JP2020066772A - Spheroidal graphite cast iron - Google Patents

Abstract

To provide a spheroidal graphite cast iron showing more excellent strength and ductility than an existing spheroidal graphite cast iron by improving the ratio in an elemental composition to be blended with the spheroidal graphite cast iron.SOLUTION: In a composition containing C:3.70-4.10 mass%, Si:1.40-2.00 mass%, Mn:0.30 mass% or less, Cu:0.30-0.51 mass%, Mg:0.027-0.045 mass%, S:0.006-0.009 mass%, P:0.015-0.021 mass% and Cr:0.06 mass% or less, and having a residue comprising Fe and inevitable impurities, a tensile strength of the composition is 600 MPa or more, an elongation of the composition is 5% or more, and an impact value of the composition at a room temperature and at -40°C is 6 J/cmor more.SELECTED DRAWING: Figure 7

Description

  TECHNICAL FIELD The present invention relates to spheroidal graphite cast iron, and particularly to cast iron having both strength and ductility by containing spherical graphite.

  In order to improve fuel efficiency of automobiles and reduce carbon dioxide gas, weight reduction of body parts is required more than ever. When reducing the weight of vehicle body parts, the use of light metal materials such as aluminum alloys and magnesium alloys is being considered in place of iron castings.

  However, aluminum alloys and magnesium alloys have lower bendability (Young's modulus) than cast iron. Therefore, when applying the above-mentioned light metal material to parts related to the suspension of the automobile body, suspension parts such as connection points between the wheel shaft and the vehicle body, etc., in order to secure the rigidity of the part itself, the cross-sectional area of the part should be changed. Need to be bigger. Therefore, even if a light metal material is used, sufficient weight reduction has not been achieved. In addition, since the light metal material is expensive, its use range as a vehicle body component is limited.

  Next, when the metal plate is processed to manufacture the parts related to the underbody of the vehicle body, even if the weight can be reduced, it is difficult to finish the desired shape of the part due to the restrictions associated with the processing and forming of the sheet metal. Furthermore, since a plurality of parts are combined and joined by welding or the like, it is inevitable that the strength of the joining part is reduced and the number of parts is increased. Therefore, it is not realistic to manufacture parts related to the underbody etc. by sheet metal working.

  In light of this point, at present, spheroidal graphite cast iron (ductile cast iron) containing spheroidal graphite is widely used as a part for weight reduction of cast iron as parts related to underbody of a vehicle body (Patent Documents 1, 2, 3). 4)).

  At present, as spheroidal graphite cast iron for parts related to underbody of automobile body, FCD400 material (tensile strength 400 MPa) and FCD450 material (tensile strength 450 MPa) conforming to JIS G 5502 (2007) are widely used. There is. Furthermore, FCD500 material (tensile strength is 500 MPa) and FCD600 material (tensile strength is 600 MPa) are used in order to reduce the weight of parts using spheroidal graphite cast iron. As a result, the cross-sectional area of the component is reduced and the weight can be reduced.

  However, when spheroidal graphite cast iron is applied to parts related to the underbody of an automobile body, these parts are subject to a low temperature environment such as −40 ° C. and a load during running in view of the running conditions of the vehicle. Exposed to extremely harsh conditions. Therefore, along with the demand for weight reduction, there is a demand for spheroidal graphite cast iron having higher strength and ductility.

JP, 2002-194479, A JP, 2004-232008, A JP, 2014-181356, A JP, 2005-10255, A

  The inventors have conducted intensive studies on the composition of spheroidal graphite cast iron in order to improve the strength and ductility of spheroidal graphite cast iron, and as a result, have found a composition that exhibits superior strength and ductility to the conventional spheroidal graphite cast iron. .

  The present invention has been made in view of the above points, by improving the ratio of the elemental composition to be blended in spheroidal graphite cast iron, spheroidal graphite cast iron that exhibits superior strength and ductility over existing spheroidal graphite cast iron. provide.

  That is, the 1st form of this invention is C: 3.70-4.10 mass%, Si: 1.40-2.00 mass%, Mn: 0.30 mass% or less, Cu: 0.30- 0.51% by mass, Mg: 0.027 to 0.045% by mass, S: 0.006 to 0.009% by mass, P: 0.015 to 0.021% by mass, Cr: 0.06% by mass or less The present invention relates to spheroidal graphite cast iron, characterized in that it is a composition containing Fe, and the balance being Fe and inevitable impurities.

