JPS6347774B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to an improvement in a heat treatment method for tough spheroidal graphite cast iron. The strength of cast iron has improved dramatically with the invention of spheroidal graphite cast iron, but its elongation and impact value are still not as good as steel, so various attempts have been made to improve this, but none have had satisfactory results. I haven't reached the point,
Furthermore, there are problems such as requiring special molten metal treatment or increasing raw material costs. In order to improve these problems, we have previously proposed a strong spheroidal graphite cast iron having a matrix structure consisting of fine ferrite grains and martensite grains or bainite grains, and a heat treatment method for obtaining the same. No. 85995 and Japanese Patent Application No. 55-32463). These inventions have the effect of increasing the eutectoid transformation temperature range by segregation in the vicinity of graphite in spheroidal graphite cast iron.
In order to correct the unevenness of the eutectoid transformation temperature range due to the micro-segregation of Si and Mn, which segregates at the eutectic cell boundary and its vicinity and has the effect of lowering the eutectoid transformation temperature range, the Mn content is reduced to 1% or less. and contains one or both of Cu and Ni, which segregate near graphite and have the effect of lowering the eutectoid transformation temperature range.
Invention relating to spheroidal graphite cast iron having a mixed matrix structure consisting of fine ferrite grains and martensite grains or fine ferrite grains and bainite grains, which reduces the action of Mn and offsets the action of Si, and homogenizes the matrix structure. , and spheroidal graphite cast iron having the above-mentioned predetermined chemical composition is heated from a base structure containing no free ferrite to a temperature within the eutectoid transformation temperature range to form a coexistence structure of ferrite, austenite, and graphite, and then rapidly cooled from this temperature. A heat treatment method that transforms into austenite and martensite to form a matrix structure in which ferrite grains and martensite grains are finely mixed; The present invention relates to a heat treatment method in which austenite is transformed into bainite by maintaining the temperature to form a matrix structure in which ferrite grains and bainite grains are finely mixed. All of the above spheroidal graphite cast irons have excellent toughness, but the ferrite and bainite or martensite in the base should be in an appropriate quantitative relationship, for example, the amount of ferrite in the base should be 30 to 70% in terms of area ratio. The heating temperature range within the eutectoid transformation temperature range required for
The problem is that the range is around 27°C, which is relatively narrow for industrial purposes, and it is important to avoid uneven temperatures inside the cast product during heat treatment due to local variations in analysis values of chemical components, especially Si, and differences in wall thickness. Taking this into account, it is desired that the above temperature range be wider. In addition, in the invention of the above-mentioned prior application, since spheroidal graphite cast iron containing no free ferrite was used as the starting material, the as-cast material in which free ferrite is present is first normalized to eliminate the free ferrite, and then subjected to a prescribed heat treatment. There has been a demand for a heat treatment method that does not require these microstructure inspections and normalization treatments. The purpose of the present invention is to provide an improved heat treatment method that meets the above-mentioned needs, and the first invention includes C3-4%, Si2.2-3.7%, Mn 1% or less, P0.1%.
Below, S0.02% or less, graphite nodularization treatment element 0.07%
The following, and Cu0.4-2% or Ni0.7-3% or A (Cu0.4%, Ni0%), B shown in attached Figure 1
(Cu2%, Ni0%), C (Ni0.7%, Cu0%), D (Ni3
%, Cu 0%), E (Ni 1%, Cu 2%), containing Cb and Ni within the range, with the remainder essentially consisting of Fe, heated and maintained at a temperature within the eutectoid transformation temperature range. A method for heat treatment of tough spheroidal graphite cast iron, in which a structure is formed in which ferrite, austenite, and graphite coexist, and then cooled to obtain a structure in which spheroidal graphite is crystallized in a matrix of martensite grains or a mixture of bainite grains and ferrite grains. Holding within the eutectoid transformation temperature range is a first stage holding at an arbitrary temperature within the temperature range, and then, depending on the holding temperature, the solid line of 30% ferrite amount in the attached Figure 2 and the ferrite The amount obtained corresponding to the Si content on the dashed line is 70% to 21 °C depending on the Cu and Ni content, respectively.
