JP2007021532A - Electron beam welded joint having excellent resistance to generation of brittle fracture - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To form a welded joint having a sufficiently high fracture toughness value δc by butt-welding a high-strength steel sheet for electron beam welding, which has a fracture toughness value of ≥355 MPa class and has a sheet thickness of >50 mm. <P>SOLUTION: A welded structure is formed of a butt-welded joint and has the following characteristics: (a) the hardness of a weld metal part is ≥110% and ≤220% of that of the base material; if necessary, (b) the width of the weld metal part is ≤20% of the sheet thickness of the base material; (c) the width of the region of a weld heat-affected zone, whose hardness is softened to ≤95% of that of the base material not affected by heat, is ≥3 mm; and/or (d) the former austenite grain size of the weld heat-affected zone (HAZ) being in contact with a weld fusion line is ≤100 μm. This butt-welded joint is an electron beam welded joint with an excellent resistance to generation of brittle fracture. <P>COPYRIGHT: (C)2007,JPO&INPIT

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 steel plate having a thickness of more than 50 mm.

  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.

  Therefore, the basic portion of the wind power generation tower has a tube structure having a large 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.

    Thus, as described above, an unprecedented need has arisen to weld an extremely thick steel plate having a thickness of 100 mm with high efficiency and on-site welding.

  In general, the electron beam welding method is a welding method that can be efficiently welded by a high-density / high-energy beam. However, since it is necessary to perform welding while maintaining a high vacuum state in a vacuum chamber, conventionally, such as automobile parts, etc. It has been used to weld relatively small parts.

  On the other hand, in recent years, an electron beam welding method (RPEBW: Reduced Pressure Electron Beam Welding) that can be applied under low vacuum is a welding method that can be used to efficiently weld an extremely thick steel plate having a thickness of about 100 mm. ) Has been developed and proposed at a welding laboratory in the UK (see, for example, Patent Document 1).

  By using this RPEW 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.

  As a countermeasure against this, when welding using the RPEBW method, there is also a method of securing toughness such as Charpy absorbed energy of the weld metal part by inserting an insert metal containing an element such as Ni into the weld groove part in advance. Proposed.

  However, in this method, since an element such as Ni in the insert metal does not diffuse uniformly to the weld heat affected zone, a new problem has arisen that the toughness of the weld heat affected zone varies particularly.

  Generally, as an index for quantitatively evaluating the safety of a welded structure, a fracture toughness value δc value based on fracture mechanics obtained by a CTOD test is known. A welded joint obtained by welding by the conventional RPEBW method has a large variation in the toughness of the weld heat-affected zone. Therefore, it has been difficult to sufficiently secure the fracture toughness value δc.

  On the other hand, in order to ensure the fracture toughness value Kc in a high heat input welded joint such as electrogas welding, the weld metal and the base metal are controlled so that the hardness ratio of the weld metal and the base metal is 110% or less. A method for improving the fracture toughness Kc of the boundary (FL part) of the part has been proposed (see, for example, Patent Document 2).

  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. If the thickness is reduced to 110% or less, the fracture toughness value of the weld metal part in the electron beam welded joint cannot be secured.

  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. Like a high heat input arc welding method such as electrogas welding, a welding wire is used. It is difficult to easily control properties such as the hardness of the weld metal and the fracture toughness value δc due to the above.

WO99 / 16101 JP-A-2005-144552

  In view of the above prior art, the present invention provides fracture toughness of both the weld metal part in the electron beam welded joint and the boundary (FL part) between the weld metal part where the local stress increases and the weld heat affected zone. It is an object to provide a method for improving the value δc and stably improving the fracture toughness of the welded joint.

  In order to solve the above problems, the present inventor investigated the mechanical properties of the base material and the welded joint. As a result, the present inventors have increased the strength and hardness of the weld metal part due to the presence of the insert metal used to improve the toughness of the weld metal part, which is significantly higher than the strength and hardness of the base metal. As a result, it has been found that the local stress increases in the vicinity of the boundary with the weld heat affected zone (HAZ portion) in contact with the weld metal portion, and therefore the fracture toughness value δc of the FL portion decreases.

  And based on the said knowledge, the novel joint design technique regarding the electron beam welding part was discovered.

  That is, in the joint design of the electron beam welded joint, the hardness of the weld metal part is controlled to be 220% or less of the hardness of the base metal, and preferably the hardness of the weld metal part is set to the hardness of the base metal. It has been found that by reducing the width of the weld metal part to 110% or more and 220% or less and 20% or less of the base metal plate thickness, a decrease in joint toughness due to overmatching can be prevented.

