JP2024047300A - Low thermal expansion alloy production method and hot-forged component - Google Patents

Abstract

To provide a low thermal expansion alloy production method which can sufficiently reduce crystal grains while improving workability upon hot working.SOLUTION: A low thermal expansion alloy production method has: a step of hot-working a low thermal expansion alloy 1 which comprises an Fe-Ni alloy or an Fe-Ni-Co alloy; and a step of refining crystal grains by subjecting the hot-worked low thermal expansion alloy to reheating treatment. It is preferable that the hot working step is executed under the temperature conditions of 950°C or more to 1,120°C or less, and the crystal grain refining step is executed under the temperature conditions of 800°C or more to 1,000°C or less.SELECTED DRAWING: Figure 1

Description

The present invention relates to a manufacturing method for low thermal expansion alloys and hot forged parts.

Conventionally, methods for manufacturing low thermal expansion alloys are known (for example, Patent Document 1).

The above-mentioned Patent Document 1 discloses a manufacturing method for producing a low thermal expansion alloy by producing an ingot from molten steel (iron-based alloy) containing Ni, Co, and Fe, and hot working the produced ingot. The hot working is hot forging.

JP 2017-172045 A

Although not disclosed in the above Patent Document 1, according to Hall-Petch's law, the smaller the crystal grains, the greater the yield stress (tensile strength) of an iron-based alloy. For this reason, it is desirable to make the crystal grains smaller in order to increase the yield stress. On the other hand, it is known that dynamic recrystallization occurs when the temperature at which an iron-based alloy is hot worked is higher, resulting in larger crystal grains.

Therefore, the hot working temperature has been lowered to make the crystal grains smaller. However, lowering the hot working temperature causes the iron-based alloy to not be softened sufficiently, which reduces the workability during hot working. In addition, studies have been conducted to increase the processing rate during hot working while lowering the hot working temperature in order to make the crystal grains smaller, but there is a problem in that cracks occur during hot working when the processing rate is increased.

This invention was made to solve the above problems, and one of the objects of this invention is to provide a method for manufacturing a low thermal expansion alloy that can make the crystal grains sufficiently small while improving the workability during hot working.

In order to achieve the above object, the inventors of the present application conducted extensive research and discovered that the crystal grains can be made smaller by performing a reheating treatment after hot working. That is, the method for producing a low thermal expansion alloy in a first aspect of the present invention comprises a step of hot working a low thermal expansion alloy including an Fe-Ni alloy or an Fe-Ni-Co alloy, and a step of reheating the hot worked low thermal expansion alloy to make the crystal grains finer.

In the manufacturing method of a low thermal expansion alloy according to the first aspect of the present invention, as described above, the process includes a step of reheating the hot-worked low thermal expansion alloy to refine the crystal grains. This allows the crystal grains to be made sufficiently small. Furthermore, by including a step of refining the crystal grains after the hot working step, it is not necessary to make the crystal grains small during hot working, and therefore it is not necessary to lower the hot working temperature to make the crystal grains small. This allows the temperature during hot working to be set at a temperature that allows for improved workability. As a result, the crystal grains can be made sufficiently small while improving workability during hot working.

In the manufacturing method of the low thermal expansion alloy according to the first aspect, the step of refining the crystal grains is preferably carried out at a temperature lower than that of the hot working step. In this manner, the reheating temperature is low, so that the crystal grains can be easily made sufficiently small.

In the manufacturing method of the low thermal expansion alloy according to the first aspect, the Fe-Ni-Co alloy preferably contains 28% by mass to 30% by mass of Ni and 15% by mass to 17% by mass of Co. The inventors of the present application have learned from the experiments (examples) described below that by performing hot working and reheating treatment (crystal refinement treatment) on an Fe-Ni-Co alloy having such a composition, it is possible to refine the crystal grains while improving the workability during hot working.

In this case, the hot working step is preferably carried out at a temperature of 950°C to 1120°C, and the grain refinement step is carried out at a temperature of 800°C to 1000°C. The inventors of the present application have learned from the experiments (examples) described below that by carrying out hot working and reheating treatment (grain refinement treatment) under such temperature conditions, it is possible to refine the grains while improving the workability during hot working.

In the manufacturing method of the low thermal expansion alloy in which the hot working step is carried out under a temperature condition of 950°C or more and 1120°C or less, the grain refinement step is preferably carried out under a temperature condition of 800°C or more and less than 950°C. The inventors of the present application have learned from the experiments (examples) described below that by configuring in this way, the grains can be further refined.

