High-strength and high-toughness low-temperature nodular cast iron and preparation method thereof

Abstract

The application provides a QT450-10LT high-strength and high-toughness low-temperature nodular cast iron and a preparation method thereof, and belongs to the technical field of nodular cast iron. The nodular cast iron comprises the following components in percentage by weight: C: 3.65-3.85%, Si: 1.8-2.1%, Mn: less than 0.025%, P: less than or equal to 0.01%, S: less than or equal to 0.01%, Ni: less than 0.8%, Mg (residual): less than or equal to 0.045%, the total content of other impurity elements is less than 0.01%, and the rest is iron. The preparation method adopts a shell-type buried steel shot + multi-stage heat treatment process, namely high-temperature annealing treatment, three-phase zone rapid cooling treatment and stress relief annealing treatment, so that the prepared low-temperature nodular cast iron can ensure that the low-temperature impact absorption energy at-20 DEG C, -40 DEG C and -50 DEG C is greater than or equal to 12J on the basis of improving the tensile strength and yield strength, and the application field of the low-temperature nodular cast iron is widened.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of nodular cast iron, and particularly relates to a QT450-10LT high-strength and high-toughness low-temperature nodular cast iron and a preparation method thereof. BACKGROUND

[0002] Nodular cast iron is a high-strength cast iron material developed in the late 1940s, and its comprehensive performance is close to that of steel. Due to its excellent mechanical properties, it has been successfully used to cast parts with complex stress, high strength, toughness and wear resistance requirements.

[0003] Nodular cast iron is a cast iron material containing spheroidal graphite obtained by spheroidizing and inoculating the original molten iron. Since the stress concentration caused by the graphite form is eliminated to the maximum extent, the mechanical properties of the cast iron, especially the strength and plastic toughness, are effectively improved. So far, according to the strength from low to high, various grades of nodular cast iron material series have been formed to meet the different performance and special use in the engineering field. For example, QT400-18LT is mainly used for the production of high-iron, wind power and automobile parts due to its high low-temperature impact toughness, and its low-temperature impact absorbing energy can reach more than 12J, which can meet the use requirements under high-cold climate conditions. However, with the promulgation of relevant regulations on low-carbon, energy-saving and environmental protection in various countries in the world, lightweight and green manufacturing of mechanical equipment has become the general trend and has become an important feature of new production forces. Although QT450-10 is a common material grade in the nodular cast iron standard and is widely used in the engineering field, it is only suitable for the use of engineering components under ordinary working conditions due to its low low-temperature toughness, and cannot meet the use performance under low-temperature working conditions. The existing QT400-18LT material is difficult to meet the design and production needs of lightweight rail transit equipment such as high-speed rail due to its low tensile strength and weak carrying capacity. Therefore, it is urgent to develop a higher-grade high-strength and high-toughness low-temperature nodular cast iron material with a tensile strength of not less than 450MPa and a low-temperature impact absorbing energy of not less than 12J at -20℃, -40℃ and -50℃ to meet the use requirements of high-end nodular cast iron parts under low-temperature working conditions in the lightweight manufacturing of mechanical equipment.

[0004] At present, more researches are carried out on nodular cast iron. For example, Chinese patent CN109930058A discloses a kind of -40 DEG C low temperature high strength high toughness nodular cast iron and its preparation method and railway locomotive parts, the chemical composition of the nodular cast iron and its weight percentage are as follows: C 3.6%-3.9%, Si 1.9%-2.1%, Mn<0.2%, P<0.03%, S 0.003%-0.012%, Cu 0.35%-0.5%, Ni 1.1%-1.5%, Re≤0.03%, Mg 0.025%-0.05%, the balance is iron and inevitable impurities. The composition of the nodular cast iron material of the invention realizes the organic integration of high strength, high toughness and low temperature impact toughness on nodular cast iron, can meet higher use requirements, and has the foresight of technology.

