EP4786070A1

EP4786070A1 - Method for producing hot-forged material

Info

Publication number: EP4786070A1
Application number: EP24928409.2A
Authority: EP
Inventors: Yoji Yamashita; Shinichi Kobayashi; Chuya Aoki
Original assignee: Proterial Ltd
Current assignee: Proterial Ltd
Priority date: 2024-03-06
Filing date: 2024-11-14
Publication date: 2026-08-05
Also published as: AU2024431915A1; WO2025187130A8; WO2025187130A1; CN122514433A; JP7782758B1; JPWO2025187130A1

Abstract

When hot forging is performed on a hot-forging material having a surface to which a heat-resistant insulating material has been bonded, occurrence of defects on the surface of the resultant hot-forged product, the defects caused due to the bonding of the heat-resistant insulating material, is suppressed.

A hot-forged product manufacturing method includes: a heating step that heats an unheated material to be hot-forged to a hot forging temperature in a heating furnace, to obtain a heated material; a heat-resistant insulating material bonding step that bonds a heat-resistant insulating material made of inorganic fibers to at least part of a surface of the heated material extracted from the heating furnace, to obtain a hot-forging material; and a hot forging step that compresses the hot-forging material partially or entirely by using a die, an anvil, or a tool, to form the hot-forging material into a predetermined shape, wherein after a liquid glass lubricant that is free of styrene acrylic resin or that contains styrene acrylic resin of less than 5 mass% is applied to the surface of the inorganic-fiber heat-resistant insulating material, the surface to be bonded to the heated material, and is dried, the inorganic-fiber heat-resistant insulating material is bonded to the heated material.

Description

TECHNICAL FIELD

The present invention relates to hot-forged product manufacturing methods, in particular, to manufacturing methods for a hot-forged product made of a poor workability alloy.

BACKGROUND ART

When a hot-forging material that has been heated to a hot forging temperature is hot-forged, if the temperature of the hot-forging material drops, the hot forgeability deteriorates. Thus, various countermeasures to prevent such temperature drop have been conventionally proposed.
For example, in a proposed hot-forging technique (Patent Document 1), first, a heat-resistant insulating material made of inorganic fibers is bonded to the surface of a heated material extracted from a heating furnace, so as to obtain a hot-forging material. Next, hot forging is performed on this hot-forging material. In relation to this, another technique (Patent Document 2) has also been proposed in which first, a glass lubricant containing glass particles is applied (or sprayed) to a surface of the heat-resistant insulating material, the surface to be bonded to a heated material, and next, the heat-resistant insulating material to which the glass particles have been attached is dried. For example, a "liquid glass lubricant" containing: glass particles composed of SiO₂, B₂O₃, or the like; a binder component (a resin component); and water is commonly used as the above-described glass lubricant containing glass particles. There has also been proposed a hot-working lubricant to which various kinds of minute amounts of components have been added (Patent Document 3).
Regarding bonding a heat-resistant insulating material to which the liquid glass lubricant has been applied to a surface of a heated material, another technique (Patent Document 4) has also been proposed. In this technique, the above-described heat-resistant insulating material is bonded to the surface of the heated material extracted from a heating furnace, this heated material is returned into the heating furnace, and "reheating" is performed, so as to improve the adhesion of the heat-resistant insulating material to the surface of the heated material.

REFERENCE DOCUMENT LIST

PATENT DOCUMENTS

Patent Document 1: WO 2021/182606 A
Patent Document 2: JP 2016-215275 A
Patent Document 3: JP 2005-343948 A
Patent Document 4: CN 105478643 A

SUMMARY OF THE INVENTION

PROBLEM TO BE SOLVED BY THE INVENTION

The technique described in Patent Document 1 is effective in suppressing temperature drop of a hot-forging material during hot forging.
A hot-forged product obtained upon completion of hot forging is subjected to machining such as grinding or polishing, so that a surface of the hot-forged product is adjusted to be approximately smooth. When hot forging is performed on a hot-forging material having a surface to which a heat-resistant insulating material has been bonded, part of the heat-resistant insulating material has peeled off or been abraded by the time of completion of the hot forging, and there are cases in which "particular defects" are observed, the occurrence mechanism of which is different from that of normal forging defects, on the surface of the hot-forged product to which the heat-resistant insulating material has been bonded. If significant particular defects are observed, the cost of the above-described grinding needs to be increased, for example.
It is an object of the present invention to provide a hot-forged product manufacturing method in which hot forging is performed on a hot-forging material having a surface to which a heat-resistant insulating material has been bonded, and this manufacturing method suppresses occurrence of defects on the surface of the resultant hot-forged product, the defects caused due to the bonding of the heat-resistant insulating material.

