JP2011246806A - Electron beam welded joint, electron beam welding steel material, and manufacturing method therefor - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an electron beam welding steel material optimum for building a foundation portion of an ocean wind power generation steel tower, and balanced properly with fracture toughness of a base material, a thermal affection part and a molten metal part, and to provide an electron beam welded joint formed in the steel material.SOLUTION: This electron beam welding steel material has a prescribed component composition, wherein an electron beam welding hardenability index CeEB defined by expression (1) is 0.49-0.60, and wherein a CTOD value δof the molten metal part after electron-beam-welded, a CTOD value δof the thermal affection part and a CTOD value δof the base material satisfy expression (2) and Expression (3). The expressions represent: CeEB=C+9/40Mn+1/15Cu+1/15Ni+1/5Cr+1/5Mo+1/5V (1), 0.3≤δ/δ≤1.1 (2), and 0.3≤δ/δ≤1.1 (3).

Description

  The present invention relates to a steel material for electron beam welding that is irradiated and irradiated with an electron beam to a welded portion, and a manufacturing method thereof, and further to an electron beam welded joint formed by irradiating the welded portion of the steel material with an electron beam. It is.

In recent years, the use of renewable natural energy has been actively attempted in order to cope with the reduction of CO ₂ gas that contributes to global warming and the future depletion of fossil fuels such as oil. Wind power generation is another promising renewable energy, and large-scale wind power plants are being built.

  The region most suitable for wind power generation is a region where strong winds can be expected constantly, and therefore, offshore wind power generation is planned and realized on a global scale (see Patent Documents 1 to 4).

  In order to construct a tower for wind power generation on the ocean, it is necessary to drive the foundation of the tower into the seabed ground. In order to sufficiently secure the height of turbine blades for wind power generation from the sea level, the foundation portion also needs to have a sufficient length.

  Therefore, the structure of the foundation part of the steel tower is a steel pipe structure having a large cross section with a plate thickness of more than 50 mm, for example, about 100 mm and a diameter of about 4 m, and the height of the steel tower reaches 80 m or more. In recent years, it has been demanded that a huge steel structure such as a tower for wind power generation be assembled easily and efficiently by electron beam welding on the coast near the construction site.

  That is, an unprecedented technical request has been made to weld an extremely thick steel plate having a thickness of 100 mm at a construction site and with high efficiency.

  Generally, high energy density beam welding such as electron beam welding and laser beam welding is efficient welding. However, there is a limit to the plate thickness that can be welded with a laser beam, and conventional electron beam welding has to be performed in a vacuum chamber maintained in a high vacuum state. Therefore, conventionally, the thickness and size of the steel plate that can be welded by high energy density beam welding are limited by the capability of the welding apparatus and the size in the vacuum chamber.

  On the other hand, in recent years, an electron beam welding method has been proposed in which the vicinity of the welded portion is depressurized and an extremely thick steel plate having a thickness of about 100 mm can be efficiently welded on site. For example, a welding method (RPEBW: Reduced Pressure Electron Beam Welding) that can be applied under a low vacuum has been developed at a welding laboratory in the UK (see Patent Document 5).

  By using reduced pressure electron beam welding (RPEBW), even when constructing a large steel structure such as a steel tower for wind power generation, a portion to be welded can be locally placed in a vacuum state and efficiently welded. . The RPEBW method is a welding method in which the degree of vacuum is low compared to a method of welding in a vacuum chamber, but an improvement in toughness of the molten metal part (WM) can be expected as compared with conventional arc welding.

  Generally, as an index for quantitatively evaluating the safety of a welded structure, a fracture toughness value δc based on fracture mechanics obtained by a CTOD (Crack Tip Opening Displacement) test is known. The fracture toughness is affected by the size of the specimen, so even if good results are obtained in a small test such as the conventional V-notch Charpy impact test, it is good in the CTOD test for welded joints of large steel structures. The fracture toughness value δc is not always obtained.

  The electron beam welding method is a method in which the base metal of the welded portion is once melted and solidified by the energy of the electron beam and is welded, and the component composition of the welded portion is usually almost the same as that of the base metal. Therefore, it is difficult to adjust the hardness of the molten metal part and the mechanical characteristics such as the fracture toughness value δc with a welding wire or the like, as in a high heat input arc welding method such as electrogas welding.

Therefore, in order to improve the fracture toughness value δc of the electron beam welded joint, a method of optimizing the hardness and cleanliness of the molten metal part (WM) has been proposed (see, for example, Patent Documents 6 and 7). In Patent Document 6, the hardness of the molten metal part is set to be more than 110% and less than or equal to 220% of the hardness of the base material, and the width of the molten metal part is set to 20% or less of the thickness of the base material part Proposed. Patent Document 7 proposes that the amount of O in the weld metal is 20 ppm or more and the amount of oxide having a particle size of 2.0 μm or more is 10 pieces / mm ² or less.

JP 2008-111406 A JP 2007-092406 A JP 2007-322400 A JP 2006-037397 A International Publication No. 99/16101 Pamphlet JP 2007-21532 A JP 2008-88504 A

  In the construction of offshore wind power generation towers, the steel materials are butted and welded, and the welded parts are used as they are without being heat-treated. Therefore, they are excellent for molten metal parts (WM) and heat-affected parts (HAZ). Toughness is required. In the case of electron beam welding, since no welding wire is used, the toughness of the molten metal part and the heat-affected zone is controlled by adjusting the component composition of the base material.

  Conventionally, there have been proposed methods for controlling inclusions in the molten metal part, the relationship between the hardness of the molten metal part and the hardness of the base metal, or the width of the molten metal part, but the toughness of the heat affected zone is insufficient. If there is, the fracture toughness value of the welded joint is lowered.

  In addition, plate-shaped or foil-shaped Ni (insert metal) can be stretched on the welding surface and electron beam welding can be performed to increase the toughness of the molten metal portion (WM) to be higher than the toughness of the base material. However, in this case as well, if the composition of the base material is not appropriate, the difference between the hardness of the molten metal portion and the hardness of the heat-affected zone becomes significant, and the toughness greatly varies in the weld zone.

