JP4751027B2 - High strength welded steel pipe with excellent weld brittle cracking characteristics - Google Patents

Description

  The present invention relates to a welded steel pipe that is free from embrittlement cracks that occur in a welded portion in a high-strength steel pipe that is welded from the inner and outer surfaces whose tensile strength exceeds 850 MPa used for a natural gas / crude oil transportation line pipe or the like.

  In recent years, laying of ultra-high-strength large-diameter line pipes having a tensile strength exceeding 850 MPa has been studied from the viewpoint of improving transportation efficiency and cost reduction of incidental facilities in long-distance pipelines for transporting natural gas. Such a line pipe is usually formed by the UOE method or the bending roll method, and the seam welded portion that becomes a joint is performed by submerged arc welding in the order of inner surface welding and outer surface welding. However, in the nondestructive inspection after the outer surface welding, cracks in the direction perpendicular to the steel pipe axis direction, so-called transverse cracks, are sometimes found in the weld.

  Steel pipes where such cracks remain can be destroyed by tensile stress that exceeds the yield strength of the pipe body in the axial direction in the frozen land zone, and cracks have progressed due to penetration or repeated stress loading. In some cases, the transport fluid may leak, leading to a major accident. For this reason, the generation of cracks during production must be prevented or the generated cracks must be reliably removed by nondestructive inspection.

  Brittle cracks in high strength materials are generally caused by hydrogen, and the brittle cracks are less likely to occur as the strength decreases. However, if the strength of the seam welded portion is reduced, embrittlement cracks are less likely to occur, but it is also assumed that deformation from the seam welded portion is selectively promoted during internal pressure loading, leading to fracture from the welded portion. Therefore, a method for preventing embrittlement cracks while keeping the weld strength higher than the base metal strength is required.

  Since hydrogen embrittlement cracks depend on the hydrogen concentration, load stress, material properties, and particularly strength, it is necessary to control these combined effects below the crack limit occurrence value. As a method for reducing the hydrogen concentration, after welding, it is heated to 100 ° C. or higher, preferably 200 ° C. or higher, and held for an appropriate time to diffuse hydrogen to below the critical hydrogen amount.

  From this point of view, Patent Document 1 discloses a technique for preventing hydrogen cracking in a seam welded portion of a high-strength material by suppressing the seam welded portion strength, base material strength, and welding conditions of the UOE steel pipe in a composite manner. Although this invention describes that the transverse crack of the weld frequently occurs in the preceding seam weld, it does not disclose a technique for reducing the welding residual stress intended by the present invention.

  Similarly, Patent Document 2 discloses a technique for preventing cracking of a high-strength UOE steel pipe. Although the present invention describes a method for preventing toughness and solidification cracking by quenching and tempering the entire steel pipe after welding, there is a problem that enormous cost increases are caused due to heat treatment.

  Other methods of reducing the stress caused by hydrogen cracking include a method of reducing the residual stress by applying plastic deformation to the welded portion by so-called stress-relieving tempering that is heated to about 700 ° C. after welding and hammering. However, these methods need to be performed immediately after welding, and are not necessarily appropriate methods for seam welds in consideration of the manufacturing process and manufacturing cost.

  Further, as an improvement of the welding material, there are a method of reducing diffusible hydrogen harmful to cracking by creating a hydrogen trap site such as VN, and a method of reducing residual stress at normal temperature by a low temperature transformation melt. Trap sites are not always a useful method for high-strength materials, and low-temperature transformation melts cause a significant cost increase.

JP 2003-313121 A JP 57-35636 A

  The present invention is intended to prevent hydrogen-induced lateral cracking in a welded portion that occurs in a high-strength welded steel pipe that performs seam welding from the inner and outer surfaces. Technology for preventing hydrogen cracking in welds: heat treatment for diffusing hydrogen, heat treatment for reducing residual stress, imparting plastic deformation to reduce residual stress, imparting hydrogen trap sites, and controlling residual stress However, all of them involve adding a new process such as heat treatment equipment to the conventional pipe making process or increasing the cost due to an increase in alloy components. The present invention relates to a technique capable of preventing hydrogen-induced cracking by optimizing the welding conditions of a high-strength steel pipe.

The present invention has been made to solve the above problems, the tensile strength is less 1100MPa or more 850 MPa, from the inner surface of the seam welding by submerged arc welding, from subsequently the outer surface, the welded steel pipe to perform by more prime respectively, the height of the weld metal to lead in the inner surface of the steel pipe: height of the weld metal to follow with Wi before Kigai surface: the ratio of the Wo,
0.6 ≦ Wo / Wi ≦ 1.0, or 1.7 ≦ Wo / Wi ≦ 2.5
It is a welded steel pipe excellent in the embrittlement cracking property of the weld metal.

