JPH09201604A - Manufacturing method of seamless steel pipe - Google Patents

Abstract

(57) Abstract: A method for producing a seamless steel pipe having no carburized layer on the inner surface with high efficiency is provided. SOLUTION: The rough rolling by a mandrel mill using a graphite-based lubricant, the reheating and the finish rolling by a styrech reducer are continuously performed, and then the inner surface of the pipe is subjected to a descaling treatment to reduce the residual oxide scale thickness. 10μ
m or less and then 1100 to 1250 in an oxidizing atmosphere
A decarburization heat treatment of heating and holding at 3 ° C for 20 minutes is performed. [Effect] It is possible to manufacture a product having excellent inner surface properties as well as machinability and corrosion resistance with high productivity. Further, it is possible to prevent the generation of heat energy loss. Furthermore, since it is not necessary to install equipment dedicated to pipe inner surface descaling in the mill line, equipment costs can be reduced.

Description

Detailed Description of the Invention

[0001]

TECHNICAL FIELD The present invention relates to a method for producing a seamless steel pipe by a Mannesmann-mandrel mill system,
More specifically, the present invention relates to a method for manufacturing a seamless steel pipe that prevents internal carburization caused by a graphite-based lubricant used during rough rolling by a mandrel mill.

[0002]

2. Description of the Related Art Manufacture of a seamless steel pipe by the Mannesmann-mandrel mill system is usually carried out as follows.

A round billet is heated to a predetermined temperature (usually 1150 to 1250 ° C.) by using a heating furnace such as a rotary hearth type, and the round billet is passed through an inclined roll piercing and rolling mill typified by Mannesmann piercer. To form a hollow hollow shell. Next, a mandrel bar coated with a lubricant is inserted into the hollow shell in a skewered form, and is passed through a mandrel mill consisting of 7 to 9 stands to roughly roll into a blank for finish rolling of a predetermined size in one pass. After this rough rolling, the mandrel bar is pulled out from the pipe by using a burst ripper device, and the pipe end portion whose shape has deteriorated is cut with a hot saw or the like. The tube is then placed in a reheat furnace and reheated (usually 900-1000).
After descaling by spraying high-pressure water only on the outer surface of the pipe, the product is passed through a finish rolling machine such as a stretch reducer to reduce the outer diameter and slightly reduce the wall thickness, and finish rolling to a predetermined product size. After that, the finish-rolled pipe is transferred to a cooling floor, cooled to room temperature, cut into a predetermined product length with a cold saw or the like, and then transferred to a finishing line including a final heat treatment step.

When manufacturing a seamless steel pipe by the Mannesmann-mandrel mill system, a mandrel bar used during rough rolling by a mandrel mill is inserted into a hollow shell in a high temperature state (usually 1100 to 1200 ° C.) and seized. It is exposed to easy conditions. Further, the cross-sectional shape and wall thickness of the pipe after the mandrel mill rolling are influenced by the roll rotation speed and the roll hole shape at the time of rolling, and are also influenced by the friction between the mandrel bar and the material (hollow shell). Therefore, in order to prevent the mandrel bar from sticking to the hollow shell, and to ensure proper friction with the material, as described above, the outer surface of the mandrel bar is coated with a lubricant prior to insertion into the hollow shell. It

However, when the applied lubricant has a large friction coefficient, the pipe is not plastically deformed uniformly in the circumferential direction and the longitudinal direction during rolling, and a stable cross-sectional shape and wall thickness distribution cannot be obtained. Further, when the lubricity of the applied lubricant is poor, the rolled pipe and the mandrel bar are seized and the bar cannot be pulled out, resulting in impaired workability. Therefore, as the lubricant applied to the mandrel bar, it is necessary to use a lubricant for hot rolling, which has excellent lubrication characteristics during hot rolling.

