JP3526702B2 - Steel plate for high tensile welded structure excellent in fatigue strength of welded joint and method for producing the same - Google Patents

Description

Detailed Description of the Invention

[0001]

TECHNICAL FIELD The present invention relates to shipbuilding, construction structures,
The present invention relates to a steel sheet having excellent fatigue strength of a welded portion used in fields such as offshore structures and bridges, and particularly having a tensile strength of 60 to 80 kgf / mm ² class high-strength welded structural steel sheet and a method for producing the same.

[0002]

2. Description of the Related Art With the increase in size of welded structures and the increasing demand for environmental protection, structural members are required to have higher reliability than ever before. Fatigue fracture, brittle fracture, ductile fracture, etc. are possible fracture modes assumed in a welded structure. Of these, fatigue fracture is the most frequently occurring fracture mode in an actual operating environment. Is the most important issue for improving the reliability of the. There are many cases in which fatigue failure has become a problem, such as the recent occurrence of fatigue cracks in large tankers and collapse starting from fatigue cracks in offshore structures.

Up to now, many technologies for improving fatigue fracture strength have been disclosed, but most of them are related to improving the fatigue strength of a base material of a thin steel plate or a spot weld. For example, JP-A-61-96057 discloses that the fatigue strength can be improved by setting the area ratio of bainite in the welded portion to 5 to 60%.

Although the relationship between the microstructure of the heat affected zone (HAZ) and the fatigue strength has not been clarified so far, in JP-A-5-34592, the fatigue strength of the HAZ structure is the formation of island martensite. It has been revealed that That is, it is described that when hard island martensite is present in the HAZ structure, microfatigue cracks once generated are prevented or delayed from propagating and the fatigue strength is substantially improved.

Further, the inventors of the present invention have found that fatigue crack initiation and
As a result of conducting a systematic experiment on propagation and its microstructure dependence, it was revealed that the HAZ microstructure that most effectively suppresses the initiation and propagation of fatigue cracks is ferrite. That is, a method has been previously proposed in which the HAZ ferrite structure fraction is increased by limiting the carbon equivalent value (hereinafter abbreviated as Ceq) to improve the fatigue strength of the welded joint (Japanese Patent Laid-Open No. 8- 73983).

Further, the present inventors have found that 60 to 80 kgf / mm ²
When HAZ structure becomes bainite like high strength steel,
It has been clarified that the addition of Si and the limitation of Ceq are effective in suppressing the initiation and propagation of fatigue cracks. That is, Si addition has the effect of strengthening the lath boundary by solid-solution strengthening the ferrite in bainite while suppressing martensitic transformation.
It was proposed that the limitation of Ceq can improve the fatigue strength of the welded portion by strengthening the entire bainite (Japanese Patent Application No. 7-15450).

[0007]

DISCLOSURE OF THE INVENTION Problems to be Solved by the Invention
No. 7 publication improves the fatigue strength by limiting the bainite area ratio of the base metal to a specific range, which relates to the improvement of the fatigue strength of the thin steel plate base metal, and the thickness targeted by the present application. It is not effective in improving the fatigue strength of welded joints in butt welding of steel plates or fillet welding.

In Japanese Laid-Open Patent Publication No. 5-34592, in order to generate island-shaped martensite, the welded portion is welded with A after welding.
Since a special post-welding heat treatment of heating and cooling to an intermediate temperature range of c _{1 to} Ac ₃ is performed, the fatigue strength cannot be improved as it is.

Japanese Unexamined Patent Publication (Kokai) No. 8-73983 discloses improving the fatigue strength of a welded joint by limiting the Ceq value and increasing the HAZ ferrite microstructure fraction. Fig. 2 shows the relationship between the tensile strength and the fatigue limit ratio of reproduced HAZ materials in various steel materials. HAZ fatigue strength can be improved by using ferrite as the HAZ structure, that is, fatigue strength can be obtained even when using welded joints. It shows that it can be improved.

However, this method is applied to the case where the HAZ structure is ferrite with a mild steel plate for welded structure and a high strength steel plate having a tensile strength of 50 kgf / mm ² , and the welding heat input is small and the cooling rate is fast, or 60 to 80 kgf. / mm ^{2 In} high-strength steel, the HAZ microstructure is not particularly aimed at improvement when it becomes bainite or especially martensite.

