JP2016027188A - Low-thermal expansion cast steel product and method for producing the same - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain a cast steel product with a low thermal expansion coefficient.SOLUTION: A low-thermal expansion cast steel product has a component composition (mass%) comprising C: 0.04% or less, Si: 0.3% or less, Mn: 0.5% or less, Al: 0.2% or less, Ni: 31-34%, and Co: 2-6%, with the balance being Fe and inevitable impurities, where the austenite structure has an average crystal grain size of 200 μm or less, and a thermal expansion coefficient at 18-28°C is 0.2×10/°C or less.SELECTED DRAWING: Figure 3

Description

  The present invention relates to a low thermal expansion cast steel product having a low thermal expansion coefficient and a method for producing the same.

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

Patent Document 1 includes Ni: 29.5 to 35%, Co: 2.0 to 7.0%, Cr: 0.001 to 2.0%, and a thermal expansion coefficient of 0.5 × 10 ^{−. 6 /℃~2.0×10 -6} / ℃ low thermal expansion alloy is disclosed. This alloy is obtained by quenching after homogeneous solution treatment, or by annealing at a rate of 1 ° C./sec or less and then performing a cold rolling process of 10% or more.

  Patent Document 2 discloses a low thermal expansion line containing Co: 65% or less, Ni: 30% or less, Cr: 10% or less, and a total content of Co and Ni of 25 to 65%. Patent Document 2 discloses a method of transforming a low thermal expansion line into a part of a work-induced martensite phase of an austenite phase by cold working.

  Patent Document 3 discloses a low thermal expansion alloy containing Ni: 0.03 to 1.5%, Ni and Co: 53 to 55%, and Cr: 9 to 10%. Patent Document 3 discloses a method of annealing an alloy at 650 to 900 ° C. and then cooling the alloy at a rate of less than 20 ° C./min in a furnace.

Japanese Patent No. 2796966 JP-A-6-279945 Japanese Patent No. 5534150

Casting Technology Course Editorial Committee (1967), Steel Castings, Nikkan Kogyo Shimbun, p. 3-32,

  In general, a cast steel product is used for a member having a complicated shape instead of machining or welding because of ease of manufacture. Cast steel products have the advantage of being easy to manufacture because any shape from small to large parts can be obtained by pouring molten metal into the mold.

  So far, as described above, various inventions have been made for invar alloys at the specimen level. However, no reports have been made on large cast steel products that show low values of thermal expansion coefficients for all parts, in particular, that have a weight of 10 kg or more. This is because in solidification with a mold, a temperature gradient is generated in a direction substantially perpendicular to the mold wall surface, and solidification proceeds from the wall surface to the center. This is thought to be because it is difficult to make the entire product uniform.

  This invention solves said problem and makes it a subject to provide the cast-steel goods which have a low thermal expansion coefficient in all the parts, and its manufacturing method, even if it is large sized so that a weight may be 10 kg or more.

  The present inventors have intensively studied a method for producing a cast steel product that exhibits a low coefficient of thermal expansion for all the parts even when the weight is particularly large such as 10 kg or more. As a result, part or most of the cast steel product after casting is forcibly cooled to the Ms point or less to transform it into martensite, and then heated again to austenite the martensite structure, and then cooled at a predetermined rate. Therefore, if it is a normal cast steel product, the structure which becomes an austenite crystal grain of the magnitude | size about 1-10 mm (for example, refer nonpatent literature 1) cannot be obtained by the normal solidification structure control over the whole cast steel product. As a result, it has been found that even if the cast steel product is large, a cast steel product having a low thermal expansion coefficient can be produced at any part. The present invention has been made based on the above findings, and the gist thereof is as follows.

(1) Component composition is mass%, C: 0.04% or less, Ni: 31-34% and Co: 2-6%, the balance is Fe and inevitable impurities, the average of austenite structure A low thermal expansion cast steel product having a crystal grain size of 200 μm or less and a thermal expansion coefficient at 18 to 28 ° C. of 0.2 × 10 ⁻⁶ / ° C. or less.

  (2) The low thermal expansion cast steel product according to (1), wherein the weight is 10 kg or more.

  (3) The low thermal expansion cast steel product according to (1) or (2) above, wherein the maximum thickness is 35 mm or more.

