JP2020122188A - Low thermal expansion casting and method for manufacturing the same - Google Patents

Abstract

To provide a low thermal expansion casting which has sufficient strength even at high temperature, and has a low coefficient of thermal expansion.SOLUTION: A low thermal expansion casting that has 0.2% bearing force in a tensile test at 400°C of 100 MPa or more, an average thermal expansion coefficient at 25-350°C of 6.0 ppm/°C or less and a Curie temperature of 350°C or higher is manufactured by subjecting a cast having a component composition containing, by 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 and %Co are each contents of Ni and CO (mass%), and the balance Fe with inevitable impurities to appropriate heat treatment.SELECTED DRAWING: None

Description

The present invention relates to a low thermal expansion casting, and particularly to a low thermal expansion casting excellent in high temperature strength.

With the development of communication technology in recent years, parabolic antennas used for transmitting and receiving equipment have become extremely large and have low thermal expansion properties, as well as their processing precision, that is, castability, machinability, vibration absorption capacity and mechanical strength. Higher ones are required. For example, as the antenna reflector, carbon fiber reinforced plastic (CFRP) having high rigidity and corrosion resistance is generally used.

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 alloy or Super Invar alloy is selected as the material of the molding die.

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

Patent Document 2 discloses C: 0.1 wt.% as a member of ultra-precision equipment including a CFRP mold. % Or less, Si: 0.1 to 0.4 wt. %, Mn: 0.15 to 0.4 wt. %, Ti: more than 2 to 4 wt. %, Al: 1 wt. %, Ni: 30.7-43.0 wt. % And Co: 14 wt. % Or less, and the Ni and Co contents satisfy the following equation (1), have a composition of the balance Fe and unavoidable impurities, and heat in the temperature range of −40° C. to 100° C. It is disclosed that 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 and having excellent thermal shape stability and rigidity is used.

37.7≦Ni+0.8×Co≦43 (1)

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

The Invar alloy and the Super Invar alloy used in the conventional CFRP molding die have low strength at high temperature, which is the operating temperature range of the die, and therefore have a problem that the die is 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 a CFRP mold, and has a low thermal expansion coefficient in the range of 25 to 400°C. This is an issue.

The present inventors diligently studied a method for increasing the yield strength at high temperature in a low thermal expansion casting. As a result, in the Fe-Ni-Co alloy, the content of Ni and Co is controlled within an appropriate range, and an appropriate heat treatment is performed after casting to use expensive alloy elements such as Nb, Ti, and Al. However, it has been found that it is possible to increase the yield strength at high temperatures.

The present invention has been made based on the above findings, and the summary thereof is as follows.

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

(2) A cryoprocessing step of cooling the casting having the composition of the above (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, which is characterized by sequentially comprising a recrystallization treatment step of heating to ~1200°C, holding for 0.5 to 5 hours, and then rapidly cooling.

(3) A first cryoprocessing step of cooling the casting having the component composition of the above (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 of heating the casting to 800 to 1200° C. and holding it for 0.5 to 5 hours and then rapidly cooling it, cooling the casting from room temperature to the Ms point or lower, and holding it at a temperature of the Ms point or lower for 0.5 to 3 hours, Manufacture of a low thermal expansion casting characterized by sequentially comprising a second cryoprocessing step of raising the temperature to room temperature and a reverse transformation processing step of heating the casting to 600 to 750° C., holding it for 0.5 to 5 hours and then rapidly cooling. Method.

(4) Cryogenic treatment step in which the casting having the composition of the above (1) is cooled from room temperature to the Ms point or lower, is held at the temperature of the Ms point or lower for 0.5 to 3 hours, and is heated to room temperature. A method for producing a low thermal expansion casting, which comprises sequentially performing a reverse transformation treatment step of heating to 750° C., holding for 0.5 to 5 hours, and then rapidly cooling.

According to the present invention, it is possible to obtain a low thermal expansion casting having a high yield strength in a high temperature range and a low thermal expansion coefficient, and therefore, the present invention can be applied to a member of an ultra-precision equipment such as a CFRP mold used under high temperature.