  According to the present invention, it is possible to prevent the occurrence of embrittlement cracks in a welded portion of a high-strength steel pipe having a tensile strength exceeding 850 MPa used for natural gas / crude oil transportation line pipes and the like by optimizing the welding conditions. Become.

In the UOE pipe making process where the tensile strength exceeds 850 MPa, the vicinity of the plate material edge is bent with the C press, bent into a U shape with the U press, and then formed into a steel tube with the O press, and then, usually from the outer surface. Then, inner welding is performed by submerged arc welding, followed by outer surface welding, and roundness is further adjusted by pipe expansion to form a UOE steel pipe. However, transverse cracks were found in the ultrasonic flaw inspection after outer surface welding, although the frequency was low. As a result of the investigation, it was found that the crack was inherent in the internal weld. As a result of the fracture surface observation, it was concluded that the fracture surface was peculiar to hydrogen cracking, and it was hydrogen cracking due to hydrogen incorporated into the weld metal from the flux, dew condensation on the groove, moisture in the atmosphere, etc. and the residual welding stress. .

  Therefore, the present inventors measured the amount of diffusible hydrogen in the welded inner surface where cracking occurred, and found that the maximum amount reached 2 ppm after 4 hours after the outer surface post-welding.

  Next, in order to grasp the relationship between the amount of hydrogen and the crack limit stress, hydrogen was enclosed by electrolytic charging in a round bar tensile test cut out from the seam weld inner surface. As shown in FIG. 1 and FIG. 2, it was found that the weld metal having a tensile strength of 950 MPa and 850 MPa has a certain relationship between the hydrogen concentration and the critical stress. Since the maximum amount of hydrogen in actual production is 2 ppm, an indicator is obtained that hydrogen cracking does not occur at a load stress of 750 MPa or less for a weld metal with a tensile strength of 950 MPa and 670 MPa or less for a weld metal with a tensile strength of 850 MPa or less. It was.

  The present inventors pay attention to the fact that hydrogen cracks are generated from the inner surface weld metal. The axial stress generated in the seam welded portion by the finite element method (hereinafter referred to as FEA) is considered in consideration of thermal stress and transformation stress, and the tensile strength is A 950 MPa weld metal was analyzed (FIG. 3). As a result, the axial stress of the inner surface weld near the fusion line reached 800 MPa, which coincided with the crack occurrence position specified from the crack observation. Furthermore, the residual stress predicted by FEA coincided with the residual stress of the inner and outer surfaces measured by X-ray diffraction.

  From the hydrogen crack limit stress shown in FIG. 1 and the residual stress distribution shown in FIG. 3, it was found that lateral cracks occurred at the peak stress portion generated at the inner surface weld. Since cracks do not occur at a stress of 750 MPa or less at a substantial maximum hydrogen content of 2 ppm, a welding method for controlling the peak stress to 750 MPa or less was examined.

FIG. 4 shows the difference in residual stress distribution when the height ratio between the inner surface welding and the outer surface welding of the weld metal is changed. The height here to add slave, respectively it as shown in FIG. 5 weld metal, is defined as Wo, and Wi the height of the weld metal prior row. The Wo / Wi of the cracked steel pipe was 1.03, and when Wo / Wi exceeded 1.4, it was predicted that the peak stress decreased toward the surface. Moreover, when Wo / Wi was less than 1.0, the peak stress decreased.

The relationship of FIG. 4 is shown in FIG. 6 as the relationship between Wo / Wi and peak stress. From this, Wo / Wi ≦ 1.0, or 1.7 ≦ Wo / Wi
The peak stress was less than 750 MPa, and it was predicted that no hydrogen cracking occurred.

FIG. 7 shows the same analysis performed on a welding material having a tensile strength of 850 MPa. The crack initiation limit stress is as shown in Fig. 2, and the safety range for this is the same,
Wo / Wi ≦ 1.0 or 1.7 ≦ Wo / Wi
It was foreseen.

  Next, the allowable maximum value and minimum value of Wo / Wi were examined. It goes without saying that if one of the weld metal parts becomes too small, an area that cannot be welded due to cracking or the like will occur, but if the heat input of the large weld metal becomes too large, the heat affected zone ( HAZ) The width is increased, and the toughness and weld joint strength are adversely affected. It is considered that the decrease in toughness is due to the fact that the HAZ width increases with an increase in heat input, leading to coarsening of crystal grains, and the probability that the crack initiation point at the notch tip of the Charpy test piece hits the coarse grains increases.