  (4) A cast steel product whose component composition is mass%, C: 0.04% or less, Ni: 31-34% and Co: 2-6%, the balance being Fe and unavoidable impurities, Ms A cryotreatment step of cooling to a temperature below the point, holding at 0.5 to 3 hr at a temperature below the Ms point, and then raising the temperature to room temperature, and heating the cast steel product subjected to the cryotreatment to 800 to 1100 ° C. Low heat, characterized by comprising a recrystallization treatment step for 5 to 5 hours and a cooling step for rapidly cooling the recast cast steel product at a cooling rate of 650 to 300 ° C. at 30 ° C./min or more. A method for producing an expanded cast steel product.

  (5) The low thermal expansion of (4) above, further comprising a solution treatment step of heating the cast steel product to 800 to 1100 ° C. and holding it for 0.5 to 5 hours before the cryo treatment step. Manufacturing method of cast steel products.

  (6) The method for producing a low-thermal-expansion cast steel product according to (3), further comprising a diffusion treatment step of holding the cast steel product at 1100 to 1300 ° C. for 5 to 50 hours before the cryotreatment step.

  (7) The method for producing a low-thermal-expansion cast steel product according to (5), further comprising a diffusion treatment step of holding the cast steel product at 1100 to 1300 ° C. for 5 to 50 hours before the solution treatment step.

  (8) The above (4) to (7), further comprising a tempering treatment step of holding a cast steel product at 300 to 400 ° C. for 1 to 10 hours between the cryotreatment step and the recrystallization treatment step. ) Any one of the methods for producing a low thermal expansion cast steel product.

  According to the present invention, by performing a predetermined heat treatment on the cast steel product after casting, the entire cast steel product has a structure with a small crystal grain size, so that a cast steel product having a low coefficient of thermal expansion can be obtained at all sites. Therefore, it can be applied to a large component that is thermally stable and requires a complicated shape.

It is an example of the structure | tissue after performing a cryo process to a cast steel product. It is an example of the structure | tissue which gave the recrystallization process to the cast steel article. It is an example of the structure | tissue after performing solution treatment to a cast steel product.

  Hereinafter, the present invention will be described in detail. Hereinafter, “%” regarding the component composition represents “% by mass” unless otherwise specified. First, the component composition of the cast steel product of the present invention will be described. In the present invention, particularly for large cast steel products, the component composition must be strictly controlled in order to make the thermal expansion coefficient of the entire cast steel product low.

  Ni is an element that lowers the thermal expansion coefficient. If the amount of Ni is too large or too small, the thermal expansion coefficient does not become sufficiently small. Moreover, when there is too much Ni amount, it will become difficult to produce a martensitic transformation by cooling. In order to make the thermal expansion coefficient low throughout the large cast steel product, it is necessary to control the amount of Ni within a narrow range. Specifically, Ni is set to a range of 31 to 34%.

  Co contributes to a decrease in the thermal expansion coefficient when combined with Ni. In order to obtain a desired thermal expansion coefficient, the range of Co is 2 to 6%.

C dissolves in austenite and contributes to an increase in strength. However, C forms a solid solution in the matrix in the recrystallization treatment process, precipitates during cooling, and increases the thermal expansion coefficient. In order to set the thermal expansion coefficient to 0.2 × 10 ⁻⁶ / ° C. or less, it is necessary to reduce the amount of precipitated C. Therefore, the amount is 0.04% or less.

  Si, Mn, and Al are added as deoxidizers. In the present invention, in order to produce a sound cast steel product without blowing, the minimum amount necessary for deoxidation may be added. Addition amounts are set to Si: 0.3% or less, Mn: 0.5% or less, and Al: 0.2% or less, respectively.

  The balance of the component composition is Fe and inevitable impurities. Inevitable impurities are those that are inevitably mixed rather than intentionally included in the steel from the raw materials and manufacturing environment, etc., when industrially manufacturing steel having the component composition specified in the present invention. Say.

  Specifically, S, P, Cu, Cr, etc. are mentioned. When these elements are inevitably mixed, the contents are P: 0.01% or less, S: 0.01% or less, Cu: 0.01% or less, and Cr: 0.01% or less. C, Si, Mn, and Al may be mixed as inevitable impurities even if not intended to be contained, but there is no problem as long as the content is within the above range.