[0005] For another example, Chinese patent CN103194660A discloses a kind of low temperature ferrite nodular cast iron material manufacturing method, the chemical composition of the material is C: 3.5-3.9, Si: 1.9-2.3, Mn≤0.2, P≤0.03, S≤0.02, Mg: 0.04-0.06, Ni: 0.25-0.95;The performance of the material reaches: -40 DEG C, -50 DEG C, -60 DEG C under impact energy is all ≥12J, the tensile strength at room temperature is all ≥400MPa, the elongation is all ≥18%, and other main mechanical property indexes are qualified. It can be widely used in cold regions and the manufacturing of mechanical equipment products with low temperature impact resistance and high toughness requirements.

[0006] However, there are few researches on nodular cast iron with QT450-10LT grade at present, and the mechanical properties of the reported QT450-10LT nodular cast iron, especially the low temperature impact absorption energy, cannot meet the requirements well, so the application field of the QT450-10LT nodular cast iron is greatly limited, and therefore it is necessary to develop a kind of QT450-10LT high strength, high toughness low temperature nodular cast iron and its preparation method. SUMMARY

[0007] Based on the deficiencies in the prior art, the present application aims to provide a kind of QT450-10LT high strength, high toughness low temperature nodular cast iron, which has excellent tensile strength, yield strength, elongation after fracture, Brinell hardness and impact energy at -20 DEG C, -40 DEG C and -50 DEG C.

[0008] In order to achieve the above-mentioned purpose, the technical scheme adopted by the present application is as follows:

[0009] In one aspect, the present application provides a high-strength and high-toughness QT450-10LT low-temperature ductile cast iron, comprising the following components by mass percentage: C: 3.65%-3.85%, Si: 1.8%-2.1%, Mn <0.025%, P≤0.01%, S≤0.01%, Ni <0.8%, Mg (residual) ≤0.045%, the total content of other impurity elements being less than 0.01%, and the balance being iron.

[0010] As a preferred embodiment, the QT450-10LT high-strength and high-toughness low-temperature ductile cast iron comprises the following components by mass percentage: C: 3.75%, Si: 2.0%, Mn: 0.017%, P: 0.007%, S: 0.003%, Ni: 0.71%, Mg (residual): 0.034%, the total content of other impurity elements being less than 0.01%, and the balance being iron.

[0011] As another preferred embodiment, the QT450-10LT high-strength and high-toughness low-temperature ductile cast iron comprises the following components by mass percentage: C: 3.81%, Si: 2.05%, Mn: 0.017%, P: 0.005%, S: 0.005%, Ni: 0.75%, Mg (residual): 0.037%, the total content of other impurity elements being less than 0.01%, and the balance being iron.

[0012] As still another preferred embodiment, the QT450-10LT high-strength and high-toughness low-temperature ductile cast iron comprises the following components by mass percentage: C: 3.85%, Si: 2.0%, Mn: 0.021%, P: 0.006%, S: 0.007%, Ni: 0.74%, Mg (residual): 0.038%, the total content of other impurity elements being less than 0.01%, and the balance being iron.

[0013] In another aspect, the present application also provides a preparation method of the above-mentioned QT450-10LT high-strength and high-toughness low-temperature ductile cast iron, comprising the following steps:

[0014] Step one, smelting of the original molten iron:

[0015] The original molten iron is smelted in a medium-frequency induction furnace using high-purity pig iron and carbon scrap steel, and the molten iron is discharged at a temperature of 1500-1520℃;

[0016] Step two, spheroidizing and inoculation treatment:

[0017] The spheroidizing and inoculation treatment is performed using the cover package method; specifically, low-rare earth spheroidizing agent, silicon-based inoculant and covering agent are sequentially added to the bottom of the spheroidizing package, and then the original molten iron smelted in step one is quickly injected into the spheroidizing package for spheroidizing and inoculation treatment to obtain the molten ductile cast iron;

[0018] Step 3: Mold Preparation

[0019] The shell-shaped backfill steel shot molding process is used; the specific method is to assemble the mold in a special sand box, and at the same time fill the shell mold with steel shot to obtain a shell-shaped backfill steel shot mold with a high cooling rate and high rigidity.

