JPH11197717A - Plug mill plug for manufacturing seamless steel tube - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a plug mill plug for manufacturing a seamless steel tube having excellent durability capable of improving the quality of an inner surface of a high alloy steel tube manufactured by especially Mannesmann plug mill rolling and reducing its manufacturing cost. SOLUTION: In the plug mill plug for manufacturing the seamless steel tube wherein a scale adding heat treatment is applied after melting steel and casting it into a plug shaped mold, the steel is composed of, by weight, >=0.3% to <0.9% C, 0.1-3%, Si, 0.2-2% Mn, <=0.05% P, <=0.06% S, 0.5-15% Ni, 10-30% Cr, 3-6% Mo, 3-6% W, 0.5-10% Co, 0.015-0.6% Ti, 0.05-2% Nb, 0.01-0.1% sol. Al, 0.005-0.02% B and the balance Fe with inevitable impurities.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a tool for manufacturing a high alloy steel pipe manufactured by Mannesmann plug mill rolling, and more particularly to a tool having a high durability for the purpose of improving the quality of the inner surface of the steel pipe and reducing the manufacturing cost. It relates to a good plug mill plug for manufacturing seamless steel pipes.

[0002]

2. Description of the Related Art In recent years, in the mining of oil and gas, mining in deeper and more corrosive gas wells has increased.
AISI4 used for seamless pipes for these oil country tubular goods
The production of 13Cr steel equivalent to 20 has been increasing. Among the methods for producing seamless steel pipes in the past, the Mannesmann plug mill method has been widely practiced because of its high productivity.In this method, a round steel slab heated and soaked at a predetermined temperature is first used. After forming a hollow shell with a piercer rolling mill as a first punch, the hollow shell is stretched with an elongator rolling mill and a plug mill as a second punch. Although the plugs used for each are subjected to heat treatment for scaling to prevent seizure with the material to be rolled, the amount of scale generated on the surface of the material to be rolled during heating before rolling and during rolling is high for high Cr steel. It has less resistance than carbon steel and high deformation resistance of high Cr steel compared to carbon steel when hot, and the scale of the plug surface is easily worn and damaged, effectively preventing seizure between the plug and the material to be rolled. It becomes difficult to do. In particular, seizure between the plug mill plug and the material to be rolled is likely to occur, increasing the cost of the plug due to a shortened life of the plug, and further deteriorating the quality of the inner surface of the product due to deformation of the plug surface. This tends to increase costs.

As means for increasing the life of a plug mill plug during rolling of a high alloy steel, Japanese Patent Application Laid-Open No. Sho 59-204 discloses a method.
No. 63, JP-B-62-14632, a plug made of a cast alloy is subjected to immersion treatment and scaling heat treatment, so that even when the surface scale is consumed, the immersion formed on the plug surface is reduced. Methods have been reported in which the layer prevents seizure of the plug and the material to be rolled. In this method, Mo, W, Co,
A method for adding V, Nb, and Zr is disclosed. In addition, Kawasaki Steel Technical Report vol. 13, N
o. In No. 13, from 1.5 to 3.
It has been reported that 0% Ni addition is effective.

[0004]

However, in order to extend the life of the plug mill plug, it is essential that the scale formed on the plug surface remains, and with the disappearance of this scale, even if the scale is burned down on the plug surface. Even if the immersion layer is formed for the purpose, the life of the plug cannot be significantly improved. That is, the strength of the plug is reduced by increasing the heating temperature of the plug surface due to the disappearance of the surface scale having the heat insulating effect. In addition, the effect of preventing burning of the plug and the material to be rolled is smaller in the case of the immersion layer than in the case of the scale, and the immersion layer alone cannot sufficiently prevent the burning. Also, to increase the hot strength of the plug mill plug,
Although the addition of Mo, W, Co, V, Nb, Zr, O, Co, and Ni alone can suppress the deformation of the plug surface layer, the impact stress applied to the plug during rolling and the plug surface at that time are reduced. During rapid heating of the plug and rapid cooling of the plug surface after use, the surface of the plug is peeled off due to the occurrence of cracks on the surface of the plug mill plug, and the life of the plug mill plug cannot be improved.

Further, as a result of intensive studies by the present inventors,
The steel used so far for plug mill plugs is a high C-high Cr stainless steel casting alloy. The cast structure of this alloy is such that carbides crystallize between dendrite dendrites, and the dendrite dendrites have carbides in the martensite structure. Is a distributed organization. Since the turtle cracks described above propagate along the crystallized carbide between the dendrite dendrites, it is difficult to prevent the generation and progress of the turtle cracks if the crystallized carbides are present.

SUMMARY OF THE INVENTION An object of the present invention is to solve the above-mentioned problems, in particular, to improve the quality of the inner surface of a high-alloy steel pipe manufactured by Mannesmann plug mill rolling and reduce the manufacturing cost. An object of the present invention is to provide a good plug mill plug for manufacturing a seamless steel pipe.

