JP2002224711A - Manufacturing method of seamless pipe - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a manufacturing method of seamless pipes excellent in properties of the inner wall and the internal body. SOLUTION: Circular billets hot-rolled from slabs made at continuous casters whose segregation displacement ratio P(%) is 5 to 40% are used. The definition of P is as the following: P(%)=(Δd/D)×100 where P: segregation displacement ratio (%), Δd: the position of segregation from the center of the circular billet (mm), and D: the diameter of the circular billet. Hollow raw pipes are made by piercing circular billets that are heated at predetermined are stretched temperature, and seamless pipes and as necessary reduced from the hollow raw pipes.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for manufacturing a seamless steel pipe, and more particularly to an improvement in the inner surface and internal properties of a seamless steel pipe made of a difficult-to-work material such as stainless steel.

[0002]

2. Description of the Related Art Conventionally, a Mannesmann-type pipe manufacturing method has been widely used for manufacturing a seamless steel pipe. In the method of manufacturing a seamless steel pipe by the Mannesmann method, a rolled material (round billet) heated to a predetermined temperature is first formed into a hollow material through a piercing and rolling step by a piercing mill, and then is used as an elongator, plug mill, or mandrel mill. This is a method in which a pipe is expanded by an elongation rolling mill to reduce the wall thickness, and further reheated if necessary, and thereafter the outer diameter is reduced mainly by a reduction rolling mill such as a stretch reducer or a sizer to obtain a seamless steel pipe having a predetermined size.

Generally, as a rolling material, a continuously cast round billet is used as it is, or a billet (round billet) in which a continuously cast slab is formed into a round shape by hot rolling. In the piercing and rolling step, a round billet, which is a rolled material, is pierced with a plug at the center of the cross section to form a hollow shell. At that time, a defect may occur on the inner surface of the hollow shell. This is because when the round billet to be used is made of continuous casting, the center of the cross section of the round billet almost coincides with the final solidification position at the time of continuous casting, so that cavities, porosity and center segregation are near the center of the cross section. Exists (FIG. 2A). Also, when the continuous cast slab is formed into a round billet by hot rolling, the vicinity of the central portion of the thickness of the continuous cast slab is the final solidification position. (FIG. 2 (b). At the time of perforation, these porosity,
It is considered that a defect occurs on the inner surface of the hollow shell due to the cavity and the center segregation. In particular, there is a problem that the occurrence frequency is high in stainless steel which is a difficult-to-process material.

As a measure for preventing such an inner surface defect of the seamless steel pipe, first, it is conceivable to reduce the center segregation of the rolled material. Many methods have been proposed to reduce center segregation. For example, reduction of segregation components such as S and P, reduction of molten steel temperature (superheat degree of molten steel) during continuous casting, electromagnetic stirring of molten steel in a mold, Forging pressure of slab, soaking heat treatment of slab, etc. However, to date, these measures have reduced center segregation, but have not yet completely eliminated center segregation, especially in seamless steel pipes of difficult-to-work materials. There remains a problem that porosity, cavities, and inner surface defects due to center segregation occur.

[0005] Separately, Japanese Patent Application Laid-Open No. 8-52555 discloses a round billet in which the final solidification position of a slab is shifted from the center of the slab by 1% to 3% of the slab diameter in the round billet continuous casting.
A method for producing a seamless steel pipe having no inner surface flaws has been proposed in which a hollow billet is produced by heating to a rolling temperature and piercing and rolling the center of the round billet as a center.

[0006]

SUMMARY OF THE INVENTION
According to the technique described in Japanese Patent Application Laid-Open No. 8-52555, there is a problem that even if the generation of inner surface flaws can be prevented, a two-plate defect is easily generated inside the steel pipe wall thickness. A position shifted from the center of the slab by 1 to 3% of the slab diameter is a place where severe shear deformation occurs at the time of drilling by the plug, and if strong segregation, porosity, or a cavity exists at such a position, porosity and crimping of the cavity are performed. In addition to this, it is considered that a two-plate defect is caused by cracking of a strong segregation part.
(FIG. 3) It is an object of the present invention to advantageously solve the above-mentioned problems of the prior art and to propose a method for manufacturing a seamless steel pipe having excellent inner surface properties and inner properties.

[0007]

Means for Solving the Problems In order to achieve the above-mentioned object, the present inventors have intensively studied the relationship between the center segregation position of a rolled material and the occurrence of a two-plate defect. As a result, the present inventors obtained the following equation (1): P (%) = (Δd / D) × 100 (1) (where P: segregation displacement (%), Δd: rolling material (round) By setting the segregation displacement (P) defined by the segregation position (mm) from the thickness center of the billet) and D: the total thickness (outer diameter) (mm) of the rolled material (round billet) to 5 to 40%. It was found that both the inner surface properties and inner properties of steel pipes were improved.