[0020] Step 4: Pouring molten iron:

[0021] The molten ductile iron prepared in step two is transferred into a casting ladle for secondary inoculation. Then, the molten iron is poured into the mold cavity of the coated sand shell prepared in step three. After solidification, cooling and sand removal, ductile iron with a cast ferrite matrix is ​​obtained.

[0022] Step 5: Microstructure regulation and treatment:

[0023] The microstructure control treatments include high-temperature annealing, three-phase region rapid cooling, and stress-relief annealing.

[0024] in,

[0025] The high-temperature annealing treatment is as follows: the ductile iron obtained in step four is subjected to high-temperature annealing treatment to eliminate grain boundary carbides or eutectic carbides and to homogenize the composition. The heating temperature is 890-920℃, the holding time is 1-2 hours, and then it is cooled with the furnace.

[0026] The aforementioned three-phase rapid cooling process involves heating the high-temperature annealed ductile iron to 720-800℃, holding it at that temperature for 1-2 hours, and then rapidly cooling it. The cooling method includes mist cooling or salt bath cooling to obtain low-temperature ductile iron with a dual-phase matrix structure.

[0027] The stress-relief annealing process involves heating the low-temperature ductile iron to 450-600℃, holding it at that temperature for 30-60 minutes, and then removing it from the furnace and cooling it to room temperature.

[0028] The aforementioned low rare earth spheroidizing agent is a low rare earth magnesium silicon iron spheroidizing agent with a rare earth content of 0.2%-0.5%.

[0029] The silicon-based inoculant is a barium-containing ferrosilicon-based inoculant with a barium content of 1.8%-2.1%.

[0030] The amount of the low rare earth spheroidizing agent added is 1.0-1.2% of the total mass of the original molten iron; the amount of the silicon-based inoculant added is 0.3-0.5% of the total mass of the original molten iron.

[0031] The inoculant used in the secondary inoculation is a low rare earth magnesium silicon iron spheroidizing agent with a rare earth content of 0.2%-0.5%; the amount of the low rare earth magnesium silicon iron spheroidizing agent added is 0.3-0.5% of the total mass of ductile iron molten iron.

[0032] The shell backfilling steel ball molding process comprises the following steps:

[0033] After the shell is bonded and assembled, it is transported to a steel ball embedding line, and is placed in a special sand box. After the steel ball embedding line automatically embeds the steel ball, it is transported to a pouring area, and then molten iron is poured into the mold. The heating temperature of the core shooter is 200 DEG C, the solidification time is 4 minutes, and the sand shooting time is 8 seconds. The diameter of the steel ball is 3-8 mm.

[0034] Compared with the prior art, the beneficial effects are that:

[0035] (1) The shell can obtain a good appearance of the casting, and the shell has high strength, prevents the type wall from moving out due to the impact of high-temperature molten iron during pouring, and is beneficial to the graphitization expansion of the ductile iron for self-feeding.

[0036] (2) The present application can obtain the ductile iron with better tensile strength, yield strength, and elongation at break by reasonably controlling the weight percentage of various metal elements and the temperature parameters in the preparation process, and can significantly improve the minimum impact absorption energy at-20 DEG C, -40 DEG C and -50 DEG C, thereby widening the application field of low-temperature ductile iron. DETAILED DESCRIPTION

[0037] The following non-limiting examples can make those skilled in the art more fully understand the present application, but do not limit the present application in any way. The following content is only an exemplary description of the scope of the present application, and those skilled in the art can make various changes and modifications to the present application according to the disclosed content, and it should also belong to the scope of the present application.

