JP2005256088A - Spheroidal graphite cast-iron member - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a spheroidal graphite cast iron member whose surface layer is a structure mainly composed of ferrite in an as-cast state, and whose inside is composed of a structure having a pearlite area rate higher than that of the surface layer and which is strong to a notch effect, has high elongation and impact value and high tensile strength even when the member is not separately subjected to a heat treatment. <P>SOLUTION: The spheroidal graphite cast iron contains at least 1.0 to 2.5wt% Ni and is so formed that the surface layer when placed in the as-cast state is the base structure mainly composed of the ferrite and that the internal base has higher pearlite area rate than that of the surface layer. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  The present invention relates to a spheroidal graphite cast iron member, and in particular, the surface layer is a structure mainly composed of ferrite in the as-cast state, and the inside is a structure having a higher pearlite area ratio than the surface layer, and a notch effect is performed without performing a separate heat treatment. The present invention relates to a spheroidal graphite cast iron member having a high tensile strength and a high tensile strength.

  The conventional spheroidal graphite cast iron members FCD370 to 450 having a ferrite base have a problem that they are strong against a notch effect and have high elongation and impact values, but low tensile strength. Further, spheroidal graphite cast iron members FCD500 to 800 mainly composed of pearlite bases are contrary to spheroidal graphite cast iron members FCD370 to 450 which are high in tensile strength but weak against notch effect and low in elongation and impact value. There's a problem.

  In order to solve this problem, there is a spheroidal graphite cast iron member in which the surface layer is made of ferrite and the inside is made of pearlite (see, for example, Patent Document 1). This spheroidal graphite cast iron member is made by adding a pearlite element such as Mn, Cu, Sn, Pb, Cr and Sb, and then performing a heat treatment to make the surface layer ferritic, making the surface layer a soft and stretchable material, This method utilizes the phenomenon that when the bending stress or impact is applied, the surface layer becomes difficult to crack and is difficult to break, but there is a problem that a separate heat treatment is required later.

JP-A-9-296215

  The present invention has been made in view of the above problems, and in the as-cast state, the surface layer has a structure mainly composed of ferrite, and the inside has a structure having a higher pearlite area ratio than the surface layer, and is subjected to a separate heat treatment. The object of the present invention is to provide a spheroidal graphite cast iron member that is strong against the notch effect, has high elongation and impact value, and high tensile strength.

  The spheroidal graphite cast iron member of the present invention contains at least 1.0 to 2.5% by weight of Ni, is placed in an as-cast state, and the surface layer is a ferrite-based matrix structure, and the inner matrix is a pearlite area than the surface layer. The rate was high.

  Further, in addition to Ni, 3.0 to 4.0% by weight of C, 1.7 to 3.5% by weight of Si, less than 0.7% by weight of Mn, less than 0.02% by weight of S, 0.0. The element of less than 05% by weight of P and 0.02 to 0.1% by weight of Mg was included.

  Further, the element is contained, the balance is made of iron and inevitable impurities, and the value of C weight% + 0.23 Si weight% for the C and Si is 3.8 to 4.4.

  In addition, the elongation of the internal base is 8% or more.

  Further, the depth of the surface layer and the elongation of the inner base are adjusted by adjusting the content of one or both of the elements of Ni and Si, and the depth of the surface layer and the The internal base growth was set to a predetermined value.

  Less than 0.7% by weight of Mn, less than 0.5% by weight of Cu, less than 0.35% by weight of Cr, less than 0.02% by weight of Sn and less than 0.02 by weight of Sb. The depth of the surface layer and the internal elongation are adjusted by adjusting the content of at least one element among these elements within a range satisfying weight%, and the depth of the surface layer and the The internal base growth was set to a predetermined value.

  Further, by adjusting the cooling rate at the eutectoid transformation temperature passage in the cooling process of the casting of the spheroidal graphite cast iron member, the depth of the surface layer and the internal elongation are adjusted, the depth of the surface layer and the The internal base growth was set to a predetermined value.

  Further, the ratio of the antimony amount to the rare earth content in the melt of the spheroidal graphite cast iron member exceeds 0.4, and the contained Sb is less than 0.02% by weight.

  The spheroidal graphite cast iron member is used for a movable main mold, a fixed main mold, a holder, and / or a die base of a die casting mold.

