JP2006082100A - Method for improving service life of mandrel bar - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method by which the service life of a mandrel bar is improved without bringing about the deterioration of the manufacturing efficiency and thickness accuracy of a seamless steel tube. <P>SOLUTION: This method is a method by which the service life of the mandrel bar which is used for rolling with a mandrel mill is improved and which includes a first process for removing a material which is coagulated on the outer surface of the mandrel bar used for rolling and of which a tube stock is composed and a second process for forming an iron hydroxide film on the outer surface of the mandrel bar. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to a method for improving the life of a mandrel bar used when producing a seamless steel pipe using a mandrel mill, and in particular, a mandrel bar life that does not cause a decrease in the production efficiency and wall thickness accuracy of the seamless steel pipe. It relates to an improvement method.

  In the production of a seamless steel pipe by the so-called Mannesmann-mandrel mill method, first, a round steel piece heated by a rotary hearth type heating furnace is punched by a punching machine to form a hollow thick-walled raw pipe. Next, a mandrel mill comprising a plurality of stands each having a plurality of perforated rolling rolls disposed in each stand in a state where a mandrel bar (hereinafter referred to as a bar as appropriate) having a lubricant applied to the surface is inserted into the raw tube. 1 is rolled to reduce the outer diameter and the wall thickness to form a long element tube having a predetermined size.

  Here, after the rolling by the mandrel mill is completed, the mandrel bar is pulled out from the raw tube by a burst slipper and is used in a circulating manner. That is, the bar pulled out from the raw tube is sent to the bar cooling zone and cooled to an appropriate temperature, and then the surface is coated with a lubricant and used for the next rolling.

  The raw tube formed by rolling with a mandrel mill is reheated in a reheating furnace as necessary, and then subjected to drawing with an outer diameter by a drawing mill such as a sizer or stretch reducer, so that the product is seamless. Made of steel pipe.

  In the manufacture of seamless steel pipes by the Mannesmann-Mandrel mill method described above, the ratio of the mandrel bar cost to the manufacturing cost is large. Therefore, it is extremely important to reduce the manufacturing cost of the seamless steel pipe by improving the life by regenerating the bar whose surface condition has deteriorated by repeated rolling.

  More specifically, as shown in FIG. 7, at the initial stage of rolling (FIG. 7A), the mandrel bar 10 is sufficiently lubricated on the surface of the mandrel bar 10 (in practice, the scale coating 20 is lubricated). Although the agent is applied (not shown in FIG. 7), the scale film 20 is gradually scraped by being repeatedly subjected to rolling, and the final stage of rolling (FIG. 7). At the stage where many of the same mandrel bars are used as shown in (b)), the scale film 20 becomes very thin. When continuously used in this state (overused state shown in FIG. 7 (c)), a convex portion in the rolling direction of the raw tube starts from the corner of the heat crack 11 generated on the bar 10 surface. Will give rise to. When the mandrel bar 10 in which this biting has occurred is subjected to the next rolling, if the rolling direction of the raw tube is opposite to the direction of the convex portion, seizure occurs between the bar 10 and the raw tube, Since part of the constituent material adheres to the surface of the bar 10, the mandrel bar 10 that has reached such a surface state can no longer be used for rolling. Therefore, it is possible to regenerate the mandrel bar whose surface state has deteriorated in this way (return the surface state to a state close to the initial stage of rolling), and to be able to use it repeatedly (improve its life). It is extremely important in reducing.

  Various methods have been proposed as methods for improving the service life of the mandrel bar. For example, after removing seizure generated on the outer surface of the mandrel bar used for rolling, the original tempering temperature of the mandrel bar is removed. There has been proposed a method characterized in that a scale film is formed by performing a heat treatment (see, for example, Patent Document 1).

  According to the method described in Patent Document 1, although the life of the mandrel bar can be improved, it has the following problems.

  That is, since the method described in Patent Document 1 is a method of generating a scale film by performing heat treatment, there is a problem that the production efficiency of the seamless steel pipe is affected by the capacity of the heat treatment furnace. In other words, unless the capacity of the heat treatment furnace for heat treatment is increased, the amount of heat treatment per unit time of the mandrel bar is restricted, resulting in a decrease in mandrel bar circulation efficiency and, in turn, a decrease in production efficiency of seamless steel pipes. There is a problem of being invited.

In addition, since the bar surface is softened by heat-treating the mandrel bar, when rolling in the mandrel mill, especially in a stand having a large rolling load, the outer diameter of the mandrel bar is reduced by the reduction, so that the raw tube to be rolled However, there is a problem that the desired thickness is not obtained (the thickness varies in the axial direction of the raw tube).
JP 2002-361305 A

  The present invention has been made to solve such a problem of the prior art, and provides a method capable of improving the life of a mandrel bar without causing a decrease in the production efficiency and thickness accuracy of seamless steel pipes. The issue is to provide.

