JP4398751B2 - High strength weld metal with excellent low temperature toughness - Google Patents

Description

  The present invention relates to a weld metal formed by gas shielded arc welding, and more particularly to a weld metal having high strength and good low temperature toughness.

  In recent years, steel materials used in structures have been increasingly strengthened, and it is desired to increase the strength of joints that join steel materials together. There are various methods for joining steel materials, and welding is generally used. Therefore, high strength is also demanded for the weld metal formed when the steel materials are welded together.

  By the way, as a material of a structure used in a low temperature environment such as an offshore structure, a liquefied gas tank, or a line pipe, low temperature toughness is strongly required in addition to high strength. Therefore, good low temperature toughness is also demanded for the weld metal.

  However, unlike steel materials, weld metal cannot be increased in toughness by heat treatment, so the low temperature toughness of the weld metal is inferior to that of steel. It is known that the low temperature toughness of the weld metal is greatly affected by the structure of the weld metal, and as a method for improving the toughness, it is effective to generate a microstructure called acicular ferrite. It is known. Since this acicular ferrite is a structure that grows by using Ti-based oxides contained in the weld metal as nuclei, it is considered that controlling the presence of the oxide is effective in improving the toughness of the weld metal. .

  Moreover, if Ti and B (boron) are added in combination, a Ti-based oxide (that is, an acicular ferrite formation nucleus) is secured, and coarse ferrite generated from the prior austenite grain boundaries is improved by the hardenability by B. It has also been reported that the generation of acicular ferrite can be made uniform. However, it is also known that if a large amount of B is added, the crack resistance of the weld metal deteriorates.

  On the other hand, the present inventors have also conducted various studies to improve the low temperature toughness of the weld metal, and have previously proposed a weld metal having excellent toughness as part of the results. For example, Patent Document 1 proposes a technique for improving the toughness of a weld metal by generating residual austenite having a lower deformation resistance than that of a matrix in the weld metal. However, the weld metal obtained by this technique sometimes has insufficient toughness at a low temperature range of −60 ° C. level.

Patent Document 2 proposes a technique for improving the toughness of the entire weld metal by incorporating solute Ti into the weld metal. Further, in Patent Document 3, the total amount of martensite and residual austenite structure existing in the as-welded region that is not affected by the heat of the next pass in the weld metal is defined, thereby concentrating solid solution N in the residual austenite. Thus, the diffusion of solid solution N to the entire matrix was suppressed, and as a result, a technique for preventing a decrease in toughness by weakening the transfer fixing action of the solid solution N was proposed. However, in these techniques, since the oxygen content contained in the weld metal has not been sufficiently considered, the control thereof is insufficient and there is room for improvement.
Japanese Patent Laid-Open No. 2000-61687 (see [Claims] and [0010]) JP 2000-263283 A (see [Claims] and [0009]) JP 2001-254141 A (see [Claims] and [0010])

  The present invention has been made in view of such a situation, and an object thereof is to provide a weld metal having a high strength of 590 MPa or more and excellent toughness in a low temperature range of −60 ° C. It is in.

The high-strength weld metal excellent in low-temperature toughness according to the present invention that has solved the above problems is a weld metal formed by gas shielded arc welding, and is substantially free of B and Mo. C: 0.04 to 0.12%, Si: 0.10 to 0.80%, Mn: 3.0% or less (not including 0%), Ni: 3.5% or less (including 0%) No), Ti: 0.005 to 0.1% and O: 0.015 to 0.060%, respectively, and TP calculated by the following formula (1) is 350 to 600, and (2 ) The gist is that the EP calculated by the formula is 20 to 220.
TP = 240 × [Mn] + 50 × [Ni] (1)
EP = 1300 × [Ti] −600 × [O] (2)
In the formula, [] indicates the content of each element, and the unit is mass%.

However, when B: 0.01% or less (excluding 0%) is contained in the weld metal, it is calculated by the following equation (4) instead of the EP calculated by the above equation (2). The EP must be 20-220.
EP = 1300 × [Ti] −600 × [O] + 17000 × [B] (4)
In the formula, [] indicates the content of each element, and the unit is mass%.