[0038] When the examples give numerical ranges, it should be understood that, unless otherwise specified by the present application, each numerical range has two endpoints and any number between the two endpoints can be selected. Unless otherwise defined, all technical and scientific terms used in the present application have the same meaning as generally understood by those skilled in the art to which the present application belongs.

[0039] The present application will be further described in the following specific examples. The various chemical reagents used in the examples of the present application are obtained by conventional commercial routes unless otherwise specified.

[0040] Example 1: A QT450-10LT high-strength and high-toughness low-temperature ductile iron and a preparation method thereof

[0041] The weight percentage components include C: 3.75%, Si: 2.0%, Mn: 0.017%, P: 0.007%, S: 0.003%, Ni: 0.71%, Mg (residual): 0.034%, and the rest is iron and inevitable impurities.

[0042] The preparation method comprises the following steps:

[0043] Step one, smelting of the original iron liquid:

[0044] The original iron liquid is smelted in a medium-frequency induction furnace by using high-purity pig iron and carbon scrap steel, and the temperature of the iron liquid when discharged from the furnace is 1500-1520℃.

[0045] Step two, spheroidizing and inoculation treatment:

[0046] The spheroidizing and inoculation treatment is performed by using the cover package method; specifically, low-rare earth spheroidizing agent, silicon-based inoculant and covering agent are sequentially added to the bottom of the spheroidizing package, and then the original iron liquid smelted in step one is quickly injected into the spheroidizing package to perform spheroidizing and inoculation treatment, thereby obtaining the spheroidal graphite cast iron liquid.

[0047] The low-rare earth spheroidizing agent is a low-rare earth magnesium-silicon-iron spheroidizing agent, and the rare earth content is 0.2%-0.5%; the addition amount of the low-rare earth spheroidizing agent is 1.0% of the total mass of the original iron liquid.

[0048] The silicon-based inoculant is a barium-containing silicon-iron-based inoculant, and the barium content is 1.8%-2.1%; the addition amount of the silicon-based inoculant is 0.3% of the total mass of the original iron liquid.

[0049] Step three, preparation of the mold:

[0050] The molding is performed by using the shell backfilling steel shot process; specifically, after the shell is bonded and assembled, it is transported to a steel shot embedding box production line, and then is placed into a special sand box; after the steel shot is automatically embedded by the steel shot embedding box production line, it is transported to a pouring area; the heating temperature of the core shooter is 200℃, the solidification time is 4 min, and the sand shooting time is 8 s; the diameter of the steel shot is 3-8 mm.

[0051] Step four, pouring of the iron liquid:

[0052] The spheroidal graphite cast iron liquid prepared in step two is transferred into a pouring ladle, and secondary inoculation is performed in the pouring ladle; then the iron liquid is poured into the film-coated sand shell mold cavity prepared in step three, and after solidification, cooling and sand removal cleaning, the spheroidal graphite cast iron with a cast ferrite matrix is obtained.

[0053] The inoculant used in the secondary inoculation is a low-rare earth magnesium-silicon-iron spheroidizing agent, and the rare earth content is 0.2%-0.5%; the addition amount of the secondary inoculant is 0.3% of the total mass of the spheroidal graphite cast iron liquid.

[0054] Step five, microstructure regulation and control treatment:

[0055] The microstructure regulation and control treatment comprises high-temperature annealing treatment, three-phase zone rapid cooling treatment and stress relief annealing treatment.

[0056] wherein,

[0057] The high-temperature annealing treatment is that the spheroidal graphite cast iron obtained in step four is subjected to high-temperature annealing treatment to eliminate grain boundary carbide or eutectic carbide and homogenize the composition, the heating temperature is 890-920℃, the holding time is 1-2 hours, and the furnace is cooled after that.

[0058] The three-phase zone rapid cooling treatment is that the spheroidal graphite cast iron after high-temperature annealing is heated to 720-800℃, held for 1-2 hours, and then rapidly cooled, and the cooling mode includes fog cooling or salt bath cooling, so as to obtain the low-temperature spheroidal graphite cast iron with a dual-phase matrix structure.

