JPH10128412A - Manufacture of seamless 13-chromium base stainless steel tube covered with mill scale - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a manufacturing method of 13 Cr base stainless steel tube covered with mill scale excellent in surface property and corrosion resis tance by controlling scale in descaling and rolling. SOLUTION: About a hollow tube stock of stainless steel containing 11-15wt.% Cr, the outer layer scale of scale consisting of an outer layer and inner layer is removed with a descaler. The thickness of 0.1-50μm in the inner layer scale consisting essentially of a spinel type oxide FeCr2 O4 is left and elongating is executed at a draft of >=40% or, about a tube stock for finish rolling, 0.1-50μm of the inner layer scale containing the spinel type oxide as a main component and Fe2 SiO4 as an assistant composition is left and finish rolling is executed.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing a black scale-coated 13Cr stainless steel seamless steel pipe having excellent surface properties and corrosion resistance. In the present specification, 11 to 15% by weight
The stainless steel containing Cr is referred to as 13Cr stainless steel.

[0002]

2. Description of the Related Art A conventional method of manufacturing a seamless steel pipe is as follows.

[0003] The material billet is heated from 1100 ° C to 1300 ° C
Then, a hollow shell is manufactured by piercing and piercing, and the hollow shell is stretched and rolled by a mandrel mill without cooling. There are various methods of elongation rolling, and a mandrel mill rolling method, which is excellent in dimensional accuracy and productivity, is widely used.

In mandrel mill rolling, elongation rolling is performed with a mandrel bar having a surface coated with a hot rolling lubricant inserted into the inner surface of a hollow shell. The temperature of the tube in the mandrel mill is from 1050 ° C to 1200 on the inlet side of the mandrel mill.
The temperature generally ranges from 800 ° C. to 1000 ° C. at the outlet side. The tube rolled by the mandrel mill is generally called a finish rolling tube.

[0005] If necessary, the raw tube for finish rolling is reheated from 850 ° C to 1100 ° C, which is lower than that before the mandrel mill, by a reheating furnace, and then finished by a finish rolling machine such as a stretch reducer. The outer diameter is rolled into a seamless steel pipe of a predetermined size. After that, it is reheated to 900 ° C. or higher and quenched, and then tempered at around 700 ° C.

As described above, in the production of a seamless steel pipe,
The billet, the hollow shell, and the finish rolling shell, which are the materials to be rolled at each stage, are heated at 1300 ° C to 700 ° C, and scales are inevitably formed on the inner and outer surfaces of the tube. The oxide scale is removed by shot blasting after finish rolling and further by pickling and descaling of sulfuric acid or nitric acid, and the seamless steel pipe is shipped without scale. In recent years, speeding up of pickling descaling, reduction of the amount of pickling solution used, and the like have been demanded.
Shipment with scale of Cr stainless steel seamless steel pipe is under study. In the following explanation, "shipment with scale"
This is referred to as "black-scaled steel pipe", and "scaled steel pipe" as-finished and as-heated after rolling is referred to as "black-scale coated steel pipe".

Black scale coated 13Cr manufactured by the current method
The stainless steel pipe has the following problems.

[0008] Since the surface irregularities of the scale are remarkable, it becomes difficult to determine the surface flaw of the ultrasonic flaw detection performed at the time of shipment.

[0009] Since many scale cracks, that is, microcracks are present and the environmental barrier effect of the scale is insufficient, rust is generated from the base metal. This is a particular problem when seawater adheres during sea transport.

[0010] Rust during storage due to the difficulty of evenly applying the rust preventive oil.

Scale peeling during bending.

Appearance, that is, poor appearance. Poor appearance mainly corresponds to the occurrence of red rust.

Among the above defects, a combination of the two factors of "surface irregularities" and "red rust generation" is referred to as "defective surface properties".

As a method for controlling the scale of 13Cr stainless steel, a method has been disclosed in which the scale on the surface is removed before the quenching treatment of the sheet material (Japanese Patent Laid-Open No. Sho 57-19757).
19329). However, since this method employs an acid pickling treatment as a scale removing method, it is difficult to apply the method to a continuously manufactured steel pipe. Except for this example, there has been almost no proposal for optimizing the hot rolling conditions for the purpose of scale control during hot rolling of steel pipes.

[0015]

SUMMARY OF THE INVENTION It is an object of the present invention to provide a method for producing a black scale coated 13Cr stainless steel seamless pipe having excellent surface properties and corrosion resistance by controlling the scale in descaling and rolling. It is in.

