JP4513515B2 - Welded joint with excellent corrosion resistance - Google Patents

Description

  The present invention relates to a welded joint of steel material having excellent fatigue crack growth resistance characteristics and a structure using the welded joint. More particularly, the present invention relates to a welded joint excellent in corrosion resistance (local corrosion resistance) of a welded portion and a structure using the welded joint.

  In recent years, the tendency to increase the size of welded structures has become remarkable, and it is desired to increase the strength and weight of structural members. However, when high strength steel is used, the design stress increases, so that fatigue failure is likely to occur from the welded portion, and its improvement is an important issue. Thick steel plates such as structural steel materials are generally welded. Therefore, if fatigue cracks that develop from the weld and propagate can be retained by the steel materials, it is effective in extending the fatigue life of the structure. For this reason, various steel plates having an effect of suppressing the progress of fatigue cracks have been proposed.

  As a typical example, Patent Document 1 has a structure in which the structure is mainly composed of one or more of ferrite, pearlite, and bainite, and the average existence interval is 20 μm or less and the average flatness ratio is 5 or more. A steel sheet excellent in fatigue crack propagation resistance, in which island martensite is present in a volume ratio of 0.5 to 5%, is shown.

  Patent Document 2 has a structure composed of a base of a hard part and a soft part dispersed in the base, and the hardness difference between the two parts is 150 or more in terms of Vickers hardness. A steel sheet having a progress suppressing effect is disclosed.

  However, these steel plates need to contain various alloy elements in order to ensure high strength or to control the structure. Therefore, when these steel plates are welded, the chemical composition differs between the base material and the weld metal depending on the welding material, and the welded portion may corrode locally compared to the base material. The local corrosion of the welded part is a big problem because it leads to stress concentration and also causes destruction.

  The local corrosion of the weld is caused by so-called electrochemical galvanic corrosion as a result of the formation of a local battery due to the potential difference between the steel base metal and the weld metal where the composition and structure differ. There is a cause. Therefore, in order to prevent this corrosion, it is necessary to use a weld metal suitable for the base material.

  In the case of local corrosion occurring between the base metal and the welded part, for example, when there is a cause for the difference in structure between the base metal and the welded part, the local corrosion is reduced by making the welded part structure almost the same as the base metal structure. It can be prevented to some extent. As a specific method, there is a method in which the welded portion is post-heat-treated. However, in the case of on-site welding, post-heat treatment is time-consuming, so studies are being made in the direction of minimizing post-heat treatment. This method is therefore not desirable.

  When galvanic corrosion is considered, local corrosion of the weld metal can be prevented by making the chemical composition of the weld metal electrochemically more noble than the base metal. For this reason, there is an attempt to prevent corrosion by defining the chemical composition of the steel material and the weld metal and applying a potential difference between them. For example, Patent Document 3 discloses that Ni in the chemical composition of the weld metal is higher than that of the base material, such as welding a steel material having a predetermined composition with a welding material containing 3 to 6% by weight of Ni. ing. However, in this method, since the weld metal is too noble with respect to the base material, there is a problem that the potential difference from the base material becomes too large, and this time the base material is corroded.

  As a method for preventing local corrosion of the weld metal, for example, Patent Document 4 uses a welding wire, a flux raw material, and a fired flux in which metal powders such as Cr, Cu, and Ni are mixed in submerged welding of bainite-based steel. Thus, it is disclosed to prevent local corrosion of a welded portion used in a seawater environment.

Patent Document 5 discloses a welded joint in which local corrosion of the welded portion is prevented by maintaining an optimal balance between Cu, Ni, Cr, Mo in the weld metal and the Cr content of the base metal.
However, in any of the methods disclosed in the above-mentioned patent documents, it is necessary to adjust the components by putting a special element into the welding rod or flux, which increases the cost and handles a special welding rod for the welder. As a result, there is a problem that workability deteriorates.
JP-A-6-271985 JP 7-242992 A JP-A-1-142024 JP-A-5-8042 JP-A-7-155951

  An object of the present invention is to provide a welded joint that is excellent in local corrosion resistance of a welded portion, particularly when a steel material that is excellent in fatigue crack growth characteristics is used as a steel material that constitutes a welded structure.

Based on the premise that the cause of local corrosion of the welded portion is the potential difference between the weld metal and the base metal, the inventors have investigated a large number of welded portions of various steel materials and have obtained the following knowledge.
1) Since the weld metal is a solidified structure in the carbon steel commingled weld, the natural potential is somewhat lower than that of the base metal which is not so generally. Therefore, initially, the noble base material is the cathode, the base weld metal is the anode, a battery is formed, and corrosion occurs in the weld metal that is the anode.

