JP5833966B2 - Welded joint with excellent fatigue characteristics - Google Patents

Description

  The present invention relates to a welded joint excellent in fatigue properties used for structures requiring excellent fatigue strength such as shipbuilding, buildings, bridges, construction machines, and the like, in particular, a steel material having a tensile strength of 590 MPa to less than 780 MPa. The present invention relates to a welded joint having excellent fatigue characteristics used as a material.

  In recent years, welded structures belonging to technical fields such as shipbuilding, buildings, bridges, and construction machinery have been increased in size and accompanying weight reduction, and at the same time, the strength of steel used is also increasing. In general, it is known that when the strength of steel is increased, the fatigue strength of the steel itself is improved, but the fatigue strength of the welded joint is not improved. When the strength of the steel material is increased, in the welded joint, a stress concentration is generated in the weld toe portion 4 shown in FIG. 2, so that fatigue cracks are likely to occur, and conversely, the fatigue strength is reduced. That is, there is a problem that the fatigue strength of the welded structure is reduced by increasing the strength of the steel material.

  Conventionally, to solve such problems, the weld toe is processed smoothly with a grinder, etc. to reduce the stress concentration and improve the fatigue strength. Measures have been taken. However, if such measures are taken, a new problem has arisen that the work efficiency will be reduced this time. Moreover, if the toe treatment is insufficient, the stress concentration on the weld toe part is further strengthened, and the fatigue strength of the welded structure may be further reduced.

  In such a situation, a welded joint that can secure sufficient fatigue strength without requiring a toe treatment after welding is required, and is proposed by Patent Documents 1 to 3 and the like.

  The technique described in Patent Document 1 is a technique that suppresses the generation and propagation of cracks by controlling the structure of the HAZ and the base material, and can obtain a steel sheet with a long fatigue life of the welded part. In order to suppress crack initiation / propagation in HAZ, it is effective to increase the ferrite area ratio of HAZ, and the ferrite fraction is defined as 15 to 80% in area ratio. However, when the ferrite fraction of HAZ is increased as described in Patent Document 1, it becomes difficult to ensure the tensile strength of the welded joint, and the restriction on the strength becomes very large. Therefore, in the actual welded structure, when the technique described in Patent Document 1 is applied, the applicable parts are limited.

  The technique described in Patent Document 2 is a technique that was developed on the assumption that a bainite-based base metal structure containing martensite is suitable for improving joint fatigue characteristics, and had a component in which the HAZ structure exceeded 60%. By using a welded joint, the fatigue characteristics of the welded joint are improved. However, as described in Patent Document 2, in order to make the base material structure as described above, it is necessary to add a large amount of alloy elements, and there is a problem that the manufacturing cost increases. Moreover, the addition of a large amount of alloy elements as described in Patent Document 2 not only deteriorates the weldability but also has the problem of degrading workability because the steel material itself is hardened.

  The technique described in Patent Document 3 is a technique in which both the weldability and high strength of a welded joint can be achieved by setting the metal structure to be a ferrite and bainite-based structure and the area ratio of pearlite to be 10% or less. In addition, the technology is said to be able to suppress the occurrence of cracks by controlling the heat input during welding and limiting the hardness distribution in the toe weld line direction and suppressing the generation of stress concentration sources It is.

  According to this technique, by restricting the hardness distribution in the toe weld line direction, the hardness difference between the weld metal part and the HAZ part can be suppressed, but the weld metal part and the HAZ part can be suppressed. The stress concentration due to the hardness difference has not been eliminated, and it is unclear how much the fatigue characteristics can be improved. Further, in order to obtain a welded joint having excellent fatigue characteristics, it is necessary to use a steel material in which the recuperation temperature range during rolling is strictly controlled as a base material. Furthermore, since it is necessary to strictly control the amount of heat input during welding, there is still a problem in terms of workability during welding.

JP-A-9-95754 JP-A-10-1743 JP 2010-24467 A

  The present invention has been made as a solution to the above-described conventional problems, and can use a steel material that does not require special management during production as a base material, has excellent workability during welding, and has fatigue characteristics. It is an object to provide an excellent welded joint.