  Further, the spheroidal graphite cast iron member is used for automobile undercarriage parts, press machines or machine tools.

  The spheroidal graphite cast iron member of the present invention contains at least 1.0 to 2.5% by weight of Ni, is placed in an as-cast state, and the surface layer is a ferrite-based matrix structure, and the inner matrix is a pearlite area than the surface layer. Since the ratio is high, it is strong against the notch effect, has a high elongation and impact value, and can have high tensile strength without performing a separate heat treatment.

  Further, in addition to Ni, 3.0 to 4.0% by weight of C, 1.7 to 3.5% by weight of Si, less than 0.7% by weight of Mn, less than 0.02% by weight of S, 0.0. Since the element of less than 05% by weight of P and 0.02 to 0.1% by weight of Mg is included, the predetermined high performance can be achieved more reliably.

  Further, since the element is contained, the balance is made of iron and inevitable impurities, and the value of C weight% + 0.23 Si weight% for the C and the Si is 3.8 to 4.4. It is possible to achieve a predetermined high performance.

  Further, since the elongation of the internal base is 8% or more, destruction can be reliably prevented.

  Further, the depth of the surface layer and the elongation of the inner base are adjusted by adjusting the content of one or both of the elements of Ni and Si, and the depth of the surface layer and the Since the elongation of the internal base is set to a predetermined value, the desired high performance can be easily achieved.

  Less than 0.7% by weight of Mn, less than 0.5% by weight of Cu, less than 0.35% by weight of Cr, less than 0.02% by weight of Sn and less than 0.02 by weight of Sb. The depth of the surface layer and the internal elongation are adjusted by adjusting the content of at least one element among these elements within a range satisfying weight%, and the depth of the surface layer and the Since the elongation of the internal base is set to a predetermined value, the desired high performance can be easily achieved.

  Further, by adjusting the cooling rate at the eutectoid transformation temperature passage in the cooling process of the casting of the spheroidal graphite cast iron member, the depth of the surface layer and the internal elongation are adjusted, the depth of the surface layer and the Since the elongation of the internal base is set to a predetermined value, the desired high performance can be easily achieved.

  Further, since the ratio of the antimony amount to the rare earth content in the molten metal of the spheroidal graphite cast iron member exceeds 0.4 and the contained Sb is less than 0.02% by weight, generation of chunky graphite And can be effective particularly for large castings.

  Further, since the spheroidal graphite cast iron member is used for a movable main mold, a fixed main mold, a holder and / or a die base of a die casting mold, it can exhibit high strength and high toughness and can be used effectively.

  Further, since the spheroidal graphite cast iron member is used for undercarriage parts of automobiles, press machines or machine tools, even if the impact is intermittently repeated, the spheroidal graphite cast iron member is hardly broken and can have a long life.

  The spheroidal graphite cast iron member according to the embodiment of the present invention will be described below.

  The spheroidal graphite cast iron member according to the first embodiment of the present invention will be described. The spheroidal graphite cast iron member according to the first embodiment of the present invention has a predetermined high performance due to the Ni containing effect.

  Table 1 shows chemical components of the samples of the spheroidal graphite cast iron member and the comparative member according to the first embodiment of the present invention.

表１に示すように、試料３〜６は、本発明の第１の実施の形態の球状黒鉛鋳鉄部材の試料を示し、試料１、２、７、８は比較部材の試料を示し、本発明の球状黒鉛鋳鉄部材の試料３〜６は、Ｎｉ含有量の範囲を１．０〜２．５重量％とし、比較部材の試料１、２、７、８は、Ｎｉ含有量の範囲を１．０〜２．５重量％外としている。

As shown in Table 1, Samples 3 to 6 are samples of the spheroidal graphite cast iron member according to the first embodiment of the present invention, Samples 1, 2, 7, and 8 are samples of the comparative member, and the present invention. Samples 3 to 6 of the spheroidal graphite cast iron member have a Ni content range of 1.0 to 2.5% by weight, and samples 1, 2, 7, and 8 of the comparative member have a Ni content range of 1. It is outside 0 to 2.5% by weight.

