JPS6321728B2 - - 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 attempts have been made to make the graphite finer and more uniform and to add alloying elements to improve this. However, sufficient results have not been achieved, and there are problems such as requiring special molten metal processing 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 Patent Application No. 1983-32463). 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.
Fine ferrite grains and martensite grains (patent application No. 85995-1986) or fine ferrite grains and bainite grains (patent application No. 1985-85995) are designed to reduce the action of Mn and offset the action of Si, thereby making the matrix structure uniform. 55-
No. 32463), and the invention relates to spheroidal graphite cast iron having a mixed matrix structure consisting of the above-mentioned predetermined chemical composition, which is heated to a temperature within the eutectoid transformation temperature range from a matrix structure containing no free ferrite to produce ferrite. , a heat treatment method that creates a coexistence structure of austenite and graphite, and then rapidly cools the austenite to transform it into martensite to form a matrix structure in which ferrite grains and martensite grains are finely mixed, or from a temperature within the eutectoid transformation temperature range. 250～
It relates to a heat treatment method in which the material is rapidly cooled in a 370° C. heat bath and kept at that temperature to transform austenite into bainite to form a matrix structure in which ferrite grains and bainite grains are finely mixed. All of the above-mentioned spheroidal graphite cast irons have excellent toughness.
The problem is that the heating temperature range within the eutectoid transformation temperature range required to achieve 30 to 70% is about 20 to 27 degrees Celsius, which is relatively narrow for industrial purposes. Considering the non-uniformity of temperature inside the cast product during heat treatment, especially due to variations in Si and wall thickness, the above temperature range is desired to be wider. 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, as well as Cu0.4-2% or Ni0.7-3% or A (Cu0.4%, Ni0%) shown in Figure 1 of the attached drawings,
B (Cu2%, Ni0%), C (Ni0.7%, Cu0%), D
(Ni3%, Cu0%), E (Ni1%, Cu2%), and the remainder is substantially
Spheroidal graphite cast iron made of Fe is heated and held at a temperature within the eutectoid transformation temperature range to form a structure in which ferrite, austenite, and graphite coexist, and then rapidly cooled to form a matrix containing martensite grains or bainite grains and ferrite grains. A method for heat treatment of a structure in which spheroidal graphite is crystallized and strong spheroidal graphite cast iron, wherein the average heating rate to a temperature within the eutectoid transformation temperature range is 10°C/min or less in a temperature range of 600°C or higher,
And the holding temperature within the eutectoid transformation temperature range is determined from the temperature corresponding to the Si content on the straight line W ₁ - X ₁ and the straight line Y ₂ - Z ₂ shown in the attached Figure 2, depending on the Cu or Ni content.
21℃×(Cu%−1.0%) or 21℃×(Ni%−0.5
The second invention relates to a heat treatment method for tough spheroidal graphite cast iron, characterized in that the temperature is between the temperatures where is a heat treatment similar to that in the first invention (however, eutectoid transformation The present invention relates to a heat treatment method for tough spheroidal graphite cast iron, characterized in that the holding temperature within the temperature range is further increased by 28°C per 1% (Mo + Cr) according to the content of Mo or Cr. 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%. If it is less than 3%, chill tends to enter the cast product, and if 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 it exceeds 3.7%, the effect of the embrittlement effect of Si becomes significant, resulting in a significant drop in elongation and impact value, which is undesirable, so the upper limit is set at 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 becomes too low, causing Cu to be described later. Since the eutectoid transformation temperature range cannot be completely corrected by Ni and Ni, it is set to 1% or less. P is normally contained as an impurity, but if too much it has the property of embrittling cast iron, so it is best to keep it at 0.1% or less. Similarly, S is usually contained as a lower purity substance, but it is a harmful element that has a strong property of inhibiting graphite spheroidization, so the less it is, the better. The content is
If it exceeds 0.02%, the amount of graphite 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, if the S content of the molten metal exceeds 0.02%, it is necessary to carry out a desulfurization treatment prior to the graphite spheroidization treatment to reduce the S content 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 the content is used, and the amount remaining during casting is usually 0.07% or less. Cu segregates around graphite and has the effect of lowering the eutectoid transformation temperature range in 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 increasing the eutectoid transformation temperature range at the eutectic cell boundary. By correcting the non-uniformity of the local eutectoid transformation temperature section in the base structure, which is based on the effect of segregation in the vicinity of the eutectoid transformation temperature section and lowering the eutectoid transformation temperature section of that part, the temperature section 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 its content is less than 0.7%, the effect will not be sufficiently recognized.
