JP2021059752A - Spheroidal graphite cast iron excellent in strength and toughness and having low hardness - Google Patents

Abstract

To provide spheroidal graphite cast iron which is suitable as a material that constitutes various machines and automotive parts, in particular a material constituting a suspension component, excellent in the strength and toughness, and having low hardness.SOLUTION: Spheroidal graphite cast iron having a composition made of, by mass ratio, C: 3.5 to 3.9%, Si: 2.45 to 2.85%, Mn: 0.2 to 0.6%, Cu: 0.70 to 1.20%, Mg: 0.02 to 0.06%, P: 0.04% or smaller, S: 0.02% or smaller, and the balance made of Fe and inevitable impurities, and having a two-phase mixed base tissue comprising, by area rate, a fine ferrite phase of 15 to 50% and a fine perlite phase of 50 to 85%, and in which the number of crystallites of a base texture obtained by a J-shaped method is 250 pieces/mm or larger.SELECTED DRAWING: None

Description

The present invention relates to spheroidal graphite cast iron having excellent strength and toughness and low hardness.

Spheroidal graphite cast iron is widely used in various machine and automobile parts because of its excellent mechanical properties and good castability. In particular, suspension arm parts such as automobile suspension arms and steering knuckles are required to have static strength and fatigue strength to support the vehicle body, as well as impact resistance so that they will not be damaged even if there is an impact due to an accident or the like. Will be done. Therefore, spheroidal graphite cast iron used for suspension system parts is required to have toughness such as elongation in addition to tensile strength and proof stress. In order to satisfy such a requirement, FCD400, FCD450 and the like specified in JIS G5502 have been conventionally used as spheroidal graphite cast iron having a base structure mainly composed of a ferrite phase and having toughness.

_{In recent years, there has been a strong demand for reduction of CO 2} emissions of automobiles in order to prevent global warming. For that purpose, it is necessary to improve the fuel efficiency of automobiles. Is required to be lighter. In order to reduce the weight of parts while ensuring the required strength, it is effective to reduce the size and thickness of the parts. For this purpose, it is conceivable to use pearlite-based spheroidal graphite cast iron such as FCD600 and FCD700, which have higher strength than FCD400 and FCD450. However, since strength and toughness of spheroidal graphite cast iron are contradictory, FCD600, FCD700 and the like have toughness. Is not suitable for suspension equipment parts that require low impact resistance. In order to reduce the weight of suspension system parts while ensuring strength and toughness, spheroidal graphite cast iron having excellent both strength and toughness is required.

Various proposals have been made conventionally in order to obtain spheroidal graphite cast iron having excellent strength and toughness. For example, Patent Document 1 (a) has a mass ratio of C: 3.4 to 4%, Si: 1.9 to 2.8%, Mg: 0.02 to 0.06%, Mn: 0.2. ~ 1%, Cu: 0.2-2%, Sn: 0-0.1%, (Mn + Cu + 10 × Sn): 0.85-3%, P: 0.05% or less, S: 0.02% or less It has a composition of the balance Fe and unavoidable impurities, and (b) has a two-phase mixed matrix structure consisting of a fine ferrite phase of 2 to 40% and a fine pearlite phase of 60 to 98% in terms of area ratio. We propose a spheroidal graphite cast iron having a maximum length of a ferrite phase of 300 μm or less and (c) having the pearlite phase formed around graphite dispersed in the two-phase mixed matrix structure and having excellent strength and toughness.

By the way, various parts of machines and automobiles made of spheroidal graphite cast iron are rarely assembled to machines and automobiles as cast materials, and are generally assembled by machining the materials. For example, in the case of steering knuckles for suspension system parts, after casting, machines such as cutting machines for mounting surfaces with peripheral parts such as axles, shock absorbers, steering devices and braking devices, connecting parts such as mounting holes, and parts requiring high dimensional accuracy. After being processed, it must have high machinability because it can be assembled into an automobile.