Invention of Claim 1 is the mass%, C: 0.01-0.06%, Si: 1.0% or less (including 0%), Mn: 1.25-2.5%, Cr: 0.1 to 2.0%, Mo: 1.5% or less (including 0%), V: 0.04% or less (including 0%), Ti: 0.005 to 0.030%, B: 0.0006 to 0.005%, P: 0.02% or less (not including 0%), S: 0.01% or less (not including 0%), N: 0.0020 to 0.010%, Cu: 0.01 to 2.0% and / or Ni: 0.01 to 5.0%, the balance is Fe and inevitable impurities, and the metal structure is 90% or more in area ratio with ferrite a bainite, further satisfying the following formula (1) and (2) a tensile strength of welded joints der using steel to less than 590 MPa 780 MPa Te, and an excellent welded joint fatigue characteristics characterized by having a HAZ portion satisfies the following equation (3).

Kp = [Mn] +1.5 [Cr] +2 [Mo] = 2.4 to 4.5 (1)
[Ti] / [N] = 1.5 to 4.0 (2)
ΔHv / L <150 (3)
However, in the formulas (1) and (2), [] indicates the content (% by mass) of each element, and in the formula (3), ΔHv is the maximum hardness at a depth position of 100 μm from the surface in the HAZ part . The difference between the value and the minimum value, L is the length (mm) of the HAZ part.

  The invention according to claim 2 is the welded joint having excellent fatigue characteristics according to claim 1, wherein the steel material further contains Ca: 0.005% or less by mass%.

  Invention of Claim 3 is a welded joint excellent in the fatigue characteristics of Claim 1 or 2 in which the said steel material contains Nb: 0.04% or less further by the mass%.

  Invention of Claim 4 is a welded joint excellent in the fatigue characteristics in any one of Claim 1 thru | or 3 in which the said steel material contains Al: 0.2% or less further by the mass%.

  According to the present invention, it is possible to use a steel material that does not require special management at the time of production as a base material, and it is not necessary to strictly control the amount of heat input during welding. In addition, a welded joint having excellent fatigue characteristics can be obtained.

The load transmission type cross joint fatigue test piece used for the fatigue test and the hardness test is shown, (a) is a front view, and (b) is a plan view. It is the front view which expanded the toe part vicinity of a load transmission type cross joint fatigue test piece. It is explanatory drawing for demonstrating the method of calculating | requiring the hardness gradient in a HAZ part, Comprising: It is the front view which expanded further the toe part vicinity of a load transmission type cross joint fatigue test piece.

  In order to obtain a steel material used as a base material when using conventional techniques including the technique described in Patent Document 3 in obtaining a welded joint having excellent fatigue characteristics, the present inventors have strict management during manufacturing. Because there is a situation that it is necessary to strictly control the amount of heat input even during welding, it has excellent fatigue characteristics even without special management during steel production and welding In order to find out the conditions under which welded joints can be obtained, we conducted diligent studies, experiments and research.

  The present inventors have used various chemical component compositions (hereinafter sometimes simply referred to as component compositions) and metal structures on the premise that steel materials that can be obtained without special management at the time of production are used. Using the steel material as a base material, the welding conditions were appropriately changed, and a load transmission type cross joint fatigue test piece (hereinafter sometimes simply referred to as a test piece) as shown in FIG. 1 was produced. A fatigue test was conducted using the load transmission type cross joint fatigue test piece prepared under these various conditions, and the following knowledge was obtained by conducting detailed observation and analysis of the test piece after the fatigue test. .