  2) At the stage where corrosion progresses and grows, the anode has a low pH and is covered with corrosion products, resulting in a lack of oxygen. On the other hand, the supply of oxygen is sufficient at the base material cathode. Therefore, an oxygen concentration cell is formed between the two, and the corrosion of the weld metal generated in 1) further grows.

3) In order to prevent the occurrence and growth of such local corrosion, first, it is necessary to make the natural potential of the weld metal as noble as the base metal.
4) Cu, Ni, and Mo are conceivable as alloy elements that make the natural potential of steel noble. Addition of these elements to the weld metal also enhances the corrosion resistance under a reduced pH environment (anode). Therefore, if Cu, Ni, and Mo are added to the weld metal so as to be slightly higher than the content in the base material, the natural potential difference between the weld metal and the base material is reduced or eliminated. However, the amount of Cu added to the weld metal is limited by the problem of weldability, and the cost of adding Ni is high. Therefore, it is not sufficient to adjust the content of these elements, but also share them with other elements. It is possible.

5) Therefore, the inventors focused on Si and Cr. It has been found that addition of Si has a function of lowering the natural potential of steel, and Cr also has a function of lowering the natural potential of steel in a solution having a lowered pH. Therefore, local corrosion can be avoided by appropriately controlling Si and Cr in the weld metal.

  Based on this knowledge, the amount of Cu, Ni, and Mo is higher in the weld metal than in the base metal, and the amount of Cr and Si is appropriately controlled in the weld metal so that the base metal and the weld metal are controlled. Can be brought close to the natural potential.

The present invention has been completed based on the above findings, and the composition of the steel material is, by mass, C: 0.01 to 0.20%, Si: 0.60% or less, Mn: 0.50 to 2. 0%, P: 0.02% or less, S: 0.02% or less, Al: 0.003 to 0.10%, Cu: 0.01 to 1.5%, Ni: 0.02 to 1.5 %, Cr: 0.02 to 1.2%, Mo: 0.01 to 1.0%, Ti: 0.010 to 0.1% and Nb: 0.005 to 0.1% , the balance Is the iron and impurities, and the maximum strain of the first time when the steel material is given a maximum tensile / compressive strain of ± 0.012, a repetition rate of 0.5 Hz, and a gradual increase / decrease of wave number 12 until the maximum strain is applied 15 times. The repetitive softening parameter indicated by the ratio σ15 / σ1 between the stress σ1 at the time and the stress σ15 at the 15th maximum strain is 0.65 to 0.95, A welded joint characterized by satisfying the following expressions (1) and (2) or expressions (1) and (3) at the same time:
(ΔCu + ΔNi + ΔMo) ≦ 0.0 (1) Equation (ΔSi + 1.5 × ΔCr) ≧ 0.2 (2) Equation 0.0 ≧ (ΔSi + 1.5 × ΔCr) ≧ −0 .2 Formula (3) where ΔCu = CuM-CuW, ΔNi = NiM-NiW, ΔMo = MoM-MoW,
ΔSi = SiM-SiW, ΔCr = CrM-CrW,
CuW, NiW, SiW, CrW, and MoW are each element content (mass%) of a weld metal,
CuM, NiM, SiM, CrM, and MoM are each element content (mass%) of a base material.
The composition of the steel material further includes, in mass%, V having a content not exceeding 1.5%, W having a content not exceeding 1.0%, and REM having a content not exceeding 0.01%, You may have 1 type, or 2 or more types.

  The present invention also relates to a steel welded structure having such a welded joint.

  According to the present invention, in a steel material particularly excellent in fatigue crack growth characteristics, a welded portion with excellent local corrosion resistance of the welded portion in which the corrosion of the welded portion falls within the range of corrosion assumed for the base metal. A welded joint is achieved without the use of special welding rods or expensive elements. Therefore, since the welded structure having the welded joint of the present invention only needs to consider the corrosion life of the base material, a welded structure having a long fatigue life constructed from a steel material having good fatigue crack growth characteristics is obtained. Occurrence of a situation in which the welded portion breaks and the life becomes shorter than the design life can be avoided.

  In the welded joint in which steel materials are welded to each other, the present invention satisfies the following formula (1) for the Cu, Ni, and Mo contents of the weld metal and the base metal, and further, the Si and Cr contents are the following (2 ) Or a welded joint that satisfies either of the formulas (3).