[0059] The stress relief annealing treatment is that the low-temperature spheroidal graphite cast iron is heated to 450-600℃, held for 30-60 minutes, and then taken out of the furnace and cooled to room temperature, and the sample taken out is the low-temperature spheroidal graphite cast iron.

[0060] Example 2: A QT450-10LT high-strength and high-toughness low-temperature spheroidal graphite cast iron and a preparation method thereof

[0061] The weight percentage components include C: 3.81%, Si: 2.05%, Mn: 0.017%, P: 0.005%, S: 0.005%, Ni: 0.75%, Mg (residual): 0.037%, and the rest is iron and inevitable impurities.

[0062] The preparation method comprises the following steps:

[0063] Step one, smelting of the original molten iron:

[0064] High-purity pig iron and carbon scrap steel are used to smelt the original molten iron in a medium-frequency induction furnace, and the molten iron is discharged at a temperature of 1500-1520℃;

[0065] Step two, spheroidizing and inoculation treatment:

[0066] The spheroidizing and inoculation treatment is performed by the cover package method; specifically, low-rare earth spheroidizing agent, silicon-based inoculant, and covering agent are sequentially added to the bottom of the spheroidizing package, and then the original molten iron smelted in step one is quickly injected into the spheroidizing package for spheroidizing and inoculation treatment to obtain the spheroidal graphite cast iron molten iron;

[0067] The low-rare earth spheroidizing agent is a low-rare earth magnesium-silicon-iron spheroidizing agent, and the rare earth content is 0.2%-0.5%; the addition amount of the low-rare earth spheroidizing agent is 1.2% of the total mass of the original molten iron.

[0068] The silicon-based inoculant is a barium-containing silicon-iron-based inoculant, and the barium content is 1.8%-2.1%; the addition amount of the silicon-based inoculant is 0.5% of the total mass of the original molten iron.

[0069] Step three, mold preparation:

[0070] The shell backfilling steel shot process is used for molding; specifically, the shell is bonded and assembled, then transported to a steel shot embedding line, and placed in a special sand box; after the steel shot is automatically embedded by the steel shot embedding line, it is transported to a pouring area; the heating temperature of the core shooter is 200 DEG C, the solidification time is 4 min, and the sand shooting time is 8 s; the diameter of the steel shot is 3-8 mm;

[0071] Step four, molten iron pouring:

[0072] The molten iron prepared in step two is transferred into a pouring ladle, and secondary inoculation is performed in the pouring ladle; then the molten iron is poured into the coated sand shell mold cavity prepared in step three; after solidification, cooling and sand cleaning, the as-cast ferrite matrix nodular cast iron is obtained.

[0073] The inoculant used in the secondary inoculation is a low-rare earth magnesium-silicon-iron spherulitic agent, and the rare earth content is 0.2%-0.5%; the addition amount of the secondary inoculant is 0.5% of the total mass of the nodular cast iron molten iron.

[0074] Step five, microstructure regulation treatment:

[0075] The microstructure regulation treatment includes high-temperature annealing treatment, three-phase zone rapid cooling treatment and stress relief annealing treatment.

[0076] Among them,

[0077] The high-temperature annealing treatment is that the nodular cast iron obtained in step four is subjected to high-temperature annealing treatment to eliminate grain boundary carbides or eutectic carbides and homogenize the composition; the heating temperature is 890-920 DEG C, the holding time is 1-2 hours, and the post-furnace cooling is performed.

[0078] The three-phase zone rapid cooling treatment is that the nodular cast iron after high-temperature annealing is heated to 720-800 DEG C, held for 1-2 hours, and then rapidly cooled, and the cooling method includes fog cooling or salt bath cooling, so as to obtain low-temperature nodular cast iron with a dual-phase matrix structure.

