A method for preparing high-strength high-toughness nodular cast iron

Abstract

The application relates to the technical field of nodular cast iron, and particularly provides a preparation method of high-strength and high-toughness nodular cast iron, wherein basic raw materials are weighed according to the mass percentage, the basic raw materials including 48-52% of pig iron, 28-32% of scrap steel and 18-22% of return-furnace materials, 0.002-0.008% of antimony accounting for the total mass of molten iron, and 0.015-0.025% of copper accounting for the total mass of molten iron; after smelting, the molten iron is transferred into a spheroidizing package, Mg-Ce-Y composite spheroidizing agents and inoculants are added for inoculation; after the inoculation is completed, casting is carried out, and the castings are sent into an electric resistance furnace for normalizing, quenching and tempering treatment. The application realizes the mechanical property combination of tensile strength >=800MPa and elongation rate >=10%.

Description

Technical Field

[0001] This invention relates to the field of ductile iron technology, and specifically provides a method for preparing high-strength and high-toughness ductile iron. Background Technology

[0002] Conventional QT800-10 cannot simultaneously meet the requirements of high strength and high elongation, especially in thick-section castings where performance inconsistencies are likely to occur. Traditional processes have limitations in carbon equivalent control, spheroidizing agent / inoculant selection, or heat treatment parameters, leading to high costs or unstable performance. For example, conventional spheroidizing processes can easily cause graphite morphology deterioration, affecting toughness; adding expensive alloying elements to improve strength also increases costs. Summary of the Invention

[0003] To resolve the contradiction between strength and toughness in existing QT800-10 technology, this invention provides a method for preparing high-strength, high-toughness ductile iron, specifically achieved through the following technical solution:

[0004] A method for preparing high-strength, high-toughness ductile iron, characterized by comprising the following steps:

[0005] Step 1: Weigh the basic raw materials as follows by mass percentage: 48-52% pig iron, 28-32% scrap steel, and 18-22% recycled material. Process the pig iron, scrap steel, and recycled material into blocks. Then weigh out antimony at 0.002-0.008% of the total mass of molten iron and copper at 0.015-0.025% of the total mass of molten iron.

[0006] Step 2: Add pig iron into the electric furnace. After the pig iron has completely melted, add scrap steel and recycled materials. After the recycled materials have completely melted, add antimony and copper.

[0007] Step 3: Transfer the molten iron to a spheroidizing ladle, add 1.4-1.6% of Mg-Ce-Y composite spheroidizing agent by weight of the total molten iron, and then add 0.5-0.7% of silicon-barium inoculant by weight of the total molten iron for the first inoculation; after the first inoculation is completed, load 0.1-0.7% of silicon-barium inoculant by weight of the total molten iron into an in-flow inoculation device for in-flow inoculation;

[0008] Step 4: Casting. The initial casting temperature shall not be lower than 1350℃, and the final casting temperature shall not be lower than 1342℃.

[0009] Step 5: After casting, cool, demold, shot blast, and polish.

[0010] Step 6: The casting is placed in an electric resistance furnace and heated from room temperature to 900°C, held for 6 hours, and then cooled to room temperature to complete the normalizing process. After normalizing, the casting is heated from room temperature to 860°C and held for 6 hours. After holding, the casting is quickly removed and placed in 20°C circulating water for quenching. After quenching, the casting is placed in a furnace and heated from room temperature to 560°C, held for 6 hours.

[0011] The pig iron in step 1 contains 3.5-3.8% C, 1.8-2.0% Si, ≤0.3% Mn, ≤0.04% P, and ≤0.02% S.

[0012] The scrap steel in step 1 contains C 0.1-0.2%, Mn 1.2-1.5%, Si ≤0.3%, P ≤0.03%, and S ≤0.02%.

[0013] In step 1, the pig iron is derusted, has slag inclusions and blocky impurities removed, and is then processed into regular blocks with a side length of 50-80mm. The scrap steel is derusted of its surface impurities and then cut into blocks with a side length of 150-200mm using a flame cutting device. The recycled material is descaled and then crushed into blocks with a side length of 80-120mm.

[0014] When used, the antimony is mixed evenly with ferrosilicon powder at a mass ratio of 1:5.

[0015] The composition of the Mg-Ce-Y composite spheroidizing agent is 8-10% Mg, 2-3% Ce, 1-2% Y, 40-45% Si, and the remainder is Fe; the amount added is 1.4-1.6% of the total mass of molten iron.

[0016] In step 4, the casting speed is controlled by the tilt angle of the spheroidizing ladle: 10° in the initial stage; 20° in the middle stage; and 5° in the later stage.

[0017] The casting has a tensile strength ≥800MPa and an elongation ≥10%.

[0018] The technical solution of this invention has the following advantages:

[0019] This invention belongs to the field of metal materials technology, specifically relating to the improvement of the chemical composition design, smelting process, spheroidizing treatment or heat treatment method of ductile iron (QT800-10) to achieve a combination of mechanical properties with tensile strength ≥800MPa and elongation ≥10%.

[0020] This invention effectively solves the technical problems in the prior art, such as the difficulty of simultaneously achieving high strength and high elongation with conventional QT800-10, the tendency for uneven performance of thick cross-section castings, and the high cost and poor performance stability caused by unreasonable carbon equivalent control, single selection of spheroidizing agent / inoculant, or limited heat treatment parameters in traditional processes. In terms of the raw material system, by optimizing the basic raw material ratio and precisely controlling the addition form and amount of trace elements Sb and Cu, the carbon content of the molten iron is ensured to meet the spheroidization requirements and avoid graphite floating by using pig iron, while Mn is added by high-manganese scrap steel to control the pearlite ratio. Sb is used to inhibit ferrite formation and promote uniform pearlite precipitation, and Cu is used to enhance the ductility of ferrite through solid solution strengthening. At the same time, the introduction of recycled materials significantly reduces raw material costs and reduces compositional fluctuations. In terms of the smelting process, a medium-frequency induction furnace is used and the furnace lining is preheated at 800-850℃. The process involves adding pig iron in batches, adding scrap steel and recycled materials in batches, and adding premixed trace elements in sequence, combined with a stirring process after each batch. This not only reduces the loss of the furnace lining caused by thermal shock and the risk of scrap steel bridging, but also avoids local component segregation in the molten iron, ensuring that the molten iron has good fluidity and slag removal effect at the tapping temperature of 1500-1550℃, laying a foundation for high-quality molten iron in subsequent processes. In the spheroidization and inoculation treatment stages, Mg-C is used. The e-Y composite spheroidizing agent replaces the traditional single Mg spheroidizing agent, combining a dual inoculation mode of primary inoculation and in-flow inoculation to significantly refine the size of graphite spheroids, inhibit spheroidization fading, and effectively prevent the formation of white cast iron. During casting, the resin sand mold bottom-pouring gate design, 80-100℃ mold preheating, and staged control of casting temperature and speed ensure stable filling of the mold cavity with molten iron, avoiding defects such as air entrapment, cold shuts, and shrinkage cavities. In the post-casting treatment stage, 24-hour natural cooling prevents cracks or abnormal growth of graphite spheroids in the casting due to improper cooling rate. Shot blasting removes surface impurities while forming a work-hardened layer to help improve strength. Grinding ensures the dimensional accuracy of the casting. The heat treatment process strictly follows the rules of classifying large and small parts for furnace loading, not stacking them in the furnace, and not placing anything within 100mm on both sides. With precise parameters for normalizing, quenching, and tempering, the temperature uniformity in the furnace is controlled within ±5℃, effectively eliminating casting internal stress and allowing austenite to fully transform into tempered sorbite, ultimately achieving a balance between strength and toughness. Testing revealed that the ductile iron castings prepared by this invention have a tensile strength of 820–850 MPa, an elongation of 11.5–13.5%, a yield strength of 650–690 MPa, and a hardness of HB250. Their mechanical properties are significantly superior to those of conventional QT800-10, and the process exhibits high stability, ensuring the uniformity of performance in thick-section castings. It balances high performance with low cost and possesses outstanding industrial application value. Detailed Implementation

[0021] The technical solution of the present invention will be clearly and completely described below. Obviously, the described embodiments are only some embodiments of the present invention, not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0022] This invention provides a method for preparing high-strength and high-toughness ductile iron. The initial raw materials for preparing high-strength and high-toughness ductile iron are 48-52% pig iron, 28-32% scrap steel, and 18-22% recycled material. Then, trace elements of less than one-thousandth are added. Among them, high-manganese scrap steel is selected, and in this invention, automobile steel beams are used.