In Japanese Patent Application No. 7-015450, 0.6%
The above Si addition strengthens the ferrite in bainite by solid solution to strengthen the lath boundary and suppresses fatigue crack initiation and propagation. However, less than 0.6% is required to improve HAZ toughness. Is. Therefore, when less than 0.6%, it is not considered to obtain good HAZ toughness while improving fatigue strength.

The present invention does not improve 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,
In the welding heat input intermission heat from low heat input, the fatigue strength improvement of the welded joint or fillet welded joint butt while welding meaning
While obtaining the desired HAZ toughness, tensile strength 60 ~
And its object is to provide a 80 kgf / mm ² high tensile welding structural steel plate and a process for their preparation.

[0013]

DISCLOSURE OF THE INVENTION The present inventors have found that 60 to 8
As a result of a detailed study for the purpose of improving fatigue strength, using the HAZ structure of 0 kgf / mm ² high strength steel as bainite, it was found that the addition of high Nb or high V and the limitation of Ceq are effective. That is, it was clarified that the addition of high Nb strengthens the lath boundary in bainite, and the restriction of Ceq strengthens the entire bainite, thereby improving the fatigue strength of the welded joint.

That is, the gist of the present invention is as follows. (1) % by mass, C: 0.02 to 0.20%, Si: 0.1
Less than 1.6%, Mn: 0.5 to 2.0%, P: 0.05
0% or less, S: 0.050% or less , and Nb and V : Nb: more than 0.05%, 0.30% or less, V: 0.05
% Or less, or Nb: 0.05% or less, V: 0.05
%, 0.30% or less , 0.46 ≦ Ceq (%) ≦ 0.65, and the balance Fe and unavoidable components , the base metal structure is martensite. Including bainite
Main structure, welded HAZ structure is bainite 60%
A high-strength welded structural steel sheet with excellent fatigue strength for welded joints made of ultra . However, Ceq (%) = C + Mn / 6 + (Cu + Ni) / 15 + (Cr + Mo + V) / 5 + Nb / 3

(2) In addition to the above-mentioned components, the mass%
And, Cu: 0.1-2.5%, Ni: 0.1-5.0
%, Cr: 0.1 to 1.0%, Mo: 0.1 to 1.5
%, One or two or more of them are contained, and the base metal structure contains martensite as described in (1) above .
It is mainly composed of inite and HAZ structure of welded part is bainai.
A steel plate for high-strength welded structures, which has a fatigue strength of welded joints of over 60% .

(3) In the above item (1) or (2), which further contains Al: 0.01 to 0.08% by mass% in addition to any of the components described above. Mainly described bainite whose base metal structure contains martensite
Is the structure, and the weld HAZ structure is more than 60% bainite?
Excellent high tensile welding structural steel fatigue strength of Ranaru welded joint.

(4) Bainite in which the matrix structure further contains martensite in addition to the components of any of the above items
It is a main structure, and in mass%, Ti: 0.001 to 0.0
The welded HAZ structure according to any one of the above items (1) to (3) is characterized by containing 5%.
High-strength welded structural steel sheet with excellent fatigue strength of welded joints consisting of over 60% inite .

(5) A steel ingot containing the component described in any one of (1) to (4) above is hot-rolled and cooled, and then reheated to Ac ₃ or more and 1000 ° C. or less, followed by water cooling or The base material structure is martensa, which is characterized by being air-cooled and subsequently tempered in a temperature range of 400 ° C. to Ac _1.
It is a bainite-based structure containing iron and has a welded part HAZ.
A method for producing a high-strength welded structural steel sheet excellent in fatigue strength of a welded joint having a structure of more than 60% bainite .

(6) A steel ingot containing the component described in any one of (1) to (4) above is Ac _{3 or} above 120.
After the hot rolling is completed at a temperature of 0 ° C. or lower, it is water-cooled or air-cooled and then tempered in the temperature range of 400 ° C. to Ac ₁ , and the base metal structure is martensite.
It is a bainite-based structure containing
A method for producing a high-strength welded structural steel sheet having excellent fatigue strength of a welded joint having a weave of more than 60% bainite .