On the other hand, when Mo: 1.0% or less (not including 0%) is contained in the weld metal, it is calculated by the following equation (5) instead of TP calculated by the above equation (1). TP must be 350-600.
TP = 240 × [Mn] + 50 × [Ni] + 45 × [Mo] (5)
In the formula, [] indicates the content of each element, and the unit is mass%.

Furthermore, the said subject is a weld metal formed by gas shielded arc welding, Comprising: C: 0.04-0.12%, Si: 0.10-0.80%, Mn: 3.0% or less ( 0% not included), Ni: 3.5% or less (not including 0%), Mo: 1.0% or less (not including 0%), Ti: 0.005 to 0.1%, O: 0.015 to 0.060% and B: 0.01% or less (not including 0%), respectively, and TP calculated by the following formula (7) is 350 to 600, and the following (8) Even a high-strength weld metal excellent in low-temperature toughness having an EP calculated by the equation of 20 to 220 can be solved.
TP = 240 × [Mn] + 50 × [Ni] + 45 × [Mo] (7)
EP = 1300 × [Ti] −600 × [O] + 17000 × [B] (8)
In the formula, [] indicates the content of each element, and the unit is mass%.

The weld metal of the present invention is preferably one containing any of the following elements (a) to (c) alone or in combination of two or more selected arbitrarily as another element. .
(A) Cu: 2.0% or less (excluding 0%)
(B) Cr: 1% or less (excluding 0%)
(C) Nb: 0.20% or less (including 0%) and / or V: 0.20% or less (including 0%)

When the cross section of the weld metal of the present invention is observed with an electron microscope at a magnification of 6000, there are 50 or more oxide inclusions containing Ti having a particle size of 0.2 to 2 μm in terms of 1 mm ² observation field. Those that do are preferred. It is more preferable that the oxide inclusion is mainly composed of M ₂ O ₃ type.

  According to the present invention, it is possible to provide a weld metal having high strength of 590 MPa or more and excellent low temperature toughness in a −60 ° C. region.

  The present inventors have studied from various angles to solve the above problems. As a result, in order to achieve high strength of 590 MPa or more, the balance of Mn, Ni and Mo contained in the weld metal may be appropriately controlled, and in order to improve low temperature toughness in the −60 ° C. region. Knowing that the balance of Ti, O and B contained in the weld metal should be properly controlled, the present invention has been completed based on these findings. Hereinafter, the function and effect of the present invention will be described.

  The present inventors conducted various studies from the viewpoint of increasing the strength of the weld metal. As a result, it has been found that it is important to control the balance of Mn, Ni and Mo contained in the weld metal in order to achieve a high strength of 590 MPa or more. That is, Ni affects the structural change of the weld metal and increases the toughness of the matrix itself. Therefore, as a result of studying from the viewpoint of maximizing the action of Ni, the present inventors have shown that the above action can be effectively achieved by adjusting the amount of Mn and Mo, and the strength of the weld metal can be dramatically improved. Revealed. Conventionally, since the component composition of the weld metal is affected by the composition of the base metal, the relationship between the balance of these elements contained in the weld metal and the strength of the weld metal has not yet been studied.

Next, when the influence of the above-mentioned elements contained in the weld metal on the strength improvement of the weld metal was examined, the contribution of each element is the following expression (5) [meaning including the above expression (7). same as below. ] If the TP value calculated by substituting each element content in the weld metal into this equation is 350 to 600, it is possible to achieve a high strength of 590 MPa or more. I understood. If the TP value is less than 350, the hardenability is insufficient and sufficient strength cannot be obtained. On the other hand, if the TP value exceeds 600, the hardenability increases too much, a supercooled structure is formed, and the strength becomes too large. A preferred range for the TP value is 360-500.
TP = 240 × [Mn] + 50 × [Ni] + 45 × [Mo] (5)
In the formula, [] indicates the content of each element, and the unit is mass%.

However, when the relationship between each element contained in the weld metal and the strength was further studied, if the TP value satisfies the above range, the strength of the weld metal can be sufficiently increased without including Mo. found. That is, since Mo has the effect of increasing the softening resistance due to tempering, it is recommended to contain Mo when heat treatment is performed to remove residual stress after welding, but it increases the strength of the weld metal. From the viewpoint, it is not necessarily an essential element. Therefore, when Mo is not substantially contained in the weld metal, the Mn and Ni contents are expressed by the following formula (1) [meaning including the above formula (3). same as below. ] To calculate the TP value, and this value should satisfy the above range.
TP = 240 × [Mn] + 50 × [Ni] (1)
In the formula, [] indicates the content of each element, and the unit is mass%.

