Preparation method of nodular cast iron and nodular cast iron

Abstract

The preparation method of the nodular cast iron comprises the following steps that a nodular cast iron blank is prepared, and the blank is heated and subjected to heat preservation; after heat preservation is finished, the blank is subjected to quenching treatment in a quenching medium, nodular cast iron is obtained, the quenching temperature of quenching treatment is 250-400 DEG C, the quenching time is 2-5 h, and the quenching medium adopted for quenching treatment comprises, by mass, 25%-50% of KNO3, 25%-50% of NaNO2, 10%-35% of NaNO3 and 3%-20% of H2O; after quenching treatment, ausferrite with a certain depth is formed on the surface layer of the nodular cast iron, the internal structure of the nodular cast iron is pearlite, and the mass ratio of the pearlite is 90-97%. According to the preparation method of the nodular cast iron and the nodular cast iron, the alloy content in a nodular cast iron material is reduced, and the pearlite-based nodular cast iron with ultrahigh strength performance is obtained by optimizing a heat treatment process.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of metal material preparation, in particular to a preparation method of nodular cast iron and the nodular cast iron. BACKGROUND

[0002] For the thick nodular cast iron parts with wall thickness of 60mm-200mm and mainly pearlite matrix, the mechanical properties of the thick nodular cast iron parts are currently improved by adding manganese, copper, nickel and tin alloys, but the improvement of the mechanical properties of the material is limited, and the general tensile strength is less than 900MPa; and the addition of a large amount of alloy increases the material cost, resulting in a relatively complex manufacturing process.

[0003] For example, it is found by searching that the quenching nodular cast iron composition includes: carbon 3.40-3.80%, silicon 2.30-2.70%, manganese ≤0.35%, phosphorus ≤0.04%, sulfur ≤0.02%, magnesium 0.03-0.05%, copper 0.3-0.5%, nickel 0.50-0.80%. The composition of the nodular cast iron proposed in patent CN119530638A is carbon: 3.0-3.5%, silicon: 4.3-4.7%, manganese 0.1-0.3%, phosphorus ≤0.05%, sulfur ≤0.02%, molybdenum 0.7-1.0%, chromium 0.6-0.8%, nickel 0.4-0.7%, niobium 0.3-0.6%, magnesium 0.03-0.06%, rare earth 0.05-0.1%, antimony 0.006-0.012%, and the balance is iron.

[0004] The components of the above two schemes have the problems of too many alloy contents and types and high cost, and in view of the above technical problems, the present application scheme is proposed. SUMMARY

[0005] The main purpose of the present application is to overcome the defects of the prior art, provide a preparation method of nodular cast iron and nodular cast iron, reduce the alloy content in the nodular cast iron material, and prepare a pearlite-based nodular cast iron with ultra-high strength performance by combining a heat treatment process.

[0006] To achieve the above purpose, the present application provides the following technical scheme: the present application provides a preparation method of nodular cast iron, comprising the following steps: Preparation of nodular cast iron blank, heating and holding the blank; After holding, the blank is quenched in a quenching medium to obtain nodular cast iron, the quenching temperature of the quenching treatment is 250-400℃, the quenching time is 2-5h, the mass percentage of the salt component in the quenching medium used for quenching treatment is KNO3 25-50%, NaNO2 25-50%, NaNO3 10-35%, and the water component in the quenching medium accounts for 3-20% of the salt component; The surface layer of the spheroidal graphite cast iron after quenching treatment forms a certain depth of austenite, and the internal structure of the spheroidal graphite cast iron is pearlite, and the mass percentage of the pearlite is 90-97%.

[0007] Preferably, the mass percentage of the salt component in the quenching medium is KNO3 42-52%, NaNO2 40-50%, and NaNO3 1-10%, and the H2O in the quenching medium accounts for 3-10% of the total mass of the salt component.

[0008] Preferably, the preparation method of the spheroidal graphite cast iron blank comprises the following steps: heating raw materials including scrap steel and pig iron to 1530-1560℃, adding ferromanganese and ferrosilicon when the temperature is lowered to 1500-1530℃, and smelting to obtain molten iron; pouring the spheroidal graphite cast iron blank after adding spheroidizing agent and inoculant to the molten iron for spheroidizing and multiple inoculation.

