JPH1157924A - Round groove anvil and method for hot-forging round steel billet by using the anvil - Google Patents

Abstract

PROBLEM TO BE SOLVED: To produce a round steel billet extremely excellent in roundness without causing any crack flow in a short time by providing a round groove satisfying a specific condition between an inlet side part for feeding a work and an outlet side part for discharging. SOLUTION: In this round groove anvil 10, the round groove 1 satisfying the following condition is arranged between the inlet side part for feeding the work and the outlet side part for discharding. As for this condition, the depth D at the center of a groove bottom and the curvature radius Rg in the groove bottom part 1a are equal except both end parts at the inlet and the outlet side, the width H1 in the groove bottom part at the inlet side part is narrower than the width H in the finishing part at the outlet side and constant toward the inlet side, and an introducing part 1g in which escaping parts 1g having the constant width through the flat, surface 1h having the constant width and apex angle θ2 in both sides toward the outlet side are succesively formed is provided. The width in the groove bottom part between the introducing part and the finishing part is enlarged from H1 toward the outer side and a connecting part 1e successively forming the escaping part through the flat surface 1d having the width and the apex angle θ1 in both sides gradually shrunk toward the outlet side is provided. The length L2 of the connecting part is longer than the curvature radius of the groove bottom part.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to carbon steel, alloy steel,
A round steel slab made of stainless steel or high alloy steel and used for obtaining a round steel slab having a circular cross section from a material having a rectangular cross section, and a hot forging method for the round steel slab using the round steel slab About.

[0002]

2. Description of the Related Art In order to produce a round slab by hot forging, the following three methods are usually used.

[0003] The first method is to use a flat anvil to alternately press the opposite surfaces of a square or rectangular rectangular cross-section material (steel ingot, bloom, etc.) into small rectangular cross-sections, specifically a substantially square shape. Form into an intermediate material of cross section. Next, in the same manner as described above, through a forming process of alternately pressing down the corners of the intermediate material using flat anvils to make a substantially regular octagonal cross-section, to a roughly finished hexagonal or rough finished rough round material having a polygonal cross-section of more than it Rough molding. Thereafter, a method of forming and finishing a round steel slab having a predetermined outer diameter with a circular cross section using a finishing anvil with a round groove formed thereon (hereinafter referred to as a round groove anvil) by spiral forging.

In a second method, an intermediate material roughly formed into a substantially square cross section in the first method is roughly formed by using a rough molding anvil in which a V-groove is formed (hereinafter referred to as a V-groove anvil). After performing spiral forging to roughly form a rough finish coarse round material with a regular hexagonal or more polygonal cross section, in the same way as above, using a round groove anvil to a round steel piece with a circular cross section having a predetermined outer diameter A method of forming and finishing by spiral forging.

[0005] A third method is to use a large round cast slab having a circular cross section manufactured by continuous casting as a material, and using a V-groove anvil or a flat anvil as the material, a polygonal cross section of approximately a regular hexagon or more. A method of forming a round steel piece having a predetermined outer diameter by spiral forging using a round groove anvil in the same manner as described above, after performing a spiral forging for roughly forming a rough finished coarse round material.

[0006] Spiral forging is a process of continuously feeding an object to be processed in an axial direction and simultaneously rotating the anvil by a predetermined angle around its axis at the time of opening operation of the anvil which is opened and closed. It refers to the forging method to be performed (the same applies hereinafter).

[0007] In any of the hot forging methods that go through the above-described steps, a round groove anvil as described below is usually used for finish molding in any of the steps.

FIGS. 4A and 4B show a typical example of a conventional round groove anvil commonly used. FIG. 4A is a plan view, and FIG. Sectional view taken along the line, FIG.
FIG. 2 is a sectional view taken along line F-F of FIG.

[0009] As shown in FIG.
Is the offset amount G such that the depth at the center of the groove bottom is D.
A groove bottom 1a having a radius of curvature Rg and a width H formed with
And a round groove 1 having a semicircular hole type having a relief portion 1b formed with a radius of curvature Rc on both sides of the groove bottom portion 1a and having a constant depth D excluding both end portions in the longitudinal direction of the anvil. ing. A chamfered portion 1c having a radius of curvature Rb is formed at both ends in the longitudinal direction of the anvil, and a perch surface 1f is formed at both sides in the width direction. And the hot finished outer diameter D of the round slab to be obtained
b is adjusted to “2 (R
g-G) to 2Rg ".

