Low thermal expansion castings and methods of making the same

Abstract

The present invention relates to a low thermal expansion casting having sufficient strength even at high temperatures and a low thermal expansion coefficient. According to the present invention, a low thermal expansion casting is obtained by subjecting the casting to appropriate heat treatment, the casting containing, in mass%, C: 0 to 0.100%, Si: 0 to 1.00%, Mn: 0 to 1.00%, Co: 8.0 to 13.0%, and Ni satisfying -2.5 x %Ni + 85.5 ≤ %Co ≤ -2.5 x %Ni + 90.5 (%Ni and %Co are the contents of Ni and Co, respectively, in mass%), with the remainder being Fe and unavoidable impurities, the low thermal expansion casting having a 0.2% yield strength in a tensile test at 300°C of 125 MPa or more, an average thermal expansion coefficient of 4.0 ppm / °C or less in the range of 25 to 300°C, and a Curie temperature of 250°C or more.

Description

TECHNICAL FIELD

[0001] The present application relates to a low thermal expansion casting, and more particularly to a low thermal expansion casting having excellent high temperature strength. BACKGROUND

[0002] With the development of communication technology in recent years, the parabolic antenna and the like used in the transmission / reception equipment thereof is becoming very large, and in addition to low thermal expansion, the machining precision, i.e., castability, machinability, vibration absorbing ability, and mechanical strength and the like need to be high. For example, as an antenna reflector, carbon fiber reinforced plastic (CFRP) having high rigidity and corrosion resistance is generally used.

[0003] The thermal expansion coefficient of CFRP is smaller than that of steel, and in order to ensure high dimensional precision after forming, the forming mold needs to be formed of a material having the same degree of thermal expansion coefficient. Therefore, ferroin and super ferroin are selected as the material of the forming mold.

[0004] Patent Document 1 discloses that a low thermal expansion cast iron having a graphite structure in an austenite base iron is used as a forming mold, and as a component composition shown in weight %, it contains 0.09% or more and 0.43% or less of solid solution carbon, less than 1.0% of silicon, 29% or more and 34% or less of nickel, 4% or more and 8% or less of cobalt, and the remaining portion is formed of iron, and the thermal expansion coefficient in the temperature range of 0 to 200°C is 4 x 10 -6 / °C or less.

[0005] Patent Document 2 discloses that an alloy steel having excellent thermal shape stability and rigidity is used as a member of a super precision equipment including a CFRP mold, and the alloy steel has a component composition containing C: 0.1 wt.% 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.% or less, Ni: 30.7 to 43.0 wt.%, and Co: 14 wt.% or less, and the content of the Ni and Co satisfies the following (1) formula, the remaining portion is formed of Fe and inevitable impurities, and the thermal expansion coefficient in the temperature range of -40 to 100°C is 4 x 10 -6 / °C or less, and the Young's modulus is 16100 kgf / mm 2 or more.

[0006] 37.7 ≤ Ni + 0.8 x Co ≤ 43 (1)

[0007] Prior Art Documents

[0008] Patent Documents

[0009] Patent Document 1: Japanese Patent Application Laid-Open No. 6-172919

[0010] Patent Literature 2: Japanese Patent Application Laid-Open (JP-A) No. 11-293413 SUMMARY

[0011] PROBLEMS TO BE SOLVED BY THE INVENTION

[0012] The conventional inconel alloy and super-inconel alloy used for the CFRP forming mold have the following problems to be solved: the strength at a high temperature in the use temperature range of the mold is low, and thus the mold is easily damaged.

[0013] The present invention has been made in view of the above circumstances, and an object thereof is to provide a low thermal expansion casting which has sufficient strength even at 300°C, which is the use temperature range of a CFRP mold, and has a low thermal expansion coefficient in the range of 25 to 300°C.

[0014] MEANS OF SOLVING THE PROBLEMS

[0015] The present inventors have intensively studied a method for improving the yield strength at a high temperature in a low thermal expansion casting. As a result, it has been found that the yield strength at a high temperature can be improved without using expensive alloy elements such as Nb, Ti, and Al by controlling the contents of Ni and Co in a Fe-Ni-Co alloy within an appropriate range and performing appropriate heat treatment after casting.

[0016] The present invention has been made based on the above findings, and the gist thereof is as follows.

