JP2000042609A - Manufacture of seamless steel tube and seamless steel tube having excellent inner surface quality - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a manufacturing method of a seamless steel tube capable of stably obtaining a product tube of 0.2 mm or less in depth of a flaw in an inner surface and the seamless steel tube having excellent inner surface quality which is obtained by the method. SOLUTION: When thinning is applied to a hollow tube stock after piercing and rolling with a stretch mill which is provided witch an inner surface control tool, the inner surface of the hollow tube stock is purged with inert gas, a lubricant whose main component is univalent alkali metal borate and which does not incorporate phosphorus is uniformly coated at 50-300 g/m2 onto the inner surface of the tube just after that, and thereafter stretch 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 plug mill, a mandrel mill, and a hollow shell formed by a piercing mill represented by a mannes man piercer or a press piercing mill.
A method for manufacturing a hot seamless steel pipe that performs wall-thinning with a stretching rolling mill represented by an Assel mill and a Pilger mill,
The present invention relates to a seamless steel pipe having an excellent inner surface quality obtained by the method.

[0002]

2. Description of the Related Art When a hollow shell formed by a piercing mill is subjected to wall-thinning processing by an elongating mill, an inner surface regulating tool is used. Specifically, in a mandrel mill, an assell mill, and a pilger mill, a bar-shaped elongated inner surface regulating tool called a mandrel bar is used. In the plug mill, a shell-shaped integral type or plug-type inner surface regulating tool divided into two in the axial direction is used. Among them, the mandrel mill is a multi-stand elongation rolling mill, so that the length of the inner surface regulating tool is the longest.

[0003] Of the above-mentioned inner surface regulating tools, an inner surface regulating tool of a mandrel mill (hereinafter, referred to as a mandrel bar) will be described as an example.

A plurality of mandrel bars (usually JIS
G 4404) is prepared and recycled. Prior to insertion into the hollow shell, a lubricant is applied to the surface thereof. This is because a relative slip occurs between the inner surface of the hollow shell and the surface of the mandrel bar during elongation rolling.If the boundary surface is not sufficiently lubricated, the hollow shell and the mandrel bar are seized and the inner surface quality is good. This is because it is not possible to obtain a good product tube. Therefore, a lubricant is applied to the surface of the mandrel bar in order to prevent seizure of the hollow shell and the mandrel bar and to secure a stable low coefficient of friction.

As the above-mentioned lubricant, a water-dispersed graphite-based lubricant in which a main component is a mixture of graphite and a resin-based organic binder and the mixture is dispersed in water is generally used. As described above, the graphite-based lubricant is applied to the surface of the mandrel bar before being inserted into the hollow shell, and then dried and used. That is, the above graphite-based lubricant is used as a dry solid film (hereinafter, this lubrication method is referred to as "method A").

However, when the hollow shell is made of an alloy steel such as Cr-Mo steel or stainless steel containing a large amount of Cr, specifically 4.5 to 30% by weight of Cr, these steels are made of SKD.
Easy to seize with 6 mandrel bars. Therefore, even in the above-mentioned Method A, a graphite-based lubricant having improved seizure resistance by mixing iron oxide and glass powder in addition to graphite and a resin-based organic binder may be used as the lubricant. (See Japanese Patent Application Laid-Open No. 7-118687).

Further, the lubricating property is insufficient only by the above-mentioned method A, and it is necessary to further improve the lubricating property in some cases.
In this case, for example, JP-A-60-64720 and JP-A-61-37989 and JP-A-1-27890.
The method disclosed in Japanese Patent Publication No. 3 is sometimes adopted. That is, before rolling (before inserting the mandrel bar) or in a hollow shell during rolling, a mixture called a descaling agent or a pickling agent containing borate as a main component using air, steam or an inert gas as a medium ( Hereinafter, simply referred to as borate-based lubricant). Then, a film having good lubricating properties is formed on the inner surface, specifically, a lubricating film composed of a reaction product of the oxide scale formed and adhered to the inner surface of the tube and the borate-based lubricant introduced is formed. (Hereinafter, this lubrication method is referred to as “method B”).

