JP2002309339A - Welded joint having heat affected zone with excellent toughness and fatigue resistance - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a welded joint which has heat affected zone with excellent toughness and fatigue resistance. SOLUTION: The welded joint consists of high tensile strength steel having a composition containing, by mass, 0.01 to 0.1% C, 0.1 to 1.6% S, 0.5 to 2% Mn, <=0.01% P, <=0.005% S, 0.05 to 0.3% Nb, 0.001 to 0.05% Al, 0.001 to 0.05% Ti, 0.0002 to 0.006% Mg, 0.0001 to 0.008% O and 0.002 to 0.008% N, and the balance Fe with inevitable impurities, and satisfying Ceq <=0.45% and Pcm <=0.2%. One or more kinds of the single or combined particles of carbides, nitrides and oxides having a particle size of 0.002 to 5 μm are dispersed into the steel, and the previous γ particle size in the structure of the heat affected zone is 10 to 200 μm without depending on weld heat input. The final pass of the weld zone is formed by the weld metal whose Ms temperature reaches <=350 to >=150 deg.C.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

TECHNICAL FIELD The present invention relates to construction, shipbuilding, bridges,
Welded joints used for construction machines, marine structures, etc., more specifically, tensile strengths of 590 to 780 MPa (60 to 80 MPa)
kgf / mm ²⁾ weld heat affected zone comprised of the welding structural steel (HA
Z) It relates to a welded joint having excellent toughness and fatigue properties.

[0002]

2. Description of the Related Art With the increase in size of welded structures and the increasing demand for environmental protection, structural members have been required to have ever higher reliability. Fracture modes assumed for welded structures include fatigue fracture, brittle fracture, and ductile fracture. Of these, fatigue fracture is the most frequently occurring fracture mode in an actual use environment. Is the most important issue for improving the reliability of There have been many cases where fatigue failure has become a problem, such as the recent occurrence of fatigue cracks in large tankers and collapse starting from fatigue cracks in marine structures. A number of techniques for improving the fatigue strength of steel have been proposed so far, most of which are related to a base material of a thin steel sheet.
For example, Japanese Patent Application Laid-Open No. 61-96057 discloses that the fatigue strength can be improved by setting the area ratio of bainite to 5 to 60%. According to a study on fatigue fracture of a thick steel plate welded joint, a fatigue crack is generated at a stress concentrated portion of a weld. Since residual stress is also acting on this portion, it has been clarified that the occurrence of fatigue cracks is facilitated by the superposition of stress concentration and residual stress.

Although the relationship between the microstructure and the fatigue strength of the heat affected zone has not been clarified so far, in Japanese Patent Application Laid-Open No. Hei 5-34528, the fatigue strength of the HAZ structure is improved by the formation of island martensite. Is described. That is, it is described that when hard island-like martensite is present in the HAZ structure, the micro fatigue crack once generated is prevented or delayed from propagating, and the fatigue strength is substantially increased. As a result of conducting a systematic experiment on the microstructure dependence of poor fatigue initiation / propagation of the weld, JP-A-6-207794 discloses that the HAZ structure that suppresses the initiation / propagation of fatigue cracks most effectively is ferrite. It has been revealed that That is, it is described that the fatigue strength of the welded portion is improved by limiting the carbon equivalent value (hereinafter, Ceq) and increasing the HAZ ferrite structure fraction.

[0004] 60 to 80 kgf / mm ² HA like high tensile steel
When the Z structure is bainite, the suppression of fatigue crack initiation / propagation is effectively limited by the addition amount of Si or Nb and the Ceq, as described in JP-A-7-15450. That is, the limitation of the amount of added Si has the effect of strengthening the lath boundary by solid solution strengthening of ferrite in bainite while suppressing the addition of martensite, and the limitation of the amount of added Nb has the effect of strengthening the lath boundary by solid solution or precipitation strengthening of Nb carbide.
Limiting Ceq improves the fatigue strength of the weld by strengthening the entire bainite. On the other hand, attempts to improve the mechanical conditions of the joint in order to improve the fatigue properties of the welded joint have been made for some time. The portion where the fatigue crack is likely to occur is the welded portion. The reason for this is that the welded portion has a stress concentration portion, a tensile residual stress is generated, and the like. Solving these causes is effective for realizing a welded joint having high fatigue strength. Therefore, as a high fatigue strength welded joint in the prior art, a joint in which stress concentration is reduced by applying a decorative welding by a mechanical method or TIG welding, or compressive residual stress is introduced into a part where fatigue occurs by using peening, and at the same time, There were joints with reduced stress concentration. Since these joints directly increase the cost of manufacturing a structure, a welded joint having improved fatigue strength other than such a joint has been desired.

[0005] Recently, attention has been paid to a method of utilizing a transformation expansion of a weld metal to reduce residual stress and thereby improve fatigue strength. For example, Ohta et al. In the 61st meeting of the Japan Welding Society, 520-521,
We report on the improvement of fatigue strength of square turning welded joints by utilizing transformation expansion of weld metal. According to this report,
By lowering the temperature at which the transformation from austenite to martensite (Ms temperature) is started, the expansion accompanying the transformation becomes larger than the heat shrinkage after the transformation, and as a result, residual stress of compression is introduced, and a high fatigue strength welded joint is obtained. Will be obtained. According to Ohta et al., The main plate (flat plate) of the turning welding joint is preheated, and the additional material (vertical plate) is welded at room temperature, and the improvement in fatigue strength is confirmed. The welding joints reported by Ohta et al. Have many problems from the viewpoint of construction cost, such as preheating must be performed from the viewpoint of construction work, and the vertical plates are kept at room temperature.

[0006]

Problems to be Solved by the Invention JP-A-61-9605
In the invention described in Japanese Patent Publication No. 7 (1995), the fatigue strength is improved by limiting the bainite area ratio of the base material to a specific range. This relates to the improvement of the fatigue strength of a thin steel sheet base material. It has no effect on improving the fatigue strength of the welded joint in butt welding or fillet welding of the target thick steel plate. Further, in the inventions described in JP-A-59-110490 and JP-A-01-301823,
Special work must be performed after welding, and the fatigue strength cannot be improved as it is. JP-A-5-34592
In the invention described in the publication, in order to generate island-like martensite, a special post-weld heat treatment of heating and cooling the welded portion to an intermediate temperature range of Ac ₁ to Ac ₃ after welding is performed, and the welding is performed as it is. Fatigue strength cannot be improved.
It is well known that the formation of island-like martensite significantly lowers toughness, and no consideration is given to obtaining good HAZ toughness.

In the invention described in JP-A-6-207794, the fatigue strength of a weld is improved by limiting the Ceq value and increasing the HAZ ferrite fraction. And the tensile strength is 50kg
f / mm ² high-strength steel sheet with a HAZ structure of ferrite, welding heat input is small and cooling rate is high, or 60 to 8
It is not intended to particularly improve the case where the microstructure of the HAZ of a 0 kgf / mm ² high strength steel sheet becomes bainite or martensite. In the invention relating to the limitation of components described in JP-A-7-15450, etc., 0.6% or more of S
i addition strengthens the solid solution of ferrite in bainite,
The addition strengthens the lath boundary by solid solution or precipitation strengthening of Nb carbide to suppress the generation and propagation of fatigue cracks. However, the addition of 0.6% or more of Si or the addition of high Nb causes HAZ embrittlement. Therefore, neither method considers obtaining good HAZ toughness while maintaining improvement in fatigue strength.

Further, it is an existing finding that the lower the transformation temperature is, the lower the residual stress tends to be, and it is easily presumed that the fatigue strength is affected by the residual stress. However, high fatigue strength welded joints that can be manufactured using a simple construction method applicable to construction work have not yet been established. Although the method of Ota et al. Uses a technique of reducing residual stress, the adopted construction method is not practical,
It is hard to say that it is a welded joint suitable for construction work. On the other hand, conventional joints on which decorative welding such as peening or TIG welding is performed are themselves a factor of increasing the construction cost of a welded structure. If a joint that achieves high fatigue strength is established by introducing compressive residual stress into a weld by simple construction and using it, the effect will be enormous from the viewpoint of improving the reliability of the welded structure.

According to the present invention, not only the improvement of the fatigue strength by the additional welding method for realizing the reduction of the stress concentration and the reduction of the welding residual stress, but by controlling the steel material composition,
Tensile strengths of 590-7 that can obtain a significant improvement in fatigue strength while maintaining good HAZ toughness of butt welded joints or fillet welded joints as they are, without depending on welding heat input.
80MPa high-strength welded structural steel sheet as the base metal, and a welded joint with excellent weld heat affected zone toughness and fatigue properties by a simple welding construction method utilizing low-temperature transformation expansion, and a particularly advantageous joint for its implementation It is intended to provide a structure.

