CN113897536A

CN113897536A - Preparation method of nodular cast iron with excellent toughness and high-temperature performance

Info

Publication number: CN113897536A
Application number: CN202111172633.7A
Authority: CN
Inventors: 帅德国; 帅莉
Original assignee: Xiangyang Jinnaite Machinery Co ltd
Current assignee: Xiangyang Jinnaite Machinery Co ltd
Priority date: 2021-10-06
Filing date: 2021-10-06
Publication date: 2022-01-07
Anticipated expiration: 2041-10-06
Also published as: CN113897536B

Abstract

The invention relates to a preparation method of ductile cast iron with excellent toughness and high-temperature performance, which is characterized by comprising the steps of smelting, spheroidizing, inoculating and pouring, wherein the ductile cast iron obtained by pouring comprises the following components: 3.5-3.9%, Si: 2.3-2.6%, Mn: 0.3-0.6%, Mg: 0.02-0.08%, RE: 0.01-0.05%, Al: 0.01-0.05%, Cr: 1-3%, Cr/Al is 50-200, and the rest is Fe and inevitable impurities.

Description

Preparation method of nodular cast iron with excellent toughness and high-temperature performance

Technical Field

The invention relates to a preparation method of ductile cast iron with excellent toughness and high-temperature performance, which can prepare the ductile cast iron with excellent high-temperature strength and high-temperature oxidation resistance and excellent room-temperature mechanical properties (particularly toughness).

Background

Compared with steel, the cost of the nodular cast iron is lower, the performance of the nodular cast iron is similar to that of the steel, and therefore the nodular cast iron is regarded as an ideal material for replacing the steel by the iron.

In high temperature conditions, such as piston rings, exhaust manifolds, boilers, turbocharger components, etc., a large amount of ductile iron has been used. With higher and higher working requirements, the high-temperature load and high-temperature environment borne by the nodular cast iron material are worse and worse, so that higher requirements are provided for the high-temperature performance of the nodular cast iron. The excellent high-temperature strength and high-temperature oxidation resistance are regarded as important material selection bases for the nodular cast iron applied under high-temperature working conditions.

As known in the art, Al and Cr are elements capable of improving the high-temperature oxidation resistance of nodular cast iron, but the addition amount of Al is generally more than 5%, the addition amount of Cr is higher, and the addition amount of Cr can even be more than 15%, although the high-temperature oxidation resistance can be improved by adding the elements into the nodular cast iron, the toughness of the nodular cast iron is rapidly reduced when the contents of Cr and Al are higher, the material becomes brittle and is easy to crack in the using process, the high-temperature resistant nodular cast iron added with Cr and Al does not consider the strength characteristic at high temperature, the reliability of the high-temperature operation cannot be ensured, and the application use of the nodular cast iron material is limited.

Based on the above knowledge, the inventors of the present invention propose a method for preparing ductile cast iron with both toughness and high temperature performance, especially with excellent high temperature strength and excellent high temperature oxidation resistance, as well as excellent toughness, so as to meet the increasingly severe and complex use requirements under high temperature conditions.

Disclosure of Invention

The invention provides a preparation method of a nodular cast iron material with high-temperature oxidation resistance, high-temperature strength and toughness. The material is not only suitable for the application of the conventional nodular cast iron, but also has good adaptability to parts for high-temperature application.

The technical object of the present invention is achieved by the following means.

The invention provides a preparation method of ductile cast iron with excellent toughness and high-temperature performance, which comprises the steps of smelting, spheroidizing, inoculating and pouring, wherein the ductile cast iron obtained by pouring comprises the following components: 3.5-3.9%, Si: 2.3-2.6%, Mn: 0.3-0.6%, Mg: 0.02-0.08%, RE: 0.01-0.05%, Al: 0.01-0.05%, Cr: 1-3%, Cr/Al is 50-200, and the rest is Fe and inevitable impurities.

As a preferred technical scheme, the nodulizer in the preparation process is a rare earth magnesium silicon nodulizer, more than one inoculation is adopted in the inoculation process, and more than one of stokehole inoculation, ladle inoculation, stream inoculation and in-mold inoculation can be selected by combining the inoculation effect.

The beneficial effects of the invention are mainly obtained by the components of the nodular cast iron obtained by final pouring. The design basis of the components of the nodular cast iron obtained by pouring in the invention is described below.