  A second aspect relates to the spheroidal graphite cast iron according to the first aspect, in which the content of Cu in the composition is Cu: 0.45 to 0.51 mass%.

  A third aspect relates to the spheroidal graphite cast iron according to the first or second aspect, wherein the composition has a tensile strength of 600 MPa or more and the composition has an elongation of 5% or more.

A fourth aspect relates to the spheroidal graphite cast iron according to any one of the first to third aspects, wherein the impact value of the composition at room temperature and −40 ° C. is 6 J / cm ² or more.

A fifth aspect relates to the spheroidal graphite cast iron according to any one of the first to fourth aspects, wherein the composition has a Vickers hardness at −30 ° C. of 170 kgf / mm ² or more.

  A sixth aspect relates to the spheroidal graphite cast iron according to any one of the first to fifth aspects, in which the composition is used as a vehicle body material.

  According to the spheroidal graphite cast iron of the present invention, C: 3.70 to 4.10 mass%, Si: 1.40 to 2.00 mass%, Mn: 0.30 mass% or less, Cu: 0.30 to 0. 51% by mass, Mg: 0.027 to 0.045% by mass, S: 0.006 to 0.009% by mass, P: 0.015 to 0.021% by mass, Cr: 0.06% by mass or less However, by using a composition in which the balance is Fe and inevitable impurities, strength and ductility superior to those of the existing spheroidal graphite cast iron are exhibited. Physical properties are improved especially at low temperatures.

4 is a cross-sectional photograph of spheroidal graphite cast iron of Prototype Example 3. 9 is a photograph of a cross section of spheroidal graphite cast iron of Prototype Example 9. 9 is a cross-sectional photograph of spheroidal graphite cast iron of Prototype Example 11. 16 is a photograph of a cross section of spheroidal graphite cast iron of Prototype Example 14. 20 is a photograph of a cross section of spheroidal graphite cast iron of Prototype Example 18. It is a photograph of a cross section of spheroidal graphite cast iron of Prototype Example 25. It is a graph which shows the relationship between hardness and tensile strength. It is a graph which shows the relationship between temperature and an impact value. It is a graph which shows the relationship between hardness and an impact value.

  In the description and examples of the present invention, the percentage (%) indicating the blending ratio is “mass percentage”. The spheroidal graphite cast iron of the present invention will be described first from the composition elements. The preferred composition of spheroidal graphite cast iron is C: 3.70 to 4.10 mass%, Si: 1.40 to 2.00 mass%, Mn: 0.30 mass% or less, S: 0.006 to 0.009. % By weight, Cu: 0.30 to 0.51% by weight, Mg: 0.027 to 0.045% by weight, P: 0.015 to 0.021% by weight, Cr: 0.06% by weight or less. The balance is a composition containing Fe and inevitable impurities. In particular, the feature of the spheroidal graphite cast iron of the present invention is a composition in which the amount of Si is positively suppressed and the amount of Cu is increased as compared with the conventional spheroidal graphite cast iron.

  C (carbon) is an element that forms a graphite structure. C is preferably contained in the range of 3.7 to 4.10 mass% in the total mass of the spheroidal graphite cast iron. When the C content is less than 3.7% by mass, the graphite particles of the spheroidal graphite cast iron are relatively reduced and the pearlite structure is increased in the spheroidal graphite cast iron. As a result, the strength is improved, but the elongation and impact value are decreased. If the C content exceeds 4.10% by mass, the graphite particles become large and the spheroidization tends to be impeded. Therefore, C is contained in the range of 3.70 to 4.10% by mass, and more preferably in the range of 3.80 to 4.10% by mass.

  Si (silicon) is an element that promotes crystallization of graphite. Si is preferably contained in the range of 1.40 to 2.00 mass% with respect to the total mass of the spheroidal graphite cast iron. When the Si content is less than 1.40% by mass, the elongation of the spheroidal graphite cast iron is improved, but the strength is undeniably reduced. When the Si content exceeds 2.00 mass%, the impact value tends to decrease. Therefore, Si is contained in the range of 1.40 to 2.00 mass%. The content of Si is a content relatively smaller than the content of Si in conventional spheroidal graphite cast iron such as FCD500 material (based on JIS G 5502 (2007)).