In the manufacturing method of the low thermal expansion alloy in which the above-mentioned grain refinement process is carried out under a temperature condition of 800°C to 1000°C, the grain refinement process is preferably carried out under a time condition of 2 hours to 8 hours. The inventors of the present application have learned from the experiments (examples) described below that the grains can be made smaller by carrying out a reheating treatment (grain refinement process) under such time conditions.

In this case, the process of refining the crystal grains is preferably carried out at a temperature of 800°C or higher and lower than 850°C for a time period of 2 hours or higher and 4 hours or lower. The inventors of this application have learned from the experiments (examples) described below that this configuration makes it possible to further reduce the crystal grains.

In the manufacturing method of the low thermal expansion alloy according to the first aspect, the average grain size of the low thermal expansion alloy is preferably adjusted to 100 μm or less by a grain refinement process. With this configuration, the yield stress can be increased due to the small grain size.

In this case, the average grain size of the low thermal expansion alloy is preferably adjusted to 50 μm or less by a process of refining the grains. By configuring in this way, the grains can be made even smaller.

In the manufacturing method of the low thermal expansion alloy according to the first aspect, the Vickers hardness of the low thermal expansion alloy is preferably adjusted to 160 HV or less by a process of refining the crystal grains. The inventors of this application have learned from the experiments (examples) described below that in such low thermal expansion alloys, the processing strain during hot treatment is eliminated by reheating treatment, resulting in small crystal grains.

In the manufacturing method of the low thermal expansion alloy according to the first aspect, the hot working step preferably includes a step of hot forging the low thermal expansion alloy, and the grain refinement step involves reheating the hot forged low thermal expansion alloy to form a hot forged part with refined grains. This configuration makes it possible to manufacture a hot forged part with a small average grain size and a large yield stress.

The hot forged part in the second aspect of the present invention is made of a low thermal expansion alloy, including an Fe-Ni alloy or an Fe-Ni-Co alloy, whose average crystal grain size is adjusted to 100 μm or less.

In the hot forged part according to the second aspect of the present invention, as described above, the average grain size of the crystal grains of the hot forged part is adjusted to 100 μm or less, so that the yield stress can be increased.

According to the present invention, as described above, it is possible to provide a method for producing a low thermal expansion alloy that can improve workability during hot working while making the crystal grains sufficiently small.

1A to 1C are diagrams for explaining a method for manufacturing a low thermal expansion alloy according to an embodiment. 1 is a photograph of the Fe—Ni—Co alloy of Example 1. 1 is a photograph of the Fe—Ni—Co alloy of Example 2. 1 is a photograph of the Fe—Ni—Co alloy of Example 3. 1 is a photograph of the Fe—Ni—Co alloy of Example 4. 1 is a photograph of the Fe—Ni—Co alloy of Example 5. 1 is a photograph of the Fe—Ni—Co alloy of Example 6. 1 is a photograph of the Fe—Ni—Co alloy of Example 7. 1 is a photograph of the Fe—Ni—Co alloy of Example 8. 1 is a photograph of the Fe—Ni—Co alloy of Example 9. 1 is a photograph of the Fe—Ni—Co alloy of Comparative Example 1. 1 is a photograph of the Fe—Ni—Co alloy of Comparative Example 2. 1 is a photograph of the Fe—Ni—Co alloy of Comparative Example 3. 1 is a photograph of the Fe—Ni—Co alloy of Comparative Example 4. 1 is a photograph of the Fe—Ni—Co alloy of Comparative Example 5. 1 is a photograph of the Fe—Ni—Co alloy of Comparative Example 6. 1 is a photograph of the Fe—Ni—Co alloy of Comparative Example 7. 1 is a photograph of the Fe—Ni—Co alloy of Comparative Example 8. 1 is a photograph of the Fe—Ni—Co alloy of Comparative Example 9. 1 is a photograph of the Fe—Ni—Co alloy of Comparative Example 10. 1 is a photograph of the Fe—Ni—Co alloy of Example 10. 1 is a photograph of the Fe—Ni—Co alloy of Example 11. 1 is a photograph of the Fe—Ni—Co alloy of Example 12. 1 is a photograph of the Fe—Ni—Co alloy of Example 13. 1 is a photograph of the Fe—Ni—Co alloy of Example 14. 1 is a photograph of the Fe—Ni—Co alloy of Example 15. 1 is a photograph of the Fe—Ni—Co alloy of Example 16. 1 is a photograph of the Fe—Ni—Co alloy of Example 17. 1 is a photograph of the Fe—Ni—Co alloy of Example 18. 1 is a photograph of the Fe—Ni—Co alloy of Example 19. 1 is a photograph of the Fe—Ni—Co alloy of Example 20. 1 is a photograph of the Fe—Ni—Co alloy of Example 21. 1 is a photograph of the Fe—Ni—Co alloy of Example 22. 1 is a photograph of the Fe—Ni—Co alloy of Example 23.