[0079] The stress relief annealing treatment is that the low-temperature nodular cast iron is heated to 450-600 DEG C, held for 30-60 minutes, and then taken out of the furnace and cooled to room temperature; the sample taken out is the low-temperature nodular cast iron.

[0080] Example 3: QT450-10LT high-strength and high-toughness low-temperature nodular cast iron and a preparation method thereof

[0081] It comprises the following components by weight percentage: C: 3.85%, Si: 2.0%, Mn: 0.021%, P: 0.006%, S: 0.007%, Ni: 0.74%, Mg (residual): 0.038%, and the rest is iron and unavoidable impurities.

[0082] The preparation method comprises the following steps:

[0083] Step one, smelting of the original iron liquid:

[0084] The original iron liquid is smelted in a medium-frequency induction furnace by using high-purity pig iron and carbon scrap steel, and the temperature of the iron liquid when discharged from the furnace is 1500-1520℃.

[0085] Step two, spheroidizing and inoculation treatment:

[0086] The spheroidizing and inoculation treatment is performed by using the cover package method; specifically, low-rare earth spheroidizing agent, silicon-based inoculant and covering agent are sequentially added to the bottom of the spheroidizing package, and then the original iron liquid smelted in step one is quickly injected into the spheroidizing package to perform spheroidizing and inoculation treatment, thereby obtaining the spheroidal graphite cast iron liquid.

[0087] The low-rare earth spheroidizing agent is a low-rare earth magnesium-silicon-iron spheroidizing agent, and the rare earth content is 0.2%-0.5%; the addition amount of the low-rare earth spheroidizing agent is 1.1% of the total mass of the original iron liquid.

[0088] The silicon-based inoculant is a barium-containing silicon-iron-based inoculant, and the barium content is 1.8%-2.1%; the addition amount of the silicon-based inoculant is 0.4% of the total mass of the original iron liquid.

[0089] Step three, preparation of the mold:

[0090] The molding is performed by using the shell backfilling steel shot process; specifically, after the shell is bonded and assembled, it is transported to a steel shot embedding box production line, and then is placed into a special sand box, and after the steel shot is automatically embedded by the steel shot embedding box production line, it is transported to a pouring area; the heating temperature of the core shooter is 200℃, the solidification time is 4 min, and the sand shooting time is 8 s; the diameter of the steel shot is 3-8 mm.

[0091] Step four, pouring of the iron liquid:

[0092] The spheroidal graphite cast iron liquid prepared in step two is transferred into a pouring ladle, and secondary inoculation is performed in the pouring ladle, and then the iron liquid is poured into the film-coated sand shell mold cavity prepared in step three, and after solidification, cooling and sand removal cleaning, the spheroidal graphite cast iron with a cast ferrite matrix is obtained.

[0093] The inoculant used in the secondary inoculation is a low-rare earth magnesium-silicon-iron spheroidizing agent, and the rare earth content is 0.2%-0.5%; the addition amount of the secondary inoculant is 0.4% of the total mass of the spheroidal graphite cast iron liquid.

[0094] Step five, microstructure regulation and control treatment:

[0095] The microstructure regulation and control treatment comprises high-temperature annealing treatment, three-phase zone rapid cooling treatment and stress relief annealing treatment.

[0096] wherein,

[0097] The high-temperature annealing treatment is that the spheroidal graphite cast iron obtained in step four is subjected to high-temperature annealing treatment to eliminate grain boundary carbide or eutectic carbide and homogenize the composition, the heating temperature is 890-920℃, the holding time is 1-2 hours, and the furnace cooling is followed.

[0098] The three-phase zone rapid cooling treatment is that the spheroidal graphite cast iron after high-temperature annealing is heated to 720-800℃, and then subjected to rapid cooling after holding for 1-2 hours, and the cooling mode includes fog cooling or salt bath cooling, so as to obtain the low-temperature spheroidal graphite cast iron with a dual-phase matrix structure.