[0009] Preferably, the molten iron is inoculated at least four times.

[0010] Preferably, the spheroidizing and inoculating method comprises: adding spheroidizing agent and first inoculant, and then adding at least part of the molten iron to the ladle for spheroidizing and first inoculation; adding second inoculant to the ladle for second inoculation, and adding the remaining molten iron.

[0011] Preferably, the spheroidizing and inoculating method further comprises third inoculation treatment: adding third inoculant to the ladle after the second inoculation treatment.

[0012] Preferably, the spheroidizing agent is rare earth magnesium silicon ferroalloy, and the mass percentage of the components is 0.1%≤Re≤0.7%, 1%≤Mg≤5%, and the addition amount of the spheroidizing agent is 1.0%-1.5% of the total mass of the molten iron.

[0013] Preferably, the first inoculant, the second inoculant and the third inoculant are independently selected from at least one of silicon-barium inoculant, silicon-calcium inoculant or silicon-calcium-barium inoculant.

[0014] Preferably, the addition amount of the first inoculant and the second inoculant is 0.5-0.7% of the total mass of the molten iron.

[0015] Preferably, the addition amount of the third inoculant is 0.2-0.4% of the total mass of the molten iron.

[0016] Preferably, it is characterized in that, during the pouring process, a fourth inoculant is added for pouring and stream inoculation, the fourth inoculant is selected from sulfur-oxygen inoculant or silicon-barium inoculant, and the addition amount of the fourth inoculant is 0.2-0.4% of the total mass of the molten iron.

[0017] In addition, the application further provides a nodular cast iron, wherein the mass percentage of each component of the nodular cast iron is as follows: C 3.0% to 3.8%, Si 1.5% to 2.7%, 0 < Mn ≤ 0.5%, 0 < P < 0.03%, S 0.004% to 0.01%, 0 < Re ≤ 0.03%, Mg 0.025% to 0.05%, and the balance is iron and impurities; the nodular cast iron has a certain depth of austenite in a surface layer, and the internal structure of the nodular cast iron is pearlite, and the mass percentage of the pearlite is 90 to 97%.

[0018] Preferably, the depth of the austenite of the nodular cast iron is 1 to 5 mm.

[0019] The preparation method and the nodular cast iron have the following technical effects: (1) the low-alloy components are used to obtain high-performance nodular cast iron without adding additional alloys, thereby reducing the material cost; (2) the heat treatment process and the quenching medium component are optimized to obtain a good quenching intensity, so that the grain of the blank quenching structure is refined, and the mechanical properties are improved; (3) the inoculation process adopts multi-stage inoculation, including ladle inoculation, pouring inoculation, pouring inoculation and stream inoculation, the silicon-based inoculant is added for multiple times, the nodular cast iron structure uniformity is improved, the grain is refined, the graphite ball roundness and the graphite ball quantity are improved, and the blank metallurgical quality is improved; and (4) the quality of the conventional nodular cast iron grade is improved, and the nodular cast iron can be popularized to thick products such as crankshafts, wind power planet carriers and nodular cast iron ingots. BRIEF DESCRIPTION OF DRAWINGS

[0020] Figure 1 The microstructure of the nodular cast iron material is shown.

[0021] Figure 2 The internal microstructure of the pearlite (100x) is shown, wherein the left graph is the graphite morphology, and the right graph is the matrix structure.

[0022] Figure 3 The surface microstructure of the austenite (100x) is shown, wherein the left graph is the graphite morphology, and the right graph is the matrix structure.

[0023] wherein, Figure 1 The dark part is the austenite AF, and the other white part is the pearlite P. DETAILED DESCRIPTION

[0024] The technical scheme of the application will be described in detail below with reference to the drawings, but the protection scope of the application is not limited to the embodiments.

[0025] The application provides a preparation method of the nodular cast iron with low-alloy content, the preparation process is optimized to improve the nodular cast iron structure uniformity, refine the grain, improve the graphite ball roundness and the graphite ball quantity, and improve the mechanical strength; the nodular cast iron material is "elemental" without adding additional alloys, thereby reducing the material cost.