One embodiment of the present invention will be described below with reference to the drawings.

[Embodiment]
(Low thermal expansion alloy)
As shown in Fig. 1, the low thermal expansion alloy 1 constitutes a hot forged part 10. In other words, the hot forged part 10 is manufactured using the low thermal expansion alloy 1. The hot forged part 10 is used as a hermetic sealing member or a transport member for transporting a semiconductor. Furthermore, the hot forged part 10 of this embodiment is brazed to another member by a brazing material.

The low thermal expansion alloy 1 in this embodiment includes an Fe-Ni alloy or an Fe-Ni-Co alloy.

An example of an Fe-Ni alloy is the so-called 42 alloy, which contains 42% by weight of Ni and the remainder is Fe. Another example of an Fe-Ni alloy is the so-called Invar, which contains 36% by weight of Ni and the remainder is Fe, or the so-called Super Invar, which contains 32% by weight of Ni and 4% by weight of Co and the remainder is Fe.

The Fe-Ni-Co alloy contains, for example, 28% by mass to 30% by mass of Ni and 15% by mass to 17% by mass of Co. The Fe-Ni-Co alloy is, for example, ASTM standard F15-95 (ASTM F15-95), which is composed of 29% by mass of Ni, 17% by mass of Co, trace elements, and the balance being iron.

(Method of manufacturing low thermal expansion alloy)
As shown in FIG. 1, the method for producing a low thermal expansion alloy 1 of this embodiment includes a step of hot working the low thermal expansion alloy 1 and a step of reheating the alloy to refine the crystal grains.

The process of hot working the low thermal expansion alloy 1 is a process of forging the prepared low thermal expansion alloy 1 while heating it. The temperature condition for heating during hot working is 950°C or higher and 1120°C or lower.

The process of refining the crystal grains is carried out while reheating the hot-worked low thermal expansion alloy 1 in a furnace. The temperature condition for reheating is 800°C or higher and 1000°C or lower, and in order to further refine the crystal grains, it is preferable for the temperature condition for reheating to be 800°C or higher and lower than 950°C. It is preferable for the temperature condition for reheating to be lower than the temperature condition for hot working.

The grain refinement process is carried out by reheating for a time period of 2 hours or more and 8 hours or less. In order to make the grains smaller, the grain refinement process is more preferably carried out by reheating for a time period of 2 hours or more and 4 hours or less.

The average grain size of the low thermal expansion alloy 1 is adjusted to preferably 100 μm or less, and more preferably 50 μm or less, by a process of refining the grains, in order to increase the yield stress (mechanical strength). The grain size of the grains can be made smaller as the reheating temperature is lower. In other words, according to the Hall-Petch law, which states that the smaller the grain size of the grains, the greater the yield stress, the lower the reheating temperature, the greater the yield stress and the improved mechanical strength.

The Vickers hardness of the low thermal expansion alloy 1 is adjusted to 160 HV or less by a process of refining the crystal grains. The higher the reheating temperature, the smaller the Vickers hardness can be. It is believed that the crystal grains of the hot-worked low thermal expansion alloy 1 become larger due to the phenomenon of dynamic recrystallization, and processing strain accumulates. Then, by reheating, the processing strain is removed.

After the grain refinement process, the low thermal expansion alloy 1 is cooled. The cooling method may be water cooling by immersion in water, furnace cooling by natural cooling in the furnace, or air cooling by removing from the furnace and cooling.

The low thermal expansion alloy 1 may be used as the hot forged part 10 as it is, or may be subjected to bending or the like after cooling to form the hot forged part 10, for example.
(Effects of the embodiment)
In this embodiment, the following effects can be obtained.

In this embodiment, as described above, a process for refining the crystal grains is provided by reheating the hot-worked low thermal expansion alloy 1. This allows the crystal grains to be made sufficiently small. Furthermore, by providing a process for refining the crystal grains after the hot working process, it is not necessary to make the crystal grains small during hot working, and therefore it is not necessary to lower the hot working temperature to make the crystal grains small. This allows the temperature during hot working to be set at a temperature that allows for improved workability. As a result, the crystal grains can be made sufficiently small while improving workability during hot working.

