JP2003027179A - Steel for welding structure having excellent low temperature toughness - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide steel for welding structures which has excellent low temperature toughness in a welded joint by high heat input welding of >=400 kJ/cm such as electro-gas arc welding. SOLUTION: The steel has a composition containing, by mass, 0.04 to 0.12% C, 0.01 to 0.5% Si, 0.5 to 2% Mn, 0.04 to 0.08% sol.Al, 0.0005 to 0.003% B, 0.001 to 0.01% S, 0.04 to 0.08% sol.Al, 0.004 to 0.007% N and 0.001 to 0.005% O, and, if required, containing one or more kinds selected from the group consisting of Cu, Ni, Cr, Mo, V and Nb, and further containing Al2 O3 precipitates having a diameter of an equivalent circle of 0.5 to 3 μm, and further satisfying 0.9×1.3B<=N<=1.2×1.3B; wherein B is contents (%).

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to high-strength steel used for large welded structures such as marine structures, pressure vessels, ships, bridges, constructions and lipipes, and particularly for high heat input HAZ toughness at low temperatures. Regarding excellent things.

[0002]

2. Description of the Related Art In recent years, steel structures such as marine structures, pressure vessels, and ships have become large in size, and the steel materials used have become thicker. For thick materials, low temperature toughness due to low temperature cracking during small heat input welding such as tack welding, and large heat input welding such as electrogas welding (EGW) and submerged arc welding (SAW) to improve work efficiency. May be a problem.

Therefore, in the production of thick-walled materials, in order to prevent cold cracking during small heat input welding, low Ceq and low Pcm are required.
In addition, the component design is usually performed to prevent deterioration of toughness of the high heat input welding HAZ part.

However, recently, depending on the structure, -40
It can be seen that low temperature toughness at ℃ is required.
The component design is far more difficult than before.

Various proposals have been made as a method for improving the HAZ toughness of large heat input welding. For example, Japanese Patent Publication No. 55-2616.
No. 4 discloses that fine TiN is precipitated to suppress coarsening of γ crystal grains, and Japanese Patent No. 2950076 proposes a method of suppressing coarsening of austenite by a large amount of fine Al2O3 precipitates. There is.

Further, Japanese Patent Laid-Open No. 61-79745 discloses a method for improving the HAZ toughness by forming intragranular ferrite by using Ti oxide particles as nucleation sites to refine the structure, and Japanese Laid-Open Patent Publication No. 5-287374. The publication proposes a method of forming intragranular acicular ferrite by using Ca oxide or Ca oxysulfide as a nucleus to refine the structure.

Further, in JP-A-9-20955,
A method has been proposed in which the amount of N is adjusted according to the amounts of Ti and B, and the toughness of the bond part is improved by TiN and BN.

However, according to the method disclosed in Japanese Patent Publication No. 55-26164, most of TiN is melted at the bond portion at about 1400 ° C., so that the structure becomes coarse and
Bond part toughness is not improved.

Further, even in the method described in Japanese Patent No. 2950076, it is difficult to suppress coarsening of the austenite grain size by Al2O3 when it is held at high temperature for a long time.

In the method described in Japanese Patent Laid-Open No. 61-79745, Al, which is a strong deoxidizing element, is 0.007% or less, which is an extremely small special component for finely dispersing Ti oxide in steel. The steel type knitting in melting becomes complicated.

Similarly, in the method described in Japanese Patent Laid-Open No. 5-287374, O is 0.0040% or less and Al, which is a strong deoxidizing element, is 0.0 in order to stably secure Ca oxide.
The content is limited to 07% or less, and precise control is required by a deoxidizing method, addition of component elements, and the like, and depending on the welding material, there is a concern that the toughness of the weld metal part may be reduced.

In the method described in Japanese Patent Application Laid-Open No. 9-20955, since the amount of N added is equal to or more than the amount fixed as Ti or B nitride, the HAZ whose heating temperature is 1000 to 1200 ° C. becomes brittle due to solid solution N. To do.

In Japanese Patent No. 2931065, the heat input amount of TiN in the heat-affected zone of welding is 500 kJ / cm-10.
The manufacturing conditions are specified so that the dimensions that do not melt even when exposed to high temperature for a long time with an extremely large heat input of 00 kJ / cm and the number required for miniaturization are obtained. It has been proposed to improve the low temperature toughness in the entire HAZ including.

