阅读说明：本技术 一种球墨铸铁的制备方法 (Preparation method of nodular cast iron ) 是由 帅德国 帅德军 于 2021-09-07 设计创作，主要内容包括：本发明涉及一种球墨铸铁的制备方法,包括熔炼、球化、孕育、浇注步骤,并且孕育过程中使用含Bi孕育剂；浇注得到的球墨铸铁的化学成分按重量百分比计为:碳:3.45-3.70％,硅:3.6-4.3％,锰:0.31-0.58％,硫:0.05％以下,磷:0.05％以下,铋:0.0012-0.005％,余量为Fe以及不可避免的杂质元素,并且硅含量[Si]与铋含量[Bi]满足750≤[Si]:[Bi]≤3000,制备得到的球墨铸铁具有优异的强度、韧性、延伸率,并且硬度适中,球化级别为1-3级,石墨大小等级为6-7级,基体组织为铁素体和珠光体。(The invention relates to a preparation method of nodular cast iron, which comprises the steps of smelting, spheroidizing, inoculation and pouring, wherein a Bi-containing inoculant is used in the inoculation process; the cast nodular cast iron comprises, by weight, 3.45-3.70% of carbon, 3.6-4.3% of silicon, 0.31-0.58% of manganese, less than 0.05% of sulfur, less than 0.05% of phosphorus, 0.0012-0.005% of bismuth and the balance of Fe and inevitable impurity elements, wherein the silicon content [ Si ] and the bismuth content [ Bi ] satisfy 750 ≤ Si ] and [ Bi ] 3000 ≤ Bi, and the prepared nodular cast iron has excellent strength, toughness and elongation and moderate hardness, the spheroidization grade is 1-3 grade, the graphite size grade is 6-7 grade, and the matrix tissue is ferrite and pearlite.)

1. The preparation method of the nodular cast iron is characterized by comprising the steps of smelting, spheroidizing, inoculating and pouring, wherein a Bi-containing inoculant is used in the inoculating process; the cast nodular cast iron comprises, by weight, 3.45-3.70% of carbon, 3.6-4.3% of silicon, 0.31-0.58% of manganese, less than 0.05% of sulfur, less than 0.05% of phosphorus, 0.0012-0.005% of bismuth and the balance of Fe and inevitable impurity elements, wherein the silicon content [ Si ] and the bismuth content [ Bi ] satisfy 750-3000% of [ Si ] and the bismuth content [ Bi ].

2. A method for the preparation of nodular cast iron according to claim 1, characterised in that it comprises the following steps:

first step, raw material preparation: selecting raw materials according to design components and raw material components and calculating the mixture ratio;

step two, smelting: smelting molten iron by adopting a medium-frequency induction furnace;

thirdly, treating with a nodulizer;

step four, inoculant treatment and pouring: and (2) adopting secondary inoculation, firstly carrying out primary ladle inoculation, then pouring and carrying out stream inoculation, and using the Bi-containing inoculant during stream inoculation to obtain the nodular cast iron after pouring.

3. The method for preparing ductile iron according to any of claims 1-2, wherein the bismuth content of the ductile iron is 0.002-0.006%.

4. A method of producing spheroidal graphite cast iron according to any one of claims 1 to 3, wherein the spheroidal graphite cast iron has a silicon content [ Si ] and a bismuth content [ Bi ] satisfying 1000 ≦ Si ≦ 2000.

5. The method for producing ductile iron according to any one of claims 1-4, wherein the ductile iron further has a carbon content [ C ], a silicon content [ Si ], and a bismuth content [ Bi ] satisfying 1500 ≦ ([ C ] +1/3[ Si ]): 2500 ≦ Bi.

6. The method according to any one of claims 1 to 5, wherein the microstructure matrix of the nodular cast iron is pearlite and ferrite, the volume fraction of pearlite is 40 to 60%, and the volume fraction of ferrite is 40 to 60%.

7. The method according to any one of claims 1 to 6, wherein the spheroidal graphite cast iron has a spheroidization grade of 1 to 3 and a graphite size grade of 6 to 7.

8. The method according to any one of claims 1 to 7, wherein the ductile iron has a tensile strength of 550MPa or more, an elongation of 14% or more, and a room temperatureThe thermal impact toughness is 50J/cm²Above, hardness 170-230 HB.

