JP4719118B2 - Electron beam welded joint with excellent brittle fracture resistance - Google Patents

Description

  The present invention relates to an electron beam welded joint excellent in brittle fracture resistance of a welded structure, particularly a welded structure formed by butt welding a thick steel plate having a thickness of more than 50 mm with an electron beam.

Social needs to move away from fossil energy such as oil and utilize renewable natural energy are extremely increasing, and large-scale wind power generation is also spreading worldwide.
The most suitable area for wind power generation is the area where strong winds can always be expected, and offshore wind power generation has also been realized on a global scale. In order to construct a wind power tower on the ocean, it is necessary to drive the foundation of the tower into the ground at the bottom of the sea. In order to secure the height of the turbine blades of the wind power generation from the sea level, the foundation is also sufficient. Length is needed.

For this reason, the basic portion of the wind power generation tower has a pipe structure having a large cross section with a plate thickness of about 100 mm and a diameter of about 4 m, and the overall height of the tower is 80 m or more. It is required to weld and assemble such a huge structure easily and efficiently on the coast near the construction site.
Therefore, an unprecedented need has arisen to weld an extremely thick steel plate having a thickness of 100 mm as described above with high efficiency and on-site welding.

  Generally, the electron beam welding method is a welding method that can efficiently weld a thick steel plate with a high-density, high-energy beam. However, since it is necessary to perform welding while maintaining a high vacuum state in a vacuum chamber, The size of the steel plate that can be welded was limited.

On the other hand, recently, an electron beam welding method (RPEBW: Reduced Pressure Electron Beam Welding), which can be applied under low vacuum, is a welding method that can be used to efficiently weld ultra-thick steel sheets in the field. It has been developed and proposed in a laboratory (see, for example, Patent Document 1).
By using this RPEBW method, even when welding a large structure such as a wind power tower, it is expected that only the part to be welded is locally evacuated and welding can be performed efficiently.

  However, on the other hand, in this RPEBW method, a molten metal portion (hereinafter referred to as a weld metal) that is melted by an electron beam and then solidified in order to perform welding in a state where the degree of vacuum is lowered as compared with a method of welding in a vacuum chamber. A new problem has emerged that it is difficult to ensure the toughness of the part.

For such problems, conventionally, by inserting an insert metal such as plate-like Ni onto the welding surface and performing electron beam welding, the Ni content of the weld metal is set to 0.1 to 4.5%, and It is known in Patent Document 2 and the like to improve toughness such as Charpy impact value.
However, when welding using the RPEBW method, in this method, elements such as Ni in the insert metal are not uniformly diffused to the weld heat affected zone, and the weld metal and the weld heat affected zone (hereinafter referred to as HAZ zone). However, the problem that the toughness of the HAZ part varies greatly has been clarified.

In recent years, the fracture toughness value δc based on fracture mechanics, which is required by CTOD (Crack Tip Opening Displacement) test, has come to be emphasized as an index for quantitatively evaluating the safety of welded structures. It is coming. Even if a small test such as the conventional V-notch Charpy impact test shows a good result, the CTOD test of a welded joint of a large structure does not always show a good fracture toughness value δc.
The welded joint obtained by welding by the RPEBW method has a large variation in the toughness of the HAZ part even by the method using the insert metal as described above, and thus it has been difficult to sufficiently secure the fracture toughness value δc.

  In order to solve such a problem, the present inventors used an insert beam for a high-strength thick steel plate having a yield strength of 355 MPa class or more and a thickness of more than 50 mm (preferably more than about 50 mm to 100 mm) using an electron beam. When welding, the presence of the insert metal used to improve the toughness of the weld metal part increases the strength and hardness of the weld metal part, which is significantly higher than the strength and hardness of the base metal. It has been found that the local stress increases in the vicinity of the fusion weld line (hereinafter referred to as the FL portion) at the boundary between the metal portion and the HAZ portion in contact with the metal portion, and therefore the fracture toughness value δc of the FL portion decreases. .

