Low thermal expansion steel casting product and method for manufacturing same

Abstract

The object of the present invention is to provide a low thermal expansion cast steel product with excellent ultrasonic flaw detection properties. The low thermal expansion cast steel product of the present invention has a chemical composition comprising by mass%, of C: 0.040% or less, Si: 0.30% or less, Mn: 0.05 to 0.50%, S: 0.005% or less, Ni: 31.00 to 34.00%, Co: 2.00 to 6.00%, and Al: 0.035 to 0.100%, and the balance of Fe and unavoidable impurities, an average grain size of the austenite structure of 200µm or less, and an attenuation coefficient of the ultrasonic bottom echo of 2.0dB / cm or less.

Description

FIELD

[0001] The present invention relates to a low thermal expansion cast steel product with a low coefficient of thermal expansion and a method for manufacturing same.BACKGROUND

[0002] Thermally stable Invar alloys are widely used as component materials in electronics, semiconductor-related equipment, laser processing machines, and ultra-precision processing equipment.

[0003] PLT 1 discloses a low thermal expansion alloy comprising Ni: 29.5 to 35%, Co: 2.0 to 7.0%, Cr: 0.001 to 2.0%, having a coefficient of thermal expansion 0.5×10 -6< / °C to 2.0×10 -6< / °C. This alloy is obtained by homogenizing solution treatment, followed by quenching or annealing with cooling at a rate of 1° / sec or less, and then performing cold rolling of 10% or more.

[0004] PLT 2 discloses a low thermal expansion wire comprising Co: 65% or less, Ni: 30% or less, Cr: 10% or less, a total content of Co and Ni being 25 to 65%. PLT 2 discloses a method for transforming a part of the austenite phase into a strain-induced martensite phase by cold working the low thermal expansion wire.

[0005] PLT 3 discloses a low thermal expansion alloy comprising Ni: 0.03 to 1.5%, a total of Ni and Co: 53 to 55%, Cr: 9 to 10%. PLT 3 discloses a method of annealing the alloy at 650 to 900°C and then cooling it at a rate of less than 20°C / min in the furnace.[CITATIONS LIST][PATENT LITERATURE]

[0006] [PLT 1] JP 2796966 B2 [PLT 2] JP H6-279945 A [PLT 3] JP 5534150 B2 SUMMARY[TECHNICAL PROBLEM]

[0007] In the low thermal expansion cast steel products, austenite crystal grains tend to coarsen, resulting in inferior ultrasonic flaw detection properties compared to forged products. In other words, since ultrasound scatters and attenuates at the grain boundaries, it becomes difficult to detect defects using ultrasonic flaw detection, a non-destructive testing.

[0008] The object of the present invention is to solve the above problems and provide a low thermal expansion cast steel product with excellent ultrasonic flaw detection properties.[SOLUTION TO PROBLEM]

[0009] The inventors have diligently studied methods to improve ultrasonic flaw detection properties and have made the present invention. The present invention includes the following embodiments.

[0010] (1) A low thermal expansion cast steel product, wherein a chemical composition of the cast steel product comprises, by mass%, C: 0.040% or less, Si: 0.30% or less, Mn: 0.05 to 0.50%, S: 0.005% or less, Ni: 31.00 to 34.00%, Co: 2.00 to 6.00%, Al: 0.035 to 0.100% and balance of Fe and unavoidable impurities, the average grain size of the austenite structure is 200µm or less, and the attenuation coefficient of the ultrasonic bottom echo is 2.0dB / cm or less. (2) The low thermal expansion cast steel product of (1), wherein an average coefficient of thermal expansion is 1.0×10 -6< / °C or less at 25 to 100°C. (3) A method for manufacturing the low thermal expansion cast steel product of (1) or (2), comprising: a cryogenic treatment step of cooling a cast steel product comprising, by mass%, C: 0.040% or less, Si: 0.30% or less, Mn: 0.05 to 0.50%, S: 0.005% or less, Ni: 31.00 to 34.00%, Co: 2.00 to 6.00%, Al: 0.035 to 0.100% and balance of Fe and unavoidable impurities to below Ms point, holding the cast steel product at a temperature below the Ms point for 0.5 to 3 hours, and then heating the cast steel product to room temperature; and a recrystallization treatment step of heating the cryogenic treated cast steel product to 800 to 1100°C, and holding the cast steel product for 0.5 to 5 hours. [ADVANTAGEOUS EFFECTS OF INVENTION]