As the amount increases, the effect gradually approaches saturation and the cost increases, so 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 part of their content with each other, but the effect is somewhat weaker with Ni, and the addition of a large amount of Cu In consideration of inhibiting the spheroidization of graphite, A, B, C, and
It is preferable to set it within the range surrounded by each point 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 cooled, it undergoes isothermal transformation in a heat bath and finally becomes a matrix structure consisting of a mixture of bainite grains and ferrite grains in which austenite has been transformed. In the case of Japanese Patent Application No. 85995/1987, 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 subsequent research revealed that the heating rate to a temperature within the eutectoid transformation temperature range was slowed down. It was found that spheroidal graphite cast iron containing free ferrite can be used as a starting material. Therefore, in the invention according to the above-mentioned earlier application, it is necessary to normalize the spheroidal graphite cast iron containing free ferrite in its structure to form a pearlite base structure, but in the method according to the present invention, it is necessary to normalize the spheroidal graphite cast iron containing free ferrite in the structure, but in the method according to the present invention, the If chill occurs, there is no need to pre-heat treat the spheroidal graphite cast iron other than annealing to eliminate the chill, and even spheroidal graphite cast iron that contains free ferrite in its structure in the as-cast state can be heated at a heating rate within a specified range. It can be used as a starting material as it is. If the amount of ferrite in the base of the 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 it exceeds 70%, the tensile strength will be The sheath strength decreases significantly. Therefore, the amount of ferrite in the base should be 30 to 70%, and the amount of martensite or bainite should be 70 to 30%. Regarding the average heating rate to the temperature within the eutectoid transformation temperature range, it is 10℃/
minutes or less. There is virtually no effect on the change in free ferrite due to the slow heating rate at temperatures below 600°C. If the average heating rate of 600°C or higher is too fast, when spheroidal graphite cast iron containing free ferrite in its structure is used as a starting material, the free ferrite portion tends to become a ferrite-rich structure, making the base structure non-uniform. As a result, toughness is impaired. Further, according to experimental results, by setting the heating rate to 10° C./min or less, it is possible to widen the heating temperature range (hereinafter referred to as MD range) in which the amount of ferrite in the base is 30 to 70%. Figure 2 shows the approximate Cu1 in the experimental example described later.
%, and the chemical composition of the spheroidal graphite cast iron according to the present invention containing 0.5% Ni, a matrix structure of 30% ferrite or 70% ferrite is obtained.The heating rate and eutectoid transformation temperature range shown by Si content are FIG. 3, which shows the relationship between holding temperatures, has been redrawn with Si content and holding temperature set on the horizontal and vertical axes, respectively. The holding temperature in the first invention is a straight line W _{1 -}
The straight line Y ₂ with a heating rate of 10°C/min, which forms the base structure of
The temperature is between Z ₂ and modifications as described below depending on the content of Cu and Ni, respectively.
If the holding temperature is higher than this temperature range, the amount of ferrite in the base will be less than about 30%, while if the holding temperature is lower than this temperature range, the amount of ferrite in the base will be more than 70%. It becomes like this. In addition to being increased by Si, the eutectoid transformation temperature range
Cu or Ni
The above straight line W ₁ - X ₁ and straight line Y ₂ according to the content of
- It is necessary to make corrections to the temperature indicated by Z ₂ .
Both Cu and Ni lower the eutectoid transformation temperature range by approximately 20°C per 1%. Figure 2 shows Cu1% and Ni0.5
Since this is an example of %, the above straight line W ₁ - X ₁ and the straight line
Depending on the content of Cu and Ni, the holding temperature obtained from Y ₂ - Z ₂ is 21℃ x (Cu% - 1.0%) or 21℃ x (Ni
% - 0.5%). In addition, in order to keep the amount of ferrite in the base within the range of 30 to 70% even if the above heating rate fluctuates within the range of 10°C/min or less, the line W ₁ - X ₁ in the figure should be replaced with Heating rate to form a base structure of 30% ferrite shown in
Replace _the straight _line W _{2 -}
It is preferable to adopt the straight line Y ₁ - Z ₁ . If the holding time is 5 minutes or more, the above-mentioned base structure can be obtained, but in the case of mass production, the holding time is set to 30 minutes to 2 hours in consideration of the difference in wall thickness of the cast product and economic efficiency. is desirable. From Figure 2, the heating rate is 2℃/min or 10℃/min.