[0008] In a stretching rolling mill other than a mandrel mill, the above method A is rarely used, and the method B is used exclusively. Particularly, in the plug mill, only the B method is often used because the length of the plug, which is an inner surface regulating tool, is extremely short as compared with the mandrel bar of the mandrel mill.

[0009] However, in the plug mill, in addition to the lubrication treatment by the above-mentioned method B, usually, in the case of an integrated plug, only graphite or the main component is directed toward the rolling section from the tip portion of the integrated plug, and from the split portion of the split plug. The elongation rolling is performed while supplying a graphite lubricant of graphite.

The lubricant used in the above-mentioned method A may be a graphite-based lubricant to which a carburizing inhibitor is added or a non-graphite-based lubricant containing no graphite at all. It is used in the same manner as the graphite-based lubricant.

[0011]

According to the findings of the present inventors, when the hollow shell is an alloy steel containing a large amount of alloy components or the temperature of the elongation rolling mill is low, during the rolling, The lubrication treatment for obtaining a product tube having an inner surface flaw depth of 0.3 mm or less by a mandrel mill, in which seizure is likely to occur when the bar is pulled out after rolling, is performed by using a hollow shell made of carbon steel, S
If the surface of the mandrel bar made of KD6 is an oxide scale adhered skin, in most cases, only the above-mentioned method A is sufficient.

On the other hand, when using a mandrel bar on which the surface of SKD6 is plated with a metal such as Cr or Ni, especially a Cr plating for the purpose of improving the service life, even if the hollow shell is made of carbon steel, only method A is used. The lubrication treatment is not sufficient, and it is necessary to use the method B as an auxiliary lubrication means.

The hollow shell is made of SKD6 compared to carbon steel.
Mandrel bar and 4.5-30% by weight which are easy to seize
In the case of alloy steel such as Cr-Mo steel or stainless steel containing Cr, regardless of the surface condition of the mandrel bar, the improvement of seizure resistance as described in the above-mentioned Japanese Patent Application Laid-Open No. Hei 7-118687 is improved. Even when the intended graphite-based lubricant is used, the lubrication treatment using only the method A is not sufficient, and the method B must be used as the auxiliary lubrication means. Above all, when a metal-plated mandrel bar is used, it is necessary to apply the B method.

However, in any case, a product pipe having better inner surface quality, specifically, a product pipe having an inner surface flaw depth of 0.2 mm or less cannot be stably obtained. It is remarkable in the case of alloy steel such as Cr-Mo steel or stainless steel containing 4.5 to 30% by weight of Cr, and there is a problem that using a metal-plated mandrel bar at this time makes it extremely unstable. .

More specifically, for example, the hollow shell is made of SUS304, and the mandrel bar is made of SKD6 having a surface plated with Cr having an average thickness of 50 μm. The method A is disclosed in Japanese Patent Application Laid-Open No. Hei 7-118687. Even when the graphite-based lubricant shown was used, inner surface flaws having a depth of more than 0.2 mm occurred frequently, and a product pipe with an inner surface flaw having a depth of 0.2 mm or less was not obtained.

Further, in addition to the above-mentioned Method A, even if the lubrication treatment of Method B using a borate-based lubricant disclosed in the above-mentioned JP-A-61-37989 is performed, the inner surface exceeding 0.2 mm is obtained. Flaws were partially generated, and a product pipe with inner flaws having a depth of 0.2 mm or less could not be obtained stably.

Further, in a plug mill to which only the method B is applied, the same applies to the above description, and especially, a 4.5 mm hollow shell is used.
In the case of alloy steels such as Cr-Mo steel and stainless steel containing up to 30% by weight of Cr, internal flaws exceeding 0.2 mm occur frequently in some parts, and product pipes with internal flaw depths of 0.2 mm or less are It was not obtained stably.