[0011] According to the present invention, it is possible to obtain a low thermal expansion cast steel product with excellent ultrasonic flaw detection properties.BRIEF DESCRIPTION OF DRAWINGS

[0012] [Fig.1] Fig.1 shows an example of the structure after cryogenic treatment of the cast steel product. [Fig.2] Fig.2 shows an example of the structure after recrystallization treatment of the cast steel product. DESCRIPTION OF EMBODIMENT

[0013] The following is a detailed description of the present invention.<Chemical composition>

[0014] First, the chemical composition of the cast steel product of the present invention will be described. Hereinafter, "%" in relation to the chemical composition represents "mass %" unless otherwise specified.(C: 0.040% or less)

[0015] C contributes to the increase in strength by dissolving in austenite. However, C dissolves in the matrix during the recrystallization treatment step and precipitates during cooling, increasing the coefficient of thermal expansion. In order to achieve the average coefficient of thermal expansion of 1.0×10 -6< / °C or less at 25 to 100°C, it is necessary to reduce the amount of C precipitation. Therefore, the C content is set to 0.040% or less. The C content may be 0.035% or less, 0.030% or less, 0.025% or less, or 0.020% or less. C is not an essential element, and the C content may be 0%. The C content may be 0.001% or more, 0.002% or more, 0.004% or more, 0.008% or more, 0.010% or more, or 0.015% or more.(Si: 0.30% or less)

[0016] Si is added as a deoxidizer. In the present invention, to produce good cast steel products without blowing, it is sufficient to add the minimum amount necessary for deoxidation. The final cast steel product does not need to comprise Si, and the lower limit of the Si content is 0%. Si may be comprised in a range of 0.30% or less. Deoxidation can also be performed with Al, and therefore Si does not need to be added. The Si content may be 0.25% or less, 0.20% or less, 0.15% or less, or 0.10% or less. The Si content may be 0.01% or more, 0.02% or more, 0.03% or more, 0.04% or more, 0.05% or more, or 0.07% or more.(Mn: 0.05 to 0.50%)

[0017] Mn is added as a deoxidizer. Mn is also added to fix S by reacting with S to form MnS. In the present invention, in order to achieve this effect, the Mn content is set to 0.05% or more. If the Mn content is too high, the effect will only saturate, so the Mn content is set to 0.50% or less. The Mn content may be 0.10% or more, 0.15% or more, 0.20% or more, 0.25% or more, or 0.30% or more. The Mn content may be 0.48% or less, 0.45% or less, 0.40% or less, or 0.35% or less.(S: 0.005% or less)

[0018] S is contained as an impurity. If S segregates at the grain boundaries, it inhibits recrystallization and ultrasonic flaw detection properties are deteriorated, and therefore the S content should be low. In the low thermal expansion cast steel products of the present invention, the S content is set to 0.005% or less. The S content may be 0.004% or less, or 0.003% or less. The lower the S content, the better, and the lower limit of S content is 0%. However, setting the S content to 0% increases costs, so the S content may be 0.001% or more, or 0.002% or more.(Ni: 31.00 to 34.00%)

[0019] Ni is an element which decreases the coefficient of thermal expansion. If the Ni content is too high or too low, the coefficient of thermal expansion does not become sufficiently small. Further, if the Ni content is too high, it becomes difficult to induce martensite transformation by cooling, and the ratio of equiaxed grains after recrystallization decreases. In order to achieve the average coefficient of thermal expansion of 1.0×10 -6< / °C or less at 25 to 100°C, it is necessary to control the Ni content within a narrow range. In particular, the Ni content is set to a range of 31.00 to 34.00%. The Ni content may be 31.10% or more, 31.20% or more, 31.30% or more, 31.40% or more, 31.50% or more, 31.60% or more, or 31.70% or more. The Ni content may be 33.80% or less, 33.50% or less, 33.00% or less, or 32.50% or less.(Co: 2.00 to 6.00%)

[0020] Co contributes to decrease in the coefficient of thermal expansion by being combined with Ni. In order to achieve a coefficient of thermal expansion at 25 to 100°C of 1.0×10 -6< / °C or less, the Co content is set to a range of 2.00 to 6.00%. The Co content may be 2.30% or more, 2.50% or more, 2.70% or more, 3.00% or more, 3.50% or more, or 4.00% or more. The Co content may be 5.80% or less, 5.60% or less, 5.40% or less, 5.20% or less, or 5.00% or less.

