JP7237345B2 - Low thermal expansion casting and its manufacturing method - Google Patents

Description

TECHNICAL FIELD The present invention relates to a low thermal expansion casting, and more particularly to a low thermal expansion casting having excellent high-temperature strength.

With the recent development of communication technology, the parabolic antennas used in the transmission and reception equipment have become very large, and not only low thermal expansion, but also their processing accuracy, that is, castability, machinability, vibration absorption capacity, mechanical strength, etc. high is required. For example, carbon fiber reinforced plastics (CFRP), which have high rigidity and corrosion resistance, are commonly used as antenna reflectors.

The coefficient of thermal expansion of CFRP is smaller than that of steel, and in order to ensure high dimensional accuracy even after molding, the molding die must be made of a material having a similar coefficient of thermal expansion. Therefore, invar alloys and super invar alloys are selected as materials for molding dies.

Patent Document 1 describes, as a molding die, a cast iron having a graphite structure in an austenitic base iron, in which the component composition expressed in weight% is 0.09% or more and 0.43% or less of solid solution carbon, silicon 1 Less than 0.0%, containing 29% to 34% nickel, 4% to 8% cobalt, and the balance iron, and having a thermal expansion coefficient of 4 × 10 ^-6 /°C or less in the temperature range of 0 to 200°C. The use of expanded cast iron is disclosed.

Patent Document 2 describes C: 0.1 wt. % or less, Si: 0.1 to 0.4 wt. %, Mn: 0.15-0.4 wt. %, Ti: greater than 2 to 4 wt. %, Al: 1 wt. % or less, Ni: 30.7 to 43.0 wt. % and Co: 14 wt. % or less, the content of Ni and Co satisfies the following formula (1), the balance is Fe and inevitable impurities, and the heat in the temperature range of -40 ° C. to 100 ° C. It discloses the use of an alloy steel having an expansion coefficient of 4×10 ⁻⁶ /° C. or less and a Young's modulus of 16100 kgf/mm ² or more, which is excellent in thermal shape stability and rigidity.

37.7≦Ni+0.8×Co≦43 (1)

JP-A-6-172919 JP-A-11-293413

Invar alloys and super-invar alloys used in conventional CFRP molding dies have low strength at high temperatures, which is the operating temperature range of the dies, and therefore have the problem that the dies are easily damaged.

In view of the above circumstances, the present invention provides a low thermal expansion casting that has sufficient strength even at 400 ° C., which is the operating temperature range of CFRP molds, and has a low coefficient of thermal expansion in the range of 25 to 400 ° C. The challenge is to

The present inventors diligently studied methods for increasing yield strength at high temperatures in low-thermal-expansion castings. As a result, in the Fe-Ni-Co alloy, by controlling the contents of Ni and Co in an appropriate range and by performing an appropriate heat treatment after casting, it is possible to use expensive alloying elements such as Nb, Ti, and Al. It was found that it is possible to increase the yield strength at high temperatures without

The present invention was made based on the above findings, and the gist thereof is as follows.

(1) The component composition is, in mass%, C: 0 to 0.10%, Si: 0 to 1.00%, Mn: 0 to 1.00%, Co: 13.00 to 17.50%, and Ni satisfying -3.5 × % Ni + 118 ≤ % Co ≤ -3.5 × % Ni + 121 (% Ni, % Co are the contents of Ni and Co, respectively (% by mass)), and the balance is Fe and unavoidable It has a 0.2% yield strength of 100 MPa or more in a tensile test at 400 ° C., an average thermal expansion coefficient of 6.0 ppm / ° C. or less at 25 to 350 ° C., and a Curie temperature of 350 ° C. or more. Low thermal expansion casting.

(2) A cryo treatment step in which the casting having the composition of (1) is cooled from room temperature to the Ms point or lower, held at the Ms point or lower for 0.5 to 3 hours, and heated to room temperature, and the casting is heated to 800°C. A method for producing a low thermal expansion casting characterized by comprising, in order, a recrystallization treatment step of heating to 1200° C., holding for 0.5 to 5 hours, and then quenching.

(3) A first cryo-treatment step of cooling the casting having the composition of (1) from room temperature to below the Ms point, maintaining the temperature below the Ms point for 0.5 to 3 hours, and raising the temperature to room temperature;
A recrystallization treatment step in which the casting is heated to 800 to 1200 ° C., held for 0.5 to 5 hours, and then rapidly cooled, the casting is cooled from room temperature to the Ms point or lower, and held at the Ms point or lower for 0.5 to 3 hours, Manufacture of low thermal expansion castings characterized by comprising in order a second cryo treatment step in which the temperature is raised to room temperature, and a reverse transformation treatment step in which the casting is heated to 600 to 750 ° C., held for 0.5 to 5 hours, and then quenched. Method.

