Low-thermal-expansion casting and manufacturing method therefor

Abstract

The present invention relates to a low thermal expansion cast steel having high strength, high rigidity, and low coefficient of thermal expansion. The low thermal expansion cast steel of the present invention comprises is characterized in that, the chemical composition comprises, by mass%, C:0 to 0.040%, Si:0.05 to 0.70%, Mn:0.10 to 0.70%, Ni:33.0 to 34.5%, Co:4.40 to 6.20%, Ti:1.00 to 2.50 %, and the balance of Fe and unavoidable impurities, the average grain diameter of the austenitic structure is 200µm or less, the tensile strength at 23°C is 700MPa or more, and the average coefficient of thermal expansion at 18 to 28°C is 2.0×10-6 / °C or less.

Description

FIELD

[0001] The present invention relates to a low thermal expansion cast steel.BACKGROUND

[0002] As an antenna reflector, carbon fiber reinforced plastic (CFRP) having high rigidity and high corrosion resistance is generally used. The coefficient of thermal expansion of CFRP is extremely small as about 1.5×10 -6< / °C. Therefore, in order to ensure high dimensional accuracy even after molding, it is required that a molding die be composed of a material having a coefficient of thermal expansion of the same degree as that of CFRP. Therefore, an Invar alloy or a super Invar alloy is used as the material of the molding die.

[0003] In addition, a low thermal expansion cast steel which can be produced by casting is used, which is suitable for CFRP mold material having complex shapes, spindles, and the like.

[0004] Patent Literature 1 discloses a high rigidity low thermal expansion cast steel having high Young's modulus even when it is as cast and having lower Ms point than a conventional low thermal expansion cast steel, which is invented for producing a member having a complicated form requiring high rigidity with low thermal expansion as a casting.

[0005] On the other hand, with respect to the low thermal expansion alloy having high strength at high temperature, Patent Literature 2 discloses using, as a molding die, a low thermal expansion alloy, which is a cast iron having a graphite structure in the austenite, comprises, by weight%, solid C: 0.09% or more and 0.43% or less, Si: less than 1.0%, Ni: 29% or more and 34% or less, Co: 4% or more and 8% or less and balance: Fe, and having a coefficient of thermal expansion at a range of temperature of 0 to 200°C is 4.0×10 -6< / °C or less.

[0006] Patent Literature 3 discloses the use of an alloyed steel excellent in thermal shape-stability and stiffness as a member of an ultra-precision apparatus including a CFRP mold. The alloyed steel comprises C:0.1wt.% or less, Si:0.1 to 0.4wt.%, Mn:0.15 to 0.4wt.%, Ti: more than 2 to 4wt.%, Al:1wt.% or less, Ni:30.7 to 43.0wt.%, Co:14wt.% or less and the balance of Fe and unavoidable impurities, and the content of Ni and Co satisfies the formula (1). The alloyed steel has a coefficient of thermal expansion of 4× 10 -6< / °C or less in a temperature-range of -40 to 100°C and a Young's modulus of 16100kgf / mm 2< or more.

[0007] Patent Literature 4 discloses a low thermal expansion cast steel having high strength and rigidity at room temperature. The cast steel comprises, by mass %, C: 0 to 0.1%, Si: 0 to 0.5%, Mn: 0 to 0.5%, S: 0 to 0.05%, Ni: 29.0 to 34.0%, Co: 0 to 8.0%, the balance of Fe and unavoidable impurities and has an average grain diameter of the austenitic structure of more than 200µm and less than 600µm, a 0.2% proof stress at 23°C of 350MPa or more, a Young's modulus of 130GPa or more, and an average coefficient of thermal expansion at 18 to 28°C of 2.0×10 -6< / °C or less.[CITATIONS LIST][PATENT LITERATURE]

[0008] [PLT 1] JP 2016-027187 A [PLT 2] JP H6-172919 A [PLT 3] JP H11-293413 A [PLT 4] JP 2020-122180 A SUMMARY[TECHNICAL PROBLEM]

[0009] An object of the present invention is to provide a low thermal expansion cast steel having higher tensile strength, higher rigidity, and lower coefficient of thermal expansion.[SOLUTION TO PROBLEM]

[0010] The inventors have intensively studied a method of obtaining a low thermal expansion cast steel having both high rigidity and high strength, and found that both rigidity and tensile strength can be increased by utilizing intermetallic compounds of Ni and Ti.

[0011] The present invention has been made based on the above knowledge, and includes the following embodiments. (1) A low thermal expansion cast steel, the chemical composition of the cast steel, comprising, by mass%, C: 0 to 0.040%, Si: 0.05 to 0.70%, Mn: 0.10 to 0.70%, Ni: 33.0 to 34.5%, Co: 4.40 to 6.20%, Ti: 1.00 to 2.50%, and the balance of Fe and unavoidable impurities, an average grain size of an austenite structure of the cast steel being 200µm or less, a tensile strength at 23°C of the cast steel being 700MPa or more, an average coefficient of thermal expansion at 18 to 28°C being 2.0×10 -6< / °C or less. (2) The low thermal expansion cast steel according to embodiment (1), wherein a ratio of equiaxed grains in the austenitic structure is 60% or more in area ratio. (3) The low thermal expansion cast steel according to embodiment (1) or (2), wherein a Young's modulus of the cast steel is 140GPa or more. (4) A method for producing the low thermal expansion cast steel according to embodiment (1), the method comprising: a Ni depletion step of heating a cast steel to 600 to 750°C, holding for 2 to 24 hours and cooling to room temperature, the chemical composition of the cast steel comprising, by mass%, C: 0 to 0.040%, Si: 0.05 to 0.70%, Mn: 0.10 to 0.70%, Ni: 33.0 to 34.5%, Co: 4.40 to 6.20%, Ti: 1.00 to 2.50%, and the balance of Fe and unavoidable impurities; a cryogenic treatment step of cooling the cast steel from room temperature to a temperature Mf point (martensitic transformation finish temperature) and Ms point (martensitic transformation start temperature), holding for 0.5 to 3 hours, and raising the room temperature; a recrystallization step of heating the cast steel to 800 to 1000 °C, holding for 0.5 to 5 hours, and quenching; and an aging step of heating the cast steel to 600 to 750°C, holding for 2 to 24 hours, and quenching. [ADVANTAGEOUS EFFECTS OF INVENTION]

[0012] According to the present invention, it is possible to obtain a low thermal expansion cast steel having high tensile strength, high rigidity, and low coefficient of thermal expansion.BRIEF DESCRIPTION OF DRAWINGS

[0013] [Fig. 1] Fig. 1 is a schematic diagram showing a temperature change in the method of producing the low thermal expansion cast steel of the present invention. [Fig. 2] Fig.2 is a diagram showing an example of a metal structure after a cryogenic treatment step, and (a) is an example in which a cryogenic treatment is performed without a Ni depletion step and (b) is an example in which a cryogenic treatment is performed after a Ni depleting step. DESCRIPTION OF EMBODIMENTS≪ Low thermal expansion cast steel ≫

[0014] In the low thermal expansion cast steel of the present invention, the chemical composition of the cast steel comprises, by mass%, C: 0 to 0.040%, Si: 0.05 to 0.70%, Mn: 0.10 to 0.70%, Ni: 33.0 to 34.5%, Co: 4.40 to 6.20%, Ti: 1.00 to 2.50%, and the balance of Fe and unavoidable impurities, the average grain size of the austenite structure of the cast steel is 200µm or less, the tensile strength at 23°C of the cast steel is 700MPa or more, and the average coefficient of thermal expansion at 18 to 28°C is 2.0×10 -6< / °C or less.≪Chemical composition≫

[0015] First, the chemical composition of the low thermal expansion cast steel of the present invention is described in detail. Hereinafter, "%" regarding the chemical composition represents "% by mass" unless otherwise specified.(C: 0 to 0.040%)

[0016] C solidifies in austenite and contributes to increase in strength. When the content of C is increased, the coefficient of thermal expansion is increased, and it becomes difficult to refine the crystal grains since the martensitic transformation is suppressed. Therefore, the content of C is set to 0.040% or less. The content of C is not essential and may be 0. The content of C may be 0.001% or more, 0.002% or more, 0.003% or more, or 0.005% or more. The content of C may be 0.030% or less, 0.025% or less, 0.022% or less, or 0.020% or less.(Si: 0.05 to 0.70%)