  In order to control the TP value calculated by the above formula [5] [or the above formula (1)] to satisfy a predetermined range, the component composition of the welding wire used at the time of welding may be adjusted. That is, since Mn, Ni, and Mo are hardly affected by the deoxidation process during welding, the component composition of the welding wire is almost the same as that of the weld metal. In addition, when these elements are contained in a base material, although it is influenced also by the component composition of a base material, if it considers welding conditions, it can estimate how much it is influenced.

  Next, the present inventors advanced research to improve the toughness in a low temperature range of −60 ° C. with respect to a weld metal that could achieve a high strength of 590 MPa or more. As a result, it was found that it is important to control the balance of Ti, O and B contained in the weld metal in order to improve the low temperature toughness in the temperature range. That is, it is considered that the toughness of the weld metal in a low temperature environment is remarkably improved by specifying the form of oxides of Ti and O and further limiting the sites of the acicular ferrite transformation by B. In addition, since the influence of oxygen contained in the weld metal has not been sufficiently elucidated in the past, the present inventors consider that the relationship between the balance of these elements and the low temperature toughness has not been studied.

Next, the degree of contribution of these elements contained in the weld metal to the low temperature toughness of the weld metal was examined. The degree of contribution of each element is the following expression (4) [meaning including the above expression (8). same as below. If the EP value calculated by substituting the amount of each element in the weld metal into this equation is 20 to 220, good low temperature toughness is exhibited in the −60 ° C. region. I knew that. When the EP value is less than 20, the acicular ferrite is coarsened or the amount of coarse products growing from the grain boundaries is increased, so that low temperature toughness cannot be secured. On the other hand, when the EP value exceeds 220, the balance between Ti and O is poor, and the oxides aggregate and coarsen to coarsen the acicular ferrite, or the amount of B becomes excessive and inhibits the formation of the acicular ferrite, resulting in low temperature toughness. Deteriorates. A preferred range for the EP value is 40-150.
EP = 1300 × [Ti] −600 × [O] + 17000 × [B] (4)
In the formula, [] indicates the content of each element, and the unit is mass%.

However, as a result of further studies on the relationship between each element contained in the weld metal and the low temperature toughness, it has been found that the toughness of the weld metal can be improved without containing B. That is, if the EP value satisfies the above range, good low temperature toughness can be obtained without containing B. Therefore, when B is not contained in the weld metal, the Ti and O contents are expressed by the following formula (2) [meaning including the above formula (6). same as below. ] To calculate the EP value, and this value should be within the above range.
EP = 1300 × [Ti] −600 × [O] (2)
In the formula, [] indicates the content of each element, and the unit is mass%.

  In order to control the EP value calculated by the above formula (4) [or the above formula (2)] to satisfy a predetermined range, the component composition of the welding wire and the composition of the shield gas used at the time of welding are adjusted. Just do it. That is, since Ti and O are greatly affected by the deoxidation process during welding, the amount contained in the weld metal varies greatly depending on the welding conditions, and the content of Ti and O contained in the weld metal is strictly controlled. It is difficult to do. However, if the component composition of the welding wire and the composition of the shield gas used during welding are controlled as described below, the Ti and O contents in the weld metal can be adjusted to a suitable range. On the other hand, since B is hardly affected by the deoxidation process during welding, the B content contained in the weld metal can be adjusted only by controlling the B content contained in the welding wire used during welding. Hereinafter, the EP value control method will be described more specifically.

(I) When the Ti content contained in the welding wire exceeds 0.1% by mass, the composition of the shielding gas used at the time of welding may be CO ₂ : Ar = 5 to 80% by volume: 95 to 20% by volume. If the CO ₂ fraction is less than 5% by volume, the amount of oxygen contained in the weld metal becomes too small. If it exceeds 80% by volume, the amount of oxygen contained in the weld metal becomes too large. Production is inhibited.

