Title:
Method for producing high chromium seamless pipe or tube
Kind Code:
A1


Abstract:
A billet containing, by mass %, 8.0 to 16.0% of Cr with a value represented by the formula of “Cr+4Si−(22C+0.5Mn+1.5Ni+30N)” of not more than 9.0%, wherein the symbols of the elements represent the contents, by mass %, of the elements in the steel, is heated by means of a heating furnace conditioned so as to have a soaking temperature of 1100 to 1300° C. and an in-furnace time (min) of not less than “0.5×Diameter of the billet (mm)”. Thereafter, by means of a centering machine capable of forming a center hole having a diameter of not less than “0.34×Diameter of the billet (mm)” at one end face of the said billet where piercing starts by punch of 60 to 110° punching angle, a center hole is formed at the said face of the billet, followed by piercing and rolling by an inclined roll type piercing mill of not more than 8.0% plug tip draft rate. The thus-produced high-Cr seamless pipe or tube has excellent internal surface properties with minimized internal surface defects.



Inventors:
Ohe, Taro (Osaka, JP)
Nakakaike, Koji (Wakayama-shi, JP)
Application Number:
12/230497
Publication Date:
01/01/2009
Filing Date:
08/29/2008
Primary Class:
International Classes:
C21D9/08
View Patent Images:



Primary Examiner:
KESSLER, CHRISTOPHER S
Attorney, Agent or Firm:
CLARK & BRODY (Alexandria, VA, US)
Claims:
What is claimed is:

1. A method for producing a high-Cr seamless pipe or tube, comprising the steps of: heating a billet in a heating furnace; forming a center hole, by means of a centering machine, at one end face of the heated billet where piercing starts; and piercing and rolling the billet by using an inclined roll type piercing mill, wherein: the said billet containing, by mass %, 8.0 to 16.0% of Cr, with the value of Cr* represented by the following formula (1) of not more than 9.0%; the said heating step being carried out at a soaking temperature of 1100 to 1300° C., and an in-furnace time from the charging of the billet into the heating furnace to the discharging therefrom being set to satisfy the following formula (2); the said center hole being formed by using a punch with a punching angle of 60 to 110° in the center hole forming step being set to have a diameter represented by the following formula (3); and the said piercing and rolling step is carried out at a plug tip draft rate represented by the following formula (4) being set to not more than 8.0%:
Cr*=Cr+4Si−(22C+0.5Mn+1.5Ni+30N) (1),
In-furnace time (min)>0.5×Diameter of the round billet (mm) (2),
Diameter of the center hole (mm)>0.34×Diameter of the round billet (mm) (3),
Plug tip draft rate (%)={(Diameter of the round billet (mm)−Roll gap at the foremost end of the plug (mm))/Diameter of the round billet (mm)}×100 (4), wherein, in the above formula (1), the symbols of the elements represent the contents, by mass %, of the elements in the high-Cr seamless pipe or tube.

Description:

This application is a continuation of the international application PCT/JP2007/053916 filed on Mar. 1, 2007, the entire content of which is herein incorporated by reference.

TECHNICAL FIELD

The present invention relates to a method for producing a high-Cr seamless pipe or tube used for oil or gas wells, for various industrial plants, for building structures, and so on. The present invention relates in particular to a method for producing a high-Cr seamless pipe or tube containing, by mass %, 8.0 to 16.0% of Cr.

BACKGROUND ART

High-Cr seamless pipes or tubes containing, by mass %, 8.0 to 16.0% of Cr have been widely used for oil or gas wells, for various industrial plants and for building structures.

In order to effectively produce high-Cr seamless pipes or tubes, it is preferable to use an inclined roll type piercing mill (hereinafter often referred to as a “piercing mill”) in the piercing and rolling process. Specifically, a hollow pipe or tube stock is produced from a solid steel stock with a round section (hereinafter referred to as a “billet”) by use of a piercing mill. Then, the pierced pipe or tube stock is elongated and rolled by an elongator such as a mandrel mill, a plug mill, an Assel mill, or a push bench, and the outer diameter thereof is then narrowed by use of a stretch reducing mill or a sizing mill.