However, it is feared that coarse TiN will remain in the base material and the HAZ when the heat input is lowered, and the cleanliness of the steel material will be reduced. In the case of producing the steel sheet, the cooling rate of the slab at 5 ° C./min or less is not preferable from the viewpoint of productivity.

[0015]

As described above, it is said that at present, a steel plate manufacturing technique capable of obtaining excellent low temperature toughness in the entire HAZ of high heat input welding without impairing productivity is said to be well established. It is difficult to provide local embrittlement of a structure, and in particular, there is no technique provided for sufficiently reducing the Charpy impact value fluctuation range, which is not preferable because it lowers safety.

The present invention is excellent in productivity and has a large heat input welding (4
00kJ / cm or more) whole HAZ (Bon, Bon
(d + 1 mm, 3 mm, 5 mm) provides a welded structural steel having a Charpy impact value (vE-40) at −40 ° C. of 100 J or more for each test piece.

[0017]

Means for Solving the Problems The inventors of the present invention have studied the influence of the composition of components on the super-high heat input welding HAZ toughness based on the prior art.

As a result, Japanese Patent No. 2931065, as described above, produces coarse TiN which does not melt and disappear even at a long high temperature residence time characteristic of ultra-large heat input welding, and H
In order to prevent the HAZ toughness from deteriorating due to excessive N that is not fixed in AZ, the N content is specified in relation to the Ti and B contents. In the above, when the Ti addition amount is suppressed to a low level in consideration of the surface properties and cleanliness of the steel sheet, the effect of B improving the toughness is not always stably obtained, and it is not conventionally experienced as in the case of −40 ° C. At low temperatures, toughness fluctuations, which are presumably due to the effect of improving the hardenability, were observed depending on the notch position, and although it was an excellent technique, it was found that further improvement in toughness was necessary.

Therefore, the inventors of the present invention have earnestly studied a method of further improving the toughness improving effect by the intragranular ferrite formation without impairing the productivity by slowing the cooling rate in the casting solidification process. As new knowledge,
When an appropriate amount of Al ₂ O ₃ precipitates obtained in the usual casting and solidification process is secured in the steel with a certain size, the same effect as coarse TiN as TiN and BN precipitation nuclei can be obtained.
By optimizing the addition amounts of Al, B, and N, and particularly by dissolving solid solution N that does not form a nitride with B in the welded portion as AlN with a sufficient amount of Al, and reducing the solution solution N, -40 ° C Also in the entire HAZ (Bond, Bon
It was found that the fluctuation range is small and stable and good toughness can be obtained at d + 1 to 5 mm).

The present invention was made through further studies based on the above findings, that is, the present invention is: % By mass, C: 0.04 to 0.12%, Si:
0.01-0.5%, Mn: 0.5-2%, S: 0.0
01-0.01%, sol. Al: 0.04 to 0.08
%, B: 0.0005 to 0.003%, O: 0.001
.About.0.005%, N: 0.004 to 0.007%, and 0.
9 × 1.3B ≦ N ≦ 1.2 × 1.3B (where B is the content (%)) is contained, and the equivalent circle diameter is 0.5 μm or more and 3 μm or less. Of Al ₂ O ₃ precipitates of 1 × 10 ³ / mm ² or more
Welded structural steel with excellent low temperature toughness consisting of e.

2. Cu ≦ 0.5%, Ni ≦ 1.0%,
Cr ≦ 0.5%, Mo ≦ 0.5%, V ≦ 0.1%, Nb
The welded structural steel excellent in low-temperature toughness according to 1, containing one or more selected from the group of ≦ 0.03%.

[0022]

BEST MODE FOR CARRYING OUT THE INVENTION The reasons for limiting the components of the present invention will be described in detail below.

C C is necessary to secure the strength, and in order to obtain its effect, it is added at 0.04% or more. On the other hand, if added in excess of 0.12%, high-carbon island martensite is formed, and HA
Since Z toughness and weldability decrease, 0.04 to 0.1
2% (0.04% or more and 0.12% or less). still,
If it is less than 0.04%, a large amount of hardenability-improving elements must be added in order to secure strength, resulting in an increase in production cost and deterioration in toughness and weldability.

Si Si is necessary as a deoxidizing agent in the steelmaking process and ensures strength, and is added in an amount of 0.01% or more in order to obtain the effect. On the other hand, if added in excess of 0.5%, high-carbon island martensite is likely to be generated and HAZ toughness deteriorates, so the content is made 0.01 to 0.5%.

