JPH04344804A - Rolling method for mandrel mill - Google Patents

Abstract

PURPOSE:To manufacture a seewless steel tube without generating a part whose carbon concentration is high due to carburizing on the surface side in a tube stock, and without deteriorating the productivity, even if mandrel mill rolling is performed by inserting a mandrel bar to which a lubricant containing graphite is applied. CONSTITUTION:At the time of executing mandrel mill rolling by inserting a mandrel bar 7 to which a lubricant containing graphite is applied into a tube stock, bubbles having oxidizing gas as foam in the inside is applied to the outside surface of the lubricant applied to the surface of the mandrel bar 7, and thereafter, the mandrel bar 7 is inserted into the tube stock and rolling is executed.

Description

[Detailed description of the invention]

0001

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for manufacturing seamless steel pipes using a Mannesmann-mandrel mill system.

0002

[Prior Art] Seamless steel pipes are generally manufactured by a rolling method such as a mandrel mill method or a plug mill method, or a hot extrusion method such as a Eugene Sejournet method or an Erhardt push bench method. For pipe manufacturing, mandrel mill rolling method is widely used because it is excellent in terms of productivity and dimensional accuracy.

[0003] In the mandrel mill method, for example, as shown in FIG. 2, a billet material 1 is heated in a rotary hearth type heating furnace 2 to a predetermined temperature (generally 1100°C to 1300°C), and then a Mannesmann piercer is used. 3 to form a hollow tube 4A. Incidentally, such hollow shell tube 4A may be directly manufactured by the continuous casting machine 5 for manufacturing hollow shell tubes. Since the hollow tube 4A has a thick wall and a short length, it is stretched to reduce its thickness by a mandrel mill 6, which is a stretching mill. The mandrel mill 6 is a rolling mill that stretches and rolls a hollow tube 4A with a mandrel bar 7 whose surface is coated with a hot rolling lubricant, and usually consists of 6 to 8 roll stands. Each roll stand is equipped with a pair of grooved rolls 8, and between adjacent roll stands, the rotation axes of the grooved rolls 8 are aligned with each other at 90 degrees in a plane perpendicular to the rolling axis.
They are arranged offset by 0 degrees. The raw tube temperature in the mandrel mill 6 is 1050°C to 1200°C at the entrance of the rolling mill;
The temperature at the exit side of the rolling mill is generally 800°C to 1000°C. The hollow blank tube 4A is stretched to a length 2 to 4 times its original length by a mandrel mill 6, and becomes a blank tube 4B for a finishing rolling mill.

[0004] The raw tube 4B for the finishing rolling mill is heated to a predetermined temperature (generally 85
After being reheated to a temperature of 0° C. to 1000° C., finish rolling is performed by a finish rolling mill, such as a stretch reducer 12. The outer diameter of the raw pipe is reduced by up to 75% by the stretch reducer 12, and the length of the raw pipe billet is reduced by 4%.
Since the tube is stretched by a factor of 0 or more, and its outer surface is fixed in diameter by several stands of perfect circular hole rolls on the final side of the stretch reducer 10, a finished tube 13 with relatively excellent external dimensional accuracy can be obtained.

By the way, in the stretching process using a mandrel mill, if the mandrel bar surface is rolled with no lubricant applied or with a lubricant with poor lubrication performance applied,
The rolling load increases, which not only increases wear and tear on the rolls and mandrel bars, but also, in some cases, causes material to bite into the roll flange, making rolling impossible. Therefore, it is necessary to apply a hot rolling lubricant with excellent hot lubrication performance to the surface of the mandrel bar. Such lubricants are inexpensive and have very good lubricating performance, and are used, for example, in Japanese Patent Publication No. 59-3731.
A water-soluble lubricant containing graphite as a main component and an organic substance as a binder, as shown in Japanese Patent No. 7, is most commonly used.

[0006]

[Problem to be solved by the invention] However, SUS3
When a relatively low carbon concentration material such as austenitic stainless steel such as 04 or SUS316 is rolled by a mandrel mill by inserting a mandrel bar coated with a graphite-containing lubricant, there will be no raw material during rolling. Carburization occurs on the inner surface of the tube, creating areas with high carbon concentration on the inner surface of the tube.

[0007] In this high carbon concentration region, carbon diffuses through subsequent reheating, finish rolling, and further heat treatment after rolling, and as the carbon concentration decreases, it expands, but portions with high carbon concentration still remain, such as austenite. In the case of stainless steels, there is a problem in that the intergranular corrosion resistance of the inner surface deteriorates to the extent that cracks occur in the intergranular corrosion test using sulfuric acid-copper sulfate specified in JIS G0575.

