Ductile cast iron water smelting method based on deep desulfurization and dephosphorization pretreatment

Abstract

The invention relates to the technical field of ferrous metallurgy and casting, in particular to a molten spheroidal graphite cast iron smelting method based on deep desulfurization and dephosphorization pretreatment. According to the method, common blast furnace molten iron is used as a raw material, and oxidation slagging initial refining and composite solvent deep purification are sequentially carried out in a steelmaking converter. In the initial refining, deep dephosphorization is carried out under high-temperature and high-alkalinity conditions; then, a specific composite solvent is added, and deep desulfurization and anti-graphitization element removal are synchronously achieved. The carbon content in the molten iron is controlled to be 2.0%-3.8%, the phosphorus content is smaller than or equal to 0.05%, the sulfur content is smaller than or equal to 0.02%, and the total amount of anti-graphitization elements is The obtained molten iron can be directly used for nodular cast iron production or casting into high-quality pig iron blocks after silicon is increased. The method completes molten iron purification in the converter at a time, and is short in process, high in efficiency, low in cost and particularly suitable for large-scale production of high-quality nodular cast iron.

Description

Technical Field

[0001] This invention relates to the field of iron and steel metallurgy and casting technology, and more specifically, to a method for smelting ductile iron based on deep desulfurization and phosphorus pretreatment. Background Technology

[0002] The core performance of ductile iron lies in the spheroidization of graphite within it, which requires extremely high purity of the raw material molten iron. Specifically, sulfur (S) and phosphorus (P) severely degrade the toughness of the material, while anti-graphitizing elements such as titanium (Ti), vanadium (V), chromium (Cr), lead (Pb), and antimony (Sb) seriously interfere with the formation and growth of graphite spheres, resulting in substandard product performance.

[0003] Currently, the industrial production of ductile iron raw materials mainly relies on two technical routes: one is to directly smelt selected low-phosphorus, low-sulfur high-quality iron ore in a blast furnace, which is limited by scarce and expensive ore resources; the other is to smelt and carbonize pure scrap steel in an induction furnace, which requires extremely high-quality scrap steel, has large fluctuations in composition, and is very costly to carbonize. Both routes suffer from fundamental drawbacks such as high raw material costs, unstable supply, long process flow, and difficulty in controlling product composition.

[0004] Therefore, developing a method to produce high-purity ductile iron raw materials using readily available and inexpensive common raw materials through an efficient and stable short-process technology has become an urgent need for the industry. Summary of the Invention

[0005] The purpose of this invention is to provide a method for smelting ductile iron based on deep desulfurization and phosphorus pretreatment. By combining two stages—oxidative slag formation and deep purification with composite solvents—this method resolves the contradiction between retaining high carbon content and deeply removing sulfur, phosphorus, and degraphitizing trace elements within a single converter process. This invention utilizes readily available ordinary blast furnace molten iron and, through modification and utilization of existing steelmaking converter processes, ultimately produces high-purity ductile iron raw materials that can be directly used for casting or cast into standard pig iron blocks, thus solving the problems mentioned in the background section.

[0006] To achieve the above objectives, the present invention provides a method for smelting ductile iron based on deep desulfurization and phosphorus pretreatment, comprising the following steps:

[0007] S1. Raw material preparation and furnace loading:

[0008] The raw material is ordinary blast furnace hot metal, with a typical chemical composition (mass percentage) of: carbon (C) 3.8-4.5%, silicon (Si) 0.3-1.5%, phosphorus (P) 0.08-0.25%, and sulfur (S) 0.03-0.10%. This hot metal is added to a steelmaking converter, preferably a converter with top and bottom blowing function to enhance the stirring of the molten pool.

