JPH08232040A - Spheroidal graphite cast iron with high toughness and its production - Google Patents

Abstract

PURPOSE: To produce a spheroidal graphite cast iron having high strength and high toughness. CONSTITUTION: A mixed structure of ausferritic structure and retained austenite is selectively formed by 10-60vol.% around spheroidal graphite and also this mixed structure of the ausferritic structure and the retained austenite is formed by <=5vol.% in the intermediate region of the mutually separated spheroidal graphite, and the balance consists essentially of ferritic structure. The formation of crack originated from graphite can be practically prevented, and also the propagation of crack in the intermediate region of the mutually separated spheroidal graphite cast iron can be practically prevented. This structure can be obtained by subjecting a spheroidal graphite cast iron having a matrix structure containing ferritic structure and <=5vol.% pearlitic structure to temp. rise at a rate of <=16.7K/sec and to holding at 1153-1373K to selectively austenitize the vicinity of the spheroidal graphite and then subjecting the above 10-60vol.% austenitic structure around the spheroidal graphite cast iron to isothermal transformation by austempering.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a high-toughness spheroidal graphite cast iron useful as a material for mechanical structural parts, for example, lower arms used in suspensions of automobile bodies, and a method for producing the same.

[0002]

2. Description of the Related Art Spheroidal graphite cast iron is characterized in that its matrix structure can be arbitrarily changed by heat treatment and the like, and a ferrite matrix material is required when ductility is required, and a pearlite matrix material is required when strength and wear resistance are required. To use. For example, J
When it is desired to be soft and have a large elongation like FCD400 specified in IS, graphitization annealing is performed to form a ferrite matrix. When strength is required as in FCD500 and FCD700, annealing is performed at 800 to 900 ° C. and then air cooling is performed to obtain a pearlite base.

As described above, the ferrite-based spheroidal graphite cast iron has a high toughness value because it is soft and has a large elongation, but it has a drawback of low strength. On the other hand, perlite-based spheroidal graphite cast iron has high strength but insufficient toughness,
It has the drawback of low impact value. In order to obtain a spheroidal graphite cast iron having both strength and toughness, JP-A-3-264644 discloses an invention of a high toughness spheroidal graphite cast iron and a method for producing the same.

The high-toughness spheroidal graphite cast iron disclosed in JP-A-3-264644 has a matrix structure composed of a bainite structure of 10 to 80% by volume and a ferrite structure of 20 to 90% by volume, and is bainitic around the spheroidal graphite. It is characterized in that a bainite structure composed of ferrite and retained austenite and having an average thickness of 3 μm or more is formed.

[0005]

However, in the high-toughness spheroidal graphite cast iron of JP-A-3-264644, a bainite structure is deposited in a large amount in the intermediate region of the spheroidal graphite apart from the periphery of the spheroidal graphite. The bainite structure precipitated in the intermediate region accelerates the propagation of cracks, resulting in a problem that effective toughening cannot be achieved.

The present invention has been made to solve the above-mentioned problems in the method of strengthening the matrix structure of spheroidal graphite cast iron, and has a high toughness spheroidal graphite cast iron having both high strength and high toughness and a method for producing the same. The purpose is to provide.

[0007]

Means for Solving the Problems The inventors of the present invention first conceived to adopt a ferrite structure as a matrix structure in order to secure desired toughness. Next, in view of the fact that the occurrence of cracks often originates from spheroidal graphite, an intensive study was conducted on reinforcing the spheroidal graphite. As a result, it was newly found that the temperature around a spheroidal graphite can be selectively austenitized by increasing the temperature at a specific heating rate, and by austempering the ausferrite structure (ferrite and residual austenite (3
The present invention has been completed by successfully obtaining a mixed structure composed of a fine mixed structure of 0 to 40% by volume) and retained austenite.

In the high toughness spheroidal graphite cast iron according to claim 1 of the present invention, a mixed structure composed of an aus ferrite structure and retained austenite is selectively formed around the spheroidal graphite in an amount of 10 to 60% by volume, and the aus ferrite A gist is that a mixed structure composed of a structure and retained austenite is formed in an amount of 5% by volume or less in an intermediate region of spheroidal graphite separated from each other, and the balance is substantially a ferrite structure.

The high toughness spheroidal graphite cast iron according to claim 2 of the present invention is the high toughness spheroidal graphite cast iron according to the invention of claim 1, wherein the composition of the high toughness spheroidal graphite cast iron is C: 3.5 to 4.0% by weight ratio, Si; 2.
2 to 2.8%, Mn; 0.2 to 0.5%, P; 0.05
% Or less, S; 0.02% or less, Mg; 0.03 to 0.0
The gist is that it contains 6% and the balance is substantially Fe.