[0020]

BEST MODE FOR CARRYING OUT THE INVENTION The present inventors first performed a detailed microscopic observation of the state of crack initiation / propagation in a fatigue test piece of a welded joint. As a result, most fatigue cracks occur at the weld metal / HAZ interface, or fusion fusion line.
ne; weld metal and HAZ boundary), it propagates in the HAZ, and further penetrates into the base metal part, leading to the total destruction of the test piece. 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, it has been clarified that the fatigue crack is generated near the welding fusion line and propagates in the HAZ, so that the fatigue strength is greatly affected by the microstructure of the HAZ.

As described above, the fatigue crack initiation portion is near the weld fusion line, and further, at the initial stage of crack propagation, HA
Within Z. These regions correspond to the stress concentration part. Therefore, by reproducing both factors of the HAZ microstructure and the stress concentration, it is possible to investigate the influence of the HAZ microstructure on the fatigue strength.

That is, a test piece provided with stress concentration was processed from a steel material subjected to a welding heat cycle and subjected to a fatigue test to determine the relationship between HAZ microstructure and fatigue strength. The outer dimensions of the test piece are 10 x 10 x 55 mm, and the notch depth is 2 m.
m, the tip radius was 0.75 mm, the distance between the fulcrums was 40 mm, and a 3-point bending repeated load was applied to cause fatigue failure. The stress concentration factor is 2.6.

FIG. 1 shows the results of fatigue tests conducted on the following three types, in which the fatigue limit ratio (fatigue limit / tensile strength of reproduced HAZ material) is plotted against Ceq of various steel materials. Welding reproducible heat of mild steel and laboratory molten steel with tensile strength of 50 kgf / mm ² with maximum heating temperature of 1400 ° C and cooling time of 800 to 500 ° C corresponding to small heat input of actual welding for 5 seconds. The HAZ structure was made to exceed 60% bainite by applying a cycle.

Using a laboratory melting steel having a tensile strength of 60 to 80 kgf / mm ² containing Si of about 0.2%, the maximum heating temperature is 1400 ° C., and it corresponds to a small heat input to a medium heat input in actual welding. Cooling time at 800-500 ° C for 10-30
The HAZ structure was made to exceed 60% of bainite by giving a simulated weld heat cycle for seconds.

Using a laboratory molten steel having a tensile strength of 60 to 80 kgf / mm ² containing 0.1% Nb and 0.2% Si, the maximum heating temperature is 1400 ° C. Welding reproduction heat cycle with cooling time of 10 to 30 seconds at 800 to 500 ° C corresponding to heat to medium heat input is given,
The structure of HAZ is such that bainite exceeds 60%.
The Ceq shown here is based on the following equation in consideration of the hardenability increasing effect of Nb or V in the carbon equivalent equation of IIW which is generally used. Ceq (%) = C + Mn / 6 + (Cu + Ni) / 15 + (Cr + Mo + V) / 5
＋ Nb / 3

As is clear from the figure, (1) the fatigue limit ratio is 60 to 80 containing 0.1% Nb and 0.2% Si.
kgf / mm ² class steel, 60~80kgf / mm ² containing Si 0.2%
Grade steel, 50kgf / mm ² grade steel, higher in order, (2) Same as 6
0 to 80 kgf / mm ² grade steel, 0.1% Nb, 0.2% Si
The fatigue limit ratio of those containing 0.2% Si is higher than that of those containing 0.2% Si, though (3) H
When the AZ structure is more than 60% bainite, Ceq is 0.4.
It can be seen that the fatigue limit ratio increases when the content exceeds 6 %.

HA with over 60% bainite with the same fatigue strength
Despite the Z-structure, the cause of dependence on Nb and Ceq has not been fully clarified, but (1) In the region where Ceq is large, the strength difference between the cementite truss and the lath boundary in bainite is small, and (2) Addition of high Nb strengthens the lath boundary by segregating and precipitating Nb and fine Nb (CN) at the lath boundary in bainite. The reason may be to delay the occurrence.

FIG. 2 shows the results of fatigue tests on the following three types, in which the fatigue limit ratio (fatigue limit / tensile strength of reproduced HAZ material) is plotted against Ceq of various steel materials. . For and, the same materials as and of FIG. 1 were used. Is V 0.1%, Si 0.2
%, The maximum heating temperature is 1400 ° C, and the cooling time is 800-500 ° C, which corresponds to the small to medium heat input of actual welding, using laboratory molten steel with a tensile strength of about 60% to 80 kgf / mm ^2. The HAZ structure was made to exceed 60% of bainite by applying a welding reproduction heat cycle of 10 to 30 seconds. Ceq shown here is based on the same formula as that used in FIG.