  Further, according to the study by the present inventors, in the electron beam welded joint, the component composition for improving toughness does not necessarily match between the molten metal portion and the heat affected zone (base material). Therefore, even if the conventional high-HAZ toughness steel for arc welding is subjected to electron beam welding as it is, high toughness cannot be obtained at the molten metal portion. On the other hand, even if the component composition of the steel material for arc welding is optimized in consideration of the toughness of the molten metal portion formed by electron beam welding, high toughness cannot be obtained in the heat affected zone.

  That is, since electron beam welding and arc welding are basically different in terms of the welding technique and the joint structure to be formed, the problem relating to electron beam welding cannot be solved by the problem solving technique relating to arc welding.

  The present invention has been made in view of such circumstances, and an object of the present invention is a steel material for electron beam welding having a plate thickness of 45 mm or more, which constitutes a basic portion of an offshore wind power generation tower, and has high strength. And a steel material capable of forming an electron beam welded joint in which the fracture toughness of the molten metal part (WM), the heat-affected zone (HAZ), and the base material (BM) is appropriately balanced, and its manufacturing method, and Another object of the present invention is to provide an electron beam welded joint having excellent fracture toughness formed by irradiating an welded portion of the steel material with an electron beam.

  In the present invention, in order to solve the above-mentioned problem, 1.5% by mass or more of Mn is added to ensure hardenability, and Mg and Ca, which are strong deoxidizing elements, are simultaneously added to contain Mg. A fine oxide (Mg-containing oxide) is generated, and the oxide is used as a pinning particle for suppressing grain growth or a production nucleus for intragranular transformation, and a base material part (BM), a heat-affected part ( The basic idea is to properly balance the fracture toughness of the molten metal part (WM)).

  In particular, in electron beam welding where a welding wire is not used, the WM width and the HAZ width are narrow, and the heat input is low, the Mg-containing oxide finely dispersed in the molten metal portion (WM) and the heat affected zone (HAZ) , Suppresses the coarsening of austenite grains in the heat affected zone (HAZ), and promotes the formation of intragranular ferrite.

  In the present invention, the newly introduced electron beam weld hardenability index CeEB is controlled, and the fracture toughness of the base metal part (BM), the molten metal part (WM), and the heat affected zone (HAZ) is Balance properly to ensure the required fracture toughness at the weld. Furthermore, in this invention, in order to improve hardenability, Mn amount is increased, On the other hand, each amount of Cr, Mo, Cu, Ni, and / or Nb is reduced, and the manufacturing cost of the steel material for electron beam welding is reduced. Reduce.

  The electron beam weld hardenability index CeEB is an index newly introduced by the present inventors in order to improve the fracture toughness of the electron beam weld joint. The technical significance of the indicator CeEB will be described later together with the technical significance of the indicator (ratio) “C / CeEB” (C: C content) introduced together.

  The gist of the present invention is as follows.

(1) A welded joint formed by irradiating a welded portion of a steel material with an electron beam, wherein the steel material is in mass%, C: 0.02 to 0.1%, Si: 0.05 to 0.00. 30%, Mn: 1.5 to 2.5%, P: 0.015% or less, S: 0.010% or less, Ti: 0.005 to 0.015%, N: 0.0020 to 0.0060 %, O: 0.0010 to 0.0045%, and Mg: 0.0003 to 0.0027% and Ca: 0.0003 to 0.0027%,
0.0006% ≦ Mg + Ca ≦ 0.0040%
Al, Nb, and / or V is limited to Al: 0.015% or less, Nb: 0.020% or less, V: 0.030% or less, with the balance being iron and An electron beam welded joint comprising an inevitable impurity and further having an electron beam weld hardenability index CeEB defined by the following formula (1) of 0.49 to 0.60.

CeEB = C + 9 / 40Mn + 1 / 15Cu + 1 / 15Ni
+ 1 / 5Cr + 1 / 5Mo + 1 / 5V (1)
Here, C, Mn, Cu, Ni, Cr, Mo, and V are content (mass%) of a steel material component, respectively.

  (2) The electron beam weld joint according to (1) above, wherein a ratio of C amount to the electron beam weld hardenability index CeEB (C / CeEB) is 0.04 to 0.18.

  (3) The steel material is further in mass%, Cr: 0.50% or less, Mo: 0.50% or less, Cu: 0.25% or less, and Ni: 0.50% or less, or The electron beam welded joint according to (1) or (2) above, which contains two or more kinds.

  (4) The electron beam welded joint according to any one of (1) to (3) above, wherein the steel material has a thickness of 45 to 150 mm.

(5) In the electron beam welded joint, the CTOD value δ _WM of the molten metal part, the CTOD value δ _HAZ of the heat affected _zone , and the CTOD value δ _{BM of the} base material after welding at a vacuum degree of 10 Pa or less are as follows: (2) The electron beam welded joint according to any one of (1) to (4), which satisfies the expressions (2) and (3)

0.3 ≦ δ _WM / δ _BM ≦ 1.1 (2)
0.3 ≦ δ _HAZ / δ _BM ≦ 1.1 (3)
However, (delta) _WM , (delta) _HAZ , and (delta) _BM are the minimum values of CTOD value when a three-point bending CTOD test is performed 6 times at 0 degreeC.

(6) A steel material for forming the electron beam welded joint according to (1) above, in mass%, C: 0.02 to 0.1%, Si: 0.05 to 0.30%, Mn: 1.5 to 2.5%, P: 0.015% or less, S: 0.010% or less, Ti: 0.005 to 0.015%, N: 0.0020 to 0.0060%, O: 0 .0010-0.0045%, and Mg: 0.0003-0.0027%, Ca: 0.0003-0.0027%,
0.0006% ≦ Mg + Ca ≦ 0.0040%
Al, Nb, and / or V is limited to Al: 0.015% or less, Nb: 0.020% or less, V: 0.030% or less, with the balance being iron and A steel material for electron beam welding comprising an inevitable impurity and further having an electron beam welding hardenability index CeEB defined by the following formula (1) of 0.49 to 0.60.

CeEB = C + 9 / 40Mn + 1 / 15Cu + 1 / 15Ni
+ 1 / 5Cr + 1 / 5Mo + 1 / 5V (1)
Here, C, Mn, Cu, Ni, Cr, Mo, and V are content (mass%) of a steel material component, respectively.