  Samples 1 to 8 are spheroidal graphite having chemical composition elements of C, Si, Mn, P, S, Ni, Cu, Mg, Sb, Cr, and Sn, the balance being iron and unavoidable impurities. Cast iron was melted in a high-frequency induction furnace, and a cylindrical test die with a casting weight of 45 kg having a diameter of 200 mm × height of 200 mm and made of an acid-cured phenol self-hardening mold, and a thickness of 25 mm × length of 200 mm × height of 150 mm Cast into a Y block with a casting weight of 7Kg, cast, cast, and then naturally cool in the mold, cast at a position 30mm from the bottom for cylindrical test dies, and JIS for Y block A specimen for a mechanical property test was cut out at the position, and the hardness, tensile strength, 0.2% proof stress and elongation were measured by the method specified in JIS. Further, a small piece with a cast surface was cut out from the cylindrical test die and the Y block, and after mirror polishing, corrosion was applied so that ferrite and pearlite in the matrix could be distinguished, and the matrix was observed. In the observation sample, the thickness of the surface layer whose surface is completely ferrite (surface ferrite layer thickness) and the distance from the casting surface until the ratio of ferrite and pearlite becomes constant (distance to the internal uniform structure) were measured. . Further, the internal structure was observed with an optical microscope, and the pearlite area ratio in the base was determined.

  In the optical microstructure of cast iron, if it is in a state before corrosion, only graphite appears black, and the base ferrite and pearlite become white, and cannot be distinguished. On the other hand, when corrosion is performed so that ferrite and pearlite can be distinguished, graphite and pearlite appear black, and only ferrite becomes white. By utilizing this phenomenon, the pearlite area ratio in the base can be obtained by the image processing apparatus. That is, the area ratio of graphite in the observation visual field before corrosion and the area ratio of graphite and pearlite in the observation visual field after corrosion were determined, and the pearlite area ratio occupying the base was calculated using equation (1).

面積率の測定に際しては、観察部位よるバラツキをなくすため、腐食前、腐食後それぞれ３０視野以上を測定し、平均の値を用いた。

In measuring the area ratio, 30 fields or more were measured before and after corrosion and average values were used in order to eliminate variations due to the observation site.

  FIG. 1 shows a photograph of a base structure near the casting surface of a spheroidal graphite cast iron member according to the first embodiment of the present invention.

  FIG. 1 shows a representative photograph of samples 3 to 6 of the spheroidal graphite cast iron member of the present invention. As shown in FIG. 1, the spheroidal graphite cast iron member of the present invention has a black portion in the base. The white ferrite part is the ferrite in the base, and the surface ferrite layer 11 is formed on the casting surface without performing any special heat treatment, which is a feature of the present invention. I understand. In a conventional spheroidal graphite cast iron member, a two-layer structure in which the surface of the spheroidal graphite cast iron member of the present invention is ferrite and the inside is pearlite is not formed as it is, and in order to obtain such a two-layer structure Although a special heat treatment was required, the spheroidal graphite cast iron member of the present invention can have a two-layer structure even in an as-cast state, and in the intermediate layer 12 from the end of the surface ferrite layer 11 to the inside of the casting. The pearlite gradually increases, and from the predetermined position, the inside becomes an internal uniform structure 13 of a uniform base structure in which the ratio of pearlite and ferrite is a constant ratio. The portion of the internal uniform structure 13 has mechanical properties having both high strength and high toughness as described later.

  FIG. 2 is a graph showing the relationship between the thickness of the surface ferrite layer with respect to the Ni content and the distance to the internal uniform structure, and FIG. 3 is the surface ferrite with respect to the Ni content of the sample using the Y block. The relationship figure of layer thickness and the distance to an internal uniform structure | tissue is shown.

  As shown in FIGS. 2 and 3, it can be seen that a ferrite layer is formed on the surface of the casting even in the Y block having a cooling rate lower than that of the cylindrical test die. Furthermore, in both the cylindrical test type and the Y block, the thickness of the surface ferrite layer and the distance to the internal uniform structure change according to the amount of Ni, and both are maximum when Ni is 1.5% by weight. You can also see that. This result shows that the ferrite layer is formed on the surface regardless of the size of the casting, and the thickness of the surface ferrite layer is adjusted by adjusting the constituent elements constituting the spheroidal graphite cast iron member of the present invention within a certain range. This indicates that can be adjusted intentionally.

  The spheroidal graphite cast iron member of the present invention is strong against the notch effect just by making the surface layer ferrite and the inside of the pearlite two-layer spheroidal graphite cast iron member, but the elongation and impact value are high and the tensile strength is high. In order to make it a high material, the inside is made of a stretchable material.