[0023] Trace elements include 0.002–0.008% Sb and 0.015–0.025% Cu.

[0024] Sb inhibits ferrite formation, increases the pearlite ratio, and maintains toughness.

[0025] Cu improves ductility.

[0026] The pig iron was purchased from Benxi, Liaoning.

[0027] The scrap steel is used for automobile beams.

[0028] The smelting process uses a medium-frequency induction furnace, and the furnace exit temperature is controlled at 1500-1550℃.

[0029] The spheroidizing treatment uses a Mg-Ce-Y composite spheroidizing agent combined with in-flow inoculation. The amount of spheroidizing agent added is 1.4% to 1.6% of the total mass of molten iron, and the amount of barium silicon inoculant added is 0.8% to 1.2% of the total mass of molten iron.

[0030] During smelting, pig iron is added first because it is relatively regular; after the pig iron has melted, scrap steel and recycled materials are added.

[0031] After smelting, the molten iron is poured into a spheroidizing ladle to carry out the spheroidizing reaction.

[0032] The first inoculation is carried out during spheroidization, and the amount of inoculation added is 0.5% to 0.7% of the total mass of molten iron.

[0033] After spheroidization is complete, in-flow inoculation begins during the casting process, with the remaining inoculant added. For the initial casting, the temperature should not be lower than 1350℃, and for the final casting, the temperature should not be lower than 1342℃.

[0034] After casting, the process involves cooling, demolding, shot blasting, and polishing.

[0035] After polishing, heat treatment is performed.

[0036] During heat treatment, large and small parts should be placed in the furnace separately, and the components in the same furnace should be as uniform in size as possible, i.e., the furnace should contain either all large or all small parts. If large and small parts are placed in the furnace at the same time, the large parts should be placed on the inside, i.e., the side that heats up and dissipates heat more slowly. Products should not be stacked in the furnace, and it is best not to place products on the sides of the furnace.

[0037] The heat treatment process is as follows: normalizing at 900℃ and holding for 6 hours; then quenching at 860℃ and holding for 6 hours; then tempering at 560℃ and holding for 6 hours.

[0038] After tempering, allow it to cool naturally to room temperature.

[0039] After testing, the final product of this invention after heat treatment has a tensile strength of 820 MPa, an elongation of 12%, and a graphite spheroidization rate of 95%.

[0040] The present invention specifically includes the following steps:

[0041] 1. Raw material preparation and pretreatment

[0042] 1.1 Selection and Pretreatment of Basic Raw Materials

[0043] Pig iron: Foundry pig iron from Benxi, Liaoning Province is selected, with a composition of C 3.5–3.8%, Si 1.8–2.0%, Mn ≤0.3%, P ≤0.04%, and S ≤0.02%. During pretreatment, manual screening is used to remove surface rust, inclusions, and lumpy impurities, followed by processing into regular blocks with sides of 50–80 mm. This specification ensures rapid bottom placement of the pig iron in the induction furnace, preventing subsequent scrap steel from directly contacting the furnace lining and causing localized overheating damage, while also reducing component segregation during the smelting process.

[0044] Scrap steel: High-manganese scrap steel from dismantled automobile beams is selected, with a composition of C 0.1-0.2%, Mn 1.2-1.5%, Si ≤0.3%, P ≤0.03%, and S ≤0.02%. During pretreatment, non-metallic impurities such as paint and rubber attached to the surface are removed first, and then it is cut into blocks with a side length of 150-200mm using flame cutting equipment. Controlling the size of the scrap steel can prevent it from "bridging" during the smelting process. At the same time, the high manganese content can help increase the pearlite content of ductile iron, laying the foundation for strength improvement.

[0045] Recycled material: Select QT800-10 waste castings or casting surplus produced in the early stage of this process. After removing the surface oxide scale with shot blasting equipment, crush it into blocks with a side length of 80-120mm. The addition of recycled material can reduce raw material costs. At the same time, its composition is consistent with the target casting, which can reduce composition fluctuations during the smelting process. The content of non-metallic inclusions in the recycled material should be controlled to ≤0.5% to avoid affecting the toughness of the final casting.

[0046] 1.2 Basic Raw Material Ratio

[0047] Weigh the basic raw materials by mass percentage: 48-52% pig iron, 28-32% scrap steel, and 18-22% recycled material. The proportion design is based on the following: about 50% pig iron can ensure that the carbon content of the molten iron meets the requirements for spheroidization, about 30% high-manganese scrap steel can supplement Mn element to control the pearlite ratio, and about 20% recycled material can balance the fluctuation of composition. This proportion can ensure strength while avoiding excessive carbon equivalent that causes graphite to float.

[0048] 1.3 Trace Element Preparation

[0049] Weigh two trace elements: weigh antimony (Sb) powder with a purity ≥99.9% and accounting for 0.002-0.008% of the total mass of molten iron, and weigh copper (Cu) wire with a purity ≥99.5% and accounting for 0.015-0.025% of the total mass of molten iron; mix the Sb powder and ferrosilicon powder evenly at a mass ratio of 1:5, and cut the Cu wire into segments with a length of 50mm; the role of Sb is to inhibit ferrite formation and promote uniform precipitation of pearlite, but the Sb content must be strictly controlled to ≤0.008%, because exceeding 0.01% will cause the casting to become brittle. 0.02% Cu can improve the ductility of ferrite through solid solution strengthening, without affecting the strength contribution of pearlite.

[0050] 2 Smelting process

[0051] 2.1 Smelting Equipment and Preliminary Preparations

[0052] A medium-frequency induction furnace is used. Before melting, the furnace lining is inspected to ensure it is free of cracks and spalling, and its thickness is ≥100mm. 2kg of graphite powder is added to the furnace, and then the furnace power is set to 80% to preheat the lining for 30 minutes, raising the preheating temperature to 800–850℃. Preheating prevents cracking of the lining caused by contact between the cold lining and the high-temperature molten iron, and also shortens the subsequent melting time.

[0053] 2.2 Feeding sequence and smelting control

[0054] Step 1: Adding pig iron: Add the pretreated pig iron to the induction furnace in three batches, with a 5-minute interval between each batch. Add the next batch only after the previous batch has completely melted. The reason for adding pig iron first is that its regular, blocky shape allows it to spread quickly at the furnace bottom, forming a molten iron cushion layer. The subsequently added scrap steel and recycled materials can then be placed on this cushion layer, avoiding direct contact with the furnace lining. This operation reduces furnace lining wear and minimizes the risk of scrap steel bridging.

[0055] Step 2: Add scrap steel and recycled materials: After the pig iron has completely melted, add the scrap steel in two batches, stirring the molten iron for 1 minute after each batch to ensure the scrap steel is completely submerged in the molten iron. After the scrap steel has completely melted, add the recycled materials, also in batches, stirring for 1 minute after each batch. Stirring promotes uniform composition and avoids microstructure segregation caused by excessively high Mn content in certain areas.

[0056] Step 3: Adding trace elements: After the recycled material is completely melted, the premixed Sb-Si powder mixture is fed into the middle of the molten iron through a special feeding hopper and stirred for 2 minutes; then Cu wire is evenly sprinkled onto the surface of the molten iron, and the Cu wire is quickly melted and stirred for another 1 minute.

[0057] 2.3 Furnace temperature control

[0058] Increase the power of the induction furnace to 100% and raise the temperature of the molten iron to 1500–1550℃, holding it at that temperature for 10 minutes. Holding this temperature allows inclusions in the molten iron to rise fully to the surface, where slag can be skimmed off with a graphite spoon. Before tapping, check the temperature again to ensure it remains stable at 1500–1550℃. This temperature range ensures sufficient fluidity of the molten iron, preventing incomplete casting and avoiding burn-out of the spheroidizing agent due to excessively high temperatures.

