JPH0734926B2 - Method for manufacturing austenitic stainless steel seamless steel pipe - Google Patents

Description

TECHNICAL FIELD The present invention relates to a method for producing an austenitic stainless seamless steel pipe by a Mannesmann-mandrel mill system.

[Conventional technology]

Seamless steel pipe is generally rolled by a mandrel mill system, a plug mill system, or a Eugene Sejournet system.
Although manufactured by a hot extrusion method such as the Erhard pushbench method, a mandrel mill method, which is excellent in productivity and dimensional accuracy, is widely used for relatively small diameter pipe making.

The mandrel mill system is, for example, as shown in FIG.
The material billet 1 is heated to a predetermined temperature (generally 1100 ° C. to 1300 ° C.) in the rotary hearth heating furnace 2 and then pierced and rolled by the Mannesmann piercer 3 to form a hollow shell 4A. The hollow shell 4A may be directly manufactured by the continuous casting machine 5 for manufacturing the hollow shell. Since such a hollow shell 4A is thick and short, it is thinned and drawn by the mandrel mill 6 which is a drawing and rolling machine. The mandrel mill 6 includes a mandrel bar 7 whose surface is coated with a lubricant for hot rolling, and a hollow shell 4A.
It is a rolling mill that stretches and rolls when inserted into
It consists of eight roll stands. Each roll stand is provided with a pair of hole-type rolls 8, and the rotation axes of the hole-type rolls 8 are arranged so as to be offset from each other by 90 degrees in a plane perpendicular to the rolling axis between adjacent roll stands. The temperature of the raw tube in the mandrel mill 6 is 1050 ℃ on the rolling mill entrance side.
It is generally 1200 ° C, and 800 ° C to 1000 ° C on the delivery side of the rolling mill. The hollow shell 4A is stretched by the mandrel mill 6 to a length 2 to 4 times as long as the original length, and becomes the finish rolling shell 4B.

This finish rolling stock tube 4B is reheated to a predetermined temperature (generally 850 to 1000 ° C.) by a reheating furnace 11 if necessary, and then finish rolling by a finish rolling machine, for example, a stretch reducer 12. It The stretch reducer 12 reduces the outer diameter of the tube by up to 75% and stretches it to 40 times or more the length of the material billet, and the outer surface of the stretch reducer 12 has round holes on the final side of several stands. Since it is standardized by the mold roll, the finished tube 13 having relatively excellent outer diameter dimensional accuracy can be obtained. After that, the finished tube 13 is subjected to solution heat treatment, so that it is heat-treated at a predetermined solution temperature in the heat treatment furnace 14, and the tube 15 after the treatment is subjected to an inner surface polishing machine 17 to polish the inner surface as necessary, and a pickling bath 16 Finished with acid pickling to become a product.

By the way, in the stretching process using the mandrel mill 6,
When a lubricant is applied to the surface of the mandrel bar 7 and rolled, the rolling load increases, not only the roll and mandrel bar wear and seizure increase, but in some cases, the material sticks out to the roll flange. Therefore, it cannot be rolled. Therefore, the surface of the mandrel bar needs to be coated with a lubricant for hot rolling, which has excellent lubrication performance during hot rolling. As such a lubricant, one that is inexpensive and has very excellent lubricating performance, for example, a water-soluble lubricant containing graphite as a main component as shown in Japanese Patent Publication No. 59-37317 is most often used. There is.

[Problems to be Solved by the Invention]

However, austenitic stainless steel typified by SUS304 and SUS316, when mandrel mill rolling is performed by inserting a mandrel bar coated with such a graphite-based lubricant, carburization occurs on the inner surface of the raw pipe during rolling, A portion with a high carbon concentration is generated on the inner surface of the pipe. In this high carbon concentration region, carbon is diffused by subsequent reheating, finish rolling, and solution heat treatment after rolling, and although the carbon concentration decreases and spreads, a portion with a high carbon concentration still remains, for example, specified in JIS G0575. However, there is a problem that the intergranular corrosion resistance of the inner surface is deteriorated to the extent that cracking occurs in the intergranular corrosion test with sulfuric acid-copper sulfate. Therefore, in order to secure the intergranular corrosion resistance, it is necessary to remove the carburized portion on the inner surface of the rolled product by polishing or the like. For this reason, there have been problems such as a significant increase in cost and a decrease in productivity. Further, in the case of a product having a small diameter, it may not be possible to insert a grindstone for polishing into the pipe due to its small inner diameter, and it may be practically impossible to manufacture.

