High-strength cast iron waste steel recarburization smelting process

Abstract

The present application belongs to the technical field of cast iron smelting processing, and discloses a high-strength cast iron waste steel carbon-increasing smelting process method, which comprises the following steps: pretreatment of raw and auxiliary materials and preparation of equipment and tooling, batching and proportioning and weighing, electric furnace sectional smelting temperature control, furnace discharge and in-pot stream inoculation, cast pouring forming, and cast performance detection. The present application overcomes the high cost problem caused by the dependence of traditional high-strength gray cast iron on copper, chromium, molybdenum and other precious metal alloys, and builds a complete waste steel carbon-increasing smelting process without precious metal alloys, with parameter standardization, stable and controllable quality, and consideration of manufacturing cost, cast mechanical property and batch production adaptability, and has outstanding economic value and engineering practical value. The present application is suitable for thick-walled high-strength gray cast iron blank casting production of internal combustion engine flywheel, precision machine tool bed, heavy load box and the like, and is used for large-scale smelting and manufacturing of low-cost, high-quality and high-strength gray cast iron parts in the scene of medium-frequency induction electric furnace.

Description

Technical Field

[0001] This invention belongs to the field of cast iron smelting and processing technology, and relates to a smelting process method, specifically a high-strength cast iron scrap steel carbonization smelting process method. Background Technology

[0002] In the current large-scale production system of machinery manufacturing, automotive parts, and precision machine tool castings, the mainstream production scheme for high-strength gray cast iron parts (high-grade products such as HT300 and HT350) has long relied on full-volume alloying control. The industry's common logic is to stabilize the pearlitic matrix of the casting, refine the graphite structure, and improve the tensile strength and hardness by adding functional alloying elements such as copper, chromium, molybdenum, and nickel in batches to the molten iron, thereby meeting the mechanical performance standards of key components such as cylinder blocks, machine tool beds, and heavy-duty housings. However, this mature process has unavoidable economic shortcomings: copper, molybdenum, and nickel are all precious metallurgical raw materials with volatile commodity prices, and the procurement costs of various alloy irons have remained high for a long time. Coupled with the high-temperature burning loss of alloying elements during the smelting process and the redundant use of auxiliary materials due to precise composition control, the unit material cost of castings is directly increased. Industry test data shows that the comprehensive cost of metal materials per ton of finished castings produced using the traditional full alloying process generally exceeds 8,000 yuan. In a market environment where the profit of the casting industry is continuously compressed and the price of raw materials fluctuates frequently, this significantly erodes the profit margin of manufacturing enterprises. At the same time, it also gives rise to a chain of process defects such as difficulty in composition control, fluctuating scrap rate, and reduced subsequent processing performance, which have become the core bottleneck restricting the large-scale cost reduction and efficiency improvement of high-grade gray cast iron. The four existing patent documents address the cost pain points of the traditional full alloying process from four dimensions: low alloy microalloying, high proportion of scrap steel synthesis smelting, high carbon equivalent low-cost control, and new inoculation replacement alloy strengthening, clearly demonstrating the differentiated solutions for industry improvement technologies.

