WO2007110930A1

WO2007110930A1 - Process for producing seamless pipe

Info

Publication number: WO2007110930A1
Application number: PCT/JP2006/306275
Authority: WO
Inventors: Akihiro Yamanaka; Manabu Adachi; Tomio Yamakawa; Hirotsugu Nakaike; Kazuhiro Shimoda; Toshiro Anraku
Original assignee: Sumitomo Metal Industries, Ltd.
Priority date: 2006-03-28
Filing date: 2006-03-28
Publication date: 2007-10-04
Also published as: CN101410195B; EP2008733A1; CN101410195A; EP2008733B1; EP2008733A4

Abstract

A process for producing a seamless pipe characterized by including a continuous casting step in which a round billet is cast in which the whole region in a central part having a diameter of at least 60 mm in the cross section is constituted of an isometric crystal structure and which has a carbon content of 0.1 mass% or lower and a diameter in terms of cross-section diameter beyond 300 mm. In the process for seamless-pipe production, the round billet produced in the continuous casting step may be pierced/rolled with a piercer under conditions in which the ratio of the billet rotation speed to the billet outer-diameter draft is a value in a range suitable for the ratio of the gorge part diameter to the inlet diameter in the tilting rolls, thereby giving a seamless pipe reduced in defects in the inner surface. As a piercing plug may be used a plug of a suitable shape which is approximately cylindrical and has a spherical rolling part at the plug tip. Thus, Mannesmann fracture and circumferential-direction shear strain are significantly inhibited and no billet nipping failure occurs at all. This process for seamless-pipe production enables a seamless pipe having even higher inner-surface quality to be produced with exceedingly high productivity.

Description

Specification

Seamless pipe manufacturing method

Technical field

The present invention relates to a method for manufacturing a seamless pipe. More specifically, it is produced by a continuous forging process of round pieces that can reduce the axial center cracking that tends to occur at the center of the piece, which is one of the causes of internal flaws in the product, and the forging process described above. It is equipped with a piercing-rolling process that prevents the occurrence of flaws inside the hollow shell and does not cause rolling failure.

Further, the present invention relates to a method for manufacturing a seamless pipe that can produce a seamless pipe with less internal flaws.

Background art

[0002] In a process of producing a seamless pipe from a continuously forged piece by the Mannesmann method or the like without going through a rolling or forging process, there is a defect that is unique to the freeze solidification at the center of the continuously forged piece. Some axial center cracks are likely to occur. Therefore, when this piece is used as a billet for pipe making as it is, it tends to generate internal flaws in the piercing and rolling process, and that flaw often becomes a fatal defect of the product.

[0003] A secondary cooling method for improving the quality of the central portion of a continuous forged piece using heat shrinkage during the cooling of the piece has been disclosed for the purpose of reducing internal defects in the continuous forged piece. In Japanese Patent Laid-Open No. 7-1096, when the solid fraction in the central part of the piece reaches 0.1 to 0.3, the water density is 25 to L00LZ (min'm ² ). The center porosity generated in the central part of the steel piece is reduced by starting the surface cooling of the steel piece by means of, and continuing the cooling of the above water density until the solid phase ratio in the central part of the steel piece reaches 0.8 or more. A method is disclosed. Here, the solid phase ratio means the fraction occupied by the solid phase in the solid-liquid coexisting phase. This prior document does not clearly indicate the mechanism of shaft center cracking or the conditions for reducing shaft center cracking.

[0004] Further, a method for controlling the cooling of the flakes by controlling the specific amount of cooling water and improving the quality of the flakes is disclosed in JP-A-8-332556 and JP-A-2001-62550 It is disclosed. [0005] The method disclosed in the above-mentioned Japanese Patent Application Laid-Open No. 8-332556 reduces the center porosity at the center of the flank and prevents the cracking of the axial center portion. In the steel that is produced, axial center cracks may occur. Further, according to the method disclosed in Japanese Patent Laid-Open No. 2001-62550, the shrinkage rate of the surface can be made larger than the shrinkage rate of the central part of the flange, and the center porosity or center generated at the central part of the flange is obtained. Force that can reduce segregation As in the method disclosed in Japanese Patent Laid-Open No. 8-332556, in steel that forms a ferrite phase as a primary crystal during solidification, cracking of the axial center may occur. . Therefore, the methods disclosed in both of the above publications have room for further improvement in these respects.

[0006] Japanese Patent Laid-Open No. 2003-117643 discloses macro segregation, semi-macro segregation by utilizing the shrinkage of the solidified shell on the surface of the slab by cooling the surface of the slab by secondary cooling at the end of solidification. A method for reducing the center cavity is disclosed. However, in steels that produce ferrite phases as primary crystals during solidification, axial center cracks may occur, and further improvements are necessary.

[0007] Further, the present inventors disclosed in JP-A-2004-330252, a steel having a C content of 0.1% by mass or less, or a Cr content of 1% by mass or more, and a C content. However, the secondary cooling is performed immediately after the piece comes out of the bowl shape, and then the piece surface temperature is 950 to 1100 °. We proposed a continuous forging method of round round pieces that continues the secondary cooling at the end of solidification until the center of the piece is completely solidified even when the force at the point of reaching C range is reached. However, in these cooling methods, when the diameter of the piece is large, the thermal resistance of the solidified shell increases, so that the cooling effect on the shaft center part and the sufficient crack improvement effect cannot be obtained.

On the other hand, the Mannesmann plug mill type pipe manufacturing method, which is used as a representative method for producing seamless pipes, sends a solid billet heated to a predetermined temperature to a piercer, and A hollow shell is manufactured by piercing and rolling the shaft center. The hollow shell then becomes a product seamless pipe through a drawing and rolling process using a mandrel mill, reheating, or a direct diameter rolling process using a stretch reducer or sizer mill, and a refining process. [0009] In the piercing and rolling process, since the billet is rolled along the pass line, a pair of inclined rolls having a barrel shape (barrel shape) or a cone shape in which the roll axis is inclined with respect to the pass line are arranged opposite to each other. is doing. Further, a plug held by a mandrel disposed on the pass line is positioned between these inclined rolls.

As the piercer roll, a cone-shaped inclined roll may be used.

FIG. 1 is a diagram schematically illustrating the arrangement of cone-shaped inclined rolls used in the piercing and rolling process. Further, FIG. 2 is a view for explaining the arrangement of the cone-shaped inclined rolls indicated by arrows AA in FIG.

The inclined roll 1 has a gorge portion la having a roll diameter Dg in the middle portion thereof, and an entrance surface lb having a substantially truncated cone shape that is reduced in diameter toward the entrance end portion of the gorge portion la, and toward the exit end surface. And an exit face lc having a substantially frustoconical shape which is expanded in accordance with the shape, and has a cone-like shape as a whole.

[0011] The inclined roll 1 is arranged symmetrically with respect to the pass line X-X so that the roll axis is at an intersection angle γ. Further, as shown in FIG. 2, the inclined roll 1 is arranged so as to have an inclination angle j8 with respect to the pass line Χ-Χ. On the other hand, the other inclined rolls 1 not shown in FIG. 2 are also opposed to each other while being inclined in opposite directions at an inclination angle across the pass line XX.

The inclined roll 1 is directly joined to each drive device 4. Thus, each tilt roll 1 can rotate around the roll axis while securing the crossing angle γ and the tilt angle j8, and imparts a swiveling motion to the billet 3.

The plug 2 has a bullet-like shape as a whole, and its rear end is supported by the front end of the mandrel bar M, and the rear end of the mandrel bar M is connected to a thrust block device (not shown).

[0013] The billet 3 fed to the piercer configured as described above is rolled by the inclined roll 1 and the plug 2 while being swung while passing through the gap between the inclined rolls. Become.

[0014] A mechanism of generation of holo-shell inner surface defects in the piercing and rolling process will be described. The billet is swept between the inclined rolls and reaches a tip of the plug until a pair of inclined Compressed while being swirled by an oblique roll. At this time, due to the so-called “rotary forging effect”, the center portion of the billet becomes brittle and is easily perforated. If the effect of this rotational forging effect is too great, a void is generated at the center. In extreme cases, the center is destroyed and radial cracks occur. Even if the billet before piercing and rolling has no internal cracks in the center, the voids and cracks in the center caused by the "rotary forging effect" as described above can become the inner surface of the hollow shell after piercing and rolling. .