However, high-strength pearlite-based spheroidal graphite cast iron such as FCD600 and FCD700 is a difficult-to-cut material, and in particular, spheroidal graphite cast iron having a tensile strength of 700 MPa or more such as FCD700 has high strength and high hardness, so it is to be machined. Inferior in sex. For this reason, when cutting a part made of spheroidal graphite cast iron with a tensile strength of 700 MPa or more, a relatively expensive cutting tool for difficult-to-cut materials is required, and since the tool life is short, the tool is frequently replaced and the machining cost is high. There is a problem that the efficiency and productivity are inferior, for example, the machining efficiency is low because the cutting is forced to be performed at a low speed and the cutting takes a long time. FCD600, FCD700 and the like are not suitable for suspension system parts because they are not only low in toughness but also inferior in machinability due to high hardness of the material. As described above, it is desired that the material constituting the suspension device component has low hardness and high machinability in addition to strength and toughness. Although the spheroidal graphite cast iron described in Patent Document 1 has excellent strength and toughness, studies on lowering the hardness in order to obtain high machinability are not sufficient, and there is room for improvement.

International Publication No. 2013/100148 Pamphlet

Therefore, an object of the present invention is to provide spheroidal graphite cast iron having excellent strength and toughness and low hardness.

In view of the above object, as a result of diligent studies based on the spheroidal graphite cast iron of Patent Document 1, the present inventors limit the content of Si, Mn and Cu in the alloy composition of the spheroidal graphite cast iron to a relatively narrow appropriate range. At the same time, it was discovered that low hardness can be realized while ensuring excellent strength and toughness by applying the heat treatment conditions proposed in the method for producing spheroidal graphite cast iron in Patent Document 1, and the present invention was conceived.

That is, the spheroidal graphite cast iron having excellent strength and toughness and low hardness of the present invention can be used.
By mass ratio,
C: 3.5-3.9%
Si: 2.45 to 2.85%
Mn: 0.2 to 0.6%
Cu: 0.70 to 1.20%
Mg: 0.02 to 0.06%
P: 0.04% or less S: 0.02% or less It has a composition consisting of the balance Fe and unavoidable impurities.
It has a two-phase mixed matrix structure consisting of a fine ferrite phase of 15 to 50% and a fine pearlite phase of 50 to 85% in area ratio.
It is characterized in that the number of crystal grains of the matrix structure determined by the line of intersection method is 250 grains / mm or more. The number of crystal grains in the present invention is the number of crystal grain boundaries per unit length obtained by the line of intersection method as described later.

The spheroidal graphite cast iron of the present invention has a tensile strength of 700 MPa or more as an index of strength, a room temperature elongation of 10% or more as an index of toughness, and a Brinell hardness of 210 to 240 HBW as an index of hardness. It is preferable to have it.

The spheroidal graphite cast iron of the present invention preferably has a tensile strength of 700 MPa or more, and the tensile strength and Brinell hardness satisfy the following formula (1).
Brinell hardness ≤ (tensile strength / 5) +82 ... (1)
However, in the formula (1), the value of the unit HBW is substituted for the Brinell hardness, and the value of the unit MPa is substituted for the tensile strength.

The spheroidal graphite cast iron of the present invention is excellent in strength and toughness, and is suitable for automobile parts, particularly suspension device parts that require impact resistance, and contributes to fuel efficiency of automobiles by reducing the weight of the parts. In addition, the spheroidal graphite cast iron of the present invention has high machinability because it has low hardness, and can improve economic efficiency and productivity in machining of suspension device parts.

The spheroidal graphite cast iron of the present invention will be described in detail below. Unless otherwise specified, the content of constituent elements of the alloy is shown in% by mass.