(4) A cryo treatment step in which the casting having the composition of (1) is cooled from room temperature to below the Ms point, held at a temperature below the Ms point for 0.5 to 3 hours, and heated to room temperature, and the casting is heated to 600 to A method for producing a low thermal expansion casting characterized by comprising, in order, a reverse transformation treatment step of heating to 750° C., holding for 0.5 to 5 hours, and then quenching.

INDUSTRIAL APPLICABILITY According to the present invention, a low thermal expansion casting having a high yield strength in a high temperature range and a low coefficient of thermal expansion can be obtained, so it can be applied to members of ultraprecision equipment such as CFRP molds used at high temperatures.

The present invention will be described in detail below. Hereinafter, "%" relating to component composition represents "% by mass" unless otherwise specified. First, the component composition of the casting of the present invention will be described.

In the present invention, Ni and Co are essential elements that, when added in combination, contribute to lowering the coefficient of thermal expansion. In particular, in the present invention, in order to set the Curie temperature to 350 ° C. or higher, a certain amount or more of Co is contained, and in order to sufficiently reduce the thermal expansion coefficient in a wide temperature range, appropriate Ni contain the amount. If the amounts of Ni and Co are too large, the Ms point becomes too low and it becomes difficult to cause martensitic transformation by cooling, which will be described later.

The Co content is 13.00 to 17.50%, and the Ni content is 13.00 to 17.50% in order to set the Curie temperature to 350° C. or higher and to sufficiently reduce the thermal expansion coefficient over a wide temperature range. (% by mass), and when the content of Ni is %Ni (% by mass), the range satisfies −3.5×%Ni+118≦%Co≦−3.5×%Ni+121.

The reason for setting the Curie temperature to 350° C. or higher is to obtain a low coefficient of thermal expansion even at high temperatures. There is a close relationship between the Curie temperature and the coefficient of thermal expansion. In Invar alloys, the coefficient of thermal expansion is close to 0 below the Curie temperature, but increases rapidly above the Curie temperature. . The low-expansion casting of the present invention is assumed to be used in the CFRP mold operating temperature range of around 400°C. and

C dissolves in austenite and contributes to an increase in strength, so it may be contained as necessary. Although this effect can be obtained even with a small amount, it is effective and preferable to set the amount of C to 0.010% or more. When the C content increases, the coefficient of thermal expansion increases, the ductility decreases, and casting cracks are likely to occur, so the content is 0.10% or less, preferably 0.050% or less, more preferably is 0.020% or less. In the low thermal expansion casting of the present invention, C is not an essential element and its content may be zero.

Si may be added as a deoxidizer. Also, the fluidity of the molten metal can be improved. Although this effect can be obtained even with a small amount, it is effective and preferable to set the amount of Si to 0.05% or more. If the Si content exceeds 1.00%, the coefficient of thermal expansion increases, so the Si content should be 1.00% or less, preferably 0.50% or less, and more preferably 0.20% or less. In the low thermal expansion casting of the present invention, Si is not an essential element, and the content may be zero.

Mn may be added as a deoxidizer. In addition, it contributes to strength improvement by solid-solution strengthening. Although this effect can be obtained even with a small amount, it is effective and preferable to set the amount of Mn to 0.10% or more. Even if the Mn content exceeds 1.00%, the effect is saturated and the cost increases, so the Mn content is 1.00% or less, preferably 0.50% or less. In the low thermal expansion casting of the present invention, Mn is not an essential element, and the content may be zero.

The balance of the component composition is Fe and unavoidable impurities. The unavoidable impurities are those that are inevitably mixed from raw materials, production environment, etc. when industrially producing steel having the chemical composition specified in the present invention. Specifically, 0.02% or less of P, S, O, N, and the like are included.

Next, the manufacturing method of the low thermal expansion casting of the present invention will be explained.

First, casting produces a casting having the desired chemical composition. The mold used for casting, the device for pouring molten steel into the mold, and the method of pouring are not particularly limited, and known devices and methods may be used.

The obtained casting is subjected to any one of the following heat treatments.

[1] First cryo treatment step → recrystallization treatment step [2] First cryo treatment step → recrystallization treatment step → second cryo treatment step → reverse transformation treatment step [3] First cryo treatment step → reverse transformation treatment step

Each step will be explained.

