JP5842585B2 - Metal-based flux cored wire and multi-electrode submerged arc welding method using the same - Google Patents

Description

  The present invention includes a low hydrogen flux for submerged arc welding used in pipe manufacturing welding of high-strength large-diameter welded steel pipes used for applications such as natural gas, crude oil transportation line pipes, and structural pipes. The present invention relates to a wire and a multi-electrode submerged arc welding method using the same.

  In recent years, in order to reduce the total cost of natural gas or crude oil transportation pipelines, the application of high-strength large-diameter steel pipes that can reduce the amount of steel used and reduce the cost of on-site welding as well as increasing the operating pressure has begun. . Such welded steel pipes are generally submerged due to their superior weld quality, beautiful bead appearance, and high-speed welding with high heat input. Manufactured by single-sided double-sided welding from inside and outside by arc welding.

  However, in a welded portion of high-strength steel, a minute crack that is perpendicular to the welding direction often occurs in the weld metal portion. This minute crack is a serious weld defect that is detected at a certain frequency even in the nondestructive inspection after the outer surface welding and the visual inspection of the weld bead surface, and the cause is said to be a low temperature crack in the weld metal part. It is known that the cold cracking of weld metal parts is more likely to occur as the strength of the weld metal increases, and depending on the degree of stress load after laying the steel pipe, there is a risk of causing the destruction of the entire steel pipe starting from this micro crack. Of course, it must of course be completely removed in the steel pipe manufacturing stage.

  By the way, cold cracking of weld metal is generally interpreted as hydrogen embrittlement cracking, and the concentration of hydrogen (diffusible hydrogen) in the weld metal, the stress applied to the weld metal part, and the structure of the weld metal part The resistance to hydrogen cracking is considered to be the dominant factor. The reason why this low-temperature crack often occurs in welds of high-strength steel is that the weld metal part is formed of a hard structure such as martensite and bainite, which are metal structures that are not susceptible to hydrogen cracking. is there. The formation of such a hard structure is attributed to the fact that the alloy components in the wire are greatly increased in order to ensure the required material strength.

  Under such technical background, a method for preventing the occurrence of low-temperature cracking in a high-strength weld metal has been proposed so far. For example, Patent Document 1 controls the chemical composition of the weld metal and the cooling rate after welding. Thus, a method for reducing the amount of diffusible hydrogen which is the main cause of the hydrogen cracking is disclosed. However, limiting the chemical composition of the weld metal is effective in preventing cold cracking of the weld metal, but it adversely affects other properties such as the low temperature toughness and deformation characteristics of the weld metal. Therefore, it is desired to develop a method for increasing the degree of freedom in designing the weld metal component as much as possible.

  Patent Document 2 discloses a method of reducing diffusible hydrogen by subjecting a weld metal to a dehydrogenation heat treatment. In this method, the amount of diffusible hydrogen is reduced to about 0.2 cc / 100 g weld metal by heating at 100 to 200 ° C. Therefore, this disclosed method is an extremely effective method for preventing the occurrence of cold cracking. However, since it is necessary to add a new heat treatment process to the steel pipe manufacturing process, there is a problem of greatly reducing the productivity of steel pipe manufacturing. Arise.

  On the other hand, as described above, as a basic characteristic as a welding material for high-strength steel pipes, the alloy component in the wire is increased, and the strength of the formed weld metal part is generally required to be higher than the base metal strength. However, when increasing the amount of alloy added to the weld metal to increase the strength of the weld metal and manufacturing a solid wire that is generally used for submerged arc welding, the wire itself has high strength due to work hardening during wire drawing. In some cases, problems such as excessive breakage and easy breakage during wire drawing may occur.

  Patent Documents 3 and 4 disclose the use of a flux cored wire for submerged arc welding in order to solve this problem. Since the flux-cored wire is manufactured by filling a hollow steel outer shell with metal (alloy) powder, flux components, etc., there is no problem of work hardening at the time of wire drawing as in the case of solid wire production. The alloy components can be adjusted and added. However, the amount of diffusible hydrogen in the weld metal using this flux cored wire is disclosed, for example, in Patent Document 3 as being about 3 to 7 cc in 100 g weld metal, and low temperature cracking occurs in high strength steel pipes. A reduction to the level of diffusible hydrogen to prevent this has not been achieved.

JP 2003-33876 A JP 2006-263814 A JP 2008-105036 A JP 2006-142377 A

  As described above, in order to suppress the low temperature cracking of the high strength steel pipe welded portion containing a lot of alloy components, it is necessary to take measures against the following three factors. That is, reduction of diffusible hydrogen, relaxation of load stress, and improvement of weld metal structure. However, as described above, none of the conventional techniques proposes sufficient improvement measures, and some problems to be solved are included. In particular, the adjustment of the weld metal composition and the dehydrogenation heat treatment are materials. Narrowing the degree of freedom in design and causing a significant increase in manufacturing costs due to an increase in new manufacturing processes.