[0010]

Means for Solving the Problems The inventors of the present invention have 590-78.
A detailed study was conducted with the aim of improving toughness and fatigue strength using the HAZ structure of a high-strength steel of 0 MPa as bainite. As a result, the addition of high Nb and the restriction of Ceq, as well as the uniform dispersion of fine oxides or nitrides, We have found that refinement is effective. Furthermore, various studies have been made on techniques for reducing the residual stress of the welded part and improving the fatigue properties, and as a result of intensive research, the present invention has been completed. It is.

(1) In mass%, C: 0.01 to 0.1
%, Si: 0.1 to 1.6%, Mn: 0.5 to 2%,
P: 0.01% or less, S: 0.005% or less, Nb:
0.05-0.3%, Al: 0.001-0.05%,
Ti: 0.001-0.05%, Mg: 0.0002-
0.006%, O: 0.0001 to 0.008%, N:
0.002 to 0.008%, the balance being Fe and unavoidable impurities, and C defined by the following formula:
eq and Pcm satisfy Ceq: 0.45% or less and Pcm: 0.2% or less, and further, one or two of carbide, nitride and oxide single or composite particles having a particle size of 0.002 to 5 μm. A high-strength steel welded joint in which more than one species is dispersed in steel and the former γ grain size of the weld heat-affected zone structure is 10 to 200 μm regardless of welding heat input, and transformation from austenite to martensite is started. Temperature is below 350 ℃ 150 ℃
A weld joint having excellent toughness and fatigue properties in a heat affected zone of a weld, wherein a final pass of the weld is formed by the above weld metal. However, Ceq (%) = C + Mn / 6 + (Cu + Ni) / 15 +
(Cr + Mo + V) / 5 + Nb / 3 Pcm (%) = C + Si / 30 + (Mn + Cu + Cr) /
20 + Ni / 60 + Mo / 15 + V / 10 In the formulas, the symbol of each element represents mass% of each element. (2) The high-tensile steel is, in mass%, Cu: 0.1-2.
5%, Ni: 0.1 to 5% or less, Cr: 0.1 to 1%,
Mo: 0.1 to 1.5%, V: 0.005 to 0.05%
The welded joint excellent in toughness and fatigue properties of the weld heat-affected zone according to the above (1), further comprising one or more of the following. (3) The yield strength of the weld metal is 390 to 390 at a temperature at which transformation from austenite to martensite starts.
(1) or (2), wherein the welded joint is excellent in toughness and fatigue properties in a heat affected zone. (4) The weld metal contains one or more of Ni, Cr and Mo and C, and Pa defined by the following formula is P
a: The welded joint excellent in toughness and fatigue properties of the weld heat-affected zone according to any one of (1) to (3), which satisfies 0.85 to 1.3%. However, Pa (%) = C + Ni / 12 + Cr / 24 + Mo / 19 In the formula, the symbol of the element represents the mass% of each element. (5) C: 0.01 to 0.1% by mass of the weld metal.
2%, Si: 0.1-0.5%, Mn: 0.01-1.
5%, P: 0.03% or less, S: 0.02% or less, N
i: A welded joint excellent in toughness and fatigue properties in a weld heat-affected zone according to the above (3) or (4), containing 8 to 12%, with the balance being Fe and unavoidable impurities. (6) The weld metal is, in mass%, Ti: 0.01 to
0.4%, Nb: 0.01 to 0.4%, V: 0.1 to 1
%. The welded joint according to the above item (5), further comprising one or more of one or more of the above-described%. (7) The weld metal is Cu: 0.05-% by mass.
0.4%, Cr: 0.1-3%, Mo: 0.1-3%,
Co: The welded joint excellent in toughness and fatigue properties of the weld heat-affected zone according to (5) or (6), further comprising one or more of 0.1 to 2% of Co. (8) The weld metal is represented by mass%, C: 0.001 to
0.05%, Si: 0.1 to 0.7%, Mn: 0.4 to
2.5%, P: 0.03% or less, S: 0.02% or less,
Ni: 4 to 8%, Cr: 8 to 15%, N: 0.001
(3) or (4), characterized in that it contains 0.05%, C + N: 0.001 to 0.06%, and the balance is Fe and inevitable impurities. Excellent welded joint. (9) The weld metal is Cu: 0.05-% by mass.
0.4%, Mo: 0.1-2%, Ti: 0.005-
0.3%, Nb: 0.005 to 0.3%, V: 0.05
The welded joint excellent in the toughness and fatigue properties of the weld heat-affected zone according to the above (8), further comprising at least one kind of 0.5% or more. (10) Any one of the above (1) to (9), wherein one or more of out-of-plane gussets, cover plates, and studs are welded to a structural member subjected to a fatigue load as an auxiliary member. 4. A welded joint excellent in toughness and fatigue properties of a weld heat affected zone according to claim 1. (11) The welded joint according to the above (10), wherein the auxiliary member has a groove of 5 mm or more at the end to be welded, and has excellent toughness and fatigue properties in a weld heat affected zone. (12) The welded joint excellent in toughness and fatigue properties of the weld heat-affected zone according to the above (10) or (11), wherein the out-of-plane gusset is rotary-welded. (13) The welded joint according to any one of the above (10) to (12), wherein the out-of-plane gusset has scallops.

[0012]

BEST MODE FOR CARRYING OUT THE INVENTION The present inventors first studied a steel material to be a base material of a welded joint. In particular,
The state of crack initiation and propagation in fatigue test specimens of welded joints was observed in microscopic detail. As a result, most fatigue cracks occur at the boundary between the weld metal and the HAZ, that is, near the weld fusion line (fusion line: the boundary between the weld metal and the HAZ), propagate through the HAZ, and further enter the base metal. It was found that the test piece was totally destroyed. This is because the vicinity of the weld fusion line coincides with the weld toe, and the stress concentration is highest at this portion. As described above, the fatigue crack is generated near the weld fusion line and propagates in the HAZ.
It has been found that Z has a significant effect on the microstructure.

[0013] As described above, the portion where the fatigue crack occurs is near the weld fusion line, and at the initial stage of crack propagation, the HAZ is formed.
Is within. These regions correspond to the stress concentration parts.
By reproducing both the HAZ microstructure and the stress concentration factor, the effect of the HAZ microstructure on the fatigue strength can be investigated. That is, a test piece provided with stress concentration was processed from a steel material to which a reproduction welding heat cycle was applied, and subjected to a fatigue test to determine the relationship between the HAZ microstructure and the fatigue strength.
Dimension of test piece 10 × 10 × 55mm, notch depth 2
mm, the notch tip radius is 0.75mm, and the distance between fulcrums is 40
A three-point bending load was applied repeatedly as mm to cause fatigue fracture.
The stress concentration factor is 2.6.

FIG. 1 is a graph showing a maximum heating temperature of 1400.degree. C. and a cooling time of 800 to 500.degree. C. for a mild steel and a laboratory vacuum molten steel having a tensile strength of 490 MPa.
Reproduced HAZ given a reproducible heat cycle of up to 30 seconds
It is a plot of the fatigue limit ratio of the material (fatigue limit / tensile strength of the reproduced HAZ material) with respect to the tensile strength of the reproduced HAZ material. As is clear from the figure, the fatigue limit ratio greatly depends on the HAZ microstructure, and increases in the order of martensite, lower bainite, a mixed structure of lower bainite + upper bainite, upper bainite, and ferrite. That is, in a fatigue test having stress concentration, the HAZ structure has a higher fatigue limit ratio as the structure is transformed at a higher temperature, and becomes lower as the structure is transformed at a lower temperature.

Although the cause of the fatigue strength depending on the microstructure has not been completely elucidated, the dislocation density introduced at the time of transformation is higher in a low-temperature transformation structure, and the dislocations are rearranged when subjected to repeated stress. Therefore, the dislocation strengthening does not contribute much to the fatigue strength.In the case of a low-temperature transformation structure, the lath interface of bainite or martensite or the strength of the former austenite grain boundary becomes relatively lower than the strength of the intragranular structure. Fatigue cracks easily occur at interfaces and former austenite grain boundaries, and in the ferrite structure, plastic deformation at the crack tip that propagates is remarkable, plastic absorption energy increases, and as a result, crack propagation is delayed. There may be a reason.
It is known that in a smooth test piece with less stress concentration, the microstructure dependence of fatigue strength is small, and rather, it has a high correlation with static tensile strength.