  Table 2 shows the internal mechanical properties and internal pearlite area ratio of the spheroidal graphite cast iron member and the comparative member sample of the present invention.

図４は、円筒形試験型による試料ついて、Ｎｉ含有量に対する引張強さと伸びの関係図を示し、図５は、Ｙブロックによる試料ついて、Ｎｉ含有量に対する引張強さと伸びの関係図を示す。

FIG. 4 shows the relationship between the tensile strength and the elongation with respect to the Ni content for the sample of the cylindrical test mold, and FIG. 5 shows the relationship between the tensile strength and the elongation with respect to the Ni content for the sample with the Y block.

  In general, the spheroidal graphite cast iron member has a contradictory relationship in that the tensile strength and the elongation are low when the elongation is high and the tensile strength is low when the tensile strength is high.

  As shown in Table 2 and FIGS. 4 and 5, in the case of the member of the present invention including the comparative member, the tensile strength increases as the Ni content increases, but the elongation decreases on the other hand. As can be seen, the spheroidal graphite cast iron member of the present invention determines the Ni content in which both the tensile strength and the elongation are high values, and makes the two layers stronger against the notch effect, while increasing the tensile strength and elongation. Realize that.

The higher the elongation of the structure, the better. However, generally, if there is an elongation of 8% or more, deformation occurs before a sudden brittle fracture occurs, and the structure does not break. In the cylindrical test mold, it is understood from Table 2 and FIG. 4 that the Ni content must be 2.5% by weight or less in order to obtain a value of 8% or more in elongation. Moreover, since the tensile strength of carbon steel cast steel SC480 often used as a structural material is 480 N / mm ² or more and the tensile strength of FCD500 is 500 N / mm ² or more, if the lower limit of strength is 500 N / mm ² , the cylindrical shape In order to obtain a value of 500 N / mm ² or more in the test mold, it can be seen from Table 2 and FIG. 4 that the Ni content must be 1.0 wt% or more. That is, the range of Ni content satisfying the elongation of 8% or more and the tensile strength of 500 N / mm ² or more is 1.0 to 2.5% by weight, and this range is the Ni content of the spheroidal graphite cast iron member in the present invention. The amount range was determined. Furthermore, it can also be seen that the Ni content range needs to be 1.0 to 2.5% by weight in order to ensure elongation of 10% or more.

In the Y block, when the Ni content is 1.0 to 2.5% by weight, the elongation is 9% or more and the tensile strength is 640 N / mm ² or more. The value is obtained. This is because the Y block, which has a faster cooling rate than the cylindrical test die, does not cause coarsening of the structure and segregation of impurity elements between grain boundaries, which are factors that reduce elongation and tensile strength. For large castings with a slow cooling rate, Ni 1.0 to 2.5% by weight is a necessary condition, but for small castings with a fast cooling rate, Ni 0.5 to 2.5% by weight has no problem. Moreover, in the range of Ni content in the spheroidal graphite cast iron member of the present invention, the pearlite area ratio occupying the inner base structure is 24 to 78% in the cylindrical test mold, but 48 to 84% in the Y block. This difference is due to the cooling rate.

  Next, other elements contained in the spheroidal graphite cast iron member of the present invention will be described.

  If C is less than 3.0% by weight, carbides are generated and elongation is reduced, and shrinkage cavities are likely to occur. On the other hand, if it exceeds 4.0% by weight, carbon flotation in which explosive graphite or graphite floats occurs. In order to reduce the mechanical properties, it is necessary to contain 3.0 to 4.0% by weight.

  When Si is less than 1.7% by weight, carbides are generated and elongation is reduced and shrinkage cavities are easily formed. On the other hand, when it is 3.5% by weight or more, Si dissolved in the ferrite matrix weakens the ferrite matrix. In order to reduce the mechanical properties such as elongation and impact value, it is necessary to contain 1.7 to 3.5% by weight.

  C wt% + 0.23 Si wt% indicates the carbon equivalent on the hypoeutectic side of the spheroidal graphite cast iron member, and is within the composition range of C and Si described above, and C wt% + 0.23 Si wt% However, it is necessary to cast a spheroidal graphite cast iron member satisfying 3.8 to 4.4. When C weight% + 0.23 Si weight% is less than 3.8, carbides are generated and elongation tends to be reduced and shrinkage cavities are easily formed. On the other hand, when it is 4.4 or more, primary eutectic graphite is crystallized. Since the primary graphite that has come out and crystallized floats and causes intervening carbon flotation, mechanical properties such as elongation and impact value are lowered.