[0015] Further, if the billet is segregated in the center, the continuous forging material is likely to cause center porosity, the stainless steel containing 5% or more of Cr, which is likely to generate δ ferrite, and the non-ferrous structure such as copper or copper alloy. In the case of a material with poor workability that remains, when it is pierced and rolled, it promotes the generation of inner shell flaws in the hollow shell.

[0016] By the way, in order to prevent the occurrence of flaws on the inner surface of the holo-shell in the piercing and rolling process, various methods have been proposed for the conventional force. For example, JP 03-13222, JP 61-3605, 2000-140911, billet, plug and inclined roll geometry such as billet outer diameter reduction rate, inclined roll opening, etc. A method for optimizing target position conditions is disclosed.

[0017] However, the method disclosed in the above-mentioned Japanese Patent Application Laid-Open No. 03-13222 places greater emphasis on the squeezing property than the prevention of inner surface flaws at the top of the billet. Although the method disclosed in Japanese Patent Laid-Open No. 61-3605 can prevent internal flaws occurring in the material to be rolled, there is a risk of causing a rolling failure (hereinafter, also referred to as “misroll”) due to poor penetration. is there. Further, the method disclosed in Japanese Patent Laid-Open No. 2000-140911 cannot sufficiently prevent the generation of internal flaws at the top portion of the billet, similar to the method disclosed in Japanese Patent Laid-Open No. 03-13222.

Disclosure of the invention

[0018] The present invention has been made with the primary aim of preventing the occurrence of internal flaws in the continuous production process and the piercing and rolling process of round pieces in the seamless pipe manufacturing process. The continuous forging process of round slabs that significantly reduces the cracks that cause cracks, and the prevention of misrolls and the occurrence of surface creases in the holo shell when piercing and rolling the fabricated round billets It has a piercing and rolling process that can achieve both effects of For the purpose of providing a seamless pipe manufacturing method that can manufacture high quality products!

[0019] The present inventor has studied a seamless pipe manufacturing method capable of manufacturing a holo-shell with less internal flaws with high productivity, and obtained the following knowledge (a) to (f), The present invention has been completed.

[0020] (a) The ferrite phase has a lower strength than the austenite phase, and the C content is 0.1% by mass (hereinafter, “mass%” is also simply referred to as “%”). It is easy for cracks in the shaft center to occur due to the solidification of the bright phase. In addition, in the piece having the above-mentioned C content, as the diameter increases, the axial center cracking occurs, and when the piece diameter exceeds 300 mm immediately, the effect of forced cooling of the piece surface by secondary cooling is effective. Forced cooling that can be reduced simply leads to an increase in axial cracks. Therefore, it is appropriate to perform slow cooling including radiation cooling from the surface of the sepal.

[0021] (b) In the case of a large-diameter piece, the surface of the solid piece is slowly cooled and the equiaxed crystal ratio of the central part is reduced within the range of more than 1.0 and 1.0 or less at the solidification rate at the end of solidification. By increasing it, shaft center cracking is reduced. Also, in the case of a piece having a diameter of more than 300 mm, the center part of the cross section has a radius of 15 mm from the center of the piece cross section by making all regions within a diameter of 60 mm have an equiaxed crystal structure. It is possible to suppress to the area within.

[0022] (c) Even if the round flakes produced by the method of (b) above are pierced and rolled without undergoing a bulk rolling process, the occurrence of internal flaws in the holo-shell is suppressed. The

[0023] (d) The roll diameter Dg of the inclined roll gorge shown in FIG. 5, which will be described later, and the roll diameter at the billet contact start position on the roll at the same roll inlet (hereinafter also referred to as “inlet roll diameter”) D1 Ratio, that is, (DgZDl) force, in the range, the value of the ratio (NZDf) of the billet rotation number N and billet outer diameter reduction ratio Df after the billet is squeezed into the inclined roll to reach the plug tip Regardless, internal flaws are likely to occur. In the range where the roll diameter ratio (DgZD 1) is large, the generation of inner surface flaws can be suppressed. However, if the ratio (N / Dg) between the billet rotation number N and the billet outer diameter reduction ratio Df is small, misroll Incidence rate increases. Furthermore, when the ratio of the inlet roll diameter D1 to the billet outer diameter Bd (DlZBd) is less than 2.5, the stagnation state of the billet tends to be unstable and misrolling tends to occur frequently.

[0024] (e) When using a piercing-rolling plug having the shape shown in FIG. The outer diameter d of the part is 0.35 times or less of the billet outer diameter Bd, the sum of the axial length of the tip spherical surface and the length of the cylindrical part (L1 + L2) is 0.5 times or more of d, and If the shape is such that the axial length L3 formed by the radius of curvature R and the arc rotation plane satisfies the parameter “(dZ2Bd) / (R / L3)” of 0.046 or less, the billet outer diameter reduction ratio Df Even if it is made smaller, no stagnation failure occurs, and Mannesmann fracture and circumferential shear strain are suppressed, and the occurrence of flaws on the inner surface of the holo-shell can be prevented.

[0025] If d is set to less than 0.12 times Bd while the force is applied, the rolled end portion is liable to be melted and the plug life is shortened. Furthermore, if (L1 + L2) is more than 3 times d, the rolled end part will be easily deformed, and the total length of the plug will be too long.

In addition, if R and L3 have a value of less than 0.020 in the parameter “(dZ2Bd) / (R / L3)”, the occurrence of circumferential shear strain cannot be sufficiently suppressed.

Here, the billet outer diameter reduction rate Df (hereinafter also referred to as “plug tip draft rate Df”) means the reduction rate of the billet outer diameter at the plug tip position, as will be described later. It is a value represented by {(Bd-Rpg) / Bd} X 100 where the inclined roll gap is Rpg.

[0026] (f) Ensuring high-temperature strength of the plug tip rolling portion, out of the dimensions of each part of the plug having the shape shown in Fig. 4, the outer diameter d of the tip rolling portion is 0.12 times or less of the billet outer diameter Bd. If the shape is more than 0.5 times the axial length (L1 + L2) force and the curvature radii R and L3 satisfy the parameter “(dZ2BdZ (RZL 3)” of 0.046 or less, the billet Even if the outer diameter reduction ratio Df is reduced, no stagnation occurs.

[0027] On the other hand, when d is less than 0.06 times Bd, as described in the above (e), even if the tip rolling portion is strengthened, the heat capacity is small, so that it is easy to melt. Furthermore, if (L1 + L2) is more than 3 times d, the rolled end part is likely to be deformed and the entire plug length becomes too long.

In addition, if R and L3 have a parameter “(dZ2Bd) / (R / L3)” of less than 0.020, the occurrence of circumferential shear strain cannot be sufficiently suppressed.

[0028] The present invention has been completed based on the above findings, and the gist of the present invention is a method for producing a seamless pipe having a continuous forging step shown in the following (1), and a circle shown in (2). Sepal and (3) It exists in the manufacturing method of a seamless pipe provided with the piercing-rolling process shown to (7).

[0029] (1) At least a region within a diameter of 60 mm in the central part of the cross section of the round piece has an equiaxed crystal structure, and the solid part in the central part has a solid fraction of more than 0 and 1.0 or less. It is equipped with a continuous forging process that forges round pieces with a carbon content of 0.1% by mass or less and a cross-sectional diameter of more than 300mm while performing slow cooling at a surface cooling rate of 10 ° CZ or less. A seamless pipe manufacturing method characterized by the above (hereinafter, also referred to as “first invention”).

[0030] (2) A round rod piece produced by the continuous forging process described in (1) above, wherein an axial center crack generated in the center portion of the rod piece has a radius from the center of the rod cross section. A round piece (hereinafter, also referred to as “second invention”) characterized by being within an area of 15 mm or less.

[0031] (3) A seamless pipe-making method (hereinafter referred to as "third invention"), characterized by comprising a step of piercing and rolling the round piece as described in (2) above without rolling it in pieces. Also noted).