As described above, the reproduced HAZ material fatigue strength is affected by the microstructure. In particular, an increase in the fatigue limit ratio in a ferrite-based structure is a phenomenon that occurs specifically in a stress-concentrated portion. The effect of improving the fatigue strength by using as a ferrite-based structure is particularly remarkable when stress concentration exists as in a welded joint. Therefore, it is most desirable to make the HAZ microstructure a ferrite-based structure in order to improve the fatigue strength.
It is difficult to obtain a 100% ferrite structure by continuous cooling transformation that AZ receives continuously, especially in small-to-medium heat input welding with a high cooling rate, and the present invention is directed to a tensile strength of 590 to 780 MPa class. Since it is difficult for a ferrite-based structure to have the same, the upper bainite-based structure has the second highest fatigue limit ratio after ferrite.

FIG. 2 shows that the mild steel has a tensile strength of 490 MPa (50 kgf / mm ² ) and a tensile strength of about 590 to 780 containing about 0.2% of Si.
MPa (60-80 kgf / mm ² ), Tensile strength containing about 0.1% Nb is 590-780
MPa (60-80 kgf / mm ² ), containing about 0.1% of Nb, deoxidized with Mg, and refined to obtain a tensile strength of 590-780 MPa (60-80 kgf / mm ² ).
mm ² ), the maximum heating temperature was 1400 ° C. and the cooling time at 800 to 500 ° C. was 1 to
This is a result of performing a fatigue test on a steel sheet having a HAZ structure of more than 60% bainite by giving a welding reproduction heat cycle of 30 seconds and plotting the fatigue limit ratio with respect to Ceq of various steel materials. The Ceq shown here is based on the following equation, which takes into account the effect of increasing the hardenability of Nb on the commonly used IIW carbon equivalent equation. Ceq (%) = C + Mn / 6 + (Cu + Ni) / 15 +
(Cr + Mo + V) / 5 + Nb / 3 As is evident from FIG. 2, it can be seen that the addition of 0.1% Nb increases the fatigue limit ratio and further increases due to the fineness by Mg deoxidation. When Ceq is 0.3% or more, the fatigue limit ratio increases.

HA having the same fatigue strength of more than 60% bainite
Despite the Z structure, the cause of the dependence on Nb, grain refinement, and Ceq has not been fully elucidated. However, in the region where Ceq is large, the difference in strength between cementite and lath boundary in bainite becomes small, Suppression of fatigue cracking, high Nb addition strengthens the lath boundary by precipitation or segregation of Nb and fine Nb (C, N) on lath boundary in bainite, and suppresses fatigue crack generation The reason is that, together with the effect of reducing the concentration of stress due to the grain refinement, the grain boundary or lath boundary having an increased area acts as an obstacle to suppress the occurrence of fatigue cracks. Also, Nb
The HAZ toughness, which decreases due to precipitation of, increases to a level higher than that of a normal steel material due to grain refinement, and has good HAZ toughness irrespective of heat input conditions. As is clear from the above examination results, the skeleton of the present invention has a HAZ structure of more than 60% of bainite, high Nb addition, limitation of Ceq, and refinement of grain size, thereby improving the fatigue strength while maintaining the toughness of the weld. Things. The reason for limiting the range of each alloy element based on the above basic idea will be described below.

C is contained as an effective component for improving the strength of steel. If it is less than 0.01%, it is difficult to secure the strength of the base material. If it exceeds 0.1%, the toughness and weld cracking resistance of the base metal and the welded part are reduced, so
0.1%.

Si is an essential element as a deoxidizing element in addition to securing strength. In order to strengthen ferrite in bainite by solid solution to strengthen the lath boundary and improve fatigue strength, Si is added in an amount of 0.6% or more. Is required, and if it exceeds 1.6%, a crack is likely to be generated from the lath boundary. On the other hand, good H
In order to obtain AZ toughness, it is necessary to reduce the content to 0.6% or less.
It can be achieved by addition and fine graining by fine oxides or nitrides. However, even if the content of Nb is less than 0.1%, improvement in fatigue strength cannot be ensured. Therefore, the amount is set to 0.1 to 1.6%, and especially when good HAZ toughness is required.
~ 0.6% is desirable.

Mn is an essential element for increasing the strength, but if it is less than 0.5%, the strength of the base material cannot be secured. On the other hand, if it exceeds 2%, the HAZ structure becomes mainly martensite and the fatigue strength is reduced.
And

P is an element that affects the toughness of steel. If it exceeds 0.01%, it significantly impairs not only the base material but also the toughness of the HAZ. It was as follows. S is preferably as low as P, and if it exceeds 0.005%, MnS precipitation becomes remarkable, hindering the HAZ toughness of the base material and reducing ductility in the thickness direction. Furthermore, when MnS inclusions are present in large amounts, they become the starting points of fatigue cracks and cause variations in fatigue strength. Therefore, the amount was made 0.005% or less.

Nb is one of the main elements as a component of the present invention, and fine carbide or nitride of Nb precipitates on the lath boundary in bainite to strengthen the lath boundary and improve the fatigue strength of the weld joint. Even if the addition of Si is less than 0.6%, the fatigue strength can be sufficiently improved. More than 0.05% is required to obtain the effect. On the other hand, if the content exceeds 0.30%, the deposited portion becomes coarse, which becomes a starting point, easily causes cracks, and lowers the fatigue strength. Therefore, the addition amount is set to 0.05 to 0.3%.

Al is an element effective for deoxidation, reduction of austenite grain size, and the like. Also, as described later, HA
It contributes to the fine dispersion of MgO and Mg-containing oxide necessary for improving the Z toughness. In order to exhibit the effect, it is necessary to contain 0.001% or more. On the other hand, if the content exceeds 0.05%, a coarse oxide is formed, the ductility is extremely deteriorated, and a starting point of a fatigue crack is caused.
-0.05%.

Ti is an element that contributes to the improvement of the base metal strength by precipitation strengthening and is also effective in reducing the austenite grain size by heating by forming stable TiN even at high temperatures. Further, as described later, MgO required for improving HAZ toughness,
It contributes to fine dispersion of Mg-containing oxide. In order to exhibit the effect, it is necessary to contain 0.001% or more. on the other hand,
If it exceeds 0.05%, a coarse oxide is formed, the ductility is extremely deteriorated, and a fatigue crack is caused. Therefore, the amount is set to 0.001 to 0.05%.

Mg is one of the main alloying elements of the present invention, and is mainly added as a deoxidizing agent. However, if it exceeds 0.006%, coarse oxides are easily formed, and the base metal and This leads to a decrease in HAZ toughness. However, 0.0
If the addition is less than 002%, the intragranular transformation and the generation of oxides required as pinning particles cannot be expected sufficiently, so the amount was made 0.0002 to 0.006%.

O is an essential element for generating an Mg-containing oxide. If the amount of oxygen finally remaining in the steel is less than 0.0001%, the number of oxides is not sufficient, so the lower limit is 0.0001%. on the other hand,
When the content exceeds 0.008%, coarse oxides increase, resulting in a decrease in base material and HAZ toughness. Therefore, the upper limit was made 0.008%.

Since N effectively combines with Al and Ti to reduce the size of austenite grains, a small amount of N contributes to the improvement of mechanical properties. Further, it is impossible to industrially completely remove N in steel, and it is not preferable to reduce N more than necessary because an excessive load is applied to a manufacturing process. Therefore, the lower limit is set to 0.002% as a range in which industrial control is possible and the load on the manufacturing process is acceptable. If it is contained excessively, the amount of dissolved N increases, which may adversely affect ductility and toughness.
8%.

The above is the basic component system in the present invention. Further, in the present invention, the addition amount of the above components, the hardening property Ceq which becomes the quenchability of the weld heat affected zone, and the low temperature crack susceptibility Pcm considering the weldability are considered. Between Ceq ≦ 0.45% and P
It is important to satisfy cm ≦ 0.2% to improve weldability and toughness and fatigue strength of a welded portion. In other words, Ceq is preferably as high as possible for improving the strength, but if it exceeds 0.45%, the structure changes from bainite to martensite and the fatigue strength decreases. Therefore, the upper limit was set to 0.45%. In order to secure the strength with a bainite-based structure, it is preferable that Ceq not be less than 0.24%. In addition, Pcm is 0.2%
If it exceeds, low temperature cracking may occur, and it becomes difficult to improve HAZ toughness and fatigue strength as it is welded. Therefore, the upper limit was set to 0.2%.