[0009] S2, Oxidative Slagging and Initial Refining (Dephosphorization, Decarbonization, Preliminary Desulfurization):

[0010] Molten iron is added to a steelmaking converter with good stirring capabilities (preferably top and bottom blowing), along with an alkaline slagging agent primarily composed of lime (CaO), and oxygen blowing begins. Under high temperature, high oxidizing power, and high basicity (R=CaO / SiO2> 3.0) slag conditions, the following core reactions occur:

[0011] Decarburization and heating: [C] + 1 / 2 {O2} = CO↑. This reaction is violently exothermic and is the main heat source for heating the molten pool (to 1450-1600℃), providing the necessary thermodynamic driving force for all subsequent endothermic reactions. Simultaneously, by controlling the oxygen blowing rate, the carbon content is precisely adjusted from pig iron levels to the intermediate range (2.0%-3.8%) required for ductile iron, rather than the extremely low carbon content needed for steelmaking.

[0012] Deep dephosphorization: Under a strong oxidizing atmosphere, phosphorus is oxidized: 2[P] + 5(FeO) = (P2O5) + 5[Fe]. The product P2O5 is an acidic oxide, which is unstable at high temperatures, but it will immediately combine with the high concentration of free CaO in the slag to form stable calcium phosphate dissolved in the slag: (P2O5) + 4(CaO) = (4CaO·P2O5). The high alkalinity of the slag ensures that the reaction proceeds to the right, and the phosphorus is deeply removed to ≤0.05%.

[0013] Preliminary desulfurization: Under conditions of high alkalinity and the presence of iron oxide (FeO), gasification desulfurization and slag-gold interface desulfurization occur: [S] + O 2- =(S 2- The reaction is (CaS) + (O) and (S) + (CaO) = (CaS) + (O), during which a portion of the sulfur can be initially removed.

[0014] S3, Deep purification with composite solvents (extreme desulfurization and removal of de-graphitized elements):

[0015] In the later stages of S2, when dephosphorization is basically complete and the molten pool temperature rises above 1500℃, a specially formulated composite solvent is introduced into the converter. This step utilizes the high temperature, strong stirring, and the chemical environment already formed in the early stages of the converter's operation to introduce functional components and achieve deep purification.

[0016] Solvent composition: The composite solvent contains at least two functional components, preferably a combination of sodium carbonate (Na2CO3) and calcium carbide (CaC2), or a combination of sodium carbonate and rare earth ferrosilicon alloy.

[0017] The role of Na2CO3 is as follows: Na2CO3 decomposes rapidly at high temperatures: Na2CO3 = Na2O + CO2↑. The generated Na2O is an extremely strong desulfurizing agent, with a Gibbs free energy change far lower than that of CaO, enabling maximal desulfurization: Na2O(l) + [S] = Na2S(l). Simultaneously, the generated sodium vapor possesses extremely high chemical reactivity, capable of reacting with low-melting-point, high-vapor-pressure harmful elements such as lead (Pb) and antimony (Sb), or carrying them to the gas or slag phase for removal through gasification.

[0018] The role of CaC2 is as follows: CaC2 is a powerful desulfurizing and reducing agent. It reacts with sulfur in molten iron: CaC2(s) + [S] = CaS(s) + 2[C]. This reaction not only further desulfurizes the iron but also replenishes the carbon content. More importantly, CaC2 has extremely strong reducing properties, which can reduce the oxides of anti-graphitizing elements such as titanium, vanadium, and chromium dissolved in molten iron, or react directly with these elements to form high-melting-point carbides (such as TiC, VC, Cr3C2) or intermetallic compounds.

[0019] The role of rare earth elements (such as Ce and La) is that if rare earth ferrosilicon is used, rare earth elements have a strong affinity for sulfur, oxygen and various harmful elements (such as Pb, Sb, Bi and As), and can form stable compounds with high melting points and low densities (such as Ce2O2S, CeS, CePb3, etc.). These compounds are easy to float into the slag.

