JP2007160325A - Electric resistance welded tube excellent in brittle fracture resistance occurrence characteristic - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an electric resistance welded tube having excellent brittle fracture resistance occurrence characteristic and capable of obtaining the excellent a CTOD (Critical Crack Opening Displacement) value. <P>SOLUTION: A bond part in a section in the radial direction consists of one to three oblique lines having the angle θ with respect to the radial direction, preferably, 3 to 30°. The yield strength of a weld is not higher than the yield strength of a base material part, and the yield ratio of the weld is ≤85%. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to an electric resistance welded steel pipe, and particularly relates to an excellent weld brittle fracture resistance.

  In recent years, ERW steel pipes have been applied to oil well pipes, line pipes, etc. because their performance has been significantly improved due to improvements in manufacturing technology and material characteristics. Therefore, it is necessary to further improve the strength and toughness.

  In particular, the improvement of the reliability of seam welds where the base metal structure adjusted for high toughness disappears and the characteristics are likely to deteriorate is desired, and various proposals have been made.

  Patent document 1 relates to a method for producing an electric resistance welded pipe, and describes that after electric resistance welding, the weld is reheated to recover the toughness in order to improve the toughness of the seam weld.

  Patent Document 2 describes that the structure of the seam weld is mainly composed of ferrite and improves toughness. Patent Document 3 heats the entire steel pipe to 800 ° C. or higher after ERW welding so that it does not become martensite. It is described that the composite characteristics of low yield ratio, low temperature toughness and sour resistance are improved by quenching and not tempering.

On the other hand, Patent Document 4 relates to a technique called a penetrator for preventing defects in which a high melting point oxide remains in a welded part, and irradiating a laser beam near the welding point and performing seam welding while evaporating the metal in the irradiated part. Prevents the generation of penetrators and stabilizes the toughness obtained by the Charpy test.
Japanese Patent Publication No. 7-42509 JP 2002-302716 A Japanese Patent Publication No. 6-63040 Japanese Patent Publication No.61-29830

  By the way, in the case of a structure in which the use environment is severe and the environment is severely damaged, the welded portion is often required to have brittle fracture resistance. Brittle fracture occurs from the microscopic embrittlement part of the weld, for example, the micro area of the reheated heat affected zone in multi-layer welding, so the average toughness in the microstructure at the bottom of the notch is reduced. In the Charpy impact test to be evaluated, sufficient evaluation cannot be performed, and the evaluation is based on a critical CTOD value (critical crack opening displacement amount) by a CTOD test such as BS7448 standard.

  Therefore, the resistance to brittle fracture occurrence of ERW steel pipe welds according to Patent Documents 1 to 3 in which the toughness of the weld is evaluated by Charpy impact test and Patent Document 4 on the premise of the toughness evaluation by Charpy impact test is unknown. A method for producing an ERW steel pipe excellent in brittle fracture resistance characteristics characterized by a one-pass welding thermal cycle by energizing the die butt groove is not clarified.

  Accordingly, an object of the present invention is to provide a method for producing an electric resistance welded steel pipe excellent in brittle fracture resistance, in which an excellent critical CTOD value can be obtained in a CTOD test of a seam weld.

  The present inventors conducted a number of fracture toughness tests (CTOD tests) on ERW steel pipes with various changes in groove shape and welding conditions in order to grasp the fracture behavior of seam welds of ERW steel pipes, Moreover, the stress state in the seam weld when the groove shape was changed was investigated by FEM analysis.

  As a result, 1. Brittle fracture occurs from the bond part, and the microscopic embrittlement region that is the starting point of the brittle fracture exists in the bond part formed along the groove part. It has been found that when the groove shape is changed, the stress state in the bond portion changes.

The present invention was obtained by further study based on the obtained knowledge, that is, the present invention,
1. The bond portion in the radial cross section is composed of one to three hypotenuses having an angle θ with respect to the radial direction, the yield strength of the welded portion is equal to or less than the yield strength of the base metal portion, and the yield ratio of the welded portion is 85. % Electric resistance welded steel pipe with excellent brittle fracture resistance.
2. 2. The electric resistance welded steel pipe having excellent brittle fracture resistance according to 1, wherein the angle θ is 3 to 30 °.

  According to the present invention, an ERW steel pipe excellent in brittle fracture resistance is obtained, which is extremely useful industrially.

  The electric resistance welded steel pipe having excellent brittle fracture resistance according to the present invention is such that the bond part in the radial section of the welded part is composed of a hypotenuse inclined with respect to the radial direction, and further defines the mechanical properties of the welded part. To do.

  First, the shape of the bond portion in the welded portion of the ERW steel pipe, which is composed of a hypotenuse inclined by an angle θ with respect to the radial direction, will be described.