Hereinafter, the present invention will be described in detail. Hereinafter, “%” regarding the component composition represents “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 contribute to the reduction of the coefficient of thermal expansion when added in combination. In particular, in the present invention, in order to set the Curie temperature to 350° C. or higher, Co is contained in a certain amount or more, and further, in order to make the thermal expansion coefficient sufficiently small in a wide temperature range, an appropriate Ni content is set according to the Co amount. Include the amount. If the amounts of Ni and Co are too large, the Ms point becomes too low, and it becomes difficult to cause martensite transformation by the cooling described below. Therefore, the range of the amounts of Ni and Co is determined in consideration thereof.

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

The Curie temperature is set to 350° C. or higher in order 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 the Invar alloy, the coefficient of thermal expansion is close to 0 below the Curie temperature, but the coefficient of thermal expansion increases sharply above the Curie temperature. .. The low expansion casting of the present invention is assumed to be used in the vicinity of the operating temperature range of the CFRP mold, which is 400° C., and the Curie temperature is 350° C. or higher in order to make the thermal expansion coefficient low in this temperature range. And

Since C forms a solid solution in austenite and contributes to an increase in strength, C may be contained if necessary. Although this effect can be obtained even in a small amount, it is effective and preferable if the C amount is 0.010% or more. When the content of C increases, the thermal expansion coefficient increases, and further, the ductility decreases, and casting cracks easily occur, so the content is 0.10% or less, preferably 0.050% or less, and more preferably Is 0.020% or less. In the low thermal expansion casting of the present invention, C is not an essential element, and the content may be 0.

Si may be added as a deoxidizing material. Further, the fluidity of the molten metal can be improved. Although this effect can be obtained even in a small amount, it is effective and preferable if the Si amount is 0.05% or more. When 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 0.

Mn may be added as a deoxidizer. It also contributes to the strength improvement by solid solution strengthening. Although this effect can be obtained even with a small amount, it is effective and preferable if the Mn content is 0.10% or more. Even if the Mn content exceeds 1.00%, the effect is saturated and the cost becomes high. Therefore, 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 0.

The balance of the component composition is Fe and inevitable impurities. The unavoidable impurities are those that are inevitably mixed from the raw materials, the manufacturing environment, and the like when industrially manufacturing steel having the component composition defined in the present invention. Specifically, 0.02% or less of P, S, O, N and the like can be mentioned.

Next, a method for manufacturing the low thermal expansion casting of the present invention will be described.

First, a casting having a desired composition is produced by casting. The mold used for casting, the apparatus for injecting the molten steel into the mold, and the method for injecting the molten steel are not particularly limited, and known apparatuses and methods may be used.

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

[1] First cryoprocessing step→recrystallization processing step [2] First cryoprocessing step→recrystallization processing step→second cryoprocessing step→reverse transformation processing step [3] First cryoprocessing step→reverse transformation processing step

Each step will be described.

(First cryoprocessing step)
The casting is cooled to a temperature below the Ms point, held at a temperature below the Ms point for 0.5 to 3 hours, and then heated to room temperature. The cooling method is not particularly limited. The Ms point mentioned here is the Ms point before the effects of the present invention are exhibited. Since the cooling temperature may be a temperature sufficiently lower than the Ms point, it is not necessary to know the exact Ms point at this stage. Generally, the Ms point can be estimated by the following formula using the composition 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 (mass %) of each element. The elements not contained are 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 is about -100°C or less from room temperature depending on the Ni content, so that the cooling medium is -80°C or less. Dry ice and methyl alcohol or ethyl alcohol can be used. A method of immersing in liquid nitrogen or a method of spraying liquid nitrogen can be used up to a low temperature of -196°C. As a result, a structure containing martensite is formed. Further, the temperature may be raised by pulling it into the atmosphere at room temperature.

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

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

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

The martensite structure returns to austenite when heated to 600° C. or higher. However, when the heating temperature exceeds 750° C., austenite is recrystallized using dislocation as a driving force, so the heating temperature is set to 750° C. or lower. The size of the austenite crystal grains does not change due to the cryoprocessing step and the subsequent reverse transformation processing step.

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 process→recrystallization treatment process, and a higher 0° C. at 400° C. can be obtained by the cryo treatment process→reverse transformation treatment process. Since 2% proof stress can be obtained, the above steps [1] to [3] may be selected according to the required characteristics.