[0003] CN103088250A, "A High-Strength Gray Cast Iron with High Carbon Content and Low Alloying and Its Smelting Method," pioneered a cost-reduction route using a high carbon equivalent + trace chromium-tin composite low alloy, directly addressing the cost drawbacks of traditional processes that heavily rely on expensive alloys such as copper and molybdenum. Traditional alloying processes typically add 0.5%~0.6% copper and ferromolybdenum as needed to ensure tensile strength above 300MPa. This patent completely abandons expensive alloys like copper, molybdenum, and nickel, using only two low-cost trace alloys: 0.2%~0.5% chromium and 0.07%~0.12% tin. The furnace charge is a blend of scrap steel, pig iron, recycled materials, and retained molten iron. Silicon-barium inoculants refine graphite, and carbon raisers regulate the carbon equivalent to a high carbon range of 3.25%~3.35%. This high carbon equivalent improves the fluidity of the molten iron and reduces the sensitivity of the casting's cross-section, eliminating the need to compensate for the performance defects caused by low carbon equivalent with large amounts of expensive alloys. From a cost structure comparison perspective, the procurement cost of copper and molybdenum alloy auxiliary materials per ton of castings using traditional processes can reach 1200-1800 yuan, while this patent uses only trace amounts of chromium and tin, reducing the cost of alloy auxiliary materials to less than 300 yuan. At the same time, the high carbon ratio reduces shrinkage porosity and cracking defects in castings, lowers scrap losses, and further reduces unit costs. The tensile strength of the finished products in the three sets of examples is stable at 305-320 N / mm², which is completely comparable to the mechanical properties of traditional fully alloyed HT300 castings. This proves that the low trace amount alloy scheme can replace the high-cost multi-element precious alloy system, solving the core problem of high cost of traditional processes from the source of raw material formulation.

[0004] CN103540701A, "Smelting Process Applicable to Ultra-High Strength Gray Cast Iron", focuses on the cost pain point of traditional alloying of HT350 ultra-high strength gray cast iron. Traditional mass-produced HT350 castings are usually equipped with a chromium and copper dual alloy system, with copper added at 0.50%~0.60% and chromium at 0.35%~0.45%. The simultaneous use of two high-priced alloys significantly increases material costs. Although the patent retains chromium and copper alloying elements, it significantly reduces the ineffective loss of alloying elements through smelting process reconstruction, and reduces the waste of alloying auxiliary materials from the process end: the furnace charge is completely replaced by scrap steel and recycled materials instead of expensive new pig iron, and low-nitrogen graphite carburizing agent and silicon carbide are used to replace ferrosilicon for carbon and silicon control, eliminating the alloy burn-off problem caused by traditional batch feeding; the smelting stage is set to high temperature superheating and holding at 1550~1580℃ for 3~5 minutes to eliminate the heritability of the furnace charge, refine the graphite, and improve the utilization rate of alloying elements. Under the same strength, the amount of alloying added can be reduced by more than 20%; the tapping process adopts the in-flow barium silicon inoculant added in batches to avoid the uneven structure and waste of alloy efficiency caused by the one-time addition of inoculant. Compared with traditional processes, this solution reduces the proportion of newly produced pig iron procurement on the one hand, and reduces the redundancy of copper-chromium alloy input on the other hand, thus doubly compressing the cost of metal materials. Even when producing HT350 castings with higher tensile strength, the material cost per ton of castings can be reduced by more than a thousand yuan compared with the traditional all-alloy process. At the same time, high-temperature refining improves the purity of molten iron, reduces scrap caused by composition fluctuations, and indirectly reduces the overall production cost.

[0005] CN103882279A, "A Smelting Method for High-Strength Gray Cast Iron Parts," proposes a low-cost route using less alloy, more scrap steel, and novel inoculation strengthening techniques. This directly avoids the traditional design approach that relies on precious metals such as copper and molybdenum, completely reducing the input of expensive alloys at the formulation level. Traditional high-strength gray cast iron smelting generally relies on copper to stabilize pearlite and molybdenum to enhance strength. This patent, however, does not add any precious alloying elements such as copper, molybdenum, or nickel throughout the entire process. It relies solely on 0.8%~1% ferromanganese base alloy for matrix strengthening. The furnace charge consists of 60% high-proportion Q235 scrap steel, 10% low-grade Q10 pig iron, and 30% recycled material. Silicon carbide pretreatment of the molten iron and the use of silicon-zirconium inoculant optimize the graphite morphology, achieving a metallographic structure with over 80% uniform type A graphite. The finished product has a stable tensile strength of 276~277 MPa, meeting the requirements for medium- and high-strength gray cast iron. From a cost perspective, the unit price of scrap steel is much lower than that of pig iron and copper-molybdenum alloy. The 60% high scrap steel ratio significantly reduces the cost of main materials. Eliminating copper-molybdenum alloy directly saves hundreds to thousands of yuan per ton in alloy auxiliary material expenses. Traditional processes require additional costs such as precise temperature control and multiple spectral detection to match copper-molybdenum alloy smelting. This patent simplifies the alloy management process, reduces labor and testing material consumption, and alleviates the industry problem of excessively high costs in traditional full alloying processes from multiple dimensions. It is especially suitable for cost-reduction production scenarios of small and medium batches of medium-strength castings.

