JP3397172B2 - How to use a mandrel bar for hot seamless pipe rolling - Google Patents

Description

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for using a mandrel bar used in a method for producing a hot seamless pipe by a mandrel mill rolling method. 2. Description of the Related Art Mannesmann mandrel mill manufacturing method is widely adopted as a method for manufacturing a hot seamless tube. In this pipe making method, after a heated round steel piece (billet) is punched by a punching machine to form a thick hollow shell, a mandrel bar as a rolling tool is inserted into the inner surface of the hollow shell,
Thickening is performed by a mandrel mill provided with a plurality of rows of stands each composed of opposed hole rolls to obtain a thin hollow shell. [0003] The hollow shell obtained in such a mandrel mill is reheated as necessary, and then is drawn or fixed by a stretch reducer or sizer to finish the outer diameter to the final product diameter. Rolled. By the way, a mandrel bar used for mandrel mill rolling is previously formed with a lubricant film composed mainly of a solid lubricant on the surface thereof, and the contact surface between the mandrel bar and the blank tube during rolling by this lubricant film. The frictional force generated above is reduced to prevent wrinkles from occurring on the tool surface and the inner surface of the raw tube. However, since the lubricating film disappears after being subjected to one rolling, the mandrel bar once subjected to rolling forms a lubricating film on the surface again for the next rolling. There is a need to. Therefore,
A mandrel bar once subjected to rolling is cooled to an appropriate surface temperature by a water-cooled shower or the like, and then a lubricant is sprayed onto the surface and sufficiently dried to form a lubricating film. A mandrel bar is generally a JIS SKD.
6 and SKD61, etc., which are manufactured by subjecting a round bar steel made of hot tool steel to appropriate machining, quenching, and tempering, but the surface of the mandrel bar during rolling is very large. Since it is exposed to surface pressure and heat load, it is not easy to always maintain a complete lubrication state. In addition, since it is rapidly cooled by a water-cooled shower or the like after the completion of rolling, it is easy to cause wear, seizure, rough skin, and generation / progress of cracks during repeated use. In order to alleviate this problem, a method (use of a scale treatment bar) is generally employed in which an oxide scale film is formed on the surface to provide a lubricating effect and a lubricating film retaining effect by the scale itself. But
Recently, a method for improving wear resistance by forming a hard chromium plating film (use of a chromium plating bar) has been proposed and adopted. In addition, the mandrel bar in which damage that is considered to be harmful on use has occurred on the surface is machined on the outer peripheral surface, and the mandrel bar is reduced in size to a smaller outer diameter. On the other hand, the mandrel bar occupies a high ratio in the production cost of seamless pipes in the Mannesmann mandrel mill pipe manufacturing method. Effort has been made. For example, Japanese Patent Laid-Open No. 8-243610 proposes a method of repeatedly reusing substantially the same size by polishing a mandrel bar whose surface roughness has deteriorated due to rolling and then attaching a scale. Yes (scale processing bar). In Japanese Patent Laid-Open No. 8-71618,
There has been proposed a mandrel bar that can ensure excellent wear characteristics and life when rolling a high alloy steel pipe such as 13Cr steel by forming a plurality of chromium plating films having a predetermined roughness on the surface (chromium plating bar). . In Japanese Patent Application Laid-Open No. 8-71619,
There has been proposed a mandrel bar that can ensure excellent wear characteristics and life when rolling high alloy steel pipes such as 13Cr steel by forming a multi-layered film including a chromium plating film on the surface (chromium plating bar). . In JP-A-7-214116, the mandrel bar body is covered with a plurality of sleeves, and the damaged sleeve can be reused by appropriately replacing the damaged sleeve. Mandrel bars that can improve the basic unit have been proposed (chrome plating bars). However, since the method proposed in Japanese Patent Laid-Open No. 8-243610 is intended for a scale processing bar, it cannot be applied directly to a chromium plating bar. In addition, surface roughening becomes a problem with a scale treatment bar, whereas with a chrome plating bar, seizure due to partial peeling of the plating due to the development of heat cracks and seizure due to wear of the chromium plating film becomes a problem. . Further, in the mandrel bar proposed in Japanese Patent Laid-Open No. 8-71618, it is practically difficult to maintain a predetermined surface roughness due to friction and wear during rolling. Moreover, in the mandrel bar proposed in JP-A-8-71619, a multi-layered film is formed.