  (7) A steel material for forming the electron beam weld joint according to (2) above, wherein a ratio of C amount to the electron beam weld hardenability index CeEB (C / CeEB) is 0.04 to 0.18. The steel material for electron beam welding according to (6) above, wherein

  (8) A steel material for forming the electron beam welded joint according to the above (3), and further, in mass%, Cr: 0.50% or less, Mo: 0.50% or less, Cu: 0.25% The steel material for an electron beam welded joint according to the above (6) or (7), which contains 1 type or 2 types or more of Ni: 0.50% or less.

  (9) The electron according to any one of (6) to (8) above, which is a steel material forming the electron beam welded joint according to (4) and has a thickness of 45 to 150 mm. Steel for beam welded joints.

  (10) A method for producing a steel material for electron beam welding according to any one of (6) to (9) above, wherein the steel material having the component composition according to any one of (6) to (8) above, A method for producing a steel material for electron beam welding, comprising heating to 950 to 1150 ° C. and then performing a heat treatment.

  In an electron beam welded joint, in order to ensure a predetermined CTOD value (fracture toughness value), the fracture toughness values of the base metal part (BM), the molten metal part (WM), and the heat affected zone (HAZ) are: It is important to balance appropriately.

  That is, even if the fracture toughness of the base metal part and the fracture toughness of the heat-affected zone are excellent, if the fracture toughness of the molten metal part is inferior, the molten metal part becomes the starting point of fracture, and the fracture toughness as a welded joint deteriorates. To do. Even if the fracture toughness of the molten metal portion is excellent, if the fracture toughness of the heat affected zone is inferior, the breakage proceeds from the heat affected zone.

  Brittle fracture at welds (molten metal part and heat-affected zone) of steel with a yield strength of 355 MPa using high heat input welding is caused by coarse grain boundary ferrite generated around the former austenite and lath form inside the former austenite. The upper bainite, ferrite side plate, and the like that are generated at the beginning of the fracture occur.

  The fracture surface unit at the time of brittle fracture starting from coarse ferrite generated from upper bainite or prior austenite grain boundaries depends on the grain size of prior austenite. Therefore, the fracture toughness of the welded portion can be improved by reducing the grain size of the prior austenite in the molten metal portion and the heat affected zone using the pinning effect and intragranular transformation caused by the precipitates.

  Therefore, in the present invention, Mg and Ca, which are powerful deoxidizing elements, are simultaneously added to the steel, so that not only the base material (BM) but also the molten metal part (WM) and the heat-affected part (HAZ) are fine. An Mg-containing oxide is dispersed.

  Fine Mg-containing oxide functions as pinning particles, suppresses grain growth in the heat affected zone, functions as a nucleus for intragranular transformation, and generates intragranular ferrite in the molten metal and heat affected zone Let As a result, the structure of the molten metal part and the heat affected zone becomes finer, and the fracture toughness g of the base metal part, the heat affected zone and the molten metal part is improved, and the balance of these three fracture toughness is improved.

  According to the present invention, in an electron beam welded joint of a steel material having a yield strength of 355 MPa, it is possible to suppress the deterioration of fracture toughness in the molten metal part and the heat affected zone, and the base metal part, the heat affected zone, and the molten metal part. It is possible to provide an electron beam welded joint in which fracture toughness is appropriately balanced, and to provide a steel material that can form the welded joint at low cost.

It is a figure which shows qualitatively the relationship between the strength and toughness of a steel material, and a metal structure. It is a figure which shows qualitatively the relationship between hardenability and another parameter | index. (A) qualitatively shows the relationship between hardenability and crystal grain size of the molten metal part, and (b) qualitatively shows the relation between hardenability and the amount of high carbon martensite in the heat affected zone. It is a figure which shows qualitatively the relationship between the ratio of the hardness of the molten metal part with respect to the hardness of a base material, and the fracture toughness of a molten metal part and a heat affected zone. It is a figure which shows qualitatively the relationship between CeEB, the fracture toughness value ((delta) c) of a molten metal part, and a heat affected zone. It is a figure which shows the relationship between a fracture toughness value and C / CeEB qualitatively. (A) qualitatively shows the relationship between the fracture toughness value of the molten metal portion and C / CeEB, and (b) qualitatively shows the relationship between the fracture toughness value of the heat affected zone and C / CeEB. It is a figure which shows the test piece which introduce | transduced the notch.

  In the construction of offshore wind power generation towers, steel materials are used as they are without being subjected to heat treatment after welding. Therefore, excellent toughness is required for the molten metal portion and the heat-affected zone. In the case of electron beam welding, since no welding wire is used, the toughness of the molten metal part and the heat-affected zone is controlled by adjusting the component composition of the base material.

  Conventionally, electron beam welding has been applied to steel materials in which generation of oxides in a molten metal part is a problem, such as high-strength steel containing Cr or Mo (Cr-Mo high-strength steel) or stainless steel. This means that no brittle phase is generated in the heat-affected zone of stainless steel, and the structure of the heat-affected zone of Cr-Mo high-strength steel is lower bainite having excellent toughness as qualitatively shown in FIG. This is because toughness is significantly improved by controlling the oxide of the molten metal portion.

  On the other hand, the steel used for offshore wind power generation towers and the like is structural steel with a YP of about 355 MPa, and its strength is lower than that of Cr-Mo high-strength steel. As shown, the upper bainite has low toughness. When such a steel material is subjected to electron beam welding, particularly in the heat-affected zone, coarse structures such as grain boundary ferrite and upper bainite develop, and high carbon martensite is likely to be generated. Therefore, when the structural steel is electron beam welded, it is not easy to ensure the toughness of the heat affected zone.

  Regarding the relationship between structure and toughness, it is known that refinement of the crystal grain size is particularly effective for improving the toughness of the molten metal part, and that high carbon martensite reduces the toughness of the heat-affected zone particularly. . Further, regarding the relationship between the component and the structure, when the hardenability index Ceq is increased, the particle size of the molten metal portion becomes fine as shown in FIG. 2A, and the heat as shown in FIG. 2B. It is known that high carbon martensite in the affected area increases.

  Further, in order to increase the toughness of the molten metal part and the heat-affected zone, a balance between the hardness of the molten metal part and the hardness of the base material is important. That is, as shown in FIG. 3, when the hardness of the molten metal part is increased with respect to the hardness of the base material, the toughness of the molten metal part is improved, but the toughness of the heat affected zone is Reduced by the effect of curing. Therefore, if the hardenability is increased in order to prevent the formation of upper bainite having poor toughness, the problem arises that the toughness of the heat affected zone is impaired due to the effect of hardening of the molten metal portion.

