JP7583271B2 - Piercer Plug - Google Patents

Description

This disclosure relates to a piercer plug used in piercing and rolling seamless steel pipes.

Conventionally, piercer plugs have been used with a scale film formed on the surface to ensure the surface's heat insulation, lubricity, and seizure resistance (see, for example, JP-B-4-8498, JP-A-4-74848, JP-A-4-270003, JP-B-1-7147, JP-A-63-203205, etc.).

The scale film gradually wears away with each piercing and rolling. If the scale film wears away completely and the base material (plug body) is exposed, it can melt and cause seizure with the mating material. When drilling hard-to-process materials such as stainless steel, the scale film wears away significantly and can wear away after just a few passes. Each time this happens, heat treatment is required to re-form the scale film, but this heat treatment takes anywhere from several hours to several tens of hours, which is problematic in terms of efficiency.

Patent No. 4279350 proposes a technology for forming a thermal spray coating made of iron and oxides on the surface of the base material of a piercer plug. Thermal spray coatings have better adhesion to the base material and better wear resistance than scale coatings, and can be formed in a few minutes to a few tens of minutes. As a result, thermal spray coatings have a longer life than scale coatings, and can be regenerated in a short time even if they wear down.

Japanese Patent No. 5,339,016 discloses that in a manufacturing method for a plug for piercing and rolling in which a coating is formed on the surface of the base metal of the plug by arc spraying, a cored wire containing particles (specifically, _ZrO2 particles) having a lower thermal conductivity than iron oxide and solid lubricant particles (specifically, BN particles) is used as the spray wire.

Although not related to piercer plugs, Japanese Patent Laid-Open Publication No. 2003-147503 discloses a bearing in which a thermal spray coating made of a low-melting-point metal (specifically, tin, lead, zinc, etc.) or an alloy thereof and a solid lubricant is formed on the sliding surface of the base material. In addition, Japanese Patent Laid-Open Publication No. 2013-526655 discloses a thermal spray wire for coating the sliding surface of a bearing, which includes an inner layer, a powder material layer surrounding the inner layer, and an outer layer surrounding the powder material layer.

Special Publication No. 4-8498 Japanese Patent Application Publication No. 4-74848 Japanese Patent Application Publication No. 4-270003 Special Publication No. 1-7147 Japanese Unexamined Patent Publication No. 63-203205 Patent No. 4279350 Patent No. 5339016 JP 2003-147503 A Special Publication No. 2013-526655

As mentioned above, the thermal spray coating has better adhesion to the base material, i.e., the plug body, and is more abrasion resistant than the scale coating. Therefore, the life of the thermal spray coating is longer than that of the scale coating. However, when piercing a low-alloy billet at a relatively low temperature or when piercing a high-strength billet such as a high-alloy billet, peeling of the coating occurs, and melting and seizing can occur from the exposed base material, damaging the plug body.

The main properties required for a thermal spray coating to suppress damage to the plug body are lubricity, spalling resistance, and wear resistance. However, it is difficult to simultaneously satisfy all of these properties. In other words, in an arc spray coating made from iron wire, if the unoxidized Fe ratio in the coating is increased, the drilling efficiency (≒ lubricity) tends to decrease as shown in Figure 1, but the shear adhesion strength (≒ spalling resistance) of the thermal spray coating improves as shown in Figure 2, and the amount of wear decreases, that is, wear resistance improves, as shown in Figure 3. In addition, when the thermal spray coating is melted between the billet and the plug body, the friction coefficient decreases and lubricity improves, but the more the coating melts, the lower the wear resistance and spalling resistance become. In this way, there is a trade-off between lubricity, spalling resistance, and wear resistance.

The objective of this disclosure is to provide a long-life piercer plug that appropriately improves lubrication without compromising peel resistance and wear resistance.

A piercer plug according to an embodiment of the present disclosure may include a plug body and a first thermal spray coating formed on a surface of the plug body. The plug body may include a tip portion, a bottom portion rearward of the tip portion, and a rolled portion formed between the tip portion and the bottom portion. The first thermal spray coating may include a tip region formed on the surface of the tip portion, a bottom region formed on the surface of the bottom portion, and a rolled region formed on the surface of the rolled portion. The tip region may be composed of Fe, O, and impurities. The rolled region may be composed of Fe, O, W, and impurities. The content of W excluding O in the rolled region may be 4.0 wt % or less.

According to the present disclosure, it is possible to obtain a piercer plug with a long life by appropriately improving lubrication without compromising peel resistance and wear resistance.

FIG. 1 is a graph showing the measurement results of the drilling efficiency of a thermal spray coating. FIG. 2 is a graph showing the measurement results of the adhesion strength of the thermal spray coating. FIG. 3 is a graph showing the measurement results of the wear amount of the thermal spray coating. FIG. 4 is a longitudinal cross-sectional view of a piercer plug according to one embodiment of the present disclosure. FIG. 5 is a diagram showing an example of an apparatus used for forming a thermal spray coating. FIG. 6 shows a cross section of a cored wire. FIG. 7 is a longitudinal cross-sectional view showing a modified example of a piercer plug according to the present disclosure. FIG. 8 is a longitudinal cross-sectional view showing a modified example of a piercer plug according to the present disclosure. FIG. 9 is a graph showing the relationship between the W content in the rolled region and the wear rate in Experiment 2. FIG. 10 is a graph showing the relationship between the W content in the rolled region and the friction coefficient in Experiment 3.