[0032] (4) A plug is disposed along a path line between a pair of cone-shaped inclined rolls arranged opposite to each other around the path line, and the billet made of round rod pieces as described in (2) above is swung. A roll diameter Dg (mm) of the inclined roll gorge part and a roll diameter Dl (mm) at the position where the billet starts to contact the roll at the inclined roll inlet DgZDl, and the billet Squeezing force The number of billet rotations N to the plug tip The ratio of billet outer diameter reduction ratio Df (%) NZDf satisfies the deviation of the following formulas (1) to (3). A method for producing a seamless pipe, wherein the ratio DlZBd between the D1 and the billet outer diameter Bd (mm) satisfies the following expression (4) (hereinafter also referred to as “fourth invention”).

When Dg / DK l. 1

23≤N / (Df / 100) ≤40 (1)

1. l≤Dg / DK l.

20≤N / (Df / 100) ≤44 (2)

1. 5≤Dg / Dl≤l.

20≤N / (Df / 100) ≤48 (3)

Dl / Bd≥2.5 (4)

Where Ld is the bite squeezing point force, the distance in the pass line direction to the plug tip (mm), β is the tilt angle of the tilt roll (°), and Rpg is the tilt roll at the plug tip position. When the gap (mm) is assumed, the following relations (5) and (6) are satisfied.

N = 2LdZ (7u .Bd'tan iS) (5)

Df = {(Bd-Rpg) / Bd} X 100 (6)

[0033] (5) The solid round billet having the outer diameter Bd (mm) having the round rod side force as described in (2) above, wherein the outer diameter d (mm) is the same diameter over the axial direction or the outer diameter d is the axial direction. A rolled end with a cylindrical shape with an axial length L2 (mm) that increases toward the rear end, and its tip surface is formed into a spherical shape with a radius of curvature r (mm) and an axial length L1 (mm) Then, the workpiece part having an axial length L3 (mm) formed by an arc rotation surface having a radius of curvature R (mm) so that the outer diameter increases continuously toward the rear end in the axial direction continuously from the tip rolling part. And a tapered cylindrical shaft formed with a taper angle of 2 Θ (°) so that the outer diameter increases continuously toward the maximum outer diameter D (mm) at the rear end in the axial direction. A plug having a reeling portion with a length L4 (mm) is provided, and includes a step of piercing and rolling with an inclined roll type piercing and rolling machine, and the plug has an outer diameter d, a radius of curvature R, and an axial length. The seamless pipe manufacturing method (hereinafter referred to as “No. 1”) characterized in that the relationship between Ll, L2 and L3 and the outer diameter Bd of the solid round billet satisfies the deviations of the following equations (7) to (9). Also referred to as “5 inventions”).

0. 12≤d / Bd≤0. 35 (7)

0.020≤ (d / 2Bd) / (R / L3) ≤0. 046 (8)

0. 5d≤Ll + L2≤3d (9)

[0034] (6) The solid round billet having the outer diameter Bd (mm) having the round rod side force as described in (2) above, wherein the outer diameter d (mm) is the same diameter or the outer diameter d is the axial direction. A rolled end with a cylindrical shape with an axial length L2 (mm) that increases toward the rear end, and its tip surface is formed into a spherical shape with a radius of curvature r (mm) and an axial length L1 (mm) Then, the workpiece part having an axial length L3 (mm) formed by an arc rotation surface having a radius of curvature R (mm) so that the outer diameter increases continuously toward the rear end in the axial direction continuously from the tip rolling part. And a tapered cylindrical shaft formed with a taper angle of 2 Θ (°) so that the outer diameter increases continuously toward the maximum outer diameter D (mm) at the rear end in the axial direction. Using a plug that has a reeling part with a length of L4 (mm) and at least the tip rolled part has a tensile strength at 1100 ° C of 50 MPa or more, and is pierced by a tilt roll type piercing and rolling mill. Provided with a step of hole rolling, the outer diameter d of the plug, the half curvature Diameter Axial length Ll, L2 and L3 and solid round billet outer diameter Bd relationship between the following formulas (8) to (10): (Hereafter referred to as “Sixth Invention”).

0. 06≤d / Bd≤0. 12 (10)

0.020≤ (d / 2Bd) / (R / L3) ≤0. 046 (8)

0. 5d≤Ll + L2≤3d (9)

[0035] (7) The method for manufacturing a seamless pipe as described in (6) above, wherein the tip rolling portion of the plug is replaceable (hereinafter also referred to as "seventh invention").

[0036] In the present invention, the "center part solid phase ratio" refers to the fraction of the solid phase occupied by the total of the solid phase and the liquid phase in the center part of the sepal.

“Slow cooling” means cooling at a slow cooling rate, including radiation cooling from the surface of the blade, and cooling at a surface temperature of 10 ° CZ or less.

“A cylindrical shape with an axial length L2 (mm) that increases as the outer diameter d increases toward the rear end in the axial direction” means that the outer diameter d increases toward the rear end in the axial direction. ° It means a cylindrical shape with an axial length L2 (mm) that increases below.

Brief Description of Drawings

FIG. 1 is a diagram schematically illustrating the arrangement of cone-shaped inclined rolls used for piercing and rolling.

FIG. 2 is a view for explaining the arrangement of the cone-shaped inclined rolls indicated by the arrows AA in FIG.

FIG. 3 is a diagram showing an example of a plug having a simple shell shape force.

FIG. 4 is a diagram showing a plug shape used in the present invention B. FIG.

FIG. 5 is a diagram schematically illustrating a state in which a billet is pierced and rolled by placing a plug between a pair of inclined rolls opposed to each other around a pass line.

Fig. 6 is a diagram schematically showing the solidified structure and axial center crack in the cross-section of the flake, and Fig. 6 (a) shows that the central portion of the flake where the axial crack occurs is filled with equiaxed crystals. Fig. 2 (b) shows the case where the central part is not filled with equiaxed crystals.

FIG. 7 schematically shows a longitudinal section of a continuous forging apparatus used in the continuous forging process of the present invention. FIG.

FIG. 8 is a diagram for explaining the plug lead in the billet piercing and rolling, and the billet outer diameter rolling reduction at the plug tip position.

BEST MODE FOR CARRYING OUT THE INVENTION

[0038] 1. Modes for carrying out the first to third inventions

1-1. Best Mode of Inventions 1 to 3

As described above, the method for producing a seamless pipe according to the present invention has an equiaxed crystal structure in all regions within a diameter of 60 mm at the center of the cross section of the round rod piece, and the solid phase ratio in the center exceeds zero. 1. 1. While the cooling rate of the round rod surface is 10 ° CZ or less in the range of 0 or less, the carbon content is less than 0.1% by mass and the diameter of the rod cross section exceeds 300mm. A method for producing a seamless pipe, comprising a continuous forging step of forging a round piece. Further, the present invention is a seamless pipe manufacturing method comprising a step of piercing and rolling round slabs forged by the above-mentioned continuous forging step, without performing ingot rolling. In the following, the method of the present invention will be described in more detail.

[0039] 1-1-1. Relationship between the solidified structure of the piece and the inner surface

As a result of detailed observation of the flakes and the hollow shell, there is a close relationship between the axial cracks in the flakes and the distribution of equiaxed crystals in the central part. It has been found that there is an important relationship between the axial crack accompanying the cracks and the degree of internal flaws after piercing and rolling.

[0040] FIG. 6 is a diagram schematically showing a solidified structure and a shaft center crack in the cross-section of the scissors.

(a) shows the case where the center part of the flakes where the axial center crack occurs is filled with equiaxed crystals, and (b) shows the case where the same center part is not filled with equiaxed crystals. To express.

[0041] In the case of (a) in the figure (hereinafter also referred to simply as “(a)”), the opening of the axial center crack is large, but the presence of the axial center crack from the center of the rod piece exists. The range is small. On the other hand, in the case of (b) in the figure (hereinafter also referred to simply as “(b)”), the opening of the axial center crack is small, but the axial center crack has a large force at the center of the flank. Exists. In this case, the axial cracks are generated along the grain boundaries of the columnar crystals, and the appearance is similar to that of the fine cracks.