  In order to explain the observation results, first, a schematic view of the vicinity of the toe portion of the welded joint is shown in FIG. 1 is a steel material (base material), 2 is a HAZ part (heat affected zone), 3 is a weld metal, 4 is a weld toe part (hereinafter also referred to simply as a toe part), and 5 is a gusset (base material). And the same component composition and metal structure). As a result of carrying out the fatigue test using these various test pieces, fatigue cracks occurred in all the test pieces from the weld toe part 4 and progressed through the HAZ part 2 and the base material 1, leading to fracture. I confirmed. It was also confirmed that the entire life of the welded joint accounted for most of the life from the occurrence of cracks to the HAZ part 2. Further, it was also confirmed that the generation of cracks is governed by the strain concentration generated in the toe portion 4 and the strain concentration changes depending on the hardness distribution in the HAZ portion 2.

  From these test results, the present inventors have shown that it is possible to reduce the strain concentration of the toe portion 4 and to improve the fatigue life of the welded joint by optimizing the hardness distribution of the HAZ portion 2. I found it.

  Regarding the hardness distribution of the HAZ part 2, it is known that not only the composition of the steel material 1 but also the heating / cooling rate during welding has a great influence. In particular, the maximum ultimate temperature and the cooling rate during welding vary greatly depending on the distance from the weld line 6, and thus the metal structure varies greatly depending on the distance from the weld line 6. As a result, a hardness gradient (hardness distribution) is generated in the HAZ portion 2.

  Therefore, the present inventors do not depend on the distance from the welding line 6, that is, the change in the metal structure is small even if the cooling rate at the time of welding changes, and there is no sudden change in hardness in the HAZ part 2. The component composition of the steel material 1 and the welding conditions for making a welded joint were examined.

  As a result, a steel material having a Ti / N balance that appropriately controls the content of the hardenability improving element and appropriately precipitates TiN having an action of suppressing the coarsening of the prior austenite grain size during welding is used. In addition, it has been found that by producing a welded joint by performing welding under appropriate welding conditions, a hardness gradient generated in the HAZ portion can be suppressed, and a welded joint having good fatigue characteristics can be obtained. It was completed.

  Hereinafter, the present invention will be described in more detail based on embodiments.

(Metal structure of steel)
The steel plate constituting the welded joint according to the present invention is preferably a steel material having a tensile strength of 590 MPa or more and less than 780 MPa, and the metal structure of the steel plate is mainly composed of ferrite and bainite in order to obtain a high strength welded joint. To do. Specifically, the total area ratio obtained by adding ferrite and bainite is 90% or more of the metal structure. The bainite described here includes structures such as upper bainite, lower bainite, acicular ferrite, and granular bainite. The remaining metal structure is a metal structure usually contained in a steel material such as pearlite or martensite-austenite mixture (MA).

(Chemical composition of steel)
In the present invention, after making the content of C extremely small, Mn and Cr, which are hardenability improving elements, and Mo in some cases are positively added, and determined by the content of these hardenability improving elements. The Kp value to be controlled is controlled to an appropriate range, and an appropriate amount of B is added. By adding such an element to the steel material, the continuous cooling curve (CCT curve) of bainite moves to the low temperature side for a short time, and the CCT curve of ferrite moves to the long time side.

  Conventionally, martensite is generated at a high cooling rate and ferrite is generated at a low cooling rate. Therefore, the hardness is highly sensitive to the cooling rate, and martensite is generated near the weld line where the maximum temperature is high and the cooling rate is high. However, since a ferrite having low hardness is generated at a position away from the weld line, the hardness gradient in the HAZ portion is high. However, in the present invention, in order to appropriately control the content of the hardenability improving element, the metal structure of the HAZ part is stably bainite regardless of the distance from the weld line, thereby suppressing the occurrence of a hardness gradient. Can do.

  In addition to the appropriate control of the hardenability improving element content, the Ti / N balance is appropriately controlled to suppress the strength reduction due to the coarsening of the prior austenite grain size during welding and the martensitic transformation during cooling. Thus, it is possible to more reliably suppress the occurrence of a hardness gradient.

  Hereinafter, the component composition of the steel plate which comprises the welded joint which concerns on this invention is demonstrated. All units are described as%, but indicate mass%.