× (Cu% - 1%) or 21℃ × (Ni% - 0.5%) at a temperature that differs from the first stage holding temperature by 5℃ or more within the eutectoid temperature range. It consists of a second stage of holding, and the area ratio of the base organization is 30.
A second invention relates to a method for heat treatment of tough spheroidal graphite cast iron characterized by forming a mixed structure containing ~70% ferrite, and the second invention further includes one or two of Mo and Cr in addition to the chemical composition of the first invention. seeds in total
The above-mentioned No. 1 is applied to spheroidal graphite cast iron containing 0.05 to 0.5%.
The same heat treatment as in the invention (however, the second stage holding temperature within the eutectoid transformation temperature range is a temperature in which 28°C per 1% (Mo + Cr) is added depending on the content of Mo or Cr). The present invention relates to a heat treatment method for tough spheroidal graphite cast iron, which is characterized by: In this specification, the chemical composition is expressed in weight %, and the proportion of metallographic components is expressed in area % measured by line integral method on a microscopic sample. Next, the chemical composition of the spheroidal graphite cast iron according to the present invention will be explained. The content of C is 3 to 3, similar to ordinary spheroidal graphite cast iron.
4%. When the amount is less than 3%, chill tends to enter the cast product, and when it exceeds 4%, carbon dross is likely to be generated and become entangled in the cast product, resulting in defects. Since Si has the effect of expanding the eutectoid transformation temperature range, in order to make heat treatment easier, the higher the content, the better; in the present invention, the content is set at 2.2% or more.
If the amount exceeds 3.7%, the effect of the embrittlement effect of Si becomes significant and the elongation and impact value decrease significantly, which is not preferable, so the upper limit is set to 3.7%. As mentioned above, Mn has the property of segregating at the eutectic cell boundary and its vicinity, and when the content exceeds 1%, the segregation becomes severe, and the eutectoid transformation temperature range in that area falls too low, causing Cu and Since the eutectoid transformation temperature range cannot be completely corrected by Ni, it is set to 1% or less. P is normally contained as an impurity, but if the amount is large, it has the property of embrittling cast iron, so it is reduced to 0.1%.
The following should be used. Similarly, S is usually contained as an impurity, but it is a harmful element that particularly inhibits graphite spheroidization, so the less it is, the better.
If the amount exceeds 0.02%, the amount of spheroidizing agent used increases, resulting in increased generation of dross, which tends to cause defects in cast products and may even make it difficult to spheroidize graphite. Therefore, the amount of S in the molten metal is 0.02%.
If the content exceeds 0.02%, it is necessary to desulfurize the graphite prior to spheroidizing it to 0.02% or less. The graphite nodularization treatment agent is usually Mg or Mg.
In addition to alloys, one or two of Ce, Y, Ca, etc.
It is well known that more than 100% of carbon dioxide is used, and the amount remaining in cast iron is usually 0.07% or less. Cu segregates around graphite and has the effect of lowering the eutectoid transformation temperature range of that part, Si segregates around graphite and has the effect of raising the eutectoid transformation temperature range, and Mn has the effect of segregating around graphite and lowering the eutectoid transformation temperature range, and Mn has the effect of lowering the eutectoid transformation temperature range of that part. By correcting the non-uniformity of the local eutectoid transformation temperature range in the base structure, which is caused by segregation in the vicinity and lowering the eutectoid transformation temperature range of that part, the temperature range is made uniform. This prevents uneven base structure. However, if the content is less than 0.4%, the effect is insufficient.