[0042] The difference in the solidified structure and the axial center crack between the cases (a) and (b) is as follows. Presumed to be caused by the reason. That is, when the central part of the piece reaches the final solidification zone, tensile stress due to thermal stress is generated in the circumferential direction. This tensile stress is greatest at the central part of the steel piece where the temperature difference with the outer periphery of the steel piece is the largest, and when this tensile stress exceeds the strength of the material, radial cracks in the central part of the steel piece, that is, the axial center. Partial cracking occurs.

[0043] When the circumferential stress is generated in the central part of the crumb, the stress is likely to be dispersed if the central part of the crumb is filled with an equiaxed crystal structure, that is, fine crystal grains. On the other hand, when the central part of the flake has a columnar crystal structure, the columnar crystal structure has large crystal grains, so that stress tends to concentrate on the surface of the crystal grain boundary. Also, cracking occurs.

[0044] The thermal stress increases as solidification progresses to the center of the slab, but in the columnar crystal region, fine cracks occur with relatively small stress, and the cracks reach the center while releasing the stress. As a result, the center does not have a large open crack. On the other hand, in the equiaxed crystal region, cracks do not occur at low stress generated near the outer periphery of the center, and therefore strain energy due to thermal stress is not released and accumulates over time. Therefore, when the thermal stress is maximized at the center, this stress significantly exceeds the strength of the material and cracking occurs. In this case, the stored energy is released at once, resulting in large open cracks.

[0045] Further, in the case of a piece having a large cross-sectional diameter, if the surface of the piece is forcibly cooled by spraying or the like, the cooling effect does not reach the inside of the piece. As a result, it was found that a large tensile strain was generated in the inside of the slab, and an axial center crack with a larger opening was formed.

[0046] Furthermore, it was found that when round round pieces having different internal solidification structures and axial cracks as described above were pierced and rolled, the occurrence of internal flaws in the holo-shell was different. In other words, the central part of the flakes is filled with equiaxed crystals as in (b). Compared to the case, the central part of the flakes is filled with equiaxed crystals as in (a). If this is the case, the generation of internal flaws in the holo shell will be significantly reduced.

[0047] In the piercing-rolling process of round flakes, even if the size of the cracks is very small, as shown in (b), when the cracks are in a wide range of the central force of the flakes, the inner surface flaws Prone to occur Is shown. This fact is a new finding found by the present inventors that is completely different from the conventional one.

[0048] 1-1-2. Reasons for limiting the scope of the first invention to the third invention and preferred ranges

The reason why the present invention is limited to the above range and the preferable range of the present invention will be described below.

[0049] 1) C content of the piece

C is an austenite stabilizing element, and it is well known that the C content largely controls the quantity ratio of ferrite and austenite. In general, according to the present inventors' investigation that the ferrite phase has a lower strength than the austenite phase, the axis as described above is caused by the ferrite phase in a piece having a C content of 0.1% or less. It was found that cracks in the core are likely to occur.

For the reasons described above, the present invention is effective even when forging steel pieces using molten steel having a C content of 0.1% or less in steel where shaft center cracks are likely to occur. Assumptions.

[0050] 2) Size of the piece and cooling method

As the diameter of the piece increases, the occurrence of axial cracks tends to increase. When the diameter of the piece exceeds 3 OOmm, not only the effect of secondary cooling to forcibly cool the surface of the piece is reduced, but also the force It turned out that it led to the expansion of the axial crack. Therefore, for large-diameter pieces, forced cooling such as secondary cooling at the end of solidification is avoided as much as possible, and the surface cooling rate is 10 ° CZ min including radiation cooling from the piece surface. The following slow cooling is required.

Note that the cooling rate of the slow cooling is preferably adjusted to 8 ° CZ or less. Moreover, unless heating or heat retention is performed, it is realistic to set a cooling rate of about 4 ° CZ or more under radiation cooling conditions.

[0051] 3) Position of slow cooling and range of equiaxed crystallization

The cracks in the shaft center part of the flakes slowly cool the surface of the flakes at the end of solidification, specifically when the solid fraction of the central part of the flakes exceeds 0 and 1.0 or less. It can be reduced by increasing the equiaxed crystal region at the center of the cross section. The above slow cooling is In addition, it is preferable that the surface temperature of the flank is in the range of 1050-850 ° C.

[0052] For a flake with a cross-sectional diameter of more than 300 mm, by making all regions within 60 mm in diameter at the center of the cross-section into an equiaxed crystal structure, cracks in the axial center part and the central force of the cross-section of the flake also have a radius It can be suppressed to an area within 15 mm. Therefore, in the continuous fabrication process of the present invention, at least the region within the diameter of 60 mm in the central portion of the cross section of the piece has an equiaxed crystal structure, and the solid fraction in the central portion exceeds 0 and is 1.0 or less. It was decided to cool the surface of the piece slowly within the range. It is more preferable to perform slow cooling in the range where the central solid phase ratio is 0.3 or more and 0.8 or less.

[0053] Further, it has been found that the width of the equiaxed crystal region can be controlled by changing the position and strength of electromagnetic stirring, the forging temperature, and the like. In particular, the forging temperature is important, the molten steel temperature in the tundish is preferably low, and ΔT (the temperature of the target steel, the liquidus temperature of the target steel) is preferably 70 ° C or less. However, if ΔΤ is too small, the problem of nozzle clogging will cause the problem of skinning that solidifies the molten steel surface in the mold, so it is preferable to set the value to 20 ° C or higher.

Furthermore, as a result of various tests, even when the above-mentioned flakes are reheated without going through a block rolling process and pierced and rolled as they are, the obtained holo-shell has almost no internal flaws. It became clear that it became power.

[0054] In the third invention, the preferred condition when the obtained billet made of round bar is pierced and rolled by the Mannesmann plug mill method is that the roll inclination angle / 3 is 6 to 16 °, and the billet The outer diameter reduction is in the range of 3-7%.

[0055] 1-2. Examples relating to the first to third inventions

In order to confirm the effect of the present invention, the following continuous forging test was conducted, and a piercing and rolling test was conducted using the obtained round piece, and the results were evaluated.

[0056] (Test method)

FIG. 7 is a diagram schematically showing a longitudinal section of a continuous forging apparatus for realizing the continuous forging process of the present invention. As a continuous forging device, a curved continuous forging machine for round billet forging was used. The molten steel 23 injected from the tundish 211 through the immersion nozzle 21 into the vertical mold 22 is cooled by a top zone secondary cooling device 27 installed immediately below the vertical mold 22 and While being supported by a single roll 28, a solidified shell 25 is generated and pulled out by a pinch roll 29 to form a flange piece 26. The piece 26 that has produced the solidified shell 25 is cooled by the top zone secondary cooling device, and further cooled by the end-solidification secondary cooling device 210 to be completely solidified.

[0057] Here, the top zone secondary cooling device 27 is thin in the thickness of the solidified shell 25! In the region, the solid piece 26 is cooled to promote its solidification and prevent deformation due to bulging. Has the effect of The top zone secondary cooling device 27 is composed of an air mist spray with a length of 2 m connected directly below the vertical 22 and the air / water ratio is about 50 (NLZmin—air Z (L / min water)). . The water density can be adjusted to any value within the range of maximum 500LZ (min · m ² )!

[0058] The end-solidification secondary cooling device 210 is composed of a cooling device in which 5 blocks each having a length of 1.2 m are combined, and is installed at a position 30 to 36 m from the meniscus 24. Yes. An air mist spray was also used for this secondary cooling device, and the air / water ratio was fixed at about 30 (NLZmin—Air Z (LZmin—Water)) regardless of the amount of water. The amount of water can be adjusted to any value within the range of maximum water density of 100LZ (min-m ² ).

The central solid fraction and the temperature distribution in the solidified shell 25 of the lug piece 26 were obtained by unsteady heat transfer calculation.

[0059] By comparing this calculation result with the results of the temperature measurement and the strike test on the surface of the scissors, it was confirmed in advance that the above calculation results had high estimation accuracy. Therefore, this calculation method can represent the exact solidification state of the piece for each forging condition.