In this embodiment, as described above, the process of refining the crystal grains is carried out at a temperature lower than that of the hot working process. This allows the crystal grains to be easily made sufficiently small because the reheating temperature is low.

In this embodiment, the Fe-Ni-Co alloy contains 28% by mass to 30% by mass of Ni and 15% by mass to 17% by mass of Co. The inventors of this application have learned from the experiments (examples) described below that by performing hot working and reheating treatment (crystal refinement treatment) on an Fe-Ni-Co alloy having such a composition, it is possible to refine the crystal grains while improving the workability during hot working.

In this embodiment, as described above, the hot working process is performed at a temperature of 950°C to 1120°C, and the grain refinement process is performed at a temperature of 800°C to 1000°C. The inventors of this application have learned from the experiments (examples) described below that by performing hot working and reheating treatment (grain refinement treatment) under such temperature conditions, it is possible to refine the grains while improving the workability during hot working.

In this embodiment, as described above, the process of refining the crystal grains is carried out at a temperature of 800°C or higher and lower than 950°C. The inventors of the present application have learned from the experiments (examples) described below that this makes it possible to further refine the crystal grains.

In this embodiment, as described above, the process of refining the crystal grains is carried out for a time period of 2 hours to 8 hours. The inventors of this application have learned from the experiments (examples) described below that the crystal grains can be made smaller by carrying out the reheating treatment (crystal refinement treatment) under such time conditions.

In this embodiment, as described above, the process of refining the crystal grains is carried out at a temperature of 800°C or higher and lower than 850°C for a time period of 2 hours or higher and 4 hours or lower. The inventors of this application have learned from the experiments (examples) described below that this makes it possible to further reduce the crystal grains.

In this embodiment, as described above, the average grain size of the low thermal expansion alloy 1 is adjusted to 100 μm or less by the process of refining the grains. This allows the yield stress to be increased due to the small grains.

In this embodiment, as described above, the average grain size of the low thermal expansion alloy 1 is adjusted to 50 μm or less by the process of refining the grains. This allows the grains to be made even smaller.

In this embodiment, as described above, the Vickers hardness of the low thermal expansion alloy 1 is adjusted to 160 HV or less by the process of refining the crystal grains. The inventors of this application have learned from the experiments (examples) described below that in such a low thermal expansion alloy 1, the processing strain during hot treatment is eliminated by reheating treatment, and the crystal grains become small.

In this embodiment, as described above, the hot working process includes a process of hot forging the low thermal expansion alloy 1, and the grain refinement process involves reheating the hot forged low thermal expansion alloy 1 to form a hot forged part 10 with refined grains. This allows the manufacture of a hot forged part 10 with a small average grain size and a large yield stress.

In this embodiment, as described above, the average grain size of the crystal grains of the hot forged part 10 is adjusted to 100 μm or less, so the yield stress can be increased.

[Example]
(Relationship between temperature conditions and time during reheating)
The inventors of the present application forged and hot worked a low thermal expansion alloy Fe-Ni-Co alloy (ASTM F15-95) at 1080°C in order to obtain the optimal temperature conditions and reheating time during reheating. The forging ratio (working rate) was 2. The hot worked Fe-Ni-Co alloy was reheated under the conditions shown in Table 1 and water-cooled to produce an Fe-Ni-Co alloy. In addition, a number of samples were taken from the produced Fe-Ni-Co alloy, and the average grain size and Vickers hardness were measured. The hot working conditions, reheating temperature conditions, average grain size, and Vickers hardness of Examples 1 to 9 and Comparative Examples 1 to 10 are shown in Table 1 below. The mixed grain state shown in Table 1 refers to a state in which crystal grains with a minimum grain size of 50 μm or less and crystal grains with a maximum grain size of more than 200 μm are mixed together.

[Table 1]

In Example 1, reheating was performed at 1000°C for 2 hours. In Example 2, reheating was performed at 1000°C for 4 hours. In Example 3, reheating was performed at 1000°C for 8 hours. In Example 4, reheating was performed at 900°C for 2 hours. In Example 5, reheating was performed at 850°C for 2 hours. In Example 6, reheating was performed at 850°C for 4 hours. In Example 7, reheating was performed at 800°C for 2 hours. In Example 8, reheating was performed at 800°C for 4 hours. In Example 9, reheating was performed at 800°C for 8 hours.

