JPH0617186A - Spheroidal graphite cast iron member and manufacture thereof - Google Patents

Abstract

PURPOSE:To obtain a spheroidal graphite cast iron member having a double structure of a surface layer part somewhat deteriorated in hardness and improved in elongation and an inner part having high strength and in general, showing excellent mechanical strengths. CONSTITUTION:The spheroidal graphite cast iron member is a one having a surface layer part with at least 1mm thickness substantially consisting of a ferritic phase and an inner part consisting of a pearlitic phase and a ferritic phase, and in which the ferritizing rate of the surface layer part is regulated to >=70% which is the value higher than the ferritizing rate of the inner part at least by about 15%. Molten metal is poured into a mold and knock-out is executed in a state in which the temp. of the approximately whole body of the casting is still lies in the A1 transformation point or above after the completion of the solidification of the molten metal. When the temp. difference between the surface layer part and inner part of the obtd. casting reaches 40 to 60 deg.C, the casting is charged to a soaking furnace held to 750 to 900 deg.C and is held in the soaking furnace for the time in which the same ferritizing rate is attained, and the casting is cooled at 15 to 100 deg.C/min cooling rate.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a spheroidal graphite cast iron member having excellent mechanical strength and a method for producing the same.

[0002]

BACKGROUND OF THE INVENTION Since spheroidal graphite cast iron has high mechanical strength and elongation, it is widely used as parts of various machines and automobiles. In particular,
JIS G5502 5 types (FCD700) and 6 types (FCD800) are used for parts that require mechanical strength, and 0 type (FCD3) for parts that require elongation.
70) and 1 type (FCD400).
Above all, important parts such as automobile suspension systems are tensile strength,
Since it is required to have good elongation, fatigue strength, impact strength, etc., the spheroidal graphite cast iron member constituting the member must sufficiently satisfy the above-mentioned required level of strength. However, the black skin on the surface of the cast product has fine irregularities due to sand biting, slag biting, and the like, and the concave portion may act as a notch and become a starting point of fracture. Therefore, it has been found that the spheroidal graphite cast iron member has a problem that the original strength cannot be sufficiently exhibited.

Under the above circumstances, the present inventors have proposed a particularly thin spheroidal graphite cast iron member having good mechanical strength (Japanese Patent Laid-Open No. 2-290943).
Specifically, this thin-walled tough spheroidal graphite cast iron member is one in which graphite particles are dispersed in a ferrite matrix having an area ratio of 10% or less, and fine voids are present between the graphite particles and the ferrite matrix. It is characterized by being substantially absent. Further, this thin-walled, tough spheroidal graphite cast iron member is prepared by pouring a molten metal having a spheroidal graphite cast iron composition into a mold, and after the completion of solidification of the molten metal, almost all of the cast product is still in a state of A ₃ transformation point or higher. The obtained cast product was immediately put in a soaking zone of a continuous furnace maintained at a temperature of A ₃ transformation point or higher, and the cast product was held for 30 minutes or less in order to decompose cementite in the base therein, Then, the cast product can be manufactured by transferring the cast product to the cooling zone of the continuous furnace and cooling the cast product at a cooling rate that achieves ferritic conversion of the matrix.

However, unlike the case of the thin-walled spheroidal graphite cast iron member, in the case of the spheroidal graphite cast iron member having a relatively thick wall for use in a component requiring higher mechanical strength, the pearlite structure remains and the tensile strength remains. It is necessary to improve the bending strength while maintaining the bending strength. For that purpose, it has been found that heat treatment in which the whole or most of the spheroidal graphite cast iron member is made into ferrite is not good.

[0005] Therefore, an object of the present invention is to provide a spheroidal graphite cast iron member which has a blunted cutout sensitivity in the surface black skin layer and which is excellent in mechanical properties, particularly elongation.

Another object of the present invention is to provide a method of manufacturing such a spheroidal graphite cast iron member.