[A] Composition of spheroidal graphite cast iron (1) C: 3.5 to 3.9%
C is necessary for lowering the solidification start temperature to improve castability, crystallizing graphite, and precipitating a pearlite phase. If the C content is less than 3.5%, chilling is likely to occur and the toughness is lowered, and if it exceeds 3.9%, abnormal graphite is likely to be generated and the strength of spheroidal graphite cast iron is lowered. Therefore, the C content is set to 3.5 to 3.9%. The preferred C content is 3.6-3.8%.

(2) Si: 2.45 to 2.85%
Si is necessary for promoting the crystallization of graphite and increasing the fluidity of the molten metal. In particular, in the present invention, Si promotes the precipitation of the ferrite phase, which is softer than the pearlite phase, and the hardness of the spheroidal graphite cast iron. Has the effect of improving machinability. Further, since Si has an action of solid-solving in the ferrite phase to strengthen the matrix structure, it suppresses a decrease in the strength of spheroidal graphite cast iron due to an increase in the ferrite phase. If the Si content is less than 2.45%, the precipitation of the ferrite phase is insufficient and the hardness of the spheroidal graphite cast iron does not decrease. On the other hand, if it exceeds 2.85%, the effect of suppressing pearlite formation becomes high, the ferrite phase becomes excessive, the strength of the spheroidal graphite cast iron decreases, and the toughness of the ferrite phase also deteriorates. Therefore, the Si content is set to 2.45 to 2.85%. The preferred Si content is 2.50 to 2.85%, more preferably 2.55 to 2.85%.

(3) Mn: 0.2 to 0.6%
Mn is an element that is inevitably mixed from the raw material, but has an action of precipitating a pearlite phase that contributes to strength improvement as a pearlite phase stabilizing element. If the Mn content is less than 0.3%, the pearlite phase cannot be sufficiently generated, and the required strengths such as tensile strength and proof stress cannot be obtained. The Mn content that promotes pearlite formation is acceptable up to 1%, but if it exceeds 0.6%, chilling is promoted, the spheroidal graphite cast iron becomes high in hardness, and machinability and toughness are deteriorated. Therefore, the Mn content is set to 0.2 to 0.6%. The preferred Mn content is 0.3-0.6%.

(4) Cu: 0.70 to 1.20%
Cu is required to precipitate a pearlite phase that contributes to strength improvement as a pearlite phase stabilizing element. During the heat treatment, Cu suppresses the diffusion of carbon from the austenite phase to the graphite particles due to the barrier effect at the interface between graphite and the matrix, thereby delaying the transformation from the austenite phase to the ferrite phase and causing the ferrite phase to change. It is thought to suppress precipitation and growth. In addition, Cu is required for spheroidal graphite cast iron due to the pinning effect of precipitating Cu precipitates from the matrix structure during heat treatment, which suppresses the growth of crystal grains in the austenite phase and refines the crystal grains. Ensure toughness. In the spheroidal graphite cast iron of the present invention, the refinement of the crystal grains of the matrix structure, that is, the increase in the number of crystal grains is obtained by the above-mentioned action of suppressing the growth of the crystal grains of the austenite phase by Cu. If the Cu content is less than 0.70%, the pearlite phase cannot be sufficiently formed, the fineness of the crystal grains of the austenite phase is not promoted, and the tensile strength and toughness of the spheroidal graphite cast iron are lowered. On the other hand, if Cu exceeds 1.20%, the effect of improving the above characteristics is saturated, which is economically disadvantageous. Therefore, the Cu content is set to 0.70 to 1.20%. The Cu content is preferably 0.80 to 1.10%, more preferably 0.80 to 1.00%.

(5) Mg: 0.02 to 0.06%
Mg is an element required for graphite spheroidization, but if the content is less than 0.02%, the effect of graphite spheroidization is insufficient. On the other hand, when the Mg content exceeds 0.06%, chills are likely to be generated, and the machinability and toughness of the spheroidal graphite cast iron are lowered. Therefore, the Mg content is set to 0.02 to 0.06%. The preferred Mg content is 0.03 to 0.05%.