  When Mn is 0.7% by weight or more, carbide is likely to be generated, and mechanical properties such as elongation and impact value are deteriorated, and shrinkage cavities are likely to be generated. There is a need.

  When P is 0.05% by weight or more, a hard steadite phase is generated, the elongation and impact value are remarkably lowered, and shrinkage cavities are liable to occur. Therefore, it is necessary to contain P <0.05% by weight.

  S reacts with Mg, which is a graphite spheroidizing element, to produce MgS. If it is 0.02% by weight or more, the amount of Mg acting on the graphite spheroidization decreases and spheroidization failure occurs, so S <0.02 wt. % Content must be included.

  As described above, Ni can increase the tensile strength and elongation as well as strengthen the notch effect by forming two layers by setting the content range to 1.0 to 2.5% by weight. .

  Since Cu is a pearlite-promoting element, the strength increases as the amount added increases. However, when Cu is 0.5% by weight or more, the effect of promoting pearlite becomes too strong and a ferrite layer near the black skin is not generated, so Cu < It is necessary to contain 0.5% by weight.

  Mg is an element that spheroidizes graphite. Therefore, if the Mg content is less than 0.02% by weight, the spheroidization rate of the graphite is reduced. On the other hand, if it is 0.1% by weight or more, shrinkage cavities are likely to occur due to free Mg, and the elongation and impact value decrease. It is necessary to make it contain from 0.1% to 0.1% by weight.

  When Cr is 0.35% by weight or more, carbide is likely to be generated, and mechanical properties such as elongation and impact value are deteriorated. In addition, since Cr is an element that promotes pearlization, if 0.35% by weight or more, ferrite near the black skin is formed. Since no layer is formed, it is necessary to contain Cr <0.35% by weight.

  Sn is a strong pearlite-promoting element. Therefore, if Sn is 0.02% by weight or more, the effect of promoting pearlite becomes too strong to prevent the formation of a ferrite layer near the black skin, and the elongation and impact value also decrease. It is necessary to contain <0.02% by weight.

  Since the spheroidal graphite cast iron member of the present invention contains Ni, abnormal graphite called chunky graphite is generated in a casting with a slow cooling rate. In order to solve this problem, the amount ratio Sb / RE of antimony (Sb) and rare earth (RE) added to the material was examined. As a result, generation of chunky graphite was prevented when Sb / RE> 0.4 or more. I understood that I could do it. Chunky graphite is rarely generated in a small casting because the cooling rate is high, and Sb / RE> 0.4 is effective for a large casting with a slow cooling rate. However, since Sb is a strong pearlite-promoting element, the elongation and impact value decrease as the content increases. Therefore, it is necessary to contain Sb <0.02% by weight.

  As described above, the spheroidal graphite cast iron member of the present invention has a structure in which the surface layer is mainly ferrite in the as-cast state, and the inside has a structure having a higher pearlite area ratio than the surface layer, without performing a separate heat treatment, It is strong against the notch effect, has high elongation and impact value, and high tensile strength.

  Note that the spheroidal graphite cast iron member of the present invention requires high strength and high toughness, such as a die-cast main mold, because it has high strength and high elongation of 8% or more only with internal mechanical characteristics. It is suitable for the member made. Furthermore, the spheroidal graphite cast iron member of the present invention has a ferrite layer that has a high elongation on the surface and excellent shock absorption capability. Therefore, even if the impact applied to the member is repeated intermittently, it is very small like a crack. Since it is difficult for a defect to enter, and even if a micro defect is entered, it is difficult to break, so that a long service life can be achieved for a machine part subjected to intermittent impact.

  The application of the spheroidal graphite cast iron member of the second embodiment of the present invention to a large casting will be described.

  Table 3 shows chemical components of the samples of the spheroidal graphite cast iron member and the comparative member according to the second embodiment of the present invention.

