JPH10291008A - Tool for hot making tube and its manufacture - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain an inexpensive tool for hot making tubes which is used for a tool for rolling the inside surface at the time of manufacturing a long-size thin-wall seamless tube of carbon steel and low-alloy steel and its manufacturing method. SOLUTION: This tool is the tool for hot making the tubes which has a scale layer on the surface of the substrate made of steel having composition consisting of 0.10 to <0.20% C, 0.01-1.0% Si, 0.3-1.0% Mn, 0.5-4.0% Ni, 1.0-4.0% Cr, 0.05-1.0% Mo, 0-0.040% Al, 0-1.0% W, 0-1.0% Ti, 0-0.01% B, <=0.035% P, <=0.035% S and the balance Fe with impurities and the inner layer of the scale layer which is a spinel type multiple oxide of 50-300 μm thick. In the manufacturing method thereof, the steel having the above-mentioned composition is made into a tool shape by casting, machining after casting and machining after hot- working and, after that, heat treatment for scaling is executed. Shot-peening may be executed before the heat treatment for scaling.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a hot pipe making tool used as an inner rolling tool in piercing and elongation rolling when manufacturing a seamless pipe by a Mannesmann pipe making method, and a method for manufacturing the same. Is a seamless pipe made of carbon steel or low alloy steel by the Mannesmann pipe method,
The present invention relates to a hot pipe making tool which is used as an inner rolling tool for producing a long seamless pipe and has excellent wrinkle and rough surface generation resistance, heat crack generation resistance and crack propagation resistance, and a method of manufacturing the same.

[0002]

2. Description of the Related Art The process of manufacturing a seamless pipe by the Mannesmann pipe method generally uses a rolling machine such as an inclined roll type two-roll piercer, a three-roll piercer, or a roll-type press roll piercing mill. The piercing and rolling process is followed by a stretching and rolling process using an inclined roll type two-roll elongator (stretching machine) or the like. In these rolling steps, an internal rolling tool (a piercer plug, a mandrel, or the like, hereinafter, the “internal rolling tool” is also simply referred to as a “tool”) cannot be omitted.

[0003] Conventionally, as a tool for inner surface rolling when producing a seamless pipe of carbon steel or low alloy steel, for example, a piercer plug which is a tool for piercing and rolling, conventionally, 0.3% by weight of C-3 is used.
% Cr-1% Ni-based steel (hereinafter referred to as 3Cr-1
A steel substrate obtained by cutting a cast material of Ni steel) and forming an oxide scale on the surface thereof has been used. This is because the oxide scale has the function of increasing the heat insulation of the tool surface, preventing the tool surface from being exposed to high temperatures, and improving the lubricity at high temperatures. In the following description of the present specification, the “outer surface” is simply referred to as “surface”, and the “inner surface” is particularly referred to as “inner surface”.

[0004] However, when a seamless pipe of stainless steel or high Cr steel is manufactured, the deformation resistance at the pipe forming temperature is high.
Therefore, if a tool in which an oxide scale is formed on the surface of the above 3Cr-1Ni steel base is used for the production of a seamless tube of stainless steel or high Cr steel, the tool life becomes extremely short and the cost is reduced. Will increase. For this reason, the present inventors have previously proposed, in Japanese Patent Application Laid-Open No. 7-60314, "a tool for hot pipe making and a method of manufacturing the same" used for manufacturing a seamless pipe of stainless steel or high Cr steel. .

[0005] In addition, C: 0.
20-0.40%, Si: 0.30-1.00%, M
n: 0.30 to 1.00%, Cr: 1.00 to 4.00
%, Ni: 0.50 to 4.00% is known as a general 3Cr-1Ni steel.

On the other hand, recently, a thin and long seamless pipe made of carbon steel or low alloy steel has been increasingly manufactured by the Mannesmann pipe system. In the case of manufacturing a thin and long seamless pipe, the surface pressure applied to the tool increases due to the increase in the perforation length, and the surface temperature also increases, so that the tool life is significantly reduced. For this reason, when manufacturing a thin-walled and long-seamless seamless pipe of carbon steel or low alloy steel, the above-mentioned Japanese Patent Application Laid-Open No.
It is conceivable to use the tool proposed in Japanese Patent No. 0314 to improve the tool life.