As a result of intensive research, the inventors have found that the regions of the thermal spray coating formed on the surface of the plug body that require lubricity, peeling resistance, and wear resistance are not necessarily the same. Specifically, the inventors divided the plug body 2 into the tip portion 21, the rolled portion 22, and the bottom portion 23 with reference to FIG. 4, and examined the required characteristics of the thermal spray coating 3 formed on the surface of these three portions. As shown in Table 1, the tip region 31 of the thermal spray coating 3 formed on the surface of the tip portion 21, the rolled region 32 of the thermal spray coating 3 formed on the surface of the rolled portion 22, and the bottom region 33 of the thermal spray coating 3 formed on the surface of the bottom portion 23 were examined. In Table 1, "A" indicates a strongly required characteristic, "B" indicates a required characteristic, and "C" indicates a characteristic that is preferably high but not particularly required. As a result, it was found that lubricity is strongly required especially in the rolled region 32, and is not particularly required in the tip region 31 and the bottom region 33. That is, it was found that for the tip region 31 and bottom region 33 other than the rolling region 32, emphasis should be placed on wear resistance and peeling resistance, and by selectively forming a highly lubricating thermal spray coating 3 in the rolling region 32, a highly lubricating and long-lasting piercer plug 1 can be efficiently obtained. The present invention was completed based on the above findings. The following describes the embodiments of the present disclosure.

A piercer plug according to an embodiment of the present disclosure may include a plug body and a first thermal spray coating formed on the surface of the plug body. The plug body may include a tip portion, a bottom portion rearward of the tip portion, and a rolled portion formed between the tip portion and the bottom portion. The first thermal spray coating may include a tip region formed on the surface of the tip portion, a bottom region formed on the surface of the bottom portion, and a rolled region formed on the surface of the rolled portion. The tip region may be composed of Fe, O, and impurities. The rolled region may be composed of Fe, O, W, and impurities. The content of W excluding O in the rolled region may be 4.0 wt.% or less. This sufficiently reduces the wear rate of the rolled region during piercing and rolling of the billet, and the rolled region interposed between the plug body and the billet melts, thereby appropriately improving the lubricity that is strongly required for the rolled region. As a result, a piercer plug with a long life can be obtained.

The W content excluding O in the rolled region may be 0.5% by weight or more. This sufficiently reduces the friction coefficient of the rolled region. As a result, a piercer plug with a long life can be obtained.

The bottom region may be composed of the same elements as the rolled region. This allows the thermal spray coating to divide the surface region into two regions: the tip region and a region behind the tip region (the rolled region and the bottom region). Even when the surface region is divided into two regions, the lubricity of the region corresponding to the rolled region can be improved, resulting in a piercer plug with a long life.

The piercer plug may include a second sprayed coating formed between the plug body and the first sprayed coating. The second sprayed coating may be composed of Fe, O, and impurities. Fe oxides have excellent peeling resistance. This improves the adhesion between the surface of the plug body and the first sprayed coating, and suppresses peeling of the first sprayed coating. As a result, a piercer plug with a long life can be obtained.

The following describes in detail an embodiment of the present invention with reference to the drawings. The same or corresponding parts in the drawings are given the same reference numerals and their description will not be repeated. The dimensional ratios between the components shown in each drawing do not necessarily represent the actual dimensional ratios.

[Piercer plug structure]
4 is a longitudinal sectional view of the piercer plug 1 according to one embodiment of the present invention. The piercer plug 1 includes a plug body 2 and a thermal spray coating 3. The entire plug body 2 is integrally formed of the same material. The material of the plug body 2 may be, for example, carbon steel or stainless steel.

The plug body 2 has a bullet shape. The cross section of the plug body 2 has a circular shape. The plug body 2 has a leading edge 2a and a face at the end opposite the leading edge 2a, i.e., a rear end face 2b. The plug body 2 has a shape in which the outer diameter increases from the leading edge 2a to the rear end face 2b of the plug body 2. The boundary between the rear end face 2b of the plug body 2 and the other faces forms a circular ridge. The line passing through the center of the outer circumferential circle of the multiple cross sections of the plug body 2 is the plug axis C. The plug axis C is the direction of travel of the billet during piercing and rolling.

