JP5171007B2 - 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, as an index for quantitatively evaluating the safety of welded structures, the fracture toughness value δc value based on fracture mechanics obtained by CTOD (Crack Tip Opening Displacement) test is emphasized. It has become to. 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.
In the welded joint obtained by welding by the RPEBW method, it is difficult to sufficiently secure the fracture toughness value δc because the toughness of the weld heat affected zone varies greatly in the method using the insert metal as described above.

On the other hand, it is also known to improve the toughness of an electron beam welded joint without using insert metal. For example,
Patent Document 3 contains, as a base material, Al: 0.007% or less, Ti: 0.003-0.05%, O: 0.0010-0.0100%, and a particle size of 0.05. It describes that the microstructure of the HAZ part and the weld metal part is refined using a steel in which Ti oxide of ˜5.0 μm is uniformly dispersed.

Patent Document 4 describes that when the Al content of the base material is less than 0.005%, a weld metal having a fine microstructure can be obtained even with a small number of non-metallic inclusions.
In Patent Document 5, as a base material, Mn: 0.8 to 1.5%, P: 0.010% or less, S: 0.001% or less, Ti: 0.005 to 0.03%, Al: 0.005% or less, Ca: 0.001 to 0.004%, N: 0.001 to 0.005%, O: steel containing 0.003% or less, S is 0 in the weld metal part with inclusion .002~0.01%, Ti of 0.1 to 5 [mu] m, Al, a composite of oxide and MnS mainly of Ca be contained 5-1000 pieces / mm ² have been described .

Patent Document 6 contains Ti: 0.005 to 0.025%, Al: 0.020 to 0.050%, O: 0.0030% or less as a base material, and [O] <0. 0.19 × [Si] + 0.009 × [Al] The composition consists of steel with 3 μm or more inclusions in steel of 5.0 pieces / mm ² or less, and a weld metal part of 3 μm or more in steel. It is described that the number of existing objects is 3.0 pieces / mm ² or less.

  The above technology forms a large number of fine oxide-based non-metallic inclusions in the cooling process after welding, and uses these inclusions as the nucleus of the transformation in the transformation from austenite to ferrite. By forming a microstructure containing a large amount of acicular ferrite, a weld metal with excellent toughness is obtained. In both cases, the fracture toughness value δc value in the CTOD test is the same as when using the above insert metal. Is not studied, and both are electron beam welding using a high vacuum, that is, a technique based on the premise that oxygen is not supplied into the weld metal from the atmosphere, and the RPEBW has a low degree of vacuum. Further study is needed on the application of.

  As another method for improving toughness, in a high heat input welded joint such as electrogas welding, the hardness ratio of the weld metal to the base metal is set to 110 in order to ensure the fracture toughness value Kc based on the deep notch test. Patent Document 7 proposes a method for improving the fracture toughness Kc of the boundary between the weld metal part and the base metal part (hereinafter referred to as the FL part) by controlling it to be less than or equal to%.

  However, in order to ensure the fracture toughness value δc of the electron beam welded joint, it is necessary to satisfy the fracture toughness value δc of both the FL part and the weld metal part. Similarly, when the hardness is lowered to 110% or less of the base metal hardness, there arises a problem that the fracture toughness value δc of the weld metal portion in the electron beam welded joint cannot be secured.

  In addition, the electron beam welding method is a method in which the base metal of the welded portion is once melted and re-solidified by the energy of the electron beam and then welded, and the weld metal portion is hardened by a welding wire or the like, such as electrogas welding. It is difficult to easily control characteristics such as the thickness and fracture toughness value δc.

WO99 / 16101 Japanese Patent Laid-Open No. 3-247873 JP-A-62-64486 Japanese Patent Laid-Open No. 64-34598 JP 2001-207242 A JP 2003-201535 A JP-A-2005-144552

  In view of the above prior art, the present invention provides the fracture toughness value δc of both the weld metal part and the FL part where local stress increases in a welded joint which is electron beam welded without using an insert metal. It is an object of the present invention to provide a method for improving and stably improving the fracture toughness of a welded joint.

  When the present inventor performs electron beam welding using an insert metal, 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, and the strength of the base metal It has been found that when the hardness is significantly higher than the hardness, local stress increases in the vicinity of the boundary with the HAZ portion in contact with the weld 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 material, and preferably the hardness of the weld metal part is 110% or more and 220 of the hardness of the base material. %, Less than 20% of the base metal plate thickness, the joint toughness can be prevented from lowering due to over-matching of the base metal and weld metal thickness, and a patent application was filed first. (Japanese Patent Application No. 2005-207261).