(ΔCu + ΔNi + ΔMo) ≦ 0.0 (1) Equation (ΔSi + 1.5 × ΔCr) ≧ 0.2 (2) Equation 0.0 ≧ (ΔSi + 1.5 × ΔCr) ≧ −0 .2 Formula (3) where ΔCu = CuM-CuW, ΔNi = NiM-NiW, ΔMo = MoM-MoW,
ΔSi = SiM-SiW, ΔCr = CrM-CrW,
CuW, NiW, SiW, CrW, and MoW are each element content (mass%) of a weld metal,
CuM, NiM, SiM, CrM, and MoM are each element content (mass%) of a base material.

  The inventors performed carbon dioxide arc welding and covered arc welding on steel materials (see FIG. 4) provided with groove-like groove shapes for testing using welding materials having various compositions. Two samples of welding materials with the same composition were prepared and subjected to an accelerated corrosion test and a seawater immersion test in flowing water. The weld metal (depo is shown in FIGS. 1 to 3) and the HAZ part (heat affected zone) The presence or absence of selective corrosion in FIGS. In the selective corrosion, the average corrosion depth a in each part is obtained, and when the ratio (a / b) to the average corrosion depth b of the base material is larger than 1.2, the selective corrosion in that part is It was determined that there was. 1 and 2 are graphs in which the X-axis is ΔSi + 1.5 × ΔCr and the Y-axis is ΔCu + ΔNi + ΔMo.

  FIG. 1 shows the results of the accelerated corrosion test. From FIG. 1, the samples that did not cause selective corrosion of the welded portion tended to concentrate in the lower half of the figure (region where ΔCu + ΔNi + ΔMo is negative). On the other hand, in the region where ΔSi + 1.5 × ΔCr is negative, there is a strong tendency for selective corrosion to occur in the depo portion when the absolute value increases. On the contrary, in the region where ΔSi + 1.5 × ΔCr is positive, there is a tendency that selective corrosion occurs in the HAZ portion when it is close to zero.

  FIG. 2 shows the results of a seawater immersion test in flowing water. In FIG. 2, the same tendency as in FIG. 1 was observed. That is, samples that do not cause selective corrosion of the welded portion tend to concentrate in the lower half of the figure (ΔCu + ΔNi + ΔMo is negative), and in the region where ΔSi + 1.5 × ΔCr is negative, the absolute value increases. There was a strong tendency for selective corrosion to occur in the depo part, and in the region where ΔSi + 1.5 × ΔCr was positive, there was a tendency for selective corrosion to occur in the HAZ part when it was close to zero.

  FIG. 3 is a diagram in which FIGS. 1 and 2 are superimposed. As can be seen from this, (ΔCu + ΔNi + ΔMo) is negative (that is, the above equation (1) is satisfied) and (ΔSi + 1.5 × ΔCr) is 0.2 or more (the above equation (2) is It is understood that corrosion is unlikely to occur in both the depo part and the HAZ part in the case of satisfying (equal to (3)).

  Here, when ΔCu + ΔNi + ΔMo is negative, that is, when the expression (1) is satisfied, it is difficult for corrosion to occur as described in 4) above. On the other hand, when ΔSi + 1.5 × ΔCr is larger than −0.2, the tendency that corrosion hardly occurs is the same as described in 5) above.

  However, even when ΔSi + 1.5 × ΔCr is larger than −0.2, corrosion was particularly observed in the HAZ portion in the region where ΔSi + 1.5 × ΔCr was 0 to 0.2. In this regard, since the HAZ part microstructure is a bainite-containing structure and is different from the base ferrite-pearlite structure, the HAZ part potential is often more than 20 mV lower than the weld metal potential, so the HAZ part is selected. It is thought to be corroded.

  The weld metal refers to a material region that has been heated to a molten state once during welding. The welding method may be any method such as manual welding or SAW (submerged arc welding), but the chemical component of the weld metal is not determined solely by the chemical component of the filler metal, and the base metal is melted with melting. Also affected by ingredients. That is, the degree of dilution of the base material is affected by many factors such as the groove shape, the welding target position, the welding conditions, and the welding posture. For this reason, it is necessary to select a welding material in consideration of the degree of dilution of the chemical component of the weld metal from the base material.