[0007] However, the tool proposed by the present inventors in the above-mentioned gazette contains at least one of Mo and W in weight% for a total of 1.%.
The material cost is high because it contains 5 to 8.0%, and the cost increases because the heat treatment conditions for scaling are complicated. Therefore, the life of a tool made of a steel base having an inexpensive composition based on a conventional type 3Cr-1Ni steel is improved, and a thin and long carbon steel or low alloy steel made of a carbon steel or a low alloy steel is manufactured by using the tool. It is desired to produce a seamless tube.

[0008]

DISCLOSURE OF THE INVENTION The present invention has been made in view of the above-mentioned circumstances, and has been developed to manufacture a seamless pipe of carbon steel or low alloy steel, especially a thin-walled and long seamless pipe by a Mannesmann pipe manufacturing method. An object of the present invention is to provide an inexpensive hot pipe making tool excellent in wrinkle and rough surface generation characteristics, heat crack generation characteristics, and crack growth resistance used as an internal rolling tool when performing the inside rolling tool, and a method for manufacturing the same.

[0009]

The gist of the present invention resides in a method for manufacturing a hot pipe making tool shown in the following (1) and a hot pipe making tool shown in the following (2) to (4).

(1) By weight%, C: 0.10% or more.
Less than 20%, Si: 0.01 to 1.0%, Mn: 0.3
1.0%, Ni: 0.5 to 4.0%, Cr: 1.0 to
4.0%, Mo: 0.05-1.0%, Al: 0-0.
040%, W: 0 to 1.0%, Ti: 0 to 1.0%,
B: 0 to 0.01%, P as an impurity: 0.035%
Hereinafter, S: 0.035% or less, the balance having a scale layer on the surface of a steel substrate having a composition of Fe and unavoidable impurities, and an inner layer of the scale layer being a spinel-type composite oxide having a thickness of 50 to 300 μm. A hot pipe making tool characterized by the following.

(2) C: 0.10% to 0.1% by weight.
Less than 20%, Si: 0.01 to 1.0%, Mn: 0.3
1.0%, Ni: 0.5 to 4.0%, Cr: 1.0 to
4.0%, Mo: 0.05-1.0%, Al: 0-0.
040%, W: 0 to 1.0%, Ti: 0 to 1.0%,
B: 0 to 0.01%, P as an impurity: 0.035%
In the following, S: 0.035% or less, the balance being steel having a composition of Fe and unavoidable impurities is machined by casting or after casting into a tool shape, and then heat treatment for scaling is performed. Manufacturing method for hot pipe making tools.

(3) By weight%, C: 0.10% or more.
Less than 20%, Si: 0.01 to 1.0%, Mn: 0.3
1.0%, Ni: 0.5 to 4.0%, Cr: 1.0 to
4.0%, Mo: 0.05-1.0%, Al: 0-0.
040%, W: 0 to 1.0%, Ti: 0 to 1.0%,
B: 0 to 0.01%, P as an impurity: 0.035%
In the following, S: 0.035% or less, the balance being steel having a composition of Fe and unavoidable impurities is machined after hot working to form a tool shape, and then heat treatment for scaling is performed. A method for manufacturing tools for pipe making.

(4) The method for producing a hot pipe making tool according to the above (2) or (3), wherein shot peening is performed before heat treatment for scaling after forming the tool shape.

Hereinafter, the above (1) to (4) may be referred to as the inventions of (1) to (4), respectively.

[0015] As described above, the above "surface" refers to the "outer surface".

The spinel-type composite oxide is an oxide having a spinel-type crystal structure, and has a composition of F
In addition to e, for example, those containing other alloy elements such as Cr.

[0017]

BEST MODE FOR CARRYING OUT THE INVENTION In order to solve the above-mentioned problems, the present inventors have first made a tool (hereinafter simply referred to as 3Cr-1Ni steel) having an oxide scale formed on the surface of a 3Cr-1Ni steel substrate.
Using a plug as a steel tool), various thin and long seamless pipes of carbon steel and low alloy steel were manufactured by the Mannesmann pipe method, and the process until the plug reached the end of its life was investigated.