In the electron beam welding of a high-strength thick steel plate having a yield strength of 355 MPa class or higher and a plate thickness of more than 50 mm (preferably, more than about 50 mm to about 100 mm), the inventors of the present invention From the viewpoint of preventing reduction in joint toughness due to over-matching of hardness, the technology for improving the microstructure of the weld metal part using the fine oxide as described in Patent Documents 3 to 6 is further developed. Thus, a technique for realizing a welded joint that can improve the fracture toughness value δc of both the weld metal part and the FL part when insert metal is not used and can stably secure the fracture toughness of the welded joint was studied.
And in the process, when the inclusion of a specific size was present in the weld metal at a certain frequency or more, it was found that the fracture toughness value δc varies, and the present invention was completed.

The gist of the present invention for solving the above problems is as follows .
(1 ) By mass%, C: 0.02 to 0.2%, Mn: 0.8 to 3.5%, S: 0.0005 to 0.0025%, Al: less than 0.02%, Ti: A butt weld joint of a welded structure using a steel material containing 0.01 to 0.05% and having a Pcm value represented by the above formula (1) of 0.12% to 0.5%. And
The amount of O contained in the weld metal of the weld joint is 20 ppm or more and 70 ppm or less, and the amount of oxide having a particle size of 2.0 μm or more is 10 pieces / mm ² or less, and the particle size is 0.1 μm or more. An electron beam welded joint excellent in brittle fracture resistance, characterized in that the amount of Ti oxide less than 2.0 μm is 30 to 600 pieces / mm ² .
Pcm = C + Si / 30 + Mn / 20 + Cu / 20 + Ni / 60 + Cr / 20 + Mo / 15 + V / 10 + 5B
(1)

( 2 ) The electron beam welded joint having excellent brittle fracture resistance as described in (1 ) above, wherein the hardness of the weld metal part is more than 110% and 220% or less of the hardness of the base metal.
( 3 ) The electron beam welded joint having excellent brittle fracture resistance according to (1) or (2) , wherein the welded structure is a butt-welded steel plate having a thickness of more than 50 mm.

  According to the present invention, when a high-strength steel sheet having a yield strength of 355 MPa class and a thickness of more than 50 mm is welded by electron beam welding to form a welded structure, a weld joint having a sufficiently high fracture toughness value δc can be formed. it can.

  As a result of investigating in detail the failure occurrence point in the CTOD test of an electron beam welded joint formed using a steel plate in which Ti is added and fine oxide is dispersed, It has been found that oxides of a certain size or more can be obtained, and variation in the fracture toughness value δc in the CTOD test can be reduced by reducing the presence frequency of such oxides.

Hereinafter, an experiment in which the above knowledge is obtained will be described.
A 70 mm thick steel plate containing C: 0.04%, Mn: 1.8%, S: 0.003%, Al: 0.006%, Ti: 0.02% was butted. In order to examine the difference between the welded joints depending on the presence or absence, one was (a) Ni foil inserted into the butt, and the other was (b) welded by the RPEBW method without inserting Ni foil.
At each welded joint after welding, specimens were taken from two positions in the steel sheet thickness direction 1/4 and 3/4, and the weld metal part (WM part), the boundary between the weld metal part and the base metal part A CTOD test was performed with notches in the (FL part) and the HAZ part, and the hardness change of the welded joint part was examined.

A CTOD test result is shown in FIG. 1, and the hardness change of a welded joint part is shown in FIG.
In the case of (a) in which Ni foil is inserted into the butt portion, the hardness of the weld metal portion is high and the fracture toughness value δc in the FL portion is reduced, whereas in the case of not inserting Ni foil (b) The weld metal part has a low hardness and the degree of overmatching of the hardness is relaxed. The fracture toughness value in the FL part is the same level as the weld metal part, and the fracture toughness value of the weld metal part The value of δc was also somewhat lower than when Ni foil was inserted.

Next, the state of oxide dispersion in the weld metal part in the cases (a) and (b) was investigated.
In the case of (b), the amount of Ti oxide having a particle size of 0.1 μm or more and less than 2.0 μm is 400 pieces / mm ² , and the fine Ti oxide is uniformly dispersed in the weld metal. The number of oxides having a particle size of 2 μm or more was 2 / mm ² , and the number was small.
On the other hand, also in the case of (a), the oxide dispersion state was the same as in the case of (b), and no special difference was observed between the two. However, in the case of (a), since the hardness of the weld metal part was as high as 260% of the hardness of the FL part, it is considered that the local stress in the Fl part increased and δc became low.