Further, when the borate-based lubricant used in the method B contains phosphorus (specifically, phosphate),
Phosphorus generated by reducing part of the steel during hot rolling at 900 ° C. or higher reacts with iron in the steel to form a low melting point compound, which causes grain boundary cracking, 0.2mm
There is also a problem that landslide-like inner surface flaws having a depth exceeding 0.1 mm are frequently generated. For this reason, there has been a strong demand for the development of a method capable of stably obtaining a product tube having an inner surface flaw depth of 0.2 mm or less.

An object of the present invention is to stably obtain a product tube having an inner surface flaw depth of 0.2 mm or less regardless of the material of the hollow shell and the surface condition of the mandrel bar or plug used. It is an object of the present invention to provide a seamless steel pipe manufacturing method and a seamless steel pipe excellent in inner surface quality obtained by the method.

[0020]

In order to achieve the above object, the present inventors have conducted many production experiments using actual mandrel and plug mill pipe lines.
As a result, the following was found.

In any of the above methods, prior to the application of the method B, the inside of the hollow shell is purged with a weak oxidizing gas or an inert gas, and the main agent is made of a monovalent alkali metal immediately after the purging. 50-3 boric acid-free lubricants
By applying elongation and rolling after applying 00 g / m ^2, a product tube having an inner surface flaw depth of 0.2 mm or less is reliably and stably regardless of the material of the hollow shell and the surface condition of the mandrel bar used. It was found that it could be obtained.

Here, when adopting the above-mentioned means, the details of the reason why the product pipe having the inner surface flaw having a depth of 0.2 mm or less can be stably obtained are unknown, but are presumed as follows. You.

The high temperature (about 1250) formed by the piercing mill
C), cooling water such as a main roll of a piercing mill enters and remains inside the hollow shell. Therefore, when a borate-based lubricant is introduced into the tube, the cooling water remaining in the tube is taken into the lubricant and the melting thereof is delayed. As a result, the lubricating performance is not sufficiently exhibited, and an inner surface flaw having a depth exceeding 0.2 mm occurs on the inner surface of the product pipe after the elongation rolling.

On the other hand, when the borate-based lubricant is charged immediately after the inside of the pipe is purged with a weak oxidizing gas or an inert gas, since the cooling water does not exist in the pipe, the charged lubricant is quickly melted. Since the lubrication performance is sufficiently exhibited, it is estimated that no inner surface flaw exceeding 0.2 mm is generated on the inner surface of the product pipe after the elongation rolling.

The gist of the present invention based on the above findings is the following method (1) for producing a hot seamless steel pipe and the following (2) a seamless steel pipe having excellent inner surface quality.

(1) A method for producing a seamless steel pipe in which a hollow shell formed by a piercing mill is subjected to a wall-thinning process by a drawing mill equipped with an inner surface regulating tool, wherein the inner surface of the hollow shell is formed Purging with a weakly oxidizing gas or an inert gas, and immediately after the inner surface of the tube, a monovalent alkali metal borate as a main component,
A method for producing a seamless steel pipe in which a phosphorus-free lubricant of 50 to 300 g / m ² is uniformly applied, and then subjected to a wall-thinning process by a draw rolling mill.

(2) A seamless steel pipe manufactured by the method described in (1) above, wherein the inner surface has a flaw depth of 0.2 m.
m seamless steel pipe with excellent inner surface quality.

In the method (1) of the present invention, the lubricating film is preferably formed on the surface of the inner surface regulating tool (mandrel bar) when the elongating mill is a mandrel mill. The hollow shell has a C content of 4.5 to 30% by weight.
It is preferable that the surface of the inner surface regulating tool (mandrel bar) in the case of alloy steel such as Cr-Mo steel or stainless steel containing r is coated with metal plating, specifically with Cr plating. Further, the seamless steel pipe of the present invention (2) is
Cr containing 4.5 to 30% by weight of Cr, not limited to carbon steel
-It may be made of alloy steel such as Mo steel or stainless steel.

[0029]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The method of the present invention and a seamless steel pipe will be described below. First, the reason why the manufacturing conditions are determined as described above in the method of the present invention will be described.