  Mn (manganese) is an element that stabilizes the pearlite structure in spheroidal graphite cast iron. The Mn content is preferably 0.30 mass% or less in the total mass of the spheroidal graphite cast iron. If the Mn content is less than 0.15% by mass, strength reduction cannot be denied. When the Mn content exceeds 0.30 mass%, the pearlite structure increases and the elongation and impact value decrease. Therefore, the Mn content is 0.30 mass% or less, and more preferably 0.25 mass% or less.

  Cu (copper) is an element that stabilizes the pearlite structure in spheroidal graphite cast iron. As the Cu content increases, the proportion of the pearlite structure in the spheroidal graphite cast iron also increases and contributes to the strength improvement. Therefore, the Cu content is preferably 0.30 to 0.51% by mass based on the total mass of the spheroidal graphite cast iron. The present invention increases the Cu content for the purpose of improving strength and ductility. Therefore, if the Cu content is less than 0.30% by mass, the desired strength is not satisfied. When the content of Cu exceeds 0.51% by mass, the pearlite structure becomes excessive and the elongation and the reduction of impact value are likely to occur. Therefore, from the balance of both properties, the Cu content is 0.30 to 0.51% by mass, more preferably 0.45 to 0.51% by mass, and even more preferably 0.47 to 0.51% by mass. The content is% by mass. The Cu content is higher than the Cu content in conventional spheroidal graphite cast iron such as FCD500 material (based on JIS G 5502 (2007)). In the conventional spheroidal graphite cast iron, heat treatment is performed in order to improve the hardness (Vickers hardness: Hv). On the other hand, by increasing the Cu content as in the present invention, it became possible to improve the hardness in the as-cast state.

  Mg (magnesium) is an element that contributes to the spheroidization of graphite in spheroidal graphite cast iron. Mg is preferably contained in the range of 0.027 to 0.045 mass% in the total mass of the spheroidal graphite cast iron. When the content of Mg is less than 0.027% by mass, the spheroidization rate of graphite decreases, and the strength and elongation of spheroidal graphite cast iron decrease. If the Mg content exceeds 0.045 mass%, carbides are likely to precipitate and the elongation and impact value of spheroidal graphite cast iron are reduced. Therefore, Mg is contained in the range of 0.027 to 0.045 mass%.

  S (sulfur) is an element that controls the number of graphite particles in spheroidal graphite cast iron. S is preferably contained in the range of 0.006 to 0.009 mass% in the total mass of the spheroidal graphite cast iron. If the S content is less than 0.006% by mass, the graphite particles are reduced and the pearlite structure is increased. Therefore, the elongation and impact value of spheroidal graphite cast iron decrease. When the content of S exceeds 0.009 mass%, graphitization is hindered and the spheroidization rate of graphite also decreases. Therefore, the elongation and impact value of spheroidal graphite cast iron decrease. Therefore, S is contained in the range of 0.006 to 0.009 mass%.

  P (phosphorus) is an element derived during the production of spheroidal graphite cast iron. P is preferably contained in the range of 0.015 to 0.021 mass% in the total mass of the spheroidal graphite cast iron. As the P content increases, steadite (phosphorus eutectic) occurs. As steadite increases, the elongation and impact value of spheroidal graphite cast iron decrease. Therefore, since the inclusion of P should be avoided as much as possible, P is contained in the range of 0.015 to 0.021 mass% excluding unavoidable inclusion.

  Cr (chromium) is an element mainly derived from the raw material iron and the like during the production of spheroidal graphite cast iron. The raw material iron includes various raw material iron such as scrap iron and stainless steel. Therefore, since Cr is also contained in these iron products, it can be finally included in the spheroidal graphite cast iron. If the Cr content exceeds 0.1% by mass, carbides tend to precipitate. Therefore, from the viewpoint of suppressing precipitation, the Cr content is suppressed to 0.06 mass% or less.