C: 0.01-0.06%
C is an element that is important for ensuring the hardness of the HAZ part and the strength of the base material. If C exceeds 0.06%, not bainite but martensite is generated in the vicinity of the weld line, leading to a decrease in fatigue strength. Preferably, it is 0.055% or less. On the other hand, if it is less than 0.01%, the necessary minimum base material strength cannot be obtained. Preferably, it is 0.015% or more.

・ Si: 1.0% or less (including 0%)
Si is an element useful as a deoxidizing material, but if it exceeds 1.0%, the fatigue strength decreases. Therefore, the upper limit is made 1.0%. Preferably it is 0.6% or less.

Mn: 1.25 to 2.5%, Cr: 0.1 to 2.0%, Mo: 1.5% or less (including 0%)
These elements have the effect of improving the hardenability, make it easy to form bainite throughout the HAZ, and add an appropriate amount of B after making the content of C extremely small as described above. Thus, it has the effect of ensuring both the strength of the HAZ portion and the strength of the base material at the time of small heat input. Mn and Cr must be contained. Mn is required to be 1.25% or more, and Cr is required to be 0.1% or more. If Mn and Cr are less than these contents, the desired hardenability improving effect is not exhibited and the base material strength is insufficient. Preferably, Mn is 1.3% or more, Cr is 0.5% or more, and Cr is more preferably 1.0% or more. However, if the content of these elements exceeds 2.5% for Mn, 2.0% for Cr, and 1.5% for Mo, martensite tends to be formed and the fatigue strength decreases. Preferably, Mn is 2.2% or less, Cr is 1.5% or less, and Mo is 1.3% or less.

・ V: 0.04% or less (including 0%)
V has the effect of improving the hardenability by adding a small amount. However, if added over 0.04%, martensite tends to occur and the hardness gradient increases, resulting in a decrease in fatigue strength. Preferably it is 0.03% or less.

Ti: 0.005-0.030%
Ti forms nitrides with N and suppresses γ grain coarsening in the HAZ part during welding, thereby making it difficult for martensite to occur and reducing the hardness gradient to ensure fatigue strength. However, if the Ti content exceeds 0.030%, coarse TiN is generated, and the fatigue strength decreases. Preferably it is 0.020% or less. On the other hand, if the content is less than 0.005%, the effect of suppressing γ grain coarsening becomes insufficient. Preferably it is 0.007% or more.

・ N: 0.0020-0.010%
As described above, N is a useful element by forming a nitride with Ti and contributing to the suppression of γ grain coarsening in the HAZ part during welding. However, N combines with B to reduce the solid solution B, inhibits the hardenability improving effect of B, and has the effect of reducing fatigue strength, and the N content exceeds 0.010%. And the effect becomes remarkable. Preferably it is 0.0080% or less. On the other hand, if the content is less than 0.0020%, the effect of suppressing γ grain coarsening becomes insufficient. Preferably it is 0.0035% or more.

・ B: 0.0006 to 0.005%
B is an element that improves hardenability, and easily forms bainite even at a low cooling rate. As described above, by adding an appropriate amount of B after making the content of C extremely small, Mn, Cr, and Mo are added. By adding, it has the effect of reducing the hardness gradient of the HAZ part. If the B content is less than 0.0006, the effect of improving hardenability cannot be expected, and the strength of the base material is insufficient. Preferably it is 0.0007% or more, More preferably, it is 0.0010% or more. On the other hand, if the content exceeds 0.005%, the hardenability decreases, and the hardness gradient increases. Preferably it is 0.003% or less.

Cu: 0.01-2.0% and / or Ni: 0.01-5.0%
Cu and Ni have the effect of improving fatigue strength by solid solution strengthening. However, if Cu is added in an amount of 2.0% and Ni exceeds 5.0%, the HAZ portion is significantly hardened, resulting in a decrease in fatigue strength. Preferably, Cu: 1.5% or less, Ni: 4 0.0% or less. On the other hand, if the content of both Cu and Ni is less than 0.01%, the fatigue strength cannot be improved.