There is a problem that the manufacturing cost and maintenance cost of the mandrel bar increase significantly. Further, the mandrel bar proposed in Japanese Patent Laid-Open No. 7-214116 has a structure in which the body of the mandrel bar is covered with a plurality of sleeves, so that the manufacturing cost of the mandrel bar is greatly increased. There is a risk that the sleeve may be deformed or fall off during rolling. The present invention has been made in view of the above-mentioned conventional problems. When a chromium plating bar is used, it is not always necessary to maintain a specific surface roughness, and a multi-layered film can be formed. Without causing significant increase in manufacturing cost and maintenance cost due to the sleeve structure, suppressing the occurrence of seizure due to plating peeling due to the development of heat cracks and wear of the chromium plating film,
An object of the present invention is to provide a method for using a mandrel bar that can achieve a longer life and an improved unit consumption. In order to achieve the above-mentioned object, the method of using the mandrel bar for hot seamless pipe rolling according to the present invention has a depth of heat cracks generated during use. Discontinue use before the maximum thickness of 8 mm or before the residual plating thickness reaches a minimum of 5 μm, and after removing the plating film, finish or polish the surface of the base material, re-plat and reuse I am going to do that. And
By doing so, peeling of the plating around the crack opening is suppressed, tube inner surface flaws due to seizure are less likely to occur, and base metal exposure due to plating wear is less likely to occur, resulting in seizure. It becomes difficult. DETAILED DESCRIPTION OF THE INVENTION The present inventor has investigated in detail the plating peeling caused by the development of heat cracks due to repeated use and the wear of the chromium plating film, which are factors that determine the life of the chromium plating bar. As a result, the following knowledge was obtained. A) When the depth of heat cracks generated by rolling exceeds 8 mm, plating peeling starting from the periphery of the crack opening rapidly progresses, and the base material is exposed to cause tube inner surface flaws due to seizure. Becomes prominent. b) The rolling use is temporarily stopped before the depth of the heat crack reaches 8 mm at the maximum, and after removing the plating, the surface of the base material is polished, and after re-plating, the rolling is used again. Plating peeling is suppressed and tube inner surface flaws due to seizure are less likely to occur. C) When it is worn by rolling and the remaining plating thickness reaches about 5 μm, the base material is exposed during rolling,
The frequency of seizure is extremely high. d) If the rolling use is temporarily stopped before the plating thickness reaches the minimum of 5 μm, and after removing the plating, the surface of the base material is polished and re-plated. Does not occur and seizure hardly occurs. The method of using the mandrel bar for hot seamless pipe rolling according to the present invention is based on the above-mentioned knowledge, and a chromium plating film is used on the surface for use in mandrel mill rolling in the Mannesmann pipe manufacturing method. In the method of using the formed mandrel bar, after the use is stopped and the plating film is removed before the depth of the heat crack generated during use reaches a maximum of 8 mm or before the residual plating thickness reaches a minimum of 5 μm. The surface of the base material is finish-polished or surface-finished, re-plated, and reused. The mandrel bar used in the method of the present invention is:
The base material may be a material conventionally used for a mandrel bar for hot seamless tube rolling, but considering the load during rolling applied to the mandrel bar, JIS SKD6 and SKD6
A hot tool steel such as 1 is desirable. In addition, when a mandrel bar used in the method of the present invention is newly manufactured, an appropriate surface roughness is obtained by machining the base material to an outer diameter determined in consideration of the final chrome plating thickness and then polishing or the like. 10 to 200 after realizing the degree
A chromium plating film having a thickness of about μm is formed. In carrying out the method of the present invention, whether or not the heat crack depth of the mandrel bar reaches a maximum of 8 mm or the residual plating thickness reaches a minimum of 5 μm is measured by appropriate inspection work or the like. By investigating the relationship between the rolling performance (rolling material, number of rollings, rolling length, etc.) and measurement data in advance, it may be determined by managing only the rolling performance without actual measurement work. In carrying out the method of the present invention,
