JP2001303207A - Martensitic seamless stainless steel pipe and its producing method - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a martensitic seamless stainless steel pipe whose surface is provided with scale excellent in adhesion and to provide its producing method. SOLUTION: This martensitic seamless stainless steel pipe contains, by mass, 0.2 to 0.5% Ni and 9 to 16% Cr, and has a boundary between scale on the surface and the steel which is provided with an Ni concentrated layer containing >=3% Ni, and in this producing method, the surface of a hollow pipe is coated with an oxidizer containing boric acid, and heating is performed.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a martensitic stainless steel seamless steel pipe having an oxide scale with excellent adhesion on the surface and a method for producing the same.

[0002]

2. Description of the Related Art A typical method of manufacturing a seamless steel pipe is a Mannesmann-mandrel mill pipe manufacturing method. This manufacturing method is widely used because of its excellent dimensional accuracy and productivity. Typical production processes of this method include a billet heating process, a piercing rolling process using a piercing mill (piercer), a stretching rolling process using a mandrel mill, a reheating process, a drawing rolling process using a reducer, a quenching and a tempering heat treatment process. . First, add a round billet of 110
After heating to 0 ° C. to 1300 ° C., a hollow shell is manufactured by piercing and rolling (piercing), and the hollow shell is subjected to elongation rolling by a mandrel mill. There are various methods for elongation rolling, and a mandrel mill rolling method, which is excellent in dimensional accuracy and productivity, is widely used. In mandrel mill rolling, a mandrel bar having a surface coated with a lubricant for hot rolling is stretched and rolled in a state where the mandrel bar is inserted into a hollow shell to mainly reduce the wall thickness. The temperature of the raw tube is generally 1050 ° C to 1150 ° C at the entrance of the mandrel mill, and 800 ° C to 1000 ° C at the exit side. A tube rolled by a mandrel mill is generally called a finish tube for finish rolling. The finish rolling tube is 850 ° C by reheating furnace.
After being reheated to 1100 ° C., rolling is performed by a finishing rolling machine such as a stretch reducer to make the tube outer diameter to a predetermined size. Thereafter, quenching is performed from 900 ° C. or higher, and then tempering is performed at around 700 ° C.

As described above, in the production of a seamless steel pipe by the Mannesmann-Mandrel pipe manufacturing method, since the raw pipe is heated at 1300 ° C. to 700 ° C. in each step, an oxide scale layer is inevitably formed on the pipe surface. Is generated. Usually, the scale is removed by shot blasting or pickling after finish rolling, and the seamless steel pipe is shipped without an oxide scale.

[0004] However, in recent years, there has been a demand for speeding up production and reducing the amount of pickling solution used, and seamless stainless steel pipes of martensitic stainless steel used as oil well pipes and line pipes have oxidized surfaces. Shipment with a scale (hereinafter simply referred to as a scale) is being considered. Hereinafter, the steel pipe with the scale attached is referred to as a black scale coated steel pipe. The black scale-coated martensitic stainless steel pipe manufactured by the conventional method has the following problems.

[0005] 1) The scale is inferior in adhesiveness and is partially peeled off, and the peeled off part has an insufficient environmental blocking effect and becomes a starting point of rusting even during storage. In addition, since the surface of the partially scale-removed portion is made uneven, it is difficult to uniformly apply the rust-preventive oil, which causes red rust during transportation by sea.

[0006] 2) When a steel pipe having red rust is used as an oil country tubular good or a line pipe, the red rust portion becomes a starting point of hole corrosion due to CO ₂ and H ₂ S.

[0007] 3) Ultrasonic flaw inspection is performed when the product is shipped as a product. However, surface flaw inspection becomes difficult in a scale state having many surface irregularities. Japanese Patent Application Laid-Open No. 57-19329 discloses an invention of a quenching treatment method for a 13Cr stainless steel sheet for quench hardening. This method is a method of quenching after removing a certain amount of scale by pickling or the like after hot rolling and annealing in order to prevent generation of nodular scale by quenching. This method has to remove scale by pickling and cannot be applied to a continuous manufacturing process such as a manufacturing process of a seamless steel pipe.

[0008]

SUMMARY OF THE INVENTION An object of the present invention is to provide a martensitic stainless steel seamless steel pipe provided with an oxide scale having excellent adhesion on the surface and a method for producing the same.

[0009]

Means for Solving the Problems The present inventors have thought that the adhesion of the scale should be enhanced in order to prevent the peeling of the scale, and as a result of intensive studies on a scale composition having excellent adhesion, the following findings were obtained. I came to.

A) The scale formed on the surface of martensitic stainless steel in the conventional process of manufacturing a seamless steel pipe is as follows:
The outer layer scale mainly composed of Fe ₂ O ₃ and Fe ₃ O ₄ and F
It has a multilayer structure of eCr ₂ O ₄ and an inner layer scale made of Fe ₃ O ₄ .

