JPH07136702A - Manufacture of cr containing seamless steel tube - Google Patents

Abstract

PURPOSE:To suppress the occurrence of a defect generated on the inner surface of a tube by setting the absolute value of a strain defined by a specific formula within a specific range at the time of piercing a round billet by inclined rolls and a plug. CONSTITUTION:A continuous casting ingot containing 2 to 27wt.% Cr is used as a base stock. The continuous casting ingot having the sectional shape of a rectangle whose ratio of a long side to a short side is 1.6 to 3.0 is used as the base stock. This ingot is formed to a round billet 1 by normal rolling. When this round billet 1 is pierced by inclined rolls 2 and a plug 3, the absolute value ¦epsilonn¦ of a strain epsilonn during piercing, defined by a formula, is controlled to be ¦epsilonn¦ <=0.015 within a range form the position 10mm behind the tip part of a plug to the rear end of the plug. Consequently, a Cr contg. seamless steel tube with less defect generation factor on an inner surface can be manufactured. A seamless stainless steel tube such as SUS304, etc., to be widely used for boiler, oil well, further, for machine structural use, etc., can be obtained.

Description

Detailed Description of the Invention

[0001]

The present invention relates to a Cr content of 2% or more by weight.
Related to a method for producing a Cr-containing seamless steel pipe containing Cr, particularly used in various fields such as boilers, oil wells, gas wells, chemical plants, and mechanical structures, 2 1 / 4Cr steel, SUS 30
The present invention relates to a method for manufacturing a stainless steel seamless steel pipe such as 4, 316, 321.

[0002]

2. Description of the Related Art A seamless steel pipe comprises a solid round billet pierced by means of inclined rolls and plugs, and thereafter rolled by a mandrel bar called a mandrel mill rolling system and three hole-shaped rolls. Formed from a combination of diameter reduction and elongation by a hot drawing device, or rolling with two hole-shaped rolls and plugs called plug mill method, inclined rolls and a tube with plugs, and two hole-shaped rolls It is generally formed by reducing the diameter.

The solid round billet used as the material is currently filled with a material produced by a continuous casting method, and the method for filling the bill is to use a rectangular raw material (hereinafter referred to as bloom) to roll it into a round shape. , Or a round material (hereinafter referred to as a round billet) can be used as cast. When producing seamless steel pipes by the above method, ordinary carbon steel or low alloy steel can be piped with almost no problem, but in the case of high Cr materials with a Cr content of 2% or more by weight, continuous casting Larger cavities are more likely to occur in the center of blooms or round billets than plain steel or low alloy steel, and they remain as they are for round billets. When a round billet is used for drilling, the center part of the billet will be cracked by the inclined roll before being drilled by the plug (hereinafter referred to as Mannesmann cracking).
A large void is generated due to, and even if this portion is rolled by the plug and the inclined roll, it often remains as an inner surface defect.

Due to such Mannesmann cracking during perforation, it has been conventionally difficult to perforate a continuously cast material having a high Cr content without defects, and a steel ingot material that is unlikely to cause a cavity in the center has been used. It was However, due to recent advances in continuous casting technology, the generation of cavities in the central part has been considerably suppressed, so continuous casting materials are now being applied. In this case, it is generally unavoidable that a very high proportion of internal surface defects are generated, and these are removed by maintenance to obtain the final product.

In producing a seamless steel pipe having such a stainless steel composition by the Mannesmann rolling method, as a method for preventing internal surface defects during perforation when applying a continuous casting material, for example, Japanese Patent Laid-Open No. 61-140301. Is disclosed in. The content is C: 0.32 wt% or less, N: 0.4
Due to the stainless steel composition limited to wt% or less, a round billet for a seamless stainless steel pipe to be subjected to inclined rolling pipe making has a rectangular cross section that is three times or more the cross-sectional area of the round billet, and its long and short sides The length ratio is 2.5 or more, the long side length is 1100 mm or less, and the short side length is at least 40 relative to the outer diameter of the round billet to be made.
mm from a large continuous casting slab, by hot rolling under the condition that the reduction amount in the long side direction is 120 mm / pass or more, a bloom with a rectangular cross section having a short side that is at least 30 mm larger than the outer diameter of the round billet is created. This is a method for producing a round billet for a seamless stainless steel pipe by an inclined rolling method, which comprises subjecting this bloom to hot rolling to form a round billet through an intermediate stage.