[0017] (1) A low thermal expansion casting, characterized by comprising, in mass%, C: 0 to 0.100%, Si: 0 to 1.00%, Mn: 0 to 1.00%, Co: 8.0 to 13.0%, and Ni satisfying -2.5 x %Ni + 85.5 < %Co < -2.5 x %Ni + 90.5 (%Ni and %Co are the contents (mass%) of Ni and Co, respectively), and the remainder being Fe and unavoidable impurities, the 0.2% yield strength in a tensile test at 300°C being 125 MPa or higher, the average thermal expansion coefficient in the range of 25 to 300°C being 4.0 ppm / °C or lower, and the Curie temperature being 250°C or higher.

[0018] (2) A method for producing a low thermal expansion casting, characterized by comprising, in order: a low-temperature treatment step of cooling a casting having the composition of (1) from room temperature to a temperature below Ms point, holding the temperature below Ms point for 0.5 to 3 hours, and then increasing the temperature to room temperature; and a recrystallization treatment step of heating the casting to 800 to 1200°C, and then quenching after holding for 0.5 to 5 hours.

[0019] (3) A method for producing a low thermal expansion casting, characterized by sequentially comprising: a first low temperature treatment step of cooling a casting having the composition of the (1) from room temperature to a temperature below Ms point, holding at a temperature below Ms point 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 performing quenching after holding for 0.5 to 5 hours; a second low temperature treatment step of cooling the casting from room temperature to a temperature below Ms point, holding at a temperature below Ms point for 0.5 to 3 hours, and raising the temperature to room temperature; and a reverse phase transformation treatment step of heating the casting to 550 to 750°C, and performing quenching after holding for 0.5 to 5 hours.

[0020] (4) A method for producing a low thermal expansion casting, characterized by sequentially comprising: a low temperature treatment step of cooling a casting having the composition of the (1) from room temperature to a temperature below Ms point, holding at a temperature below Ms point for 0.5 to 3 hours, and raising the temperature to room temperature; and a reverse phase transformation treatment step of heating the casting to 550 to 750°C, and performing quenching after holding for 0.5 to 5 hours.

[0021] Effects of the Invention

[0022] According to the present application, a low thermal expansion casting having high yield strength in a high temperature region, and further having a low thermal expansion coefficient can be obtained, and thus can be applied to a member of an ultra-precision device such as a CFRP mold used at a high temperature. DETAILED DESCRIPTION

[0023] Hereinafter, the present application will be described in detail. Hereinafter, unless otherwise specified, "%" relating to the composition means "mass %". First, the composition of the casting of the present application will be described.

[0024] In the present application, Ni and Co are essential elements which are added in combination so as to contribute to the reduction of the thermal expansion coefficient. In particular, in the present application, in order to make the Curie temperature 250°C or higher, Co is contained in an amount of 5% or more, and further, in order to sufficiently reduce the thermal expansion coefficient in a wide temperature range, an appropriate amount of Ni is contained depending on the amount of Co. When the amounts of Ni and Co are too large, the Ms point becomes too low, and it is difficult to cause the martensite transformation by cooling as described later, and thus the range of the amounts of Ni and Co is determined also taking this into consideration.

[0025] To set the Curie temperature to 250°C or higher, and further to sufficiently reduce the coefficient of thermal expansion in a wide temperature range, the content of Co is set to 8.0 to 13.0%, and the content of Ni is set to satisfy the range of -2.5 x %Ni + 85.5 < %Co < -2.5 x %Ni + 90.5 when the content of Co is expressed as %Co (mass %) and the content of Ni is expressed as %Ni (mass %). The upper limit of the amount of Co is preferably 12.0%, and more preferably 11.0%. The content of Ni preferably satisfies -2.5 x %Ni + 86.5 < %Co < -2.5 x %Ni + 89.5, and more preferably -2.5 x %Ni + 87.0 < %Co < -2.5 x %Ni + 89.0.

[0026] The Curie temperature is made 250°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, and in Invar alloys, the coefficient of thermal expansion becomes close to 0 below the Curie temperature, but the coefficient of thermal expansion sharply increases when the Curie temperature is exceeded. Assuming that the low-thermal-expansion casting of the present application is used at around 300°C, which is the temperature range of use as a CFRP mold, in order to make the coefficient of thermal expansion in this temperature range a low value, the Curie temperature is made 250°C or higher. The Curie temperature is preferably 280°C or higher, more preferably 300°C or higher, and further preferably 310°C or higher.