[0021] Furthermore, in order to decrease the coefficient of thermal expansion, it is preferable that [Ni], the Ni content (mass%), and [Co], the Co content (mass%) satisfy 34.00≤[Ni]+0.8×[Co]≤38.00. [Ni]+0.8×[Co] may be 34.20% or more, 34.50% or more, 34.70% or more, 35.00% or more, or 35.50% or more. [Ni]+0.8×[Co] may be 37.80% or less, 37.60% or less, 37.20% or less, 36.80% or less, or 36.50% or less.(Al: 0.035 to 0.100%)

[0022] Al is added as a deoxidizer. If the Al content is too low, insufficient deoxidation leads to the formation of Mn oxides, preventing MnS formation, and S tends to segregate at the grain boundaries, inhibiting recrystallization. As a result, ultrasonic flaw detection decreases. Therefore, the Al content is set to 0.035% or more. If the Al content is too high, the effect will saturate, so the Al content is set to 0.100% or less. The Al content may be 0.040% or more, or 0.045% or more. The Al content may be 0.090% or less, 0.080% or less, 0.070% or less, or 0.060% or less.

[0023] The balance of the chemical composition is Fe and unavoidable impurities. The unavoidable impurities refer to those substances that are unintentionally contained in the steel from raw materials or the manufacturing environment when producing steel with the chemical composition specified in the present invention. The unavoidable impurities include elements such as P and Cu, which are not intentionally added during the manufacturing process. The content of impurities is not limited as long as it does not affect the effects of the invention. The content of impurities may be, in mass%, 0.50% or less, 0.40% or less, 0.30% or less, 0.20% or less, 0.15% or less, 0.10% or less, 0.05% or less, or 0% in total.<Microstructure of cast steel >

[0024] The microstructure of the cast steel of the present invention is an austenitic structure with an average grain size of 200 µm or less. The structure is mainly fine equiaxed structure. If the ratio of equiaxed structure is low, the average crystal grain size of austenite increases. It is not necessary for the structure to be equiaxed structure entirely. However, it is preferable that the ratio of equiaxed structure is 60% or more in terms of area ratio. The ratio of equiaxed structure may be 70% or more, 80% or more, 85% or more, 90% or more, or 95% or more in terms of area ratio.

[0025] The average crystal grain size of austenite is determined as the average diameter of the equivalent circle of the crystal grains observed with an optical microscope after cutting out a sample for structural observation from near the center of the cast steel and etching with Marble's reagent. Here, among the crystal grains observed with an optical microscope, those with a ratio of the long axis to the short axis of 3 times or more are judged as columnar structure, and those with a ratio of less than 3 times are judged as equiaxed structure.<Attenuation coefficient>

[0026] The cast steel of the present invention has an attenuation coefficient of the ultrasonic bottom echo of 2.0 dB / cm or less. Here, the attenuation coefficient is defined as the value (dB / cm) obtained by (B1-B2) / L, where B1(dB) is the first bottom echo height, B2(dB) is the second bottom echo height B2, and L(cm) is the distance from the surface of the cast steel sample to the bottom when the output is adjusted so that the first bottom echo height is 75-85% of the ultrasonic output at 100%. The attenuation coefficient is preferably 1.8 dB / cm or less, 1.5 dB / cm or less, 1.2 dB / cm or less, or 1.0 dB / cm or less. Due to the small attenuation coefficient, it is advantageous for defect detection by non-destructive ultrasonic testing. Such an attenuation coefficient can be obtained by producing low thermal expansion cast steel with the chemical composition within the above range and by the manufacturing method described below.<Coefficient of thermal expansion>