[0017] Si is added as a deoxidizer. Si is also an element which improves fluidity of the molten metal. In order to obtain these effects, the content of Si is set to 0.05% or more. The content of Si may be 0.07% or more, 0.10% or more, 0.12% or more, or 0.15% or more. When the content of Si exceeds 0.70%, the coefficient of thermal expansion is increased. Therefore, the content of Si is set to 0.70% or less. The content of Si may be 0.60% or less, 0.50% or less, or 0.40% or less.(Mn: 0.10 to 0.70%)

[0018] Mn is added as a deoxidizer. Further, Mn also contributes to strength improvement by solid solution strengthening. In order to achieve these effects, the content of Mn is set to 0.10% or more. The content of Mn may be 0.12% or more, 0.15% or more, 0.20% or more, or 0.25% or more. When the content of Mn exceeds 0.70%, the effects are saturated, and due to the martensitic transformation suppressed, it becomes difficult to refine the crystal grains and the coefficient of thermal expansion is increased. Therefore the content of Mn is set to 0.70% or less. The content of Mn may be 0.60% or less, 0.50% or less, or 0.40% or less.(Ni: 33.0 to 34.5%)

[0019] Ni is an element which lowers the coefficient of thermal expansion. Even if the content of Ni is too small or too large, the coefficient of thermal expansion increases. Further, Ni forms an intermetallic compound Ni 3 Ti with Ti and the intermetallic compound precipitates and distributes in the crystal grains, thereby increasing the rigidity and strength. In the low thermal expansion cast steel of the present invention, the content of Ni is set to 33.0 to 34.5% in order to precipitate intermetallic compounds for obtaining the desired rigidity and strength. Note that generally, when the content of Ni is increased, it becomes difficult to cause martensitic transformation by cooling. In the present invention, the crystal grain refinement by the martensitic transformation is achieved by the heat treatment described later. However, if the content of Ni is too large, the crystal grains cannot be refined. The content of Ni may be 33.2% or more, 33.4% or more, or 33.6% or more. The content of Ni may be 34.4% or less, 34.3% or less, or 34.2% or less.(Co: 4.40 to 6.20%)

[0020] Co contributes to lowering the coefficient of thermal expansion in combination with Ni. Even if the content of Co is too small or too large, the coefficient of thermal expansion increases. In order to obtain the desired coefficient of thermal expansion, Co is set to 4.40 to 6.20%. The content of Co may be 4.60% or more, 4.80% or more, 5.00% or more, or 5.20% or more. The content of Co may be 6.10% or less, 6.00% or less, 5.90% or less, 5.80% or less, or 5.60% or less.(Ti: 1.00 to 2.50%)

[0021] Ti is an element which increases strength by forming an intermetallic compound Ni 3 Ti with Ni and precipitating and distributing the intermetallic compound in the crystal grains. The content of Ti is set to 1.00% or more in order to improve strength. If the content of Ti is too large, the coefficient of thermal expansion is increased. Therefore, the content of Ti is set to 2.50% or less. The content of Ti may be 1.10% or more, 1.20% or more, 1.30% or more, 1.40% or more, or 1.50% or more. The content of Ti may be 2.40% or less, 2.30% or less, 2.20% or less, 2.10% or less, or 2.00% or less.

[0022] The balance of the chemical compositions consists of Fe and unavoidable impurities. The unavoidable impurities are elements which are unavoidably mixed from the raw material and the producing environment or the like when industrially producing the cast steel having a chemical composition in the present invention, and contained within a range which does not harm the required characteristics for the low thermal expansion casting of the present invention. Examples thereof include P, S, and Al contained in the raw material, and Nb, B, Mg, Ce, and La contained in a molten metal as an inoculum. The content of the element contained as an impurity may be 0.100% or less, 0.050% or less, 0.030% or less, 0.020% or less, 0.010% or less, 0.008% or less, or 0.005% or less for each element. The content of impurities may be 0%.(Average grain size of austenite structure)

[0023] In the low thermal expansion cast steel of the present invention, the austenite grain size is refined by a heat treatment described later. Specifically, the average grain size of the austenite structure is 200µm or less. The average grain size of the austenite structure may be 180µm or less, 160µm or less, 140µm or less, 120µm or less, or 100µm or less.

[0024] The average grain size of the austenite structure is measured by observation by an optical microscope. In particular, the observation surface is polished to Emery Paper #1200, and the surface is mirror-finished by buffing, and a sample 20s corroded at room temperature using a marble liquid is observed by an optical microscope. The average grain size is measured by a quadrature method according to JIS G 0551:2020 on the obtained microstructure.

[0025] The structure of the cast steel of the present invention mainly comprises the equiaxed grain having various crystal orientations, and therefore, a crystal having high Young's modulus (111) or (110) and the like is included at more than a constant ratio. As a result, higher tensile strength and Young's modulus can be obtained compared to conventional low thermal expansion cast steel, which mainly comprises columnar grains having a crystal orientation (100) with a low Young's modulus. In the present invention, among the crystal grains observed by the optical microscope, the ratio of the long to short sides of the crystal grains is judged to be three times or more for columnar grains and less than three times for equiaxed grain.

[0026] It is not necessary that all of the structures are equiaxed grains, but it is preferable that the ratio of equiaxed grains is 60% or more in area ratio. The ratio of the equiaxed grains may be 70% or more, 80% or more, 85% or more, 90% or more, or 95% or more by area ratio. The equiaxed grain and the columnar grain can be distinguished by the above-described observation with the optical microscope. In the present invention, the aspect ratio is determined to be 3.0 or less crystal grains for equiaxed grain.(Tensile strength at 23°C)

[0027] The strength of the low thermal expansion cast steel of the present invention can be evaluated by the results of the tensile test at 23°C. In particular, the low thermal expansion cast steel has a tensile strength measured in a tensile test at 23°C of 700MPa or more. The tensile strength may be 720MPa or more, 750MPa or more, 780MPa or more, 800MPa or more, 820MPa or more, or 850MPa or more. There is no upper limit on the tensile strength. However, the tensile strength may be, for example, 1100MPa or less. Tensile strength is measured by the offsetting method of tensile test according to JIS Z 2241:2011 using a tensile test piece according to JIS G 0567:2020.(Average coefficient of thermal expansion at 18 to 28°C)

[0028] The low thermal expansion cast steel has lower coefficient of thermal expansion, with an average coefficient of thermal expansion at 18 to 28°C of 2.0×10 -6< / °C or less. The coefficient of thermal expansion is measured by taking a cylindrical thermal expansion test piece (φ6.0×25L) from the center of the steel ingot and using a thermal expansion measuring machine. The average coefficient of thermal expansion at 18 to 28°C may be 1.9×10 -6< / °C or less, 1.8×10 -6< / °C or less, 1.7× 10 -6< / °C or less, or 1.6× 10 -6< / °C or less. There is no lower limit of the average coefficient of thermal expansion at 18 to 28°C, and the average coefficient of thermal expansion at 18 to 28°C may be a negative value. The mean coefficient of thermal expansion at 18 to 28°C may be, for example, 0 or more, 0.5× 10 -6< / °C or more, or 1.0× 10 -6< / °C or more.(Young's modulus)

[0029] Even in conventional low thermal expansion cast steel, the Young's modulus and the coefficient of thermal expansion can be adjusted to some extent by adjusting the chemical composition. However, the Young's modulus and the coefficient of thermal expansion are approximately in a trade-off relationship. In other words, as the Young's modulus increases, the coefficient of thermal expansion also increases. In the low thermal expansion cast steel of the present invention, the Young's modulus of 140GPa or more can be obtained. The Young's modulus may be 142GPa or more, 145GPa or more, 147GPa or more, or 150GPa or more. There is no upper limit of the Young's modulus. However, the Young's modulus may be, for example, 180GPa or less.