[0016]

Means for Solving the Problems The present inventors have made intensive studies to achieve the above object, and as a result, have confirmed the following matters.

(A) The scale formed on the surface of 13Cr stainless steel has the following two-layer structure. That is, hematite (Fe ₂ O ₃ ) and magnetite (Fe
₃ O ₄₎ Porous poor outer layer and the spinel adhesion by consisting of FeCr ₂ O ₄ (hereinafter, referred to as "spinel type oxide")
Consisting of a dense inner layer with excellent adhesion and a main component of
This is the scale of the layer structure.

The inner layer scale contains 20 to 70% by volume of a spinel type oxide FeCr ₂ O ₄ as a main component, and also contains FeO, Fe ₃ O ₄ and Fe ₂ SiO ₄ as subcomponents. Among them, Fe ₂ SiO ₄ is concentrated in the range from the interface between the base material and the scale to about 50 μm on the scale side.

(B) Defects in the surface properties of the current black-skin-coated 13Cr stainless steel seamless steel pipe, that is, unevenness of the scale and red rust, are caused by the outer layer oxide partially removed during descaling before elongation rolling, especially hematite. (Fe ₂ O ₃ )
Caused by

On the other hand, Cr, which is an element for improving the corrosion resistance, is concentrated in the inner layer oxide, so that the inner layer scale acts as a protective film against oxidation. Both spinel oxides FeCr ₂ O ₄ and Fe ₂ SiO ₄ increase the adhesion of the scale to the base material. Fe ₂ SiO ₄ is formed at the grain boundaries of the scale, melts depending on the heating temperature, solidifies with the temperature drop during rolling, or solidifies from the time of heating, plastically deforms with high deformability during rolling, and after rolling is completed. Acts as a binder between the base material and the scale.

(C) When descaling with high-pressure water or the like, if the total scale thickness of the outer layer and the inner layer is within a certain range, only the outer layer scale can be easily removed. That is, there is no strong bonding force between the outer scale and the inner scale, and the outer scale can be clearly separated and removed from the inner scale by the external force.

(D) Among the problems of the above-described black scale-coated 13Cr stainless steel seamless steel pipe, the occurrence of microcracks can be suppressed by increasing the rolling reduction in elongation rolling to a certain value or more.

The present invention has been completed on the basis of the above-mentioned items, and has been completed through repeated experiments for producing various types of 13Cr stainless steel seamless steel pipes at the production site.
The gist is a method for manufacturing an r-type stainless seamless steel pipe.

(1) After heating and billet-rolling a 13Cr stainless steel billet to produce a hollow shell, the outer layer scale formed of the outer layer and the inner layer formed in the hollow shell is removed by a descaler, and the spinel type is removed. Oxide FeC
The inner layer scale containing r ₂ O ₄ as a main component has a thickness of 0.1 to 50.
A method for producing a black skin-coated 13Cr stainless steel seamless steel pipe having excellent surface properties and corrosion resistance, including a method of producing a raw tube for finish rolling by elongating at a reduction ratio of 40% or more and elongating and rolling at 40% or more ([Invention 1] And).

(2) After heating the finish rolling base tube of 13Cr stainless steel, the outer layer scale formed of the outer layer and the inner layer formed on the finish rolling base tube is removed by a descaler, and the spinel oxide FeCr is removed. _The inner layer scale containing ₂ O ₄ as a main component and Fe ₂ SiO ₄ as one of the sub-components has a thickness of 0.1 to 50 μm, and is subjected to finish rolling to produce a seamless steel pipe. A method for producing an excellent black scale coated 13Cr stainless steel seamless steel pipe (referred to as [Invention 2]).

In [Invention 1], "hollow shell"
An intermediate material for elongation rolling in which a billet is pierced by a piercer or the like. The hollow shell produced by the piercer is usually stretched and rolled without cooling, but may be cooled and then reheated and stretched and rolled.

[Finish Rolling Tube] in [Invention 2]
The term refers to an intermediate material that has been subjected to elongation rolling. The finish rolling tube manufactured by elongation rolling is usually cooled once, reheated, and then subjected to finish rolling.

The "scale" in [Invention 1] or [Invention 2] is a scale formed on the outer surface of a hollow shell or a finish rolling shell, and comprises an outer layer scale and an inner layer scale. The scale on the inner surface of the pipe is usually completely removed by shot blasting. In [Invention 1] or [Invention 2], since descaling is performed immediately before elongation rolling or immediately before finish rolling, it is hot descaling.

