JPH04367307A - Manufacture of two-phase stainless seamless steel pipe excellent in quality of outside pipe surface - Google Patents

Abstract

PURPOSE:To provide a manufacturing method for a two-phase stainless steel pipe excellent in the quality of an outside pipe surface with less rolling scratches. CONSTITUTION:By using a square bloom whose corner parts are preliminarily ground to have the radius of curvature of >=15mm and a raw material forming an aluminum coating of <=0.1mm thickness to a round billet applied with a blasting treatment a tube is made by a normal mannessmann rolling method. In such a manner, the outside scratches of the pipe are greatly reduced since alloy layer excellent in hot machineability is uniformly formed.

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 seamless duplex stainless steel pipes with fewer rolling defects and excellent outer surface quality.

0002

BACKGROUND OF THE INVENTION In recent years, in the oil and gas energy development industry, the environment of the oil and gas fields that are the targets of development has become increasingly harsh. In other words, as the well becomes deeper, H2
The content of highly corrosive gases such as S and CO2 is increasing. High alloy steel, typified by duplex stainless steel, has come to be used as a seamless oil country tubular material or line pipe material that can cope with such environmental changes.

[0003] Duplex seamless stainless steel pipes are usually manufactured by hot extrusion using the Eugene method, so-called Mannesmann rolling using the plug mill method or mandrel mill method, but hot extrusion imposes restrictions on the size of the product due to limitations in extrusion force. Furthermore, since the production efficiency is lower than that of the rolling method, manufacturing by the rolling method is becoming more common. However, when using the rolling method, there is a problem in that rolling defects occur not only inside the tube but also on the outside surface of the tube. In particular, from the viewpoint of corrosion resistance, the problem of rolling defects becomes more serious as the steel type has a higher amount of alloys such as Cr, Mo, and Ni added.

The following techniques are conventionally known as measures to prevent or reduce rolling defects in this type of steel. (1) As seen in Japanese Patent Publication No. 1-19465, Ca, REM, Al, etc., which reduce the amount of S and O in steel that are harmful to hot workability, and indirectly affect the reduction of the amount of S and O A technology to add a specific amount of to steel. (2) As seen in Japanese Patent Application Laid-Open No. 1-228603,
A technology that specifies the billet heating temperature and controls the operating conditions of the piercing rolling mill within a specific range to obtain good quality inside and outside the tube. (3) As seen in Japanese Patent Application Laid-Open No. 1-233001,
A technology that applies an appropriate lubricant to materials to prevent scratches due to seizure and scale entrainment.

However, (1) is not very effective in downstream mills where the material temperature decreases and the ductility of the material inevitably decreases, and (2) is not very effective in preventing the occurrence and growth of flaws after piercing rolling. Unable to do so, (3
) is effective for ordinary steel, but Cr, Ni, Mo
This alone cannot be a sufficient measure against duplex stainless steel containing a large amount of. That is, when manufacturing two-phase seamless stainless steel pipes by the Mannesmann rolling method, the problem of rolling defects is unavoidable, and therefore, it has been essential to clean the defects by grinding. However, the reality is that large maintenance costs pose problems in terms of productivity, cost, and yield.

[0006]

[Problems to be Solved by the Invention] In view of the above-mentioned problems, an object of the present invention is to reduce rolling defects that cause problems in cost and productivity when manufacturing two-phase stainless steel seamless pipes by the Mannesmann rolling method. That is.

[0007]

[Means for Solving the Problems] The present inventors conducted research and development on the mechanism by which rolling scratches occur, and found that rolling scratches are generated starting from surface defects in the material at the elongator.
We found that it grows in downstream mills (plug mills and reeler mills). From this, since the depth of rolling flaws depends on the growth process, it is thought that if the controlling factors of this growth process can be controlled, the flaw depth can be reduced. It has been found that for the growth of flaws, the reduction in processing temperature, represented by the material surface temperature, is more important than the added strain caused by rolling. In other words, when the processing temperature decreases, the hot deformability of the material's surface layer decreases, leading to the progression of fracture, that is, the growth of flaws, but the conclusion is that the flaw growth process is determined by the hot deformability of the material's surface layer. Obtained. Therefore, there are basically only the following two approaches to suppressing flaw growth. ■Improvement of hot deformability of material surface layer, ■Suppression of temperature drop in material surface layer.

The present inventors mainly pursued the former approach method, and as a result, found that it is possible to modify the surface layer of a material without causing flaws to grow even when the material temperature decreases. In other words, when a material with an aluminum film applied to its surface is heated, aluminum, which is a ferrite-forming element, diffuses into the material, resulting in the formation of an α single-phase alloy layer with excellent workability, which reduces hot deformation. It was found that the growth of scratches could be suppressed by improving the performance.