  The remaining component in spheroidal graphite cast iron is Fe (iron), as is obvious. Furthermore, when procuring raw iron for spheroidal graphite cast iron, various raw iron such as scrap iron and stainless steel can be included in addition to pure iron. Therefore, in addition to the above-mentioned elements other than Fe, various unavoidable impurities are mixed. The unavoidable impurities are, for example, Al (aluminum), Ce (cerium), Zn (zinc), Sn (tin), and the like. Since the amount of unavoidable impurities is much smaller than the above-mentioned various elements and the detection limit, it is not possible to specify a specific amount.

  The production of spheroidal graphite cast iron is similar to existing casting methods. Raw iron, a spheroidizing material, and an inoculant, which are raw materials, are put into a known high-frequency electric furnace or the like. Various elements are further added and uniformly melted, and the components are adjusted. Then, the molten metal in the molten state is taken out of the furnace, and the elemental composition of the contained components is measured by a method such as spark discharge optical emission spectroscopy. Then, the molten metal is poured into a predetermined mold (test piece for measurement), cooled, and taken out. Thus, spheroidal graphite cast iron corresponding to each component is completed.

  The composition prepared based on the aforementioned elemental composition, that is, the spheroidal graphite cast iron of the present invention has a tensile strength of 600 MPa or more. At the same time, the elongation (breaking elongation) of the composition is 5% or more. These indexes are measured according to the standard of JIS Z 2241 (2011) regarding ductility. The spheroidal graphite cast iron is processed or cast into, for example, a No. 4 test piece having the same standard.

  By increasing the tensile strength, the strength of the spheroidal graphite cast iron of the present invention is improved. Therefore, the tensile strength of the spheroidal graphite cast iron is 600 MPa or more, preferably 750 MPa or more, more preferably 800 MPa or more. Elongation decreases with increasing tensile strength. Therefore, in view of the physical properties desired for spheroidal graphite cast iron, the elongation is preferably 5% or more, and more preferably 7% or more, as a range of good elongation.

The impact value at room temperature and −40 ° C. of the composition, that is, the spheroidal graphite cast iron of the present invention, is 6 J / cm ² or more. The impact value relates to strength and is measured in accordance with JIS Z 2242 (2005) Charpy impact test method for metallic materials. In the measurement, the spheroidal graphite cast iron is processed or cast into the shape of a U-notch impulse test piece (No. 3 test piece). The spheroidal graphite cast iron of the present invention is required to have resistance even in a low temperature environment. In particular, the spheroidal graphite cast iron of the present invention is assumed to be used for parts related to the underbody of an automobile body. Therefore, considering the running conditions of the automobile, impact resistance under the condition of -40 ° C is also required. Therefore, it becomes possible to control the quality of the spheroidal graphite cast iron by grasping the impact value in the range of room temperature to -40 ° C described above.

Further, the composition, that is, the spheroidal graphite cast iron of the present invention, has a Vickers hardness of 170 kgf / mm ² or more, preferably 200 kgf / mm ² or more, further measured at -30 ° C according to JIS Z 2244 (2009). It is 230 kgf / mm ² or more. As described above, the spheroidal graphite cast iron of the present invention is assumed to be used in a low temperature environment. Therefore, the hardness of spheroidal graphite cast iron is required at low temperatures. Therefore, the above range is preferably defined.

  Here, in the physical properties of spheroidal graphite cast iron, hardness and impact value have an inverse correlation, and hardness and elongation also have an inverse correlation. It is also known that the elongation and the impact value have a direct correlation. Therefore, the spheroidal graphite cast iron having the optimum physical property value is selected in consideration of the purpose, use, conditions and the like of the parts to which the spheroidal graphite cast iron of the present invention is applied. As described in Examples below, the spheroidal graphite cast iron satisfies the resistance while maintaining the hardness at an appropriate level.

  The composition having the composition and physical properties described in the series, that is, the spheroidal graphite cast iron of the present invention is used as a vehicle body material for automobiles. It is a component mainly related to the underbody of an automobile body, specifically, a steering knuckle, a lower arm, an upper arm, a suspension and the like.