(First cryo treatment step)
The casting is cooled to below the Ms point, held at the temperature below the Ms point for 0.5 to 3 hours, and then heated to room temperature. A cooling method is not particularly limited. The Ms point referred to here is the Ms point before the effects of the present invention are exhibited. Since the cooling temperature should be sufficiently lower than the Ms point, it is not necessary to know the exact Ms point at this stage. In general, the Ms point can be estimated by the following formula using the components of steel.

Ms=521-353C-22Si-24.3Mn-7.7Cu-17.3Ni
-17.7Cr-25.8Mo
Here, C, Si, Mn, Cu, Ni, Cr, and Mo are contents (% by mass) of each element. Elements not contained are set to 0.

In the case of the component composition of the low thermal expansion casting of the present invention, the Ms point calculated by the above formula depends on the amount of Ni, and is about -100 ° C. or lower from room temperature. Dry ice and methyl alcohol or ethyl alcohol can be used. Furthermore, a method of immersing in liquid nitrogen or a method of spraying liquid nitrogen can be used for lower temperatures down to -196°C. Thereby, a structure containing martensite is formed. Also, the temperature may be raised by raising the temperature to room temperature in the air.

(Recrystallization treatment step)
The casting is reheated to 800-1200°C, held at 800-1200°C for 0.5-5 hours, and rapidly cooled to room temperature. As a result, the structure in which martensite was formed returns to an austenite structure. The crystal grain size of the structure formed by normal solidification is about 1 to 10 mm, but by going through the above cryo treatment process and the subsequent recrystallization process, the austenite grain size is refined and the crystal The structure is composed of equiaxed crystal centers with random orientations, and the structure after quenching is a fine equiaxed crystal structure with an average grain size of about 30 to 800 μm. Thereby, Young's modulus can be increased, and high 0.2% proof stress at 400° C. can be obtained. Although the method of quenching is not particularly limited, water cooling is preferred.

(Second cryo treatment step)
Following the recrystallization treatment, the casting is again cooled to below the Ms point, held at the temperature below the Ms point for 0.5 to 3 hours, and then heated to room temperature. Cooling and heating in the second cryo-treatment process may be performed in the same manner as in the first cryo-treatment process. By this treatment, the structure of the casting becomes a structure containing martensite again.

(Reverse transformation treatment step)
Following the cryo-treatment, the casting is heated to 600-750° C., held for 0.5-5 hours, and then rapidly cooled to room temperature to convert the structure to austenite. Plastic deformation occurs when the structure undergoes martensite transformation during the cryo treatment process. The strain (dislocation) at that time remains in the structure that has become austenite by the reverse transformation treatment. Thereby, a higher 0.2% proof stress at 400° C. can be obtained.

The martensitic structure returns to austenite by heating to 600°C or higher, but if the heating temperature exceeds 750°C, the austenite recrystallizes with the driving force of dislocations. Note that the size of the austenite grains does not change due to the cryo treatment process and the subsequent reverse transformation process.

As described above, a high Young's modulus and a high 0.2% proof stress at 400 ° C. can be obtained by the cryo treatment step → recrystallization treatment step, and a higher 0 at 400 ° C. by the cryo treatment step → reverse transformation treatment step. Since a 2% yield strength can be obtained, the above steps [1] to [3] may be selected according to the required properties.

Before the first cryo treatment step, a solution treatment step may be provided in which the casting is heated to 800 to 1200° C., held for 0.5 to 5 hours, and rapidly cooled to room temperature. By solution treatment, precipitates precipitated during casting dissolve into a solid solution, improving ductility and toughness. Although the method of quenching is not particularly limited, water cooling is preferred.

When producing castings, the molten metal may contain Nb, Ti, B, Mg, Ce, and La as inoculants to facilitate formation of solidification nuclei. Also, an inoculant such as Co(AlO ₂ ), CoSiO ₃ , Co-borate, etc. may be applied to the surface of the mold together with the mold coating material that is usually applied to the mold to facilitate formation of solidification nuclei. Furthermore, the molten metal in the mold may be stirred and flowed by a method using an electromagnetic stirrer, a method of mechanically vibrating the mold, a method of vibrating the molten metal with ultrasonic waves, or the like. By applying these methods, the structure of the casting is more likely to be equiaxed, so that the low thermal expansion casting of the present invention can be produced more efficiently.
The excellent high temperature strength of the low thermal expansion casting of the present invention can be evaluated by the results of a tensile test at 400°C. Specifically, the low thermal expansion casting of the present invention has a 0.2% proof stress of 100 MPa or more as measured by a tensile test at 400°C.