As shown in FIG. 4, the plug body 2 has a tip portion 21, a bottom portion 23 opposite to the tip portion 21, and a rolled portion 22 positioned between the tip portion 21 and the bottom portion 23. The rolled portion 22 is a rear portion (right side in the figure) continuing from the tip portion 21. The bottom portion 23 has a reeling portion 23a and a relief portion 23b. The reeling portion 23a is a rear portion continuing from the rolled portion 22. The relief portion 23b is a rear portion continuing from the reeling portion 23a and is a portion close to the rear end face 2b. The tip portion 21 and the rolled portion 22 are portions that are responsible for most of the wall thickness reduction in piercing and rolling. The bottom portion 23 is a portion that finishes the wall thickness of the hollow blank tube (also called a shell) in piercing and rolling. In other words, the tip portion 21 is a portion on which the force for piercing the billet mainly acts during piercing and rolling. The rolling section 22 is the section where the force for expanding the diameter of the billet during piercing and rolling mainly acts. The reeling section 23a is the section where the force for uniformizing the wall thickness dimension of the hollow blank pipe created by piercing or expanding the diameter of the billet during piercing and rolling mainly acts to smooth the inner surface of the hollow blank pipe.

The outer diameter of the plug body 2 increases from the front to the rear of the plug axis C. That is, the outer diameter of the plug body 2 changes in the direction of the plug axis C. From the tip 21 to the rolled portion 22, the rate of change of the outer diameter of the plug body 2 in the direction of the plug axis C decreases as it approaches from the tip 2a to the reeling portion 23a. That is, in the longitudinal section, the angle θ2 of the tangent of the outer edge line from the tip 21 to the rolled portion 22 to the plug axis C decreases as it approaches from the tip 2a to the reeling portion 23a. The rate of change of the outer diameter of the plug body 2 in the direction of the plug axis C in the reeling portion 23a is the same as or smaller than the minimum value of the rate of change from the tip 21 to the rolled portion 22. That is, in the longitudinal section, the angle θ1 of the tangent of the outer edge line of the reeling portion 23a to the plug axis C is the same as or smaller than the minimum value of the angle θ2 of the tangent of the outer edge line from the tip 21 to the rolled portion 22 to the plug axis C. In the example shown in FIG. 4, the rate of change of the outer diameter of the reeling portion 23a in the direction of the plug axis C is approximately constant.

The front end of the rolled portion 22 should be located 0.2 x D [mm] or less from the front end 2a, where D [mm] is the overall length of the plug body 2 (the dimension of the plug axis C from the front end 2a to the rear end face 2b). The rear end of the rolled portion 22 should be located 0.4 x D [mm] or more from the front end 2a. In other words, the rolled portion 22 is located between the front end 21 and the bottom 23, within a range from 0.2 x D [mm] to 0.4 x D [mm] starting from the front end 2a with respect to the overall length D of the plug body 2.

The sprayed coating 3 is formed on the surface of the plug body 2. The sprayed coating 3 covers the surfaces of the tip portion 21, the rolled portion 22, and the bottom portion 23, except for the rear end surface 2b of the plug body 2. That is, as shown in FIG. 4, the sprayed coating 3 has three regions: a tip region 31 covering the tip portion 21, a rolled region 32 covering the rolled portion 22, and a bottom region 33 covering the bottom portion 23. The three regions are divided into each region and cover the surface of the plug body 2. The constituent elements contained in each of the tip region 31, the rolled region 32, and the bottom region 33 are determined according to the characteristics (wear resistance, lubricity, and peeling resistance) required for each region. That is, the constituent elements contained in each region differ according to the characteristics required for each region.

As shown in Table 1 above, tip region 31 is required to have high wear resistance. Therefore, tip region 31 is made of Fe, O, and impurities. This improves wear resistance. Note that tip region 31 may contain other substances as long as they improve wear resistance.

As shown in Table 1 above, the rolling region 32 is required to have a high level of lubricity. Therefore, the rolling region 32 is made of Fe, O, W, and impurities. This allows the melting point of the rolling region 32 to be lower than that of the tip region 31. That is, when the billet is pierced and rolled, the rolling region 32 of the thermal spray coating 3 can be melted in the operating temperature range of the piercer plug 1. During piercing and rolling, the surface temperature of the piercer plug 1 is usually, for example, 850 to 1300°C. It is preferable that the rolling region 32 is determined so that it melts in this temperature range. The friction coefficient decreases as the rolling region 32 interposed between the plug body 2 and the billet melts. This allows the lubricity of the rolling region 32 to be improved. In addition, the rolling region 32 may be composed of Fe, O, one or more elements selected from the group consisting of Na, Si, P, Ca, V, Nb, and W, and impurities, as long as it can improve lubricity, and may also contain other substances.

As shown in Table 1 above, bottom region 33 does not have any particularly strongly required properties. However, bottom region 33 is required to have peeling resistance. Therefore, the constituent elements of bottom region 33 should preferably contain Fe and O. This can improve the peeling resistance of bottom region 33. Note that since bottom region 33 does not have any strongly required properties, lubricity may be improved as in the modified example described below.