[Production of spheroidal graphite cast iron]
In the production of spheroidal graphite cast iron, a 200 kg high-frequency melting furnace was used, and a shredder of raw material iron (crushed iron raw material), carburizing material, and 10 g of FeS were added to obtain a molten amount of 100 kg (molten metal), and 1560 ± 30 ° C. The molten material was tapped into the crucible. Prior to tapping, the weight ratio of spheroidizing material was 1.6%, cover material was 0.6%, inoculum was 0.4%, CaSi was 0.1%, and inoculum was 0.4%. Placed in. At the same time, the components were adjusted by adding metals such as Cu so as to obtain the elemental composition ratios of the prototypes shown in the table below. At this point, the spheroidizing treatment was allowed to proceed to remove (think) impurities floating on the surface of the melt.

  When the spheroidizing treatment of the melt proceeded, an appropriate amount was taken and the elemental composition of the components contained in the melt was measured by spark discharge emission spectroscopy. In this way, the melts of Prototype Examples 1 to 26 having different compositions were prepared. Then, in order to obtain test pieces necessary for measuring various physical properties, the melt of each prototype was poured into the mold at 1440 ± 30 ° C. Through a series of preparations, rod-shaped articles of a prescribed length of spheroidal graphite cast iron having different compositions were obtained by casting. Then, the following physical properties were measured.

[Measurement of Vickers hardness]
The Vickers hardness of the test piece of spheroidal graphite cast iron under the temperature was measured based on the measurement based on JIS Z 2244 (2009). At the time of measurement, AVK-C2 manufactured by Akashi Seisakusho Co., Ltd. was used. The measured values are Hv items in Tables 1 to 6. Hereinafter, simply “hardness” refers to Vickers hardness.

[Measurement of tensile strength]
The tensile strength was measured according to the standard of JIS Z 2241 (2011). In addition, elongation and 0.2% proof stress were also measured according to the same standard. The spheroidal graphite cast iron rods of each trial example were processed into a JIS No. 4 test piece shape by a lathe. 20ton autograph (AG-X250kN) made by Shimadzu Corporation was used for the measurement of the test piece. The temperature at the time of measurement was room temperature, and the tensile speed was 1 mm / min.

[Measurement of impact value]
The impact value was measured according to the Charpy impact test method of JIS Z 2242 (2005) metal material. The spheroidal graphite cast iron rods of each prototype were processed by a lathe into the shape of No. 3 test piece of the same standard. A 5 kgf Charpy impact tester (CI-50D-SC) manufactured by Tokyo Kenki Co., Ltd. was used to measure the impact value using the test piece. The temperature at the time of measurement was -30 ° C. The test pieces of some prototypes were measured under a plurality of temperatures of 20 ° C, -20 ° C, -30 ° C, and -40 ° C.

[Tissue observation and results]
Six representative examples of spheroidal graphite cast iron as prototypes were selected, and the cross section of each example was observed with an optical microscope at a magnification of 100 times. In order to emphasize the lightness and darkness, a nital etching process was performed. 1 is a prototype example 3, FIG. 2 is a prototype example 9, FIG. 3 is a prototype example 11, FIG. 4 is a prototype example 14, FIG. 5 is a prototype example 18, and FIG. 6 is a prototype example 25.

  In prototype example 3, since there is a large amount of Si, there are many crystalline portions (white portions) derived from Si. In Prototype Example 9, the amount of Cu is relatively small and the formation of graphite particles is undeveloped. In Prototype Examples 11, 14 and 18, Si is suppressed and Cu is increased, so that a good grain structure of spheroidal graphite is observed, and the crystalline portion derived from Si is also suppressed. In Prototype Example 25, both Si and Cu were suppressed, and although graphite particles were seen, the crystalline portion derived from Si was increased in total.

[Results of elemental composition and physical properties]
Tables 1 to 6 show the elemental compositions of the spheroidal graphite cast irons of Prototype Examples 1 to 26. The elemental composition in the table is% by mass (mass%). In addition, the measurement results of Vickers hardness (Hv in the table) are shown. The measurement results of tensile strength (unit: MPa), 0.2% proof stress (unit: MPa), and elongation (unit:%) for some prototypes are also shown. The same unit system will be used hereinafter. In measuring each item, an average value was obtained by measuring 3 or 5 times for each prototype.