  The present invention advantageously responds to the above problems from a technical and economic viewpoint, and reduces the amount of diffusible hydrogen that can be said to be the main cause of cold cracking of weld metal without relying on heat treatment before welding or after welding. It is an object of the present invention to provide a flux cored wire for submerged arc welding and a multi-electrode submerged arc welding method using the same in order to reduce extremely efficiently.

The present invention has been made to solve the above problems, and the gist of the present invention is as follows. That is,
(1) A flux-cored wire used for submerged arc welding, wherein the flux component filled in the wire has a melting point of 1000 ° C. or less and 0.1 to 10% by mass of a powdery fluorine compound. A flux-cored wire for submerged arc welding, characterized by containing less than%.
(2) The flux-cored wire for submerged arc welding according to (1) above, wherein the metal powder not containing an oxide is 80% by mass or more of the filler component in the flux-cored wire.
(3) In multi-electrode submerged arc welding of three or more electrodes, the flux cored wire for submerged arc welding described in (1) or (2) above is used for the first electrode, or the first and second electrodes, and the residual electrode A multi-electrode submerged arc welding method using a solid wire.

  According to the present invention, natural gas, high-strength line pipes for crude oil transportation, structural high-strength steel pipes, and the like are caused by diffusible hydrogen generated in weld metals of high-strength steel pipes containing alloy components that are highly susceptible to hydrogen cracking resistance. It becomes possible to prevent the occurrence of cold cracking.

The figure which shows the relationship between the amount of diffusible hydrogen in a submerged weld metal, and melting | fusing point of the added fluoride.

  Embodiments of the present invention will be described below. The presence or absence of cold cracking in the weld metal depends on the presence or absence of diffusible hydrogen. That is, if the amount of diffusible hydrogen is reduced as much as possible, cold cracking does not occur even when the load stress generated in the cooling process or the pipe expansion process after the end of welding reaches a considerably high level. Here, the hydrogen source of diffusible hydrogen in the weld metal can be broadly divided into residual moisture in the flux, moisture present on the groove surface for reasons such as dew condensation, and contamination from the atmosphere during welding. Moisture. For this reason, in order to prevent cold cracking of the weld metal, such as drying a sintered flux with relatively high moisture absorption in the atmosphere before welding and preheating the weld groove surface before welding. Measures taken in an effort to eliminate the cause of diffusible hydrogen from the ground up. However, in spite of such measures, diffusible hydrogen still remains in the weld metal although it is in a small amount, causing cold cracking. For this reason, low-temperature cracking of high-strength weld metal could not be suppressed unless further measures such as dehydrogenation heat treatment and load stress removal were taken.

Therefore, the present invention focuses on eliminating hydrogen taken into the weld metal by positively using a chemical reaction occurring when an arc is generated in the submerged arc welding process, unlike the prior art. The hydrogen mixed from the hydrogen source described above originates from atomic hydrogen that water (H ₂ O) originally entered the welding arc in a molecular state and dissociated. That is, H ₂ O dissociates under the high temperature environment of the arc, and further exists in the form of hydrogen plasma or hydrogen ions (H ⁺ ) in the arc and is taken into the molten pool. In the present invention, gaseous hydrogen fluoride (HF) is formed by chemically bonding H ⁺ existing in the arc with fluoride ions (F ⁻ ) generated from low melting point fluoride added from the flux-cored wire. The technical idea is to reduce the concentration of hydrogen that is originally taken into the molten pool by generating the gas and discharging it outside the arc.

In general, the spraying flux used in submerged arc welding, often CaF ₂ is added which is a kind of fluoride. CaF ₂ is generally added for the purpose of improving the low temperature toughness of the weld metal by increasing the basicity of the flux and reducing the oxygen concentration in the weld metal. This CaF ₂ also has the effect of dissociating in the high-temperature environment in the arc and generating fluoride ions, but its melting point is relatively high at 1403 ° C., so most of the added CaF ₂ leads to dissociation. However, it remains in the welding slag, and the above-described effects are hardly obtained.

Therefore, the present inventors have made a detailed study on the type of fluoride that is more easily dissociated in the arc and promotes the H ⁺ + F ⁻ → HF reaction, and the optimum addition method thereof. As a result, using a fluoride having a relatively low melting point as a flux, the method of adding is not simply spraying, but a flux-cored wire, and the first electrode of multi-electrode submerged arc welding, or The present inventors have found a new finding that by applying to the first and second electrodes, the diffusible hydrogen concentration in the weld metal can be greatly reduced.

