A method for producing a high carbon equivalent gray cast iron

Abstract

The present application relates to the technical field of cast iron alloy, and particularly relates to a preparation method of high-carbon-equivalent gray cast iron, which comprises the following steps: taking scrap steel as main material, taking scrap castings, silicon carbide and carbon additive as auxiliary materials, and smelting according to the determined raw material ratio and the value range of carbon equivalent; sampling the molten iron to obtain the first cooling curve of the molten iron, extracting the characteristic points to perform thermal analysis and determine the metallurgical state of the molten iron, adjusting the content of silicon carbide of pre-inoculation or analyzing the possibility of excessive inoculation; pre-inoculating the molten iron, sampling the molten iron again to obtain the second cooling curve of the molten iron, extracting the characteristic points, determining the pearlite morphology, and calculating the sensitivity index of the molten iron to inoculation according to the characteristic point change of the first cooling curve and the second cooling curve. The present application solves the problem that high carbon equivalent easily leads to coarse graphite and performance decline by pre-inoculation adjustment and tapping inoculation.

Description

Technical Field

[0001] This invention relates to the field of cast iron alloy technology, and in particular to a method for preparing high carbon equivalent gray cast iron. Background Technology

[0002] Gray cast iron is widely used in key components such as machine tool beds due to its excellent casting fluidity, wear resistance, and vibration damping properties. Carbon equivalent (CE=C+1 / 3(Si+P)) is the core indicator for measuring the casting performance of cast iron. A high carbon equivalent means better fluidity and filling capacity, which can effectively reduce casting defects such as cold shuts and incomplete filling, and improve the product qualification rate.

[0003] In order to achieve high mechanical properties, traditional gray cast iron production typically employs a design scheme with medium to low carbon equivalent. By reducing the carbon and silicon content, more pearlite matrix and fine graphite are obtained to ensure strength. However, this approach has significant shortcomings:

[0004] (1) The lower carbon equivalent leads to a decrease in the fluidity of molten iron, which increases the difficulty of casting and is not friendly to complex thin-walled castings;

[0005] (2) To compensate for insufficient fluidity, it is often necessary to increase the pouring temperature, which will increase energy consumption and smelting costs, and exacerbate the thermal shock to the mold.

[0006] (3) Graphite coarsening: Excessive carbon equivalent will cause graphite to grow excessively during solidification, forming coarse A-type or even C-type (flowering) graphite, which will severely rupture the matrix;

[0007] (4) Reduced pearlite content: High carbon equivalent widens the solidification range, making it easier to form ferrite, which leads to a decrease in pearlite content in the matrix. The coarsening of graphite and the reduction of pearlite together result in a significant decrease in key mechanical properties of the material, such as tensile strength, hardness and elastic modulus, making it difficult to meet the performance requirements of high-end equipment.

[0008] Therefore, there is an irreconcilable contradiction in the existing technology: it is difficult to achieve both high carbon equivalent and high strength. Developing a preparation method that can precisely control the microstructure of high carbon equivalent gray cast iron, effectively refine graphite and stabilize the pearlite matrix while maintaining its excellent casting fluidity, thereby obtaining superior mechanical properties, has become a technical problem that urgently needs to be solved in this field. Summary of the Invention

[0009] Therefore, the present invention provides a method for preparing high carbon equivalent gray cast iron to overcome the technical problem in the prior art that the process cannot be adjusted based on the characterization curve and corresponding characterization index during the preparation process, which leads to the easy coarsening of graphite and the decline in performance of gray cast iron under high carbon equivalent.

[0010] To achieve the above objectives, the present invention provides a method for preparing high-carbon equivalent gray cast iron, comprising:

[0011] The process involves smelting scrap steel as the main material, with scrap castings of the same material, silicon carbide, and carbon raisers as auxiliary materials, according to a determined raw material ratio and a range of carbon equivalent values.

[0012] The first cooling curve of the molten iron is obtained by sampling and smelting. The characteristic points of the first cooling curve are extracted for thermal analysis to determine the metallurgical state of the molten iron, and the silicon carbide content of the pre-inoculated iron is adjusted or the possibility of over-inoculation is analyzed.

[0013] The molten iron is pre-inoculated, and the molten iron is sampled a second time to obtain the second cooling curve of the molten iron. The characteristic points of the second cooling curve are extracted to determine the pearlite morphology, and the sensitivity index of the molten iron to inoculation is calculated based on the changes in the characteristic points of the first and second cooling curves.

[0014] The sensitivity index reflects the sensitivity of molten iron to inoculation. The demand for barium silicon inoculant is determined based on the eutectic undercooling and the sensitivity index, thereby adjusting the risk of over-inoculation in subsequent inoculation processes.

[0015] Alternatively, the reasons for the insensitivity of molten iron to inoculation can be determined based on the characteristic points of the first cooling curve, and it may be necessary to determine whether the risk of over-inoculation in subsequent inoculation processes is within the normal range.

[0016] Based on the characteristic points of the first cooling curve and the changes in another characteristic point of the first and second cooling curves, determine whether the risk of over-inoculation in the subsequent inoculation process exceeds the normal range, and adjust the content of silicon barium inoculant added in iron tapping inoculation and casting inoculation accordingly.

[0017] After high-temperature static pre-inoculation, the molten iron is sampled and poured into test blocks to test the white iron width of the fracture surface of the test blocks. The judgment criteria for the white iron width are adjusted, and the cooling control process is determined based on the pearlite morphology.

[0018] A multi-level inoculation process is adopted, using silicon-barium inoculant for in-flow inoculation of molten iron for tapping and casting, and cooling control of molten iron during the casting process.