If the Cu content increases, it becomes difficult to make graphite spheroidal, and embrittlement occurs due to the precipitation of Cu-rich ε phase, so the upper limit is preferably 2%. Ni has the same effect as Cu, but if the amount is less than 0.7%, the effect will not be fully recognized, and if the amount increases, the effect will gradually approach saturation, and it will also increase the cost. Therefore, it is best to set the upper limit to 3%. Since Cu and Ni have similar effects as mentioned above, it is possible to replace a portion of their content with each other, but the effect is somewhat weaker for Ni and that a large amount of Cu Considering that addition inhibits the spheroidization of graphite, A, B, C,
It is preferable to set it within the range surrounded by points D and E. Mo and Cr will be described later. Next, the heat treatment of the first invention will be explained. According to the invention disclosed in Japanese Patent Application No. 55-32463, when spheroidal graphite cast iron is heated to a temperature within its eutectoid transformation temperature range, the cementite in the base pearlite dissolves in the surrounding ferrite, and the ferrite becomes austenite. As the cementite disappears, the triple points of the austenite particles generated by the growth of austenite become nuclei and a ferrite phase appears, which grows into a mixed base structure of austenite and ferrite grains. It is thought that when rapidly cooled, a matrix structure consisting of a mixture of bainite grains and ferrite grains, in which austenite is transformed, is thought to be formed through isothermal transformation in a hot bath. In the case of Japanese Patent Application No. 54-85995, in which the material is rapidly cooled from a temperature in the eutectoid transformation temperature range to room temperature, it is thought that a matrix structure in which ferrite grains and martensite grains are mixed is formed by the same mechanism as described above. In the inventions related to these earlier applications, spheroidal graphite cast iron with a matrix structure containing no free ferrite was used as the starting material, but as a result of subsequent research, it was found that in order to maintain the temperature within the eutectoid transformation temperature range, The temperature is maintained at an arbitrary temperature _t1 within the temperature range (hereinafter referred to as the first temperature).
The release process is carried out by carrying out two-stage holding (hereinafter referred to as second-stage holding), followed by holding at _t2 , which is at least ±5°C different from _t1 and within the temperature range (hereinafter referred to as second-stage holding). It has been found that not only can spheroidal graphite cast iron containing ferrite be used as a starting material, but also that the holding temperature range (hereinafter referred to as MD range) in which the amount of ferrite in the base is 30 to 70% can be widened. The reason why the MD range expands is thought to be as follows.
In other words, the austenite grains (or ferrite grains) produced by being held at the above temperature t ₁ are partially converted into ferrite by being held at the above temperature _{t 2 which} is lower (or higher) than the above t 1. However, once formed, austenite grains or ferrite grains are highly stable, and this transformation progresses at a slower rate than the transformation from pearlite to ferrite and austenite at temperatures within the eutectoid transformation temperature range. Therefore, when using the method according to the present invention, that is, the method of holding in two stages at temperatures t ₁ and t ₂ , if the holding temperature is within the eutectoid transformation temperature range, the temperature at which the amount of ferrite in the matrix deviates from 30 to 70% It is thought that the MD range will widen because the amount of ferrite in the matrix will still be in the range of 30 to 70% within the retention time in industrial heat treatment even if it is maintained at Since the stability of the austenite grains or ferrite grains produced by being held at the above temperature t ₁ is greater, it is better to set the temperature t ₂ to be lower than the temperature t ₁ than to reduce the MD range. The effect of expansion is large and advantageous. Therefore, in this case, if chill is generated during solidification, there is no need to pre-heat treat the material other than annealing to eliminate the chill, and it is possible to form a spherical shape containing free ferrite in the structure in the as-cast state. Graphite cast iron can also be used as a starting material. If the amount of ferrite in the matrix of spheroidal graphite cast iron obtained by the method of the present invention is less than 30%, the elongation and impact value will be lower than desired values, which is undesirable, and if the amount exceeds 70%. When this happens, the tensile strength and yield strength decrease significantly. Therefore, the amount of ferrite in the base should be 30 to 70%, and the amount of martensite or bainite should be the remaining 70 to 30%. The maintenance within the eutectoid transformation temperature range is as follows. Figure 2 shows a matrix structure with 30% ferrite or ferrite for spheroidal graphite cast iron containing approximately 1% Cu and 0.5% Ni in the experimental example described later.
The first stage holding temperature t ₁ at which 70% base structure is obtained
2 is a graph showing the relationship between the second stage holding temperature t ₂ and the Si content. In Fig. 2, the solid line represents the relationship between temperature t ₁ and temperature t ₂ at which a matrix structure of 30% ferrite is obtained, and the broken line represents the relationship between temperature t 1 and temperature t ₂ at which a matrix structure of 70% ferrite is obtained.