In Comparative Example 1, reheating was not performed. In Comparative Example 2, reheating was performed at 750°C for 2 hours. In Comparative Example 3, reheating was performed at 750°C for 4 hours. In Comparative Example 4, reheating was performed at 750°C for 8 hours. In Comparative Example 5, reheating was performed at 700°C for 2 hours. In Comparative Example 6, reheating was performed at 650°C for 2 hours. In Comparative Example 7, reheating was performed at 600°C for 2 hours. In Comparative Example 8, reheating was performed at 500°C for 2 hours. In Comparative Example 9, reheating was performed at 500°C for 4 hours. In Comparative Example 10, reheating was performed at 500°C for 8 hours.

Figure 2 is a photograph of a sample taken from the Fe-Ni-Co alloy of Example 1, magnified 150 times. The Fe-Ni-Co alloy of Example 1 had an average grain size of 150 μm and a Vickers hardness of 152.6 HV. The crystal grains were approximately uniform in size and were not in a mixed grain state.

Figure 3 is a photograph of a sample taken from the Fe-Ni-Co alloy of Example 2, magnified 150 times. The Fe-Ni-Co alloy of Example 2 had an average grain size of 200 μm and a Vickers hardness of 151.7 HV. The crystal grains were approximately uniform in size and were not in a mixed grain state.

Figure 4 is a photograph of a sample taken from the Fe-Ni-Co alloy of Example 3, magnified 150 times. The Fe-Ni-Co alloy of Example 3 had an average grain size of 200 μm and a Vickers hardness of 146.4 HV. The crystal grains were approximately uniform in size and were not in a mixed grain state.

Figure 5 is a photograph of a sample taken from the Fe-Ni-Co alloy of Example 4, enlarged 150 times. The Fe-Ni-Co alloy of Example 4 had an average grain size in the range of 60 μm to 70 μm, and a Vickers hardness of 151.8 HV. The crystal grain size was also approximately uniform, and the sample was not in a mixed grain state.

Figure 6 is a photograph of a sample taken from the Fe-Ni-Co alloy of Example 5, magnified 150 times. The Fe-Ni-Co alloy of Example 5 had an average grain size of 60 μm and a Vickers hardness of 150 HV. The crystal grains were approximately uniform in size and were not in a mixed grain state.

Figure 7 is a photograph of a sample taken from the Fe-Ni-Co alloy of Example 6, magnified 150 times. The Fe-Ni-Co alloy of Example 6 had an average grain size of 70 μm and a Vickers hardness of 147 HV. The crystal grains were approximately uniform in size and were not in a mixed grain state.

Figure 8 is a photograph of a sample taken from the Fe-Ni-Co alloy of Example 7, magnified 150 times. The Fe-Ni-Co alloy of Example 7 had an average grain size of 45 μm and a Vickers hardness of 159.6 HV. The crystal grains were approximately uniform in size and were not in a mixed grain state.

Figure 9 is a photograph of a sample taken from the Fe-Ni-Co alloy of Example 8, enlarged 150 times. The Fe-Ni-Co alloy of Example 8 had an average grain size in the range of 45 μm to 50 μm, and a Vickers hardness of 151 HV. The crystal grains were approximately uniform in size and were not in a mixed grain state.

Figure 10 is a photograph of a sample taken from the Fe-Ni-Co alloy of Example 9, enlarged 150 times. The Fe-Ni-Co alloy of Example 9 had an average grain size in the range of 45 μm to 50 μm, and a Vickers hardness of 149 HV. The crystal grains were approximately uniform in size and were not in a mixed grain state.

Figure 11 is a photograph of a portion of the Fe-Ni-Co alloy of Comparative Example 1, magnified 150 times. The Fe-Ni-Co alloy of Comparative Example 1 was in a mixed grain state, with crystal grains having a minimum grain size of 50 μm or less and crystal grains having a maximum grain size of more than 200 μm. The Vickers hardness was 183.8 HV.

Figure 12 is a photograph of a sample taken from the Fe-Ni-Co alloy of Comparative Example 2, enlarged 150 times. The Fe-Ni-Co alloy of Comparative Example 2 had a mixed grain state, with crystal grains having a minimum grain size of 50 μm or less and crystal grains having a maximum grain size of more than 200 μm. The Vickers hardness was 161 HV.

Figure 13 is a photograph of a sample taken from the Fe-Ni-Co alloy of Comparative Example 3, enlarged 150 times. The Fe-Ni-Co alloy of Comparative Example 3 had a mixed grain state, with crystal grains having a minimum grain size of 50 μm or less and crystal grains having a maximum grain size of more than 200 μm. The Vickers hardness was 154 HV.