[0006] CN109898010A, "An Electric Furnace Melting Technology for High-Strength Gray Cast Iron Machine Tool Castings," addresses the dual shortcomings of traditional low-carbon equivalent + high-alloy processes for high-end machine tool castings. It proposes an improved scheme using high-carbon equivalent, low-chromium single microalloying, and two-stage inoculation to solve the dual cost burden of "high-alloy input + additional costs derived from low-carbon equivalent" in traditional processes. Traditional HT300 machine tool castings generally use low-carbon equivalent formulations. To compensate for the insufficient strength and stiffness caused by low carbon content, large amounts of copper, molybdenum, and chromium composite alloys must be added. Furthermore, low-carbon equivalent molten iron has poor fluidity, high shrinkage stress in the castings, and is prone to deformation and shrinkage porosity defects. Subsequent rework and machining losses further increase the overall cost. This patent elevates the carbon equivalent to a high-carbon range of 3.8% on average, adding only 0.2%~0.3% inexpensive chromium as a single microalloy, eliminating the need for expensive alloys such as copper, molybdenum, and nickel. It relies on high-temperature overheating at 1500~1530℃ and two-stage in-flow silicon-barium inoculation to refine graphite and eutectic clusters. The high carbon equivalent significantly improves the filling capacity of molten iron, reducing shrinkage cavities, deformation, and scrap rates in castings, lowering machining cutting resistance, and simultaneously reducing hidden costs such as tool wear and rework time. Traditional machine tool castings cost over 8000 yuan per ton of metal, with the core increase coming from copper-molybdenum alloys and defect losses. This patent uses only a trace amount of chromium alloy, with the main material consisting of over 50% scrap steel, resulting in a 15%~25% reduction in overall material costs. Simultaneously, the castings exhibit superior dimensional stability and vibration damping performance compared to traditional all-alloy low-carbon equivalent products, balancing cost control with the performance requirements of high-end machine tool castings, providing a low-cost alternative process for high-value-added, high-strength castings.

[0007] A review of the technological innovation logic of the four patents reveals that existing improvement technologies in the industry all focus on overcoming the core pain points of traditional full alloying processes: Traditional processes rely heavily on expensive alloys such as copper, molybdenum, and nickel to achieve high strength, which is the fundamental reason why the cost of metal materials per ton of casting exceeds 8,000 yuan. Furthermore, the combination of low carbon equivalent and high alloys also generates hidden costs such as casting defects and processing losses, further amplifying the production burden on enterprises. The four patents, however, employ four technical paths—"micro-volume low-cost chromium-tin composite microalloying," "process optimization to reduce alloy loss," "complete elimination of expensive alloys," and "high carbon equivalent to reduce alloy dependence"—to significantly reduce the input of expensive alloy auxiliary materials while ensuring that the tensile strength, hardness, and metallographic structure of castings meet standards. Simultaneously, they optimize furnace charge structure, melting temperature control, and inoculation processes, reducing the cost of main materials and scrap losses. These patents address the common industry problem of high costs associated with traditional alloying processes from the entire process of formulation, melting, and post-processing, providing casting enterprises with diversified and feasible technological transformation directions for balancing product mechanical properties and production costs. Summary of the Invention

[0008] To address the shortcomings of existing technologies, this invention aims to provide a high-strength cast iron scrap steel carbon-enhancing smelting process, which completely eliminates the addition of high-priced alloying elements such as copper, chromium, and molybdenum, significantly reduces the production cost of casting metal materials, stably optimizes the metallographic structure and mechanical properties of castings, avoids the problem of high material cost per ton of castings in traditional alloying processes, and simultaneously establishes a standardized and implementable full-process smelting control system.

