JPH05305A - Manufacture of seamless tube of high chromium steel excellent in quality of outer surface of tube - Google Patents

Abstract

PURPOSE:To provide means for preventing rolling flaws on the outer surface of tube that bring troubles of cost and productivity in the time of manufacturing a seamless tube of high chromium steel by Mannesmann rolling process. CONSTITUTION:When an aluminum film is preliminarily formed on the surface of a rolling base stock (round billet or square bloom), an alloy layer excellent in hot workability is formed the outer layer by diffusing aluminum to the outer layer of the base stock when heating the base stock. And, burning between a tool and the material to be rolled is reduced by presence of this alloy layer. Further, aluminum prevents delta ferrite precipitation due to decarburization when heating the base stock. By combined action of them, rolling flaws on the outer surface of tube can be prevented.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing a high Cr steel seamless pipe which is free from rolling defects and has an excellent outer surface quality.

[0002]

2. Description of the Related Art In the energy development industries such as oil and gas, the environment of oil and gas fields to be developed has become severe in recent years. That is, the deeper the well is, the more the content of highly corrosive gas is increasing. 13% as a seamless oil well pipe material that can respond to such environmental changes
High Cr steel represented by Cr steel (mainly SUS420) has come to be used. Further, in the boiler for thermal power generation, the application of various ferritic heat resistant steel pipes based on 9% Cr is increasing due to the needs of high temperature and high pressure operation.

13Cr steel (SUS420) and 9Cr steel seamless pipes are generally manufactured by a rolling method. However, the rolling method has a problem that rolling flaws and cracks occur on the outer surface of the pipe as well as inside the pipe. Conventionally, as a flaw countermeasure in the Mannesmann rolling of this steel,
The following techniques have been proposed for targeting shoe mark defects caused by burning with a guide shoe. That is, Japanese Unexamined Patent Publication No. 1-30311 discloses a technique for preventing the generation of shoe marks by rolling while supplying an anti-seizure agent to the surface of the guide shoe that comes into contact with the material to be rolled, and Japanese Patent Laid-Open No. 60-184410. As can be seen in the publication, a technique has been proposed in which a lubricant is applied to the outer surface of a material to be rolled in advance and then rolled to prevent the generation of shoe marks.

However, only the shoe marks are not all of the rolling flaws. Scale biting flaws cannot be ignored as rolling flaws. Furthermore, even with the same 13% Cr steel, the C content is SU
In the case of SUS410 steel, which is less than S420 and causes δ ferrite to precipitate during hot working and cracking, there are cases where it cannot be used as a product pipe.

[0005]

SUMMARY OF THE INVENTION The present invention provides a means for preventing rolling defects that cause cost and productivity problems in producing a high Cr steel seamless pipe by the Mannesmann rolling method from the above problems. The purpose is to do.

[0006]

Means for Solving the Problems The inventors of the present invention have conducted research and development back to the mechanism of occurrence of rolling flaws in order to achieve such an object. As a result, the cause of rolling flaws is formed on the surface during heating of the material. It was found that it was in δ ferrite. That is, the δ-ferrite formed by decarburizing the surface layer of the material during heating easily causes seizure on the tool, which causes seizure flaws. Moreover, once the small piece of the material to be rolled attached to the tool is transferred to the material to be rolled again, it becomes a flaw. Furthermore, the δ ferrite / austenite boundary becomes the starting point of cracking due to the difference in deformability and deformation resistance with respect to the matrix austenite. In particular, in SUS410 steel, since δ ferrite exists not only in the surface decarburized layer but also inside, the crack depth reaches several mm. Therefore, in order to prevent rolling flaws and cracks, it is important to prevent the formation of δ ferrite.
As a result of searching various techniques for overcoming such a problem, it was found that it is effective to previously form an Al layer on the surface of the material. That is, the gist of the present invention is that a high Cr steel seamless pipe characterized by being manufactured by the Mannesmann rolling method using a round billet or square bloom of high Cr steel having an aluminum coating formed on the surface thereof. Is a manufacturing method.

The present invention will be described in detail below. After the molten high-Cr steel melted in a normal melting furnace such as an electric furnace or a converter is formed into a round billet or a square billet having a shape generally used by the continuous casting method or the decomposition rolling method, the surface of the billet is formed. The aluminum coating is coated by an arbitrary method such as a plating method or a thermal spraying method. After that, the billet is heated to a high temperature, processed into a thick hollow shell with a Mannesmann perforator, and rolled, a tube polisher, a standard machine, and a Mannesmann rolling method of further reheating to finish the drawing rolling machine. After that, it becomes a tube product.

In the pipe manufacturing process of the Mannesmann rolling method, the aluminum coating film coated on the surface of the round billet or the square billet has the following effects. That is, during heating, the aluminum coating prevents decarburization of high Cr steel and prevents precipitation of δ ferrite. The diffusion of aluminum into the surface layer of the material during heating forms an alloy layer with excellent hot deformability on the surface layer of the material to be rolled, so the surface layer deforms well following the additional shear strain in rolling, resulting in Does not cause defects such as cracks and scratches. Further, the scale formed on the outermost layer of the aluminum diffusion layer effectively prevents metal-to-metal contact between the tool and the material to be rolled and prevents seizure, so that the shoe marks are drastically reduced and diving scratches are also reduced.

The high Cr steel seamless steel pipe produced according to the present invention as described above can obtain a pipe outer surface quality free from rolling flaws without causing seizure damage of tools.

[0010]

EXAMPLES Next, the present invention will be further described based on examples. Test rolling was performed by the Mannesmann rolling method.
As the test materials, as shown in Table 1, three kinds of high Cr steels A to C were used. Steel A is SUS410 equivalent steel,
Steel B is SUS420 equivalent steel. These alloys, which were tapped in the converter, were formed into blooms having a cross section of 215 x 215 mm, and aluminum was sprayed to obtain raw materials for rolling. In this example, the aluminum coating was formed by thermal spraying,
It may be plated. Table 2 shows the test results evaluated by visually observing the outer surface of the pipe after pickling the rolled material.

As is clear from Table 2, No. According to the present invention of 1 to 3, good tube outer surface quality can be obtained. On the other hand, Comparative Example No. Nos. 4 to 6 had many defects and cracks.

[0012]

[Table 1]

[0013]

[Table 2]

[0014]

According to the present invention as described above, it is possible to manufacture a high Cr steel seamless pipe having a good pipe outer surface quality by the Mannesmann rolling method without causing rolling defects. Further, such an effect extends to steel having a smaller amount of Cr and easy to work. Furthermore, according to the present invention, since the direct contact between the metal and the tool can be prevented by the scale mainly composed of the aluminum alloy layer and the alumina,
It is also possible to prevent seizure damage to the tool as a side effect.

Claims (1)

Claims: 1. A pipe made by the Mannesmann rolling method using a round billet or a square bloom of high Cr steel having an aluminum coating formed on the surface, which is excellent in outer surface quality of the pipe. High Cr steel seamless pipe manufacturing method.