In addition, in order to ensure stable equiaxed crystals of the slabs, a saddle type electromagnetic stirring device 212 was installed at a position approximately 200 mm below the meniscus. The magnetic stirring device 212 has a frequency of 4 Hz, a maximum current of 600 A, and a magnetic flux density of 0.6 T (Tesla). By changing the value of the current flowing through the coil of the electromagnetic stirring device 212, the magnetic flux density can be changed to change the stirring intensity. In this fabrication test, the rotation frequency of the magnetic field was in the range of 3 to 6 Hz.

[0060] Further, detailed test conditions will be described. For the forging test, C: 0.05-0.07%, Si: 0.05-0.3%, Mn: l. 2-1.5%, P: 0.080-0.015%, S : 0. 001 to 0.0 Using molten steel with a component composition of 06%

Table 1 shows the test conditions.

[0061] [Table 1]

[0062] In the same table, the cooling rate represents the maximum value (° CZmin) of the cooling rate on the surface of the slab when the solid fraction at the center of the slab exceeds 0 and is 1.0 or less.

The diameters of the forged pieces were 310 mm and 360 mm. The degree of equiaxed crystal filling in the center of the cross section of the slab (diameter when the equiaxed crystal is approximated by a circle) can be changed by changing the molten steel temperature and electromagnetic stirring conditions during forging. It was. Liquid phase of steel The wire temperature was 1520 ° C, and the value of (molten steel temperature – liquidus temperature) was defined as the superheat (° C) of the molten steel in the tundish.

[0063] Further, by changing the forging speed and the secondary cooling conditions in the top zone, the range of the surface temperature of the flakes and the solid fraction at the central part of the flakes in the secondary cooling region at the end of the solidification phase was adjusted. A portion of the steady forging speed region was cut out from the obtained round piece (round billet) and subjected to an internal investigation of the piece and a piercing and rolling test.

[0064] A 2 m long piece was taken for internal investigation of the piece, and 10 crossed sample plates were taken at equal intervals in the longitudinal direction, mirror-polished, and then acid-corroded to cause cracks in the axial center and the like. The state of axial crystal formation was investigated. The equiaxed crystal filling condition was evaluated by obtaining the diameter (mm) of the region where the solidified structure is occupied only by the equiaxed crystal structure by a circle at the center of the cross section of the flake, and evaluating the diameter as the equiaxed crystal region diameter. .

[0065] Ten pieces of 6 m long pieces were cut out for each forging condition for the piercing and rolling test. These slabs for piercing and rolling test were heated to 1200 ° C in a heating furnace and then rolled with a piercing and rolling mill (tilted roll type piercing and rolling mill), with a roll inclination angle of 3: 8 to 16 ° and a billet outer diameter reduction rate. : From 3 to 7%, pierced and tilted, rolled from 310 mm diameter round shell, outer shell 330 mm outer diameter, and from 360 mm diameter round shell, outer shell 370 mm outer shell Were manufactured respectively. The inner shell of the hollow shell thus obtained was examined by ultrasonic flaw detection.

[0066] Table 1 shows the equiaxed crystal region diameter, the axial center crack length, and the number of inner surface defects. Here, the axial crack length is represented by the maximum diameter (mm) of the area where the axial crack is present in all the observed cross-sectional samples, and the number of internal defects is 10 pieces. The average number of occurrences (individual Z pieces – pieces) determined based on the number of internal flaw occurrences investigated for the sample was displayed.

[0067] (Test results)

Test Nos. A1 to A8 are tests for examples of the present invention that satisfy the conditions specified in the first invention of the present invention A, and test Nos. A9 to A20 indicate at least one of the conditions specified in claim 1. We are not satisfied with this test.

[0068] In test numbers A1 to A8, the secondary cooling at the end of solidification was slowly cooled at a cooling rate of less than 10 ° CZ. As a result, the diameter of the equiaxed crystal region at the center of the cross-section of the flake was 60 mm or more, and the crack length of the flake shaft center part was as low as 30 mm or less, indicating good flake properties. .

In addition, the hollow shells obtained by piercing and rolling round pieces produced by these forging tests showed a low strength of 0.1 (piece Z-round pieces) or less.

[0069] On the other hand, test numbers A9 to A16 are tests in which the tundish molten steel superheat degree and the stirring intensity of electromagnetic stirring were changed with respect to test numbers A1 to A8. As a result, the diameter of the equiaxed crystal region at the center of the cross-section of the flank became less than 60 mm, which did not satisfy the conditions specified in the first invention. Numerous axial cracks of the flakes occur along the crystal grain boundaries of the columnar crystals distributed around the outer periphery of the equiaxed crystal, and the axial cracks have the form shown in Fig. 1 (b). did. In addition, the crack length of the axial center part was also significantly longer than that of the inventive examples of test numbers A1 to A8.

[0070] Further, the number of inner surface flaws generated in the hollow shell obtained by piercing and rolling these round pieces is 15

It was an extremely high value (individual z per piece).

[0071] In addition, in test numbers A17 to A20, the stirring intensity of electromagnetic stirring was particularly increased, and the solid phase fraction in the center part of the sepal exceeded 0 and was 1.0 or less by the end-stage secondary cooling device. Forced cooling with spray was performed, and the surface of the chip was cooled at a cooling rate of 25 ° CZ or more. As a result, the filling condition of equiaxed crystals at the center of the cross section of the flakes was good, and the shape of the axial center crack was the shape shown in Fig. 1 (a), but the result of strong cooling The crack in the shaft center has expanded.

Furthermore, although the number of inner surface flaws generated in the hollow shell obtained by piercing and rolling these round pieces is lower than that of test numbers A9 to A16, it is compared with the present invention examples of test numbers A1 to A8. It became a high value.

[0072] 2. Mode for carrying out the fourth invention

2- 1. Best Mode for Fourth Invention

In the fourth invention, as described above, a plug is arranged along a pass line between a pair of cone-shaped inclined rolls arranged opposite to each other around the pass line, and the billet, which is a round rod single force of the second invention, is swung. A ratio DgZDl of a roll diameter Dg (mm) of the inclined roll gorge portion and a roll diameter Dl (mm) at a position where the billet starts to contact the roll at the inclined roll inlet, and the billet Squeezing force The ratio of the billet rotation number N to the billet outer diameter reduction ratio Df (%) NZDf satisfies any of the above formulas (1) to (3), and D1 and the billet outer diameter Bd (mm The ratio D1Z Bd satisfies the above formula (4). Hereinafter, the method of the present invention will be described in more detail.

[0073] 2- 1-1. Correlation of major factors affecting piercing and rolling

Prevent misroll occurrence, prevent internal flaws due to temperature drop at the top of the hollow shell, and prevent internal flaws due to excessive rotary forging over the entire length including the top! As a result of investigating how to satisfy, the ratio of the inlet roll diameter to the roll diameter of the inclined roll gorge at the position where the billet starts to contact the inclined roll, and the billet rotation until the billet squeezing force reaches the plug tip position. It was found that the ratio of the number of times to the outer diameter reduction rate of the billet was a very important factor.

FIG. 5 is a diagram schematically illustrating a situation in which a billet is pierced and rolled by placing a plug between a pair of inclined rolls arranged to face each other around a pass line. In the figure, the tilt angle j8 of the tilt roll 1 is set to zero. The gorge portion la of the cone-shaped inclined roll 1 is a position where the inlet surface lb and the outlet surface lc of the inclined roll 1 cross each other, and the gap between the pair of inclined rolls 1 and 1 is the minimum. In the roll gorge part la, the roll diameter is Dg (mm). The shape of the inlet surface lb of the inclined roll 1 may be a cross-sectional shape having two or more gradient angles, or may be a curved cross-sectional shape.

Further, in the geometric two-dimensional plane shown in FIG. 5, the inclined roll diameter at the point A where the billet 3 starts to contact the inclined roll inlet surface lb is shown as the inlet roll diameter Dl (mm). The distance in the direction parallel to the pass line X—X (distance in the pass line direction) to the tip position of the same A point force plug 2 is indicated by Ld (mm). The gap between the inclined rolls at the plug tip position is R pg (mm), and the angle between the pass line X—X and the inclined roll inlet face lb is indicated by θ 1.

[0076] Next, when the outer diameter of the billet 3 is Bd (mm) and the inclination angle of the inclined roll is j8 (°), the squeezing force N the number of billet rotations up to the plug tip and the billet The outer diameter reduction rate Df can be expressed by the above equations (5) and (6).