The low thermal expansion casting of the present invention further has an average thermal expansion coefficient of 6.0 ppm/°C or less at 25 to 400°C, which is a low thermal expansion coefficient over a wide temperature range. When the components are adjusted so that the average coefficient of thermal expansion is 4.0 to 6.0 ppm, it matches the coefficient of thermal expansion of CFRP, and is therefore suitable as a member of a CFRP molding die.

Since the low thermal expansion casting of the present invention has a high Curie temperature, the thermal expansion coefficient does not increase greatly even at high temperatures, and it has high high temperature yield strength. can also prevent damage.

Using a high-frequency melting furnace, molten metal adjusted to have the chemical composition shown in Table 1 was poured into a mold to produce a Y block. After that, the following heat treatment was performed.

Processing no. 1:
First cryo treatment step→recrystallization treatment step Treatment no. 2:
First cryo treatment step→recrystallization treatment step→second cryo treatment step→reverse transformation treatment step Treatment No. 3:
First cryo treatment step → reverse transformation treatment step Treatment No. 0:
No heat treatment

In the first cryo-treatment step, the Y block was immersed in liquid nitrogen, cooled to below the Ms point, held for 1.5 hours, then taken out from the liquid nitrogen, left at room temperature, and heated to room temperature.

In the recrystallization treatment step, the Y block was heated to the temperature shown in Table 1, held for 3 hours, and then cooled with water.

In the second cryo-treatment process, the same treatment as in the first cryo-treatment process was performed.

In the reverse transformation process, the Y block was heated to the temperature shown in Table 1, held for 3 hours, and then cooled with water.

Two samples were taken from the obtained casting, a tensile test (JIS G 0567 compliant) at 400° C. was performed, 0.2% yield strength was measured by the offset method, and two average values were used as the measured values. Similarly, a test piece for thermal expansion coefficient measurement was taken, and the average thermal expansion coefficient from 25 to 400° C. and the Curie temperature were measured. As the Curie temperature, the inflection point obtained from the elongation-temperature chart at the time of measurement was used.

Table 1 shows the results.

The low thermal expansion casting of the present invention has a low coefficient of thermal expansion and also exhibits a high 0.2% yield strength in a tensile test at 400°C.

On the other hand, in the comparative example, at least one of the 0.2% yield strength at 400° C. and the thermal expansion coefficient could not be obtained.

Claims (4)

The component composition is mass%,
C: 0 to 0.10%,
Si: 0 to 1.00%,
Mn: 0 to 1.00%,
Co: 13.00 to 17.50%, and Ni that satisfies -3.5 × % Ni + 118 ≤ % Co ≤ -3.5 × % Ni + 121 (% Ni, % Co are the contents of Ni and Co, respectively (mass %))
containing, the balance being Fe and unavoidable impurities,
0.2% proof stress in a tensile test at 400 ° C. is 100 MPa or more,
an average thermal expansion coefficient of 6.0 ppm/°C or less at 25 to 350°C;
A low thermal expansion casting characterized by having a Curie temperature of 350°C or higher. A method for producing a low thermal expansion casting according to claim 1,
A cryo treatment step in which the casting having the composition according to claim 1 is cooled from room temperature to the Ms point or less, held at the temperature below the Ms point for 0.5 to 3 hours, and heated to room temperature, and the casting is heated to 800 to 1200. A method for producing a low thermal expansion casting characterized by comprising, in order, a recrystallization treatment step of heating to ° C., holding for 0.5 to 5 hours, and then quenching. A method for producing a low thermal expansion casting according to claim 1,
A first cryo treatment step of cooling the casting having the composition according to claim 1 from room temperature to the Ms point or lower, maintaining the temperature at the Ms point or lower for 0.5 to 3 hours, and raising the temperature to room temperature;
A recrystallization treatment step in which the casting is heated to 800 to 1200 ° C., held for 0.5 to 5 hours, and then rapidly cooled.
A second cryo treatment step in which the casting is cooled from room temperature to below the Ms point, held at a temperature below the Ms point for 0.5 to 3 hours, and heated to room temperature, and the casting is heated to 600 to 750 ° C., 0.5 A method for producing a low-thermal-expansion casting characterized by comprising, in order, a reverse transformation treatment step of holding for ~5 hours and then quenching. A method for producing a low thermal expansion casting according to claim 1,
A cryo treatment step of cooling the casting having the composition according to claim 1 from room temperature to the Ms point or lower, maintaining the temperature at the Ms point or lower for 0.5 to 3 hours, and raising the temperature to room temperature,
A method for producing a low thermal expansion casting, characterized in that the casting is heated to 600 to 750° C., held for 0.5 to 5 hours, and then quenched after being rapidly cooled.