  As described above, when the Ni foil is not inserted, the degree of overmatching between the weld metal part and the HAZ part can be reduced by obtaining a weld metal in an appropriate oxide dispersion state. , HAZ part, it was confirmed that a high fracture toughness value δc can be obtained, and the relationship between the oxide in the weld metal part and the fracture toughness value δc of the weld metal part and the FL part when no Ni foil is inserted. investigated.

  C: 0.04%, Mn: 1.8%, S: 0.003%, Al: 0.006%, Ti: 0.02%, oxygen content is different from 10-250 ppm, thickness After butt welding a 70 mm steel plate by the RPEBW method, similarly, specimens are taken from the weld metal parts at two positions 1/4 and 3/4 in the thickness direction of the steel plate to measure the fracture toughness value and the number of oxides. went.

  The number of inclusions is obtained by obtaining the area of each oxide by image processing of an optical microscope image, and the diameter of the circle equivalent to that area (circle equivalent diameter) as the particle diameter of the oxide, and the particle diameter is 2 μm. The number of oxides per unit area was determined.

The results are shown in FIG. 3, and the number of oxides having a size of 2 μm or more is 10 pieces / mm ² or less, so that the variation in fracture toughness value δc of the weld metal is greatly reduced and a sufficiently high value is obtained. I understand that.

Furthermore, through the same experiment, the kind of Ti oxide and the dispersion condition for obtaining a weld metal having a good fracture toughness value were obtained.
As a result, it was found that a weld metal part having a good fracture toughness value δc can be obtained when the amount of Ti oxide having a particle size of 0.1 μm or more and less than 2.0 μm is 30 to 600 pieces / mm ² .

  And this invention was made | formed as a result of examining further about the chemical composition of the base material from which the dispersion condition of such an oxide was obtained.

Hereinafter, the present invention will be sequentially described.
In the present invention, as a base material constituting the welded structure, at least by mass, C: 0.02-0.2%, Mn: 0.8-3.5%, S: 0.0005-0. A steel material containing 0025%, Al: less than 0.02%, Ti: 0.01 to 0.05% and having a Pcm value of 0.12% to 0.5% is used.

C needs to be at least 0.02% in order to ensure the strength as a welded structure, but if it exceeds 0.2%, solidification cracks are likely to occur.
Mn needs to be at least 0.8% in order to ensure strength and toughness, but if it exceeds 3.5%, the hardenability increases too much and the toughness decreases.
S is an element that lowers toughness, and should be 0.0025% or less. However, in order to form MnS and use a complex of oxide and MnS as an intragranular transformation nucleus, it is preferable to contain 0.0005% or more.

Al is usually added as a deoxidizer in the production of steel. However, since Al oxide has a very low ability to produce ferrite transformation nuclei, in the present invention, deoxidation with Ti is performed, so the Al content is reduced to 0. 0.02% or less. If it is 0.005% or less, it is more preferable, and it does not need to be contained in particular.
In the present invention, Ti is an element essential for use as a deoxidizer, to generate Ti oxide, and to improve the fracture toughness of the weld metal and the HAZ part by refining the microstructure with the Ti oxide. In order to form the necessary Ti oxide, at least 0.01 or more is necessary. However, if it exceeds 0.05%, the amount and size of the oxide become excessive, which may be a starting point of destruction.

O is also necessary in the base material to form Ti oxide. In order to satisfy the conditions of the particle diameter and number of Ti oxides in the weld metal, it is necessary to contain at least 20 ppm, more preferably 40 ppm or more in the weld metal. The amount of oxygen in the weld metal varies not only with the steel content of the base metal, but also with the degree of vacuum in electron beam welding, so the content in the base material cannot be specified uniformly. The O content is preferably 40 ppm or more in normal high vacuum electron beam welding, and 30 ppm or more in the above-described RPEBW having a low degree of vacuum. Since the O content in the weld metal is preferably 250 ppm or less in order to satisfy the conditions of the particle size and number of oxides described later, the upper limit of the O content in the base material is preferably the same. In addition, the upper limit of O content in a weld metal shall be 70 ppm or less based on an Example.