However, since the hot workability of high-Cr steel is lower than that of a so-called “common steel”, the piercing and rolling by a piercing mill thereof tends to cause defects on the internal surface of the resulting pipe or tube stock. Typical examples of the defects on the internal surface of the pipe or tube stock include a so-called internal fracture flaw, which may be called often “internal scab” or “lapping mark”.

In order to eliminate the internal surface defects on pipes or tubes, it is necessary to cut or grind flawed points in an additional process that is separate from the pipe or tube production process. Since the said treatment needs a lot of man-hours, it leads to a remarkable reduction in production efficiency. Further, a deep flaw leads to disposal of the pipe or tube itself, resulting in deterioration of production yield.

The following techniques in the Patent Documents 1 to 6 are proposed for suppress internal surface defects on pipe or tube stock in the piercing and rolling stage.

The Patent Document 1 discloses a technique for enhancing the hot workability of a steel stock in the piercing and rolling process by a piercing mill while minimizing the contents of P and S that are impurity elements in a steel, thereby suppressing internal fracture flaws.

The Patent Document 2 discloses a technique for suppressing the production of δ-ferrite by reducing the heating temperature of a billet and also by suppressing the work heat generation through reducing the average strain rate in the piercing and rolling process by a piercing mill.

The Patent Document 3 discloses a method for producing a martensitic seamless steel pipe or tube, capable of improving the microstructure which is formed in the hot working process, by regulating the contents of the specified alloy components and also by controlling the annealing heating time and setting the billet heating temperature to lower.

The Patent Document 4 discloses a technique for performing piercing and rolling while adjusting, in the piercing and rolling process by a piercing mill with disk roll-type guide shoes, the diameter of a steel stock, the clearance between guide shoes, the inclined roll gap, and so on.

The Patent Document 5 discloses a method for producing a high-Cr seamless steel pipe or tube, capable of improving the microstructure which is formed in the hot working process, by regulating the content of Cr, the contents of S and P as impurity elements, the contents of elements to be added, and also by adjusting the heating or soaking time of the billet, thereby preventing internal surface defects.

The Patent Document 6 discloses a method for producing a martensitic stainless seamless steel pipe or tube, capable of improving the microstructure which is formed in the hot working process, by regulating the contents of specified alloy components and adjusting the cross angle and the feed angle at the time of piercing and rolling, thereby preventing internal surface defects.

The Patent document 7 discloses a method for producing a seamless pipe or tube with an inclined roll type piercing mill and a mandrel mill, by using a billet whose rear end part is processed into a depression prior to piercing and rolling process, thereby avoiding occurrence of flaws on pulling out the mandrel bar in the elongating and rolling process.

Patent Document 1: Japanese Laid-Open Patent Publication No. 59-208055,

Patent Document 2: Japanese Laid-Open Patent Publication No. 63-281705,

Patent Document 3: Japanese Laid-Open Patent Publication No. 4-224659,

Patent Document 4: Japanese Laid-Open Patent Publication No. 5-69011,

Patent Document 5: Japanese Laid-Open Patent Publication No. 2003-3212,

Patent Document 6: Japanese Laid-Open Patent Publication No. 2004-43935.

Patent Document 7: Japanese Laid-Open Patent Publication No. 2004-82174.

DISCLOSURE OF THE INVENTION

Problems to be Solved by the Invention

Even if the techniques proposed by the Patent Documents 1 to 7 described above are applied, the internal surface defects in high-Cr pierced pipe stocks containing 8.0 to 16.0% of Cr could not effectively be suppressed.

Therefore, the present inventors observed internal fracture flaws on the high-Cr seamless pipes or tubes in detail in order to examine the causal relationship between occurring the flaws and the various conditions, such as the billet heating condition, preliminary work condition on the billet prior to piercing and rolling, or various conditions of the piercing and rolling.

This resulted in findings that internal fracture flaws can be substantially suppressed by forming a center hole at one end face of the billet where the piercing and rolling starts, under specified conditions, in advance of the piercing and rolling.

