EP2839891B1

EP2839891B1 - Method for producing plug for heat formed pipe

Info

Publication number: EP2839891B1
Application number: EP13777664.7A
Authority: EP
Inventors: Yasuyoshi Hidaka; Yasuto Higashida; Kazuhiro Shimoda
Original assignee: Nippon Steel and Sumitomo Metal Corp
Current assignee: Nippon Steel Corp
Priority date: 2012-04-19
Filing date: 2013-03-06
Publication date: 2016-08-24
Anticipated expiration: 2033-03-06
Also published as: JP5566417B2; EP2839891A4; WO2013157312A1; EP2839891A1; US20150125630A1; JP2013248619A

Description

Technical Field

The present invention relates to a method for producing a plug for hot tube-making (hereafter simply referred to as a plug), and more particularly to a method for producing a plug included in a piercer and an elongator.

Background Art

The Mannesmann tube-making process is widely adopted as the method for producing seamless tubes. In the Mannesmann tube making process, a round billet heated to around 1200°C is piercing-rolled by a piercer. The piercer includes a pair of inclined rolls and a plug. The plug is disposed between the inclined rolls of the pair and on a pass line. The piercer presses the round billet onto the plug while rotating the round billet in the circumferential direction by means of the inclined rolls, to piercing-roll the round billet into a hollow shell. Further, as needed, the elongator elongates the hollow shell to expand the diameter and reduce the thickness thereof. The elongator has a similar configuration to that of the piercer, that is, a pair of inclined rolls and a plug.
As so far described, since the plug is used for piercing, and elongating a round billet of high temperature to expand the diameter thereof, the plug is subject to high temperature and high interfacial pressure from the round billet. As a result, the surface of the plug is subject to wear and scoring. Such wear and scoring will reduce the life of the plug.
To improve the life of the plug, a technique to form a sprayed coating on the surface of the plug is proposed. For example, in WO2009/057471 (Patent Document 1), arc spraying is performed with iron wire rod onto the surface of the plug to form a sprayed coating. Patent Document 1 describes that the sprayed coating improves the scoring resistance of the plug thereby increasing the life thereof.

Disclosure of the Invention

However, the sprayed coating may have a poor adhesiveness to the plug body. Poor adhesiveness will cause the sprayed coating to peel off. If the sprayed coating peels off, inner surface flaws of the hollow shell become more likely to occur. Moreover, the life of the plug will also decline.
It is an object of the present invention to provide a method for producing a plug having a sprayed coating which is resistant to peeling.
A method for producing a plug for hot tube-making according to the present embodiment includes a step of preparing a plug whose surface includes a sprayed coating formed thereon, the sprayed coating containing iron and an iron oxide; and a heat treatment step during which the plug is kept at 400°C to 550°C for 5 to 60 minutes.
The method for producing a plug according to the present embodiment can produce a plug whose sprayed coating is less likely to peel off.

Brief Description of the Drawings

Figure 1 is a longitudinal sectional view of a plug according to the present embodiment;
Figure 2 is a diagram showing the relationship between the heat treatment temperature and the proportions of iron and iron oxide in the coating;
Figure 3 is a cross sectional view of a specimen to be used for a test to measure tensile residual stress in the sprayed coating of the plug; and
Figure 4 is a schematic diagram to explain the method of measuring tensile residual stress by use of the specimen shown in Figure 3.

