JP2008036698A - Manufacturing method of austenitic stainless steel large forgings - Google Patents

Abstract

A method for producing a large austenitic stainless steel forged product capable of reliably destroying a huge cast structure and capable of suppressing the coarsening of crystal grains caused by repeated forging and reheating. To provide.
A first heating step of heating an ingot made of austenitic stainless steel to a temperature of 1250 ° C. or higher and Tmp (° C.) or lower (where Tmp (° C. is a melting point of the austenitic stainless steel)); Until the temperature of the austenitic stainless steel drops below the recrystallization start temperature (Trex (° C.)), the maximum strain (ε) per pass is 0.2 or more, and the forging ratio is 2S or more. The manufacturing method of the austenitic stainless steel large forged product provided with the 1st forge process which forges the said austenitic stainless steel so that it may become.
[Selection figure] None

Description

  The present invention relates to a method for producing a large austenitic stainless steel forged product. More specifically, the present invention relates to an austenitic stainless steel for forging a large steel ingot made of austenitic stainless steel to produce a bar having a predetermined cross-sectional area. The present invention relates to a method for manufacturing a large forged product.

Forging refers to a processing method in which a metal material is compression-molded between dies to finish it into a desired shape. Forging is roughly classified into die forging using a closed die and free forging using an open die. Die forging is mainly used for products that are small and mass-produced. On the other hand, the free forging is applied to a small one having a weight of several kg to a huge one having a weight of 100 tons or more.
Forging may be performed on a material (for example, a rolled material, a forged steel piece, etc.) subjected to plastic working after casting, but in the case of a large product, forging is generally performed directly from a steel ingot. In addition to the development of a large cast structure, a steel ingot may have cracks, voids, component segregation, and the like between these structures. Therefore, the forging of large-sized products is not only to obtain the desired shape, but also to change the microstructure to a good workability such as fracture and refinement of the cast structure, crimping of cracks and voids, diffusion of segregation components, etc. As done.

Also, forging depends on the forging temperature,
(1) Hot forging in which forging is performed by heating the material to a temperature above the recrystallization temperature,
(2) Cold forging forging at a temperature near room temperature,
(3) Warm forging in which forging is performed in a temperature range between hot and cold,
are categorized. Forging large products is performed exclusively hot to reduce deformation resistance and to obtain a fine structure.
When performing hot forging, optimum conditions are selected for the heating temperature, forging temperature, and processing rate of the material according to the composition of the material. In general, the lower the heating temperature, the lower the forging temperature, and / or the higher the processing rate, the finer the crystal grains. For example, when austenitic stainless steel is hot forged, it is generally said that the heating temperature is 1150 to 1250 ° C. and the forging temperature is 1250 to 950 ° C. (see Non-Patent Document 1).

"Stainless Steel Handbook (3rd edition)", edited by Stainless Steel Association, page 899, Table 9.4

  Austenitic stainless steel has excellent corrosion resistance, oxidation resistance, and heat resistance, and has a high temperature strength of 600 ° C or higher compared to ferritic stainless steel, so it is used for large parts for various large plants such as nuclear power plants. Yes. This type of large component is generally manufactured by hot forging a steel ingot using an open die. In addition, due to restrictions of industrially used heating furnaces and press machines, conventionally, when forging large forgings industrially, the forging temperature is a maximum of 1200 ° C, and multiple forgings and reheatings are repeated. Generally, it is finished to a predetermined product size.

  On the other hand, austenitic stainless steel has high hot deformation resistance. Therefore, it is difficult to increase the amount of reduction per forging at a forging temperature of around 1200 ° C. Further, since the temperature of the material falls below the recrystallization temperature in a relatively short time after the start of forging, the number of forgings per heat is relatively small. Therefore, when manufacturing a large forged product, it was necessary to repeat light reduction and reheating.

  However, in a large forged product, when the amount of reduction per one time decreases, it becomes difficult to give strain to the inside of the steel ingot. Even when the strain is applied to the inside of the steel ingot, the strain is removed during reheating because the amount of strain applied to the inside of the material is small under light pressure. For this reason, in a large forged product, forging may be completed without applying sufficient strain as a driving force for starting recrystallization inside the material, and a huge cast structure may remain in the center. Moreover, the reheat performed repeatedly causes the crystal grains refined by forging to become coarse.

