Isothermal quenching nodular cast iron and preparation method thereof

Abstract

The invention belongs to the technical field of nodular cast iron manufacturing, and particularly relates to isothermal quenching nodular cast iron and a preparation method thereof.The CEL of the isothermal quenching nodular cast iron is 4.3-4.33, and the isothermal quenching nodular cast iron comprises, by mass, 3.4%-4.02% of C, 2.2%-2.8% of Si, smaller than or equal to 0.2% of Mn, smaller than or equal to 0.02% of S and smaller than or equal to 0.03% of P. The preparation method comprises the steps of raw material selection, burdening, smelting, spheroidizing inoculation, pouring, isothermal quenching treatment and the like. According to the isothermal quenching nodular cast iron, the mechanical property of the national standard ADI brand QTD800-10 is improved and greatly surpassed, the tensile strength of the isothermal quenching nodular cast iron is larger than or equal to 1000 MPa, the elongation is larger than or equal to 10%, the spheroidizing grade is larger than or equal to grade 2, the graphite grade is larger than or equal to grade 6, a base body is ausferrite, and on the premise that the high plasticity and toughness of the material are guaranteed, the strength of the material is greatly improved. The high-degree improvement of the strength can greatly meet and expand the application range and field of the material.

Description

Technical Field

[0001] The invention belongs to the technical field of ductile iron manufacturing, and particularly relates to isothermal quenching ductile iron and a preparation method thereof. Background Art

[0002] Ductile iron is carbon in the form of spherical graphite. Its mechanical properties are far superior to those of gray iron and close to those of steel. It has excellent casting, cutting and wear resistance, and has a certain degree of elasticity. It is widely used in the manufacture of high-end castings such as crankshafts, gears, pistons, automobile wheel hub brackets, and various mechanical parts.

[0003] The isothermal quenched ductile iron with the grade QTD800-10 in GBT24733-2009 has a tensile strength of ≥800MPa and an elongation of ≥10%. It is a material with high strength and high plasticity and toughness. It is generally suitable for automotive parts and various engineering connectors. It can replace alloy steel and high manganese steel connectors. However, since the material tensile strength is only 800MPa, the existing conventional cast ductile iron stipulates that the spheroidization grade is not less than grade 3, which is greatly limited in the production of some parts that require high strength and high toughness. In addition, the conventional isothermal quenching process uses a salt bath cooling system that is air-cooled or water-cooled outside the bath, and the cooling effect is slow, which is not conducive to further improving the tensile strength of the material.

[0004] Therefore, those skilled in the art urgently need to develop an isothermal quenched ductile iron and a preparation method thereof, which can ensure that the material has a tensile strength of more than 1000 MPa and a spheroidization grade of more than level 2 while ensuring high plasticity and toughness. Summary of the invention

[0005] The purpose of the present invention is to provide an isothermal quenched ductile iron and a preparation method thereof which can exceed the mechanical properties of the national standard ADI grade QTD800-10, so that the material has a tensile strength of more than 1000MPa, an elongation of ≥10%, and a spheroidization grade of more than level 2 while ensuring high plasticity and toughness, so as to greatly meet and expand the application scope and field of the material.

[0006] To achieve the above object, the present invention is implemented according to the following technical solutions:

[0007] An austempered ductile iron having a CEL of 4.3-4.33 and a chemical composition measured in mass percentage comprising:

[0008] C: 3.4% to 4.02%, Si: 2.2% to 2.8%, Mn≤0.2%, S≤0.02%, P≤0.03%.

[0009] Preferably, the chemical composition of the austempered ductile iron measured in mass percentage includes:

[0010] C: 3.60% to 4.02%, Si: 2.28% to 2.45%, Mn: 0.10% to 0.18%, S: 0.014% to 0.019%, P: 0.021% to 0.027%.