Figure 14 is a photograph of a sample taken from the Fe-Ni-Co alloy of Comparative Example 4, enlarged 150 times. The Fe-Ni-Co alloy of Comparative Example 4 had a mixed grain state, with crystal grains having a minimum grain size of 50 μm or less and crystal grains having a maximum grain size of more than 200 μm. The Vickers hardness was 147 HV.

Figure 15 is a photograph of a sample taken from the Fe-Ni-Co alloy of Comparative Example 5, enlarged 150 times. The Fe-Ni-Co alloy of Comparative Example 5 had a mixed grain state, with crystal grains having a minimum grain size of 50 μm or less and crystal grains having a maximum grain size of more than 200 μm. The Vickers hardness was 159 HV.

Figure 16 is a photograph of a sample taken from the Fe-Ni-Co alloy of Comparative Example 6, enlarged 150 times. The Fe-Ni-Co alloy of Comparative Example 6 had a mixed grain state, with crystal grains having a minimum grain size of 50 μm or less and crystal grains having a maximum grain size of more than 200 μm. The Vickers hardness was 169 HV.

Figure 17 is a photograph of a sample taken from the Fe-Ni-Co alloy of Comparative Example 7, enlarged 150 times. The Fe-Ni-Co alloy of Comparative Example 7 had a mixed grain state, with crystal grains having a minimum grain size of 50 μm or less and crystal grains having a maximum grain size of more than 200 μm. The Vickers hardness was 167 HV.

Figure 18 is a photograph of a sample taken from the Fe-Ni-Co alloy of Comparative Example 8, enlarged 150 times. The Fe-Ni-Co alloy of Comparative Example 8 had a mixed grain state, with crystal grains having a minimum grain size of 50 μm or less and crystal grains having a maximum grain size of more than 200 μm. The Vickers hardness was 164 HV.

Figure 19 is a photograph of a sample taken from the Fe-Ni-Co alloy of Comparative Example 9, enlarged 150 times. The Fe-Ni-Co alloy of Comparative Example 9 had a mixed grain state, with crystal grains having a minimum grain size of 50 μm or less and crystal grains having a maximum grain size of more than 200 μm. The Vickers hardness was 167 HV.

Figure 20 is a photograph of a sample taken from the Fe-Ni-Co alloy of Comparative Example 10, enlarged 150 times. The Fe-Ni-Co alloy of Comparative Example 10 had a mixed grain state, with crystal grains having a minimum grain size of 50 μm or less and crystal grains having a maximum grain size of more than 200 μm. The Vickers hardness was 164 HV.

From the above results, the inventors of the present application have found that by reheating an Fe-Ni-Co alloy in which some of the crystal grains have become coarse due to heating during hot working at a temperature of 800°C or higher for 2 to 8 hours, the grain size of the crystal grains can be made uniform rather than mixed. The inventors of the present application have also found that by setting the reheating temperature to 1000°C or lower, preferably less than 950°C, and more preferably less than 850°C, the crystal grains can be made small, less than 100 μm. Based on this, the inventors of the present application have found that, contrary to the conventional understanding that reheating promotes the coarsening of crystal grains based on the phenomenon of dynamic recrystallization in which crystal grains become coarse due to hot working, it is possible to make the crystal grains smaller by reheating. The inventors of the present application have also found that the longer the time for reheating, the larger the crystal grains become, so that it is preferable to perform reheating for a time condition of 2 to 4 hours. The inventors of the present application have also found that the higher the temperature for reheating, the lower the Vickers hardness. This suggests that processing strain accumulated during hot processing, but that reheating removed the processing strain, resulting in smaller crystal grains.

(Reheating at different temperatures)
The inventors of the present application produced Examples 10, 11 and 12 by reheating the Fe-Ni-Co alloys of Comparative Examples 5, 7 and 8 under the reheating temperature conditions of the present embodiment. Specifically, the Fe-Ni-Co alloys of Comparative Examples 5, 7 and 8 were further overheated at 800°C for 2 hours to produce Examples 10, 11 and 12. In addition, a plurality of samples were taken from the produced Fe-Ni-Co alloys, and the average grain size and Vickers hardness were measured for each of the plurality of samples. The temperature conditions during hot working, the temperature conditions of the first reheating, the time conditions of the first reheating, the temperature conditions of the second reheating, the time conditions of the second reheating, the average grain size and the Vickers hardness of Examples 10 to 12 are shown in Table 2 below.

[Table 2]

Figure 21 is a photograph of a sample taken from the Fe-Ni-Co alloy of Example 10, enlarged 150 times. In Example 10, the average grain size was 35 μm, and no crystal grains larger than 200 μm were observed. The size of the crystal grains was also approximately uniform, and they were not in a mixed grain state. The Vickers hardness was also 154 HV.