However, when the CO ₂ fraction is 5 to 50% by volume, it is not necessary to contain B in the weld metal, and the EP value only needs to satisfy the range defined in the present invention. On the other hand, when the CO ₂ fraction is more than 50% by volume and 80% by volume or less, it is necessary to add B as an essential element to the weld metal.

(II) When the Ti content contained in the welding wire is more than 0.06% by mass to 0.1% by mass, the composition of the shielding gas used at the time of welding is CO ₂ : Ar = 5 to 60% by volume: 95 to 40% by volume. good. If the CO ₂ fraction is less than 5% by volume, the amount of oxygen contained in the weld metal will be too small. If it exceeds 60% by volume, the amount of oxygen contained in the weld metal will be too large. Production is inhibited.

Further, when the Ti content is in the above range, it is necessary to add B as an essential element to the weld metal. For example, when a gas mixture of CO ₂ : Ar = 20% by volume: 80% by volume is used as the shielding gas, and welding is performed at a current of 280 A, a voltage of 30 V, and a welding speed of about 30 cm / min, B is added to the weld metal. It is necessary to add 10 ppm or more.

(III) When the Ti content contained in the welding wire is 0.06% by mass or less, the composition of the shielding gas used during welding may be CO ₂ : Ar = 5 to 30% by volume: 95 to 70% by volume. If the CO ₂ fraction is less than 5% by volume, the amount of oxygen contained in the weld metal becomes too small. If it exceeds 30% by volume, the amount of oxygen contained in the weld metal becomes too large. Production is inhibited.

Further, when the Ti content is in the above range, it is necessary to add B as an essential element to the weld metal. For example, when a gas mixture of CO ₂ : Ar = 20% by volume: 80% by volume is used as the shielding gas, and welding is performed at a current of 280 A, a voltage of 30 V, and a welding speed of about 30 cm / min, B is added to the weld metal. Should be added at 25 ppm or more.

Conventionally, CO ₂ is generally used alone as a shielding gas at the time of gas shielded arc welding. However, when forming the weld metal of the present invention, as described above, CO _{2 is used} as the shielding gas. It is preferable to use a mixed gas of Ar and Ar.

  Next, the component composition in the weld metal of the present invention will be described.

  The weld metal of the present invention includes, as essential components, C: 0.04 to 0.12%, Si: 0.10 to 0.80%, Mn: 3.0% or less (not including 0%), Ni: 3.5% or less (not including 0%) , Ti: 0.005 to 0.1% and O: 0.015 to 0.060%, respectively. In addition to these elements, Mo and B are contained. The reason why these ranges are determined will be described below.

C: 0.04-0.12%
C is an element indispensable for ensuring the strength of the weld metal. In order to exert this effect effectively, it is necessary to contain 0.04% or more, preferably 0.05% or more (more preferably 0.06%). Above) It is good to contain. However, if the C content exceeds 0.12%, the strength of the weld metal excessively increases, leading to toughness deterioration. Therefore, the C content needs to be suppressed to 0.12% or less. It is recommended that the content be 0.10% or less.

Si: 0.10 to 0.80%
Si is an element having a deoxidizing action, and cleans the weld metal and, when yielded, strengthens the solid solution of ferrite. In order to exert this effect, the content should be 0.10% or more, preferably 0.20% or more. However, if the Si content exceeds 0.80%, the strength of the weld metal increases excessively and causes a decrease in toughness. Moreover, a hard 2nd phase is produced | generated and it becomes a cause which deteriorates the toughness of a weld metal. Accordingly, the Si content must be suppressed to 0.80% or less, and preferably 0.50% or less.

Mn: 3.0% or less (excluding 0%)
Mn is an element indispensable for ensuring the strength and toughness of the weld metal, and 0.3% or more (more preferably 0.5% or more, more preferably 1.0% or more) is contained in order to effectively exhibit these effects. It is recommended. However, if it exceeds 3.0%, the hardenability increases too much and the toughness is lowered. Moreover, a toughness fall is caused also by producing | generating a hard 2nd phase by segregation. Therefore, the Mn content needs to be suppressed to 3.0% or less, preferably 2.0% or less, more preferably 1.6% or less.