  Thus, the relationship between the hardenability of steel and the grain size of WM and the high carbon martensite of HAZ, the ratio of the hardness of WM to the hardness of the base metal, and the toughness of welded joints are qualitatively Was known. However, conventionally, there has been no concept of controlling the balance of fracture toughness of welded joints by the components of steel materials. Therefore, for example, when electron beam welding is performed on a base material with improved hardenability, the toughness of the WM is improved, but the toughness of the HAZ is significantly reduced.

  In view of this, the inventors of the present invention have studied an index for indicating hardenability suitable for electron beam welding in order to ensure required toughness in an electron beam welded joint, and newly devised and introduced “CeEB”. That is, the “electron beam hardenability index CeEB” defined by the following equation (1) focuses on the hardenability that greatly affects the formation of the structure in order to further increase the fracture toughness of the electron beam welded joint. New indicators are needed that take into account ensuring production.

CeEB = C + 9 / 40Mn + 1 / 15Cu + 1 / 15Ni
+ 1 / 5Cr + 1 / 5Mo + 1 / 5V (1)
Here, C, Mn, Cu, Ni, Cr, Mo, and V are content (mass%) of a steel material component, respectively.

  CeEB defined by the above equation (1) is based on the known carbon equivalent Ceq (= C + 1 / 6Mn + 1 / 15Cu + 1 / 15Ni + 1 / 5Cr + 1 / 5Mo + 1 / 5V) correlated with hardness, and Mn is evaporated during electron beam welding. Therefore, the index was devised in consideration of the decrease in hardenability. The coefficient of Mn was set to 9/40 (> 1/6) based on the degree of decrease in hardenability obtained empirically.

  The index CeEB is an upper bainite that ensures hardenability within the required range in the base metal before electron beam welding, promotes the formation of fine ferrite in the molten metal part, and lowers toughness in the heat affected part. It is an index for suppressing the formation of high carbon martensite and the like.

  FIG. 4 qualitatively shows the relationship between CeEB and the fracture toughness value (δc) of the molten metal part (WM) and the heat-affected zone (HAZ) in the electron beam welded joint. The solid line curve is the fracture toughness value (δcmn) of the molten metal part, and the broken line curve is the fracture toughness value (δcha) of the heat affected zone. A two-dot chain line curve is a fracture toughness value (predicted value of HAZ toughness) of a virtual heat-affected zone ignoring a change in the hardness of the WM. Such a predicted value of HAZ toughness is a fracture toughness value obtained when a fracture toughness test is performed using a test piece that has been subjected to heat treatment that simulates the thermal history of HAZ.

  When the index CeEB increases, the WM structure becomes finer and δcmn improves, and in HAZ, the predicted value of HAZ toughness decreases due to the increase in high carbon martensite and the hardening of HAZ. In addition, when CeEB increases, WM hardens, and due to the influence, δcha falls below the predicted value of HAZ toughness. Thus, it becomes possible to comprehensively evaluate the fracture toughness of the molten metal part and the heat-affected zone by the index CeEB, and if CeEB is set within an appropriate range, the fracture toughness values of the molten metal part and the heat-affected zone can be obtained. It can be set to be equal to or more than the target value indicated by a one-dot chain line. When utilizing pinning particles and intragranular transformation described later, δcmn and δcha are improved according to the effect.

  Next, the present inventors examined the relationship between the amount of C and CeEB of the base material, and the toughness of the base material, the molten metal portion, and the heat affected zone. As a result, it has been found that it is preferable to adjust the ratio “C / CeEB” between the amount of C in the base material and CeEB within a specific range. The technical significance of the ratio “C / CeEB” will be described below.

  The ratio “C / CeEB” is an index for preventing the hardenability of the weld metal part and the hardenability of the heat-affected part from being extremely biased. FIG. 5 (a) shows the relationship between CeEB and the fracture toughness value (δc) of the molten metal portion, and FIG. 5 (b) shows the relationship between CeEB and the fracture toughness value of the heat affected zone.

  Since CeEB is an index of hardenability, when CeEB increases, the fracture toughness value increases because the particle size becomes finer in the molten metal part, and the formation of high carbon martensite is promoted in the heat-affected part, resulting in a fracture toughness value. Decreases. In electron beam welding, a part of Mn in the molten metal part evaporates, and the amount of Mn decreases.

  Therefore, as shown in FIG. 5A, in order to improve the fracture toughness of the molten metal part, it is preferable to increase the C / CeEB to ensure the hardenability. On the other hand, in the heat-affected zone, the generation of high carbon martensite is promoted by an increase in the C content. Therefore, as shown in FIG. 5B, in order to ensure the fracture toughness value, it is preferable to limit C / CeEB.

  Furthermore, the present inventors examined a method for improving the balance between the fracture toughness value of the molten metal portion and the fracture toughness value of the heat-affected zone. As a result, when an appropriate amount of Mg and Ca are simultaneously added to generate “fine oxide containing Mg” (Mg-containing fine oxide) that functions as a nucleus for generating pinning particles and intragranular transformation, It was found that the toughness of the molten metal part was improved.

  In the present invention, the amount of C, CeEB, and C / CeEB of the base material are controlled within an appropriate range, and a small amount of elements such as Mg and Ca are added, and the molten metal part with respect to the fracture toughness value of the base material and An electron beam welded joint in which the ratio of fracture toughness values of HAZ is improved and variation in fracture toughness value δc is suppressed as much as possible, and a steel material capable of forming the welded joint.

The steel material of the present invention is mass%, C: 0.02 to 0.1%, Si: 0.05 to 0.30%, Mn: 1.5 to 2.5%, P: 0.015% or less , S: 0.010% or less, Ti: 0.005 to 0.015%, N: 0.0020 to 0.0060%, O: 0.0010 to 0.0045%, and Mg: 0 .0003-0.0027%, Ca: 0.0003-0.0027%,
0.0006% ≦ Mg + Ca ≦ 0.0040%
Al, Nb, and / or V is limited to Al: 0.015% or less, Nb: 0.020% or less, V: 0.030% or less, with the balance being iron and Consists of inevitable impurities.

  Hereinafter, the reason and amount of each element added will be described. In addition,% means the mass%.