[0006]

However, when the round billet is manufactured by the method disclosed in Japanese Patent Laid-Open No. 61-140301, it is possible to prevent the inner surface defects of the pipe during drilling. Since the lower limit of the ratio a / b between the length a and the length b of the short side is as large as 2.5, the problem of buckling during width reduction during round billet rolling remains unsolved, and continuous casting There is a problem that the size of the round billet to be manufactured is limited because there is a limit to the reduction ratio when rolling from the raw material to the round billet.

The present invention solves the problems described above by solving the problems described above.
It is an object of the present invention to provide a method for producing a seamless steel pipe containing.

[0008]

Means for Solving the Problems The inventors of the present invention have conducted extensive experiments and researches to solve the above-mentioned problems, and as a result, the strain distribution of the material to be perforated by the inclined roll and the plug during perforation was found to be a pipe inner surface defect. The present invention has been completed based on this finding. That is,
Where the gist of the present invention is, when producing a Cr-containing seamless steel pipe using a continuously cast slab containing Cr: 2 to 27% by weight as a material, the ratio of the long side to the short side of the material is Is 1.6
A continuous cast slab with a rectangular cross-section of ~ 3.0 is used to form this billet into a round billet by normal rolling, and when this round billet is punched with an inclined roll and a plug, the perforation defined by the following formula Absolute value of strain ε _n in
_A method for producing a Cr-containing seamless steel pipe, wherein _n | is | ε _n | ≤ 0.015 at a position from the position 10 mm from the plug tip toward the plug rear end.

Ε _n = l _n (t _n / t _n-1 ) where t _n-1 : wall thickness (mm) at an arbitrary position P _n-1 from the position 10 mm from the plug tip toward the plug rear end t _n : wall thickness (mm) at the position P _n, which is 1 mm away from the position P _n-1 toward the rear end of the plug.

The continuous cast slab has a weight ratio of Cr: 2 to 27.
%, C: 0.22% or less, Si: 1.00% or less, Mn: 2.00% or less, P: 0.040%, S: 0.030%, and may contain the balance Fe and unavoidable impurities, and Ni: 16.00% or less. , Mo: 3.50% or less may be contained in one kind or two kinds.

[0011]

[Operation] FIG. 1 is a schematic diagram for explaining the definition of strain during perforation of a material to be perforated. In the figure, when a solid round billet 1 is punched using a pair of inclined rolls 2 and 2 and a plug 3, a wall thickness t at the time of punching at a position P ₀ of a tip portion 3a of the plug 3 on a pass line PL. as There is t _0, when the 2 position P _n-1 and the wall thickness at P _n t any 1mm intervals on the path line PL and t _n-1, t _n, the strain epsilon _n at position P _n It is defined by the following equation (1).

Ε _n = l _n (t _n / t _n-1 ) ... (1) According to the experiments conducted by the present inventors, all paths from the tip portion 3 a of the plug 3 to the rear end portion 3 b are shown. It was found that the maximum strain amount of the absolute value | ε _n | of the strain ε _{n at} the position on the line PL has a deep causal relationship with the occurrence of the inner surface defects of the pipe. That is, by optimizing the plug shape and setting during drilling, if the ratio of the long side to the short side of the continuous casting material is 1.6 or more, 2 to 27% of Cr can be obtained regardless of the reduction ratio during round billet rolling. It was confirmed that it is possible to manufacture the contained seamless steel pipe with a remarkably low internal defect rate.

The reasons for limiting the Cr content of the present invention will be described below. Cr is added in a wide range to improve the corrosion resistance, high temperature oxidation resistance and high temperature hydrogen attack resistance, and due to the inclusion of Cr, a continuous casting material with a conventional cross-sectional shape is used, and it is perforated by the Mannesmann rolling method in the usual way. Therefore, if rolled, the inner surface defect is likely to occur in the seamless steel pipe, but if it is less than 2%, the inner surface defect does not occur even by the conventional cross-sectional shape and the drilling method, so that it is not necessary to apply the present invention.
If it exceeds 27%, thermal stress cracking different from that in the cavity tends to occur, and this defect cannot be prevented by the present invention, so the range is limited to 2 to 27%.

Next, the reason for limiting the ratio a / b of the long side a and the short side b of the continuously cast rectangular section material will be described. Figure 2 shows the weight ratio of C: 0.20%, Si: 0.45%, Mn: 0.50%, C
Using a continuously cast rectangular cross-section material containing r: 13.1%, P: 0.018%, S: 0.002% and different a / b consisting of the balance Fe and inevitable impurities, rolled into a round billet with a diameter of 110 mm epsilon _n | | maximum value of perforations at 0.0139 absolute value of the skew rolls and the at every position toward the front end of the plug to the plug rear end than the position of 10mm in a drilling with plug (1) ε _n defined by the equation Then, by mandrel mill rolling and hot drawing of the outer diameter, the outer diameter is 42.7 mmφ and the wall thickness is 4.5 mm.
It shows the relationship between a / b and the inner surface defect occurrence rate when a seamless steel pipe of t was manufactured.

First, the lower limit of a / b is limited to 1.6, as shown in FIG.
This is because the occurrence of inner surface defects is drastically reduced when the value is 1.6 or more, but the inner surface defects occur frequently when the value is less than 1.6. Next, the reason that the upper limit of a / b is limited to 3.0 is, as is clear from FIG.
The effect of a / b on internal defects saturates at 2.4, a / b
When the value exceeds 3.0, wrinkle flaws due to buckling easily occur at the center of the width when the long side is width-reduced in the step of rolling this into a round billet.

For the above two reasons, a / b is 1.6 to 3.
Limited to 0 range. Furthermore, when drilling a round billet with an inclined roll and a plug, the absolute value | ε _n | of strain ε _n during drilling defined by the above equation (1) is 10
｜ ε at all positions from the mm position to the rear end of the plug
| <The reason for limiting such that 0.0150, as shown in FIG. 3 | epsilon | incidence of <inner surface defects from 0.0150 is depleted, |
This is because the inner surface defect occurrence rate is high when ε |> 0.015.

Here, in FIG. 3, C: 0.20% by weight ratio, S
i: 0.45%, Mn: 0.50%, Cr: 13.1%, P: 0.018%,
S: 0.002% included, balance Fe and unavoidable impurities a
/B=1.87 continuous casting material, diameter 110 by rolling
As a round billet of mmφ, the absolute value of ε _n defined by the above formula (1) at all positions from the tip of the plug to the rear end of the plug from 10 mm from the tip of the plug during perforation with inclined rolls and plugs | ε a / b and inner surface defect occurrence rate when a seamless steel pipe with a diameter of 42.7mmφ and a wall thickness of 4.5MMt is manufactured by mandrel mill rolling and hot outer diameter drawing and stretching after changing the maximum value of _n | And the maximum value of | ε _n |.

Further, the reason for limiting the position at the time of drilling showing the maximum value of | ε _n | from the position 10 mm from the tip of the plug toward the rear end of the plug is that the temperature at the time of drilling is 10 mm from the tip of the plug. This is because even if the strain in this range is considerably large, it does not lead to the occurrence of inner surface defects. However, in the portion of the plug rear end from the position 10 mm from the plug tip, the heat of the plug lowers the temperature of the material to be perforated during perforation, and therefore the deformability deteriorates. Within the range of distortion that is
ε _n | ≦ 0.0150 must be satisfied.

[0019]

EXAMPLES Examples of the present invention will be described below.
After continuously casting a material having the chemical composition shown in Table 1 into an a / b rectangular or round steel slab shown in Table 2, a rectangular cross section is rolled, and a round cross section is as-cast. A material billet for a seamless steel pipe with a diameter of 110 mmφ, and when drilling this billet with an inclined roll and plug,
The maximum value of the absolute value | ε _n | of strain ε _n defined by the equation (1) at all positions from the position 10 mm from the plug tip to the plug rear end is set to the value shown in Table 2. Perforation was performed, and then mandrel mill rolling and hot outer diameter drawing processing were carried out to produce a seamless steel pipe having an outer diameter of 42.7 mmφ and a wall thickness of 4.5 mmt.

[0020]

[Table 1]

[0021]

[Table 2]

[0022]

[Table 3]

[0023]

[Table 4]

Table 3 shows the shape and setting of the plug used during drilling and the maximum value of the above | ε _n |.

[0025]

[Table 5]

The dimensions shown in Table 3 are shown schematically in FIG. Table 2 also shows the cavities during continuous casting that occurred in these seamless steel pipes and the internal surface defect occurrence rate due to segregation. Among the preferable conditions of the present invention, | ε _n |
The maximum value of is within the range of the present invention, but a / b is outside the range of the present invention as Comparative Example I, and a / b is within the range of the present invention, but the maximum value of | ε _n | Those outside the range are shown in Table 2 as Comparative Example II.

As is clear from Table 2, in the examples of the present invention in which both the cross-sectional shape of the continuously cast steel piece and the maximum value of | ε _n | are within the scope of the present invention, all internal surface defects caused by cavities and segregation. It can be seen that is significantly reduced. Therefore, even if the reduction ratio is determined to be defective due to the inner surface defects of the pipe in the above-mentioned JP-A-61-140301, the inner surface defects are reduced to the extent that there is no problem by implementing the method of the present invention. I understand.

[0028]

As described above, according to the present invention,
When drilling a round billet having a Cr content of 2 to 27%, it is possible to suppress defects that occur on the inner surface of the pipe. Therefore, for energy-related steel pipes such as boilers, oil wells, gas wells, and chemical plants, where corrosive fluid transportation steel pipes and machine structure steel pipes, which are expected to increase in demand in the future,
The industrial value of the present invention, which enables the seamless steel pipe to be provided at a low manufacturing cost, is great.

[Brief description of drawings]

FIG. 1 is a schematic diagram for explaining the definition of strain during perforation of a material to be perforated used in the present invention.

FIG. 2 is a characteristic diagram showing the influence of the cross-sectional shape of the continuous casting material on the occurrence rate of internal surface defects due to cavities and segregation of the continuous casting material at the time of seamless steel pipe piercing and rolling.

FIG. 3 is a characteristic diagram showing the influence of the maximum value of | ε _n | on the occurrence rate of internal surface defects due to cavities and segregation during seamless steel pipe piercing and rolling.

FIG. 4 is a schematic diagram showing a piercer plug shape and a dimensional relationship of each part.

[Explanation of symbols]

 1 Round billet 2 Inclined roll 3 Plug

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Takuya Atsumi 1 Kawasaki-cho, Chuo-ku, Chiba-shi, Chiba Kawasaki Steel Corporation Technical Research Division (72) Inventor Toshikazu Masuda 1-chome, Kawasaki-cho, Handa-shi, Aichi No. 1 Kawasaki Iron & Steel Co., Ltd. Chita Works (72) Inventor Ryosuke Mochizuki 1-1, Kawasaki-cho, Handa-shi, Aichi Kawasaki Steel Co., Ltd. Chita Works

Claims (3)

[Claims] 1. When producing a Cr-containing seamless steel pipe from a continuously cast slab containing Cr: 2 to 27% by weight, the material has a long side to short side ratio of 1.6 to 3.0. Using a continuously cast slab having a rectangular cross-sectional shape, which is made into a round billet by normal rolling, when piercing this round billet with an inclined roll and a plug, in the perforation defined by the following formula: A method for producing a Cr-containing seamless steel pipe, wherein the absolute value of the strain ε _n | ε _n | is | ε _n | ≦ 0.015 at a position 10 mm from the plug tip toward the plug rear end. ε _n = l _n (t _n / t _n-1 ) where t _n-1 : wall thickness (mm) t _{n at an} arbitrary position P _n-1 from the position 10 mm from the plug tip toward the plug rear end: Wall thickness (mm) at position P _n 1 mm away from position P _n-1 to the rear end of the plug 2. The continuously cast slab has a weight ratio of Cr: 2 to 27.
%, C: 0.22% or less, Si: 1.00% or less, Mn: 2.00% or less, P: 0.040%, S: 0.030%, and the balance Fe and unavoidable impurities. Steelless pipe manufacturing method. 3. The continuously cast slab further comprises Ni: 1 by weight ratio.
The method for producing a Cr-containing seamless steel pipe according to claim 2, which comprises one or two of 6.00% or less and Mo: 3.50% or less.