JP2016047546A - Method for manufacturing seamless steel pipe - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method for manufacturing seamless steel with excellent inner surface quality by performing piercing-rolling using a plug for piercing-rolling on seamless steel pipe capable of providing excellent durability as well as self-lubricating property when used to perform piercing-rolling on a seamless pipe, particularly on a high alloy seamless steel pipe.SOLUTION: A method for manufacturing seamless steel pipe comprises: performing, by a piercing mill including a roll and a plug, piercing-rolling on a heated round billet to change it to a hollow tube stock; performing a diameter-reducing and thickness-reducing processing on the hollow tube stock to change it to a steel pipe with predetermined dimensions, using a plug for piercing-rolling on seamless steel pipe comprising: a vapor-deposited film 5 made of Al-Cr-based nitride or Al-Cr-B-based nitride, coating surfaces 4a, 4b of metal matrices; and a tip portion 4a of the metal matrix, made of heat resistant alloy, preferably, Mo-based alloy, Ni-based alloy, or Co-based alloy.SELECTED DRAWING: Figure 1

Description

  TECHNICAL FIELD The present invention relates to a method for producing a seamless steel pipe, and particularly exhibits seamless durability when producing a high alloy seamless steel pipe, and can produce a seamless steel pipe excellent in inner surface quality. The present invention relates to a method of manufacturing a seamless steel pipe that performs piercing and rolling using a plug for use.

  As a representative method for producing a seamless steel pipe in a hot state, a hollow billet is produced by piercing and rolling a round billet heated by a piercing and rolling machine called a Mannesmann Piercer, and a plug mill or a mandrel is produced on the hollow shell. There is a method in which a diameter-reducing process is performed by a drawing mill called a mill to finish a seamless steel pipe having a predetermined size.

  As shown in FIG. 3, the piercing and rolling mill is supported by two inclined rolls 1 that are opposed to each other while being inclined in opposite directions and are rotated in the same direction, and a mandrel 3 that is arranged on a pass line. A plug 2 is provided.

  In the piercing and rolling process by the piercing and rolling machine configured as described above, a Mannesmann crack is generated by the rotary forging effect in the center of the solid round billet B rolled by the inclined roll 1, and this crack is pushed by the plug 2. The hollow shell H is obtained by spreading.

  In the hot piercing and rolling process described above, the plug 2 is constantly exposed to high temperature and high load due to constant contact with the heated round billet B and the hollow shell H, and thus is very easily worn or melted. Therefore, in general, a steel plug is subjected to a scaling process at a high temperature of 900 to 1000 ° C., and a scale film of several tens to several hundreds of μm is formed on the plug surface to prevent wear. However, the above-mentioned steel plugs, when used for piercing and rolling of high alloy steels containing 5% or more of Cr, which has been increasing in demand in recent years, are extremely worn and can only be used for piercing and rolling several round billets. I can't stand it.

  The reason why plug wear is conspicuous in the piercing and rolling of high alloy steel as described above is that the tip of the plug is mainly at the Mannesmann cracked part during the piercing and rolling of the round billet of high alloy steel according to the conventional investigation. It has been clarified that the scale formed on the surface layer is peeled off locally and seizure occurs at the tip of the plug.

  Furthermore, since the scale has a heat insulating action, the plug base material is prevented from being exposed to high temperatures, but the plug tip from which the scale has been stripped is subjected to heat conduction from the round billet, processing heat and The temperature rises due to frictional heat and the like, and high temperature deformation becomes significant. As a result, it is considered that the wear of the plug tip portion proceeds rapidly.

  In order to prevent the seizure and high-temperature deformation that occur at the plug tip as described above, the high-temperature strength of the plug has been improved by changing the plug material or modifying the surface of the plug. Many methods have been proposed for controlling and improving the plug life.