[0011] Preferably, the tensile strength of the austempered ductile iron is ≥1000 MPa, the elongation is ≥10%, the spheroidization grade is ≥2, the graphite grade is ≥6, and the matrix is ​​ausferrite.

[0012] The present invention also proposes a method for manufacturing the austempered ductile iron, comprising the following steps:

[0013] S1. Raw materials: including ductile iron, low carbon scrap steel, petroleum coke recarburizer and silicon carbide;

[0014] S2, batching: the raw materials in step S1 are batched according to a certain ratio, and then smelted in a medium frequency furnace, the CEL and chemical composition of the molten iron are detected by a molten iron quality controller, and the CEL and chemical composition of the molten iron are adjusted to meet the CEL and chemical composition requirements of the austempered ductile iron by adjusting the batching ratio;

[0015] S3, smelting and spheroidizing inoculation: adding raw materials for smelting according to the adjusted proportion of ingredients that meet the CEL and chemical composition requirements of isothermal quenching ductile iron, controlling the smelting temperature at 1500-1550°C, and after standing at this temperature for 3 minutes, pouring the molten iron into the spheroidizing bag for spheroidizing inoculation treatment;

[0016] S4, pouring, after the spheroidization incubation in step S3 is completed, the slag is removed, and then the molten iron is transferred from the spheroidizing ladle to the pouring ladle, and then transferred to the pouring area to pour the clean molten iron into the sand mold to complete the pouring and obtain the casting;

[0017] S5. Isothermal quenching treatment: the casting is placed in a box furnace, and the temperature is increased at a rate of 2°C / min for 2 hours. After the furnace temperature reaches 900°C, the casting is kept warm for 2 hours. After the insulation is completed, the casting is quenched into an isothermal salt bath within 12 seconds or less. The salt bath material in the isothermal salt bath is 50% sodium nitrate and 50% potassium nitrate. When the casting is quenched into the isothermal salt bath, cooling water is evenly added to the bottom of the isothermal salt bath for temperature control due to the rapid increase in temperature. The amount of cooling water added is 2-3% of the salt bath material. The temperature of the isothermal salt bath is rapidly reduced within 5 minutes and controlled at 380°C±5°C by rolling and stirring the cooling water in the isothermal salt bath. The casting is taken out of the furnace after being kept warm in the salt bath for 1 hour, and the isothermal quenched ductile iron is obtained.

[0018] Preferably, in step S3, the spheroidization inoculation treatment comprises the following steps:

[0019] According to the quality of the molten iron to be spheroidized and inoculated, a certain mass of spheroidizer, inoculant and ductile iron chips are placed in the bottom of the spheroidizing ladle from bottom to top, and then the molten iron is flushed into the spheroidizing ladle by a one-time flushing method; wherein, the spheroidizer is 2-6 spheroidizer, the inoculant is silicon-barium-calcium inoculant, the spheroidizing temperature is 1480-1520℃, and the spheroidizing reaction is ensured to be detonated after a delay of 20-30 seconds, the spheroidizing time is not less than 1 minute, and the slag is removed after the spheroidizing reaction stops.

[0020] Preferably, in step S3, the mass of molten iron added to the spheroidizing bag is 200 kg (after the molten iron is injected, the liquid surface is at a safe distance of 10-15 cm from the upper edge of the spheroidizing bag). The amount of spheroidizing agent added is 1.1% of the mass of molten iron, the amount of inoculant added is 0.4% of the mass of molten iron, and the amount of iron filings added is 1-2 kg.

[0021] Preferably, in step S3, the particle size of the 2-6 spheroidizing agent is 3-8 mm, and the particle size of the silicon-barium-calcium inoculant is 1-3 mm.

[0022] Preferably, during pouring in step S4, a flow inoculant is used for flow inoculation, the flow inoculant is a Bi-containing inoculant with a particle size of 40-140 mesh, the addition amount of the Bi-containing inoculant is 70 g / min, and the pouring temperature is 1350-1380°C.