[0008] Therefore, in the case of austenitic stainless steel, in order to ensure intergranular corrosion resistance, it is necessary to remove the carburized portion on the inner surface of the rolled product pipe by polishing, etc., which significantly increases costs. This caused problems such as a decrease in productivity. Furthermore, in the case of small-diameter products, it may be impossible to insert a grindstone for polishing due to the small inner diameter, and it may become virtually impossible to manufacture the product.

[0009] In order to solve these problems, Sb
, Japanese Patent Application Laid-Open No. 63-27474 discloses that intergranular corrosion resistance is ensured by using austenitic stainless steel with improved carburization resistance by adding Sn as a material.
1 is disclosed. However, in this conventional method,
In order to completely provide carburization resistance, it is necessary to add a large amount of Sb and Sn, and as a result, hot workability is reduced, which may cause rolling defects or impede production. There are problems such as narrowing the range of wall thickness.

The present invention was developed in view of the above-mentioned circumstances, and even if a mandrel bar coated with a graphite-containing lubricant is inserted and mandrel mill rolling is carried out,
The purpose is to manufacture at low cost without reducing productivity without creating a portion with high carbon concentration due to carburization on the inner surface of the raw tube.

[0011]

[Means for Solving the Problems] In order to solve the above-mentioned problems, the present inventor has repeatedly investigated and researched the inner carburization situation in mandrel mill rolling of seamless steel pipes. As a result, during mandrel mill rolling, in the gap between the raw tube and the mandrel bar, graphite, which is the main component of the graphite-based lubricant applied to the surface of the mandrel bar, and organic substances contained as a binder are present in the gap. Reacts with oxygen and burns, producing CO
It was discovered that a large amount of gas is generated, and that these combustion gases promote carburizing reactions, and that these carburizing gases cause carburization of the inner surface of the tube.

Therefore, after further research on this point, it was found that CO2 gas or O2 gas, water vapor, or
Alternatively, by filling the material with an oxidizing gas such as a mixed gas of these gases, it is possible to prevent the carburizing reaction even if a mandrel bar coated with a graphite-based lubricant is inserted and rolled in a mandrel mill, resulting in finish rolling without carburizing. It was discovered that a raw pipe for use could be obtained.

However, in order to fill the gap between the raw tube and the mandrel bar with oxidizing gas, when the raw tube is filled with oxidizing gas in advance and then the mandrel bar is inserted, at the same time, outside air is sucked in from the tube end. It becomes impossible to fill the gap between the raw tube and the mandrel bar with the amount of oxidizing gas necessary to prevent the carburizing reaction. Therefore, after inserting the raw tube, it is necessary to blow oxidizing gas from the tube end to fill the gap between the raw tube and the mandrel bar, but generally the gap between the raw tube and the mandrel bar is several times. Even if oxidizing gas is blown into the gap at high pressure, the pressure loss of the blown gas is large and it is difficult to blow the gas effectively, making it difficult to fill the gap with the amount necessary to prevent the carburizing reaction. It takes a very long time to replace the oxidizing gas with the oxidizing gas, which has the drawback of significantly hindering productivity.

Therefore, after extensive research on this point, we found that when a mandrel bar coated with a lubricant containing graphite is inserted into a blank tube and rolled in a mandrel mill, the outer surface of the lubricant coated on the mandrel bar surface After coating the foam with oxidizing gas inside, the mandrel bar is inserted into the raw pipe to effectively and quickly prevent the carburizing reaction in the gap between the raw pipe and the mandrel bar. They discovered that it is possible to fill the required amount of oxidizing gas to achieve this, and have completed the present invention.

That is, in the present invention, when a mandrel bar coated with a lubricant containing graphite is inserted into a raw pipe and rolled in a mandrel mill, an oxidizing agent is added to the outer surface of the lubricant coated on the surface of the mandrel bar. This is a mandrel mill rolling method characterized in that after applying gas bubbles inside, the mandrel bar is inserted into the raw tube and rolling is performed.

[0016]

[Function] Hereinafter, the present invention will be specifically explained along with its function.

[0017] In the present invention, CO2 gas, O2 gas, water vapor, or a mixed gas thereof can be used as the oxidizing gas to fill the gap between the mandrel bar and the raw tube immediately before mandrel mill rolling. The effects of these oxidizing gases are as shown below.

[0018] When a mandrel bar coated with a lubricant containing graphite and an organic substance as a binder is inserted into the raw tube and mandrel mill rolling is performed, the graphite in the lubricant burns due to contact with the hot raw tube. , CO gas, CO2
Generates gas. At this time, since the gap between the mandrel bar and the raw tube is generally very narrow, there is not enough oxygen to completely burn the graphite, so most of the combustion gas generated during rolling under normal conditions is It becomes CO gas.