[0007]

As a result of earnest research in view of the above object, the inventors of the present invention have found that the as-cast spheroidal graphite cast iron member has a surface layer portion with extremely large irregularities, so that the surface irregularities are In order to discover that it becomes the starting point of fracture, and to prevent fracture due to bending stress, etc., the hardness of the surface layer part is reduced somewhat while the internal pearlite structure remains substantially unchanged (ferrite in the surface layer part). The present invention was conceived by discovering that a heat treatment for improving the conversion rate should be performed.

That is, the spheroidal graphite cast iron member of the present invention is
Most of the surface layer portion is composed of a ferrite phase and has a thickness of at least 1 mm, and the interior is composed of a pearlite phase and a ferrite phase. The surface layer portion has a ferritization rate of 70% or more. At least about 15
% High.

The first method for producing the spheroidal graphite cast iron member of the present invention is to inject a molten metal having a spheroidal graphite cast iron composition into a mold, and after completion of solidification of the molten metal, almost all of the cast product is still A ₁
When the temperature difference between the surface layer and the inside of the obtained cast product is 40 to 60 ° C., the cast product is held at 750 to 900 ° C. Put in a soaking furnace, and the rate of ferritization of the surface layer is 70%
At least 1 from the above-mentioned internal ferrite ratio
It is characterized in that it is held in the soaking furnace for a time period which increases by 5%, then the cast product is transferred to a cooling furnace and cooled at a cooling rate of 15 to 100 ° C./min.

The second method for producing the spheroidal graphite cast iron member of the present invention is to prepare a pearlitic spheroidal graphite cast iron member.
It is placed in a soaking furnace maintained at 0 to 870 ° C., and is kept in the soaking furnace for a time at which the ferritization rate of the surface layer portion is 70% or more and is at least 15% higher than the internal ferritization rate, Then transfer the casting to a cooling furnace,
It is characterized by cooling at a cooling rate of 15 to 100 ° C./min.

The present invention will be described in detail below. [A] Composition of spheroidal graphite cast iron

The spheroidal graphite cast iron member of the present invention generally contains the following chemical components: C: 3.40 to 3.90 wt% Si: 1.9 to 2.5 wt% Mn: 0.5 wt% or less P: 0.05 wt% or less S: 0.02 % By weight Mg: 0.02 to 0.06% by weight Cu: 0.8% by weight or less The balance has a composition consisting essentially of Fe and inevitable impurities.

(1) C: 3.40 to 3.90% by weight If the content of C is less than 3.40% by weight or exceeds 3.90% by weight, the castability is deteriorated.

(2) Si: 1.9 to 2.5% by weight If the Si content is less than 1.9% by weight, the tendency to form carbides increases, and if it exceeds 2.5% by weight, it becomes difficult to control the amount of pearlite and the content of pearlite becomes uniform. It becomes difficult to obtain perlite.

(3) Mn: 0.5% by weight or less Mn is a pearlite stabilizing element and a carbide forming element. If the Mn content exceeds 0.5% by weight, these effects are reduced.

(4) P: 0.05% by weight or less P is a raw material for inhibiting spheroidization, and its content is 0.05% by weight.
Should not be exceeded.

(5) S: 0.02% by weight or less S is a spheroidization inhibiting material, and its content is 0.02% by weight.
Should not be exceeded.

(6) Mg: 0.02 to 0.06% by weight When the content of Mg is less than 0.02% by weight, the yield for obtaining spheroidal graphite cast iron decreases. On the other hand, if it exceeds 0.06% by weight, chilling may occur.

(7) Cu: 0.8% by weight or less Cu is a pearlite stabilizing element having no tendency to form carbides, and makes pearlite in the structure uniform. But C
Even if the content of u exceeds 0.8% by weight, its effect cannot be expected to be further increased.

The preferred composition of the spheroidal graphite cast iron member is C: 3.60 to 3.80% by weight Si: 2.0 to 2.5% by weight Mn: 0.4% by weight or less P: 0.05% by weight or less S: 0.015% by weight or less Mg: 0.02 to 0.05% by weight % Cu: 0.7 wt% or less The balance is essentially Fe and unavoidable impurities.

