JP4348703B2 - Seamless steel pipe manufacturing method - Google Patents

Description

  The present invention relates to a method for manufacturing a seamless steel pipe using a mandrel mill and a drawing rolling apparatus, and more particularly to a method for manufacturing a seamless steel pipe that can easily and effectively suppress carburization that may occur on the inner surface of the pipe.

  As a method for producing a seamless steel pipe, various methods such as a mandrel mill method, a plug mill method, a Eugene Segerne method, and an Erhard push bench method are known. Among these methods, a mandrel mill method that is excellent in all aspects such as productivity, dimensional accuracy, and inner and outer surface quality is widely used.

  In the manufacturing method of the seamless steel pipe by the mandrel mill method, as shown in FIG. 1, after heating the billet 1 to a predetermined temperature (generally 1100 to 1300 ° C.) in the heating furnace 2, The hollow shell 4 is punched and the hollow shell 4 is stretched and rolled by a mandrel mill 5.

  In the mandrel mill 5, a mandrel bar 6 coated with a rolling lubricant containing graphite is inserted and the hollow shell 4 is stretched and rolled. The hollow shell 4 stretched and rolled by the mandrel mill 5 is reheated to a predetermined temperature (generally 850 to 1000 ° C.) in a reheating furnace 7 as necessary, and then stretch stretcher, sizer, etc. Finish rolling is performed by the drawing rolling device 8.

  As the rolling lubricant, a binder containing an organic substance mainly composed of graphite is generally used.

  Here, when the hollow shell 4 is made of a low carbon concentration material such as austenitic stainless steel (SUS304, SUS316, etc.), the mandrel bar 6 coated with a lubricant containing graphite is inserted and stretch-rolled. (And reheating the blank 4 after rolling) causes carburization, and a carburized layer having a carbon concentration higher than that of the base material is formed on the inner surface of the blank 4. Hereinafter, the mechanism in which carburization occurs will be described more specifically with reference to FIG.

  First, when starting rolling in the mandrel mill 5, if the mandrel bar 6 is inserted into the hollow shell 4, the graphite (C) contained in the lubricant applied to the surface of the mandrel bar 6 is converted into the inner surface of the shell 4. Adhere to. In this state, as shown in FIG. 2A, the graphite (C) is only adhered on the scale on the inner surface of the base material, and the carburization does not occur because the graphite and the base material are not in direct contact.

  However, by starting rolling in the mandrel mill 5 and by pulling out the mandrel bar 6 from the raw tube 4, the scale on the inner surface of the base material is destroyed, and as shown in FIG. ) And the inner surface of the base material are in direct contact with each other.

  Accordingly, carburization occurs from the contact portion between the graphite (C) and the base material (and carburization proceeds by reheating the raw tube 4 in this state in the reheating furnace 7), and the carbon concentration is higher than that of the base material. A high carburized layer will be formed.

  If the carburized layer described above remains in the seamless steel pipe as a product, for example, when the base pipe 4 is made of carbon steel, a local abnormally hardened portion is generated on the inner surface of the seamless steel pipe and the machinability is lowered. There is a problem. For example, when the raw tube 4 is made of austenitic stainless steel, there is a problem that the corrosion resistance such as intergranular corrosion resistance is lowered.

  Therefore, conventionally, various methods for suppressing carburization of the inner surface of the seamless steel pipe have been proposed.

For example, it has been proposed to control the amount of graphite adhering to the mandrel bar surface to 100 mg / m ² or less when drawing and rolling with a mandrel mill (see, for example, Patent Document 1).

However, as a result of intensive investigations by the inventors of the present invention, as proposed in Patent Document 1, suppressing the graphite adhesion amount to a very small amount of 100 mg / m ² or less usually uses a graphite lubricant together. It is very difficult for a simple production line (due to the effects of mandrel bars, adhesion from the production line, grease present in the production line, etc.), and even if it can be achieved, it is very costly and not effective. is there.