Since it shows the relationship of temperature t ₂ to the same first stage holding temperature t ₁ , if there is a second stage holding temperature between the temperatures t ₂ obtained on the graph from the broken line and the solid line for the same first stage holding temperature t 1, ferrite will be 30 to 70% A base organization within the range of will be obtained. For example, in the case of 3.4% Si, if _t1 is 810℃, the holding temperature _t2 at which 70% ferrite is obtained from point A on the broken line is 787℃, and the temperature at which 30% ferrite is obtained from point B on the solid line. as t ₂
Since 842℃ can be obtained, the second stage holding is 787℃ and 842℃.
If the temperature is between 30 and 70%, a matrix structure containing ferrite in the range of 30 to 70% will be obtained. These holding temperatures t ₁ and t ₂ are lowered by Cu and Ni. Each 1% of Cu and Ni lowers the eutectoid transformation temperature range by approximately 21°C, so the temperature t ₁ and t ₂ are respectively 21°C x (Cu% - 1%) depending on their content.
Or it is necessary to correct by subtracting 21℃×(Ni%-0.5%). Note that if the difference between temperatures t ₁ and t ₂ is too small, it will be substantially the same as the heat treatment of the previous application in which the temperature is maintained at a constant temperature in the eutectoid transformation temperature range, and the amount of ferrite in the starting material may be limited or the influence of the heating rate may be reduced. Therefore, it is preferable to provide a difference of at least 5° C. or more. Experimentally, if the holding time is kept at the holding temperatures t ₁ and t ₂ for 5 minutes or more, a structure with 30 to 70% ferrite can be obtained according to Fig. 2, but in actual operation, differences in wall thickness of the cast product or economical 20~ each considering gender
Preferably it is 60 minutes. Next, the spheroidal graphite cast iron that has been heated and maintained at the above-mentioned first and second stage holding temperatures to form a mixed matrix structure of ferrite and austenite is rapidly cooled to transform the austenite grains into martensite grains, or heated in a salt bath, etc. The austenite grains are transformed into bainite grains by immersion and holding in a bath to perform isothermal transformation. In isothermal transformation, if the heat bath temperature is lower than 250°C, it will take an extremely long time to complete the transformation, and there is a risk that the material will become brittle. On the other hand, if the heat bath temperature is higher than 400°C, some Ar' transformation occurs during the rapid cooling process to the heat bath temperature, resulting in primary troostite, which impairs toughness. Therefore, the temperature of the hot bath is preferably in the range of 250 to 400°C. A second invention further includes the chemical composition of the first invention.
A total of 0.05 to 0.5 of one or two types of Mo and Cr
%. If the wall thickness of the cast product is large, the cooling rate from the temperature within the eutectoid transformation temperature range will be slow, and some Ar′ transformation may occur during the rapid cooling process, resulting in primary troostite.
Or, if a small amount of Cr or both is contained
By delaying the start of Ar′ transformation, the formation of primary troostite can be prevented, so in thick-walled castings or castings with large differences in wall thickness, a small amount of Mo or Cr or It is desirable to contain both. In this case, if the total content is less than 0.05%, the above effects will be insufficient, and if it exceeds 0.5%, the time required to be kept in hot melt during isothermal transformation treatment will become longer, making it uneconomical. The content of Mo or Cr or both is
It is preferably 0.05 to 0.5%. In the range of 0.5% or less, Mo and Cr both have the effect of increasing the eutectoid transformation temperature range by approximately 28°C per ₁ %. It is necessary to add 28° C.×(Mo%+Cr%) to the obtained temperature t ₂ in addition to the above-mentioned correction according to the contents of Cu and Ni. The rest is the same as described for the first invention. Next, an experimental example will be explained. () Experiments on matrix structure Spheroidal graphite cast iron pig iron, steel scrap, ferrosilicon,
Nickel and steel are used as raw materials, melted in a 50Kg high-frequency induction electric furnace, subjected to graphite spheroidization treatment by adding Fe-Si-Mg alloy and late inoculation by adding ferrosilicon, and cast into a shell mold to form No. A Y.
A block was cast, and the feeder portion was removed to prepare a test material. Its chemical composition is shown in Table 1.