[0030] Young's modulus is measured by a two-point support transverse resonance method at room temperature conforming to JIS Z 2280:1993 by using a Young's modulus test piece (7t× 16w× 125L) taken from the center of a steel ingot.≪ Method for Producing Low Thermal Expansion Cast ≫

[0031] Next, with reference to Fig. 1, a method for producing the low thermal expansion cast steel of the present invention is described. Fig. 1 is a diagram showing an outline of a temperature change of a cast steel in each step described below, a horizontal axis being time, and a vertical axis being temperature.

[0032] The method for producing the low thermal expansion cast steel of the present invention comprises in sequence a Ni depletion step of heating a cast steel having the above-described chemical composition to 600 to 750°C, holding for 2 to 24 hours and cooling to room temperature; a cryogenic treatment step of cooling the cast steel from room temperature to a temperature between Mf point (martensitic transformation finish temperature) and Ms point (martensitic transformation start temperature), holding for 0.5 to 3 hours, and raising the room temperature; a recrystallization step of heating the cast steel to 800 to 1000 °C, holding for 0.5 to 5 hours, and quenching; and an aging step of heating the cast steel to 600 to 750°C, holding for 2 to 24 hours, and quenching.

[0033] The mold used in the producing the low thermal expansion cast steel of the present invention, the pouring device of molten steel into the mold, and the pouring method are not limited, and may be used known apparatus, or methods. The structure of the cast steel produced by the mold becomes a structure comprising mainly the columnar grain.

[0034] When the cast steel is produced, the molten metal may contain Nb, B, Mg, Ce, or La as an inoculant in order to facilitate the formation of solidification nuclei. In addition, solidification nuclei may be easily formed by applying an inoculant such as Co (AlO 2 ), CoSiO 3 , Co-borate or the like to the mold surface together with a coating material which is usually applied to the mold. Further, the molten metal in the mold may be stirred and flowed by using an electromagnetic stirring apparatus, mechanically vibrating the mold, vibrating the molten metal by ultrasonic waves, or the like. By applying these methods, the structure of the casting tends to become equiaxed grains.

[0035] The produced cast steel is, in sequence, subjected to the following heat treatment.(Ni depletion step)

[0036] In Ni depletion step, the content of Ni in the matrix phase is reduced by precipitating Ni and Ti in the steel as intermetallic compounds. In Ni depletion process, cast steel produced by casting is heated to 600 to 750°C, held for 2 to 24 hours, and cooled to room temperature (S1 in Fig. 1). The heating rate is not limited, and may be, for example, 1 to 100°C / hr. During the holding of 2 to 24 hours, the temperature may be constant between 600 and 750°C. The temperature may vary in the range of 600 to 750°C and need not be constant. The cooling rate is not limited and may be, for example, 1 to 100°C / hr.

[0037] In the low thermal expansion cast steel of the present invention, the content of Ni is increased in order to improve the rigidity and strength by forming an intermetallic compound Ni 3 Ti and precipitating and distributing the intermetallic compound inside and outside the crystal grains. However, in general, when the content of Ni is increased, Ms point and Mf point are lowered, and thus the martensite content obtained in the cryogenic treatment is decreased. Therefore, in the present invention, heating the cast steel produced by casting to 600 to 750°C, holding for 2 to 24 hours, and cooling to room temperature, Ni and Ti in the steel are precipitated as an intermetallic compound and Ni content in the matrix phase is reduced. As a result, Ms point and Mf point of the matrix phase are increased, and thus the martensite content obtained by the subsequent cryogenic treatment step is increased.(Cryogenic treatment step)

[0038] In the cryogenic treatment step, the cast steel through the Ni depletion step is cooled from room temperature to a temperature between Mf point and Ms point, held for 0.5 to 3 hours, and heated to room temperature (S2 of Fig. 1). It is not necessary to know the precise Ms and Mf points at this stage, and Ms and Mf points may be estimated by the following equation using the chemical composition of the matrix phase:         Ms(°C)=521-353C-22Si-24.3Mn-7.7Cu-17.3Ni-17.7Cr-25.8Mo         Mf(°C)=410.5-407.3C-7.3Si-37.8Mn-20.5Cu-19.5Ni-19.8Cr-4.5Mo

[0039] Here, C Si, Mn, Cu, Ni, Cr, and Mo are contents (by mass%) of the respective elements. For element, which is not contained, 0 is assigned.

[0040] The methods of cooling to a temperature between Mf point and Ms point and the cooling rate are not limited. Dry ice, and methyl alcohol or ethyl alcohol may be used as the cooling medium for cooling up to -80°C. For cooling further low temperature of -196°C, a method of immersing in liquid nitrogen or a method of spraying liquid nitrogen may be used. The cooling temperature is preferably as close to Mf point as possible. As a result, a structure containing martensite is formed.

[0041] The temperature may be constant between Mf point and Ms point during holding of 0.5 to 3 hours at a temperature range between Mf point and Ms point. The temperature may vary in the range of Mf point to Ms point and need not be constant. The heating rate up to room temperature is not limited. The temperature increase may be performed by, for example, pulling up a cast steel cooled to a temperature between Mf point and Ms point to the atmosphere at room temperature.

[0042] Fig. 2 shows examples of the structure of cast steels after cryogenic treatment having the same compositional composition, which contains Ni of 34 mass%, when the cryogenic treatment is performed without the Ni depletion step, and when the cryogenic treatment is performed after the Ni depletion step. (a) is a case where the cryogenic treatment is performed without the Ni depletion step, and (b) is a case where the cryogenic treatment is performed after the Ni depletion step. Conditions of the cryogenic treatment are the same as those of (a) and (b). In the structure, the black portion is martensite. The formed martensite rate when the cryogenic treatment is performed without the Ni depletion step of (a) is about 30%. The formed martensite rate when the cryogenic treatment is performed after the Ni depletion step of (b) is about 70%, and it can be seen that a large amount of martensite is formed as compared with (a). As a result, (b), in which a larger amount of martensite is generated, has smaller average grain size due to austenite refinement in the following recrystallization step.(Recrystallization step)

[0043] In the recrystallization step, the cast steel through the cryogenic treatment step is heated to 800 to 1000°C, held for 0.5 to 5 hours, and then quenched (S3 in Fig. 1). As a result, the structure in which the martensite is formed returns to the austenite structure. The crystal grain size of the structure formed by ordinary solidification is about 1 to 10mm. According to the Ni depletion treatment step, the cryogenic treatment step, since a large amount of martensite is formed through the recrystallization step, recrystallization is accelerated, austenite grain size is refined, and the structures mainly become equiaxed grain structures having random crystal orientations. Further, by this heat treatment, Ni and Ti precipitated as the intermetallic compound is dissolved in the structure, and even after quenching, Ni and Ti remain in solid solution in the austenite structure. The structure after quenching has an average grain size of 200µm or less. The method of quenching is not limited. However, water cooling is preferable.(Aging step)

[0044] In the aging, the cast steel through the recrystallization step is heated to 600 to 750°C, held for 2 to 24 hours, and then quenched (S4 in Fig. 1). Thus, by making Ni and Ti, which are diffused into the structure, precipitate the grains as the intermetallic compound Ni 3 Ti, the strength increases, and by reducing Ni content in the matrix, the coefficient of thermal expansion decreases.EXAMPLES

[0045] Hereinafter, specific examples of the present invention will be described with reference to Examples. The following example is merely an example included in the present invention, and is not intended to limit the present invention.<No. 1>

[0046] The molten metal was poured into a mold and a cast steel (Y-block) having the components listed in No.1 of Table-1 was produced. Next, in the Ni depletion step, the cast steel was heated at 700°C, held for 12 hours, and cooled to room temperature.

[0047] Next, in the cryogenic treatment step, the cast steel was immersed in liquid nitrogen and held for 2 hours, and then pulled up to the atmosphere at room temperature and heated to room temperature. Subsequently, in the recrystallization step, the cast steel was heated to 900°C, held for 3 hours, and water cooled to room temperature. Thereafter, in the aging step, the cast steel was heated to 700°C, held for 12 hours, and water cooled to room temperature. Mf and Ms points shown in Table 1 are values obtained by substituting the values of the chemical composition shown in Table 1 into the formula described above. Note that Mf point was assumed to -273°C, if the value obtained by substituting the above formula was lower than - 273°C.