Technical Field

The invention relates to a preparation method of nodular cast iron, which is suitable for high-silicon solid solution strengthened nodular cast iron, and the prepared nodular cast iron has the advantages of strength, toughness, elongation and hardness, and can be applied to products such as vehicle axle housings, differential mechanism housings, speed reducer housings, steering knuckles, planet carriers and the like.

Background

With the increasing demand of the society for energy conservation and emission reduction, the light weight of vehicles becomes an important development trend, and the materials for vehicle parts are required to have higher mechanical properties to ensure the lightness and thinness of parts.

At present, nodular cast iron materials such as QT450-10 and QT500-7 are selected as materials such as axle housings and differential housings of vehicles. However, the materials are difficult to consider strength, elongation and toughness, for example, QT450-10 can meet the requirement but the strength does not meet the requirement, QT500-7 can meet the requirement but the extension difference is far, the parts are large in size and high in weight due to the fact that the parts are made of the two materials, the requirements on light weight of the whole vehicle are not facilitated, and the toughness of the two materials is low, so that the parts cannot meet the requirements on light weight, heavy load and high-power working conditions. Although the addition of alloying elements such as Cu, Ni, and Mo can improve the strength of the material, the balance among the mechanical properties cannot be taken into consideration, and the alloying elements are generally expensive. Therefore, the development of a preparation method of the ductile cast iron material with high strength, high elongation, high toughness, ideal hardness and low cost is required to meet the requirements of light weight and high performance of parts.

Disclosure of Invention

The invention provides a preparation method of nodular cast iron which has the advantages of strength, toughness, elongation and hardness and low cost, the material prepared by the method can be applied to products such as vehicle axle housings, differential mechanism housings, speed reducer housings, steering knuckles, planet carriers and the like, and the high performance of the material and the light weight of parts can be realized without adding expensive elements such as Ni, Cu, Mo and the like.

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

The preparation method of the nodular cast iron is characterized by comprising the steps of smelting, spheroidizing, inoculating and pouring, wherein an inoculant containing Bi is used in the inoculating process; the cast nodular cast iron comprises, by weight, 3.45-3.70% of carbon, 3.6-4.3% of silicon, 0.31-0.58% of manganese, less than 0.05% of sulfur, less than 0.05% of phosphorus, 0.0012-0.005% of bismuth and the balance of Fe and inevitable impurity elements, wherein the silicon content [ Si ] and the bismuth content [ Bi ] satisfy 750-3000% of [ Si ] and the bismuth content [ Bi ].

The technical purpose of the invention is realized mainly by selecting the inoculant in the preparation process and adjusting the components and the content of the nodular cast iron.

Si is used for solid solution strengthening, the strength and hardness of the nodular cast iron can be improved, but the strength improvement tends to bring about the loss of elongation, Mn is also an element for strengthening and improving the hardness, the effect of Mn is similar to that of Si, but Mn is easy to generate carbide, and the elongation and the toughness are deteriorated, so that the strength and the hardness can be basically ensured by controlling 3.6-4.3% of Si and 0.31-0.58% of Mn, namely adopting the design of high silicon and low manganese, and the alloying elements are relatively cheap. Preferably, Si is 3.7-4.2%, more preferably 3.75-4.1%; mn is preferably 0.35 to 0.55%, and Mn is more preferably 0.40 to 0.50%.

However, when the content of Si is high, graphite fragments of the nodular cast iron are formed, and the performance of the nodular cast iron is seriously influenced. The inventor of the present invention found that high toughness and elongation can be obtained while preventing graphite from being crushed by controlling the contents of Si and Bi within a certain range of ratio based on the use of a Bi-containing inoculant in the production process.

Based on the above knowledge, the inventors have completed the present invention. The strength and hardness of the material are ensured by controlling the carbon content of 3.45-3.70%, the silicon content of 3.6-4.3% and the manganese content of 0.31-0.58% in the nodular cast iron, the graphite can be prevented from being broken into blocks by adding 0.0012-0.005% of Bi in the inoculation process, and the excellent matching of elongation and toughness can be obtained by limiting the specific proportion of [ Si ] to [ Bi ] to 750-3000.