  Based on the above findings, stable and excellent toughness is ensured in electron beam welding of high-strength steel plates with yield strength of 355 MPa class or higher and plate thicknesses of more than 50 mm (preferably more than 50 mm to 100 mm). The present invention has been completed as a technique for embodying a welded joint.

  The gist of the present invention is as follows.

(1) In butt weld joints of welded structures,
(A) An electron beam welded joint excellent in brittle fracture resistance, characterized in that the hardness of the weld metal part is more than 110% and less than 220% of the hardness of the base metal part.

(2) In butt weld joints of welded structures,
(A) The hardness of the weld metal part is more than 110% and less than or equal to 220% of the hardness of the base metal part, and (b) the width of the weld metal part is 20% or less of the plate thickness of the base metal part. An electron beam welded joint with excellent brittle fracture resistance.

(3) In butt weld joints of welded structures,
(A) the hardness of the weld metal part is more than 110% and 220% or less of the hardness of the base metal part,
(B) the width of the weld metal part is 20% or less of the thickness of the base metal part, and
(C) Excellent resistance to occurrence of brittle fracture, characterized in that the width of the weld-affected zone softened to 95% or less of the hardness of the base metal that is not affected by heat is 3 mm or more. Electron beam welded joint.

(4) In butt weld joints of welded structures,
(A) the hardness of the weld metal part is more than 110% and less than or equal to 220% of the hardness of the base material;
(C) the width of the weld heat affected zone region softened to a hardness of 95% or less of the hardness of the base material portion not affected by heat is 5 mm or more, and
(D) An electron beam welded joint excellent in brittle fracture resistance, characterized in that the prior austenite grain size of the weld affected zone (HAZ) in contact with the weld melting line is 100 μm or less.

(5) In butt weld joints of welded structures,
(A) the hardness of the weld metal part is more than 110% and 220% or less of the hardness of the base metal part,
(B) the width of the weld metal part is 20% or less of the thickness of the base metal part,
(C) the width of the weld affected area softened to 95% or less of the hardness of the base metal part not affected by heat is 3 mm or more, and
(D) An electron beam welded joint excellent in brittle fracture resistance, characterized in that the prior austenite grain size of the weld-affected zone in contact with the weld melt line is 100 μm or less.

  (6) The weld structure is excellent in brittle fracture resistance according to any one of (1) to (5), wherein the welded structure is obtained by butt welding a high-strength steel sheet having a thickness of more than 50 mm. Electron beam welded joint.

  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 subjected to electron beam welding, a weld joint having a sufficiently high fracture toughness value δc can be formed.

  In a general electron beam welded joint, it is difficult to secure a required fracture toughness δc in a weld metal part formed by melting and resolidifying a part of a base material part. For this reason, conventionally, during electron beam welding, an insert metal such as nickel foil is inserted into the weld groove to improve the hardenability of the weld metal, and this synergistic effect ensures a fracture toughness value δc. Are known.

  However, the present inventor has found that in this method, the fracture toughness value δc of the weld heat affected zone, particularly the boundary between the weld metal zone and the weld heat affected zone (FL zone) in the electron beam welded joint is significantly reduced. It was found that the fracture toughness value δc of the welded joint could not be secured sufficiently.

  Therefore, the present inventor made a prototype steel sheet of 460 MPa in yield strength, inserted an insert metal having a Ni content of 4% into the welding groove, performed electron beam welding, and was obtained by a CTOD test. The fracture toughness value δc of the welded joint was measured and evaluated.

  As a result of the CTOD test of the above welded joint, the fracture toughness value δc of the weld metal part was a sufficiently high value of 0.2 mm or more, but the fracture toughness value δc of the boundary part (FL part) between the weld metal part and the HAZ part. Was found to be a very low value of 0.02 mm or less.

Therefore, as a result of detailed investigation of the fracture occurrence point in the CTOD test of the above welded joint,
(I) The fracture occurrence position is the boundary (weld fusion line [FL]) between the weld metal part (WM) and the weld heat affected zone (HAZ), and in the CTOD test of the weld joint, As a result of analyzing the distribution form of local stress that becomes driving force by the three-dimensional finite element method,
(Ii) It was found that the local stress in the FL part is significantly affected by the hardness of the adjacent weld metal part (WM).