[0059] 3. Spheroidization and Incubation Treatment

[0060] 3.1 Preparation of spheroidizing package

[0061] A 500kg acidic spheroidizing ladle was selected, with the composite spheroidizing agent placed at the bottom of the ladle as a pre-set dam. The composite spheroidizing agent was a Mg-Ce-Y composite spheroidizing agent, with the composition of 8-10% Mg, 2-3% Ce, 1-2% Y, 40-45% Si, and the remainder being Fe. The addition amount was 1.4-1.6% of the total mass of molten iron. Mg was the main spheroidizing element, which could cause graphite to precipitate in a spherical shape. Ce and Y were auxiliary spheroidizing elements, which could refine the size of graphite spheres and inhibit spheroidization fading. The spheroidizing effect of this composite spheroidizing agent was better than that of a single Mg spheroidizing agent.

[0062] 3.2 First pregnancy

[0063] The surface of the molten iron is covered with a 0.5–0.7% (by mass) silicon-barium inoculant, consisting of 70–75% Si, 2–3% Ba, and the remainder Fe. Then, 2 kg of perlite is added as insulation to reduce the burn-off of the inoculant under the impact of the molten iron. The first inoculation occurs when the molten iron is poured into the spheroidizing ladle. The impact of the molten iron triggers the spheroidizing reaction, while the silicon-barium inoculant rapidly melts. Ba can delay the decay of Si, and Si can promote graphite spheroidization, preventing the formation of white iron (cementite). This stage of inoculation can initially increase the graphite spheroidization rate to over 85%.

[0064] 3.3 Control of spheroidization reaction

[0065] Slowly pour qualified molten iron into the spheroidizing ladle through a chute to avoid excessive impact that could cause the spheroidizing agent to splash. Mg vapor will be generated during the spheroidizing reaction, which lasts for 8-10 minutes. During this time, cover the spheroidizing ladle opening with a graphite cap to reduce Mg vapor escape. After the reaction, check the temperature of the molten iron after spheroidizing. It needs to be controlled between 1420-1450℃. Too low a temperature will reduce fluidity, while too high a temperature will cause the inoculant to burn off. At the same time, use a sampling spoon to take a small amount of molten iron to prepare a rapid metallographic sample.

[0066] 3.4 Flow-based gestation

[0067] The amount of barium silicon inoculant added is 0.1-0.7% of the total mass of molten iron, and it is loaded into the in-flow inoculation device. The in-flow inoculation device is installed in the middle of the casting chute. When molten iron flows from the spheroidizing ladle into the casting chute, the in-flow inoculant falls evenly into the molten iron and flows into the mold at the same time as the molten iron. In-flow inoculation can supplement Si elements before the molten iron enters the mold, further refine the graphite spheroids, and at the same time inhibit the decline of graphite morphology during the casting process, so that the final graphite spheroidization rate is stable at more than 95%.

[0068] 4. Casting operation

[0069] 4.1 Preparation of mold and casting equipment

[0070] Sand casting is adopted, and the mold is a resin sand mold. The gating system is designed as a bottom pouring type. The bottom pouring type gating can make the molten iron fill the cavity smoothly and avoid air entrapment. Before casting, the mold is heated by a hot air gun to avoid casting cracks caused by contact between the cold mold and the high temperature molten iron. The preheating temperature is 80-100℃. At the same time, the mold cavity is checked for foreign matter.

[0071] 4.2 Casting Temperature Control

[0072] Infrared thermometers were used to detect the temperature of molten iron during the casting process: the initial pouring temperature was between 1350℃ and 1390℃, with the initial pouring temperature for small parts weighing less than 100kg between 1350℃ and 1380℃, and the initial pouring temperature for large parts weighing over 100kg between 1360℃ and 1390℃. This temperature ensures that the molten iron does not solidify in the gating system, avoiding incomplete pouring; the final pouring temperature was not lower than 1342℃, because there is heat loss in the molten iron during the casting process, and a low final pouring temperature will cause cold shut defects at the bottom of the casting; the temperature was recorded once after each mold was cast to ensure that the casting temperature of all molds met the above requirements.

[0073] 4.3 Casting speed and operation control

[0074] The casting speed is controlled by the tilt angle of the spheroidizing ladle: in the initial stage, the tilt angle is 10° and the flow rate is 30 kg / min to avoid overflow from the pouring cup; in the middle stage, the tilt angle is 20° and the flow rate is 50 kg / min to quickly fill the cavity and reduce oxidation; in the later stage, the tilt angle is 5° and the flow rate is 20 kg / min to slowly fill the cavity and avoid air entrapment at the top of the cavity; the flow should not be interrupted during the casting process to prevent delamination defects; if the process is paused, a small amount of molten iron should be kept flowing in the gating system; after each mold is finished casting, an insulating riser is placed on top of the pouring cup to reduce shrinkage defects at the top of the casting.

[0075] 5. Post-casting cleaning

[0076] 5.1 Cooling and Demolding

[0077] After casting, place the mold in a well-ventilated and dry place for natural cooling. The cooling time should be controlled within 24 hours. If the cooling rate is too slow, the graphite spheres will grow. If the cooling rate is too fast, thermal stress will be generated inside the casting, which will easily cause cracks. After cooling, demold the casting and check the surface of the casting to ensure that there are no obvious cracks, cold shuts, incomplete pouring, or other defects.

[0078] 5.2 Shot blasting

[0079] After demolding, the casting is fed into a crawler-type shot blasting machine. Shot blasting can remove oxide scale and residual sand particles from the surface of the casting. At the same time, the impact of the shot forms a work-hardened layer on the surface of the casting, which helps to improve the strength of the casting. After shot blasting, compressed air is used to blow away the steel shot residue on the surface of the casting.

[0080] 5.3 Polishing process

[0081] An angle grinder is used to grind the gating gates and flash of the casting: the gating gates are first cut off with a band saw and then ground until they are flush with the surface of the casting; the flash is ground to a width of ≤0.5mm; water cooling is used during the grinding process to avoid overheating and oxidation of the casting surface; after grinding, the dimensions of the casting are checked with a ruler to ensure that they meet the design requirements.

[0082] 6. Heat treatment process

[0083] 6.1 Furnace Loading Rules and Preparation

[0084] The heat treatment is carried out in a box-type resistance furnace. Before loading the furnace, the castings are classified: they are divided into large and small parts according to weight. If they are all small or large parts, they are loaded into the same furnace. If large and small parts are mixed, the large parts should be placed inside the furnace chamber. Since the heating rate inside is slower than outside, it can match the heat capacity of the large parts and avoid stress caused by the temperature difference between inside and outside. Castings must not be stacked in the furnace, and no castings should be placed within 100mm on both sides of the furnace chamber. This loading rule can control the temperature uniformity inside the furnace within ±5℃ and avoid fluctuations in the performance of the castings after heat treatment.

[0085] 6.2 Normalizing treatment

[0086] The heating rate of the resistance furnace after loading was set to 10℃ / min, raising it from room temperature to 900℃. The furnace temperature was recorded every 30 minutes during the heating process to ensure stable heating and avoid cracking of the casting due to excessively rapid heating. After the temperature reached 900℃, it was held for 6 hours. The purpose of normalizing treatment is to ensure uniform precipitation of pearlite inside the casting and to eliminate internal stress generated during casting. Holding for 6 hours ensures that the core of the casting reaches the normalizing temperature completely. After holding, the furnace door was opened, and the casting was removed and placed in the air to cool to room temperature.

[0087] 6.3 Quenching treatment

[0088] After normalizing, the casting was reloaded into the furnace. The heating rate of the resistance furnace was set to 8℃ / min, raising the temperature from room temperature to 860℃. After reaching 860℃, the temperature was held for 6 hours. The quenching temperature was chosen to be 860℃, which is lower than the upper limit of the austenitizing temperature, thus avoiding coarse grains. Holding for 6 hours ensured that the casting was fully austenitized, with the austenite grain size controlled below 50μm. After holding, the casting was quickly removed and quenched in circulating water at 20℃ for 5 minutes. Quenching can transform austenite into martensite, improving the strength of the casting, but it will lead to a decrease in toughness, which needs to be improved by subsequent tempering.