[0027] C is solid-solved in austenite, and contributes to the increase in strength, and thus can be contained as needed. This effect is obtained even in a small amount, but it is effective and preferable when the amount of C is 0.010% or more. When the content of C is increased, the coefficient of thermal expansion increases, and further, the ductility decreases, and thus casting cracks are easily generated, and thus the content is set to 0.100% or less, preferably 0.050% or less, and more preferably 0.020% or less. In the low-thermal-expansion casting of the present application, C is not an essential element, and the content can be 0.

[0028] Si can also be added as a deoxidizing material. In addition, the fluidity of the melt can be improved. This effect is obtained even in a small amount, but it is effective and preferable when the amount of Si is 0.05% or more. When the amount of Si exceeds 1.00%, the coefficient of thermal expansion increases, and thus the amount of Si is set to 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 application, Si is not an essential element, and the content can be 0.

[0029] Mn can also be added as a deoxidizing material. In addition, it is also effective to increase the strength by solid solution strengthening. This effect is obtained even in a small amount, but it is more effective and is preferred when the amount of Mn is set to 0.10% or more. Even if the content of Mn exceeds 1.00%, the effect is saturated and the cost becomes high, so the amount of Mn is set to 1.00% or less, preferably 0.80% or less, more preferably 0.60% or less, and further preferably 0.50% or less. In the low thermal expansion cast product of the present application, Mn is not an essential element and the content can be 0.

[0030] The remaining portion of the component composition is Fe and inevitable impurities. By the term "inevitable impurities", it means impurities that are inevitably mixed from raw materials or manufacturing environments and the like when a steel having the component composition defined in the present application is industrially manufactured. Specifically, P, S, O, N and the like at 0.02% or less can be mentioned.

[0031] Next, the manufacturing method of the low thermal expansion cast product of the present application will be described.

[0032] First, a cast product having a desired component composition is manufactured by casting. The mold for casting or the injection device, the injection method for injecting molten steel into the mold are not particularly limited, and known devices and methods can be used.

[0033] The obtained cast product is subjected to any one of the following heat treatments.

[0034] [1] 1st low temperature treatment step → recrystallization treatment step

[0035] [2] 1st low temperature treatment step → recrystallization treatment step → 2nd low temperature treatment step → reverse transformation treatment step

[0036] [3] 1st low temperature treatment step → reverse transformation treatment step

[0037] The respective steps will be described.

[0038] (1st low temperature treatment step)

[0039] After the cast product is cooled to the Ms point or less, the temperature is maintained at the Ms point or less for 0.5 to 3 hours, and then warmed to room temperature. The method of cooling is not particularly limited. In addition, the Ms point referred to here is the Ms point in the stage before the effect of the present application is exhibited. Since the cooling temperature can be set to a sufficiently low temperature compared to the Ms point, it is not necessary to know the accurate Ms point in this stage. Generally, the Ms point can be estimated from the composition of the steel by the following formula.

[0040] Ms = 521 - 353C - 22Si - 24.3Mn - 7.7Cu - 17.3Ni - 17.7Cr - 25.8Mo

[0041] Here, C, Si, Mn, Cu, Ni, Cr, Mo are the contents (mass %) of each element. Elements not contained are set to 0.

[0042] In the case of the composition of the low thermal expansion cast product of the present application, the Ms point calculated by the above formula depends on the amount of Ni and becomes from room temperature to around -100°C or less, and therefore, as a cooling medium, dry ice and methanol or ethanol can be used up to -80°C. Furthermore, up to a low temperature of -196°C, a method of immersion in liquid nitrogen or a method of spraying liquid nitrogen can be used. Thereby, a structure containing martensite is formed. Further, the temperature increase is performed by lifting into the atmosphere at room temperature.

[0043] (Recrystallization treatment step)

[0044] The cast product is reheated to 800 to 1200°C, held at 800 to 1200°C for 0.5 to 5 hr, and quenched to room temperature. Thereby, the structure having martensite is restored to an austenite structure. The crystal grain size of the structure formed by ordinary solidification is about 1 to 10 mm, but by going through the above low temperature treatment step and the subsequent recrystallization treatment step, the austenite grain size is refined, and becomes a structure of equiaxed crystal centers with random crystal orientation, and the structure after quenching becomes a fine equiaxed crystal structure with an average grain size of about 30 to 800 μm. Thereby, the Young's modulus can be improved, and further, a higher 0.2% yield strength at 300°C can be obtained. The method of quenching is not particularly limited, but water cooling is preferred.