Best Mode for Carrying Out the Invention

Hereafter, referring to the drawings, an embodiment of the present invention will be described in detail. The like or corresponding parts in the drawings are given the like reference characters, without repeating the description thereof.
The method for producing a plug according to the present embodiment includes a step of preparing a plug whose surface includes a sprayed coating formed thereon, the sprayed film containing iron and an iron oxide; and a heat treatment step during which the plug is kept at 400°C to 550°C for 5 to 60 minutes.
The iron oxide in the prepared sprayed coating of the plug is composed dominantly of wustite (FeO). Wustite has a relatively low adhesiveness. Retaining the plug, on which such sprayed coating is formed, at 400°C to 550°C for 5 to 60 minutes causes the wustite in the sprayed coating to transform into magnetite (Fe₂O₃). Magnetite has a higher adhesiveness than that of wustite. Therefore, the adhesiveness of the sprayed coating to the plug body is improved, and the sprayed coating becomes less likely to peel off.
Further, tensile residual stress remains in the sprayed coating, which has been formed on the surface of the plug body by spraying. If a plug in which tensile residual stress still remains in the sprayed coating is used for piercing-rolling, the sprayed coating becomes more likely to peel off due to the tensile residual stress. In the present embodiment, the plug including a sprayed coating formed thereon is subjected to a heat treatment at the above described conditions (at 400°C to 550°C for 5 to 60 minutes). As a result, the tensile residual stress in the sprayed coating is reduced, and the sprayed coating becomes less likely to peel off.
Hereafter, the details of the method for producing a plug according to the present embodiment will be described.

[Method for producing a plug]

The method for producing a plug according to the present embodiment produces a plug for hot tube-making. The plug for hot tube-making can be utilized for a piercer and an elongator. The method for producing a plug of the present embodiment includes a step (preparation step) of preparing a plug having a sprayed coating, and a step (heat treatment step) of heat treating the plug.

[Preparation step]

In the preparation step, a plug 10 as shown in Figure 1 is prepared.
The plug 10 includes a plug body 11 and a sprayed coating 12. The plug body 11 has, for example, a well-known shape and material.
The sprayed coating 12 contains iron (Fe) and an iron oxide. Preferably, the sprayed coating 12 consists of iron, an iron oxide, and impurities. The sprayed coating 12 consisting of iron, an iron oxide, and impurities is formed by, for example, arc spraying as follows. An iron wire rod which provides a spraying material, and an arc spraying apparatus are prepared. The arc spraying apparatus includes a spraying nozzle. The spraying nozzle blows out the spraying material which is melted by arc, with compressed air or nitrogen gas.
The iron wire rod is arc sprayed by the arc spraying apparatus to form a sprayed coating 12 on the surface of the plug body 11. By adjusting the distance (hereafter referred to as a spraying distance) between the spraying nozzle of the arc spraying apparatus and the plug body 11, it is possible to adjust the content of the iron oxide in the sprayed coating. The spraying distance is, for example, 200 to 1000 mm. The arc spraying apparatus can adjust the position of the spraying nozzle. Note that the sprayed coating 12 may be formed by other spraying processes than the above described arc spraying.
Since the sprayed coating 12 contains iron, which provides the matrix, and an iron oxide, it has wear resistance and thermal insulation performance. Thereby, the wear and melting loss of the plug body 11 are suppressed.
However, the iron oxide in the sprayed coating 12 formed by spraying is mostly wustite (FeO). Wustite has poor adhesiveness to the plug body 11. Therefore, the sprayed coating 12 may peel off from the plug body 11. Further, since the sprayed coating 12 is formed by spraying, tensile residual stress remains in the sprayed coating 12 after it is cooled. The tensile residual stress reduces the adhesiveness of the sprayed coating 12. Thereby, the tensile residual stress makes the sprayed coating 12 more likely to peel off from the plug body 11.
Accordingly, to further increase the adhesiveness of the sprayed coating to the plug body 12, the following heat treatment is conducted.

[Heat treatment step]

Heat treatment is conducted on the prepared plug 10, that is, the plug 10 after the sprayed coating 12 is formed by spraying. To be specific, the plug 10 is put into a heat treatment furnace. The internal temperature of the heat treatment furnace is kept at 400°C to 550°C. As a result, the temperature of the plug 10 put into the heat treatment furnace will become 400°C to 550°C. In the heat treatment furnace, the temperature of the plug 10 is retained at 400°C to 550°C for 5 to 60 minutes. The plug 10 is taken out from the heat treatment furnace after the retention time has passed.
In the sprayed coating 12 of the plug 10, which is produced by the above described step, the transformation from wustite to magnetite takes place, and the ratio of magnetite in the sprayed coating 12 increases. Further, the tensile residual stress decreases. Thereby, the adhesiveness of the sprayed coating 12 to the plug body 11 increases, and the sprayed coating 12 becomes less likely to peel off. Hereafter, such operation will be described in detail.