[0019] Furthermore, the determined raw material ratios and the range of values ​​for carbon equivalent are as follows:

[0020] Mn 0.5~0.7%, P≤0.04%, S 0.07~0.1%, Cr 0.25~0.45%, Sn 0.06~0.08%, Ti≤0.03%, N 90~110ppm, carbon equivalent calculated according to the formula CE=C+1 / 3(Si+P) and set to a carbon equivalent range of 3.75%~3.90%;

[0021] The carbon raiser is added in the range of 0.6-2%, scrap steel 60%-80%, silicon carbide 0.8%-1.2%, and particle size 1-5mm.

[0022] Furthermore, the processes of pre-inoculation, tapping inoculation, and casting inoculation include:

[0023] Pre-inoculation involves adding 0.05%~0.15% silicon carbide with a particle size of 0.2~1mm before tapping.

[0024] During iron tapping, a barium silicate inoculant with a particle size of 3-10 mm and a content of 0.3-0.5% is added in the stream. The barium silicate inoculant contains 2%-4% Ba and is added in the stream, accounting for 80% of the iron tapping time.

[0025] During casting inoculation, a barium silicon inoculant with a particle size of 0.2-0.8mm and a content of 0.1%-0.2% is added in the flow during casting, and the in-flow addition accounts for 80% of the iron tapping time.

[0026] Furthermore, the process of determining the metallurgical state of molten iron includes: extracting characteristic points from the first cooling curve and calculating the primary eutectic undercooling of the molten iron sample;

[0027] If the first eutectic undercooling is less than the target value, the molten iron is judged to be in excellent metallurgical condition, and the possibility of over-inoculation needs to be analyzed.

[0028] If the first eutectic undercooling is greater than or equal to the target value, the molten iron is judged to be in poor metallurgical condition and not within the set range, and the silicon carbide content of the pre-inoculated iron is increased.

[0029] Furthermore, the process of determining the pearlite morphology includes extracting characteristic points from the second cooling curve and calculating the eutectoid undercooling.

[0030] When the eutectoid supercooling is less than the critical value, it indicates that austenite decomposes at a higher temperature, the transformation driving force is small, and it tends to form coarser pearlite.

[0031] When the eutectoid supercooling is greater than or equal to the critical value, it indicates that the supercooling is large, the transformation driving force is large, and it tends to form fine lamellar pearlite.

[0032] Furthermore, the process of calculating the sensitivity index of molten iron to inoculation includes: calculating the sensitivity index of molten iron to inoculation based on the primary eutectic undercooling and the pre-inoculation eutectic undercooling;

[0033] When the sensitivity index is greater than the calibration index, it is determined that the undercooling of the molten iron after pre-inoculation changes significantly, and the molten iron is sensitive to the introduction of foreign cores. The demand for silicon barium inoculant is determined based on the secondary eutectic undercooling and the sensitivity index.

[0034] When the sensitivity index is less than the calibration index, it is preliminarily determined that the molten iron is not sensitive to the introduction of foreign cores, and the reason for the insensitivity is determined based on the primary eutectic undercooling.

[0035] Furthermore, when the secondary eutectic undercooling is within the calibrated range and the sensitivity index is within the calibrated index range, inoculation is carried out using barium silicon inoculant according to the added content;

[0036] When the secondary eutectic undercooling exceeds the calibrated range and the sensitivity index is lower than the calibrated range, it is determined that the demand for barium silicon inoculant is high. The amount of barium silicon inoculant added is increased based on the average ratio of the eutectic undercooling and the sensitivity index to the corresponding calibrated values.

[0037] Furthermore, when the eutectic undercooling is less than the calibrated temperature, it is determined that there are a large number of graphite precipitation nuclei in the molten iron. Therefore, it is not sensitive to the introduction of foreign nuclei. It is necessary to determine whether the over-inoculation risk in the subsequent inoculation process exceeds the normal range.

[0038] When the eutectic undercooling is greater than the rated temperature, it is determined that the molten iron is not sensitive to the introduction of foreign cores, the molten iron has high tolerance for inoculation, and the risk of over-inoculation that may exist in subsequent inoculation processes is within the normal range.

[0039] Furthermore, when the eutectic undercooling is less than the calibrated temperature and the change in eutectic re-glow temperature is greater than the critical temperature, it is determined that the risk of over-inoculation in subsequent inoculation processes may exceed the normal range, and the content of barium silicon inoculant added in iron tapping inoculation and casting inoculation is adjusted accordingly.

[0040] Furthermore, the initial evaluation width of the white iron width test is increased. When the white iron width at the fracture tip is less than the increased initial evaluation width, it is judged that the molten iron has a high carbon equivalent and strong graphitization ability.

[0041] If the width of the white iron at the fracture tip is greater than the increased initial evaluation width, the fracture of the test block with excessively large white iron is deemed unqualified.

[0042] When the eutectoid undercooling is less than the critical value, the flow rate of compressed air in the cooling control process is increased according to the ratio of the eutectoid undercooling to the critical value.

[0043] Compared with existing technologies, the beneficial effects of this invention are as follows: the synthetic cast iron technology uses scrap steel as the main material, without pig iron, and has no coarse graphite inheritance, resulting in high purity of molten iron and fine and uniform graphite. It utilizes the high nitrogen content of scrap steel to improve strength, has a low shrinkage tendency, and balances casting performance and strength under high carbon equivalent. Furthermore, by adding silicon carbide with different particle sizes and range parameters at different times in the smelting process, and by adding silicon carbide at the bottom of the furnace, the low temperature of less than 1300°C in the early stage of melting allows silicon carbide to decompose slowly, fully utilizing its nucleation potential to maximize its role as a heterogeneous nucleus. The purpose of adding silicon carbide for pre-inoculation before tapping is to compensate for the lack of graphite precipitation nuclei caused by the absence of pig iron. The two additions respectively serve to supplement graphitization nuclei and rapidly dissolve to adjust metallurgical quality.