Figure 22 is a photograph of a sample taken from the Fe-Ni-Co alloy of Example 11, enlarged 150 times. In Example 11, the average grain size was 45 μm, and no crystal grains larger than 200 μm were observed. The size of the crystal grains was also approximately uniform, and they were not in a mixed grain state. The Vickers hardness was 153 HV.

Figure 23 is a photograph of a sample taken from the Fe-Ni-Co alloy of Example 12, enlarged 150 times. In Example 12, the average grain size was 50 μm, and no crystal grains larger than 200 μm were observed. The size of the crystal grains was also approximately uniform, and they were not in a mixed grain state. The Vickers hardness was 153 HV.

From the above results, the inventors of the present application found that when reheating is performed at a temperature below 800°C, the crystal grains can be refined by further reheating at a temperature of 800°C or higher.

(Hot working temperature)
In order to obtain the optimum temperature conditions for hot working, the inventors of the present application produced Fe-Ni-Co alloys by reheating and cooling Fe-Ni-Co alloys that had been hot worked under different temperature conditions. In addition, a number of samples were taken from the inside near the center of the produced Fe-Ni-Co alloys and from the outside near the outer periphery, and the average grain size was measured for each of the samples. The temperature conditions for hot working, the temperature conditions for reheating, the time conditions for reheating, and the average grain size are shown in Table 3 below.

[Table 3]

In Example 14, after hot working at 1120°C, reheating was performed at 800°C for 4 hours. In Example 15, after hot working at 1120°C, reheating was performed at 800°C for 8 hours. In Example 16, after hot working at 1080°C, reheating was performed at 800°C for 2 hours. In Example 17, after hot working at 1080°C, reheating was performed at 800°C for 4 hours. In Example 18, after hot working at 1080°C, reheating was performed at 800°C for 8 hours. In Example 19, after hot working at 1000°C, reheating was performed at 800°C for 2 hours. In Example 20, after hot working at 1000°C, reheating was performed at 800°C for 4 hours. In Example 21, after hot working at 1000°C, reheating was performed at 800°C for 8 hours. In Example 22, after hot working at 950°C, reheating was performed at 800°C for 2 hours. In Example 23, after hot working at 950°C, reheating was performed at 800°C for 4 hours. In Example 24, after hot working at 950°C, reheating was performed at 800°C for 8 hours.

Figure 24 is a photograph of a sample taken from the Fe-Ni-Co alloy of Example 14, enlarged 150 times. In the Fe-Ni-Co alloy of Example 14, the average grain size was 60 μm. In addition, the crystal grain size was approximately uniform and was not in a mixed grain state.

Figure 25 is a photograph of a sample taken from the Fe-Ni-Co alloy of Example 15, enlarged 150 times. In the Fe-Ni-Co alloy of Example 15, the average grain size was 60 μm. In addition, the crystal grain size was approximately uniform and was not in a mixed grain state.

Figure 26 is a photograph of a sample taken from the Fe-Ni-Co alloy of Example 16, enlarged 150 times. In the Fe-Ni-Co alloy of Example 16, the average grain size was 45 μm. In addition, the crystal grain size was approximately uniform and was not in a mixed grain state.

Figure 27 is a photograph of a sample taken from the Fe-Ni-Co alloy of Example 17, enlarged 150 times. In the Fe-Ni-Co alloy of Example 17, the average grain size was 45 μm or more and 50 μm or less. In addition, the crystal grain size was approximately uniform and was not in a mixed grain state.

Figure 28 is a photograph of a sample taken from the Fe-Ni-Co alloy of Example 18, enlarged 150 times. In the Fe-Ni-Co alloy of Example 18, the average grain size was 45 μm or more and 50 μm or less. In addition, the crystal grain size was approximately uniform and was not in a mixed grain state.

Figure 29 is a photograph of a sample taken from the Fe-Ni-Co alloy of Example 19, enlarged 150 times. In the Fe-Ni-Co alloy of Example 19, the average grain size was 45 μm. In addition, the crystal grain size was approximately uniform and was not in a mixed grain state.

Figure 30 is a photograph of a sample taken from the Fe-Ni-Co alloy of Example 20, enlarged 150 times. In the Fe-Ni-Co alloy of Example 20, the average grain size was 45 μm or more and 50 μm or less. In addition, the crystal grain size was approximately uniform and was not in a mixed grain state.