As is clear from the figure, (1) the fatigue limit ratio is 60 to 80 kg including 0.1% V and 0.2% Si.
f / mm ² class steel, 60~80kgf / mm ² class steel containing Si 0.2%, higher in the order of 50 kgf / mm ² class steel, (2) the same 60
~80kgf / mm 0.1% V in ^secondary steel, including the 0.2% Si
Despite the high Ceq, the fatigue limit ratio is higher than that containing 0.2% Si. (3) HAZ
When the structure is more than 60% bainite, Ceq is 0.46 %
It can be seen that the fatigue limit ratio increases with the above.

HA with the same fatigue strength of more than 60% bainite
Despite the Z structure, the cause of dependence on V and Ceq has not been fully clarified, but (1) In the region where Ceq is large, the strength difference between the cementite truss and the lath boundary in bainite is small, and (2) Addition of high V strengthens the lath boundary by segregating and precipitating V and fine V (CN) or V ₄ (CN) at the lath boundary in bainite. However, it is possible to delay the initiation of fatigue cracks.

As is clear from the above-mentioned examination results, in the skeleton of the present invention, in order to improve the fatigue strength of the weld where the HAZ structure exceeds 60% of bainite, the addition of high Nb or V causes the lath boundary in bainite to increase. By strengthening and limiting Ceq, the entire bainite is strengthened to improve the fatigue strength of the welded joint. That is, according to the present invention, the HAZ structure has a composition of 60% to 80% bainite.
The improvement of the fatigue strength of the welded joint of kgf / mm ² high strength steel was achieved by strengthening the lath boundary in bainite and further strengthening the entire bainite.

Based on the above basic idea, the reasons for limiting the basic components will be described below. C is an essential element for increasing strength, but if it is less than 0.02%, 60 to 80 kgf /
The base metal strength of mm ² cannot be secured. On the other hand, if it exceeds 0.20%, the HAZ toughness is extremely deteriorated and cracks easily occur from the lath boundary, so the amount was made 0.02 to 0.20%.

Si is an element that is essential as a deoxidizing element in addition to securing strength, and is added in an amount of 0.6% or more in order to strengthen the solid solution of ferrite in bainite to strengthen the lath boundary and improve the fatigue strength. However, if it exceeds 1.6%, on the contrary, cracks are likely to occur from the lath boundary. On the other hand, good HA
In order to obtain Z toughness, it is necessary to reduce the content to 0.6% or less. Therefore, the improvement of fatigue strength can be maintained by high Nb as shown below.
Alternatively, it can be achieved by adding high V. However,
Even if high Nb or high V is added, if it becomes less than 0.1, improvement in fatigue strength cannot be secured. Therefore, the amount is 0.
1 to 1.5%, and 0.1 to 0.6% is preferable when particularly good HAZ toughness is required.

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 2.0%, the HAZ structure mainly contains martensite and the fatigue strength is lowered.
It was set to 2.0%.

P has a high segregation rate and forms a low-melting point substance to cause solidification cracking.
The amount was set to 0.05% or less. If S is also large like P, solidification cracking occurs, so the amount is made 0.05% or less.

Nb and V are the most important elements as components of the present invention, and Nb and fine Nb (CN) or V and fine V (CN) or V are present at the lath boundary in bainite.
₄ (CN) segregates and precipitates to strengthen the lath boundary and improve the fatigue strength of the welded joint, and the fatigue strength can be sufficiently improved even when Si addition is less than 0.6%. More than 0.05% is required to obtain the effect. On the other hand, if it exceeds 0.30%, coarse Nb (CN) or V (CN) precipitates, which easily becomes a starting point for cracking, and the fatigue strength decreases. Therefore, the addition amount is more than 0.05 to 0.
30%.

Further, Nb and V form carbon / nitride during tempering and increase the strength by precipitation hardening . However,
If both elements are added in excess of 0.05%, the HAZ toughness will be reduced. From the above, the addition amount of Nb and V is N
When b is more than 0.05 to 0.3%, the addition amount of V is
0.05% or less, conversely V exceeds 0.05 to 0.3%
In that case, the amount of Nb added is preferably 0.05% or less .