[0020] The composite solvent forms a localized strong reducing and strong chemical affinity zone in the molten pool. The generated Na2S, CaS, rare earth compounds, and various anti-graphitizing element carbides / compounds, due to their lower density than molten iron or immiscibility, rapidly aggregate, grow, and float to the surface under the intense stirring dynamics of the converter. They are effectively captured and fixed by the high-alkalinity slag, thereby achieving the goal of deep and simultaneous purification, reducing the sulfur content to ≤0.02% (preferably ≤0.015%) and the total amount of anti-graphitizing elements to ≤0.10% (preferably ≤0.05%).

[0021] S4, Endpoint Control:

[0022] Through online monitoring or sampling analysis, smelting is controlled to stop at the following composition and temperature endpoints: carbon (C) content: 2.0%-3.8%; phosphorus (P) content: ≤0.05%; sulfur (S) content: ≤0.02%; total anti-graphitizing elements: ≤0.10%; temperature: 1480℃-1550℃. This endpoint ensures that the molten iron has both the chemical composition required for casting and a suitable superheating temperature.

[0023] S5. Product diversion processing:

[0024] Based on the final product form, choose one of the following paths for processing:

[0025] Path A (Production of commercial ductile iron ingots): Silicon-iron alloy is directly added to the converter to increase silicon content to the standard range for cast iron (e.g., 1.5%-3.5%), and then the iron is tapped and cast into ingots.

[0026] Path B (Production of ready-to-use ductile iron): The qualified pure molten iron is directly tapped into a medium-frequency induction furnace, where precise composition fine-tuning (silicon, manganese, etc.) and temperature homogenization are carried out, and then it is directly used for spheroidizing, inoculation and casting processes.

[0027] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0028] This smelting method for ductile iron based on deep desulfurization and phosphorus pretreatment first uses a combination of oxidation slag formation for dephosphorization, followed by deep reduction purification with a composite solvent. In the converter, phosphorus is stably removed from the molten iron to ≤0.05% and sulfur to ≤0.015% (as low as 0.005%) in one go. At the same time, the total amount of anti-graphitizing elements such as titanium and vanadium is reduced by more than 90%. This solves the core problem of raw material purity in the production of high-end ductile iron and lays an irreplaceable raw material foundation for obtaining high-quality spheroidization effect.

[0029] The ductile iron produced by molten iron provided by this invention has a stable graphite spheroidization rate of over 95%, achieving a fine and rounded spherical graphite morphology. This directly translates into superior mechanical properties, enabling castings to maintain high strength (tensile strength ≥ 850 MPa) while obtaining higher plasticity and toughness (elongation ≥ 6%). The overall performance indicators far exceed those of ductile iron products produced using conventional purified molten iron.

[0030] Meanwhile, this invention integrates the deep purification task, which traditionally requires multiple stations (such as desulfurization stations, converters, and refining furnaces), into a single converter process. This not only simplifies the production process and shortens the smelting cycle, but also reduces the temperature drop and handling of molten iron; at the same time, it provides two product paths: commercial pig iron blocks and ready-to-use molten iron, offering high production flexibility.

[0031] Although a special composite solvent is used, the method of this invention eliminates the need for external desulfurization treatment and significantly reduces the additional consumption of spheroidizing agent and inoculant during subsequent spheroidizing treatment due to impurities in the molten iron. From the perspective of the entire process, the overall cost is optimized, and due to the shortened process, energy consumption and emissions are reduced, resulting in better environmental friendliness. Attached Figure Description

[0032] Figure 1 This is a flowchart illustrating the overall process of the present invention. Detailed Implementation

[0033] The technical solutions in the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0034] Example 1: Production of ductile iron casting blocks using Na2CO3-CaC2 composite solvent (see...) Figure 1 ).

[0035] Raw materials: Ordinary molten iron supplied by the steel plant's blast furnace, with a temperature of 1320℃ and a quantity of 105 tons.

[0036] Equipment: 130-ton top and bottom combined blowing converter, equipped with a secondary lance temperature measurement and carbon determination system and an automatic feeding system.

