouring time and unpacking temperature are controlled to obtain the QT500--7 casting blank. Step 6: The QT500--7 casting blank is subjected to stabilized graphitization heat treatment, which includes high temperature homogenization holding, slow cooling ferritization holding and stress relief cooling, to obtain QT500--7 casting with improved elongation. The resulting QT500--7 casting has a matrix structure that is mainly composed of ferrite and contains a small amount of pearlite. The graphite spheroidization rate is not less than 85%, the tensile strength is not less than 500MPa, and the elongation after fracture is not less than 8.5%, preferably not less than 10.0%, and more preferably not less than 12.5%.

2. The method for improving the elongation of QT500-7 castings according to claim 1, characterized in that, In step one, the mass ratio of pig iron, low-manganese scrap steel, and recycled material in the furnace charge is 35-55:25-45:10-25; the low-manganese scrap steel contains no more than 0.20wt% Mn, 0.020wt% Ti, 0.025wt% P, and 0.020wt% S; the recycled material is ductile iron recycled material of the same or similar grade, which is shot-peened or shot-blasted before being added to reduce the amount of sand inclusions, oxide scale, and oil contamination introduced.

3. The method for improving the elongation of QT500-7 castings according to claim 1, characterized in that, In steps one and two, the sulfur content of the raw molten iron before spheroidizing is controlled at 0.006-0.018 wt%, the phosphorus content is controlled at no more than 0.035 wt%, and the manganese content is controlled at 0.10-0.25 wt%. When the sulfur content of the raw molten iron is higher than 0.018 wt%, sodium carbonate, calcium carbide, magnesium-based desulfurizer or a combination thereof are used for pre-desulfurization outside the furnace, and the desulfurization slag is removed before spheroidizing.

4. The method for improving the elongation of QT500-7 castings according to claim 1, characterized in that, In step two, the carbon equivalent (CE) of the molten iron after spheroidization inoculation is controlled at 4.35-4.65, where CE = C + 1 / 3 Si; the Si content is controlled at 2.45-2.85 wt%, which is used to improve the strength of the ferrite matrix through silicon solid solution strengthening, so that the volume fraction of pearlite in the final casting matrix is ​​not higher than 22%, providing a microstructure basis for obtaining high elongation.

5. The method for improving the elongation of QT500-7 castings according to claim 1, characterized in that, In step three, the low rare earth magnesium silicon iron spheroidizing agent is FeSiMgRE spheroidizing agent, which contains 5.0-6.5% Mg, 0.5-1.5% RE and 42-48% Si by mass percentage, and its addition amount is 1.0-1.5% of the mass of the molten iron; the spheroidizing treatment temperature is 1430-1480℃, the spheroidizing reaction time is 60-120s, and after the spheroidizing reaction is completed, the surface slag is removed, and the residual Mg content in the treated molten iron is 0.035-0.055wt% and the residual RE content is 0.006-0.018wt%.

6. The method for improving the elongation of QT500-7 castings according to claim 1, characterized in that, In step four, the long-term inoculation in the ladle uses a ferrosilicon inoculant containing Ba, Ca, and Al, with an addition amount of 0.25-0.45% of the molten iron mass; the supplementary inoculation in the transfer ladle uses a 75 ferrosilicon inoculant or a Si-Ca inoculant, with an addition amount of 0.08-0.20% of the molten iron mass; the instantaneous inoculation in the flow uses a ferrosilicon inoculant containing Zr, Ca, and Ba, with an addition amount of 0.04-0.12% of the molten iron mass; after the graded composite inoculation, the time from the last inoculation to the completion of casting is controlled to be 3-8 minutes.

7. The method for improving the elongation of QT500-7 castings according to claim 1, characterized in that, In step five, the pore density of the ceramic filter is 10-20 ppi; the pouring temperature is 1350-1410℃; the pouring time per box is 8-45s; the casting is opened after cooling in the sand mold to 350-550℃, and heat-insulating risers or chills are set for casting parts with a wall thickness greater than 40mm to adjust the cooling rate, so as to reduce the solidification temperature difference of different wall thickness areas of the casting.

8. The method for improving the elongation of QT500-7 castings according to claim 1, characterized in that, In step six, the stabilized graphitization heat treatment specifically involves: heating the QT500--7 casting blank to 880-920℃ at a heating rate of 60-120℃ / h and holding it at that temperature for 1.5-3.0h; then furnace cooling to 720-760℃ at a rate of 20-50℃ / h and holding it at that temperature for 2.0-5.0h; and then furnace cooling to 580-650℃ at a rate of 10-40℃ / h before air cooling. This process eliminates possible carbide and pearlite segregation in the as-cast microstructure, transforming the matrix microstructure into a mixed matrix dominated by ferrite and containing a small amount of pearlite, thereby obtaining the QT500--7 casting with improved elongation.

9. The method for improving the elongation of QT500-7 castings according to claim 1, characterized in that, The obtained QT500--7 castings have a graphite spheroidization rate of 85-95%, a graphite spheroid number of 120-260 per mm², a graphite size of 6-8, a ferrite volume fraction of 75-90%, a pearlite volume fraction of 8-22%, a carbide volume fraction of no more than 1.0%, and a hardness difference of no more than 25 HBW between different test locations on the casting body.

10. A high elongation QT500-7 casting prepared by the method according to any one of claims 1-9, characterized in that, The QT500--7 casting has a tensile strength of 500-580MPa, a yield strength of not less than 320MPa, an elongation after fracture of not less than 8.5%, preferably not less than 10.0%, more preferably not less than 12.5%, and a Brinell hardness of 160-210HBW.

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