[0044] Furthermore, while high-purity scrap steel provides a foundation for molten iron with low impurities and trace elements, it lacks graphitization nuclei. This method utilizes the decomposition of SiC in molten iron to provide carbon and silicon, using incompletely dissolved SiC particles and their decomposition products as effective heterogeneous nuclei for graphite precipitation. Simultaneously, the SiC decomposition process has a deoxidizing effect, reducing the oxygen content of the molten iron, purifying the melt, and significantly improving the graphitization ability of the molten iron, thus enhancing its "metallurgical quality." The focus shifts from "achieving the target chemical composition" to "achieving the target composition while obtaining the optimal metallurgical state of the molten iron." Pearlite provides the necessary strength to compensate for the strength reduction that may result from high carbon equivalent. In this embodiment, the Mn content is much greater than S, and Sn is also added. The balanced Mn and Sn elements effectively inhibit ferrite formation, resulting in more pearlite in the as-cast state. Adjusting and increasing the casting cooling rate allows for the formation of fine lamellar pearlite. The unique feature of this invention lies in shifting from "passive inheritance" to "active construction" of the metallurgical quality of the molten iron. It is closer to the principle of metallurgical reaction, has stronger control over the final structure, and can produce higher quality molten iron.

[0045] Furthermore, a small undercooling value indicates strong graphitization ability and numerous internal crystal nuclei in the molten iron; a large undercooling value indicates a severe tendency towards white iron formation, requiring strong inoculation. This method, through online thermal analysis, transforms the abstract concept of "molten iron quality" into a quantifiable "eutectic undercooling" index, enabling rapid and objective assessment of the metallurgical state of molten iron, replacing traditional subjective judgments based on experience. Based on the measured values, a direct correlation logic is established with the amount of inoculant added. This allows for advance prediction and precise control of the inoculation treatment intensity before casting, achieving "on-demand inoculation" and avoiding defects caused by insufficient or excessive inoculation from the source. This makes process adjustments more scientific and refined, effectively improving the consistency and stability of molten iron quality and reducing the tendency towards white iron formation.

[0046] Furthermore, pre-inoculation plays a role in regulating the metallurgical quality of molten iron in the application scenario of artificially manufactured graphite precipitation cores in this embodiment. Due to the high carbon equivalent in the molten iron, the goal of the finished gray cast iron is to provide the necessary strength to obtain a pearlitic matrix to compensate for the strength reduction that may be caused by the high carbon equivalent. At the same time, good inoculation treatment ensures that the graphite morphology is good and in the A-type graphite morphology. Even if the matrix is ​​hard pearlite, the casting still has good machinability because the "notch effect" of graphite is weakened.

[0047] Furthermore, this method transforms the abstract "sensitivity" of molten iron to inoculation into a quantifiable indicator by calculating the "sensitivity index" and introducing the "change in eutectic reglow temperature," achieving a leap from experience-based judgment to data-driven approaches. A multi-condition decision tree constructed based on the sensitivity index, the supercooling baseline value, and the change in reglow temperature can accurately distinguish whether molten iron is in a state of high-quality sensitivity, standard demand, or sluggish insufficiency, and dynamically adjust the inoculant dosage to effectively avoid the risks of over-inoculation or under-inoculation. The closed-loop control system ensures that different batches of molten iron receive precise inoculation "on demand," significantly improving microstructure consistency and product quality stability, while simultaneously reducing costs and increasing efficiency by avoiding inoculant waste or defective scrap.

[0048] Furthermore, the gray casting comprises a matrix structure and lamellar graphite; in this embodiment, the matrix structure needs to be a pearlitic matrix. A faster cooling rate increases the cooling speed of the casting, which helps to form fine lamellar pearlite. The casting is heated to the austenitizing temperature, approximately 850-900°C, held at that temperature, and then cooled in air. This faster cooling rate ensures a high proportion, or even 100%, of pearlitic microstructure. Attached Figure Description

[0049] Figure 1 This is a schematic flowchart of the preparation method of high carbon equivalent gray cast iron in an embodiment of the present invention;

[0050] Figure 2 This is a schematic diagram of the cooling curve of molten iron in an embodiment of the present invention;

[0051] Figure 3 This is a schematic diagram of the process for determining the metallurgical state of molten iron in an embodiment of the present invention;

[0052] Figure 4 This is a schematic diagram of the process in an embodiment of the present invention, which reflects the sensitivity of molten iron to inoculation based on the sensitivity index. Detailed Implementation

[0053] To make the objectives and advantages of the present invention clearer, the present invention will be further described below with reference to embodiments; it should be understood that the specific embodiments described herein are merely for explaining the present invention and are not intended to limit the present invention.

[0054] Preferred embodiments of the present invention will now be described with reference to the accompanying drawings. Those skilled in the art should understand that these embodiments are merely illustrative of the technical principles of the present invention and are not intended to limit the scope of protection of the present invention.

[0055] It should be noted that in the description of this invention, the terms "upper", "lower", "left", "right", "inner", "outer", etc., which indicate directions or positional relationships, are based on the directions or positional relationships shown in the accompanying drawings. This is only for the convenience of description and is not intended to indicate or imply that the device or element must have a specific orientation, or be constructed and operated in a specific orientation. Therefore, it should not be construed as a limitation of this invention.

[0056] Furthermore, it should be noted that, in the description of this invention, unless otherwise explicitly specified and limited, the terms "installation," "connection," and "linking" should be interpreted broadly. For example, they can refer to a fixed connection, a detachable connection, or an integral connection; they can refer to a mechanical connection or an electrical connection; they can refer to a direct connection or an indirect connection through an intermediate medium; and they can refer to the internal connection of two components. Those skilled in the art can understand the specific meaning of the above terms in this invention according to the specific circumstances.