The high temperature (approximately 1250) formed by the piercing mill
℃), cooling water such as the main roll of a piercing mill remains inside the hollow shell, and if this hollow shell is directly lubricated by the method B, as described above, The supplied borate-based lubricant takes in the cooling water, causing a delay in melting of the lubricant itself. Therefore, in the present invention, the inside of the pipe is purged using a weak oxidizing gas or an inert gas in order to remove the cooling water in the pipe that causes the melting delay of the borate-based lubricant.

Here, the weak oxidizing gas mentioned in the present invention is air or carbon dioxide (CO ₂ ), and the inert gas is N ₂ gas, Ar gas or He gas. . That is, when a gas other than the above, for example, oxygen gas or water vapor is used, the oxide scale on the inner surface of the tube grows and becomes thicker, which is one of the causes of the generation of inner surface flaws. In addition, a part of the water vapor may remain in the pipe, causing a delay in melting of the borate-based lubricant. Therefore, a weak oxidizing gas or an inert gas is used as the purge gas.

In the case where the hollow shell is made of low alloy steel or carbon steel containing less than 13% by weight of Cr, the air of the weak oxidizing gas grows an oxide scale on the inner surface of the tube. It is desirable to use carbon dioxide gas (CO ₂ ) or an inert gas because it may cause flaws. As the inert gas, it is preferable to use N ₂ gas which is the cheapest and easily available from the viewpoint of economy.

The purge pressure of the weak oxidizing gas or the inert gas is sufficient to be about 4 to ²⁰ kgf / cm ² , but from the viewpoint of surely removing the cooling water remaining in the pipe, it is preferable to set the purge pressure as high as possible. preferable.

The composition and application amount of the borate-based lubricant to be charged into the pipe immediately after purging with the weak oxidizing gas or the inert gas are determined as described above for the following reasons. That is, the melting point of the monovalent alkali metal borate is 1000
The melting point of the divalent alkali metal borate is as high as 1150 ° C. or higher. For this reason, the monovalent alkali metal borate melts very easily after being put into a tube at a high temperature (about 1250 ° C.), but the divalent alkali metal borate does not melt easily and is lubricated. Inferior in sex, 0.
It is not possible to prevent the occurrence of inner surface flaws exceeding 2 mm.

When phosphorus is present in the lubricant introduced into the pipe, as described above, a part of the phosphorus reacts with iron in the steel (rolled material) to form a compound having a low melting point. Causes cracks at the grain boundaries, resulting in frequent occurrence of rough inner surface flaws having a depth exceeding 0.2 mm.

Further, when the coating amount is less than 50 g / m ² , the lubricating film breaks during elongation rolling, and the
Inner surface flaws. Conversely, 300g
/ M ² or more, the lubricant adheres to the inner surface and remains on the inner surface, and the appearance of the product tube deteriorates. Therefore, not only the removal process is required, but also the basic unit of the lubricant is reduced, and the production cost is increased. Invite. For this reason, the application amount of the borate-based lubricant to be introduced into the pipe was set to 50 to less than 300 g / m ² .

Here, the borate-based lubricant to be introduced into the tube is a monovalent alkali metal borate (for example, Na ₂ B _{4) as} long as it does not contain phosphorus or a phosphorus compound such as phosphate.
_{O 7 · 10H 2 O, Na} 2 B 4 O 7 · 5H 2 O, anhydrous Na ₂ B ₄
Sodium borate, potassium borate, and lithium borate represented by O ₇ and the like)
It may be a mixture of one or more selected from the following or / and groups.

For example, monovalent alkali metal carbonates such as sulfate, sodium carbonate, sodium sulfate, potassium carbonate and lithium carbonate.

For example, metal soaps such as sodium stearate, sodium palmitate and sodium oleate.

The monovalent alkali metal carbonate described above has a melting point of 1 like the monovalent alkali metal borate.
000 ° C. or lower. Therefore, high temperature (about 1250 ° C)
It melts very easily after being put into the tube, and acts as a fluid lubricating film. On the other hand, the metal soap described in (1) burns explosively the moment it is put into a tube at a high temperature (about 1250 ° C.), and the flame disperses the monovalent alkali metal borate and carbonate. It acts so as to be uniformly applied to the inner surface of the tube.