Ni: 3.5% or less (excluding 0%)
Ni, like Mn, is an essential element for ensuring the strength and toughness of the weld metal, but it works particularly effectively in improving low-temperature toughness. In order to effectively exhibit such an action, it is recommended to contain 0.1% or more (more preferably 0.2% or more, still more preferably 0.5% or more, particularly preferably 0.6% or more). However, if the content exceeds 3.5%, the hardenability increases and the toughness of the weld metal is deteriorated, so the Ni content must be suppressed to 3.5% or less. The content is preferably 3.0% or less, more preferably 2.5% or less.

Ti: 0.005-0.1%
Ti is important as an element constituting an oxide inclusion that becomes a nucleus for generating acicular ferrite in the weld metal, and includes Ti that becomes an acicular ferrite formation nucleus when the Ti content is less than 0.005%. Oxide inclusions are not sufficiently formed, and acicular ferrite is hardly formed, and satisfactory toughness cannot be obtained. Therefore, in order to generate oxide inclusions containing Ti, it is necessary to contain 0.005% or more of Ti, preferably 0.008% or more. However, if the Ti content exceeds 0.1%, carbide (TiC) precipitates and remarkably increases the strength of the weld metal and degrades the toughness. Therefore, the Ti content should be suppressed to 0.1% or less. Preferably it is 0.08% or less.

O: 0.015-0.060%
O is an extremely important element for generating oxide inclusions containing Ti which is a nucleus of acicular ferrite in the weld metal. In order to exert its effect effectively, O must be contained in an amount of 0.015% or more. Don't be. It is recommended to contain 0.020% or more preferably. However, if the content exceeds 0.060%, the oxide inclusions containing Ti are coarsened, making it difficult to form nuclei for acicular ferrite, leading to deterioration of toughness. Therefore, the O content needs to be suppressed to 0.060% or less, preferably 0.050% or less, more preferably 0.04% or less.

  The weld metal of the present invention contains these elements as essential components, but Mo: 1.0% or less (not including 0%) or B: 0.01% or less (not including 0%) as necessary. May be included. The reason for limiting to such a range will be described.

Mo: 1.0% or less (excluding 0%)
If the Mo content exceeds 1.0%, the strength of the weld metal becomes too high and the toughness deteriorates, so the Mo content should be suppressed to 1.0% or less. It is desirable to suppress it to 0.7% or less, more preferably 0.5% or less, and still more preferably 0.45% or less. However, Mo has the effect | action which suppresses that a ferrite produces | generates from a grain boundary, and the intensity | strength of a weld metal can be raised by making it contain. Mo also acts to increase the softening resistance due to tempering. In order to exhibit these effects effectively, it is recommended to contain 0.01% or more (more preferably 0.1% or more, and still more preferably 0.2% or more).

B: 0.01% or less (excluding 0%)
If the B content exceeds 0.01%, the toughness deteriorates and it is easy to cause weld cracking. Therefore, it is necessary to suppress the B content to 0.01% or less. Preferably, it is preferably suppressed to 0.005% or less. However, B has an effect of fixing N dissolved in the weld metal and an effect of suppressing the formation of ferrite from the grain boundary, and is an element contributing to improvement of toughness. In order to exhibit these effects effectively, it is desirable to contain 0.0005% or more (more preferably 0.0010% or more).

  The constituent elements of the weld metal according to the present invention are as described above, and as other elements, (a) Cu: 2.0% or less (excluding 0%), (b) Cr: 1% or less (0% (C) Nb: 0.2% or less (including 0%) and / or V: 0.2% or less (including 0%), etc. are effective, but the balance is Fe and inevitable impurities. It is. The reasons for setting these ranges are as follows.

Cu: 2.0% or less (excluding 0%)
Cu has the same action as Ni, and the inclusion of Cu can increase the strength without impairing the low temperature toughness of the weld metal. In order to exhibit such an action effectively, it is preferable to contain 0.1% or more (more preferably 0.2% or more). However, if the content exceeds 2.0%, the hardenability of the weld metal is increased and the toughness is deteriorated, so the Cu content needs to be suppressed to 2.0% or less. Preferably 1.5% or less is recommended.