[0099] The stress relief annealing treatment is that the low-temperature spheroidal graphite cast iron is heated to 450-600℃, and then taken out after holding for 30-60 minutes, and the sample is cooled to room temperature, and the low-temperature spheroidal graphite cast iron is obtained.

[0100] Comparative Example 1

[0101] The difference from Example 1 is that the QT450-10LT high-strength and high-toughness low-temperature spheroidal graphite cast iron comprises the following weight percentage components: C: 3.75%, Si: 1.72%, Mn: 0.017%, P: 0.007%, S: 0.003%, Ni: 0.71%, Mg (residual): 0.034%, and the rest is iron and inevitable impurities, and the preparation method is the same as that of Example 1.

[0102] Comparative Example 2

[0103] The difference from Example 1 is that the QT450-10LT high-strength and high-toughness low-temperature spheroidal graphite cast iron comprises the following weight percentage components: C: 3.75%, Si: 2.25%, Mn: 0.017%, P: 0.007%, S: 0.003%, Ni: 0.71%, Mg (residual): 0.034%, and the rest is iron and inevitable impurities, and the preparation method is the same as that of Example 1.

[0104] Comparative Example 3

[0105] The difference from Example 1 is that the three-phase zone rapid cooling treatment temperature in the preparation process is 850℃, and the others are the same as those of Example 1.

[0106] Comparative Example 4

[0107] The difference from Example 1 is that the stress relief annealing treatment temperature in the preparation process is 400℃, and the others are the same as those of Example 1.

[0108] Effect data

[0109] Experimental method:

[0110] (1) Tensile strength, yield strength and elongation at break: according to GB / T 228.1-2010 "Metallic materials - Tensile testing - Part 1: Method of test at room temperature" to detect;

[0111] (2) Brinell hardness: according to GB / T 231.1-2002 "Metallic materials - Brinell hardness test - Part 1: Test method" to detect;

[0112] (3) Minimum impact energy: according to GB / T 229-2007 "Metallic materials - Charpy pendulum impact test method" to detect.

[0113] The detection data is shown in Table 1 below.

[0114] Table 1

[0115]

[0116] According to the detection data in Table 1 above, it can be seen that the low-temperature nodular cast iron prepared in Examples 1-3 ensures that the tensile strength and yield strength are improved, and the low-temperature impact absorption energy at -20℃, -40℃ and -50℃ is ≥12J, thereby widening the application field of low-temperature nodular cast iron.

[0117] The weight percentage of silicon (Si) in Comparative Example 1 is 1.72%, which is not within the protection range of the present application (1.8-2.1%), and the tensile strength of the obtained nodular cast iron cannot meet the standard requirement of 450 MPa, which is unqualified.

[0118] The weight percentage of silicon (Si) in Comparative Example 2 is 2.25%, which is not within the protection range of the present application (1.8-2.1%), and the tensile strength of the obtained nodular cast iron cannot meet the standard requirement of 450 MPa, which is unqualified.

[0119] The three-phase zone rapid cooling treatment temperature of the nodular cast iron in Comparative Example 3 is 850℃, which is not within the protection range of the present application (720-800℃) and does not meet the process requirement, and the low-temperature impact absorption energy of the obtained nodular cast iron does not meet the standard requirement of ≥12J, which is unqualified.

[0120] The stress relief annealing treatment temperature of the nodular cast iron in Comparative Example 4 is 400℃, which is not within the protection range of the present application (450-600℃) and does not meet the process requirement, and the tensile strength of the obtained nodular cast iron is greatly improved, but the low-temperature impact absorption energy does not meet the standard requirement of ≥12J, which is unqualified.

[0121] Finally, it should be noted that the above content is only used to illustrate the technical solutions of the present application, and is not a limitation on the protection scope of the present application. Simple modifications or equivalent replacements of the technical solutions of the present application made by ordinary skilled in the art do not deviate from the essence and scope of the technical solutions of the present application.