By controlling the addition of Bi in the inoculation process, the reduction of toughness and elongation is avoided on the basis of inhibiting graphite crumbling. If the content of Bi is too high, the effect of preventing graphite from being lumped reaches a limit, and it sharply deteriorates the toughness and elongation of nodular cast iron, adversely affecting the properties of nodular cast iron; if Bi is too low, the effect of preventing the graphite from being crushed into lumps cannot be effectively obtained, so that the Bi content is limited to 0.0012 to 0.005%, preferably 0.002 to 0.006%, more preferably 0.002 to 0.004%.

By controlling the Si content in the raw materials (including iron-containing raw materials such as pig iron, nodulizer and inoculant) and the addition amount of Bi in the inoculation process, and controlling the proportion of [ Si ] and [ Bi ], if the proportion of [ Si ] and [ Bi ] exceeds 3000 or is less than 750, even if Bi is in the range of 0.0012-0.005%, the obtained nodular cast iron material can not satisfy both the elongation and the toughness required by the invention, so that the [ Si ] and [ Bi ] of the nodular cast iron are limited to be in the range of 750-3000, and as a preferable technical scheme, the silicon content [ Si ] and the bismuth content [ Bi ] of the nodular cast iron are controlled to be more than or equal to 1000 and less than or equal to 2000, and more excellent elongation and toughness performance can be obtained.

Specifically, the preparation method of the nodular cast iron comprises the following steps:

first step, raw material preparation: selecting raw materials according to design components and raw material components and calculating the mixture ratio, wherein the raw materials are selected from pig iron, scrap steel, foundry returns and the like, and the pig iron is Q10 or Q12 pig iron with low sulfur, phosphorus and manganese contents;

step two, smelting: smelting molten iron in a medium-frequency induction furnace, discharging at 1500 +/-20 ℃,

thirdly, treating with a nodulizer; the nodulizer can be FeSiMg8RE3 nodulizer, and is subjected to nodulizing treatment by adopting an impact method;

step four, inoculant treatment and pouring: adopting secondary inoculation, firstly, using 75SiFe to perform in-ladle inoculation, then, pouring and performing stream inoculation, using a ferrosilicon bismuth inoculant when performing stream inoculation, wherein the Bi content in the ferrosilicon bismuth inoculant is 0.1-2.5%, and obtaining the nodular cast iron after pouring.

As a further improvement, the carbon content [ C ], the silicon content [ Si ] and the bismuth content [ Bi ] of the nodular cast iron are controlled to further satisfy 1500 & ltC ] +1/3[ Si ], [ Bi ] 2500 & ltC ] +1/3[ Si ] actually represents the carbon equivalent value CE of the nodular cast iron, and the inventor finds that by controlling the proportion of CE and [ Bi ], the nodular cast iron can ensure excellent casting performance and graphitization capability while ensuring excellent elongation and toughness performance, and the improvement of the casting performance can effectively reduce casting defects and improve various mechanical properties of castings.

The microstructure matrix of the nodular cast iron is pearlite and ferrite, preferably, the volume fraction of the pearlite is 40-60%, the volume fraction of the ferrite is 40-60%, the spheroidization grade of the nodular cast iron is 1-3 grade, and the graphite size grade is 6-7 grade.

Through the optimization of the components and the content and proportion relation, the inventionThe obtained nodular cast iron has tensile strength of more than 550MPa, elongation of more than 14 percent and room-temperature impact toughness of 50J/cm²Above, the hardness of 170-230HB is preferably 600MPa or more in tensile strength, preferably 16% or more in elongation, preferably 70J/cm in room-temperature impact toughness²As above, the yield strength is preferably 380MPa or more.

The preparation method of the invention has the following beneficial effects.

Fully utilizes cheap alloying elements and reasonably matches the content, and ensures excellent strength and hardness performance. Si is used for solid solution strengthening, so that the strength and hardness of the nodular cast iron are improved, and the content of a strengthening element Mn is reduced, namely, the high-silicon low-manganese design is adopted, expensive metals such as Cu, Ni and Mo are not required to be added, and the strength and hardness performance can be ensured.