In order to solve such problems, it is possible to secure intergranular corrosion resistance by using austenitic stainless steel with Sb and Sn added to improve carburization resistance as a material. No. 63-274741. In that method, in order to completely have carburization resistance performance, it is necessary to add a large amount of Sb and Sn, and to reduce hot workability, rather cause rolling defect flaws, or There are problems such as narrowing the manufacturable thickness range.

The present invention was developed in view of the above circumstances,
Even if mandrel mill rolling is carried out by inserting a mandrel bar coated with a graphite-based lubricant, rolling defects of austenitic stainless seamless steel pipes that do not deteriorate intergranular corrosion resistance will be generated. The purpose is to manufacture at low cost without lowering productivity.

[Means for Solving the Problems]

In order to solve the above-mentioned problems, the present inventor repeatedly conducted an investigation and research on the internal carburizing condition in the mandrel mill rolling of the austenitic stainless seamless steel pipe. as a result,
The depth of the high carbon concentration layer due to carburization of the graphite-based lubricant in the final rolling stock pipe immediately after the mandrel mill rolling is at most about 50 μm below the inner surface, and the graphite-based lubricant remains on the inner surface. This high carbon concentration layer and residual lubricant diffuse, carburize and spread during the subsequent reheating before finish rolling, finish rolling, and solution heat treatment performed on the finished pipe, degrading the intergranular corrosion resistance of the product. I found that.

Therefore, further research on this point revealed that at least the inner surface of the blank for finish rolling immediately after the mandrel mill rolling was completed.
By removing 20 μm or more and 50 μm or less, it is possible to completely remove the residual lubricant and high carbon concentration layer, and then perform reheating, finish rolling, and solution heat treatment to prevent intergranular corrosion. It was found that it is possible to obtain an austenitic stainless seamless steel pipe whose performance does not deteriorate.

Furthermore, by immersion in nitric hydrofluoric acid pickling with a suitable composition range,
The lubricant and high carbon concentration layer remaining on the inner surface of the pipe can be preferentially dissolved and removed, and when the residual lubricant and high carbon concentration layer are polished and removed by polishing with a grindstone, no wrinkles are generated on the inner surface during finish rolling. Therefore, they have found that it is sufficient to carry out finish polishing using an abrasive having a particle size smaller than a specific grain size, and have completed the present invention.

That is, according to the present invention, the inner surface of the final rolling stock pipe immediately after the mandrel mill rolling is at least 20 μm or more and 50 μm in the thickness direction
The method for producing an austenitic stainless seamless steel pipe is characterized by performing finish rolling after removing m or less. The inner surface of the tube can be removed by immersing it in a nitric hydrofluoric acid solution containing 1 to 10% by weight of hydrofluoric acid and 3 to 20% by weight of nitric acid, or by corroding the inner surface of the tube. It is advisable to finish-polish the inner surface of the blank tube for finish rolling with the above abrasive.

[Action]

 Hereinafter, the present invention will be specifically described together with its operation.

The present invention includes SUS304 series, SUS310 series, and 316 series.
It can be applied to the production of all austenitic stainless seamless steel pipes such as 316 series, 321 series and 347 series.

First, in the present invention, at least 20 μm or more and 50 μm or less are removed from the inner surface of the finishing rolling stock pipe immediately after the mandrel mill rolling, for the following reasons.

The present inventor has used C: 0.045% by weight Si: 0.51% by weight Mn: 1.54% by weight Cr: 18.1% by weight Ni: 8.6% by weight after heating a round billet of SUS304 having an outer diameter of 1100 mm and a length of 1300 mm to 1240 ° C. , Mannesmann piercer outer diameter 110m
m, wall thickness of 11.25 mm, length of 3200 mm by hollowing and rolling,
Then 28% graphite, 10.4% organic binder, and 61.6 water on the surface.
% Mandrel bar with a lubricant composition of 90% is inserted, and the mandrel mill uses an outer diameter of 90 mm, a wall thickness of 3.75 mm, and a length.
An 11700 mm tube for finish rolling was manufactured. A sample was taken from the central portion in the longitudinal direction of this finishing rolling tube, and the average carbon concentration was analyzed every 0.01 mm from the inner surface of the tube. As a result, the third
The results shown in the figure were obtained.