MEANS FOR SOLVING THE PROBLEM

That is, the present invention is a hot-forged product manufacturing method including: a heating step that heats an unheated material to be hot-forged to a hot forging temperature in a heating furnace, to obtain a heated material; a heat-resistant insulating material bonding step that bonds a heat-resistant insulating material made of inorganic fibers to at least part of a surface of the heated material extracted from the heating furnace, to obtain a hot-forging material; and a hot forging step that compresses the hot-forging material partially or entirely by using a die, an anvil, or a tool, to form the hot-forging material into a predetermined shape, wherein after a liquid glass lubricant that is free of styrene acrylic resin or that contains styrene acrylic resin of less than 5 mass% is applied to the surface of the inorganic-fiber heat-resistant insulating material, the surface to be bonded to the heated material, and is dried, the inorganic-fiber heat-resistant insulating material is bonded to the heated material.
In the heating step, oxygen concentration in the heating furnace may be 3.0 vol% or greater.
In addition, the hot-forged product manufacturing method according to the present invention, the hot forging step is open-die forging, and in the heat-resistant insulating material bonding step, the heat-resistant insulating material is bonded to at least part of a surface of a freely deformed portion of the heated material, the part not in contact with any one of the die, anvil, and tool in the open-die forging.

EFFECTS OF THE INVENTION

According to the present invention, in a hot-forged product manufacturing method in which hot forging is performed on a hot-forging material having a surface to which a heat-resistant insulating material has been bonded, it is possible to suppress occurrence of defects on the surface of the resultant hot-forged product, the defects caused due to the bonding of the heat-resistant insulating material.

BRIEF DESCRIPTION OF THE DRAWINGS

[FIG. 1] This is a photograph of the external appearance of an example of defects on a surface of a hot-forged product, the defects having been caused by glass corrosion.
[FIG. 2] This is a photograph of the external appearance of an example of a surface of a hot-forged product obtained according to the present invention.

MODE FOR CARRYING OUT THE INVENTION

Hereinafter, processes according to the present invention will be described. The term "unheated material" refers to a material that has not been charged into a heating furnace, and the term "heated material" is a material that has been heated to a hot forging temperature in the heating furnace. In addition, the term "hot-forging material "refers to a material that has a predetermined portion to which a heat-resistant insulating material has been bonded, and that is ready for hot forging, and the term "hot-forged product" is a product that has been formed into a predetermined shape by a hot forging apparatus.