Cu, Ni, Cr, M selectively added
o and V are elements that enhance the quenchability Ceq, and it is effective to include one or more of them in the basic components. The reasons for limiting the components of each element are described below. Cu is
It is an element effective for increasing strength without reducing toughness,
If it is less than 0.1%, there is no effect, and if it exceeds 2.5%, cracks are likely to occur during heating of the slab or welding. Therefore,
The amount is set to 0.1 to 2.5%. Ni is an element effective for improving toughness and strength. To obtain the effect, addition of 0.1% or more is necessary. However, addition of 5% or more not only lowers weldability but also reduces HAZ. The structure is changed from bainite to a structure mainly composed of martensite to reduce the fatigue strength. Therefore, the amount was set to 0.1 to 5%.
Cr is 0.1% for enhancing hardenability and securing strength.
It is necessary. On the other hand, if it exceeds 1%, it is not preferable for the same reason as Ni. Therefore, the amount is 0.1-1%
And Mo is an element effective for improving hardenability, improving strength, resistance to tempering embrittlement, and suppressing recrystallization. To obtain the effect, it is necessary to add 0.1% or more. If it exceeds, toughness and weldability deteriorate. Therefore, the amount was set to 0.1 to 1.5%. V is charcoal
A nitride is formed to increase the strength by precipitation strengthening. Therefore, it is necessary to add 0.005% or more.
If it exceeds 5%, the HAZ toughness decreases. Therefore,
The amount was made 0.005 to 0.05%.

Next, a preferred method for producing a steel sheet will be described. The present invention provides a tensile strength 59 excellent in weld fatigue strength.
The present invention provides a steel plate for a high-strength welded structure of 0 to 780 MPa. First, the method of adding Mg at the steel making stage is as described above. First, after adding Si and Mn, first,
After adjusting the amount of oxygen in the molten steel by adding Ti, a small amount of Mg
Is added gradually, or after adding Ti and a small amount of Mg at the same time, Mg is added again in the final stage. The optimal amount of added Mg depends on the amount of oxygen existing in the molten steel after the addition of Ti. In the experiment, the oxygen concentration at that time depends on the amount of Ti added and the time until the addition of Mg.
What is necessary is just to control the addition amount in an appropriate range. The appropriate amount of dissolved oxygen at the final addition of Mg is about 0.1 to 50 ppm. The minimum of 0.1 ppm is the minimum amount that can form fine Mg oxides. If it exceeds 50 ppm, coarse Mg oxides can be formed and the pinning force is weakened. The reason for limiting the temperature of the reheating treatment after hot rolling and cooling to not less than Ac _{3 and not} more than 1000 ° C. is as follows.
The base material strength is a temperature range for changing the base material structure to a bainite-based structure containing martensite and obtaining a balance between strength and toughness of the base material. This is because sufficient strength cannot be obtained below this temperature.

It is possible to adopt the ordinary hot rolling method as described above, but when the carbon equivalent value is low or the sheet thickness is large, the required strength can be obtained by the ordinary rolling method. May not be possible. In such a case, the base material strength can be increased by controlled rolling and accelerated cooling. The reason why the hot rolling temperature is increased to a high temperature range of 1200 ° C. is that the graining effect by the rolling treatment can be expected and the strength and toughness are improved as compared with the reheating treatment.

Tempering treatment to be performed after rolling and cooling
Is intended to improve the toughness of the base metal structure by recovery.
Therefore, the heating temperature is a temperature range where reverse transformation does not occur
Ac ₁Must be: Recovery is the disappearance of dislocations
Body to reduce the lattice defect density.
It is necessary to heat to 300 ° C or more to realize
Therefore, the lower limit was set to 300 ° C. In the present invention,
Contains Cu, Mo, Nb, and V precipitation elements for heat
Increases base metal strength by generating fine precipitates during processing
Can be up. Note that precipitation hardening works most effectively.
Temperature depends on precipitation hardening element
However, the range of 400 to 600 ° C. is preferable.

Next, a description will be given of a study on a welded joint having excellent fatigue strength by making full use of the characteristics of the high-tensile steel. First, the technical concept will be described. The first technical idea is that a high fatigue strength welded joint can be obtained by reducing the residual stress at the bead toe where a fatigue crack occurs. When the weld metal transforms from austenite to martensite during the cooling process, the volume increases, ie expands. At this time, compressive stress is generated in the weld metal because it is constrained from surrounding parts. However, the introduction of the compressive stress accompanying the transformation expansion also returns to the tensile stress state during cooling to room temperature if the subsequent thermal contraction is large. Except for some stainless steel materials, welding materials used for ordinary steel materials always undergo transformational expansion at a certain temperature, but due to the high temperature, subsequent thermal shrinkage ultimately generates tensile residual stress. I do. Since the thermal shrinkage is obtained by multiplying the temperature change by the thermal expansion coefficient, in order to make the residual stress as small as possible and, in some cases, into a compressed state, the temperature change may be made small.

There are two methods for reducing this temperature change. One is to use a material that lowers the Ms temperature, and the other is to preheat the plate. Since the method of preheating the plate eventually cools to the outside air temperature, it is considered that the change in temperature does not seem to be small at first glance.
However, if preheating is performed, the temperature distribution of the plate becomes uniform in a temperature region higher than room temperature, and in the subsequent cooling process, a uniform temperature is maintained, so that no thermal stress is generated. The temperature difference in this case is the temperature change in this case. This is because if the temperature distribution of the plate is uniform,
Due to the fact that thermal stress does not occur on heating or cooling.

In the case of ordinary materials, the Ms temperature is around 450 ° C., but it is not practical to set the preheating to a value close to that. Therefore, as a method of reducing the temperature change, that is, the heat shrinkage after the transformation, it is indispensable to select a material having a low Ms temperature. However, if the value of the Ms temperature is inappropriate, or if the material selection is inappropriate, the residual stress cannot be reduced by using only the material, and additional measures such as the use of preheating are required. In fact, Ota et al. Achieve high fatigue strength welded joints by preheating. When trying to apply a low Ms temperature material to the work being performed, it is desirable to be able to achieve a high fatigue strength welded joint by reducing the residual stress by performing without preheating. It is not only the construction cost, but unlike the case of fatigue test specimens, the preheating of the actual structure must be performed only in the vicinity of the welded part, so-called local preheating, and the introduction of residual stress and deformation due to preheating construction itself This is because there is concern. Therefore, an object of the present invention is to provide a welded joint in which residual stress is reduced to such an extent that high fatigue strength can be expected without preheating.

The purpose of the present invention is to improve the fatigue strength by using the transformation expansion caused by the lowering of the Ms temperature of the weld metal as described above.
In order to further reduce the residual stress, a second technical idea of the present invention is to set the yield strength of the weld metal at an Ms temperature to an appropriate value. In general,
It is necessary to add C, Ni, Cr and the like to the material having a low Ms temperature, and it is considered that a certain level of strength is secured. However, in order to reliably achieve a high fatigue strength welded joint, it is desirable to set the strength in an appropriate range. This technical idea of controlling the strength is based on the fact that even if transformation expansion occurs, there is a limit to the compressive elastic strain caused by the expansion, and the value is the value obtained by dividing the yield strength by the Young's modulus. It is.

Here, for example, consider the case where the transformation expansion amount of the weld metal is 3%. Assuming that the weld metal is completely constrained from the surroundings, a 3% transformation expansion introduces a 3% compressive strain, resulting in a total strain of 0%. At this time, the compressive strain of 3% can be classified into plastic strain and elastic strain, but the elastic strain has a limit as described above, and the rest must be plastic strain. The weld metal then undergoes thermal shrinkage, which in turn introduces tensile strain into the weld metal. Due to this tensile strain, the amount of elastic compressive strain introduced during the transformation expansion decreases, and depending on the amount of thermal shrinkage, it may become tensile strain. It can be understood from this consideration that even if the heat shrinkage amount is reduced, that is, Ms
Even if the temperature is lowered, if the compression elastic strain limit (maximum value) is small, the residual stress cannot be reduced. This means that by increasing the compressive elastic strain limit, the residual stress can be surely reduced and, consequently, can be brought into a compressed state, thereby achieving the high fatigue strength welded joint which is the object of the present invention. Means you can do it. The reason why such discussions always hold is that
It comes from the fact that the transformation expansion strain is always greater than the elastic strain limit. To increase the elastic strain limit,
The yield strength may be increased. For that purpose, the yield strength of the weld metal must be set to an appropriate value. This is the second technical concept of the present invention.