For example, stainless steel pipe for piping specified in JIS G3459
In SUS304TP-SH, the carbon content is 0.08% by weight or less, but in this result, the carbon concentration is significantly higher than 0.14% by weight up to 0.02 mm from the inner surface, and 0.05 mm from the inner surface. When it exceeds the limit, the carbon concentration of the base material is about the same as 0.045% by weight. Therefore, if at least 0.02 mm or more is removed from the inner surface, the high concentration carbon region due to carburization can be removed. Further, if it is removed up to 0.05 mm, it is possible to remove all the effects of carburization and maintain good intergranular corrosion resistance.

Next, when the inner surface of the finishing rolling tube is corroded and removed by immersion in a nitric hydrofluoric acid solution, the hydrofluoric acid contained in the nitric hydrofluoric acid solution,
The reason for limiting the concentration of nitric acid to 1 to 10% by weight of hydrofluoric acid and 3 to 20% by weight of nitric acid will be described.

Hydrofluoric acid is generally the most commonly used acid when pickling stainless steel and has the function of dissolving the base metal of stainless steel, but if the content in the acid solution is less than 1% by weight, the result is 1% by weight or more, since the dissolution rate is low and the pickling efficiency is poor. On the other hand, if it exceeds 10% by weight, the dissolution action becomes too strong, the passivation film formed by effective Cr is destroyed, and local pitting occurs, so the surface quality deteriorates and the corrosion resistance decreases, so the upper limit is set. 10% by weight.

Nitric acid has an effect of forming a passive film by effective Cr in stainless steel, and has an effect of suppressing an unnecessary dissolution effect by hydrofluoric acid, but if the amount is less than 3% by weight, the effect is not sufficient. % Or more is required. On the other hand, 20% by weight
If it exceeds, the total acidity may become unnecessarily high, which may promote local pitting with hydrofluoric acid.
The upper limit is 20% by weight.

Next, in the case of polishing and removing the inner surface of the pipe with an abrasive, the reason why the final finishing polishing is limited to polishing with an abrasive having a grain size smaller than # 80 will be described below.

A material such as austenitic stainless steel, which is inferior in hot workability to ordinary steel, is rolled by a stretch reducer, which is a finish rolling mill, and then rolled on the inner surface of the pipe 13 as shown in Fig. 4. Longitudinal wrinkles 20 called so-called wrinkles are generated. If this wrinkle is sharp and deep,
The scale bitten in the wrinkle marks remains without being removed by subsequent pickling, and not only the deterioration of corrosion resistance,
For example, it may cause troubles such as seizure of tools during secondary processing such as cold drafting.

In order to solve such a problem, the present inventor has proposed a wrinkle flaw prevention method disclosed in Japanese Patent Laid-Open No. 63-290606. That is, the surface roughness of the mandrel bar used during mandrel mill rolling is set to Rmax of 30 μm or less,
By smoothing the inner surface of the blank for finishing rolling and setting the average value of the stret coefficient (tensile stress / yield stress) given to the blank by the rolling rolls of each stand to the blank during rolling in a stretch reducer to 0.58 or more, the wrinkles are set. It suppresses the occurrence of flaws.

However, even if the inner surface of the blank tube for finish rolling is smoothed during the mandrel mill rolling, wrinkles will occur if the roughness of the inner surface becomes rough due to the subsequent inner surface polishing. The present inventor is an abrasive grain size at the time of final finishing of the inner surface of the finishing rolling polishing tube, for example JISR
The relationship between the grain size number specified in 6001 and the wrinkle flaw depth after finish rolling was investigated, and the results shown in FIG. 5 were obtained. If the wrinkle flaw depth is 10 μm or less, the scale in the wrinkle flaw can be easily removed by pickling, and troubles such as seizure on the tool in the secondary processing do not occur. Therefore, if the grain size of the abrasive grains in the final finish polishing is # 80 or more, the inner surface of the blank for finish rolling becomes smooth, and harmful wrinkles are not generated in the subsequent finish rolling.