As a result, the life of a plug as a tool made of 3Cr-1Ni steel in the case of manufacturing a conventional seamless pipe is limited to the opening of a cracked portion starting from wrinkle flaws and rough skin (hereinafter, simply referred to as wrinkles). The life of the plug when manufacturing a thin and long seamless pipe is not only due to surface seizure due to wrinkles, but also to the fine It has been found that, in many cases, the life is extended based on a thermal crack as a starting point, which further becomes a fatigue crack and propagates to open a crack or break the plug body.

In other words, in order to improve the life of a 3Cr-1Ni steel tool when manufacturing a thin and long seamless pipe, it is necessary to suppress the formation of wrinkles, the heat crack initiation characteristics and the crack resistance. It turns out that progress is necessary.

Then, various carbon steels and low carbon steels are manufactured by a Mannesmann tube method using a plug having an oxide scale formed on the surface of a steel base having various chemical compositions based on 3Cr-1Ni steel as a basic composition. A thin and long seamless pipe of alloy steel was manufactured, and the surface portion and the inside (hereinafter, referred to as a base material portion) of the plug were investigated in detail. As a result, the following (a) to
The matter of (i) became clear.

(A) When the toughness and ductility of the steel used as the material of the plug are increased, the progress of cracks due to thermal cracks can be suppressed, so that the life of the plug is improved.

(B) If the high-temperature deformation resistance of the steel used as the material of the plug is increased, the generation of wrinkles can be suppressed, so that the life of the plug can be extended.

(C) The surface of the plug is locally aged by the rise in temperature during pipe production, particularly the surface at the tip of the plug is austenitized, and is hardened significantly by burning in the subsequent cooling process. Due to this curing, the strength during hot work is increased, so that wrinkles can be avoided. However, on the other hand, when the room temperature tensile strength of the hardened portion is higher than 1600 MPa, ductility and toughness are reduced, so that thermal cracks are more likely to occur,
Once a thermal crack has occurred, the crack will grow rapidly.

(D) On the other hand, except for the hardened layer at the surface of the plug, the base material portion, which has been subjected to a temperature rise during pipe production on the inner surface side of the plug, undergoes temper embrittlement and the toughness of the portion is significantly reduced. . Incidentally, the toughness of the base material portion where the tempering embrittlement has occurred is even lower than the toughness of the hardened layer on the plug surface.

(E) The crack in the plug penetrates through the hardened layer on the surface and reaches the embrittled layer on the base material. In an extreme case, the crack reaches the inner surface.

(F) To increase the toughness of the hardened surface layer,
It is most effective to lower the content of C to reduce the hardness as-quenched.

(G) To improve the toughness of the base material embrittlement layer described in (d) above, an appropriate amount of Mo is added to steel, or the contents of P and S as impurity elements are reduced. Is effective.

(H) From the viewpoint of the recyclability of tools (steel materials), it is desirable to add Mo which can increase the toughness without reducing P and S. The addition of Mo not only improves toughness but also increases high-temperature strength, and is therefore effective in reducing wrinkles caused by plastic flow on the plug surface.

(I) A tool which has been subjected to hot working under appropriate conditions and processed into a tool shape (that is, a tool manufactured according to the invention of (3)) has reduced segregation and refined crystal grains. Tool that has been cast into a tool shape by casting or tool that has been tool shaped by casting and machining (ie (2)
The tool and the ductility produced according to the invention of the present invention are excellent in toughness and ductility. In the case of the invention (3) in which hot working is performed when working into a tool shape, Si as a steel composition is used.
, A better toughness can be secured.

The present invention has been completed based on the above findings.

Hereinafter, each requirement of the present invention will be described in detail. In addition, “%” of the component content means “% by weight”.

(A) Chemical composition of steel C: 0.10% or more and less than 0.20% C has the effect of increasing the high-temperature strength and hardenability of steel. Further, it has an effect of forming a hardened portion on the surface of the tool to enhance wear resistance. However, its content is 0.10%
If it is less than 1, the tensile strength at room temperature of the hardened surface layer is 1000M.
Since the pressure is lower than Pa, the surface of the tool is greatly deformed during pipe production, and a desired pipe production operation may not be performed. On the other hand, when the C content is 0.20% or more, the tensile strength at room temperature of the hardened surface portion layer exceeds 1600 MPa, so that both ductility and toughness deteriorate. Therefore, the content of C is 0.1
0% or more and less than 0.20%.