The present invention includes two technical ideas described above, namely, the use of a weld metal having a low Ms temperature and an increase in the elastic strain limit by increasing the yield strength of the weld metal. Heat affected zone (HAZ)
There is a third technical idea that the residual stress can be reduced by utilizing the transformation expansion of the weld metal. The fatigue crack does not necessarily propagate from the weld toe where fatigue occurs to the steel HAZ, but rather to the weld metal, but in the present invention, the steel itself is not necessarily a low Ms temperature material. However, in the present invention, it is considered that by forming a low Ms temperature weld metal on the bead forming the toe, a residual stress equivalent to that of the weld metal can be introduced into the steel material HAZ as a reaction force. Since the residual stress is a stress distribution in a situation where no external force acts, it has a feature that the resultant force is zero as a whole. Thus, introducing compressive residual stress also implies introducing tensile residual stress somewhere else in the weld. However, the occurrence of fatigue mainly occurs from the stress concentration portion (bead toe) on the surface, and the value of the residual stress there is important. If it is distributed in a portion where there is no danger of high fatigue strength, a high fatigue strength welded joint can be realized.

FIG. 3 shows a turning welding joint which is considered to be the most severe in view of the fatigue problem. When the hatched weld bead of FIG. 3 undergoes transformation expansion at a low temperature, as can be seen from the figure, a compressive stress is introduced as a reaction force into the steel material HAZ, for example, the regions A and B in FIG. It can be understood. Here, it should be noted that the compressive stress of the steel material HAZ is not introduced because the steel material itself has undergone transformation expansion, but is a reaction force against the transformation expansion of the weld metal. Therefore, the residual stress in the direction perpendicular to the weld bead can be reduced in the region A in FIG. 3, and the residual stress in both the weld bead direction and the direction perpendicular to the bead can be reduced in the region B. Since the fatigue crack occurs from the bead toe, the residual stress here is reduced. On the other hand, the HAZ located under the weld bead, that is, existing inside the steel material, is conversely subjected to tensile stress due to expansion of the bead. However, since this portion is not a fatigue crack initiation site, improvement in fatigue strength of the welded joint as a whole can be expected.

Next, a fourth technical concept of the present invention will be described. In the present invention, the fatigue strength of the bead toe is improved by the first, second, and third technical ideas described above. However, even if the fatigue strength of the bead toe is improved in the weld joint as a whole, if fatigue cracks occur in other parts, the weld joint as a whole is determined by the fatigue strength there. Normally, fatigue cracks originate from the bead toes because of the lowest fatigue strength there, and therefore, in the present invention, the first, second,
The third technical concept aimed at improving the fatigue strength of the bead toe. With this alone, the improvement of the fatigue strength of the welded part can be expected, but the improvement in the fatigue strength of the bead toe may cause the fatigue strength of other parts to determine the fatigue strength of the welded joint as a whole. . In order to further improve the fatigue strength of the welded joint, it is desirable to improve the fatigue strength of a portion where a fatigue crack may occur outside the toe portion. In the joints dealt with in the present invention, the portion where the fatigue crack is likely to occur at portions other than the bead toe portion is an unwelded portion existing inside the rotary welded portion. For example, the unwelded portion existing between the gusset and the structural member when the out-of-plane gusset is attached to the structural member by welding and welding, particularly the unwelded portion near the corner turning portion, itself forms a stress concentration portion. Therefore, there is a risk that fatigue cracks may be generated therefrom. Therefore, in the present invention, a fourth technical idea is to reduce the unwelded portion near the corner turning portion and improve the fatigue strength there.

Since the fourth technical concept does not necessarily improve the fatigue strength of the bead toe where a fatigue crack occurs in a normal welded joint, the first, second and third techniques of the present invention are not considered. This technology can be expected to be effective when used in combination with thought. Conversely, by using these technical ideas in combination, it is possible to reliably realize a high fatigue strength welded joint. The present inventors have come to discover a mechanism for reducing the residual stress at the fatigue crack initiation site as described above, and further conducted intensive research on the relationship with the weld joint fatigue strength, and finally found a practical high fatigue This has led to the completion of a strength welded joint.

The reason why the Ms temperature range of the weld metal and the yield strength range of the weld metal at the Ms temperature are limited will be described below. The Ms temperature shows a value of 500 ° C. or less even in ordinary steel materials and weld metals.
0 ° C. or less. This value depends on the component. For example, as can be seen from the welding CCT diagram of welding structural steel issued by the Iron and Steel Institute of Japan, the addition of about 5% of Ni can lower the Ms temperature to about 350 ° C. . However, when the Ms temperature is higher than 350 ° C., the effect of reducing the residual stress is not sufficient, and the effect of improving the fatigue strength cannot be expected. On the other hand, setting the Ms temperature to 150 to 350 ° C. can be realized with a material having an industrial value, and is within a range in which improvement in fatigue strength by reduction of residual stress can be expected. Ms
The lower limit of the temperature, 150 ° C., was set as the lower limit that could be realized with a material of industrial value. Upper limit of Ms temperature 350
According to the second technical idea of the present invention, even if the value is higher than 350 ° C., the residual stress reduction effect can be expected if the yield strength is sufficiently high, and as a result, the improvement of the fatigue strength can be expected, but the temperature is too high. Since the yield strength also has a problem of whether it can be realized with a material of industrial value, the upper limit was set to 350 ° C. Since it is preferable that the Ms temperature is lower to reduce the residual stress, the Ms temperature is preferably 300 ° C.
It is desirable to set as follows.

Next, the reason for limiting the range of the yield strength will be described. When the yield strength is less than the lower limit of 390 MPa, the Ms temperature must be lower than 150 ° C. in order to ensure that the residual stress reduction effect can be expected. If the Ms temperature is lower than this, the material is limited to a material having low practicality, and this is contrary to the intention of the present invention. Therefore, the lower limit is set to 390 MPa. Preferably, the lower limit of the yield strength is 490 MPa or more. The upper limit of 1180 MPa requires that many special alloying elements be added in order to obtain a higher yield strength, which not only impairs the economic efficiency but also restricts the welding method. Because it is contrary to the intention, the upper limit is set to 1180 MPa.

Next, the parameter Pa expressed by the following equation
Is introduced, and the reason for limiting the value range is described. Pa (%) = C + Ni / 12 + Cr / 24 + Mo / 19 In the above formula, the symbol of the element represents the mass% of each element. The parameter Pa is calculated from the mass% of C, Ni, Cr and Mo. These components have the function of improving strength and lowering the Ms temperature by being added to the weld metal. In particular, in the sense of an element that reduces the Ms temperature,
These C, Ni, Cr and Mo are the elements to be used most effectively. From the viewpoint of improving the strength, Ti,
Although the effective use of carbide-forming elements such as Nb and V can be considered, adding Ti, Nb, V, and the like so that the Ms temperature becomes sufficiently low undesirably causes a large problem in joint characteristics. On the other hand, the functions of lowering the Ms temperature of C, Ni, Cr, and Mo and lowering the residual stress are not necessarily the same, so that coefficients corresponding to the respective functions are determined.
The index Pa indicating the effect is determined for the four elements as a whole.

Regarding the appropriate range of the value of Pa, for example, if the value of Pa is too small, it is difficult to reduce the Ms temperature, and even if it becomes possible by adding another element, the characteristics of the weld joint can be secured. It is not preferable from the point of view.
Conversely, a large Pa means that the Ms temperature is lower, but a Pa that is too large is uneconomical because the addition of alloying elements must be increased accordingly. Thus, the range of Pa is set to 0.85% or more and 1.3% or less. In addition, in order to secure a higher fatigue strength welded joint, it is desirable to set the lower limit of Pa to 0.95. Further, in the present invention, since the technique of increasing the yield strength of the weld metal and reducing the residual stress more reliably is used together, when the yield strength of the weld metal at the Ms temperature is 490 MPa or more, welding is performed. From the viewpoint of the possibility of the existence of retained austenite in the metal and economics, P
The upper limit of a is preferably set to 1.25. Further, the yield strength of the weld metal at Ms temperature is 5
When the pressure is 90 MPa or more, the upper limit of Pa is preferably set to 1.2 from the viewpoint of economy.