[0006] As such a lubricant, for example, there is a water-soluble lubricant containing graphite as a main component, which is inexpensive and has very excellent lubricating properties, as disclosed in Japanese Patent Publication No. 59-37317, and this graphite-based lubricant is used. Conventionally, many lubricants have been used. However, when rough rolling is performed using a mandrel bar coated with a lubricant containing graphite as a main component, a carburizing phenomenon occurs during rolling, and a carburized layer having a carbon concentration higher than that of the base metal is generated on the inner surface side of the pipe. .

The carburized layer generated on the inner surface side of the pipe is diffused into the base metal during subsequent reheating, during finish rolling with a stretch reducer, and during heat treatment such as solution treatment after finish rolling, so that the carbon concentration itself. However, the depth of the carburized layer becomes deeper on the contrary, and the carburized layer portion having a high carbon concentration exceeding the predetermined standard remains.

The residual carburized layer portion exceeding the predetermined standard causes a local abnormally hardened portion on the inner surface of the product pipe in carbon steel or the like to reduce the machinability of the product. Further, in the case of stainless steel, the corrosion resistance such as intergranular corrosion resistance of the inner surface of the product is reduced.

Therefore, in order to obtain a product having the above-mentioned machinability and corrosion resistance of a predetermined performance, a removing step such as polishing and removing the carburized layer portion on the inner surface of the pipe with a grindstone driven to rotate is essential in the refining step. Will be required as a process. As a result, productivity and yield are reduced and the product cost is significantly increased.In the case of a product with a small diameter, there is a problem that the grinding wheel cannot be inserted into the pipe and cannot be practically manufactured. .

As a method for solving such a problem,
For example, it is conceivable to use a non-graphite type lubricant as proposed in JP-A-64-16894. However, this non-graphite type lubricant has insufficient lubricity, especially when stainless steel is the object of rolling, and rolling defects such as streaks frequently occur on the inner surface of the pipe. As a result, not only a great number of man-hours are required for maintenance, but also the product yield is lowered, and the lubricant itself is expensive and not economical.

Further, in Japanese Unexamined Patent Publication No. 4-111907, an inner surface layer of a rolling mill tube for finishing after mandrel mill rolling using a graphite-based lubricant is provided in an amount of 20 to 50 from the inner surface.
A method has been proposed in which the carburized layer is completely removed by pickling or polishing over a thickness of μm, and then reheating and finish rolling. However, in this method, it is practically impossible to completely remove the carburized layer by the above-mentioned pickling or polishing means while the pipe is in a hot state at a high temperature, and it is necessary to cool the pipe to room temperature once. It has the drawback of large energy loss. Further, since the rough rolling and the finish rolling are performed in two steps, there is a drawback that the productivity is significantly reduced.

Further, in Japanese Unexamined Patent Publication No. 6-182427, similar to Japanese Unexamined Patent Publication No. 4-111907, one end of the finishing raw pipe after mandrel mill rolling using a graphite-based lubricant to the other A method has been proposed in which the lubricant component present on the inner surface of the raw pipe is removed and then reheated by performing mechanical descaling processing in which silica sand is injected into the pipe toward the pipe end. However, although this method can solve the drawbacks of the method proposed in Japanese Patent Laid-Open No. 4-111907, it blows hard silica sand onto the inner surface of the pipe in a hot state at high temperature, so that the inner surface of the pipe is covered with silica sand. It has a drawback that the inner surface quality is deteriorated due to the inclusion of the product and the product quality is inferior. In addition, it is necessary to additionally install a dedicated equipment for inner surface descaling in the traverse table on the exit side of the mandrel mill, which not only adds to the equipment cost but also causes a loss of heat energy in the portion. .

[0013]

SUMMARY OF THE INVENTION The present invention has been made in view of the above circumstances, and its object is to improve the machinability of a product even when mandrel mill rolling is performed using a graphite-based lubricant. In addition, there is no need to install special equipment for internal descaling in the mill line, as well as corrosion resistance and deterioration of internal surface properties, heat energy loss does not occur at the time of internal descaling, and productivity is reduced. An object of the present invention is to provide a cheap and seamless steel pipe manufacturing method.