(6) P: 0.04% or less, S: 0.02% or less,
Since both P and S are graphite spheroidizing inhibitor elements that are inevitably mixed from the raw material, their contents are set to 0.04% or less for P and 0.02% or less for S, respectively.

[B] Structure of spheroidal graphite cast iron In the matrix structure of spheroidal graphite cast iron of the present invention, the fine ferrite phase and the fine pearlite phase are distributed in a camouflage pattern (or the fine ferrite phase is island-like in the pearlite phase. It is a (dispersed) two-phase mixed matrix structure. The area ratio of the ferrite phase in the matrix structure is preferably 15 to 50%, and the area ratio of the pearlite phase is preferably 50 to 85%. Although the ferrite phase is formed by finely dispersing due to the action and effect described later, the precipitation area ratio is relatively larger than that of the ferrite phase of spheroidal graphite cast iron of Patent Document 1, and the lower limit thereof is 15%. By setting the area ratio of the ferrite phase in the matrix structure to 15 to 50%, the hardness of the spheroidal graphite cast iron can be reduced and the machinability can be improved.

The fine ferrite phase is formed at the grain boundaries of the pearlite phase by the action of suppressing the precipitation and growth of the ferrite phase by the pearlite phase stabilizing element in the heat treatment and suppressing the growth of the crystal grains of the austenite phase by the pinning effect of Cu precipitates. It is formed by fine dispersion along the line. The fine ferrite phase is not reticulated but has an elongated shape divided by pearlite crystal grains. The shape of such a ferrite phase may be called "dendritic". Further, the fine pearlite phase is a pearlite-transformed matrix of fine crystal grains (austenite crystal grains) completely austenitized by austenitizing heat treatment without being coarsened by lowering the temperature.

In the spheroidal graphite cast iron of the present invention, both the ferrite phase and the pearlite phase are finely dispersed and precipitated by the above-mentioned effect of suppressing the precipitation and growth of the ferrite phase, so that the crystal grains of the matrix structure are finely divided. The degree of grain refinement can be expressed by the number of crystal grains. The larger the number of crystal grains, the better the toughness. Specifically, the number of crystal grains in the matrix structure is preferably 250 grains / mm or more. When the number of crystal grains is less than 250 / mm, spheroidal graphite cast iron does not have sufficient toughness because coarse crystal grains are present. The number of crystal grains in the matrix structure is more preferably 270 grains / mm or more, and most preferably 300 grains / mm or more.

[C] Characteristics of spheroidal graphite cast iron (1) Strength and toughness Spheroidal graphite cast iron has excellent strength and toughness because high tensile strength and toughness are required in addition to high tensile strength and toughness. preferable. Specifically, the spheroidal graphite cast iron of the present invention preferably has a tensile strength of 700 MPa or more at room temperature and an elongation of 10% or more at room temperature.

If the tensile strength of spheroidal graphite cast iron at room temperature is 700 MPa or more, the suspension system parts can be miniaturized and thinned to reduce the weight while ensuring the strength required to support the vehicle body. It becomes. The tensile strength of the spheroidal graphite cast iron is more preferably 720 MPa or more, and most preferably 740 MPa or more. The upper limit of the tensile strength is not particularly limited, but it is preferable that the hardness is 800 MPa or less so that the hardness described later does not become excessive.

If the room temperature elongation of the spheroidal graphite cast iron is 10% or more, the suspension device parts have impact resistance, and damage can be suppressed even if there is an impact due to an accident such as a collision. The room temperature elongation of spheroidal graphite cast iron is more preferably 11% or more, and most preferably 12% or more.

(2) Hardness It is desirable that suspension device parts have high machinability because they are machined such as cutting on the mounting surface with peripheral parts, connecting parts such as mounting holes, and parts that require high dimensional accuracy after casting. Is done. Generally, spheroidal graphite cast iron having a tensile strength of 700 MPa or more is inferior in machinability because of its high hardness. It is preferable that the tensile strength is 700 MPa or more and the hardness is low in order to have high machinability. Specifically, the spheroidal graphite cast iron of the present invention preferably has a Brinell hardness of 210 to 240 HBW. The Brinell hardness of spheroidal graphite cast iron is more preferably 210 to 235 HBW.