[Consideration of relationship between hardness and tensile strength, relationship between hardness and 0.2% proof stress]
Next, the prototypes with different Si contents were selected, and the relationship between the hardness (Hv) and the tensile strength (MPa) of each example and the relationship between the hardness (Hv) and the 0.2% proof stress (MPa) of each example were It is Table 7. In addition to classifying according to the Si content (approximately 2.8, 2.3, 2.0, 1.5 mass%), we present a trial example of individual hardness at each Si content, and show its tensile strength and Shows 0.2% proof stress. In addition, the relationship between the hardness (Hv) and the tensile strength (MPa) of each example and the relationship between the hardness (Hv) and the 0.2% proof stress (MPa) of each example were plotted and graphed (see FIG. 7). See the graph).

  From Table 7 and FIG. 7, there is a positive correlation between hardness and tensile strength, and hardness and 0.2% proof stress. Furthermore, it was possible to produce spheroidal graphite cast iron having different hardness while suppressing the amount of Si. That is, it is a prototype example “24, 14, 25”. In spheroidal graphite cast iron, the tensile strength and the 0.2% proof stress are specified depending on the hardness rather than the Si content. Therefore, the hardness of the spheroidal graphite cast iron produced by suppressing the amount of Si can be organized using the hardness as an index.

[Relationship between temperature and shock value]
Impact values were measured for the spheroidal graphite cast irons of different prototypes having different Si contents under different temperature conditions. As shown in Tables 8 to 10, it is summarized for each hardness (Hv250, Hv230, Hv170) that is almost common, and the Si content corresponds to 2.8, 2.3, 2.0, and 1.5 mass%. It is summarized by a prototype example. The numerical values in the table are impact values (J / cm ² ). Further, the temperature (° C.) and impact value (J / cm ² ) in each table are graphed as shown in FIG. At the time of measurement, an average value was obtained by measuring three times per one prototype example.

[Study on the relationship between temperature and shock value]
From the graphs of Tables 8 to 10 and FIG. 8, the spheroidal graphite cast iron of the trial example having a low hardness showed a high impact value at any temperature (see the group of Hv170). Further focusing, in the lower temperature range, the spheroidal graphite cast iron of the prototype example in which the Si content was suppressed exhibited a high impact value (see prototype example 12). Next, paying attention to the spheroidal graphite cast iron of the trial example of medium hardness and high hardness, the impact value tends to decrease in a temperature-dependent manner. However, in Prototype Examples 10 and 14 in which the Si content was suppressed, the impact value did not decrease much as compared with the other Prototype Examples.

  Therefore, when the impact value of spheroidal graphite cast iron in a low temperature environment is taken into consideration, it has been found that the suppression of the Si content is extremely effective. Since the hardness of spheroidal graphite cast iron is required depending on the parts, the hardness cannot be simply lowered. However, the impact value can be controlled by suppressing the Si content. Therefore, it is possible to provide spheroidal graphite cast iron according to the purpose.

[Relationship between hardness and impact value]
The spheroidal graphite cast irons of the trial production examples having different Si contents described above were classified according to hardness and the relationship with the impact value (J / cm ² ) at −30 ° C. was obtained. As shown in Table 11, the hardness (Hv) was roughly divided into 250, 230, and 170 depending on the Si content (1.5, 2.0, 2.3, and 2.8 mass%). The number of the prototype example corresponding to this and the impact value of -30 ° C are also given. Further, it was graphed as shown in FIG.

[Study on the relationship between hardness and impact value]
From Table 11 and the graph of FIG. 9, it was revealed that the impact value was improved as the content of Si was suppressed in any hardness. Therefore, it was confirmed that although the hardness required for the parts of spheroidal graphite cast iron differs, the tendency is that the more the content of Si is suppressed, the more the impact value can be improved. As is clear from the prototype, it is possible to provide a spheroidal graphite cast iron of appropriate hardness and parts thereof, while improving the impact value therein.

[Study on elemental composition]
From a series of studies and discussions, it can be said that suppressing the Si content in spheroidal graphite cast iron is effective in improving the impact value. However, as indicated by the relationship between hardness and tensile strength (“Table 7 and FIG. 7”) and hardness and impact value (“Table 11 and FIG. 9”) described above, the index of hardness alone indicates the desired tensile strength. It becomes difficult to obtain a shock value. Therefore, it is necessary to uniformly satisfy the impact value, the tensile strength and the 0.2% proof stress. Therefore, focusing on the elemental composition of each prototype example in Tables 1 to 6, it can be seen that the hardness can be adjusted depending on the amount of Cu.