  Furthermore, it has also been clarified that the alloy composition and the amount of the welding wire can be easily adjusted by applying the flux-cored wire, leading to the establishment of a multi-electrode submerged arc welding method suitable for pipe making welding of high-strength steel pipes. It was.

  Hereinafter, the reason for limitation of the low hydrogen type submerged welding wire and the multi-electrode welding method of the present invention will be described.

Melting point of added fluoride: 1000 ° C. or less FIG. 1 shows the amount of diffusible hydrogen in a submerged weld metal having a strength exceeding a tensile strength of 800 (N / mm ² ) and the added fluoride. It is the figure (in the figure; with fluoride addition) which showed the relationship with melting | fusing point. In the figure, with fluoride addition, a fluoride-added flux-cored welding wire manufactured by adding various fluorides having different melting points to the flux-cored welding wire flux was used. The figure also shows the measurement results when no fluoride is added (in the figure; no fluoride added). This is the same as the production of a flux-added welding wire without adding fluoride from the flux component of the above-mentioned fluoride-added flux-containing welding wire, and welding it under the same conditions, and measuring the amount of diffusible hydrogen. It is a thing.

  The amount of diffusible hydrogen was measured using a test method based on JIS Z 3118, a method for measuring the amount of hydrogen in steel welds, and the measurement of hydrogen gas released from the test piece was based on a gas chromatograph method. Moreover, each welding wire and the flux with which it fills in a wire use a commercial material, and perform the drying process according to a conventional method before use.

From this figure, regardless of the type of wire and the type of flux filled in the original wire, when the melting point of fluoride is relatively low compared to CaF ₂ (1406 ° C), it is compared with the case where no fluoride is added. Thus, the effect of reducing the amount of diffusible hydrogen in the weld metal is recognized. This effect is remarkable when the melting point of the fluoride added in the flux filled in the wire is 1000 ° C. or less. This means that when the melting point of the fluoride added to the flux added in a composite is 1000 ° C. or less, the dissociation reaction and the hydrogen fluoride generation reaction were further promoted in the arc. It can be achieved by setting the melting point of the fluoride to be added to 1000 ° C. or less. For this reason, the melting point of the added fluoride is regulated to 1000 ° C. or less. As the fluoride having a melting point of 1000 ° C. or lower, for example, the fluorides shown in Table 1 can be used.

  Next, a description will be given of the limiting conditions for achieving the above-described effects and without adversely affecting the weldability, particularly the welding workability and the weld bead appearance, without hindering the production process of various fluxes.

Mass% of fluoride added: 0.1% or more and less than 10% The fluoride added to the flux filled in the wire dissociates almost 100% in the arc atmosphere. From this, the fluoride added to a flux is also positioned as a gas generating agent. Of these gas components, [F ⁻ ] combines with [H ⁺ ] as described above to contribute to the dehydrogenation reaction in the arc and reduce the amount of diffusible hydrogen in the weld metal. To obtain, at least 0.1% addition is necessary. Therefore, the lower limit of addition is defined as 0.1%. On the other hand, when excessively added, the amount of gas generated becomes excessive, and there is a risk that a pock mark is generated. For this reason, the upper limit of the addition amount is specified to be less than 10%.

  In the present invention, a predetermined effect can be obtained even if different types of fluorides are added in combination. However, at least one of the fluorides added in combination must have a melting point within the scope of the claims, and the mass% of the total fluoride must be less than 10% of the total flux mass.

Metal powder mass ratio not including oxide among filler components in flux-cored wire : 80% or more The metal powder in the flux component filled in the wire is the main component of the filler and is pure metal, iron-based The sum of alloys, metal alloys, and metal carbides is added to ensure the strength and toughness of the weld metal, and remains as an alloy component in the weld metal after welding.

  In general flux cored wire, a relatively large amount of various oxides are added to the filler for the purpose of exhibiting an arc stabilizing effect. However, in the present invention, the applicable welding is limited to submerged arc welding. The above effect is borne by the flux dispersed during welding, and the addition of a large amount of oxide in the flux component filled in the wire increases the oxygen content of the weld metal depending on the welding conditions. Since the low temperature toughness of the part may be lowered, it is preferable that the lower limit of addition of the non-oxide component as the main component of the filler is 80%.