The above is the basic component system in the present invention. Further, in the present invention, between the addition amount of the above components and the Ceq of the welding heat affected zone, 0.46≤Ceq (%) ≤
It is important to satisfy 0.65,
It is intended to improve the fatigue strength of the welded portion.

That is, if it is less than 0.46%, the strength difference between the cementite truss in bainite and the lath boundary in the HAZ structure becomes large, and fatigue cracks easily occur at the lath boundary. Therefore, the higher the Ceq, the better, but if it exceeds 0.65%, bainite becomes a martensite-based structure and the fatigue strength decreases. Therefore, the range is 0.46
Limited to ~ 0.65%.

Next, Cu, Ni, C selectively added
Each of r and Mo is an element that enhances hardenability and enhances Ceq, and it is effective to contain one or two or more of the basic components. The reasons for limiting the components to the strength of each element and other effects will be described below.

Cu is effective as an element for improving the strength, but if it is less than 0.1%, the effect of improving the strength is not observed.
On the other hand, if it exceeds 2.5%, hot cracking tends to occur during welding, so the addition amount is made 0.1 to 2.5%.

Ni enhances hardenability and strength.
In order to obtain this effect, the addition amount is required to be 0.1% or more, but if it exceeds 5.0%, the HAZ structure becomes mainly martensite from bainite and reduces the fatigue strength. Therefore, the addition amount is set to 0.1 to 5.0%. Considering alloy cost, it is preferably 0.1 to 3.0%.

Cr is required to be 0.1% or more in order to enhance the hardenability and secure the strength. On the other hand, if 1.0% is exceeded, N
Similar to i, the HAZ structure mainly changes from bainite to martensite and reduces fatigue strength. Therefore, the addition amount is set to 0.1 to 1.0%.

Mo increases the strength and prevents temper embrittlement. In addition, the non-recrystallization temperature range is expanded to promote the grain refining effect of low temperature rolling. These effects do not sufficiently appear below 0.1%. On the other hand, if it exceeds 1.5%, coarse carbides are formed to reduce toughness, and the HAZ is significantly hardened. Therefore, the addition amount is set to 0.1 to 1.5%.

Although the above are the constituents of the present invention, it is effective in the present invention to add one or both of Al and Ti. Al is also necessary as a deoxidizing element in steelmaking, and it fixes N existing as an impurity in steel, and H
Improve the toughness of AZ. To obtain this effect, sol.Al
0.01% or more is required, and the amount is 0.0
If it exceeds 8%, a large amount of Al ₂ O ₃ -based clusters are generated in the steel, causing a problem of cracking during welding, so the range was made 0.01 to 0.08%.

By fixing N existing as an impurity in the steel, Ti forms TiN to suppress coarsening of austenite grains in the HAZ and lowers the solid solution N to improve the toughness of the welded portion. . The amount is 0.001%
If it is less than 0.05%, it is not effective, and if it is too much, TiC is generated and the toughness of the base material is deteriorated.
A small amount of 0.02% is preferable.

Next, the reasons for limiting the steel sheet manufacturing conditions will be described. The present invention has a tensile strength of 6 which is excellent in the fatigue strength of the welded portion.
The present invention provides a high-strength welded steel plate having a strength of 0 to 80 kgf / mm ² . First, the reason for limiting the temperature of the reheating treatment after hot rolling / cooling to Ac ₃ or more and 1000 ° C. or less is that the base material composition is mainly bainite containing martensite,
Moreover, it is a temperature range for obtaining the strength-toughness balance of the base material. This is because sufficient strength cannot be obtained below this temperature, and austenite grains become coarse and toughness deteriorates at reheating temperatures above this temperature. Also, the upper limit temperature of hot rolling is 120
The reason why the temperature was expanded to the high temperature range of 0 ° C. is that the rolling treatment can be expected to have a grain-refining effect and the deterioration of toughness is suppressed as compared with the reheating treatment.

The tempering treatment carried out subsequent to rolling and cooling is intended to improve the toughness of the base material structure by recovery, and the upper limit of the heating temperature is set to Ac ₁ or less. Further, in the present invention, since the precipitation elements of Cu, Mo, Nb, and V are contained, the base material strength can be improved by forming fine precipitates by heat treatment, so the lower limit is made 400 ° C. The heating temperature for causing the precipitation hardening to act most effectively depends on the precipitation hardening element, but is 400 to 650.
The range of ° C is preferred.