  In order to solve the above-mentioned problems, the inventors of the present invention have intensively studied. As a result, if an iron hydroxide film is formed on the outer surface of the mandrel bar, the life equivalent to that when a scale film (iron oxide film) is formed is obtained. It was found that the production efficiency and thickness accuracy of seamless steel pipes were not reduced. The present invention has been completed based on the knowledge of the inventors.

  That is, the present invention is a method for improving the life of a mandrel bar used for rolling in a mandrel mill, and is a first method for removing a material constituting an element tube adhered to the outer surface of a mandrel bar used for rolling. The present invention provides a method for improving the life of a mandrel bar, comprising a step and a second step of forming an iron hydroxide film on the outer surface of the mandrel bar.

  According to such an invention, as will be described later, it is possible to obtain the same life as when a scale coating (iron oxide coating) is formed on the outer surface of the mandrel bar. In addition, unlike the case of forming the scale coating, no heat treatment is required, so that there is an advantage that the circulation efficiency of the mandrel bar is not lowered, and hence the production efficiency of the seamless steel pipe is not lowered. Further, since no heat treatment is required, it is possible to obtain the advantages that the surface of the mandrel bar is not softened and the thickness accuracy of the raw tube to be rolled is not lowered.

  In the second step, chloride or bromide may be applied to the outer surface of the mandrel bar in order to form an iron hydroxide film on the outer surface of the mandrel bar. However, if the temperature of the mandrel bar when it is allowed to stand after applying chloride or bromide is too high (for example, a temperature exceeding 60 ° C.), there is a problem that stress corrosion cracking occurs and the mandrel bar breaks. . Moreover, even if the temperature of the mandrel bar is set to room temperature to prevent stress corrosion cracking, if sufficient time is not secured (for example, less than 6 hours), a sufficiently thick iron hydroxide film is formed. However, when used in a mandrel mill, there is a problem that seizure is likely to occur as a result of an increase in the coefficient of friction with the base tube. Therefore, it is preferable to apply chloride or bromide to the outer surface of the mandrel bar and leave it at a temperature of 60 ° C. or less for 6 hours or more.

  Alternatively, in the second step, an aqueous solution containing an alkali metal chloride, an alkali metal bromide, an alkaline earth metal chloride, an alkaline earth metal bromide, hydrogen chloride and an aqueous solution containing hydrogen bromide on the outer surface of the mandrel bar It is also possible to form an iron hydroxide coating by applying any of the above. Also in this case, for the same reason as described above, the outer surface of the mandrel bar has an aqueous solution and an odor containing alkali metal chloride, alkali metal bromide, alkaline earth metal chloride, alkaline earth metal bromide, hydrogen chloride. It is preferable to apply any one of the aqueous solutions containing hydrogen fluoride and leave it at a temperature of 60 ° C. or less for 6 hours or more.

  According to the mandrel bar life enhancement method of the present invention, not only can a life equal to the case where a scale coating (iron oxide coating) is formed on the outer surface of the mandrel bar, but also a scale coating is formed. In contrast, since heat treatment is not required, it is possible to obtain an advantage that the circulation efficiency of the mandrel bar is not lowered, and consequently the production efficiency of the seamless steel pipe is not lowered. Further, since no heat treatment is required, it is possible to obtain the advantages that the surface of the mandrel bar is not softened and the thickness accuracy of the raw tube to be rolled is not lowered.

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

<Offline test>
An off-line rolling test was performed using a plate material made of the same material as the mandrel bar to which the life enhancement method according to an embodiment of the present invention was applied. FIG. 1 is a schematic diagram showing a schematic configuration of an offline testing machine used in the present embodiment. As shown in FIG. 1, a rolling roll of the off-line testing machine 1 is placed on a material to be rolled W in a state where a plate-like material to be rolled W made of 13% Cr-containing steel is placed on a plate material 10 made of the same material as the mandrel bar. A total of 8 passes were rolled by R (the first to third passes and the fifth to seventh passes were rolled in the same direction, and the fourth pass and the eighth pass were rolled in the reverse direction).

More specifically, after the contact surface of the plate material 10 with the material to be rolled W is a machined surface, a 5% magnesium chloride (MgCl ₂ ) solution is applied to the contact surface with a brush, and the normal temperature ( The iron hydroxide film was formed by leaving it to stand at 60 ° C. or less for 1 day, and the rolling material W was placed on the iron hydroxide film coated with the lubricant L, and a rolling test was performed. As a comparative example, the contact surface of the plate material 10 with the material to be rolled W is subjected to heat treatment to form a scale film, and the material W to be rolled is placed on the scale film coated with the lubricant L. A rolling test was conducted. Furthermore, as another comparative example, the material to be rolled is obtained by polishing the contact surface of the plate 10 with the material to be rolled W with a GF (Grinding Flap) wheel or the like, and applying the lubricant L to the polished contact surface. W was placed and a rolling test was performed.