Accordingly, the present inventors produced billets with outer diameters of 70 mm and 60 mm including the center using continuous forged pieces of 0.2% C steel, and pierced and rolled under the conditions shown in Table 2. The line Investigate the occurrence of misrolls such as stagnation defects and the presence of internal flaws

[0077] Further, the above-mentioned relational force is calculated, and the billet outer diameter reduction ratio Df at the plug tip position, billet squeezing force, the number of billet rotations N up to the plug tip position, and various roll shapes are changed. Then, a piercing and rolling test was performed. Ratio of roll diameter Dg of roll gorge to roll diameter D1 at the billet contact start position to roll DgZDl (hereinafter also simply referred to as “roll diameter ratio DgZDl”) and billet squeezing force Up to plug tip The relationship between the billet rotation number N and the billet outer diameter reduction rate Df at the plug tip position NZDf (hereinafter simply referred to as “the ratio of billet rotation number N and billet outer diameter reduction rate Df NZDf”), and Table 3 shows the relationship of the ratio DlZBd between the roll diameter D1 (hereinafter simply referred to as “inlet roll diameter D1J”) and the billet outer diameter Bd at the position where the billet starts to contact the roll at the inclined roll inlet.

[0078] [Table 2] Table 2

[0079] [Table 3] Table 3

[0080] In the evaluation in Table 3, the circles indicate the case where no inner surface flaws occurred and no misroll occurred over the entire length of the holo shell. The thumbprint indicates the case where an internal flaw occurs on the holo shell. X indicates when 3 of 20 piercing rolls have misrolled, ▲ indicates when 2 to 3 of 20 piercing rolls are misrolled, and △ indicates piercing and rolling. This shows the case where a misroll occurs in one of the twenty.

[0081] From the results shown in Table 3, the following were found and the effects of the invention could be confirmed. In other words, in the range where the roll diameter ratio Dg / Dl is small, the ratio of the billet rotation number N to the billet outer diameter reduction ratio Df is likely to cause internal flaws even when the ratio NZDf is small or large. . In the range where the roll diameter ratio DgZDl is large, the generation of inner surface flaws can be suppressed, but the ratio N / Df between the billet rotation number N and the billet outer diameter reduction ratio Df is small, and the misroll occurrence ratio increases.

Furthermore, if the ratio DlZBd force between the inlet roll diameter D1 and the billet outer diameter Bd is less than the range, for example, less than 2.5, the stagnation state of the billet becomes unstable and misrolls occur frequently.

[0082] The fourth invention is made on the basis of the above-described knowledge. As described above, the billet which is also a round-shaped piece of the second invention is used, and any one of the formulas (1) to (3) is used. This is a seamless pipe manufacturing method including a piercing-rolling process that satisfies the condition of formula (4).

[0083] 2—1 2. Reason for limiting the preferred range of the fourth invention

In the seamless pipe manufacturing method of the fourth invention, the entire length including the top portion of the hollow shell is In order to prevent the occurrence of inner surface flaws, a piercing and rolling process that satisfies any of the above formulas (1) to (3) is provided according to the range of the roll diameter ratio DgZDl. Normally, an increase in the roll diameter ratio DgZDl is effective in preventing the occurrence of internal flaws, but the upper limit is limited due to equipment restrictions.

[0084] For example, when the roll diameter Dg of the roll gorge portion is increased, the facility scale is also increased, and the facility cost is increased. On the other hand, when the inlet roll diameter D1 of the inclined roll is reduced, there are equipment problems such as reduced bearing strength on the rolling entry side. At the same time, as shown in Table 3, the roll diameter ratio DgZDl increases. Since the ratio DlZBd between the inlet roll diameter D1 and the billet outer diameter Bd decreases and misroll tends to occur frequently, the roll diameter ratio DgZDl also has an upper limit, and the upper limit is set to 1.8.

[0085] In piercing and rolling in an actual mill, the billet outer diameter reduction ratio Df is operated within an appropriate range of 4 to 8%. Therefore, when the ratio NZDf of billet rotation number N and billet outer diameter reduction ratio Df at the time of squeezing is adapted to the deviation of the above formulas (1) to (3), the outer diameter reduction ratio Df is It is also preferable to have a condition of 4 to 8%.

[0086] Further, in the seamless pipe manufacturing method of the fourth invention, the ratio DlZBd of the inlet roll diameter D1 to the billet outer diameter Bd is It is characterized by comprising a piercing and rolling process that satisfies the formula (4). If the value of DlZBd falls below the lower limit, slipping will occur when the billet is swallowed, resulting in an unstable swallowing condition. In the present invention, the upper limit of DlZBd is not defined, but the surface power of the facility is limited, and it is preferably 6.5 or less.

Dl / Bd≥2.5 (4)

[0087] The seamless pipe manufacturing method of the fourth invention is directed to the use of a cone-shaped inclined roll. The reason why barrel-shaped inclined rolls are not targeted is that the diameter ratio DgZDl is limited to 1.03 or less for barrel-shaped inclined rolls that are merely different in terms of quality and efficiency. 4) It is difficult to apply to the manufacturing method of the invention.

[0088] In the manufacturing method of the fourth invention, center porosity is easily generated when billet is segregated at the center, continuous forging material, stainless steel containing 5% or more of Cr that is likely to generate δ ferrite, and copper and copper for non-ferrous metals. Even if the material has poor structure, such as an alloy, it has a remarkable effect. The fruit can be demonstrated.

[0089] 2- 2. Examples relating to the fourth invention

In order to confirm the effect of the fourth invention, a hollow shell was manufactured by piercing and rolling, and the results were investigated.

[0090] Using the piercing-rolling machine having the configuration shown in Fig. 1 and Fig. 2, the round bar obtained in the test No. A1 of the first invention and the round bar obtained in the test No. A5 Using a billet, a piercing and rolling test was conducted under the conditions shown in Table 4. The steel composition is C: 0.05 to 0.07%, Si: 0.05 to 0.3%, Mn: l. 2 to 1.5%, and the outer diameter of each round piece is 3 10mm and 360mm.

[0091] [Table 4] Table 4

[0092] Table 5 shows the results of producing a holo-shell by piercing and rolling. The ◎ mark in the occurrence of internal flaws in Table 5 indicates that the number of internal flaws per unit length lm of the holo seal is 1 or less, and the 〇 mark per unit length lm of the holo shell. X indicates the case where three or fewer inner surface defects occur, and X indicates the case where more than three inner surface defects occur per unit length lm of the hollow shell. The misroll occurrence rate (%) is indicated by the ratio of the number of rolls generated as a result of piercing and rolling using 20 billets for each roll setting and rolling conditions.

[0093] [Table 5]

According to the results shown in Table 5, in test number B1 B5, either of the above formulas (1) to (3) is satisfied according to the roll diameter ratio DgZDl, and the condition of formula (4) is also satisfied. Since it satisfied, it was possible to completely prevent the generation of internal flaws over the entire length of the holoshell where there was no misroll. [0095] On the other hand, in test numbers B6 to B9, either of the above formulas (1) to (3) or the condition of formula (4) could not be satisfied. A defect occurred.

[0096] 3. Modes for carrying out the fifth to seventh inventions

3- 1. Best Mode of Fifth Invention to Seventh Invention

As described above, the fifth aspect of the invention is the solid round billet of the outer diameter Bd, which is the round rod piece force of the second aspect of the invention. Increasing axial length L2 in the shape of a cylinder, the tip surface of which has a radius of curvature !: A tip-rolled section formed into a spherical shape with an axial length L1, and an outer diameter that is continuous with the tip-rolled section. A workpiece part with an axial length L3 formed by a circular arc rotating surface with a radius of curvature R so as to increase toward the rear end in the direction, and the maximum outer diameter at the rear end in the axial direction. The process of piercing and rolling with a tilting roll type piercing and rolling machine using a plug having a tapered cylindrical axial length L4 and a reeling part formed so as to increase with force S against D The outer diameter d of the plug, radius of curvature R, axial length Ll, L2 and L3 and the outside of the solid round billet Wherein the relation between bd (7) to (9), a method of manufacturing a seamless pipe which satisfies even the deviation.