  And based on this knowledge, the hardness of the weld metal part is controlled to be 220% or less of the hardness of the base metal part, and preferably the hardness of the weld metal part is 110% of the hardness of the base metal part. It has been found that by reducing the width of the weld metal part to 220% or less and 20% or less of the base metal plate thickness, it is possible to prevent a decrease in joint toughness due to the overmatching of the hardness of the base metal part and the weld metal part. (Patent application No. 2006-207261).

By the way, steel materials containing 2.5% by mass or more of Ni and having higher strength and excellent toughness at low temperatures have been used so that they can be used in places with more severe natural conditions.
In such a welded joint using a steel material having a high Ni content, the fracture toughness of the welded joint is also good by means of adjusting the ratio of the hardness of the weld metal part and the hardness of the base material proposed in the previous patent application. There was a case where the value δc could not be secured.

WO99 / 16101 Japanese Patent Laid-Open No. 3-247873

  Therefore, the present invention improves the fracture toughness value δc of both the weld metal part and the FL part where local stress increases in an electron beam welded joint in a welded structure using a steel material having a high Ni content, It is an object to provide a method for stably improving the fracture toughness of a joint.

  When the present inventors perform electron beam welding of a steel material having a high Ni content by placing an insert metal at a butt portion, the joint toughness is reduced by overmatching the hardness of the base metal and the weld metal as described above. From the viewpoint of prevention, further investigation was made on a means for relaxing the local stress in the FL portion and obtaining a high fracture toughness value δc.

  As a result, after adjusting the Ni concentration of the weld metal to a specified range, the fracture toughness of the welded joint can be stably improved by applying the above-mentioned means for adjusting the ratio of the hardness of the weld metal to the hardness of the base metal. It has been found that it can be improved.

  Further, the local stress in the FL part is influenced by the strength change in the narrow region on both sides sandwiching the weld melt line (FL) in addition to the hardness of the weld metal part, and if the change is small It is found that the concentration of local stress in the FL part can be alleviated, and by adjusting the hardness ratio before and after the FL and the Ni concentration of the above-mentioned weld metal, the fracture toughness of the welded joint of the steel material having a high Ni content is also obtained. Has been found to be able to improve stably.

The present invention has been made based on the above findings, and the gist thereof is as follows.
(1) A butt weld joint of a welded structure using a steel material containing Ni of 2.5% by mass or more, wherein the content of Ni contained in the weld metal of the weld joint is more than 4% by mass 8 An electron beam welded joint excellent in brittle fracture resistance, wherein the weld metal part has a hardness of 110% to 220% of the hardness of the base metal part.

  (2) A butt weld joint of a welded structure using a steel material containing Ni of 2.5% by mass or more, wherein the content of Ni contained in the weld metal of the weld joint is more than 4% by mass 8 And the hardness of the weld metal part 0.5 mm from the weld melt line is 100% to 180% of the hardness of the base metal part 0.5 mm from the weld melt line. Electron beam welded joint with excellent brittle fracture resistance.

(3) A butt weld joint of a welded structure using a steel material containing 2.5 mass% or more of Ni,
The content of Ni contained in the weld metal of the weld joint is more than 4% by mass and less than 8%, and the hardness of the weld metal part is more than 110% and less than 220% of the hardness of the base material part, The brittle fracture resistance is characterized in that the hardness of the weld metal part 0.5 mm from the weld melt line is 100% to 180% of the hardness of the base metal part 0.5 mm from the weld melt line. Electron beam welded joint with excellent generation characteristics.
(4) The electron beam welding having excellent brittle fracture resistance according to any one of (1) to (3), wherein the width of the weld metal portion is 20% or less of the thickness of the steel material. Fittings.
(5) The brittle fracture resistance according to any one of (1) to (4), wherein the weld joint of the welded structure is a butt weld of a high-strength steel plate having a thickness of more than 50 mm Excellent electron beam welded joint.

  According to the present invention, a high-strength steel plate containing 2.5% by mass or more of Ni and having a yield strength of 420 MPa class or more and having a thickness of more than 50 mm is electron beam welded to obtain a welded structure. At this time, a welded joint having a sufficiently high fracture toughness value Δc can be formed.