  FIG. 1 schematically shows the shape of the bond portion in the radial cross section of the weld. In the figure, the dotted line indicates the radial direction as a line perpendicular to the steel plate surface and toward the center of the steel pipe, the thick line indicates the bond portion, BM indicates the base material, and HAZ indicates the weld heat affected zone.

  FIG. 1A shows the case where the hypotenuse inclined by the angle θ with respect to the radial direction is one side, FIG. 1B shows the case of two sides, FIG. 1C shows the case of three sides, and FIG. In the case of two or more sides, the bond portion has a sawtooth shape. The angle θ may be different on each side.

  In the electric resistance welded steel pipe excellent in brittle fracture resistance according to the present invention, the bond portion in the radial cross section of the welded portion is composed of a hypotenuse inclined at an angle θ with respect to the radial direction, and the number of hypotenuses is 1 to 3 sides. To do.

  When the bond part is tilted, during the CTOD test, the fatigue crack of the CTOD test piece is introduced across the high stress area of the welded part (around the bond part), and the tip of the fatigue crack is in the bond part. The limit CTOD value is improved as compared with the case along.

  The angle θ is 3 to 30 °. When the angle θ is less than 3 °, the above-described effect cannot be obtained. On the other hand, when the angle θ exceeds 30 °, shear fracture tends to occur along the bond portion, and the tensile strength of the welded portion (perpendicular to the seam welded portion). Since the tensile strength in the direction is reduced, the angle is set to 3 to 30 °. In order to stably obtain the desired tensile strength of the welded portion, the angle θ is preferably 3 to 15 °.

  The hypotenuse is between 1 and 3 sides. As shown in FIG. 1 (a), if the bond part is a hypotenuse consisting of one side and is inclined, the fatigue crack of the CTOD test piece crosses the high stress area of the welded part (around the bond part) during the CTOD test. As a result, the limit CTOD value is improved as compared with the case where the tip of the fatigue crack is along the bond portion.

  On the other hand, as shown in FIG. 1 (d), when the hypotenuse is set to four sides, the number of times the tip of the fatigue crack crosses the bond portion increases, and the above-described effect cannot be obtained. FIG. 1 (d) shows a case where the hypotenuse has four sides, but the same applies to cases where there are five or more hypotenuses. Therefore, in the present invention, the length is 1 to 3 sides.

  As shown in FIGS. 1B and 1C, when the hypotenuse has two sides and three sides, the bond portion has a sawtooth shape. In this case, the angle θ indicates a narrow angle among the angles formed by the dotted line and the hypotenuse on the left and right in the radial direction indicated by the dotted line.

  Furthermore, in the present invention, in the tensile properties of the welded portion, the yield strength is not more than the base metal, and the yield ratio is not less than 85%.

  When the yield strength of the welded portion is higher than that of the base material, the stress at the crack tip of the CTOD test piece is increased and the critical CTOD value is decreased.

  If the yield ratio of the welded portion is smaller than 85%, the stress at the crack tip of the CTOD specimen increases and the critical CTOD value decreases, so the yield ratio is set to 85% or more.

  Table 1 shows the results of the CTOD test conducted on the welded portion of the APIX 80 grade ERW steel pipe having the shape of the bond portion shown in FIGS. The CTOD test piece was a side notch test piece in which a fatigue notch was introduced into the dotted line portion of FIGS. 1A to 1D, and three pieces were tested under each test condition to obtain the lowest limit CTOD value. The test was based on BS7448Part2.

  From Table 1, when the bond part is a hypotenuse inclined at an angle of 3 to 30 ° with respect to the radial direction and the number of hypotenuses is 1 to 3 (No. 1 to 6), the number of the angle or hypotenuse However, it is superior in the limit CTOD value compared to the case where it deviates from the definition of the present invention (No. 7 to 9).

  Further, in the tensile properties of the welded portion, when the yield ratio is as low as 75% (No. 10), when the yield strength exceeds the base metal (No. 11), the shape of the bond portion satisfies the provisions of the present invention. However, the critical CTOD value is low.

  The tensile properties of the base metal and the welded part were determined according to JIS Z2241 using test pieces according to JIS Z2201. The joint strength test was performed according to JIS Z3121.

The figure which shows the cross-sectional shape in case the number of sides is 1-5 in the bond part which consists of a hypotenuse of angle (theta), (a) is one, (b) is two, (c) is three, (d) Is a diagram showing the case of four.

Claims (2)

  The bond portion in the radial cross section is composed of one to three hypotenuses having an angle θ with respect to the radial direction, the yield strength of the welded portion is equal to or less than the yield strength of the base metal portion, and the yield ratio of the welded portion is 85. % Electric resistance welded steel pipe with excellent brittle fracture resistance.   2. The electric resistance welded steel pipe having excellent brittle fracture resistance according to claim 1, wherein the angle [theta] is 3 to 30 [deg.].

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