  C is an element that contributes to improving the strength. In order to ensure the strength as a welded structure, 0.02% or more is added. Moreover, when there is little C amount, the hardenability of a molten metal part may be insufficient and toughness may be impaired. A preferable lower limit is 0.03%, and more preferably 0.04%. On the other hand, if C exceeds 0.1%, the hardenability increases, and particularly the toughness of the molten metal part and the heat-affected zone decreases, so the upper limit is made 0.1%. A preferable upper limit is 0.08%, and more preferably 0.06%.

  Si is a deoxidizing element and is also an effective element for securing the strength of the steel sheet. Therefore, 0.05% or more is added. However, when Si is added excessively, a large amount of high carbon martensite is generated particularly in the heat-affected zone and the toughness is lowered, so the upper limit is made 0.30%. A preferable upper limit is 0.20%, and more preferably 0.15%.

  Mn is an element effective for securing toughness and enhancing the hardenability to ensure the strength of the steel sheet. If it is less than 1.5%, the toughness, strength, and hardenability of the steel material cannot be sufficiently secured, and Mn evaporates from the molten metal part during electron beam welding, and the hardenability of the molten metal part decreases. . Therefore, 1.5% or more of Mn is added in order to increase the toughness, strength, and hardenability of the steel material, and further to ensure the toughness by increasing the hardenability of the molten metal part.

  The minimum with preferable Mn is 1.7%, More preferably, it is 1.8%. However, if Mn exceeds 2.5%, the hardenability increases, and particularly the toughness of the heat-affected zone decreases, so the upper limit is made 2.5%. A preferable upper limit is 2.4%, and more preferably 2.3%.

  P is an impurity, which adversely affects the toughness of the base material (BM), the molten metal part (WM), and the heat affected zone (HAZ). In particular, in order to ensure the toughness of the molten metal part (WM) and the heat-affected zone (HAZ), it is preferable that P is small, and is suppressed to 0.015% or less. Preferably it is 0.010% or less. From the viewpoint of manufacturing cost, P is preferably 0.001% or more.

  S is an element that forms MnS. MnS precipitates using fine TiN or Mg-containing fine oxides as nuclei, forms a Mn-diluted region, and promotes the formation of intragranular ferrite (intragranular transformation). In order to promote intragranular transformation, it is preferable to contain 0.0001% or more of S. A preferred lower limit is 0.001%. On the other hand, when S is contained excessively, the toughness of the molten metal part (WM) and the heat-affected zone (HAZ) is particularly lowered, so it is suppressed to 0.010% or less. Preferably, the amount of S is 0.005% or less.

  Ti is an element that combines with N to form fine nitrides that contribute to the refinement of crystal grains. In an electron beam welded joint having a low heat input, if fine TiN is present in the heat affected zone (HAZ), it functions as a nucleus for intragranular transformation.

  In order to improve the toughness of the heat-affected zone and the molten metal zone by suppressing grain growth or intragranular transformation, 0.005% or more of Ti is added. A preferred lower limit is 0.007%. On the other hand, if Ti is excessive, coarse TiN is generated and toughness deteriorates, so the upper limit is made 0.015%. A preferable upper limit is 0.012%.

  N is an element that combines with Ti to form fine nitrides. To improve the toughness of the molten metal part and heat-affected zone by refining the crystal grains of the base metal, suppressing the coarsening of the grain size in the heat-affected zone due to the pinning effect, and reducing the grain size due to intragranular transformation 0.0020% or more is added. A preferred lower limit is 0.0030%.

  On the other hand, if the amount of N is excessive, it adversely affects the toughness of the molten metal part and the heat affected zone, so the upper limit is made 0.0060%. A preferable upper limit is 0.0050%.

  O is an element that generates a Mg-containing fine oxide, and is set to 0.0010% or more. Since the O amount of the molten metal portion formed by electron beam welding is smaller than the O amount of the base material, the lower limit is preferably 0.0015%. More preferably, it is 0.0020%. However, if the amount of O is excessive, the oxide becomes coarse and becomes the starting point of destruction, so the upper limit is made 0.0045%.

  Mg is an extremely important element in the present invention. Mg forms a Mg-containing fine oxide and contributes to the promotion of intragranular transformation. In order to use the Mg-containing fine oxide as pinning particles, 0.0003% or more is added. In order to promote intragranular transformation, 0.0005% or more is preferably added.

  On the other hand, if Mg exceeds 0.0027%, a coarse oxide is generated and the toughness of the base material and the heat-affected zone decreases, so the upper limit is made 0.0027%. A preferred upper limit is 0.0025%.

  Ca is a strong deoxidizing element, and 0.0003% or more is added in order to suppress the coarsening of the Mg oxide and secure the Mg-containing fine oxide. Moreover, Ca produces | generates CaS and suppresses the production | generation of MnS extended | stretched in a rolling direction. In order to improve the properties in the plate thickness direction of the steel material, particularly lamellar resistance, it is preferable to add 0.0005% or more.

  On the other hand, if Ca exceeds 0.0027%, a coarse oxide is generated, and the toughness of the base material and the heat-affected zone decreases, so the upper limit is made 0.0027%. A preferred upper limit is 0.0025%.

  In the present invention, Ca and Mg are added simultaneously in order to strengthen deoxidation by adding Ca and suppress the coarsening of Mg oxide. That is, since Ca forms an oxide preferentially over Mg, the coarsening of the Mg oxide is suppressed, and the production of the Mg-containing fine oxide is promoted. The Mg-containing fine oxide functions as pinning particles and intragranular transformation nuclei, and also serves as a TiN production nucleus. In the present invention, Mg and Ca are added in a total amount of 0.0006% or more in order to reinforce the nucleation of ferrite in the prior austenite grains, refine the structure in the prior austenite grains, and suppress the formation of coarse austenite. To do.

  On the other hand, if the total amount of Mg and Ca is excessive, the oxides aggregate and coarsen, adversely affecting the toughness of the base material and the heat-affected zone, so the upper limit of the total amount is set to 0.0040%. The upper limit with preferable total amount is 0.0030%, More preferably, it is 0.0025%.

  The steel material of the present invention may further contain Al, Nb, and / or V within a certain limit for the following reason.