B) Since the inner layer scale is a dense crystal, if it is in close contact with steel without cracking or peeling, it is effective as a corrosion-resistant film in an oil well environment and a marine transportation environment.

C) However, the scale of the surface of the conventional martensitic stainless steel seamless steel pipe is in an uneven state in which cracks and peeling are partially caused due to poor adhesion.

D) When Ni is concentrated at the interface between the scale and the steel, Ni acts as a binder between the steel and the scale, and the adhesion of the scale is remarkably improved. e) As a means for concentrating Ni, it is effective to apply an antioxidant containing boric acid to the surface of the high-temperature tube, or to apply an antioxidant and heat. The present invention has been made based on such findings, and the gist is as follows.

(1) Ni: 0.2-0.5% by mass
%, Cr: 9-16% is a martensitic stainless steel pipe provided with an oxide scale on the surface, and 3% by mass of Ni at the interface between the oxide scale and the steel on the surface of the steel pipe.
A martensitic stainless steel seamless steel pipe having a Ni-enriched layer containing the above.

(2) Ni: 0.2-0.5% by mass
%, Cr: 9 to 16%, after forming a hollow tube by piercing and rolling a billet of martensitic stainless steel using a piercer, finishing it into a steel tube by hot working and subjecting it to a quenching treatment. A method for producing a martensitic stainless steel seamless pipe in which an oxidizing agent containing boric acid is applied to the surface of the hollow pipe.

Here, the Ni content of the Ni enriched layer is ED
X is the Ni concentration (mass%) obtained by line analysis using an electron beam having a probe diameter of 1 μm so as to cross the interface between the steel and the scale layer in the cross section of the steel pipe. Further, the seamless steel pipe of the present invention is a steel pipe having a scale on one or both of the inner surface and the outer surface thereof.

[0017]

BEST MODE FOR CARRYING OUT THE INVENTION Hereinafter, the present invention will be described specifically.

The martensitic stainless seamless steel pipe of the present invention has a mass percentage of Ni: 0.2-0.5%, Cr:
Contains 9-16%. Ni is an important element serving as a Ni source of a Ni-enriched layer formed between the scale and the steel. Ni contributes to the improvement of corrosion resistance and mechanical properties. If the content is less than 0.2%, the Ni concentration of the concentrated layer cannot be increased to 3% or more. On the other hand, if it exceeds 0.5%, mechanical properties may be degraded due to retained austenite. Cr must be contained in an amount of 9% or more in order to ensure carbon dioxide gas corrosion resistance in an oil well environment. On the other hand, if it exceeds 16%, a large amount of δ ferrite is precipitated, resulting in poor hot workability and rash. Scratches will occur. For these reasons, the contents of Ni and Cr are regulated as described above.

The chemical composition of the seamless steel pipe of the present invention is Ni and C
The content other than r is not particularly limited, but if C is contained at 0.6% or more, burning cracks may occur in the manufacturing process. Therefore, the upper limit is preferably 0.5%. Si is effective as a deoxidizing agent and is preferably 1% or less. M
o has no particular adverse effect on the scale properties and can be contained as needed. If it is 0.5 to 7%, it is effective in improving corrosion resistance. Mn is effective for fixing S as MnS, but becomes a spinel oxide when oxidized. Since this Fe—Mn spinel type oxide embrittles the entire scale, the allowable upper limit of Mn is 1.5%.
Is preferred.

The scale provided on the surface of the seamless steel pipe of the present invention is composed of an outer layer scale and an inner layer scale, and the boundary between these layers is a mixed layer having a complicated structure of the inner layer scale and the ground iron. The outer layer scale is Fe ₂ O ₃ and Fe ₃
O ₄ , the inner layer scale is FeCr ₂ O ₄ and Fe ₃ O
_{Consists of four} . And the inner layer scale is F
It contains spinel type oxides of eCr ₂ O ₄ and Fe ₃ O ₄ , and 3% by mass or more of N
There is a Ni-enriched layer containing i. The Ni-enriched layer is in a state where Ni is concentrated in the spinel-type scale {(Fe, Cr) ₂ O ₄ or Fe ₂ SiO ₄ } of the inner layer.

When the amount of Ni in the Ni enriched layer is less than 3%, the adhesion of the scale is not improved. If the thickness of the Ni-enriched layer is 1 μm or more, it is preferable to set the thickness to 1 μm or more because adhesion is developed.

The Ni concentration is a Ni concentration (% by mass) determined by performing line analysis using an EPMA with an electron beam having a probe diameter of 0.1 μm as described above so as to cross the interface between the scale layer and the steel. It shall be quantified.

Next, the manufacturing method will be described.

In order to concentrate Ni at the interface between the scale and the steel, the surface of the hollow tube is formed between the step after the hollow pipe manufactured by piercing and rolling the billet with a piercer and before the quenching treatment. Is coated with an antioxidant containing boric acid.
The coating may be performed during a high temperature, or may be performed after being applied to a tube that has been cooled to a normal temperature. It is preferably applied to any of a hollow tube after piercing and rolling, a hollow tube after elongation rolling by a mandrel mill, and a hollow tube after finish rolling by a stretch reducer or the like. In some cases, it may be applied two or more times.