C: carbon is an element constituting the graphite structure, and is advantageous for improving the molten iron fluidity of graphite cast iron. The carbon is beneficial to improving the strength, the performances such as strength, toughness and elongation and the like are simultaneously reduced due to graphite segregation when the carbon is too high, the strength of the nodular cast iron cannot be guaranteed when the carbon is too low, the fluidity of molten iron is poor, casting defects are increased, and various mechanical properties are reduced. Suitable C contents of the present invention are 3.5-3.9%, preferably 3.6-3.8%, more preferably 3.65-3.75%.

Si: silicon is a graphitization promoting element and a solid solution strengthening element, the influence of the content of Si on the fluidity of molten iron is large, and the high-temperature oxidation resistance of Si can be improved. If the Si content is too low, the graphitization effect and the strengthening effect of the nodular cast iron are obtained, and if the Si content is too high, the plasticity and the elongation of the nodular cast iron are remarkably reduced, and the toughness is remarkably deteriorated, and the Si content in the present invention is limited to 2.3 to 2.6%, preferably 2.4 to 2.5%.

Mn: manganese can be combined with sulfur to generate MnS, so that the deterioration of the performance of nodular cast iron caused by sulfur impurity is reduced, a proper amount of Mn is beneficial to ensuring the strength of the nodular cast iron, the plasticity and toughness of the nodular cast iron are reduced due to overhigh Mn, and the desulfurization effect and the strengthening effect are insufficient due to overlow Mn. The Mn content in the present invention is set to 0.3 to 0.6%, preferably 0.4 to 0.5%.

Mg: magnesium is an element that promotes spheroidization, O and S are impurities that hinder spheroidization of graphite, and Mg can react with O and S to promote nodular crystallization. If the content of Mg is too low, the spheroidization promoting effect is not remarkable, and if the content of Mg is too high, impurities are easy to form or precipitate during crystallization, so that the nodular cast iron becomes brittle, and the plasticity and the toughness are reduced. The content of Mg in the present invention is set to 0.02 to 0.08%, preferably 0.03 to 0.06%, more preferably 0.04 to 0.05%.

RE: rare earth is an element promoting spheroidization, which can react with O, S, ensure spheroidization effect of graphite, and contribute to refinement of spheroidal graphite. Too low results in insufficient refining and ensuring spheroidization effect, too high a cost rises and the cost of the preparation process increases. The rare earth content is set to 0.01 to 0.05%, preferably 0.02 to 0.04%, more preferably 0.025 to 0.035% in the present invention.

Al: aluminum is an element for improving the high-temperature oxidation resistance, but when the aluminum content is higher, the graphite loses the spherical shape, the abnormal graphite is increased, the nodular cast iron is quickly embrittled, and the plasticity and toughness are rapidly deteriorated, in the common high-temperature nodular cast iron, the aluminum content reaches about 5%, the material is very brittle, and the toughness and the plasticity are extremely low.

Cr: chromium is a commonly used additive element in cast iron, has a strengthening effect, has positive significance for improving high-temperature oxidation resistance, high-temperature strength and high-temperature creep resistance, and is easy to form carbide once the content of Cr is too high, so that the toughness and plasticity of the nodular cast iron are reduced rapidly. According to the invention, by adding low contents of Al and Cr, excellent toughness can be achieved while excellent high-temperature performance is ensured. The Cr content in the present invention is 1 to 3%, preferably 1.5 to 2.5%, more preferably 1.7 to 2.3%.

Cr/Al: the chromium-aluminum ratio is important for obtaining the technical effect of the invention, and Cr and Al are elements for improving the high-temperature oxidation resistance of the nodular cast iron, but in the prior art, the beneficial high-temperature oxidation resistance effect can be obtained only by adding Al or Cr with higher content, and the toughness must be sacrificed. The inventor of the invention finds that by controlling the proportion of Cr and Al within a certain range, excellent high-temperature strength and high-temperature oxidation resistance can be obtained under the condition of lower addition amount of Al and Cr, toughness can be considered, and various mechanical properties of room temperature can not be lost. When Cr/Al is out of the range of the invention, the nodular cast iron with high temperature performance, toughness and other mechanical properties cannot be obtained. In order to obtain the above technical effects, the present invention limits Cr/Al to the range of 50 to 200, preferably 55 to 150, more preferably 60 to 100, particularly preferably 65 to 90, 70 to 80.

In the present invention, a non-exhaustive list of the inevitable impurity elements is P, S, and the contents of the inevitable impurity elements are defined as P: 0.05% or less, S: below 0.03%, the lower the content of impurities, the better, but the higher the requirements for raw materials and smelting, the higher the cost, and the preferred P: 0.03% or less, S: less than 0.02%.