[0037] Process steps and specific operations:

[0038] Charging and Initial Smelting: 105 tons of blast furnace hot metal were added to the converter. 7500 kg of lime, 1500 kg of lightly calcined dolomite, and 300 kg of fluorite were added through the high-level hopper as slag-forming materials. Oxygen blowing was initiated using a constant pressure, variable lance position operation, with slag formation initially at a lower lance position. The oxygen supply intensity was set at 3.2 Nm³. 3 / (min·t).

[0039] Oxidative dephosphorization and heating: After approximately 8 minutes of blowing, the molten pool temperature rose to approximately 1480℃, and the carbon content decreased to approximately 3.0%. During this stage, sampling via the secondary lance confirmed that the phosphorus content had decreased from the initial 0.152% to 0.045%, and the sulfur content to 0.025%, essentially achieving the initial dephosphorization and desulfurization targets. The slag basicity (CaO / SiO2) remained around 3.8.

[0040] Deep purification with composite solvent: After 10 minutes of blowing, the molten pool temperature reaches 1520℃. At this point, composite solvent A is added to the furnace in one go via the feeding system. The composition of composite solvent A is: 850kg sodium carbonate (Na2CO3) + 450kg calcium carbide (CaC2). After addition, the bottom blowing gas is switched to pure argon, and the flow rate is increased to 0.08 Nm³. 3 / (min·t), intensify stirring of the molten pool for 3 minutes to ensure that the solvent and molten iron react fully.

[0041] Endpoint control and tapping: Blowing was stopped after a total blowing time of 13 minutes. The endpoint composition and temperature were determined using a secondary lance: Carbon (C) 3.18%, Silicon (Si) 0.65%, Phosphorus (P) 0.021%, Sulfur (S) 0.006%, Titanium (Ti) 0.005%, Vanadium (V) 0.002%. The molten pool temperature was 1545℃. The endpoint composition fully met the preset requirements.

[0042] Product processing: 1900 kg of ferrosilicon alloy containing 75% silicon is added to the furnace to increase the silicon content of the molten iron to approximately 2.0%. The iron is tapped and cast into pig iron blocks, which is the product of this embodiment.

[0043] Product Performance: Sampling and analysis of the cast iron blocks. Testing showed that the cast iron blocks fully meet the requirements of grade Q12 in GB / T 1412-2018 "Pig Iron for Ductile Iron," and the content of sulfur, phosphorus, and anti-graphitizing elements is far superior to the standard specifications, making them suitable as raw materials for the production of high-quality ductile iron.

[0044] Example 2: Direct production of molten iron for casting using Na2CO3-rare earth silicon-iron composite solvent (see...) Figure 1 ).

[0045] Raw materials: Ordinary molten iron supplied by the steel plant's blast furnace. The molten iron temperature is 1305℃, and the amount used is 90 tons.

[0046] Equipment: A 130-ton converter of the same model as in Example 1, and a 60-ton medium-frequency induction holding furnace.

[0047] Process steps and specific operations:

[0048] Charging and smelting: 90 tons of molten iron are added to the converter. 6500 kg of lime and 1200 kg of lightly calcined dolomite are added, and oxygen smelting begins.

[0049] Intermediate operation: After blowing for 7 minutes, when the temperature was about 1495℃, a rapid analysis of the sample showed that the phosphorus content had dropped to 0.038%, and oxygen blowing continued.

[0050] Deep purification by adding composite solvent: After blowing for 9 minutes and reaching a temperature of 1515℃, add composite solvent B. The composition of composite solvent B is: 600kg sodium carbonate (Na2CO3) + 250kg rare earth ferrosilicon alloy (RE30%). Strengthen bottom blowing argon stirring for 3 minutes.

[0051] Endpoint control: Stop blowing after 12 minutes of total blowing. The final molten iron composition is: C 2.88%, P 0.026%, S 0.009%, lead (Pb) <0.0005%, antimony (Sb) <0.0005%, and temperature 1528℃.

[0052] Subsequent processing (Path B): Transfer all of this pure molten iron to a 60-ton medium-frequency induction holding furnace. Add 500 kg of ferrosilicon containing 75% silicon and an appropriate amount of ferromanganese alloy to the furnace. Slightly adjust the composition to: Si 2.25%, Mn 0.35%. Hold at 1510℃ for later use.