  For example, Patent Document 1 proposes a seamless steel pipe rolling plug in which the entire plug or a plug portion including a part of the outermost surface of the plug is made of ceramic. Patent Document 2 proposes a seamless steel pipe piercing and rolling plug having a niobium alloy containing niobium at 50% by weight or more at the tip and having a silicide layer on the surface thereof.

JP-A-10-180315 JP-A-10-156410

  However, the above-described conventional techniques have the following problems.

  Since the lubricity of the ceramic material and silicide described in Patent Document 1 or 2 is inferior to the lubricity of the scale formed on the surface layer of a conventional steel plug, the plug for seamless steel pipe piercing and rolling is used. When applied to the plug, the lubricity of the plug surface is lowered. Decreasing lubricity on the plug surface causes a decrease in the advance rate of piercing and rolling, and increases the number of round billets that stagnate during piercing and rolling, so that Mannesmann cracks are excessively increased and the internal properties of the hollow shell are deteriorated. . Further, since the frictional heat generation increases due to an increase in the coefficient of friction between the plug surface and the material to be rolled, the temperature of the inner surface of the material to be rolled becomes extremely high particularly in piercing and rolling of high alloy steel, etc. There was a problem that wrinkles such as fogging were likely to occur on the inner surface of the tube.

  Also, in conventional plugs, the scale covers the surface to ensure lubricity, but oxide protrusions are formed on the plug surface, and these protrusions press the inner surface of the material to be rolled, so that after rolling In some cases, a concave wrinkle is formed on the inner surface of the hollow shell.

  Once the above-mentioned concave wrinkles are generated, they remain until the subsequent process and cause deterioration of the inner surface quality of the product pipe. For this reason, there is a problem in that the number of maintenance steps for removing the concave ridges on the inner surface by blasting or the like in the refining process increases, and the manufacturing cost of the steel pipe increases.

  The present invention performs piercing and rolling using seamless steel pipe piercing and rolling plugs that exhibit excellent durability and excellent self-lubricity when piercing and rolling seamless steel pipes, particularly high alloy seamless steel pipes. Thus, an object of the present invention is to provide a method for producing a seamless steel pipe having excellent inner surface quality.

  The inventors conducted hot piercing and rolling of a high billet steel round billet using a seamless steel pipe piercing and rolling plug in which various hard coatings were formed on the surface of a metal base material, and the durability of the plug As a result of intensive studies on the lubricity and hardness of the hard coating in the high temperature range, which is a factor of the inner surface quality of the manufactured steel pipe, the following knowledge was obtained.

  In general, there are many cases in which high melting point ceramics, cermets, or cemented carbides are applied to tool materials in order to have high hardness in a high temperature range and to prevent seizure when machining steel materials.

  However, since the above hard material is inferior in impact resistance, when it is applied to a plug for seamless steel pipe piercing and rolling, it is exposed to high loads such as thermal shock and impact load, and thus it is easily broken. . In particular, it was found that a plug made of a hard material alone was damaged during piercing and rolling, and the main starting point of the damage was the joint between the plug and the mandrel.

  In order to prevent the above problem of breakage, the plug base material is made of a steel material, and the surface of the base material is covered with a hard material, so that the break starting from the joint between the plug and the mandrel In addition, the surface that contacts the round billet and the hollow shell is made a plug having the characteristics of a hard material.

  There are many types of hard coating materials, and among them, CrN-based coating materials are well known to have excellent frictional properties with steel materials and high hardness. In addition, as a film formation method, the physical vapor deposition method does not need to heat the base material to a high temperature, so there is less damage to the base material, and unlike the thermal spraying method, impurities are mixed and internal defects are generated in the film. Therefore, it is widely used as a surface modification technique for cold cutting tools.

  However, since the heat resistant temperature of CrN is generally about 700 ° C., it is not suitable for use in a high temperature environment of 1000 ° C. or higher such as piercing and rolling of seamless steel pipes. Therefore, as a result of studying a method for improving the heat-resistant temperature without losing the excellent characteristics of CrN, it has been found that it is effective to add Al to CrN. Al-Cr nitrides in which Al is added to CrN in a range of 25 to 50% have a heat resistant temperature higher than that of CrN, can be used in a high temperature atmosphere of 1000 ° C or higher, and are hard at room temperature. Increases to about 3000 in Vickers hardness.