According to a third aspect of the present invention, in the method for producing a high-toughness spheroidal graphite cast iron, a spheroidal graphite cast iron having a matrix structure containing a ferrite structure and a pearlite structure of 5% by volume or less is 16.7.
10-60 volume% austenite around the spheroidal graphite cast iron is austempered by austempering, by heating at K / sec or more and holding at 1153-1373K to selectively convert 10-60 volume% austenite around the spheroidal graphite. The gist is to perform a constant temperature transformation of the structure to form a mixed structure composed of an ausferrite structure and retained austenite.

The method for producing a high-toughness spheroidal graphite cast iron according to a fourth aspect of the present invention is the method according to the third aspect, wherein the composition of the high-toughness spheroidal graphite cast iron is C: 3.5 to 4.0% by weight. S
i; 2.2 to 2.8%, Mn; 0.2 to 0.5%, P;
0.05% or less, S; 0.02% or less, Mg; 0.03
The gist of the present invention is to contain 0.06%, with the balance being substantially Fe. The method for producing a high-toughness spheroidal graphite cast iron according to claim 5 of the present invention is the method according to claim 3 or 4, wherein the isothermal transformation temperature by the austemper is 543 to 6
The gist is that it is 83K.

[0012]

In the high-toughness spheroidal graphite cast iron according to claim 1 of the present invention, a mixed structure consisting of an aus ferrite structure and retained austenite is selectively formed around the spheroidal graphite in an amount of 10 to 60% by volume, and the starting point is graphite. A ferrite structure existing in the intermediate region of the spherical graphite in which the mixed structure composed of the ausferrite structure and the retained austenite formed in the intermediate region of the spherical graphite separated from each other is set to 5% by volume or less while making the occurrence of cracks less likely. Is increased to make crack propagation less likely to occur in the intermediate region of the spheroidal graphite, so that high strength and high toughness can be obtained.

FIG. 1 is a diagram showing the relationship between the volume% in the matrix and the impact value of the mixed structure consisting of the ausferrite structure selectively formed around spheroidal graphite and the retained austenite. If the mixed structure is less than 10% by volume, a sufficient improvement of the impact value cannot be obtained, and if it is between 10 and 60% by volume, the improved impact value is maintained, and if it exceeds 60% by volume, the impact value is rather decreased. As described above, the volume% of the structure composed of the ausferrite structure selectively formed around the spheroidal graphite and the retained austenite is limited to 10 to 60% because the impact value is sufficiently improved when the volume ratio is less than 10% by volume. Is not obtained, and if it exceeds 60% by volume, the impact value is rather lowered.

The spheroidal graphite cast iron as a raw material of the material of the present invention contains a ferrite structure and a pearlite structure of 5% by volume or less. This is because it hardly affects the toughness.

Next, the reason why the chemical composition of the high-toughness spheroidal graphite cast iron of the present invention is limited as in claim 2 or 4 will be explained. The range of C and Si is used for general spheroidal graphite cast iron, and if it is less than this range, it is difficult to prevent the spheroidization of graphite and the occurrence of chill, and if it exceeds this range, the castability is deteriorated and the mechanical properties are deteriorated. Properties, especially toughness, are reduced.

Mn is 0.2 to 0.5%, and if it exceeds 0.5%, the toughness is deteriorated due to segregation, and if it is less than 0.2%, the heat treatment property is deteriorated, so this range is set. P improves the castability, but if it exceeds 0.05%, the toughness is greatly reduced due to segregation, and it is set to 0.05% or less. S is a spheroidization-inhibiting element, and forms Sulfide to reduce toughness, so 0.0
It was set to 2% or less. Mg is in the range used for general spheroidal graphite cast iron.

In the method for producing spheroidal graphite cast iron according to claim 3 of the present invention, spheroidal graphite cast iron having a matrix structure substantially a ferrite structure is heated at a temperature rising rate of 16.7 K / sec or more to 1153 to 1373K. By holding it at, the surroundings of the spheroidal graphite are selectively austenitized by 10 to 60% by volume. When the heating rate is less than 16.7 K / sec, the austenitizing transformation temperature is lowered and the austenitizing time at low temperature is prolonged, so that austenitization between graphite grains occurs and the desired structure cannot be obtained. If the holding temperature is less than 1153K, austenitization between graphite grains occurs and the desired structure cannot be obtained. If the holding temperature exceeds 1373K, the austenitizing rate is high and 10-60% by volume.
It is difficult to control the transformation of and the target structure cannot be obtained. The holding time in austenitization is 5 to 120.
sec, which is appropriately selected depending on the temperature rising rate, holding temperature, and chemical composition.