Next, the reason why the components of the weld metal are limited will be described. Actually, the number of component systems for obtaining the Ms temperature and the yield strength described above is not necessarily one. The weld metal in the present invention is composed of two component systems mainly using Ni described in the above (5) to (7) and a component system mainly using Cr described in the above (8) and (9). In the following, the former is Ni-based weld metal and the latter is C
We will call it r-based weld metal.

First, the reason for limiting the component range of the Ni-based weld metal will be described. C functions to lower the Ms temperature by being added. When C is not added, martensite is hardly obtained, and reduction of residual stress must be achieved only by other expensive elements, which is not economical. The effect of C addition can be obtained at 0.01% or more, but on the other hand, excessive addition causes problems of toughness deterioration of the weld metal and cracks in the weld metal, so the upper limit was made 0.2%. The upper limit of C is preferably set to 0.15% from the viewpoint of weld metal cracking.

Si has the effect of lowering the oxygen level of the weld metal as a deoxidizing element. Particularly, in welding work, it is extremely important to control the amount of Si to an appropriate value because there is a risk of air being mixed during welding. First, regarding the lower limit of Si, when the amount of Si added to the weld metal is less than 0.1%, the deoxidizing effect is weakened, the oxygen level in the weld metal becomes too high, and the mechanical properties and
In particular, the lower limit of the weld metal is set to 0.1% because there is a risk of causing deterioration in toughness. On the other hand, the excessive addition of Si also causes toughness degradation, so the upper limit was made 0.5%.

Mn is an element for increasing the strength. Therefore, it is an element that should be used effectively from the viewpoint of ensuring yield strength during transformation expansion. The lower limit of Mn, 0.01%, was set as the minimum value at which the effect of securing strength was obtained. On the other hand, excessive addition causes toughness degradation of the base metal and the weld metal, so the upper limit was made 1.5%.

P and S are impurities in the present invention,
If there is a large amount in the weld metal, the toughness deteriorates, so it is desirable to reduce the toughness as much as possible.
% And 0.02%.

Ni is a single element of austenite, that is, a metal having a face-centered structure, and is an element that makes the state of austenite more stable by adding it to the weld metal. Iron itself has an austenitic structure in a high temperature range, and has a ferrite or body core structure in a low temperature range. Ni
Is added to make the face-centered structure of iron in a high-temperature range more stable, so that it becomes a face-centered structure even in a lower temperature range as compared with the case of no addition. This means that the temperature at which the structure transforms into a body-core structure becomes lower. The lower limit of Ni, 8%, was determined in terms of the minimum amount of addition in which the transformation temperature was controlled and the residual stress reduction effect was exhibited.
On the other hand, even if it is added in excess of 12%, the effect is saturated from the viewpoint of reducing residual stress, which is economically disadvantageous.

Cu is an effective element for improving the welding workability, since it has the effect of improving the electrical conductivity by plating the welding wire. Further, since Cu is also a hardenable element, an effect of promoting martensitic transformation by adding it to the weld metal can be expected. Cu
The lower limit of 0.05% was set as a minimum value necessary for improving workability and promoting martensitic transformation. But,
Excessive addition not only saturates the effect of workability improvement, but also increases the wire manufacturing cost and is economically disadvantageous.
0.4% was made the upper limit.

Nb combines with C in the weld metal,
Form carbides. Nb carbide functions to increase the strength of the weld metal in a small amount. In addition, the merit of addition is significant from the viewpoint of increasing the yield strength at Ms temperature. Nb
Is set to 0.01% as a minimum value at which a carbide is formed and a strength increasing effect can be expected. However, on the other hand, since excessive carbide formation causes toughness deterioration, a value of 0.1 is set as a value that does not impair the reliability of the welded portion due to the toughness deterioration.
The upper limit was 4%. V is an element having the same function as Nb. However, unlike Nb, in order to expect the same precipitation effect, it is necessary to increase the amount of Nb added. V
The lower limit of 0.1% of the addition was set as a minimum value at which precipitation hardening can be expected by adding the same. On the other hand, when a large amount is added, precipitation hardening becomes too remarkable and toughness is deteriorated, so that the upper limit of V is set to 1%. Like Ti and Nb, Ti also forms carbides and causes precipitation hardening. However, just as the precipitation hardening of V differs from that of Nb, the precipitation hardening of Ti is also different from Nb and V. Therefore, the range of the addition amount of Ti is set to a range different from Nb and V. The lower limit of 0.01% of the amount of Ti added was determined as a minimum amount at which the effect can be expected, and the upper limit of 0.4% was determined in consideration of deterioration in toughness.

Cr is a precipitation hardening element like Nb, V and Ti. Further, Cr is an element to be effectively utilized because it also has the effect of reducing the Ms temperature. However, the Ni-based weld metal in the present invention is mainly composed of Ms
Since the temperature is reduced, the amount of Cr added should be smaller than that of Ni. Excessive addition of Cr does not necessarily improve the effect of reducing residual stress, and is not preferable because Cr is expensive. The lower limit of 0.1% of the amount of Cr added was set as the minimum value at which the effect of adding residual Cr to obtain the residual stress reduction effect was obtained. On the other hand, as for the Ni-based weld metal, the Ms temperature has already been reduced by the addition of Ni, and the strength is secured by other precipitated elements. Since not only saturation but also deterioration of toughness becomes remarkable, the upper limit of the added amount of Cr was set to 3%.
Mo is also an element having the same effect as Cr. But M
o is an element in which precipitation hardening can be expected more than Cr. Therefore, the addition range was set narrower than Cr. 0 for the lower limit.
1% was set as the minimum value at which the effect of Mo addition can be expected. The upper limit of 3% was set because, if added more than this, the composition would be excessively hardened and the toughness would be significantly deteriorated.

Co, unlike Ti and the like, is not an element that causes strong precipitation hardening. However, from the viewpoint of increasing the strength by the addition and securing the toughness while expecting the increase in the strength, it is an element to be effectively used because it is a more preferable element than Ni. But N
Since i is added to the weld metal in order to secure a low Ms temperature at which a residual stress reduction effect can be expected, the lower limit of 0.1% of the Co addition amount is the minimum value at which the effect of Co addition can be expected. Set as a value. On the other hand, excessive addition results in excessive increase in strength and deterioration of toughness.
And

Next, the reason for limiting the component range of the Cr-based weld metal will be described. C functions to lower the Ms temperature by being added. When C is not added, martensite is hardly obtained, and reduction of residual stress must be achieved only by other expensive elements, which is not economical. The effect of adding C can be obtained by adding 0.001% or more. On the other hand, excessive addition causes problems of weld cracking and deterioration of toughness.
%.

Si has the effect of lowering the oxygen level of the weld metal as a deoxidizing element. Particularly, in welding work, it is extremely important to control the amount of Si to an appropriate value because there is a risk of air being mixed during welding. First, regarding the lower limit of Si, when the amount of Si added to the weld metal is less than 0.1%, the deoxidizing effect is weakened, the oxygen level in the weld metal becomes too high, and the mechanical properties and
In particular, the lower limit of the weld metal is set to 0.1% because there is a risk of causing deterioration in toughness. On the other hand, excessive addition of Si also causes toughness degradation, so the upper limit was set to 0.7%.

Mn is an element for increasing the strength. Therefore, it is an element that should be used effectively from the viewpoint of ensuring yield strength during transformation expansion. The lower limit of Mn, 0.4%, was set as the minimum value at which the effect of securing the strength was obtained. on the other hand,
Excessive addition causes toughness degradation of the weld metal, so the upper limit was made 2.5%.

P and S are impurities in the present invention,
If there is much in the weld metal, the toughness deteriorates, so the upper limits were set to 0.03% and 0.02%, respectively.

Ni is a metal having austenitic, that is, a face-centered structure by itself. Iron itself has an austenitic structure in a high temperature range, and has a ferrite or body core structure in a low temperature range. Addition of Ni makes the face-centered structure of iron in a high-temperature range more stable, so that it has a face-centered structure even in a lower temperature range as compared with the case without addition. This means that the temperature at which the structure transforms into a body-core structure becomes lower. Ni has the effect of improving the toughness of the weld metal by adding Ni.
The lower limit of 4% of the amount of Ni added to the Cr-based weld metal was determined from the viewpoint of the minimum amount of addition in which the effect of reducing residual stress appears and securing of toughness. On the other hand, in Cr-based weld metal,
From the viewpoint of reducing the Ms temperature to some extent by the addition of Cr described below and reducing the residual stress, Ni was added by 8%.
Even if added in excess of the above, the effect is saturated and economically disadvantageous.