Furthermore, the wear resistance in a high-temperature environment was improved by adding B to the Al—Cr-based nitride in a range of 3 to 25%. This improvement in wear resistance is due to the formation of a B ₂ O ₃ layer having self-lubricating properties on the surface of the nitride film by dispersing B in the Al—Cr nitride and being oxidized on the surface of the film. it is conceivable that. Therefore, a CrN film heated to 1000 ° C. by a high-temperature sliding test, an Al—Cr-based nitride film (45% Al-55% Cr in terms of atomic percentage of only the metal component), and an Al—Cr—B-based nitride ( The friction coefficient of the coating of 45% Al-45% Cr-10% B) in terms of atomic% of only the metal component was evaluated. The set load was 5N, and the counterpart material was SUS304. As a result, as shown in Table 1, the coefficient of friction of the CrN film was about 0.45, whereas the coefficient of friction of the Al—Cr nitride film was about 0.3, and Al—Cr— The friction coefficient of the B-based nitride coating was about 0.25. From this result, compared with the CrN coating, the Al—Cr nitride coating has an effect of improving lubricity and hardness, and further Al—Cr in which Al is added in the above range. It is considered that the wear of the Cr—B-based nitride film is further improved because the lubricity is further improved.

  Further, by covering the plug surface with the above Al-Cr nitride or Al-Cr-B nitride, the advance efficiency at the time of piercing and rolling of the seamless steel pipe can be improved without depending on the scale. It is thought that the inner surface quality of the pipe is improved.

  Further, the plug for seamless steel pipe piercing and rolling has a high temperature at the tip of the plug due to an increase in load load and temperature rise due to processing heat generation of the material to be rolled during piercing and rolling of a high alloy steel billet such as 13Cr steel having a large deformation resistance. Deformation increases. At this time, the hard coating coated on the plug surface cannot follow the deformation of the plug base material and may break down. It is clear that it is more desirable for such a breakage of the coating to manufacture the plug tip portion with a heat-resistant alloy such as a Mo base, a Ni base, or a Co base to reduce the high temperature deformation of the plug base material. It was.

  The present invention has been completed on the basis of the above findings and comprises the following gist.

  (1) A heated round billet is pierced and rolled by a piercing and rolling mill composed of a roll and a plug to form a hollow shell, and the hollow shell is subjected to reduction and thickness reduction processing by a drawing mill and a constant diameter rolling mill. In the method of manufacturing a seamless steel pipe having a steel pipe of a predetermined size, the plug is a seamless steel pipe piercing and rolling plug in which a vapor-deposited coating of Al-Cr nitride is formed on the surface of a metal base material. A method for producing a seamless steel pipe.

  (2) The method for manufacturing a seamless steel pipe according to (1), wherein a tip portion of the metal base material is made of a heat-resistant alloy.

  (3) The method for producing a seamless steel pipe according to (1) or (2), wherein the plug has a surface hardness of 3000 to 3500 in terms of Vickers hardness.

  (4) The seamless steel pipe according to any one of (1) to (3), wherein the coating contains Al: 25 to 50% and Cr: 50 to 75% in atomic% of only a metal component. Manufacturing method.

  (5) The method for producing a seamless steel pipe according to any one of (1) to (4), wherein the coating film is formed by a PVD method.

  (6) The method for producing a seamless steel pipe according to any one of (1) to (5), wherein the coating contains B in an amount of 3 to 25% in terms of atomic% of only the metal component.