[0048] A Young's modulus test piece (7t×16w×125L), a tensile test piece (according to JIS G 0567), and a thermal expansion test piece (φ6×25L) were taken from the obtained Y-block, and austenite grain size, Young's modulus, tensile strength at 23°C, and average coefficient of thermal expansion at 18 to 28°C were measured by the above described methods.<Nos. 2 to 17>

[0049] Casting steels were produced in the same manner as in No.1, except for the change the chemical composition and the production conditions as described in Tables 1. In Table 1, "depletion", "cryogenic", "recrystallization", "aging" means, respectively, the "Ni depletion step", the "cryogenic treatment step", the "recrystallization step", and the "aging step" described above , "∘" means that the process was performed in the range of the above-mentioned temperature and time, blank means that the process was omitted. For the obtained cast steels, the austenite grain size, Young's modulus, tensile strength at 23°C, and average coefficient of thermal expansion of 18 to 28°C were measured in the same manner as in No. 1.

[0050] Table 2 shows the results. In this examples, if the tensile strength at 23°C was 700MPa or more, Young's modulus was 140GPa or more, and the average coefficient of thermal expansion of 18 to 28°C was 2.0×10 -6< / °C or less, the obtained cast steel was judged to have high tensile strength, have rigidity, and have low coefficient of thermal expansion, and the problem to be solved by the invention was solved.

[0051] Nos. 1, 5 and 7 are inventive examples, and cast steel with Young's modulus of 140GPa or more, tensile strength at 23°Cof 700MPa or more, and average coefficient of thermal expansion of 18 to 28°C of 2.0×10 -6< / °C or less were obtained.

[0052] No. 2 is a comparative example in which the cast steel was produced without a Ni depletion step, and as a result, the amount of martensite produced in the cryogenic treatment step was small, and, the austenite grain size was reduced in the subsequent recrystallization step, and the Young's modulus was lowered.

[0053] No. 3 is a comparative example in which the cast steel was produced without a Ni depletion step, a cryogenic treatment step, and a recrystallization step, and as a result, the austenite grain size cannot be refined, and the Young's modulus is lowered.

[0054] No. 4 is a comparative example in which the cast steel was produced without a Ni depletion step, a cryogenic treatment step, a recrystallization step, and an aging step, and as a result, the austenite grain size could not be refined, and the Young's modulus was lowered. In addition, Ni content in the matrix phase did not decrease, and the coefficient of thermal expansion increased. Furthermore, Ni 3 Ti did not precipitate, and the tensile strength is lowered.

[0055] No. 6 is a comparative example in which the cast steel was produced without a Ni depletion step, and as a result, the amount of martensite produced in the cryogenic treatment step was small, and the austenite grain size was reduced in the subsequent recrystallization step, and the Young's modulus was lowered.

[0056] No. 8 is a comparative example in which the cast steel was produced without a Ni depletion step, and as a result, the austenite grain size could not be refined, and Young's modulus is lowered. Further, since Ni content is relatively large and has not passed through Ni depletion step, the austenite grain size of the matrix phase is increased, the tensile strength is lowered.

[0057] No. 9 is a comparative example in which the content of C was large, and as a result, the coefficient of thermal expansion was increased. In addition, the tensile strength was lowered. It is assumed that the decrease in tensile strength was due to the precipitation of TiC.

[0058] No. 10 was a comparative example in which the content of Si was large, and as a result, the coefficient of thermal expansion was increased.

[0059] No. 11 was a comparative example in which the content of Mn was large, and as a result, the coefficient of thermal expansion was increased. Further, it is assumed that the martensitic transformation was suppressed, and as a result, the austenite grain size could not be refined, and the Young's modulus was lowered.

[0060] No. 12 was a comparative example in which the content of Ni was small, and as a result, the coefficient of thermal expansion was increased.

[0061] No. 13 was a comparative example in which the content of Ni was large, and as a result, the coefficient of thermal expansion was increased. In addition, the austenite grain size could not be refined, and the Young's modulus was lowered.

[0062] No. 14 was a comparative example in which the content of Co was small, and as a result, the coefficient of thermal expansion was increased.

[0063] No. 15 was a comparative example in which the content of Co was large, and as a result, the coefficient of thermal expansion was increased.

[0064] No. 16 was a comparative example in which the content of Ti was small, and as a result, the tensile strength was low.

[0065] No. 17 was a comparative example in which the content of Ti was large, and as a result, the coefficient of thermal expansion was increased. [Table 1]No.Chemical Composition (mass%), Balance: Fe and impuritiesMf point (°C)Ms point (°C)Production ProcessCSiMnNiCoTidepletioncryogenicrecrystallizationaging10.0050.160.2133.85.202.10-260-74○○○○Inv. ex.20.0050.160.2133.85.202.10-260-74○○○Comp. ex.30.0050.160.2133.85.202.10-260-74○Comp. ex.40.0050.160.2133.85.202.10-260-74Comp. ex.50.0210.150.5933.16.012.48-267-77○○○○Inv. ex.60.0210.150.5933.16.012.48-267-77○○○Comp. ex.70.0060.520.2934.44.511.32-273-95O○O○Inv. ex.80.0060.520.2934.44.511.32-273-95○○○Comp. ex.90.0510.350.4034.05.081.97-273-103○○○○Comp. ex.100.0121.020.5633.45.722.31-273-97O○○○Comp. ex.110.0210.250.9533.95.511.42-273-101○○○○Comp. ex.120.0070.230.3032.85.211.35-245-61O○○○Comp. ex.130.0050.250.3235.25.321.68-273-103○○○○Comp. ex.140.0090.240.2833.54.052.39-259-74○○○○Comp. ex.150.0050.310.2533.46.342.42-255-71O○○○Comp. ex.160.0110.280.2233.94.980.85-265-81O○○○Comp. ex.170.0130.190.3134.34.793.13-273-89○○○○Comp. ex.* Underline means that the value is outside of the scope of the present invention. [Table 2] No.Ratio of equiaxed crystalsAustenite grain sizeTensile strengthAverage coefficient of thermal expansionYoung's modulus%µmMPa×10- 6< / °CGPa196889081.9149Inv. ex.2452508001.9135Comp. ex.358067751.9110Comp. ex.457603202.5108Comp. ex.599559641.8155Inv. ex.6613028511.9138Comp. ex.7861897301.6142Inv. ex.8367506881.7110Comp. ex.9801806502.5145Comp. ex.10861647502.2140Comp. ex.11525007232.6115Comp. ex.1291807593.5153Comp. ex.13256988123.9106Comp. ex.14871018012.4146Comp. ex.1589967692.8144Comp. ex.1690746701.5153Comp. ex.1784989063.1151Comp. ex. * Underline means that the desired property of the present invention was not obtained.

[0066] According to the present invention, it was confirmed that a low thermal expansion cast steel having high strength, high rigidity, and low coefficient of thermal expansion can be obtained.

Examples

examples

[0045]Hereinafter, specific examples of the present invention will be described with reference to Examples. The following example is merely an example included in the present invention, and is not intended to limit the present invention.

[0046]The molten metal was poured into a mold and a cast steel (Y-block) having the components listed in No.1 of Table-1 was produced. Next, in the Ni depletion step, the cast steel was heated at 700°C, held for 12 hours, and cooled to room temperature.

[0047]Next, in the cryogenic treatment step, the cast steel was immersed in liquid nitrogen and held for 2 hours, and then pulled up to the atmosphere at room temperature and heated to room temperature. Subsequently, in the recrystallization step, the cast steel was heated to 900°C, held for 3 hours, and water cooled to room temperature. Thereafter, in the aging step, the cast steel was heated to 700°C, held for 12 hours, and water cooled to room temperature. Mf and Ms points shown in Table 1 are value...

FIELD

[0001] The present invention relates to a low thermal expansion cast steel.BACKGROUND

[0002] As an antenna reflector, carbon fiber reinforced plastic (CFRP) having high rigidity and high corrosion resistance is generally used. The coefficient of thermal expansion of CFRP is extremely small as about 1.5×10 -6< / °C. Therefore, in order to ensure high dimensional accuracy even after molding, it is required that a molding die be composed of a material having a coefficient of thermal expansion of the same degree as that of CFRP. Therefore, an Invar alloy or a super Invar alloy is used as the material of the molding die.