[0089] 6.4 Tempering treatment

[0090] The quenched castings are loaded into the furnace, and the resistance furnace is heated at a rate of 5℃ / min from room temperature to 560℃. Slow heating prevents cracking of the martensite structure due to a sudden temperature increase. After reaching 560℃, the temperature is held for 6 hours. The purpose of tempering is to partially decompose the martensite into tempered sorbite; 560℃ is the optimal tempering temperature. Temperatures higher than this will result in excessive strength loss, while temperatures lower than this will not adequately improve toughness. Holding for 6 hours ensures sufficient microstructure transformation. After holding, the resistance furnace power is turned off, allowing the castings to cool naturally to room temperature to avoid stress residue caused by rapid cooling. Natural cooling further releases internal stress in the castings, ultimately achieving a balance between strength and toughness.

[0091] Samples were taken from the heat-treated castings, and tensile test specimens with a size of Φ10mm×50mm were taken according to the GB / T1348-2022 "Ductile Iron Castings" standard. The tensile test was carried out using a universal testing machine.

[0092] Test results: The tensile strength is 820-850 MPa, the elongation is 11.5-13.5%, the yield strength is 650-690 MPa, and the hardness is HB250. This combination of properties is superior to that of conventional QT800-10. The main reasons are that the composite spheroidizing agent refines the graphite spheres, Sb-Cu microalloying controls the pearlite ratio, and the optimized heat treatment process balances the ratio of martensite to tempered sorbite.

[0093] In this invention, large parts are castings weighing over 100 kg, and small parts are castings weighing between 15 kg and 100 kg.

[0094] The present invention will now be described in detail through specific embodiments.

[0095] Example 1

[0096] 1. Raw material preparation and pretreatment

[0097] 1.1 Selection and Pretreatment of Basic Raw Materials

[0098] Pig iron: Foundry pig iron produced in Benxi, Liaoning Province is selected, with a composition of C 3.5%, Si 1.8%, Mn 0.2%, P 0.03%, and S 0.01%. During pretreatment, manual screening is used to remove surface rust, slag inclusions, and blocky impurities, and then it is processed into regular blocks with a side length of 50mm.

[0099] Scrap steel: High-manganese scrap steel from dismantled automobile beams is selected, with a composition of C 0.1%, Mn 1.2%, Si 0.2%, P 0.02%, and S 0.01%. During pretreatment, non-metallic impurities such as paint and rubber attached to the surface are removed, and then it is cut into blocks with a side length of 150mm using flame cutting equipment.

[0100] Recycled material: Select QT800-10 waste castings or casting surplus produced in the early stage of this process, remove the surface oxide scale by shot blasting equipment, and then crush them into blocks with a side length of 80mm.

[0101] 1.2 Basic Raw Material Ratio

[0102] Weigh the basic raw materials by mass percentage: 48% pig iron, 30% scrap steel, and 22% recycled material. The proportion design is based on the following: 48% pig iron can ensure that the carbon content of the molten iron meets the requirements for spheroidization, 30% high-manganese scrap steel can supplement Mn element to control the pearlite ratio, and 22% recycled material can balance the fluctuation of composition. This proportion can ensure strength while avoiding excessive carbon equivalent that causes graphite to float.

[0103] 1.3 Trace Element Preparation

[0104] Weigh two trace elements: weigh antimony (Sb) powder with a purity of 99.9% and accounting for 0.002% of the total mass of molten iron; weigh copper (Cu) wire with a purity of 99.5% and accounting for 0.015% of the total mass of molten iron; mix the Sb powder and ferrosilicon powder evenly at a mass ratio of 1:5, and cut the Cu wire into segments with a length of 50mm.

[0105] 2 Smelting process

[0106] 2.1 Smelting Equipment and Preliminary Preparations

[0107] A medium-frequency induction furnace is used. Before melting, the furnace lining is inspected to ensure that there are no cracks or peeling and that the thickness is 100mm. 2kg of graphite powder is added to the furnace, and then the power of the medium-frequency furnace is set to 80% to preheat the furnace lining for 30 minutes, and the preheating temperature is raised to 800℃.

[0108] 2.2 Feeding sequence and smelting control

[0109] Step 1: Add pig iron: Add the pretreated pig iron to the medium frequency furnace in 3 batches, with an interval of 5 minutes between each batch. Add the next batch only after the previous batch has completely melted.

[0110] Step 2: Add scrap steel and recycled materials: After the pig iron has completely melted, add scrap steel in two batches, stirring the molten iron for 1 minute after each batch to ensure that the scrap steel is completely submerged in the molten iron; after the scrap steel has completely melted, add recycled materials in the same batches, stirring for 1 minute after each batch.

[0111] Step 3: Adding trace elements: After the recycled material is completely melted, the premixed Sb-Si powder mixture is fed into the middle of the molten iron through a special feeding hopper and stirred for 2 minutes; then Cu wire is evenly sprinkled onto the surface of the molten iron, and the Cu wire is quickly melted and stirred for another 1 minute.

[0112] 2.3 Furnace temperature control

[0113] Increase the power of the induction furnace to 100% and raise the temperature of the molten iron to 1500℃, then hold for 10 minutes. Skim off the surface slag with a graphite spoon; check the temperature again before unloading to ensure it is stable at 1500℃.

[0114] 3. Spheroidization and Incubation Treatment

[0115] 3.1 Preparation of spheroidizing package

[0116] A 500kg acidic spheroidizing ladle is selected, and the composite spheroidizing agent is placed at the bottom of the spheroidizing ladle with a pre-set dam. The composite spheroidizing agent is a Mg-Ce-Y composite spheroidizing agent with the composition of 8% Mg, 2% Ce, 1% Y, 40% Si, and the remainder being Fe. The amount added is 1.4% of the total mass of molten iron.

[0117] 3.2 First pregnancy

[0118] A silicon-barium inoculant with a total mass of 0.5% of molten iron was applied to the surface of the composite spheroidizing agent. The silicon-barium inoculant consisted of 70% Si, 2% Ba, and the remainder Fe. Then, 2 kg of perlite was applied to the surface.

[0119] 3.3 Control of spheroidization reaction

[0120] The qualified molten iron is slowly poured into the spheroidizing ladle through a chute to avoid excessive impact from the molten iron, which would cause the spheroidizing agent to splash. Mg vapor will be generated during the spheroidizing reaction, and the reaction time will last for 8 minutes. During this time, the mouth of the spheroidizing ladle is covered with a graphite cap to reduce the escape of Mg vapor. After the reaction is completed, the temperature of the molten iron after spheroidizing is detected and controlled at 1420℃. At the same time, a small amount of molten iron is taken with a sampling spoon to prepare a rapid metallographic sample.

[0121] 3.4 Flow-based gestation

[0122] The amount of inoculant added is 0.3% of the total mass of molten iron, and it is loaded into the in-flow inoculation device; the in-flow inoculation device is installed in the middle of the casting chute.

[0123] 4. Casting operation

[0124] 4.1 Preparation of mold and casting equipment

[0125] Sand casting is adopted, and the mold is a resin sand mold. The gating system is designed as a bottom pouring type. The bottom pouring type gating can make the molten iron fill the cavity smoothly and avoid air entrapment. Before casting, the mold is heated by a hot air gun to avoid casting cracks caused by contact between the cold mold and the high temperature molten iron. The preheating temperature is 80℃. At the same time, the mold cavity is checked for foreign matter.

[0126] 4.2 Casting Temperature Control

[0127] Infrared thermometers were used to detect the temperature of molten iron during the casting process: the initial pouring temperature was 1370℃ and the final pouring temperature was 1342℃; the temperature was recorded once for each mold during the casting process to ensure that the casting temperature of all molds met the above requirements.