From the viewpoint of the above-mentioned content, it is an objective of the present invention to provide a method for producing a high-Cr seamless pipe or tube, particularly a high-Cr seamless pipe or tube containing, by mass %, 8.0 to 16.0% of Cr, capable of suppressing, internal surface defects especially internal fracture flaws caused in the piercing and rolling process by a piercing mill.

Means for Solving the Problem

The gist of the present invention is a method for producing a high-Cr seamless pipe or tube described in the following:

“A method for producing a high-Cr seamless pipe or tube, comprising the steps of

heating a billet in a heating furnace;

forming a center hole, by means of a centering machine, at one end face of the heated billet where piercing starts; and

piercing and rolling the billet by using an inclined roll type piercing mill,

wherein:

the said billet containing, by mass %, 8.0 to 16.0% of Cr, with the value of Cr* represented by the following formula (1) of not more than 9.0%;

the said heating step being carried out at a soaking temperature of 1100 to 1300° C., and an in-furnace time from the charging of the billet into the heating furnace to the discharging therefrom being set to satisfy the following formula (2);

the said center hole being formed by using a punch with a punching angle of 60 to 110° in the center hole forming step being set to have a diameter represented by the following formula (3); and

the said piercing and rolling step is carried out at a plug tip draft rate represented by the following formula (4) being set to not more than 8.0%:


Cr*=Cr+4Si−(22C+0.5Mn+1.5Ni+30N) (1),


In-furnace time (min)>0.5×Diameter of the round billet (mm) (2),


Diameter of the center hole (mm)>0.34×Diameter of the round billet (mm) (3),


Plug tip draft rate (%)={(Diameter of the round billet (mm)−Roll gap at the foremost end of the plug (mm))/Diameter of the round billet (mm)}×100 (4),

wherein, in the above formula (1), the symbols of the elements represent the contents, by mass %, of the elements in the high-Cr seamless pipe or tube”.

EFFECT OF THE INVENTION

According to the method of the present invention, in producing the high-Cr seamless pipes or tubes containing 8.0 to 16.0% of Cr, the internal fracture flaws among the internal surface defects, which are caused in the piercing and rolling process by a piercing mill, can be suppressed.

BEST MODE FOR CARRYING OUT THE INVENTION

Research studies by the present inventors on the internal fracture flaws which are caused in the piercing and rolling process by a piercing mill revealed the following.

It was found that the internal fracture flaws can be remarkably suppressed by preventing the occurrence of δ-ferrite in the billet by controlling the billet soaking conditions, and also by the reducing roll forging frequency in the piercing and rolling process by a piercing mill, as shown in the following (a) to (c).

(a) Prior to the piercing and rolling process by a piercing mill, the billet containing 8.0 to 16.0% of Cr and having a value of Cr* represented by the said formula (1) of not more than 9.0% is heated in a heating furnace at a soaking temperature of 1100 to 1300° C. for an in-furnace time which satisfies the said formula (2), whereby the occurrence of δ-ferrite can be suppressed by the soaking effect. Therefore, the major factor in causing internal fracture flaws which occur in the piercing and rolling process by a piercing mill can be suppressed.

(b) The internal fracture flaws which occur in the piercing and rolling process by a piercing mill can be suppressed by reducing the roll forging frequency N represented by the following formula (5).


N=(2L×Brps)/{(Circumferential speed at the roll gouge position×sin β×Piercing efficiency)/Piercing ratio} (5);

wherein, in the above formula (5), the meanings of L, Brps and Piercing ratio, β and Piercing efficiency are as follows:

L: Distance from the position where the rolls bite the billet to the tip of the plug,

Brps: Rotating number of the billet, that is to say, “Rotating speed component of the roll circumferential speed at the position where the rolls bite the billet/Circumferential length of the billet”,

Piercing ratio: “Length of the pierced pipe or tube stock/Length of the billet”.

B: the feed angle (°) of the roll, and,

Piercing efficiency: the advance efficiency (%) in the piercing and rolling process.

(c) It is apparent from the said formula (5) that the reduction in the roll forging frequency “N” of the billet can be attained by reducing the value of “L” or increasing the value of the piercing efficiency. The value of “L” can be easily attained by reducing the “plug tip draft rate”. Although it was considered to increase the feed angle of the roll “β”, this could cause defective biting of a billet to piercing rolls.