[0003] In addition, a low thermal expansion cast steel which can be produced by casting is used, which is suitable for CFRP mold material having complex shapes, spindles, and the like.

[0004] Patent Literature 1 discloses a high rigidity low thermal expansion cast steel having high Young's modulus even when it is as cast and having lower Ms point than a conventional low thermal expansion cast steel, which is invented for producing a member having a complicated form requiring high rigidity with low thermal expansion as a casting.

[0005] On the other hand, with respect to the low thermal expansion alloy having high strength at high temperature, Patent Literature 2 discloses using, as a molding die, a low thermal expansion alloy, which is a cast iron having a graphite structure in the austenite, comprises, by weight%, solid C: 0.09% or more and 0.43% or less, Si: less than 1.0%, Ni: 29% or more and 34% or less, Co: 4% or more and 8% or less and balance: Fe, and having a coefficient of thermal expansion at a range of temperature of 0 to 200°C is 4.0×10 -6< / °C or less.

[0006] Patent Literature 3 discloses the use of an alloyed steel excellent in thermal shape-stability and stiffness as a member of an ultra-precision apparatus including a CFRP mold. The alloyed steel comprises C:0.1wt.% or less, Si:0.1 to 0.4wt.%, Mn:0.15 to 0.4wt.%, Ti: more than 2 to 4wt.%, Al:1wt.% or less, Ni:30.7 to 43.0wt.%, Co:14wt.% or less and the balance of Fe and unavoidable impurities, and the content of Ni and Co satisfies the formula (1). The alloyed steel has a coefficient of thermal expansion of 4× 10 -6< / °C or less in a temperature-range of -40 to 100°C and a Young's modulus of 16100kgf / mm 2< or more.

[0007] Patent Literature 4 discloses a low thermal expansion cast steel having high strength and rigidity at room temperature. The cast steel comprises, by mass %, C: 0 to 0.1%, Si: 0 to 0.5%, Mn: 0 to 0.5%, S: 0 to 0.05%, Ni: 29.0 to 34.0%, Co: 0 to 8.0%, the balance of Fe and unavoidable impurities and has an average grain diameter of the austenitic structure of more than 200µm and less than 600µm, a 0.2% proof stress at 23°C of 350MPa or more, a Young's modulus of 130GPa or more, and an average coefficient of thermal expansion at 18 to 28°C of 2.0×10 -6< / °C or less.[CITATIONS LIST][PATENT LITERATURE]

[0008] [PLT 1] JP 2016-027187 A [PLT 2] JP H6-172919 A [PLT 3] JP H11-293413 A [PLT 4] JP 2020-122180 A SUMMARY[TECHNICAL PROBLEM]

[0009] An object of the present invention is to provide a low thermal expansion cast steel having higher tensile strength, higher rigidity, and lower coefficient of thermal expansion.[SOLUTION TO PROBLEM]

[0010] The inventors have intensively studied a method of obtaining a low thermal expansion cast steel having both high rigidity and high strength, and found that both rigidity and tensile strength can be increased by utilizing intermetallic compounds of Ni and Ti.

[0011] The present invention has been made based on the above knowledge, and includes the following embodiments. (1) A low thermal expansion cast steel, the chemical composition of the cast steel, comprising, by mass%, C: 0 to 0.040%, Si: 0.05 to 0.70%, Mn: 0.10 to 0.70%, Ni: 33.0 to 34.5%, Co: 4.40 to 6.20%, Ti: 1.00 to 2.50%, and the balance of Fe and unavoidable impurities, an average grain size of an austenite structure of the cast steel being 200µm or less, a tensile strength at 23°C of the cast steel being 700MPa or more, an average coefficient of thermal expansion at 18 to 28°C being 2.0×10 -6< / °C or less. (2) The low thermal expansion cast steel according to embodiment (1), wherein a ratio of equiaxed grains in the austenitic structure is 60% or more in area ratio. (3) The low thermal expansion cast steel according to embodiment (1) or (2), wherein a Young's modulus of the cast steel is 140GPa or more. (4) A method for producing the low thermal expansion cast steel according to embodiment (1), the method comprising: a Ni depletion step of heating a cast steel to 600 to 750°C, holding for 2 to 24 hours and cooling to room temperature, the chemical composition of the cast steel comprising, by mass%, C: 0 to 0.040%, Si: 0.05 to 0.70%, Mn: 0.10 to 0.70%, Ni: 33.0 to 34.5%, Co: 4.40 to 6.20%, Ti: 1.00 to 2.50%, and the balance of Fe and unavoidable impurities; a cryogenic treatment step of cooling the cast steel from room temperature to a temperature Mf point (martensitic transformation finish temperature) and Ms point (martensitic transformation start temperature), holding for 0.5 to 3 hours, and raising the room temperature; a recrystallization step of heating the cast steel to 800 to 1000 °C, holding for 0.5 to 5 hours, and quenching; and an aging step of heating the cast steel to 600 to 750°C, holding for 2 to 24 hours, and quenching. [ADVANTAGEOUS EFFECTS OF INVENTION]

[0012] According to the present invention, it is possible to obtain a low thermal expansion cast steel having high tensile strength, high rigidity, and low coefficient of thermal expansion.BRIEF DESCRIPTION OF DRAWINGS

[0013] [Fig. 1] Fig. 1 is a schematic diagram showing a temperature change in the method of producing the low thermal expansion cast steel of the present invention. [Fig. 2] Fig.2 is a diagram showing an example of a metal structure after a cryogenic treatment step, and (a) is an example in which a cryogenic treatment is performed without a Ni depletion step and (b) is an example in which a cryogenic treatment is performed after a Ni depleting step. DESCRIPTION OF EMBODIMENTS≪ Low thermal expansion cast steel ≫

[0014] In the low thermal expansion cast steel of the present invention, the chemical composition of the cast steel comprises, by mass%, C: 0 to 0.040%, Si: 0.05 to 0.70%, Mn: 0.10 to 0.70%, Ni: 33.0 to 34.5%, Co: 4.40 to 6.20%, Ti: 1.00 to 2.50%, and the balance of Fe and unavoidable impurities, the average grain size of the austenite structure of the cast steel is 200µm or less, the tensile strength at 23°C of the cast steel is 700MPa or more, and the average coefficient of thermal expansion at 18 to 28°C is 2.0×10 -6< / °C or less.≪Chemical composition≫

[0015] First, the chemical composition of the low thermal expansion cast steel of the present invention is described in detail. Hereinafter, "%" regarding the chemical composition represents "% by mass" unless otherwise specified.(C: 0 to 0.040%)

[0016] C solidifies in austenite and contributes to increase in strength. When the content of C is increased, the coefficient of thermal expansion is increased, and it becomes difficult to refine the crystal grains since the martensitic transformation is suppressed. Therefore, the content of C is set to 0.040% or less. The content of C is not essential and may be 0. The content of C may be 0.001% or more, 0.002% or more, 0.003% or more, or 0.005% or more. The content of C may be 0.030% or less, 0.025% or less, 0.022% or less, or 0.020% or less.(Si: 0.05 to 0.70%)

[0017] Si is added as a deoxidizer. Si is also an element which improves fluidity of the molten metal. In order to obtain these effects, the content of Si is set to 0.05% or more. The content of Si may be 0.07% or more, 0.10% or more, 0.12% or more, or 0.15% or more. When the content of Si exceeds 0.70%, the coefficient of thermal expansion is increased. Therefore, the content of Si is set to 0.70% or less. The content of Si may be 0.60% or less, 0.50% or less, or 0.40% or less.(Mn: 0.10 to 0.70%)

[0018] Mn is added as a deoxidizer. Further, Mn also contributes to strength improvement by solid solution strengthening. In order to achieve these effects, the content of Mn is set to 0.10% or more. The content of Mn may be 0.12% or more, 0.15% or more, 0.20% or more, or 0.25% or more. When the content of Mn exceeds 0.70%, the effects are saturated, and due to the martensitic transformation suppressed, it becomes difficult to refine the crystal grains and the coefficient of thermal expansion is increased. Therefore the content of Mn is set to 0.70% or less. The content of Mn may be 0.60% or less, 0.50% or less, or 0.40% or less.(Ni: 33.0 to 34.5%)