The nodular cast iron obtained by casting of the invention can further comprise 0.3 to 0.8 percent of Cu and also can further comprise 0.01 to 0.2 percent of Sn and/or Sb on the basis of the elements.

Cu: copper is a strengthening element of nodular cast iron, but causes a reduction in elongation. If Cu is too low, the strengthening effect is not significant, and if Cu is too high, the elongation property is seriously deteriorated. The Cu content in the present invention is limited to 0.3 to 0.8%, preferably 0.4 to 0.7%, more preferably 0.5 to 0.6%.

Sb and/or Sn: sb and Sn are elements for increasing the number of graphite particles, so that the spheroidization rate of the nodular cast iron can be improved, and the toughness and the elongation rate of the nodular cast iron are improved. If the addition amount is too low, the spheroidization promoting effect is not obvious, and if the addition amount is too low, the generation of flaky graphite is easily caused, so that the toughness and the elongation property of the nodular cast iron are remarkably deteriorated. In the present invention, Sb or Sn may be added alone or together with Sb and Sn, and the amount of Sb or Sn added alone or together is 0.01 to 0.2%, preferably 0.02 to 0.15%, more preferably 0.05 to 0.1%.

The room temperature performance of the nodular cast iron obtained by casting in the invention is as follows: tensile strength of 800-²The above; the high temperature performance is as follows: tensile strength at 400 ℃ of 400-200MPa, and the thickness of an oxide layer after heat preservation for 120h at 900 ℃ in an air atmosphere is not more than 150 mu m.

The nodular cast iron obtained by the casting method can be applied to conventional nodular cast iron, such as crankshafts, camshafts, connecting shafts, connecting rods, gears and the like, and is also particularly suitable for high-temperature resistant parts applied to high-temperature working conditions of engine parts, turbocharging parts, piston rings, exhaust manifolds, grates, boilers, sintering trolleys and the like.

The beneficial effects of the invention are as follows.

Compared with the prior art of high-aluminum high-chromium heat-resistant nodular cast iron, the preparation method of the invention matches the micro-aluminum low-chromium on the component design of the nodular cast iron, and the ratio of Cr/Al is controlled, excellent high-temperature mechanical property and room-temperature toughness property are obtained, and the mechanical property at room temperature is not lost, by compositely adding Cr and Al and controlling the ratio of Cr/Al, so that the nodular cast iron can obtain excellent high-temperature strength and high-temperature oxidation resistance even if the sum of the contents of the oxidation resisting elements Cr and Al is below 5 percent, and has excellent room temperature toughness, when the Cr and the Al are controlled in a proper range and the proportion range is proper, the nodular cast iron can easily form a thin oxide layer rich in Cr and Al at high temperature, and the oxide layer is not obviously thickened at high temperature for a long time, the alloy performance is excellent, and the cost of the elements is lower. Through the content control of other low-price basic alloy elements and the coordination with the content of Cr and Al, the nodular cast iron with excellent high-temperature strength, high-temperature oxidation resistance, room-temperature toughness, room-temperature strength, room-temperature elongation and other properties is finally prepared.

Detailed Description

In order to make those skilled in the art fully understand the technical scheme and the beneficial effects of the present invention, the following further description is made in combination with specific test examples.

And preparing the nodular cast iron according to the design components, wherein all P elements are qualified when being controlled to be 0.02% +/-0.002%, and all S elements are qualified when being controlled to be 0.015% +/-0.002%. The specific preparation method is as follows.

The first step, chemical composition design: selecting pig iron, scrap steel, foundry returns and the like with low sulfur, phosphorus and manganese contents as raw materials according to target chemical components, and calculating the consumption of each raw material.

Step two, smelting in an intermediate frequency furnace: and smelting each component into molten iron by adopting a medium-frequency induction furnace.

Step three, spheroidizing: the nodulizer is a rare earth magnesium silicon nodulizer, and the nodulizing method is to perform nodulizing treatment by adopting a flushing method.

Step four, inoculation treatment and pouring: adopting a ferrosilicon inoculant to perform inoculation treatment in a ladle; and sand casting is adopted to obtain the nodular cast iron.