The “inner layer scale mainly composed of a spinel oxide” in [Invention 1] refers to the inner layer scale in the inner layer scale.
0 to 70% by volume is a spinel-type oxide.
O, Fe ₃ O ₄ , Fe ₂ SiO _{4 and the} like may be included.

The elongation rolling refers to rolling of a hollow shell and is mainly performed by a mandrel mill, but is not limited to the mandrel mill as long as the elongation rolling is performed. The rolling reduction in elongation rolling is {(thickness before elongation rolling-thickness after elongation rolling) / thickness before elongation rolling} × 100.
(%).

The “inner layer scale containing spinel-type oxide as a main component and Fe ₂ SiO ₄ as one of the sub-components” in [Invention 2] refers to the scale component formed by heating the finish rolling tube. Point. The content of the spinel oxide as a main component is 20 to 70% by volume even in the inner layer scale before the finish rolling in [Invention 2]. It goes without saying that, during heating before finish rolling, the raw tube for finish rolling is heated with the scale that it had at the end of elongation rolling.

[0032]

Next, the reasons for limitation of [Invention 1] and [Invention 2] will be described.

1.13 Cr-based stainless steel The material of the 13Cr-based stainless steel pipe, which is the object of the present invention, is mainly martensitic stainless steel containing 11 to 15% by weight of Cr, but is ferritic stainless steel. You may. If Cr is contained in an amount of 11 to 15% by weight, a desired scale layer is formed in a normal heating atmosphere as described later.

In [Invention 2], since the inner layer scale must contain Fe ₂ SiO ₄ as a subcomponent, the 13Cr stainless steel must contain Si. However, the Si content need not be particularly high.

As an example of the chemical composition in [Invention 1] and [Invention 2], Cr: 11 to 15%, C: 0.4%
Hereinafter, Si: 0.01 to 1%, Mn: 1% or less, Ni:
9% or less, Mo: 5% or less, and trace amounts of Al and C
a, Ti, rare earth elements and the like.

2. The descalers of [Invention 1] and [Invention 2] are both ordinary high-pressure water descalers.
It is desirable that the device can be set to 0 to 300 kgf / cm ² G. Here, “G” is a symbol indicating that it is a pressure measured by a gauge. With such an apparatus, high-pressure water is sprayed from the entire circumference or a predetermined range of the steel pipe while rotating the steel pipe to separate and remove the outer scale from the inner scale. High water pressure is 100kgf / cm ² G
If it is less than 300, the separation of the outer layer scale is insufficient, and 300
Since the inner layer scale exceeds kgf / cm ² G is sometimes peeled off is preferably in the range of 100~300kgf / cm ² G.

3. Inner layer scale 3-1: Main component In both [Invention 1] and [Invention 2], among the scales formed on the surface of 13Cr-based stainless steel, the spinel oxide, which is the main component of the inner layer scale, improves the corrosion resistance. It contains about 60% by weight of Cr and is dense and acts as a protective coating on the base material. If the main component of the inner layer scale is not a spinel oxide, both the surface properties and the corrosion resistance will be poor, so the main component of the inner layer scale is a spinel oxide. If the content of the spinel oxide is less than 20% by volume, the surface properties and corrosion resistance are insufficient, so that it cannot be shipped as black scale. On the other hand, if the spinel-type oxide exceeds 70% by volume, the surface properties are rather deteriorated, so that the content of the spinel-type oxide in the inner layer scale is set to 20 to 70% by volume.

Other compounds in the inner layer scale include F
eO, Fe ₃ O ₄ , and Fe ₂ SiO ₄ . Of these, Fe ₂
SiO ₄ must be included as an auxiliary component in [Invention 2]. Fe ₂ Si on scale before finish rolling
This is because if O ₄ is not contained, the adhesion of the scale will not be good. In order to further increase the adhesion, it is desirable that Fe ₂ SiO ₄ be contained in the inner layer scale in an amount of 0.5% by volume or more. FeO and Fe ₃ O ₄ are
Since the adhesiveness of the scale is reduced and the surface properties are also deteriorated, it is better to use as little as possible.