Applicable electrode of flux-cored wire at the time of multi-electrode welding of 3 or more electrodes: only the first electrode, or the first and second electrodes Generally, in the construction of the longitudinal seam welding of the welded steel pipe to which the present invention is applied, In order to increase the tube speed and improve the productivity, multi-electrodes of 3 or more electrodes and high-speed welding are often applied. For welding employing multiple electrodes of three or more electrodes, the type of welding wire applied to each electrode can be selected as appropriate. However, in the present invention, as described above, the application electrode for flux-cored wire is mainly used as the leading electrode. It is necessary to apply to the center. As described above, in the present invention, it is necessary for [F ⁻ ] generated by the fluoride dissociation reaction to effectively capture [H ⁺ ], but when the flux-cored wire is applied only to the trailing electrode. Since the welding speed is relatively high, there is a possibility that the welding is completed without the capture reaction sufficiently progressing.

Further, even when a flux-cored wire is applied after the third electrode, in addition to the saturation effect of [H ⁺ ] being saturated, the integrated amount of the oxide component contained in each electrode welding wire becomes too large, This is because the oxygen content of the weld metal may increase.

Therefore, from the viewpoint of generating [F ⁻ ] as much as possible on the front surface of the molten pool, the applicable electrode of the flux-cored wire is preferably only the first electrode or the first and second electrodes.

  Therefore, a general solid wire is used for the remaining electrode, which is an electrode other than only the first electrode or the first and second electrodes.

Hereinafter, the present invention will be described in detail based on examples. The steel sheet having high tensile strength shown in Table 2 was piped to the outer diameter shown in the same table using the UOE process, and the assembling part was subjected to submerged arc welding of 3 electrodes and 4 electrodes to form inner and outer surfaces. Longitudinal seam welding was performed in this order. Table 3 shows the flux-cored wires used in this example. In the table, “added fluoride in wire”
It means the added fluoride in the flux component filled in the wire. The “composite added fluoride in the wire” means that the added fluoride is added in a composite manner.

Incidentally, were subjected during welding, flux is sprayed into the groove uses the melt flux, the component is composed mainly of _{SiO 2} -CaO-CaF 2, it was dried in 1hr at 300 ° C. prior to welding . The melting type flux is used because it is excellent in high-speed weldability.

  After completion of welding on each of the inner and outer surfaces, the welding workability is evaluated by visual observation of the welded part. Further, in order to detect low temperature cracks in the welded metal part, after leaving the outer surface welded for 72 hours, the joint part is subjected to JIS. The ultrasonic flaw detection test based on G0584 was performed, and the presence or absence of the generation | occurrence | production of the low temperature crack of a weld metal part was confirmed. It should be noted that no dehydrogenation heat treatment is applied to the base metal part and the weld metal part before or after the start of pipe making welding.

  Table 4 shows the evaluation results of Examples and Comparative Examples. As shown in Examples 1 to 16, for the welded steel pipe to which an appropriate amount of the low melting point fluoride according to the present invention is added, the appearance of the welded portion is very good, and the diffusible hydrogen content of the inner and outer surface weld metal As a result, no cold cracking occurred in the weld metal.

On the other hand, in the comparative example using the flux B0 to which a low melting point fluoride other than CaF ₂ is not added, although the weld bead appearance is good, the amount of diffusible hydrogen in the weld metal is high, and the inner and outer surfaces Cold cracking has occurred in the weld metal part.

  Further, even in the case of a low melting point fluoride, the amount of diffusible hydrogen is low under the influence of the low melting point fluoride in Comparative Examples 17 and 18 where the addition amount exceeds 10% by mass, and no low temperature cracking occurs. However, since the addition amount is too large, welding defects such as meandering of the weld bead or slag entrainment due to destabilization of the welding arc are recognized, and the appearance is not good.

  Further, in Comparative Examples 19 to 22 where the melting point of the added fluoride is higher than 1000 ° C., cold cracking occurs in the weld metal portion regardless of the addition amount, and the addition amount exceeds 10% by mass. The weld bead appearance is not good.

  Further, in Comparative Examples 23 and 24 in which low melting point fluorides are added in combination, although the low temperature cracking does not occur, the total amount is too large, so the appearance of the weld bead is not good and is not suitable for practical use.

Claims (2)

It is a flux-cored wire used for submerged arc welding, and the flux component filled in the wire contains a melting point of 1000 ° C. or lower and a powdery fluorine compound of 1.3 mass% or more and less than 10 mass%. ,
And the total fluoride is less than 10% by mass of the total flux mass,
Metal flux for submerged arc welding containing 80% by mass or more of powder composed of at least one of pure metal, iron-base alloy, metal alloy, and metal carbide among flux components in flux- cored wire Wire. In multi-electrode submerged arc welding of three or more electrodes, the metal flux-cored wire for submerged arc welding according to claim 1 is used for the first electrode or the first and second electrodes, and a solid wire is used for the remaining electrode. A feature of the multi-electrode submerged arc welding method.

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