  The structure of the cast steel product of the present invention is an austenite structure having an average particle size of 200 μm or less. The structure is centered on fine equiaxed crystals. It is not necessary for all the structures to be equiaxed crystals, but the proportion of equiaxed crystals is preferably 60% or more in terms of area ratio. The ratio of equiaxed crystals is more preferably 90% or more in terms of area ratio, and even more preferably 95% or more.

  In the case of a large cast steel product, it is important that all the structures of the cast steel product have an austenite structure with an average particle size of 200 μm or less. If there is a structure that does not satisfy this requirement, a low thermal expansion coefficient cannot be obtained for the entire cast steel product.

  Next, the manufacturing method of the low thermal expansion cast steel product of this invention is demonstrated.

  The mold used for producing the low thermal expansion cast steel product of the present invention, the apparatus for injecting molten steel into the mold, and the injection method are not particularly limited, and any known apparatus or method may be used. The structure of the cast steel product manufactured by the mold is a structure centered on columnar crystals. The cast steel product is subjected to the following heat treatment.

  First, a cast steel product is rapidly cooled to the Ms point or lower, held at a temperature lower than the Ms point for 0.5 to 3 hours, and then heated to room temperature (cry treatment process). The cooling method is not particularly limited. In general, the Ms point can be estimated by the following equation using steel components.

Ms = 521-353C-22Si-24.3Mn-7.7Cu-17.3Ni
-17.7Cr-25.8Mo
Here, C, Si, Mn, Cu, Ni, Cr, and Mo are content (mass%) of each element. The element not contained is 0.

  In the case of the component composition of the low thermal expansion cast steel product of the present invention, the Ms point calculated by the above formula is about −10 ° C. to −70 ° C., particularly depending on the amount of Ni. A method of immersing in methyl alcohol or ethyl alcohol, liquid nitrogen, or a method of spraying liquid nitrogen can be used. Accordingly, a structure containing fine martensite is formed. Further, the temperature increase may be performed by raising the temperature in the atmosphere at room temperature. In FIG. 1, the example of the structure | tissue after a cryo processing process is shown.

  Next, the cast steel product is reheated to 800 to 1100 ° C. and held at 800 to 1100 ° C. for 0.5 to 5 hours (recrystallization treatment step). Then, the cooling rate between 650-300 degreeC shall be 30 degrees C / min or more, and a cast steel product is cooled (cooling process). Thereby, the structure in which martensite is formed returns to the austenite structure. The crystal grain size of the structure formed by normal solidification is about 1 to 10 mm, but the austenite structure is equiaxed by passing through the cryo treatment step, the subsequent recrystallization treatment step, and the cooling step. It becomes a fine structure with an average particle diameter of 200 μm or less. FIG. 2 shows an example of the structure after the recrystallization treatment step.

In the present invention, the cooling rate of the recrystallization process is extremely important. In order to diligently examine the influence of the cooling rate, the inventors have embedded a thermocouple in a part of a large cast steel product and a part having a different thickness, and obtained the cooling rate by actually measuring the temperature during cooling, and the measurement part The thermal expansion coefficient of the test specimen collected from the sample was measured. As a result, when the chemical component is in the above range, the average particle size of the austenite structure is 200 μm or less, and the cooling rate between 650 to 300 ° C. is 30 ° C./min or more, the heat at 18 to 28 ° C. It has been found that the expansion coefficient can be 0.2 × 10 ⁻⁶ / ° C. or lower.

  Prior to the cryo treatment, a solution treatment step of heating the cast steel product to 800 to 1100 ° C., holding it for 0.5 to 5 hours, and quenching may be provided. Due to the solution treatment, precipitates deposited at the time of casting become a solid solution, and ductility and toughness are improved. FIG. 3 shows an example of the structure after the solution treatment. The structure at this stage is an austenite structure mainly composed of columnar crystals, as in a normal cast steel product.