[0057] Please see Figures 1-4 As shown, Figure 1 This is a schematic flowchart of the preparation method of high carbon equivalent gray cast iron in an embodiment of the present invention; Figure 2 This is a schematic diagram of the cooling curve of molten iron in an embodiment of the present invention; Figure 3 This is a schematic diagram of the process for determining the metallurgical state of molten iron in an embodiment of the present invention; Figure 4 This is a schematic diagram of the process in an embodiment of the present invention, which reflects the sensitivity of molten iron to inoculation based on the sensitivity index.

[0058] This invention provides a method for preparing high carbon equivalent gray cast iron, comprising:

[0059] Step S1: Using scrap steel as the main material and scrap castings of the same material and silicon carbide as auxiliary materials, smelting is carried out according to the determined raw material ratio and the range of carbon equivalent values.

[0060] Step S2: Sample the molten iron to obtain the first cooling curve of the molten iron, extract the characteristic points of the first cooling curve for thermal analysis to determine the metallurgical state of the molten iron, adjust the silicon carbide content of the pre-inoculation or analyze the possibility of over-inoculation.

[0061] Step S3: Pre-inoculate the molten iron, take a second sample of the molten iron to obtain the second cooling curve of the molten iron, extract the characteristic points of the second cooling curve, determine the pearlite morphology, and calculate the sensitivity index of the molten iron to inoculation based on the changes of the characteristic points of the first cooling curve and the second cooling curve.

[0062] Step S4: Based on the sensitivity index reflecting the sensitivity of molten iron to inoculation, determine the demand for barium silicon inoculant based on eutectic undercooling and the sensitivity index, and adjust the risk of over-inoculation in subsequent inoculation processes.

[0063] Step S5, or determine the reason for the insensitivity of molten iron to inoculation based on the characteristic points of the first cooling curve, whether it is necessary to determine whether the risk of over-inoculation in subsequent inoculation processes is within the normal range;

[0064] Step S6: Based on the characteristic points of the first cooling curve and the changes in another characteristic point of the first and second cooling curves, determine whether the risk of over-inoculation in the subsequent inoculation process exceeds the normal range, and adjust the content of silicon barium inoculant added in the iron tapping inoculation and casting inoculation accordingly.

[0065] Step S7: After the molten iron is pre-inoculated at high temperature, a sample is poured into a test block to check the white iron width of the fracture surface of the test block. The judgment criteria for the white iron width are adjusted, and the cooling control process is determined according to the pearlite morphology.

[0066] Step S8 involves employing a multi-level inoculation process, using a barium silicon inoculant for in-flow inoculation of molten iron during tapping and casting, and controlling the cooling of the molten iron during the casting process.

[0067] Specifically, the synthetic cast iron technology of this invention uses scrap steel as the main material, without pig iron, and has no coarse graphite inheritance. The molten iron has high purity, and the graphite is fine and uniform. The high nitrogen content of the scrap steel is used to improve strength and reduce shrinkage tendency, while taking into account casting performance and strength under high carbon equivalent. By adding silicide with different particle sizes and range parameters at different times in the smelting process, and adding silicon carbide at the bottom of the furnace, the low temperature of less than 1300°C in the early stage of melting allows silicon carbide to decompose slowly, making full use of the nucleation potential of silicon carbide to maximize its effect as a heterogeneous nucleus. The purpose of adding silicon carbide for pre-inoculation during the smelting process is to compensate for the lack of graphite precipitation nuclei caused by not using pig iron. The two additions respectively play the roles of supplementing graphitization nuclei and rapidly dissolving to adjust metallurgical quality.

[0068] Raw material ratio and carbon equivalent range: Mn 0.5~0.7%, P≤0.04%, S 0.07~0.1%, Cr 0.25~0.45%, Sn 0.06~0.08%, Ti≤0.03%, N 90~110ppm.

[0069] The carbon equivalent (CE) value is calculated using the formula CE=C+1 / 3(Si+P) and is designed to be in the high carbon equivalent range of 3.75%~3.90%.

[0070] The smelting process uses a medium-frequency induction furnace for smelting, adds silicon carbide for inoculation, adds alloys to control the range of trace element content in the molten iron, and adds a carbonizer to supplement the carbon equivalent in the molten iron.

[0071] During implementation, the carbon raiser addition range is 0.6% to 2%.

[0072] Specifically, the smelting process uses scrap steel as the main material and does not use pig iron. 60% to 80% of the material is scrap steel, and the rest is scrap castings of the same material. The scrap castings need to be shot blasted to ensure that the surface is free of rust and sand, and 0.8% to 1.2% of silicon carbide with a particle size of 1 to 5 mm is added.

[0073] In the pre-inoculation process, SiC is used for pre-inoculation in the furnace, and 0.05%~0.15% silicon carbide (particle size 0.2~1mm) is added before exiting the furnace.

[0074] The high-temperature settling process improves the metallurgical quality of molten iron by setting it at high temperatures, purifies impurities, and increases tensile strength, modulus of elasticity, and supercooling.

[0075] Specifically, within a certain range, increasing the superheat temperature of molten iron and extending the high-temperature settling time can purify the molten iron, thereby refining the graphite and matrix structure in cast iron. However, further increasing the superheat temperature will reduce the nucleation ability of cast iron. Therefore, increasing the undercooling in the high-temperature settling melting process may have adverse effects. There is a "critical temperature" that, once exceeded, will gradually show negative effects on the performance of cast iron.

[0076] In the tapping process, molten iron from the medium-frequency electric furnace is poured into a ladle. During tapping, a silicon-barium inoculant with a particle size of 3-10mm is added in slurry. The silicon-barium inoculant is SiBaCa, containing 2%-4% Ba. The silicon-barium inoculant is added to the molten iron in slurry, with an addition content of 0.3-0.5%. The inoculant is placed in an inoculation device and flows out evenly. The silicon-barium inoculant slurry accounts for 80% of the tapping time.

