JP2006144109A - Electric resistance welded tube having excellent crack resistance in weld zone and its production method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a welded steel tube having excellent crack resistance in a weld zone, and to provide its production method. <P>SOLUTION: In the cross-section of the weld zone, the number of oxide based inclusions with a grain size of ≥3 μm is controlled to ≤2 pieces/mm<SP>2</SP>, the number of the oxide based inclusions is measured, preferably, by an electron beam microprobe analysis process, and, regarding the method for controlling welding conditions, as the atmosphere at the time of welding, inert gas such as N<SB>2</SB>is used, and the concentration of oxygen in the gas is controlled to ≤60 ppm. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to an electric resistance welded steel pipe and a method for producing the same, and more particularly to a material excellent in crack resistance of a weld.

  Steel pipes are used for various purposes such as pipelines, machine structural members, heat exchangers, automotive parts, and various plants. Seamless steel pipes and welds manufactured by various methods according to their use or size. Steel pipe is used.

  Among them, a steel strip is continuously formed into a tubular shape to form a tubular body (open pipe), and both opposing edge portions are heated by high-frequency heating or resistance heating and welded to be electroformed. Steel pipes have high dimensional accuracy, can be manufactured with higher efficiency than other manufacturing methods, and are relatively inexpensive, so that they are conventionally manufactured and used in large quantities.

  However, when using ERW pipes in fields where high material properties and high reliability are required, there are many oxide inclusions in the weld and cause cracking. Become.

  The reason why there are more oxide inclusions in welded parts of ERW pipes compared to other welded pipes is that ERW welding is an intermediate welding method between fusion welding such as arc welding and pressure welding. This is because a clear molten pool is not formed, and a considerable amount of oxide formed by oxidation during welding is difficult to be removed from the inside of the steel.

  A weld with a high oxygen content deteriorates mechanical properties and causes cracking. For this reason, various methods, for example, shielding the weld with a non-oxidizing gas and performing electric resistance welding, upsetting the steel strip to a tubular body during welding, and discharging inclusions as much as possible to the outside. It has been proposed to reduce the oxygen content of the weld.

  Patent Document 1 discloses a method in which the joint end is shielded with a non-oxidizing gas and electro-welding welding is performed, the upset amount is regulated with respect to the welded pipe wall thickness, and the oxygen amount and inclusions in the welded portion are reduced. It is disclosed.

  Nitrogen and argon are used as a non-oxidizing atmosphere, and a seal box is provided to keep the welded portion non-oxidizing to suppress the formation of oxides. Further, poor bonding and non-metallic inclusions appear on the surface of the welded portion. It is characterized in that upset is performed so as to prevent cracking of inclusions in the welded portion.

  In Patent Document 2, the oxygen content of the welded portion is set to 200 ppm or less, but the edge portion where the tubular body is attached is preheated within a specific temperature range, irradiated with a laser beam, and welded after melting. In addition, the oxide occupying most of the oxygen content of the steel is discharged to the outside of the steel pipe.

Patent Document 3 shields the weld with a non-oxidizing gas such as nitrogen or argon, irradiates the welded part during high-frequency heating with a laser beam or a plasma arc, and raises the pressure-contacted part to a higher temperature. It is characterized by facilitating discharge and effectively preventing welding defects.
JP 63-241116 A JP-A-9-194998 JP-A-5-23867

  However, the method of shielding the weld with a non-oxidizing gas does not clearly show the relationship between the shielding situation and the effect of preventing the formation of oxides, and the weld is heated with a laser beam or the like to discharge oxides by upset. Even if easy, inclusions and the like are not completely eliminated, but a considerable amount of inclusions remain and increase the amount of oxygen in the steel, so the mechanical properties such as the strength and toughness of the welded part are the base material. Conventionally, the ERW welded steel pipe, which is inferior to the parts, is considered to be impossible to manufacture in the past, which has been a major cause of limiting its application.

  Therefore, the present invention clarifies a quantitative guideline indicating a shield state in which a sound welded portion can be obtained, and has the same mechanical properties as the base metal part, such as the strength and toughness of the welded part, and has less restrictions on applications. And it aims at providing the manufacturing method.

  As for the cause of the mechanical properties of the ERW welded part being inferior to that of the base metal part, the inventors of the present invention have the following points: 1) the amount of oxygen in the welded part, 2) the amount of inclusions, 2) the heat history of the welded part From these researches, we found that the effects of oxygen and inclusions in one weld were particularly significant, and repeated research.

As a result, it has been found that the total number of oxide inclusions in the weld can be reduced by limiting the number of large oxide inclusions having a specific dimension or more in the weld. The present invention has been made based on further findings based on the knowledge obtained, that is, the present invention
1. An ERW steel pipe excellent in crack resistance of a welded portion, characterized in that the number of oxide inclusions having a particle size of 3 μm or more in a welded section is 2 pieces / mm ² or less.
2 The number of the oxide inclusions is obtained by an electron beam microprobe analysis method, wherein the welded steel pipe is excellent in crack resistance of the welded portion.

3. ERW steel pipe with excellent crack resistance in welds, characterized by adjusting welding conditions so that the number of oxide inclusions with a particle size of 3 μm or more in the weld cross section is 2 / mm ² or less Manufacturing method.

  4 The welding method according to 3 above, wherein the welding conditions are adjusted by using an inert gas atmosphere and the oxygen concentration in the gas is 60 ppm or less. Steel pipe manufacturing method.

