JP3460608B2 - Method of manufacturing iron-based high Cr seamless steel pipe - Google Patents

Description

Detailed Description of the Invention

[0001]

TECHNICAL FIELD The present invention relates to a Cr content of 7 to 16%.
The present invention relates to a method for producing an iron-based high Cr-based seamless steel pipe, in particular, a technique for directly punching a round billet by continuous casting without rolling, and then making the pipe with a rolling machine such as a mandrel mill or a plug mill. Is.

[0002]

2. Description of the Related Art The manufacturing process of high Cr type seamless steel pipe is
After adjusting to a predetermined composition in steel making, cast into a bloom or slab with a steel ingot or continuous casting machine, then finish by hot working into a predetermined round billet shape in the slab and rolling process, and then enter the pipe manufacturing process. Generally, in carbon steel or low alloy steel, as described in JP-A-6-172859, after adjusting to predetermined components in steel making, it is directly cast into a round billet by a continuous casting machine, and thereafter, It is possible to immediately start the pipe manufacturing process.

The difference between the two is that, as described in JP-A-3-248744, in a billet of high Cr steel containing Cr in an amount of 5% or more, voids are generated due to solidification shrinkage, and the inner surface is formed in the punching process. Because it becomes the starting point of the rash-like scratches.

As a method for solving such a problem in high Cr steel, a manufacturing process which requires a hot rolling process for forming a round billet from a continuously cast bloom or slab having a rectangular cross section is disclosed in Japanese Patent Application Laid-Open No. Hei 3 No. Japanese Patent No. 248744 proposes continuous casting with a steel ingot having an aspect ratio of 1.5 to 4.0.

Further, Japanese Patent Laid-Open No. 58-215251 discloses a method of using electromagnetic stirring for unsolidified crater end molten steel in bloom continuous casting, and Japanese Laid-Open Patent Publication No. 61-229441 discloses a bar wire at the center of the bloom in continuous bloom casting. Has been proposed to prevent the central cavity from being continuously supplied. As described above, in the case of high Cr steel, it is necessary to secure the soundness of the ingot center portion in the previous step of continuous casting of blooms and slabs even when including a hot rolling step of forming a round billet as a subsequent step. is there. Therefore, in the case of directly forming a round billet by continuous casting, it is more difficult to obtain a sound property of the center of the ingot because the hot rolling step after continuous casting is not performed as a post step. Therefore, a method has been proposed in which defects in the center of the round billet are directly reduced during continuous casting to form a billet having a predetermined shape.

Japanese Unexamined Patent Publication No. 9-201201 and Japanese Unexamined Patent Publication No. 9-2012
-2012602 discloses that when continuously casting a round billet,
We propose a method to reduce porosity at the center by applying a light reduction in a continuous casting machine after solidification is completed. Further, JP-A-9-295113 and JP-A-9-300053 propose methods of applying a static magnetic field at the final stage of coagulation and applying a slight reduction. In order to apply a static magnetic field under light pressure, new equipment is required, which cannot be achieved with the current continuous casting machine. Japanese Unexamined Patent Publication No. 8-52555 proposes a method of changing the cooling rate in the circumferential direction outside the billet during continuous casting of a round billet so that the position of the final solidified portion is eccentric from the center of the billet. Requires new cooling control equipment.

[0007]

[Problems to be Solved by the Invention] As described above,
The production of a seamless steel pipe of steel containing 7 to 16% of Cr is a method of producing a round billet for pipe production by rolling after casting a bloom or a slab by continuous casting, or a case of directly casting a round billet. However, it is difficult to say that it is an economical method because it requires complicated processing and equipment at the casting stage. The present invention has been made in view of the above points, and contains 7 to 16% Cr.
An object of the present invention is to provide a method for economically producing a seamless steel pipe of a steel containing.

[0008]

[Means for Solving the Problems] In order to achieve the above object, the present inventors have studied a manufacturing process of a seamless steel pipe. Using the test steels shown in Table 1, Process 1: A round billet is manufactured by continuous casting and punched as it is,
Process 2: drilling after rolling, and Process 3: scratching the outer surface of the billet after rolling,
A seamless steel pipe was manufactured in each process of drilling. Table 2
Shows the occurrence of scratches in each process and the comprehensive evaluation.

In the conventional processes 2 and 3 in which rolling is performed before perforation, internal scratches are small, but none of them can be eliminated, and some cases require maintenance.
Further, the rolling deteriorates the properties of the outer surface, and requires care for the outer surface of the billet. In the process 2 in which no care is taken for scratches, scratches on the outer surface, which is a quality problem, occur. On the other hand, in the case of the process 1, since rolling is not performed before perforation, there is no occurrence of external scratches due to rolling, and the manufacturing cost for repairing scratches is unnecessary. In terms of quality, scratches are generated on the inner surface, but some of them require care, and some alloy compositions require no care at all. In the comprehensive evaluation including not only the quality but also the manufacturing cost, the process 1 in which the cast billet was perforated without passing through the rolling step was the best.