[Transformation from wustite to magnetite]

The iron oxide includes wustite (FeO), magnetite (Fe₃O₄), and hematite (Fe₂O₃). Among these, since magnetite is more consistent with iron than wustite and hematite are, magnetite has a highest adhesiveness with the plug body 11. Further, the heat insulation performance of magnetite is equal to those of wustite and hematite. Therefore, to increase the adhesiveness of the sprayed coating of the plug 10, it is preferable to increase the ratio of magnetite in the iron oxide of the sprayed coating.
Employing a heat treatment temperature of 400°C to 550°C and a heat treatment time of 5 to 60 minutes will make it possible to reduce the ratio of wustite and increase the ratio of magnetite in the sprayed coating.
Figure 2 is a diagram showing the relationship between the heat treatment temperature and the proportions of iron, which is the matrix, and each iron oxide (wustite, magnetite, and hematite) in the coating. Figure 2 was resulted from the following method.
A plurality of thick plate test specimens (each of which has dimensions of 20 mm × 50 mm × 10 mm) were prepared. For each test specimen, arc spraying was performed at the same condition by using an iron wire rod having the same composition, to form a sprayed coating of the same thickness (500 µm) in each test specimen.
After the sprayed coating was formed, the composition of the sprayed coating of each test specimen was analyzed. To be specific, by an X-ray diffraction method (XRD method), peak intensities of iron and each oxide (wustite, magnetite, and hematite) in the sprayed coating of each test specimen were determined. In the present discussion, for the sake of convenience, the heights of maximum peaks out of the obtained peak intensities were used as the indices for the ratios of iron and each iron oxide in the sprayed coating. The ratios of iron and each iron oxide in the sprayed coating of each test specimen as sprayed were the same for every test specimen.
Next, each test specimen was subjected to heat treatment. The heat treatment temperature was 400°C to 650°C, and the heat treatment time was 60 minutes for every test specimen. The ratios of iron and iron oxides in the sprayed coating were analyzed by an XRD method on the sprayed coating of the test specimens after heat treatment to obtain Figure 2. The ordinate in Figure 2 indicates peak intensity ratios of iron and each iron oxide. The peak intensity ratio was defined by the following formula.
Peak intensity ratio = Each maximum peak height of iron and iron oxide/ total of maximum peak heights of iron and each iron oxide
A higher peak height ratio means a larger ratio of the content in the sprayed coating. With reference to Figure 2, when the heat treatment temperature is not less than 400°C, the peak intensity ratio of magnetite remarkably increased and also the peak intensity ratio of iron (Fe) increased as the temperature rose.
Here, the chemical reaction formula of the transformation from wustite to magnetite is as follows.

4FeO → Fe + Fe₃O₄
Referring to the above described chemical reaction formula and Figure 2, when the heat treatment temperature is not less than 400°C, the transformation from wustite to magnetite is facilitated, and the iron ratio and magnetite ratio in the spray coating increase. On the other hand, the wustite ratio decreases.
Further, when the heat treatment temperature is higher than 550°C, the iron ratio rapidly decreases and the hematite ratio increases as the heat treatment temperature rises. The decrease of the iron ratio is probably due to that the oxidation of the sprayed coating itself has progressed causing the iron in the sprayed coating to transform into hematite.
As so far described, if the heat treatment temperature is 400°C to 550°C, the magnetite ratio and the iron ratio, which increase the adhesiveness with the plug body 11, will increase in the sprayed coating 12. As a result, the adhesiveness of the sprayed coating 12 to the plug body 11 will increase.

[Reduction of tensile residual stress]

Conducting heat treatment at 400°C to 550°C will further reduce tensile residual stress in the sprayed coating. Table 1 shows the level of tensile residual stress (a deflection) when heat treatment is conducted at each condition (heat treatment temperature and heat treatment time). Table 1 was resulted from the following method.