[0077] In the casting process, the treated molten iron is poured from the ladle into the mold. During casting, a silicon barium inoculant with a particle size of 0.2~0.8mm is added in the flow, with a content of 0.1%~0.2%. The inoculation device is placed to flow out evenly. The silicon barium inoculant in the flow accounts for 80% of the casting time.

[0078] Cooling control: The sand mold has a built-in hollow cold iron tube for compressed air temperature control, combined with a uniform cooling pouring system and simulated solidification software to ensure uniform cooling, with an opening temperature ≤300℃.

[0079] Specifically, high-purity scrap steel provides a low-impurity, low-trace-element basis for molten iron, but lacks graphitization nuclei. This method provides carbon and silicon through the decomposition of SiC in molten iron, using incompletely dissolved SiC particles and their decomposition products as effective heterogeneous nuclei for graphite precipitation. Simultaneously, the SiC decomposition process has a deoxidizing effect, reducing the oxygen content of the molten iron, purifying the melt, and significantly improving the graphitization ability of the molten iron, thus enhancing its "metallurgical quality." The focus shifts from "achieving the target chemical composition" to "achieving the target composition while obtaining the optimal metallurgical state of the molten iron." Pearlite provides the necessary strength to compensate for the strength reduction that may result from high carbon equivalent. In this embodiment, the Mn content is much greater than S, and Sn is also added. The balanced Mn and Sn elements effectively inhibit ferrite formation, resulting in more pearlite in the as-cast state. Adjusting and increasing the casting cooling rate can form fine lamellar pearlite. The unique feature of this invention lies in shifting from "passive inheritance" to "active construction" of the metallurgical quality of molten iron. It is closer to the principle of metallurgical reaction, has stronger control over the final structure, and can produce higher quality molten iron.

[0080] Samples of molten iron are taken during the smelting process, and the temperature of the molten iron samples is measured in real time to plot a temperature-time curve, which is the first cooling curve of the molten iron.

[0081] When molten iron cools below the liquidus line, it begins to solidify and releases latent heat of crystallization. The release of latent heat compensates for or slows down the temperature drop, thus creating a eutectic plateau, or inflection point, on the cooling curve.

[0082] Extract the characteristic points of the first cooling curve and calculate the primary eutectic undercooling of the molten iron sample. Primary eutectic undercooling = theoretical eutectic temperature - minimum eutectic temperature.

[0083] In this embodiment, the theoretical eutectic temperature is 1154°C, and the minimum eutectic temperature is the lowest point of the eutectic plateau on the first cooling curve.

[0084] If thermal analysis shows that the primary eutectic undercooling is less than the target value, the molten iron is judged to be in excellent metallurgical condition, and the silicon carbide pretreatment has played a powerful role. Only a small amount or a conventional dose of inoculation treatment is needed. Over-inoculation may lead to coarse graphite or other defects, and the possibility of over-inoculation needs to be analyzed.

[0085] If thermal analysis shows that the primary eutectic undercooling is greater than or equal to the target value, it is determined that the metallurgical state of the molten iron is poor and not within the set range, and the content of silicon carbide in the pre-inoculation is increased.

[0086] Specifically, the amount of silicon carbide in the pre-inoculated culture is increased based on the ratio of the supercooling to the target value.

[0087] The target value is 15°C.

[0088] Specifically, a small undercooling value indicates strong graphitization ability and numerous internal crystal nuclei in the molten iron; a large undercooling value indicates a severe tendency towards white iron formation, requiring strong inoculation. This method, through online thermal analysis, transforms the abstract concept of "molten iron quality" into a quantifiable "eutectic undercooling" index, enabling rapid and objective assessment of the metallurgical state of molten iron, replacing traditional subjective judgments based on experience. Based on the measured values, a direct correlation logic is established with the amount of inoculant added. This allows for advance prediction and precise control of the inoculation treatment intensity before casting, achieving "on-demand inoculation" and avoiding defects caused by insufficient or excessive inoculation from the source. This makes process adjustments more scientific and refined, effectively improving the consistency and stability of molten iron quality and reducing the tendency towards white iron formation.

[0089] After pre-inoculation and adjustment, a second sample of molten iron is taken to generate a second cooling curve. The characteristic points of the second cooling curve are extracted to calculate the eutectoid undercooling. Eutectoid undercooling = theoretical eutectoid temperature - minimum eutectoid temperature.

[0090] In practice, the theoretical eutectoid temperature is 738℃, and the lowest eutectoid temperature is the lowest point of the eutectoid plateau at the bottom of the identified cooling curve.

[0091] When the eutectoid undercooling is less than the critical value, it indicates that the austenite decomposes at a higher temperature, the transformation driving force is small, and coarser pearlite is usually formed.

[0092] When the eutectoid supercooling is greater than or equal to the critical value, it indicates that the supercooling is large, the transformation driving force is large, and it tends to form fine lamellar pearlite.

[0093] The critical value is a preset value set based on historical data of the cooling curve produced by gray cast iron.

[0094] Specifically, pre-inoculation serves to regulate the metallurgical quality of molten iron in the application scenario of artificially manufactured graphite precipitation cores in this embodiment. Due to the high carbon equivalent in the molten iron, the goal of the finished gray cast iron is to provide the necessary strength to compensate for the strength reduction that may be caused by the high carbon equivalent to obtain a pearlitic matrix. At the same time, good inoculation treatment ensures that the graphite morphology is good, that is, the graphite morphology is A-type graphite. Even if the matrix is ​​hard pearlite, the casting still has good machinability because the "notch effect" of graphite is weakened.

[0095] The core function of the traditional 75SiFe inoculant is to provide a large number of silicate (SiO2) and manganese sulfide (MnS) particles as heterogeneous nuclei for graphite precipitation.