Cr, unlike Ni, stabilizes the ferrite phase. However, by adding Cr, although it is ferrite in a high temperature range, it forms austenite in a medium temperature range, and forms ferrite again when the temperature is further lowered. In the case of a welded part, ferrite on the lower temperature side is generally not obtained in the heat history due to the welding heat input, and martensite is obtained. Therefore, the effect of increasing hardenability by adding Cr is effective. can get. That is, the martensitic transformation by adding Cr is
Two effects are obtained, namely, no ferrite transformation occurs due to an increase in hardenability, and a lower Ms temperature itself, and the transformation expansion for reducing the residual stress is effectively used while satisfying both of these effects. The lower limit of 8% was set as the Cr addition range. The upper limit of 15% was set because the effect would be saturated even if an amount exceeding this amount was added, and it would be economically disadvantageous.

N is an austenite stabilizing element.
Since addition of N also makes it easier to obtain martensite, a minimum addition is necessary. N, 0.
001%, like C, was determined as the minimum value for obtaining a low Ms temperature. However, an excessive addition forms a nitride and causes a problem of deterioration in toughness and ductility. Therefore, the upper limit is set to 0.05%.

C and N have similar functions, such as forming carbides and nitrides and stabilizing austenite, respectively, and the sum of them, that is, the amount of C + N, also needs to have upper and lower limits. C + N lower limit, 0.001%
Is the minimum value for facilitating obtaining martensite and lowering the Ms temperature, and 0.06% of the upper limit.
Is defined as a limit value at which the problems of toughness deterioration and ductility deterioration due to carbides and nitrides do not occur.

Cu is an element effective for improving the welding workability, because it has an effect of improving the electrical conductivity by plating the welding wire. Further, since Cu is also a hardenable element, an effect of promoting martensitic transformation by adding it to the weld metal can be expected. Cu
The lower limit of 0.05% was set as a minimum value necessary for improving workability and promoting martensitic transformation. But,
Excessive addition not only saturates the effect of workability improvement, but also increases the wire manufacturing cost and is economically disadvantageous.
0.4% was made the upper limit.

Nb combines with C in the weld metal,
Form carbides. Nb carbide functions to increase the strength of the weld metal in a small amount. In addition, the merit of addition is significant from the viewpoint of increasing the yield strength at the Ms temperature. Nb
Is set to 0.005% as a minimum value at which a carbide is formed and an effect of increasing strength can be expected. But,
On the other hand, since excessive carbide formation causes toughness degradation, the upper limit is set to 0.3% as a value that does not impair the reliability of the welded portion due to toughness degradation. V is an element that functions similarly to Nb. However, unlike Nb, in order to expect the same precipitation effect, it is necessary to increase the amount of Nb added.
The lower limit of 0.05% of the addition of V was set as the lowest value at which precipitation hardening can be expected by adding V. On the other hand, if too much is added, precipitation hardening becomes too remarkable, and toughness is deteriorated. Therefore, the upper limit of V is set to 0.5%. Like Ti and Nb, Ti also forms carbides and causes precipitation hardening. But,
Just as the precipitation hardening of V differs from that of Nb, the precipitation hardening of Ti is also different from Nb and V. Therefore, the range of the addition amount of Ti is set to a range different from Nb and V. Ti
The lower limit of 0.005% of the addition amount is determined as the minimum amount at which the effect can be expected, and the upper limit of 0.3% is determined in consideration of toughness deterioration. Mo is also an element in which precipitation hardening can be expected like Nb, V and Ti. However, in order to obtain the same effect as Nb, V, and Ti, it is necessary to add Mo to Nb, V, and Ti. The lower limit of 0.1% of the Mo content was set as the lowest value at which an increase in yield strength due to precipitation hardening can be expected.
Further, the upper limit of 2% was determined in consideration of toughness degradation similarly to Nb, V, and Ti.

The reasons for limiting the range of the components of the weld metal have been described above. As a method of controlling the components of the weld metal within these ranges, a method of controlling the components of the welding wire, a component of the welding wire and flux, and the like. Or the method of controlling the components of the welding core wire and the coating flux, etc., but in the present invention, regardless of these methods, if the components of the weld metal are set within the above-mentioned range, high fatigue Strength welding joints can be realized. Further, a welding wire forming a weld metal that is a component range in the present invention, a combination of a welding wire and a flux, or a combination of a welding core wire and a coating flux can be easily formed by those skilled in the art. is there.

When the weld metal of the present invention is formed on the weld bead forming the weld toe, a high fatigue strength welded joint is realized. However, after the weld bead is formed, another bead is formed. If so, the distribution of the residual stress may change. When this bead is newly formed as a bead forming a weld toe, a welded joint may be produced by selecting a material for the bead so as to be a weld metal according to the present invention. However, if this is not the case, the residual stress distribution may change, so that the weld bead forming the weld toe is finally solidified, i.e. becomes the final bead, compared to other weld beads nearby. It is desirable to have a welded joint with a selected welding order.

Next, the reason why the shape of the joint is limited will be described. In the present invention, as described above, the Ms temperature of the weld metal is reduced, and further, by controlling the strength range, the residual stress is reduced and the fatigue strength is improved. The reaction force against the transformation expansion of the weld metal is used to reduce the residual stress of the weld metal. In order to effectively use this reaction force, it is preferable to limit the shape of the welded joint itself. This technique is effective in a joint shape as shown in FIG. That is, as an auxiliary member, an out-of-plane gusset, a cover plate, a joint in which one or more of studs are welded to a structural member subjected to fatigue load, an out-of-plane gusset has scallops, or an out-of-plane gusset Is a joint having a preferred shape, which is welded by turning welding to a structural member subjected to fatigue load. A welded joint for determining the fatigue strength of the entire welded structure may have such a joint shape in order to improve the fatigue characteristics, and extremely high fatigue characteristics can be expected due to a synergistic effect with the requirements of the present invention.

In welding the auxiliary member, it is preferable to limit the range of the groove from the end to be welded as described below. There are two stress concentration portions existing at the ends, that is, a bead toe portion and an unwelded portion existing inside the turning welded portion. From the viewpoint of fatigue, the bead toe portion is usually a more severe portion, but the present invention has achieved improvement in fatigue strength at this portion. Therefore, it is apparent that a higher fatigue strength can be realized by improving the fatigue strength of the unwelded portion near the end, which is another stress concentration portion, that is, the root portion. The reason for forming a groove from the end of the structural member having the gusset and scallops is to reduce the unwelded portion and to reduce the stress concentration.

Therefore, since the groove is not intended to improve the static strength of the joint, it is not always necessary to make a groove for the entire welded portion. However, if the range of the groove is too small, the concentration of stress cannot be suppressed, and the fatigue strength may be the same as when no groove is formed. The groove range of 5 mm or more was set as the lowest value at which the effect of forming a groove could be expected. In addition,
From the viewpoint of suppressing stress concentration, it is desirable that this groove range is preferably set to 2 cm or more.

When providing a groove at the end to be welded in this way, attention must be paid to the amount of reduction in the area of the unwelded portion inside the turning welded portion in the region where the groove is formed, compared to the case where the groove is not formed. . That is, a groove is formed at the welded end because the first, second, and third technical ideas of the present invention achieve the high fatigue strength of the bead toe, so that the fatigue strength is relatively low. This is because the fatigue strength of the stress concentrated portion formed by the unwelded portion inside the turned welding portion, that is, the root portion, is improved. Therefore, unless the stress concentration is suppressed until the effect of improving the fatigue strength becomes remarkable, this object cannot be achieved. For this reason, it has already been mentioned that the invention has limited the groove range. However, if the unwelded part remains at the same level as when the groove is not formed due to the inappropriate groove shape, stress concentration cannot be suppressed even if the groove range is appropriate. It is not enough to further increase the overall fatigue strength. It is preferable to set the lower limit of the reduction in the area of the unwelded portion to 10% with respect to the case where no groove is formed. If the amount of reduction is 10% or more, the effect of forming a groove is sufficiently recognized. In addition, from the viewpoint of suppressing stress concentration and improving the fatigue strength of the root portion, it is more preferable that the lower limit of the amount of reduction be 20%.

[0073]

Embodiments of the present invention will be described below. A steel plate having a thickness of 20 to 40 mm was manufactured from a slab manufactured by continuous casting. Table 1 shows the chemical components. Steels A and B are inventive steels, and steel C is a comparative steel.