Mn Mn is added in an amount of 0.5% or more in order to secure the strength.
On the other hand, if it exceeds 2%, the hardenability increases, and the weldability and HA
In order to deteriorate Z toughness, it is set to 0.5 to 2%.

S S is the nucleation site of ferrite in the HAZ part, M
It is necessary to generate nS and is made 0.001% or more.
On the other hand, if it exceeds 0.01%, the toughness of the base material and the welded portion is reduced, so the content is made 0.001 to 0.01%.

BB is added in an amount of 0.0005% or more in order to form BN which becomes a nucleation site of ferrite. On the other hand, 0.00
If added over 3%, the HAZ toughness will decrease, so
It is set to 0.0005 to 0.003%.

Sol. Al sol. Al is a solid solution N detrimental to deoxidation and HAZ toughness.
To reduce the amount of Al ₂ O ₃ to form Al ₂ O ₃ . On the other hand, if it exceeds 0.08%, coarse Al-based inclusions start to occur and the toughness decreases, so 0.0
4 to 0.08%.

FIG. 2 shows the sol. Impact on the Charpy impact value (vE-40) over the entire HAZ. It shows the influence of Al. Including components other than Al within the scope of the present invention,
sol. Two types of steels were used in which only the amount of Al was changed within the scope of the present invention and outside the scope of the present invention. As a result, in the steel of the present invention, excellent Charpy impact test results are stably obtained over the entire HAZ. The test results are average values.

FIG. 3 shows the individual impact value (vE-40) and the average value in the Charpy impact test in which the notch position is HAZ + 1 mm in the test result of FIG.
ol. In the case of a sample steel having a low Al content outside the range of the present invention, it exhibits unstable behavior in which an extremely low impact value that is far from the average value is generated, resulting in concern about safety due to local embrittlement. .

Sol. In the case of the sample steel with a low Al content, if the maximum heating temperature is lower than the bond, HAZ + 1mm, Al
By the solid solution N that does not form N and does not form a nitride with B,
It seems that the toughness of the ferrite ground structure deteriorated.

NN is 0.004% or more in order to suppress coarsening of austenite crystal grains in HAZ and to generate BN which becomes a nucleation site of ferrite. On the other hand, 0.0
If it exceeds 07%, the amount of solute N will be excessive due to the formation of Al nitrides and the toughness will decrease, so 0.004 to 0.
007%.

In the present invention, the N content is 0.004 to 0.
Within the range of 007%, 0.9 × 1.3B ≦ N ≦
It is necessary to specify 1.2 × 1.3B (B is the content (%)) and most of the contained N be BN.

The amount of O in the O steel is 0.001 to 0.005 in order to sufficiently secure Al ₂ O ₃ precipitates which become precipitation sites of BN and to prevent the formation of coarse inclusions due to excessive addition. %.

Al ₂ O ₃ Precipitate The Al ₂ O ₃ precipitate has a diameter corresponding to a circle of 0.5 μm or more in order to precipitate BN which serves as a ferrite precipitation nucleus and form a fine ferrite structure in the HAZ portion to improve low temperature toughness. When the thickness is 3 μm or less, the number is defined as 1 × 10 ³ or more. In the present invention, Al ₂ O ₃ precipitates mean Al ₂ O ₃ precipitates and Al ₂ O ₃ precipitates.
₂ O ₃ as a main component and other oxides (eg, SiO ₂ , Ca
It is a composite precipitate containing O).

In FIG. 1, the equivalent circle diameter is 0.5 μm or more, 3
The sol. effect on the number of Al ₂ O ₃ precipitates of μm or less. The influence of the amount of Al is shown. The test steel is the steel of the present invention having the chemical composition according to claim 1, and in the steel of the present invention, only N content or so.
l. A comparative steel was prepared in which only the amount of Al was outside the range of the present invention.

As a result, sol. For steels in which the amount of Al falls within the range of the present invention, the equivalent circle diameter is 0.5 μm or more,
Although the number of Al ₂ O ₃ precipitates having a size of 3 μm or less is 1 × 10 ³ or more, among these, the comparative steels having an N content outside the scope of the present invention have Charpy impact test results at −40 ° C. It was unstable.