  In the welded joint of the present invention, it is preferable that the steel material as the base material is excellent in fatigue crack growth characteristics. Specifically, the maximum tensile / compressive strain ± 0.012, the repetition rate 0.5 Hz, and the stress σ1 at the time of the first maximum strain when applying a gradual increase / decrease load of wave number 12 up to the maximum strain 15 times A steel material having fatigue characteristics indicated by a repeated softening parameter indicated by a ratio σ15 / σ1 to a stress σ15 at the time of the 15th maximum strain of 0.65 to 0.95 is preferable.

  As described in Japanese Patent Application Laid-Open No. 2001-41868 by the present inventors, the repeated softening parameter σ15 / σ1 when a steel material having a hardened structure is repeatedly softened has a stress intensity factor range (ΔK) that is normally used of 20 MPa. -Good correlation with fatigue crack growth rate (da / dN) at √m.

  When this repeated softening parameter is less than 0.65, the crack growth rate becomes slow, but the toughness and weldability of the steel material deteriorates, and the use as a steel for welded structures is remarkably limited. On the other hand, if the repeated softening parameter exceeds 0.95, not only does the crack growth rate increase, but it also causes a decrease in strength. When the repeated softening parameter is 0.65 or more and 0.95 or less, the fatigue crack propagation resistance characteristics of the steel material are excellent, and deterioration of toughness and weldability does not occur.

  Examples of the steel material having such a repeated softening parameter include those having the following composition in mass%: C: 0.01 to 0.20%, Si: 0.60% or less, Mn: 0.8. 50 to 2.0%, P: 0.02% or less, S: 0.02% or less, Al: 0.003 to 0.10%, Cu: 0 to 1.5%, Ni: 0 to 1.5 %, Cr: 0 to 1.2%, Mo: 0 to 1.0%, V: 0 to 1.5%, Ti: 0 to 0.1%, Nb: 0 to 0.1%, W: 0 ~ 1.0%, REM: 0 ~ 0.01% and balance iron and impurities.

The reason for limiting the steel composition in the present invention will be described next. All percentages relating to steel composition are mass%.
C: 0.01 to 0.20%
C is an element effective for ensuring the strength of the structural member. However, if the content is less than 0.01%, it is difficult to obtain an effect of improving the strength. On the other hand, if the C content exceeds 0.20%, the weldability is lowered, so that welding is difficult, and the use area as structural steel is remarkably limited. In order to ensure high strength and weldability, the C content is preferably 0.04 to 0.15%.

Si: 0 to 0.60%
Si has a deoxidizing action. However, if its content exceeds 0.60%, the toughness deteriorates. A desirable content is 0.05 to 0.5%.

Mn: 0.50 to 2.0%
Mn is an element effective for securing strength. However, if the content is less than 0.50%, the effect is not sufficient. On the other hand, if the Mn content exceeds 2.0%, the toughness deteriorates. A desirable content is 0.70 to 1.8%.

P: 0.02% or less P exists as an impurity, and if its content is large, it affects the toughness of steel. In particular, if it exceeds 0.02%, not only the base material but also the toughness of the HAZ is significantly reduced.

S: 0.02% or less S is present as an impurity, and if its content is large, the HAZ toughness of the base metal is inhibited, and the ductility in the thickness direction is also reduced. Furthermore, it also causes generation of MnS inclusions and also a starting point of fatigue cracks, so the content was made 0.02% or less.

Al: 0.003 to 0.10%
Al has a deoxidizing action. However, if the content is less than 0.003%, the effect is not sufficient, and the toughness deteriorates because the oxide in the steel increases. On the other hand, if the Al content exceeds 0.10%, the toughness decreases. A desirable content is 0.010 to 0.050%.

Cu: 0 to 1.5%
Cu is an element effective for securing strength and improving corrosion resistance. However, if the Cu content exceeds 1.5%, deterioration of toughness is caused. A desirable content is 0.01 to 1.0%.

Ni: 0 to 1.5%
Ni is an element effective for securing strength and improving toughness. However, even if Ni is contained so as to exceed 1.5%, the effect is saturated and the cost is increased. A desirable content is 0.02 to 1.3%.

Cr: 0 to 1.20%
Like Cu, Cr is an element effective for securing strength and improving corrosion resistance. However, when the Cr content exceeds 1.20%, the toughness is deteriorated. A desirable content is 0.02 to 1.0%.

Mo: 0 to 1.0%
Mo is an element effective for enhancing the hardenability and improving the strength. However, if the Mo content exceeds 1.0%, not only the toughness is deteriorated but also the cost is increased. A desirable content is 0.01 to 0.8%.