[0009] To achieve the above objectives, the technical solution adopted by the present invention is as follows: A high-strength cast iron scrap steel carbonization smelting process includes the following steps: S1. Pre-treatment of raw and auxiliary materials and preparation of equipment and tooling: Select medium-sized low-carbon scrap steel conforming to GB4223-2004 standard, use JB / T14236-2023 graphite electrode particles as carbon raiser, use homogeneous casting risers as remelting material, FeSi75 ferrosilicon, FeMn82C1.0 ferromanganese, and BS-I type barium silicon as inoculant; remove paint, rust, oil, and mud from the scrap steel surface, and remove sealed pipes and containers; place the inoculant in a 300-350℃ oven for drying and holding for 1-2 hours; check the medium-frequency electric furnace, thermocouple temperature measuring instrument, direct-reading spectrometer, and the integrity of the drying and preheating ladle, clean the furnace wall and taphole residue, and bake the ladle until dark red.

[0010] S2. Ingredient proportioning and weighing: Materials are prepared according to weight percentages; when producing HT300 cast iron, scrap steel: 60-65%, remelted material: 35-40%, graphite recarburizer: 2-5%, FeSi75 ferrosilicon: 0.4-0.8%, FeMn82C1.0 ferromanganese: 0.6-1.0%, BS-I inoculant: 0.6-0.8%; when producing HT350 cast iron, scrap steel: 65-70%, remelted material: 30-35%, graphite recarburizer: 3-8%, FeSi75 ferrosilicon: 0.5-1.0%, FeMn82C1.0 ferromanganese: 0.8-1.2%, BS-I inoculant: 0.7-0.9%; no ferrochrome, electrolytic copper, or ferromolybdenum are added throughout the process.

[0011] S3. Electric furnace segmented melting temperature control: The recycled materials, graphite recarburizer, and low-carbon scrap steel were fed into the medium-frequency electric furnace in sequence, with the materials added as they melted at low temperatures. After the furnace charge was completely melted, the temperature was raised to 1400-1450℃, and the composition of the molten iron was analyzed using a direct-reading spectrometer. HT300 molten iron with a wall thickness of 30-40mm: C: 3.1-3.3%, Si in the furnace: 1.2-1.5%, Mn: 0.9-1.1%, P≤0.05%, S≤0.05%; For HT300 molten iron with a wall thickness ≥40mm, the following parameters should be controlled: C: 3.0~3.2%, Si in the furnace: 1.1~1.3%, Mn: 0.9~1.2%, P≤0.05%, S≤0.05%; HT350 molten iron with a wall thickness of 30-40mm: C: 3.0-3.2%, Si in the furnace: 1.1-1.3%, Mn: 1.0-1.2%, P≤0.05%, S≤0.05%; For HT350 molten iron with a wall thickness ≥40mm, the following parameters should be controlled: C: 2.9~3.2%, Si in the furnace: 1.0~1.2%, Mn: 1.0~1.2%, P≤0.05%, S≤0.05%; If the composition does not meet the standard, ferrosilicon, ferromanganese and carbon raiser should be added as needed in the 1450-1500℃ range to adjust, and a second sample should be taken and retested until the composition meets the standard. After the composition is qualified, heat to 1500-1510℃, apply a covering agent, turn off the power and let stand for 3-8 minutes, and remove the floating slag and impurities.

[0012] S4. Baking and subsequent incubation within the bread: The electric furnace is tilted to tap iron, and the tapping temperature is controlled at 1490-1500℃. BS-I barium silicon inoculant is added in the ladle using the in-ladle pouring method, ensuring a smooth and uninterrupted flow of molten iron. After tapping, the molten iron is stirred, and slag is thoroughly removed a second time. The silicon content of the molten iron in the ladle after inoculation is tested before the furnace. The final silicon content of the casting is controlled at 1.5-1.8%, and the silicon increase during inoculation is 0.40-0.53%.