In a general electron beam welded joint, a part of the base metal is melted and re-solidified to form a weld metal, so that it is difficult to secure a required fracture toughness value δc in the weld metal part.
Therefore, as described above, during electron beam welding, an insert metal such as nickel foil is inserted into the butt portion, the hardenability of the weld metal portion is improved, and the toughness is ensured by a synergistic effect with the addition of Ni. However, even with this method, the toughness of the FL portion varies widely, so it was difficult to sufficiently secure the fracture toughness value δc.

  On the other hand, as described above, the hardness of the weld metal part is set to 110% or more and 220% or less of the hardness of the base material, and the deterioration of joint toughness due to the overmatching of the hardness of the base material and the weld metal part can be prevented. In the case of a welded joint using a steel material having a high Ni content, a fracture toughness value of a good welded joint cannot be ensured even by such means.

  Therefore, in order to investigate the influence of the Ni content of the weld metal, two types of steel plates, namely, a steel plate containing 3% by mass of Ni and a steel plate not containing Ni, were manufactured, and a plurality of Fe-Ni alloys having different Ni contents or An insert metal foil made of pure Ni was inserted into each welding butt, and electron beam welding was performed. Then, specimens are taken from each welded joint after welding, and a notch is provided on the HAZ side (FL, HAZ part) of the weld metal part (WM) and FL part, and a CTOD test is performed to obtain a fracture toughness value δc. (Hereinafter, also simply referred to as CTOD value) was measured, and the Ni concentration of the weld metal part was measured.

FIG. 1 shows a result of plotting the fracture toughness value δc of the WM part, FL, and HAZ part against the Ni content in the weld metal based on the obtained measurement results.
As shown in FIG. 1, in the case of a steel sheet with a Ni content of 3%, the weld metal (WM) with a Ni content in the range of more than 4% to 8% is shown in the WM part (◯) and FL, HAZ part (● ), A CTOD value of 0.15 mm or more can be ensured, but it can be seen that only a low CTOD value of less than 0.15 mm is obtained in either the WM part or FL, HAZ part in the other range.

In the case of a steel sheet not containing Ni, none of the WM part (Δ) and FL, HAZ part (black Δ) showed a CTOD value of 0.15 mm or more.
The higher the δc value, the better. However, the Norwegian Maritime Association (DNV) and other standards require a value of about 0.1 to 0.2 mm at the design temperature. The δc value was set to 0.15 mm or more.

Furthermore, when the hardness of the weld metal part and the base metal part of the example in which the CTOD value of 0.15 mm or more was secured in both the WM part and the FL, HAZ part was measured, the hardness of the weld metal part was that of the base metal part. It was found that the hardness was in the range of more than 110% and 220% or less.
From the above results, in the electron beam welded joint of steel material having a high Ni content, it is possible to relieve local stress in the FL portion and to make the Ni content of the weld metal more than 4% to 8%. It proved effective for securing.

Next, the present inventors examined a means for further relaxing the local stress in the FL portion when electron beam welding is performed using an insert metal, and obtaining a higher fracture toughness value δc.
As a result, the local stress in the FL part is influenced by the strength change in the narrow area on both sides of the weld metal line (FL) in addition to the hardness of the weld metal part. It was found by the following experiment that the concentration of local stress in the FL portion can be relaxed if it is small.

  Two electron beam welded joints A and B having different hardness distributions shown in Table 1 were prepared, and the fracture toughness value δc was measured by conducting a CTOD test in the FL part. Hv (WM average) / Hv (HAZ average) As shown in Table 1, the δc value of the FL portion was higher in the B joint having a larger value of). The Hv distribution in the vicinity of HL was examined. As shown in FIG. 2, the B joint has a smaller hardness distribution gradient in the vicinity of the FL, and the increase in local stress caused by the strength matching is that of the joint B. Was found to be smaller than the joint A.

As described above, the reason why the fracture toughness value δc is higher when the change in hardness near the FL portion is smaller is considered as follows.
In general, in a region that becomes a weld metal, solidification starts from the boundary with the base material and proceeds to the inside. During solidification, some alloy elements concentrate in the liquid phase because the solubility differs between the solid phase and the liquid phase. Due to the presence of such elements, the chemical composition of the weld metal may differ between the early and late stages of solidification, and as a result, the weld metal may vary in hardness. It is considered that the fracture toughness value δc decreases as the value increases.