Furthermore, in the present invention, in order to ensure the hardenability of the weld metal part and prevent the formation of coarse ferrite in the weld metal part, the Pcm value represented by the following formula (1) in the base metal is set to 0. .12% by mass or more. Moreover, since the hardness of a weld metal part will become high too much when Pcm value exceeds 0.5 mass%, an upper limit shall be 0.5 mass%. From the viewpoint of setting the hardness of the weld metal part to 220% or less of the hardness of the base material, 0.38% or less is preferable.
Pcm = C + Si / 30 + Mn / 20 + Cu / 20 + Ni / 60 + Cr / 20 + Mo / 15 + V / 10 + 5B
(1)

  The steel material used as the base material of the electron beam welded joint according to the present invention may be a known welding steel as long as the above-described components are satisfied. Based on steel containing 0% by mass or less and P of 0.01% by mass or less with the balance being Fe and inevitable impurities, depending on required properties such as improvement of base metal strength and joint toughness, Ni, Cr , Mo, Cu, W, Co, V, Nb, Zr, Ta, Hf, REM, Y, Ca, Mg, Te, Se, B, or a total of 8 mass% or less of one or more of them The steel contained in is preferred.

In the present invention, Ti oxide is finely dispersed and used as a nucleus of the transformation in the transformation from austenite to ferrite, and by forming a microstructure containing a lot of fine acicular ferrite exhibiting good toughness, In order to obtain a weld metal with excellent toughness, it is necessary to prevent the amount of oxide having a particle size of 2.0 μm or more from exceeding 10 pieces / mm ² as shown in FIG. is there. When it exists in steel exceeding it, it becomes a starting point of the fracture | rupture in a CTOD test, and causes the dispersion | variation in the fracture toughness value in a weld metal part.

The particle size of the Ti oxide functioning as an intragranular transformation nucleus is 0.1 μm or more and less than 2.0 μm, and the amount of Ti oxide having a particle size in the range is 30 to 600 pieces / mm ^2. Thus, a microstructure containing a large amount of fine acicular ferrite can be formed.
Some fine Ti oxides form a composite with MnS by precipitation of MnS around them. This composite is more effective as an intragranular transformation nucleus, and the Ti oxide of the present invention includes such a composite.

In the weld metal part, the amount of oxide having a particle size of 2.0 μm or more is made not to exceed 10 pieces / mm ^2, and the amount of Ti oxide having a particle size of 0.1 μm or more and less than 2.0 μm is 30 to 600. In order to achieve the number of pieces / mm ² , it is preferable to use a steel material whose oxide size is suppressed to 2.0 μm or less as a base material.

For that purpose, it is necessary to carefully control the inclusions in the deoxidation process of the steel material as the base material.
Al is usually used for deoxidizing steel, but if Al, which is a strong deoxidizing element, is added, the deoxidation reaction proceeds rapidly and a large oxide of 2 μm or more is generated. Therefore, a relatively small oxide is generated by deoxidizing with Ti having a smaller deoxidizing ability than Al. However, if a large amount of Ti is added at once, a coarse oxide is likely to be formed. Therefore, the timing of introducing Ti is controlled so that the amount of oxygen in the molten steel decreases stepwise, or Ti, which is a weak deoxidizing element, is added. By adding a very small amount of a strong deoxidizing element such as Al, Ca, Mg, etc., the generation of coarse oxides of 2 μm or more is suppressed, and a large number of fine oxides of 0.1 to 2 μm are generated. Can be made.

The present invention is more effective when combined with the technique disclosed in the previous application (Japanese Patent Application No. 2005-207261).
That is, the hardness Hv (WM) of the weld metal part is set to be more than 110% and 220% or less of the hardness Hv (BM) of the base material.
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. To do. 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.

  In order to keep the ratio of the hardness of the weld metal part and the hardness of the base metal within the above range without using an insert metal, it is necessary to control the generation of pro-eutectoid ferrite formed in the weld metal as much as possible. . Therefore, in this invention, the hardenability of steel materials is ensured by restricting the Pcm value of the steel material used as a base material to 0.12% or more.

In the present invention, the plate thickness of the steel plate is not particularly limited, but the above-mentioned problem becomes apparent in a high-strength steel plate having a plate thickness of more than 50 mm.
Electron beam welding is usually performed under conditions of a voltage of 175 V, a current of 120 mA, and a welding speed of about 125 mm / min when the plate thickness is 80 mm. Usually, welding is performed under a high vacuum of 10 ⁻³ mbar or less, but the present invention is applicable even to a joint welded under a low vacuum degree such as the above-described RPEBW method, for example, a vacuum of about 1 mbar. be able to.