Si: 0.01 to 1.0% Si reduces the width and depth of the austenitic region on the tool surface due to the deoxidizing action of steel and the Ac ₁ point, thereby increasing the temperature during pipe making. It has the effect of preventing the deterioration of toughness and ductility due to hardening in the subsequent cooling process. Further, it has the effect of densifying the oxide scale layer on the tool surface. However, if the content is less than 0.01%, the effect of addition is poor. on the other hand,
If it exceeds 1.0%, the toughness is deteriorated and the thickness of the scale layer on the tool surface is reduced, so that the lubricity is reduced. Therefore, the Si content is set to 0.01 to 1.0%.

Incidentally, the deoxidizing action of steel, the action of raising the Ac ₁ point,
In order to sufficiently secure the effect of densifying the oxide scale layer on the tool surface, the content of Si is preferably 0.3% or more.

In a steel having a basic composition of 3Cr-1Ni steel, when the content of Si is less than 0.15%, the effect of improving toughness is particularly large. For this reason, when excellent toughness is particularly required for the tool, the Si content is set to 0.1%.
It may be less than 5%. In addition, the Si content is 0.15%
In the case where the shape is suppressed to less than 1, it is preferable to adopt the method according to the invention of (3) in which hot working is performed when forming the tool shape.

Mn: 0.3 to 1.0% Mn is an element effective for increasing the hardenability of steel and increasing the thickness of a hardened surface layer. In order to secure the effect, it is necessary to use 0.
Requires a content of 3% or more. However, when the content exceeds 1.0%, remarkable deterioration of toughness is caused. Therefore,
The Mn content was set to 0.3 to 1.0%.

Ni: 0.5 to 4.0% Ni enhances the hardenability of steel to increase the thickness of the hardened surface layer. Unlike other elements, Ni has the effect of increasing the toughness of the hardened surface layer and the base material. Have. Further, it remains as metal particles in the scale layer on the tool surface, and also has the effect of increasing the peel resistance of the scale. However, if the content is less than 0.5%, it is difficult to obtain the above effect. On the other hand, if the content exceeds 4.0%, the transformation point (Ac ₁ point) is greatly reduced to a temperature of 700 ° C. or less, and the high-temperature strength is reduced. Therefore, Ni
Was set to 0.5 to 4.0%.

Cr: 1.0 to 4.0% Cr enhances the hardenability of steel to increase the thickness of a hardened surface layer, and at the same time, densifies the oxide scale layer on the tool surface to increase the adhesion to the base material. It is an effective element. However, if the content is less than 1.0%, it is difficult to form a surface hardened layer. In addition, since the scale becomes porous and peels off, the tool life is shortened. On the other hand, when Cr is contained in excess of 4.0%, toughness is deteriorated. Furthermore, since the oxidation resistance is excessively improved, the thickness of the oxide scale layer does not become sufficiently large, and the protection effect of the tool by the oxide scale layer cannot be secured. Therefore, the content of Cr is set to 1.0 to 4.0.
%.

Mo: 0.05 to 1.0% Mo has the effect of improving the hardenability and high-temperature strength of steel and improving the toughness. Further, Mo also has the effect of increasing the adhesion of the oxide scale layer on the tool surface to the base material. In particular, a steel having a basic composition of 3Cr-1Ni steel is effective in preventing embrittlement of a portion that has been subjected to a temperature rise during tempering or pipe making. However, the content is 0.0
If it is less than 5%, the effect of addition is poor. On the other hand, if it exceeds 1.0%, the effect of improving high-temperature strength and toughness is saturated because it remains as undissolved carbide, and coarse carbides rather deteriorate toughness. Therefore, the content of Mo is set to 0.05 to 1.0%. In order to surely obtain the above-mentioned effects, the content of Mo is preferably set to 0.10% or more.

Al: 0 to 0.040% Al may not be added. If added, it has the effect of deoxidizing the steel and increasing the toughness, especially the toughness of the surface hardened layer. To ensure this effect, the content of Al is preferably set to 0.005% or more. However, when its content is 0.1.
If it exceeds 040%, the strength at high temperatures is reduced. Therefore, the content of Al is set to 0 to 0.040%. Here, the content of Al refers to the content as so-called “sol. Al”.