And (4) carrying out chemical component analysis on the nodular cast iron obtained by pouring, and detecting the strength, elongation, toughness and high-temperature oxidation resistance. The analysis of chemical components and the test of various mechanical properties at room temperature are carried out according to the national standard GB/T1348-2009 and the standard requirements cited by the national standard GB/T228.2-2015, the high-temperature mechanical properties are carried out according to the national standard GB/T228.2-2015, and the high-temperature oxidation resistance is characterized by observing and measuring the thickness of an oxide layer by using a microscope after the sample is kept at 900 ℃ in the air atmosphere for 120 hours. The results of the analysis of the chemical components of the spheroidal graphite cast irons of test nos. 1 to 24 are recorded in table 1, and the results of the various mechanical property tests are shown in table 2.

Table 1 (the components are in percentage by mass, and the balance is Fe)

Numbering	C	Si	Mn	Mg	RE	Al	Cr	Cr/Al	Cu	Type (B)
											1	3.53	2.51	0.32	0.056	0.042	0.018	1.2	66.7	0	Examples of the invention
2	3.58	2.32	0.54	0.032	0.018	0.012	1.6	133.3	0	Examples of the invention
											3	3.63	2.45	0.48	0.027	0.025	0.025	1.9	76.0	0	Examples of the invention
4	3.72	2.38	0.37	0.071	0.031	0.034	2.5	73.5	0	Examples of the invention
											5	3.82	2.57	0.44	0.065	0.048	0.041	2.8	68.3	0	Examples of the invention
6	3.88	2.53	0.58	0.047	0.014	0.031	2.2	71.0	0	Examples of the invention
											7	3.66	2.43	0.55	0.039	0.022	0.047	2.9	61.7	0	Examples of the invention
8	3.78	2.35	0.39	0.042	0.037	0.021	1.8	85.7	0	Examples of the invention
											9	3.61	2.36	0.34	0.072	0.044	0.024	2.1	87.5	0.43	Examples of the invention
10	3.74	2.49	0.51	0.058	0.035	0.045	2.7	60.0	0.71	Examples of the invention
											11	3.53	2.51	0.32	0.056	0.042	0.03	1.2	40.0	0	Comparative example
12	3.53	2.51	0.32	0.056	0.042	0.018	3.3	183.3	0	Comparative example
											13	3.53	2.51	0.32	0.056	0.042	0.008	1.2	150.0	0	Comparative example
14	3.53	2.51	0.32	0.056	0.042	0.018	3.7	205.6	0	Comparative example
											15	3.53	2.51	0.32	0.056	0.042	0.07	1.2	17.1	0	Comparative example
16	3.53	2.51	0.32	0.056	0.042	0.018	0.78	43.3	0	Comparative example
											17	3.58	2.32	0.54	0.032	0.018	1.61	0	/	0	Comparative example
18	3.58	2.32	0.54	0.032	0.018	0	1.61	/	0	Comparative example
											19	3.21	2.45	0.48	0.027	0.025	0.025	1.9	76.0	0	Comparative example
20	4.28	2.45	0.48	0.027	0.025	0.025	1.9	76.0	0	Comparative example
											21	3.72	2.19	0.37	0.071	0.031	0.034	2.5	73.5	0	Comparative example
22	3.72	2.77	0.37	0.071	0.031	0.034	2.5	73.5	0	Comparative example
											23	3.82	2.57	0.25	0.065	0.048	0.041	2.8	68.3	0	Comparative example
24	3.82	2.57	0.71	0.065	0.048	0.041	2.8	68.3	0	Comparative example

In the above test examples, the numbers 1 to 10 are all inventive examples of the present invention. The element contents of Nos. 11 to 24 or Cr/Al do not meet the requirements of the present invention, and therefore, test examples Nos. 11 to 24 are comparative examples of the present invention.

Table 2 shows the mechanical properties and high-temperature oxidation resistance of test examples Nos. 1 to 24.

TABLE 2

The invention examples of the present invention are further analyzed and explained below with reference to the chemical components of table 1 and the mechanical properties and high temperature oxidation resistance of table 2.

The numbers 1-10 in table 1 are all inventive examples of the present invention, which satisfy the requirements of the present invention for the content of each element and the Cr/Al ratio, and each inventive example in table 2 can satisfy the performance requirements of the present invention, i.e., room temperature performance: tensile strength of 800-²The above; high temperature performance: the tensile strength is 400-600MPa at 400 ℃, the tensile strength is 100-200MPa at 900 ℃, and the thickness of an oxide layer after heat preservation for 120h at 900 ℃ in an air atmosphere is not more than 150 mu m. Particularly, in the invention examples 3, 4 and 6 in which Cr/Al is 70 to 80 and the invention examples 8 to 9 in which Cr/Al is 65 to 90, the oxide layer thickness after heat preservation at 900 ℃ for 120 hours in the air atmosphere is 100 μm or less.