[0128] 4.3 Casting speed and operation control

[0129] The casting speed is controlled by the tilt angle of the spheroidizing ladle: 10° in the initial stage and 30 kg / min in the middle stage; 20° in the middle stage and 50 kg / min in the middle stage; and 5° in the later stage and 20 kg / min in the later stage. After each mold is finished casting, an insulating riser is placed on top of the pouring cup to reduce shrinkage defects at the top of the casting.

[0130] 5. Post-casting cleaning

[0131] 5.1 Cooling and Demolding

[0132] After casting, the mold is placed in a well-ventilated and dry place for natural cooling, with the cooling time controlled within 24 hours.

[0133] 5.2 Shot blasting

[0134] After demolding, the casting is fed into a crawler-type shot blasting machine. Shot blasting can remove oxide scale and residual sand particles from the surface of the casting. At the same time, the impact of the shot forms a work-hardened layer on the surface of the casting, which helps to improve the strength of the casting. After shot blasting, compressed air is used to blow away the steel shot residue on the surface of the casting.

[0135] 5.3 Polishing process

[0136] An angle grinder was used to grind the gating gates and flash of the casting: the gating gates were first cut off by a band saw and then ground until they were flush with the surface of the casting; the flash was ground to a width of 0.3mm; water cooling was used during the grinding process; after grinding, the dimensions of the casting were checked with a ruler to ensure that they met the design requirements.

[0137] 6. Heat treatment process

[0138] 6.1 Furnace Loading Rules and Preparation

[0139] The heat treatment is carried out in a box-type resistance furnace. Before loading the furnace, the castings are classified: they are divided into large and small parts according to weight. In this embodiment, all castings are small parts and are loaded into the same furnace. Castings must not be stacked in the furnace and no castings are placed within 100mm on both sides of the furnace.

[0140] 6.2 Normalizing treatment

[0141] The heating rate of the resistance furnace after loading was set to 10℃ / min, and the temperature was increased from room temperature to 900℃. The furnace temperature was recorded every 30 minutes during the heating process to ensure stable heating and avoid cracking of the casting due to excessive heating. After the temperature reached 900℃, it was held for 6 hours.

[0142] 6.3 Quenching treatment

[0143] After normalizing, the casting is reloaded into the furnace. The heating rate of the resistance furnace is set to 8℃ / min, and the temperature is raised from room temperature to 860℃. After reaching 860℃, the temperature is held for 6 hours. The quenching temperature is selected at 860℃, which is lower than the upper limit of the austenitizing temperature to avoid grain coarsening. Holding for 6 hours can make the casting completely austenitized. After holding, the casting is quickly taken out and placed in 20℃ circulating water for quenching for 5 minutes.

[0144] 6.4 Tempering treatment

[0145] The quenched castings are loaded into the furnace, and the heating rate of the resistance furnace is set to 5℃ / min, raising the temperature from room temperature to 560℃. Slow heating avoids cracking of the martensitic structure due to a sudden temperature increase. After reaching 560℃, the temperature is held for 6 hours. After holding, the power to the resistance furnace is turned off, allowing the castings to cool naturally to room temperature with the furnace, avoiding stress residue caused by rapid cooling. Natural cooling allows for further release of internal stress in the castings, ultimately achieving a balance between strength and toughness.

[0146] 7. Performance Testing and Result Analysis

[0147] Samples were taken from the heat-treated castings, and tensile test specimens with a size of Φ10mm×50mm were taken according to the GB / T1348-2022 "Ductile Iron Castings" standard. The tensile test was carried out using a universal testing machine.

[0148] Test results: The tensile strength is 820 MPa, the elongation is 11.5%, the yield strength is 650 MPa, and the hardness is HB250. This combination of properties is superior to that of conventional QT800-10.

[0149] Example 2

[0150] 1. Raw material preparation and pretreatment

[0151] 1.1 Selection and Pretreatment of Basic Raw Materials

[0152] Pig iron: Foundry pig iron produced in Benxi, Liaoning Province is selected, with a composition of C 3.65%, Si 1.9%, Mn 0.25%, P 0.035%, and S 0.015%. During pretreatment, manual screening is used to remove surface rust, slag inclusions, and blocky impurities, and then it is processed into regular blocks with a side length of 65mm.

[0153] Scrap steel: High-manganese scrap steel from dismantled automobile beams is selected, with a composition of C 0.15%, Mn 1.35%, Si 0.25%, P 0.025%, and S 0.015%. During pretreatment, non-metallic impurities such as paint and rubber attached to the surface are removed, and then it is cut into blocks with a side length of 175mm using flame cutting equipment.

[0154] Recycled material: Select QT800-10 waste castings or casting surplus produced in the early stage of this process, remove the surface oxide scale by shot blasting equipment, and then crush them into blocks with a side length of 100mm.

[0155] 1.2 Basic Raw Material Ratio

[0156] Weigh the basic raw materials by mass percentage: 50% pig iron, 30% scrap steel, and 20% recycled material; the proportion design is based on the following: 50% pig iron can ensure that the carbon content of the molten iron meets the requirements for spheroidization, 30% high-manganese scrap steel can supplement Mn element to control the pearlite ratio, and 20% recycled material can balance the fluctuation of composition.

[0157] 1.3 Trace Element Preparation

[0158] Weigh two trace elements: weigh antimony (Sb) powder with a purity of 99.9% and accounting for 0.005% of the total mass of molten iron; weigh copper (Cu) wire with a purity of 99.5% and accounting for 0.02% of the total mass of molten iron; mix the Sb powder and ferrosilicon powder evenly at a mass ratio of 1:5; cut the Cu wire into segments with a length of 50 mm.

[0159] 2 Smelting process

[0160] 2.1 Smelting Equipment and Preliminary Preparations

[0161] A medium-frequency induction furnace was used. Before melting, the furnace lining was inspected to ensure that there were no cracks or peeling and that the thickness was 110 mm. 2 kg of graphite powder was added to the furnace, and then the power of the medium-frequency furnace was set to 80% to preheat the furnace lining for 30 minutes, and the preheating temperature was raised to 825℃.

[0162] 2.2 Feeding sequence and smelting control

[0163] Step 1: Adding pig iron: Add the pretreated pig iron to the induction furnace in three batches, with a 5-minute interval between each batch. Add the next batch only after the previous batch has completely melted. The reason for adding pig iron first is that its regular, blocky shape allows it to spread quickly at the furnace bottom, forming a molten iron cushion layer. The subsequently added scrap steel and recycled materials can then be placed on this cushion layer, avoiding direct contact with the furnace lining. This operation reduces furnace lining wear and minimizes the risk of scrap steel bridging.

[0164] Step 2: Add scrap steel and recycled materials: After the pig iron has completely melted, add scrap steel in two batches, stirring the molten iron for 1 minute after each batch to ensure that the scrap steel is completely submerged in the molten iron; after the scrap steel has completely melted, add recycled materials in the same batches, stirring for 1 minute after each batch.

[0165] Step 3: Adding trace elements: After the recycled material is completely melted, the premixed Sb-Si powder mixture is fed into the middle of the molten iron through a special feeding hopper and stirred for 2 minutes; then Cu wire is evenly sprinkled onto the surface of the molten iron, and the Cu wire is quickly melted and stirred for another 1 minute.

[0166] 2.3 Furnace temperature control

[0167] Increase the power of the induction furnace to 100% and raise the temperature of the molten iron to 1525℃, then hold for 10 minutes. Skim off the surface slag with a graphite spoon; check the temperature again before unloading to ensure it is stable at 1525℃.

[0168] 3. Spheroidization and Incubation Treatment

[0169] 3.1 Preparation of spheroidizing package

[0170] A 500kg acidic spheroidizing ladle is selected, and the composite spheroidizing agent is placed at the bottom of the spheroidizing ladle with a pre-set dam. The composite spheroidizing agent is a Mg-Ce-Y composite spheroidizing agent with the following composition: Mg 9%, Ce 2.5%, Y 1.5%, Si 42.5%, and the remainder being Fe. The amount added is 1.5% of the total mass of molten iron.