  FIG. 2 shows the results of the offline test described above. Here, the horizontal axis in FIG. 2 indicates the number of passes, and the vertical axis indicates the friction coefficient between the material to be rolled W and the plate material 10. Note that the friction coefficient on the vertical axis in FIG. 2 is a value calculated by dividing the force F applied in the axial direction to fix the position of the plate 10 by the rolling load P of the rolling roll R. As shown in FIG. 2, a plate material (hereinafter referred to as “iron hydroxide coating material” as appropriate) in which magnesium chloride is applied to the contact surface with the material W to be rolled to form an iron hydroxide coating is used (FIG. 2). (Data plotted in FIG. 2) in the case of using a plate material (hereinafter referred to as “scale coating material” as appropriate) that has been subjected to heat treatment until the seventh pass rolling (hereinafter referred to as “scale coating material”). The friction coefficient is almost the same as that of the data plotted in Fig. 2), and a plate material (hereinafter referred to as "abrasive material" where appropriate) is used (the data plotted with "O" in Fig. 2). The coefficient of friction was smaller than If the coefficient of friction between the material to be rolled W and the plate material 10 decreases, seizure hardly occurs between the plate material 10 and the material to be rolled W, and as a result, the life of the plate material 10 is improved. Become. In addition, when the abrasive was used, the friction coefficient was further increased by rolling in the reverse direction in the fourth pass, but this is due to the rolling in the third pass as shown in FIG. When the sheet material 10 has a raised portion having a convex portion in the rolling direction of the material W, the rolling contact surface of the material to be rolled W hits the raised convex portion of the plate material 10 when rolling in the reverse direction in the next fourth pass. This is thought to be due to the increased friction coefficient.

  Furthermore, as shown in FIG. 2, during the eighth pass rolling, the scale coating material had a large friction coefficient, whereas the iron hydroxide coating material was stable at a low value. The friction coefficient of the scale coating material was increased because the scale coating was scraped off as the number of rolling passes increased, and the surface state similar to the surface state shown in FIG. When rolling in the reverse direction, the contact surface of the material to be rolled W is considered to be in contact with the raised convex portion of the plate material 10 to increase the coefficient of friction. On the other hand, as shown in FIG. 4, the iron hydroxide coating material is less prone to flaking because the corners of the heat crack are rounded by applying magnesium chloride to the surface of the plate 10. It is considered that the friction coefficient is stable at a low value.

  As described above, a scale coating (iron oxide coating) is formed on the surface of a plate material by an offline test using a plate material of the same material as the mandrel bar to which the mandrel bar life enhancement method according to the present invention is applied. It has been found that the friction coefficient between the rolled material and the plate material is stable at a low value (that is, the life is improved) equivalently or more. Therefore, when applied to an actual mandrel mill, even if the material to be rolled is a raw tube and the plate material is a mandrel bar, it is equal to or more than the case where a scale film (iron oxide film) is formed on the outer surface of the mandrel bar. In addition, it can be expected that the coefficient of friction between the raw tube and the bar is stabilized at a low value (that is, the life is improved).

<Online test>
Using a mandrel bar to which the life enhancement method according to an embodiment of the present invention was applied, a rolling test was performed with a mandrel mill (7-stand, two-roll type retained mandrel mill). The base tube to be rolled was made of 13% Cr-containing steel and rolled so that the wall thickness at the outlet side of the mandrel mill was 6.50 mm. More specifically, first, when the material constituting the raw tube adheres to the outer surface of the mandrel bar used for rolling in the mandrel mill, the outer surface of the bar is fixed by a wire brush or the like. It was removed by rubbing and rubbing. Next, a 5% strength magnesium chloride (MgCl ₂ ) solution is applied to the outer surface of the bar with a brush and left at room temperature (60 ° C. or lower) for 1 day to form an iron hydroxide film. A rolling test was carried out by inserting a lubricant on the coating and passing it through a raw tube.