However, since the width of the round groove 1 is constant in the longitudinal direction of the anvil, the round groove anvil 20 as described above has a cover on the inlet side for feeding a rough finished rough round material having a polygonal cross section of 16 or more hexagons. Insufficient width spread of processing material is not enough. For this reason, when the cross-sectional dimension of the rough finished coarse round material that fluctuates due to wear of the flat metal anvil or the V-groove anvil and a change in friction coefficient during the rough forming becomes larger than a predetermined dimension, the width spread amount of the material to be processed becomes round. The material exceeds the allowable width expansion of the grooved anvil 20 and the material starts to bite between the escape portions 1b of the circular grooved anvil 20 used in the opposed arrangement, and in the case of extreme cases, between the perch surfaces 1f. I will be.

Therefore, in the actual finish forging with the round groove anvil 20, in order to avoid the occurrence of the above-mentioned biting, a slight underfill state, that is, the finish forging is performed in anticipation of the dimensional variation of the rough finished coarse round material. The forging is performed in a state where the rolling allowance is reduced and the groove bottom 1a is not completely filled with the material. As a result, in the round steel slab after the finish forming, there is a problem that a concave portion composed of a local flat surface remains in the circumferential direction and the axial direction, and a round steel slab having satisfactory roundness cannot be obtained. . This problem is particularly prominent when forging ferritic stainless steel, duplex stainless steel, or the like, which has a large width expansion during rolling.

In addition, in the above-mentioned conventional forging method, since the above-mentioned characteristics of the round groove anvil 20 are rough-formed into a rough finished coarse round material having a hexagonal or more polygonal cross section before the finish forming by the round groove anvil 20, it is necessary. As described above, the processing time becomes longer, and a decrease in the temperature of the material to be processed is inevitable. Therefore, the deformability is poor.
In the case of difficult-to-work materials such as duplex stainless steel and high alloy steel containing 25% by weight or more of Cr and 35% by weight or more of Ni (hereinafter simply referred to as high alloy steel), cracks other than those caused by biting occur frequently. There was also a problem that it was easy to do.

[0013] For this reason, the development of a round groove anvil capable of finish forming round steel slabs having satisfactory roundness even with materials such as ferritic stainless steel and duplex stainless steel having a large width spread during rolling down and It has been desired to develop a forging method capable of suppressing the occurrence of cracks even in difficult-to-work materials such as duplex stainless steel and high alloy steel having poor deformability.

[0014]

SUMMARY OF THE INVENTION The present invention has been made in view of the above situation, and has as its object to provide a round groove anvil capable of obtaining a round steel piece excellent in roundness, Provides a hot forging method for round steel slabs that can complete processing in a short time and can suppress the occurrence of cracks even when forging materials with poor deformability such as duplex stainless steel and high alloy steel. Is to do.

[0015]

SUMMARY OF THE INVENTION The gist of the present invention resides in the following (1) a round groove anvil and a (2) hot forging method of a round slab.

(1) A round groove anvil 10 provided with a round groove 1 which satisfies the following conditions between an inlet side where a material to be processed is fed and an outlet side where a material is discharged.

The depth D at the center of the groove bottom and the radius of curvature Rg of the groove bottom 1a are the same in the longitudinal direction of the anvil except for the chamfered portion 1c on the entrance side and the exit side.

At the entry side, a groove bottom 1a having a radius of curvature Rg is provided.
Width H smaller than the width H of the finishing unit ₁ is output side, and is constant toward the inlet side, via a predetermined flat surface 1h toward the outlet side width and apex angle theta ₂ on both sides An introduction portion 1 in which a relief portion 1b having a constant radius of curvature Rc is continuously formed.
g.

[0019] Between the intro 1g and exit side of the finishing unit are sequentially enlarged from H ₁ to H according to the width of the groove bottom portion 1a of the curvature radius Rg is directed to the exit side, the width and the apex angle on both sides comprising sequentially reduced connecting portion 1e which relief portion 1b is continuous molding consisting of a radius of curvature Rc of approximately constant width over a flat surface 1d according theta ₁ is directed toward the outlet side.

The length L ₂ of the connecting portion 1e is equal to the groove bottom 1
1 times or more of the curvature radius Rg of a.

(2) A hot forging method in which a material having a rectangular cross-sectional shape is finish-formed into a round steel slab having a circular cross-section, and the material is roughly formed into a substantially octagonal polygonal cross-sectional shape. Using a round groove anvil 10 described in (1) above to form a round section in a hot forging method.