Next, the austenite structure of 10 to 60% by volume selectively formed around this spheroidal graphite cast iron is subjected to isothermal transformation by austemper, and 10 to 60% by volume of aus ferrite is selectively formed around the spheroidal graphite cast iron. A mixed structure composed of the structure and retained austenite is obtained.
The isothermal transformation temperature is limited to 543 to 683K because when the isothermal transformation temperature is lower than 543K, a structure other than the ausferrite structure and the retained austenite structure is generated and becomes brittle, and when it exceeds 683K, a pearlite structure is generated and the toughness is deteriorated. Because.

[0019]

EXAMPLES Examples of the present invention will be described in comparison with comparative examples to clarify the effects of the present invention. As a raw material of spheroidal graphite cast iron containing 5% by volume of pearlite structure subjected to ferritic annealing with the chemical components shown in Table 1, the heat treatment according to the method of the present invention shown in Table 2 was performed as Examples 1 and 2. . As Comparative Example 1 and Comparative Example 2, heat treatment was performed under the conditions shown in Table 2. Comparative Example 1 is a comparative example in which the temperature rising rate in austenitization is extremely slower than that of the method of the present invention and the holding temperature is low, and Comparative Example 2 has a temperature rising rate in austenitization slightly lower than that of the method of the present invention. This is a slow comparative example.

[0020]

[Table 1]

[0021]

[Table 2]

Then, Examples 1 and 2 and Comparative Example 1
Regarding 2 and 2, A: thickness of (ausferrite + retained austenite) mixed structure (1 in FIG. 2) around graphite, B: ratio of (ausferrite + retained austenite) mixed structure in matrix structure, C: mutual Formed in the intermediate region of spaced spheroidal graphite (ausferrite +
The ratio of the retained austenite) mixed structure (2 in FIG. 2) in the matrix structure was measured, and is shown in Table 3.

[0023]

[Table 3]

FIG. 3 is a 100 × micrograph showing the metal structure of Example 1. As is clear from the photograph of FIG. 3, a mixed structure of about 15 μm of ausferrite and retained austenite is formed around the spheroidal graphite cast iron. The whole matrix structure is composed of 20% by volume of a mixed structure composed of aus ferrite structure and retained austenite and 80% by volume of a ferrite structure, and 5% by volume of the mixed structure is formed in an intermediate region of spheroidal graphite separated from each other. Has been done. The amount of retained austenite was about 8% by volume. Microscopic observation of the metal structure of Example 2 revealed that
A mixed structure of about 8 μm of ausferrite and retained austenite is formed around the spheroidal graphite cast iron. The volume fraction of the mixed structure of the ausferrite structure and the retained austenite in the matrix structure is about 30%, and 5% by volume of the mixed structure is formed in the intermediate region of spheroidal graphite separated from each other. By the heat treatment according to the method of the present invention, a mixed structure composed of an ausferrite structure and retained austenite is selectively formed around the spherical graphite in an amount of 10 to 60% by volume, and a mixed structure composed of the ausferrite structure and the retained austenite is mutually formed. It was found that a spheroidal graphite cast iron was formed in which 5 vol% or less was formed in the intermediate region of the separated spheroidal graphite particles, and the balance was substantially a ferrite structure.

On the other hand, FIG. 4 is a 100 × micrograph showing the metallographic structure of Comparative Example 1. As is apparent from the photograph of FIG. A mixed structure of volume% ausferrite structure and retained austenite is formed. Further, FIG. 5 is a 100 × micrograph showing the metal structure of Comparative Example 2. As is apparent from the photograph of FIG. 5, about 5 μm of ausferrite and retained austenite are formed around the spheroidal graphite cast iron. A mixed structure is formed, and a mixed structure composed of about 30% by volume of ausferrite structure and retained austenite is formed in an intermediate region of spheroidal graphite separated from each other. As a result, the comparative example in which the temperature rising rate in austenitization is extremely slower than that of the method of the present invention and the holding temperature is low, or the comparative example in which the temperature rising rate in austenitization is slightly slower than that of the method of the present invention, are separated from each other. It was found that the desired high toughness could not be obtained because a mixed structure composed of about 30% by volume of ausferrite structure and retained austenite was formed in the intermediate region of spheroidal graphite.

In order to further confirm this result, Examples 1 and 2 obtained by the heat treatment of this Example and Comparative Examples 1 and 2 were obtained.
Further, the tensile strength and impact value of spheroidal graphite cast iron (hereinafter referred to as Comparative Example 3) including 5% by volume of pearlite structure and the balance of ferrite structure were measured, and the obtained results are shown in FIG.