  (7) Mo: 99.0 to 99.5% Mo-based alloy, Ni: 45 to 65% Ni-based alloy, or Co: 43 to 50% Co-based alloy. The method for producing a seamless steel pipe according to any one of (2) to (6), wherein:

  According to the present invention, even when producing a high alloy seamless steel pipe, piercing and rolling is performed using a seamless steel pipe piercing and rolling plug that exhibits excellent durability and has excellent self-lubricity. Seamless steel pipes with excellent inner surface quality can be manufactured at low cost.

Sectional schematic diagram showing an example of a seamless steel pipe piercing and rolling plug according to the present invention Comparison of inner surface roughness of hollow shell after piercing and rolling Schematic diagram showing the rolling mode by piercing and rolling mill

  Hereinafter, a seamless steel pipe piercing-rolling plug (sometimes simply referred to as a “plug”) according to the present invention will be described with reference to the drawings. FIG. 1 is a schematic cross-sectional view showing an example of a seamless steel pipe piercing and rolling plug according to the present invention. The plug is composed of a tip portion 4a, a main body portion 4b, and an Al—Cr nitride deposited film 5. 1 shows a case where the metal base material of the plug is divided into the main body portion 4b and the tip portion 4a, and the respective parts are made of different materials. However, in the present invention, the main body portion 4b and the tip portion 4a may be integrated. included.

  As described above, the material of the main body portion 4b (including the front end portion 4a when the main body portion 4b and the front end portion 4a are integrated) of the metal base material of the plug prevents damage at the joint portion with the mandrel. The steel material having toughness is used, and the surfaces of the tip portion 4a and the main body portion 4b are covered with hard Al-Cr nitride. When the thickness of the coating is less than 1 μm, the wear resistance is inferior, and when it exceeds 10 μm, chipping and film peeling tend to occur and the life is reduced, so that the thickness is preferably 1 to 10 μm. More preferably, it is 2 to 5 μm, but is not particularly specified.

  Usually, various methods such as chemical vapor deposition (CVD) and physical vapor deposition (PVD) can be applied to form a film such as Al-Cr nitride. In the case of a piercing-rolling plug, since the metal base material hardness decreases and the film adhesion problem occurs depending on the treatment temperature at which the film is formed, an ion plating method which is a kind of PVD method is preferred.

  Moreover, in order to improve the adhesion between the metal base material of the plug and the deposited film of the Al—Cr nitride, the surface hardness of the metal base material of the plug may be increased to 1500 to 1800 in terms of Vickers hardness Hv. preferable. When the metal base material is made of steel, it is common to perform nitriding as a method for hardening the surface. For example, in the case of nitriding such as a salt bath nitriding method, a low temperature side of about 500 to 550 ° C. Can be processed. Therefore, if high-speed steel (SKH) or alloy die steel (SKD) having a tempering temperature of 550 ° C. or higher is selected as the material of the metal base material of the plug, the mechanical properties of the metal base material are obtained by the nitriding treatment. (Strength, toughness, etc.) will not change.

  Furthermore, the surface of the plug tip 4a and the body 4b has a surface roughness of 0.2 μm or less in terms of arithmetic average roughness Ra in order to suppress peeling of the deposited film 5 of Al—Cr nitride. Is preferably finished. When a high load is applied like a plug for seamless steel pipe piercing and rolling, stress concentrates on the uneven part of the interface between the vapor-deposited coating and the metal base material of the plug, in particular, the convex part on the surface of the metal base material. Since the deposited film is destroyed at the starting point, it is preferable to smooth the surface roughness of the metal base material (the tip portion 4a and the main body portion 4b) of the plug.

  As described above, the tip portion 4a of the plug is preferably made of a heat-resistant alloy because it requires a high-temperature strength having a yield strength of 100 MPa or more at the rolling temperature (about 1200 ° C.). More preferably, the Ni-based alloy containing 45 to 65% by mass of Ni, the Mo-based alloy containing 99.0 to 99.5% by mass of Mo, and 43 to 50% by mass of Co. A heat-resistant alloy such as a Co-base alloy is desirable. Ni is less than 45% by mass and more than 65% by mass in the Ni-based alloy, Mo is less than 99.0% and 99.5% by mass in the Mo-based alloy, Co is less than 43% by mass and 50% by mass in the Co-based alloy In the case of exceeding, the high temperature strength decreases.