Therefore, the boundary between the weld metal and the base metal, that is, the yield strength on the weld metal side in contact with the FL, is an important requirement for determining the local stress on the HAZ part side in contact with the FL. Therefore, the hardness of the weld metal side in contact with FL is important as an index that can express the yield strength.
And when there is a crack in FL, the region that affects the crack tip is in the range of ± 0.5 mm, so the position (FL + 0.5) that is 0.5 mm from the weld melt line to the HAZ side, The local stress of the FL portion is determined by the hardness Hv at a position (FL-0.5) 0.5 mm from the weld melt line to the weld metal side.

FIG. 3 shows the relationship between the local stress σ _yy (vertical axis) of the FL part obtained by the finite element method (FEM) and the hardness Hv (horizontal axis) of the weld metal (WM). It is shown that the correlation between Hv and σ _yy at the position (●) is the best. Although there is a temporary correlation between Hv and σ _yy even at the position ½ of the WM portion (◯) and the position FL of 1 mm (Δ), the variation is large, and (FL ± 0.5) It can be seen that the hardness at the position represents the magnitude of the local stress.

  As a result of our investigation, the hardness Hv (FL-0.5) of the weld metal part 0.5 mm from the weld melt line is 0.5 mm from the weld melt line (HAZ part). It was found that if the hardness Hv (FL + 0.5) of the steel is 100% or more and 180% or less, the local stress in the FL portion can be relaxed and a weld joint having a sufficiently high fracture toughness value δc can be formed.

  As described above, when a steel material containing 2.5% by mass or more of Ni is butt welded by electron beam welding, the content of Ni contained in the weld metal of the welded joint is more than 4% by mass 8%. %, And the hardness of the weld metal part 0.5 mm from the weld melt line is 100% to 180% of the hardness of the base metal part 0.5 mm from the weld melt line. By welding in such a manner, it is possible to relax the local stress in the FL portion and form a weld joint having a sufficiently high fracture toughness value δc.

The present invention based on the above findings will be sequentially described below.
The present invention is directed to a high-strength steel material containing 2.5% by mass or more of Ni as a steel material forming a welded structure. The high-strength steel plate to be used may be one manufactured from a structural steel for welding having a known component composition.
For example, in mass%, C: 0.02 to 0.20%, Si: 0.01 to 1.0%, Mn: 0.3 to 2.0%, Al: 0.001 to 0.20%, N: 0.02% or less, P: 0.01% or less, S: 0.01% or less, Ni: 2.50 to 9.0% as basic components, requirements such as improvement of base metal strength and joint toughness One or two of Cr, Mo, Cu, W, Co, V, Nb, Ti, Zr, Ta, Hf, REM, Y, Ca, Mg, Te, Se, B Steel containing the above in a total of 8% or less can be used.

  The plate thickness of the steel plate is not particularly limited, but it is a high-strength steel plate having a plate thickness of more than 50 mm that makes the above-mentioned problems manifest.

Further, during welding, an insert metal made of Ni is disposed at the butt portion. In this invention, it is necessary to weld so that Ni may contain more than 4%-8% (mass%) in the weld metal of a welded joint. As the insert metal, it is easy to use a foil made of pure Ni.
Calculate the thickness of the pure Ni foil necessary to achieve the target Ni content from the Ni content of the steel material used as the base material, the Ni content in the target weld metal, and the dimensions of the steel material, and so on. An insert metal is prepared by preparing a foil having a proper thickness or by stacking a plurality of thin foils to a required thickness.

  In the present invention, as disclosed in the previous application (Japanese Patent Application No. 2006-207261), the hardness of the weld metal part is set to be more than 110% and less than or equal to 220% of the hardness of the base material part. The weld metal part needs to have a certain degree of hardness in order to ensure hardenability and prevent the formation of coarse ferrite. The hardness of the weld metal part is more than 110% of the hardness of the base metal. There is a need to. However, if it is too hard, the fracture toughness value δc is lowered due to an increase in local stress, so it is suppressed to 220% or less.