The problem to be solved by the present invention is to provide a method for producing an austenitic stainless steel large forged product capable of reliably destroying a huge cast structure.
In addition, another problem to be solved by the present invention is that when producing a large forged product made of austenitic stainless steel, it is possible to suppress the coarsening of crystal grains caused by repeated forging and reheating. Another object of the present invention is to provide a method for producing a large austenitic stainless steel forged product.

In order to solve the above-mentioned problems, the method for producing an austenitic stainless steel large forged product according to the present invention comprises an ingot made of austenitic stainless steel of 1250 ° C. or higher and Tmp (° C.) or lower (provided that Tmp (° C.) Between the first heating step of heating to the temperature of the melting point of the austenitic stainless steel) and the maximum temperature per pass until the temperature of the austenitic stainless steel falls below the recrystallization start temperature (Trex (° C.)). The gist of the present invention is to include a first forging step for forging the austenitic stainless steel so that the strain (ε) is 0.2 or more and the forging ratio is 2S or more. In this case, the training ratio in the first forging step is preferably 4S or more.
A second heating step of heating the austenitic stainless steel to a temperature of 1050 ° C. or higher and 1150 ° C. or lower after the temperature of the austenitic stainless steel is lowered to less than the recrystallization start temperature (Trex (° C.)); A second forging step of forging the austenitic stainless steel so that the maximum strain (ε) per pass is 0.2 or more and the forging ratio is 2S or more. good.

When a large forged product made of austenitic stainless steel is manufactured by forging, the hot deformation resistance of the material can be reduced by setting the heating temperature to 1250 ° C. or higher and Tmp (° C.) or lower. Therefore, even when a press having a relatively small pressing ability is used, the maximum strain (ε) per pass can be set to 0.2 or more. Moreover, the number of passes per heat can be increased by increasing the heating temperature. Furthermore, in addition to setting the maximum strain (ε) to 0.2 or more, if the forging ratio is 2S or more, a huge cast structure can be reliably destroyed.
Further, by performing forging at a relatively high temperature, it is possible to reduce the number of times of heat performed until a predetermined product size is obtained. Therefore, the coarsening of the crystal grains resulting from reheating can be prevented, and the forging efficiency is greatly improved.
Furthermore, if the forging process is divided into two stages, a first forging process and a second forging process, and the heating temperature at the time of performing the second forging process is relatively low, grain growth is suppressed and a fine grain structure is obtained. Is obtained.

Hereinafter, an embodiment of the present invention will be described in detail.
The manufacturing method of the austenitic stainless steel large forged product which concerns on the 1st Embodiment of this invention is equipped with the 1st heating process and the 1st forge process.

The first heating step is a step of heating an ingot made of austenitic stainless steel to a temperature of 1250 ° C. or higher and Tmp (° C.) or lower (where Tmp (° C. is the melting point of austenitic stainless steel)).
The method according to the present invention can be applied to any austenitic stainless steel. Specific examples of the austenitic stainless steel to which the present invention is applicable include SUS201, SUS202, SUS301, SUS302, SUS303, SUS303Se, SUS303Cu, SUS304, SUS304L, SUS304N1, SUS304N2, SUS304LN, SUS304J3, SUS305, SUS309S, SUS310S, SUS316, SUS316L, SUS316N, SUS316LN, SUS316Ti, SUS316J1, SUS316J1L, SUS316F, SUS317, SUS317L, SUS317LN, SUS317J1, SUS836L, SU890L, SUS321, SX34
In addition, the method according to the present invention can naturally be applied to materials (for example, rolled materials, forged steel pieces, etc.) that have been previously subjected to plastic working, or small products, but forging the ingot directly. When applied to a large product manufactured by the above, a high effect is obtained. The size of the forged product is preferably 7 t or more in terms of weight.