[0026] The mass percentage of each component of the spheroidal graphite cast iron mentioned in the embodiment is as follows: C 3.0% to 3.8%, Si 1.5% to 2.7%, Mn≤0.5%, P<0.03%, S 0.004% to 0.01%, Re≤0.03%, Mg 0.025% to 0.05%, and the balance is iron and impurities. The spheroidal graphite cast iron in the embodiment includes the base elements C, Si, Mn, P, and S, and does not add additional alloys, and the rare earth and magnesium elements are introduced by a spheroidizing agent. Compared with the existing spheroidal graphite cast iron with ultra-high strength, the alloy content and types are reduced, thereby reducing the material cost.

[0027] The spheroidal graphite cast iron with ultra-high strength in the embodiment satisfies the tensile strength Rm≥900Mpa and the elongation A≥2%.

[0028] In order to obtain the spheroidal graphite cast iron with ultra-high strength, the preparation process is optimized in the embodiment. The preparation method of the spheroidal graphite cast iron includes the following steps: The spheroidal graphite cast iron blank is prepared, and the blank is heated and kept. The heating temperature and the holding time of the blank are not specifically limited by the wall thickness and the mechanical property requirements of the cast iron. The heating range of the spheroidal graphite cast iron blank is 800~1000℃, and the holding time is 1~5h.

[0029] The preparation method of the spheroidal graphite cast iron blank includes the following steps: the raw materials including scrap steel and pig iron are heated to 1530~1560℃, and the manganese iron and silicon iron are added when the temperature is reduced to 1500~1530℃, and the melting is performed to obtain the molten iron; the spheroidizing agent and the inoculant are put in, and the molten iron is spheroidized and inoculated multiple times; and the spheroidal graphite cast iron blank is obtained by pouring. The molten iron is inoculated at least four times in the embodiment, which involves ladle inoculation, bottom inoculation, pouring ladle inoculation, and stream inoculation. The silicon is added to the spheroidal graphite cast iron by adding the silicon-containing inoculant, and the inoculation process is controlled by multiple inoculation treatments to improve the uniformity of the spheroidal graphite cast iron structure and refine the grains.

[0030] The specific inoculation step method is as follows: the spheroidizing agent and the first inoculant are put in, and then at least a part of the molten iron is put in to perform the spheroidization and the first inoculation in the ladle; the second inoculant is added to the ladle to perform the second inoculation, and the remaining molten iron is put in; the third inoculant is added to the ladle after the second inoculation treatment. In addition, the fourth inoculant is put in during pouring to perform the pouring stream inoculation.

[0031] The spherification agent in the scheme of the embodiment adopts a rare earth magnesium silicon ferroalloy, wherein the component mass percentage is 0.1%≤Re≤0.7%, 1%≤Mg≤5%, and the adding amount of the spherification agent is 1.0% to 1.5% of the total mass of the molten iron. The first inoculant, the second inoculant and the third inoculant are selected from a silicon-barium inoculant, and the barium content in the silicon-barium inoculant accounts for 9 to 15%. The adding amount of the first inoculant and the second inoculant is 0.5 to 0.7% of the total mass of the molten iron. The adding amount of the third inoculant is 0.2 to 0.4% of the total mass of the molten iron. The fourth inoculant is selected from a sulfur-oxygen inoculant, and the adding amount of the fourth inoculant is 0.2 to 0.4% of the total mass of the molten iron.

[0032] After the holding is completed, the blank is quenched in a quenching medium to obtain the nodular cast iron, the quenching temperature of the quenching treatment is 250 to 400℃, the quenching time is 2 to 5h, the mass percentage of the salt component in the quenching medium used in the quenching treatment is KNO3 42 to 52%, NaNO2 40 to 50%, NaNO3 1 to 10%, and the H2O in the quenching medium accounts for 3 to 10% of the total mass of the salt component. After the quenching treatment, the surface layer of the nodular cast iron forms a certain depth of austenite, and the internal structure of the nodular cast iron is pearlite, and the mass percentage of the pearlite is 90 to 97%.