V: 0 to 1.5%
V has the effect of increasing the strength and toughness of the steel. However, if it exceeds 1.5%, toughness is deteriorated. A desirable content is 0.05 to 1.0%.

Ti: 0 to 0.1%
Ti is an element effective for ensuring toughness. However, if its content exceeds 0.1%, the toughness is rather lowered. A desirable Ti content is 0.01 to 0.05%.

Nb: 0 to 0.1%
Nb is an effective element for ensuring toughness. However, if the content exceeds 0.1%, the toughness is rather lowered. A desirable content is 0.005 to 0.05%.

W: 0 to 1.0%
W has an effect of improving the strength of steel and also effective in suppressing fatigue crack growth. Furthermore, since it is effective in corrosion resistance, it is effective in suppressing fatigue crack growth even in corrosive environments such as in sour crude oil. However, if its content exceeds 1.0%, deterioration of toughness is caused. A desirable content of W is 0.05 to 0.5%.

REM: 0 to 0.01%
REM refines the structure and is effective in improving toughness. However, if the content exceeds 0.01%, the toughness deteriorates. A desirable content of REM is 0.0005 to 0.01% .

  A steel ingot having the composition shown in Table 1 was melted and a steel plate having a thickness of 25 to 50 mm was manufactured by hot rolling. From this steel plate, as shown in FIG. 4, a test material for welding was prepared in which a trapezoidal groove was provided as a groove at the center of 95 × 160 mm. The plate thickness was reduced to 20 mm for the steel types 1 and 4, but the remaining steel plates remained at the original thickness. Welding was performed on the groove of the test material using a filler material (welding rod or wire) having the composition shown in Table 2. Two weld samples were prepared for each combination of steel type and filler metal.

When the filler metal is a CO ₂ wire indicated by A in Table 2, the welding is a two-layer, two-pass carbon dioxide arc welding (both the first and second layers are 200 A × 25 V × 30 cm / min, CO ₂ gas flow rate: 25 L / min) In the case of other filler materials, two-layer, two-pass manual welding (first layer: 160 A × 24 V × 20 cm / min, second layer: 160 A × 24 V × 15 cm / min).

  For each test material after welding (weld sample), the composition side of the surface side of the weld metal is analyzed by spectroscopic analysis to determine the content of Si, Cr, Cu, Ni, and Mo. The difference (Δ) from the content was calculated. The results are shown in Table 3. In Table 3, symbols A to D in the test material symbols mean the filler materials (symbols in Table 2) used for welding.

From each of these weld test materials, test pieces having the dimensions shown in FIG. 5 were collected and subjected to a corrosion test. The corrosion test was performed in two ways: an accelerated corrosion test and a seawater immersion test in flowing water.
In the accelerated corrosion test, the surface on the welded surface side of the test piece was automatically polished and degreased in # 600, and then a 12 × 42 mm range including a weld bead was left, and the others were coated with a silicone resin. The test solution was artificial seawater (ASTM D-1141) based on the ASTM standard maintained at 30 ° C., and the test piece was immersed in this test solution for 72 hours.

  In the flowing seawater immersion test, the surface of the test piece was similarly automatically polished with # 600, degreased, and the test piece covered in the same manner as above was immersed in natural raw seawater kept in a fluid state for 3 months. did.

  After each corrosion test, the test piece was taken out of the seawater and descaled with ammonium citrate. Then, for each part of the weld bead part (welded metal), HAZ part, and base metal, the average corrosion depth of each part is obtained as an average value of the corrosion depth at five points from the optical micrograph of the test piece cross section. It was. The corrosion depth can be measured as the difference from the thickness of the coated non-corroded part.

  Corrosion also occurs in the base material. Normally, a structure is constructed by predicting the corrosion depth of a steel material (corresponding to the base material) used in the structure and adding a corrosion allowance, so that the amount of corrosion in the base material part is HAZ and welded. If it is greater than the amount of corrosion on the metal, it is not a problem. However, when the amount of corrosion at the HAZ portion or the weld metal is larger than that of the base material, the lifetime of the structure becomes shorter than the lifetime prediction, which is a serious problem.

  Therefore, on the basis of the average corrosion depth b of the base material, how many times the average corrosion depth a of the HAZ part or the weld metal is larger than the average corrosion depth b of the base material (that is, the a / b ratio). If the value of a / b ratio is 1.2 or more, it is determined that the part is selectively corroded, so that the average corrosion depth of the HAZ part and at least one part of the weld metal is determined. When the thickness is 1.2 times or more of the base metal, the corrosion characteristics of the welded portion (including the HAZ portion and the weld metal) are poor (there is selective corrosion of the welded portion).