In the first heating step, the heating temperature of the ingot is preferably 1250 ° C. or higher. When the heating temperature is less than 1250 ° C., the hot deformation resistance increases, so it is difficult to increase the maximum strain (ε) per pass. Further, since the material temperature falls below the recrystallization temperature in a relatively short time, the number of passes per heat cannot be increased. The heating temperature is more preferably 1270 ° C. or higher.
On the other hand, if the heating temperature becomes too high, the material may melt. In addition, the crystal grains become coarse after recrystallization, and a fine structure cannot be obtained. Therefore, the heating temperature is preferably Tmp (° C.) or less. The heating temperature is more preferably 1300 ° C. or lower. The “melting point” refers to a temperature at which a liquid phase starts to appear (solidus temperature).
The heating time is not particularly limited as long as the entire material is at a uniform temperature. However, heating more than necessary causes the structure to become coarse. The optimum heating time is usually about 10 to 15 hours, although it depends on the size of the ingot.

In the first forging process, the maximum strain (ε) per pass is 0.2 or more while the temperature of the austenitic stainless steel is lowered below the recrystallization start temperature (Trex (° C.)). And it is the process of forging and stretching austenitic stainless steel so that a forge ratio may be 2S or more.
“Forging” is a type of free forging that reduces the cross-sectional area of the material between the upper and lower anvils and increases the length in the axial direction, and is also called solid forging. Anvils include flat, round, and Yakken types. In the first forging process, the kind of anvil used is not particularly limited, and these anvils are used according to the cross-sectional shape of the forged product to be produced.

For example, when manufacturing a forged product with a circular cross section from a prismatic ingot, the following method is generally used.
First, the heated ingot is compressed from one direction (0 ° direction) perpendicular to the axis using a flat die while stepping in the longitudinal direction (first pass). After completion of the first pass forging along the longitudinal direction, the forged product is rotated by 90 ° around the axis, and the forged product is stepped in the longitudinal direction in the same manner while using the flat die for the first pass. Compression is performed from a direction (90 ° direction) different from the direction by 90 ° (second pass). Thereafter, forging from the 0 ° direction and forging from the 90 ° direction are repeated a plurality of passes as necessary, the cross-sectional area is reduced while maintaining a substantially square cross-sectional shape.

Next, the compression direction is switched to a direction different from the compression direction of the first pass by 45 ° (45 ° direction) and a direction different from 135 ° (135 ° direction), forging from 45 ° direction and forging from the 135 ° direction. Stretch (replace face angle). When the face angle is exchanged, strain can be uniformly introduced into the forged product. Thereafter, forging from the 45 ° direction and forging from the 135 ° direction are each repeated a plurality of passes as necessary, and the cross-sectional area is further reduced while maintaining a substantially square cross-sectional shape.
When the cross-sectional shape approaches the product size, the forged product is stepped in the longitudinal direction, and the cross-section of the forged product is made into an octagonal cross section using a flat die. In this case, forging from 0 ° direction, 90 ° direction, 45 ° direction, and 135 ° direction is repeated multiple times as necessary, and the cross-sectional area is further reduced while maintaining an almost octagonal cross-sectional shape. Let Finally, while rotating the forged product around its axis, compression is performed using a round die to finish the forged product into a circular cross section.
In the following description, forging that is performed mainly for the purpose of breaking the cast structure and reducing the cross section of the ingot is referred to as “rough forging”. In the example of, “finishing forging” is a process in which an ingot having a square cross section is finished from an octagonal cross section to a final product shape.

In the present invention, “strain (ε)” means true strain (= ln (h / h ₀ ), h = height after deformation, h ₀ = height before deformation). The “maximum strain per pass (ε)” is the amount of strain introduced into the material by one compression and refers to the maximum value introduced into the material. Generally, when compressively deforming a material, the end surface is restrained by friction,
(1) On the upper and lower end faces, an indeformable band in which deformation is constrained,
(2) In the diagonal direction (direction inclined by approximately 45 ° with respect to the compression direction), a main deformation band that undergoes large shear deformation,
(3) Around the main deformation band, a uniform deformation band that receives substantially uniform compression in the compression direction,
Is formed. As a result, the strain is non-uniform within the material and is greatest at about the center of the main deformation zone.