  FIG. 3 shows notches at the boundary (FL) between the weld metal part (WM) and the weld heat affected zone (HAZ) and the weld heat affected zone (HAZ) for a test piece having a thickness of 70 mm. Analysis of crack opening stress distribution at each position distant from the notch tip in the crack propagation direction when CTOD (Crack Tip Opening Displacement) at 0.05 mm is 0.05 mm An example of the results is shown.

  From this figure, (iv) When the plate thickness exceeds 50 mm and reaches about 70 mm, the degree of restraint (force) in the plate thickness direction increases remarkably, and the strength of the weld metal part (WM) increases to the base material (BM) and welding heat. It is understood that when the strength of the affected zone (HAZ) is higher (in the case of WM-H), the local stress is remarkably increased at the boundary (FL) between the weld metal zone (WM) and the weld heat affected zone (HAZ). (Refer to □ [WM-H] and black square mark [WM-L] in the figure).

  On the other hand, even if the strength of the weld metal part (WM) is higher than that of the base material (BM) or the weld heat affected zone (HAZ) (in the case of WM-H), the weld heat affected zone (HAZ). Then, the local stress does not increase, and is almost the same as when the strength of the weld metal part (WM) is low (in the case of WM-L).

  From this, the reason why the δc value decreases is when the strength of the weld metal part (WM) is higher than the strength of the base material (BM) or the weld heat affected zone (HAZ) (in the case of WM-H). It is thought that this is because local stress increases at the boundary (FL) between the weld metal part (WM) and the weld heat affected zone (HAZ).

  That is, as a result of the above analysis, the present inventor (v) suppresses a significant increase in local stress at the boundary portion (FL) between the weld metal portion (WM) and the weld heat affected zone (HAZ), and increases the δc value. In order to improve, it discovered that it was necessary to make the intensity | strength of a weld metal part (WM) as low as possible.

  However, it has been found that when the hardness of the weld metal part is lowered, the hardenability of the weld metal part (WM) cannot be ensured, so that coarse ferrite is generated and, as a result, the CTOD value is lowered.

  Here, based on the above analysis results, the hardness [Hv (WM)] of the weld metal part is changed variously, the CTOD value δc of the FL part is measured, and the δc value is set to “the hardness of the weld metal part [Hv (WM)] / Hardness [Hv (BM)] ”of the base metal, and as a result of plotting the hardness [Hv (WM)] of the weld metal part as shown by“ ● ”in FIG. It was found that if the hardness [Hv (BM)] is suppressed to 220% or less, the fracture toughness value δc can be prevented from decreasing due to an increase in local stress.

  The higher the δc value, the better, but the standard such as the Norwegian Maritime Association (DNV) requires a value of about 0.1 to 0.2 mm at the design temperature, and the target δc value in the present invention. Was 0.15 mm or more.

  In the electron beam welded joint by the conventional method, it was difficult to stably ensure the fracture toughness value δc at −20 ° C. of 0.15 mm or more.

  Thus, by making the hardness [Hv (WM)] of the weld metal part lower than the hardness [Hv (BM)] of the base material, the δc of the FL part is improved, but the hardness of the weld metal part is increased. When [Hv (WM)] is excessively decreased, the δc value of the weld metal portion is decreased, and as a result, the fracture toughness value δc of the electron beam welded joint cannot be ensured.

  As a result of the study by the present inventor, as shown by a circle in FIG. 1, if the hardness [Hv (WM)] of the weld metal part is 110% or more of the hardness [Hv (BM)] of the base metal, It was found that the required CTOD value can be secured in the weld metal part.

  FIG. 2 shows the influence of the hardness ratio of the weld metal part and the base metal and the γ grain size on the relationship between the HAZ softening width and the CTOD value of the FL part. As the HAZ width increases, the CTOD value of the FL portion tends to improve.

  This is because the influence of strength matching is mitigated by the HAZ softening, and the HAZ width is desirably 3 mm or more.

  Further, the present inventor has found that the occurrence or distribution of local stress in the weld melt line (FL) in contact with the weld metal part is governed by the hardness of the weld metal part, but “softens” in the HAZ region in contact with the FL. It has been found that when the “region” is large, the local stress of FL tends to be relaxed.

  According to the experimental results shown in FIG. 2, the above-mentioned relaxation phenomenon is recognized as the HAZ softening width becomes wider, and particularly when 3 mm or more is present, the HAZ softening width is preferably 3 mm or more.

  In principle, the local stress of the FL part decreases as the hardness of the HAZ part becomes lower than the hardness of the base material. However, according to the results of experiments by the inventors, the local stress reduction effect of the FL part is clearly recognized. This was the case where the hardness of the HAZ part was 5% or more lower than the hardness of the base material.