Therefore, the condition for improving the “piercing efficiency” in the piercing and rolling process by a piercing mill was further examined in detail. Consequently, the following findings (d) and (e) were obtained.

(d) The “piercing efficiency” can be improved by setting a punching angle of a centering machine to a specified range in the process of forming a center hole, since the interval between the time the rolls bite the billet and the time when piercing and rolling stabilizes is shortened.

(e) The stable piercing and rolling of the billet can be achieved by making the center hole diameter equal to or larger than a specified value determined relative to the billet diameter, even in the case of a low “plug tip draft rate” without the occurrence of defective biting of a billet to piercing rolls.

The present invention has been accomplished on the basis of the above findings.

All of the requirement of the present invention will next be described in detail. In the following description, the symbol “%” of the content of each element represent “% by mass”.

(A) Chemical Composition of a Billet

A billet, that is a steel stock for a high-Cr seamless pipe or tube, according to the present invention needs to be made of high-Cr steel which has a chemical composition containing 8.0 to 16.0% of Cr, with a value of Cr* represented by the above-mentioned formula (1) of not more than 9.0%.

Cr: 8.0 to 16.0%

In order to obtain corrosion resistance, the content of Cr must be set to 8.0% or more. However, if the content of Cr be a large amount, δ-ferrite is easily occurred at the time of high-temperature working since Cr is a ferrite forming element, and the hot workability is impaired. Particularly, if the content of Cr exceeds 16.0%, the hot workability is impaired remarkably. Moreover, an excessive addition of Cr leads to an increase in manufacturing cost of seamless pipes or tubes. Therefore, the content of Cr is set to 8.0 to 16.0%.

Value of Cr*: not more than 9.0%

When the value of Cr* represented by the above-mentioned formula (1) exceeds 9.0%, δ-ferrite is easily occurred even if the content of Cr is within the above range of 8.0 to 16.0%, and so, the sulfide stress cracking resistance and hot workability are deteriorated. Therefore, the value of Cr* represented by the above-mentioned formula (1) is set to not more than 9.0%.

From the above-mentioned reason, a high-Cr steel which has a chemical composition containing 18.0 to 16.0% of Cr and having a value of Cr* represented by the above-mentioned formula (1) of not more than 9.0% is used as a billet in the present invention.

Only the regulation of Cr content and the value of Cr* represented by the above-mentioned formula (1) as the chemical composition suffices for the billet that is the steel stock for a high-Cr seamless pipe or tube of the present invention.

As a preferable chemical composition of the billet that is the steel stock for a high-Cr seamless pipe or tube of the present invention, for example, a high-Cr steel which contains C: 0.15 to 0.22%, Si: 0.1 to 1.0%, Mn: 0.10 to 1.00%, Cr: 12.0 to 14.0%, P: not more than 0.020%, S: not more than 0.010%, N: not more than 0.05%, 0 (oxygen): not more than 0.0060%, one or more elements selected from 0.005 to 0.200% each of V, Nb and Ti and 0.0005 to 0.0100% of B (a total of 0.005 to 0.200% in combination of two or more thereof), Al: 0 to 0.1%, Ni: 0 to 0.5%, Cu: 0 to 0.25%, Ca: 0 to 0.0050% and the balance being Fe and impurities, with a value of Cr* represented by the formula (1) of not more than 9% can be recommended.

As another preferable chemical composition of the billet that is the steel stock for a high-Cr seamless pipe or tube of the present invention, for example, a high-Cr steel which contains C: 0.003 to 0.050%, Si: 0.05 to 1.0%, Mn: 0.10 to 1.50%, Cr: 10.5 to 14.0%, P: not more than 0.035%, S: not more than 0.010%, N: not more than 0.070%, 0 (oxygen): not more than 0.0060%, V: 0 to 0.200%, Ti: 0 to 0.300%, Mo: 0.2 to 3.0%, Ni: 0 to 7.0%, Zr: 0 to 0.580% and the balance being Fe and impurities, with a value of Cr* represented by the formula (1) of not more than 9% can also be recommended.