Graphite blocking of the high-silicon nodular cast iron can be avoided by adopting the inoculant containing Bi and controlling the inoculant within a certain range. The inventors of the present invention have found that high toughness and elongation can be obtained while preventing graphite particles from being broken into pieces by controlling the contents of Si and Bi within a certain range of ratio.

The method of the invention designs elements with high silicon and low manganese and introduces micro-elements with micro bismuth in the inoculation process, the silicon, the manganese and the bismuth are mutually matched and limit the specific [ Si ] to [ Bi ] ratio, finally, the nodular cast iron material with high strength, high elongation, high toughness and proper hardness is obtained, the nodular cast iron material is particularly suitable for the working conditions of light weight, high power and heavy load, and the prepared nodular cast iron has the characteristics of excellent performance and low cost.

The tensile strength of the nodular cast iron obtained by the invention is above 550MPa, the elongation is above 14 percent, and the room-temperature impact toughness is 50J/cm²Above, hardness 170-230 HB.

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 (3) preparing the nodular cast iron according to the design components, wherein all P elements are qualified if controlled to be 0.025% +/-0.002%, and all S elements are qualified if controlled to be 0.02% +/-0.002%. The specific preparation method is as follows.

The first step, chemical composition design: selecting and using Q10 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, wherein the tapping temperature of the molten iron is 1500 +/-10 ℃.

Step three, spheroidizing: the nodulizer is FeSiMg8RE3 nodulizer, and the nodulizing method is implemented by adopting a flushing method, wherein the adding amount of the nodulizer is 1.5%.

Step four, inoculation treatment and pouring: adopting secondary inoculation, firstly, adopting 75SiFe inoculant in the ladle, wherein the addition amount is 0.3 percent, and the granularity is 4-8 mm; simultaneously, ferromanganese required by the final chemical composition of the material is added; then pouring and stream inoculation are carried out, wherein the stream inoculant is a Bi-containing ferrosilicon bismuth inoculant, the mass percent of Si is more than or equal to 75 percent, and the Bi accounts for 1.0 percent; and sand casting is adopted, and the casting temperature is 1400 +/-10 ℃, so that the nodular cast iron is obtained.

The nodular cast iron prepared by the method is subjected to chemical component analysis, the detection of strength, elongation, toughness and hardness, and the observation and analysis of microstructure. The analysis of chemical components, the test of various mechanical properties and the observation and analysis of microstructures are carried out according to the national standard GB/T1348-2009 and the cited standard requirements thereof. The results of the analysis of the chemical components of the spheroidal graphite cast irons of test nos. 1 to 21 are recorded in table 1, and the results of the mechanical property test and the observation and analysis of the microstructure are shown in table 2.

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

Numbering	C	Si	Mn	Bi	P	S	[Si]:[Bi]	([C]+1/3[Si]):[Bi]	Type (B)
										1	3.62	3.77	0.42	0.0013	0.025	0.021	2900.00	3751.28	Examples of the invention
2	3.62	3.77	0.42	0.0024	0.024	0.019	1570.83	2031.94	Examples of the invention
										3	3.62	3.77	0.42	0.0045	0.026	0.02	837.78	1083.70	Examples of the invention
4	3.62	3.64	0.42	0.0024	0.026	0.022	1516.67	2013.89	Examples of the invention
										5	3.62	4.1	0.42	0.0024	0.025	0.019	1708.33	2077.78	Examples of the invention
6	3.48	4.21	0.42	0.0033	0.024	0.018	1275.76	1479.80	Examples of the invention
										7	3.57	3.65	0.42	0.0018	0.023	0.018	2027.78	2659.26	Examples of the invention
8	3.5	3.89	0.42	0.0026	0.025	0.019	1496.15	1844.87	Examples of the invention
										9	3.68	4.02	0.42	0.003	0.026	0.021	1340.00	1673.33	Examples of the invention
10	3.5	3.89	0.35	0.0026	0.024	0.019	1496.15	1844.87	Examples of the invention
										11	3.5	3.89	0.55	0.0026	0.023	0.021	1496.15	1844.87	Examples of the invention
12	3.5	3.89	0.26	0.0026	0.025	0.019	1496.15	1844.87	Comparative example
										13	3.5	3.89	0.64	0.0026	0.023	0.021	1496.15	1844.87	Comparative example
14	3.62	4.1	0.42	0.0013	0.025	0.019	3153.85	3835.90	Comparative example
										15	3.62	3.64	0.42	0.0049	0.026	0.021	742.86	986.39	Comparative example
16	3.62	4.38	0.42	0.0024	0.026	0.019	1825.00	2116.67	Comparative example
										17	3.62	3.45	0.42	0.0024	0.024	0.021	1437.50	1987.50	Comparative example
18	3.62	4.1	0.42	0.0054	0.023	0.019	759.26	923.46	Comparative example
										19	3.62	4.1	0.42	0.0011	0.025	0.022	3727.27	4533.33	Comparative example
20	3.38	3.64	0.42	0.0024	0.024	0.02	1516.67	1913.89	Comparative example
										21	3.77	3.64	0.42	0.0024	0.026	0.018	1516.67	2076.39	Comparative example