[0013] S5. Casting and molding: Once the molten iron reaches the specified temperature, it is poured. During the pouring process, slag is continuously removed, ignition is initiated, and liquid is replenished to ensure that the pouring cup is continuously filled with molten iron without interruption or splashing.

[0014] S6. Casting performance testing: Simultaneous casting of standard tensile test specimens was performed. After the casting cooled and the sand was removed, body specimens were prepared to test tensile strength, Brinell hardness, and metallographic structure. HT300 had a tensile strength ≥300MPa and a hardness of 210~240HB; HT350 had a tensile strength ≥350MPa. The metallographic structure was mainly composed of uniform type A graphite, without defects such as white iron, shrinkage porosity, or gas pores. If the performance was not up to standard, the batching ratio, melting temperature control, and inoculant addition were adjusted and the material was remelted.

[0015] As a limitation of the present invention, the graphite electrode particles in S1 have a carbon-reinforcing agent particle size of 0.3-5 mm, a fixed carbon content of ≥98.5%, a sulfur content of ≤0.3%, and a carbon absorption rate of greater than 85%.

[0016] As another limitation of the present invention, the temperature range for adding ferromanganese and ferrosilicon in S3 is 1350 to 1400°C.

[0017] As a third limitation of the present invention, the particle size of BS-I barium silicon inoculant in S4 is 3-10 mm, and the composition is Si: 55-60%, Ca: 1-2%, Ba: 4-6%.

[0018] As a fourth limitation of the present invention, the inner wall of the ladle and the spout of the S5 pouring ladle are free of sand, water, and rust before pouring, and are kept dry throughout the process.

[0019] By adopting the above-described technical solution, the beneficial effects achieved by this invention compared to the prior art are as follows: (1) This invention does not add precious metal alloy auxiliary materials such as copper, chromium, and molybdenum throughout the process. It relies entirely on a large proportion of low-carbon scrap steel combined with high-absorption graphite carbonizer to control the carbon equivalent, eliminating the expenditure on high-priced alloy procurement. The cost of metal materials per ton of casting is significantly lower than RMB 8,000, greatly reducing the raw material production cost of casting enterprises.

[0020] (2) The present invention is equipped with standardized furnace charge quality standards, fixed batching ratios, segmented precise temperature control ranges, and standardized in-package incubation processes, forming a complete set of implementable process parameters, solving the problems of discrete technical parameters and difficult production control in existing technologies. The process is highly replicable and suitable for stable batch production.

[0021] (3) The present invention uses high temperature static purification of molten iron, fully floats to remove slag impurities, and is combined with preheated and dried silicon barium inoculant to inoculate during tapping, which refines the graphite structure. The casting is mainly composed of uniform type A graphite, without white iron, and the scrap rate of shrinkage porosity, slag inclusions and deformation defects is greatly reduced. The tensile strength and hardness of the casting are stable and meet the technical requirements of HT300 and HT350 high-strength gray cast iron.

[0022] (4) This invention utilizes scrap steel and recycled materials in a high proportion, reducing the consumption of new pig iron. The raw materials are widely available and easy to obtain. At the same time, it simplifies the alloy control process before the furnace, reduces the manpower hours for spectral detection and auxiliary material control, and further reduces the overall hidden production cost.

[0023] In summary, this invention completely overcomes the high cost pain point caused by the reliance on copper, chromium, and molybdenum precious metal alloys in traditional high-strength gray cast iron. It constructs a complete set of scrap steel carbonization smelting processes that are free of precious metal alloys, have standardized parameters, and have stable and controllable quality. It takes into account manufacturing costs, casting mechanical properties, and mass production adaptability, and has outstanding economic and engineering practical value.