[0171] 3.2 First pregnancy

[0172] A silicon-barium inoculant with a total mass of 0.6% of molten iron was applied to the surface of the composite spheroidizing agent. The silicon-barium inoculant had a composition of 72.5% Si, 2.5% Ba, and the remainder Fe. Then, 2 kg of perlite was applied to the surface.

[0173] 3.3 Control of spheroidization reaction

[0174] The qualified molten iron is slowly poured into the spheroidizing ladle through a chute to avoid excessive impact from the molten iron, which could cause the spheroidizing agent to splash. Mg vapor is generated during the spheroidizing reaction, which lasts for 9 minutes. During this time, the ladle opening is covered with a graphite cap to reduce the escape of Mg vapor. After the reaction, the temperature of the molten iron after spheroidizing is monitored and controlled at 1435℃. This temperature ensures the fluidity of the molten iron and avoids either a decrease in fluidity due to excessively low temperature or burn-off of the inoculant due to excessively high temperature. At the same time, a small amount of molten iron is taken with a sampling spoon to prepare a rapid metallographic sample.

[0175] 3.4 Flow-based gestation

[0176] The amount of inoculant added is 0.5% of the total mass of molten iron, and it is loaded into the in-flow inoculation device; the in-flow inoculation device is installed in the middle of the casting chute.

[0177] 4. Casting operation

[0178] 4.1 Preparation of mold and casting equipment

[0179] Sand casting is adopted, and the mold is a resin sand mold. The gating system is designed as a bottom pouring type. The bottom pouring type gating can make the molten iron fill the cavity smoothly and avoid air entrapment. Before casting, the mold is heated by a hot air gun to avoid casting cracks caused by contact between the cold mold and the high temperature molten iron. The preheating temperature is 90℃. At the same time, the mold cavity is checked for foreign matter.

[0180] 4.2 Casting Temperature Control

[0181] Infrared thermometers were used to detect the temperature of molten iron during the casting process: the initial pouring temperature was 1380℃, which ensured that the molten iron did not solidify in the gating system and prevented incomplete pouring; the final pouring temperature was 1350℃, which prevented cold shut defects at the bottom of the casting due to heat loss during the casting process; the temperature was recorded once after each mold was poured to ensure that the pouring temperature of all molds met the above requirements.

[0182] 4.3 Casting speed and operation control

[0183] The casting speed is controlled by the tilt angle of the spheroidizing ladle: in the initial stage, the tilt angle is 10° and the flow rate is 30 kg / min to avoid overflow from the pouring cup; in the middle stage, the tilt angle is 20° and the flow rate is 50 kg / min to quickly fill the cavity and reduce oxidation; in the later stage, the tilt angle is 5° and the flow rate is 20 kg / min to slowly fill the cavity and avoid air entrapment at the top of the cavity; the flow should not be interrupted during the casting process to prevent delamination defects; if the process is paused, a small amount of molten iron should be kept flowing in the gating system; after each mold is finished casting, an insulating riser is placed on top of the pouring cup to reduce shrinkage defects at the top of the casting.

[0184] 5. Post-casting cleaning

[0185] 5.1 Cooling and Demolding

[0186] After casting, the mold is placed in a well-ventilated and dry place for natural cooling, with the cooling time controlled within 24 hours.

[0187] 5.2 Shot blasting

[0188] After demolding, the casting is fed into a crawler-type shot blasting machine. Shot blasting can remove oxide scale and residual sand particles from the surface of the casting. At the same time, the impact of the shot forms a work-hardened layer on the surface of the casting, which helps to improve the strength of the casting. After shot blasting, compressed air is used to blow away the steel shot residue on the surface of the casting.

[0189] 5.3 Polishing process

[0190] An angle grinder was used to grind the gating gates and flash of the casting: the gating gates were first cut off by a band saw and then ground until they were flush with the surface of the casting; the flash was ground to a width of 0.4 mm; water cooling was used during the grinding process; after grinding, the dimensions of the casting were checked with a ruler to ensure that they met the design requirements.

[0191] 6. Heat treatment process

[0192] 6.1 Furnace Loading Rules and Preparation

[0193] The castings are classified into large and small parts according to weight before being loaded into the furnace. In this embodiment, all castings are large parts and are loaded into the same furnace. Castings must not be stacked in the furnace and no castings should be placed within 100mm on both sides of the furnace. This loading rule can control the temperature uniformity in the furnace within ±5℃ and avoid fluctuations in the performance of the castings after heat treatment.

[0194] 6.2 Normalizing treatment

[0195] The heating rate of the resistance furnace after loading was set to 10℃ / min, and the temperature was raised from room temperature to 900℃. The furnace temperature was recorded every 30 minutes during the heating process to ensure stable heating. After the temperature reached 900℃, it was held for 6 hours. After the holding period, the furnace door was opened and the castings were taken out and placed in the air to cool to room temperature.

[0196] 6.3 Quenching treatment

[0197] After normalizing, the castings were reloaded into the furnace. The heating rate of the resistance furnace was set to 8℃ / min, and the temperature was raised from room temperature to 860℃. After reaching 860℃, the temperature was held for 6 hours. After the holding period, the castings were quickly removed and placed in circulating water at 20℃ for quenching for 5 minutes.

[0198] 6.4 Tempering treatment

[0199] The quenched castings are loaded into the furnace, and the heating rate of the resistance furnace is set to 5℃ / min, raising the temperature from room temperature to 560℃. After the temperature reaches 560℃, it is held for 6 hours. After the holding period, the power supply to the resistance furnace is turned off, and the castings are allowed to cool naturally to room temperature with the furnace to avoid stress residue caused by rapid cooling.

[0200] 7. Performance Testing and Result Analysis

[0201] Samples were taken from the heat-treated castings, and tensile test specimens with a size of Φ10mm×50mm were taken according to the GB / T1348-2022 "Ductile Iron Castings" standard. The tensile test was carried out using a universal testing machine.

[0202] Test results: The tensile strength is 835 MPa, the elongation is 12.5%, the yield strength is 670 MPa, and the hardness is HB250. This combination of properties is superior to that of conventional QT800-10.

[0203] Example 3

[0204] 1. Raw material preparation and pretreatment

[0205] 1.1 Selection and Pretreatment of Basic Raw Materials

[0206] Pig iron: Foundry pig iron produced in Benxi, Liaoning Province is selected, with a composition of C 3.8%, Si 2.0%, Mn 0.3%, P 0.04%, and S 0.02%. During pretreatment, manual screening is used to remove surface rust, slag inclusions, and blocky impurities, and then it is processed into regular blocks with a side length of 80mm.

[0207] Scrap steel: High-manganese scrap steel from dismantled automobile beams is selected, with a composition of C 0.2%, Mn 1.5%, Si 0.3%, P 0.03%, and S 0.02%. During pretreatment, non-metallic impurities such as paint and rubber attached to the surface are removed, and then it is cut into blocks with a side length of 200mm using flame cutting equipment.

[0208] Recycled material: Select QT800-10 waste castings or casting surplus produced in the early stage of this process, remove the surface oxide scale by shot blasting equipment, and then crush them into blocks with a side length of 120mm.

[0209] 1.2 Basic Raw Material Ratio

[0210] Weigh the basic raw materials by mass percentage: 52% pig iron, 28% scrap steel, and 20% recycled material. The proportion design is based on the following: 52% pig iron can ensure that the carbon content of the molten iron meets the requirements for spheroidization, 28% high-manganese scrap steel can supplement Mn element to control the pearlite ratio, and 20% recycled material can balance the fluctuation of composition. This proportion can ensure strength while avoiding excessive carbon equivalent that would cause graphite to float.

[0211] 1.3 Trace Element Preparation

[0212] Weigh two trace elements: weigh antimony (Sb) powder with a purity of 99.9% and accounting for 0.008% of the total mass of molten iron; weigh copper (Cu) wire with a purity of 99.5% and accounting for 0.025% of the total mass of molten iron; mix the Sb powder and ferrosilicon powder evenly at a mass ratio of 1:5, and cut the Cu wire into segments with a length of 50 mm.