The W content in the rolling region is 4.0% by weight or less. The W content in the rolling region is the W content relative to the total of Fe, W, and impurities contained in the rolling region (W/(Fe+W+impurities)×100), i.e., the content excluding O (hereinafter, the same applies to the W content in the tip region and bottom region). As will be described later, when the W content in the rolling region is 4.0% by weight or less, the wear rate of the rolling region can be reduced. As a result, a piercer plug 1 with a long life can be obtained. From the viewpoint of reducing the wear rate of the rolling region, the W content in the rolling region should be 4.0% by weight or less, preferably 3.0% by weight, and more preferably 2.4% by weight or less.

The W content in the rolling region is 0.5% by weight or more. As described later, by making the W content in the rolling region 0.5% by weight or more, the friction coefficient of the rolling region can be reduced. As a result, a piercer plug 1 with a long life can be obtained. From the viewpoint of sufficiently reducing the friction coefficient of the rolling region, the W content in the rolling region is preferably 0.5% by weight or more, more preferably 0.8% by weight or more, and even more preferably 1.2% by weight or more.

In this way, according to the present disclosure, by forming a highly lubricating spray coating 3 (rolled region 32) on the optimal portion, i.e., the surface of the rolled portion 22, and appropriately improving lubricity, it is possible to obtain a piercer plug 1 with excellent lubricity and a long life without compromising peel resistance and wear resistance.

[Manufacturing method of piercer plug]
Hereinafter, an example of a method for manufacturing the piercer plug 1 will be described. The method described below is merely an example, and the method for manufacturing the piercer plug 1 is not limited to this.

Prepare the plug body 2. The plug body 2 is made of carbon steel or stainless steel, including at least a portion of the surface excluding the rear end face 2b.

A thermal spray coating 3 is formed on the plug body 2. The thermal spray coating 3 can be formed using an arc spraying device 100 shown in FIG. 5.

The arc spraying device 100 includes a spray gun 101 and a rotating table 104. The spray gun 101 generates an arc at the tip of the continuously supplied spray wires 102 and 103, and sprays the molten metal using compressed air.

In this embodiment, the tip region 31, rolled region 32, and bottom region 33 of the thermal spray coating 3 are formed separately on the surfaces of the tip portion 21, rolled portion 22, and bottom portion 23 of the plug body 2, respectively.

First, the bottom region 33 of the spray coating 3 is formed on the surface of the bottom 23. The spray wires 102 and 103 used to form the bottom region 33 are not particularly limited, but it is preferable to improve the peeling resistance. Therefore, the spray wires 102 and 103 used to form the bottom region 33 are iron wires. The iron wires are arc sprayed to coat the surface of the bottom 23 with iron oxide. In other words, the constituent elements of the bottom region 33 are Fe and O. This improves the peeling resistance of the bottom region 33.

Next, the rolled region 32 of the thermal spray coating 3 is formed on the surface of the rolled portion 22 of the plug body 2. The thermal spray wire 102 used to form the rolled region 32 is a so-called cored wire 110. Figure 6 is a cross-sectional view perpendicular to the axial direction of the cored wire 110. The cored wire 110 includes an iron shell 111 and a filler material 112 sealed inside the shell 111.

The filler 112 contains Fe and W. The filler 112 may contain one or more elements selected from the group consisting of Na, Si, P, Ca, V, Nb, and W, which are capable of forming a thermal spray coating having a lower melting point than the tip region 31. The filler 112 may further contain other substances.

In this embodiment, the cored wire 110 is melted by the thermal spray gun 101 to form the rolled region 32 on the surface of the rolled portion 22. At this time, the above-mentioned substance contained in the filler 112 can make the melting point of the rolled region 32 lower than that of the tip region 31. The substance contained in the filler 112 may be the same substance as the substance in the rolled region 32, or it may be a mixture of multiple substances that will become the same compound after thermal spraying.

The substance contained in the filler 112 is not limited to one or more selected from the group consisting of Na, Si, P, Ca, V, Nb, and W. It can be appropriately determined so that the rolling region 32 can melt in the operating temperature range of the piercer plug 1 during piercing and rolling. More specifically, during piercing and rolling, the surface temperature of the piercer plug 1 is usually 850 to 1300°C, for example, 1150°C or 1200°C. It is preferable that the rolling region 32 is determined so that it melts in this temperature range.

The sprayed wire 103 forming the rolling region 32 may be a cored wire 110 having the same configuration as the sprayed wire 102, or may be an ordinary iron wire. The iron wire is, for example, a carbon steel (ordinary steel) wire, and more specifically, a wire containing iron as the main component and carbon, silicon, manganese, impurities, etc. In the process of forming the rolling region 32 in this embodiment, at least one of the sprayed wires 102 and 103 may be a cored wire 110 containing a substance capable of forming a sprayed coating having a lower melting point than the tip region 31.

Next, the tip region 31 of the spray coating 3 is formed on the surface of the tip portion 21. The spray wires 102 and 103 used to form the tip region 31 are iron wires. The iron wires are arc sprayed to coat the surface of the tip portion 21 with iron oxide. In other words, the constituent elements of the tip region 31 are Fe and O. This improves the wear resistance of the tip region 31.