From the above results, the present inventors have found that after casting,
In the method of drilling Mannesmann without going through the rolling step, conventionally, it was found that the internal surface damage at the time of drilling, which has been a problem, can be suppressed by a detailed examination of the alloy composition and manufacturing conditions, and the above manufacturing method is the most It was completed as an inexpensive manufacturing method.

That is, the present invention is as follows. In a method for producing an iron-based high Cr-based seamless steel pipe, which comprises casting a billet containing 7 to 16% by weight of Cr by a continuous casting method and then performing Mannesmann drilling without a rolling step, Is 340 mm or less, and the Mannesmann perforation is at least 1473K or more,
And the billet heating temperature that satisfies the expression (1): BT (K)
1. A method for producing an iron-based high Cr-based seamless steel pipe, characterized in that

Ni + 0.2Mn + 30 (C + N) -Cr-1.5Mo-1.5Si + 5.88 * 10 ⁴ /BT≧28.2 (1) However, each element is wt%.

2. The billet is further C:
0.005-0.3%, Ni: 7% or less, Mo: 3% or less, N: 0.2% or less, The manufacturing method of the iron-based high Cr type | system | group seamless steel pipe of 1 characterized by the above-mentioned.

3. The billet is further in wt%, S:
0.005% or less, H: 0.001% or less, B: 0.0
02% or less and O: 0.005% or less, The manufacturing method of the iron-based high Cr type | system | group seamless steel pipe of 1 or 2 characterized by the above-mentioned.

[0015]

[Table 1]

[0016]

[Table 2]

[0017]

BEST MODE FOR CARRYING OUT THE INVENTION According to the present invention, the occurrence of inner surface scratches at the time of punching Mannesmann is first suppressed by shrinkage and elimination of segregation in the central portion of the billet to secure soundness.
Further, in order to make the effect prominent, the formation of δ-ferrite at the center of the billet during drilling is prevented by limiting the heating temperature during drilling by Mannesmann according to the alloy composition.

1. Billet diameter: 340 mm or less When a billet is manufactured by continuous casting, the properties of the central part are governed by the cooling rate. Large billet diameter,
If the cooling rate is reduced, segregation and contraction of the central portion are likely to occur, which induces internal surface defects after pipe making.
Below. If the billet diameter is too small, the manufacturability is lowered, so 170 mm or more is preferable.

2. Billet heating temperature during perforation: Satisfying the following formula (1) and 1200 ° C. or higher Ni + 0.2Mn + 30 (C + N) -Cr-1.5Mo-1.5Si + 5.88 * 10 ⁴ /BT≧28.2 (1 However, BT: billet heating temperature (unit: K), and each element is wt%.

When δ-ferrite is present at the center of the billet during drilling, internal surface defects are likely to occur, so it is necessary to prevent its formation. As a result of detailed studies including the influence of segregation in the central portion, the present inventors have found that δ-ferrite does not occur when the alloy components and the heating temperature satisfy the expression (1). On the other hand, the billet heating temperature is set to 1200 ° C. or higher in order to reduce the deformation resistance during hot working.
The billet heating temperature at the time of drilling is specified in the above range from the viewpoint of workability of drilling and prevention of δ-ferrite formation. The maximum heating temperature is 1 to ensure ductility.
It is desirable that the temperature be 350 ° C. or lower.

3. Chemical composition The present invention is intended for high Cr steel containing 7 to 16% of Cr, but enhances suitability as an oil country tubular good with excellent corrosion resistance and hot workability (generation characteristics of slight internal surface damage during drilling). In order to improve the heat treatment property and heat treatment property, further components can be specified.

C: 0.005 to 0.3% C forms a carbide with Cr in steel and is added in order to increase the strength which is important as an oil country tubular good. If it is less than 0.005%, sufficient strength cannot be obtained, and if it exceeds 0.3% and is excessively added, the amount of Cr effective for corrosion resistance is consumed for carbide formation and corrosion resistance deteriorates, so 0.005-0 And 3%.

Cr: 7 to 16% Cr is a fundamentally important element necessary for corrosion resistance and is added. If it is less than 7%, sufficient corrosion resistance cannot be obtained, and if it exceeds 16%, the amount of δ-ferrite produced is increased by adjusting other alloying elements, and the strength and toughness deteriorate.
16%.

Ni: 7% or less Ni improves corrosion resistance and is an extremely effective element for forming austenite. It suppresses δ-ferrite,
It is added to improve the pipe forming property.

When the content increases, the transformation point (Ac ₁ point) is lowered to limit the tempering temperature, and since it is expensive, the upper limit is set to 7%.

Mo: 3% or less Mo is an element which is particularly effective for stress corrosion cracking resistance and corrosion resistance, but if it exceeds 3%, excessive δ-ferrite is produced, so the upper limit is made 3%.

N: 0.2% or less N is an austenite-forming element, which suppresses δ-ferrite and improves pipe forming properties. Further, when it forms a solid solution, it improves the corrosion resistance, but as with C, forming a compound with Cr lowers the effective Cr and deteriorates the corrosion resistance.
The upper limit is 2%.

In the present invention, in the high Cr steel, the following trace elements are specified in combination with the above elements or alone.
The effect can be made more remarkable.