In the case of forging a large product, the larger the maximum strain (ε) per pass, the easier the fracture of the huge cast structure. In order to reliably destroy a huge cast structure, the maximum strain (ε) per pass is preferably 0.2 or more. The maximum strain (ε) per pass is more preferably 0.35 or more, and still more preferably 0.5 or more.
The amount of reduction necessary to obtain such maximum strain (ε) varies depending on the size of the ingot. In general, the larger the ingot, the greater the amount of reduction required. Further, the maximum strain (ε) increases as the amount of reduction increases. For example, when forging an ingot of about 900 mm square to 1300 mm square, the maximum strain (ε) can be set to 0.2 or more when the reduction amount is 100 mm or more.
In the first forging process, usually, a plurality of passes are forged. Moreover, as will be described later, heating and forging may be repeated a plurality of times. In this case, it is only necessary that the maximum strain (ε) per pass satisfies the above-described conditions in at least one pass. The more paths that satisfy the above conditions, the better. Moreover, it is preferable that the above conditions are satisfied in all passes except when the cross-sectional shape of the forged product approaches the final product shape and it is physically difficult to ensure the above conditions.

The “forging ratio” in the first forge process is the material in the first forge process (when the first heating process and the first forge process are repeated a plurality of times, a plurality of first forge processes). The total amount of deformation to be applied, which is the ratio (A / a) of the cross-sectional area (A) before the first forge process to the cross-sectional area (a) after all the forge processes in the first forge process. Say.
In the case of forging a large product, the larger the forging ratio in the first forge process, the easier the destruction of the huge cast structure. In order to reliably destroy a huge cast structure, the forging ratio in the first forging process is preferably 2.0S or more. The training ratio is more preferably 3S or more, and further preferably 4S or more.
Depending on the cross-sectional shape of the ingot and the final product dimensions, the maximum strain (ε) and / or the forging ratio may not satisfy the above conditions. In such a case, it is preferable to perform upset forging before forging to increase the cross-sectional area of the ingot.

“Recrystallization start temperature (Trex (° C.)” refers to the lowest temperature at which softening due to recrystallization or recovery occurs while the material is at a high temperature during or after deformation of the material. Forging is performed in a temperature interval from the heating temperature of the material to the recrystallization start temperature (Trex (° C.)). When the temperature of the material is lower than the recrystallization start temperature, since recrystallization does not occur, hot deformation resistance increases. Accordingly, when the forging to the target dimension cannot be performed by one heating, the heating and forging are repeated as many times as necessary. However, if heating is repeated more than necessary, grain growth proceeds and crystal grains become coarse. Therefore, it is preferable that the amount of processing per one heating be as large as possible and forge-stretching with the minimum number of heating times.
In addition, when heating and forging are repeated a plurality of times, each of the first heating step and each of the first forging steps may be performed under the same conditions, or the conditions may be determined for each repetition. It may be different.

  The recrystallization start temperature (Trex (° C.)) depends on the maximum strain (ε) applied in the material. Generally, the recrystallization start temperature (Trex (° C.)) decreases as the maximum strain (ε) per pass increases and / or the initial crystal grain size before forging decreases.

Next, the manufacturing method of the austenitic stainless steel large forged product which concerns on the 2nd Embodiment of this invention is demonstrated. The manufacturing method according to the present embodiment includes a first heating step, a first forging step, a second heating step, and a second forging step. Of these, since for the first heating step及beauty first forging step is the same as in the first embodiment, description thereof will be omitted.

The second heating step is a step of heating the austenitic stainless steel to a temperature of 1050 ° C. or higher and 1150 ° C. or lower after the temperature of the austenitic stainless steel is lowered below the recrystallization start temperature (Trex (° C.)).
A 2nd heating process and the 2nd forge process mentioned later are mainly performed in order to refine | miniaturize the crystal grain in material. Therefore, when forging is performed only for the purpose of destroying a huge cast structure, these steps can be omitted.
The heating temperature in the second heating step may be equal to or higher than the temperature at which the work region is recrystallized under the forging conditions in the second forging step. Specifically, the heating temperature is preferably 1050 ° C. or higher.
On the other hand, if the heating temperature is too high, grain growth proceeds during heating, and the initial grain size before forging becomes coarse. In addition, fine crystal grains generated by recrystallization cause grain growth while the material is at a high temperature after forging. Accordingly, the heating temperature is preferably 1150 ° C. or lower, more preferably 1100 ° C. or lower.