Comparative examples of the present invention are analyzed one by one in conjunction with tables 1 and 2 below.

Comparative examples 11 to 16 are comparative examples of inventive example 1, and the content of Cr or Al is adjusted so that at least one of Cr, Al and Cr/Al does not meet the requirements of the present invention. Specifically, although both Cr and Al of comparative example 11 meet the requirements of the present invention, Cr/Al is out of the scope of the present invention and cannot synergistically exert the combined action of Cr and Al, so that the thickness of the oxide layer after 900 ℃ x 120h cannot meet the requirements of the present invention; the comparative example 12 has too high Cr content, and although Cr/Al falls within the range of the present invention, the oxide layer thickness after 900 c x 120h could not meet the inventive requirements; comparative example 13 has too low Al, and although Cr/Al falls within the range of the present invention, its oxide layer thickness after 900 c x 120h could not meet the inventive requirements; the Cr content of the comparative example 14 is too high, the Cr content and the Cr/Al do not meet the requirements of the invention, and the combined action of the Cr and the Al cannot be synergistically exerted, so that the room-temperature elongation, the room-temperature toughness and the thickness of an oxide layer after 900 ℃ multiplied by 120h cannot meet the requirements of the invention; the comparative example 15 has too high Al, the Al content and the Cr/Al do not meet the requirements of the invention, and the combined action of the Cr and the Al can not be exerted synergistically, so that the room-temperature tensile strength, the room-temperature yield strength, the room-temperature toughness, the room-temperature elongation, the 400-DEG C tensile strength, the 900-DEG C tensile strength and the oxide layer thickness after the temperature of 900-DEG C multiplied by 120h can not meet the requirements of the invention; the Cr content of the comparative example 16 is too low, the Cr content and the Cr/Al do not meet the requirements of the invention, and the combined action of the Cr and the Al cannot be cooperatively exerted, so that the room-temperature tensile strength, the room-temperature yield strength, the 400-DEG C tensile strength, the 900-DEG C tensile strength and the oxide layer thickness after 900-DEG C x 120h cannot meet the requirements of the invention; comparison of comparative examples 11 to 16 with inventive example 1 shows that controlling reasonable Al content is important for obtaining room temperature strength, room temperature toughness, room temperature plasticity, high temperature strength, and high temperature oxidation resistance, and controlling reasonable Cr content is important for obtaining room temperature strength, room temperature toughness, room temperature plasticity, high temperature strength, and high temperature oxidation resistance; proper control of Cr/Al is important to achieve excellent high temperature oxidation resistance.

Comparative examples 17 to 18 were comparative examples of inventive example 2, in which Al or Cr was added alone in an amount similar to the Al + Cr content of inventive example 2. Specifically, the Al content of comparative example 17 is out of the range of the present invention, which causes the room temperature tensile strength, room temperature yield strength, room temperature toughness, room temperature elongation, 400 ℃ tensile strength, 900 ℃ tensile strength to be out of the inventive requirements, and since no Cr is contained, Al and Cr cannot synergistically exert the high temperature oxidation resistance effect, which causes the oxide layer thickness to be out of the inventive requirements after the temperature of 900 ℃ x 120 h; the comparative example 18 has a Cr content within the range of the present invention, but does not contain Al, so that Al and Cr cannot synergistically exert the high-temperature oxidation resistance, and the oxide layer thickness after 900 ℃ x 120h cannot meet the invention requirements. By comparison, it is shown that adding Al and Cr simultaneously and ensuring their contents and proportions within certain ranges is important for obtaining room temperature strength, room temperature toughness, room temperature elongation, high temperature strength, high temperature oxidation resistance.

Comparative examples 19 to 20 comparative examples of inventive example 3 were prepared, and the C content was adjusted. Specifically, the comparative example 19 has a C content lower than the requirements of the present invention, resulting in that the room temperature tensile strength and room temperature yield strength as well as the high temperature tensile strength thereof do not satisfy the requirements of the present invention, and the comparative example 20 has a C content higher than the requirements of the present invention, and has a room temperature strength and a high temperature strength thereof too high, resulting in that the room temperature plasticity and room temperature toughness thereof do not satisfy the requirements of the present invention. By comparison, it is shown that controlling the appropriate C content is important for obtaining excellent room temperature strength, room temperature elongation, room temperature toughness, and high temperature strength.