  The plug tip 4a and main body 4b are mounted in such a manner that the concave and convex portions as shown in FIG. 1 are shrink-fitted, screwed, press-fitted, or press-contacted so that they cannot be rotated around the axis. It may be performed by any of a method of fixed connection or a method of fitting the concave and convex portions to the loose using an appropriate drop-off preventing mechanism and mounting them so that they can rotate around their axes. .

  In the present invention, the deposited film of Al-Cr nitride is required to exhibit high hardness and durability even under a high load environment. It is preferable that 25% or more and 50% or less and Cr be 50% or more and 75% or less.

Furthermore, it is possible to improve the self-lubricating property on the surface of the deposited film by adding B to the Al—Cr-based nitride to obtain a deposited film of the Al—Cr—B based nitride. In this case, B is dispersed as BN in the deposited film. When this BN is oxidized on the surface of the vapor deposition coating, a B ₂ O ₃ layer having self-lubricating properties is formed on the surface of the vapor deposition coating, thereby improving the self-lubricity of the surface of the vapor deposition coating. If B is too small, the self-lubricating property of B ₂ O ₃ cannot be fully expressed. If B is excessive, borides of Cr and Al are precipitated and the deposited film becomes brittle. The ratio is preferably 3% or more and 30% or less in atomic%.

  The seamless steel pipe piercing and rolling plug of the present invention described above is used by being set in a piercing and rolling machine according to a conventional method. By using the plug configured as described above, it is possible to suppress the occurrence of damage such as erosion or erosion in the plug surface layer portion, and therefore it becomes possible to perform piercing and rolling of difficult-to-work materials such as high Cr steel. In addition, since the oxide layer generated on the plug surface has very good lubricity, it is possible to manufacture a seamless steel pipe having excellent advancement efficiency in piercing and rolling and having good inner surface properties.

  Hereinafter, based on an Example, this invention is demonstrated concretely.

  The seamless steel pipe piercing and rolling plug used in the example of the present invention is a plug having a composite structure as shown in FIG. 1, the tip portion 4a of which is a heat resistant alloy having the composition shown in Table 2, and the main body portion 4b of a salt bath. Alloy die steel (SKD61), the surface of which was hardened by nitriding by a nitriding method, and the concave and convex portions of the tip portion 4a and the main body portion 4b were shrink-fitted and fixedly connected. Thereafter, by changing the target material by an ion plating method, an Al-Cr-based nitride vapor-deposited film or an Al-Cr-B-based nitride having a component composition as shown in Table 2 and a film thickness of about 3 μm is used. A plug in which a deposited film was formed on the surface of the main body portion 4b and the tip portion 4a was used as an example of the present invention.

  Further, the plug used as Comparative Example 1 is a low alloy steel integrated plug, similar to the conventional seamless steel pipe piercing and rolling plug, and a scale having a thickness of about 200 μm is formed on the surface thereof by heat treatment. It is a plug.

  Each plug is set in a piercing and rolling machine, and a round billet made of SUS420J2 having an outer diameter of 58 mm and a length of 250 mm heated to 1200 ° C. is pierced and rolled into a hollow shell having an outer diameter of 69 mm, an inner diameter of 54 mm, and a length of 450 mm. Thus, the performance of each plug and the inner surface properties of the hollow shell were evaluated.

  In order to evaluate the durability of the plug, each round billet is subjected to piercing and rolling while visually observing the plug every time one piercing and rolling is completed, and judging whether or not to continue use depending on the degree of plug wear. did.

  Further, as an evaluation index of lubricity, from the length of the hollow shell and the piercing and rolling time, as shown in the formula <1>, the forward efficiency of the piercing and rolling is determined by the piercing direction component of the rolling speed of the rolling roll and the hollow shell. It was calculated as a ratio with the rolling speed. The higher the forward efficiency, the better the lubricity of the plug.