  For example, in the comparison between Prototype Examples 10 and 11 and Prototype Example 12, the hardness is improved as the spheroidal graphite cast iron having a clearer Cu content is obtained. A similar tendency is confirmed in the comparison between the prototype examples 24 and 26 and the prototype example 25. In addition, the prototype examples 14 and 18 have the same tendency. From this, it is possible to control the hardness of spheroidal graphite cast iron by adjusting the contents of Si and Cu, and finally satisfy the impact value and the physical properties such as tensile strength and 0.2% proof stress appropriately. It can be considered that the obtained spheroidal graphite cast iron can be produced.

  Based on each prototype, the preferable Si content in the spheroidal graphite cast iron is 1.40 to 2.00 mass%.

  The content of Cu in the spheroidal graphite cast iron is 0.30 to 0.51% by mass, more preferably 0.45 to 0.51% by mass, and even more preferably 0.47 to 0. The content is 51% by mass. For example, Prototype Examples 1, 3, 4, 7, 21, 23 and the like are examples in which the contents of Si and Cu are both increased. In this case, hardness develops higher. Further, in Prototype Example 25 in which the contents of both Si and Cu were suppressed, the hardness decreased. The prototype examples 11 and 13 have substantially the same Si content, but different Cu contents. The hardness is obviously different between the two prototypes. Therefore, considering the good physical properties of the spheroidal graphite cast iron, the content can be within the above range. Examples in which both Si and Cu converge within the specified range are prototype examples 10, 11, 14, 18, 24, and 26.

  From the results of the prototype example, other elemental compositions of spheroidal graphite cast iron are as follows: C is 3.70 to 4.10 mass%, Mn is 0.30 mass% or less, S is 0.006 to 0.009 mass%, Mg Is 0.027 to 0.045% by mass, P is 0.015 to 0.021% by mass, and Cr is 0.06% by mass or less.

The tensile strength of spheroidal graphite cast iron can be derived from 600 MPa or more and 780 MPa or less from the measurement results of a series of prototype examples (see each table and FIG. 7). Similarly, the elongation is 5% or more, preferably 7% or more. The impact value at room temperature and −40 ° C. is 6 J / cm ² or more in consideration of the Si content (see each table and FIGS. 7 and 8). In addition, the Vickers hardness is 170 kgf / mm ² or more, preferably 230 kgf / mm ² or more (see each table).

  As understood from the physical properties, the spheroidal graphite cast iron of the present invention is suitable for use as a vehicle body material from the indicators of impact value, tensile strength, 0.2% proof stress and elongation, and has resistance even at low temperatures. Therefore, it is suitable for parts related to the underbody of an automobile body where a load is more easily applied.

  INDUSTRIAL APPLICABILITY The spheroidal graphite cast iron of the present invention exhibits good physical properties and has resistance to low temperatures in cold regions (for example, Russia and North America). Suitable for Therefore, it is a promising alternative to the conventional spheroidal graphite cast iron used for parts.

Claims (6)

C: 3.70 to 4.10 mass%,
Si: 1.40 to 2.00 mass%,
Mn: 0.30 mass% or less,
Cu: 0.30 to 0.51% by mass,
Mg: 0.027 to 0.045 mass%,
S: 0.006 to 0.009 mass%,
P: 0.015 to 0.021% by mass,
Cr: contains 0.06 mass% or less,
A spheroidal graphite cast iron characterized in that the balance is a composition comprising Fe and unavoidable impurities.   The spheroidal graphite cast iron according to claim 1, wherein the content of Cu in the composition is Cu: 0.45 to 0.51% by mass.   The spheroidal graphite cast iron according to claim 1 or 2, wherein the tensile strength of the composition is 600 MPa or more, and the elongation of the composition is 5% or more. The spheroidal graphite cast iron according to any one of claims 1 to 3, wherein an impact value of the composition at room temperature and -40 ° C is 6 J / cm ² or more. The spheroidal graphite cast iron according to any one of claims 1 to 4, wherein the composition has a Vickers hardness at -30 ° C of 170 kgf / mm ² or more.   The spheroidal graphite cast iron according to any one of claims 1 to 5, wherein the composition is used as a vehicle body material.

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