W: 0 to 1.0% W may not be added. If added, it has the effect of increasing the high-temperature strength. To ensure this effect, W should be equal to 0.
The content is preferably 1% or more. However, if the content exceeds 1.0%, the effect of improving high-temperature strength and toughness is saturated because it remains as undissolved carbide, and coarse carbides rather deteriorate toughness. Therefore, the content of W is set to 0 to 1.0%.

Ti: 0 to 1.0% Ti need not be added. If added, it has the effect of refining the structure of the steel and increasing the room temperature strength and the high temperature strength.
To ensure this effect, the content of Ti is preferably set to 0.05% or more. However, its content is 1.
If it exceeds 0%, the toughness is remarkable and lowers. Therefore,
The content of Ti was set to 0 to 1.0%.

B: 0 to 0.01% B may not be added. If added, it has the effect of refining the structure of the steel and increasing the room temperature strength and the high temperature strength. To ensure this effect, the content of B is preferably set to 0.0020% or more. However, when its content is 0.1.
If it exceeds 01%, the toughness decreases. Therefore, B
Was set to 0 to 0.01%.

In the present invention, the contents of P and S as impurity elements are limited as follows.

P: not more than 0.035% P deteriorates the toughness of the surface hardened layer and the base material, and particularly when the content exceeds 0.035%, the toughness is remarkably deteriorated.
Therefore, the content of P as an impurity element is set to 0.03.
5% or less. If the content of P is 0.01% or less, extremely good toughness can be obtained. Therefore, when extremely good toughness is required for the tool, the content of P may be restricted to 0.01% or less.

S: 0.035% or less S also deteriorates the toughness of the surface hardened layer and the base material, and particularly when the content exceeds 0.035%, the toughness is significantly reduced.
Therefore, the content of S as an impurity element is set to 0.03
5% or less. When the S content is 0.01% or less, extremely good toughness can be obtained. Therefore, when extremely good toughness is required for the tool, the content of S may be restricted to 0.01% or less.

Since N as an impurity element causes solidification defects during smelting, its content is preferably regulated to 0.02% or less.

(B) Scale of the Tool Surface The tool surface (the surface of the steel substrate of the hot pipe making tool) is oxidized for scaling. In this case, the scale layer is generally divided into two layers. That is, the outer layer has a relatively porous scale mainly composed of FeO. On the other hand, the inner layer is a dense scale layer made of a composite oxide having a spinel-type crystal structure and having excellent exfoliation resistance. The interface between the inner layer scale and the base material exhibits a grain boundary oxidation type.

Of the above-mentioned scale layers, the outer layer scale is easily peeled off and does not contribute to the tool life, and the above-mentioned inner layer scale affects the tool life. And, when the thickness of the dense inner layer scale made of the spinel-type composite oxide is 50 to 300 μm, the tool life is improved.

When the thickness of the inner layer scale is less than 50 μm, the effect of increasing the lubricity of the scale at a high temperature and the effect of increasing the heat insulating property of the tool surface to prevent the tool surface from being exposed to a high temperature as described above. , The tool life cannot be improved. On the other hand, if the thickness of the inner layer scale exceeds 300 μm and is too thick, the inner layer scale is porous and easily peeled, so that the tool life cannot be improved.
Therefore, in the present invention, the inner layer of the scale layer has a thickness of 5 mm.
It was defined as a spinel / composite oxide type scale of 0 to 300 μm.

As the heat treatment for performing the above-mentioned scaling on the tool surface, for example, steam:
5 to 25%, CO ₂ : 10 to 15%, O ₂ : 5% or less in an atmosphere of 900 to 1200 ° C. in an atmosphere of 1 to 1%.
It is preferable to perform a process of holding for 0 hour and gradually cooling at a cooling rate of 100 ° C./h or less so that the scale does not crack or peel off.

(C) Forming into Tool Shape In the production of hot pipe making tools (piercer piercing plugs, elongator rolling plugs, plug mill rolling, etc.), shaping into a tool shape is performed by the following method.

The steel having the chemical composition of the above (A) is melted and cast, and is cast as it is, or is machined after casting to finish to a required shape (the method of the invention of (2)).