According to the round groove anvil 10 described in the above (1), since the introduction portion 1g having the above-described configuration is provided on the entrance side, the rough molding material having a substantially octagonal cross-sectional shape is to be finished. The part 1g can be formed into a round steel slab having a circular cross section at a stretch while forming into a rough finished rough round material having a polygonal cross section of a regular hexagon or more. And, in this case, even if the cross-sectional dimension of the roughly formed octagonal rough formed material is large and the material width spread amount at the time of reduction is increased, the material of the increased portion is the flat surface 1h portion formed on both sides of the groove bottom 1a. Then, the material is gradually stepped by the flat surface 1d of the connecting portion 1e, so that the material does not bite between the flank portions 1b of the finished portion. Therefore, it is not necessary to perform forging in the underfill state, and it is possible to perform forging that completely fills the groove bottom 1a with the material, and it is possible to prevent the occurrence of a local flat surface recess. As a result, it is possible to form a round steel slab having excellent roundness and no flaws caused by biting.

Further, according to the hot forging method for round slabs described in the above (2), a step of roughly forming a rough finished rough round material having a polygonal cross section of a regular hexagon or more is omitted, and a substantially regular octagon is obtained. The rough-formed material roughly formed into the polygonal cross-sectional shape is finish-formed at once using the round groove anvil 10 described in (1) above, so that it is roughly formed into a rough-finished rough round material having a polygonal cross section of a regular hexagon or more. The processing time is shortened, and a decrease in the temperature of the material is suppressed. As a result, in combination with the function and effect of the round groove anvil 10, even if the material is inferior in deformability, cracks are not generated or the number of cracks is suppressed as much as possible. Not only that, round steel pieces without roundness and flaws caused by biting can be formed in a short time.

Japanese Patent Application Laid-Open Nos. Hei 5-212486 and Hei 6-71377 disclose a round groove anvil provided with an introduction portion similar to the connecting portion 1e of the present invention described in the above (1) on the entrance side. It is shown. However, the round groove anvils shown therein are merely the ones in which the depth of the center of the groove bottom of the introduction portion is gradually increased toward the entry side.

In both publications, the depth D at the center of the groove bottom 1a and the radius of curvature Rg of the groove bottom 1a are set to absorb the width spread of the material which occurs in the case of the same circular groove anvil in the longitudinal direction of the anvil. No specific means are described, and these are not helpful at all for implementing the present invention.

In the round groove anvils disclosed in the above publications, the amount of reduction in the same cross section of the work material differs for each shot during spiral forging on the entry side, and the outer diameter of the same cross section changes stepwise. Therefore, it cannot be axially symmetric, and the amount of width expansion at the output side fluctuates. For this reason, the roundness desired by the present invention cannot be secured with ferrite stainless steel, duplex stainless steel, or the like having a large width spread.

[0027]

BEST MODE FOR CARRYING OUT THE INVENTION A round groove anvil according to the present invention and a method of hot forging a round steel slab using the round groove anvil will now be described in detail with reference to the accompanying drawings.

First, the round groove anvil according to the present invention will be described. The same parts as those in the related art are denoted by the same reference numerals, and detailed description thereof will be omitted.

FIG. 1 is a schematic view showing an example of a round groove anvil according to the present invention. FIG. 1 (a) is a plan view, and FIG. 1 (b) is a sectional view taken along the line II-II of FIG. (C) is a sectional view taken along the line I-I of FIG. (A), FIG. (D) is a sectional view taken along the line i- (a) of FIG. a) It is a sectional view taken along the line II-II in the figure.

As shown in FIG. 1, a round groove anvil 10 of the present invention is provided.
Is similar to the conventional round groove anvil 20 (see FIG. 4 described above).
A groove bottom 1a having a constant radius of curvature Rg formed with an offset amount G such that the depth at the center of the groove bottom is D, and an escape portion 1b formed with a constant radius of curvature Rc on both sides of the groove bottom 1a. And a round groove 1 having a constant depth D excluding both ends in the longitudinal direction of the anvil. Further, at both ends in the longitudinal direction of the anvil, there are chamfered portions 1c formed with a constant radius of curvature Rb.

[0031] Then, its inlet side, the curvature and radius Rg constant groove bottom portion 1a which is a narrow H ₁ constant than the width H of the finishing part of the length L ₁ of the region over out wide side, the groove bottom portion 1a, flat surfaces 1h and 1h having a constant apex angle θ ₂ formed on both sides of the flat surface 1h,
An introduction portion 1g is provided, which is composed of escape portions 1b and 1b each having a constant width and having a constant curvature radius Rc.