The present invention has been completed based on the above-mentioned findings and further studies. That is, in the present invention, a continuous cast slab is formed into a round billet by hot rolling, and then the round billet is heated to a predetermined temperature and pierced to form a hollow shell, and then the hollow shell is stretch-rolled, or Further, in the method of manufacturing a seamless steel pipe which is drawn and rolled into a steel pipe having a predetermined size, the following round formula (1) is used as the round billet. P (%) = (Δd / D) × 100 (1) P: segregation displacement (%), Δd: segregation position (mm) from the center of round billet thickness, D: segregation displacement P (%) defined by round billet total thickness (outer diameter) (mm)) 5-40
The present invention relates to a method for producing a seamless steel pipe having excellent inner surface properties and inner properties, characterized by using a round billet of which the ratio is%.

[0009]

DESCRIPTION OF THE PREFERRED EMBODIMENTS In the present invention, first, molten steel having a desired composition is formed into a slab by a continuous casting method. The obtained continuously cast slab is further hot-rolled into a round billet to obtain a rolled material for a seamless steel pipe. In the continuous cast slab used in the present invention, it is preferable that the final solidification position is shifted to the front side or the back side by 5 to 25% of the sheet thickness from the center in the sheet thickness direction. The cavity, porosity, and segregation in the round billet cross section largely deviate from the perforation position, thereby preventing deterioration of the inner surface property and lowering of the inner property.

As a method of shifting the final solidification position of the slab from the center in the thickness direction, for example, the cooling (secondary cooling) of the slab coming out of the continuous casting mold is made uneven on the front and back surfaces of the slab. This can be achieved by making the cooling on the front side or the back side stronger or weaker than the other. The molten steel injected into the mold solidifies from the surface layer in contact with the mold surface due to heat removal from the mold surface, forms a solidified shell, and is then withdrawn from the mold. After leaving the mold, it is cooled by secondary cooling such as water spray, and while further developing a solidified shell,
Solidifies sequentially. If the cooling from the front and back surfaces is uniform, the final solidification position is substantially at the center in the slab thickness direction. In the present invention, the secondary cooling is performed unevenly on the front and back surfaces of the slab, and the final solidification position is shifted to the front side or the back side by 5 to 25% of the plate thickness from the center in the plate thickness direction. If the amount of displacement of the final solidification position is less than 5% of the plate thickness, shear deformation acts during piercing and rolling, so that the pressure at the final solidification position such as cavities and segregation becomes insufficient, and two-plate defects are likely to occur. Become. On the other hand, if the amount of displacement exceeds 25% of the plate thickness, there is a difficulty in operating the actual machine such as warpage or cracking of the slab.

Thus, the slab whose final solidification position is shifted from the center in the thickness direction is further subjected to hot rolling to form a round billet of a predetermined size. Also, a round billet having a large diameter may be eccentric, and the segregation may be shifted from the center by a process such as cutting. According to such a method, the following equation (1) P (%) = (Δd / D) × 100 (1) (where P: segregation displacement (%), Δd: Segregation position (mm), D: Segregation displacement degree P (%) defined by round billet total thickness (outer diameter) (mm) is 5 to 40
% Billet. Segregation displacement P
Is illustrated in FIG.

By setting the degree of segregation displacement P in the range of 5 to 40%, during piercing and rolling, the cavity and the area where segregation is present are removed from the inner surface to prevent the occurrence of inner surface defects and improve the inner surface properties. At times, the shear deformation in the cavity and the area where segregation exists can be reduced, thereby preventing the occurrence of splits and improving the internal properties.

When P is less than 5%, the location of the cavity and the segregation corresponds to a region where a large shear deformation occurs at the time of drilling, and there is a concern that an inner surface defect and a double crack may occur in the steel pipe wall thickness. On the other hand, a shift such as segregation in which P exceeds 40% conversely promotes defects on the outer surface. For this reason, the segregation displacement P of the round billet is limited to 5 to 40%. In addition, it is preferably 5 to 25%.

The round billet having the above segregation displacement P is heated to a predetermined temperature and pierced and rolled to form a hollow shell. The heating temperature is preferably in the range of 1200 to 1300 ° C. If the temperature is lower than 1200 ° C, the deformation resistance increases and rolling becomes difficult.On the other hand, if the temperature exceeds 1300 ° C, hot working due to δ ferrite and grain boundary melting is performed. There is a problem that the properties are reduced and the amount of oxide scale generated is increased. The conditions for piercing and rolling are not particularly limited, and there are no problems under conventionally known conditions.