The Si content was varied by setting Cu to approximately 1% and Ni to 0.5%. The table also shows the amount of free ferrite for each sample material.

[Table] Regular hexahedral samples of 10 x 10 x 10 mm were taken from these test materials and the following tests were conducted. (-1) Investigation of the relationship between eutectoid transformation temperature interval holding temperature and matrix structure. The average heating rate of the sample above 600℃ is 2
There are two types: ℃/min and 40℃/min, and 738 to 880℃.
Heat to a temperature between and hold for 30 minutes, then
Cool or heat to a temperature between 720-870℃,
After being held at that temperature for 60 minutes, it was cooled with water, and the amount of ferrite and martensite in the matrix of the microscopic sample was measured by the line integral method. From the measurement results, the relationship between the first stage holding temperature and the second stage holding temperature at which ferrite in the base is 30% or 70% can be determined by changing the amount of Si and heating rate 2.
FIG. 3 describes the temperature at ℃/min and 40℃/min. From Fig. 3, the second-stage holding temperature at which a matrix structure with 30% or 70% ferrite content can be obtained at a heating rate of 40°C/min or less is determined for each first-stage holding temperature, and the solid line is shown in Fig. 2. 30% ferrite, the broken line indicates 70% ferrite. Note that a heating rate of 40° C./min or less is a heating rate that can be obtained without special consideration in actual operation. As mentioned above in Figure 2, if the second stage holding temperature is maintained between the solid line and the broken line depending on the Si content and the first stage holding temperature, the ferrite in the base will be 30 to 70%. be able to. For example, even if Si is 3.2% and the first stage holding temperature varies between 790 and 820°C (difference R = 30°C), the second stage holding temperature will be 789°C shown by point C in the figure and point D. If the temperature is between 827℃ (R=38℃) shown in the figure, the amount of ferrite in the base can be reduced by 30 to 70%.
It can be seen that the MD range is expanded compared to the invention related to the earlier application. Note that when Si is between 3.0 and 3.2% (difference R = 0.2%), the first stage holding temperature is between 790 and 820°C (R = 30°C).
Even if the temperature varies between It can be seen that there is no particular difficulty in industrial heat treatment even if variations in the component composition during manufacturing, especially variations in the amount of Si, are considered. As described above, it is necessary to modify the first-stage holding temperature and second-stage holding temperature in FIG. 2 depending on the content of Cu, Ni, Mo, or Cr. (-2) Investigation of the effect of heat treatment on free ferrite. In order to investigate whether the structure of spheroidal graphite cast iron obtained by the method of the present invention is affected by the amount of free ferrite before heat treatment, test materials with the chemical composition shown in Table 2 were used as described in the previous experiment. It was manufactured in the same manner and the following experiments were conducted.

[Table] Figure 4 is a micrograph (100x magnification) showing the structure of the test material in the as-cast state, where a shows the structure of test material P, b shows the structure of test material Q, and the amount of free ferrite. was measured at 17% for P and 6% for Q. Since sample material Q contains Mo, the amount of free ferrite precipitated in a bull's eye shape is smaller than that in sample material P. These test materials were heated to 820℃ and held for 30 minutes, then lowered to 805℃ and held for 60 minutes.
Next, it was transferred to a nitrite-based salt bath at 340°C, held for 2 hours, and then cooled in air, and its structure was examined. FIG. 5 is a micrograph (400x magnification), and a and b show the structures of test materials P and Q, respectively, as in FIG. 4. The amount of ferrite in the base is 50 in a
%, b is 47%, and there is virtually no difference between the two. As mentioned above, from FIGS. 4 and 5, even though there is a difference in the amount of free ferrite between specimens P and Q before heat treatment, there is no difference in the amount of ferrite in the structure after heat treatment. It can be seen that in the present invention, spheroidal graphite cast iron containing free ferrite may be used as the starting material. () Experiments on mechanical properties (-1) Investigation of the relationship between the amount of ferrite in the base and mechanical properties. Test materials having the chemical compositions shown in Table 3 were produced in the same manner as in the experiment () above.