[0007]

In order to solve the above problems and achieve the object, the present invention uses the following means. (1) The plug of the present invention is a plug mill plug for producing a seamless steel pipe obtained by melting steel, casting it into a plug-shaped mold, and then performing heat treatment for scaling.
And C: 0.3% or more and less than 0.9%, Si: 0.1 to
3%, Mn: 0.2 to 2%, P: 0.05% or less, S: 0.06% or less, Ni: 0.5 to 15%,
Cr: 10 to 30%, Mo: 3 to 6%, W: 3 to 6
%, Co: 0.5 to 10%, and Ti: 0.015
0.6%, Nb: 0.05-2%, sol. Al:
A plug mill plug for seamless steel pipe production, comprising 0.01 to 0.1% and B: 0.005 to 0.02%, the balance being Fe and unavoidable impurities. . (2) The plug of the present invention is further selected as a steel component from the group of V: 0.05 to 2%, Zr: 0.05 to 2%, and N: 0.2% or less by weight%. The plug mill plug for producing a seamless steel pipe according to the above (1), characterized in that it contains one or more kinds.

[0008]

BEST MODE FOR CARRYING OUT THE INVENTION The inventors of the present invention have conducted intensive studies to solve the above-mentioned problems, and as a result, the generation of cracks in the surface layer of a plug during the Mannesmann plug mill rolling and the suppression of the progress thereof have been suppressed. In order to achieve this, the C content is reduced to less than 0.9% to reduce the amount of crystallized carbide generated in the dendrite structure, and Ti is added to the molten steel before casting to form TiN.
Alternatively, TiC is precipitated to reduce the size of the cast structure to suppress the occurrence and progress of turtle cracks. In addition, the addition of Co increases the amount of Nb, W, and Mo carbides to be precipitated, thereby increasing the room temperature to about It has been found that increasing the strength of the plug in the temperature range of 1100 ° C. is effective. Based on the above findings, the present inventors have developed a plug mill plug made of a low C-high Cr stainless cast alloy and having its surface subjected to a heat treatment for scaling, at the heating temperature of the plug surface layer during rolling. In order to impart high temperature strength and toughness to an impact load, and to suppress the progress of cracks on the plug surface when the plug is used, C, Co, Ni,
By properly controlling the amount of Ti, a plug mill plug for producing a seamless steel pipe having good durability was found, and the present invention was completed.

[0009] That is, the present invention, by limiting the steel composition to the following range, it is possible to improve the quality of the inner surface of high alloy steel pipes produced especially by Mannesmann plug mill rolling and to reduce the production cost. Of the present invention can provide a plug mill plug for producing a seamless steel pipe.

The reasons for adding the components of the present invention and the reasons for limiting the components will be described below. (1) Component composition range C: 0.3% or more and less than 0.9% C is an essential element for increasing the room-temperature strength and the high-temperature strength of the plug, and it is necessary to contain 0.3% or more of C. Yes. On the other hand, if it is added in an amount of 0.9% or more, the toughness is reduced, and the crystallized carbide is crystallized at the time of casting. Therefore, the content is 0.3% or more and less than 0.9%.

Si: 0.1 to 3% Si is a deoxidizing element. If its content is less than 0.1%, its effect is small, and if its content exceeds 3%, the oxide scale thickness on the plug surface is reduced. And reduce the life of the plug. Therefore, its content is 0.1-3%. Mn: 0.2 to 2% Mn is added to increase the high-temperature strength. However, if it is less than 0.2%, its effect is small, and if it exceeds 2%, a martensitic structure cannot be obtained in the plug, and the plug strength is reduced. descend.
Therefore, its content is 0.2-2%. P: 0.05% or less P is an unavoidable impurity element and segregates at the grain boundary during austenite heating, lowering the grain boundary strength and remarkably lowering the plug strength. Therefore, its upper limit is 0.0
5%.

S: 0.06% or less S is an unavoidable impurity element and segregates at the grain boundary during austenite heating, lowering the grain boundary strength and remarkably deteriorating the plug strength. Therefore, its upper limit is 0.0
6%.