[0024] This invention is applicable to the casting production of HT300 and HT350 grade thick-walled high-strength gray cast iron blanks for internal combustion engine flywheels, precision machine tool beds, heavy-duty housings, etc., and is used for the large-scale smelting and manufacturing of low-cost, high-quality, high-strength gray cast iron parts in medium-frequency induction furnace scenarios. Detailed Implementation

[0025] The preferred embodiments of the present invention will be described below. It should be understood that the preferred embodiments described herein are for illustrative purposes only and are not intended to limit the scope of the invention.

[0026] Example 1: A process for carbonizing and smelting scrap steel for high-strength cast iron parts This embodiment relates to a high-strength cast iron scrap steel carbon-enhancing smelting process, taking the smelting of HT3005# flywheel castings as an example. The wall thickness of the casting is 45mm, and a 0.5t medium-frequency electric furnace is used. The specific steps include: S1. Pretreatment of raw and auxiliary materials: Select 202A low carbon scrap steel and clean the surface oil and rust; use graphite carburizing agent with a particle size of 0.5-3mm; dry and keep warm at 320℃ for 1.5h with BS-Ⅰ inoculant; clean the furnace wall residue in a 0.5t medium frequency electric furnace and bake the ladle until it turns dark red.

[0027] S2. Batching and weighing (total molten iron 500kg): 300kg low-carbon scrap steel, 200kg HT300 remelting material, 11kg graphite carbonizer, 2kg FeSi75 ferrosilicon, 3kg FeMn82C1.0 ferromanganese, 3kg BS-Ⅰ inoculant, without adding ferrochrome, copper, or ferromolybdenum.

[0028] S3. Electric furnace smelting temperature control: Feed the recycled material in the order of → carbon raiser → scrap steel and melt until clean. Take a sample for spectral analysis at 1420℃. Control the composition of the molten iron: C: 3.0~3.2%, Si in the furnace: 1.1~1.3%, Mn: 0.9~1.1%, P≤0.05%, S≤0.04%. After the composition is qualified, raise the temperature to 1505℃, sprinkle a covering agent and let it stand for 6 minutes, and remove the slag.

[0029] S4. Tapping and In-flow Inoculation: The tapping temperature of the electric furnace is 1495℃. During the tapping process, 3kg of dry BS-Ⅰ inoculant is added in-flow. After tapping, the casting is stirred and slag is removed. After inoculation, the silicon content of the casting is 1.72%.

[0030] S5. Casting and Molding: The flywheel mold is cast smoothly and continuously, with continuous ignition and slag removal during the casting process.

[0031] S6. Performance testing: The tensile strength of the single-cast sample is 312MPa, the Brinell hardness is 225HB, the metallographic graphite is uniform and fine, and there are no white iron or shrinkage defects, which meets the technical requirements of HT300.

[0032] Example 2: A process for carbonizing and smelting scrap steel for high-strength cast iron parts This embodiment relates to a high-strength cast iron scrap steel carbon-enhancing smelting process, taking the smelting of an HT350 heavy-duty machine tool housing casting as an example. The casting has a wall thickness of 35mm and is smelted using a 1t medium-frequency electric furnace. The specific steps include: S1. Pretreatment of raw and auxiliary materials: cleaning and removing impurities from 203A low-carbon scrap steel; graphite carbonizer with a particle size of 0.3-3mm; drying and holding BS-Ⅰ inoculant at 330℃ for 2 hours; overhauling and cleaning a 1t medium-frequency electric furnace, and fully baking the double ladles.

[0033] S2. Batching and weighing (total molten iron 1000kg): 680kg low-carbon scrap steel, 320kg HT350 remelting material, 6kg graphite recarburizer, 8kg FeSi75 ferrosilicon, 10kg FeMn82C1.0 ferromanganese, 8kg BS-Ⅰ inoculant, with no chromium, copper, or molybdenum alloys added.