[Table] The free ferrite of sample material 20 is 10%, and the first stage was held at the temperature shown in Table 4 (30
minutes) and second-stage holding (60 minutes),
Subsequently, it was transferred to a nitrite salt bath maintained at 330°C for 1.5 hours, and then cooled in air. The amount of ferrite in Table 4 is the amount of ferrite in the base after heat treatment. In addition, in the heat treatments marked with * at the beginning of the rows in the table, either the first stage holding temperature or the second stage holding temperature is such that the amount of ferrite is 30% or 70% for the Si content of sample 20 of 3.35%. The first stage holding temperature and the second stage shown in Figure 2
This shows that the obtained ferrite amount is outside the range of 30 to 70% because it is out of combination with the stage holding temperature, and is included for comparison.

[Table] Sample materials M-2, C-2, and MC-3 had zero free ferrite, but after heat treatment, 50%
The holding temperature in the first stage was set to 824℃, and the holding temperature in the second stage was
The stage holding temperature was 809° C., and the heat treatment was otherwise performed in the same manner as in the case of sample material 20 (however, the holding time in the salt bath was 2 hours). Tensile test pieces and impact test pieces (No. 3) with a parallel part diameter of 6 mm and a gauge length of 25 mm were manufactured from the above heat-treated specimens, and tested at a strain rate of 1 mm/min using an Instron type tensile tester. A tensile test and an impact test were conducted using an impact testing machine with a capacity of 5 kg·m.The impact test pieces were then examined under a microscope to measure the amount of ferrite, and then the hardness was measured. The test results are shown in Figure 6. As usual, tensile strength and yield strength decrease as the amount of ferrite in the base increases, and elongation increases as the amount of ferrite increases. The impact value increases as the amount of ferrite increases, but reaches a peak at approximately 50%, and tends to decrease as the amount of ferrite increases further. As expected, the hardness decreases as the amount of ferrite increases.
No particular difference was observed between the test materials containing Mo or Cr and the results of the test materials not containing these, other than a slightly lower impact value. From the above results, it is desirable to keep the amount of ferrite in the base to 70% or less in order to have high strength, and also to prevent the increase in hardness from impairing machinability in order to have sufficient toughness. It can be seen that it is desirable to set it to 30% or more in order to (-2) Investigation of the relationship between heat bath temperature and mechanical properties. Test materials having the chemical compositions shown in Table 5 were produced in the same manner as in the experiment () above. The amount of free ferrite in each sample material is as noted above.

[Table] Following the first stage of holding these test materials at 825℃ for 30 minutes, P-2 was heated to 795℃.
and 800℃ for Q-2 and QR-2.
Next, for R-2, a second stage of holding at 803°C for 60 minutes was performed, followed by a holding temperature of 250 to 400°C.
Transfer to a nitrite-based salt bath maintained at 2°C.
It was held for a time and cooled in the air. Test pieces were taken from these heat-treated test materials in the same manner as in the experiment (-1) above and used for testing. The test results are shown in Figure 7. The amount of ferrite in the base is 47-53% for specimens P-2 and QR-2 (the amount of bainite is
53-47%), 45-50 for Q-2 and R-2
% (the amount of bainite was 55-50%). The higher the salt bath temperature, the lower the tensile strength and hardness, and the higher the yield strength. Elongation and impact value are 350~
The highest value is shown at 375℃. However, even at a salt bath temperature of 250℃, the elongation is still over 10.5%, and the impact value is over 1.4Kg・m/ ^cm2 , and even at a salt bath temperature of 400℃, the elongation is still over 13%, and the impact value is 1.3Kg・m/cm2.
Indicates m/cm ² or more. From the above results, it can be seen that the salt bath temperature at which excellent toughness can be obtained is in the range of 250 to 400°C, with a particularly preferable range being 300 to 378°C. As explained above, according to the heat treatment method of the present invention, the amount of ferrite in the starting material is not affected and the temperature has a wide allowable range. Since it is not necessary to strictly control the heat treatment temperature, it is suitable for mass production, and has extremely high industrial value. Further, the spheroidal graphite cast iron obtained by the method of the present invention has excellent toughness, and its mechanical properties are not inferior to those of the spheroidal graphite cast iron of the prior application.

[Brief explanation of the drawing]

Figure 1 shows Ni and Cu of spheroidal graphite cast iron according to the present invention.