[B] Layer structure of spheroidal graphite cast iron member (1) Surface layer part The metallographic structure of the surface layer part needs to have a ferrite rate of 70% or more. Here, the "ferritization rate" refers to the rate of the ferrite phase in the matrix. Ferritization rate is 70%
If it is less than, the hardness of the surface layer is not sufficiently reduced,
The action of preventing destruction is insufficient. A preferable ferritic conversion rate is 80% or more. The rest of the base in the surface layer is mostly pearlite phase.

The surface layer has a Rockwell hardness HR of less than 93.
Have B. When the Rockwell hardness HRB is 93 or more, there is a high possibility that the unevenness will be the starting point of the fracture.

The surface layer has a thickness of at least 1 mm. If the surface layer portion is less than 1 mm, the effect of preventing breakage cannot be sufficiently obtained. The upper limit of the thickness depends on the total thickness of the spheroidal graphite cast iron member, but is generally 10% or less of the total thickness. If the thickness of the surface layer portion exceeds 10% of the total thickness, the mechanical strength of the spheroidal graphite cast iron member decreases.

(2) Inside The metal structure inside is composed of a pearlite phase and a ferrite phase. The ratio of the internal ferrite phase (ferritization ratio) is lower than that of the surface layer portion by at least about 15%. The mechanical strength of the entire spheroidal graphite cast iron member is insufficient unless the internal ferrite ratio is at least about 15% lower than that of the surface layer portion (the ratio of the pearlite phase is less than about 45%). Although it depends on the ferriticity of the surface layer, the internal ferriticity is generally preferably about 0 to 45%.

Due to the above structure, the inside has a higher Rockwell hardness HRB than the surface layer portion. Generally, it is preferable that the internal Rockwell hardness HRB is higher than the surface layer portion by about 10.

The spheroidal graphite cast iron member having the double structure as described above has a surface layer portion of at least 1 mm, and is therefore relatively thick. Specifically, the effect is great when the wall thickness is 12 mm or more, preferably 15 mm or more.

[C] Method for producing spheroidal graphite cast iron member (1) First method

The first method for producing the spheroidal graphite cast iron member having such a layered structure is to subject the cast product to heat treatment in a soaking furnace and a cooling furnace within a short time after the mold is separated. . A continuous furnace having a soaking zone and a cooling zone is preferably used. In the first method, a molten metal having a spheroidal graphite cast iron composition is injected into a mold, and after completion of solidification of the molten metal, mold casting is performed when almost all of the cast product is still in a state of A ₁ transformation point or higher, When the temperature difference between the surface layer and the inside of the cast product reached 40 to 60 ° C,
It is placed in a soaking furnace maintained at 0 to 900 ° C., and kept in the soaking furnace for a time at which the ferritization rate of the surface layer portion is 70% or more and is at least 15% higher than the internal ferritization rate, Next, the cast product is transferred to the cooling furnace and cooled at a cooling rate of 15 to 100 ° C./min to make the surface layer portion more ferrite.

The first method will be described in detail with reference to FIGS.

First, mold casting is performed at a temperature T ₁ (point A) after casting. Mold separation temperature T ₁ is A ₁ transformation point (about 720 ° C)
It must be above. When the temperature T ₁ is lower than the A ₁ transformation point, not only the heating and holding time thereafter becomes long, but also the temperature difference between the inner and outer layers becomes small due to long-time heating, and as a result, the double structure cannot be obtained. Specifically, the temperature T _{1 is set} to 800
It is preferable to set the temperature to 900 ° C.

After separating the mold, the cast product is left in the atmosphere for a short time. During the cooling to the point B, the temperature difference in the surface layer portion is larger than that in the inside, so that the temperature difference ΔT occurs. This temperature difference Δ
T needs to be 40-60 degreeC. If it is lower than 40 ° C, a sufficient double structure cannot be obtained. On the other hand, if it exceeds 60 ° C., the temperature of the surface layer portion becomes too low. However,
The surface temperature is the surface temperature of the cast product, and the internal temperature is the temperature of the central part of the cast product. The time t ₁ required for obtaining the temperature difference ΔT depends on the wall thickness of the cast product, but is usually 2 to 30 minutes. The preferred holding time is 5 to 20
Minutes.