The second forging step is a step for forging the austenitic stainless steel so that the maximum strain (ε) per pass is 0.2 or more and the forging ratio is 2S or more.
In the second forging process, in order to obtain a fine-grained structure, it is important to increase the amount of processing and impart strain to the entire material to promote recrystallization. For this purpose, the maximum strain (ε) per pass is preferably 0.2 or more. The maximum strain (ε) is more preferably 0.35 or more, and further preferably 0.5 or more.
The training ratio in the second forging process is preferably 2.0S or more. The training ratio is more preferably 3.0S or more, and still more preferably 4.0S or more.
In addition, the “forging ratio” in the second forging process is the second forging process (when the second heating process and the second forging process are repeated a plurality of times, a plurality of second forging processes). The total amount of deformation applied to the material, the ratio (B / b) of the cross-sectional area (B) before the second forge process to the cross-sectional area (b) after all the forge processes in the second forge process are completed. ).

In the second forging process, forging of a plurality of passes is usually performed, it is only necessary that the maximum strain (ε) per pass satisfies the above-described conditions in at least one pass. The more paths that satisfy the above conditions, the better. Moreover, it is preferable that the above conditions are satisfied in all passes except when the cross-sectional shape of the forged product approaches the final product shape and it is physically difficult to ensure the above conditions.
In addition, the transition from the first heating process and the first forging process to the second heating process and the second forging process is not particularly limited, and is optimal according to the final product shape and the degree of forging progress. At a certain time. Moreover, you may repeat a 2nd heating process and a 2nd forge process in multiple times. However, in order to obtain a fine grained structure, it can be forged to the final product shape by one heating, the above-mentioned training ratio can be obtained by one heating, and in at least one pass, It is preferable to shift from the first forging process to the second forging process so that the maximum strain (ε) per pass satisfies the above-described conditions. Specifically, it is preferable to perform rough forging in the first forging process and finish forging in the second forging process.

Next, the effect | action of the manufacturing method of the austenitic stainless steel large forged product which concerns on this invention is demonstrated.
Free forging is a forging method in which an open die is used to plastically deform a material. However, because the end face is constrained by friction, strain occurs mainly in a diagonal direction. In this case, the greater the amount of reduction, the greater the strain generated in the material.
FIG. 1 shows a numerical analysis result of strain when a square member of 700 mm square is compressed and deformed. When the amount of reduction is 50 mm, as shown in FIG. 1A, the maximum strain generated in the material is 0.19. On the other hand, when the amount of reduction is 100 mm, the maximum strain generated in the material reaches 0.5 as shown in FIG.

  When manufacturing a large forged product made of austenitic stainless steel by forging, if the heating temperature is relatively low, the hot deformation resistance increases, so the maximum strain per pass cannot be increased. In addition, since the temperature of the material falls below the recrystallization start temperature in a relatively short time and the number of passes per heat decreases, it is necessary to increase the number of reheats. Increasing the number of reheats reduces the cumulative strain that is the driving force for recrystallization. For this reason, under the conditions of low temperature and light pressure, strain necessary for recrystallization cannot be introduced into the central portion, and recrystallization hardly proceeds in the central portion. As a result, the initial structure (that is, a huge cast structure) remains in the center.

On the other hand, when the heating temperature is 1250 ° C. or higher, the hot deformation resistance of the material can be reduced. Moreover, the number of passes per heat can be increased by increasing the heating temperature. Therefore, even when a press having a relatively small pressing ability is used, the maximum strain (ε) per pass can be set to 0.2 or more. Further, when forging is performed under conditions of high temperature and high pressure so that the maximum strain (ε) is 0.2 or more and the forging ratio is 2S or more, a large forged product is produced. However, a huge cast structure can be destroyed reliably.
Further, by performing forging at a relatively high temperature, it is possible to reduce the number of times of heat performed until a predetermined product size is obtained. Therefore, the coarsening of the crystal grains resulting from reheating can be prevented, and the forging efficiency is greatly improved.