  According to the present invention, the number of oxide inclusions in the welded portion is small, the welded portion is excellent in crack resistance, and an ERW steel pipe having a welded portion having mechanical properties comparable to the base metal portion is obtained. It is extremely useful in industry.

According to the present invention, the number of oxide inclusions having a particle size of 3 μm or more is measured by measuring the electron inclusions in the cross section of the weld with an electron beam microprobe analyzer (EPMA). It is a condition that is 2 pieces / mm ² or less.

  FIG. 1 is a schematic explanatory view showing an example of a pipe making facility for carrying out the present invention. In the figure, 1 is a tubular body that is electro-welded and becomes an electric-welded steel pipe, 2 is a shield device, 3 is a work coil, and 4 is a squeeze. The roll 5 is an edge portion of the tubular body 1, and a is a welding point formed by the edge portion 5 of the tubular body 1 striking first.

In the illustrated pipe making equipment, a steel strip is formed into a tubular body 1 by a multi-stage roll (not shown), and a portion where the edge portion 5 of the tubular body 1 is abutted in the shield device 2 is heated and melted by a work coil 3. Thereafter, the squeeze roll 4 is used for upsetting to produce an ERW steel pipe.
In the present invention, welding conditions when the tubular body 1 is heated and melted by the work coil 3 in the shield device 2 are measured by EPMA for oxide inclusions in a welded section cut out from the ERW steel pipe after welding. The condition is such that the number of oxide inclusions having a particle size of 3 μm or more is 2 / mm ² or less.

  The oxide inclusions are preferably measured by EPMA. FIG. 2 shows a schematic diagram of EPMA. The electron beam applied to the sample 12 from the electron gun 11 generates secondary electrons, reflected electrons, and fluorescent X-rays in the sample 12. Secondary electrons and reflected electrons are detected by the secondary electron detector 13 and the reflected electron detector 14. The fluorescent X-ray is dispersed by the spectroscopic crystal 15 and detected by the detector 16 to identify the sample 12.

  The measurement conditions for oxide inclusions in the welded portion of the ERW steel pipe are preferably an analysis region of 4 mm × 4 mm, a voltage of 15 kV, a current of 300 nA, a beam diameter of 0.5 μm, and an integration time of 30 sec. Instead of EPMA, an elemental analysis may be performed by mounting an energy dispersive X-ray detector (EDX).

The welding conditions for the ERW steel pipe according to the present invention are such that the number of oxide inclusions having a particle diameter of 3 μm or more is 2 pieces / mm ² or less in the cross section of the weld cut out from the ERW steel pipe. The welding conditions can be achieved by setting the atmosphere during welding to an inert gas such as N ₂ and setting the oxygen concentration in the gas to 60 ppm or less. In the present invention, the cross section of the welded portion is a position where the edge portions 5 on both sides of the tubular body 1 are abutted in the electro-welded welded portion.

  In the present invention, the material of the ERW steel pipe may be steel as long as the chemical composition is not specified. The present invention can be applied to an ERW pipe made of steel mainly composed of ordinary alloy elements and unavoidable impurities used for iron and steel materials.

  The effects of the present invention will be described in detail with reference to examples. Using steel strips of various compositions as test materials, ERW steel pipes were manufactured using the manufacturing apparatus shown in FIG. The ERW steel pipe was welded under a plurality of conditions determined in advance so that the size and number of oxide inclusions in the weld cross section varied.

  Table 1 shows the component composition of the test material, and Table 2 shows the manufacturing conditions and the size and number of oxide inclusions observed in the weld cross section.

Specimen No. Examples 1 to 8 are Si-Mn steels, and Nos. 1 to 4 in Table 2 are examples of the present invention in which the number of oxide inclusions having a particle size of 3 μm or more is 2 / mm ² or less in the weld cross section. Thus, a sound welded portion excellent in crack resistance of the welded portion was obtained.

On the other hand, Nos. 5 to 8 in Table 2 are comparative examples in which the number of oxide inclusions having a particle size of 3 μm or more in the cross section of the weld is 3 pieces / mm ² or more, and the crack resistance of the weld is poor. It was. The oxygen concentration in the present invention example was 60 ppm or less, and the oxygen concentration in the comparative example was 79 ppm or more.

    In the crack resistance test, as shown in Fig. 3, the ERW steel pipe is sandwiched with a press and compressed until there is no space inside the pipe. It was.

The figure which shows an example of the manufacturing apparatus of an electric resistance steel pipe. The figure explaining the measuring method by EPMA. The figure explaining a cracking resistance test method.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Tubular body 2 Shielding device 3 Work coil 4 Squeeze roll 5 Edge part a Welding point 11 Electron gun 12 Sample 13 Secondary electron detector 14 Reflected electron detector 15 Spectroscopic crystal 16 Detector 21 ERW steel pipe 22 Welded part 23 Press

Claims (4)

An electric resistance welded steel pipe excellent in crack resistance of a welded portion characterized in that the number of oxide inclusions having a particle size of 3 μm or more in a welded section is 2 / mm ² or less. The ERW steel pipe excellent in crack resistance of a welded portion according to claim 1, wherein the number of oxide inclusions is obtained by an electron beam microprobe analysis method. An ERW steel pipe excellent in crack resistance of a welded portion characterized by adjusting welding conditions so that the number of oxide inclusions having a particle size of 3 μm or more is 2 pieces / mm ² or less in the welded section. Production method. The method for adjusting the welding conditions is characterized in that the atmosphere during welding is an inert gas, and the oxygen concentration in the gas is 60 ppm or less. Manufacturing method of sewn steel pipe.

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