Here, in general, in an iron-based material such as spheroidal graphite cast iron, the strength and the hardness are in a direct proportional relationship, and the hardness of the material increases as the strength of the material increases. If the rate of increase in hardness is small with respect to the increase in strength, it can be said that the material has low hardness and excellent machinability while ensuring strength. Although the strength and hardness of the spheroidal graphite cast iron of the present invention are in a direct proportion to each other, the increase in hardness is gradual with respect to the increase in strength, and the material has a tensile strength of 700 MPa or more and a low hardness. Specifically, it is preferable that the spheroidal graphite cast iron of the present invention has a tensile strength of 700 MPa or more, and the tensile strength and the Brinell hardness satisfy the following formula (1).
Brinell hardness ≤ (tensile strength / 5) +82 ... (1)
However, in the formula (1), the value of the unit HBW is substituted for the Brinell hardness, and the value of the unit MPa is substituted for the tensile strength. In the equation (1), the intercept value 82 on the right side is more preferably 80, and most preferably 78.

[D] Method for producing spheroidal graphite cast iron The method for producing spheroidal graphite cast iron of the present invention is as follows.
(1) After casting and solidifying the molten metal having the composition of the spheroidal graphite cast iron of the above-mentioned [A],
(2) (i) A heat treatment having a step of producing fine austenite crystal grains (transformed into pearlite crystal grains after lowering the temperature) by keeping the entire matrix at a temperature at which austenite is formed.
as well as,
(Ii) In a predetermined temperature section within the temperature range where eutectoid transformation occurs, a heat treatment is performed, which comprises a step of cooling at a cooling rate in which a fine ferrite phase is generated.
With
(A) It has a two-phase mixed matrix structure consisting of a fine ferrite phase of 15 to 50% and a fine pearlite phase of 50 to 85% in terms of area ratio.
(B) A spheroidal graphite cast iron having a structure in which the number of crystal grains of the matrix structure determined by the line of intersection method is 250 grains / mm or more is produced.
In a temperature range lower than the eutectoid transformation temperature range, the temperature is normally cooled to room temperature.
Hereinafter, the methods for producing the spheroidal graphite cast irons (1) and (2) and the matrix structure of the spheroidal graphite cast irons (a) and (b) obtained thereby will be described in detail.

(1) Austenitic heat treatment conditions [step (i)]
By keeping the entire matrix structure at a temperature at which it completely austenites, fine austenite crystal grains (transformed into pearlite crystal grains after temperature reduction) are generated. The austenitizing temperature is preferably 800 to 865 ° C. If this temperature is less than 800 ° C., the pearlite phase remains, and the ferrite phase is formed and grown from the pearlite phase after the temperature is lowered to the eutectoid transformation temperature range, so that the crystal grains are coarsened and the strength is lowered. On the other hand, when this temperature exceeds 865 ° C., the austenite crystal grains (transformed into pearlite crystal grains after the temperature is lowered) become coarse, the toughness deteriorates, and the heat treatment strain becomes large. The time for holding at the austenitizing temperature varies depending on the holding temperature, but is preferably 5 to 30 minutes. In less than 5 minutes, it is difficult to completely austenite, the ferrite phase grows and the strength decreases, and in more than 30 minutes, the austenite crystal grains become coarse, and fine pearlite phase cannot be obtained after the temperature is lowered, and the toughness deteriorates. In addition, the heat treatment strain becomes large. The austenitizing heat treatment temperature is preferably 800 to 860 ° C, more preferably 800 to 855 ° C. The austenitizing heat treatment time is preferably 10 to 25 minutes.