Here, the application conditions of the magnesium chloride (MgCl ₂ ) were determined based on the following knowledge. That is, as shown in Table 1, when the temperature of the mandrel bar is too high when magnesium chloride is applied (70 ° C. column shown in Table 1), as a result of structural analysis by X-ray diffraction, It has been found that there is a problem that stress corrosion cracking (SCC) occurs in the bar, and the mandrel bar breaks. Moreover, even if the temperature of the mandrel bar is set to room temperature (10 ° C. to 60 ° C. shown in Table 1) in order to prevent stress corrosion cracking, the sufficient time is not secured (4 hr, 5 hr columns shown in Table 1). ), A sufficient thickness of the iron hydroxide film cannot be formed, and when used in a mandrel mill, the coefficient of friction with the base pipe increases (becomes greater than 0.065), and seizure is likely to occur. I found out there was a problem. The friction coefficient shown in Table 1 means the friction coefficient obtained by rolling in the first pass immediately after applying magnesium chloride to the outer surface of the bar. The friction coefficient is a value calculated by dividing the thrust force applied to the retainer that grips the rear end of the mandrel bar by the average rolling load of all the stands of the perforated rolling roll, and the average rolling load is the mandrel mill This is a value obtained by measuring the rolling load at each stand for 5 seconds at a pitch of 0.2 seconds after the rolling in the steady state (about 1 second after the start of rolling), and averaging this.

According to the results described above, the application condition of magnesium chloride (MgCl ₂ ) is preferably left at a temperature of 60 ° C. or less after application for 6 hours or more, and in this embodiment also at room temperature (60 after application). (1 ° C. or lower) for 1 day. In addition, as a comparative example, after rubbing and rubbing the outer surface of the bar with a wire brush or the like, heat treatment is performed to form a scale coating, and a lubricant coated on the scale coating is inserted into the raw tube W. A rolling test was conducted.

  FIG. 5 shows the results of the online test described above. Here, the horizontal axis of FIG. 5 indicates the number of passes using the same mandrel bar repeatedly, and the vertical axis indicates the coefficient of friction between the blank tube and the bar. Further, an iron hydroxide film or a scale film was formed on the outer surface of the bar at the timing shown as “surface treatment” in FIG. Note that the friction coefficient on the vertical axis in FIG. 5 is calculated by the same method as described above for Table 1. As shown in Fig. 5, when using a bar coated with magnesium chloride on the outer surface to form an iron hydroxide coating (data plotted with "●" in Fig. 5), heat treatment is applied to form a scale coating. When the bar was used (data plotted with “◯” in FIG. 5), substantially the same friction coefficient was obtained.

  As described above, an on-line test using the mandrel bar life enhancement method according to the present invention can provide the same coefficient of friction as when a scale coating (iron oxide coating) is formed on the outer surface of the mandrel bar. (In other words, the lifetime is improved).

  In addition, in the above online test, the formation of an iron hydroxide film or a scale film on the outer surface was repeated every 30 passes of rolling, and the mandrel bar used for rolling a total of 30 passes × 10 times had a hardness of the outer surface (Rockwell Hardness HRc) was measured. The result is shown in FIG. As shown in FIG. 6, for the bar having a scale film formed on the outer surface, the bar surface was softened by heat treatment, and HRc = 28 to 35, whereas iron hydroxide was formed on the outer surface. About the bar | burr in which the film was formed, since heat processing was not required, the surface hardness was high compared with the bar | burr in which the scale film was formed, and it was HRc = 33-40. Therefore, it can be expected that a seamless steel pipe with good wall thickness accuracy can be produced without causing a decrease in wall thickness accuracy of the rolled pipe due to softening of the bar surface.

FIG. 1 is a schematic diagram showing a schematic configuration of an offline testing machine that performs an offline rolling test using a mandrel bar to which the life enhancement method according to the present invention is applied. FIG. 2 is a graph showing the results of the off-line test. FIG. 3 is a diagram showing an example of the result of measuring the surface roughness of a conventional mandrel bar. FIG. 4 is a diagram showing an example of a result of measuring the surface roughness of a mandrel bar to which the life enhancement method according to the present invention is applied. FIG. 5 is a graph showing the results of the online test. FIG. 6 is a graph showing the results of measuring the hardness of the outer surface of the mandrel bar in the online test. FIG. 7 is an explanatory diagram for explaining the transition of the surface state of a conventional mandrel bar.

Explanation of symbols

10 ... Mandrel bar W ... Raw tube

Claims (3)

A method for improving the life of a mandrel bar used for rolling in a mandrel mill,
A first step of removing the material constituting the raw tube adhered to the outer surface of the mandrel bar used for rolling;
And a second step of forming an iron hydroxide coating on the outer surface of the mandrel bar.   The iron hydroxide film is formed by applying chloride or bromide on the outer surface of the mandrel bar in the second step and leaving it at a temperature of 60 ° C or lower for 6 hours or longer. To improve the mandrel bar life.   In the second step, among the alkali metal chloride, alkali metal bromide, alkaline earth metal chloride, alkaline earth metal bromide, an aqueous solution containing hydrogen chloride and an aqueous solution containing hydrogen bromide on the outer surface of the mandrel bar The method for improving the mandrel bar life according to claim 1, wherein the iron hydroxide film is formed by applying any of the above and leaving it to stand at a temperature of 60 ° C or lower for 6 hours or longer.

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