Cr: 1% or less (excluding 0%)
Cr has the effect of increasing the hardenability of the weld metal, suppressing the formation of ferrite from the grain boundaries, and refining the structure. In order to effectively exhibit such an action, it is recommended to contain 0.1% or more (more preferably 0.2% or more). However, if the Cr content exceeds 1%, the strength of the weld metal becomes too high and the toughness deteriorates, so the Cr content must be suppressed to 1% or less. Preferably 0.8% or less is recommended.

Nb: 0.2% or less (including 0%) and / or V: 0.2% or less (including 0%)
Nb and V are effective elements for improving the hardenability of the weld metal and improving the strength. However, if the Nb content exceeds 0.2% and the V content exceeds 0.2%, carbides precipitate in the weld metal. In order to deteriorate toughness, it is preferable to keep the Nb content to 0.2% or less and the V content to 0.2% or less. More preferably, the Nb content is 0.15% or less, the V content is 0.15% or less, more preferably the Nb content is 0.03% or less, the V content is 0.08% or less, particularly preferably the Nb content is 0.15% or less, and the V content is 0.15% or less. It is good to do. When Nb and V are used in combination, the total content is preferably 0.3% or less, more preferably 0.2% or less, still more preferably 0.05% or less, and particularly preferably 0.04% or less.

The weld metal of the present invention satisfies the above component composition range and the parameters of TP and EP satisfy a predetermined range. However, oxide inclusions containing Ti are dispersed in the weld metal. It is preferable. That is, when the cross section of the weld metal is observed with an electron microscope at a magnification of 6000, it is preferable that 50 or more oxide inclusions containing Ti and having a particle diameter of 0.2 to 2 μm are present in terms of 1 mm ² observation field.

  The reason why oxide-based inclusions containing Ti are defined is that they serve as nuclei for forming acicular ferrite and controlling the presence of the inclusions is effective in improving toughness. Therefore, the generation of acicular ferrite can be controlled by defining the number of oxide inclusions containing Ti.

The number of oxide-based inclusions containing Ti is measured by observing the weld metal at 6000 times with an electron microscope, and the number when converted per observation field area 1 mm ² is 50 or more. preferable. This is because a large amount of oxide inclusions containing Ti having an appropriate size are dispersed in the weld metal, whereby acicular ferrite is efficiently generated and low temperature toughness is enhanced. As the electron microscope, a known microscope such as a scanning electron microscope or a transmission electron microscope may be used. Note that the number of viewing fields is at least five, and the average of the obtained results is the number of oxide inclusions containing Ti. Moreover, the position observed with an electron microscope will not be specifically limited if it is a cross section of a weld metal.

  When measuring the number of oxide-based inclusions containing Ti, the object of measurement is one having a particle size of 0.2 to 2 μm. In oxide inclusions containing Ti with a particle size of less than 0.2 μm, the inclusions are too fine to function as nuclei for acicular ferrite, while oxide inclusions containing Ti with a particle size of more than 2 μm This is because the material is too coarse and deteriorates toughness. Therefore, in the weld metal of the present invention, a large number of oxide inclusions containing Ti having a particle size of less than 0.2 μm may exist, but coarse oxide inclusions having a particle size exceeding 2 μm. Is preferably not present as much as possible.

When 50 or more oxide inclusions containing Ti and having a particle diameter of 0.2 to 2 μm are present in the observation field of 1 mm ² , there are almost no oxide inclusions having a particle diameter exceeding 2 μm. The present inventors have confirmed that this is not the case. When coarse oxide inclusions with a particle size exceeding 2 μm are present in the weld metal, the weld metal is observed with an electron microscope at a magnification of 6000 times, and this is measured when converted to an observation field of 1 mm ² It is acceptable if the number is up to 5.

The oxide inclusions containing Ti are not limited to Ti oxides, but include, for example, (Mn ₂ Ti) O ₃ , EDX (energy dispersive X-ray detector; energy dispersive X -ray spectrometer) refers to inclusions in which Ti and O peaks are observed when the component composition is measured. At this time, in addition to Ti and O, peaks such as Al, Si, and Mn may be observed.

  Note that oxide inclusions that do not contain Ti do not form nuclei for acicular ferrite and do not contribute to toughness improvement, and are therefore excluded from the measurement target.

  In order to actively generate oxide inclusions containing Ti having a particle size of 0.2 to 2 μm in the weld metal, the Ti content contained in the welding wire used during welding and the composition of the shielding gas may be adjusted.