  A diffusion treatment step of holding the cast steel product at 1100 to 1300 ° C. for 5 to 50 hours may be further provided before the cryotreatment step (before the solution treatment step when the solution treatment step is provided). This eliminates the segregation of Ni and impurities in the steel, and even a large-sized cast steel product can more stably produce a cast steel product having an extremely low coefficient of thermal expansion.

In order to make the recrystallized austenite crystal grains finer between the cryoprocessing step and the recrystallization processing step, 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 martensite may be tempered (tempering treatment step).

  The molten metal adjusted to have the component composition shown in Table 1 was poured into a mold to produce a cast steel product having the weight and maximum wall thickness shown in Table 1. It was confirmed that the contents of elements other than Fe, which are not described, are inevitable impurity levels.

For manufactured cast steel products,
(A) Cryo treatment → recrystallization treatment (b) Solution treatment → cry treatment → recrystallization treatment (c) Diffusion treatment → cry treatment → recrystallization treatment (d) Diffusion treatment → solution treatment → cry treatment → recrystallization treatment (E) Diffusion treatment → Solution treatment → Cryo treatment → Refining treatment → Recrystallization treatment (f) Cryo treatment → Refining treatment → Recrystallization treatment (g) Perform any one of the heat treatments of solution treatment, and finally A cast steel product was obtained.

  A test piece was sampled from the central portion of the wall thickness at each position of the manufactured cast steel product, subjected to the heat treatment, and each test was performed. The thermal expansion coefficient was obtained as an average thermal expansion coefficient of 18 to 28 ° C. using a thermal expansion measuring machine. The average crystal grain size of the austenite structure was determined as the average value of the equivalent circle diameters of the observed crystal grains. No. 3, no. 8 and no. For 16, measurements were taken at a plurality of positions. The results are shown in Table 1.

  As shown in Table 1, it was confirmed that the cast steel product of the example of the present invention had a small crystal grain size and a low thermal expansion coefficient was obtained over the entire cast steel product.

Claims (8)

Ingredient composition is mass%,
C: 0.04% or less,
Si: 0.3% or less,
Mn: 0.5% or less,
Al: 0.2% or less,
Ni: 31-34%, and Co: 2-6%
And the balance is Fe and inevitable impurities,
The average crystal grain size of the austenite structure is 200 μm or less,
A low thermal expansion cast steel product having a thermal expansion coefficient at 18 to 28 ° C. of 0.2 × 10 ⁻⁶ / ° C. or less.   The low thermal expansion cast steel product according to claim 1, wherein the weight is 10 kg or more.   3. The low thermal expansion cast steel product according to claim 1 or 2, wherein the maximum thickness is 35 mm or more. Ingredient composition is mass%,
C: 0.04% or less,
Si: 0.3% or less,
Mn: 0.5% or less,
Al: 0.2% or less,
Ni: 31-34%, and Co: 2-6%
A cryoprocessing step of cooling the cast steel product containing Fe and the inevitable impurities to the Ms point or less, holding the solution at a temperature of the Ms point or less for 0.5 to 3 hours, and then raising the temperature to room temperature;
A recrystallization treatment step in which the cryotreated cast steel product is heated to 800 to 1100 ° C. and held for 0.5 to 5 hours;
A method for producing a low-thermal-expansion cast steel product comprising a cooling step of rapidly cooling a recast-treated cast steel product at a cooling rate of 650 to 300 ° C at 30 ° C / min or higher.   5. The low thermal expansion cast steel product according to claim 4, further comprising a solution treatment step of heating the cast steel product to 800 to 1100 ° C. and holding for 0.5 to 5 hours before the cryotreatment step. Manufacturing method.   The method for producing a low-thermal-expansion cast steel product according to claim 3, further comprising a diffusion treatment step of holding the cast steel product at 1100 to 1300 ° C for 5 to 50 hours before the cryotreatment step.   The method for producing a low thermal expansion cast steel product according to claim 5, further comprising a diffusion treatment step of holding the cast steel product at 1100 to 1300 ° C for 5 to 50 hours before the solution treatment step.   The tempering process which hold | maintains a cast steel product for 1 to 10 hours at 300-400 degreeC is further provided between the said cryo process process and the said recrystallization process process, The any one of Claims 4-7 characterized by the above-mentioned. A method for producing a low thermal expansion cast steel product according to claim 1.