3-2. Inner Scale Thickness In [Invention 1] and [Invention 2], when the inner scale thickness exceeds 50 μm, cracking or peeling occurs at the time of elongation rolling regardless of the type of the inner scale, which causes red rust as a secondary scale. Fe ₂ O _3, which is the cause, is generated, and the surface properties of the black scale are significantly deteriorated. On the other hand, when the scale thickness is less than 0.1 μm, the base iron is locally exposed by rolling and becomes a starting point of rust, so the inner layer scale thickness is set to 0.1 μm or more. Therefore, the inner layer scale thickness just before elongation rolling ([Invention 1]) or just before finish rolling ([Invention 2]) is 0.1 to 50 μm so that a dense black scale excellent in surface properties and corrosion resistance is formed. I do.

The heating atmosphere, conditions, etc., in which the scale is composed of two layers, an inner layer and an outer layer, the outer layer scale is easily removed with a descaler, and the thickness of the inner layer scale is in the range of 0.1 to 50 μm are known empirically. For example, the following atmosphere and conditions are applicable.

[Atmosphere] A heavy oil combustion atmosphere (containing H ₂ O), the composition of which is O ₂ : 5% by volume, CO ₂ : 10
Vol%, H ₂ O: 10~20 vol%, N _2: the balance.

[Heating Conditions] Temperature: 950 to 1100
° C, time: about 60 minutes.

4. Elongation rolling (in the case of [Invention 1]) 4-1. Elongation Rolling Temperature Elongation rolling is mainly performed by a mandrel mill, but need not always be a mandrel mill. Wherein the temperature of the elongation rolling is no problem in a temperature range of conventional _{800 ℃ ~1200 ℃, Fe 2 O} 3 ( red rust) 950 ° C. In view of the product to 11
It is desirable to perform in a temperature range of 50 ° C.

4-2. Reduction ratio of elongation rolling The ratio of the portion where microcracks have occurred to the total surface area covered by the scale immediately after elongation rolling (hereinafter, microcrack ratio) is about 10% when the reduction ratio in elongation rolling is less than 40%. On the other hand, if it is 40% or more, it can be suppressed to several% or less. For this reason, in order to maintain corrosion resistance during storage and sea transportation, the rolling reduction of elongation rolling is set to 40% or more.

The upper limit of the rolling reduction is not particularly limited. However, considering the performance of the current mandrel mill and the like, 90% is considered.
It is desirable to make the following.

5. Finish Rolling (In the case of [Invention 2]) Finish rolling is usually performed by a stretch reducer generally used, but finishing may be performed by a processing method instead of the stretch reducer. The rolling temperature range of the finish rolling is desirably 900 ° C. or higher from the viewpoint that the plastic deformation ability of Fe ₂ SiO ₄ is sufficiently large.

5. Relationship between Elongation Rolling ([Invention 1]) and Finish Rolling ([Invention 2]) The most desirable method for producing a 13Cr stainless steel pipe having excellent surface properties and corrosion resistance is [Invention 1] and [Invention 2]. It is a combined method. That is, [Invention 1]
In this method, the finish rolling of [Invention 2] is performed on the finish rolling base tube manufactured by performing the elongation rolling. However, it is not always necessary to use a method combining [Invention 1] and [Invention 2] to produce a 13Cr stainless steel pipe having excellent surface properties and corrosion resistance.

For example, although the elongation rolling of [Invention 1] is performed and the finish rolling is performed by any method, the elongation rolling other than [Invention 1] is performed, and the finish rolling is also performed by the finishing rolling other than [Invention 2]. A 13Cr stainless steel seamless steel pipe having better surface properties and corrosion resistance than those manufactured by the method can be manufactured. This is because, in [Invention 1], the inner layer scale having good adhesion is refined by rolling and pressed to the base material, which is maintained until the final product.

In the case of [Invention 2], since the scale control is performed in the final stage, even when only the finish rolling of [Invention 2] is applied without performing the elongation rolling of [Invention 1], it is extremely excellent. It is possible to manufacture a 13Cr stainless steel seamless steel pipe having surface properties and corrosion resistance.

[0050]

Next, the effect of [Invention 1] will be described by [Example 1], and the effect of [Invention 2] will be described by [Example 2]. [Example 1] The material billet was made of API standard 13Cr stainless steel (hereinafter referred to as steel code "API") and 13
Modified steel of Cr-based stainless steel
Was used.

Table 1 shows the chemical composition of these steels.