[0019] Ni is an element which lowers the coefficient of thermal expansion. Even if the content of Ni is too small or too large, the coefficient of thermal expansion increases. Further, Ni forms an intermetallic compound Ni 3 Ti with Ti and the intermetallic compound precipitates and distributes in the crystal grains, thereby increasing the rigidity and strength. In the low thermal expansion cast steel of the present invention, the content of Ni is set to 33.0 to 34.5% in order to precipitate intermetallic compounds for obtaining the desired rigidity and strength. Note that generally, when the content of Ni is increased, it becomes difficult to cause martensitic transformation by cooling. In the present invention, the crystal grain refinement by the martensitic transformation is achieved by the heat treatment described later. However, if the content of Ni is too large, the crystal grains cannot be refined. The content of Ni may be 33.2% or more, 33.4% or more, or 33.6% or more. The content of Ni may be 34.4% or less, 34.3% or less, or 34.2% or less.(Co: 4.40 to 6.20%)

[0020] Co contributes to lowering the coefficient of thermal expansion in combination with Ni. Even if the content of Co is too small or too large, the coefficient of thermal expansion increases. In order to obtain the desired coefficient of thermal expansion, Co is set to 4.40 to 6.20%. The content of Co may be 4.60% or more, 4.80% or more, 5.00% or more, or 5.20% or more. The content of Co may be 6.10% or less, 6.00% or less, 5.90% or less, 5.80% or less, or 5.60% or less.(Ti: 1.00 to 2.50%)

[0021] Ti is an element which increases strength by forming an intermetallic compound Ni 3 Ti with Ni and precipitating and distributing the intermetallic compound in the crystal grains. The content of Ti is set to 1.00% or more in order to improve strength. If the content of Ti is too large, the coefficient of thermal expansion is increased. Therefore, the content of Ti is set to 2.50% or less. The content of Ti may be 1.10% or more, 1.20% or more, 1.30% or more, 1.40% or more, or 1.50% or more. The content of Ti may be 2.40% or less, 2.30% or less, 2.20% or less, 2.10% or less, or 2.00% or less.

[0022] The balance of the chemical compositions consists of Fe and unavoidable impurities. The unavoidable impurities are elements which are unavoidably mixed from the raw material and the producing environment or the like when industrially producing the cast steel having a chemical composition in the present invention, and contained within a range which does not harm the required characteristics for the low thermal expansion casting of the present invention. Examples thereof include P, S, and Al contained in the raw material, and Nb, B, Mg, Ce, and La contained in a molten metal as an inoculum. The content of the element contained as an impurity may be 0.100% or less, 0.050% or less, 0.030% or less, 0.020% or less, 0.010% or less, 0.008% or less, or 0.005% or less for each element. The content of impurities may be 0%.(Average grain size of austenite structure)

[0023] In the low thermal expansion cast steel of the present invention, the austenite grain size is refined by a heat treatment described later. Specifically, the average grain size of the austenite structure is 200µm or less. The average grain size of the austenite structure may be 180µm or less, 160µm or less, 140µm or less, 120µm or less, or 100µm or less.

[0024] The average grain size of the austenite structure is measured by observation by an optical microscope. In particular, the observation surface is polished to Emery Paper #1200, and the surface is mirror-finished by buffing, and a sample 20s corroded at room temperature using a marble liquid is observed by an optical microscope. The average grain size is measured by a quadrature method according to JIS G 0551:2020 on the obtained microstructure.

[0025] The structure of the cast steel of the present invention mainly comprises the equiaxed grain having various crystal orientations, and therefore, a crystal having high Young's modulus (111) or (110) and the like is included at more than a constant ratio. As a result, higher tensile strength and Young's modulus can be obtained compared to conventional low thermal expansion cast steel, which mainly comprises columnar grains having a crystal orientation (100) with a low Young's modulus. In the present invention, among the crystal grains observed by the optical microscope, the ratio of the long to short sides of the crystal grains is judged to be three times or more for columnar grains and less than three times for equiaxed grain.

[0026] It is not necessary that all of the structures are equiaxed grains, but it is preferable that the ratio of equiaxed grains is 60% or more in area ratio. The ratio of the equiaxed grains may be 70% or more, 80% or more, 85% or more, 90% or more, or 95% or more by area ratio. The equiaxed grain and the columnar grain can be distinguished by the above-described observation with the optical microscope. In the present invention, the aspect ratio is determined to be 3.0 or less crystal grains for equiaxed grain.(Tensile strength at 23°C)

[0027] The strength of the low thermal expansion cast steel of the present invention can be evaluated by the results of the tensile test at 23°C. In particular, the low thermal expansion cast steel has a tensile strength measured in a tensile test at 23°C of 700MPa or more. The tensile strength may be 720MPa or more, 750MPa or more, 780MPa or more, 800MPa or more, 820MPa or more, or 850MPa or more. There is no upper limit on the tensile strength. However, the tensile strength may be, for example, 1100MPa or less. Tensile strength is measured by the offsetting method of tensile test according to JIS Z 2241:2011 using a tensile test piece according to JIS G 0567:2020.(Average coefficient of thermal expansion at 18 to 28°C)

[0028] The low thermal expansion cast steel has lower coefficient of thermal expansion, with an average coefficient of thermal expansion at 18 to 28°C of 2.0×10 -6< / °C or less. The coefficient of thermal expansion is measured by taking a cylindrical thermal expansion test piece (φ6.0×25L) from the center of the steel ingot and using a thermal expansion measuring machine. The average coefficient of thermal expansion at 18 to 28°C may be 1.9×10 -6< / °C or less, 1.8×10 -6< / °C or less, 1.7× 10 -6< / °C or less, or 1.6× 10 -6< / °C or less. There is no lower limit of the average coefficient of thermal expansion at 18 to 28°C, and the average coefficient of thermal expansion at 18 to 28°C may be a negative value. The mean coefficient of thermal expansion at 18 to 28°C may be, for example, 0 or more, 0.5× 10 -6< / °C or more, or 1.0× 10 -6< / °C or more.(Young's modulus)

[0029] Even in conventional low thermal expansion cast steel, the Young's modulus and the coefficient of thermal expansion can be adjusted to some extent by adjusting the chemical composition. However, the Young's modulus and the coefficient of thermal expansion are approximately in a trade-off relationship. In other words, as the Young's modulus increases, the coefficient of thermal expansion also increases. In the low thermal expansion cast steel of the present invention, the Young's modulus of 140GPa or more can be obtained. The Young's modulus may be 142GPa or more, 145GPa or more, 147GPa or more, or 150GPa or more. There is no upper limit of the Young's modulus. However, the Young's modulus may be, for example, 180GPa or less.

[0030] Young's modulus is measured by a two-point support transverse resonance method at room temperature conforming to JIS Z 2280:1993 by using a Young's modulus test piece (7t× 16w× 125L) taken from the center of a steel ingot.≪ Method for Producing Low Thermal Expansion Cast ≫

[0031] Next, with reference to Fig. 1, a method for producing the low thermal expansion cast steel of the present invention is described. Fig. 1 is a diagram showing an outline of a temperature change of a cast steel in each step described below, a horizontal axis being time, and a vertical axis being temperature.

[0032] The method for producing the low thermal expansion cast steel of the present invention comprises in sequence a Ni depletion step of heating a cast steel having the above-described chemical composition to 600 to 750°C, holding for 2 to 24 hours and cooling to room temperature; a cryogenic treatment step of cooling the cast steel from room temperature to a temperature between Mf point (martensitic transformation finish temperature) and Ms point (martensitic transformation start temperature), holding for 0.5 to 3 hours, and raising the room temperature; a recrystallization step of heating the cast steel to 800 to 1000 °C, holding for 0.5 to 5 hours, and quenching; and an aging step of heating the cast steel to 600 to 750°C, holding for 2 to 24 hours, and quenching.

[0033] The mold used in the producing the low thermal expansion cast steel of the present invention, the pouring device of molten steel into the mold, and the pouring method are not limited, and may be used known apparatus, or methods. The structure of the cast steel produced by the mold becomes a structure comprising mainly the columnar grain.