[Table 1]

TABLE 1

Heat treatment temperature (°C)	Heat treatment time (minutes)
Heat treatment temperature (°C)	5	20	40	60	70
400	0.3	0.25	0.2	0.1	0.1
450	0.35	0.3	0.22	0.12	0.12
500	0.3	0.3	0.2	0.16	0.16
550	0.3	0.25	0.22	0.2	0.15
600	0.25	0.25	0.2	0.15	0.15
650	0.3	0.2	0.15	0.1	0.1
Non-heat treatment	0.55

A plurality of specimens 20 shown in Figure 3 were prepared. Each specimen 20 included a base 21, a base metal 22, a sprayed coating 23, and a plurality of bolts 24. The base 21 had dimensions of 20 mm width, 50 mm length, and 10 mm thickness. The base metal 22 was disposed on the upper face of the base 22. The base metal 22 had a shape of 20 mm width, 50 mm length, and 1 mm thickness. As shown in Figure 3, each end portion (opposite side portions) of the base metal 22 was secured to the base 21 with a bolt 24. A sprayed coating 23 was formed on the upper face of the base metal 22. The sprayed coating 23 was formed by arc spraying, the condition of which was the same as in Figure 1, and the thickness thereof was 500 µm.
Heat treatment was not conducted for one specimen 20, and heat treatment was conducted at various heat treatment temperatures (400 to 650°C) and heat treatment times (5 to 70 minutes) shown in Table 1 for the other plurality of specimens 20. One of the bolts 24, which were secured to both end portions of the specimen 20 which was not subjected to heat treatment and the specimen 20 subjected to heat treatment, was detached as shown in Figure 4. At this moment, as shown in Figure 4, the base metal 22 and the sprayed coating 23 were deflected due to tensile residual stress. Then, as shown in Figure 4, the distance from the upper face of the base 21 to the edge of the lower face of the base metal 22 was defined as a deflection FL (mm). It was judged that the deflection FL was an index of tensile residual stress, and the larger the deflection, the larger the tensile residual stress.
Referring to Table 1, in the specimens 20 which were subjected to heat treatment, the deflection FL was smaller than in the specimen 20 which was not subjected to heat treatment, and thus tensile residual stress was reduced. Further, in the specimens 20 subjected to heat treatment, the tensile residual stress decreased as the heat treatment time increases.
As so far described, retaining the sprayed coating 12 at a heat treatment temperature of 400°C to 550°C for 5 to 60 minutes will result in an increase in the magnetite ratio in the sprayed coating 12, thereby increasing the adhesiveness of the sprayed coating 12 to the plug body 11. Further, the tensile residual stress of the sprayed coating 12 also decreases. Therefore, in the plugs 10 subjected to heat treatment at the above described condition, the sprayed coating 12 becomes less likely to peel off from the plug body 11.
When the heat treatment time is too short, the transformation from wustite to magnetite will not proceed so that the adhesiveness of the sprayed coating 12 is not likely to increase. On the other hand, when the heat treatment time is too long, further oxidation will proceed in the sprayed coating 12 so that the ratio of iron which is the matrix, declines and the ratio of iron oxides such as hematite increases. Therefore, the adhesiveness of the sprayed coating 12 to the plug body 11 decreases on the contrary. Accordingly, the heat treatment time is 5 to 60 minutes. A preferable lower limit of the heat treatment time will be 10 minutes, and more preferably 20 minutes.
The heat treatment temperature is preferably 500°C ± 25°C (475°C to 525°C).
In the plugs 10 which are produced in the above described steps (the preparation step and the heat treatment step), the adhesiveness of the sprayed coating 12 to the plug body 11 increases. Therefore, the sprayed coating 12 is less likely to peel off.
In the plugs 10 prepared in the above described preparation step, the sprayed coating 12 contains iron and iron oxides. However, the sprayed coating 12 may further contain other oxides. For example, the sprayed coating 12 may contain W oxide (WO₃) along with iron oxides. For example, by arc spraying an iron wire rod containing W, the sprayed coating 12 will contain iron, iron oxides, and W oxide. Even with a plug on which such a sprayed coating is formed, the result of conducting the above described heat treatment step will be that the magnetite ratio in the sprayed coating increases, and the tensile residual stress decreases. Therefore, the sprayed coating is less likely to peel off.