表３に示すように、試料９は本発明の第２の実施の形態の球状黒鉛鋳鉄部材の試料を示し、試料１０、１１は比較部材の試料を示す。試料９〜１１は、実際の産業上利用性を考慮して縦５００ｍｍ×横５００ｍｍ×高さ５００ｍｍの鋳物重量９００Ｋｇの５００角ブロックの鋳造を試みた。この試料９〜１１は、Ｃ、Ｓｉ、Ｍｎ、Ｐ、Ｓ、Ｎｉ、Ｃｕ、Ｍｇ、Ｓｂ、Ｃｒ、Ｓｎの化学成分の元素と、残部が鉄および不可避的不純物からなり、この組成を有する球状黒鉛鋳鉄を高周波誘導炉にて溶製し、酸硬化フェノールの自硬性鋳型で作製した鋳型にて５００角ブロックを鋳造したものである。合わせて、比較部材としてＦＣＤ５００相当材、ＦＣＤ８００相当材を５００角ブロックに鋳造した。鋳造後、鋳型内で鋳放しの自然冷却をして、鋳造した試料９〜１１の５００角ブロックのほぼ中央部から機械的性質試験用試験片を切り出し、ＪＩＳに規定された方法で、硬度、引張強さ、０．２重量％耐力および伸びを測定した。また、本発明部材およびＦＣＤ５００相当材については鋳肌付きの小片を切り出し、第１の実施の形態の実施例１と同様に基地中のフェライトとパーライトを区別できるように腐食を施し、基地組織を観察した。

As shown in Table 3, sample 9 represents a sample of the spheroidal graphite cast iron member according to the second embodiment of the present invention, and samples 10 and 11 represent samples of the comparative member. In consideration of actual industrial applicability, Samples 9 to 11 were tried to cast a 500 square block having a casting weight of 900 kg and a length of 500 mm × width of 500 mm × height of 500 mm. Samples 9 to 11 are spherical particles having chemical composition elements of C, Si, Mn, P, S, Ni, Cu, Mg, Sb, Cr, and Sn, the balance being iron and unavoidable impurities. A graphite cast iron is melted in a high frequency induction furnace, and a 500 square block is cast with a mold made of a self-hardening mold of acid-cured phenol. In addition, FCD500 equivalent material and FCD800 equivalent material were cast into 500 square blocks as comparative members. After casting, the mold is naturally cooled in the mold, and a test piece for mechanical property test is cut out from a substantially central portion of 500 square blocks of the cast samples 9 to 11, and the hardness, Tensile strength, 0.2 wt% yield strength and elongation were measured. In addition, for the member of the present invention and the FCD500 equivalent material, a small piece with a cast surface is cut out, and in the same manner as in Example 1 of the first embodiment, corrosion is performed so that ferrite and pearlite in the matrix can be distinguished, Observed.

  Table 4 shows the mechanical properties inside the samples 9 to 11 of the spheroidal graphite cast iron member and the comparative member of the present invention.

比較部材の試料１０（ＦＣＤ５００相当材）の基地組織はフェライトとパーライトの混合組織であり、伸びを重視した材質である。一方、試料１１（ＦＣＤ８００相当材）の基地組織は完全にパーライトであり、強度を重視した材料である。しかし、本発明の球状黒鉛鋳鉄部材の試料９は、比較部材の試料１０、試料１１のいずれと比べても引張強さ、伸びともに優れていることが分かる。一般に球状黒鉛鋳鉄鋳物の場合、鋳物の肉厚が厚なるほど冷却速度が遅くなり、結晶粒の粗大化と粒界間に炭化物が生成するため、強度や伸びは低下するのが通常である。本発明の球状黒鉛鋳鉄部材の場合、炭化物を生成する元素を極力低減していることおよびＮｉがフェライトに固溶することによって靭性を損なわずに強度を高めていることから、肉厚が厚い鋳物になっても強度と伸びの低下が一般の球状黒鉛鋳鉄部材に比べて少ない。本発明の球状黒鉛鋳鉄部材は、ダイキャスト主型のような大物鋳物に適用された場合でも、機械的性質が低下せず、高いレベルで強度と伸びのバランスを維持することを示すものである。