As shown in FIG. 6, in Comparative Example 3, the impact value was 28 J / cm ² , but the tensile strength was extremely low.
It was 20 MPa. Further, in Comparative Examples 1 and 2, the tensile strength was improved to 600 to 730 MPa as compared with the spheroidal graphite cast iron that was not subjected to heat treatment, but about 30% by volume of aus ferrite was contained in the intermediate region of the spheroidal graphites which were separated from each other. Since the mixed structure consisting of the structure and the retained austenite was formed, the improvement in impact value was low and it was J / cm ² at 23 to 27. On the other hand, the first embodiment of the present invention
And 2 had tensile strengths of 680 to 700 MPa and impact values of 32 to 35 J / cm ² , and both the tensile strength and impact value were significantly improved, confirming the effect of the present invention.

[0028]

As described above, in the high-toughness spheroidal cast iron of the present invention, the mixed structure composed of the ausferrite structure and the retained austenite is selectively contained around the spheroidal graphite in an amount of 10-6.
0% by volume, a mixed structure composed of the ausferrite structure and the retained austenite is formed in an amount of 5% by volume or less in an intermediate region of spheroidal graphite separated from each other, and the balance is substantially a ferrite structure. By making it difficult to generate cracks starting from graphite and preventing cracks from propagating in an intermediate region of spheroidal graphite cast iron that is separated from each other, high strength and high toughness can be obtained. The method for producing a high-toughness spheroidal graphite cast iron according to the present invention comprises 16.7K of spheroidal graphite cast iron having a matrix structure containing a ferrite structure and a pearlite structure of 5% by volume or less.
The temperature rises at a temperature increase rate of not less than / sec, and is 1153 to 1373.
By holding K for 10 to 60% by volume austenite around the spheroidal graphite, and then subjecting the austenite structure of 10 to 60% by volume around the spheroidal graphite cast iron to austenite by an austempered transformation to obtain an ausferrite structure. And a retained austenite.

[Brief description of drawings]

FIG. 1 is a diagram showing a relationship between a volume percentage of a mixed structure composed of aus ferrite around a graphite grain and retained austenite and an impact value.

FIG. 2 is a diagram schematically showing a metallographic structure obtained in an example.

FIG. 3 is a 100 × micrograph showing a metal structure of an example of the present invention.

FIG. 4 is a 100 × micrograph showing a metal structure of a comparative example.

FIG. 5 is a 100 × photomicrograph showing the metal structure of another comparative example.

FIG. 6 is a bar graph showing the tensile strength and impact value of Examples and Comparative Examples of the present invention.

[Explanation of symbols]

1 ・ ・ ・ ・ (Ausferrite + retained austenite) mixed structure around graphite 2 ・ ・ ・ ・ (Ausferrite + retained austenite) mixed structure formed in the intermediate region of spheroidal graphite separated from each other

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[Submission date] March 8, 1995

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Claims (5)

[Claims] 1. A mixed structure composed of an aus ferrite structure and retained austenite is selectively formed around the spheroidal graphite.
0 to 60% by volume, a mixed structure consisting of the ausferrite structure and the retained austenite is formed in an amount of 5% by volume or less in an intermediate region of spheroidal graphite separated from each other, and the balance is substantially a ferrite structure. High toughness spheroidal graphite cast iron characterized by. 2. The composition of the high-toughness spheroidal graphite cast iron is C: 3.5-4.0%, Si: 2.2-2.8% by weight ratio.
Mn; 0.2-0.5%, P; 0.05% or less, S;
The high-toughness spheroidal graphite cast iron according to claim 1, containing 0.02% or less, Mg; 0.03 to 0.06%, and the balance being substantially Fe. 3. A spheroidal graphite cast iron whose matrix structure contains a ferrite structure and a pearlite structure of 5% by volume or less.
The temperature is raised at 7 K / sec or more and maintained at 1153 to 1373 K to selectively austenite the periphery of spheroidal graphite in an amount of 10 to 60% by volume, and then austemper the austenite in an amount of 10 to 60% by volume around the spheroidal graphite cast iron. A method for producing a high-toughness spheroidal graphite cast iron, which comprises isothermally transforming a structure into a mixed structure composed of an ausferrite structure and retained austenite. 4. The composition of the high-toughness spheroidal graphite cast iron is C: 3.5-4.0%, Si: 2.2-2.8%, in a weight ratio.
Mn; 0.2-0.5%, P; 0.05% or less, S;
The method for producing a high-toughness spheroidal graphite cast iron according to claim 3, wherein the content is 0.02% or less, Mg; 0.03 to 0.06%, and the balance is substantially Fe. 5. The method for producing a high-toughness spheroidal graphite cast iron according to claim 3, wherein the isothermal transformation temperature by the austemper is 543 to 683K.