[0023] The working principle of the present invention is as follows: in the present invention, carbon is used to increase the graphitization degree of ductile iron and reduce the tendency of white cast iron. When the carbon content is high, it is beneficial to increase the spheroidization rate and graphite grade and improve the toughness of the material; silicon is an element that promotes graphitization, can improve the tensile strength, yield strength and hardness of ductile iron, and can increase the brittle transition temperature of ductile iron; manganese is an element with white cast iron tendency, which has a particularly adverse effect on impact toughness and elongation, so the present application controls the manganese content to a minimum level; sulfur is an anti-spheroidization element; phosphorus is a phosphorus eutectic forming element, which greatly affects the plasticity and toughness of ductile iron and improves the brittle transition temperature; the present invention directly adds 2-3% of water into the isothermal salt bath to achieve the dual effects of cooling and stirring, the cooling effect is fast and the composition of the cooling medium is changed to a certain extent, so that the material has a greatly improved tensile strength under the premise of ensuring high plasticity and toughness, which far exceeds the range specified by the national standard.

[0024] The beneficial effects of the present invention are:

[0025] The austempered ductile iron of the present invention improves and greatly exceeds the mechanical properties of the national standard ADI grade QTD800-10, has a tensile strength of ≥1000MPa, an elongation of ≥10%, a spheroidizing grade of ≥2, a graphite grade of ≥6, and a matrix of ausferrite. Under the premise of ensuring high plasticity and toughness, the high degree of strength improvement can greatly meet and expand the application scope and field of the material. BRIEF DESCRIPTION OF THE DRAWINGS

[0026] Figure 1 This is a metallographic photograph of the material in Example 1.

[0027] Figure 2 This is a photo of the material matrix in Example 1. DETAILED DESCRIPTION

[0028] The present invention will be further described below with reference to specific embodiments in conjunction with the accompanying drawings. The illustrative embodiments and descriptions of the present invention are used to explain the present invention but are not intended to limit the present invention.

[0029] All raw materials of the present invention are not particularly limited in their sources and can be purchased from the market or prepared according to conventional methods well known to those skilled in the art.

[0030] There is no particular limitation on the purity of all raw materials in the present invention, and the present invention preferably adopts the conventional purity used in the art.

[0031] The devices used in the present invention are not particularly limited and can be any of the commonly used devices in the art.

[0032] Example 1

[0033] A kind of austempered ductile iron, with chemical composition of C: 3.60%, Si: 2.35%, Mn: 0.17%, S: 0.019%, P: 0.026%, and the balance is Fe, and is used for making end bearings.

[0034] Its metallographic and matrix photos (magnified 100 times) are as follows Figure 1 , Figure 2 As shown, it can be seen that the spheroidization rate of the material reaches 95%, the graphite grade reaches level 8, the whole is ausferrite, and there is no phosphorus eutectic.

[0035] Example 2

[0036] A kind of austempered ductile iron, with chemical composition of C: 3.68%, Si: 2.32%, Mn: 0.15%, S: 0.014%, P: 0.021%, and the balance of Fe, is used for making Jiaolong.

[0037] Example 3

[0038] A isothermal quenching ductile iron has the following chemical compositions: C: 3.72%, Si: 2.28%, Mn: 0.17%, S: 0.018%, P: 0.023%, Cu: 0.6%, Mo: 0.12%, and the balance is Fe, and is used for making a plug plate.

[0039] Example 4

[0040] A isothermal quenching ductile iron has the following chemical compositions: C: 3.69%, Si: 2.42%, Mn: 0.11%, S: 0.014%, P: 0.023%, and the balance is Fe, and is used for making a coupler.

[0041] Example 5

[0042] A kind of austempered ductile iron, with chemical composition of C: 3.65%, Si: 2.38%, Mn: 0.10%, S: 0.015%, P: 0.025%, and the balance of Fe, is used for making sprockets.