In order to form the tip region 31, the rolled region 32, and the bottom region 33 separately, the regions that are not to be arc sprayed may be masked. Also, the spray angle of the spray gun 101 may be changed relatively so that the molten metal is sprayed onto a predetermined region, and the region to be sprayed may be changed as appropriate. In other words, as long as the spray coating 3 can be formed separately for each surface region on the surfaces of the tip, rolled portion, and bottom portion of the plug body 2, the method is not limited.

The longer the spraying distance, the higher the ratio of oxides in the sprayed coating 3. This is because the oxidation of the metal sprayed from the tip of the spray gun 101 progresses according to the spraying distance. The spraying distance is not limited to this, but is, for example, 100 to 1400 mm. In addition, by spraying while gradually increasing the spraying distance, it is possible to increase the ratio of metal components near the plug body 2 and increase the ratio of oxides toward the surface.

While rotating the plug body 2 around its axis using the turntable 104, spraying is performed until the surface region of the thermal spray coating 3 reaches a predetermined thickness. The thickness of the surface region is not limited to this, but is, for example, 200 to 3000 μm.

After the thermal spray coating 3 is formed, it is preferable to carry out a heat treatment for diffusion. This allows the plug body 2 and the thermal spray coating 3 to be more closely attached to each other. The heat treatment for diffusion is, for example, performed at 700 to 1200°C for 5 minutes or more. The heat treatment time is more preferably 10 minutes or more. The heat treatment temperature is more preferably a temperature lower than the melting point of a material capable of forming a thermal spray coating having a lower melting point than the tip region 31 described above. For example, the heat treatment temperature can be 1100°C or lower, more preferably 1000°C or higher. The heat treatment temperature may also be 800°C or higher, more preferably 900°C or higher.

In this way, according to the manufacturing method of the piercer plug 1 disclosed herein, by separately forming each of the tip region 31, the rolled region 32, and the bottom region 33, it is possible to efficiently obtain a piercer plug 1 with excellent lubricity and long life without compromising peel resistance and wear resistance.

[Modification]
In the above embodiment, the surface region of the thermal spray coating 3 is divided into three regions (the tip region 31, the rolled region 32, and the bottom region 33), but as shown in FIG. 7, it may be divided into two regions, the tip region 31 and a region behind the tip region 31 (one region including the rolled region 32 and the bottom region 33). Hereinafter, these two regions may be collectively referred to as the trunk region. In this case, the thermal spray coating 3 is formed so as to improve the wear resistance of the tip region 31 and improve the lubricity of the region behind the tip region 31. This improves the lubricity of the region corresponding to the rolled region 32, so that a piercer plug 1 with high lubricity can be obtained without impairing the peeling resistance and the wear resistance. In addition, since it is sufficient to divide the surface region into two, the work of forming the thermal spray coating in each region can be simplified. The surface region of the thermal spray coating 3 may be divided into four or more regions. Even when the surface region is divided into a plurality of regions, the process of forming each region at a position corresponding to the surface of the plug body 2 is the same as the above-mentioned process of forming the surface region. That is, the surface layer region is formed by arc spraying each divided region.

Also, as shown in FIG. 8, a sprayed coating 4 of the sprayed coating 3 may be formed between the plug body 2 and the sprayed coating 3. As shown in the figure, the sprayed coating 4 can be divided into three regions at positions corresponding to the respective regions of the sprayed coating 3. The sprayed coating 4 may be one region, or may be divided into two or more regions. The sprayed coating 4 is preferably composed of Fe and O. This improves the adhesion between the surface of the plug body 2 and the sprayed coating 3, suppresses peeling of the sprayed coating 3, and provides a piercer plug 1 with a longer life.

The thermal spray coating 4 can be formed on the surface of the plug body 2 using the arc spraying device 100, similar to the thermal spray coating 4. After the thermal spray coating 4 is formed on the surface of the plug body 2, the thermal spray coating 3 is formed on the surface of the thermal spray coating 4. The process of forming the thermal spray coating 4 is the same as the process of forming the thermal spray coating 3 described above, so a detailed description will be omitted.

In this way, the thermal spray coating 3 can be divided into regions not only in the direction of the plug axis C of the plug body 2, but also in the thickness direction of the thermal spray coating 3. This makes it possible to more appropriately form the thermal spray coating 3 according to the characteristics required for each part of the plug body 2 (the tip portion 21, the rolled portion 22, and the bottom portion 23).

The present invention will be explained in more detail below based on examples. The present invention is not limited to these examples.

(Experiment 1)
In experiment 1, a thermal spray coating was formed on the surface of a plug body by arc spraying using multiple types of wires (thermal spray wire rods), and a billet piercing test was performed. The plug body used in this test was made of an iron-based alloy. As shown in Table 2, a plug body was used in which the maximum diameter in the longitudinal section of the plug body was 70 mm and the total length of the plug axial center was 230 mm, or a plug body in which the maximum diameter in the longitudinal section of the plug body was 57 mm and the total length of the plug axial center was 120 mm. On the surfaces of these plug bodies, thermal spray coatings were formed in the tip region, the rolled region, and the bottom region by arc spraying. In this case, the thickness of the tip region was 1200 to 1500 μm, the thickness of the rolled region was 500 μm, and the thickness of the bottom region was 300 μm.