[0033] The application forms a certain depth of austenite in the surface layer of the blank and a structure mainly composed of refined pearlite in other positions by optimizing the components of the quenching medium and the quenching process parameters, and the nodular cast iron with such a base structure can obtain super-high strength.

[0034] The salt solution used in the heat treatment process of the application is different from the conventional quenching medium, the water content is reduced, the components and proportions of nitrate and nitrite are optimized, a good quenching intensity is obtained, the quenching structure of the blank is refined, and the mechanical strength of the nodular cast iron is improved. In addition, the ideal base structure is achieved by controlled cooling and speed, that is, a certain depth of austenite in the surface layer of the blank and a structure mainly composed of refined pearlite in other positions.

[0035] In addition, the application also protects the nodular cast iron material obtained by the preparation process, the mass percentage of each component of the nodular cast iron is C 3.0% to 3.8%, Si 1.5% to 2.7%, Mn≤0.5%, P<0.03%, S 0.004% to 0.01%, Re≤0.03%, Mg 0.025% to 0.05%, and the balance is iron and impurities. The nodular cast iron material proposed in the application does not add additional alloy, and a special structure is formed by controlling and optimizing the process, so that the performance of the nodular cast iron material exceeds that of high-alloy nodular cast iron, and the material cost is reduced.

[0036] This application obtains ductile iron with a certain depth of austenite on its surface and pearlite internal structure. Furthermore, different morphologies of austenite can be obtained by adjusting the process, thereby acquiring products with different performance requirements. This application characterizes the metallographic structure of the obtained ductile iron material according to the national standard GB / T 9441-2021 "Metallographic Examination of Ductile Iron", as shown in the attached figure. Figure 1 As shown, the mass percentage of pearlite is 90-97%, and the depth of ductile iron austenite is 1-5 mm.

[0037] This embodiment tested the mechanical properties of ductile iron, such as tensile strength, yield strength, elongation, and hardness, based on the composition and preparation process.

[0038] Table 1. Preferred ductile iron composition in this embodiment.

[0039] Table 2 Quenching process parameters and spheroidizing agent addition amounts used in different embodiments of this application

[0040] Table 3. Mechanical property test results obtained from different embodiments of this application.

[0041] Table 4. Graphite morphology and matrix structure of different embodiments of this application

[0042] Combining the above table and appendix Figure 2 and 3 The embodiments of this application use the above-mentioned components and preparation process to obtain ductile iron with low spheroidization grade, and the matrix structure is mainly composed of pearlite and ferrite, with pearlite accounting for 95%. The surface layer is austenite and the interior is pearlite. The obtained ductile iron exhibits excellent performance in terms of tensile strength, yield strength, elongation and hardness.

[0043] In addition, a comparative example is included in this embodiment to determine the effects of quenching process and spheroidizing agent addition amount on the microstructure and mechanical properties of ductile iron. The composition of the ductile iron in the comparative example adopts the composition range defined in this application.

[0044] Table 5 Comparative Example Process Parameters

[0045] Table 6. Graphite morphology and matrix structure of ductile iron obtained in the comparative examples.

[0046] Based on the above embodiments, in Comparative Example 1, the KNO3 salt content in the salt component was too high, resulting in an insufficient cooling rate and a pearlite content of less than 95%. In Comparative Example 2, the water content added to the salt bath was too high, leading to an excessively fast salt quenching rate and the formation of martensite, which deteriorated the performance. In Comparative Example 3, the amount of spheroidizing agent added was insufficient, resulting in poor spheroidization, a high spheroidization grade, and a pearlite content of less than 95% in the microstructure. In Comparative Example 4, the quenching temperature was too low, resulting in an excessively fast cooling rate and the formation of martensite, which also deteriorated the performance.

[0047] The following specific example illustrates the preparation process of ductile iron.

[0048] Step 1, Furnace charge preparation: Weigh the required amounts of pig iron, scrap steel, carbon raiser, and ferromanganese according to the chemical composition of ductile iron.