[0027] The cast steel product of the present invention preferably has an average coefficient of thermal expansion at 25 to 100°C of 1.0×10 -6< / °C or less. The coefficient of thermal expansion is measured by taking a thermal expansion test specimen from near the center of the cast steel product, heating it from 0°C to 130°C at a heating rate of 3°C / min using a thermal expansion measuring device, and measuring the average coefficient of thermal expansion from 25°C to 100°C. As the thermal expansion measuring device, the NETZSCH DIL 402C can be used. The average coefficient of thermal expansion at 25 to 100°C may be 1.00×10 -6< / °C or less, 0.90×10 -6< / °C or less, 0.80× 10 -6< / °C or less, 0.70×10 - 6< / °C or less, 0.60× 10 - 6< / °C or less, or 0.50×10 - 6< / °C or less.<<Manufacturing method>>

[0028] Next, the manufacturing method of the low thermal expansion cast steel product of the present invention will be described. The low thermal expansion cast steel product of the present invention can be obtained through the casting and heat treatment described below. In the manufacturing method of the low thermal expansion cast steel product of the present invention, forging is not performed.(Casting)

[0029] First, molten steel which is adjusted to have the above chemical composition for the cast steel product is manufactured. The method for manufacturing molten steel is not limited, and known equipment and methods may be used. Next, the molten steel is poured into a mold to solidify, resulting in a cast steel product. The mold, the molten steel pouring devise device for the mold, and the pouring method are not particularly limited, and known devices and methods can be used. The microstructure of cast steel products manufactured in molds is mainly a columnar structure. The following heat treatment to the obtained cast steel product is applied.(Cryogenic treatment step)

[0030] Next, the cast steel is rapidly cooled to below the Ms point, maintained at a temperature below the Ms point for 0.5 to 3 hours, and then heated to room temperature (cryogenic treatment process). The cooling method is not particularly limited. Generally, the Ms point can be estimated using the chemical composition of steel with the following equation. Ms C <mprescripts / > <none / > ∘ = 521 - 353 C - 22 Si - 24.3 Mn - 7.7 Cu - 17.3 Ni - 17.7 Cr - 25.8 Mo

[0031] Here, C, Si, Mn, Cu, Ni, Cr, and Mo show the content (mass%) of each element. Elements not contained are considered as 0.

[0032] In the case of the chemical composition of the low thermal expansion cast steel product of the present invention, the Ms point calculated by the above formula depends particularly on the Ni content and ranges from about -10°C to -70°C. Therefore, as a cooling medium, methods such as immersion in dry ice and methyl alcohol or ethyl alcohol, immersion in liquid nitrogen, or spraying liquid nitrogen can be used. This results in the formation of a microstructure containing fine martensite. Further, the temperature increase can be achieved by pulling it up into the atmosphere at room temperature. Figure 1 shows an example of the microstructure after the cryogenic treatment step. In the microstructure photograph, the black areas are martensite. The formation of a large amount of martensite leads to the refinement of austenite grain size in the subsequent recrystallization.(Recrystallization step)

[0033] Next, the cast steel product is reheated to 800 to 1100°C, held at 800 to 1100°C for 0.5 to 5 hours, and then cooled. As a result, the microstructure in which martensite was formed returns to an austenite structure. The crystal grain size of the microstructure formed by general solidification is about 1 to 10 mm, but through the above cryogenic treatment step and subsequent recrystallization step, the austenite structure becomes a fine structure which is mainly equiaxed structure with an average grain size of 200µm or less. Figure 2 shows an example of the microstructure after the recrystallization step. It can be confirmed that the microstructure after the recrystallization step is a fine structure with an average grain size of 200µm or less.(Solution treatment step)

[0034] After cryogenic treatment, a solution treatment may be performed where the cast steel is heated to 800 to 1100°C, held for 0.5 to 5 hours, and then rapidly cooled. The solution treatment process is not an essential step and can be performed as necessary. Through solution treatment, precipitates formed during casting dissolve, and ductility and toughness are improved.(Diffusion treatment step)