P: 0.02% or less (not including 0%), S: 0.01% or less (not including 0%)
Since P and S are impurity elements, their content is preferably as small as possible. Content which does not have a bad influence on fatigue strength improvement is P: 0.02% or less, S: 0.01% or less. The lower limit of the contents of P and S is not particularly specified, but it is difficult to industrially make P and S in steel 0%.

* Kp = [Mn] +1.5 [Cr] +2 [Mo] = 2.4-4.5
The Kp value determined by the above-described formula depending on the contents of Mn, Cr, and Mo that are hardenability improving elements needs to be 2.4 to 4.5. When the Kp value is less than 2.4, the effect of improving the hardenability cannot be exhibited, and ferrite is generated in the HAZ portion, the hardness gradient is increased, and the fatigue strength is reduced. A preferable Kp value is 2.7 or more. However, if the Kp value exceeds 4.5, martensite is likely to be generated in the HAZ part. In this case, too, the hardness gradient increases and the fatigue strength decreases. In addition, [] shows content (mass%) of each element.

[Ti] / [N] = 1.5 to 4.0
As described above, the Ti content needs to be 0.005 to 0.030%, and the N content needs to be 0.0020 to 0.010%. When satisfied, the Ti / N balance, that is, [Ti] / [N] needs to be in the range of 1.5 to 4.0. When the Ti / N balance is less than 1.5, the N content in the steel increases, so that the HAZ hardness increases, the hardness gradient increases, and the fatigue strength decreases. A preferable Ti / N balance is 2.0 or more. On the other hand, when the Ti / N balance exceeds 4.0, the effect of suppressing the coarsening of HAZ by γ grains by TiN cannot be obtained, so that martensitic transformation is likely to occur, the hardness gradient increases, and the fatigue strength decreases. A preferable Ti / N balance is 3.5 or less. In addition, [] shows content (mass%) of each element.

  The above is the reason for limiting the component range of the essential additive elements of the steel sheet constituting the welded joint according to the present invention, and the balance is Fe and inevitable impurities. Inevitable impurities include O, H and the like, and these elements may be contained to the extent that they do not impair various properties of the steel material.

  It is more effective if the steel material of the present invention contains the following elements. The reason for limiting the component range when these elements are contained will be described below.

-Ca: 0.005% or less Ca is a useful element by contributing to the suppression of the coarsening of γ grains because Ca has the effect of refining Ti nitride. In order to exert such an effect, it is preferable to add 0.0005% or more. More preferably, it is 0.0010% or more. However, if added in excess of 0.005%, the base material toughness is lowered and the fatigue strength is lowered. When added, the content is made 0.005% or less. More preferably, it is 0.004% or less.

Nb: 0.04% or less Nb has the effect of greatly improving the hardenability by adding a small amount. However, if added over 0.04%, the HAZ part is cured, resulting in a decrease in fatigue strength. More preferably, it is 0.03% or less.

Al: 0.2% or less Al is a deoxidizing element, and at the same time, it is a preferable element to be added in order to enhance the hardenability improvement effect based on B by fixing N and increasing solid solution B. However, if added over 0.2%, the fatigue strength decreases, so when added, the content is made 0.2% or less. More preferably, it is 0.1% or less.

(Hardness gradient in HAZ part)
・ ΔHv / L <150
The hardness gradient in the HAZ part (heat affected zone) can be obtained by dividing the difference ΔHv between the maximum and minimum hardness values in the HAZ part by the length L (mm) of the HAZ part. The slope (ΔHv / L) must be less than 150. When the hardness gradient (ΔHv / L) is 150 or more, strain concentration occurs at the weld toe. Therefore, in order to ensure fatigue strength, the hardness gradient (ΔHv / L) needs to be less than 150. is there.

  Note that the hardness gradient in the HAZ portion (heat affected zone) can be determined by, for example, the measuring method shown in FIG. The weld joint shown in FIG. 3 is a load transmission type cross joint fatigue test piece. Although it can be assumed that the hardness distribution on the very surface of the specimen has a large effect on crack initiation, it is difficult to accurately measure the hardness in the vicinity of the surface with the current technology, so the depth position is 100 μm from the surface. This can be done by measuring the hardness of.