[0213] 2 Smelting process

[0214] 2.1 Smelting Equipment and Preliminary Preparations

[0215] A medium-frequency induction furnace was used. Before melting, the furnace lining was inspected. 2 kg of graphite powder was added to the furnace, and then the power of the medium-frequency furnace was set to 80% to preheat the furnace lining for 30 minutes. The preheating temperature was raised to 850℃.

[0216] 2.2 Feeding sequence and smelting control

[0217] Step 1: Adding pig iron: Add the pretreated pig iron to the induction furnace in three batches, with a 5-minute interval between each batch. Add the next batch only after the previous batch has completely melted. The reason for adding pig iron first is that its regular, blocky shape allows it to spread quickly at the furnace bottom, forming a molten iron cushion layer. The subsequently added scrap steel and recycled materials can then be placed on this cushion layer, avoiding direct contact with the furnace lining. This operation reduces furnace lining wear and minimizes the risk of scrap steel bridging.

[0218] Step 2: Add scrap steel and recycled materials: After the pig iron has completely melted, add scrap steel in two batches, stirring the molten iron for 1 minute after each batch to ensure that the scrap steel is completely submerged in the molten iron; after the scrap steel has completely melted, add recycled materials in the same batches, stirring for 1 minute after each batch.

[0219] Step 3: Adding trace elements: After the recycled material is completely melted, the premixed Sb-Si powder mixture is fed into the middle of the molten iron through a special feeding hopper and stirred for 2 minutes; then Cu wire is evenly sprinkled onto the surface of the molten iron, and the Cu wire is quickly melted and stirred for another 1 minute.

[0220] 2.3 Furnace temperature control

[0221] Increase the power of the induction furnace to 100% and raise the temperature of the molten iron to 1550℃, then hold for 10 minutes. Skim off the surface slag with a graphite spoon; check the temperature again before unloading to ensure it is stable at 1550℃.

[0222] 3. Spheroidization and Incubation Treatment

[0223] 3.1 Preparation of spheroidizing package

[0224] A 500kg acidic spheroidizing ladle is selected, and the composite spheroidizing agent is placed at the bottom of the spheroidizing ladle with a pre-set dam. The composite spheroidizing agent is a Mg-Ce-Y composite spheroidizing agent with the composition of 10% Mg, 3% Ce, 2% Y, 45% Si, and the remainder being Fe. The amount added is 1.6% of the total mass of molten iron.

[0225] 3.2 First pregnancy

[0226] A silicon-barium inoculant with a total mass of 0.7% of molten iron was applied to the surface of the composite spheroidizing agent. The silicon-barium inoculant consisted of 75% Si, 3% Ba, and the remainder Fe. Then, 2 kg of perlite was applied to the surface.

[0227] 3.3 Control of spheroidization reaction

[0228] The qualified molten iron is slowly poured into the spheroidizing ladle through a chute to avoid excessive impact from the molten iron, which would cause the spheroidizing agent to splash. Mg vapor will be generated during the spheroidizing reaction, and the reaction time will last for 10 minutes. During this time, the mouth of the spheroidizing ladle is covered with a graphite cap to reduce the escape of Mg vapor. After the reaction is completed, the temperature of the molten iron after spheroidizing is detected and controlled at 1450℃. At the same time, a small amount of molten iron is taken with a sampling spoon to prepare a rapid metallographic sample.

[0229] 3.4 Flow-based gestation

[0230] The inoculant is added at a rate of 0.5% of the total mass of molten iron and is loaded into the in-flow inoculation device. The in-flow inoculation device is installed in the middle of the casting chute. When molten iron flows from the spheroidizing ladle into the casting chute, the in-flow inoculant falls evenly into the molten iron and flows into the mold synchronously with the molten iron.

[0231] 4. Casting operation

[0232] 4.1 Preparation of mold and casting equipment

[0233] Sand casting is adopted, the mold is a resin sand mold, and the gating system is designed as bottom pouring. Before casting, the mold is heated by a hot air gun to a preheating temperature of 100℃, and at the same time, the mold cavity is checked for foreign matter.

[0234] 4.2 Casting Temperature Control

[0235] Infrared thermometers were used to detect the temperature of molten iron during the casting process: the initial pouring temperature was 1380℃, and the final pouring temperature was 1360℃. The temperature was recorded once for each mold during the casting process to ensure that the casting temperature of all molds met the above requirements.

[0236] 4.3 Casting speed and operation control

[0237] The casting speed is controlled by the tilt angle of the spheroidizing ladle: in the initial stage, the tilt angle is 10° and the flow rate is 30 kg / min to avoid overflow from the pouring cup; in the middle stage, the tilt angle is 20° and the flow rate is 50 kg / min to quickly fill the cavity and reduce oxidation; in the later stage, the tilt angle is 5° and the flow rate is 20 kg / min for slow filling. After each mold is finished casting, an insulating riser is placed on top of the pouring cup.

[0238] 5. Post-casting cleaning

[0239] 5.1 Cooling and Demolding

[0240] After casting is completed, place the mold in a well-ventilated and dry place for natural cooling. The cooling time is controlled within 24 hours. After cooling, demold the casting and inspect the surface of the casting.

[0241] 5.2 Shot blasting

[0242] After demolding, the casting is fed into a crawler-type shot blasting machine. Shot blasting can remove oxide scale and residual sand particles from the surface of the casting. At the same time, the impact of the shot forms a work-hardened layer on the surface of the casting, which helps to improve the strength of the casting. After shot blasting, compressed air is used to blow away the steel shot residue on the surface of the casting.

[0243] 5.3 Polishing process

[0244] An angle grinder is used to grind the gating gates and flash of the casting: the gating gates are first cut off by a band saw and then ground until they are flush with the surface of the casting; the flash is ground to a width of 0.5mm; water cooling is used during the grinding process, and the dimensions of the casting are checked by a ruler after grinding to ensure that they meet the design requirements.

[0245] 6. Heat treatment process

[0246] 6.1 Furnace Loading Rules and Preparation

[0247] A box-type resistance furnace is used for heat treatment. Before loading the furnace, the castings are classified into large and small parts according to weight. In this embodiment, large and small parts are mixed and large parts are placed inside the furnace. Castings must not be stacked in the furnace, and no castings are placed within 100mm on both sides of the furnace. This loading rule can control the temperature uniformity in the furnace within ±5℃ and avoid fluctuations in the performance of the castings after heat treatment.

[0248] 6.2 Normalizing treatment

[0249] The heating rate of the resistance furnace after loading was set to 10℃ / min, and the temperature was raised from room temperature to 900℃. The furnace temperature was recorded every 30 minutes during the heating process to ensure stable heating and avoid cracking of the casting due to excessive heating. After the temperature reached 900℃, it was held for 6 hours. After the holding period, the furnace door was opened and the casting was taken out and placed in the air to cool to room temperature.

[0250] 6.3 Quenching treatment

[0251] After normalizing, the castings were reloaded into the furnace. The heating rate of the resistance furnace was set to 8℃ / min, and the temperature was raised from room temperature to 860℃. After reaching 860℃, the temperature was held for 6 hours. After the holding period, the castings were quickly removed and placed in circulating water at 20℃ for quenching for 5 minutes. Quenching can transform austenite into martensite and improve the strength of the castings, but it will lead to a decrease in toughness, which needs to be improved by subsequent tempering.

[0252] 6.4 Tempering treatment

[0253] After quenching, the casting is loaded into the furnace. The heating rate of the resistance furnace is set to 5℃ / min, and the temperature is raised from room temperature to 560℃. Slow heating can prevent the martensitic structure from cracking due to a sudden temperature rise. After the temperature reaches 560℃, it is held for 6 hours. After the holding time is completed, the power supply of the resistance furnace is turned off, and the casting is allowed to cool naturally to room temperature with the furnace to avoid stress residue caused by rapid cooling.

[0254] 7. Performance Testing and Result Analysis

[0255] Samples were taken from the heat-treated castings, and tensile test specimens with a size of Φ10mm×50mm were taken according to the GB / T1348-2022 "Ductile Iron Castings" standard. The tensile test was carried out using a universal testing machine.