The cast product obtained with this temperature difference ΔT is introduced into a soaking furnace. The temperature T _{2 of the} soaking furnace is maintained at 750 to 900 ° C. When the temperature of the soaking furnace is less than 750 ° C,
The temperature of the casting is too low and needs to be reheated. Further, when the temperature exceeds 900 ° C, the energy loss becomes large. The preferable heating and holding temperature T ₂ is 780 to 850 ° C.

In the soaking furnace at the above temperature, the surface layer portion immediately reaches the heating and holding temperature T ₂ (point C), but the internal temperature remains lower than that.

The time t ₂ for holding the cast product in the soaking furnace is
This is the time when the ferritic conversion rate of the surface layer portion is 70% or more and is higher than the internal ferritic conversion rate by at least 15%.
The reason for this is as follows.

As shown in FIG. 2 (a), since the surface layer portion is cooled relatively rapidly, the graphite particles are miniaturized and the matrix is in the γ phase. When the surface layer portion of such a structure is heated and held at the temperature T ₂ , carbon in the matrix is absorbed in the graphite particles, and the carbon content of the matrix decreases. Therefore, the matrix is ferriticized by the subsequent slow cooling.

On the other hand, since the inside is cooled more slowly than the surface layer, graphite particles grow as shown in FIG. 2 (b). Further, the base is in the γ phase, like the surface layer. When the inside of such tissue is heated to a temperature lower than the surface layer,
The amount of carbon in the matrix absorbed in the graphite particles is relatively small, and a phase in which the carbon content is reduced is formed only in the area around the graphite particles. This low carbon phase becomes ferritic in the subsequent slow cooling step. However, the phase that has not been reduced to carbon becomes pearlite. Therefore, the internal matrix is a mixture of ferrite phase and pearlite phase.

After heating and holding, the cast product was transferred to the cooling zone,
Gradually cool at a cooling rate of 15 to 100 ° C./minute (point D). If the cooling rate is less than 15 ° C./minute, the rate of internal pearlite decreases. On the other hand, if it exceeds 100 ° C./min, pearlite tends to remain in the matrix and the surface layer cannot be softened. A preferable cooling rate is 20 to 40 ° C / minute. This gradual cooling causes ferrite formation in the surface layer portion and the matrix in the inside, but since there is a difference in the structure between the surface layer portion and the inside as described above, the rate of ferrite formation in the surface layer portion becomes higher. In addition, it is not necessary to perform gradual cooling to room temperature, and at least about 65
It may be performed up to 0 ° C. (temperature T ₃ ) (point E). Below the temperature T ₃ , no phase change of the matrix occurs.

By the above heat treatment, only the surface layer of the spheroidal graphite cast iron member is transformed into ferrite, and a cast product having a double structure composed of a high hardness inside and a low hardness surface layer is obtained.

(2) Second Method The second method for producing the spheroidal graphite cast iron member of the present invention is a method of producing a pearlitic spheroidal graphite cast iron member at 780 to 870 ° C.
In the soaking furnace held in the above step, and holding in the soaking zone for a time such that the ferritization rate of the surface layer portion is 70% or more and at least 15% higher than the internal ferritization rate, and then the cast product is By transferring to the cooling zone of the continuous furnace and cooling at a cooling rate of 15 to 100 ° C./min, the surface layer portion is made more ferrite.

The spheroidal graphite cast iron member which is the starting material is pearlite. Specifically, it is preferable to use a spheroidal graphite cast iron member having a pearlite area ratio of 30% or more. Such a pearlitic spheroidal graphite cast iron member can be produced by subjecting spheroidal graphite cast iron having the above composition to a known pearlite treatment.
Also in the spheroidal graphite cast iron member that has been subjected to the pearlite treatment, the graphite particles are finely divided in the surface layer portion, and the graphite particles are relatively coarsely grown inside.