〔Example〕

FIG. 1 shows a flow sheet of an embodiment of the method of the present invention.
Reference numerals 1 to 8 and 11 to 16 are the same as those in FIG. 2 already described. In the present invention, the finishing rolling stock tube 4B stretched by the mandrel mill 6 is treated in the pickling tank 9 or the polishing machine 10 to remove the inner surface of the tube from 20 μm to 50 μm in the thickness direction.

The austenitic stainless steel of each composition shown in Table 1 was heated to 1240 ℃ in a rotary hearth furnace with a round vitret of 110mm in outer diameter, and then Mannesmann piercer was used to outer diameter: 110mm, wall thickness: 12.25mm , Length: 3200 mm hollow shell is pierced and rolled, and then a mandrel bar coated with a lubricant composition of graphite: 28% by weight, organic binder: 10.4% by weight, water: 61.6% by weight is inserted into the mandrel. Outside diameter on mill: 9
A blank for finish rolling having a thickness of 0 mm, a wall thickness of 3.75 mm and a length of 11700 mm. After removing the inner surface of each of the obtained finishing rolling tubes by the method shown in Table 2, reheating to 960 ° C and then using a stretch reducer, outer diameter 60.5 mm, wall thickness 3.5 mm, length 18600
The finished tube was mm. The product was further heated to 1080 ° C., subjected to solid solution treatment, and then cooled with water to obtain a product.

A sample is taken from the center of the product in the longitudinal direction, JIS G0575
The intergranular corrosion test with sulfuric acid-copper sulfate specified in 1. was carried out, and the intergranular corrosion resistance was evaluated by the presence or absence of cracks. The results are shown in Table 2.

As is clear from Table 2, the austenitic stainless seamless steel pipe produced according to the present invention has excellent intergranular corrosion resistance, and the wrinkle flaw depth on the inner surface is also within a range without any problem. A product without pitting is obtained.

[Effects of the Invention] According to the present invention, even if mandrel mill rolling is performed by inserting a mandrel bar coated with a graphite-based lubricant, harmful wrinkle flaws on the inner surface, intergranular corrosion resistance without pitting An austenitic stainless seamless steel pipe having excellent performance can be easily manufactured.

[Brief description of drawings]

FIG. 1 is a process chart showing an embodiment of the present invention, FIG. 2 is a process chart showing a conventional method, FIG. 3 is a graph showing a carbon concentration distribution on the inner surface of a finish rolling stock pipe, and FIG. 4 is a finished pipe. FIG. 5 is a schematic diagram showing a surface wrinkle flaw generation state, and FIG. 5 is a graph showing a relationship between an inner surface wrinkle flaw of a finished pipe and an abrasive grain size number of final finish polishing at the time of inner surface polishing of a blank for finish rolling. 1 …… Material billet 2 …… Rotary hearth heating furnace 3 …… Mannesmann piercer 4A …… Hollow tube 4B …… Finish rolling tube 5 …… Continuous casting machine for manufacturing hollow tube 6 …… Mandrel mill 7 …… Mandrel bar 8 …… Pore roll 9 …… Nitrofluoric acid pickling tank 10 …… Internal polishing machine 11 …… Reheating furnace 12 …… Stretch reducer 13 …… Finishing tube 14 …… Heat treatment furnace 15 …… Finishing Pickling pipe 16 …… Finishing pickling tank 17 …… Internal polishing machine

Claims (3)

[Claims] 1. After removing 20 μm or more and 50 μm or less in the thickness direction of the inner surface of the blank for finish rolling immediately after the mandrel mill rolling,
A method for producing an austenitic stainless seamless steel pipe, which comprises performing finish rolling. 2. A pipe for finish rolling is immersed in a nitric hydrofluoric acid solution containing 1 to 10% by weight of hydrofluoric acid and 3 to 20% by weight of nitric acid to corrode and remove the inner surface of the pipe. A method for producing the austenitic stainless steel seamless steel pipe described. 3. The method for producing an austenitic stainless steel seamless pipe according to claim 1, wherein the final finish polishing is carried out by polishing with an abrasive having a grain size finer than # 80 to remove the inner surface of the finishing rolling pipe. .