(2) Heat treatment conditions in the eutectoid transformation temperature range [Step (ii)]
When completely austenitic spheroidal graphite cast iron is cooled at a cooling rate at which a ferrite phase is finely formed in a predetermined temperature section within a temperature range where eutectoid transformation occurs, the matrix structure in the present invention is 15 to 50% in area ratio. It has a two-phase mixed structure consisting of a fine ferrite phase and a 50 to 85% fine pearlite phase, and has a structure in which the number of crystal grains of the matrix structure determined by the crossing method is 250 grains / mm or more. Here, the temperature range in which the eutectoid transformation occurs (eutectoid transformation temperature range) is such that austenite transforms into ferrite or ferrite and cementite from _{the temperature Ar 3 at which the transformation from austenite to ferrite starts in the cooling process in the heat treatment.} Completion temperature The temperature range up to Ar ₁ (evaporative transformation temperature). The predetermined temperature interval within the temperature range where the eutectoid transformation occurs is preferably 670 to 750 ° C. When cooled at a predetermined cooling rate described later in a temperature range of 670 to 750 ° C., a two-phase mixed structure and a matrix structure having 250 crystal grains / mm or more can be obtained. The upper limit of the predetermined temperature section may be 730 ° C.

The cooling rate in a predetermined temperature section within the temperature range where the eutectoid transformation occurs is important for making the matrix structure a two-phase mixed structure and for the number of crystal grains of the matrix structure to be 250 grains / mm or more. Is preferably 5 to 20 ° C./min. If the cooling rate is less than 5 ° C./min, ferrite formation is promoted, a fine ferrite phase cannot be obtained, the strength is lowered, the number of crystal grains is reduced, and the toughness is lowered. On the other hand, if the cooling rate exceeds 20 ° C./min, the formation of the ferrite phase at the pearlite grain boundaries is insufficient, the impact characteristics deteriorate, sufficient toughness cannot be obtained, and the hardness is not low and high machinability. Cannot be obtained. A more preferable cooling rate is 5 to 15 ° C./min. The temperature history in a predetermined temperature section within the temperature range where eutectoid transformation occurs is as long as fine ferrite phases are generated at the pearlite grain boundaries in just proportion and the number of crystal grains in the matrix structure is 250 grains / mm or more. , Continuous cooling at a constant rate or intermittent cooling may be used. After heat treatment in the eutectoid transformation temperature range, it is cooled to room temperature. The cooling rate from the austenitizing temperature to the eutectoid transformation temperature range is preferably 2 to 20 ° C./min.

The present invention will be described in more detail with reference to the following examples, but the present invention is not limited to those examples. Unless otherwise specified, the content of each element constituting the alloy is shown in% by mass.

Pig iron, steel plate scraps, and spheroidal graphite cast iron return scraps, which are the raw materials, are melted in a high-frequency melting furnace with a capacity of 100 kg, and a molten metal containing a carbonizing material, a pearlite phase stabilizing element, and an Fe-Si alloy is added to melt the molten metal. did. Using this molten metal as a graphite spheroidizing agent, hot water was poured into a ladle in which a Fe-Si-Mg alloy and a covering material made of steel plate scrap covering the molten metal were installed at about 1500 ° C., and spheroidizing treatment was performed by a sandwich method. The spheroidized molten metal was poured into a sand mold at about 1400 ° C., and a plurality of vertically cast stepped Y blocks including a test portion having a wall thickness of 12 mm as a test material were cast. At the time of pouring, Fe—Si alloy powder was added to the pouring flow and inoculated. In this way, spheroidal graphite cast iron having the composition shown in Table 1 was obtained. Examples 1 to 11 are spheroidal graphite cast iron within the composition range of the present invention, and Comparative Examples 1 to 9 are spheroidal graphite cast iron outside the composition range of the present invention. Further, Comparative Example 10 corresponds to FCD700 having a pearlite phase matrix, and is the same as conventional spheroidal graphite cast iron as it is cast.

注：（１）残部はＦｅ及び不可避的不純物である。

Note: (1) The balance is Fe and unavoidable impurities.