(Forward efficiency) = (L / t) / (Vr × sin θ) <1>
Where L: the length of the hollow shell
t: piercing and rolling time
Vr: Rolling speed of the rolling roll
θ: Inclination angle of rolling roll These calculation results and the results of plug wear after piercing and rolling are also shown in Table 2. Table 2 also shows the results of measuring and averaging the roughness of the plug surface before use.

  As is apparent from Table 2, the plug used in the example of the present invention was able to pierce and roll 10 round billets without damaging the plug, and was able to continue use thereafter. Furthermore, it was found that the plug of the present invention has higher piercing and rolling advancement efficiency than the comparative example and excellent lubricity.

  On the other hand, with the conventional low alloy steel perforated plug as a comparative example, the second round billet could be pierced and rolled without damaging the plug, but after the third round billet was pierced and rolled, The wear was confirmed and could not be used continuously.

  Moreover, the result of having measured inner surface roughness with respect to each hollow shell after piercing-rolling is shown in FIG.

  As is apparent from FIG. 2, in the present invention example, even when the piercing and rolling were repeated, the generation of rice granular protrusions made of oxide on the plug surface was not observed, and the inner surface roughness of all ten hollow core tubes that were pierced and rolled. The arithmetic average roughness Ra was 0.55 μm or less, and no concave wrinkles occurred on the inner surface of the hollow shell.

  On the other hand, the inner surface roughness of the hollow shell obtained by piercing and rolling using the plug of the scale coating as a comparative example is about 5.0 μm in terms of arithmetic average roughness Ra, and the uneven shape of the inner surface of the hollow shell is remarkable. Met.

  As described above, according to the manufacturing method of the present invention, it is possible to efficiently manufacture a high-alloy seamless steel pipe having excellent inner surface quality.

DESCRIPTION OF SYMBOLS 1 Inclined roll 2 Plug for piercing-rolling 3 Mandrel B Round billet H Hollow base tube 4a Tip part 4b of the seamless steel pipe piercing-rolling plug according to the present invention Main body 5 of the plug for seamless steel piercing-rolling according to the present invention Al- Cr-based nitride deposition coating

(6) The method for producing a seamless steel pipe according to any one of (1) to (5), wherein the coating film contains B in an amount of 3 to 30 % in terms of atomic% of only the metal component.

Claims (7)

  A heated round billet is pierced and rolled by a piercing and rolling mill composed of a roll and a plug to form a hollow shell, and the hollow shell is subjected to a reduction in diameter by a drawing mill and a constant diameter rolling mill to obtain a predetermined dimension. In the method for producing a seamless steel pipe, the plug is a seamless steel pipe piercing-rolling plug in which an Al-Cr nitride deposited film is formed on the surface of a metal base material. Manufacturing method of steelless pipe.   The method for manufacturing a seamless steel pipe according to claim 1, wherein a tip portion of the metal base material is made of a heat-resistant alloy.   The method for producing a seamless steel pipe according to claim 1 or 2, wherein the plug has a surface hardness of 3000 to 3500 in terms of Vickers hardness.   The method for producing a seamless steel pipe according to any one of claims 1 to 3, wherein the coating contains Al: 25 to 50% and Cr: 50 to 75% in atomic% of only a metal component.   The method for manufacturing a seamless steel pipe according to any one of claims 1 to 4, wherein the coating is formed by a PVD method.   The method for producing a seamless steel pipe according to any one of claims 1 to 5, wherein the coating contains B in an amount of 3 to 25% in terms of atomic% of only the metal component.   The heat-resistant alloy is either Mo: 99.0 to 99.5% Mo-based alloy, Ni: 45 to 65% Ni-based alloy, or Co: 43 to 50% Co-based alloy by mass%. The method for producing a seamless steel pipe according to any one of claims 2 to 6.

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