The base structure of the sample 10 (material equivalent to FCD500) of the comparative member is a mixed structure of ferrite and pearlite, and is a material that places importance on elongation. On the other hand, the base structure of sample 11 (a material equivalent to FCD800) is completely pearlite, and is a material that emphasizes strength. However, it can be seen that sample 9 of the spheroidal graphite cast iron member of the present invention is superior in both tensile strength and elongation compared to either sample 10 or sample 11 of the comparative member. In general, in the case of spheroidal graphite cast iron castings, the thicker the casting, the slower the cooling rate, and the coarsening of the crystal grains and the formation of carbides between the grain boundaries, the strength and elongation are usually reduced. In the case of the spheroidal graphite cast iron member of the present invention, since the elements that generate carbides are reduced as much as possible and the strength is increased without impairing the toughness due to Ni being dissolved in ferrite, the casting is thick. Even if it becomes, the fall of intensity | strength and elongation is few compared with a general spheroidal graphite cast iron member. The spheroidal graphite cast iron member of the present invention shows that, even when applied to a large casting such as a die-cast main mold, the mechanical properties are not deteriorated and the balance between strength and elongation is maintained at a high level. .

  FIG. 6 shows a photograph of the base structure near the casting surface of the sample of the spheroidal graphite cast iron member of the present invention, and FIG. 7 shows a photograph of the base structure near the casting surface of the sample of the comparative material.

  As shown in FIG. 6, it can be seen that Sample 9 of the spheroidal graphite cast iron member of the present invention appears even when the surface ferrite layer observed in Example 1 is a 500 square block. On the other hand, as shown in FIG. 7, in the sample 10 of the comparative member, the surface ferrite layer as in the sample 9 of the spheroidal graphite cast iron member of the present invention is not observed, and becomes a mixed structure of pearlite and ferrite from directly below the casting surface. Yes. Accordingly, it can be seen that the spheroidal graphite cast iron member of the present invention may have a surface ferrite layer that cannot be obtained with a general spheroidal graphite cast iron member even when applied to a large casting with a slow cooling rate.

  FIG. 8 shows a graph in which the pearlite area ratio occupying the base at an interval of 5 mm from the casting surface is measured for the sample of the spheroidal graphite cast iron member of the present invention, and FIG. 9 shows the sample of the spheroidal graphite cast iron member of the present invention. The graph which measured the hardness which occupies in a base at intervals of 5 mm from a casting surface is shown.

  Sample 9 of the spheroidal graphite cast iron member of the present invention is a large casting, and the pearlite in the base appears from a position 15 mm from the casting surface, and the pearlite area ratio increases toward the inside, from a position 30 mm from the casting surface. The inside became a uniform structure with a constant pearlite area ratio. The hardness also increased with an increase in the pearlite area ratio, and the inside became stable from around 30 mm from the casting surface around HB185. Moreover, although the microstructure was observed over the entire cast product, the occurrence of abnormally shaped graphite called chunky graphite was not observed, which is a very desirable structure.

  It can be seen that the thickness of the surface ferrite layer of the spheroidal graphite cast iron member of the present invention is 15 mm, and the thickness of the surface ferrite layer varies depending on the product thickness from the results shown in Examples 1 and 2. When considering an actual cast iron-based structural member, the thickness of the product varies from product to product, and even in the same product, the thickness varies from part to part. Therefore, the thickness of the ferrite layer may be different from part to part. In addition, there is a possibility that the ferrite layer is lost at a certain portion because a process of cutting the casting surface may be performed in order to obtain a predetermined dimension. However, the spheroidal graphite cast iron member of the present invention has a cooling rate when passing through the eutectoid transformation temperature even if the addition amount of Ni, Si, which is a ferrite solidifying element, Mn, Cu, Sb, which is a pearlite element, is adjusted. Even if is adjusted, the thickness of the ferrite layer can be adjusted. Here, the cooling rate when passing through the eutectoid transformation temperature may be the cooling process in the mold after pouring or the cooling process during heat treatment. Therefore, even when a processing step is added to the cast product of the spheroidal graphite cast iron member of the present invention, it is possible to adjust the material so that the ferrite layer remains.

  As described above, the spheroidal graphite cast iron member of the present invention has a two-layer structure having a uniform structure in which the matrix structure on the surface is ferrite and the inner matrix structure has a certain ratio of pearlite and ferrite. Even if only the mechanical properties are seen, it has high strength and high toughness, and can be applied to large-sized, high-strength members such as die-cast main molds, which are subjected to repeated impact loads. In addition, by having a ferrite layer with high elongation on the surface, it has excellent notch strength and increased impact resistance, so there is less fatigue degradation even when impact is intermittently applied to the member, and it is more durable than conventional products The period is extended to extend the service life (extension of the service life).