The thermal spray coating was applied separately to the tip region, the rolled region, and the bottom region using an Fe wire or an Fe-W cored wire. The composition of the filler for the Fe-W cored wire was adjusted so that the W content after the thermal spray coating was 0 to 10.5% by weight. Hereinafter, the thermal spray coating applied using an Fe wire will be referred to as an Fe wire thermal spray coating, and the thermal spray coating applied using an Fe-W cored wire will be referred to as an Fe-W thermal spray coating. As shown in Table 2, the melting point of the Fe wire thermal spray coating is 1370°C, and the melting point of the Fe-W wire thermal spray coating is 1120°C. In other words, the Fe-W thermal spray coating can have a lower melting point than the Fe wire thermal spray coating.

Using the various piercer plugs thus prepared, piercing and rolling of SUS304 billets heated to 1200°C or 1150°C was carried out. Piercing was repeated until each piercer plug was damaged and unusable, and the number of billets that could be pierced and rolled before each piercer plug became unusable was compared as the "life number of passes." The results are shown in Table 2. In Table 2 below, Test Nos. 5, 6, 9, 10, and 11 are examples, and Test Nos. 1, 2, 3, 4, 7, 8, 12, 13, 14, 15, and 16 are comparative examples.

In Test No. 1, a piercer plug with an Fe wire spray coating applied to the tip region, rolling region, and bottom region was used to pierce a SUS304 billet at 1200°C. As a result, after three passes, the spray coating in the rolling region wore away and peeled off, exposing the plug body. In this case, it is presumed that there is a risk of the plug body melting. This is thought to be due to poor lubricity in the rolling region. In the remarks column of Table 2, "rolling region wear to melting" refers to a case where the plug body is exposed and there is a risk of the plug body melting.

In Test No. 2, a piercer plug with an Fe-W wire sprayed coating with a W content of 2.4 wt.% applied to the tip region and an Fe wire sprayed coating applied to the rolling region and bottom region was used to pierce a SUS304 billet at 1200°C. As a result, as in Test No. 1, after three passes, the sprayed coating in the rolling region wore away and peeled off, exposing the plug body.

In test No. 3, a piercer plug with an Fe-W wire spray coating, each with a W content of 2.4% by weight, applied to the tip region and rolled region, and an Fe wire spray coating applied to the bottom region was used to pierce a SUS304 billet at 1200°C. As a result, the life was longer than that of the piercer plugs in tests No. 1 and 2, but the spray coating in the tip region was lost after four passes.

In test No. 4, a piercer plug with an Fe-W wire spray coating, with a W content of 2.4% by weight in the tip, rolled and bottom regions, was used to pierce a SUS304 billet at 1200°C. As a result, as in test No. 3, the spray coating in the tip region was lost and melted after four passes.

In test No. 5, a piercer plug with an Fe wire spray coating applied to the tip and bottom regions and an Fe-W wire spray coating with a W content of 2.4% by weight applied to the rolled region was used to pierce a SUS304 billet at 1200°C. As a result, the spray coating was not damaged even after piercing for up to eight passes, and the life was extended.

In test No. 6, a piercer plug with an Fe wire spray coating on the tip region and an Fe-W wire spray coating with a W content of 2.4 wt% on the rolling region and bottom region was used to pierce a SUS304 billet at 1200°C. As a result, as in test No. 5, the spray coating was not damaged even with piercing up to eight passes, and the life was extended.

In Test No. 7, a piercer plug with an Fe wire spray coating applied to the tip region and the rolling region and an Fe-W wire spray coating with a W content of 2.4 wt% applied to the bottom region was used to pierce a SUS304 billet at 1200°C. As a result, as in Test No. 1, after three passes, the spray coating in the rolling region wore away and peeled off, exposing the plug body.

In Test No. 8, the size of the piercer plug was different from that in Test No. 0.1 (the same applies to Test Nos. 9 to 16 below), and a piercer plug with an Fe wire spray coating applied to the tip region, rolling region, and bottom region was used to pierce a SUS304 billet at 1,150°C. As a result, as in Test No. 0.1, after three passes, the spray coating in the rolling region wore away and peeled off, exposing the plug body.

In test No. 0.9, a piercer plug with an Fe wire spray coating applied to the tip region and an Fe-W wire spray coating with a W content of 0.6% by weight applied to the rolling region and bottom region was used to pierce a SUS304 billet at 1150°C. As a result, the spray coating was not damaged even after piercing for up to five passes, and a relatively long life was achieved.

In test No. 0.10, a piercer plug with an Fe wire spray coating applied to the tip region and an Fe-W wire spray coating with a W content of 1.2% by weight applied to the rolling region and bottom region was used to pierce a SUS304 billet at 1150°C. As a result, the spray coating was not damaged even after piercing for up to seven passes, and the life was extended.