Heating Step

First, according to the present invention, an unheated material to be hot-forged is heated to a hot forging temperature in a heating furnace. The unheated material is not limited to a particular material, and examples of the unheated material include an ingot, a billet, a preform, and a powder compact. Because an ingot, billet, or the like can be formed into a desired shape by open-die forging, the advantageous effects of the present invention can be obtained most when an ingot, billet, or the like is used. This unheated material is heated to a hot forging temperature in a heating furnace, to obtain a heated material.
Although the hot forging temperature varies depending on the material of the unheated material, it is easy to determine the hot forging temperature as appropriate. For example, 950°C to 1180°C is set as the hot forging temperature for a nickel-based alloy known as a poor workability alloy. When a nickel-based alloy contains a gamma-prime phase (γ') of 20 vol% or greater, 1010°C to 1180°C can be set as the hot forging temperature. 900°C to 1180°C can be set as the hot forging temperature for a titanium alloy.
According to the present invention, after this heating step, a heat-resistant insulating material bonding step, which will be described below, is performed. In this heat-resistant insulating material bonding step, a heat-resistant insulating material is bonded to the heated material extracted from the heating furnace. It is preferable that the temperature drop of the heated material be zero until this heat-resistant insulating material is bonded to the heated material. However, in reality, the temperature of the heated material drops by a non-negligible extent. Thus, when the unheated material is heated in the heating step, the unheated material may be heated to a temperature higher than a forging temperature (a forging start temperature), which is set at the start of hot forging, by approximately 5°C to 100°C (for example, a temperature obtained by adding approximately 5°C to 100°C to the above hot forging temperature). The temperature of the heated material may drop more than 100°C from the forging start temperature after the heated material is extracted from the heating furnace, without performing the heat-resistant insulating material bonding step. Even in this case, by heating the unheated material at a higher temperature, the temperature drop can be suppressed, and the temperature during the hot forging can be maintained high.
It is preferable that the surface roughness of the unheated material be rougher than a standard finish. In this way, when the heat-resistant insulating material is bonded to the surface of the heated material in the next heat-resistant insulating material bonding step, a slight space is formed between the heat-resistant insulating material and the heated material, and the air in this space is expected to function as an insulating layer. In addition, the glass particles attached to the heat-resistant insulating material more easily fuse to the asperities of the surface of the heated material.
Of course, the as-cast or as-plastic-worked surface finish may be acceptable. However, in the case of a poor workability alloy, because cracks or the like may occur on the surface due to added elements, it is preferable that surface defects that could cause these cracks in hot forging be removed by machining such as grinding or polishing. Even when cracks or the like are not observed, it is preferable that a portion of the surface of the unheated material, the portion to which the heat-resistant insulating material is to be bonded in the next heat-resistant insulating material bonding step (that is, the portion to which the glass lubricant is to fuse), be adjusted to a standard finish or rougher by machining.