As the heat crack depth increases, the rate at which the heat crack progresses also increases. Therefore, considering the maintenance time and cost of the mandrel bar, the rolling is interrupted at a stage where the heat crack depth is as small as possible, for example, about 1 to 5 mm. It is preferable. On the other hand, regarding the residual plating thickness, it is desirable to interrupt the rolling after using it as close to the limit value (5 μm) as possible. Further, in the method of the present invention, the chromium plating bar whose rolling use has been interrupted is pickled with hydrochloric acid or the like, or is subjected to electrochemical plating dissolution with an alkaline solution such as caustic soda, or mechanically with a polishing / grinding machine or the like. The plating film on the surface is removed by the method. At this time, from the viewpoint of plating removal efficiency, it is desirable to remove the lubricant on the mandrel bar surface in advance in the dissolution of the plating by pickling or the like. The lubricant removing method may be spraying of high-temperature steam, or a mechanical method by polishing or shot blasting. The mandrel bar from which the plating has been removed is finished and polished for the purpose of ensuring good plating adhesion during re-plating. However, polishing and plating removal
When a mechanical method such as a grinding machine is employed, the removal of plating and this polishing operation can be combined. Surface processing such as shot blasting may be employed instead of polishing work. The finish polishing is used to care for damage to the surface of the base material or to reduce the depth of the heat crack after re-plating, and to suppress the progress of the heat crack at the time of reuse. It may be sharpened. Of course, you may grind | polish until a heat crack is removed completely. The mandrel bar whose surface has been surface-finished by finish polishing or shot blasting is subjected to chrome plating in the same manner as in the case of new production, and then subjected to rolling again.
The mandrel bar subjected to rolling again is removed from the plating again after the rolling use is continued in the range where the heat crack depth or the residual chromium plating thickness does not exceed the limit value,
After finishing such as polishing, re-plating is performed. The results of experiments conducted to confirm the effect of the method of using the mandrel bar for hot seamless pipe rolling according to the present invention will be described below. [Example 1] The material is SKD61 of JIS.
And the outer diameter is 2 with 70μm chrome plating on the surface.
About 30 mandrel bars with a dimension of 00 to 450 mm and a length of 24 m, the occurrence rate of inner surface flaws of the rolled material (product) corresponding to each mandrel bar was investigated from the rolling usage results for one month. For the target mandrel bar, the heat crack depth of the entire surface is measured by leakage magnetic flux inspection, and 1
The maximum heat crack depth for each book was determined. In addition, 90% or more of the material of the subject rolled material was ordinary steel or low alloy steel (Cr content: 5% by weight or less). The result is shown in FIG. From FIG. 1, it is clear that the rate of occurrence of internal flaws increases rapidly from the vicinity where the maximum heat crack depth exceeds 8 mm. In addition, when the surface of the mandrel bar was observed, the maximum heat crack depth was 8 m.
For mandrel bars exceeding m, significant plating delamination occurred, whereas the maximum heat crack depth was 8 m.
In the mandrel bar not exceeding m, no significant plating peeling occurred. [Example 2] The material is JIS SKD61.
And the outer diameter of 150 μm chromium plating on the surface is 2
60 mm, mandrel bar with a length of 24 m, 20
The occurrence rate of inner surface flaws of products (pipe) corresponding to each mandrel bar was investigated from the actual rolling usage for one month. For the target mandrel bar, the residual plating thickness was measured using an electromagnetic film thickness meter at intervals of 1 m in the length direction and at four points in the circumferential direction. The plating thickness was determined. In addition, 90% or more of the material of the rolling material used as the object was stainless steel (Cr content: 12% or more). The result is shown in FIG. From FIG. 2, it is clear that the incidence of internal diseases increases rapidly when the minimum plating thickness is 5 μm or less. When the surface of the mandrel bar was observed, noticeable seizure occurred in the mandrel bar with a minimum plating thickness of less than 5 μm, whereas there was almost no surface damage in the mandrel bar with a minimum plating thickness of 5 μm or more. Met. [Embodiment 3] The material is JIS SKD61.
And the outer diameter is 2 with 70μm chrome plating on the surface.
Two new mandrel bars each having a dimension of 60 mm and a length of 24 m were rolled and used under the four conditions shown below, and the life (total number of rolled bars) was investigated.