[0074]

[Table 1]

Table 2 shows the component values of Ni-based and Cr-based weld metals used for examining residual stress and fatigue strength. Table 1 shows the Ms temperature (° C.) and the yield strength at the Ms temperature, which were obtained by directly collecting a Formaster test piece and a tensile test piece from each weld metal, measuring the Ms temperature first, and then measuring the Ms temperature. It is the result of conducting a tensile test at that temperature.

[0076]

[Table 2]

FIG. 4 shows a view of a corner turning welded joint prepared for measuring the residual stress at the bead toe. 4 is 5 cm from both ends of the out-of-plane gusset.
Turning welding is performed within the range of the groove shown in FIG. The main bead at the welded portion uses a normal welding material, but is a joint formed with a weld metal of WF, WA, WB, and WH shown in Table 1 as an additional bead (final pass) after the main welding. For residual stress, a 2 mm gauge gage was attached to the weld toe as shown in the figure.
It was measured by a so-called cutting method in which stress was relaxed by machining. FIG. 6 shows the measurement results of the residual stress.
(D) As is clear, in the example of the present invention, WA, WB,
WH is compressive residual stress, while WF is Ni
Residual stress is tensile due to insufficient addition.

FIG. 7 shows the fatigue strength of a welded joint in which the out-of-plane gusset shown in FIG. The fatigue load application direction is the arrow direction in FIG. 4, that is, the out-of-plane gusset longitudinal direction. When investigating the fatigue strength, in order to make a comparison between the case with and without the groove, the case with and without the groove shown in Fig. 5 within 5 cm from the end of the gusset Were produced using steels A, B, and C, and a weld metal of WA was formed as an additional bead shown in FIG.

Regarding WF, WB, and WH in Table 1, an additional bead (final pass) was formed for the grooved joint. In the case of a grooved joint, fatigue occurs at the weld toe where the additional bead is formed, and the crack propagates through the steel HAZ. If the groove is not formed, the unwelded portion inside the corner turning weld is Propagated from the end where the stress concentration portion exists. FIG. 7 shows the fatigue life by a dotted line and a solid line.
As shown in FIG. 7, WA, WB,
Regarding the fatigue strength of the joint subjected to the additional bead by WH, the fatigue life and fatigue limit are clearly improved as compared with the comparative example.

[0080]

As described above, according to the present invention, it is possible to improve the fatigue strength of a joint, and to provide a welded joint having excellent weld heat-affected zone toughness and fatigue properties which can be produced only by a practical construction method. It is possible to Therefore, the present invention can effectively improve the fatigue life of a welded structure, and can be said to be an invention with extremely high industrial value.

[Brief description of the drawings]

FIG. 1 is a diagram showing the dependence of the fatigue limit ratio (fatigue limit / tensile strength of a reproduced HAZ material) on the tensile strength of a reproduced HAZ material.

FIG. 2 is a diagram showing the dependence of the fatigue limit ratio on Ceq of various steel materials.

FIG. 3 is a diagram schematically showing a steel material HAZ region in which residual stress is reduced by a transformation expansion direction of a weld metal and a reaction force thereof in an example of a welded joint to which the present invention is applied.

FIG. 4 shows, as an example of the present invention, a welded joint in which an out-of-plane gusset is attached to a structural member subjected to a fatigue load by angular welding, and an additional bead is formed as a final pass, and a residual stress measurement position in the welded joint. It is the figure which showed typically.

FIG. 5 is a view showing a groove shape at a welding portion of the auxiliary member.

FIG. 6 is a diagram showing measurement results of residual stress at respective positions of the joint in the example.

FIG. 7 is a diagram showing measurement results of fatigue strength at respective positions of the joint in the example.

 ──────────────────────────────────────────────────続 き Continued on the front page (72) Inventor Tadashi Koseki 20-1 Shintomi, Futtsu-shi, Chiba Nippon Steel Corporation Technology Development Division (72) Inventor Tadashi Kasuya 20-1 Shintomi, Futtsu-shi, Chiba Made in New Japan Inside the Technology Development Division, Steel Corporation

Claims (13)

[Claims] 1. Mass%, C: 0.01 to 0.1%, Si: 0.1 to 1.6%, Mn: 0.5 to 2%, P: 0.01% or less, S: 0.005% or less, Nb: 0.05 to 0.3%, Al: 0.001 to 0.05%, Ti: 0.001 to 0.05%, Mg: 0.0002 to 0.006%, O: 0.0001 to 0.008%, N: 0.002 to 0.008%, the balance being Fe
And Ceq and Pcm defined by the following formulas, satisfying Ceq: 0.45% or less, Pcm: 0.2% or less, and a carbide having a particle diameter of 0.002 to 5 μm; One or two or more kinds of single or composite particles of nitrides and oxides are dispersed in steel. A weld heat-affected zone, characterized in that a final pass of the weld is formed by a weld metal having a temperature at which transformation from austenite to martensite is 350 ° C. or lower and 150 ° C. or higher. Welded joint with excellent toughness and fatigue properties.
However, Ceq (%) = C + Mn / 6 + (Cu + Ni) / 15 +
(Cr + Mo + V) / 5 + Nb / 3 Pcm (%) = C + Si / 30 + (Mn + Cu + Cr) /
20 + Ni / 60 + Mo / 15 + V / 10 In the formulas, the symbol of each element represents mass% of each element. 2. The high-tensile steel is, in mass%, Cu: 0.1 to 2.5%, Ni: 0.1 to 5% or less, Cr: 0.1 to 1%, Mo: 0.1. The weld heat-affected zone is excellent in toughness and fatigue properties according to claim 1, further comprising one or more of 1.5 to 1.5% and V: 0.005 to 0.05%. Welded joints. 3. The toughness and fatigue properties of a weld heat-affected zone according to claim 1, wherein the yield strength of the weld metal is 390 to 1180 MPa at a temperature at which transformation from austenite to martensite starts. Excellent welded joint. 4. The welding metal is one of Ni, Cr, and Mo.
Containing two or more species and C, and defined by the following formula:
The welded joint excellent in toughness and fatigue properties of the weld heat-affected zone according to any one of claims 1 to 3, wherein a satisfies Pa: 0.85 to 1.3%. However, Pa (%) = C + Ni / 12 + Cr / 24 + Mo / 19 In the formula, the symbol of the element represents the mass% of each element. 5. The weld metal in mass%, C: 0.01 to 0.2%, Si: 0.1 to 0.5%, Mn: 0.01 to 1.5%, P: 0. The weld heat-affected zone toughness according to claim 3 or 4, comprising 0.03% or less, S: 0.02% or less, and Ni: 8 to 12%, with the balance being Fe and unavoidable impurities. Welded joint with excellent fatigue properties. 6. The weld metal is one or two types in mass%: Ti: 0.01 to 0.4%, Nb: 0.01 to 0.4%, V: 0.1 to 1%. The welded joint according to claim 5, further comprising: a heat-affected zone toughness and fatigue properties. 7. The weld metal is represented by mass%: Cu: 0.05 to 0.4%, Cr: 0.1 to 3%, Mo: 0.1 to 3%, Co: 0.1 to 2 7. The welded joint according to claim 5, further comprising one or two or more of the above-described heat-affected zone toughness and fatigue properties. 8. 8. The weld metal is represented by mass%, C: 0.001 to 0.05%, Si: 0.1 to 0.7%, Mn: 0.4 to 2.5%, P: 0. 0.03% or less, S: 0.02% or less, Ni: 4 to 8%, Cr: 8 to 15%, N: 0.001 to 0.05%, C + N: 0.001 to 0.06% The welded joint according to claim 3 or 4, wherein the welded joint is excellent in toughness and fatigue properties of a weld heat affected zone. 9. The weld metal is represented by mass%: Cu: 0.05 to 0.4%, Mo: 0.1 to 2%, Ti: 0.005 to 0.3%, Nb: 0.005. The weld heat-affected zone has excellent toughness and fatigue characteristics according to claim 8, further comprising one or more of 0.3 to 0.3% and V: 0.05 to 0.5%. Welded joints. 10. The method according to claim 1, wherein at least one of an out-of-plane gusset, a cover plate, and a stud is welded to a structural member subjected to a fatigue load. 2. A welded joint having excellent toughness and fatigue properties in the heat affected zone of claim 1. 11. An auxiliary member is provided at the end to be welded.
The welded joint according to claim 10, wherein the welded joint has a groove of not less than mm and has excellent toughness and fatigue properties in a heat affected zone. 12. The welded joint according to claim 10, wherein the out-of-plane gusset is rotary-welded. 13. The welded joint according to claim 10, wherein the out-of-plane gusset has scallops.