[0049] Step 2, Smelting: First, add the scrap steel and carbonizer weighed in Step 1 into the heating furnace, then add pig iron. After all the furnace charge has melted, add a slag remover to remove slag from the molten iron. When the smelting temperature reaches 1530-1560℃, allow it to stand at high temperature for slag removal. When the molten iron temperature drops to 1500-1530℃, add ferromanganese and ferrosilicon to obtain molten iron.

[0050] Step 3, spheroidizing and inoculation treatment: First, add 1.0% to 1.5% of the total weight of the molten iron obtained in Step 2 to the spheroidizing pit inside the ladle and tamp it down. Then, add 0.1% to 0.5% of the total weight of the molten iron to the bottom of the ladle as an inoculant. Finally, cover the bottom inoculant with 0.4% to 1.0% of the total weight of the molten iron using silicon steel sheets. The spheroidizing agent is a rare earth magnesium-silicon-iron alloy, with a Re content ≤0.7% and a Mg content ≤5%.

[0051] The molten iron obtained in step 2 is tapped out in two stages. First, a portion of the molten iron is directly poured into the ladle. Then, the ladle is covered with a lid to carry out spheroidization and bottom inoculation. After the spheroidization reaction in the ladle is completed, the lid is removed, and the inoculant is added to the ladle. The amount of inoculant added is 0.5% to 0.7% of the total weight of the molten iron. Then, the remaining molten iron is tapped out.

[0052] After the molten iron is tapped, the molten iron in the ladle is stirred and the slag is removed. After the slag is removed, the ladle is turned over. The amount of inoculant added during the turning over is 0.5 to 0.7% of the total weight of the molten iron.

[0053] Step 4: When the temperature of the molten iron after spheroidization and inoculation reaches 1330℃~1400℃, pour it into the mold. During the pouring process, add 0.2%~0.4% of the total weight of the molten iron inoculant above the molten iron flow for in-flow inoculation. After pouring, wait for the mold to cool naturally to a temperature ≤300℃, open the mold, remove the sand and clean it to obtain a plain ultra-high strength ductile iron blank.

[0054] Step 5, heat treatment process of ductile iron: The blank is shot blasted, ground and finished to remove defects such as sand adhering to the surface, and relevant flaw detection is carried out to confirm whether the blank is qualified. If it is qualified, it is transferred to the subsequent heat treatment. The blank is heated to 800~1000℃ in an electric furnace and held for 1~5 hours. After holding, the blank is quickly immersed in a salt solution to perform heat treatment using the quenching medium. The salt quenching temperature is 250~400℃, and the salt quenching time is 2~5 hours. In addition, a stirrer is installed in the heat treatment medium, and the stirring rate is adjusted according to the casting to ensure the uniformity of the quenching medium flow.

[0055] In summary, the preparation method and the proposed ductile iron in this embodiment achieve the following technical effects: ① High-performance ductile iron is obtained by using low-alloy components without adding additional alloys, reducing material costs. ② By optimizing the heat treatment process and quenching medium composition, better quenching intensity is obtained, refining the grain size of the quenched blank and improving mechanical properties. ③ The inoculation process employs multi-stage inoculation, including in-laminate inoculation, pouring inoculation, inverted inoculation, and in-flow inoculation. Multiple additions of silicon-based inoculants improve the uniformity of the ductile iron structure, refine the grain size, increase the roundness and number of graphite spheroids, and improve the metallurgical quality of the blank. ④ The quality of conventional ductile iron grades is improved, making it applicable to thick and large products such as crankshafts, wind turbine planetary carriers, and ductile iron ingots.

[0056] This application scheme achieves a high spheroidization rate, a large number of graphite spheroids, a high grade of graphite spheroid size, and a dense, shrinkage-free material body through the smelting process. A special heat treatment process, primarily involving different cooling medium ratios, controls the cooling rate to achieve an ideal matrix structure: a certain depth of ausferrite on the surface of the blank, with refined pearlite dominating other areas. In other words, through smelting and special heat treatment processes, a microstructure of excellent graphite morphology + ausferrite + refined pearlite is ultimately obtained. Ductile iron with this matrix structure can achieve ultra-high strength.

[0057] As described above, although the invention has been shown and described with reference to specific preferred embodiments, it should not be construed as limiting the invention itself. Various changes in form and detail may be made without departing from the spirit and scope of the invention as defined in the appended claims.