[0045] (2nd low temperature treatment step)

[0046] After the recrystallization treatment, the cast product is again cooled to below the Ms point, and after being held at a temperature below the Ms point for 0.5 to 3 hr, the temperature is increased to room temperature. The cooling and temperature increase of the 2nd low temperature treatment step can be performed similarly to the 1st low temperature treatment step. By this treatment, the structure of the cast product becomes again a structure containing martensite.

[0047] (reverse phase transformation treatment step)

[0048] After the low temperature treatment, the cast product is then heated to 550 to 750°C, and after being held for 0.5 to 5 hr,

[0049] quenched to room temperature, whereby the structure becomes austenite. In the low temperature treatment step, plastic deformation occurs when the structure is transformed to martensite. The strain (dislocation) at this time remains in the structure which becomes austenite by the reverse phase transformation treatment. Thereby, a higher 0.2% yield strength at 300°C can be obtained.

[0050] By heating to 550°C or higher, the martensite structure is restored to austenite, but when the heating temperature exceeds 750°C, the austenite recrystallizes with dislocations as the driving force, so the heating temperature is set to 750°C or lower. In addition, the size of the austenite crystalline grains does not change due to the low-temperature treatment step and the subsequent reverse phase transformation treatment step.

[0051] As described above, by the low-temperature treatment step → recrystallization treatment step, a high Young's modulus and a relatively high 0.2% yield strength at 300°C can be obtained, and by the low-temperature treatment step → reverse phase transformation treatment step, a higher 0.2% yield strength at 300°C can be obtained, so the above-mentioned [1] to [3] steps can be selected according to the necessary characteristics.

[0052] Also, a quenching and tempering treatment step can be provided after the first low-temperature treatment step and the second low-temperature treatment step, which heats the cast product to 300 to 500°C and maintains it for 2 to 6 hours. The quenching and tempering treatment step can be provided after either of the first low-temperature treatment step and the second low-temperature treatment step or after both steps. Sometimes, the temperature of the subsequent recrystallization and reverse phase transformation is lowered by quenching and tempering, and the process can sometimes be made more efficient.

[0053] Also, a solution treatment step can be provided before the first low-temperature treatment step, which heats the cast product to 800 to 1200°C and maintains it for 0.5 to 5 hours and then rapidly cools it to room temperature. By solution treatment, precipitates that have been precipitated at the time of casting are dissolved, and the ductility and toughness are improved. The method of rapid cooling is not particularly limited, but water cooling is preferred.

[0054] Also, when the cast product is manufactured, the melt can be made to contain Nb, Ti, B, Mg, Ce, La as inoculants, so that the solidification nuclei are easily generated. In addition, the cast product can be made to contain inoculants such as Co(AlO2), CoSiO3, Co-borate, etc. on the surface of the mold by being coated together with the mold coating material that is coated on the usual mold, so that the solidification nuclei are easily generated. Furthermore, the melt in the mold can be stirred and made to flow by a method using an electromagnetic stirring device, a method of mechanically vibrating the mold, a method of vibrating the melt with ultrasonic waves, etc. By applying these methods, the structure of the cast product becomes more easily equiaxed crystals, so the low-thermal-expansion cast product of the present application can be more efficiently manufactured.

[0055] The excellent high-temperature strength of the low-thermal-expansion cast product of the present application can be evaluated from the results of the tensile test at 300°C. Specifically, the low-thermal-expansion cast product of the present application has the following characteristics: the 0.2% yield strength determined by the tensile test at 300°C is 125 MPa or higher, preferably 130 MPa or higher, more preferably 140 MPa or higher, and further preferably 150 MPa or higher.

[0056] Further, the low thermal expansion cast of the present application has an average thermal expansion coefficient of 4.0 ppm / °C or less in the range of 25 to 300°C, preferably 3.5 ppm / °C or less, and more preferably 3.0 ppm / °C or less, and can have a low thermal expansion coefficient in a wide temperature range. When the average thermal expansion coefficient is adjusted to 2.0 to 4.0 ppm, it matches the thermal expansion coefficient of CFRP, and is thus preferable as a member for a CFRP molding mold.