Before the first cryotreatment step, a solution treatment step of heating the casting to 800 to 1200° C., holding it for 0.5 to 5 hours, and rapidly cooling it to room temperature may be provided. By the solution treatment, the precipitates deposited during casting form a solid solution, and the ductility and toughness are improved. The method of rapid cooling is not particularly limited, but water cooling is preferable.

At the time of producing a casting, Nb, Ti, B, Mg, Ce, and La may be contained in the molten metal as an inoculant to facilitate the formation of solidification nuclei. In addition, coagulation nuclei may be easily generated by applying an inoculum such as Co(AlO ₂ ), CoSiO ₃ , Co-borate, etc. to the mold surface together with the mold coating material usually applied to the mold. Further, the molten metal in the mold may be stirred and flowed by a method using an electromagnetic stirring device, 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 cast product is more likely to become equiaxed crystals, so that the low thermal expansion cast product of the present invention can be manufactured more efficiently.
The excellent high temperature strength of the low thermal expansion casting of the present invention can be evaluated by the result of the tensile test at 400°C. Specifically, the low thermal expansion casting of the present invention has a characteristic that the 0.2% proof stress measured by a tensile test at 400° C. is 100 MPa or more.

The low thermal expansion casting of the present invention can further obtain a low thermal expansion coefficient in a wide temperature range, with an average thermal expansion coefficient at 25 to 400°C of 6.0 ppm/°C or less. When the components are adjusted so that the average coefficient of thermal expansion becomes 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 significantly increase even at high temperatures, and has high high-temperature proof stress. Therefore, it can be used for members of ultra-precision equipment such as CFRP molds used at high temperatures. Even the damage can be suppressed.

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

Process 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 the Ms point or lower and held for 1.5 hours, then taken out from the liquid nitrogen and left at room temperature to rise to room temperature.

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

In the second cryoprocessing step, the same processing as the first cryoprocessing step was performed.

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

Two samples were taken from the obtained castings, subjected to a tensile test at 400° C. (in accordance with JIS G 0567), 0.2% proof stress was measured by the offset method, and two average values were taken as the measured values. Similarly, a test piece for measuring the thermal expansion coefficient was sampled, and the average thermal expansion coefficient of 25 to 400° C. and the Curie temperature were measured. As the Curie temperature, the bending point obtained from the elongation-temperature chart during measurement was used.

The results are shown in Table 1.

The low thermal expansion casting of the present invention has a low thermal expansion coefficient, and further exhibits a high 0.2% proof stress in a tensile test at 400°C.

On the other hand, in the comparative example, the target characteristics were not obtained in at least one of the 0.2% proof stress at 400° C. and the thermal expansion coefficient.

Claims (4)

Ingredient 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 satisfying -3.5 x% Ni + 118 ≤% Co ≤ -3.5 x% Ni +121 (% Ni and %Co are the contents of Ni and Co, respectively (mass %))
And the balance is Fe and unavoidable impurities,
0.2% proof stress of 100 MPa or more in a tensile test at 400° C.,
The average thermal expansion coefficient at 25 to 350° C. is 6.0 ppm/° C. or less,
A low thermal expansion casting having a Curie temperature of 350° C. or higher. A cryo-treatment step of cooling a casting having the component 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, and the casting to 800 to 1200. A method for producing a low thermal expansion casting, comprising a recrystallization treatment step of heating to 0°C, holding for 0.5 to 5 hours, and then rapidly cooling. A first cryo-treatment step of cooling a casting having the component composition according to claim 1 from room temperature to an 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;
The casting is cooled from room temperature to the Ms point or lower and kept at a temperature of the Ms point or lower for 0.5 to 3 hours,
Manufacture of a low thermal expansion casting characterized by sequentially comprising a second cryo-treatment step of raising the temperature to room temperature and a reverse transformation treatment step of heating the casting to 600 to 750° C., holding it for 0.5 to 5 hours and then rapidly cooling it. Method. A cryo-treatment step of cooling a casting having the component composition according to claim 1 from room temperature to an 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, comprising 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 rapidly cooled.