[0014]

Means for Solving the Problems The inventors of the present invention, when producing a seamless steel pipe by the Mannesmann-mandrel mill system, perform it after rough rolling by a mandrel mill using a graphite-based lubricant and thereafter. Detailed investigation was conducted on the occurrence of carburization on the inner surface of the pipe after reheating and finish rolling with a stretch reducer.

As a result, the carburized layer depth of the high carbon concentration exceeding the predetermined standard due to the graphite-based lubricant of the raw material tube for finish rolling immediately after rough rolling by the mandrel mill is 5 from the inner surface of the tube.
It is up to 0 μm, and a large amount of graphite-based lubricant remains on the inner surface of the pipe.

If this is charged into the reheating furnace as it is and reheated, the oxygen remaining in the tube is not sufficiently supplied, so that the graphite left behind adheres incompletely. Therefore, the CO partial pressure in the pipe increases, the carburizing phenomenon progresses, the carburized layer depth becomes deeper, and the carbon concentration in the portion also increases, and a large amount of carbon is generated in the oxide scale produced during heating. Be trapped.

Further, even if only the oxide scale on the outer surface of the pipe is removed after reheating and finish rolling with a stretch reducer, there is no change in the carburized layer depth and carbon concentration on the inner surface of the pipe and the carbon content in the oxide scale. Not be.

Furthermore, if the tube after the finish rolling is subjected to final heat treatment at a temperature higher than the above reheating temperature such as solid solution, a decarburization phenomenon occurs and the carburized layer becomes shallow to some extent. It was confirmed that the carburized layer did not disappear because the deoxidation phenomenon was suppressed by the oxide scale containing a large amount of generated carbon.

Therefore, the present inventors have proposed the above-mentioned Japanese Patent Laid-Open No. 4-1.
In order to eliminate the drawbacks of the methods proposed in JP-A-11907 and JP-A-6-182427, without interposing an internal descaling step for removing the graphite-based lubricant remaining on the inner surface of the pipe in the middle thereof. , Conducted various experimental studies to see if rough rolling and reheating with a mandrel mill and finish rolling with a stretch reducer could be performed continuously.

As a result, even after the tube after rough rolling by the mandrel mill is reheated as it is and finish-rolled by the stretch reducer, descaling is performed so that the oxidized scale on the inner surface becomes equal to or less than a predetermined thickness. After doing
It was found that the carburized layer can be eliminated by applying a predetermined heat treatment. Specifically, the thickness of the oxide scale is 10
When the particle size is less than or equal to μm and the material is heated and held at 1050 to 1250 ° C. for 3 to 20 minutes in an oxidizing atmosphere, the oxide scale does not act as a barrier and the decarburization phenomenon actively occurs to obtain a product having no carburized layer. I found out.

The outer surface of the tube is not carburized because a graphite-based lubricant is not usually used during rough rolling and finish rolling in order to avoid slip between the roll and the material. Even if a graphite-based lubricant is used, unlike the inner surface of the pipe, oxygen is sufficiently supplied, the graphite attached to the outer surface of the pipe is completely burned into CO ₂ , and a decarburization phenomenon occurs. It is not formed.

The gist of the present invention based on the above findings lies in the following method for producing a seamless steel pipe.

In a method for producing a seamless steel pipe in which rough rolling is performed using a graphite-based lubricant, and the tube after the rough rolling is reheated and finish rolling is performed, in the method for producing a seamless steel pipe, after the finish rolling, the thickness of the oxide scale on the inner surface of the pipe is After descaling to 10 μm or less, in an oxidizing atmosphere at 1050 to 1250 ° C. for 3 to
A method for producing a seamless steel pipe, which comprises performing a heat treatment of heating and holding for 20 minutes.

[0024]

DETAILED DESCRIPTION OF THE INVENTION The method of the present invention will now be described in detail with reference to the accompanying drawings.

FIG. 1 shows the method of the present invention and the above-mentioned Japanese Patent Laid-Open No. 4-1.
FIG. 11 is a process diagram showing the conventional method proposed in Japanese Patent Laid-Open No. 11907 and Japanese Patent Application Laid-Open No. 6-182427 in comparison.