[0097] Here, in the case where a plug having a tensile strength at 1100 ° C of the tip rolled portion of 50 MPa or more is used, a method for producing a seamless pipe satisfying the above (10) instead of the above formula (7) Is. Hereinafter, the methods of the fifth to seventh inventions will be described in more detail.

FIG. 8 is a diagram for explaining the billet outer diameter reduction rate of the plug lead and the plug tip in billet piercing-rolling. In the description of the present invention, the plug lead PL refers to the distance from the position of the roll gorge la of the cone type inclined roll 1 to the tip position of the plug 2 as shown in FIG. In the figure, RO is the shortest distance between the inclined rolls 1 and 1 at the position of the gorge la.

Accordingly, in FIG. 8, when the plug is set so that the plug lead PL becomes small, the value defined by the above equation (6) becomes large accordingly, so that the plug lead is set small as described above. This can be rephrased as a case where the billet outer diameter reduction rate at the plug tip is set large.

[0100] In order to achieve the effects of the present invention, the present inventors have described each of the above (e) and (f). Although it has been described that it is preferable to satisfy the conditions, it is more preferable that the following conditions are satisfied.

[0101] In the fifth and sixth inventions, the cylindrical portion having the outer diameter d of the tip rolled portion and the axial length L2 is not necessarily required to have the same diameter in the axial direction, and the cutting and heat treatment are repeated. In consideration of reuse, the tip force in the axial direction of the outer diameter d may also be a tapered columnar shape with a half angle of the taper angle increasing toward the rear end of 4 ° or less.

[0102] Furthermore, the reeling portion is a portion provided to keep the thickness of the material constant, and the thickness strength is not actively performed here. For this reason, it is preferable that the angle of the reeling section is almost the same as the surface angle on the roll exit side!

Further, it is the tip rolling portion of the plug that requires a predetermined high temperature strength. Therefore, it is effective to divide the plug into a member that uses the tip rolling part and a base material that forms the work part and the reeling part.

[0103] Normally, it is preferable to use 0.5% Cr-l.5% Ni—3.0% W steel as the plug base metal. In addition, high strength steel containing W and Mo, Nb-10% W-2.5% Zr Nb alloy, or Mo—0.5% Ti—0.08% Zr It is preferable to use a Mo alloy. These are the forces that can satisfy the required high temperature strength. In this case, the scale thickness of the base material is preferably in the range of 200 μm to 1000 μm from the viewpoint of the adhesion of the scale and the plug life.

[0104] Further, as a member used for the tip rolling portion, a member in which a thickness scale is formed on a base material can be used. By forming a thick scale and covering the surface of the member, heat resistance can be ensured and effective in suppressing melting damage, and the thick scale also exhibits an excellent effect on lubricity during piercing and rolling.

[0105] 3- 2. Examples relating to the fifth to seventh inventions

(Example 1)

In Example 1, a billet having an outer diameter of 70 mm including the center portion thereof was cut out from the round piece manufactured with test number A1 of the first invention, and this was pierced and rolled to obtain the seamless of the fifth invention. The effect of the pipe manufacturing method was confirmed. Prepare two-zone plugs with the shape shown in Fig. 3 and plugs with the shape shown in Fig. 4 as the plugs to be used. Are shown in Table 6. Here, the plug number C8 is a two-zone type plug. All plugs were made of 0.5% Cr-l.5% Mo-3.0% W alloy steel.

[Table 6]

The inclined rolls (main rolls) of the piercing and rolling mill are all set with the entrance surface of the inclined rolls with the outer diameter of the gorge portion set to 410 mm, the inclination angle β set to 0 °, and the crossing angle γ set to the angles described later. Both the incoming side angle, which is the angle formed by the straight line parallel to the pass line X—X, and the outgoing side angular force, which is the angle formed by the outgoing side of the inclined roll and the straight line parallel to the pass line X—X, are 3.5 °. A cone roll was prepared. The entrance side diameter and the exit side diameter of the inclined rolls were different for each cross angle γ (5 °, 10 °, 15 °) described later.

[0108] The above-mentioned plug and inclined roll are attached to a piercing and rolling machine as shown in Fig. 5, and from the round rod piece manufactured by the continuous forging process of the first invention, the outer diameter including its center part is 70mm and the length is 700mm. , C: 0.05 ~ 0.07%, Si: 0.05 ~ 0.3%, Mn: l. 2 ~ 1.5% steel billet after heating to 1250 ° C, outer diameter 75mm A piercing and rolling test was conducted to manufacture a hollow shell having a thickness of 6 mm and a length of 2100 mm.

[0109] In the piercing and rolling test, the inclination angles 13 of the main rolls were all 10 °, and the cross angles γ of the cone-type inclined rolls were 5 °, 10 °, and 15 °, respectively. In addition, the plug tip billet outer diameter reduction ratio Df was changed in 5 steps of 3%, 4%, 5%, 6% and 7%. Table 7 shows the set dimensions of the shortest distance RO between the inclined rolls, the plug lead PL, and the inclined roll gap Rpg at the plug tip position. The test results are shown in Table 8.

[0110] [Table 7]

Table 7

(Note) The unit of RO PL and Rpg is.

When plugs with plug numbers C1 and C4 that satisfy the conditions specified in the fifth invention are used, even if the billet outer diameter reduction rate Df at the plug tip position is reduced to 3%, there is no stagnation failure. A good quality holo-shell that does not occur, has no internal defects, or has very little defects.

On the other hand, when plugs C5, C6 and 2-zone type plug number C8 that do not satisfy the conditions specified in the fifth invention are used, the billet outer diameter reduction ratio Df is 3%. In both cases, the stagnation was poor. In addition, when the plug of V and plug number C7 that did not satisfy the conditions specified in the fifth invention was used, the tip of the plug was melted.

[0113] When a plug satisfying the conditions specified in the fifth invention is used, the maximum value of the billet outer diameter rolling reduction Df, in which no inner surface flaws occur, is 3 on a piercing mill with an inclined roll cross angle γ of 5 °. When the crossing angle is increased, the upper limit of the billet outer diameter reduction ratio Df, in which no internal flaws are generated, can be expanded.

On the other hand, when plugs that did not satisfy the conditions specified in the fifth invention were used, the generation of internal flaws could not be completely suppressed.

[0114] (Example 2)

In Example 2, the billet made of round slabs produced by the continuous forging method of the first invention was subjected to a piercing and rolling test using an actual piercing and rolling machine, so that the seamless pipes of the fifth to seventh inventions were obtained. The effect of this manufacturing method was verified.

The outer diameter of 310 mm, length of 5600 mm, C: 0.05 to 0.07%, Si: 0.05 to 0.3%, Mn: l A piercing and rolling test was conducted to produce a hollow shell having an outer diameter of 325 mm, a wall thickness of 48 mm, and a length of 10000 mm after heating the billet bowl having a steel composition of 2 to 1.5% to 1250 ^° C.

[0115] As plugs to be used, three types of plugs having the shape shown in Fig. 4 and a two-zone type plug shown in Fig. 3 were prepared. Table 9 shows the dimensions of each part of these plugs.

[0116] [Table 9]

[0117] Table 10 and Table 11 show the set dimensions of the shortest distance RO between the inclined rolls, the plug lead PL, and the inclined roll gap Rpg at the plug tip position.

[0118] [Table 10] S tsollI

Out-of-plug billet drop rate (%)

(Note) The unit of R0, PL and Rpg is (mm).

Table 1 1

(Note) The unit of R0, PL and Rpg is (mm).

[0120] The inclined roll of the piercing mill used in the test has an outer diameter of the gorge part of 1400 mm, the crossing angle γ is 20 °, the entrance side angle is 3 °, and the exit side angle is 4 °. Also, the plug tip billet outer diameter reduction ratio Df was changed in 7 steps within the range of 2.0 to 7.0%.