  In addition, a method of removing a lubricant and a carburized layer remaining on the inner surface of the raw tube after rolling with a mandrel mill using an abrasive or high-pressure water has been proposed (for example, Patent Documents 2, 3, 4, 5). reference).

  However, the method of removing the carburized layer or the like using an abrasive has the problems that the cost of the abrasive such as a grindstone increases and that the time required for polishing is reduced and the production efficiency decreases. In addition, the method of removing the lubricant and the like using high-pressure water has a problem that uneven cooling is likely to occur, and there is a possibility that an operation trouble due to bending of the raw tube may occur in a reheating furnace or the like.

  Furthermore, a method for suppressing carburization (or promoting decarburization) by regulating the furnace temperature and atmosphere in a reheating furnace or the like has also been proposed (see, for example, Patent Documents 6, 7, 8, and 9).

However, since carburization occurs at a very narrow contact portion between graphite and the inner surface of the base material, even if the inside of the base material is filled with an oxidizing atmosphere, carburization is effective unless the atmosphere at the contact portion is changed by aeration. Therefore, there is a problem that decarburization cannot be effectively promoted. In general, reheating furnaces often adopt a transverse feed method using a walking beam, so it is difficult to vent the inside of the base material as it is. It is necessary to modify the equipment. In addition, when low-temperature air or the like is ventilated, there is also a problem that the basic unit of the fuel increases, and as a result, the manufacturing cost of the seamless steel pipe increases. Furthermore, since a scale is generated on the inner surface of the base material due to the oxidizing atmosphere, there is a problem that it is difficult to promote decarburization.
JP 2000-24706 A Japanese Patent Laid-Open No. 4-111907 JP-A-6-182427 JP-A-8-224611 JP 2001-105007 A JP-A-4-168221 JP-A-8-57505 JP-A-8-90043 JP-A-9-201604

  The present invention has been made to solve the problems of the prior art, and an object of the present invention is to provide a method for producing a seamless steel pipe that can easily and effectively suppress carburization that may occur on the inner surface of the pipe. To do.

  In order to solve the above-mentioned problems, the inventors of the present invention have made intensive studies. As a result, the amount of graphite adhering to the mandrel bar surface is limited, and if the inner diameter reduction ratio of the raw tube in the drawing mill is increased, the effect is simple and effective. Thus, the present inventors have found that carburization can be suppressed (decarburization can be promoted) and the present invention has been completed.

That is, the present invention is a method for producing a seamless steel pipe using a mandrel mill and a drawing rolling apparatus, and the amount of graphite adhering on the mandrel bar surface used for rolling in the mandrel mill is set to 100 g / m ² or less. At the same time, the inner diameter reduction ratio of the raw pipe in the drawing mill is set to 15% or more, the temperature of the raw pipe on the inlet side of the drawing mill is set to 900 ° C. or more, and from the start of rolling in the drawing mill to the end of rolling. The present invention provides a method for producing a seamless steel pipe characterized in that the elapsed time is set at a rolling speed at which the elapsed time is 12 seconds or more .

Incidentally, preferably, before the start of rolling in the reducing rolling device, water or oxidizing gas is injected toward the advance element inner surface.

According to the method for manufacturing a seamless steel pipe according to the present invention, by limiting the amount of graphite adhering on the mandrel bar surface to 100 g / m ² or less, the occurrence of carburization in the mandrel mill is suppressed, and the drawing rolling apparatus In addition to being able to exfoliate part of the graphite together with the scale on the inner surface of the base material by increasing the inner diameter reduction ratio of the raw tube to 15% or more, the new surface of the base material exposed by the scale peeling is the atmosphere ( Decarburization is effectively promoted because it is in direct contact with air) and has good air permeability because of vertical feed. Further, it is not necessary to modify the equipment of the reheating furnace, and it is only necessary to adjust the reduction ratio in the existing drawing rolling apparatus, so that it can be easily implemented. Therefore, it has an excellent effect that carburization that can occur on the inner surface of the pipe can be easily and effectively suppressed (accelerating decarburization).

  Hereinafter, embodiments of the present invention will be described with reference to the accompanying drawings as appropriate.