[0052]

[Table 1]

Steel billets (outer size 192 m) shown in Table 1
m) in a rotary hearth heating furnace from 1100 ° C. to 120
After heating in a temperature range of 0 ° C., the outer shape is 192 mm, the thickness is 16 mm, and the length is 6650 mm by a Mannes Man Piercer.
Was manufactured. The water pressure of the hollow shell is kept at 1 before the elongation rolling by the mandrel mill without cooling.
It was descaled with a high-pressure water descaler set in the range of 00 to 250 kgf / cm ² G. After removing the outer scale of the hollow shell by descaling and adjusting the inner scale type and thickness, the mandrel mill stretches and rolls a finishing pipe with an outer diameter of 151 mm, a wall thickness of 6.5 mm and a length of 20 m. Was done. Scale type before elongation rolling,
The scale thickness and the microcrack ratio were measured separately by heating a hollow shell for scale measurement under the same conditions.

Thereafter, the raw tube for finish rolling was heated to 1100 ° C. for 20 minutes in a heavy oil combustion atmosphere containing water, and descaled by a descaler having a water pressure set in a range of 100 to 250 kgf / cm ² G. Using a stretch reducer, a seamless steel pipe having an outer diameter of 63.5 mm, a wall thickness of 5.5 mm, and a length of 56 m was obtained. After the descaling, the components of the remaining scale immediately before the finish rolling were not particularly investigated. Then, at 980 ° C, 6
After heating for 5 minutes and quenching with high-pressure water, 730
Tempering was performed at ℃, and linseed oil was applied to the outer surface of the tube to obtain a final product.

The surface properties of the final product were comprehensively evaluated by both:

The presence or absence of unevenness of the black scale on the surface visually.

The occurrence of red rust is regarded as a change to red, and the chromaticity value is displayed by a color difference meter for the degree of red.

The corrosion resistance test was carried out by immersing in a 100-fold diluted solution of seawater and then simulating sea transportation at a temperature of 50 ° C.
After exposure for one week in air with a humidity of 98%, the test was performed by observing the presence or absence of rusting.

Table 2 is a list showing the results of these tests.

[0060]

[Table 2]

In Table 2, the evaluation criteria for surface properties and corrosion resistance are as follows. The microcrack ratio in Table 2 is the microcrack ratio immediately after elongation rolling.

A. Surface property evaluation standard ：: No unevenness and no red rust.

Δ: There is no unevenness, but red rust is generated.

×: Both irregularities and red rust occurred.

B. Corrosion resistance evaluation standard ：: No rust was generated.

Δ: Rust is generated in the area ratio of 20% or less.

×: Rust is generated in an area ratio of 20% or more.

According to the measurement by the color difference meter, in all of the test numbers 1 to 8 of the examples of the present invention, the amount of hematite (Fe ₂ O ₃ ) produced was very small, and the chromatogram indicating the change of the black scale to red due to the generation of red rust. The Kussnes value was negative.

In the present invention, since the rolling was performed at a rolling reduction of 40% or more, the microcrack ratio on the scale surface was extremely small and no rust was generated.

On the other hand, in Test No. 9 which is a comparative example, since the scale thickness before elongation rolling was as thin as 0.05 μm, the microcrack ratio immediately after elongation / rolling was high, which affected the final product and caused the surface properties and corrosion resistance. Both were bad.

In Test Nos. 10 to 15, the microcrack ratio was high because the rolling reduction in elongation rolling was less than 40%, and rust was generated due to this.

Test Nos. 16 to 24 were poor in both corrosion resistance and surface properties regardless of the magnitude of the microcrack ratio because the thickness of the inner layer scale before elongation rolling was large.

In the comparative examples of Test Nos. 10 to 24 described above, the chromaticity value indicating the change of the black scale to red due to the generation of red rust is 1 or more, that is, the color of the black scale due to the formation of Fe ₂ O ₃ The change (red discoloration) was clear. Irregularities of the surface black scale were easily confirmed visually.

[Example 2] The material billet was API13C
Fe ₂ based on r-based stainless steel (steel code: API2)
For the purpose of investigating the effect of the volume percentage of SiO ₄ , five types of 13Cr stainless steels having different Si contents (steel code: trial 1, trial 2, trial 3, trial 4, trial 5) were used.

Table 3 shows the chemical compositions of these test steels.

[0076]

[Table 3]

A billet having an outer diameter of 192 mm was heated in a rotary hearth heating furnace in a temperature range of 1100 ° C. to 1200 ° C., and was pierced with an outer diameter of 192 mm and a thickness of 16 mm.
A 6650 mm long hollow shell was manufactured. The hollow shell is
The water pressure is 100 to 250 without cooling once.
The scale species and thickness were adjusted with a high pressure water descaler controlled between kgf / cm ² G. After that, the hollow shell was 151 mm in outer diameter and 6 in wall thickness by a mandrel mill.
It was elongated and rolled into a 5 mm, 20 m long base tube for finish rolling.