As shown in the figure, in the conventional method, the descaling of the inner surface of the pipe, which was performed after the rough rolling by the mandrel mill using the lubricant containing graphite as the main component, is finished by the stretch reducer in the method of the present invention. I'll do it later. At this time, it is necessary to perform descaling so that the oxide scale has a thickness of 10 μm or less. As described above, this is because the oxide scale thickness after descaling is 10 μm.
Even if decarburization heat treatment is performed under the conditions described later when the thickness is thicker, this oxide scale acts as a barrier to prevent the contact between oxygen in the oxygen atmosphere and the material on the pipe surface side, and the decarburization reaction is suppressed, and even until finish rolling. This is because the carburized layer that has occurred in is not lost.

As described above, according to the present invention, unlike the conventional method, the descaling of the inner surface of the pipe is performed after the hot finish rolling, so that it is not necessary to perform the hot rolling in two steps, and the heat energy loss is reduced. I have no worries. Therefore, descaling of the inner surface of the tube may be performed by any means.
That is, any means such as pickling, grinding stone grinding, or spraying hard particles such as grid blasting or shot blasting on a tube cooled to room temperature after finish rolling may be used. Further, descaling can be performed using a wire brush or the like.

In order to avoid heat energy loss, descaling may be performed by means of hard particle spraying means or grindstone grinding means while the pipe is in a hot state. However, as described above, the former means is the inner surface of the product. It is not recommended as it will deteriorate the properties. Further, the latter means is not recommended because the grindstone is clogged early and it is necessary to replace it frequently.

For descaling a pipe in a room temperature state by a pickling means so that the thickness of the remaining oxide scale on the inner surface of the pipe becomes 10 μm or less, pickling may be carried out under the following conditions, for example. That is, the tube is placed in an aqueous sulfuric acid solution containing 10 to 15% by weight of sulfuric acid and having a temperature of 50 to 70 ° C. in an amount of 20 to 4
Soak for 0 minutes. In this case, the residual oxide scale thickness after pickling can be 10 μm or less regardless of the type of tube material.

Further, similar to the above, for a pipe in a room temperature state,
In the method using the descaling means for spraying hard particles onto the surface, for example, particles such as sand or steel balls having a particle size of 0.2 to 5.0 mm are sprayed at a spraying pressure of 5 to 10 kgf / cm ² to obtain the residual oxide scale. Thickness of 1
It can be 0 μm or less.

Further, in the same manner as described above, in the method by the descaling means for grinding the surface of a pipe in a room temperature state with a grindstone, for example, a grindstone having a roughness of about # 14 to # 18 is about 800 to 900 rpm. 2-4k while rotating
It is pressed with a pressing force of gf / cm ² , which is 0.6 to 0.
By moving in the axial direction of the tube at a rate of about 8 m / min, the thickness of the residual oxide scale can be reduced to 10 μm or less.

As shown in FIG. 1, among the pipes whose inner surface has been descaled, hot-finished products are subjected to final heat treatment as they are to obtain product pipes. For those requiring higher dimensional accuracy, the cold working process (process shown by the broken line in the figure) without heat treatment of the raw pipe for softening.
A cold-drawn product with only a die or a die and a plug, or cold rolling with a cold pilger mill. It is said that

Furthermore, for the cold-finished product that becomes the raw pipe, the pipe is subjected to the heat treatment for softening and then subjected to the above-mentioned cold working with a high working degree, and then subjected to the final heat treatment to be cooled. It is considered to be a finished product tube.

Of the above heat treatments, in the present invention, the final heat treatment of the hot-finished product pipe and the semi-cold-finished product pipe and the raw pipe heat treatment of the cold-finished product pipe are carburizing the inner surface of the pipe produced in the hot rolling process. Decarburization heat treatment to remove the layer,
The conditions are 1050 to 1250 in an oxidizing atmosphere.
It is necessary to heat and hold at 3 ° C for 3 to 20 minutes. The reason is as follows.