[0035] Before the cryogenic treatment step, a diffusion treatment step may be included, where the cast steel product is held at 1100 to 1300°C for 5 to 50 hours. The diffusion treatment step is not an essential step and can be performed as necessary. By this step, the segregation of Ni and impurities in the steel is prevented, cast steel products with a low coefficient of thermal expansion can be produced more stably, even for large-sized cast steel products.(Tempering step)

[0036] Between the cryogenic treatment step and the recrystallization step, in order to further refine the recrystallized austenite grains, tempering martensite may be performed where the cast steel product is heated to 300 to 400°C just below the Ac 3 point and held at 300 to 400°C for 1 to 10 hours. The tempering step is not an essential step and may be performed as necessary.EXAMPLES<Invention Example 1>

[0037] Using a high-frequency induction furnace, the molten metal was poured into a mold to manufacture a Y-type test material adjusted to the chemical composition shown in Table 1, No. 1. The manufactured Y-type test material was subjected to cryogenic treatment by cooling it to - 196°C and holding it for 1.5 hours, followed by recrystallization treatment by reheating it to 900°C, holding it for 2.0 hours, and then air cooling, resulting in a low thermal expansion cast steel product.<Invention Examples 2-7, Comparison Examples 1-6>

[0038] The chemical composition, and temperatures of cryogenic treatment and recrystallization were set according to the conditions listed in Table 1 and Table 2, and the rest was carried out in the same manner as Example 1 to obtain a low thermal expansion cast steel product. Regarding each step temperature in Table 2, "-" indicates that the step was not performed. Noted that Comparative Example 6 is a forged product which was forged after casting.(Average grain size of austenite, and Coefficient of thermal expansion at 25 to 100°C)

[0039] For the obtained low thermal expansion cast steel product, the average grain size of austenite and the coefficient of thermal expansion at 25 to 100°C were measured using the above method.(Attenuation coefficient )

[0040] A sample cut to 25×25×100L was taken from the Y-shaped test material, and the bottom echo height was measured using the vertical method in a direction perpendicular to the longitudinal direction. The frequency of the ultrasound was set to 2MHz, and the first bottom echo height of the sample was adjusted to 80%. Then, the second bottom echo height B2(dB) was measured. From the first height B1(dB) and the sample height (L), (B1-B2) / L was calculated and used as the attenuation coefficient.

[0041] The above results are shown in Table 2. Note that "≥2.70" in the attenuation coefficient in Table 2 means that the attenuation was significant and could not be measured. Further, the Ms point is a value calculated using the above estimation formula.[Table 1]

[0042] Table 1No.Chemical Composition (mass%) balance: Fe and impurities[Ni]+0.8×[Co] (mass%)CSiMnSNiCoAlInv. Ex.10.0170.090.180.00331.833.210.04534.40Inv. Ex.20.0040.040.210.00333.165.390.03537.47Inv. Ex.30.0310.260.480.00431.375.860.08736.06Inv. Ex.40.0170.090.180.00331.534.350.04935.01Inv. Ex.50.0120.240.140.00432.305.440.04136.65Inv. Ex.60.0080.130.230.00532.702.900.04835.02Inv. Ex.70.0150.140.120.00433.872.040.05535.50Comp. Ex.10.0170.090.180.00331.833.210.04534.40Comp. Ex.20.0170.090.210.00531.985.210.02836.15Comp. Ex.30.0190.090.240.01532.414.030.02235.63Comp. Ex.40.0110.080.380.00534.336.130.00239.23Comp. Ex.50.0060.240.400.00430.801.900.01332.32Comp. Ex.60.0030.090.240.00532.535.360.04536.82* Underlines indicate outside scope of present invention. [Table 2]

[0043] Table 2No.Ms pointTemperature of cryogenic treatmentTemperature of recrystallizationCrystal grain sizeCoefficient of thermal expansion (25 to 100°C)attenuation coefficient[°C][°C][°C][µm][×10 -6< / °C][dB / cm]Inv. Ex.1-42-196900750.550.82Inv. Ex.2-60-19611001900.941.55Inv. Ex.3-50-196800600.610.85Inv. Ex.4-37-196900500.340.91Inv. Ex.5-51-1961000500.771.14Inv. Ex.6-56-196900800.821.26Inv. Ex.7-76-1969001700.911.93Comp. Ex.1-42--12000.55≥2.70Comp. Ex.2-45-1968004300.52≥2.70Comp. Ex.3-54-19610004800.70≥2.70Comp. Ex.4-88-196120012003.14≥2.70Comp. Ex.5-29-1961000702.7822.70Comp. Ex.6-51--1000.580.53~0.65* Underlines indicate outside scope of present invention or desired properties not obtained.