Furthermore, between the finishing portion of the outlet side with the introduction portion 1g, a width over a region of length L ₂ and a gradually expanding radius of curvature Rg constant groove bottom portion 1a from H ₁ to H,
A flat surface which is continuously formed on both sides of the groove bottom 1a with a vertex angle θ ₁ gradually decreasing from the introduction portion 1g toward the exit finish portion, and having a width gradually decreasing from the introduction portion 1g toward the exit finish portion. There is provided a connecting portion 1e including 1d, 1d, and escape portions 1b, 1b formed on the flat surfaces 1d, 1d with a constant radius of curvature Rc and having a substantially constant width.

The inlet-side introduction portion 1g configured as described above.
Has a groove bottom 1a having a constant width in the longitudinal direction of the anvil and flat surfaces 1h, 1h, and the total width of both is larger than the width H of the groove bottom 1a of the finished part on the output side. For this reason, an approximately regular octagonal rough forming material is to be processed, and its cross-sectional shape is roughly formed into a rough finished rough round cross-section of approximately a regular hexagon or more in which the corner is an arc surface having a radius of curvature Rg at the introduction portion 1g. can do.

At this time, since the radius of curvature Rg and the depth D of the groove bottom 1a are the same as those of the groove bottom 1a of the finishing portion and the connecting portion 1e, the introduction portion 1g is processed for each shot during spiral forging. The outer diameter (dimension between the groove bottom centers) of the same cross section of the material does not change in a stepwise manner, and the width of the material at the time of rolling down becomes constant, so that an axisymmetric hexagon is obtained.

The connecting portion 1e constructed as described above.
Is provided with sequentially enlarged groove bottom portion 1a to H of the width of the side exits H ₁ of the entry side, sequentially reduced flat surface 1d toward the finish portion of the side stenter from inlet side, and 1d The total width of the two gradually decreases toward the finishing part on the delivery side.

For this reason, even if there is a dimensional variation in the roughly formed octagonal rough material to be processed, and even if the width of the material at the time of rolling is uneven, all the material in the widened portion has a total groove width. In the groove of the large introduction portion 1g. The material of the widened portion accommodated in the groove of the introduction portion 1g is constrained by the flat surface 1d of the groove whose total groove width is gradually reduced toward the finish portion on the output side in the connection portion 1e. It is molded and plastically deforms toward the center of the groove width.

Therefore, according to the round groove anvil 10 of the present invention,
Even if the cross-sectional shape of the material to be processed is substantially a regular octagon and the cross-sectional dimensions vary, the material between the clearance portions 1b not only in the introduction portion 1g and the connection portion 1e but also in the finishing portion on the output side. Does not bite out, and the introduction part 1g
And the connecting portion 1e forms a rough finished rough round cross section closer to a perfect circle, and then finishes the groove bottom 1a having a radius of curvature Rg with a constant depth D and width H to a circular cross section at a finished portion. It is possible to finish a round steel slab with a small degree of surface, excellent roundness and no flaws.

Here, in order to obtain a round steel piece having a desired roundness, specifically, a roundness defined by the following equation of 1 mm or less, the length L ₂ of the connection 1e is determined by changing the length L ₂ of the groove bottom 1a. Must be equal to or more than one time the radius of curvature Rg. This is clear from the experimental results shown in FIG.

Roundness = maximum outer diameter−minimum outer diameter (unit: m
m) Further, the apex angle θ ₂ constituting the flat surfaces 1h and 1h of the introduction portion 1g is obtained by roughly forming the cross-sectional shape of the roughly formed octagonal rough material to be processed into a substantially regular hexagonal axisymmetric cross section. The value is determined based on the amount of reduction on the entry side of the round groove anvil 10 given at the time of spiral forging and the rotation angle of the material to be processed per shot, and is usually 90 to 1
The value is in the range of 35 °. However, since the width of the material at the time of rolling is different if the amount of rolling at the entry side of the round groove anvil 10 and the material of the material to be processed are changed, an experiment is carried out in advance for each of the rolling amount and the material, and It is preferable to determine the angle of the greatest common divisor that can obtain an axially symmetric cross-sectional shape close to a square, and determine the angle as the vertex angle θ ₂ .