Heat-Resistant Insulating Material Bonding Step

After the unheated material is heated to a hot forging temperature, a heat-resistant insulating material is bonded to a predetermined portion on at least part of the surface of the heated material extracted from the heating furnace, to obtain a hot-forging material.
First, the above-described heat-resistant insulating material is made of inorganic fibers. Examples of the "inorganic fibers" according to the present invention include glass fibers, ceramic fibers, etc. It is preferable that the heat-resistant insulating material be made of ceramic fibers having excellent heat resistance. Among various kinds of ceramic fibers, for example, KAOWOOL (registered trademark) is particularly preferable for its availability and cost. By using the inorganic-fiber heat-resistant insulating material, even if the surface roughness of the heated material is coarse to some extent, the heat-resistant insulating material can be easily bonded along the surface shape of the heated material, together with the effect of the adhesive of the glass lubricant applied to the heated material. In addition, the fibers easily engage with the asperities of the surface of the heated material. Because of its light weight, the heat-resistant insulating material can also be easily bonded to the side surface of the heated material, for example.
As in the present invention, by bonding a heat-resistant insulating material to at least part of the surface of a heated material extracted from a heating furnace, the heat-resistant insulating material is maintained in its entirety on the surface of the heated material in the initial phase of hot forging. Thus, drop in the temperature of the hot-forging material during the hot forging can be suppressed. If the heat-resistant insulating material is placed on the surface of the unheated material that has not been charged into a heating furnace, the heat-resistant insulating material can be easily crushed at the time of transfer for hot forging, although this depends on the relationship between temperature and time. As a result, it becomes difficult to suppress the above-described temperature drop. In addition, in the final phase of the hot forging, the peak at which suppression of the above-described temperature drop is desirable has already passed. According to the present invention, because part of the heat-resistant insulating material has peeled off or been abraded, it is possible to eliminate the labor for removing the heat-resistant insulating material when the surface of the hot-forged product obtained upon completion of the hot forging is machined. In addition, since part of the heat-resistant insulating material has peeled off or been abraded in the final phase of the hot forging, it is also possible to suppress excessive heat buildup (plasticity-induced heating) of the hot-forging material. To obtain these advantageous effects, it is particularly effective to use glass fibers or ceramic fibers as the inorganic fibers of the heat-resistant insulating material.
There is a known technique in which the bonding of a heat-resistant insulating material is performed easily and within a short time in a heat-resistant insulating material bonding step such as that described above. In this technique, glass lubricant is allowed to be present between the heat-resistant insulating material and a surface of a heated material, the surface to which the heat-resistant insulating material is to be bonded. That is, glass particles are attached to a surface of the heat-resistant insulating material, the surface to which the heated material is to be bonded, and the heat-resistant insulating material is bonded to a predetermined location of the heated material.
With this technique, the glass particles in the glass lubricant are softened by the heat held on the surface of the heated material, and the heat-resistant insulating material and the heated material are consequently bonded to each other. Thus, this technique is effectively applied to, for example, hot forging of a nickel-based superalloy having a high hot forging temperature. In addition, as the method for attaching the glass particles to the heat-resistant insulating material, for example, there is a method of applying a liquid glass lubricant containing glass particles to the surface of the heat-resistant insulating material, the surface to which the heated material is to be bonded, by brush coating or spraying (spreading). The application by spraying is preferable in that the glass particles can be evenly attached to the surface of the heat-resistant insulating material, the surface to which the heated material is to be bonded.
Next, after the application of the glass lubricant, the heat-resistant insulating material to which the glass particles have been attached is dried. This is preferable, for example, in that the glass particles can be sufficiently attached and in that glass corrosion of the material, which will be described below, can be prevented. In addition, by drying the heat-resistant insulating material to which the glass particles have been attached, when the heat-resistant insulating material is bonded to the surface of the heated material, sudden evaporation of a volatile component such as the binder contained in the glass lubricant can be suppressed. In this respect, if the glass lubricant is directly applied to the surface of the heated material, the above-described volatile component could suddenly evaporate.
However, the present inventors found that, even if the temperature drop of the hot-forging material can be suppressed by bonding the above-described heat-resistant insulating material to the surface of the heated material, "particular defects", the occurrence mechanism of which is different from that of normal forging defects, are observed on the surface to which the heat-resistant insulating material has been bonded when the hot-forged product obtained upon completion of the hot forging is observed. The present inventors conducted intensive research and researched the occurrence mechanism of these particular defects, and found that these defects are caused by "glass corrosion" that occurs at a three-phase interface among the material being forged, the glass lubricant, and the forging atmosphere.
First, when the present inventors observed the surface of the hot-forged product obtained immediately after completion of the hot forging, in some cases, although the present inventors did not observe a defect (that is, a normal forging defect) even on a surface to which the heat-resistant insulating material was not bonded, the present inventors observed defects (that is, particular defects) on the surface to which the heat-resistant insulating material was bonded (the portions in the circles in FIG. 1). This result shows that these particular defects are not forging cracks caused by normal temperature drop. In addition, the present inventors investigated details of these particular defects, and found that these defects are caused by the above-described glass corrosion, and that the occurrence of the glass corrosion can be suppressed by determining the kind of glass lubricant applied to the heat-resistant insulating material.