In the above test examples, the numbers 1 to 11 are all inventive examples of the present invention. Mn contents of Nos. 12 to 13 do not meet the requirements of the present invention, [ Si ]: Bi ] of Nos. 14 to 15 do not meet the requirements of the present invention, [ Si ] of Nos. 16 to 17 do not meet the requirements of the present invention, [ Bi ] of No. 18 do not meet the requirements of the present invention, [ Bi ] and [ Si ] of No. 19 do not meet the requirements of the present invention, and C contents of Nos. 20 to 21 do not meet the requirements of the present invention, so test examples Nos. 12 to 21 are comparative examples of the present invention.

Table 2 shows the mechanical properties and microstructure of test examples Nos. 1 to 21.

TABLE 2

Further analysis is described below with reference to the chemical composition of table 1 and the mechanical properties and microstructure of table 2.

The numbers 1 to 11 in Table 1 are all inventive examples of the present invention, which satisfy the requirements of the present invention for the contents of the respective elements and satisfy [ Si]:[Bi]Within the range of 750-²Above, hardness 170-230 HB; in particular, the numbers 2, 4 to 5 and 8 to 9 further satisfy [ Si ]]:[Bi]In the range of 1000-]+1/3[Si]):[Bi]Within the range of 1500-²The above. In inventive examples 10 to 11, [ Si ] was satisfied]:[Bi]In the range of 1000-]+1/3[Si]):[Bi]In the range of 1500-The Mn content in invention example 10 is low, the strength does not reach the preferable level of 600MPa or more, but the strength still satisfies the requirement of 550MPa or more, and the Mn content in invention example 11 is high, the elongation does not reach the preferable level of 16% or more, and the toughness does not reach 70J/cm²The above level, but still according to the invention, the elongation is more than 14%, and the toughness is 50J/cm²The above requirements.

The spheroidization grade, the graphite size grade and the metallographic structure of each invention example also meet the requirements of 1-3 spheroidization grade, 6-7 graphite size grade and pearlite + ferrite matrix structure.

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

Comparative example 12: although the contents of C, Si, P, S and Bi and the ratio of [ Si ] to [ Bi ] are all within the range required by the invention, the Mn content is lower than the requirement of the invention, and the mechanical property test and microstructure analysis in Table 2 prove that the elongation, toughness and microstructure can meet the requirement of the invention, but the strength and hardness can not meet the requirement of the invention because Mn is an element for improving the strength and hardness and the Mn is too low.

Comparative example 13: although the contents of C, Si, P, S and Bi and the ratio of [ Si ] to [ Bi ] are all within the range required by the invention, the Mn content is higher than the requirement of the invention, and the mechanical property test and microstructure analysis of the table 2 prove that although the strength and microstructure can meet the requirement of the invention, the elongation, toughness and hardness can not meet the requirement of the invention because the Mn can deteriorate the elongation and toughness and has the function of improving the hardness.

Comparative example 14: although the contents of C, Si, Mn, P, S and Bi are all in the range required by the invention, the content of [ Si ] Bi is higher than the requirement of the invention, and the mechanical property test and the microstructure analysis in the table 2 prove that the strength, the hardness and the microstructure can meet the requirement of the invention, but the elongation and the toughness do not meet the requirement of the invention, and the control of the content of [ Si ] Bi in a reasonable range is important for obtaining excellent elongation and toughness.