[0034] S3. Electric furnace smelting temperature control: After layered feeding and melting, the temperature is raised to 1430℃ and a sample is taken; the composition of molten iron is C: 3.05%, Si in the furnace: 1.22%, Mn: 1.12%, P: 0.042%, S: 0.045%; the temperature is raised to 1510℃ and left to stand for 7 minutes, then the slag is removed and purified.

[0035] S4. Tapping and Inoculation Treatment: The tapping temperature is 1498℃, and an inoculant is added with the flow. After inoculation, the casting contains 1.65% silicon.

[0036] S5. Casting and molding: Continuous and uninterrupted casting of the box mold.

[0037] S6. Performance testing: The sample has a tensile strength of 365MPa, a hardness of 232HB, a pearlite content of 98% in the matrix, no casting defects, and meets the HT350 standard.

[0038] Comparative example (traditional copper-chromium alloying process) The same HT300 flywheel castings were prepared with 0.3% ferrochrome and 0.55% electrolytic copper added to the batch, and the remaining furnace charge ratios were the same as in Example 1. The cost of metal materials per ton of castings was 8,300 yuan, the tensile strength of the sample was 308 MPa, the castings had local shrinkage defects, the scrap rate was 12% higher than in Example 1 of this invention, and the overall production cost was significantly higher than that of this invention.

[0039] It should be noted that the above description is merely a preferred embodiment of the present invention and is not intended to limit the present invention. Although the present invention has been described in detail with reference to the above embodiments, those skilled in the art can still modify the technical solutions described in the above embodiments or make equivalent substitutions for some of the technical features. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A process for carbonizing and smelting scrap steel for high-strength cast iron parts, characterized in that: Specifically, the following steps are included: S1. Pre-treatment of raw and auxiliary materials and preparation of equipment and tooling: Select scrap steel, carbon raiser, recycled material, FeSi75 ferrosilicon, FeMn82C1.0 ferromanganese, and inoculant; remove paint, rust, oil, and mud from the surface of the scrap steel, and remove sealed pipes and containers; place the inoculant in an oven for drying and heat preservation; check the integrity of the medium-frequency electric furnace, thermocouple thermometer, direct-reading spectrometer, and preheating ladle; clean the furnace wall and taphole residue; and bake the ladle until it turns dark red. S2. Ingredient proportioning and weighing: Materials are prepared according to weight percentage; when producing HT300 cast iron, scrap steel: 60-65%, recycled material: 35-40%, carbon raiser: 2-5%, FeSi75 ferrosilicon: 0.4-0.8%, FeMn82C1.0 ferromanganese: 0.6-1.0%, BS-Ⅰ inoculant: 0.6-0.8%; When producing HT350 cast iron, scrap steel: 65-70%, recycled material: 30-35%, carbon raiser: 3-8%, FeSi75 ferrosilicon: 0.5~1.0%, FeMn82C1.0 manganese iron: 0.8-1.2%; inoculant: 0.7-0.9%; no chromium iron, electrolytic copper, or molybdenum iron added throughout the entire process; S3. Electric furnace segmented melting temperature control: The furnace charge is fed into the medium-frequency electric furnace in the following order: recycled material, carbon raiser, and low-carbon scrap steel. The charge is added as it melts at low temperatures. After the furnace charge is completely melted, the temperature is raised to 1400-1450℃. The composition of the molten iron is then analyzed by a direct-reading spectrometer. If the composition does not meet the standard, when the temperature rises to the range of 1450-1500℃, add ferrosilicon, ferromanganese and carbon raiser to adjust the chemical composition, and take a second sample for retesting until the composition meets the standard. After the molten iron in the furnace has met the required composition, the temperature is raised to 1500-1510℃, a covering agent is applied, the power is turned off and the furnace is left to stand for 3-8 minutes, and the floating slag and impurities are removed. S4. Baking and subsequent incubation within the bread: The electric furnace is tilted to tap iron, and the inoculant is added in the ladle using the infusion method, ensuring a smooth and uninterrupted flow of molten iron. After tapping, the molten iron is stirred, and slag is thoroughly removed a second time. The silicon content of the molten iron in the ladle after inoculation is tested before the furnace. The final silicon content of the casting is controlled at 1.5-1.8%, and the silicon increase during inoculation is 0.40-0.53%. S5. Casting and molding: Once the molten iron reaches the specified temperature, it is poured. During the pouring process, slag is continuously removed, ignition is initiated, and liquid is replenished to ensure that the pouring cup is continuously filled with molten iron without interruption or splashing. S6. Casting performance testing: Simultaneous casting of standard tensile test specimens was performed. After the casting cooled and the sand was removed, body specimens were prepared to test the tensile strength, Brinell hardness, and metallographic structure. HT300 had a tensile strength ≥300MPa and a hardness of 210~240HB; HT350 had a tensile strength ≥350MPa. The metallographic structure was mainly composed of uniform type A graphite, without defects such as white iron, shrinkage porosity, or gas pores. If the performance was not up to standard, the batching ratio, melting temperature control, and inoculant addition were adjusted and the material was remelted.