A steel ingot or a steel slab having the chemical composition of (A) is hot worked by hot forging or hot rolling to be roughly formed, and then machined to finish it into a required shape (( 3) Invention method).

In the above-mentioned forming method according to the invention (2), the casting and machining methods are not particularly limited, and may be performed by ordinary methods. Furthermore, in the above-mentioned forming method according to the invention of (3), the method of manufacturing the steel ingot or the billet and the method of machining are not particularly limited, and may be performed by ordinary methods.

When hot working is performed under the following conditions in the above-described forming method, the toughness and / or crack propagation resistance is improved as compared with the case of forming by the above-described method, so that the tool life is improved.

That is, in the above-mentioned hot working, if the forming ratio at the time of working is large, the crystal grains can be refined. In particular, when the forming ratio is 1.2 or more, the crystal grain refining effect is large and the toughness and / or the crack resistance is high. Progressability is improved. For this reason,
In the above hot working, the steel ingot or the billet is
After heating to a temperature of 000 to 1250 ° C, a molding ratio of 1.
It is preferable to perform hot working so as to be 2 or more.

If the heating temperature is less than 1000 ° C., the deformation resistance of the steel is large, so that the crack susceptibility becomes high. For this reason, cracks are generated particularly in a rough formed material having a forming ratio of 1.2 or more, and a desired tool can be obtained. This is because it may not be possible. on the other hand,
When the heating temperature exceeds 1250 ° C, the high-temperature ductility deteriorates,
In particular, when processing with a molding ratio of 1.2 or more is performed, the heating temperature becomes 1
This is because, as in the case where the temperature is lower than 000 ° C., hot working into a rough molding material may not be possible.

The "forming ratio" indicates (A0 / A) when A0 is the cross-sectional area before processing and A is the cross-sectional area after processing.

(D) Shot blasting After forming into a predetermined tool shape by the method described in (C) above, a shot blasting treatment is performed, and then a heat treatment for scaling is performed. Can greatly enhance the tool's protective action by the oxide scale layer (scale inner layer). Therefore, if you want to further increase the tool life,
A shot blast treatment may be performed before the heat treatment for scaling (invention (4)).

The shot blasting method according to the invention (4) is not particularly limited, and may be performed by an ordinary method.

The hot pipe making tool according to the invention of (1) is formed by the method of (C), subjected to the shot blasting of (D) if necessary, and then scaled by (B). Being manufactured.

Hereinafter, the effect of the present invention will be described more specifically with reference to an example, taking a piercing plug for a piercer as a tool for hot pipe making.

[0064]

EXAMPLE Steel having the chemical composition shown in Table 1 was prepared by a conventional method.
0 kg atmosphere furnace melting (Steel A, Steel C, Steel D, Steel F, Steel G, Steel I and Steel KL) or 3 ton electric furnace melting (Steel B, Steel E,
Steel H, Steel J and Steel P).

Steels C to I in Table 1 are steels of the present invention,
Steel A, steel B, steels J to N and steel P are steels of comparative examples in which any of the components deviated from the content specified in the present invention. In addition, among the steels of the comparative examples, the steels A and B are the conventional 3Cr-1.
Ni steel.

[0066]

[Table 1]

Of the steel melted from a 500 kg atmosphere furnace, steel A,
Steel D, steel F, steel G and steel K to N are cast into a mold by a usual method, and steel C and steel I are machined after casting into a mold by a normal method. Maximum outer diameter 187
mm piercer plug shape.

Thereafter, the steel F, the steel G, the steel K, and the steel L were not subjected to shot blasting, and the steels A, C,
After performing shot blasting on steel D, steel I, steel M and steel N by a usual method, all of them were in a volume% and steam: 20%.
_{%, CO 2: 12.5%,} O 2: In 1% of the atmosphere, and held for 5 hours at 950 ° C., a heat treatment was carried out scale with slow cooling at a cooling rate of 50 ° C. / h.

On the other hand, electric furnace melted steel B, steel E, steel H,
Regarding steel J and steel P, a billet having a diameter of 220 mm obtained by hot forging by a usual method was heated to 1200 ° C., then hot forged at a forming ratio of 1.2, and further machined. The piercer plug had a maximum outer diameter of 187 mm. Thereafter, steel B and steel H were not subjected to shot blasting, and steel E, steel J and steel P were shot blasted in the usual manner. In an atmosphere of 12% CO _{2 and} 1% O ₂ , a heat treatment for scaling was performed in which the sample was kept at 950 ° C. for 5 hours and gradually cooled at a cooling rate of 50 ° C./h.