[0044] As shown in Table 2, the low thermal expansion cast steel product of the present invention shown in inventive examples achieved a low coefficient of thermal expansion at 25 to 100°C, and as cast, it had an attenuation coefficient as small as that of forged products. In other words, it had excellent ultrasonic flaw detection properties.

[0045] Comparative Example 1 is an example where cryogenic treatment and recrystallization were not performed, resulting in coarse austenite grains as cast. As a result, the attenuation coefficient increased.

[0046] Comparative Example 2 is an example where the Al content was low, and it is considered that MnS was not formed due to the formation of Mn oxides, making S more likely to segregate at the grain boundaries, thereby inhibiting recrystallization. As a result, the average grain size of austenite increased, and the attenuation coefficient became large.

[0047] Comparative Example 3 is an example where the S content was high and the Al content was low, resulting in the formation of Mn oxides instead of MnS, and due to the high S content, S easily segregated at the grain boundaries, which is considered to have inhibited recrystallization. As a result, the average grain size of austenite increased, and the attenuation coefficient became large.

[0048] Comparative Example 4 is an example where the Ni content was high, the Co content was high, and the Al content was low, resulting in an increased coefficient of thermal expansion due to inappropriate Ni and Co contents, and the formation of Mn oxides instead of MnS, leading to easy segregation of S at the grain boundaries, which is considered to have inhibited recrystallization. Further, the high Ni content stabilized the austenite, resulting in a small amount of martensitic transformation during cryogenic treatment, and the equiaxed structure after recrystallization were reduced. As a result, the average grain size of austenite increased, and the attenuation coefficient became large.

[0049] Comparative Example 5 is an example where the Ni content was low, the Co content was low, and the Al content was low, resulting in an increased coefficient of thermal expansion due to inappropriate Ni and Co contents, and the formation of Mn oxides instead of MnS, leading to easy segregation of S at the grain boundaries, which is considered to have inhibited recrystallization. As a result, the average grain size of austenite increased, and the attenuation coefficient became large.

[0050] Comparative Example 6 is an example of a forged product which was not subjected to cryogenic treatment or recrystallization. Since it was a forged product, it had a worked structure and is different from the low thermal expansion cast steel product of the present invention, which is cast steel.

Claims

1. A low thermal expansion cast steel product, wherein a chemical composition of the cast steel product comprises, by mass%, C: 0.040% or less, Si: 0.30% or less, Mn: 0.05 to 0.50%, S: 0.005% or less, Ni: 31.00 to 34.00%, Co: 2.00 to 6.00%, Al: 0.035 to 0.100% and balance of Fe and unavoidable impurities, the average grain size of the austenite structure is 200µm or less, and the attenuation coefficient of the ultrasonic bottom echo is 2.0dB / cm or less.

2. The low thermal expansion cast steel product according to claim 1, wherein an average coefficient of thermal expansion is 1.0× 10- 6 / °C or less at 25 to 100°C.

3. A method for manufacturing the low thermal expansion cast steel product according to claim 1 or 2, comprising: a cryogenic treatment step of cooling a cast steel product comprising, by mass%, C: 0.040% or less, Si: 0.30% or less, Mn: 0.05 to 0.50%, S: 0.005% or less, Ni: 31.00 to 34.00%, Co: 2.00 to 6.00%, Al: 0.035 to 0.100% and balance of Fe and unavoidable impurities to below Ms point, holding the cast steel product at a temperature below the Ms point for 0.5 to 3 hours, and then heating the cast steel product to room temperature; and a recrystallization treatment step of heating the cryogenic treated cast steel product to 800 to 1100°C, and holding the cast steel product for 0.5 to 5 hours.

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