The second method will be described in detail with reference to FIG. First, the pearlitic spheroidal graphite cast iron member was
The soaking furnace is placed in a soaking furnace maintained at 80 to 870 ° C. (point B), and the soaking furnace is kept for a period of time at which the ferritization rate of the surface layer portion is 70% or more and at least 15% higher than the internal ferritization rate. Hold in. Heating temperature is 780 ℃
If it is less than the above, it is difficult to make the structure uniform by diffusion of C in the matrix. On the other hand, if it is higher than 870 ° C., it is difficult to control the structure to have a double structure because the inside is affected by heat treatment. A preferable heating and holding temperature is 800 to 850 ° C.

The holding time t ₂ in the soaking zone is the time during which the ferritic conversion rate of the surface layer portion is 70% or more and at least 15% higher than the internal ferritic conversion rate. During this time, the surface layer is heated relatively quickly, so
The temperature rises by T. Therefore, the carbon in the matrix of the surface layer portion is absorbed in the graphite particles, and the carbon content of the matrix decreases.

On the other hand, since the inside is heated more gently than the surface layer portion, the amount of carbon in the matrix absorbed in the graphite particles is relatively small, and the carbon content in the region around the graphite particles is relatively small. There is a degraded phase. This low carbon phase becomes ferritic in the subsequent slow cooling step. However, the phase that has not been reduced to carbon becomes pearlite. Therefore, the internal matrix is a mixture of ferrite phase and pearlite phase.

The heating and holding time t ₂ of the cast product depends on the wall thickness of the cast product, but is usually 2 to 30 minutes, preferably 5 to 20 minutes.

After heating and holding, the cast product was transferred to the cooling zone,
Gradually cool at a cooling rate of 15 to 100 ° C./minute (point D). The reason for limiting the cooling rate is the same as above. The slow cooling does not have to be performed to room temperature, and may be performed to at least about 650 ° C. (temperature T ₃ ) (point E).

By the above heat treatment, as in the first method, the surface layer of the spheroidal graphite cast iron member becomes more ferrite, and a cast product having a double structure consisting of a high hardness inside and a low hardness surface is obtained. can get.

FIG. 4 shows a temperature pattern when the second method is performed subsequent to the pearlite treatment. In the pearlite treatment, austenite treatment at 840 to 860 ° C. for 0.5 to 2 hours is performed in step G-H, and 780 to 780 in step I-J.
A soaking treatment at 820 ° C. for 0.5 to 1 hour is performed, and then forced cooling is performed.

The present invention is explained in detail by the following examples.

Example 1, Comparative Examples 1 and 2 (1) Composition Using spheroidal graphite cast iron material having a composition consisting of iron, unavoidable impurities and components shown in Example 1 of Table 1 below, FIGS. A test piece having the shape shown in FIG.

Table 1 Weight% Example No. C Si Mn P S Mg Mg Cu 1 ⁽¹⁾ 3.68 2.26 0.31 0.035 0.011 0.037 0.56 2 ⁽²⁾ 3.71 2.40 0.21 0.021 0.007 0.035 0.18 3 ⁽³⁾ 3.60 2.40 0.30 0.022 0.008 0.031 0.52 Note: (1) Example 1. (2) FCD45. (3) FCD60.

(2) Heat treatment The spheroidal graphite cast iron melt having the above composition was poured into the mold at 1410 ° C., and the surface temperature of the obtained test piece-shaped cast product was still A ₁
When the temperature was above the transformation point (about 720 ° C.), the mold was removed. After the time (5 minutes) when the temperature difference ΔT between the surface layer portion and the inside reached 50 ° C., the cast product was put into a soaking zone of a continuous furnace heated and held at 830 ° C. and held for 15 minutes. The cast was then transferred to a cooling zone where it was cooled to 650 ° C at a cooling rate of 55 ° C / min and removed from the continuous furnace.

The cast product of Example 1 (shape shown in FIG. 6) obtained by the above heat treatment was observed with a micrograph. FIG. 8 (a) shows a photomicrograph (× 100) of the surface layer portion, and FIG. 8 (b) shows a photomicrograph (× 100) of the inside.
From each micrograph, the ferritic conversion rate of the surface layer and the inside was obtained.