  Therefore, in the present invention, it is preferable to set the width of the weld heat affected zone region softened to 95% or less of the hardness of the base material portion not affected by heat to 3 mm or more.

  Further, when the width of the weld heat affected zone is 10 mm or more, there is a concern that strain is concentrated on the softened portion from the viewpoint of securing the joint strength and fatigue strength.

  As described above, in order to ensure the predetermined CTOD value δc in the welded joint, it is important to prevent the local stress from increasing in the weld melt line (FL) which is the most fragile part of the welded joint. It is also important to improve the microscopic brittle fracture resistance in the vicinity of FL.

  As a result of investigating and investigating the mechanism of brittle fracture occurring near the FL, pro-eutectoid ferrite formed around the former austenite, upper bainite and ferrite side plates formed in the lath form inside the former austenite, etc. I found out.

  The fracture surface unit when the upper bainite or ferrite undergoes open fracture depends on the grain size of the austenite phase. It was found that the brittle fracture occurrence characteristics can be improved.

  Further, as a result of the study by the present inventor, when “hardness of weld metal part [Hv (WM)] / hardness of base material [Hv (BM)]” approaches 220% defined in the present invention, welding is performed. The decrease in fracture toughness value δc due to the strength matching between the metal and the HAZ part and the influence of the structure cannot be ignored.

  Therefore, even under such conditions, in order to stably secure the fracture toughness value δc of the joint, the prior austenite grain size of the weld heat affected zone (HAZ) in contact with the weld fusion line (FL) is set to 100 μm or less, It is preferable to suppress the coarsening of the prior austenite grain size (see FIG. 2).

  Also, if 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 welding (EBW welding) in a high vacuum state in a vacuum chamber. .

  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.

  The high-strength steel sheet of the welded structure used in the present invention may be 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 as basic components, depending on required properties such as improvement of base material strength and joint toughness, Ni, Cr, Steel containing at least one of Mo, Cu, W, Co, V, Nb, Ti, Zr, Ta, Hf, REM, Y, Ca, Mg, Te, Se, and B is preferred.

  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 manifests this problem.

  Further, in order to ensure the fracture toughness δc value of the electron beam weld metal part (melted part), even when an insert metal containing Ni element or the like is used in the groove part, it has the characteristics specified in the present invention. The component composition is not particularly limited to a specific component composition.

  For example, as a chemical component of the welding material, C: 0.01 to 0.06%, Si: 0.2 to 1.0%, Mn: 0.5 to 2.5%, Ni: 0 to 4.0% , Mo: 0 to 0.30%, Al: 0 to 0.3%, Mg: 0 to 0.30%, Ti: 0.02 to 0.25%, B: 0 to 0.050% are desirable. The chemical component of the welding material may be appropriately selected in consideration of the chemical component of the steel material.

For example, when the plate thickness is 80 mm, the electron beam welding is performed under the conditions of a voltage of 175 V, a current of 120 mA, and a welding speed of about 125 mm / min. Usually, welding is performed under a high vacuum of 10 ⁻³ mbar or less, but even a joint that is welded under a low degree of vacuum that can be constructed with simple equipment, for example, a vacuum of about 1 mbar, is within the scope of the present invention. .

  Hereinafter, the present invention will be described based on examples, but the conditions in the examples are one condition example adopted to confirm the feasibility and effects of the present invention, and the present invention is the one condition example. It is not limited to.

  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.

Example 1
Thick steel plates having a thickness of 50 to 100 mm were prepared, and the characteristics and performance of welded joints were tested and investigated. The results are shown in Table 1. The welding is electron beam welding.

  Table 2 shows the chemical components (mass%) of the steel types used for welding. Table 3 shows chemical components (mass%) of the insert metal used for welding.

  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.

  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.

  The HAZ softening width is a width of a region when a HAZ region softened by 5% from the hardness of the base material is measured from the weld melt line toward the base material.

  The prior γ grain size of HAZ is a representation of the prior austenite grains in the HAZ part in contact with the weld melting line, with an equivalent circle diameter.

  Regarding the performance of the welded joint, δc (mm) is a value obtained at a test temperature of −10 ° C. in the CTOD test described above.

  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 1, No. of the present invention example. Nos. 1 to 17 indicate that various conditions are within the range defined by the present invention, and the δc value is a sufficient value.