The oxide inclusions containing Ti have various crystal structures such as M ₃ O ₄ type and M ₂ O ₃ type. However, to improve the low temperature toughness of weld metal by refining acicular ferrite. It is also important to control the crystal structure of oxide inclusions containing Ti. That is, by controlling the crystal structure of oxide inclusions containing Ti that are the nuclei of the acicular ferrite, the growth state of the acicular ferrite can be controlled and the low temperature toughness can be improved. From this point of view, it is recommended that the crystal structure of the oxide inclusions containing Ti is mainly composed of oxide inclusions containing M ₂ O ₃ type Ti.

However, in actual operation, it is difficult to uniformly control the crystal structure of all oxide inclusions including Ti existing in the weld metal. Therefore, in practice, oxide-based inclusions containing Ti having various crystal structures are mixed in the weld metal. The crystal structure of oxide inclusions can be identified by performing an X-ray diffraction test on the residue obtained by electrolytic extraction of the weld metal part. Of the peaks indicating the crystal structure of oxide inclusions observed at this time, , where peaks indicating the M ₂ O ₃ type is greater than the peak showing another crystal structure, in the present invention it is assumed that M ₂ O ₃ type is referred to as a main component.

  In addition, among oxide inclusions containing Ti, those having a particle size of less than 0.2 μm or more than 2 μm cannot form acicular ferrite nuclei as described above. remove.

In order to make the crystal structure of oxide inclusions containing Ti contained in the weld metal mainly M ₂ O ₃ type, the content of elements having a deoxidizing action such as Ti contained in the welding wire used during welding and shielding What is necessary is just to adjust a gas composition appropriately.

  The weld metal of the present invention that satisfies the above requirements has a high strength of 590 MPa or more and is excellent in low temperature toughness in the −60 ° C. region. It can be suitably used as a weld metal when a structure used below is welded.

  Hereinafter, the present invention will be described in more detail with reference to examples. However, the following examples are not intended to limit the present invention, and may be implemented with appropriate modifications within a range that can meet the purpose described above and below. These are all possible and are within the scope of the present invention.

  Using solid wires, the steel materials were gas shielded arc welded together to form a weld metal.

  As the solid wire, one having the component composition shown in Table 1 below, which was subjected to copper plating on the melted wire and then drawn to 1.2 mmφ, was used. As the steel materials, the steel materials were welded with a plate thickness: 20 mm and a groove shape: 45 ° V-shaped groove. In addition, although the solid wire was used in the Example, the weld metal which concerns on this invention is not limited to what was obtained using the solid wire, For example, you may use a flux cored wire instead of a solid wire.

Welding conditions were welding current: 280 A, voltage: 28-30 V, welding speed: about 30 cm / min, and gas shield arc welding with 5 layers and 13 passes was performed. As the shielding gas, a mixed gas of CO ₂ and Ar was used. The types of welding wires used at the time of welding and the CO ₂ fraction (volume%) in the mixed gas are shown in Tables 2 and 3 below, respectively. Moreover, it calculated from the component composition of the obtained weld metal, the TP value calculated from the above formula (1), (3) or (5), the above formula (2), (4) or (6) The EP values are shown in Tables 2 and 3 below.

  Next, the number and crystal structure of oxide inclusions containing Ti contained in the obtained weld metal were measured.

The number of particles was measured by observing a cross section of the weld metal with a scanning electron microscope (SEM) at a magnification of 6000 times, and oxide inclusions containing Ti and having a particle diameter of 0.2 to 2 μm were measured. The number of observation visual fields was five, and the average value of the five positions was converted per observation visual field area of 1 mm ² to calculate the number of oxide inclusions containing Ti. The calculation results are shown in Table 4 below.

  On the other hand, the crystal structure was identified by conducting an X-ray diffraction test on the residue obtained by electrolytic extraction of the weld metal. Note that the main peak structure represented by X-ray diffraction was the crystal structure of oxide inclusions. The identification results are shown in Table 4 below.