Ni: 0.5 to 15% Ni is a solid solution strengthening element. If it is less than 0.5%, its effect is small, and if it exceeds 15%, the amount of scale generation is reduced and the life of the plug is remarkably increased. descend. Therefore, its content is 0.5 to 15%. Cr: 10 to 30% Cr is an element which remarkably enhances the strength of the plug as a carbide. When the content is less than 10%, a sufficient amount of Cr carbide cannot be deposited, and the normal temperature strength and the high temperature strength of the plug are insufficient. 3
If it exceeds 0%, the amount of scale generated on the plug surface will decrease, and the effect of preventing burning of the plug and the material to be rolled will not be obtained, and the life of the plug will decrease. Therefore, its content is 1
0 to 30%. Mo: 3 to 6% Mo is an element that increases the high-temperature strength by dissolving in steel or precipitating as Mo carbide. Further, it is an element that increases the toughness of steel at normal temperature or tempering temperature. If the amount of Mo is less than 3%, the desired effect cannot be obtained. On the other hand, if it exceeds 6%, the toughness is reduced. Therefore, its content is 3-6%. W: 3 to 6% W is an element which becomes carbide and remarkably enhances the strength of the plug. If it is less than 3%, a sufficient amount of carbide cannot be deposited, and the ordinary temperature strength and high temperature strength of the plug are insufficient. If it exceeds 6%, the toughness will decrease. Therefore, its content is 3-6%.

Co: 0.5 to 10% Co is an important element in the present invention. That is, the addition of Co enhances the high-temperature strength by solid solution strengthening, and the addition of Co increases the precipitation amount of Nb carbide, W carbide, and Mo carbide, thereby increasing the plug strength at room temperature and tempering temperature range. Has the effect of enhancing. In addition, Co remains in the scale on the plug surface to increase the strength of the scale and to reduce the consumption of the scale.

If the content of Co is less than 0.5%, the above-mentioned effects cannot be sufficiently obtained. On the other hand, if the content of Co exceeds 10%, the manufacturing cost of the plug is increased, but the effects are saturated. Therefore, its content is 0.5-10%. Ti: 0.015 to 0.6% Ti is the most important element in the present invention together with Co.

Ti becomes TiN or TiC in molten steel and acts as a solidification nucleus of the dendrite at the time of casting to suppress the generation of a dendrite having an orientation. Since the oriented dendrite structure promotes the progress of turtle cracks at the surface layer of the plug mill plug, the reduction of the directional property of the cast structure by adding Ti suppresses the progress of turtle cracks and the plug surface. Effectively works to secure the scale of
If less than 0.015%, this effect cannot be obtained, and if Ti is added in excess of 0.6%, TiC,
TiN becomes the starting point of cracking, and promotes cracking of the plug,
This will hinder its life. Therefore, the content is 0.015 to 0.6%. Nb: 0.05 to 2% Nb is an element that enhances room-temperature and high-temperature strength by precipitating as NbC. If it is less than 0.05%, a predetermined increase in strength cannot be obtained. On the other hand, if the addition exceeds 2%, the toughness is reduced and the plug is cracked. Therefore, its content is 0.05-2%
It is. B: 0.005 to 0.02% B is an element that becomes a nitride and increases high-temperature strength.
If less than 0.005%, the effect cannot be obtained, and 0.02%
In the case where the ratio exceeds 1, the toughness is deteriorated, and the plug is broken. Therefore, the content is 0.005 to 0.02%. sol. Al: 0.01 to 0.1% Al is a deoxidizing element and is an element necessary for securing the ductility and toughness of steel. If it is less than 0.01%, a desired effect cannot be obtained. If the content exceeds 0.1%, the effect is saturated, or conversely, the ductility and the toughness are reduced. For this reason, the content of Al is 0.01 to 0.1%.

In the present invention, the life of the plug mill plug can be improved by further containing one or more of the following contents of V, Zr, and N. V: 0.05 to 2% V is added to increase the high-temperature strength. However, if the content is less than 0.05%, the effect cannot be obtained, and if it exceeds 2%, the toughness is reduced, and the plug is broken. Is generated. Therefore, its content is 0.05-2%.

Zr: 0.05 to 2% Zr is an element which becomes a nitride and enhances the high-temperature strength. If it is less than 0.05%, the effect cannot be obtained, and if it exceeds 2%, the toughness is increased. This leads to deterioration and breakage of the plug. Therefore, its content is 0.05-2%. N: 0.2% or less N is an unavoidable impurity. If the content exceeds 0.2%, shrinkage cavities are formed in the plug during casting, thereby lowering the strength of the plug. It may cause breakage. Therefore, the upper limit is 0.2%.

The manufacturing conditions are not specified in the present invention. That is, the method of melting steel during plug production,
The casting method and the scale heat treatment method may be any conditions that are usually employed. Hereinafter, the effects of the present invention will be proved by giving examples of the present invention.

[0020]

EXAMPLE A 500 kgf steel having a chemical composition shown in Table 1 was melted in the air and cast into a plug mill plug having a maximum diameter of 168 mm. The cast plug is finished by mechanical grinding at the part that comes into contact with the material to be rolled, and then heat-treated by scaling in a steam oxidation atmosphere to produce 19 plug mill plugs of each steel, 18 of which 13
Used for rolling Cr seamless steel pipe. About the other one, Vickers hardness was measured and the thickness of the surface scale was measured with the optical microscope.