For this reason, the monovalent alkali metal borate is preferably used as a mixture with one or more members selected from the above-mentioned and / or groups. Most preferably, it is used as a mixture of species or two or more species. This is because a lubricant consisting of a monovalent alkali metal borate alone has poor uniform application properties.

However, when one or two or more selected from the above and / or each group are mixed, the mixing ratio is 50 to 80% by weight of a monovalent alkali metal borate. Is also preferably 30% by weight or less. This is because the amount of monovalent alkali metal borate is 50
If it is less than 80% by weight, the lubricity is reduced, and if it is more than 80% by weight, the uniform coatability is reduced. Also, the reason is that if the content exceeds 30% by weight, not only the effect is saturated, but also the blending amount of the monovalent alkali metal borate is reduced, and the lubricity is reduced.

The particle size of each component constituting the above borate-based lubricant is desirably 500 μm or less, preferably 100 μm or less, from the viewpoint of ensuring uniform coating properties. Further, their purity is 85% or more, preferably 9% or more.
It is desirable to use one having 0% or more.

B of the above borate lubricant for hollow shell
The injection by the method may be performed by, for example, providing a blowing nozzle at one end of the hollow shell, blowing the nozzle toward the other end of the hollow shell, or feeding the blowing nozzle into the pipe. In that case, as the blowing medium,
For the same reason as in the above-described purging, it is preferable to use an inert gas, specifically, N ₂ gas. At this time, it is more preferable that the hollow shell is rotated around its axis for the purpose of improving the uniform coating property of the borate-based lubricant charged.

When the elongating mill is a mandrel mill, it goes without saying that the surface of the mandrel bar, which is an inner surface regulating tool, is subjected to the same lubrication treatment as in the conventional method A. That is, when the hollow shell is made of carbon steel or low alloy steel, the main component is graphite and a graphite-based lubricant that is a resin-based organic binder.
In the case of alloy steel such as Mo steel or stainless steel, a dry solid film of graphite, a resin-based organic binder, iron oxide and a graphite-based lubricant that is a glass powder is formed.

Further, when the hollow shell is made of an alloy steel such as Cr-Mo steel or stainless steel containing 4.5 to 30% by weight of Cr and it is necessary to prevent carburization of the inner surface, a carburizing inhibitor is used. To form a dry solid film of a graphite-based lubricant to which is added or a non-graphite-based lubricant containing no graphite at all.

On the other hand, when the elongation rolling mill is a plug mill, it is not necessary to perform a lubrication treatment particularly using a lubricant on the surface of the plug which is an inner surface regulating tool. However,
In the plug mill, as described above, in addition to the lubrication treatment by the above-mentioned method B, usually only graphite or main material is applied from the tip portion of the integrated plug toward the rolling portion from the tip portion of the integrated plug and from the split portion of the split plug. Elongation rolling is performed while supplying a graphite-based lubricant having a graphite component.

Therefore, also in the present invention, when the elongation mill is a plug mill, in addition to lubrication treatment by the B method, only graphite or a graphite whose main component is graphite from the leading end portion or divided portion of the plug toward the rolling portion. It goes without saying that a system lubricant is supplied. At that time, when the hollow shell is made of an alloy steel such as Cr-Mo steel or stainless steel containing 4.5 to 30% by weight of Cr and it is necessary to prevent carburization of the inner surface,
It goes without saying that a graphite-based lubricant to which a carburizing inhibitor is added or a non-graphite-based lubricant containing no graphite is supplied.

[0049]

EXAMPLES Six types of borate-based lubricants having the compositions shown in Table 1 and four types of test steels having the chemical compositions shown in Table 2 were prepared.