  Al has the effect of improving the toughness of the base material by deoxidation and refinement of the microstructure, so 0.001% or more is added as necessary. Preferably, 0.003% or more is added. However, since Al oxide has a small ferrite transformation nucleation ability and hardly contributes to intragranular transformation, Al is made 0.015% or less. When the Al oxide becomes coarse, it becomes a starting point of fracture, so the preferable upper limit is 0.012%, more preferably 0.010%.

  Nb is an element effective for improving the hardenability of the base material and increasing the strength. If necessary, Nb is added in an amount of 0.001% or more. Preferably, 0.003% or more is added. However, if Nb is added excessively, the toughness of the molten metal portion and the heat-affected zone decreases, so the upper limit is made 0.020%. A preferable upper limit is 0.012%, and more preferably 0.010%.

  V is an element having an effect of enhancing hardenability and temper softening resistance by addition of a small amount, and is added in an amount of 0.005% or more as necessary. Preferably, 0.010% or more is added. However, if V is added excessively, the toughness of the molten metal part and the heat-affected zone decreases, so the upper limit is made 0.030%. A preferable upper limit is 0.025%, and more preferably 0.020%.

  The steel material of the present invention may further contain one or more of Cr, Mo, Cu, and Ni as necessary. Since these elements are effective in improving toughness, 0.05% or more of Cr, Mo, Cu, and / or Ni is added respectively.

  However, since Cr, Mo, Cu, and Ni are expensive, from an economical viewpoint, Cr: 0.50% or less, Mo: 0.50% or less, Cu: 0.25% or less, Ni: 0 50% or less. In particular, in the steel material of the present invention in which the amount of Mn is increased, if these elements are added excessively, the hardenability becomes too high and the balance of toughness may be impaired. Therefore, Preferably, the total amount of Cr, Mo, Cu, and / or Ni is 0.70% or less. More preferably, it is 0.50% or less.

  In the steel material of the present invention, the electron beam weld hardenability index CeEB defined by the following formula (1) is set to 0.49 to 0.60 based on the above component composition.

CeEB = C + 9 / 40Mn + 1 / 15Cu + 1 / 15Ni
+ 1 / 5Cr + 1 / 5Mo + 1 / 5V (1)
Here, C, Mn, Cu, Ni, Cr, Mo, and V are content (mass%) of a steel material component, respectively.

  The electron beam welding hardenability index CeEB is an index that displays the hardenability in consideration of the decrease in the amount of Mn in the molten metal part unique to electron beam welding. When CeEB is less than 0.49, the hardenability of the molten metal portion is insufficient, upper bainite is generated, and the fracture toughness of the welded joint becomes insufficient.

  When CeEB is 0.50 or more, preferably 0.51 or more, fracture toughness is further improved. However, when CeEB exceeds 0.60, the fracture toughness of the heat affected zone (HAZ) becomes insufficient. Therefore, the upper limit of CeEB is preferably 0.59, more preferably 0.58.

  The ratio of the amount of C to the electron beam welding hardenability index CeEB (C / CeEB) is an index that displays the balance between the hardenability of the molten metal part and the hardenability of the heat-affected zone and the base material, and is 0.04-0. .18 is preferred. In electron beam welding, Mn evaporates and the amount of Mn in the molten metal portion is smaller than the amount of Mn in the base material. Therefore, it is preferable to increase the amount of C in the base material to ensure hardenability. When the amount is excessive, high carbon martensite is generated in the HAZ.

  If the C / CeEB is less than 0.04, the hardenability of the molten metal part is insufficient and the fracture toughness is lowered, so the lower limit is made 0.04. A preferred lower limit is 0.05. On the other hand, if C / CeEB exceeds 0.18, the fracture toughness of the heat-affected zone may decrease, so the upper limit is set to 0.18. A preferable upper limit is 0.15, and more preferably 0.10.

In the welded joint formed by electron beam welding, the steel material of the present invention has a CTOD value δ _WM of the molten metal portion, a CTOD value δ _HAZ of the heat affected _zone , and a CTOD value δ _{BM of the} base material expressed by the following formula (2): And (3) are preferably satisfied at the same time.
0.3 ≦ δ _WM / δ _BM ≦ 1.1 (2)
0.3 ≦ δ _HAZ / δ _BM ≦ 1.1 (3)

However, (delta) _WM , (delta) _HAZ , and (delta) _BM are the minimum values of CTOD value when a three-point bending CTOD test is performed 6 times at 0 degreeC. Of the δ _BM , δ _HAZ , and δ _WM , δ _BM is the largest. However, in consideration of variations in measurement data, the upper limit of δ _WM / δ _BM and δ _HAZ / δ _BM is 1. Set to 1.

When δ _WM / δ _BM and δ _HAZ / δ _BM are less than 0.3, the balance of δ _WM , δ _HAZ , and δ _BM becomes extremely poor, and the fracture toughness of the weld is greatly reduced. The lower limit of δ _WM / δ _BM and δ _HAZ / δ _BM is 0.3. A preferred lower limit is 0.4, more preferably 0.5.

When electron beam welding is performed on steel using a fine Mg-containing oxide as in the present invention, it is difficult to increase the fracture toughness of HAZ and WM to the same level as the base material. Therefore, especially when it is necessary to increase the fracture toughness of the base material, the preferable upper limit of δ _WM / δ _BM and δ _HAZ / δ _BM is 0.6, more preferably 0.55.

  That is, according to the steel material of the present invention, the fracture toughness of the molten metal part and the heat-affected zone in the welded joint after electron beam welding is less deteriorated than the fracture toughness of the base metal, and the fracture toughness of each part is moderate. A balanced weld joint can be obtained.

Electron beam welding can be performed under a low vacuum that can be achieved with simple equipment, for example, under a reduced pressure of 10 Pa or less. The lower limit of the degree of vacuum is preferably 10 ⁻² Pa, although it depends on the capacity of the equipment. The welding conditions are determined according to the performance of the apparatus and the thickness of the steel material within the ranges of acceleration voltage 130 to 180 V, beam current 100 to 130 mA, welding speed 100 to 250 mm / min. For example, when the plate thickness is 80 mm, an acceleration voltage of 175 V, a beam current of 120 mA, and a welding speed of about 125 mm / min are recommended.