[0053] Application Results: After undergoing wire feeding spheroidization (using yttrium-based heavy rare earth magnesium wire) and in-flow inoculation treatment, this molten iron was used to cast crankshafts for a certain type of diesel engine. Metallographic and mechanical property tests were performed on samples of the casting. The results showed that the graphite spheroidization level was Grade 1, the spheroidization rate was ≥95%, the pearlite content was 85%, the tensile strength was ≥850 MPa, and the elongation was ≥5%. The performance was excellent and stable.

[0054] Example 3: Optimizing production using the slag retention method (see...) Figure 1 ).

[0055] After the smelting in Example 1 was completed, about 20% of the final slag (high basicity, high CaS content) was left in the converter and not poured out. Then, 100 tons of new ordinary blast furnace molten iron (composition similar to Example 1, P content of 0.18%) was added. Due to the slag left, the amount of lime added in this smelting was reduced to 6000 kg and lightly calcined dolomite to 1200 kg. When the blowing reached 9 minutes and the temperature was 1505°C, composite solvent A (700 kg Na2CO3 + 350 kg CaC2) was added, and the total amount added was reduced by about 18% compared to Example 1. The blowing was stopped after a total blowing time of 12.5 minutes.

[0056] Results: The final molten iron composition was: C 3.05%, P 0.024%, S 0.007%. Even with reduced solvent usage, the purity was still achieved at the same high level as in Example 1. This example demonstrates that the slag retention method can utilize the high-basicity slag from the previous furnace, reducing the consumption of new materials and further lowering production costs, showcasing the excellent economic efficiency and optimizability of the process of this invention.

[0057] Comparative Example 1: Conventional converter dephosphorization hot metal process.

[0058] 105 tons of molten iron, identical to that in Example 1, were added to the converter; 7500 kg of lime, 1500 kg of dolomite, and 300 kg of fluorite were added. Oxygen blowing was carried out according to the conventional dephosphorization process.

[0059] After blowing for 13 minutes, the blowing was stopped. The final target was mainly low carbon (<0.05%) and low phosphorus. The final molten iron composition was approximately: C 0.03%, P 0.015%, S 0.028%, Ti 0.060%, V 0.032%. The temperature was approximately 1650℃.

[0060] This molten iron is a typical ultra-low carbon steel, completely unsuitable for casting. If it were to be used in ductile iron, a large amount of carbon would have to be added in subsequent processes, resulting in high costs. Furthermore, sulfur and anti-graphitizing elements (Ti, V) are almost entirely absent, failing to meet the purity requirements for ductile iron raw materials.

[0061] Comparative Example 2: Attempt to produce "high-carbon molten iron" by adding only lime and fluorite.

[0062] An attempt was made to simulate the production of high-carbon molten iron in a converter, using the same raw materials as Comparative Example 1; lime and fluorite were added, and the amount of oxygen blown was controlled, with an attempt to stop blowing at a higher carbon content (~3.0%).

[0063] When the carbon content reached approximately 3.0% and the temperature reached approximately 1500℃, blowing was stopped and samples were taken. The results showed a phosphorus content of 0.065% (incomplete dephosphorization) and a sulfur content of 0.030% (poor desulfurization). More importantly, the content of anti-graphitizing elements such as titanium and vanadium was almost identical to that of the original molten iron, indicating no significant purification.

[0064] Without the composite solvent described in this invention, conventional oxidation slag formation alone can control carbon and phosphorus to some extent in a converter, but it cannot achieve deep desulfurization, let alone effectively remove anti-graphitizing trace elements. The purity of the resulting molten iron is insufficient for producing high-performance ductile iron. While the deep purification step using the composite solvent can retain a certain carbon content, it cannot achieve deep removal of sulfur and anti-graphitizing elements.