[0256] Test results: The tensile strength is 850 MPa, the elongation is 13.5%, the yield strength is 690 MPa, and the hardness is HB250. This combination of properties is superior to that of conventional QT800-10.

[0257] Example 4

[0258] The difference between this embodiment and Embodiment 1 is that the initial pouring temperature is 1350℃ and the final pouring temperature is 1342℃. All other operations remain the same.

[0259] Test results: tensile strength is 815 MPa, elongation is 11.2%, yield strength is 645 MPa, and hardness is HB250.

[0260] Example 5

[0261] The difference between this embodiment and Embodiment 1 is that the initial pouring temperature is 1380℃ and the final pouring temperature is 1353℃. All other operations remain the same.

[0262] Test results: tensile strength is 828 MPa, elongation is 12.0%, yield strength is 658 MPa, and hardness is HB250.

[0263] Example 6

[0264] The difference between this embodiment and Embodiment 2 is that the initial pouring temperature is 1360℃ and the final pouring temperature is 1353℃. All other operations remain the same.

[0265] Test results: tensile strength is 832 MPa, elongation is 12.4%, yield strength is 668 MPa, and hardness is HB250.

[0266] Example 7

[0267] The difference between this embodiment and Embodiment 2 is that the initial pouring temperature is 1390℃ and the final pouring temperature is 1357℃. All other operations remain the same.

[0268] Test results: Tensile strength is 834 MPa, elongation is 13%, yield strength is 678 MPa, and hardness is HB250.

[0269] This invention effectively solves the technical problems in the prior art, such as the difficulty of simultaneously achieving high strength and high elongation with conventional QT800-10, the tendency for uneven performance of thick cross-section castings, and the high cost and poor performance stability caused by unreasonable carbon equivalent control, single selection of spheroidizing agent / inoculant, or limited heat treatment parameters in traditional processes. In terms of the raw material system, by optimizing the basic raw material ratio and precisely controlling the addition form and amount of trace elements Sb and Cu, the carbon content of the molten iron is ensured to meet the spheroidization requirements to avoid graphite floating by using pig iron, while Mn is added by high-manganese scrap steel to control the pearlite ratio. Sb is used to inhibit ferrite formation and promote uniform pearlite precipitation, and Cu is used to enhance the ductility of ferrite through solid solution strengthening. At the same time, the introduction of recycled materials significantly reduces raw material costs and reduces compositional fluctuations. In terms of the smelting process, a medium-frequency induction furnace is used, and the furnace lining is preheated at 800-850℃. The process involves adding pig iron in batches, adding scrap steel and recycled materials in batches, and adding premixed trace elements in sequence, combined with a stirring process after each batch. This not only reduces the loss of the furnace lining caused by thermal shock and the risk of scrap steel bridging, but also avoids local component segregation in the molten iron, ensuring that the molten iron has good fluidity and slag removal effect at the tapping temperature of 1500-1550℃, laying a foundation for high-quality molten iron in subsequent processes. In the spheroidization and inoculation treatment stages, Mg-C is used. The e-Y composite spheroidizing agent replaces the traditional single Mg spheroidizing agent, combining a dual inoculation mode of primary inoculation and in-flow inoculation to significantly refine the size of graphite spheroids, inhibit spheroidization fading, and effectively prevent the formation of white cast iron. During casting, the resin sand mold bottom-pouring gate design, 80-100℃ mold preheating, and staged control of casting temperature and speed ensure stable filling of the mold cavity with molten iron, avoiding defects such as air entrapment, cold shuts, and shrinkage cavities. In the post-casting treatment stage, 24-hour natural cooling prevents cracks or abnormal growth of graphite spheroids in the casting due to improper cooling rate. Shot blasting removes surface impurities while forming a work-hardened layer to help improve strength. Grinding ensures the dimensional accuracy of the casting. The heat treatment process strictly follows the rules of classifying large and small parts for furnace loading, not stacking them in the furnace, and not placing anything within 100mm on both sides. With precise parameters for normalizing, quenching, and tempering, the temperature uniformity in the furnace is controlled within ±5℃, effectively eliminating casting internal stress and allowing austenite to fully transform into tempered sorbite, ultimately achieving a balance between strength and toughness. Testing revealed that the ductile iron castings prepared by this invention have a tensile strength of 820–850 MPa, an elongation of 11.5–13.5%, a yield strength of 650–690 MPa, and a hardness of HB250. Their mechanical properties are significantly superior to those of conventional QT800-10, and the process exhibits high stability, ensuring the uniformity of performance in thick-section castings. It balances high performance with low cost and possesses outstanding industrial application value.

[0270] Obviously, the above embodiments are merely illustrative examples for clear explanation and are not intended to limit the implementation. Those skilled in the art will recognize that other variations or modifications can be made based on the above description. It is neither necessary nor possible to exhaustively list all possible implementations here. However, obvious variations or modifications derived therefrom are still within the scope of protection of this invention.

Claims

1. A method for preparing high-strength, high-toughness ductile iron, characterized in that, Includes the following steps: Step 1: Weigh the basic raw materials as follows by mass percentage: 48-52% pig iron, 28-32% scrap steel, and 18-22% recycled material. Process the pig iron, scrap steel, and recycled material into blocks. Then weigh out antimony at 0.002-0.008% of the total mass of molten iron and copper at 0.015-0.025% of the total mass of molten iron. Step 2: Add pig iron into the electric furnace. After the pig iron has completely melted, add scrap steel and recycled materials. After the recycled materials have completely melted, add antimony and copper. Step 3: Transfer the molten iron to a spheroidizing ladle, add 1.4-1.6% of Mg-Ce-Y composite spheroidizing agent by weight of the total molten iron, and then add 0.5-0.7% of silicon-barium inoculant by weight of the total molten iron for the first inoculation; after the first inoculation is completed, load 0.1-0.7% of silicon-barium inoculant by weight of the total molten iron into an in-flow inoculation device for in-flow inoculation; Step 4: Casting. The initial casting temperature shall not be lower than 1350℃, and the final casting temperature shall not be lower than 1342℃. Step 5: After casting, cool, demold, shot blast, and polish. Step 6: The casting is placed in an electric resistance furnace and heated from room temperature to 900°C, held for 6 hours, and then cooled to room temperature to complete the normalizing process. After normalizing, the casting is heated from room temperature to 860°C and held for 6 hours. After holding, the casting is quickly removed and placed in 20°C circulating water for quenching. After quenching, the casting is placed in a furnace and heated from room temperature to 560°C and held for 6 hours for tempering. After tempering, the casting is allowed to cool naturally to room temperature. The scrap steel in step 1 contains C 0.1-0.2%, Mn 1.2-1.5%, Si≤0.3%, P≤0.03%, and S≤0.02%. When used, the antimony is mixed evenly with ferrosilicon powder at a mass ratio of 1:

5. The prepared ductile iron castings have a tensile strength of 820-850 MPa, an elongation of 11.5-13.5%, a yield strength of 650-690 MPa, and a hardness of HB250. The pig iron in step 1 contains 3.5-3.8% C, 1.8-2.0% Si, ≤0.3% Mn, ≤0.04% P, and ≤0.02% S. The composition of the Mg-Ce-Y composite spheroidizing agent is 8-10% Mg, 2-3% Ce, 1-2% Y, 40-45% Si, and the remainder is Fe; the amount added is 1.4-1.6% of the total mass of molten iron.

2. The method for preparing high-strength, high-toughness ductile iron according to claim 1, characterized in that, In step 1, the pig iron is derusted, has slag inclusions and blocky impurities removed, and is then processed into regular blocks with a side length of 50-80mm. The scrap steel is derusted of its surface impurities and then cut into blocks with a side length of 150-200mm using a flame cutting device. The recycled material is descaled and then crushed into blocks with a side length of 80-120mm.

3. The method for preparing high-strength, high-toughness ductile iron according to claim 1, characterized in that, In step 4, the casting speed is controlled by the tilt angle of the spheroidizing ladle: 10° in the initial stage; 20° in the middle stage; and 5° in the later stage.