For each steel, the surface scale layer and mechanical properties were investigated using a part of the piercer plug obtained as described above.

For the surface scale layer, the thickness of the inner layer was measured by observing the cross section of the tip of the piercer plug with a microscope, and the scale structure was specified. The mechanical properties were as follows: a tensile test specimen with a parallel part diameter of 6 mm from the piercer plug, J
A 2 mm V notch Charpy impact test piece of IS4 and a test piece for measuring high temperature deformation resistance of 8 mm in diameter and 10 mm in length were sampled, and the tensile strength (TS), draw (RA) and Charpy absorbed energy ( _V ) at room temperature (20 ° C.) were obtained. E _20),
In addition, the deformation resistance during compression at 900 ° C. was measured.

Further, the piercer plug obtained as described above was subjected to a piercing test on an actual machine to evaluate its life. That is, a JIS 45C round billet having an outer diameter of 225 mm and a length of 2800 mm heated to 1250 ° C. was pierced and rolled to a thickness (wall thickness) of 16 mm and a shell length of 10 m by a usual method. The life of a piercer plug is evaluated by the number of billets that can be pierced by one plug, that is, the number of piercings, and the unevenness due to cracks and wrinkles on the plug surface increases,
When it was determined that the plug was damaged the next time the hole was drilled during drilling, or when the drilling did not proceed halfway and the pipe could not be produced, the plug life was reached. The piercer plug evaluated as having reached the end of its life was visually inspected for the cause of the end of its life (life cause).

Table 2 shows the results of the investigation of the surface scale layer and the mechanical properties described above. The inner layers of the scale layers were all spinel-type composite oxides. Table 3 shows the results of a survey on the life of the piercer plug. The cause of the service life is `` heat crack '' when the crack occurs due to the heat crack and the service life is reached, and `` wrinkle '' when the unevenness due to the wrinkles at the tip becomes large and the service life is reached. When the cracks developed and penetrated and reached the end of their life, they were indicated as "penetrating cracks".

[0074]

[Table 2]

[0075]

[Table 3]

The following items are clear from Tables 2 and 3.

Conventional 3Cr-1Ni steel (steel A and steel B)
Has been used as a tool material for seamless pipes of carbon steel and low alloy steel for many years. However, when a conventional piercer plug made of 3Cr-1Ni steel is used to make a thin and long hole having a thickness (thickness) of 16 mm and a length of 10 m, the surface pressure applied to the tool (piercer plug) is high. The surface temperature also rises, so the life is shortened. After about 500 uses, cracks due to thermal cracks occur (Steel A), or unevenness due to wrinkles at the tip becomes large (Steel B), and the life is shortened. Reached.

On the other hand, in the case of a piercer plug using the steel of the present invention, RA (ductility) at room temperature and _V E ₂₀
(Toughness) is high, and the hot deformation resistance at 900 ° C. is also large. Therefore, the growth of cracks due to thermal cracks is suppressed, and the formation of wrinkles is suppressed, so that the life of the plug using the steel of the present invention is 3 times longer than the life of the conventional plug using 3Cr-1Ni steel. The number of perforations (plug life) of at least 689 was obtained.

In the case where a plug is formed by machining after hot working, a longer plug life is obtained as compared with a case in which the plug is formed as it is by casting or by further machining after casting.

On the other hand, among the steels of the comparative examples, steels K to N and steel P
When the piercer plug using the, RA (ductility) and _V E ₂₀ at room temperature or cracking (toughness) was due to the low thermal cracking is (steel K, Steel L and steel N), thermal cracking It progressed and reached through cracks (steel M and steel P), and the life was reached. Further, in the case of a piercer plug using steel J among the steels of the comparative example, plastic flow occurs on the plug surface due to low high-temperature deformation resistance at 900 ° C., and the unevenness of wrinkles caused by the flow increases, and the life is shortened. Had been reached.

Next, a tensile test piece having a diameter of 6 mm from the tip end of the piercer plug which has reached the end of its life and JIS 4
No. of harvested 2mmV notch Charpy impact test pieces were measured TS, RA and _V E ₂₀ at room temperature (20 ° C.).