If the heating atmosphere is not an oxidizing atmosphere, the decarburizing reaction does not occur, and the heating holding temperature and time are 1050 ° C.
If not more than 3 minutes, the decarburization reaction does not occur. On the contrary, if the heating and holding temperature exceeds 1250 ° C. or exceeds 20 minutes, the crystal grain size of the base material becomes coarse and desired mechanical properties cannot be obtained. Further, when cold working is performed, cold workability is significantly deteriorated. Furthermore, the thickness of the oxide scale generated during this heat treatment becomes thicker, which requires a large number of man-hours to remove it, which is uneconomical.In the case of cold working, the oxide scale peels off during the processing and the surface of the product is removed. Remarkably deteriorate.

Therefore, in the present invention, the heat treatment conditions are set as described above.

As the heating furnace for carrying out the decarburizing heat treatment, any heating furnace may be used as long as it is an oxidizing atmosphere furnace such as a heavy oil burning heating furnace or a C gas burning heating furnace.

The oxidizing atmosphere may be any atmosphere as long as it is an oxidizing atmosphere, but 5 to 30 vo
An atmosphere having an O ₂ concentration of 1% is desirable. In addition, 5
It is more preferable to set the atmosphere containing water vapor (H ₂ O) of 0 vol% or less. In this case, the decarburization reaction is further promoted and the carburized layer can be efficiently eliminated.

As mentioned above, according to the method of the present invention, the inner surface of the pipe is descaled after the hot finish rolling.
If the decarburization heat treatment of heating the residual scale thickness to 10 μm or less and then heating at 1050 to 1250 ° C. for 3 to 20 minutes in an oxidizing atmosphere, the carburized layer generated during hot rolling may be completely eliminated, or Even if it is not completely eliminated, the carbon concentration is reduced to a concentration that does not cause any problem. That is, for example, SUS304 specified in JIS
In L, SUS316L, etc., the standard upper limit value of the carbon content is 0.030% by weight, but a carburized layer having a high carbon concentration exceeding this is not formed.

In the present invention, since the final heat treatment of the hot-finished product pipe and the semi-cold-finished product pipe is the decarburization heat treatment under the above conditions, desired mechanical properties may not be secured. However, in this case, it suffices to perform an appropriate heat treatment after the decarburization heat treatment to secure desired mechanical properties and the like, and there is no problem.

[0041]

The method of the present invention will be described below with reference to examples.

A brush was applied at room temperature and then dried, and a mandrel bar having an outer diameter of 141 mm was prepared on the surface of which a graphite lubricant film having a composition shown in Table 1 and having a film thickness of about 100 μm was formed.

[0043]

[Table 1]

Then, using this mandrel bar, an outer diameter 181.
0mm, wall thickness 16.0mm, length 7000mm, temperature 1
A hollow shell of 7 steel types having the chemical composition shown in Table 2 at 100 ° C. is passed through a mandrel mill consisting of 7 stands to have an outer diameter of 151.0 mm, a wall thickness of 5.0 mm, and a length of 25300 m.
m rough rolling to a finishing blank tube of m, and the resulting finishing rolling tube to 1000 ° C. in a heavy oil-fired reheating furnace in an oxidizing atmosphere.
It was kept heated for 0 minutes.

[0045]

[Table 2]

Subsequently, the raw material tube for finish rolling extracted from the reheating furnace is supplied as it is to a stillet reducer consisting of 26 stands, and high pressure water is jetted from an annular nozzle provided in the vicinity of the inlet side to only the outer surface descale. Outer diameter 63.5mm, wall thickness 7mm, length 40000mm
Finish rolling and cooling to room temperature, then length 10,000
It was cut into 4 mm.

Thereafter, descaling was performed on the inner surface of the pipe by pickling and shot blasting to prepare pipes having various residual oxide scale thicknesses (0 to 20 μm) changed. Decarburization heat treatment was performed under various conditions shown in Fig. 1 and the carburized condition of the inner surface of the obtained hot-finished product pipe (carbon concentration on the inner surface of the pipe and its carburized depth) was investigated. In addition, the carburizing condition of the inner surface of the pipe after each process after the rough rolling by the mandrel mill was also investigated.