On the other hand, the carburization and decarburization equilibrium of steel in a CO+CO2 mixed gas is expressed by the following equation.

[0020]

[Equation 1] Here, (C) is the concentration of carbon dissolved in the steel. That is, if the partial pressure of CO in the atmosphere is high, the reaction will be (1)
As the equation progresses to the left, the equilibrium carbon concentration (C) in the steel increases, in other words, carburization occurs. Conversely, if the partial pressure of CO2 in the atmosphere is high, the reaction proceeds to the right in equation (1), indicating decarburization.

In the conventional rolling method, most of the combustion gas of the lubricant is CO gas due to incomplete combustion, so (1)
The reaction in the equation proceeds to the left, and the carbon concentration on the inner surface of the raw tube increases due to carburization. On the other hand, in advance, the atmosphere in the gap between the raw tube and the mandrel bar during mandrel mill rolling was changed to CO2.
By setting the partial pressure to a high state, the reaction of formula (1) proceeds in the right direction, so carburization of the inner surface of the raw tube can be prevented.

[0022] Furthermore, if the atmosphere in the gap between the raw tube and the mandrel bar during mandrel mill rolling is made in advance to a state with a high O2 partial pressure, the graphite in the lubricant will be completely combusted, resulting in a high CO2 partial pressure. As mentioned above, carburization will not occur.

On the other hand, the reaction in high temperature steam is irreversibly expressed by the following equation.

[0024]

##EQU00002## Further, an equilibrium reaction equation expressed by the following equation holds secondarily between each component of the gas produced by the reaction.

[0025]

[Equation 3] Here, if the water vapor partial pressure in the atmosphere is increased, the reaction in equation (4) will proceed to the right, resulting in an increase in the partial pressure of CO2, and the reaction in equation (1) will proceed in the right direction. .

That is, carburization of the inner surface of the raw tube can be prevented by setting the atmosphere in the gap between the raw tube and the mandrel bar during mandrel mill rolling in advance to a state where the partial pressure of O2 is high.

As described above, when a mandrel bar coated with a lubricant containing graphite is inserted into a raw tube and rolled in a mandrel mill, the gap between the mandrel bar and the raw tube is filled with oxidizing gas.
That is, by rolling in a state filled with CO2 gas, O2 gas, water vapor, or a mixed gas thereof, carburization of the inner surface of the mother pipe after rolling can be prevented.

On the other hand, the bubbles containing the above-mentioned oxidizing gas can be produced by adding amides, hydroxyalkylammonium salts, chloride, etc. Obtained by bubbling the above oxidizing gas into a foam stabilizer such as alkylammonium, amine oxide, ethylene oxide adduct of alkyl amine, sulfoxide, condensate of maleic acid derivative, polyhydric alcohol ester of fatty acid, etc. Can be done. It is sufficient that these bubbles stably exist after being applied to the surface of the mandrel bar until they are inserted into the raw tube. Therefore, in the present invention, after applying foam containing the oxidizing gas as described above on the outer surface of the lubricant applied to the surface of the mandrel bar, the mandrel bar is inserted into the raw pipe. By rolling,
During rolling, the above-mentioned oxidizing gas is generated from these bubbles, the gap between the mandrel bar and the raw tube is filled with the oxidizing gas, and the reaction of equation (1) above proceeds to the right.
It becomes possible to prevent the carburization reaction, and it is possible to obtain a raw pipe for finish rolling without carburization on the inner surface.

[0029]

DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 1 is a process diagram showing an example of a seamless steel pipe production line in which the present invention is implemented.

In FIG. 1, a material billet 1 is heated to a predetermined temperature (generally 1100° C.) in a rotary hearth type heating furnace 2.
After being heated to 1300° C.), it is punch-rolled by a Mannesmann piercer 3 to form a hollow tube 4A. Note that the hollow shell tube 4A may be directly manufactured by the continuous casting machine 5 for manufacturing hollow shell tubes.

The hollow tube 4A is stretched by a mandrel mill 6 to reduce its thickness. The mandrel mill 6 inserts a mandrel bar 7, whose surface is coated with a graphite-containing lubricant by a lubricant applicator 21, into the hollow shell 4A.
A is stretched and rolled. In addition, mandrel mill 6 is 6~
It consists of eight roll stands, each roll stand is equipped with a pair of grooved rolls 8, and the rotation axes of the grooved rolls 8 are mutually aligned between adjacent roll stands in a plane perpendicular to the rolling axis. They are arranged 90 degrees apart. The raw tube temperature in mandrel mill 6 is 105 at the entrance side of the rolling mill.
0℃~1200℃, 800℃~100℃ on the exit side of the rolling mill
Generally, the temperature is 0°C. The hollow raw tube 4A is stretched to a length 2 to 4 times its original length by a mandrel mill 6, and becomes a raw tube 4B for a finishing rolling mill.