Further, the same test pieces as in Example 1 were prepared from the spheroidal graphite cast iron having the compositions shown in Examples 2 and 3 of Table 1 (Comparative Examples 2 and 3), but the spheroidal graphite cast iron members of Comparative Examples 2 and 3 were manufactured. Conventional pearlite treatment (conditions: 850 ° C x 1 hour +78
Only soaking at 0 to 820 ° C + forced cooling) was performed.

(3) Strength test Using a bending tensile test piece shown in FIG. 5 (measuring portion was left as black skin and other portion was machined), tensile strength (σB) and proof stress (σ _0.2 ) were used. And elongation (δ) were measured.

The bending test piece shown in FIG. 6 (whole body was left as a black skin) was supported at a fulcrum distance of 300 mm, and a load was applied to determine the relationship between the bending load and the bending displacement.

Further, the bending impact test piece shown in FIG. 6 was supported at a distance between fulcrums of 100 mm, and a weight of 50 kg was 10 cm.
Dropped from the height of. Under these conditions, the drop position was raised by 10 cm until a crack was generated in the test piece, and the bending impact strength was determined from the drop height when the crack was generated.

Further, using the rotary bending fatigue test piece shown in FIG. 7 (the measurement portion was left as a black skin and the other portion was machined), the fatigue limit (condition: room temperature) was measured by an Ono type rotary bending fatigue tester. , Constant load, rotation speed 3600rpm in air
) Was measured. The fatigue limit is expressed by a stress limit that does not damage after repeated 10 ⁷ times (a stress limit in which a crack does not occur or does not propagate even if a crack occurs). The results of the above measurements are shown in Table 4.
Shown in.

Example 2 (1) Composition The same test piece as in Example 1 was prepared using a spheroidal graphite cast iron material having a composition consisting of iron, unavoidable impurities and the components shown in Table 2 below.

Table 2 wt% Example No. C Si Mn P S Mg Mg Cu 2 3.73 2.24 0.35 0.020 0.009 0.033 0.45

(2) Heat Treatment A spheroidal graphite cast iron melt having the above composition was poured into a mold at 1420 ° C., the mold was separated, and then pearlite treatment was performed to obtain a test piece-shaped cast product. After cooling to room temperature, it was placed in a soaking furnace heated to 800 ° C. and held for 5 minutes. Next, the cast product was placed in a cooling furnace, cooled at a rate of 20 ° C./min, and taken out at 200 ° C.

The cast product (shape shown in FIG. 6) obtained by the above heat treatment was observed with a micrograph. Figure 9
A micrograph (× 100) of the surface layer is shown in FIG.
An internal micrograph (× 100) is shown in (b). From each micrograph, the ferritic conversion rate of the surface layer and the inside was obtained. Further, a mechanical strength test was conducted in the same manner as in Example 1. The results are shown in Table 4.

Example 3 (1) Composition A spheroidal graphite cast iron material having a composition consisting of iron, unavoidable impurities and the components shown in Table 3 below was used to prepare the same test piece as in Example 1.

Table 3 wt% Example No. C Si Mn P S Mg Mg Cu 3 3.70 2.21 0.30 0.021 0.008 0.038 0.08

(2) Heat Treatment The molten spheroidal graphite cast iron having the above composition was poured into a mold at 1415 ° C., the mold was separated, and then cooled to room temperature. Then 850
It was placed in a heat treatment furnace heated and maintained at 0 ° C. and maintained for 60 minutes, and then maintained at 810 ° C. for 1 hour. After taking out from the furnace, forced air cooling was performed to perform pearlite treatment. Each of the obtained castings was placed in a soaking furnace heated and held at 810 ° C,
Hold for 5 minutes. Next, put the cast product in a cooling furnace, and
It was cooled at a rate of minutes and taken out at 200 ° C.