(B) Heating Conditions of a Billet

In the present invention, it is necessary to heat the billet, that is the steel stock for a high-Cr seamless pipe or tube, which has the above chemical composition described in the above section (A), in a heating furnace at a soaking temperature 1100 to 1300° C. so that the in-furnace time from the charging of the billet into the heating furnace to the discharging therefrom can satisfy the above-mentioned formula (2).

When a soaking temperature is below 1100° C., the temperature in the piercing and rolling process by a piercing mill, or the temperature in the elongating and rolling process by a mandrel mill or a plug mill is reduced. Therefore, in that case, the deformation resistance of the material to be rolled increases, and the piercing and rolling or the elongating and rolling becomes unstable and seizure flaws are easily caused.

When a soaking temperature is over 1300° C., δ-ferrite is occurred and grown in the billet, and then, the internal fracture flaws are also easily caused.

If the in-furnace time of the billet in the heating furnace does not satisfy the above-mentioned formula (2) even at a soaking temperature of 1100 to 1300° C., the billet center part cannot be sufficiently soaked and this make it difficult to stably pierce and roll the billet by the piercing mill. This causes a deterioration of the even wall thickness rate.

Therefore, in the present invention, the billet, that is the steel stock to be pierced and rolled by the piercing mill, is heated at a soaking temperature of 1100 to 1250° C. so that the in-furnace time from charging of the billet into the heating furnace to the discharging therefrom satisfies the above-mentioned formula (2).

In order to prevent the reduction in production yield by scale loss, the above-mentioned in-furnace time in the heating furnace is desirably set to less than “1.5×diameter of the billet (mm)”.

(C) Centering Conditions of a Billet

In the present invention, it is necessary to form a center hole having a diameter represented by the above-mentioned formula (3), at one end face of the billet where piercing is started by a piercing mill, by means of a centering machine which has a punch with a punching angle of 60 to 110°. Wherein, the billet heated under the conditions described in the above section (B) should be used.

In the step for forming a center hole, if the said center hole is formed by means of a centering machine which has a punch with a punching angle of not more than 110°, the plug does not be pressed back in the center of the billet end face in the piercing and rolling process. Moreover, the billet can be pierced and rolled quickly and stably even at a low plug tip draft rate, that is to say, even at a low roll forging frequency N represented by the above-mentioned formula (5), thereby shortening the time needed for piercing and rolling. In other words, the number of times that the billet is roll forged can be reduced, thereby preventing internal fracture flaws from occurring sufficiently. However, if the punching angle is less than 60° then the billet must be punched deeply to secure the required hollow diameter, which causes internal surface defects at that portion and often results in a lower product yield. On the other hand, when the punching angle exceeds 110°, it is difficult to pierce and roll the billet under a low plug tip draft rate condition. Moreover, it is difficult to secure a sufficient piercing efficiency when the rolls bite the billet. The said 110° or larger punching angle makes it largely impossible to prevent internal fracture flaws and in some cases defective biting of a billet to rolls is caused. Therefore, the punching angle of the centering machine is set to 60 to 110°.

If the diameter of the center hole is less than “0.34×diameter of the billet (mm)” even though the punching angle of the centering machine is 60 to 110°, it is necessary to use a thin punch. And in this case, the piercing efficiency cannot be improved because of punch breakage and failure to secure the required depth of the center hole. Therefore, the diameter of the center hole should satisfy the above-mentioned formula (3), that is to say, should be “0.34×billet diameter (mm)” or more.

(D) Piercing and Rolling by a Piercing Mill

In the present invention, the billet having a center hole formed under the conditions described in the above section (C) must be pierced and rolled at a plug tip draft rate of not more than 8.0%.

A plug tip draft rate exceeding 8.0% means that the value of “L” in the above-mentioned formula (5), that is to say, the distance from the position where the rolls bite the billet to the tip of the plug is geometrically large. Since the roll forging frequency N, represented by the above-mentioned formula (5), is increased in this case, so-called “Mannesmann fractures” are excessively caused, which leads to an easy occurrence of internal fracture flaws. Therefore, the plug tip draft rate is set to not more than 8.0%. A more preferable plug tip draft rate is not more than 6.0%. The lower limit of the plug tip draft rate, which depends on the conditions of the center hole described in the above section (C), is about 3.0% at which the billet can be geometrically bitten into the rolls of piercing mill.