[0065] Experimental Example 1: Through parallel comparative experiments, the comprehensive effect of the ductile iron smelting method based on deep desulfurization and phosphorus pretreatment of the present invention in the efficient production of high-purity ductile iron raw materials was verified. It was also compared with the current conventional process route to quantitatively evaluate its advantages in purity control, element yield, process economy and final product performance.

[0066] Experimental Groups: This experiment was designed with 4 groups of experiments, which were carried out on top and bottom blown converters of the same capacity (130 tons).

[0067] Experimental Group 1 (TG-1): The complete process of Example 1 of the present invention was adopted.

[0068] Experimental Group 2 (TG-2): The complete process of Example 2 of the present invention was adopted.

[0069] Comparative Group 1 (CG-1): The conventional converter dephosphorization steelmaking process described in Comparative Example 1 was used.

[0070] Comparative Group 2 (CG-2): The experimental process of "high carbon steelmaking" described in Comparative Example 2 was adopted.

[0071] Raw materials: All experimental groups used 100 tons of ordinary foundry iron from the same batch and the same blast furnace as initial raw materials to ensure consistency at the starting point; the composition of the initial iron was confirmed by direct reading spectrometer as follows: C: 4.28%, Si: 0.42%, Mn: 0.35%, P: 0.149%, S: 0.036%, Ti: 0.061%, V: 0.034%.

[0072] Experimental procedures and operation records:

[0073] TG-1 and TG-2: The process parameters of Examples 1 and 2 above are strictly followed. Key operation nodes (feeding, oxygen blowing time, solvent addition timing and amount, endpoint determination) are automatically recorded and controlled by the converter secondary system.

[0074] CG-1: The standard dephosphorized steel smelting procedure is implemented, and the blowing is carried out with the target of [C]≤0.05% and [P]≤0.015%.

[0075] CG-2: Try to control the amount of oxygen blown, and stop blowing steel immediately when the secondary lance detects that the molten pool [C]≈3.0% and the temperature≈1500℃.

[0076] Sampling and testing methods:

[0077] Process sampling: Use a secondary gun to take composite probe samples during the middle (about 7-8 minutes) and at the end of the blowing process.

[0078] Endpoint sampling: During the tapping process, a cylindrical sample is taken from the steel stream using a disposable molten steel sampler.

[0079] Testing equipment: After all samples were polished and prepared, the main elements (C, Si, Mn, P, S) were analyzed using an ARL direct-reading spectrometer; and trace elements such as Ti, V, Pb, and Sb were accurately determined using an inductively coupled plasma mass spectrometer (ICP-MS).

[0080] Subsequent verification: The pig iron blocks obtained from TG-1, the molten iron obtained from TG-2 and CG-2 (the molten steel from CG-1 was not included in this verification due to its composition not meeting the requirements) were spheroidized and inoculated under the same conditions, and standard Kiel test blocks (Φ30mm) were cast to test the mechanical properties and observe the metallographic structure.

[0081] To comprehensively evaluate the effects of each process, the following multi-dimensional evaluation criteria are established:

[0082] Core purity indicators:

[0083] Sulfur content ([S]): target ≤0.015%, the lower the better; Phosphorus content ([P]): target ≤0.050%; Total anti-graphitizing elements ([Ti]+[V]+[Pb]+[Sb], etc.): target total ≤0.020%.

[0084] Process efficiency and economic indicators:

[0085] Carbon yield: the degree of closeness between the final carbon content and the target carbon content (3.0%); Solvent / auxiliary material consumption: the amount of solvent and slag-forming materials consumed per unit of molten iron; Smelting cycle: the total time from iron addition to tapping.

[0086] Final product performance indicators (ductile iron):

[0087] Tensile strength (Rm) and elongation (A): tested according to GB / T 1348 standard; graphite morphology: spheroidization rate (≥90% is qualified) and spheroidization grade (1-3 is excellent).

[0088] The experimental results and data are shown in Tables 1 and 2.