The material of the target rolled material is 95% or more of normal steel or low alloy steel in conditions 1 and 2, and 95% of conditions 3 and 4
The above was performed with stainless steel. Conditions 1 and 3 (Conventional method) A new article was used after rolling was started until the surface of the mandrel bar was damaged and a product inner surface flaw occurred. Condition 2 (Method of the present invention) After starting rolling with a new product, when the maximum heat crack reaches 2 mm, the rolling is stopped, the plating is removed by dissolution with an alkaline solution, finish polishing and re-plating are performed, and then rolling is performed again. It was used for. Thereafter, when the maximum heat crack reached 5 mm, the rolling was stopped, and the plating was removed by dissolving the plating with an alkaline solution. Then, finish polishing and re-plating were performed, and the rolling was again performed. afterwards,
The mandrel bar was used until the surface of the mandrel bar was damaged and internal surface flaws occurred. Condition 4 (Method of the present invention) After starting rolling with a new product, the rolling is stopped when the minimum plating thickness reaches 10 μm, the plating is removed by dissolution with an alkaline solution, and then finish polishing and re-plating are performed. Then, it was subjected to rolling again. Thereafter, the above was repeated. As a result of the experiment, under condition 1, remarkable plating peeling occurred in the vicinity of 2400 passes, the base material was exposed, and product inner surface flaws caused by seizure occurred, so the use was stopped. That is, the life of the mandrel bar in condition 1 was 2400 passes. In condition 2, since the maximum heat crack depth reached 2 mm in about 1600 passes, the rolling use was interrupted, the plating was removed, polished, re-plated and then rolled again. The maximum heat crack depth after polishing is 1.9 mm, and the outer diameter reduction of the mandrel bar is 0.2 m.
m. After resuming rolling, the maximum heat crack depth reached 5 mm in about 1000 passes. Therefore, rolling use was interrupted, plating was removed, polished, re-plated, and rolling use was started again. The maximum heat crack depth after polishing is 4.9 mm, and the outer diameter reduction of the mandrel bar is 0.2 mm.
Met. After rolling resumed, remarkable plating peeling occurred in the vicinity of about 1000 passes, the base material was exposed, and product inner surface flaws were generated due to seizure. That is, the life of the mandrel bar in condition 2 is 1
600 + 1000 + 1000 = 3600 passes. Further, in condition 3, since the product inner surface flaw caused by remarkable bar surface seizure occurred in the vicinity of 200 passes, the use was stopped. That is, the life of the mandrel bar in condition 3 was 200 passes. In condition 4, since the minimum plating thickness reached 10 μm in about 170 passes, the rolling use was interrupted,
The plating was removed, polished, re-plated, and then rolled again. The outer diameter reduction of the mandrel bar after polishing is 0.
It was 1 mm. After the rolling was resumed, the minimum plating thickness reached 10 μm in about 170 passes, so the rolling use was stopped, the plating was removed, polished, re-plated, and then the rolling use was started again. The outer diameter reduction of the mandrel bar after polishing is 0.1 mm
Met. Thereafter, the repetition is substantially the same as described above.
Confirmed until times. The plating of the mandrel bar after repeated use four times was removed, polished and re-plated, but it was still usable. That is, the life of the mandrel bar in condition 4 was 170 × 5 = 850 passes or more. From the above results, the effect of the present invention is clear. As described above, according to the present invention,
When using a chromium plating bar, it is not always necessary to maintain a specific surface roughness, and it is not necessary to significantly increase manufacturing and maintenance costs by forming a multi-layered film or forming a sleeve structure. It is possible to suppress the occurrence of plating peeling due to the development of heat cracks and the occurrence of seizure due to the wear of the chromium plating film, and it is possible to realize a longer mandrel bar life and an increase in the basic unit.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a graph showing the relationship between the maximum heat crack depth and the rate of occurrence of internal flaws. FIG. 2 is a diagram showing a relationship between a minimum plating thickness and an inner surface flaw occurrence rate.

──────────────────────────────────────────────────── ─── Continued from front page (58) Fields surveyed (Int.Cl. ⁷ , DB name) B21B 25/00 B21B 25/06

Claims (1)

(57) [Claims] [Claim 1] In a method of using a mandrel bar having a chromium plating film formed on the surface thereof used in mandrel mill rolling in the Mannesmann pipe manufacturing method, heat cracks generated during use Discontinue use before the maximum depth of 8 mm, or before the residual plating thickness reaches 5 μm minimum, and after removing the plating film, finish polishing or surface processing of the base material surface and then re-plating A method of using a mandrel bar for hot seamless pipe rolling, characterized by being reused.