  The reason why it was determined that there is selective corrosion at the weld at 1.2 times or more of the average corrosion depth of the base metal is as follows. Although there are variations in the corrosion depth of the base metal, the minimum and maximum corrosion depth is about ± 20% of the average corrosion depth. Therefore, corrosion deeper than the maximum corrosion depth of the base metal (average corrosion depth x 1.2) is larger than the expected corrosion of the base metal, that is, local corrosion (selective corrosion) in the weld. Thus, the corrosion characteristics are judged to be poor.

  Table 3 shows the larger numerical value of the HAZ part and the a / b value of the weld metal and the part that generated the numerical value for each of the accelerated corrosion test and the flowing water seawater immersion test. In Table 3, Depo means weld metal. When the a / b value is less than 1 in both the HAZ part and the weld metal, the maximum corrosion site is the base material, and thus “maximum corrosion with the base material” is displayed. In that case, since the corrosion resistance (local corrosion resistance) of the welded portion is particularly excellent, “◯” is shown in Table 3. On the other hand, when the ratio of the a / b value of the maximum corrosion depth of the welded part is larger than 1.2, × (the corrosion resistance of the welded part is poor), and the case where it is in the range of 1.0 to 1.2 (Within tolerance).

  From Table 3, according to the present invention, the value of (ΔCu + ΔNi + ΔMo) is 0 or negative (satisfies the above expression (1)), and the value of (ΔSi + 1.5ΔCr) is −0.2 to 0, or If it is 0.2 or more (the above formula (2) or (3) is satisfied), the corrosion at the base metal is larger than the welded portion, or the corrosion at the HAZ portion or the weld metal is the largest. However, it is less than 1.2 times the average corrosion depth of the base metal, and even if it is used as a welded joint of a structure, there is no need to consider corrosion in the welded part. It is only necessary to consider the corrosion life of the steel.

  On the other hand, in the case outside the scope of the present invention, the corrosion of at least one of the HAZ part and the weld metal becomes large in at least one of the accelerated corrosion test and the flowing seawater immersion test (a / b ratio is 1.2 from 1.2). Therefore, it is not preferable to use it as a welded joint.

It is a figure which shows the result of an accelerated corrosion test. It is a figure which shows the result of the seawater immersion test of a flowing water state. It is a figure obtained by superimposing FIG. 1 and FIG. The test material used for the welding test in an Example and the shape of the groove | channel are shown. The shape of the weld specimen used for the corrosion test is shown.

Claims (3)

The composition of the steel material is% by mass : C: 0.01 to 0.20%, Si: 0.60% or less, Mn: 0.50 to 2.0%, P: 0.02% or less, S: 0 0.02% or less, Al: 0.003-0.10%, Cu: 0.01-1.5%, Ni: 0.02-1.5%, Cr: 0.02-1.2%, Mo : 0.01 to 1.0%, Ti: 0.010 to 0.1% and Nb: 0.005 to 0.1% , the balance being iron and impurities,
When the steel material is given a maximum tensile / compressive strain ± 0.012, a repetition rate of 0.5 Hz, and a gradual increase / decrease load of wave number 12 up to the maximum strain, 15 stresses σ1 and 15 at the time of the first maximum strain The repetitive softening parameter indicated by the ratio σ15 / σ1 to the stress σ15 at the time of the maximum strain at the second time is 0.65 to 0.95,
A welded joint in which the steel materials are welded together, wherein the following formulas (1) and (2) or formulas (1) and (3) are satisfied at the same time.
(ΔCu + ΔNi + ΔMo) ≦ 0.0 (1) Equation (ΔSi + 1.5 × ΔCr) ≧ 0.2 (2) Equation 0.0 ≧ (ΔSi + 1.5 × ΔCr) ≧ −0 .2 Formula (3) where ΔCu = CuM-CuW, ΔNi = NiM-NiW, ΔMo = MoM-MoW,
ΔSi = SiM-SiW, ΔCr = CrM-CrW,
CuW, NiW, SiW, CrW, and MoW are each element content (mass%) of a weld metal,
CuM, NiM, SiM, CrM, and MoM are each element content (mass%) of a base material.   The composition of the steel material further includes, in mass%, V having a content not exceeding 1.5%, W having a content not exceeding 1.0%, and REM having a content not exceeding 0.01%, The weld joint according to claim 1, wherein the weld joint has one type or two or more types. Welded structure having a welded joint according to claim 1 or 2.

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