S: 0.005% or less S is generally 0.01% or less from the viewpoint of pipe forming property and corrosion resistance. However, when a round billet directly cast by a continuous casting method is subjected to Mannesmann drilling, segregation occurs. Considering the above, the content is made 0.005% or less from the viewpoint of pipe-making property.

H: 0.001% or less H is mixed as an impurity element, but if a large amount is present in steel, hydrogen cracking occurs after martensitic transformation during quenching heat treatment after pipe making, so 0.001% or less And

B: 0.002% or less B improves the hardenability of steel, but even if added in a large amount, the effect saturates and the hot workability deteriorates, so 0.0
02% or less.

O: 0.005% or less O is an unavoidable impurity element, but from the viewpoint of toughness, O.
005% or less.

[0033]

[Examples] Table 3 shows an example of actual manufacturing results by the method of the present invention. A round billet having a diameter of 280 mm was manufactured by continuous casting, and Mannesmann punching was performed without going through a rolling process. Billet heating temperature during drilling is 1553K (128
0 ° C.), 1513 K (1240 ° C.), and the presence of internal scratches was examined.

As a result, when the Mannesmann drilling was performed at the billet heating temperature satisfying the expression (1) for suppressing the generation of δ-ferrite at the time of drilling, no internal scratch was observed. Even if the formula (1) is not satisfied, the internal scratches are slight, and it is possible to make a product with sufficient care, but this will increase the number of processes and affect the manufacturing cost. , Not preferable.

Table 4 shows the effects of the chemical components further prescribed in addition to Cr. Corrosion resistance is 720 in a 5% NaCl solution saturated with H ₂ S at 0.03 atm under load conditions equivalent to yield stress.
It was held for a period of time, and those that did not crack were rated as ◯, and those that cracked were rated as x. Regarding the heat-treatability, x indicates that cracking occurred during quenching during manufacturing, and o indicates that cracking did not occur.

The hot workability was evaluated as x when the inner or outer surface of the pipe was damaged, and ◯ when it was not damaged. However, these scratches are minor and different from the pipe-forming scratches that are the subject of the present invention.

Steel Nos. 15 to 29 satisfy all the above-mentioned regulations concerning the alloy composition for improving the properties, and show excellent corrosion resistance, heat treatment property, and hot workability. On the other hand, in steel Nos. 30 to 44, some or all of the elements that affect the corrosion resistance, heat treatment property or hot workability are out of regulation,
These properties were inferior.

[0038]

[Table 3]

[0039]

[Table 4]

[0040]

As described above, according to the manufacturing process of the present invention, in the manufacture of the seamless steel pipe of the iron-base high Cr steel, the round billet manufactured by continuous casting is subjected to Mannesmann drilling without passing through the rolling step. As a result, the manufacturing cost is reduced, and the industrially useful effect is brought about.

─────────────────────────────────────────────────── ─── Continued Front Page (72) Inventor Yasuhito Inohara 1-2-2 Marunouchi, Chiyoda-ku, Tokyo Inside Nippon Kokan Co., Ltd. (72) Inventor Tatsuro Katsumura 1-2-1 Marunouchi, Chiyoda-ku, Tokyo Japan Steel Pipe Co., Ltd. (72) Inventor Tatsuharu Oda 1-2-2 Marunouchi, Chiyoda-ku, Tokyo Japan Steel Pipe Co., Ltd. (72) Inventor Tatsuo Ono 1-2-1 Marunouchi, Chiyoda-ku, Tokyo Nippon Steel Pipe Co., Ltd. (72) Inventor Toshiro Ishige 1-2-1, Marunouchi, Chiyoda-ku, Tokyo Nippon Steel Tube Co., Ltd. (56) Reference JP-A-8-229606 (JP, A) JP-A-1-228603 (JP, A) ) (58) Fields investigated (Int.Cl. ⁷ , DB name) B21B 19/04 C21D 8/10

Claims (3)

(57) [Claims] 1. Containing 7 to 16% by weight of Cr ,
In a method for producing an iron-based high Cr-based seamless steel pipe in which a W-free billet is cast by a continuous casting method, and then a Mannesmann piercing is performed without passing through a rolling step, a billet diameter is 340 mm or less, and a Mannesmann piercing is performed at least. Billet heating temperature of 1473K or higher and satisfying the formula (1): BT (K), iron-based high C
Method for manufacturing r-type seamless steel pipe. Ni + 0.2Mn + 30 (C + N) -Cr-1.5Mo
-1.5Si + 5.88 * 10 ⁴ /BT≧28.2
(1) However, each element is% by weight. Also, Ni must be added
It is an essential component to
It is an optional ingredient . 2. The billet further comprises C: 0.0% by weight.
05-0.3%, Ni: 7% or less, Mo: 3% or less,
N: 0.2% or less, The method for producing an iron-based high Cr type seamless steel pipe according to claim 1. 3. The billet further comprises S: 0.0% by weight.
05% or less, H: 0.001% or less, B: 0.002%
Hereinafter, O: 0.005% or less, The method for producing an iron-based high Cr-based seamless steel pipe according to claim 1 or 2, wherein.