[0096] When the sulfur content in molten iron is too low, less than 0.06%, there is not enough sulfur to form sufficient MnS nuclei. Therefore, the main pathway for 75SiFe to create nuclei through MnS formation is weakened, resulting in a significant reduction in inoculation effect, manifested as an increased tendency for white iron formation and a deterioration in graphite morphology.

[0097] The unique feature of this method is that it uses barium silicon inoculant, which overcomes the adverse effects of low-sulfur environments by introducing a new, sulfur-independent nucleation mechanism.

[0098] In barium silicon inoculants, barium reacts with elements such as oxygen, sulfur, silicon, and aluminum in molten iron to form complex barium-calcium-silicon-aluminum-oxygen composites. The crystal structure of these composites matches graphite better than that of MnS, making them highly efficient graphite nucleation cores.

[0099] The formation of these composite cores mainly depends on the elements contained in the inoculant itself and the oxygen in the molten iron, and the dependence on the sulfur content of the base molten iron is greatly reduced. Therefore, even in a low-sulfur environment, a large number of cores can be formed effectively.

[0100] Barium silicon acts as an anti-aging inoculant to prevent late-stage ferritization. Combined with Sn and Mn, it inhibits carbon diffusion into graphite during the eutectoid transformation, i.e., inhibition effect one, thereby ensuring high pearlite content.

[0101] The composite core formed by barium is very stable and does not easily dissolve or passivate in molten iron at high temperatures. Moreover, the efficiency of barium silicon inoculants is usually higher than that of 75SiFe.

[0102] The anti-fading ability of inoculation using barium silicon inoculant is stronger than that of FeSi inoculant. When used in gray cast iron, it has strong graphitization ability, which can effectively eliminate hard spots and edge white iron, improve the uniformity of cross-sectional hardness, reduce the tendency of molten iron to be undercooled, increase the number of eutectic clusters, improve the strength of cast iron, and transform D and E type graphite into A type graphite.

[0103] The sensitivity of molten iron to inoculation after the addition of inoculant was calculated based on the changes in the cooling curves obtained in two separate tests.

[0104] Identify the lowest pre-inoculation eutectic temperature of the second cooling curve and calculate the secondary eutectic undercooling of the second cooling curve;

[0105] The sensitivity index of molten iron to inoculation is calculated based on the primary eutectic undercooling and the secondary eutectic undercooling, wherein the sensitivity index = (primary eutectic undercooling - secondary eutectic undercooling) / primary eutectic undercooling × 100%;

[0106] When the sensitivity index is greater than the calibration index, it is determined that the change in undercooling of the molten iron after pre-inoculation is more obvious, and the molten iron is sensitive to the introduction of external cores. Adjustment is made to address the risk of over-inoculation in subsequent inoculation processes within the existing inoculation space.

[0107] Specifically, when molten iron is sensitive to the introduction of foreign nuclei, the demand for barium silicon inoculant is determined based on the secondary eutectic undercooling and the sensitivity index.

[0108] When the secondary eutectic undercooling is below the calibration range and the sensitivity index exceeds the calibration range, it is determined that the demand for barium silicon inoculant is low. The amount of barium silicon inoculant added is reduced based on the average ratio of the secondary eutectic undercooling and the sensitivity index to the corresponding calibration values.

[0109] When the secondary eutectic undercooling is within the calibrated range and the sensitivity index is within the calibrated index range, inoculation is carried out with barium silicon inoculant according to the added content;

[0110] When the secondary eutectic undercooling exceeds the calibrated range and the sensitivity index is lower than the calibrated range, it is determined that the demand for barium silicon inoculant is high. The amount of barium silicon inoculant added is increased based on the average ratio of the secondary eutectic undercooling and the sensitivity index to the corresponding calibrated values.

[0111] The calibration range is 10-15℃, and the calibration index range is 20%-40%.

[0112] When the sensitivity index is less than the calibration index, it is preliminarily determined that the molten iron is not sensitive to the introduction of foreign cores, and the reason for the insensitivity is determined based on the primary eutectic undercooling.

[0113] Among them, when the eutectic undercooling is less than the calibration temperature, it is determined that there are a large number of graphite precipitation nuclei in the molten iron. Therefore, it is not sensitive to the introduction of foreign nuclei. It is necessary to determine whether the risk of over-inoculation in the subsequent inoculation process exceeds the normal range.

[0114] When the eutectic undercooling is greater than the rated temperature, it is determined that the molten iron is not sensitive to the introduction of foreign cores, the molten iron has high tolerance for inoculation, and the risk of over-inoculation in subsequent inoculation processes is within the normal range.

[0115] In practice, the change in eutectic re-ignition temperature is the absolute value of the difference between the secondary eutectic re-ignition temperature and the primary eutectic re-ignition temperature.

[0116] The secondary eutectic re-ignition temperature and the primary eutectic re-ignition temperature are the eutectic re-ignition temperatures of the second cooling curve and the first cooling curve, respectively, which are the temperatures reached after supercooling to the lowest point and then rising back due to the release of latent heat.

[0117] If the eutectic undercooling is less than the calibrated temperature and the change in eutectic re-glow temperature is greater than the critical temperature, it is judged that the risk of over-inoculation in the subsequent inoculation process may exceed the normal range, and the content of barium silicon inoculant added in the iron tapping inoculation and casting inoculation is adjusted accordingly.

[0118] Specifically, based on the ratio of the change in critical temperature to eutectic reglow temperature, and the ratio of primary eutectic undercooling to calibration temperature, the content of barium silicon inoculant added in iron tapping inoculation and casting inoculation is reduced year-on-year.

[0119] The calibration index is 8%, the calibration temperature is 8℃, and the critical temperature ranges from 3 to 5℃.