Comparative example 15: although the contents of C, Si, Mn, P, S and Bi are all in the range required by the invention, the content of [ Si ] and [ Bi ] is lower than the requirement of the invention, and the mechanical property test and the microstructure analysis in the table 2 prove that the strength, the hardness and the microstructure can meet the requirement of the invention, but the elongation and the toughness do not meet the requirement of the invention, and the control of [ Si ] and [ Bi ] in a reasonable range is important for obtaining excellent elongation and toughness.

Comparative example 16: although the contents of C, Mn, P, S and Bi and the ratio of [ Si ] to [ Bi ] are all within the range required by the invention, the content of Si is higher than the requirement of the invention, and the mechanical property test and the microstructure analysis of the table 2 prove that although the strength, the toughness and the microstructure can meet the requirement of the invention, the elongation and the hardness can not meet the requirement of the invention because the too high content of Si can obviously influence the elongation and is an element for improving the hardness.

Comparative example 17: although the contents of C, Mn, P, S and Bi and the ratio of [ Si ] to [ Bi ] are all in the range required by the invention, the content of Si is lower than the requirement of the invention, and the mechanical property test and the microstructure analysis in the table 2 prove that the elongation, the toughness, the hardness and the microstructure can meet the requirement of the invention, but the strength of the material is obviously reduced due to the excessively low Si, so that the strength of the material cannot meet the requirement of the invention.

Comparative example 18: although the contents of C, Si, Mn, P and S and the ratio of [ Si ] to [ Bi ] are all within the range required by the invention, the content of Bi is higher than the requirement of the invention, and the mechanical property test and the microstructure analysis in Table 2 prove that although the strength, the hardness and the microstructure can meet the requirement of the invention, the elongation and the toughness can be deteriorated due to the overhigh content of Bi, so that the elongation and the toughness can not meet the requirement of the invention.

Comparative example 19: although the contents of C, Si, Mn, P and S are all in the range required by the invention, the Bi content is lower than the requirement of the invention, and the Bi content is too low, so that the [ Si ]: Bi ] exceeds the upper limit 3000 defined by the invention, the mechanical property test and the microstructure analysis of the table 2 prove that the strength, the hardness, the spheroidization rate and the matrix structure can meet the requirement of the invention, but the inhibition on graphite blocking under high silicon conditions is insufficient due to the too low Bi content, so that the elongation, the toughness and the graphite size grade can not meet the requirement of the invention.

Comparative example 20: although the contents of Si, Mn, Bi, P and S and the ratio of [ Si ] to [ Bi ] are all within the range required by the invention, the content of C is lower than the requirement of the invention, and the mechanical property test and the microstructure analysis in Table 2 prove that the elongation, the toughness, the hardness and the microstructure can meet the requirement of the invention, but the strength of the material cannot meet the requirement of the invention because C is a main strengthening element and the C is too low.

Comparative example 21: although the contents of Si, Mn, Bi, P and S and the ratio of [ Si ] to [ Bi ] are all in the range required by the invention, the content of C is higher than the requirement of the invention, and the mechanical property test and the microstructure analysis of the table 2 prove that the strength, the hardness and the microstructure can meet the requirement of the invention, but the toughness and the elongation of the material can be reduced while the strength is improved due to C, and the toughness and the elongation of the material cannot meet the requirement of the invention due to too high C.

According to the invention examples and the comparative examples, Si has important influence on the strength and the hardness, if the hardness and the elongation rate are too high, the hardness and the elongation rate do not meet the invention requirements, and if the strength is too low, the strength does not meet the invention requirements; mn is also an element for improving strength and hardness, but too high results in a decrease in elongation and too high hardness, and too low strength and insufficient hardness; c is a strengthening element, but if the C is too high, the elongation and the toughness are adversely affected, and if the C is too low, the strength of the material is insufficient; bi is an element for preventing graphite from being crushed into blocks, and if the Bi is too high, the elongation and the toughness of the material are affected, and if the Bi is too low, the effect of preventing the graphite from being crushed into blocks cannot be achieved, so that various mechanical properties of the material are reduced; controlling the ratio of [ Si ] to [ Bi ] within a reasonable range is important to obtain excellent matching of elongation and toughness.

Moreover, through comparison of the invention examples, the nodular cast iron with [ Si ] < Bi > within the range of 1000-.

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.