The carbon concentration on the inner surface of the pipe and the carburizing depth were determined by the following methods.

That is, the carbon concentration on the inner surface of the tube was determined by measuring the average carbon concentration of the material surface from which foreign substances such as oxide scale adhering to the surface of the tube were completely removed, using an emission spectroscopy analyzer. Further, the carburizing depth is determined by grinding and removing the material surface after the oxide scale is removed at a predetermined pitch, and measuring the C average concentration of the obtained material surface using the same emission spectroscopic analyzer as above. Repeatedly, the average C concentration distribution at each position in the wall thickness direction was obtained, and the depth at which the average C concentration exceeded the upper limit specified in JIS was taken as the carburizing depth.

The results are also shown in Tables 3 to 6.

[0051]

[Table 3]

[0052]

[Table 4]

[0053]

[Table 5]

[0054]

[Table 6]

As is clear from the results shown in Tables 3 to 6, any one of the thickness of the remaining oxidized scale after descaling, the atmosphere of the decarburizing heat treatment condition, the heating temperature and the holding time is out of the range specified by the present invention. Comparative examples except those with high heating temperature or long holding time (No.
1-3, 5, 9-11, 13, 17-19, 21, 25
~ 27, 29, 33-35, 37, 41-43, 45,
49 to 51 and 53), the carbon concentration of the inner surface of the pipe was higher than the upper limit specified in JIS standard in all steel types.
Moreover, the carburized layer depth reached up to 60 μm.

Comparative examples (No. 4, 6, 12, 1) in which the heating temperature during decarburization heat treatment is high or the holding time is long
4, 20, 22, 28, 30, 36, 38, 44, 4
6, 52 and 54), no occurrence of carburized layer was observed. However, although these data are omitted, the crystal grain size is coarse and the desired mechanical properties are not obtained, but a large amount of the oxide scale adhered and peeled off when cold-rolled is peeled off. The surface property of the product pipe after rolling was poor.

On the other hand, in the case of the method of the present invention (No. 7, 8, 15, 16, 23, 24, 31, 32,
39, 40, 47, 48, 55 and 56), the carbon concentration on the inner surface of the pipe was less than or equal to the upper limit specified in JIS standard, and the carburized layer depth was 0 μm. Further, the products obtained by cold rolling these had good surface properties in the pipe.

[0058]

According to the method of the present invention, the carburization of the inner surface of the pipe, which occurs during the rough rolling by the mandrel mill using the graphite-based lubricant, is performed by hot rough rolling and finish rolling in two steps. It can be eliminated without fail, and it is possible to manufacture a product having excellent inner surface properties as well as machinability and corrosion resistance with high productivity. Further, since the inner surface of the pipe is descaled after the hot rolling is finished, it is possible to prevent the generation of thermal energy loss. Further, since it is not necessary to install a dedicated equipment for the pipe inner surface descaling in the mill line, the equipment cost does not increase.

[Brief description of drawings]

FIG. 1 is a view showing a manufacturing process of a method of the present invention and a manufacturing process of a conventional method in comparison.

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Tetsuya Nakanishi 4-533 Kitahama, Chuo-ku, Osaka City, Osaka Prefecture Sumitomo Metal Industries, Ltd. (72) Inventor Junsei Iida 4-chome, Kitahama, Chuo-ku, Osaka City, Osaka Prefecture 5th 33th Sumitomo Metal Industries, Ltd.

Claims (1)

[Claims] 1. A method for producing a seamless steel pipe in which rough rolling is performed using a graphite-based lubricant, and the tube after the rough rolling is reheated and finish rolling is performed. After descaling to a thickness of 10 μm or less, the temperature is adjusted to 1050 to 1250 ° C. in an oxidizing atmosphere for 3 to
A method for producing a seamless steel pipe, which comprises performing a heat treatment of heating and holding for 20 minutes.