Next, the hollow shell is reheated as required, and expanded to a desired size by using a stretching rolling mill such as a mandrel mill or a plug mill to reduce the wall thickness. The conditions for elongation rolling do not need to be particularly limited in the present invention, and there is no problem under conventionally known conditions. The seamless steel pipe that has been subjected to elongation rolling is further reheated if necessary and subjected to drawing rolling. In the reduction rolling, it is preferable to use a reduction rolling machine such as a stretch reducer or a sizer to reduce the outer diameter, thereby forming a product steel pipe having a predetermined size. The conditions for the reduction rolling may be rolling according to a normal pass schedule, and need not be particularly limited in the present invention.

[0016]

EXAMPLE Molten steel having the composition shown in Table 1 was smelted in a converter, subjected to vacuum refining, and then subjected to continuous casting to obtain a sheet thickness of 260 mm.
Of the slab. During continuous casting, the ratio of the water density on the slab front side to the water density on the back side of the slab in the secondary cooling zone was changed. The final solidification position was confirmed by cutting the obtained slab by gas, polishing and etching the cross section, and observing the macro structure.

The obtained slab was further rolled by hot rolling into a round billet of 140 to 260 mmφ. For some, it is eccentric from the large billet and cut by cutting.
The billet was a small diameter, and the degree of segregation displacement P was in the range of 5 to 45%. Polish the cross section of the obtained round billet,
The cavity and the segregation position were confirmed by etching and observing the macrostructure, and the degree of segregation displacement P was calculated by the above equation (1).

After heating such a round billet at 1250 ° C., it is pierced and rolled by a Mannesmann plug mill to form a hollow shell, and then drawn and rolled by a mandrel mill rolling machine to obtain a 110 to 172 mm φ (thickness). : 8mm) steel pipe, then stretch reducer (rolling mill)
To make a seamless steel pipe (product steel pipe). The inner and outer surfaces of the obtained product steel pipe and the inside of the steel pipe were inspected for defects by visual inspection, ultrasonic flaw detection, and magnetic particle flaw detection. The inner and outer surface properties and inner properties were evaluated based on the number of inner surface flaws and two cracks per 100 m of the product steel pipe. (10 compared to the conventional example
It was evaluated as a ratio relative to 0. Table 2 shows the results.

[0019]

[Table 1]

[0020]

[Table 2]

In each of the examples of the present invention, the occurrence of internal surface flaws can be prevented even in difficult-to-work materials, and the occurrence of defects inside the thickness of the steel pipe, such as cracks, can be prevented.
Evaluation of inner surface property, inner property and outer property has been remarkably improved. On the other hand, in the comparative examples outside the scope of the present invention, the occurrence of internal flaws or the occurrence of cracks inside the thickness is recognized.

[0022]

According to the present invention, of course, the occurrence of internal flaws
It can prevent the occurrence of internal defects such as splits inside the wall thickness,
It is possible to improve the yield and productivity of seamless steel pipes,
It has a remarkable industrial effect.

[Brief description of the drawings]

FIG. 1 is an explanatory diagram schematically showing a relationship between a final solidification position and a perforation position and a relationship with an internal defect position in the present invention.

FIG. 2 is an explanatory diagram schematically showing a relationship between a final solidification position and a perforation position and a relationship between internal defect positions in a conventional example.

FIG. 3 is an explanatory diagram schematically showing a relationship between a final solidification position and a perforation position and a relationship between internal defect positions in a conventional example.

FIG. 4 is an explanatory diagram illustrating a definition of a segregation displacement P.

 ──────────────────────────────────────────────────続 き Continuing from the front page (72) Inventor Mitsuo Kimura 1-1-1, Kawasaki-cho, Handa-shi, Aichi Prefecture Kawata Steel Corporation Chita Works

Claims (1)

[Claims] 1. A slab made of continuous casting is formed into a round billet by hot rolling, and then the round billet is heated to a predetermined temperature and pierced to form a hollow shell. In the method for producing a seamless steel pipe which is drawn and rolled into a steel pipe having a predetermined size, the round billet has a segregation displacement degree P (%) defined by the following equation (1) of 5:
A method for producing a seamless steel pipe having excellent inner surface and internal properties, characterized by using a round billet of up to 40%. P (%) = (Δd / D) × 100 (1) where P: degree of segregation displacement (%) Δd: segregation position (mm) from the center of thickness of round billet D: total thickness of round billet ( Outer diameter) (mm)