That is, when applied to the heat-resistant insulating material, the glass lubricant is liquid, and contains, for example, glass particles composed of SiO₂, B₂O₃ or the like, a binder component (a resin component), and water. Thus, by the time the heat-resistant insulating material to which this glass lubricant has been applied is bonded to the surface of the heated material, the above-described glass lubricant needs to be dried. However, although water has been removed from the dried glass lubricant, if a remaining resin component is flammable, this resin component may come in contact with the heated material heated to a hot forging temperature, may react with the oxygen in the environment before or during the forging (burn), and may accelerate the glass corrosion on the material. Thus, the present inventors investigated various kinds of glass lubricant, and as a result, the present inventors found that "styrene acrylic resin (copolymer)" contained in the glass lubricant is highly flammable, and that limiting the content of styrene acrylic resin is effective in suppressing the acceleration of the glass corrosion. That is, when the glass lubricant according to the present invention is in a liquid state before being applied, the glass lubricant is free of styrene acrylic resin. Alternatively, even when the glass lubricant contains styrene acrylic resin, the content thereof is less than 5 mass%, preferably, less than 4 mass%, and more preferably, less than 2 mass%.
Even if the glass lubricant according to the present invention contains a sufficiently limited content of styrene acrylic resin, because another flammable component may be contained to a non-negligible extent, after the heat-resistant insulating material to which the glass lubricant has been applied is bonded the surface of the heated material, it is preferable that this state not be maintained for a long time. That is, if the glass lubricant is in contact with the surface of the heated material at a hot forging temperature for a long time, the glass lubricant could still cause glass corrosion. Thus, it is preferable that coating the surface of the unheated material with the glass lubricant in advance in the heating step to improve the adhesion of the heat-resistant insulating material to the heated material be omitted, although the oxygen concentration in the heating furnace is maintained low. In addition, after the heat-resistant insulating material is bonded to the surface of the heated material in the heat-resistant insulating material bonding step, it is preferable that the process promptly proceed to the hot forging step, which will be described below, without heating the heated material again.
In addition, according to the present invention, since coating the surface of the unheated material with the glass lubricant can be omitted in the heating step, glass corrosion does not occur, and therefore, the oxygen concentration in the heating furnace may be maintained high to some extent. For example, the oxygen concentration in the heating furnace may be set to 3.0 vol% or greater, 4.0% or greater, or 5.0% or greater. The upper limit may be set to, for example, 15.0% or less, 10.0% or less, or 8.0% or less.
For example, when the material of the unheated material is a nickel-based superalloy, because most alloys contain Cr within a range of 10 to 35 mass%, limiting the oxygen concentration in the heating furnace is effective in suppressing the oxygen in the heating furnace reacting with Cr in the alloy during the heating step.
In the heat-resistant insulating material bonding step, the heat-resistant insulating material may be bonded to part or all surface of the heated material. In addition, the portion to which the heat-resistant insulating material is to be bonded may be selected from one of the following two methods, in view of the material and shape of the unheated material.
The first method is a method that preferentially prevents the temperature drop at a portion where a forging crack is expected. If bonding the heat-resistant insulating material to the heated material takes time, the temperature of the heated material drops, which may deteriorate the hot forgeability. Therefore, it is preferable to bond the heat-resistant insulating material to the minimum necessary surface of the heated material within a time that does not deteriorate the hot forgeability. For example, when the hot-forging material is placed on a hot forging apparatus, and when there is a concern that heat is released to, for example, a lower die (lower anvil or lower tool), the heat-resistant insulating material may be bonded to a surface that is in contact with the lower die (lower anvil or lower tool). In a case of a polygonal columnar shape, the heat-resistant insulating material may be bonded to a region including an edge portion. In a case of a cylindrical shape, the heat-resistant insulating material may be bonded to a side surface of the cylindrical shape. That is, it is preferable that the heat-resistant insulating material be bonded to a place including a portion where a defect such as a crack is easily formed by the hot forging.
The second method is a method that bonds the heat-resistant insulating material to at least part of the surface of a freely deformed portion of the heated material. For example, when the hot forging is open-die forging, a portion not in contact with an upper die (upper anvil or upper tool) or a lower die (lower anvil or lower tool) is allowed to cool in air. Thus, this method is mainly to reduce the temperature drop.
In particular, the above-described two methods are effective for a nickel-based alloy known as a poor workability alloy, in particular, to a nickel-based alloy containing a γ' phase of 20 vol% or greater. That is, the bonding of the heat-resistant insulating material can reduce fine precipitation of γ' phase associated with temperature drop of the hot-forging material, and can facilitate recrystallization of the surface layer of the hot-forging material. Therefore, occurrence of defects such as forging cracks can be reduced.
Among the nickel-based alloys, alloys such as a 718 alloy and a Waspaloy alloy having a wide temperature range in which hot forging is possible can ensure the sustainability of the heating temperature, and therefore, use of these alloys contributes to reduction of forging defects (cracks).