The above are the reasons for limiting the fatigue strength of the welded joint and the tensile strength of 60 to 80 kgf / mm ² high-strength welded structural steel sheet and the manufacturing method thereof. The preferable welding condition range in 15 to 4 corresponds to a small heat input to a medium heat input of actual welding.
The cooling time is 5 KJ and 800 to 500 ° C., and is 5 to 45 seconds.

[0050]

EXAMPLES Example 1 Steels B to S having the chemical composition shown in Table 1 were melted in a 50 ton converter to obtain a thickness of 200 mm, a width of 1,500 mm and a length of 3,000 m.
Make slabs of m, and further slab each by (1) hot rolling 20
After cooling to mm ₃ and air-cooling, it is reheated to Ac ₃ or more and 1000 ℃ or less, then water-cooled or air-cooled, and then 400 ℃
~Ac those tempering in the temperature range of ₁ (after re-heating and
Tempering treatment), (2) hot rolling at a temperature of Ac ₃ or more and 1200 ° C. or less to 20 mm, water cooling or air cooling, and subsequent tempering treatment in the temperature range of 400 ° C. to Ac ₁ (after hot rolling)・ Tempered) was used as the test steel. Table 2 shows the manufacturing conditions and the tensile properties of the steel material.

FIG. 4 shows a 3 made from a T-shaped fillet welded joint.
The shape of a point bending fatigue test piece is shown. A fatigue test was carried out under the condition that the ratio of the maximum load and the minimum load was 0.1. Table 3 shows the fatigue test results. In Table 2 the tensile strength of the steel sheet in Table 3
It is shown together HAZ structure. In the table , 2 to 7 are examples of the present invention, and 10 to 19 are comparative examples. Welded joint fatigue strength is 1
0 ⁶ times Fatigue strength and fatigue limit were used as indexes for comparison. From this, it was confirmed that in the steel sheet for high-strength welded structural steel having a tensile strength of 60 to 80 kgf / mm ^{2 in} which the HAZ structure is more than 60% of bainite, the present invention example is higher than the weld joint fatigue strength of the comparative example. .

[0052]

[Table 1]

[0053]

[Table 2]

[0054]

[Table 3]

Example 2 Steels B ′ to S ′ having the chemical composition shown in Table 4 were melted in a 50 ton converter to obtain a thickness of 200 mm × a width of 1500 mm × a length of 30.
Make slabs 300 mm, further each slab (1) a material obtained by cooling after the 20mm by hot rolling, Ac ₃ to 1000
After reheating below ℃, cool with water or air and continue 4
Those that have been tempered in the temperature range of 00 ° C to Ac ₁ (after reheating / tempering), (2) hot rolled at a temperature of Ac ₃ or more and 1200 ° C or less to 20 mm, then water-cooled or air-cooled, and then continued. What was tempered in the temperature range of 400 ° C. to Ac ₁ (after hot rolling and tempered) was used as the test steel. Table 5 shows the manufacturing conditions and the tensile properties of the steel materials.

Using these test steels, T-shaped fillet welded joints were prepared under the welding conditions shown in Table 6. Welding conditions were small heat input. Table 7 shows the fatigue test results. Table 5 shows the tensile strength of the steel sheet, and Table 6 also shows the HAZ structure. 22 in the table
27 to 27 are examples of the present invention, and 30 to 39 are comparative examples. The fatigue strength of welded joints was compared using the fatigue strength of 10 ⁶ times and the fatigue limit as indexes. From this, the HAZ structure is bainite 6
It was confirmed that in the steel sheet for high-strength welded structure having a tensile strength of more than 0% of 60 to 80 kgf / mm ² , the inventive example is improved from the welded joint fatigue strength of the comparative example.

[0057]

[Table 4]

[0058]

[Table 5]

[0059]

[Table 6]

[0060]

[Table 7]

[0061]

As described above, according to the present invention, the HAZ structure has a tensile strength of 60 to 80 in which bainite exceeds 60%.
kgf / mm ² Welded joints for high-strength welded structural steel sheets, by limiting high Nb or V addition and Ceq to suppress fatigue crack initiation / propagation, without depending on stress concentration reduction due to additional welding. The fatigue strength of the welded structure can be improved, and by using the steel of the present invention, the reliability of the welded structure against fatigue fracture can be improved.