  FIG. 2 shows a conceptual diagram of the process of recrystallization and grain growth by forging. As shown in FIG. 2 (a), when the material in the initial state where the crystal grains are developed is heated to a temperature at which recrystallization can be performed, and the strain more than the critical strain necessary for recrystallization is applied to the material, FIG. ), Recrystallization nuclei are generated mainly from the old grain boundaries. The generated nuclei gradually grow as shown in FIG. 2C, and eventually become fine recrystallized grains. When the holding is continued at a higher temperature from this state, as shown in FIG. 2 (d), the grain growth of the fine grain obtained by recrystallization proceeds. In this case, the size of the recrystallized grain depends on the grain size of the crystal grain in the initial state. The smaller the initial grain size, the smaller the grain size of the recrystallized grain. Further, the recrystallization is completed in a shorter time as the initial particle size becomes smaller. In the forging and stretching process that is repeatedly performed, the refining progresses by this repetition.

As described above, forging under high temperature and high pressure is effective for destroying a huge cast structure. However, the higher the heating temperature, the larger the initial grain size before forging and the finer recrystallized grains obtained after forging progress, and the structure becomes coarser.
On the other hand, when the forging process is divided into two stages, the first forging process for forging under high temperature and high pressure, and the second forging process for forging under low temperature and high pressure, the initial particle size is The grain size of the recrystallized grains generated by forging is also suppressed. Therefore, compared with the conventional method, a large forged product having a fine structure can be obtained.

(Example 1)
A steel bar (1250 mm square) made of austenitic stainless steel (SUS304N2) was forged under the following conditions to produce a round bar (diameter 550 mm).
Heating conditions: 1270 ° C x 2 heat (see Fig. 3 (a))
Reduction amount: 100 mm (maximum strain (ε) = 0.5)
Forging ratio: 2.4S (rough forge), 2.4S (finish forge)
When the cross section of the obtained round bar was observed, the huge cast structure was almost completely destroyed.

(Comparative Example 1)
An ingot (1250 mm square) made of austenitic stainless steel (SUS304N2) was forged under the following conditions to produce a round bar (diameter 550 mm).
Heating conditions: 1220 ° C. × 2 heats, 1180 ° C. × 2 heats, 1100 ° C. × 2 heats, total 6 heats (see FIG. 3B)
Reduction amount: 30 to 50 mm (maximum strain (ε) = 0.1 to 0.19)
Training ratio: 4.4S (coarse forge), 1.6S (finish forge)
When the cross section of the obtained round bar was observed, it was confirmed that a huge cast structure remained in the cross shape at the center of the round bar.

(Example 2)
From an ingot made of austenitic stainless steel (SUS304N2), φ15 × 22.5 (uniform compression test piece: a columnar test piece provided with slight depressions on the upper and lower surfaces) was cut out. After holding the test piece at 1200 ° C. for 1 minute, it was heated to various forging temperatures (900 to 1200 ° C.) and forged at a reduction ratio of 55% (corresponding to maximum strain (ε) = 0.8). After forging, it was further held at the forging temperature for 0.1 to 1800 seconds and quenched.
The structure of the obtained sample was observed, and the recrystallization fraction Xrex and the recrystallized grain size dγrex were measured. The “recrystallization fraction Xrex” refers to the ratio (S / S ₀ ) of the area (S) of the region where recrystallization occurs to the sample area (S ₀ ). Further, “recrystallized grain size dγrex” refers to the average value of the corresponding recrystallized grain sizes by judging whether or not they are recrystallized grains on the structure photograph (can be distinguished by size).

FIG. 4 shows the relationship between the holding time t and the recrystallization fraction Xrex. FIG. 5 shows the relationship between the holding time t and the recrystallized grain size dγrex. From FIG. 4 and FIG.
(1) As the forging temperature increases, recrystallization is completed in a short time.
(2) The higher the forging temperature, the easier the growth of recrystallized grains proceeds.
(3) When forging is performed mainly for the purpose of refining crystal grains (that is, in the case of finish forging), the forging temperature is preferably 1050 to 1150 ° C.
I understand that.