[0043] Example 6

[0044] A kind of austempered ductile iron, with chemical composition of C: 4.02%, Si: 2.29%, Mn: 0.12%, S: 0.016%, P: 0.021%, and the balance of Fe, is used for making the bottom of the ship cabin.

[0045] Example 7

[0046] A isothermal quenching ductile iron has the following chemical compositions: C: 4.02%, Si: 2.30%, Mn: 0.15%, S: 0.016%, P: 0.025%, and the balance is Fe, and is used for manufacturing an anti-collision beam.

[0047] Example 8

[0048] A isothermal quenching ductile iron has the chemical composition of 4.02% C, 2.45% Si, 0.10% Mn, 0.015% S, 0.027% P and the balance of Fe, and is used for making support beams.

[0049] Example 9

[0050] A isothermal quenching ductile iron has the following chemical components: C: 4.02%, Si: 2.41%, Mn: 0.18%, S: 0.018%, P: 0.021%, and the balance is Fe, and is used for making a V-shaped support.

[0051] Example 10

[0052] A isothermal quenching ductile iron has the following chemical compositions: C: 4.02%, Si: 2.39%, Mn: 0.13%, S: 0.016%, P: 0.027%, Cu: 0.64%, Mo: 0.15%, and the balance is Fe, and is used for making scrapers.

[0053] According to GBT228.1-2021 "Tensile test of metallic materials - Part 1 - Room temperature test method", the castings in the above Examples 1 to 10 were tested for mechanical properties, and the test results are shown in the following table:

[0054] Mechanical properties test results of austempered ductile iron test blocks in Examples 1 to 5

[0055]

[0056] The above test results show that the austempered ductile iron proposed by the present invention has a tensile strength of ≥1000MPa, an elongation of ≥10%, a spheroidization grade of ≥2, a graphite grade of ≥6, and a matrix of ausferrite.

[0057] Embodiment 11

[0058] This embodiment proposes a method for manufacturing austempered ductile iron based on the above embodiments 1 to 10, comprising the following steps:

[0059] S1. Raw materials: including ductile iron, low carbon scrap steel, petroleum coke recarburizer and silicon carbide;

[0060] S2, batching: the raw materials in step S1 are batched according to a certain ratio, and then smelted in a medium frequency furnace, the CEL and chemical composition of the molten iron are detected by a molten iron quality controller, and the CEL and chemical composition of the molten iron are adjusted to meet the CEL and chemical composition requirements of the austempered ductile iron by adjusting the batching ratio;

[0061] S3, smelting and spheroidizing inoculation: adding raw materials for smelting according to the adjusted proportion of ingredients that meet the CEL and chemical composition requirements of isothermal quenching ductile iron, controlling the smelting temperature at 1500-1550°C, and after standing at this temperature for 3 minutes, pouring the molten iron into the spheroidizing bag for spheroidizing inoculation treatment;

[0062] S4, pouring, after the spheroidization incubation in step S3 is completed, the slag is removed, and then the molten iron is transferred from the spheroidizing ladle to the pouring ladle, and then transferred to the pouring area to pour the clean molten iron into the sand mold to complete the pouring and obtain the casting;

[0063] S5. Isothermal quenching treatment: the casting is placed in a box furnace, and the temperature is increased at a rate of 2°C / min for 2 hours. After the furnace temperature reaches 900°C, the casting is kept warm for 2 hours. After the insulation is completed, the casting is quenched into an isothermal salt bath within 12 seconds or less. The salt bath material in the isothermal salt bath is 50% sodium nitrate and 50% potassium nitrate. When the casting is quenched into the isothermal salt bath, cooling water is evenly added to the bottom of the isothermal salt bath for temperature control due to the rapid increase in temperature. The amount of cooling water added is 2-3% of the salt bath material. The temperature of the isothermal salt bath is rapidly reduced within 5 minutes and controlled at 380°C±5°C by rolling and stirring the cooling water in the isothermal salt bath. The casting is taken out of the furnace after being kept warm in the salt bath for 1 hour, and the isothermal quenched ductile iron is obtained.