  Then, after adjusting the hardness in such a manner, the content of Ni contained in the weld metal of the welded joint is more than 4% and not more than 8% by mass%. When a region melted by an electron beam is re-solidified, even in the case where the crystal grain size is coarsened or the oxide is reduced in the region, in order to obtain a structure that can stably secure toughness, Ni It is necessary to contain more than 4%. Further, if Ni is contained in excess of 8% by mass, the hardness of the weld metal part increases excessively, and it becomes difficult to satisfy 220% or less of the hardness ratio of the weld metal part to the base material part.

  The difference in hardness as described above is that the Ni content of the weld metal satisfies the conditions of the present invention, and further, between the steel material used as the base material and the weld metal formed using the insert metal. This is achieved by appropriately adjusting the balance between components and adjusting the cooling rate after welding so that the hardness of the weld metal does not become too high.

In the present invention, the hardness of the weld metal part 0.5 mm from the weld melt line is more than 100% and 180% of the hardness of the base metal part (HAZ part) 0.5 mm from the weld melt line. By making the following, the local stress in the FL portion is further relaxed so that a higher fracture toughness value δc can be obtained.
If the hardness ratio is less than 100%, a pro-eutectoid ferrite structure or the like is generated, and a soft structure is often mixed locally, which may be the starting point of fracture, and the necessary strength can be secured. Moreover, when it exceeds 180%, the change in hardness is too large, and the local stress in the FL portion increases too much.

  Such a gradual change in hardness from the HAZ side to the weld metal side is the same as in the case of the above-described difference in hardness, and the Ni content of the weld metal is made to satisfy the conditions of the present invention. This is achieved by appropriately adjusting the balance between the components of the steel material used as the material and the weld metal formed using the insert metal.

In the present invention, the conditions for electron beam welding are not particularly limited. For example, in the case of a plate thickness of 80 mm, the conditions are a voltage of 175 V, a current of 120 mA, and a welding speed of about 125 mm / min. Electron beam welding is usually performed under a high vacuum of 10 ⁻³ mbar or less, and is a joint welded under a low vacuum, for example, about 1 mbar, as in the above-described RPEBW method. In addition, the present invention can be applied.

Further, when the electron beam irradiation area becomes large during electron beam welding, the amount of heat input to the steel sheet becomes excessive, the structure of the FL part becomes coarse, and it is preferable for stably securing the fracture toughness value δc of the FL part. Absent.
Moreover, when producing an electron beam welded joint using RPEBW welding, the width of the weld metal tends to increase compared to a welded joint produced by electron beam welding (EBW welding) in a high vacuum state in a vacuum chamber. is there.
For this reason, in the present invention, even when RPEBW welding is used, in order to stably secure the fracture toughness value δc of the electron beam welded joint, the width of the weld metal portion is 20% or less of the thickness of the base metal portion. Is preferable.

Next, the present invention will be described based on examples. The conditions in the examples are one example of conditions adopted to confirm the feasibility and effects of the present invention, and the present invention It is not limited to examples.
The present invention can adopt various conditions or combinations of conditions as long as the object of the present invention is achieved without departing from the gist of the present invention.

  A thick steel plate having a thickness of 50 to 100 mm containing the components shown in Table 2 and the balance Fe and unavoidable impurities is prepared, and Ni insert metal (NA) or Ni- Fe alloy insert metal (NB, NC) was inserted and butt welded by electron beam welding, and then the characteristics and performance of the formed welded joint were tested and investigated.

The results of the test are shown in Table 4 together with the weld joint conditions.
Hv (BM) is an average value of the hardness in the thickness direction of the base material measured by an indentation of 10 kg. Hv (WM) is a hardness value measured with a 10 kg indentation at the center of the plate thickness of the weld metal part.
Hv (FL + 0.5) is a hardness value at a position 0.5 mm from the weld melt line to the HAZ side, and Hv (FL-0.5) is 0.5 mm from the weld melt line to the weld metal side. It is the hardness value at the position where it enters.