(Example 3)
Φ15 × 22.5 (uniform compression test piece) was cut out from an ingot made of austenitic stainless steel (SUS304N2). The specimen was held at 1270 ° C. or 1150 ° C. for 1 minute. The initial particle size before forging was 120 μm (held at 1270 ° C.) or 35 μm (1150 ° C.). Next, the test piece was heated to various forging temperatures (900 to 1200 ° C.) and forged so that the maximum strain (ε) per pass was 0.22 to 0.52. Immediately after forging, it was water cooled.
6 and 7 show the maximum strain (ε) per pass and recrystallization when test pieces having initial grain sizes of 120 μm (held at 1270 ° C.) and 35 μm (held at 1150 ° C.) are forged at various temperatures, respectively. The relationship with the fraction is shown. From FIG. 6 and FIG.
(1) The smaller the initial grain size, the more recrystallization proceeds at low temperature and low strain.
(2) As the maximum strain (ε) per pass increases, recrystallization proceeds at a lower temperature.
(3) As the forging temperature increases, recrystallization proceeds with low strain.
(4) When forging is performed mainly for the purpose of refining crystal grains (that is, in the case of finish forging), the maximum strain (ε) per pass is 0.2 or more, and the forging temperature is 1050 to 1150 ° C. preferable,
I understand that.

  Although the embodiments of the present invention have been described in detail above, the present invention is not limited to the above embodiments, and various modifications can be made without departing from the scope of the present invention.

  The method for producing an austenitic stainless steel large forged product according to the present invention can be used as a method for producing large forged products for various large plants such as nuclear power plants.

FIG. 1A shows a numerical analysis result of strain distribution when forging with a reduction amount of 50 mm, and FIG. 1B shows a numerical analysis result of strain distribution when forging with a reduction amount of 100 mm. It is a schematic diagram which shows the process of recrystallization and grain growth. FIG. 3A and FIG. 3B are diagrams showing heating patterns for forging performed in Example 1 and Comparative Example 1, respectively. It is a figure which shows the relationship between holding time and recrystallization fraction when it forges for a predetermined time with forging temperature after forging at various temperatures. It is a figure which shows the relationship between holding time and recrystallized grain size when it forges for a predetermined time at forging temperature, after forging at various temperatures. It is a figure which shows the relationship between the maximum distortion when a test piece which is 120 micrometer of initial particle diameters (1270 degreeC holding | maintenance) is forged at various temperature, and a recrystallization fraction. It is a figure which shows the relationship between the maximum distortion when a test piece which is initial stage particle size 37micrometer (1150 degreeC holding | maintenance) is forged at various temperature, and a recrystallization fraction.

Claims (5)

A first heating step of heating an ingot made of austenitic stainless steel to a temperature of 1250 ° C. or higher and Tmp (° C.) or lower (where Tmp (° C. is the melting point of the austenitic stainless steel));
Until the temperature of the austenitic stainless steel falls below the recrystallization start temperature (Trex (° C.)), the maximum strain (ε) per pass is 0.2 or more and the forging ratio is 2S. As mentioned above, the manufacturing method of the austenitic stainless steel large forging product provided with the 1st forge process which forges the said austenitic stainless steel.   The method for producing an austenitic stainless steel large forged product according to claim 1, wherein the first heating step and the first forging step are repeated a plurality of times.   The forging ratio in the said 1st forge process is 4S or more, The manufacturing method of the austenitic stainless steel large forged product of Claim 1 or 2. A second heating step of heating the austenitic stainless steel to a temperature of 1050 ° C. or higher and 1150 ° C. or lower after the temperature of the austenitic stainless steel is lowered below the recrystallization start temperature (Trex (° C.));
A second forging step of forging the austenitic stainless steel so that the maximum strain (ε) per pass is 0.2 or more and the forging ratio is 2S or more. The manufacturing method of the austenitic stainless steel large forging in any one of 1-3. The method for producing a large austenitic stainless steel forged product according to any one of claims 1 to 4, wherein the austenitic stainless steel has a weight of 7 t or more.

Images