From the above-mentioned reason, in the present invention, the round billet, that is the steel stock for a high-Cr seamless pipe or tube, having the chemical composition described in the above section (A), is heated under the conditions described in the above section (B), formed the center hole under the conditions described in the above section (C), and pierced and rolled at a plug tip draft rate of the piercing mill of not more than 8.0%.

In the present invention, the piercing efficiency of the piercing mill is not necessarily regulated. However, when the piercing efficiency is below 50%, the roll forging frequency N represented by the above-mentioned formula (5) is increased, the said so-called “Mannesmann fractures” are excessively caused, which facilitates the occurrence of the internal fracture flaws. Consequently, the preferable piercing efficiency of the piercing mill is not less than 50%.

The upper limit of the piercing efficiency of the piercing mill is empirically about 60 to 70% in barrel type rolls (cross angle: 0°), or about 75 to 95% in cone type rolls (cross angle: 5 to 300).

The present invention will be further described in more detail in reference to the examples.

EXAMPLES

The steel ingots which have chemical compositions shown in Table 1 were hot rolled in a blooming mill by a general method and made into round billets of 191 mm or 225 mm in diameter. The steel A1 and B1 in Table 1 are the steels related to the examples with chemical compositions within the range regulated by the present invention.

[Table 1]

TABLE 1
Chemical composition (% by mass) Balance: Fe and impurities
SteelsCSiMnCrPSNOVNiMoCr*
A0.1800.100.1012.20.0200.0050.0100.00200.0050.57.5
B0.0290.051.2012.50.0180.0050.0100.00301.50.28.9
Cr* = Cr + 4Si − (22C + 0.5Mn + 1.5Ni + 30N)

The each round billet of the above-mentioned size was next charged into a heating furnace and heated at a soaking temperature of 1200° C. for an in-furnace time of 240 minutes, and then formed a center hole by means of a centering machine at one end face, pierced and rolled by a piercing mill, under conditions shown in Table 2, to produce a pipe stock with an outer diameter of mm (in case of the round billet of 225 mm diameter) or with an outer diameter of 196.0 mm (in case of the round billet of 191 mm diameter).

The in-furnace time represented by the above-mentioned formula (2) is “0.5×191 (mm)” minutes or longer, that is to say, 95.5 minutes or longer for the billet of 191 mm diameter, and “0.5×225 (mm)” minutes or longer, that is to say, 112.5 minutes or longer for the billet of 225 mm diameter. Therefore, the above 240 minute in-furnace time satisfies the in-furnace time condition regulated by the present invention.

In the case of Test Nos. 4 to 6 shown in Table 2, the piercing and rolling were all discontinued due to punch breakage.

[Table 2]

TABLE 2
Ratio of
Center HolePlug Tip
BilletPunchingCenter HoleDiameter toDraft
TestDiameterAngleDiameterBilletRateProperties of
No.Steels(mm)(°)(mm)Diameter(%)the Pipe Stocks
1A191*50800.4193.0x1
2A225*50850.3783.2x1
3B225*50850.3783.2x1
4B225*501050.4673.0
5A225*501200.5332.8
6B225*501200.5332.8
7A225 60800.3564.5
8A225 60850.3784.0
9B225 60850.3784.0
10B225 601050.4673.5
11B225 601200.5333.2
12A22511070*0.311 *9.0 x2
13A225110850.3786.0
14B225110850.3786.0
15A2251101050.4675.5
16B2251101050.4675.5
17A2251101200.5335.0
18A225*115 800.356*8.8 x2
19B225*115 800.356*8.8 x2
20B225*115 850.378*8.6 x2
21A225*115 1050.467*8.5 x2
22B225*115 1200.533*8.2 x2
In the column of “Properties of the Pipe Stocks”, the mark “∘” shows that no internal fracture flaws were observed, the mark “x1” shows that punching flaws (internal surface defects) were observed, the mark “x2” shows that internal fracture flaws were observed, the symbol “—” shows that no investigation was performed because the piercing and rolling was discontinued due to punch breakage.
The mark * indicates falling outside the condition regulated by the present invention.