Claims

1. A method for preparing ductile iron, characterized in that, It includes the following steps: Prepare a ductile iron blank, heat and hold the blank. After the holding is completed, perform quenching treatment on the blank in a quenching medium to obtain the ductile iron. The quenching temperature of the quenching treatment is 250 - 400 °C, the quenching time is 2 - 5 h. The mass percentage of the salt components in the quenching medium used for the quenching treatment is 25 - 50% KNO3, 25 - 50% NaNO2, 10 - 35% NaNO3, and the water component in the quenching medium accounts for 3 - 20% of the salt components. After the quenching treatment, austenite is formed on the surface layer of the ductile iron, and the internal structure of the ductile iron is pearlite, and the mass ratio of the pearlite is 90 - 97%.

2. The method for preparing ductile iron according to claim 1, characterized in that, The mass percentage of the salt components in the quenching medium is 42 - 52% KNO3, 40 - 50% NaNO2, 1 - 10% NaNO3, and H2O in the quenching medium accounts for 3 - 10% of the total mass of the salt components.

3. The method for preparing ductile iron according to claim 1 or 2, characterized in that, The method for preparing the ductile iron blank includes the following steps: Heat raw materials including scrap steel and pig iron to 1530 - 1560 °C, let it stand until it cools down to 1500 - 1530 °C, then add ferromanganese and ferrosilicon, and carry out melting to obtain molten iron; Add a nodulizer and an inoculant, perform nodulization and multiple inoculations on the molten iron; Pour to obtain a ductile iron blank.

4. The method for preparing ductile iron according to claim 3, characterized in that, Perform inoculation on the molten iron at least four times.

5. The method for preparing ductile iron according to claim 3, characterized in that, The nodulization and inoculation treatment method includes: Add a nodulizer and a first inoculant, then add at least a part of the molten iron, and perform nodulization and the first inoculation in a ladle; Add a second inoculant to the ladle for the second inoculation, and add the remaining molten iron.

6. The method for preparing ductile iron according to claim 5, characterized in that, The nodulization and inoculation treatment method further includes a third inoculation treatment: Add a third inoculant to the ladle after the second inoculation treatment.

7. The method for preparing ductile iron according to claim 6, characterized in that, The nodulizer component is a rare earth ferrosilicon magnesium alloy, in which the component mass percentage is 0.1% < Re ≤ 0.7%, 1% < Mg ≤ 5%, and the addition amount of the nodulizer is 1.0% - 1.5% of the total mass of the molten iron.

8. The method for preparing ductile iron according to claim 6 or 7, characterized in that, The first inoculant, the second inoculant, and the third inoculant are each independently selected from at least one of a silicon-barium inoculant, a silicon-calcium inoculant, or a silicon-calcium-barium inoculant.

9. The method for preparing ductile iron according to claim 5 or 6, characterized in that, The addition amounts of the first inoculant and the second inoculant are 0.5 - 0.7% of the total mass of the molten iron.

10. The method for preparing ductile iron according to claim 6, characterized in that, The addition amount of the third inoculant is 0.2 - 0.4% of the total mass of the molten iron.

11. The method for preparing ductile iron according to claim 3, characterized in that, During the pouring process, add a fourth inoculant for in-stream inoculation during pouring. The fourth inoculant is selected from a sulfur-oxygen inoculant or a silicon-barium inoculant, and the addition amount of the fourth inoculant is 0.2 - 0.4% of the total mass of the molten iron.

12. A type of ductile iron, characterized in that, The mass percentages of the components of the ductile iron are as follows: C 3.0% - 3.8%, Si 1.5% - 2.7%, 0 < Mn ≤ 0.5%, 0 < P < 0.03%, S 0.004% - 0.01%, 0 < Re ≤ 0.03%, Mg 0.025% - 0.05%, and the balance is iron and impurities. Austenite is formed on the surface layer of the ductile iron, and the internal structure of the ductile iron is pearlite, and the mass ratio of the pearlite is 90 - 97%.

13. The ductile iron according to claim 12, characterized in that, The depth of the ductile iron ferrometalite is 1~5mm.

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