  Among these invention examples, no. 1 to 14 are Hv (WM) / Hv (BM), and the bead width / plate thickness and the HAZ softened width are within the ranges specified in the present invention. Therefore, the δc value of the HAZ part of the welded joint and the joint tensile strength Both show sufficient values.

  In addition, this invention example No. No. 14 is an example of the present invention because the HAZ softening width is smaller than the preferred range defined in the present invention. Although the δc value is slightly lower than 1 to 13, it is a good value of 0.1 mm or more.

  Invention Example No. 15 is lower than the preferred range of Hv (WM) / Hv (BM), so the hardenability of the weld metal part is insufficient and the formation of proeutectoid ferrite could not be suppressed. The δc characteristic of the HAZ part is Invention Example No. Compared with 1-14, it is a low level.

  Invention Example No. In Nos. 16 and 17, since the bead width / plate thickness is higher than the preferable range defined in the present invention, the area of the weld metal portion is large, and the influence of matching by the weld metal having a hardness higher than that of the base metal is more remarkable. Therefore, the δc values of the HAZ part and FL part are the same as those of the present invention example No. It is a low level compared with 1-14.

  In contrast, Comparative Example No. 18, 20 to 22 and 24, Hv (WM) / Hv (BM) exceeds the range defined in the present invention, so the δc value of the weld metal part is sufficient, but the HAZ part and FL part The Δc value is low.

  In Comparative Examples 19 and 23, Hv (WM) / Hv (BM) is below the range defined in the present invention, so that sufficient hardenability cannot be ensured and the δc value of the weld metal part is low. It is.

  Therefore, the present invention is applied to secure a δc value in a high strength steel having a YP of 355 MPa or more and a thickness of 50 mm or more.

  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 influence of the hardness of a weld metal part and a base material which has on the (delta) c value of a weld metal and HAZ, FL part. It is a figure which shows the influence of the hardness ratio of a weld metal part and a base material, and (gamma) grain size on the relationship between the HAZ softening width | variety and the CTOD value of HAZ and FL part. A test piece having a thickness of 70 mm is provided with notches in the boundary (FL) between the weld metal part (WM) and the weld heat affected zone (HAZ) and the weld heat affected zone (HAZ), and CTOD ( Example of FEM (three-dimensional finite element method) analysis of crack opening stress distribution at each position distant from the notch tip in the crack propagation direction when Crack Tip Opening Displacement is 0.05 mm FIG.

Claims (6)

In butt weld joints of welded structures,
(A) An electron beam welded joint excellent in brittle fracture resistance, characterized in that the hardness of the weld metal part is more than 110% and less than 220% of the hardness of the base metal part. In butt weld joints of welded structures,
(A) The hardness of the weld metal part is more than 110% and less than or equal to 220% of the hardness of the base metal part, and (b) the width of the weld metal part is 20% or less of the plate thickness of the base metal part. An electron beam welded joint with excellent brittle fracture resistance. In butt weld joints of welded structures,
(A) the hardness of the weld metal part is more than 110% and 220% or less of the hardness of the base metal part,
(B) the width of the weld metal part is 20% or less of the thickness of the base metal part, and
(C) Excellent resistance to occurrence of brittle fracture, characterized in that the width of the weld-affected zone softened to 95% or less of the hardness of the base metal that is not affected by heat is 3 mm or more. Electron beam welded joint. In butt weld joints of welded structures,
(A) the hardness of the weld metal part is more than 110% and less than or equal to 220% of the hardness of the base material;
(C) the width of the weld heat affected zone region softened to a hardness of 95% or less of the hardness of the base material portion not affected by heat is 3 mm or more, and
(D) An electron beam welded joint excellent in brittle fracture resistance, characterized in that the prior austenite grain size of the weld affected zone (HAZ) in contact with the weld melting line is 100 μm or less. In butt weld joints of welded structures,
(A) the hardness of the weld metal part is more than 110% and 220% or less of the hardness of the base metal part,
(B) the width of the weld metal part is 20% or less of the thickness of the base metal part,
(C) the width of the weld affected area softened to 95% or less of the hardness of the base metal part not affected by heat is 3 mm or more, and
(D) An electron beam welded joint excellent in brittle fracture resistance, characterized in that the prior austenite grain size of the weld-affected zone in contact with the weld melt line is 100 μm or less.   The electron beam welded joint having excellent brittle fracture resistance according to any one of claims 1 to 5, wherein the welded structure is obtained by butt welding a high-strength steel plate having a thickness of more than 50 mm.

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