The method of manufacturing a seamless steel pipe according to an embodiment of the present invention can be implemented using existing equipment as shown in FIG. 1, but various parameters are not set conventionally. Yes. More specifically, in the present embodiment, the graphite adhesion amount on the surface of the mandrel bar 6 used for rolling in the mandrel mill 5 is set to 100 g / m ² or less, and a drawing rolling device (stretch reducer or sizer) is used. Etc.) The inner diameter reduction ratio of the raw tube 4 in 8 is set to 15% or more. In order to set the graphite adhesion amount to 100 g / m ² or less, for example, a method of using a lubricant containing a solid lubricant other than graphite to reduce the graphite concentration, or reducing the graphite concentration of the lubricant Thus, it is possible to apply a method of cleaning the mandrel bar 6 and the production line in order to prevent the adhesion of graphite from the mandrel bar 6 to be used and the production line through which the mandrel bar 6 passes. In addition, the fact that the graphite adhesion amount is set to 100 g / m ² or less means that the amount of adhesion on the inner surface of the raw tube 4 after cooling, the atmosphere on the inner surface of the raw tube 4, etc. It can be confirmed by a method or the like of 100 g / m ² or less in advance.

By limiting the amount of graphite adhering on the surface of the mandrel bar 6 to 100 g / m ² or less, the occurrence of carburization in the mandrel mill 5 is suppressed, and the inner diameter reduction ratio of the raw tube 4 in the drawing mill 8 is 15 By increasing to more than%, part of the graphite can be peeled off along with the scale on the inner surface of the base material, and the new surface of the base material exposed by the peeling of the scale is in direct contact with the atmosphere (air) and is fed vertically. Since air permeability is good for conveyance, decarburization is effectively promoted (see FIG. 3). In addition, it is not necessary to modify the equipment of the reheating furnace 7, and it is only necessary to adjust the reduction ratio in the existing drawing rolling apparatus 8, so that it can be easily implemented. Therefore, according to the manufacturing method according to the present embodiment, carburization that can occur on the inner surface of the seamless steel pipe can be easily and effectively suppressed (decarburization is promoted). The inner diameter reduction ratio is a value defined by the following expression (1), where di is the raw pipe inner diameter on the inlet side of the drawing mill 8 and do is the inner diameter on the outlet side.
Inner diameter reduction ratio = (di-do) / di × 100 (%) (1)

  In order to further promote the decarburization in the drawing mill 8, the raw tube 4 rolled by the mandrel mill 5 is heated in the reheating furnace 7, and the temperature of the raw tube 4 on the entry side of the drawing rolling device 8. Is preferably set to 900 ° C. or higher.

  Further, in the drawing mill 8, the elapsed time from the start of rolling to the end of rolling in the drawing mill 8 in order to further promote decarburization by ensuring a sufficient time for the atmosphere on the inner surface of the raw tube 4 to vent. (That is, after the front end portion of the blank 4 is caught in the rolling roll of the foremost stand of the drawing mill 8 until the rear end of the blank 4 comes out of the rolling roll of the last stand of the drawing mill 8. Rolling is preferably performed at a rolling speed at which (elapsed time) is 12 seconds or more.

Furthermore, in order to further promote decarburization in the drawing mill 8, it is preferable to inject water or an oxidizing gas in advance toward the inner surface of the raw tube 4 before starting rolling in the drawing mill 8. As the oxidizing gas, for example, O _2, CO _2, air, water vapor, it is possible to illustrate these mixed gas.

  Hereinafter, the features of the present invention will be further clarified by showing examples.

<Example 1>
Using the equipment shown in FIG. 1, the adhesion amount of graphite on the surface of the mandrel bar 6 used for rolling in the mandrel mill 5 is set to 100 g / m ² or less, and the inner diameter reduction ratio of the raw tube 4 in the drawing mill 8 is set. A seamless steel pipe was produced under the condition of 15% or more.

<Example 2>
In addition to the conditions of Example 1, the temperature of the raw tube 4 on the entry side of the drawing apparatus 8 was set to 900 ° C. or higher.