Hot Forging Step

By using a hot-forging material prepared by the above-described steps and by using dies, anvils, or tools, the hot-forging material is partially or entirely compressed and is formed into a predetermined shape. It is preferable that the forging apparatus to be used be a large-sized hot forging apparatus, which can form the hot-forging material into a predetermined shape, and of which the forging load is several thousand tons or greater, even when the hot-forging material is a poor workability alloy.
In addition, according to the present invention, it is preferable that the hot forging step be open-die forging. The hot-forging material on which open-die forging is performed has a heavy weight, a larger area through which heat is released to the air, and a large machining quantity. Thus, by bonding the heat-resistant insulating material, a greater effect of suppressing the temperature drop of the hot-forging material can be obtained. In this case, as described above, for example, when hot forging is performed on a general nickel-based alloy such as a 718 alloy or a Waspaloy alloy having a relatively wide temperature range in which hot forging is possible, it is preferable that the heat-resistant insulating material according to the present invention be bonded in advance to at least part of the surface of a freely deformed portion of the heated material, the part not in contact with any one of the above-described dies, anvils, and tools in the open-die forging.

EXAMPLES

Liquid glass lubricants 1 to 3 shown in Table 1 were each prepared by using a generally known liquid glass lubricant (a glass lubricant containing glass particles (so-called, borosilicate glass particles) dispersed in water and composed of SiO₂, B₂O₃, or the like of approximately 50 mass%), and by adjusting a binder component (a styrene acrylic resin component) contained in a corresponding one of the glass lubricants.
Next, each of the liquid glass lubricants 1 to 3 shown in Table 1 was applied to a surface of a Kaowool used as an inorganic-fiber heat-resistant insulating material, the surface to which a heated material is to be bonded. Next, the resultant heat-resistant insulating materials were dried. [Table 1]

Glass Lubricant No. Styrene Acrylic Resin (Mass%)

1 25

2 5

3 0 (None Added)

Example 1

In Example 1, the impact of glass corrosion on the surfaces of the heated materials under the following conditions A and B relating to the steps before the hot forging step was evaluated in preliminary experiments.

Condition A: In the heating step, the surfaces of the unheated materials were coated with the glass lubricants in advance.
Condition B: In the heat-resistant insulating material bonding step, heat-resistant insulating materials were bonded to the surfaces of the heated materials extracted from the heating furnace, and the resultant heated materials were heated again.

First, the peripheral surface of an individual Waspaloy cylindrical billet (cross-sectional diameter: 355 mm, height: 800 mm) was subjected to turning to achieve a standard finish, and the resultant cylindrical billets were used as the unheated materials. Next, each of the liquid glass lubricants 1 to 3 shown in Table 1 was applied to part of the peripheral surface of a corresponding one of the unheated materials by brush coating. These unheated materials were dried and charged into the heating furnace (oxygen concentration in the furnace: approximately 9.0 vol%). After the entire unheated materials reached a hot forging temperature of 1080°C, this heating temperature was maintained for 4 hours or longer (condition A). Next, the unheated materials maintained at the above-described heating temperature were temporarily extracted from the heating furnace. Next, heat-resistant insulating materials to which the above-described liquid glass lubricants have been applied were attached to peripheral surfaces of the unheated materials, the peripheral surfaces to which the liquid glass lubricants were not applied. The unheated materials were returned into the heating furnace again, and were heated for 30 minutes or longer (condition B). Next, the unheated materials heated again as described above were extracted from the heating furnace as the heated materials or hot-forging materials, and were cooled by air (allowed to cool). Finally, the surfaces of the resultant materials were observed.
As a result of the observation, first, the resultant materials obtained under condition A were evaluated. Clear discoloration was observed on the peripheral surfaces of the heated materials, the peripheral surfaces to which glass lubricants 1 and 2 were applied, and glass corrosion was formed. Slight discoloration was observed also on the peripheral surface to which the glass lubricant 3 was applied, and local glass corrosion was formed. Next, the resultant materials obtained under condition B were evaluated. Clear discoloration was observed on the entire peripheral surfaces of the heated materials, the surfaces to which the heat-resistant insulating materials were bonded, and glass corrosion was formed, regardless of the type of the glass lubricants 1 to 3. Thus, in order to suppress glass corrosion, it is preferable that the glass lubricants such as those used in Examples under conditions A and B not remain in contact with the surface of the material at a hot forging temperature for a long time.