[Brief description of drawings]

FIG. 1 is a table showing the results of an investigation of the relationship between fatigue limit ratio and Ceq in a fatigue test of a newly reproduced HAZ material with notches.

FIG. 2 is a chart showing the results of investigation of the relationship between the fatigue limit ratio and Ceq in a fatigue test of a newly reproduced HAZ material with notches.

[Fig. 3] Reproduction HAZ with a notch performed by the present inventors in the past
The chart which shows the test result regarding the tensile strength and the microstructure dependence of the fatigue limit ratio in the fatigue test of the material.

FIG. 4 is an explanatory view showing a T-shaped fillet welded joint fatigue test piece.

─────────────────────────────────────────────────── ─── Continued Front Page (72) Inventor Hidesato Mabuchi 20-1 Shintomi, Futtsu City Shin Nippon Steel Corp. Technology Development Division (72) Inventor Shuji Aiwahara 5-3 Tokai-cho, Tokai City Nippon Steel Stock company Nagoya Steel Works (56) Reference JP-A-7-171679 (JP, A) JP-A-4-210419 (JP, A) JP-A-50-54514 (JP, A) JP-A-59- 126716 (JP, A) (58) Fields surveyed (Int.Cl. ⁷ , DB name) C22C 38/00-38/60 C21D 8/02

Claims (6)

(57) [Claims] 1. In mass%, C: 0.02 to 0.20%, Si: 0.1 to less than 1.6%, Mn: 0.5 to 2.0%, P: 0.050% or less. , S: 0.050% or less , and further Nb and V : Nb: more than 0.05%, 0.30% or less, V: 0.05% or less, or Nb: 0.05% or less , V: 0.05% greater, contained in a range of 0.30% or less, and satisfy 0.46 ≦ Ceq (%) ≦ 0.65 , and wherein the balance consisting of Fe and unavoidable component, Bainite whose base metal structure contains martensite
Main structure, welded HAZ structure is bainite 60%
A high-strength welded structural steel sheet with excellent fatigue strength for welded joints made of ultra . However, Ceq (%) = C + Mn / 6 + (Cu + Ni) / 15 + (Cr + Mo + V) / 5 + Nb / 3 2. In addition to the ingredients of claim 1, further quality
% : Cu: 0.1 to 2.5%, Ni: 0.1 to 5.0%, Cr: 0.1 to 1.0%, Mo: 0.1 to 1.5%, one of Alternatively, two or more kinds are contained, and the bay in which the base metal structure contains martensite according to claim 1 .
The main structure is knight, and the HAZ structure of the weld is bainite.
A high-strength welded structural steel sheet having a fatigue strength of a welded joint of more than 60% . 3. The mother according to claim 1 or 2, further comprising: Al: 0.01 to 0.08% by mass% in addition to the components of claim 1 or 2. Mainly bainite containing martensite
Is the structure, and the weld HAZ structure is more than 60% bainite?
Excellent high tensile welding structural steel fatigue strength of Ranaru welded joint. 4. In addition to the component according to any one of claims 1 to 3, a base metal structure further contains martensite.
It is a main structure of inite and contains Ti: 0.001 to 0.05% by mass%. The HAZ structure of weld zone according to any one of claims 1 to 3, wherein the HAZ structure of bainite is 60%.
A high-strength welded structural steel sheet with excellent fatigue strength for welded joints made of ultra . 5. A steel ingot containing the component according to any one of claims 1 to 4 is hot-rolled and cooled, and then reheated to Ac ₃ or more and 1000 ° C. or less and then water-cooled or air-cooled to draw. characterized by tempering at a temperature range of more 400 ° C. to Ac _1, the parent material structure is a martensitic
It is a bainite-based structure that includes the weld HAZ structure.
A method for producing a high-strength welded structural steel sheet having excellent fatigue strength of a welded joint made of bainite of more than 60% . 6. A steel ingot containing the component according to any one of claims 1 to 4 is hot-rolled at a temperature of Ac _{3 to} 1200 ° C., water-cooled or air-cooled, and then 400 ℃ characterized by tempering at a temperature range of to Ac _1, the parent material structure is contained martensite
Bainite is the main structure and HAZ structure of welded part is bainer.
A method of manufacturing a high-strength welded structural steel sheet having a fatigue strength of a welded joint of more than 60% .