The scale on the plug surface is composed of an inner layer scale and an outer layer scale. Since the inner layer scale prevents burning of the plug and the material to be rolled, the thickness of the inner layer scale was measured.

Further, a test piece having a diameter of 10 mm and a height of 15 mm was sampled and subjected to a compression test at a test temperature of 700 ° C. at a strain rate of 10 / sec. The measurements were taken to determine the high-temperature strength of each steel.

The steam oxidation heat treatment is performed at 1050 to 1
After maintaining at 150 ° C. for about 5 hours, the temperature was gradually cooled to a temperature of 600 ° C., and then carried out of the heat treatment furnace and allowed to cool in the air.
13Cr steel rolling is 170mm in diameter and 3280mm in length
Was pierced by a first piercing machine, stretched by a second piercing machine, and further 188 mm in diameter × 12.9 by a plug mill rolling mill.
mm hollow x 11000mm long hollow shell with 184mm diameter x
The film was stretched to a thickness of 10.47 mm x 13540 mm.

The life of the plug was evaluated by the number of passes until the plug became unusable due to the amount of wear due to seizure of the plug surface, the breakage of the plug or the deformation of the surface layer of the plug for each pass. The amount of wear of the plug when it cannot be used for rolling is 1.5 to 2.0 mm.

In Table 1, No. Nos. 1 to 4 are steels of the present invention. 5 to 13 are comparative steels. Table 2 shows the Vickers hardness, the inner layer scale thickness, the average deformation resistance at 700 ° C., and the Vickers hardness of the plug mill plug of each chemical component.
The average value of the life (number of passes) of each plug mill plug when rolling a Cr seamless steel pipe is shown.

As is clear from Table 2, the steel No. of the present invention.
In Nos. 1 to 4, plug strengths at normal temperature and high temperature are obtained by adding appropriate amounts of C, Cr, Co, Mo, W, and Nb. Further, since the dendrite structure is refined by the addition of Ti, the occurrence of cracks during use is prevented, the amount of wear of the plug is reduced, and the occurrence of plug breakage is prevented.

On the other hand, the comparative steel No. The life of plugs made of steel Nos. 5 to 13 was determined according to the steel No. of the present invention. The values are smaller (1 to 16) than the life of the plugs made of steel Nos. 1 to 4.
Comparative steel No. Nos. 5, 6, and 8 to 10 have high amounts of C, Si, Nb, Ti, and W, respectively, which cause a decrease in toughness at room temperature, and cracks occur in the plug during use.

Comparative steel No. In No. 7, the Ni content was less than the claimed range of the present invention, so the toughness was insufficient, and the plug was cracked during use. Comparative steel No. 11 and 12
In this case, since the amount of C or Cr is low, the room-temperature strength of the plug is insufficient, and the surface of the plug is deformed, thereby shortening the life of the plug.

The comparative steel No. In No. 13, since the amount of Cr is high, the scale thickness of the plug surface generated at the time of heat treatment for scaling before use is insufficient, and scale lubrication during use of the plug cannot be obtained. I have.

[0030]

[Table 1]

[0031]

[Table 2]

[0032]

As described above, according to the present invention, the life of a plug mill plug for manufacturing a seamless steel pipe can be improved by specifying the steel composition. Accordingly, it is possible to provide a plug mill plug for producing a seamless steel pipe having good durability, which can improve the quality of the inner surface of a high alloy steel pipe produced by Mannesmann plug mill rolling and reduce the production cost.

 ────────────────────────────────────────────────── ─── Continued on the front page (72) Inventor Motoharu Yamazaki 1-2-1, Marunouchi, Chiyoda-ku, Tokyo Inside Nihon Kokan Co., Ltd.

Claims (2)

[Claims] 1. A plug mill plug for producing a seamless steel pipe obtained by melting steel, casting it into a plug-shaped mold, and then subjecting the steel to heat treatment for scaling, wherein the steel is expressed by weight% and C: 0.3%. % Or more and less than 0.9%,
Si: 0.1-3%, Mn: 0.2-2%, P:
0.05% or less, S: 0.06% or less, Ni: 0.
5 to 15%, Cr: 10 to 30%, Mo: 3 to 6%
And W: 3 to 6%, Co: 0.5 to 10%, and Ti:
0.015 to 0.6%, Nb: 0.05 to 2%, and s
ol. Al: 0.01 to 0.1%, B: 0.005 to
A plug mill plug for seamless steel pipe production, characterized in that it contains 0.02% and the balance consists of Fe and inevitable impurities. 2. The steel composition further comprises, by weight%, V:
0.05-2%, Zr: 0.05-2%, and N: 0.
The plug mill plug for producing a seamless steel pipe according to claim 1, wherein the plug mill plug contains one or more selected from the group of 2% or less.