  Next, the manufacturing method of the steel material of this invention is demonstrated. The steel material of the present invention is manufactured by heating a steel material such as a slab (steel piece) as a raw material and then subjecting it to a heat treatment such as hot rolling and heat treatment. Industrially, the continuous casting method is preferable as the method for producing the steel material (steel piece). According to the continuous casting method, the cooling rate after casting can be increased and the generated oxide and Ti nitride can be made finer. Therefore, the continuous casting method is preferable from the viewpoint of improving toughness.

  In general, high Mn steel is inferior in hot workability to carbon steel and low alloy steel, so it is necessary to perform thermomechanical treatment under appropriate conditions. In this invention, first, the steel material (steel piece) of the said component composition is heated to 950-1150 degreeC. When the heating temperature is less than 950 ° C., deformation resistance during hot rolling increases, and productivity decreases. On the other hand, when heating exceeds 1150 degreeC, Ti nitride of steel materials (steel piece) may coarsen and the toughness of steel materials (base material) and a heat affected zone may fall.

  After the steel material (steel piece) is heated to 950 to 1150 ° C., it is subjected to thermomechanical treatment. The thermomechanical treatment is effective in increasing the strength and toughness of the steel material. For example, (1) controlled rolling (CR), (2) controlled rolling-accelerated cooling (ACC), (3) direct quenching-tempering after rolling ( There is a method such as QT). In the present invention, (2) controlled rolling-accelerated cooling and (3) direct quenching-tempering after rolling are preferable in terms of improving fracture toughness.

The controlled rolling performed in the non-recrystallization temperature range (about 900 ° C. or less) is effective in refining the structure of the steel material and improving the strength and toughness. In the present invention, in order to prevent the formation of processed ferrite, the controlled rolling is preferably finished at a temperature equal to or higher than the Ar ₃ transformation point.

  In particular, when controlled rolling is performed, if accelerated cooling is subsequently performed, a hard phase such as bainite or martensite is generated and the strength is improved. In order to ensure strength and toughness, the stop temperature of accelerated cooling is preferably 400 to 600 ° C. The direct quenching after rolling is a method in which hot rolling is performed in a temperature range higher than the temperature range of controlled rolling, followed by quenching by water cooling or the like. According to this method, since the strength usually increases, tempering is performed to ensure toughness. The tempering temperature is preferably 400 to 600 ° C.

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

  Steel materials were produced under the conditions shown in Tables 3 and 4 using the steel materials having the component compositions shown in Tables 1 and 2. A test piece was collected from the steel material, subjected to a tensile test and a CTOD test, and the tensile strength and fracture toughness value of the base material were measured. The strength of the base material was measured on the basis of JIS Z 2241 by collecting test pieces from the 1/2 part of the plate thickness with the rolling direction as the longitudinal direction. In addition, the thing whose yield stress is 355-420 Mpa was evaluated as favorable.

  The steel material was subjected to electron beam welding to produce an I-groove butt weld joint. Electron beam welding was performed using the RPEBW method under conditions of a voltage of 175 V, a current of 120 mA, and a welding speed of about 125 mm / min under a vacuum of about 1 mbar. The weld bead width is 3.0 to 5.5 mm.

  Then, from the welded joint, (a) if the thickness is less than 60 mm, a test piece of t (plate thickness) × 2t, and (b) if the thickness is 60 mm or more, a test piece of t (plate thickness) × t. 6 samples were collected each. A 50% fatigue crack was introduced into the test piece as a notch at each position of the center of the molten metal part (WM), the fusion part (FL), and the base material (BM). The test piece which introduce | transduced the notch is shown in FIG.

In electron beam welding, since the width of the heat affected zone is narrow, the CTOD value δ _HAZ of the heat affected _zone was measured using a test piece in which a notch was introduced into the fused _zone .

A CTOD test was conducted at a test temperature of 0 ° C. to determine a fracture toughness value δc. At each notch position, the six minimum values were designated as fracture toughness values δ _WM , δ _HAZ , and δ _BM , respectively. Table 3 and Table 4, CTOD value [delta] _WM of the molten metal portion of the welded joint (WM), CTOD value [delta] _HAZ heat affected zone (HAZ), and, based on the CTOD value [delta] _BM of the base material (BM) [delta] The values of _EM / δ _BM and δ _HAZ / δ _WM are shown.

As shown in Tables 1 and 3, the steel material No. 1-30, the component composition, CeEB, and C / CeEB are all within the scope of the present invention, and δc of the base material (BM), the heat affected zone (HAZ), and the molten metal portion (WM) The ratios, δ _HAZ / δ _BM , and δ _WM / δ _BM show sufficient values.

On the other hand, as shown in Table 2 and Table 4, the steel material No. No. 31 has a small amount of C, a large amount of Mn, a high CeEB value, and a low C / CeEB. Therefore, the CTOD values of the heat affected zone (HAZ) and the molten metal portion (WM) are low, and δ _HAZ / δ _BM and δ _WM / δ _BM do not show sufficient values.

Steel No. 32 has a large amount of C and a high C / CeEB, so the CTOD values of the heat affected zone (HAZ) and the molten metal zone (WM) are low, and the values of δ _HAZ / δ _BM and δ _WM / δ _BM are insufficient. It is. Steel No. No. 34 has low Mn content and low CeEB, so the strength of the base metal (BM) is low, the hardenability of the molten metal part (WM) is insufficient, and the CTOD value of the molten metal part (WM) decreases. However, the value of δ _WM / δ _BM is insufficient.

Steel No. No. 33 has a large amount of Si, so that a brittle phase is often generated, the CTOD value of the heat affected zone (HAZ) is low, and the value of δ _HAZ / δ _BM is insufficient. Steel No. No. 35 has a large amount of Mn and a high CeEB, so the CTOD value of the heat affected zone (HAZ) is low and the value of δ _HAZ / δ _BM is insufficient.

Steel No. 36 and no. 37 has a large amount of P and S, respectively, so the CTOD values of the heat affected zone (HAZ) and the molten metal portion (WM) are low, and the values of δ _HAZ / δ _BM and δ _WM / δ _BM are insufficient. is there. Steel No. 38, no. 39 and no. 40 has a large amount of Ti, Nb, and V, respectively, so the CTOD values of the heat affected zone (HAZ) and the molten metal zone (WM) are low, and the values of δ _HAZ / δ _BM and δ _WM / δ _BM are It is insufficient.

Steel No. In No. 47, the amount of N was large and coarse nitrides were formed, so the CTOD values of the heat affected zone (HAZ) and the molten metal zone (WM) were low, and the values of δ _HAZ / δ _BM and δ _WM / δ _BM were not good. It is enough.