[0034] When the cast steel is produced, the molten metal may contain Nb, B, Mg, Ce, or La as an inoculant in order to facilitate the formation of solidification nuclei. In addition, solidification nuclei may be easily formed by applying an inoculant such as Co (AlO 2 ), CoSiO 3 , Co-borate or the like to the mold surface together with a coating material which is usually applied to the mold. Further, the molten metal in the mold may be stirred and flowed by using an electromagnetic stirring apparatus, mechanically vibrating the mold, vibrating the molten metal by ultrasonic waves, or the like. By applying these methods, the structure of the casting tends to become equiaxed grains.

[0035] The produced cast steel is, in sequence, subjected to the following heat treatment.(Ni depletion step)

[0036] In Ni depletion step, the content of Ni in the matrix phase is reduced by precipitating Ni and Ti in the steel as intermetallic compounds. In Ni depletion process, cast steel produced by casting is heated to 600 to 750°C, held for 2 to 24 hours, and cooled to room temperature (S1 in Fig. 1). The heating rate is not limited, and may be, for example, 1 to 100°C / hr. During the holding of 2 to 24 hours, the temperature may be constant between 600 and 750°C. The temperature may vary in the range of 600 to 750°C and need not be constant. The cooling rate is not limited and may be, for example, 1 to 100°C / hr.

[0037] In the low thermal expansion cast steel of the present invention, the content of Ni is increased in order to improve the rigidity and strength by forming an intermetallic compound Ni 3 Ti and precipitating and distributing the intermetallic compound inside and outside the crystal grains. However, in general, when the content of Ni is increased, Ms point and Mf point are lowered, and thus the martensite content obtained in the cryogenic treatment is decreased. Therefore, in the present invention, heating the cast steel produced by casting to 600 to 750°C, holding for 2 to 24 hours, and cooling to room temperature, Ni and Ti in the steel are precipitated as an intermetallic compound and Ni content in the matrix phase is reduced. As a result, Ms point and Mf point of the matrix phase are increased, and thus the martensite content obtained by the subsequent cryogenic treatment step is increased.(Cryogenic treatment step)

[0038] In the cryogenic treatment step, the cast steel through the Ni depletion step is cooled from room temperature to a temperature between Mf point and Ms point, held for 0.5 to 3 hours, and heated to room temperature (S2 of Fig. 1). It is not necessary to know the precise Ms and Mf points at this stage, and Ms and Mf points may be estimated by the following equation using the chemical composition of the matrix phase:         Ms(°C)=521-353C-22Si-24.3Mn-7.7Cu-17.3Ni-17.7Cr-25.8Mo         Mf(°C)=410.5-407.3C-7.3Si-37.8Mn-20.5Cu-19.5Ni-19.8Cr-4.5Mo

[0039] Here, C Si, Mn, Cu, Ni, Cr, and Mo are contents (by mass%) of the respective elements. For element, which is not contained, 0 is assigned.

[0040] The methods of cooling to a temperature between Mf point and Ms point and the cooling rate are not limited. Dry ice, and methyl alcohol or ethyl alcohol may be used as the cooling medium for cooling up to -80°C. For cooling further low temperature of -196°C, a method of immersing in liquid nitrogen or a method of spraying liquid nitrogen may be used. The cooling temperature is preferably as close to Mf point as possible. As a result, a structure containing martensite is formed.

[0041] The temperature may be constant between Mf point and Ms point during holding of 0.5 to 3 hours at a temperature range between Mf point and Ms point. The temperature may vary in the range of Mf point to Ms point and need not be constant. The heating rate up to room temperature is not limited. The temperature increase may be performed by, for example, pulling up a cast steel cooled to a temperature between Mf point and Ms point to the atmosphere at room temperature.

[0042] Fig. 2 shows examples of the structure of cast steels after cryogenic treatment having the same compositional composition, which contains Ni of 34 mass%, when the cryogenic treatment is performed without the Ni depletion step, and when the cryogenic treatment is performed after the Ni depletion step. (a) is a case where the cryogenic treatment is performed without the Ni depletion step, and (b) is a case where the cryogenic treatment is performed after the Ni depletion step. Conditions of the cryogenic treatment are the same as those of (a) and (b). In the structure, the black portion is martensite. The formed martensite rate when the cryogenic treatment is performed without the Ni depletion step of (a) is about 30%. The formed martensite rate when the cryogenic treatment is performed after the Ni depletion step of (b) is about 70%, and it can be seen that a large amount of martensite is formed as compared with (a). As a result, (b), in which a larger amount of martensite is generated, has smaller average grain size due to austenite refinement in the following recrystallization step.(Recrystallization step)

[0043] In the recrystallization step, the cast steel through the cryogenic treatment step is heated to 800 to 1000°C, held for 0.5 to 5 hours, and then quenched (S3 in Fig. 1). As a result, the structure in which the martensite is formed returns to the austenite structure. The crystal grain size of the structure formed by ordinary solidification is about 1 to 10mm. According to the Ni depletion treatment step, the cryogenic treatment step, since a large amount of martensite is formed through the recrystallization step, recrystallization is accelerated, austenite grain size is refined, and the structures mainly become equiaxed grain structures having random crystal orientations. Further, by this heat treatment, Ni and Ti precipitated as the intermetallic compound is dissolved in the structure, and even after quenching, Ni and Ti remain in solid solution in the austenite structure. The structure after quenching has an average grain size of 200µm or less. The method of quenching is not limited. However, water cooling is preferable.(Aging step)

[0044] In the aging, the cast steel through the recrystallization step is heated to 600 to 750°C, held for 2 to 24 hours, and then quenched (S4 in Fig. 1). Thus, by making Ni and Ti, which are diffused into the structure, precipitate the grains as the intermetallic compound Ni 3 Ti, the strength increases, and by reducing Ni content in the matrix, the coefficient of thermal expansion decreases.EXAMPLES

[0045] Hereinafter, specific examples of the present invention will be described with reference to Examples. The following example is merely an example included in the present invention, and is not intended to limit the present invention.<No. 1>

[0046] The molten metal was poured into a mold and a cast steel (Y-block) having the components listed in No.1 of Table-1 was produced. Next, in the Ni depletion step, the cast steel was heated at 700°C, held for 12 hours, and cooled to room temperature.

[0047] Next, in the cryogenic treatment step, the cast steel was immersed in liquid nitrogen and held for 2 hours, and then pulled up to the atmosphere at room temperature and heated to room temperature. Subsequently, in the recrystallization step, the cast steel was heated to 900°C, held for 3 hours, and water cooled to room temperature. Thereafter, in the aging step, the cast steel was heated to 700°C, held for 12 hours, and water cooled to room temperature. Mf and Ms points shown in Table 1 are values obtained by substituting the values of the chemical composition shown in Table 1 into the formula described above. Note that Mf point was assumed to -273°C, if the value obtained by substituting the above formula was lower than - 273°C.

[0048] A Young's modulus test piece (7t×16w×125L), a tensile test piece (according to JIS G 0567), and a thermal expansion test piece (φ6×25L) were taken from the obtained Y-block, and austenite grain size, Young's modulus, tensile strength at 23°C, and average coefficient of thermal expansion at 18 to 28°C were measured by the above described methods.<Nos. 2 to 17>

[0049] Casting steels were produced in the same manner as in No.1, except for the change the chemical composition and the production conditions as described in Tables 1. In Table 1, "depletion", "cryogenic", "recrystallization", "aging" means, respectively, the "Ni depletion step", the "cryogenic treatment step", the "recrystallization step", and the "aging step" described above , "∘" means that the process was performed in the range of the above-mentioned temperature and time, blank means that the process was omitted. For the obtained cast steels, the austenite grain size, Young's modulus, tensile strength at 23°C, and average coefficient of thermal expansion of 18 to 28°C were measured in the same manner as in No. 1.

[0050] Table 2 shows the results. In this examples, if the tensile strength at 23°C was 700MPa or more, Young's modulus was 140GPa or more, and the average coefficient of thermal expansion of 18 to 28°C was 2.0×10 -6< / °C or less, the obtained cast steel was judged to have high tensile strength, have rigidity, and have low coefficient of thermal expansion, and the problem to be solved by the invention was solved.