Example 2

In Example 2, in view of the results of Example 1, the surfaces of the unheated materials were not coated with any glass lubricant in the heating step, and the hot forging step was conducted without reheating the hot-forging materials to which the heat-resistant insulating materials were bonded in the heat-resistant insulating material bonding step. Next, defects on the surfaces of the hot-forged products obtained by the hot forging were observed.
First, the peripheral surface of an individual Waspaloy prismatic billet (cross section: 490 mm × 490 mm, height; 1300 mm) was polished with a grinder (abrasive grain size #16) to achieve a standard finish. The resultant prismatic billets were used as the unheated materials. These unheated materials were charged into a heating furnace (oxygen concentration in the furnace: approximately 9.0 vol%). After the unheated materials reached a hot forging temperature of 1080°C, this heating temperature was maintained for 4 hours or longer (condition A). The materials were extracted from the heating furnace as the heated materials.
In advance, the above-described glass lubricants 1 to 3 had been applied to heat-resistant insulating materials, and had been dried. Each of these heat-resistant insulating materials was attached to part of the peripheral surface of a corresponding one of the heated materials, to obtain hot-forging materials. Next, open-die forging was conducted by pressing a die on the peripheral surface of each of the hot-forging materials. The forging temperature (surface temperature) during the hot forging was approximately 850°C to 1000°C. In any one of the cases, predetermined open-die forging was completed. Finally, the surfaces of the hot-forged products obtained immediately after the open-die forging were observed.
The heat-resistant insulating materials bonded to the peripheral surfaces of the above-described hot-forged products were abraded or peeled off, and the surfaces of the hot-forged products were exposed to the outside. In addition, no defects (that is, no normal forging defects) were observed on the surfaces of the hot-forged products, the surfaces to which the heat-resistant insulating materials were not bonded. However, discoloration was observed on the portions on the above-described exposed surfaces, the portions to which the heat-resistant insulating materials to which the glass lubricants 1 and 2 had been applied were bonded. In addition, defects were observed on these portions (the portions in the circles in FIG. 1). The defects observed when the glass lubricant 1 was used had a depth of approximately 5 mm. The defects observed in the case of the glass lubricant 2 had a depth of approximately 2 mm. That is, the severity of the defects observed in the case of the glass lubricant 2 was significantly less than that observed in the case of the glass lubricant 1.
On the other hand, neither discoloration nor defects were observed on the portion to which the heat-resistant insulating material to which the glass lubricant 3 had been applied was bonded. In addition, after the portion where neither discoloration nor defects had been observed was heated to a hot forging temperature of 1080°C again in a state in which the temperature was still high after the hot forging, a heat-resistant insulating material to which the glass lubricant 3 had been applied was attached to this portion, and the same open-die forging was repeated. The forging temperature (surface temperature) during the hot forging was approximately 850°C to 1000°C. Although normal forging defects were observed after the above-described open-die forging was repeated, no defects due to glass corrosion were observed (FIG. 2 illustrates this portion of the surface after the hot-forged product was cooled by air (allowed to cool).

Claims

A hot-forged product manufacturing method comprising:
a heating step that heats an unheated material to be hot-forged to a hot forging temperature in a heating furnace, to obtain a heated material;

a heat-resistant insulating material bonding step that bonds a heat-resistant insulating material made of inorganic fibers to at least part of a surface of the heated material extracted from the heating furnace, to obtain a hot-forging material; and

a hot forging step that compresses the hot-forging material partially or entirely by using a die, an anvil, or a tool, to form the hot-forging material into a predetermined shape,

wherein after a liquid glass lubricant that is free of styrene acrylic resin or that contains styrene acrylic resin of less than 5 mass% is applied to the surface of the inorganic-fiber heat-resistant insulating material, the surface to be bonded to the heated material, and is dried, the inorganic-fiber heat-resistant insulating material is bonded to the heated material.
The hot-forged product manufacturing method according to claim 1, wherein in the heating step, oxygen concentration in the heating furnace is 3.0 vol% or greater.
The hot-forged product manufacturing method according to claim 1, wherein the hot forging step is open-die forging, and
wherein in the heat-resistant insulating material bonding step, the inorganic-fiber heat-resistant insulating material is bonded to at least part of a surface of a freely deformed portion of the heated material, the part not in contact with any one of the die, anvil, and tool in the open-die forging.