  Therefore, in the Charpy test, the relationship between the probability of occurrence of a low value in impact absorption energy and Wo / Wi was investigated. The low energy value is a phenomenon in which the absorbed energy is drastically reduced by, for example, about 20% of the average value by the test piece rather than the average value of the test in the impact test, and it is preferable that the low value occurrence probability is minimized.

FIG. 8 shows the relationship between Wo / Wi and the probability of occurrence of a low energy value. As a result,
Wo / Wi ≦ 0.6 or 2.5 ≦ Wo / Wi
It was found that the probability of low value increases in the range of.

Therefore, Wo / Wi is reduced to 0.6 ≦ Wo / Wi ≦ 1.0 or 1.7 ≦ Wo / Wi ≦ 2.5 to prevent transverse cracking and to minimize the probability of low energy generation.
It came to the conclusion that setting to is optimal.

  The above-described residual stress generation mechanism during welding is described for submerged arc welding of UOE steel pipe, and is a case where outer surface welding is performed after inner surface welding. When the inner surface welding and the outer surface welding are each performed in a single layer, the effect of the present invention appears, and when the inner surface or the outer surface is a multi-layered surface, the effect of the present invention is lost. However, usually, tack welding is performed from the outer surface side before performing inner surface welding, but a welding method that involves tacking is included in the present invention in that the main welding is performed further from the inner and outer surfaces. Further, the present invention includes the case where the outer surface welding precedes and the inner surface welding follows. Moreover, although the UOE forming process is shown as an example of the forming method of the steel pipe, it is included in the present invention when welding is performed one by one from the inner and outer surfaces by the forming method using a bending roll.

  Although the present invention is sufficiently effective even in normal molding and welding environments, it can be diffused into the weld metal due to condensation on the groove surface in the manufacturing process, adhesion of oil, excessive moisture content of the flux, etc. Although the prognostic heat of the seam part after seam welding is effective, such as when the amount of hydrogen is increased, the present invention in which the residual stress is reduced even when performing such heat treatment, the cost merit such as reduction of heating temperature and reduction of holding time The effect is great.

  The effects of the present invention will be described below with reference to examples of the present invention and comparative examples.

  Table 1 shows steel pipe size φ914 × 16t, steel pipe strength 900-1100 MPa, Table 2 shows steel pipe size φ914 × 16t, steel pipe strength 950-930 MPa, Table 3 shows steel pipe size φ711 × 12t, steel pipe strength 900-1000 MPa, Table 4 shows steel pipe size This is an embodiment when pipes are formed by a UOE forming process of φ1067 × 20 t and steel pipe strength is 900 to 1050 MPa and seam welded in the order of the inner surface and the outer surface.

  The occurrence of transverse cracking is indicated by the number of steel pipes in which transverse cracking was detected by ultrasonic flaw detection after leaving outside for 72 hours until the end of pipe expansion. The low energy value is evaluated by a Charpy impact test piece that is expanded so that the tip of a 2 mmV notch comes to a point 1 mm away from the fusion line where the lowest value is likely to occur. Was 20% or less of the average absorbed energy.

  Lateral cracks occur in the section where Wo / Wi is 1.1 to 1.4, and do not occur in other sections. On the other hand, when the energy is low, Wo / Wi = 0.4 and 2.8, the probability of occurrence increases. When 0.6 ≦ Wo / Wi ≦ 2.5, no significant difference is observed in the probability of occurrence.

The figure which shows the relationship between the hydrogen concentration and stress in the high strength steel pipe of TS: 950MPa, YS: 850MPa. The figure which shows the relationship between the hydrogen concentration and stress in the high strength steel pipe of TS: 850MPa and YS: 760MPa. The figure which shows the relationship between the position of the fusion line from a welding inner surface, and axial stress. The figure which shows distribution of the axial stress at the time of changing ratio of Wo / Wi. The figure which shows the definition of Wo and Wi. The figure which shows the relationship between ratio of Wo / Wi and axial direction stress. The figure which shows the relationship between ratio of Wo / Wi and axial direction stress. The figure which shows the relationship between inner / outer surface weld metal height ratio Wo / Wi and a transverse crack incidence.

Claims (1)

Tensile strength of less 1100MPa or more 850 MPa, from the inner surface of the seam welding by submerged arc welding, followed from the outer surface, in welded steel performed by more prime respectively, the height of the weld metal to lead in the inner surface of the steel pipe: Wi and the height of the weld metal to follow before Kigai surface: the ratio of the Wo,
0.6 ≦ Wo / Wi ≦ 1.0, or 1.7 ≦ Wo / Wi ≦ 2.5
A welded steel pipe excellent in embrittlement cracking of weld metal.

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