Claims

1. A method for preparing QT450-10LT high-strength, high-toughness low-temperature ductile iron, characterized in that: The preparation method specifically comprises the following steps: Step 1: Smelting of raw molten iron: The raw iron is molten using high-purity pig iron and carbon scrap steel in a medium-frequency induction furnace, and the molten iron tapping temperature is 1500-1520℃. Step 2: Spheroidization and incubation treatment: The spheroidizing and inoculation process is carried out using the ladle method. Specifically, a low rare earth spheroidizing agent, a silicon-based inoculator, and a covering agent are added to the bottom of the spheroidizing ladle in sequence. Then, the original molten iron smelted in step one is quickly poured into the spheroidizing ladle for spheroidizing and inoculation to obtain ductile iron molten iron. Step 3: Mold Preparation The shell-shaped backfill steel shot molding process is used; the specific method is to assemble the mold in a special sand box, and at the same time fill the shell mold with steel shot to obtain a shell-shaped backfill steel shot casting mold. Step 4: Pouring molten iron: The molten ductile iron prepared in step two is transferred into a casting ladle for secondary inoculation. Then, the molten iron is poured into the mold cavity of the coated sand shell prepared in step three. After solidification, cooling and sand removal, ductile iron with a cast ferrite matrix is ​​obtained. Step 5: Microstructure regulation and treatment: The microstructure control treatments include high-temperature annealing, three-phase region rapid cooling, and stress-relief annealing. The high-temperature annealing process is as follows: the ductile iron obtained in step four is subjected to high-temperature annealing to eliminate grain boundary carbides or eutectic carbides and homogenize the composition. The heating temperature is 890-920℃, the holding time is 1-2 hours, and then it is cooled with the furnace. The three-phase region rapid cooling treatment involves heating the high-temperature annealed ductile iron to 720-800℃, holding it at that temperature for 1-2 hours, and then rapidly cooling it. The cooling method includes mist cooling or salt bath cooling to obtain low-temperature ductile iron with a dual-phase matrix structure. The stress-relief annealing process involves heating the low-temperature ductile iron to 450-600℃, holding it at that temperature for 30-60 minutes, and then removing it from the furnace and cooling it to room temperature. The QT450-10LT high-strength, high-toughness low-temperature ductile iron comprises the following components by mass percentage: C: 3.65%-3.85%, Si: 2.0-2.05%, Mn<0.025%, P≤0.01%, S≤0.01%, Ni: 0.71-0.75%, Mg residue≤0.045%, and the total content of other impurity elements is less than 0.01%, with the remainder being iron; The QT450-10LT high-strength, high-toughness low-temperature ductile iron described above... 20℃ 40℃ Impact absorption energy at 50℃ ≥12J.

2. The preparation method according to claim 1, characterized in that: The aforementioned low rare earth spheroidizing agent is a low rare earth magnesium silicon iron spheroidizing agent with a rare earth content of 0.2%-0.5%.

3. The preparation method according to claim 1, characterized in that: The silicon-based inoculant is a barium-containing ferrosilicon-based inoculant with a barium content of 1.8%-2.1%.

4. The preparation method according to claim 1, characterized in that: The amount of the low rare earth spheroidizing agent added is 1.0-1.2% of the total mass of the original molten iron; the amount of the silicon-based inoculant added is 0.3-0.5% of the total mass of the original molten iron.

5. The preparation method according to claim 1, characterized in that: The shell-shaped backfill steel shot molding process includes the following steps: After the shell mold is bonded and assembled, it is transported to the steel shot embedding box production line and placed into a special sand box. After the steel shot embedding box line automatically fills the steel shot, it is transported to the casting area and then molten iron is poured into the mold. The core shooter heating temperature is 200℃, the curing time is 4min, and the sand shooting time is 8s. The diameter of the steel shot is 3-8mm.

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