[0051] Nos. 1, 5 and 7 are inventive examples, and cast steel with Young's modulus of 140GPa or more, tensile strength at 23°Cof 700MPa or more, and average coefficient of thermal expansion of 18 to 28°C of 2.0×10 -6< / °C or less were obtained.

[0052] No. 2 is a comparative example in which the cast steel was produced without a Ni depletion step, and as a result, the amount of martensite produced in the cryogenic treatment step was small, and, the austenite grain size was reduced in the subsequent recrystallization step, and the Young's modulus was lowered.

[0053] No. 3 is a comparative example in which the cast steel was produced without a Ni depletion step, a cryogenic treatment step, and a recrystallization step, and as a result, the austenite grain size cannot be refined, and the Young's modulus is lowered.

[0054] No. 4 is a comparative example in which the cast steel was produced without a Ni depletion step, a cryogenic treatment step, a recrystallization step, and an aging step, and as a result, the austenite grain size could not be refined, and the Young's modulus was lowered. In addition, Ni content in the matrix phase did not decrease, and the coefficient of thermal expansion increased. Furthermore, Ni 3 Ti did not precipitate, and the tensile strength is lowered.

[0055] No. 6 is a comparative example in which the cast steel was produced without a Ni depletion step, and as a result, the amount of martensite produced in the cryogenic treatment step was small, and the austenite grain size was reduced in the subsequent recrystallization step, and the Young's modulus was lowered.

[0056] No. 8 is a comparative example in which the cast steel was produced without a Ni depletion step, and as a result, the austenite grain size could not be refined, and Young's modulus is lowered. Further, since Ni content is relatively large and has not passed through Ni depletion step, the austenite grain size of the matrix phase is increased, the tensile strength is lowered.

[0057] No. 9 is a comparative example in which the content of C was large, and as a result, the coefficient of thermal expansion was increased. In addition, the tensile strength was lowered. It is assumed that the decrease in tensile strength was due to the precipitation of TiC.

[0058] No. 10 was a comparative example in which the content of Si was large, and as a result, the coefficient of thermal expansion was increased.

[0059] No. 11 was a comparative example in which the content of Mn was large, and as a result, the coefficient of thermal expansion was increased. Further, it is assumed that the martensitic transformation was suppressed, and as a result, the austenite grain size could not be refined, and the Young's modulus was lowered.

[0060] No. 12 was a comparative example in which the content of Ni was small, and as a result, the coefficient of thermal expansion was increased.

[0061] No. 13 was a comparative example in which the content of Ni was large, and as a result, the coefficient of thermal expansion was increased. In addition, the austenite grain size could not be refined, and the Young's modulus was lowered.

[0062] No. 14 was a comparative example in which the content of Co was small, and as a result, the coefficient of thermal expansion was increased.

[0063] No. 15 was a comparative example in which the content of Co was large, and as a result, the coefficient of thermal expansion was increased.

[0064] No. 16 was a comparative example in which the content of Ti was small, and as a result, the tensile strength was low.

[0065] No. 17 was a comparative example in which the content of Ti was large, and as a result, the coefficient of thermal expansion was increased. [Table 1]No.Chemical Composition (mass%), Balance: Fe and impuritiesMf point (°C)Ms point (°C)Production ProcessCSiMnNiCoTidepletioncryogenicrecrystallizationaging10.0050.160.2133.85.202.10-260-74○○○○Inv. ex.20.0050.160.2133.85.202.10-260-74○○○Comp. ex.30.0050.160.2133.85.202.10-260-74○Comp. ex.40.0050.160.2133.85.202.10-260-74Comp. ex.50.0210.150.5933.16.012.48-267-77○○○○Inv. ex.60.0210.150.5933.16.012.48-267-77○○○Comp. ex.70.0060.520.2934.44.511.32-273-95O○O○Inv. ex.80.0060.520.2934.44.511.32-273-95○○○Comp. ex.90.0510.350.4034.05.081.97-273-103○○○○Comp. ex.100.0121.020.5633.45.722.31-273-97O○○○Comp. ex.110.0210.250.9533.95.511.42-273-101○○○○Comp. ex.120.0070.230.3032.85.211.35-245-61O○○○Comp. ex.130.0050.250.3235.25.321.68-273-103○○○○Comp. ex.140.0090.240.2833.54.052.39-259-74○○○○Comp. ex.150.0050.310.2533.46.342.42-255-71O○○○Comp. ex.160.0110.280.2233.94.980.85-265-81O○○○Comp. ex.170.0130.190.3134.34.793.13-273-89○○○○Comp. ex.* Underline means that the value is outside of the scope of the present invention. [Table 2] No.Ratio of equiaxed crystalsAustenite grain sizeTensile strengthAverage coefficient of thermal expansionYoung's modulus%µmMPa×10- 6< / °CGPa196889081.9149Inv. ex.2452508001.9135Comp. ex.358067751.9110Comp. ex.457603202.5108Comp. ex.599559641.8155Inv. ex.6613028511.9138Comp. ex.7861897301.6142Inv. ex.8367506881.7110Comp. ex.9801806502.5145Comp. ex.10861647502.2140Comp. ex.11525007232.6115Comp. ex.1291807593.5153Comp. ex.13256988123.9106Comp. ex.14871018012.4146Comp. ex.1589967692.8144Comp. ex.1690746701.5153Comp. ex.1784989063.1151Comp. ex. * Underline means that the desired property of the present invention was not obtained.

[0066] According to the present invention, it was confirmed that a low thermal expansion cast steel having high strength, high rigidity, and low coefficient of thermal expansion can be obtained.

Examples

examples

[0045]Hereinafter, specific examples of the present invention will be described with reference to Examples. The following example is merely an example included in the present invention, and is not intended to limit the present invention.

[0046]The molten metal was poured into a mold and a cast steel (Y-block) having the components listed in No.1 of Table-1 was produced. Next, in the Ni depletion step, the cast steel was heated at 700°C, held for 12 hours, and cooled to room temperature.

[0047]Next, in the cryogenic treatment step, the cast steel was immersed in liquid nitrogen and held for 2 hours, and then pulled up to the atmosphere at room temperature and heated to room temperature. Subsequently, in the recrystallization step, the cast steel was heated to 900°C, held for 3 hours, and water cooled to room temperature. Thereafter, in the aging step, the cast steel was heated to 700°C, held for 12 hours, and water cooled to room temperature. Mf and Ms points shown in Table 1 are value...

Claims

1. A low thermal expansion cast steel, the chemical composition of the cast steel, comprising, by mass%, C: 0 to 0.040%, Si: 0.05 to 0.70%, Mn: 0.10 to 0.70%, Ni: 33.0 to 34.5%, Co: 4.40 to 6.20%, Ti: 1.00 to 2.50%, and the balance of Fe and unavoidable impurities, an average grain size of an austenite structure of the cast steel being 200µm or less, a tensile strength at 23°C of the cast steel being 700MPa or more, an average coefficient of thermal expansion at 18 to 28°C being 2.0×10- 6 / °C or less.

2. The low thermal expansion cast steel according to claim 1, wherein a ratio of equiaxed grains in the austenitic structure is 60% or more in area ratio.

3. The low thermal expansion cast steel according to claim 1 or 2, wherein a Young's modulus of the cast steel is 140GPa or more.

4. A method for producing the low thermal expansion cast steel according to claim 1, the method comprising: a Ni depletion step of heating a cast steel to 600 to 750°C, holding for 2 to 24 hours and cooling to room temperature, the chemical composition of the cast steel comprising, by mass%, C: 0 to 0.040%, Si: 0.05 to 0.70%, Mn: 0.10 to 0.70%, Ni: 33.0 to 34.5%, Co: 4.40 to 6.20%, Ti: 1.00 to 2.50%, and the balance of Fe and unavoidable impurities; a cryogenic treatment step of cooling the cast steel from room temperature to Mf point (martensitic transformation finish temperature) or more and Ms point (martensitic transformation start temperature), holding 0.5 to 3 hours, and raising the room temperature; a recrystallization step of heating the cast steel to 800 to 1000 °C, holding 0.5 to 5 hours, and quenching; and an aging step of heating the cast steel to 600 to 750°C, holding for 2 to 24 hours, and quenching.

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