Figure 31 is a photograph of a sample taken from the Fe-Ni-Co alloy of Example 21, enlarged 150 times. In the Fe-Ni-Co alloy of Example 21, the average grain size was 45 μm or more and 50 μm or less. In addition, the crystal grain size was approximately uniform and was not in a mixed grain state.

Figure 32 is a photograph of a sample taken from the Fe-Ni-Co alloy of Example 22, magnified 150 times. In the Fe-Ni-Co alloy of Example 22, the average grain size was 35 μm. In addition, the crystal grain size was approximately uniform and was not in a mixed grain state.

Figure 33 is a photograph of a sample taken from the Fe-Ni-Co alloy of Example 23, enlarged 150 times. In the Fe-Ni-Co alloy of Example 23, the average grain size was 45 μm. In addition, the crystal grain size was approximately uniform and was not in a mixed grain state.

Figure 34 is a photograph of a sample taken from the Fe-Ni-Co alloy of Example 24, enlarged 150 times. In the Fe-Ni-Co alloy of Example 24, the average grain size was 45 μm or more and 50 μm or less. In addition, the crystal grain size was approximately uniform and was not in a mixed grain state.

From the above results, the inventors of the present application have discovered that the grains can be made smaller by performing the hot working process at a temperature of 950°C or higher and 1120°C or lower, and by performing the reheating and grain refinement process at a temperature of 800°C or higher and 1000°C or lower.

(Modification)
The embodiments and examples disclosed herein should be considered to be illustrative and not restrictive in all respects. The scope of the present invention is indicated by the claims, not by the description of the embodiments and examples described above, and further includes all modifications (variations) within the meaning and scope of the claims.

For example, although an example has been shown in which the process of refining the crystal grains is performed under a lower temperature condition than the hot working process, the present invention is not limited to this. In the present invention, the process of refining the crystal grains may be performed under the same temperature condition as the hot working process or under a higher temperature condition than the hot working temperature.

In the above embodiment, the process of refining the crystal grains is performed for 2 hours or more and 8 hours or less, but the present invention is not limited to this. In the present invention, the process of refining the crystal grains may be performed for a time period of less than 2 hours, or may be performed for a time period of more than 8 hours.

1. Low thermal expansion alloy 10. Hot forged parts

Claims (12)

hot working a low thermal expansion alloy, the low thermal expansion alloy comprising an Fe—Ni alloy or an Fe—Ni—Co alloy;
and reheating the hot-worked low thermal expansion alloy to refine the crystal grains. The method for producing a low thermal expansion alloy according to claim 1, wherein the grain refinement step is carried out at a temperature lower than that of the hot working step. The method for producing a low thermal expansion alloy according to claim 1 or 2, wherein the Fe-Ni-Co alloy contains 28% by mass or more and 30% by mass or less of Ni and 15% by mass or more and 17% by mass or less of Co. The hot working step is carried out at a temperature of 950° C. or more and 1120° C. or less,
The method for producing a low thermal expansion alloy according to claim 3, wherein the step of refining the crystal grains is carried out under a temperature condition of 800°C or higher and 1000°C or lower. The method for producing a low thermal expansion alloy according to claim 4, wherein the process for refining the crystal grains is carried out at a temperature of 800°C or higher and less than 950°C. The method for producing a low thermal expansion alloy according to claim 4, wherein the process for refining the crystal grains is carried out for a time period of 2 hours or more and 8 hours or less. The method for producing a low thermal expansion alloy according to claim 6, wherein the step of refining the crystal grains is carried out at a temperature of 800°C or more and less than 850°C for a time period of 2 hours or more and 4 hours or less. The method for producing a low thermal expansion alloy according to claim 1 or 2, wherein the grain refinement process adjusts the average grain size of the low thermal expansion alloy to 100 μm or less. The method for producing a low thermal expansion alloy according to claim 8, wherein the grain refinement process adjusts the average grain size of the low thermal expansion alloy to 50 μm or less. The method for producing a low thermal expansion alloy according to claim 1 or 2, wherein the Vickers hardness of the low thermal expansion alloy is adjusted to 160 HV or less by the grain refinement process. The hot working step includes a step of hot forging the low thermal expansion alloy,
3. The method for producing a low thermal expansion alloy according to claim 1, wherein the step of refining the crystal grains comprises forming a hot forged part having refined crystal grains by reheating the hot forged low thermal expansion alloy. Hot-forged parts made of low thermal expansion alloys, including Fe-Ni alloys or Fe-Ni-Co alloys, with the average grain size adjusted to 100 μm or less.