Steel No. No. 41 has a small amount of Mg and steel No. 41. No. 44 has a large amount of Ca. No. 46 has a large amount of Al. 48, since the amount of O is small, the effect of the fine Mg-containing oxide cannot be obtained, the CTOD values of the heat affected zone (HAZ) and the molten metal zone (WM) are lowered, and δ _HAZ / δ _BM and δ _WM The value of / δ _BM is insufficient.

Steel No. No. 42 has a small amount of Ca and steel No. 42. No. 43 has a large amount of Mg. No. 45 has a large total amount of Mg and Ca. No. 49 has a large amount of O, so a coarse oxide is produced, and the CTOD values of the heat affected zone (HAZ) and the molten metal zone (WM) are lowered, and values of δ _HAZ / δ _BM and δ _WM / δ _BM are obtained. Is insufficient.

Steel No. 50-No. The component composition of 53 is within the scope of the present invention. No. 50 has a low CeEB, and steel material No. No. 51 has a low C / CeEB, so that the hardenability of the molten metal part (WM) is insufficient, the CTOD value of the molten metal part is lowered, and the value of δ _WM / δ _BM is insufficient. Steel No. No. 52 has a high CeEB and steel No. 53 has a high C / CeEB, the CTOD value of the heat affected _zone is low, and the value of δ _HAZ / δ _BM is insufficient.

  According to the present invention, in the molten metal part and heat-affected zone of the electron beam welded joint of a steel material with a yield strength of 355 MPa class, the fracture toughness is less deteriorated compared to the fracture toughness of the base metal. A moderately balanced electron beam welded joint and the welded joint can be formed, and a steel material suitable for construction of the foundation portion of the offshore wind power generation tower can be provided at low cost. Therefore, the present invention has high applicability in the large steel structure construction industry.

Claims (10)

An electron beam welded joint formed by irradiating a welded portion of a steel material with an electron beam, the steel material being in mass%, C: 0.02 to 0.1%, Si: 0.05 to 0.30 %, Mn: 1.5 to 2.5%, P: 0.015% or less, S: 0.010% or less, Ti: 0.005 to 0.015%, N: 0.0020 to 0.0060% , O: 0.0010 to 0.0045%, and Mg: 0.0003 to 0.0027% and Ca: 0.0003 to 0.0027%,
0.0006% ≦ Mg + Ca ≦ 0.0040%
Al, Nb, and / or V is limited to Al: 0.015% or less, Nb: 0.020% or less, V: 0.030% or less, with the balance being iron and An electron beam welded joint comprising an inevitable impurity and further having an electron beam weld hardenability index CeEB defined by the following formula (1) of 0.49 to 0.60.
CeEB = C + 9 / 40Mn + 1 / 15Cu + 1 / 15Ni
+ 1 / 5Cr + 1 / 5Mo + 1 / 5V (1)
Here, C, Mn, Cu, Ni, Cr, Mo, and V are content (mass%) of a steel material component, respectively.   2. The electron beam welded joint according to claim 1, wherein a ratio (C / CeEB) of a C amount to the electron beam weld hardenability index CeEB is 0.04 to 0.18.   The steel material further contains one or more of Cr: 0.50% or less, Mo: 0.50% or less, Cu: 0.25% or less, and Ni: 0.50% or less in mass%. The electron beam welded joint according to claim 1 or 2, characterized in that:   The electron beam welded joint according to any one of claims 1 to 3, wherein the steel material has a thickness of 45 to 150 mm. In the electron beam welded joint, the CTOD value δ _WM of the molten metal portion after welding at a vacuum degree of 10 Pa or less, the CTOD value δ _HAZ of the heat affected _zone , and the CTOD value δ _{BM of the} base material are the following (2) The electron beam welded joint according to any one of claims 1 to 4, wherein the expression (3) is satisfied.
0.3 ≦ δ _WM / δ _BM ≦ 1.1 (2)
0.3 ≦ δ _HAZ / δ _BM ≦ 1.1 (3)
However, (delta) _WM , (delta) _HAZ , and (delta) _BM are the minimum values of CTOD value when a three-point bending CTOD test is performed 6 times at 0 degreeC. It is a steel material which forms the electron beam welding joint of Claim 1, Comprising: By mass%, C: 0.02-0.1%, Si: 0.05-0.30%, Mn: 1.5- 2.5%, P: 0.015% or less, S: 0.010% or less, Ti: 0.005-0.015%, N: 0.0020-0.0060%, O: 0.0010-0 .0045% and Mg: 0.0003-0.0027% and Ca: 0.0003-0.0027%,
0.0006% ≦ Mg + Ca ≦ 0.0040%
Al, Nb, and / or V is limited to Al: 0.015% or less, Nb: 0.020% or less, V: 0.030% or less, with the balance being iron and A steel material for electron beam welding comprising an inevitable impurity and further having an electron beam welding hardenability index CeEB defined by the following formula (1) of 0.49 to 0.60.
CeEB = C + 9 / 40Mn + 1 / 15Cu + 1 / 15Ni
+ 1 / 5Cr + 1 / 5Mo + 1 / 5V (1)
Here, C, Mn, Cu, Ni, Cr, Mo, and V are content (mass%) of a steel material component, respectively.   A steel material for forming an electron beam weld joint according to claim 2, wherein a ratio of C amount to the electron beam weld hardenability index CeEB (C / CeEB) is 0.04 to 0.18. The steel material for an electron beam welded joint according to claim 6.   It is a steel material which forms the electron beam welded joint according to claim 3, and further, in mass%, Cr: 0.50% or less, Mo: 0.50% or less, Cu: 0.25% or less, and Ni: 0.50% or less of 1 type or 2 types or more, The steel material for electron beam welded joints of Claim 6 or 7 characterized by the above-mentioned.   The steel material for the electron beam welded joint according to any one of claims 6 to 8, wherein the steel material forming the electron beam welded joint according to claim 4 has a thickness of 45 to 150 mm.   It is a manufacturing method of the steel materials for electron beam welding of any one of Claims 6-9, Comprising: The steel materials which have the component composition of any one of Claims 6-8 are made into 950-1150 degreeC. A method for producing a steel material for electron beam welding, characterized by heating and then performing a heat treatment.

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