In test No. 0.11, a piercer plug with an Fe wire spray coating on the tip region and an Fe-W wire spray coating with a W content of 2.4% by weight on the rolling region and bottom region was used to pierce a SUS304 billet at 1150°C. As a result, the spray coating was not damaged even after piercing for up to seven passes, and the life was extended.

In test No. 0.12, a piercer plug with an Fe wire spray coating on the tip region and an Fe-W wire spray coating with a W content of 4.9% by weight on the rolling region and bottom region was used to pierce a SUS304 billet at 1150°C. As a result, after three passes, the spray coating on the rolling region wore away and peeled off, exposing the plug body.

In test No. 0.13, a piercer plug with an Fe wire spray coating on the tip region and an Fe-W wire spray coating with a W content of 10.5% on the rolled region and bottom region was used to pierce a SUS304 billet at 1150°C. As a result, after two passes, the spray coating on the rolled region wore away and peeled off, exposing the plug body.

In Test No. 14, a piercer plug with an Fe-W wire sprayed coating with a W content of 0.6 wt.% on the tip region and an Fe wire sprayed coating on the rolling region and bottom region was used to pierce a SUS304 billet at 1150°C. As a result, after three passes, the sprayed coating on the rolling region wore away and peeled off, exposing the plug body.

In Test No. 15, a piercer plug with an Fe-W wire sprayed coating with a W content of 1.2 wt.% on the tip region and an Fe wire sprayed coating on the rolling region and bottom region was used to pierce a SUS304 billet at 1150°C. As a result, after three passes, the sprayed coating on the rolling region wore away and peeled off, exposing the plug body.

In Test No. 16, a piercer plug with an Fe-W wire sprayed coating with a W content of 2.4 wt.% on the tip region and an Fe wire sprayed coating on the rolling region and bottom region was used to pierce a SUS304 billet at 1150°C. As a result, after three passes, the sprayed coating on the rolling region wore away and peeled off, exposing the plug body.

As a result of Test Nos. 1 to 16, the number of life passes was relatively high in Test Nos. 5, 6, 9, 10, and 11. In other words, the life of the piercer plug could be extended. In particular, the number of life passes was 7 to 8 in Test Nos. 0.5, 6, 10, and 11, and the life of the piercer plug was extended dramatically compared to the other tests. In addition, as shown by the results of Test Nos. 5 and 6, it was confirmed that the number of life passes is not affected whether the bottom region is an Fe sprayed coating or an Fe-W sprayed coating, and that the life of the piercer plug is also extended when the body region is an Fe-W sprayed coating.

On the other hand, in Test No. 12 and 13, the rolling region was an Fe-W spray coating, but the number of passes was relatively low at 3 and 2. This is thought to be because the W content was relatively high at 4.9% by weight and 10.5% by weight, which promoted melting of the rolling region and caused the rolling region to wear out significantly. In other words, it is thought that the wear progression due to the coating melting in the rolling region during piercing rolling was greater than the wear reduction associated with the reduction in shear stress due to the lubricating effect of the coating melting. Therefore, it is preferable that the W content in the rolling region be 4.0% by weight or less. In Test No. 11, where the W content is 2.4% by weight, the number of passes was high at 7, so the W content in the rolling region is preferably 3.0% by weight or less, more preferably 2.4% by weight or less.

In addition, comparing Test No. 8, 9, and 10, the number of passes in life was 5 in Test No. 9 by setting the W content to 0.6 wt%, which is more than Test No. 8, and the number of passes in life was 7 in Test No. 10 by setting the W content to 1.2 wt%, which is more than the number of passes in life. This is thought to be because the wear progression due to the film melting in the rolling area during piercing rolling was smaller than the wear reduction due to the reduction in shear stress caused by the lubricating effect of the film melting. In other words, the W content in the rolling area should be 0.5 wt% or more, preferably 0.8 wt% or more, and more preferably 1.2 wt% or more. The W content in the rolling area was further examined in the following Experiments 2 and 3 from the viewpoint of wear rate and wear coefficient.

(Test 2)
In experiment 2, the relationship between the W content in the tip region, the rolled region, and the bottom region and the wear rate of each region was evaluated. Specifically, piercer plugs having the same shape as Nos. 8 to 16 in experiment 1 were used, and piercer plugs with W contents of 0.0 wt%, 0.6 wt%, 1.2 wt%, 2.4 wt%, 4.9 wt%, and 10.5 wt% in the thermal spray coating were manufactured in the entire region including the tip region and the rolled region, respectively. In each of these piercer plugs, piercing was performed in only one pass under the same piercing conditions as Nos. 8 to 16 in experiment 1, and the wear rate per pass was evaluated by measuring the reduction in the film thickness in the tip region and the rolled region before and after piercing. Experiment 2 will be described with reference to the graph in FIG. 9. In FIG. 9, the body region refers to the rolled region and the bottom region as described above. As shown in the graph in FIG. 9, the wear rate of the tip region gradually increases as the W content increases.