[0057] Because the low thermal expansion cast of the present application has a high Curie temperature, it has a high high-temperature yield strength even at high temperatures, and the thermal expansion coefficient does not increase significantly, and thus can suppress damage even when used as a member for an ultra-precision device such as a CFRP mold that is used at high temperatures.

[0058] EXAMPLE

[0059] A Y ingot was produced by pouring a molten material having a composition adjusted as shown in Table 1 into a mold using a high-frequency melting furnace. Then, the following heat treatment was performed.

[0060] Process No. 1:

[0061] 1st low-temperature treatment step → recrystallization treatment step

[0062] Process No. 2:

[0063] 1st low-temperature treatment step → recrystallization treatment step → 2nd low-temperature treatment step → reverse phase transformation step

[0064] Process No. 3:

[0065] 1st low-temperature treatment step → reverse phase transformation step

[0066] Process No. 0:

[0067] No heat treatment

[0068] In the 1st low-temperature treatment step, the Y ingot was immersed in liquid nitrogen and cooled to a temperature below Ms, and then left for 1.5 hours, taken out of the liquid nitrogen, left at room temperature, and warmed to room temperature.

[0069] In the recrystallization treatment step, the Y ingot was heated to a temperature shown in Table 1, and then water-cooled after being left for 3 hours.

[0070] In the 2nd low-temperature treatment step, the same treatment as in the 1st low-temperature treatment step was performed.

[0071] In the reverse phase transformation treatment step, the Y ingot was heated to the temperature described in Table 1, and after being held for 3 hr, water-cooling was performed.

[0072] Two samples were extracted from the obtained cast, and a tensile test at 300°C (in accordance with JIS G0567) was performed, and the 0.2% yield strength was measured by the offset method, and the average of the two was taken as the measured value. Similarly, test pieces for the measurement of the thermal expansion coefficient were extracted, and the average thermal expansion coefficient from 25 to 300°C and the Curie temperature were measured. The Curie temperature was obtained from the inflection point of the graph of elongation versus temperature at the time of measurement.

[0073] The results are shown in Table 1.

[0074] The low thermal expansion cast of the present application has a low thermal expansion coefficient, and furthermore, in a tensile test at 300°C, shows a high 0.2% yield strength.

[0075] In contrast, in the comparative examples, at least one of the 0.2% yield strength at 300°C and the thermal expansion coefficient did not achieve the target characteristics.

[0076] [Table 1]

[0077]

Claims

1. A method for producing a low thermal expansion casting, the low thermal expansion casting having a composition consisting of, in mass%, C: 0 to 0.100%, Si: 0 to 1.00%, Mn: 0 to 1.00%, Co: 8.0 to 13.0%, and Ni satisfying -2.5 x %Ni + 85.5 < %Co < -2.5 x %Ni + 90.5, wherein, % Ni, % Co are the contents of Ni, Co in mass %, the remainder being Fe and unavoidable impurities, characterized by comprising, in this order: a first low temperature treatment step of cooling the cast having the above composition from room temperature to below Ms point, holding at a temperature below Ms point for 0.5 to 3 hours, and raising to room temperature; a recrystallization treatment step of heating the cast to 800 to 1200°C, holding for 0.5 to 5 hours, and then quenching; a second low temperature treatment step of cooling the cast from room temperature to below Ms point, holding at a temperature below Ms point for 0.5 to 3 hours, and raising to room temperature; and a reverse phase transformation treatment step of heating the cast to 550 to 750°C, holding for 0.5 to 5 hours, and then quenching.

2. A method for producing a low thermal expansion casting, the low thermal expansion casting having a composition consisting of, in mass%, C: 0 to 0.100%, Si: 0 to 1.00%, Mn: 0 to 1.00%, Co: 8.0 to 13.0%, and Ni satisfying -2.5 x %Ni + 85.5 < %Co < -2.5 x %Ni + 90.5, wherein, % Ni, % Co are the contents of Ni, Co in mass %, the remainder being Fe and unavoidable impurities, characterized by comprising, in this order: a low temperature treatment step of cooling the cast having the above composition from room temperature to below Ms point, holding at a temperature below Ms point for 0.5 to 3 hours, and raising to room temperature; and a reverse phase transformation treatment step of heating the cast to 550 to 750°C, holding for 0.5 to 5 hours, and then quenching.

Images