The cast product (shape shown in FIG. 6) obtained by the above heat treatment was observed with a micrograph. Figure 10
A micrograph (× 100) of the surface layer portion is shown in FIG.
An internal micrograph (× 100) is shown in (b). From each micrograph, the ferritic conversion rate of the surface layer and the inside was obtained. Further, a mechanical strength test was conducted in the same manner as in Example 1. The results are shown in Table 4.

Table 4 Tensile Test Ferritization Rate ⁽¹⁾ σB σ _0.2 δ Surface Layer Internal Sample (kg / mm ² ) (kg / mm ² ) (%) (%) (%) Example 1 1 64.8 43.6 6.6 82 47 2 64.0 42.4 5.7 3 67.7 43.8 7.5 Example 2 1 65.5 43.8 7.2 90 75 2 64.1 42.9 6.5 3 63.7 44.2 5.9 Example 3 1 60.5 42.5 7.3 87 68 2 59.3 41.8 6.9 3 61.4 39.6 7.6 Comparative Example 1 1 54.9 41.0 4.8 73 76 2 53.0 40.7 4.9 3 52.7 39.9 4.2 Comparative Example 2 1 62.7 44.1 2.2 42 40 2 63.5 43.8 3.4 3 62.7 43.6 2.8

Table 4 (continued) Bending test Rotating bending fatigue test Bending load Bending displacement Bending impact strength Fatigue limit ⁽¹⁾ Sample (kg) (mm) (kgf-m) (kg / mm ² ) Example 1 1 1295 65 30 19 2 1300 60 30 3 1290 62 30 Example 2 1 1320 64 30 18 2 1310 61 30 3 3 1280 62 25 Example 3 1 1300 61 25 16 2 1310 62 30 3 1300 65 25 Comparative Example 1 1 1150 22 20 16 2 1100 20 20 3 1150 23 25 Comparative example 2 1 1360 15 20 17 2 1300 15 15 3 1330 12 20 Note: (1) Average value of data of three test pieces.

As is clear from the results shown in Table 4, FCD4
5 (Comparative Example 1) and FCD60 (Comparative Example 2),
The spheroidal graphite cast iron members of the present invention (Examples 1 to 3) are excellent in any of tensile strength, elongation, bending strength, bending impact strength and fatigue limit.

With respect to the test pieces of Examples 1 to 3 (shown in FIG. 6), Rockwell hardness HR was measured at each depth from the surface.
B was measured. Depth from the surface and Rockwell hardness HR
The relationship with B is shown in FIG. As is clear from FIG. 11, since the Rockwell hardness HRB gradually increases up to a depth of about 2.5 mm, it can be seen that the surface layer portion is formed with a thickness of about 2.5 mm.

[0070]

As described above in detail, since the spheroidal graphite cast iron member of the present invention has the surface layer portion whose hardness is somewhat lowered and elongation is improved, it exhibits excellent mechanical strength as a whole. . Such a high-strength spheroidal graphite cast iron member is a cast product that requires particularly toughness and strength and has a lot of black surface (for example, parts for automobile suspension systems, joints for connecting reinforcing bars, fixing column bases of buildings. It is suitable for use as a metal fitting for joining).

[Brief description of drawings]

FIG. 1 is a graph showing a first method of the present invention.

FIG. 2 is a schematic view showing how the metallic structure of a spheroidal graphite cast iron member is changed by the first method of the present invention, (a) showing a surface layer portion, and (b) showing the inside.

FIG. 3 is a graph showing a second method of the present invention.

FIG. 4 is a graph showing a second method of the present invention having a pearlite treatment step in the first stage.

FIG. 5 is a side view showing a test piece of spheroidal graphite cast iron for performing a tensile strength test.

FIG. 6 is a partially cutaway side view showing a test piece of spheroidal graphite cast iron for a bending test and a bending impact test.

FIG. 7 is a side view showing a test piece of spheroidal graphite cast iron for a rotary bending fatigue test.

FIG. 8 is a micrograph (× 100) showing the metal structure of the test piece prepared in Example 1, where (a) shows the surface layer portion and (b) shows the inside.