When compared at the same Si%, it will be easily recognized that the MD range in which ferrite is 30 to 70% per minute is wider than the MD range at 20°C/min or 40°C/min. Next, the spheroidal graphite cast iron that has become the two-phase mixed matrix structure is rapidly cooled to transform the austenite grains into martensite grains. Alternatively, the austenite grains are transformed into bainite grains by immersing and holding them in a hot bath such as a salt bath to carry out isothermal transformation. In isothermal transformation, if the heat bath temperature is lower than 250℃, it will take an extremely long time to complete the transformation and the material may become brittle; if it is higher than 400℃, the heat bath temperature will be lower. During the rapid cooling process, some Ar′ transformation occurs, forming primary troostite, which impairs toughness. Therefore, the heat bath temperature is preferably in the range of 250 to 400°C. The second invention further includes one or two of Mo and Cr in a total of 0.05% of the chemical composition of the first invention.
~0.5%, and when the thickness of the cast product is large, the amount of ferrite in the base increases during the rapid cooling process from the temperature in the eutectoid transformation temperature range, or some Ar' transformation occurs, resulting in primary troostite. It is designed to prevent this from occurring. Therefore, for thick-walled cast products or cast products with partially thick-walled parts, it is desirable to contain a small amount of Mo or Cr. In this case, if the total content of Mo or Cr or both is less than 40.05%, the effect is insufficient, and if it is too large, the time required to hold the product in the heat bath during isothermal transformation treatment becomes long, which is uneconomical. The total content of Mo, Cr, or both is preferably 0.05 to 0.5%. Since both Mo and Cr raise the eutectoid transformation temperature range by approximately 28°C per 1% in the range of 0.5% or less, the holding temperature within the eutectoid transformation temperature range is the straight line W ₁ - X ₁ shown in Figure 2 above. And the holding temperature obtained from Y ₂ - Z ₂ is corrected according to the content of Cu or Ni, and then 28℃ × (Mo% + Cr%)
The temperature is the sum of The rest of the procedure is the same as the procedure in 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 copper were used as raw materials, melted in a high-frequency induction electric furnace with a capacity of 50 kg, and treated with graphite spheroidization by adding Fe-Si-Mg alloy. Late-stage inoculation was performed by adding ferrosilicon, and a No. A Y block was cast in a shell mold, and the riser portion was removed to prepare a test material.
Its chemical composition is approximately 1 Cu as shown in Table 1.
%, Ni was approximately 0.5%, and the Si content was varied. The amount of free ferrite is also shown in the chart.

[Table] Regular hexahedral samples of 10 x 10 x 10 mm were taken from these test materials and the following experiments were conducted. (I-1) The average heating rate above 600℃ is 2,10,
The sample was heated to a temperature between 730 and 870°C at a rate of 20 and 40°C/min, held for 60 minutes, cooled with water, and the amount of ferrite in the matrix was measured using a microscope using line integral method. Note that the portions that were austenite at the above-mentioned holding temperature are now martensite and can be easily distinguished. Figure 3 shows the relationship between the heating rate and holding temperature at which the ferrite content in the matrix becomes 30% or 70% for each sample from the measurement results. As can be seen from the change in the range between the dashed line and the solid line for the same Si content in the figure, the heating rate is low in the holding temperature range where ferrite in the base is between 30 and 70%, that is, the MD region. This tendency is particularly strong on the high Si content side. It is judged that the change in the MD region becomes gradually smaller in a range where the heating rate is slower than 10°C/min, and becomes almost constant at a heating rate of 2°C/min or less. Also, the change in holding temperature is significant in the heating rate range of approximately 10-20°C/min. Therefore, in order to obtain a wide MD range while suppressing the change in holding temperature within a small range, it is considered that the heating rate should be set to 10° C./min or less. FIG. 2 shows the relationship between the Si content and the holding temperature at which the ferrite content in the matrix becomes 30% and 70% for each heating rate shown in FIG. 3. Second
From the figure, the holding temperature at which the amount of ferrite in the base becomes 30 to 70% at a heating rate of 10℃/min or less is a heating rate of 10
℃, the temperature should be between the straight line W ₁ - X ₁ indicating 30% ferrite amount and the straight line Y ₂ - Z ₂ indicating heating rate 2℃/min or less and 70% ferrite amount (shaded range). I can understand. For example, from Figure 2
Even if the Si content varies between 3.0 and 3.2% (difference R = 0.2%), the heating rate is set to 10°C/min or less, and the holding temperature is shown as 790°C at point A and 790°C at point B.