[0050]

[Table 1]

[0051]

[Table 2]

Example 1 S of the test steels shown in Table 2
Outer diameter 1 made of US304, 13Cr steel and S45C
A billet having a length of 78 mm and a length of 2370 mm was heated to 1230 ° C. and formed into a hollow shell having an outer diameter of 181 mm, a wall thickness of 17.5 mm and a length of 6550 mm using an inclined roll type piercer. Then, these hollow shells were 151 mm in outer diameter, 6.5 mm in wall thickness, and 200 mm in length using a 7-stand mandrel mill.
After elongating to 100 mm, it was charged into a reheating furnace and
Reheat to 0 ℃, 113m outside diameter with stretch reducer
m, a wall thickness of 6.5 mm, and a length of 27140 mm.

At this time, the six types of borate-based materials shown in Table 1 were used, in which the inner surface of the hollow shell immediately before being subjected to the mandrel mill was purged with N ₂ gas at a pressure of 10 kgf / cm ² and without purging. Any one of the lubricants A to F was charged into the tube, and subjected to a lubrication treatment of Method B in which the lubricant was uniformly applied at various application amounts (0 to 350 g / m ² ).

In each case, the SK having the chemical composition shown in Table 3 was added to the mandrel bar of the mandrel mill.
It is made of D6, and its surface is coated with a 50-μm-thick Cr plate. The Cr-plated plate is lubricated according to Method A to give a graphite: resin-based organic binder having a thickness of 5:25.
One having a dry solid lubricant film of 0 μm formed thereon was used.

[0055]

[Table 3]

In each case, the inner surface temperature of the hollow shell immediately after the piercing was 1250 ° C., and the inner surface temperature of the hollow shell immediately before the introduction of the borate lubricant by the method b was 1180 ° C.

Then, the occurrence condition of the inner surface flaw of the product pipe obtained under each condition and the presence or absence of the inner surface remaining of the borate-based lubricant introduced in the lubrication treatment of the method B were examined.

The state of occurrence of inner surface flaws is determined by visually observing the inner surface of the product tube, measuring the depth of the flaws by ultrasonic inspection, and measuring the inner surface having a depth of 0.20 mm or more even in a part of the axial direction. Unsatisfactory (x) when flaws were confirmed, slightly good (△) when axial flaws were confirmed at a depth of 0.10 mm or more and less than 0.20 mm even in a part of the axial direction (△), Even if it was confirmed whether or not any inner surface flaw was confirmed, the case where the depth was less than 0.10 mm was evaluated as good (○).

On the other hand, the presence or absence of the borate-based lubricant remaining on the inner surface is evaluated as poor (x) when any part of the inner surface of the tube can be visually confirmed, and as good (）) when no residual lubricant can be confirmed. did. In addition, the overall evaluation was unsatisfactory (x) when any one of the evaluations was unsatisfactory (x), and the overall evaluation was good (o) when the other evaluations were unsatisfactory (x).

The above results are shown in Tables 4 and 5 together with the processing conditions for the hollow shell before elongation rolling by the mandrel mill.

[0061]

[Table 4]

[0062]

[Table 5]

As is evident from the results shown in Tables 4 and 5, the product pipes manufactured according to the present invention each had an inner surface flaw depth of 0.2 mm or less and a borate-based lubricant on the inner surface. No agent remained.

On the other hand, the product pipe manufactured under conditions other than those specified in the present invention has an inner surface flaw having a depth of more than 0.2 mm, The borate-based lubricant remained.

Example 2 Outer diameter 225 of SUS304 and 22Cr steel among the test steels shown in Table 2 above
The billet having a length of 3360 mm and a length of 3360 mm was heated to 1230 ° C., and formed into a hollow shell having an outer diameter of 290 mm, a wall thickness of 15.5 mm and a length of 10,000 mm by a first piercer and a second piercer of an inclined roll type. Next, these hollow shells were 280 mm in outer diameter, 13 mm in wall thickness, and 1227 in length by a plug mill.
After elongating and rolling to 0 mm, the tube was rolled with a reeler, and then the outer diameter was 275 mm, the wall thickness was 13 mm, and the length was 12 with a sizer.
Finished into a 500 mm product tube.