[0032] The above-mentioned raw tube 4B for finishing rolling mill is
1 to a predetermined temperature (generally 850 to 1000℃
) and then finish rolled by a stretch reducer 12. The outer diameter of the raw pipe is reduced by up to 75% by the stretch reducer 12 and stretched to more than 40 times the length of the raw pipe billet, and its outer surface is as perfect a circle as the last few stands of the stretch reducer 10. Since the diameter is determined by the grooved rolls, a finished tube 13 with relatively excellent external dimensional accuracy can be obtained.

However, in this embodiment, when the mandrel bar 7 coated with a graphite-containing lubricant is inserted into the hollow tube 4A and stretched and rolled in the mandrel mill 6, the lubricant is coated on the surface of the mandrel bar 7. After applying the foam supplied by the foam application device 22 onto the outer surface of the graphite-containing lubricant, the mandrel bar 7 is inserted into the hollow tube 4A to perform the mandrel rolling.

Here, the foam generated by the foam coating device 22 is as follows:
These bubbles are generated by the bubble generator 23 and have oxidizing gas inside them. The bubble generator 23 is
While containing the foaming agent aqueous solution 24, the oxidizing gas supplied by the oxidizing gas supply device 25 is aerated into the foaming agent aqueous solution 24 to generate bubbles containing the oxidizing gas as described above.

A round billet of austenitic stainless steel with an outer diameter of 110 mm having the respective compositions shown in Table 1 was heated to 1240° C. in a rotary hearth heating furnace, and then cut into an outer diameter of 110 mm and a wall thickness using a Mannesmann piercer. 12.2
A hollow tube with a diameter of 5 mm and a length of 3200 mm was punch-rolled to obtain a blank tube for mandrel mill rolling. Next, a lubricant with a composition of 28% graphite, 10.4% organic binder (polyvinyl alcohol), and 61.6% water was applied to the surface of the mandrel bar to form a dry film, and an aqueous solution of soap was applied as shown in Table 2. After applying the oxidizing gas having the composition shown, aerated and stirred into bubbles, a mandrel bar was inserted into the raw tube and immediately mandrel mill rolling was performed to obtain an outer diameter of 90 mm, a wall thickness of 3.75 mm, and a length of 3.75 mm. 1170
A raw tube for finish rolling with a diameter of 0 mm was used. After reheating the obtained raw tubes for finish rolling, they were rolled using a stretch reducer to an outer diameter of 60.5 mm, a wall thickness of 3.5 mm, and a length of 18,600 mm.
It was made into a finished tube of mm.

[0036] The mixture was further heated to 1080°C, subjected to solid solution treatment, and then cooled with water to obtain a product.

[0037] A sample was taken from the center in the longitudinal direction of the product and subjected to intergranular corrosion test using sulfuric acid-copper nitrate as specified in JIS G0575, and the intergranular corrosion resistance was evaluated based on the presence or absence of cracks, and the presence or absence of carburization was evaluated. It was confirmed.

As is clear from Table 2, according to the present invention, foam containing an oxidizing gas inside is applied to the outer surface of the lubricant applied to the surface of the mandrel bar during mandrel mill rolling, and then the mandrel bar is turned into a blank tube. When rolling is performed by inserting the steel into a steel sheet, a product without carburization can be obtained.

[0039]

[Table 1]

[0040]

[Table 2]

[0041]

Effects of the Invention According to the present invention, even if a mandrel bar coated with a graphite-based lubricant is inserted and mandrel mill rolling is carried out, productivity is not hindered and the inner surface is not carburized. can be easily manufactured.

[Brief explanation of drawings]

FIG. 1 is a process diagram showing the concept of the present invention.

FIG. 2 is a process diagram showing a conventional method.

[Explanation of symbols]

4A Hollow tube 4B Raw pipe for finish rolling 6 Mandrel mill 7 Mandrel bar 21 Lubricant applicator 22 Foam coating device

Claims (1)

[Claims] [Claim 1] When a mandrel bar coated with a lubricant containing graphite is inserted into a raw pipe and rolled with a mandrel mill,
A mandrel mill characterized in that after applying bubbles containing an oxidizing gas inside on the outer surface of a lubricant applied to the surface of the mandrel bar, the mandrel bar is inserted into a blank tube and rolled. rolling method.