2. The high-strength cast iron scrap steel carbonization smelting process according to claim 1, characterized in that: The scrap steel mentioned in S1 is medium-sized low-carbon scrap steel in block size that conforms to GB4223-2004 standard; the carbon raiser is graphite electrode particles of JB / T14236-2023; the remelting material is the riser and gating system of homogeneous castings; and the inoculant is BS-I type silicon-barium alloy inoculant.

3. The high-strength cast iron scrap steel carbonization smelting process according to claim 1, characterized in that: The drying temperature of the drying oven described in S1 is 300~350℃, and the drying time is 1~2h.

4. The high-strength cast iron scrap steel carbonization smelting process according to claim 1, characterized in that: The standards for detecting the composition of molten iron in S3 are divided into: Composition control of HT300 molten iron with a wall thickness of 30-40mm: C: 3.1-3.3%, Si in furnace: 1.2-1.5%, Mn: 0.9-1.1%, P≤0.05%, S≤0.05%; Composition control of HT300 molten iron with a wall thickness ≥40mm: C: 3.0~3.2%, Si in furnace: 1.1~1.3%, Mn: 0.9~1.2%, P≤0.05%, S≤0.05%; Composition control of HT350 molten iron with a wall thickness of 30-40mm: C: 3.0-3.2%, Si in furnace: 1.1-1.3%, Mn: 1.0-1.2%, P≤0.05%, S≤0.05%; Composition control of HT350 molten iron with wall thickness ≥40mm: C: 2.9~3.2%, Si in furnace: 1.0~1.2%, Mn: 1.0~1.2%, P≤0.05%, S≤0.05%.

5. The high-strength cast iron scrap steel carbonization smelting process according to claim 1, characterized in that: In S4, the tapping temperature is 1490–1500℃.

6. The high-strength cast iron scrap steel carbonization smelting process according to claim 1, characterized in that: The graphite electrode particles in S1 have a carbon raiser particle size of 0.3-5 mm, a fixed carbon content of ≥98.5%, a sulfur content of ≤0.3%, and a carbon absorption rate of greater than 85%.

7. The high-strength cast iron scrap steel carbonization smelting process according to claim 1, characterized in that: The addition temperature range for ferromanganese and ferrosilicon in S3 is 1350–1400℃.

8. The high-strength cast iron scrap steel carbonization smelting process according to claim 2, characterized in that: The particle size of BS-Ⅰ barium silicon inoculant in S4 is 3-10 mm, and the composition by mass fraction is Si: 55-60%, Ca: 1-2%, Ba: 4-6%.

9. The high-strength cast iron scrap steel carbonization smelting process according to claim 3, characterized in that: Before pouring S5, the inner wall of the ladle and the spout must be free of sand, water, and rust, and the entire process must be kept dry.