The raw tube for finish rolling is heated at 1100 ° C. for 20 minutes in a reheating furnace in a heavy oil combustion atmosphere containing water,
With a high pressure water descaler controlled to a range of 00 to 250 kgf / cm ² G or higher for comparison,
Scale type and thickness have been adjusted. Thereafter, the outer diameter is 63.5 mm and the wall thickness is 5.5 m by a stretch reducer.
m, 56m long seamless steel pipe
A quenching treatment of heating for 5 minutes and cooling with high-pressure water and a tempering treatment at 730 ° C. were performed.

Thereafter, linseed oil was applied to the outer surface of the steel pipe to obtain a final product. The final product was visually inspected for unevenness of the surface black scale and the presence or absence of peeled portions. Corrosion resistance is seawater 1
After being immersed in the 00-fold diluted solution, the test was performed by exposing for 1 week in an atmosphere simulating sea transportation conditions at a temperature of 50 ° C. and a humidity of 98%.

Table 4 is a list showing the test results.

[0081]

[Table 4]

The state after descaling and various evaluation criteria (surface unevenness, presence or absence of peeling, corrosion resistance) in Table 4 are as follows.

A. State after descaling P: Optimal scale (FeCr ₂ O ₄ + Fe ₂ SiO ₄ ) F: Insufficient deske (the outer layer scale also remains) N: Almost no residual scale S: Only FeCr ₂ O ₄ is formed b. Surface unevenness ○: good surface properties without unevenness ×: unevenness and lack of product quality c. Existence of peeling ○: No surface peeling and good surface properties ×: Peeling and unevenness d. Corrosion resistance ○: No rust ×: Rust generation Test numbers 12, 15, 18, 2 with insufficient deske
In Nos. 1, 24 and 29, regardless of the type of steel, surface irregularities mainly due to partial peeling of the outer layer scale are remarkably unsuitable for black scale shipping.

In Test No. 5 where the Si content was extremely low, almost no Fe ₂ SiO ₄ was formed in the inner layer, and the adhesion was reduced not only in the outer layer scale but also in the inner layer scale, regardless of the remaining state of the scale, and peeling was observed. Was.

Further, Test Nos. 13, 16, 19, 22, which were removed to the inner layer scale and had little residual scale,
In Nos. 25 and 30, corrosion resistance was poor and rust was generated due to insufficient sound black scale coating as a protective film.

Further, when the remaining scale thickness is 50 μm or more (test numbers 11, 14, 17, 20, 23, 2
8) In the portion having a thickness exceeding 50 μm, the surface properties were poor.

On the other hand, Test Nos. 1 to 1 of the examples of the present invention
In the case of 0, the scale type and scale thickness before finish rolling were in an appropriate range, and a black scale product excellent in both surface properties and corrosion resistance was obtained.

[0088]

According to the present invention, it is possible to provide a 13Cr stainless steel seamless pipe having a black scale having excellent surface properties and corrosion resistance. Since the method of the present invention can measure the corrosion resistance during use while reducing the manufacturing cost, it is a useful technique not only for the manufacturer but also for the user.

Claims (2)

[Claims] 1. A stainless steel billet containing 11 to 15% by weight of Cr is heated and pierced and rolled to produce a hollow shell, and the outer layer scale of the outer layer and the inner layer formed in the hollow shell is descalarized. And leaving an inner layer scale having a spinel type oxide FeCr ₂ O ₄ as a main component in a thickness of 0.1 to 50 μm and elongating and rolling at a rolling reduction of 40% or more to produce a raw tube for finish rolling. 13C with excellent surface properties and corrosion resistance characterized by containing
Manufacturing method of r-type stainless seamless steel pipe. 2. A stainless steel finish-rolling tube containing 11 to 15% by weight of Cr is heated, and then an outer layer scale of an outer layer and an inner layer formed on the finish-rolling tube is removed by a descaler. , Spinel-type oxide Fe
An inner layer scale containing Cr ₂ O ₄ as a main component and Fe ₂ SiO ₄ as one of sub-components having a thickness of 0.1 to 50 μm,
Black skin coated 13Cr having excellent surface properties and corrosion resistance, including a method of producing a seamless steel pipe by finish rolling.
For producing stainless steel seamless stainless steel pipes.