The tip of the plug is an austenitized region due to a rise in temperature during drilling, and is a region hardened by burning in the subsequent cooling process.

Table 4 shows the test results. Table 4
Is the target performance in
As described in the content section, TS is 1000 to 16
It means that it is 00 MPa.

[0084]

[Table 4]

Comparison between Table 4 and Table 2 shows that the RA
It can be seen that (ductility) is generally good in spite of the remarkably high strength. That is, the strength-ductility balance at room temperature of the hardened region in Table 4 was compared with the strength-ductility balance at room temperature of the plug before being subjected to the piercing test with the actual machine shown in Table 2, that is, the plug as-scaled. Then it is good.

On the other hand, it can be seen that the _V E ₂₀ (toughness) of the hardened region changes greatly depending on the strength and the alloying elements. That is, while keeping the contents of C, P and S low, M
For steel C~I invention sample which contains the o, toughness since TS is high in the cured regions _(V E ₂₀₎ is lower than the toughness of the plug before being subjected to drilling test in actual table 2 . However, the value is large, exceeding 32J.

[0087] However, in the case of steel of the comparative example one of the components is outside the content defined in the present invention, despite TS is high in curing zone, the _V E ₂₀ is the actual machine in Table 2 Some of the plugs have a toughness equivalent to or even higher than the toughness of the plug before being subjected to the piercing test. Nevertheless, the toughness of the hardened area is at most 20
J is smaller than the steel of the present invention. In the steel of the comparative example, this deterioration in toughness promotes the growth of thermal cracks at the tip and causes a reduction in plug life.

[0088]

The tool for hot pipe making of the present invention is excellent in wrinkle generation resistance, heat crack generation resistance and crack propagation resistance.
Since the tool life is long, it can be used as an inexpensive internal rolling tool when manufacturing seamless pipes of carbon steel or low alloy steel, especially thin and long seamless pipes by the Mannesmann pipe method. This hot pipe making tool can be manufactured relatively easily by the manufacturing method of the present invention.

Claims (4)

[Claims] C: 0.10% or more and 0.20% by weight%
, Si: 0.01-1.0%, Mn: 0.3-1.
0%, Ni: 0.5 to 4.0%, Cr: 1.0 to 4.0
%, Mo: 0.05 to 1.0%, Al: 0 to 0.040
%, W: 0 to 1.0%, Ti: 0 to 1.0%, B: 0 to 0%
0.01%, P as an impurity: 0.035% or less,
S: 0.035% or less, with the balance having a scale layer on the surface of a steel substrate having a composition of Fe and unavoidable impurities, and the inner layer of the scale layer being a spinel-type composite oxide having a thickness of 50 to 300 μm. A hot pipe making tool characterized by the following. 2. C: 0.10% or more and 0.20% by weight%
, Si: 0.01-1.0%, Mn: 0.3-1.
0%, Ni: 0.5 to 4.0%, Cr: 1.0 to 4.0
%, Mo: 0.05 to 1.0%, Al: 0 to 0.040
%, W: 0 to 1.0%, Ti: 0 to 1.0%, B: 0 to 0%
0.01%, P as an impurity: 0.035% or less,
S: 0.035% or less, with the balance being Fe or a composition of unavoidable impurities, cast or machined after casting to form a tool shape, and then heat-treated for scaling. Manufacturing method of pipe making tool. 3. C: 0.10% to 0.20% by weight
, Si: 0.01-1.0%, Mn: 0.3-1.
0%, Ni: 0.5 to 4.0%, Cr: 1.0 to 4.0
%, Mo: 0.05 to 1.0%, Al: 0 to 0.040
%, W: 0 to 1.0%, Ti: 0 to 1.0%, B: 0 to 0%
0.01%, P as an impurity: 0.035% or less,
S: 0.035% or less, the balance being steel made of a composition of Fe and unavoidable impurities, hot-worked, machined into a tool shape, and then subjected to heat treatment for scaling, which is characterized by hot working. Manufacturing method for pipe tools. 4. The method for producing a hot pipe making tool according to claim 2, wherein shot peening is performed before heat treatment for scaling after forming the tool shape.