At this time, the six types of borate-based lubrication shown in Table 1 were performed after purging the inner surface of the hollow shell immediately before being subjected to the plug mill with N ₂ gas at a pressure of 5 kgf / cm ² and without purging. Any one of the agents A to F was charged into a tube, and subjected to a lubrication treatment by a method B in which the coating was uniformly applied at various coating amounts (20 to 400 g / m ² ).

In each case, the plug of the Prabmill is made of Cr-Mo steel consisting of 17% Cr-5% Mo and the balance being substantially Fe, and an oxide scale film having a thickness of 70 μm is formed on the surface thereof. The two-part type was used.
Then, from the divided portion to the rolling section, 200 g of a graphite-based lubricant (3: 1 graphite and calcined salt) per one rolling tube was used.
) Was supplied.

In each case, the inner surface temperature of the hollow shell immediately after piercing was 1260 ° C., and the inner surface temperature of the hollow shell immediately before the introduction of the borate-based lubricant by the method b was 1230 ° C.

Then, the occurrence condition of the inner surface flaw of the product pipe obtained under each condition and the presence or absence of the inner surface remaining of the borate-based lubricant introduced in the lubrication treatment of the method B were examined.

The state of occurrence of internal flaws was determined by visually observing the inner surface of the product tube, measuring the depth of the flaws by ultrasonic inspection, and measuring the inner surface having a depth of 0.20 mm or more even in a part of the axial direction. Unsatisfactory (x) when flaws were confirmed, slightly good (△) when axial flaws were confirmed at a depth of 0.10 mm or more and less than 0.20 mm even in a part of the axial direction (△), Even if it was confirmed whether or not any inner surface flaw was confirmed, the case where the depth was less than 0.10 mm was evaluated as good (○).

On the other hand, the presence or absence of the borate-based lubricant on the inner surface was evaluated as poor (x) when the residual was visually confirmed even on a part of the inner surface of the pipe, and evaluated as good (○) when no residual was confirmed. did. In addition, the overall evaluation was unsatisfactory (x) when any one of the evaluations was unsatisfactory (x), and the overall evaluation was good (o) when the other evaluations were unsatisfactory (x).

The above results are shown in Table 6 together with the processing conditions for the hollow shell before elongation rolling by the plug mill.

[0073]

[Table 6]

As is evident from the results shown in Table 6, the product pipes manufactured according to the present invention all had an inner surface flaw depth of 0.2 mm or less, and the borate lubricant remained on the inner surface. I didn't.

On the other hand, a product pipe manufactured under conditions other than those specified in the present invention has an inner surface flaw having a depth of more than 0.2 mm, The borate-based lubricant remained.

[0076]

According to the method of the present invention, a product pipe having an inner surface flaw depth of 0.2 mm or less can be reliably and stably manufactured. Further, since the borate-based lubricant does not remain on the inner surface of the obtained steel pipe, it can be shipped as it is with the product pipe.

Claims (5)

[Claims] 1. A method for producing a seamless steel pipe in which a hollow shell formed by a piercing mill is subjected to a wall-thinning process by a drawing mill equipped with an inner surface regulating tool, wherein the inner surface of the hollow shell is weakly acidified. After purging with a oxidizing gas or an inert gas, immediately after applying uniformly 50 to 300 g / m ² of a phosphorus-free lubricant mainly composed of a monovalent alkali metal borate to the inner surface of the tube, A method for producing a seamless steel pipe, wherein a wall-thinning process is performed by an elongation mill. 2. The method for manufacturing a seamless steel pipe according to claim 1, wherein an inner surface regulating tool having a lubricating film formed on a surface is used. 3. The method for producing a seamless steel pipe according to claim 1, wherein at least a portion in contact with the hollow shell is covered with metal plating. 4. A seamless steel pipe produced by the method according to any one of claims 1 to 3, wherein the inner surface has a flaw depth of 0.
Seamless steel pipe excellent in inner surface quality characterized by being 2 mm or less. 5. The seamless steel pipe according to claim 4, wherein the steel pipe is made of an alloy steel containing 4.5 to 30% by weight of Cr.