[0064] Specifically, in step S3, the spheroidization inoculation treatment includes the following steps:

[0065] According to the quality of the molten iron to be spheroidized and inoculated, a certain amount of spheroidizer, inoculant and ductile iron filings are placed in the bottom of the spheroidizing ladle from bottom to top, and then the molten iron is flushed into the spheroidizing ladle by a one-time flushing method; wherein, the spheroidizer is 2-6 spheroidizer (2-6 is the brand of the spheroidizer, 2 and 6 represent the percentage content of rare earth and magnesium respectively), the inoculant is silicon-barium-calcium inoculant, the spheroidizing temperature is 1480-1520℃, and the spheroidizing reaction is ensured to be detonated after 20-30 seconds of delay, the spheroidizing time is not less than 1 minute, and the slag is removed after the spheroidizing reaction stops. wherein, the particle size of the 2-6 spheroidizer is 3-8mm, and the particle size of the silicon-barium-calcium inoculant is 1-3mm.

[0066] More specifically, in step S3, the mass of molten iron added to the spheroidizing bag is 200 kg (after the molten iron is injected, the liquid surface is at a safe distance of 10-15 cm from the upper edge of the spheroidizing bag). The amount of spheroidizing agent added is 1.1% of the mass of molten iron, the amount of inoculant added is 0.4% of the mass of molten iron, and the amount of iron filings added is 1-2 kg.

[0067] In this embodiment, during pouring in step S4, a flow inoculant is used for flow inoculation, the flow inoculant is a Bi-containing inoculant with a particle size of 40-140 mesh, the addition amount of the Bi-containing inoculant is 70 g / min, and the pouring temperature is 1350-1380°C.

[0068] The working principle of the present invention is as follows: in the present invention, carbon is used to increase the graphitization degree of ductile iron and reduce the tendency of white cast iron. When the carbon content is high, it is beneficial to increase the spheroidization rate and graphite grade and improve the toughness of the material; silicon is an element that promotes graphitization, can improve the tensile strength, yield strength and hardness of ductile iron, and can increase the brittle transition temperature of ductile iron; manganese is an element with white cast iron tendency, which has a particularly adverse effect on impact toughness and elongation, so the present application controls the manganese content to a minimum level; sulfur is an anti-spheroidization element; phosphorus is a phosphorus eutectic forming element, which greatly affects the plasticity and toughness of ductile iron and improves the brittle transition temperature; the present invention directly adds 2-3% of water into the isothermal salt bath to achieve the dual effects of cooling and stirring, the cooling effect is fast and the composition of the cooling medium is changed to a certain extent, so that the material has a greatly improved tensile strength under the premise of ensuring high plasticity and toughness, which far exceeds the range specified by the national standard.

[0069] The technical solution of the present invention is not limited to the above-mentioned specific embodiments. All technical variations made according to the technical solution of the present invention fall within the protection scope of the present invention.

Claims

1. An isothermal quenching ductile iron, characterized in that: The CEL of the austempered ductile iron is 4.3-4.33, and the chemical composition measured by mass percentage includes: C: 3.4% to 4.02%, Si: 2.2% to 2.8%, Mn≤0.2%, S≤0.02%, P≤0.03%.

2. The austempered ductile iron according to claim 1, characterized in that: The chemical composition of the austempered ductile iron is measured in mass percentage and includes: C: 3.60% to 4.02%, Si: 2.28% to 2.45%, Mn: 0.10% to 0.18%, S: 0.014% to 0.019%, P: 0.021% to 0.027%.

3. The austempered ductile iron according to claim 1 or 2, characterized in that: The tensile strength of the austempered ductile iron is ≥1000MPa, the elongation is ≥10%, the spheroidization grade is ≥2, the graphite grade is ≥6, and the matrix is ​​ausferrite.