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 1 and the balance of Fe and unavoidable impurities was prepared. After butt welding by electron beam welding, the characteristics and performance of the formed welded joint were tested. ,investigated.
The results are shown in Table 2.

In Table 2, Hv (BM) is the average value of the hardness in the plate thickness direction of the base material measured with 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.
The bead width is an average value measured at three points of the front surface, the back surface, and the center of the plate thickness of the weld metal part.
Regarding the performance of the welded joint, δc (mm) is a value obtained at a test temperature of −10 ° C. in the CTOD test.

As shown in Table 2, No. of the present invention example. Nos. 1 to 17 are those in which the chemical composition of the steel material, the amount of oxygen and the amount of oxide in the weld metal are both within the range defined by the present invention, and the δc value is a sufficient value for both the weld metal part and the FL and HAZ parts. Is shown.
In Examples 1 and 17 of the present invention, since the value of Hv (WM) / Hv (BM) is not in a more preferable range, the weld metal part, FL and HAZ part have lower δc values. In Nos. 7, 13, and 14, the number of oxides having a particle size of 2 μm or more was larger, so the δc value of the weld metal part was lower.

On the other hand, in Comparative Example 18, the amount of C of the steel material and the Pcm value are not less than the specified values of the present invention, the value of Hv (WM) / Hv (BM) is larger than the range of the present invention, and the particle size is 0.1 to Since the number of oxides of 2 μm was less than the specified value of the present invention, the δc value was insufficient in both the weld metal part and the FL and HAZ parts.
In Comparative Example 19, since the C content and Pcm of the steel material are not less than the specified values of the present invention and the number of oxides having a particle size of 0.1 to 2 μm is not more than the specified value of the present invention, both the weld metal part and the FL and HAZ parts have δc. The value was insufficient.
In Comparative Example 20, the Pcm of the steel material was not more than the specified value of the present invention, and the number of oxides having a particle size of 2 μm or more was not less than the specified value of the present invention, so the δc value of the weld metal part was insufficient.

In Comparative Example 21, the Pcm of the steel material was not more than the specified value of the present invention, and the number of oxides having a particle size of 0.1 to 2 μm was not more than the specified value of the present invention, so the δc value of the weld metal part was insufficient.
In Comparative Examples 22 and 23, the amount of Ti in the steel material was less than the specified value of the present invention, and the amount of oxygen in the weld metal was also low, so the number of oxides with a particle size of 0.1 to 2 μm was less than the specified value of the present invention. The δc value was insufficient for both the weld metal part and the FL and HAZ parts.
In Comparative Example 24, the amount of oxide having a particle size of 0.1 to 2 μm is the specified value of the present invention even though the amount of oxygen in the weld metal is sufficient because the Al amount of the steel material is not less than the specified value of the present invention. In the following, since the number of oxides having a particle size of 2 μm or more is not less than the specified value of the present invention, the δc value is insufficient for both the weld metal part 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 which shows the CTOD test result at the time of welding by RPEBW method, inserting Ni foil in a butt | matching part, or not inserting. It is a figure which shows the hardness change of the welded joint part in the case similar to FIG. It is a figure which shows the relationship between the fracture toughness value of a weld metal, and the number of oxides with a particle size of 2.0 micrometers or more.

Claims (3)

In mass%, C: 0.02-0.2%, Mn: 0.8-3.5%, S: 0.0005-0.0025%, Al: less than 0.02%, Ti: 0.01 A butt weld joint of a welded structure using a steel material containing ~ 0.05% and having a Pcm value represented by the following formula (1) of 0.12% to 0.5%,
The amount of O contained in the weld metal of the weld joint is 20 ppm or more and 70 ppm or less, and the amount of oxide having a particle size of 2.0 μm or more is 10 pieces / mm ² or less, and the particle size is 0.1 μm or more. An electron beam welded joint excellent in brittle fracture resistance, characterized in that the amount of Ti oxide less than 2.0 μm is 30 to 600 pieces / mm ² .
Pcm = C + Si / 30 + Mn / 20 + Cu / 20 + Ni / 60 + Cr / 20 + Mo / 15 + V / 10 + 5B
(1) 2. The electron beam welded joint having excellent brittle fracture resistance according to claim 1, wherein the weld metal part has a hardness of more than 110% and not more than 220% of the hardness of the base metal. The electron beam welded joint having excellent brittle fracture resistance according to claim 1 or 2 , wherein the welded structure is a butt weld of a steel plate having a thickness of more than 50 mm.

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