JPH04333340A - Method for making fine crystalline grain in non-magnetic steel cylindrical forging - Google Patents

Abstract

PURPOSE:To eliminate crack and dead metal part and further, to secure fine crystal grains by executing two process forging of ring forging line instead of mandrel forging and also, work with dies (tool). CONSTITUTION:The forging is composed of the ring forging process, by which a cylindrical raw material 12 is formed into the ring 14 having diameter larger than the finished diameter by 25-35% without restricting to circular direction and axial direction of the cylindrical raw material 12, and the forging process, by which this outer peripheral face is forged to the circular direction while rotating this ring 14 to the circular direction after this ring forging. In this ring forging process, as the cylindrical raw material 12 is forged without restricting the circular direction and the axial direction, the development of crack to the end face is restrained and the development of dead metal part is made to be as less as possible.

Description

[Detailed description of the invention]

0001

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for refining the grains of a cylindrical forged product of non-magnetic steel, and is used, for example, in the production of a retaining ring for a generator.

0002

[Prior Art] Steel exhibiting an austenitic structure at room temperature is
Since there is no transformation point between the solidified state of molten steel and room temperature, grain refinement must rely on dynamic strain recrystallization through hot forging. Here, when the fine crystal grains are used as a structural material, they are very important in order to improve the transmittance of ultrasonic waves for detecting internal defects.

On the other hand, as an austenitic steel, high MnCr
When using NiCr-based steel, the steel has poor hot workability compared to NiCr-based steel. That is, surface cracks are likely to occur, and therefore large strains cannot be applied, and crystal grains cannot be sufficiently refined. In particular, when obtaining a cylindrical forged product from a cylindrical steel ingot, it is important to suppress the occurrence of the cracks.

[0004] Conventionally, the grain size of non-magnetic steel cylindrical forged products has been refined by the procedure shown in FIGS. 3 to 5. In Fig. 3, a cylindrical steel ingot 1 with a height H0
is compression forged to a height H in the column length direction, and after forming a central hole 3A in the center of the flat forged product 2 with a wall thickness T by punching or the like, the forged product 3 with a hole is formed.
By performing the mandrel forging process shown in FIGS. 4 and 5, a cylindrical forged product 4 with refined crystal grains was obtained.

[0005] Here, the amount of compression is determined by providing an elongation allowance in advance in order to obtain a forging ratio for grain refinement, and is usually based on an empirical standard of Hf/H≧2.00. The forging work by mandrel forging is performed by inserting the mandrel 5 into the center hole 3A as shown in Figs. 4 and 5, and
(Forged product with holes) By rolling down a pair of upper and lower molds 6 having a V-shaped groove 6A on the outer circumferential surface of 3, the crystal grains are forged in the axial direction by constraining the circumferential direction and the radial direction. A cylindrical forged product 8 with a fine wall thickness Tf was manufactured. The mold 6 may have a V-shaped groove 6A on at least one side, and a so-called flat anvil may be used on the other side.

[0006]

[Problem to be Solved by the Invention] In the above-mentioned prior art, since the mandrel 5 is in contact with the material 3 over the entire length of its inner peripheral surface, the material flows (deformed) in the axial direction.
becomes constrained. Further, the material is not easily deformed because it is attempted to deform the end face 3B by applying a rolling force in the radial direction while restraining deformation in the circumferential direction so as to squeeze out the end face 3B.

In other words, the end face 3B where the temperature is most likely to drop
Tensile stress is applied to the material, and since the material is difficult to stretch, alternating bending strain is repeated in the circumferential direction of the material, resulting in cracks 7 on the end surface 3B.
There is a problem that this occurs. For this reason, increasing the number of heating cycles increases the cycle time and makes it difficult to achieve the essential refinement of crystal grains. In addition, as shown in FIG. 5, since there is a contact area between the tools 5 and 6 in the machining area of the tool (die) 6 and the mandrel 5, there is a problem that the dead metals 9 and 9A are large and crystal grains cannot be refined. Ta.

In view of the problems described above, the present invention performs a ring forging process instead of the mandrel forging process, and then performs a two-step forging process by performing processing using a die (tool). By doing so, the purpose is to eliminate cracks and dead metals, and by extension ensure the refinement of crystal grains.

[0009]

[Means for Solving the Problems] The present invention provides a cylindrical steel ingot 1
In a method of refining crystal grains by compression forging 0 in the column length direction and drilling a hole penetrating in the column length direction, the non-magnetic steel cylindrical material 12 is forged and stretched,
The following technical measures have been taken to achieve the above objectives.

That is, the present invention is a ring forging process in which the forging process is performed to form the cylindrical material 12 into a ring 14 having a diameter 25 to 35% larger than the finished diameter without constraining the circumferential direction and axial direction of the cylindrical material 12. and a step of forging the outer circumferential surface of the ring 14 in the circumferential direction while rotating the ring 14 in the circumferential direction after the ring forging.

[0011]

[Operation] In the ring forging process, the cylindrical material 12 is not restrained in the circumferential direction and the axial direction, thereby suppressing the occurrence of cracks on the end face and minimizing dead metal. Therefore,
It ensures a sufficient amount of strain, enables uniform refinement of crystal grains, and improves material yield.

[0012] In the forging process after the ring forging process, the outside diameter is finished using a tool, and dead metal at this time is also reduced.