4. A method for manufacturing austempered ductile iron according to claim 3, characterized in that: The following steps are involved: S1. Raw materials: including ductile iron, low carbon scrap steel, petroleum coke recarburizer and silicon carbide; S2, batching: the raw materials in step S1 are batched according to a certain ratio, and then smelted in a medium frequency furnace, the CEL and chemical composition of the molten iron are detected by a molten iron quality controller, and the CEL and chemical composition of the molten iron are adjusted to meet the CEL and chemical composition requirements of the austempered ductile iron by adjusting the batching ratio; S3, smelting and spheroidizing inoculation: adding raw materials for smelting according to the adjusted proportion of ingredients that meet the CEL and chemical composition requirements of isothermal quenching ductile iron, controlling the smelting temperature at 1500-1550°C, and after standing at this temperature for 3 minutes, pouring the molten iron into the spheroidizing bag for spheroidizing inoculation treatment; S4, pouring, after the spheroidization incubation in step S3 is completed, the slag is removed, and then the molten iron is transferred from the spheroidizing ladle to the pouring ladle, and then transferred to the pouring area to pour the clean molten iron into the sand mold to complete the pouring and obtain the casting; S5. Isothermal quenching treatment: the casting is placed in a box furnace, and the temperature is increased at a rate of 2°C / min for 2 hours. After the furnace temperature reaches 900°C, the casting is kept warm for 2 hours. After the insulation is completed, the casting is quenched into an isothermal salt bath within 12 seconds or less. The salt bath material in the isothermal salt bath is 50% sodium nitrate and 50% potassium nitrate. When the casting is quenched into the isothermal salt bath, cooling water is evenly added to the bottom of the isothermal salt bath for temperature control due to the rapid increase in temperature. The amount of cooling water added is 2-3% of the salt bath material. The temperature of the isothermal salt bath is rapidly reduced within 5 minutes and controlled at 380°C±5°C by rolling and stirring the cooling water in the isothermal salt bath. The casting is taken out of the furnace after being kept warm in the salt bath for 1 hour, and the isothermal quenched ductile iron is obtained.

5. The method for producing austempered ductile iron according to claim 4, characterized in that: In step S3, the spheroidization inoculation treatment includes the following steps: According to the quality of the molten iron to be spheroidized and inoculated, a certain mass of spheroidizer, inoculant and ductile iron chips are placed in the bottom of the spheroidizing ladle from bottom to top, and then the molten iron is flushed into the spheroidizing ladle by a one-time flushing method; wherein, the spheroidizer is 2-6 spheroidizer, the inoculant is silicon-barium-calcium inoculant, the spheroidizing temperature is 1480-1520℃, and the spheroidizing reaction is ensured to be detonated after a delay of 20-30 seconds, the spheroidizing time is not less than 1 minute, and the slag is removed after the spheroidizing reaction stops.

6. The method for producing austempered ductile iron according to claim 5, characterized in that: In step S3, the mass of molten iron added to the spheroidizing bag is 200 kg, the amount of spheroidizing agent added is 1.1% of the mass of molten iron, the amount of inoculant added is 0.4% of the mass of molten iron, and the amount of iron filings added is 1-2 kg.

7. The method for producing austempered ductile iron according to claim 5, characterized in that: In step S3, the particle size of the 2-6 spheroidizer is 3-8 mm, and the particle size of the silicon-barium-calcium inoculant is 1-3 mm.

8. The method for producing austempered ductile iron according to claim 4, characterized in that: In step S4, during pouring, a flow inoculant is used for flow inoculation. The flow inoculant is a Bi-containing inoculant with a particle size of 40 to 140 meshes, the addition amount of the Bi-containing inoculant is 70 g / min, and the pouring temperature is 1350 to 1380°C.

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