Figure 2 shows the relationship between the first stage holding temperature and the second stage holding temperature within the eutectoid transformation temperature range where the ferrite content is 30% or 70% as the Si content is varied. The graph shown in Figure 3 shows the second heating rate of 2°C/min and 40°C/fraction.
Figure 4 is a micrograph (x100) showing the structure of the sample material before heat treatment in the experimental example, and Figure 5 is a micrograph (x100) showing the structure after heat treatment.
400), FIG. 6 is a graph showing an example of the relationship between the amount of ferrite in the base of spheroidal graphite cast iron and mechanical properties according to the present invention, and FIG. 7 is a graph showing an example of the relationship between salt bath temperature and mechanical properties. It is a graph.

Claims (1)

[Claims] 1 C3-4%, Si2.2-3.7%, Mn1% or less, P0.1% or less, S0.02% or less, graphite nodularization treatment element 0.07% or less, and Cu0.4-2 %, or Ni0.7-3%, or A (Cu0.4%, Ni0%), B (Cu2%, Ni0%), C (Ni0.7%, Cu0%), D as shown in attached Figure 1. (Ni3%, Cu0%), E (Ni1%, Cu2%) Spheroidal graphite cast iron containing Cu and Ni within the range surrounded by E (Ni1%, Cu2%), with the balance essentially consisting of Fe, is heated to a temperature within the eutectoid transformation temperature range. Tough spheroidal graphite that is heated and held to form a structure in which ferrite, austenite and graphite coexist, and then cooled to form a structure in which spheroidal graphite is crystallized in a matrix of martensite grains or a mixture of bainite grains and ferrite grains. A heat treatment method for cast iron, in which holding within the eutectoid transformation temperature range is a first stage holding at an arbitrary temperature within the temperature range, and then the amount of ferrite is reduced to 30% as shown in the attached Figure 2 according to the holding temperature. and the temperature corresponding to the Si content on the 70% ferrite line, respectively,
The temperature is between 21℃ x (Cu% - 1%) or 21℃ x (Ni% - 0.5%) depending on the Ni content, and within the eutectoid transformation temperature range and the first stage holding temperature. A method for heat treatment of tough spheroidal graphite cast iron, comprising a second stage of holding at a temperature with a difference of 5°C or more, and forming a matrix structure into a mixed structure containing 30 to 70% ferrite in terms of area ratio. 2. The method of heat treating tough spheroidal graphite cast iron according to claim 1, wherein the temperature in the second stage holding is lower than the temperature in the first stage holding. 3 C3-4%, Si2.2-3.7%, Mn 1% or less, P 0.1% or less, S 0.02% or less, graphite nodularization treatment element 0.07% or less, one or two of Mo and Cr in total. 0.05~0.5
%, and Cu0.4-2%, or Ni0.7-3%, or A (Cu0.4%, Ni0%), B (Cu2%, Ni0%), C (Ni0. 7%, Cu0%), D (Ni3%, Cu0%), and E (Ni1%, Cu2%). After heating and holding at a temperature within the precipitation transformation temperature range to form a structure in which ferrite, austenite and graphite coexist, it is cooled and spheroidal graphite crystallizes in a matrix of martensite grains or a mixture of bainite grains and ferrite grains. A method for heat treatment of tough spheroidal graphite cast iron having a microstructure, wherein holding within the eutectoid transformation temperature range includes a first stage holding of holding at an arbitrary temperature within the temperature range, and then an attachment according to the holding temperature. Cu, Cu,
Depending on the content of Ni, Mo, and Cr, 21℃ x (Cu% - 1%), or 21℃ x (Ni% - 0.5%) subtracted, or 28℃ x (Mo% + Cr%) added. The second stage holding temperature is maintained at a temperature that is at least 5°C different from the first stage holding temperature within the eutectoid transformation temperature range, and the matrix structure contains ferrite with an area ratio of 30 to 70%. A heat treatment method for tough spheroidal graphite cast iron characterized by forming a mixed structure. 4. The method for heat treating tough spheroidal graphite cast iron according to claim 3, wherein the second stage holding temperature is lower than the first stage holding temperature.