If the temperature is between 827°C (R=37°C), the amount of ferrite in the matrix can be kept within the range of 30 to 70°C. In this experiment, Cu was approximately 1%, Ni was 0.5%, and Mo
Since this experiment was conducted on a chemical composition that does not contain Cr, if the content of Cu, Ni, Mo, or Cr is different from this, it is necessary to modify the holding temperature determined from Figure 2 according to the content. As mentioned above, there is. (-2) In order to investigate whether the structure of spheroidal graphite cast iron obtained by the present invention is affected by the amount of free ferrite before heat treatment, the second
Test materials having the chemical compositions shown in the table were prepared in the same manner as in the previous experiment, 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 and b shows the structure of test material Q. The amount of free ferrite was determined to be 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 815°C at an average heating rate of 4°C/min over 600°C and held for 2 hours, then transferred to a 340°C nitrite salt bath, held for 2 hours, and air cooled. I looked into the organization. Figure 5 is a micrograph (400x magnification) of the specimen P.
b shows the structure of sample material Q. The amount of ferrite in the base is 55% in P and 52% in Q, and there is no substantial difference in structure between the two. From Figures 4 and 5, before heat treatment, sample material P,
Even if there is a difference in the amount of free ferrite between Q, no difference is recognized in the structure after heat treatment, and in the case of the present invention, there is no problem even using spheroidal graphite cast iron with a structure containing free ferrite as a starting material. It turns out there isn't. () Experiment regarding mechanical properties (-1) In order to investigate the relationship between the amount of ferrite in the base and mechanical properties, test materials having the chemical compositions shown in Table 3 were prepared in the same manner as in the experiment () above.

[Table] After these test materials were heated to a temperature within the eutectoid transformation temperature range, they were transferred to a nitrite-based salt bath, subjected to isothermal transformation, and then heat-treated by air cooling. The heat treatment conditions are shown in Table 4. The base after heat treatment had a mixed structure of ferrite and bainite.

[Table] Tensile test pieces and Charpy impact test pieces (No. 3 test piece) with a parallel part diameter of 6 mm and a gauge length of 25 mm were manufactured from the heat-treated test materials shown in Table 4.
A tensile test was performed using an Instron type tensile tester at a strain rate of 1 mm/min, and an impact test was performed using an impact tester with a capacity of 5 kg・m.The impact test piece was then examined under a microscope to measure the amount of ferrite in the base. After that, it was subjected to a hardness test. 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, reaching a peak at approximately 50% to 60%, and tends to decrease as the amount of ferrite increases further. As expected, the hardness decreases as the amount of ferrite increases. When comparing the results of the test material containing molybdenum with the test material not containing molybdenum, no particular difference was observed other than a higher 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 provide strength, and to ensure sufficient toughness and to suppress the increase in hardness so as not to impair machinability. It can be seen that it is desirable to set it to 30% or more in order to achieve this. (-2) In order to investigate the relationship between hot bath temperature and mechanical properties, test materials having the chemical compositions shown in Table 3 were produced in the same manner as in the experiment () above. The table also shows the amount of ferrite in the base in the as-cast state.