Since Ni has a low affinity for oxygen and is not oxidized even under conditions where steel is oxidized, that is, under conditions where Fe and Cr are oxidized, Ni is concentrated on the steel surface, but conventional antioxidants are not applied. In the production method, it is almost uniformly dispersed in the scale or present in such an amount that the adhesiveness is not exhibited even when concentrated, but the concentrated layer is formed by applying a boric acid-based antioxidant. Generated. The reason why an antioxidant containing boric acid is applied to form a concentrated layer of Ni at the interface between the scale and the steel has not been clarified. Further, by applying an antioxidant containing boric acid, pores in the scale are reduced, and the adhesion is further improved.

The antioxidant containing boric acid is preferably a mixture of 80% boric acid and 20% Na stearate.

[0027]

EXAMPLES Martensitic stainless steels having the six chemical compositions shown in Table 1 were melted, formed into steel ingots, and then slab-rolled into billets having a diameter of 192 mm.

[0028]

[Table 1] Steel symbols A to F in Table 1 were obtained by varying the Ni content 13
Cr steel.

These billets were heated in a rotary hearth heating furnace at a temperature in the range of 1100 ° C. to 1200 ° C., and the outer diameter was 192 mm and the wall thickness was 16 by a Mannesmann piercer.
mm and a length of 6650 mm. At this time, a tube in which a mixture of 80% boric acid and 20% of sodium stearate was applied to the inner and outer surfaces of the hollow shell and a tube which was not applied were prepared. Was manufactured. The antioxidant was 100 g / m ² .

Then, after heating in a reheating furnace at 1100 ° C. for 20 minutes, an outer diameter of 63.5 mm was obtained by a stretch reducer.
A 5.5 mm thick, 56 m long base tube for finishing was used. After the finishing tube was heated at 980 ° C. for 65 minutes, it was quenched with high-pressure water and further tempered at 700 ° C. to obtain a product steel tube.

The analysis of the Ni concentration in the Ni-enriched layer after pipe production is as follows.
This was performed according to the method described above. Table 2 shows the analysis results.

[0032]

[Table 2] Each steel pipe has an amplitude of 10 to simulate scale peeling and falling off due to impact during handling during storage and transportation.
A vibration of 60 cycles / min in a mm cycle was applied for 1 hour. Next, as a weather resistance test, each steel pipe was immersed in an aqueous solution obtained by diluting seawater 100 times, and then exposed to an atmosphere having a temperature of 50 ° C and a humidity of 98%. 30 days after starting the corrosion test, 6
The rusting state on days 0, 90, and 120 was observed, and the area ratio covered with red rust per unit surface area was determined. Table 3 shows the test results.

[0033]

[Table 3] Since the period from shipment to use is as long as about three months, it can be said that there is no problem if there is no rust on the 90th day. As is clear from Table 3, for steel symbol A whose chemical composition is out of the range specified in the present invention, rust was observed on the 30th day regardless of the application of the antioxidant, and More than 50% were covered with red rust. In the conventional steel symbols A to F to which the antioxidant was not applied, red rust was generated on 50% of the surface on the 120th day, though the generation amount of red rust was small. No red rust was observed until the 90th day for steel symbols BF coated with the boric acid-based antioxidant of the present invention,
On day 120, only slight rust of 10% or less was observed.

[0034]

According to the present invention, a martensitic stainless steel seamless steel pipe coated with a scale having excellent adhesion can be obtained, and the scale does not peel off during handling or transport of the steel pipe. It has an excellent effect of preventing occurrence of pitting corrosion in an oil well environment without generation.

 ────────────────────────────────────────────────── ─── Continuing on the front page (72) Inventor Toshiro Anraku 4-5-33 Kitahama, Chuo-ku, Osaka-shi, Osaka F-term in Sumitomo Metal Industries, Ltd. 4K032 AA05 AA13 AA16 AA19 AA20 AA23 AA27 AA29 AA31 BA03 CA02 CB02 CF03

Claims (2)

[Claims] (1) Ni: 0.2 to 0.5% by mass, C:
r: a martensitic stainless steel tube having an oxide scale on the surface containing 9 to 16%, and a Ni-enriched layer containing 3% or more by mass of Ni at the interface between the oxide scale and the steel on the surface of the steel tube. A martensitic stainless steel seamless pipe characterized by having: 2. Ni: 0.2 to 0.5% by mass, C:
r: A billet of martensitic stainless steel containing 9 to 16% is formed into a hollow tube by piercing and rolling using a piercer, and thereafter, is turned into a steel tube by hot working and is subjected to a quenching process. A method for producing a martensitic stainless steel seamless steel pipe, characterized by applying an oxidizing agent containing boric acid to the surface of an empty pipe.