Each of the thus-obtained pipe stocks of Test Nos. 1 to 3 and 7 to 22 in Table 2 was inspected for internal fracture flaw. That is to say, the occurrence positions of internal surface defects were specified and marked by an ultrasonic flaw detecting test, and these portions were cut off and evaluated by a visual inspection. Further, an investigation for defective rolling such as the uneven wall thickness was carried out by a visual inspection and an ultrasonic flaw detecting test.

In result of each investigated property of the pipe stocks is also shown in Table 2. In the column “Properties of the pipe stocks” of Table 2, the mark “o” shows that no internal fracture flaws on pipe stock internal surface were observed, the mark “×1” shows that punching flaws (internal surface defect) on pipe stock internal surface were observed and the mark “×2” shows that internal fracture flaws were observed. The mark “−” shows that no investigation was performed because the piercing and rolling was discontinued due to punch breakage (Test Nos. 4 to 6).

It is apparent from Table 2 that each of the pipe stocks of Test Nos. 7 to 11 and 13 to 17 obtained by the method of the present invention, that is to say, by forming a center hole having a diameter represented by the above formula (3), at one end face of the round billet where piercing starts, by means of a centering machine which has a punch with a punching angle of 60 to 110°, after heating the said round billet of the high-Cr steel having the components shown in Table 1 under the conditions regulated by the present invention, and then, by piercing and rolling, within the effective biting of the billet to rolls, at a plug tip draft rate regulated by the present invention, shows a excellent internal surface properties. No defective rolling properties such as the uneven wall thickness were observed in any of these pipe stocks.

On the other hand, punching flaws (internal surface defects) or internal fracture flaws were observed in the pipe stocks of the Test Nos. 1 to 3, 12 and 18 to 22, with centering or piercing and rolling conditions out of the range regulated by the present invention, even though the billets of the steels having chemical compositions within the range regulated by the present invention, were heated in the heating conditions regulated by the present invention.

That is to say, in the case of Test Nos. 1 to 3, the punching angle of 50° was lower than the regulated lower limit of 60°. This required deep punching to secure the required hole diameter, 105.5 mm for the pipe stock of the Test No. 1 and 91.1 mm for the pipe stocks of the Test Nos. 2 and 3, resulting in punching flaws (internal surface defects).

In the case of Test No. 12, the ratio of the diameter of the center hole to the diameter of the round billet was less than 0.34 and the plug tip draft rate exceeded the regulated upper limit of 8.0%. Consequently, internal fracture flaws occurred in the pipe stock of the Test No. 12.

In the case of Test Nos. 18 to 22, the punching angle of 115°, which is higher than the regulated upper limit of 110°, made a low plug tip draft rate difficult, resulting in an 8.2 to 8.8 plug tip draft rate exceeding the regulated upper limit of 8.0%. Moreover, this plug tip draft rate resulted in defective biting of the billet to rolls, and so, internal fracture flaws occurred in the pipe stocks of the Test Nos. 18 to 22.

As described above, in the case of Test Nos. 4 to 6, the punching angle of 50°, which is lower than the regulated lower limit of 60°, caused punch breakage. Therefore, the piercing and rolling were all discontinued.

In the foregoing, the present invention has been concretely described referring to typical examples thereof, these examples are by no means limitative of the scope of the present invention. It is to be noted that any mode of practice that is not disclosed herein as an example, if it satisfies the requirements of the present invention, falls within the scope of the present invention.

INDUSTRIAL APPLICABILITY

According to the producing method of the present invention, a high-Cr seamless pipe or tube with minimized internal surface defects can be produced. Further, since it is not necessary to excessively reduce the impurities in the chemical composition of the steel, and predetermined productivity can be ensured in pipe or tube making, a high-Cr seamless pipe or tube, excellent in internal surface properties, can be efficiently produced.