9 is a micrograph (× 100) showing the metal structure of the test piece prepared in Example 2, where (a) shows the surface layer portion and (b) shows the inside.

FIG. 10 is a micrograph (× 100) showing the metal structure of the test piece prepared in Example 3, where (a) shows the surface layer portion and (b).
Indicates the inside.

FIG. 11 is a graph showing the relationship between the depth from the surface and the Rockwell hardness HRB for the test pieces prepared in Examples 1 to 3.

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Kiyoshi Suehara Kiyoshi Suehara 35 Nagahama-cho, Kanda-cho, Kyoto-gun, Fukuoka Kyushu Plant, Hitachi Metals, Ltd. (72) Toshiki Yoshida 11 Kinugaoka, Moka-shi, Tochigi Hitachi Metals (72) Inventor Kouhei Imanishi 35, Nagahama-cho, Kanda-cho, Kyoto-gun, Fukuoka Kyushu Plant, Hitachi Metals, Ltd.

Claims (6)

[Claims] 1. A spheroidal graphite cast iron member having a surface layer portion having a thickness of at least 1 mm, which is mostly composed of a ferrite phase, and an interior composed of a pearlite phase and a ferrite phase,
The spheroidal graphite cast iron member is characterized in that the surface layer has a ferritization rate of 70% or more, which is at least about 15% higher than the internal ferritization rate. 2. The spheroidal graphite cast iron member according to claim 1, wherein the surface layer has a Rockwell hardness of HRB of 93.
The spheroidal graphite cast iron member is characterized in that the hardness is lower than the internal hardness. 3. The spheroidal graphite cast iron member according to claim 1, wherein the spheroidal graphite cast iron has a weight ratio of 3.40 to 3.90.
% C, 1.9 to 2.5% Si, 0.5% or less Mn, 0.05% or less P, 0.02% or less S, 0.02 to 0.06% Mg, 0.8% or less Cu and the balance substantially Fe and unavoidable. A spheroidal graphite cast iron member having a composition of impurities. 4. A surface layer part having a thickness of at least 1 mm, which is mostly composed of a ferrite phase, and an interior composed of a pearlite phase and a ferrite phase, and the ferrite conversion rate of the surface layer part is 70% or more, In the method of manufacturing a spheroidal graphite cast iron member having a ferritic conversion rate higher than the internal ferrite content by at least about 15%, a molten metal having a spheroidal graphite cast iron composition is poured into a mold, and after completion of solidification of the molten metal, almost all of the cast product is
Mold casting is performed when it is in a state of ₁ transformation point or more, and when the temperature difference between the surface layer portion and the inside of the obtained cast product is 40 to 60 ° C, the cast product is held at 750 to 900 ° C. Put into a soaking furnace, and the ferrite rate of the surface layer is 70
% Or more and at least 15% higher than the internal ferritic rate, held in the soaking furnace, and then transferring the cast product to a cooling furnace to cool at a cooling rate of 15 to 100 ° C./min. A method characterized by: 5. A surface layer part having a thickness of at least 1 mm, which is mostly composed of a ferrite phase, and an interior composed of a pearlite phase and a ferrite phase, wherein the surface layer part has a ferritization ratio of 70% or more, A method for producing a spheroidal graphite cast iron member having a ferritic conversion rate of at least about 15% higher than that of the internal ferrite is provided.
It is placed in a soaking furnace maintained at 0 to 870 ° C., and is kept in the soaking furnace for a time at which the ferritization rate of the surface layer portion is 70% or more and is at least 15% higher than the internal ferritization rate, Then transfer the casting to a cooling furnace,
A method comprising cooling at a cooling rate of 15 to 100 ° C./min. 6. The method for manufacturing a spheroidal graphite cast iron member according to claim 4 or 5, wherein the spheroidal graphite cast iron is in a weight ratio of 3.40 to 3.90% C, 1.9 to 2.5% Si, and 0.5% or less. Mn, P less than 0.05%, S less than 0.02%, 0.02-0.06%
Of Mg, 0.8% or less of Cu and the balance substantially Fe and inevitable impurities are used.