On the other hand, the wear rate of the trunk region, i.e., the rolling region and the bottom region, was 126 μm/Pass when the W content was 0.0 wt%, 88 μm/Pass when it was 0.6 wt%, 50 μm/Pass when it was 1.2 wt%, 65 μm/Pass when it was 2.4 wt%, 128 μm/Pass when it was 4.9 wt%, and 200 μm/Pass when it was 10.5 wt%. Thus, the wear rate of the trunk region gradually slowed down until the W content was 1.2 wt%, and then gradually increased. Then, when the W content reached 4.9 wt%, the wear rate of the trunk region became 128 μm/Pass, which was almost the same as when the W content was 0 wt%. As a result, it was found that the wear rate in the rolling region is slower when the W content is at least 4.0% by weight or less than when the W content is 0% by weight, and is slowest when the W content is 1.2% by weight. In addition, when the W content is 2.4% by weight, the wear rate is almost the same as when the W content is 1.2% by weight, so it is considered that the wear rate of the trunk region can be appropriately slowed by setting the W content to 2.4% by weight or less. Thus, from the viewpoint of wear rate, it was found that the wear rate of the rolling region can be appropriately reduced by setting the W content to 4.0% by weight or less, preferably 3.0% by weight or less, more preferably 2.4% by weight or less, compared to when the W content in the rolling region is 0.0% by weight, i.e., a piercer plug with a long life can be obtained.

(Test 3)
In addition to the above-mentioned Test 2, in Experiment 3, the relationship between the friction coefficient of the thermal spray coating and the content of W was evaluated. Specifically, a high-temperature friction tester (manufactured by Yonekura Seisakusho Co., Ltd.) was used to evaluate the friction coefficient of the Fe-W wire thermal spray coating according to the content of W. A cored wire (Fe-W cored wire) was prepared in which a filler 112, which is a mixed powder containing Fe and W, was enclosed inside an outer shell 111 made of Fe, and a thermal spray coating having a W content of 0 to 14 wt % was prepared to a thickness of 500 μm on the surface of the test piece by arc spraying. The melting temperature of the thermal spray coating containing W was 1120 ° C., as in each test of Experiment 1. The test piece on which this coating was sprayed and a steel test piece heated to a predetermined temperature (1150 ° C. or 1200 ° C.) were brought into contact with each other at a surface pressure of 80 MPa and rotated and slid at a speed of 0.25 m/sec. The friction coefficient was derived from the friction force measured at the sliding distance of 5 m. The results will be described with reference to FIG. 10. The W content shown on the horizontal axis of the graph was measured by chemically analyzing the thermal spray coating. As shown in the graph of FIG. 10, when the W content in the thermal spray coating is 0.5 wt %, the friction coefficient is significantly reduced by heating at a melting temperature of 1120° C. or higher, compared to when the W content is 0.0 wt %. When the W content is 1.2 wt % or higher, the friction coefficient is significantly reduced compared to when the W content is 0.0 wt %. Thereafter, the friction coefficient gradually decreases as the W content increases. Therefore, it was found that if the W content is 0.5 wt % or higher, preferably 0.8 wt % or higher, and more preferably 1.2 wt % or higher, the friction coefficient can be effectively reduced, and the lubricating effect in the rolling region can be more effectively obtained, that is, a piercer plug with a longer life can be obtained.

The results of Experiments 1 to 3 above confirmed that, from the standpoint of wear rate, the W content in the rolling region should be 4.0% by weight or less, preferably 3.0% by weight or less, and more preferably 2.4% by weight or less, and from the standpoint of friction coefficient, the W content should be 0.5% by weight or more, preferably 0.8% by weight or more, and more preferably 1.2% by weight or more.

Reference Signs List 1 Piercer plug 2 Plug body 2a Tip 2b Rear end face 21 Tip portion 22 Rolled portion 23 Bottom portion 3 Thermal spray coating 31 Tip region 32 Rolled region 33 Bottom region 4 Thermal spray coating C Plug axis

Claims (4)

The plug body,
a first thermal spray coating formed on a surface of the plug body,
the plug body includes a front end portion, a bottom portion rearward of the front end portion, and a rolled portion formed between the front end portion and the bottom portion,
the first thermal spray coating includes a tip region formed on a surface of the tip portion, a bottom region formed on a surface of the bottom portion, and a rolled region formed on a surface of the rolled portion,
the tip region comprises Fe, O, and impurities;
The rolling region is composed of Fe, O, W, and impurities,
a W content excluding O in the rolled region is 4.0% by weight or less. 2. The piercer plug of claim 1,
a W content excluding O in the rolled region is 0.5 wt % or more. The piercer plug according to claim 1 or 2,
the bottom region being composed of the same elements as the rolled region. The piercer plug according to any one of claims 1 to 3, further comprising:
a second thermal sprayed coating formed between the plug body and the first thermal sprayed coating,
The second thermal spray coating comprises Fe, O, and impurities.

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