[0013]

Embodiments Hereinafter, embodiments of the present invention will be described with reference to the drawings. In Figure 1, 10 is a cylindrical steel ingot, specifically 18% Mn-1 used as a retaining ring for a generator.
It is 8% Cr steel (ASTM A289Cl, C steel). This steel ingot 10 has poor thermal conductivity, develops columnar crystals during solidification, has large anisotropy, and has a linear expansion coefficient of 900°C to 550°C.
Since grain boundary precipitates are formed at ℃, hot forgeability is extremely poor.

In the present invention, the above-mentioned steel ingot 10 having a height H0 is compression forged to a height H in the column length direction to obtain a flat forged material 1.
1, the material 11 is forged by punching or the like into a cylindrical material 12 with a wall thickness T having a center hole 12A, and then the cylindrical material 12 is forged and drawn. After the ring forging step, a final forging step is performed. via. In the ring forging process, as shown in Fig. 1, a mandrel 13 with a diameter sufficiently smaller than the center hole 12A is inserted into the cylindrical material 12 in a skewer shape, and a flat die (tool) is pressed down on the outer peripheral surface of the cylindrical material 12 to achieve the target. The ring 14 is rolled into a ring 14 having a diameter 25 to 35% larger than the outer diameter of the product. Note that 13A indicates a pedestal.

In this ring rolling, the cylindrical material 12 is ring rolled without being constrained in the circumferential and axial directions, so there is no cause for cracking. The length (height) Hm of the ring 14 at this stage of ring rolling is Hm/H
The ratio is 1.3 to 1.4. Further, in this rolling, although the deformation region and the dead metal region coincide, no cracks occur as described above.

The reason why the ring is rolled to a diameter 25 to 35% larger than the outer diameter of the product is as follows. That is, the amount of strain is required to be 25% in order to make the crystal grains finer, and the larger the amount of strain, the better in order to only make the grains finer. However, when the ring 14 is enlarged by more than 35%, the wall thickness of the ring 14 becomes thinner, and even if compression is applied in the circumferential direction in the next step, it will not be upset and will buckle. Therefore, it is set at 25% to 35%.

Next, the outer peripheral surface of the ring 14 is pressed down in the circumferential direction while rotating the ring 14 using a pair of molds (tools) 16 having V-shaped grooves 16A, so that the side surface 14A is formed as shown in FIG. Circumferential compressive strain (digging)
is given to a predetermined outer diameter, and its length is at least Hf
/H ratio of 1.50 or more. In this case, at least one of the tools 16 may have a V-shaped groove 16A.

Furthermore, if the length of the material is short, a mandrel 17 may be inserted as shown in FIG. 2 to assist in rotation. However, the diameter of the mandrel 17 is made sufficiently small so that the rolling force from the upper and lower tools 16 does not reach the mandrel 17. Next, non-magnetic steel cylindrical forged products (products) with an outer diameter of 820 mmφ, an inner diameter of 500 mmφ, and a length of 850 mmφ were manufactured using the conventional method and the method of the present invention, and the two were compared.

[0019]

[Table 1] Table 1 above is data confirmed by an ultrasonic flaw detection test.

[0020] Here, the ultrasonic transmittance (attenuation constant α
db/cm) indicates that the finer the crystal grains are, the smaller the value is.The noise level indicates the residual unrecrystallized coarse crystal grains, and the smaller this value is, the more uniform the noise level is. Further, the grain size (ASTM E112) indicates that the larger the value, the finer the grains.

[0021]

[Effects of the Invention] The present invention is as described above, and since ring forging is performed in advance during forging and drawing, and the circumferential and axial directions of the cylindrical material are not constrained, the occurrence of cracks on the end face of the material is suppressed. As a result, sufficient strain (deformation) can be applied, uniform refinement of crystal grains can be achieved, and the material yield can be improved.

In other words, in the conventional technology, processing recrystallization (refining) is difficult because the part to be deformed is dead metal due to contact friction with the tool surface, whereas in the present invention, the part to be deformed (inner circumferential surface) is Since there is no contact with the tool (mandrel), no dead metal is generated, and since there is no squeezing force in the axial direction and no tensile stress is generated, no cracks occur on the end surface of the material.

Furthermore, according to the present invention, the critical strain required for grain refinement is 150% to 200% because there is no contact with a tool (mandrel) as in the prior art.
%, thereby shortening forging time and reducing energy consumption.

[Brief explanation of drawings]

FIG. 1 is a process diagram of an embodiment of the present invention.

FIG. 2 is a front view of the final forging process of the present invention.

FIG. 3 is a process diagram of a conventional example.

FIG. 4 is a perspective view of a conventional forging process.

FIG. 5 is a front view of FIG. 4;

[Explanation of symbols]

10 Cylindrical steel ingot 12 Cylindrical material with holes 14 Ring 15 Tools 16 Tools

Claims (1)

[Claims] [Claim 1] After compressing and forging a cylindrical steel ingot (10) in the longitudinal direction of the column and drilling a hole penetrating the column in the longitudinal direction, the non-magnetic steel cylindrical material (12) is forged and stretched. In the method of refining crystal grains, the forging process is performed to form a ring having a diameter 25 to 35% larger than the finished diameter of the cylindrical material (12) without constraining the circumferential direction and axial direction of the cylindrical material (12). A non-magnetic steel cylinder characterized by comprising a ring forging process to form (14), and a process of forging the outer peripheral surface of the ring (14) in the circumferential direction while rotating the ring (14) in the circumferential direction after the ring forging. Grain refinement method for forged products.