[0040] Note that the inlet portion length L ₁ and finishing the length of the portion L ₃ of 1g, is not particularly limited, preferably the both connecting portions 1e length L ₂ equal to or higher than that. The width H ₁ of the groove bottom 1a having the curvature radius Rg of the introduction portion 1g.
Is uniquely determined by the tangential relationship between the radius of curvature Rg and the vertex angle θ ₂ . The groove bottom 1a having a radius of curvature Rg of the finished portion.
Is uniquely determined by the contact condition between the radius of curvature Rg and the radius of curvature Rc of the escape portion 1b.

Next, a method for hot forging round slabs according to the present invention will be described.

FIGS. 3A and 3B show the conventional manufacturing process and the manufacturing process of the present invention in comparison with each other. FIG.
(B) is the production process of the present invention.

As shown in the figure, in the method of the present invention, after a material having a rectangular cross section is roughly formed into an intermediate material having a substantially regular octagonal cross section, "process-6" in the conventional manufacturing process, that is, Is formed into a round steel slab having a circular cross section at once using the above-described round groove anvil 10 of the present invention, without going through a step of roughly forming a rough finished rough round material having a polygonal cross section of a regular hexagon or more. How to

When the “step-6” is omitted as described above, the processing time is not required, and the decrease in the temperature of the material is suppressed. Therefore, according to the method of the present invention, finish molding can be performed in a state where the material temperature is higher than before and the decrease in deformability is small. As a result, the material is easily plastically deformed, and the center of the groove bottom 1a is not only completely filled with the material, but also has a synergistic effect by the round groove anvil 10 of the present invention including the introduction portion 1g and the connection portion 1e. As a result, a round steel slab excellent in roundness with little or no flaws or cracks can be obtained. This effect is particularly remarkable when forging a material having poor deformability such as duplex stainless steel or high alloy steel.

[0045]

EXAMPLE A rectangular ingot made of SUS316 having a length of about 2000 mm and an average side length of 490 mm was used as a raw material.
This material is heated to 1290 ° C and the outer diameter is 178 in one heat.
In forging into round steel slabs of mm, a comparison was made between the conventional manufacturing process shown in FIG. 3A and the manufacturing process of the present invention shown in FIG. 3B.

At this time, in each case, the process from “Step-1” to “Step-3” is completed in 12 passes, and one side is 180 mm.
It was reduced to a rectangular cross section with corners. In two passes, "Process-
4 "and" Step-5 "are completed, and the dimension between the opposite sides is 196 m.
m (vertical dimension: 215 mm) was formed into a substantially regular octagonal intermediate material.

In the case of the conventional method, the above-mentioned intermediate material is subjected to "Step-6", and spiral forging is performed using a V-groove anvil having a vertex angle of 120 °, and a substantially regular hexagon having a dimension between opposite sides of 184 mm. After rough forming into a rough finished coarse round material,
7 "for finish molding with a round groove anvil.

The finish molding is performed by using a round groove anvil as shown in FIG. 4 having a common hot finish diameter of 172 to 184 mm.
11 mm, D = 81 mm, Rg = 92 mm, H = 15
9.3 mm, Rc = 70 mm, Rb = 40 mm,
Perforated surface 1f when rolling down using 300mm in total length
Set the distance to 19.5mm and feed amount 23mm
Spiral forging under the conditions of / shot, rotation angle 45 ° / shot, hot finish outer diameter 181.5 mm, length 1
It was finished into a 5000 mm round steel slab.

On the other hand, in the case of the method of the present invention, the above-mentioned intermediate material was not subjected to the "Step-6", but was subjected to the finish molding with the round groove anvil in "Step-7".

In the finish forming in the method of the present invention, a round groove anvil is used as the shape shown in FIG. 1 which is commonly used for a hot finish diameter of 172 to 184 mm, G = 11 mm, D = 81 mm, Rg =
92 mm, H = 159.3 mm, Rc = 70 mm, Rb
= 40 mm, L ₁ = 120 mm, L ₂ = 100 mm, L
₃ = 120 mm, θ ₁ = 135 ° → 120 °, θ ₂ = 1
35 °, H ₁ = 70.4 mm, and the total length is 420 m
19.m, and the distance between the perch surfaces 1f at the time of reduction is 19.
Spiral forging with a feed rate of 23 mm / shot and a rotation angle of 45 ° / shot,
Hot finishing A round steel slab having an outer diameter of 181.5 mm and a length of 15000 mm was finish-formed.

