Classifications

• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D6/00—Heat treatment of ferrous alloys
  • C21D6/04—Hardening by cooling below 0 degrees Celsius
• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D1/00—General methods or devices for heat treatment, e.g. annealing, hardening, quenching or tempering
  • C21D1/26—Methods of annealing
• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D6/00—Heat treatment of ferrous alloys
• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D6/00—Heat treatment of ferrous alloys
  • C21D6/001—Heat treatment of ferrous alloys containing Ni
• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D6/00—Heat treatment of ferrous alloys
  • C21D6/004—Heat treatment of ferrous alloys containing Cr and Ni
• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D6/00—Heat treatment of ferrous alloys
  • C21D6/007—Heat treatment of ferrous alloys containing Co
• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D6/00—Heat treatment of ferrous alloys
  • C21D6/02—Hardening by precipitation
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/004—Very low carbon steels, i.e. having a carbon content of less than 0,01%
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/02—Ferrous alloys, e.g. steel alloys containing silicon
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/04—Ferrous alloys, e.g. steel alloys containing manganese
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/06—Ferrous alloys, e.g. steel alloys containing aluminium
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/10—Ferrous alloys, e.g. steel alloys containing cobalt
  • C22C38/105—Ferrous alloys, e.g. steel alloys containing cobalt containing Co and Ni
• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D2211/00—Microstructure comprising significant phases
  • C21D2211/001—Austenite

Definitions

• 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.
• Thermally stable Invar alloys are widely used as component materials in electronics, semiconductor-related equipment, laser processing machines, and ultra-precision processing equipment.
• 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.
• 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.
• 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.
• austenite crystal grains tend to coarsen, resulting in inferior ultrasonic flaw detection properties compared to forged products.
• ultrasound scatters and attenuates at the grain boundaries it becomes difficult to detect defects using ultrasonic flaw detection, a non-destructive testing.
• 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.
• 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.
• C contributes to the increase in strength by dissolving in austenite.
• C dissolves in the matrix during the recrystallization treatment step and precipitates during cooling, increasing the coefficient of thermal expansion.
• 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 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 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 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.
• 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 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 contributes to decrease in the coefficient of thermal expansion by being combined with Ni.
• 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.
• [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 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.
• 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.
• the microstructure of the cast steel of the present invention is an austenitic structure with an average grain size of 200 ⁇ m 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.
• 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.
• 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.
• the cast steel of the present invention has an attenuation coefficient of the ultrasonic bottom echo of 2.0 dB/cm or less.
• 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.
• 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.
• the thermal expansion measuring device the NETZSCH DIL 402C can be used as the thermal expansion measuring device.
• 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.
• 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.
• 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.
• 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.
• 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.
• C, Si, Mn, Cu, Ni, Cr, and Mo show the content (mass%) of each element. Elements not contained are considered as 0.
• 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.
• 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.
• 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 200 ⁇ m 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 200 ⁇ m or less.
• 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.
• 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 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.

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• Crystallography & Structural Chemistry (AREA)
• Heat Treatment Of Steel (AREA)
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• 2024
  • 2024-04-11 JP JP2025515195A patent/JPWO2024219316A1/ja active Pending
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