  Specifically, as shown by x in FIG. 3, the hardness is measured every 100 μm from the position separated from the weld line (weld bond) 6 to the several hundred μm weld metal 3 side toward the base metal 1 side. . In this measurement, when a hardness equivalent to the hardness of the base material 1 is obtained for three consecutive points, the position between the position closest to the base material 1 and the weld line 6 is defined as the HAZ part 2, and the base material is continuously provided for three points. When the hardness equivalent to the hardness of 1 is obtained, the dimension between the position closest to the base material 1 and the weld line 6 is the length (thickness) L of the HAZ portion. A value ΔHv / L obtained by dividing the difference ΔHv between the maximum value and the minimum value of the hardness obtained by measurement in the HAZ part 2 by the length L (mm) of the HAZ part is defined as a hardness gradient. In addition, the hardness equivalent to the hardness of the base material 1 is a hardness at which the difference in Vickers hardness is within 10 or less. Moreover, the hardness of the base material 1 is obtained by measuring the hardness of a depth of 100 μm from the surface layer at a position sufficiently away from the welded portion.

  The hardness may be measured by a method conforming to JIS or ASTM. However, since the measurement interval is as short as 100 μm, it is preferable to use a micro Vickers hardness tester or the like for the measurement. Moreover, since it is considered that variations occur in the measurement, it is necessary to measure at least three cross sections and obtain a hardness gradient from the average value. If possible, it is preferable to measure at least 5 sections.

(Welding conditions)
Using a steel material that satisfies the above-described chemical composition and metal structure, the HAZ having a desired hardness gradient is obtained by performing welding with a heat input during welding of less than 50 kJ / cm and a welding speed of less than 100 cm / min. A welded joint having a portion can be obtained.

  If the heat input is 50 kJ / cm or more, the cooling rate becomes rapid and the effect of suppressing the coarsening of γ grains by TiN cannot be fully exhibited. For example, a steel material that satisfies the above-described chemical composition and metal structure Even if a welded joint is obtained by using martensite, martensite is likely to occur in the vicinity of the weld line. As a result, the hardness gradient increases and the fatigue strength decreases. When the welding speed is set to 100 cm / min or more, the bead shape becomes unstable, so that the shape of the weld toe portion such as occurrence of undercut becomes unstable. As a result, even if a welded joint is obtained using a steel material that satisfies the above-described chemical composition and metal structure, the hardness gradient increases and the fatigue strength decreases.

  EXAMPLES Hereinafter, the present invention will be described more specifically with reference to examples. However, the present invention is not limited by the following examples, but may be appropriately modified within a range that can meet the purpose described above and below. Of course, it is possible to implement them, and they are all included in the technical scope of the present invention.

  Steels having the chemical composition shown in Tables 1 and 2 were melted in a converter, and each slab was hot rolled under the conditions shown in Tables 3 and 4 until the plate thicknesses shown in Tables 3 and 4 were reached. Steel was obtained. A part of the steel material thus obtained was cut out, the metal structure as a base material was observed, and a tensile test was performed to obtain 0.2% proof stress (MPa) and tensile strength (MPa) of the base material. . Tables 3 and 4 show the observation results of the metal structure and the results of the tensile test.

  The metal structure was obtained by taking a photograph by observing a test piece obtained by mirror polishing a part having a thickness of 1/4 of each steel material with a 2% nital solution, and observing the part at 400 times using an optical microscope. The 10 visual fields were subjected to image analysis using “Image-Pro Plus” manufactured by Media Cybernetics, and the bainite fraction in the steel structure was measured. At this time, the lath-like structure other than the ferrite fraction, pseudopolygonal ferrite and MA was regarded as bainite.

  The tensile test was carried out by collecting JIS No. 4 test pieces from a 1/4 thickness portion of each steel material and measuring 0.2% proof stress and tensile strength using these test pieces.

  Next, the steel material after rolling was fillet welded under various welding conditions shown in Table 5 to produce a load transmission cross joint (welded joint) shown in FIG. A fatigue test piece was obtained by appropriately cutting the vicinity of the weld toe of the welded joint and subjected to a hardness test.