Next, as a mechanical property of the weld metal, tensile properties were evaluated by a tensile test, and impact properties were evaluated by a Charpy impact test. The test piece was cut out from the weld metal according to JIS Z 2202. The tensile strength (TS) is measured as a tensile property, and a value of 590 MPa or more is regarded as acceptable. In addition, the yield strength (YS) was measured as a reference value. On the other hand, as for the impact characteristics, the absorbed energy (vE- ₆₀ ) at -60 ° C is measured, and a value of 100 J or more is regarded as acceptable. As reference values, the absorption energy at −40 ° C. (vE ₋₄₀ ) and the absorption energy at −80 ° C. (vE ₋₈₀ ) were also measured. The measurement results are shown in Table 4 below.

  The following can be considered from Tables 2-4. Nos. 1 to 18 and No. 37 are examples that satisfy the requirements defined in the present invention, and have high strength (TS) of 590 MPa or more and good low-temperature toughness in the −60 ° C. region. On the other hand, Nos. 19 to 28 and Nos. 33 to 36 are examples that do not satisfy any of the requirements defined in the present invention, and at least one of strength and low temperature toughness is inferior. No. 29 to No. 32 are reference examples, and since any element selected from the group consisting of Cu, Cr, Nb, and V is excessively contained, the toughness is lowered.

Claims (6)

A weld metal formed by gas shielded arc welding,
C: 0.04 to 0.12% (meaning “mass%”, the same shall apply hereinafter),
Si: 0.10 to 0.80%,
Mn: 3.0% or less (excluding 0%),
Ni: 3.5% or less (excluding 0%),
Ti: 0.005 to 0.1%,
O: 0.015-0.060%,
Cu: each containing 2.0% or less (excluding 0%),
The balance consists of Fe and inevitable impurities, and
A high-strength weld metal having excellent low-temperature toughness, wherein TP calculated by the following formula (1) is 350 to 600 and EP calculated by the following formula (2) is 20 to 220.
TP = 240 × [Mn] + 50 × [Ni] (1)
EP = 1300 × [Ti] −600 × [O] (2)
In the formula, [] indicates the content of each element, and the unit is mass%. A weld metal formed by gas shielded arc welding,
C: 0.04 to 0.12%,
Si: 0.10 to 0.80%,
Mn: 3.0% or less (excluding 0%),
Ni: 3.5% or less (excluding 0%),
Mo: 1.0% or less (excluding 0%),
Ti: 0.005 to 0.1%,
O: 0.015-0.060%,
Cu: each containing 2.0% or less (excluding 0%),
The balance consists of Fe and inevitable impurities, and
A high-strength weld metal excellent in low-temperature toughness, wherein TP calculated by the following formula (5) is 350 to 600, and EP calculated by the following formula (6) is 20 to 220.
TP = 240 × [Mn] + 50 × [Ni] + 45 × [Mo] (5)
EP = 1300 × [Ti] −600 × [O] (6)
In the formula, [] indicates the content of each element, and the unit is mass%. A weld metal formed by gas shielded arc welding,
C: 0.04 to 0.12%,
Si: 0.10 to 0.80%,
Mn: 3.0% or less (excluding 0%),
Ni: 3.5% or less (excluding 0%),
Mo: 1.0% or less (excluding 0%),
Ti: 0.005 to 0.1%,
O: 0.015-0.060%,
B: 0.01% or less (excluding 0%),
Cu: each containing 2.0% or less (excluding 0%),
The balance consists of Fe and inevitable impurities, and
A high-strength weld metal excellent in low-temperature toughness, characterized in that TP calculated by the following formula (7) is 350 to 600 and EP calculated by the following formula (8) is 20 to 220.
TP = 240 × [Mn] + 50 × [Ni] + 45 × [Mo] (7)
EP = 1300 × [Ti] −600 × [O] + 17000 × [B] (8)
In the formula, [] indicates the content of each element, and the unit is mass%. The welding according to any one of claims 1 to 3 , further comprising Nb: 0.20% or less (including 0%) and / or V: 0.20% or less (including 0%) as other elements. metal. When the weld metal cross section is observed with an electron microscope at a magnification of 6000, 50 or more oxide inclusions containing Ti having a particle size of 0.2 to 2 μm are present in terms of an observation field of 1 mm ^2. Item 5. The weld metal according to any one of Items 1 to 4 . The weld metal according to claim 5 , wherein the oxide inclusions are mainly composed of M ₂ O ₃ type.