Regarding the performance of the welded joint, δc (mm) is a value obtained at a test temperature of −10 ° C. in the CTOD test.
The joint tensile strength (MPa) is a result of producing a NKU No. 1 test piece and conducting a joint tensile test, and indicates a fracture strength.

As shown in Table 4, No. of the present invention example. Nos. 1 to 17 have various conditions within the range defined by the present invention, and the δc value shows a sufficient value for both the weld metal part and the FL and HAZ parts.
Among them, in Examples 16 and 17 of the present invention, the value of the bead width with respect to the steel material thickness exceeds the preferable range defined in the present invention, so the δc value is a low value in both the weld metal part and the FL and HAZ parts. Yes.

On the other hand, in Comparative Examples 18, 20, 22, and 23, since the Ni content in the weld metal is as high as 8% or more, the value of Hv (WM) / Hv (BM) is 220% or more, and Hv The value of (FL−0.5) / Hv (FL + 0.5) is also 180% or more. As a result, δc of the weld metal has a sufficiently high value, but δc of the FL and HAZ portions has a very low value.
In Comparative Examples 19 and 21, since the Ni content in the weld metal is 4% or less, both Hv (WM) / Hv (BM) and Hv (FL−0.5) / Hv (FL + 0.5) are present in the present invention. However, the δc of the weld metal is an insufficient value.
In Comparative Example 24, the Ni content of the steel material is less than 2.5%, and the Ni content in the weld metal is Hv (WM) / Hv (BM), Hv (FL−0.5) / Hv (FL + 0. 5) is a value within the range of the present invention, but δc is insufficient for both the weld metal and the FL and HAZ parts.

According to the present invention, in an electron beam welded joint of a high-strength steel plate having a high strength and a large plate thickness, even if a weld defect exists or a fatigue crack is generated or grows, brittle fracture is unlikely to occur. Therefore, it is possible to prevent a fatal damage or damage that causes the welded structure to break.
Therefore, the present invention has a remarkable effect of significantly increasing the safety of the welded structure, and is an invention having high industrial utility value.

It is a figure showing the influence of Ni content in a weld metal with respect to the fracture toughness value (delta) c of a weld metal part and FL, HAZ part. It is a figure showing distribution of hardness (Hv) of FL part of an electron beam welding joint from which hardness distribution differs. It is a figure showing the relationship between the local stress of FL part, and the hardness of a weld metal part.

Claims (5)

A butt weld joint of a welded structure using a steel material containing Ni of 2.5% by mass or more,
The content of Ni contained in the weld metal of the weld joint is more than 4% and less than 8% by mass%, and the hardness of the weld metal part is more than 110% and less than 220% of the hardness of the base metal part. An electron beam welded joint with excellent brittle fracture resistance. A butt weld joint of a welded structure using a steel material containing Ni of 2.5% by mass or more,
The content of Ni contained in the weld metal of the weld joint is greater than 4% and less than 8% by mass, and the hardness of the weld metal part 0.5 mm from the weld melt line is 0. 0 from the weld melt line. An electron beam welded joint excellent in brittle fracture resistance, characterized by being 100% or more and 180% or less of the hardness of a base material portion containing 5 mm. A butt weld joint of a welded structure using a steel material containing Ni of 2.5% by mass or more,
The content of Ni contained in the weld metal of the weld joint is more than 4% by mass and less than 8%, and the hardness of the weld metal part is more than 110% and less than 220% of the hardness of the base material part, The brittle fracture resistance is characterized in that the hardness of the weld metal part 0.5 mm from the weld melt line is 100% to 180% of the hardness of the base metal part 0.5 mm from the weld melt line. Electron beam welded joint with excellent generation characteristics.   The electron beam welded joint having excellent brittle fracture resistance according to any one of claims 1 to 3, wherein the width of the weld metal portion is 20% or less of the thickness of the steel material.   The electron beam welding having excellent brittle fracture resistance according to any one of claims 1 to 4, wherein the weld joint of the welded structure is a butt weld of a high-strength steel plate having a thickness of more than 50 mm. Fittings.

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