[0120] Specifically, this method transforms the abstract "sensitivity" of molten iron to inoculation into a quantifiable indicator by calculating a "sensitivity index" and introducing a "change in eutectic regold temperature," achieving a leap from experience-based judgment to data-driven approaches. A multi-condition decision tree constructed based on the sensitivity index, undercooling baseline value, and change in regold temperature can accurately distinguish between high-quality sensitive, standard demand, and sluggish / deficient states of molten iron, and dynamically adjust the inoculant dosage, effectively avoiding the risks of over-inoculation or under-inoculation. The closed-loop control system ensures that different batches of molten iron receive precise inoculation "on demand," significantly improving microstructure consistency and product quality stability, while simultaneously reducing costs and increasing efficiency by avoiding inoculant waste or defective scrap.

[0121] Two solidification / phase transformation processes of gray cast iron:

[0122] Eutectic transformation, i.e., liquid → austenite + graphite: occurs in a relatively high temperature range of about 1150℃, and inoculation treatment mainly affects this stage.

[0123] Eutectoid transformation, i.e., austenite → ferrite + graphite / or austenite → pearlite, occurs in a relatively low temperature range of about 738℃. This stage determines whether the final matrix structure is ferrite or pearlite.

[0124] After the melting is completed and the sample is left to stand, it is taken and cast into test blocks. The white line width of the fracture surface of the test blocks is checked, and the white line width judgment standard of the test blocks is adjusted.

[0125] Specifically, increase the initial evaluation width of the white-mouth width test.

[0126] When the width of the white iron at the fracture tip is less than the increased initial evaluation width, it is judged that the molten iron has a high carbon equivalent and strong graphitization ability.

[0127] If the width of the white iron at the fracture tip is greater than the increased initial evaluation width, the fracture of the test block with excessively large white iron is deemed unqualified.

[0128] An excessively large white fracture indicates insufficient inoculation potential, requiring enhanced inoculation during tapping.

[0129] The initial evaluation width is 2-3 mm, and the increased initial evaluation width is 3-5 mm.

[0130] When the eutectoid undercooling is less than the critical value, the flow rate of compressed air in the cooling control process is increased according to the ratio of the eutectoid undercooling to the critical value.

[0131] Specifically, the gray casting comprises a matrix structure and lamellar graphite; in this embodiment, the matrix structure needs to be a pearlitic matrix. A faster cooling rate increases the cooling speed of the casting, which helps to form fine lamellar pearlite. The casting is heated to its austenitizing temperature of approximately 850-900°C, held at that temperature, and then cooled in air. This faster cooling rate ensures a high proportion, or even 100%, of pearlitic microstructure.

[0132] The carbon in gray cast iron consists of compound carbon (Fe3C) and graphitic carbon. When the compound carbon content is 0.8%, it is pearlitic gray cast iron. When the compound carbon content is less than 0.8%, it is pearlitic-ferritic gray cast iron. When all the carbon exists in the graphitic state, it is ferritic gray cast iron.

[0133] Silicon is a strong graphitizing element. It can promote the precipitation of carbon in the form of graphite and inhibit the formation of pearlite. Adding pearlite stabilizing elements can suppress ferrite. A faster cooling rate is conducive to the formation of fine lamellar pearlite.

[0134] In this embodiment of the invention, the principle of the inoculation treatment is as follows: by adding flour-like ferrosilicon alloy particles to the molten iron, a relatively high carbon equivalent value is created in the local area of ​​the molten iron, thereby obtaining more graphite nuclei. In this way, the size of the graphite is reduced, especially to prevent the graphite in the central part of the casting from growing too large due to slow heat dissipation, thereby improving the central strength of the casting and reducing the cross-sectional sensitivity of the casting.

[0135] The effects of inoculation treatment are as follows: by eliminating or reducing the tendency of white cast iron, avoiding the formation of supercooled structures, reducing the wall thickness sensitivity of cast iron parts, making the difference in microstructure and hardness between thin and thick sections of the casting smaller, which is conducive to the nucleation of eutectic clusters, increasing the number of eutectic clusters, and making the graphite in cast iron mainly fine and uniformly distributed type A graphite, thereby improving the mechanical properties of cast iron.

[0136] The technical solution of the present invention has been described above with reference to the preferred embodiments shown in the accompanying drawings. However, it will be readily understood by those skilled in the art that the scope of protection of the present invention is obviously not limited to these specific embodiments. Without departing from the principles of the present invention, those skilled in the art can make equivalent changes or substitutions to the relevant technical features, and the technical solutions after these changes or substitutions will all fall within the scope of protection of the present invention.