[Table] These test materials were heated to 810°C for P-2, Q-2 and
QR-2 was heated to 815℃ and R-2 was heated to 819℃, held for 1 hour, then transferred to a nitrite-based salt bath maintained at a specified temperature of 250 to 400℃, held for 2 hours, and then air cooled. did. A test piece was taken from this and the same test as in the experiment (-1) was conducted. The test results are shown in Figure 7. The amount of ferrite in the base is 50 to 55℃ (bainite amount 50 to 45%) for sample materials P-2, R-2, and QR-2, and Q-2.
It was 47-53% (bainite amount 53-47%). As can be seen from the figure, the higher the salt bath temperature, the lower the tensile strength and hardness, and the higher the yield strength.
The elongation peaks at a salt bath temperature of 350-375°C, and the impact value peaks at 350°C. However, even at a salt bath temperature of 400°C, the elongation was still over 12% and the impact value was over 1.3Kg·m/cm ² . From these results, the salt bath temperature at which excellent toughness can be obtained is 250.
~400℃ range, particularly preferred range is 300~
It turns out that the temperature is 375℃. As explained above, the heat treatment method of the present invention is independent of the amount of ferrite in the starting material and has a wide allowable range of heat treatment temperature, so even if there are variations in the chemical composition or wall thickness of the cast product, Accordingly, it is not necessary to strictly control the heat treatment temperature, and it is suitable for mass production and has extremely high industrial utility 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 is a graph showing the range of Si content.
A graph showing the relationship between the heating rate and the amount of ferrite, with the content and the eutectoid transformation temperature interval holding temperature as the coordinate axes, and Figure 3 is a graph showing the relationship between the heating rate and the amount of ferrite, with the coordinate axis being the heating rate to the holding temperature and the holding temperature. amount, Si
A graph showing the relationship between contents, and Figure 4 is a micrograph showing the structure of the test material before heat treatment in the experimental example (×
100), FIG. 5 is a micrograph (×400) showing the structure after heat treatment, FIG. 6 is a graph showing an example of the mechanical relationship between the amount of ferrite in the matrix of spheroidal graphite cast iron according to the present invention, and FIG. The figure is also a graph showing an example of the relationship between the holding temperature in the salt bath and the mechanical properties.

Claims (1)

[Claims] 1 C3-4%, Si2.2-3.7%, Mn 1% 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%),
Spheroidal graphite cast iron containing Cu and Ni within the range surrounded by D (Ni 3%, Cu 0%) and E (Ni 1%, Cu 2%), with the remainder essentially consisting of Fe, is heated within the eutectoid transformation temperature range. Tough spheroidal graphite that is heated and held at a certain temperature 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 the average heating rate to a temperature within the eutectoid transformation temperature range is 10°C/min or less in a temperature range of 600°C or higher, and the holding temperature within the eutectoid transformation temperature range is set as shown in the attached drawing. Si of the straight line W ₁ - X ₁ and the straight line Y ₂ - Z ₂ shown in Figure 2
From the temperature corresponding to the content to 21℃ x (Cu% - 1%) or 21℃ x (Ni
% - 0.5%), and a mixed matrix structure in which ferrite grains in the matrix have an area ratio of 30 to 70%. 2 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, one or both of Mo and Cr in total
0.05~0.5%, and Cu0.5~2% or Ni1~
3% or A (Cu0.4%,
Ni0%), B (Cu2%, Ni0%), C (Ni0.7%, Cu0
%), D (Ni 3%, Cu 0%), E (Ni 1%, Cu 2%), containing Cu and Ni within the range, with the balance essentially consisting of Fe, at the eutectoid transformation temperature. It is heated and maintained at a temperature within the range 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 tough spheroidal graphite cast iron, wherein the average heating rate to a temperature within the eutectoid transformation temperature range is 10°C/min or less in a temperature range of 600°C or higher, and the holding temperature within the eutectoid transformation temperature range is Straight line W ₁ - X ₁ and straight line Y ₂ - Z ₂ shown in attached drawing Figure 2
Depending on the content of Cu and Ni, change the Si content above from the corresponding temperature to 21℃ x (Cu% - 1%) or 21℃
(Ni% - 0.5%) or Mo at the above temperature,
Toughness characterized by a mixed matrix structure with ferrite grains in the base having an area ratio of 30 to 70% at a temperature between 28℃ x (Mo% + Cr%) depending on the Cr content. Heat treatment method for spheroidal graphite cast iron.