Furthermore, the present inventors have found that even if the rolled material is a square bloom, if it is machine ground to reduce the steepness of the corners and blasted to improve the adhesion of the Al film, the Al film thickness can be reduced to 0. It has been found that it is possible to obtain a product tube with sufficiently good external surface quality even if the diameter is .1 mm or less.

The present invention was constructed based on the above knowledge, and its gist is that the surface of a square bloom which has been mechanically ground to a corner radius of curvature of 15 mm or more and then blasted is coated with zero. .Two-phase stainless steel pipes manufactured by the Mannesmann rolling method using a material with an aluminum film formed on the surface of a blasted round billet with a thickness of 0.1 mm or less. This is a method for manufacturing seamless steel pipes.

The present invention will be explained in detail below. The present invention applies 0.1 m to the surface of a corner bloom or a blasted round billet that has been mechanically ground to a corner radius of curvature of 15 mm or more and has been subjected to a blasting process such as sandblasting, shot blasting, or grid blasting.
After applying an Al film with a thickness of less than m by coating, thermal spraying, plating, etc., pipes are manufactured according to the usual Mannesmann rolling method and its manufacturing conditions.

[0012] Al applied to the surface of square bloom and round billet
exerts the following effect. ■ Levels out unevenness on the surface of the pipe material and reduces the occurrence of surface defects during piercing and rolling. ■At high temperatures, it diffuses into the steel base and forms an α single-phase alloy layer with good workability, making it difficult for rolling defects to propagate. Moreover, this alloy layer prevents direct contact between the base iron and the tool, thereby preventing seizure.

However, unless the tube material is pretreated, the above effects cannot be obtained with an Al coating having a thickness of 0.1 mm or less. In other words, for corner blooms, in order to reduce the steepness of the corner portion, the radius of curvature is set to 1 as shown in Figure 1.
It is necessary to perform mechanical grinding of 5 mm or more and to perform blasting to improve the adhesion of the Al film. If the corner parts are not mechanically ground, even if blasting is performed, the aluminum film thickness at the corner parts will inevitably become thinner, and the thickness of the alloy layer will inevitably become thinner. Since the strain is excessive, the alloy layer is easily destroyed, leading to the occurrence of flaws at the corners. Additionally, if blasting is not performed, the adhesion between the film/material interface will be insufficient and the aluminum film will partially peel off due to thermal stress or impact, making it impossible to form a uniform alloy layer. This may cause defects. On the other hand, regarding round billets,
Since there are no sharp corners like in corner blooms, it is only necessary to perform a blasting process to improve the adhesion of the Al film.

[0014] In the present invention, the upper limit of the Al coating thickness is set to 0.1 mm because if the thickness is greater than this, a sufficient effect of reducing rolling defects can be obtained without pre-treatment of the tube material.

[0015]

EXAMPLES Next, the present invention will be further explained by examples. Test rolling was performed using the Mannesmann rolling method. The composition of the test steel is A and B2 steel types in Table 1, both of which are SUS329J.
2L equivalent steel. Both steels tapped by AOD are 215
A bloom with a cross section of 215 mm was formed, and a predetermined aluminum film shown in Table 2 was applied to the bloom to prepare a test material. In this embodiment, the aluminum film was formed by thermal spraying, but a method such as plating may also be used. The test results were evaluated by visual observation of the outer surface of the tube after pickling the rolled material, and are shown in Table 2.
It is also shown in .

As is clear from Table 2, No. According to the present inventions 1 to 8, good tube outer surface quality can be obtained. On the other hand, comparative example No. In samples No. 9 to No. 12, rolling flaws occurred at locations corresponding to the corner portions because the corners of the tube materials were steep or the adhesion of the Al film was not sufficiently obtained by blasting.

[0017]

[Table 1]

[0018]

[Table 2]

[0019]

[Effects of the Invention] As described above, according to the present invention, it is possible to produce seamless duplex stainless steel pipes with good external surface quality by Mannesmann rolling method without producing rolling defects that are detrimental to productivity and cost. . Furthermore, the effects of the present invention are as follows:
Since direct contact between the base metal and the tool is prevented, seizing is less likely to occur, and as a side effect, tool life is also improved.

[Brief explanation of drawings]

[Fig. 1] Relationship between the quality of the outer surface of a pipe after rolling and the radius of curvature of the corner of a material in which an aluminum film with a thickness of 0.05 to 0.08 mm is formed on a corner bloom whose corners are machined and sandblasted. shows. No. next to the plot in the figure. is No. in Table 2. means.

Claims (1)

[Claims] [Claim 1] The radius of curvature of the corner portion is set to 15 mm in advance.
A material in which an aluminum film is formed on the surface of a square bloom that has been mechanically ground and blasted to a thickness of 0.1 mm or less, or an aluminum film is formed on the surface of a round billet that has been previously blasted to a thickness of 0.1 mm or less. A method for manufacturing a two-phase seamless stainless steel pipe, characterized in that the pipe is manufactured using a material formed by the Mannesmann rolling method.