[0089] Table 1 Comparison of key components of molten iron at the endpoint for each experimental group (mass fraction, %)

[0090] experimental group C P S Ti V Total amount of anti-graphitizing elements raw material 4.28 0.149 0.036 0.061 0.034 0.095+ CG-1 (Comparison) 0.04 0.013 0.029 0.059 0.033 0.092 CG-2 (Comparison) 3.02 0.067 0.031 0.060 0.033 0.093 TG-1 (Invention Test Group 1) 3.15 0.022 0.006 0.005 0.002 0.007 TG-2 (Invention Test Group 2) 2.91 0.028 0.010 0.058* 0.032* <0.001**

[0091] Note: + indicates the presence of other trace elements, the actual value is slightly higher than this value; TG-2 raw material itself has low Ti and V content (<0.01%); *The total amount here is mainly Pb and Sb, which have been deeply removed; **Indicates that the total amount of this element is extremely low and has been deeply removed to below the detection limit.

[0092] Table 2 Comparison of Process Economy and Product Performance

[0093] project CG-1 CG-2 TG-1 TG-2 Carbon recovery rate (relative to target 3%) 1.3% 100.7% 105.0% 97.0% Solvent / auxiliary material cost (RMB / ton of iron) benchmark -5% +15% +12% Smelting cycle (minutes) 15 12 13 12 Tensile strength Rm (MPa) of ductile iron specimen not applicable 720 860 855 Elongation A (%) of ductile iron specimen not applicable 3 6 7 Graphite spheroidization rate not applicable 85% 95% 95%

[0094] As shown in Tables 1 and 2, while CG-1 (conventional steelmaking) can achieve deep dephosphorization, it completely loses carbon and is ineffective against sulfur and trace elements; CG-2 (attempt to retain carbon) retains carbon, but dephosphorization is incomplete, and it has almost no removal capacity for sulfur and trace elements. This indicates that the conventional converter oxidation method inherently contains an inherent contradiction in principle between "carbon reduction" and "deep removal of sulfur and trace elements."

[0095] TG-1 and TG-2 successfully controlled the carbon content within the required range (2.8-3.2%) for ductile iron while achieving deep phosphorus removal (≤0.03%). More importantly, the sulfur content was reduced by 80%-90%, and the total amount of anti-graphitizing elements was reduced by more than 90%. This demonstrates that by introducing a composite solvent for deep purification as a key step, this invention overcomes the limitations of traditional processes and achieves highly efficient removal of multiple impurity elements.

[0096] Secondly, ductile iron made from CG-2 molten iron has a high content of sulfur and trace elements, which seriously interferes with graphite spheroidization, resulting in low spheroidization rate and poor graphite morphology. Consequently, its mechanical properties (especially elongation) are far below the standard requirements.

[0097] The ductile iron made from the molten iron (TG-1 / TG-2) of this invention has high raw material purity and stable and excellent spheroidization effect, resulting in high spheroidization rate and fine and round graphite spheres. This leads to a good combination of high strength and high plasticity and toughness, and the performance indicators are superior in all aspects.

[0098] Compared to the existing complex process of "blast furnace molten iron, desulfurization station, converter dephosphorization, carbon increase / conditioning, and spheroidization treatment" with a long process and multiple steps, this invention integrates deep dephosphorization, desulfurization, and removal of trace elements into a single process in the converter, shortening the process flow and improving the production pace.

[0099] Although this invention increases the cost of special solvents, it eliminates the need for expensive desulfurizers, large amounts of carburizing agents, and the increased spheroidizing agent due to insufficient purity, making the overall cost controllable. More importantly, it provides a stable, efficient, and direct method for producing high-end ductile iron raw materials. The resulting increase in product added value and the ability to meet the demands of the high-end market far outweigh the slight increase in cost.

[0100] The foregoing has shown and described the basic principles, main features, and advantages of the present invention. Those skilled in the art should understand that the present invention is not limited to the above embodiments. The embodiments and descriptions in the specification are merely preferred examples and are not intended to limit the invention. Various changes and modifications can be made to the invention without departing from its spirit and scope, and all such changes and modifications fall within the scope of the present invention as claimed. The scope of protection of the present invention is defined by the appended claims and their equivalents.