Of the test materials made of spheroidal graphite cast iron of Examples 1 to 11 and Comparative Examples 1 to 10, all the test materials except Comparative Example 10 were subjected to the following heat treatment. That is, as austenitic heat treatment conditions, after heat treatment at a holding temperature of 850 ° C. and a holding time of 20 minutes, cooling from the austenitizing temperature to the eutectoid transformation temperature range at a cooling rate of 5 ° C./min, and then as heat treatment conditions in the eutectoid transformation temperature range. After heat treatment at a cooling rate of 10 ° C./min in a temperature range of 670 to 750 ° C., the mixture was cooled to room temperature. Further, the test material of Comparative Example 10 was left as-cast without heat treatment. The following microstructure observations and tests were performed on each test material.

(1) Structure A sample of about φ10 mm is collected from the cut surface of the grip portion of the test piece used for the tensile test described later, embedded in a resin and mirror-polished, and then the area ratio of the ferrite phase of the matrix structure and the matrix structure. The number of crystal grains was measured. For the area ratio of the ferrite phase, after corrosion etching the sample, an optical micrograph of any 5 fields of view was taken at a magnification of 100 times, and ^{graphite in a total of 4 mm 2} regions of the 5 fields of view was taken by an image analyzer for each field of view. It was determined by measuring the area ratio of the ferrite phase of the matrix structure excluding.

The matrix structure of the spheroidal graphite cast iron of the present invention is a two-phase mixed structure in which fine ferrite phases and fine pearlite phases are distributed in a camouflage pattern, and the austenite phase is subjected to austenitic heat treatment and heat treatment in the eutectoid transformation temperature range. Since the crystal grains are divided into many structural units having different orientations, it is difficult to clearly identify the structural units of the crystal grains by observing the structure with a normal optical microscope or a scanning electron microscope. Therefore, the number of crystal grains in the matrix structure was evaluated by determining the crystal orientation of the crystal grains by an electron backscatter diffraction method (EBSD) and visualizing the crystal grain boundaries.

Specifically, the number of crystal grains in the matrix structure was determined by using an electron backscatter diffraction detector (Ametec's DigiView 5 type) attached to a scanning electron microscope (Hitachi High Technologies' SU70 type). Crystal orientation mapping images are taken in a 45 × 45 μm region at 0.1 μm measurement step intervals for any of the five visual fields, and the captured crystal orientation mapping images have boundaries with crystal orientation differences of 2 ° or more at adjacent measurement points. It was determined by the crossing method, assuming that it was a grain boundary. The crossing method is based on JIS-G-0551 "Microscopic test method of steel-grain grain", and two lines with a length of 30 μm drawn so as to avoid graphite and cover the target matrix structure for each field of view. The number of grain boundaries that intersect (captured by the test line) with the test line consisting of crossed straight lines (diagonal lines) is measured, divided by the length of the test line, and the number of grain boundaries per unit length (1 mm). Was calculated. The arithmetic mean value of the number of grain boundaries per unit length of the calculated five visual fields was obtained and evaluated as the number of crystal grains. Table 2 shows the measurement results of the area ratio of the ferrite phase and the number of crystal grains in the matrix structure.

(2) Tensile test and Brinell hardness test A test piece of JIS Z 2201 No. 14A was prepared by cutting out from the test part of the vertical casting stepped Y block of each test material with a wall thickness of 12 mm, and Amsler was prepared according to JIS Z 2241. A tensile test was performed at room temperature with a tensile tester, and tensile strength and room temperature elongation were measured. Further, a test piece was prepared by cutting out from a portion of each test material at a distance of about 30 mm or more from the end face of the test portion having a wall thickness of 12 mm of the Y block and polishing the surface, and using a Brinell hardness tester according to JIS Z 2243. A Brinell hardness test was performed with a test load of 29.42 kN using an indenter of a cemented carbide ball having a diameter of 10 mm, and the Brinell hardness was measured. Table 2 shows the measurement results of tensile strength, room temperature elongation and Brinell hardness.