[Examples]

Each plug having a sprayed coating was heat treated at conditions of test numbers 1 to 20 shown in Table 2. An anti-peeling property of the sprayed coating of the each plug produced was evaluated.

[Table 2]

TABLE 2

Test number	Heat treatment temperature (°C)	Heat treatment time (minutes)	Peeling rate (%)
1	350	20	40
2	350	40	40
3	400	10	30
4	400	20	20
5	400	40	20
6	400	60	20
7	400	80	50
8	500	10	30
9	500	20	10
10	500	40	15
11	500	60	10
12	500	80	50
13	550	10	30
14	550	20	20
15	550	40	20
16	550	60	20
17	550	80	40
18	600	20	50
19	600	40	50
20	-	-	40

[Test method]

Every plug of each test number had the shape shown in Figure 1. A sprayed coating of a thickness of 600 µm was formed on the surface of each plug. The sprayed coating was formed by arc spraying an iron wire rod. The conditions of arc spraying of each plug were the same for all of them. Ten plugs were prepared for each test number.
Heat treatment was conducted on 10 plugs for each test number at the conditions (heat treatment temperature and heat treatment time) shown in Table 2. Note that the plug of test number 20 was not subjected to the heat treatment.
After the heat treatment, the plugs of test numbers 1 to 20 were used to piercing-roll round billets. The round billets had a chemical composition corresponding to SUS304 of the JIS Standard, and each had the same diameter and length. Each plug of each test number was used to piercing-roll three round billets. At this moment, the heating conditions and the piercing-roll conditions of each round billet were the same for every test number.
After piercing-rolling three round billets, the surface of each plug was visually observed to judge the presence or absence of the peeling of the sprayed coating. Then, a peeling rate (%) was determined for each test number based on the following formula. $Peeling rate = number of plugs in which peeling is confirmed / 10 \times 100$
In short, out of the 10 plugs of each test number, the rate of the plugs in which peeling is confirmed was defined as a peeling rate (%).

[Test results]

Referring to Table 2, in test numbers 3 to 6, 8 to 11, and 13 to 16, the heat treatment temperature was within a range of 400°C to 550°C and the heat treatment time was within a range of 5 to 60 minutes for each test number. As a result, the peeling rate was as low as not more than 30%.
On the other hand, in test numbers 1 and 2, the heat treatment temperature was as low as 350°C. Thereby, the peeling rate was high.
In test numbers 7, 12, and 17, although the heat treatment temperature was appropriate, the heat treatment time was as long as 80 minutes. Thereby, the peeling rate was high.
In test numbers 18 and 19, the heat treatment temperature was as high as 600°C. Thereby, the peeling rate was high.
In test number 20, no heat treatment was conducted. Thereby, the peeling rate was high.
Although an embodiment of the present invention have been described so far, the above described embodiment is merely an example for carrying out the present invention. Therefore, the present invention will not be limited to the above described embodiment, and can be carried out by appropriately varying the above described embodiment within the scope of the claims.

Claims

A method for producing a plug (10) for hot tube-making, comprising
a step of preparing a plug (10) whose surface includes a sprayed coating (12) formed thereon, the sprayed coating (12) containing iron and an iron oxide; and
a heat treatment step during which the plug (10) is kept at 400°C to 550°C for 5 to 60 minutes.
The method for producing a plug (10) according to claim 1, wherein
the step of preparing the plug (10) further comprises:
a step of preparing a plug body (11); and

a step of forming the sprayed coating (12) by performing arc spraying with an iron wire rod onto a surface of the plug body (11).
The method for producing a plug (10) according to claim 1 or 2, wherein
the sprayed coating (12) further contains W oxide (WO₃).