Comparative examples 21 to 22 comparative examples of inventive example 4 were prepared, and the content of Si was adjusted. Specifically, the Si content of the comparative example 21 is lower than the requirement of the invention, but the nodular cast iron has higher C content and Mn content, so the strength characteristic can still meet the requirement of the invention, but the Si also has important influence on the high-temperature oxidation resistance, so the oxide layer thickness after 900 ℃ multiplied by 120h can not meet the requirement of the invention; the comparative example No. 22, in which the Si content is higher than the requirement of the present invention, has too high tensile strength at room temperature and high temperature tensile strength and too low elongation at room temperature and room temperature toughness, failing to satisfy the requirement of the present invention. By comparison, the control of the proper Si content is proved to have important significance on room temperature strength, room temperature elongation, room temperature toughness, high temperature strength and high temperature oxidation resistance.

Comparative examples 23 to 24 comparative examples of inventive example 5 were prepared, and the Mn content was adjusted. Specifically, the Mn content of comparative example 23 is lower than the requirement of the present invention, and the strength property can still meet the requirement of the present invention when Mn is lower due to the higher contents of C and Si, but the sulfur-fixing effect is reduced due to the lower Mn content, so that the sulfur inclusion is easy to become the starting point of the surface oxide film destruction at high temperature, and the oxide layer thickness after 900 ℃ x 120h can not meet the requirement of the present invention; the comparative example No. 24, which has an Mn content higher than the requirements of the present invention, has room temperature strength and high temperature strength higher than the requirements of the present invention, and causes a drastic deterioration in room temperature elongation and room temperature toughness, failing to satisfy the requirements of the present invention. By comparison, the reasonable control of Mn content is shown to play an important role in obtaining room temperature strength, room temperature elongation, room temperature toughness, high temperature strength and high temperature oxidation resistance.

The above description of the embodiments is only intended to facilitate the understanding of the method of the invention and its core idea. It should be noted that, for those skilled in the art, it is possible to make various improvements and modifications to the present invention without departing from the principle of the present invention, and those improvements and modifications also fall within the scope of the claims of the present invention.

The previous description of the disclosed embodiments is provided to enable any person skilled in the art to make or use the present invention. Various modifications to these embodiments will be readily apparent to those skilled in the art, and the generic principles defined herein may be applied to other embodiments without departing from the spirit or scope of the invention. Thus, the present invention is not intended to be limited to the embodiments shown herein but is to be accorded the widest scope consistent with the principles and novel features disclosed herein.

Claims

1. The preparation method of the ductile cast iron with excellent toughness and high-temperature performance is characterized by comprising the steps of smelting, spheroidizing, inoculating and pouring, wherein the ductile cast iron obtained by pouring comprises the following components: 3.5-3.9%, Si: 2.3-2.6%, Mn: 0.3-0.6%, Mg: 0.02-0.08%, RE: 0.01-0.05%, Al: 0.01-0.05%, Cr: 1-3%, Cr/Al is 50-200, and the rest is Fe and inevitable impurities.

2. The method of claim 1, wherein the ductile iron cast further comprises 0.3-0.8% Cu.

3. The method of producing a spheroidal graphite cast iron having excellent toughness and high-temperature characteristics according to any one of claims 1 to 2, wherein the cast spheroidal graphite cast iron has an Al content of 0.02 to 0.04%, a Cr content of 1.5 to 2.5%, and a Cr/Al ratio of 60 to 100.

4. The method for producing ductile iron having excellent toughness and high temperature property according to any one of claims 1 to 3, wherein Cr/Al in the cast ductile iron is 55 to 150.

5. The method for preparing ductile iron with excellent toughness and high temperature property according to any one of claims 1-4, wherein the spheroidizing step uses rare earth magnesium silicon spheroidizing agent.

6. The method for preparing ductile iron with excellent toughness and high temperature property according to any one of claims 1-5, wherein the room temperature property of the ductile iron obtained by casting is as follows: tensile strength of 800-²The above.

7. The method for preparing nodular cast iron with excellent toughness and high temperature performance as claimed in any one of claims 1 to 6, wherein the nodular cast iron obtained by casting has 400-600MPa tensile strength at 400 ℃ and 100-200MPa tensile strength at 900 ℃.

8. The method for preparing ductile iron according to any of claims 1-7, wherein the thickness of the oxide layer of the cast ductile iron after heat preservation at 900 ℃ for 120h in air atmosphere is not more than 150 μm.

9. The method for preparing ductile iron with excellent toughness and high temperature property according to any one of claims 1-8, wherein the inoculation step comprises more than one inoculation.