<Example 3>
In addition to the conditions of Example 2, the rolling speed in the drawing apparatus 8 was set to a rolling speed at which the elapsed time from the start of rolling to the end of rolling was 12 seconds or more.

<Example 4>
In addition to the conditions of Example 2, before starting rolling in the drawing mill 8, water or steam was sprayed in advance toward the inner surface of the raw tube.

<Comparative example>
A seamless steel pipe was produced under the conditions that did not satisfy the conditions of Example 1.

<Evaluation>
About each seamless steel pipe (product) manufactured according to the conditions of Examples 1-4 and a comparative example, the carburization condition of the inner surface was evaluated. The evaluation results are shown in Table 1. In addition, in the column of “Carburization evaluation” shown in Table 1, “x” means that acid roughening due to significant carburization occurred when the produced seamless steel pipe was pickled. In addition, “△” indicates that acid roughening did not occur, but when the seamless steel pipe is made of an extremely low carbon material (SUS304L or other material with C ≦ 0.03%), heat treatment for decarburization is performed on the product or element. This means that even if it was applied to the pipe (the raw pipe after the drawing and rolling in the drawing apparatus 8), the required corrosion resistance specifications could not be satisfied. In addition, “◯” represents a case where the seamless steel pipe is made of an extremely low carbon material if heat treatment for decarburization is performed on the product or the raw pipe (the raw pipe after the drawing and rolling in the drawing apparatus 8). This also means that the required corrosion resistance specifications were satisfied. Furthermore, “◎” means that when the seamless steel pipe is made of an extremely low carbon material, the required corrosion resistance specifications can be satisfied without performing heat treatment for decarburization.

  As shown in Table 1, under the conditions of Example 1 (Example 1-1 to Example 1-6), the evaluation is “Δ”, and carburization is suppressed to a level that can satisfy the specifications except for extremely low carbon materials. I understood that I could do it. Moreover, in the conditions of Example 2 (Example 2-1 to Example 2-4), the evaluation was “◯”, and it was found that carburization can be suppressed by heat treatment even for an extremely low carbon material. Furthermore, in the conditions of Example 3 (Example 3-1 to Example 3-3) and Example 4 (Example 4-1 to Example 4-2), the evaluation is “◎”, which is an extremely low carbon material. Even so, it was found that carburization can be suppressed without heat treatment.

  On the other hand, under the conditions of the comparative example (comparative examples 1 and 2 have an inner diameter reduction ratio deviating from the conditions of example 1, and comparative examples 3 and 4 have a graphite adhesion amount deviating from the conditions of example 1) The evaluation was “x”, and carburization could not be suppressed.

Drawing 1 is an explanatory view for explaining the manufacturing process of the seamless steel pipe by a mandrel mill system. FIG. 2 is an explanatory diagram for explaining a mechanism in which carburization occurs. FIG. 3 is an explanatory diagram for explaining that decarburization is promoted by increasing the inner diameter reduction ratio of the raw tube in the drawing mill.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 ... Material billet 2 ... Heating furnace 3 ... Piercer 4 ... Hollow element pipe 5 ... Mandrel mill 6 ... Mandrel bar 7 ... Reheating furnace 8 ... Drawing rolling apparatus

Claims (2)

A method for producing a seamless steel pipe using a mandrel mill and a drawing rolling device,
While setting the graphite adhesion amount on the mandrel bar surface used for rolling in the mandrel mill to 100 g / m ² or less,
The inner diameter reduction ratio of the raw pipe in the drawing mill is set to 15% or more, the temperature of the raw pipe on the entry side of the drawing mill is set to 900 ° C. or more, and the process from the start of rolling to the end of rolling in the drawing mill A method for producing a seamless steel pipe, characterized in that the rolling speed is set at a time of 12 seconds or more . The method for producing a seamless steel pipe according to claim 1 , wherein water or an oxidizing gas is sprayed toward the inner surface of the raw pipe before starting rolling in the drawing mill.

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