In the steel containing Al outside the range of the present invention, the number of Al ₂ O ₃ precipitates having a circle equivalent diameter of 0.5 μm or more and 3 μm or less was small, and the Charpy impact test result was also unstable. .

In the present invention, sufficient characteristics can be obtained by the above construction, but Cu, N and
One, two or more of i, Cr, Mo, V, and Nb can be added. When these elements are added, Cu ≦
0.5%, Ni ≦ 1.0%, Cr ≦ 0.5%, Mo ≦
0.5%, V ≦ 0.1%, and Nb ≦ 0.03%.

In the present invention, "the balance is substantially F
“E” means that unavoidable impurities and other trace elements are contained within a range that does not impair the effects of the present invention.

Next, manufacturing conditions will be described. In the present invention, it is important to stably deposit fine Al ₂ O ₃ precipitates in the steel, but it is not necessary to make the casting solidification rate slower than 5 ° C / min, and the casting solidification process from molten steel is not always required. It may be by law. After solidification by casting, after heating under the required conditions,
The steel plate is processed by rolling and the like.

Heating conditions of the slab and working conditions such as rolling may be appropriately set according to the desired strength, and there is no problem with combining with a water cooling operation such as accelerated cooling or performing heat treatment after rolling.

[0043]

[Examples] Steels having the compositions shown in Tables 1 and 2 were melted and made into a slab by a continuous casting method, then heated to 1100 to 1250 ° C, and a plate thickness of 50 to 70 mm by TMCP, DQ-T, etc.
Manufactured steel sheet.

For these steel sheets, the mechanical properties of the base metal and electrogas arc welding (heat input 400 to 530)
The HAZ toughness of kJ / cm) was investigated. The HAZ toughness was determined by the Charpy impact test with the notch position at the bond portion and 1 mm, 3 mm, and 5 mm from the bond portion to the HAZ side, and the Charpy impact value (average value, individual value) at the test temperature of -40 ° C.
Evaluated by Only the average value was displayed.

[0045]

[Table 1]

[0046]

[Table 2]

Tables 3 and 4 show the manufacturing conditions and the results of these tests. The steel of the present invention No. Nos. 1 to 15 have a tensile strength of 510 N / mm 2 or more of the base material, a Charpy impact value of 200 J or more at -40 ° C, and -40 in the entire HAZ including the bond portion.
The Charpy impact value (average value) at 100 ° C. is 100 J or more. Further, although not shown in the table, the fluctuation range of each impact value was small, and it was within ± 20% of the average value in all the test results.

On the other hand, No. 16 to 32 are comparative steels, all of which are sol. When the amount of Al and / or N was outside the range of the present invention, the impact value (average value) at each position of the HAZ was inferior, and the fluctuation range of each impact value was large, exceeding ± 20% of the average value.

No. 16-21, 23, 25, 27-3
In No. 1, N was out of the range of the present invention, the balance of B and N was poor, and No. 1 17, 18, 20-22, 24, 26, 2
8, 32 are sol. The amount of Al is low outside the range of the present invention. Therefore, in these comparative steels, the Charpy impact value (average value) at −40 ° C. is 100 J or less at any or all positions in the entire HAZ including the bond.

[0050]

[Table 3]

[0051]

[Table 4]

[0052]

EFFECTS OF THE INVENTION According to the present invention, high heat input welded joint HA of 400 kJ / cm or more for electrogas arc welding or the like.
A welded structural steel having stable low temperature toughness in the entire Z range is obtained, which is extremely useful industrially.

[Brief description of drawings]

FIG. 1 shows the effect of sol. On the state of precipitation of Al ₂ O ₃ . The figure which shows the influence of Al amount.

FIG. 2 shows the sol. Effect on HAZ toughness (average value of vE-40: J). The figure which shows the influence of Al amount.

FIG. 3 HAZ toughness (individual impact value of vE-40: J)
On sol. The figure which shows the influence of Al amount.

   ─────────────────────────────────────────────────── ─── Continued front page    (72) Inventor Yoshiaki Murakami             1-2-1, Marunouchi, Chiyoda-ku, Tokyo             Main Steel Pipe Co., Ltd. (72) Inventor Toshifumi Kojima             1-2-1, Marunouchi, Chiyoda-ku, Tokyo             Main Steel Pipe Co., Ltd. (72) Inventor Shinichi Suzuki             1-2-1, Marunouchi, Chiyoda-ku, