JP2001096304A - Method of manufacturing martensitic stainless seamless tubes - Google Patents

Abstract

PROBLEM TO BE SOLVED: To prevent generation of flows on the outer surface of a martensitic stainless steel piece containing Cr: 10.0∼16.0 wt.%, Ni: 1.0∼8.0 wt.% in rolling it. SOLUTION: A lower layer of oxide scale produced on the surface of a martensitic stainless steel piece containing Cr: 10.0∼16.0%, Ni: 1.0∼8.0% by weight in heating it, containing the metallic piece and maintaining a scale thickness of 150 μm in which an area ratio of vacant space to the whole cross section is 20% or less and an austenitic particle diameter of 200 μm in heating is bored. This method of boring decreases an incidence rate of outer flaws, improves the quality and yield of products and diminishes repairing of flaws.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a method for producing Cr:
The present invention relates to a method for manufacturing a martensitic stainless steel seamless steel pipe containing 10.0 to 16.0% and Ni: 1.0 to 8.0%. It relates to the rolling method to reduce.

[0002]

2. Description of the Related Art In recent years, various stainless seamless steel pipes manufactured by a hot extrusion method represented by the Eugene-Sejournet method have recently been rolled by a mandrel mill method and a plug mill method which have higher productivity than the hot extrusion method. Manufactured by law. JIS standard SUS420J1 type 1
Since 3% Cr steel is used as a steel pipe for oil wells, the production amount is large. Further, with the sophistication of market requirements for material properties such as corrosion resistance, strength, and toughness, Ni, as disclosed in JP-B-3-2277 and JP-A-2-247360,
Steel grades containing a large amount of alloying elements such as Mo and Cu have been developed, and these martensitic stainless steels are generally used in Modified 1 for oil country tubular goods.
Weldable for 3Cr and line pipe applications
It is referred to by its name, such as 13Cr, and has been put to practical use as a product such as a seamless steel pipe, an electric resistance welded pipe, and a round steel.

[0003] Such stainless steel is inferior in hot workability as compared with ordinary steel, and therefore is susceptible to flaws particularly in seamless steel tube rolling, which is severely processed. JIS standard SU
In order to reduce the external flaws of the S420 series 13% Cr steel tube, the present inventors disclosed in JP-A-7-178435 that a decarburization inhibitor was applied to the surface of the rolling material, or a protective steel plate was used. After covering the entire surface, a method of rolling was proposed.

However, the above Modified 1
Ni, especially among 3Cr and Weldable 13Cr
In the steel containing 1% or more, the generation of flaws on the outer surface was remarkable, and even if the method disclosed in JP-A-7-178435 was used, the prevention was not sufficiently achieved.

In order to reduce the outer surface flaws of martensitic stainless steel containing a large amount of alloying elements such as Ni, Mo, and Cr, the inventors of the present invention disclosed in Japanese Patent Application No. A method was proposed in which the thickness of a scale layer containing few metal pieces and having few voids was reduced to 150 μm or less, and a hole was formed. In the above-described method, it is possible to reduce the outer surface flaws, but a method capable of further reducing the outer surface flaws is required.

[0006]

SUMMARY OF THE INVENTION An object of the present invention is to provide a technique for overcoming the above-mentioned problems. That is, the present invention contains a large amount of alloying elements such as Ni and Mo which are useful in product performance. An object of the present invention is to reduce the occurrence of external surface flaws during seamless rolling of a martensitic stainless seamless steel pipe.

[0007]

SUMMARY OF THE INVENTION The present invention relates to a method for preparing C
Rolling flaws on the outer surface of a martensitic stainless steel seamless steel pipe containing r: 10.0 to 16.0% and Ni: 1.0 to 8.0% were investigated and analyzed in detail.

As a result, the oxide scale on the steel surface side is
The present invention was found to contain a large amount of metal pieces and to have few voids, which was the cause of the occurrence of external flaws, and completed the present invention.

The gist of the present invention is as follows.

(1) In mass%, Cr: 10.0 to 1
The lower part of the oxide scale generated on the surface of the steel slab when the martensitic stainless steel slab containing 6.0% and Ni: 1.0 to 8.0% is heated, including the metal slab and the entire cross section A method for producing a martensitic stainless steel seamless pipe, comprising: forming a scale having a void area ratio of not more than 20% to 150 μm or less.

(2) Cr: 10.0 to 1% by mass
The lower part of the oxide scale generated on the surface of the steel slab when the martensitic stainless steel slab containing 6.0% and Ni: 1.0 to 8.0% is heated, including the metal slab and the entire cross section A method for producing a seamless martensitic stainless steel pipe, comprising: forming a scale having a void area ratio of not more than 20% to a scale thickness of not more than 150 μm and an austenite particle size of 200 μm or less during heating.

(3) Cr: 10.0 to 1% by mass
6.0%, Ni: 1.0 to 8.0%, Ti: 0.01 to
When the martensitic stainless steel slab containing 0.05% is heated, the lower part of the oxide scale generated on the steel slab surface contains the metal slab and the area ratio of voids to the entire cross section is 20% or less. A method for producing a martensitic stainless steel seamless pipe, wherein the scale thickness is set to 150 μm or less, and the austenite particle size during heating is set to 200 μm or less.

(4) Any one of the above (1) to (3), wherein the content of the metal pieces in the lower layer of the oxide scale is 30% or more in terms of the area ratio to the entire cross section. 2. The method for producing a martensitic stainless steel seamless pipe according to item 1.

[0014]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The rolling method according to the present invention will be described below in detail.

The present inventors have found that, by mass%, Cr: 10.0
In a martensitic stainless steel containing 16.0% to 16.0% and Ni: 1.0% to 8.0%, an oxide scale containing Ni-enriched metal pieces as shown in FIG. Has a great effect on As shown in FIG. 1, a part of the metal pieces in the oxide scale is connected to the ground iron as shown in FIG. 2, so that it is difficult to remove even if high-pressure water descaling of about 210 atm is performed. . It has been found that when a material to which such an oxide scale having excellent adhesion to the ground iron is adhered is punched by a press roll punch or an inclined punch, an external flaw is formed by the following mechanism. In drilling by press roll drilling machine,
As shown in FIG. 3, in the vicinity of the corner of the rectangular billet, the friction received from the roll in the direction of the roll side flange causes the scale to be pushed unevenly, resulting in a groove defect.
When the tube having the groove defects is stretched and rolled by an inclined rolling mill, the groove defects may wrap due to friction in the rotating direction received from the rolls, resulting in external flaws. On the other hand, also in the inclined rolling mill, due to the friction in the rotating direction received from the rolls, the scale is pushed unevenly, the surface becomes grooved, and the surface is wrapped to become an outer surface flaw.

Subsequently, the relationship between the scale formed on the surface of the steel material and the scale existing inside the flaw was examined. The form of the oxide scale formed on the steel material surface is as shown in FIG. 1 and is classified into two layers. It is presumed that the oxide scale on the surface layer in the range indicated by I in the drawing has many voids and few metal pieces, and thus easily peels off. The oxidation scale on the surface layer in the range indicated by I is hereinafter referred to as scale I.
I will call it. On the other hand, the oxide scale (lower portion of the oxide scale) on the side of the base iron in the range indicated by II in the figure has few voids and many metal fragments. The scale including the metal pieces and having a small number of voids shown in II is a scale in which the area ratio of the voids is 20% or less and the area ratio of the metal pieces is 30% or more with respect to the entire cross section arbitrarily cut. This II
The oxide scale containing the metal pieces and having few voids is hereinafter referred to as scale II.

Next, as a result of examining the scale existing inside the groove-like flaw, it was found that this scale was a scale containing a large amount of metal pieces and was a scale II as shown in FIG. Scale I with many voids and few metal fragments
Can be peeled off from the material to be pierced immediately after contact with the roll without causing any problem, but the scale II has good adhesion to the material to be pierced.
Although the steel tends to plastically deform together with the ground iron, the scale is cut in the ground steel / scale interface or in the scale due to shear deformation caused by friction from the rolls and is pushed unevenly, resulting in groove defects.

A rolling method for reducing the external flaws caused by the scale was studied. As described above, the starting point of external flaws is
Since the scale II is a groove-like defect generated by being pressed unevenly, it was conceived that by reducing the thickness of the scale II, the groove-like defect generated on the surface becomes shallower and the outer surface flaw can be prevented. Generally, as the heating time increases, the thickness of the oxide scale increases, but as shown in FIG.
It has been found that the thickness of I also increases as the heating time increases.

First, a square steel slab was pierced into a thick hollow shell by a press roll piercing machine, and then examined by elongating and rolling into a thin-walled tube by an inclined rolling mill. Scale II formed on the surface
Using a steel slab with a changed thickness, after piercing a thick-walled hollow tube with a press roll piercing machine, rolling it into a thin-walled tube with an inclined rolling mill,
The depth of the outer surface flaw was investigated. At that time, some steel slabs were punched out by punching with a press roll punch, and the depth of groove defects due to press roll punching was investigated.

FIG. 6 shows the relationship between the thickness of the scale II and the depth of the groove defect after drilling. As shown in FIG. 6, the depth of the groove-like defect greatly increases from around 200 μm in thickness of the scale II.

FIG. 7 shows the relationship between the thickness of the scale II and the depth of flaws on the outer surface of the thin hollow shell after elongation rolling in the inclined rolling mill. As shown in FIG. 7, by reducing the thickness of the scale II, the groove defects after punching the press roll become shallower,
It has been found that the outer surface flaws after rolling by the inclined rolling mill become shallower, and that the outer surface flaws can be prevented by setting the thickness of the scale II to 150 μm or less.

Next, the depth of the outer surface flaws of the raw tube drilled by the inclined rolling mill was investigated using a steel slab having a scale II formed on the surface layer and having a changed thickness. FIG. 8 shows the relationship between the thickness of the scale II and the depth of the outer surface flaw.

It has been found that even in the inclined rolling mill, external flaws can be prevented by setting the thickness of the scale II to 150 μm or less. Therefore, in the present invention, the thickness of the scale II is limited to 150 μm or less. And scale I
The thickness of I can be reduced to 150 μm or less by setting the heating time of the slab to 140 minutes or less as shown in FIG. Preferably, it is 100 minutes or less.

The present inventors have found that, by mass%, Cr: 10.0
In the seamless rolling of a martensitic stainless steel containing 〜16.0% and Ni: 1.0-8.0%, the outer surface flaws generated even when the measures according to the invention of claim 1 are taken. As a result of detailed investigation and analysis, the generation mechanism of another outer surface flaw described below was found.

In the above steel, Ni as shown in FIG.
As shown in FIG. 9, a wedge-shaped oxide scale is formed at the interface between the oxide scale 4 containing the metal piece 2 and the oxide 3 and the ground iron 5, as shown in FIG. May be formed at the austenite grain boundary 7. This wedge-shaped oxide scale is hereinafter referred to as a wedge-shaped scale 6. It has been found that when the hollow shell having the wedge-shaped scale is rolled by an inclined rolling mill, an outer surface flaw is formed by a mechanism described below.

FIG. 10 shows rolling in a tilt rolling mill. The process from when the material 8 to be rolled, which is a rolled material, starts to contact the roll 9 until it comes into contact with a tool on the inner surface side called a plug 10 supported by the mandrel bar 11 (hereinafter, referred to as an empty massaging process).
Then, the rolled material is pressed down by the rolls, and a tensile stress in the circumferential direction is generated on the outer surface side of the position perpendicular to the line connecting the centers of the two rolls (the position shown in FIG. 10B). As a result of investigating the rolled material after stopping the raw pipe with the wedge-shaped scale in the inclined rolling mill and examining the rolled material, the outer surface flaw was caused by cracking due to circumferential tensile stress in the process of empty kneading starting from the wedge-shaped scale I found something. Further, as described in the first aspect of the present invention, when a steel slab to which an oxide scale including a metal piece is adhered is pierced by a piercing machine such as a press roll piercing machine and an inclined piercing machine, the oxidized scale including the metal piece is pushed. In some cases, a groove defect may be generated. It has been found that when the depth of the groove-shaped defect is 200 μm or more and the wedge-shaped scale is present at the bottom thereof, an outer surface flaw is more easily generated.

Subsequently, as a result of a detailed investigation of the relationship between the material and the wedge scale generated at the interface between the oxide scale and the base iron, the wedge scale was formed at the austenite grain boundary during heating, and had a great effect on the grain size. Was found to receive FIG. 11 shows the relationship between the austenite grain size and the depth of the wedge scale of a slab heated at 1280 ° C. for 2 hours. When the austenite grain size decreases, the wedge scale decreases and the austenite grain size becomes 200 μm or less. It was found that no wedge-shaped scale was generated. In order to suppress the coarsening of austenite grains, an element having a pinning effect may be added. In the present invention, it has been found that Ti is particularly effective as an element having a pinning effect. Using. By adding 0.01% or more of Ti, the austenite particle size when heated to 1280 ° C. becomes 20%.
0 μm or less. Further, Ti is an element having a strong affinity for N and generates nitride. However, an excessively high content increases the existing density of nitride and deteriorates hot workability, so the upper limit is set to 0.05%. . Therefore, when adding Ti, T
i: 0.01 to 0.05%.

Next, the steel type to which the present invention is applied is expressed by mass% and C
r: 10.0 to 16.0%, Ni: 1.0 to 8.0%.
The reason for the limitation is described below.

When Ni is 1.0% or less, the scale containing the metal pieces as shown in FIG. 1 does not occur, or even if it occurs, the scale does not exceed 150 μm. Ni is an element effective for improving corrosion resistance, and is contained from the viewpoint of preventing the formation of δ ferrite, but is an expensive element and if contained in a large amount, increases hot deformation resistance and lowers workability, so the upper limit is set. It was set to 8.0%.

Next, the reason for limiting Cr will be described. As a result of analyzing the scale in the range shown in FIG.
· Cr was ₂ O _3. The amount of FeO.Cr ₂ O ₃ produced increases with an increase in the amount of Cr, but peaks at about 13% by mass, and decreases when Cr is further added. C
When r is less than 10% by mass or more than 16% by mass, 150
No scale larger than μm is generated. Therefore, the application range of Cr is set to 10.0 to 16.0% by mass.

[0031]

(Example 1) Side length 215 mm, length 2500
A modified 13Cr square bar having the components shown in Table 1 in mm was heated under the conditions shown in Table 2. The heated square bar is 256mm in outer diameter and 65m in thickness with a press roll punch.
m, 2960 mm in length, after drilling 2
It was stretch-rolled to 56 mm, 20 mm in thickness, and 7790 mm in length. Table 3 shows the results of inspection of the outer surface flaws of the tube after elongation rolling.
Shown in

The present invention No. In No. 1, the thickness of the scale II was 90 to 120 μm, and no generation of external flaws was observed. On the other hand, in Comparative Example No. In Sample No. 2, the thickness of the scale II was 240 to 280 μm, and flaws were observed in 38 out of 50 scales.

[0033]

[Table 1]

[0034]

[Table 2]

[0035]

[Table 3]

(Embodiment 2) The diameter is 80 mm and the length is 800
The modified 13Cr round material having the components shown in Table 1 in mm was heated under the conditions shown in Table 4. The heated round material is subjected to 80 mm outside diameter, 8 mm wall thickness, and 2 length using an inclined rolling mill.
It was pierced to 220 mm. Table 5 shows the results of examining the outer surface flaws of the pipe after drilling.

The present invention No. In No. 3, the scale II had a thickness of 80 to 120 μm, and no external surface flaw was observed. On the other hand, in Comparative Example No. In No. 4, scale II had a thickness of 240 to 280 μm, and seven out of ten had flaws.

[0038]

[Table 4]

[0039]

[Table 5]

(Embodiment 3) Side length 220 mm, length 250
Modified 13Cr of the components shown in Table 6 at 0 mm
Was heated under the conditions shown in Table 7. The heated square bar was pierced by a press roll piercing machine, subsequently rolled by an inclined rolling mill, a plug mill, and a reeler, reheated, and fixed to an outer diameter of 273 mm and a wall thickness of 15.1 mm by a sizer.
Table 7 shows the results of examination of the outer surface flaws of the raw tube after rolling.
Table 8 shows the austenite particle size after heating and the occurrence of wedge-shaped scale.

The present invention No. In No. 1, no wedge-shaped scale was generated, and no generation of outer surface flaws was observed. On the other hand, in Comparative Example No. In 2, a wedge-shaped scale was generated, and
Outer surface flaws were observed in eight of them. Comparative Example No. In No. 3, since a groove-like defect and a wedge-like scale having a depth of 350 μm were generated, an outer surface flaw was observed in 92 out of 100 lines.

[0042]

[Table 6]

[0043]

[Table 7]

[0044]

[Table 8]

[0045]

As described above, according to the present invention, Ni,
The martensitic stainless steel material containing a large amount of alloying elements useful for product performance such as Mo and Cu can significantly reduce the occurrence of external surface flaws, improve quality and yield, and reduce flaw care. Is big.

[Brief description of the drawings]

FIG. 1 is a micrograph showing a cross section of an oxide scale,
(A) is a cross-sectional view of the oxide scale, (b) is a micrograph in which the I portion is enlarged, and (c) is a micrograph in which the II portion is magnified.

FIG. 2 is a photomicrograph showing a cross section of the oxide scale at the oxide scale / iron interface.

FIG. 3 is a micrograph showing a cross section of a groove defect.

FIG. 4 is an enlarged view of FIG. 3 and is a micrograph showing an oxide scale inside a groove defect.

FIG. 5 is a diagram showing a relationship between a heating time and a thickness of a scale II.

FIG. 6 is a diagram showing the relationship between the thickness of a scale II and the depth of a groove defect after perforation.

FIG. 7 is a diagram showing the relationship between the thickness of the scale II and the depth of flaws on the outer surface of the thin hollow shell after elongation rolling in an inclined rolling mill.

FIG. 8 is a diagram showing the relationship between the thickness of the scale II and the depth of the outer surface flaw of the raw tube after drilling in the inclined rolling mill.

FIG. 9 is a schematic view showing a cross section of a wedge-shaped scale.

FIGS. 10A and 10B are schematic views of inclined rolling, in which FIG. 10A is a main part of an inclined rolling mill, and FIG.

FIG. 11 is a diagram showing a relationship between an austenite grain size and a depth of a wedge-shaped scale.

[Explanation of symbols]

 Reference Signs List 1 void 2 metal piece 3 oxide 4 oxide scale 5 ground iron 6 wedge-shaped scale 7 austenite grain boundary 8 material to be rolled 9 roll 10 plug 11 mandrel bar

 ──────────────────────────────────────────────────続 き Continuing on the front page (72) Inventor Shunji Sakamoto 1-1, Hatata-cho, Tobata-ku, Kitakyushu Nippon Steel Corporation Inside Yawata Works (72) Inventor Ken-ichi Takaku 1-1, Tobita-cho, Tobata-ku, Kitakyushu New Nippon Steel Corporation Yawata Works (72) Inventor Koichi Furusho 1-1, Tobata-cho, Tobata-ku, Kitakyushu City Nippon Steel Corporation Yawata Works, Ltd. (72) Inventor Seiji Ishibashi 1, Tobihata-cho, Tobata-ku, Kitakyushu-shi -1 Inside Nippon Steel Corporation Yawata Works

Claims (4)

[Claims] (1) Cr: 10.0 to 16.0% by mass.
%, Ni: 1.0 to 8.0%, the lower layer of the oxide scale formed on the surface of the steel slab when the martensitic stainless steel slab is heated, including the metal slab and the voids of the entire cross section. A method for producing a martensitic stainless steel seamless steel pipe, wherein a scale thickness having an area ratio of 20% or less and a thickness of 150 μm or less are perforated. 2. Cr: 10.0 to 16.0% by mass.
%, Ni: 1.0 to 8.0%, the lower layer of the oxide scale formed on the surface of the steel slab when the martensitic stainless steel slab is heated, including the metal slab and the voids of the entire cross section. A method for producing a martensitic stainless steel seamless pipe, wherein a scale thickness having an area ratio of 20% or less is set to 150 μm or less, and an austenite particle size during heating is set to 200 μm or less. 3. Cr: 10.0 to 16.0% by mass
%, Ni: 1.0 to 8.0%, Ti: 0.01 to 0.0
When the martensitic stainless steel slab containing 5% is heated, the lower part of the oxide scale formed on the steel slab surface is
The area ratio of the voids to the entire cross section including the metal pieces is 2
A method for producing a martensitic stainless steel seamless pipe, wherein a scale thickness of 0% or less is set to 150 μm or less, and an austenite particle size during heating is set to 200 μm or less. 4. The marten according to claim 1, wherein the content of the metal pieces in the lower layer of the oxide scale is 30% or more in terms of the area ratio with respect to the entire cross section. Manufacturing method of sight stainless steel seamless steel pipe.