注：(1) パーライト相の面積率は（100−フェライト相の面積率）％である。

Note: (1) The area ratio of the pearlite phase is (area ratio of 100-ferrite phase)%.

As shown in Tables 1 and 2, in the test materials of Examples 1 to 11 within the composition range of the present invention, the area ratio of the ferrite phase of the matrix structure is 15 to 50%, and the number of crystal grains of the matrix structure is It was 250 pieces / mm or more, the tensile strength was 700 MPa or more, and the room temperature elongation was 10% or more. From these data, it can be seen that the test materials of Examples 1 to 11 within the composition range of the present invention have high strength and toughness.

In addition, the test materials of Examples 1 to 11 have a Brinell hardness in the range of 210 to 240 HBW and a tensile strength of 700 MPa or more, which is high strength but low hardness. As a result, the spheroidal graphite cast iron of Examples 1 to 11 can realize high machinability.

On the other hand, in the test materials of Comparative Examples 1 to 10 outside the composition range of the present invention, any of the tensile strength, the room temperature elongation and the Brinell hardness did not satisfy the values specified in the present invention. Further, the test materials of Comparative Examples 1 to 10 did not satisfy the range of the ferrite phase area ratio of 15 to 50% specified in the present invention. Comparative Examples 1, 4 and 8 having a low Si and / or Cu content and Mn content had a low tensile strength of less than 700 MPa. Further, Comparative Example 7 having a high Si content also had a tensile strength of less than 700 MPa. Further, Comparative Example 2 having a low Si content and Comparative Examples 5, 6 and 9 having a high Cu or Mn content had a high tensile strength of 700 MPa or more, but had a low room temperature elongation of less than 10%. It did not meet the requirement of having both high strength and toughness. Further, in Comparative Example 10 as-cast, which corresponds to FCD700 having a pearlite phase matrix, although it had a tensile strength of 780 MPa, the room temperature elongation was 9.5%.

Among the comparative examples, Comparative Examples 2, 3, 5, 6, 9 and 10 having a high tensile strength of 700 MPa or more all have a Brinell hardness of more than 240 HBW and are high-strength materials having a tensile strength of 700 MPa or more. However, it did not meet the requirement of low hardness.

As described above, it was confirmed that the spheroidal graphite cast iron of the present invention is a spheroidal graphite cast iron having excellent strength and toughness as well as low hardness.

Claims (3)

Spheroidal graphite cast iron with excellent strength and toughness and low hardness.
By mass ratio,
C: 3.5-3.9%
Si: 2.45 to 2.85%
Mn: 0.2 to 0.6%
Cu: 0.70 to 1.20%
Mg: 0.02 to 0.06%
P: 0.04% or less S: 0.02% or less It has a composition consisting of the balance Fe and unavoidable impurities.
It has a two-phase mixed matrix structure consisting of a fine ferrite phase of 15 to 50% and a fine pearlite phase of 50 to 85% in area ratio.
A spheroidal graphite cast iron characterized in that the number of crystal grains of the matrix structure determined by the line of intersection method is 250 grains / mm or more. The spheroidal graphite cast iron having excellent strength and toughness and low hardness according to claim 1 is characterized in that it has a tensile strength of 700 MPa or more, a room temperature elongation of 10% or more, and a Brinell hardness of 210 to 240 HBW. Spheroidal graphite cast iron. The spheroidal graphite cast iron having excellent strength and toughness and low hardness according to claim 1 or 2, has a tensile strength of 700 MPa or more, and the tensile strength and Brinell hardness satisfy the following formula (1). Spheroidal graphite cast iron characterized by.
Brinell hardness ≤ (tensile strength / 5) +82 ... (1)
(However, the value of the unit HBW is substituted for the Brinell hardness, and the value of the unit MPa is substituted for the tensile strength.)