  In addition, the spheroidal graphite cast iron member of the present invention has the same mechanical properties as cast steel products used for large, high-strength members, but has a low melting temperature and is used for melting furnaces because of its low melting temperature. Since the refractory is less consumed, an extra process such as heat treatment is not required, and a two-layer structure can be obtained as it is, so that it can be manufactured in a short manufacturing period and the manufacturing cost can be reduced.

The photograph figure of the base organization near the cast skin of the spheroidal graphite cast iron member of a 1st embodiment of the present invention is shown. The sample of the cylindrical test mold shows the relationship between the surface ferrite layer thickness with respect to the Ni content and the distance to the internal uniform structure. The sample of the Y block shows the relationship between the surface ferrite layer thickness and the distance to the internal uniform structure with respect to the Ni content. The relationship between tensile strength and elongation with respect to Ni content is shown for a sample with a cylindrical test die. The sample of Y block shows the relationship between tensile strength and elongation with respect to Ni content. The photograph figure of the base organization near the cast skin of the sample of the spheroidal graphite cast iron member of the present invention is shown. The photograph figure of the base organization near the cast skin of the sample of a comparative material is shown. The graph which measured the pearlite area rate which occupies in a base in the space | interval 5 mm from a casting skin with respect to the sample of the spheroidal graphite cast iron member of this invention is shown. The graph which measured the hardness which occupies in a base for the sample of the spheroidal graphite cast iron member of this invention at intervals of 5 mm from a casting skin is shown.

Explanation of symbols

11 Surface ferrite layer 12 Intermediate layer 13 Internal uniform structure

Claims (10)

  A spherical shape characterized in that it contains at least 1.0 to 2.5% by weight of Ni, and is placed in an as-cast state in which the surface layer is a ferrite-based matrix structure and the inner matrix has a higher pearlite area ratio than the surface layer. Graphite cast iron member.   In addition to Ni, 3.0 to 4.0 wt% C, 1.7 to 3.5 wt% Si, less than 0.7 wt% Mn, less than 0.02 wt% S, 0.05 wt% 2. The spheroidal graphite cast iron member according to claim 1, comprising less than P and 0.02 to 0.1% by weight of Mg.   3. The element is included, the balance is made of iron and inevitable impurities, and the value of C wt% + 0.23 Si wt% is 3.8 to 4.4 for the C and Si. The spheroidal graphite cast iron member described in 1.   2. The spheroidal graphite cast iron member according to claim 1, wherein the elongation of the inner base is 8% or more.   The depth of the surface layer and the elongation of the inner base are adjusted by adjusting the content of one or both of the elements of Ni and Si, and the depth of the surface layer and the inner The spheroidal graphite cast iron member according to claim 2, wherein the base elongation is set to a predetermined value.   Less than 0.7 wt% Mn, less than 0.5 wt% Cu, less than 0.35 wt% Cr, less than 0.02 wt% Sn and less than 0.02 wt% Sb The depth of the surface layer and the internal elongation are adjusted by adjusting the content of at least one element among these elements in a range satisfying the above conditions, and the depth of the surface layer and the internal The spheroidal graphite cast iron member according to claim 2, wherein the base elongation is set to a predetermined value.   The depth of the surface layer and the internal elongation are adjusted by adjusting the cooling rate when passing through the eutectoid transformation temperature in the cooling process of the casting of the spheroidal graphite cast iron member, and the depth of the surface layer and the internal 2. The spheroidal graphite cast iron member according to claim 1, wherein the base elongation is set to a predetermined value.   The ratio of the amount of antimony to the rare earth content in the molten metal of the spheroidal graphite cast iron member exceeds 0.4, and the contained Sb is less than 0.02% by weight. 2. A spheroidal graphite cast iron member according to 2.   The spheroidal graphite cast iron according to any one of claims 1 to 8, wherein the spheroidal graphite cast iron member is used for a movable main mold, a fixed main mold, a holder, and / or a die base of a die casting mold. Element.   The spheroidal graphite cast iron member according to any one of claims 1 to 8, wherein the spheroidal graphite cast iron member is used for an undercarriage part of an automobile, a press machine, or a machine tool.

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