Then, after finishing molding, the round steel slab which had been allowed to cool to the room temperature after being air-cooled was subjected to six outer portions at a pitch of 30 ° in the circumferential direction at a distance of 4000 mm from the center in the longitudinal direction and 4000 mm from the end face. The diameter was measured and its average outer diameter was determined, while the roundness defined by the above equation was determined. Further, the entire length was observed, and the dent amount was measured for the most significant portion of the unevenness. Table 1 shows these results and the time required for completing “Step-1” to “Step-7”.

[0052]

[Table 1]

As shown in Table 1, the roundness of the obtained round slab was poor at 1.5 mm or more when the conventional method was used. 0.8m in case
m and good.

Also, the dent amount was unsatisfactory as 1.5 to 2.1 mm when the conventional method was used, but was 1.0 to 1.0 mm when the method according to the present invention was used. It was as good as 4 mm.

Furthermore, in the case of using the conventional method, it took 20 minutes to complete the forging, but in the case of using the method of the present invention, it took only 17 minutes, and the time was reduced by about 15%.

Although the description of the data is omitted, no flaws caused by biting were found in any case.
However, when the conventional method was used, the occurrence of cracks due to a decrease in deformability was slightly observed.

[0057]

According to the round groove anvil of the present invention and the forging method using the anvil, a round steel piece having extremely excellent roundness and having no crack can be manufactured in a short time.

[Brief description of the drawings]

FIG. 1 is a schematic view showing an example of a round groove anvil according to the present invention, in which FIG. 1 (a) is a plan view, FIG. 1 (b) is a sectional view taken along the line II in FIG. 1 (a), and FIG. (c) is a cross-sectional view taken along the line I-I of FIG. (a), FIG. (d) is a cross-sectional view taken along the line of the ha-a of FIG. (a), and (e) of FIG. FIG.

2 is a diagram showing the relationship between the length of the connecting portion 1e L ₂ and roundness.

3A and 3B are diagrams showing a conventional manufacturing process and a manufacturing process of the present invention in comparison with each other. FIG. 3A is a diagram showing a conventional manufacturing process, and FIG. 3B is a diagram showing a manufacturing process of the present invention.

FIG. 4 is a schematic view showing an example of a conventional round groove anvil; FIG. 4 (a) is a plan view, FIG. 4 (b) is a cross-sectional view taken along a ho-ho line in FIG. 4 (a), and FIG. FIG. 2 is a sectional view taken along line F-F of FIG.

[Explanation of symbols]

 10: round groove anvil of the present invention, 1: round groove, 1a: groove bottom, 1b: relief portion, 1c: chamfered portion, 1d: flat surface, 1e: connecting portion, 1f: perch surface, 1g: introduction portion, 1h : Flat surface.

Claims (2)

[Claims] A round groove anvil (10), comprising a round groove (1) which satisfies the following conditions between an inlet side into which a work material is fed and an outlet side from which a material is discharged. . The depth (D) of the center of the groove bottom and the radius of curvature (Rg) of the groove bottom (1a) are the same except for the chamfers (1c) on the entrance side and the exit side.
Both are the same in the longitudinal direction of the anvil. On the entry side, the width (H ₁ ) of the groove bottom (1a) having a radius of curvature (Rg) is smaller than the width (H) of the finished portion on the exit side, and is constant toward the entry side. Width and apex angle (θ
₂ ) is provided with an introduction portion (1g) in which a relief portion (1b) having a constant width and a radius of curvature (Rc) is continuously formed through a constant flat surface (1h) toward the exit side. Between the above-mentioned introduction part (1g) and the finishing part on the outlet side, the width of the groove bottom part (1a) of the radius of curvature (Rg) is sequentially increased from (H ₁ ) to (H) as going toward the outlet side, Relief portions (1b) each having a radius of curvature (Rc) having a substantially constant width were continuously formed on both sides of the flat surface (1d) whose width and apex angle (θ ₁ ) were gradually reduced as going toward the exit side. A connection portion (1e); The length (L ₂ ) of the connecting portion (1e) is equal to the length of the groove bottom (1e).
At least one time the radius of curvature (Rg) of a). 2. A hot forging method for finish-forming a material having a rectangular cross-section into a round steel slab having a circular cross-section, wherein said material is roughly formed into an approximately octagonal polygonal cross-section, A hot forging method of a round steel slab, wherein the round steel slab is subjected to finish molding into a circular cross section using the round groove anvil (10) according to claim 1.