[0137] The above description is merely a preferred embodiment of the present invention and is not intended to limit the invention. Various modifications and variations can be made to the present invention by those skilled in the art. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A method for preparing high carbon equivalent gray cast iron, characterized in that, include: The process involves smelting scrap steel as the main material, with scrap castings of the same material, silicon carbide, and carbon raisers as auxiliary materials, according to a determined raw material ratio and a range of carbon equivalent values. Sampling molten iron and obtaining the first cooling curve of the molten iron. Based on one of the characteristic points of the extracted first cooling curve, thermal analysis is performed to determine the metallurgical state of the molten iron, so as to adjust the silicon carbide content of the pre-inoculation or analyze whether there is over-inoculation. Extract one characteristic point from the first cooling curve and calculate the first eutectic undercooling of the molten iron sample. If the first eutectic undercooling is less than the target value, the molten iron is judged to be in excellent metallurgical condition, and the possibility of over-inoculation needs to be analyzed. If the first eutectic undercooling is greater than or equal to the target value, it is determined that the metallurgical state of the molten iron is poor and not within the set range, and the content of silicon carbide in the pre-inoculation is increased. Pre-inoculation of molten iron is carried out, and a second sample of molten iron is taken to obtain a second cooling curve of the molten iron. The morphology of pearlite is determined based on another characteristic point extracted from the second cooling curve. Another characteristic point of the second cooling curve was extracted to calculate the eutectoid supercooling. When the eutectoid supercooling is less than the critical value, it indicates that the transformation driving force is small and the pearlite morphology tends to form coarse pearlite. When the eutectoid supercooling is greater than or equal to the critical value, it indicates that the transformation driving force is large, and the pearlite morphology tends to form fine lamellar pearlite; The sensitivity index of molten iron to inoculation is calculated based on the changes in one characteristic point of the first cooling curve and the same characteristic point of the second cooling curve. The process of calculating the sensitivity index of molten iron to inoculation based on the change of the same characteristic point of the second cooling curve includes calculating the sensitivity index of molten iron to inoculation based on the primary eutectic undercooling and the secondary eutectic undercooling. When the sensitivity index is greater than the calibration index, it is determined that the undercooling of the molten iron after pre-inoculation changes significantly, and the molten iron is sensitive to the introduction of external cores. The demand for barium silicon inoculant is determined based on the secondary eutectic undercooling and the sensitivity index. When the sensitivity index is less than the calibration index, it is preliminarily determined that the molten iron is not sensitive to the introduction of foreign cores, and the reason for the insensitivity is determined based on the primary eutectic undercooling. The sensitivity index reflects the sensitivity of molten iron to inoculation. The amount of barium silicon inoculant required for subsequent inoculation processes is determined based on the same characteristic point of the second cooling curve and the sensitivity index. The reasons for the insensitivity of molten iron to inoculation are determined based on the sensitivity index and one of the characteristic points of the first cooling curve, so as to determine whether the risk of over-inoculation in subsequent inoculation processes is within the normal range. Based on the changes in one characteristic point of the first cooling curve and another characteristic point of the first and second cooling curves, determine whether the over-inoculation risk of the subsequent inoculation process exceeds the normal range, and adjust the content of silicon barium inoculant added in iron tapping inoculation and casting inoculation accordingly. Another characteristic point is the eutectic reglow temperature of the second cooling curve and the first cooling curve; If the eutectic undercooling is less than the calibrated temperature and the change in eutectic re-glow temperature is greater than the critical temperature, it is determined that the risk of over-inoculation in the subsequent inoculation process exceeds the normal range, and the content of barium silicon inoculant added in the iron tapping inoculation and casting inoculation is adjusted accordingly. After high-temperature static pre-inoculation, the molten iron is sampled and poured into test blocks to test the white iron width of the fracture surface of the test blocks. The judgment criteria for the white iron width are adjusted, and the cooling control process is determined based on the pearlite morphology. A multi-level inoculation process is adopted, using silicon-barium inoculant for in-flow inoculation of molten iron for tapping and casting, and cooling control of molten iron during the casting process.

2. The method for preparing high-carbon equivalent gray cast iron according to claim 1, characterized in that, The range of values ​​for the raw material ratio and carbon equivalent is as follows: Mn 0.5~0.7%, P≤0.04%, S 0.07~0.1%, Cr 0.25~0.45%, Sn 0.06~0.08%, Ti≤0.03%, N 90~110ppm, carbon equivalent calculated according to the formula CE=C+1 / 3(Si+P) and set to a carbon equivalent range of 3.75%~3.90%; The carbon raiser is added in the range of 0.6-2%, scrap steel 60%-80%, silicon carbide 0.8%-1.2%, and particle size 1-5mm.

3. The method for preparing high-carbon equivalent gray cast iron according to claim 1, characterized in that, The processes of pre-inoculation, tapping inoculation, and casting inoculation include: Pre-inoculation involves adding 0.05%~0.15% silicon carbide with a particle size of 0.2~1mm before tapping. During iron tapping, a barium silicate inoculant with a particle size of 3-10 mm and a content of 0.3-0.5% is added in an in-flow manner. The barium silicate inoculant contains 2%-4% Ba and is added in an in-flow manner. The in-flow inoculation accounts for 80% of the iron tapping time. During casting, a barium silicon inoculant with a particle size of 0.2-0.8 mm and a content of 0.1%-0.2% is added for in-flow inoculation, and in-flow inoculation accounts for 80% of the iron tapping time.

4. The method for preparing high-carbon equivalent gray cast iron according to claim 1, characterized in that, The process of determining the amount of barium silicon inoculant required for subsequent inoculation processes includes: When the secondary eutectic undercooling is within the calibrated range and the sensitivity index is within the calibrated index range, inoculation is carried out with barium silicon inoculant according to the added content; When the secondary eutectic undercooling exceeds the calibrated range and the sensitivity index is lower than the calibrated range, it is determined that the demand for barium silicon inoculant is high. The amount of barium silicon inoculant added is increased based on the average ratio of the eutectic undercooling and the sensitivity index to the corresponding calibrated values.

5. The method for preparing high-carbon equivalent gray cast iron according to claim 1, characterized in that, The process of determining the reasons for the insensitivity of molten iron to inoculation includes: When the eutectic undercooling is less than the calibration temperature, it is determined that there is an excessive amount of graphite precipitates in the molten iron. The precipitates are not sensitive to the introduction of foreign nuclei. It is necessary to determine whether the risk of over-inoculation in the subsequent inoculation process exceeds the normal range. When the eutectic undercooling is greater than the rated temperature, it is determined that the molten iron is not sensitive to the introduction of foreign cores, the molten iron has high tolerance for inoculation, and the risk of over-inoculation in subsequent inoculation processes is within the normal range.

6. The method for preparing high-carbon equivalent gray cast iron according to claim 1, characterized in that, Increase the initial evaluation width for white iron width inspection. If the white iron width at the fracture tip is less than the increased initial evaluation width, the carbon equivalent of the molten iron is judged to be high. If the width of the white iron at the fracture tip is greater than the increased initial evaluation width, the fracture of the test block with excessively large white iron is deemed unqualified. When the eutectoid undercooling is less than the critical value, the flow rate of compressed air in the cooling control process is increased according to the ratio of the eutectoid undercooling to the critical value.

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