[0121] For each plug, the base metal other than the tip rolling part was made of 3.0% Cr-l.0% Ni steel, and its strength was 1100 ° C and 30MPa. The material and physical properties of the tip rolling part are as follows. For plug numbers C9 and C10, members were used in which a scale was formed on a 3.0% Cr-l.0% Ni steel base material. Plug number C11 of the same shape as plug number C11! /, The same number for a member made of 0.5% Cr- l. 5% Mo—3.0% W alloy steel with scale formed For C-11-2, Nb-10.0% W-2.5% Zr Nb alloy, and for plug number C12-1 with the same shape as plug number 12, 0.5% Cr-1 5% Mo- 3.0% W-based alloy steel, and for the same number 12-2, Mo—0.5% Ti-0.08% Zr Mo alloy, Each was used.

[0122] Table 12 shows the measurement results of the material of the tip rolling part of the plug used, the tensile strength of the tip rolling part at 1100 ° C, and the scale thickness of the base metal. The scale treatment at this time was performed in the temperature range of 1000 ° C. to 1100 ° C., and the scale thickness was adjusted by adjusting the treatment time. Further, the plug has a structure in which the tip rolling part can be replaced.

[0123] [Table 12]

From the results in Table 12, we can see that: When plug No. C9 satisfying the conditions specified in the fifth invention is used, rolling is possible although billet penetration is poor when billet outer diameter reduction ratio Df is 2.5%. Therefore, it was possible to suppress the occurrence of internal flaws. In contrast, when using a conventional two-zone plug with plug number C10 However, it was impossible to roll with Df of 4.0% or less, and the occurrence of internal flaws could not be suppressed.

[0125] When plugs with plug numbers C11-2 and C12-2 satisfying the conditions specified in the sixth invention are used, piercing and rolling is stably performed with a billet outer diameter reduction ratio Df of 2.5% or less. It was possible to suppress the occurrence of internal flaws. In contrast, when the plug number C111 is used, the tensile strength at 1100 ° of the plug tip rolled part does not satisfy the conditions specified in the sixth invention, and the scale thickness is only normal. However, this caused damage to the plug and could not suppress the generation of internal flaws.

[0126] Although the tensile strength of the plug tip rolled part is less than 50 MPa, plug damage C12-1 with a thicker scale than usual can be used to prevent plug damage. It was possible to suppress the occurrence of internal flaws. However, when the same plug was used and the billet outer diameter reduction ratio Df was 2.0%, plug damage occurred. The reason for this is that the billet propulsive force was reduced and the rolling time was increased by reducing Df. Industrial applicability

[0127] The seamless pipe manufacturing method of the present invention includes a continuous forging process capable of forging a round piece having a significantly reduced axial center crack of the piece, which is a cause of internal flaws in the hollow shell, and the round It has a piercing and rolling process in which a round billet made of flakes is pierced and rolled to produce a holo-shell, so that both the effects of preventing the occurrence of misroll and preventing the occurrence of flaws on the inner surface of the holo-shell are achieved. It is possible to produce high-quality products with less internal defects based on productivity.

[0128] In particular, the ratio of the billet rotation frequency and the billet outer diameter reduction ratio is adjusted to a suitable range for the round billet according to the ratio between the roll diameter of the roll gorge of the tilt roll and the inlet roll diameter of the tilt roll. By piercing and rolling under the above-described conditions, a seamless pipe with fewer inner surface defects can be produced without being misrolled. In addition, by using a plug having a substantially cylindrical shape and a plug tip rolling portion having a spherical tip end surface as a perforated plug, the inner surface quality is further improved without causing any stagnation of the billet. Seamless pipes can be manufactured. Therefore, the method of the present invention can be widely used as a method capable of producing a seamless pipe with good inner surface quality with high productivity.

Claims

The scope of the claims

[1] At least the region within 60mm in diameter at the center of the cross-section of the round piece has an equiaxed crystal structure, and the surface of the round piece is cooled in the range of more than 1.0 and less than 1.0. It is equipped with a continuous forging process that forges round rods with a carbon content of 0.1% by mass or less and a cross-sectional diameter of more than 300mm while performing slow cooling at a speed of 10 ° CZ or less. A seamless pipe manufacturing method.

[2] In the round piece manufactured by the continuous forging process according to claim 1, the axial center crack generated at the center of the piece is in a region within a radius of 15 mm from the center of the cross section of the piece. A round splinter characterized by its existence.

[3] A method for producing a seamless pipe, comprising a step of piercing and rolling the round piece according to claim 2 without subjecting it to partial rolling.

[4] A plug is arranged along a pass line between a pair of cone-shaped inclined rolls arranged opposite to each other around the nose line, and the billet as described in (2) above is rotated while being swung. A roll diameter Dg (mm) of the inclined roll gorge part and a roll diameter Dl (mm) at a position where the billet starts to contact the roll at the inclined roll inlet DgZDl, and the billet radius Bending force Number of billet rotations up to plug tip N and billet outer diameter reduction ratio Df (%) NZDf satisfies the following formulas (1) to (3)! A method of manufacturing a seamless pipe, wherein the ratio DlZBd between the D1 and the billet outer diameter Bd (mm) satisfies the following formula (4) (hereinafter also referred to as “fourth invention”).

When Dg / DK l. 1

23≤N / (Df / 100) ≤40 (1)

1. l≤Dg / DK l.

20≤N / (Df / 100) ≤44 (2)

1. 5≤Dg / Dl≤l.

20≤N / (Df / 100) ≤48 (3)

Dl / Bd≥2.5 (4)

N = 2LdZ (7u .Bd'tan iS) (5)

Df = {(Bd-Rpg) / Bd} X 100 (6)

[5] The solid round billet of the outer diameter Bd (mm), which also serves as the round flange piece according to claim 2, has an outer diameter d (mm) that is the same diameter over the axial direction or an outer diameter d that is at the rear end in the axial direction. A tip rolling section that has a cylindrical shape with an axial length L2 (mm) that increases toward the end, and has a tip surface formed into a spherical shape with a radius of curvature r (mm) and an axial length LI (mm). A workpiece part having an axial length L3 (mm) formed by an arc rotation surface having a radius of curvature R (mm) so that the outer diameter increases continuously toward the rear end in the axial direction continuously to the tip rolling part, The axial length of a tapered cylinder formed with a taper angle of 2 θ (°) so that the outer diameter increases continuously toward the maximum outer diameter D (mm) at the rear end in the axial direction. A plug having a reeling part of L4 (mm) and a step of piercing and rolling with an inclined roll type piercing and rolling machine, the outer diameter d of the plug, the radius of curvature, the axial length Ll, L A method for producing a seamless pipe, characterized in that the relationship between 2 and L3 and the outer diameter Bd of the solid round billet satisfies the deviations of the following formulas (7) to (9).

0. 12≤d / Bd≤0. 35 (7)

0.020≤ (d / 2Bd) / (R / L3) ≤0. 046 (8)

0. 5d≤Ll + L2≤3d (9)

[6] The solid round billet of the outer diameter Bd (mm) of the round flange piece according to claim 2, wherein the outer diameter d (mm) is the same diameter in the axial direction or the outer diameter d is at the rear end in the axial direction. A tip rolling section that has a cylindrical shape with an axial length L2 (mm) that increases toward the end, and has a tip surface formed into a spherical shape with a radius of curvature r (mm) and an axial length LI (mm). A workpiece part having an axial length L3 (mm) formed by an arc rotation surface having a radius of curvature R (mm) so that the outer diameter increases continuously toward the rear end in the axial direction continuously to the tip rolling part, The axial length of a tapered cylinder formed with a taper angle of 2 θ (°) so that the outer diameter increases continuously toward the maximum outer diameter D (mm) at the rear end in the axial direction. Using a plug with an L4 (mm) reeling section and at least the tip rolled section with a tensile strength at 1100 ° C of 50 MPa or more, and piercing with an inclined hole type piercing and rolling mill Comprising an extended to process, the outer diameter d of the plug, the radius of curvature R, axial length Ll, the relationship between the outer diameter Bd of L2 and L3 and Nakajitsumaru billet below (8) to A method of manufacturing a seamless pipe, characterized in that the deviation of equation (10) is satisfied.

0.06≤d / Bd≤0.12 (10)

0.020≤ (d / 2Bd) / (R / L3) ≤0.046 ... (8)

0.5d≤Ll + L2≤3d (9)

7. The method for manufacturing a seamless pipe according to claim 6, wherein the rolled end portion of the plug is replaceable.