Claims

1. A method for smelting ductile iron based on deep desulfurization and phosphorus pretreatment, characterized in that, Includes the following steps: S1. Raw material preparation and charging: Provide ordinary blast furnace molten iron as raw material and mix it into the steelmaking converter; S2, Oxidation Slag Formation and Initial Refining: Add an alkaline slag-forming agent, mainly lime, into the converter and blow oxygen. Under high temperature, high oxidizing and high alkalinity slag conditions, decarburization, heating, deep dephosphorization and preliminary desulfurization are carried out, wherein the slag alkalinity R=CaO / SiO2>3.0; S3. Deep purification with composite solvent: In the later stage of S2, when dephosphorization is basically completed and the temperature of the molten pool rises to above 1500°C, a composite solvent is added into the converter. The composite solvent contains at least two functional components. The high temperature and strong stirring conditions in the converter are used to achieve deep and simultaneous purification of sulfur and anti-graphitization elements. S4. Control the smelting to stop at the following endpoints: the carbon content in the molten iron is 2.0%-3.8%, the phosphorus content is ≤0.05%, the sulfur content is ≤0.02%, the total amount of anti-graphitizing elements is ≤0.10%, and the temperature is 1480℃-1550℃, to obtain the final molten iron that meets the requirements for ductile iron production.

2. The smelting method for ductile iron based on deep desulfurization and phosphorus pretreatment according to claim 1, characterized in that, In step S3, the composite solvent is a combination of sodium carbonate and calcium carbide, or a combination of sodium carbonate and rare earth ferrosilicon alloy.

3. The smelting method for ductile iron based on deep desulfurization and phosphorus pretreatment according to claim 1, characterized in that, In step S3, after the composite solvent is added, the stirring intensity of the bottom-blown gas in the converter is enhanced, and the bottom-blown gas is argon.

4. The smelting method for ductile iron based on deep desulfurization and phosphorus pretreatment according to claim 1, characterized in that, In step S4, the anti-graphitizing element includes one or more of titanium, vanadium, chromium, lead, and antimony.

5. The smelting method for ductile iron based on deep desulfurization and phosphorus pretreatment according to claim 1, characterized in that, In step S4, the final sulfur content is controlled to be ≤0.015%.

6. The smelting method for ductile iron based on deep desulfurization and phosphorus pretreatment according to claim 1, characterized in that, In step S4, the total amount of anti-graphitization elements at the control endpoint is ≤0.05%.

7. The molten ductile iron smelting method based on deep desulfurization and phosphorus pretreatment according to any one of claims 1-6, characterized in that, After obtaining the final molten iron by completing S4, one of the following subsequent processes may be selectively performed: The final molten iron is siliconized and then cast into pig iron blocks; Alternatively, the molten iron from the endpoint can be tapped into a holding furnace for fine-tuning of its composition, and then used for spheroidization, inoculation, and casting of ductile iron.

8. The smelting method for ductile iron based on deep desulfurization and phosphorus pretreatment according to claim 7, characterized in that, The silicon-enriched and cast iron blocks are: In the converter, ferrosilicon alloy is added to the final molten iron to increase silicon content, adjusting the silicon content to 1.5%-3.5%, and then the iron is tapped and cast into ingots.

9. The smelting method for ductile iron based on deep desulfurization and phosphorus pretreatment according to claim 7, characterized in that, The composition fine-tuning of the heat preservation furnace is as follows: The molten iron is tapped into a medium-frequency induction furnace, where the silicon and manganese content is adjusted and the temperature is homogenized.

10. The smelting method for ductile iron based on deep desulfurization and phosphorus pretreatment according to claim 7, characterized in that, After completing one heat of S4 smelting, a portion of the final slag is retained in the converter for use in the next heat of S2 smelting.

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