The hardness test using fatigue test pieces obtained from various welded joints is carried out by a method according to JIS Z 2244 using a micro Vickers tester (the detailed method for determining the hardness gradient is as described above). Was measured to determine the hardness gradient (ΔHv / L) of the HAZ part. For the welded joints made, the maximum stress was kept constant at 350 MPa, and the minimum stress was changed to set the stress range (Δσ) and conduct a fatigue test. Create an SN diagram for each steel type. Then, fatigue strength of 1 × 10 ⁶ times (simply described as fatigue strength in Tables 3 and 4) was obtained.

As described above, in order to ensure fatigue strength, the hardness gradient (ΔHv / L) needs to be less than 150. In this example, the hardness gradient (ΔHv / L) is less than 150, and A fatigue joint having a fatigue strength of 1 × 10 ⁶ times of 75 MPa or more is deemed acceptable as a welded joint having excellent fatigue characteristics. These test results are shown in Tables 3 and 4.

No. taken from a welded joint that satisfies the requirements of the present invention (requirements of claim 1). The fatigue test pieces 1 to 24 all had a hardness gradient (ΔHv / L) of less than 150, and the fatigue strength after 1 × 10 ⁶ times was 75 MPa or more. A welded joint obtained by collecting 1 to 24 fatigue test pieces can be evaluated as a welded joint having excellent fatigue characteristics.

In contrast, no. Since the fatigue test specimens 25 to 36, 38 to 45 do not satisfy the requirements of the present invention (requirements of claim 1), at least one of the chemical composition of the steel material and the metal structure, the hardness gradient (ΔHv / L) At least one of the fatigue strengths of 1 × 10 ⁶ times could not satisfy the above-mentioned pass judgment condition. From the above test results, No. A welded joint obtained by collecting fatigue test pieces of 25 to 36 , 38 to 45 cannot be evaluated as a welded joint having excellent fatigue characteristics.

1 ... Steel (base material)
2 ... HAZ part (heat-affected zone)
3 ... weld metal 4 ... weld toe 5 ... gusset 6 ... weld line

Claims (4)

In mass%, C: 0.01 to 0.06%, Si: 1.0% or less (including 0%), Mn: 1.25 to 2.5%, Cr: 0.1 to 2.0% Mo: 1.5% or less (including 0%), V: 0.04% or less (including 0%), Ti: 0.005 to 0.030%, B: 0.0006 to 0.005% , P: 0.02% or less (not including 0%), S: 0.01% or less (not including 0%), N: 0.0020 to 0.010%, Cu: 0.01 to 2. Containing 0% and / or Ni: 0.01-5.0%, the balance being Fe and inevitable impurities,
The metal structure has an area ratio of 90% or more of ferrite and bainite,
Furthermore, a welded joint using a steel material satisfying the following formulas (1) and (2) and having a tensile strength of 590 MPa or more and less than 780 MPa ,
And the welded joint excellent in the fatigue characteristic characterized by having a HAZ part which satisfies the following formula | equation (3).
Kp = [Mn] +1.5 [Cr] +2 [Mo] = 2.4 to 4.5 (1)
[Ti] / [N] = 1.5 to 4.0 (2)
ΔHv / L <150 (3)
However, in the formulas (1) and (2), [] indicates the content (% by mass) of each element, and in the formula (3), ΔHv is the maximum hardness at a depth position of 100 μm from the surface in the HAZ part . The difference between the value and the minimum value, L is the length (mm) of the HAZ part.   The welded joint excellent in fatigue characteristics according to claim 1, wherein the steel material further contains Ca: 0.005% or less by mass%.   The welded joint excellent in fatigue characteristics according to claim 1 or 2, wherein the steel material further contains, by mass%, Nb: 0.04% or less.   The welded joint excellent in fatigue characteristics according to any one of claims 1 to 3, wherein the steel material further contains, by mass%, Al: 0.2% or less.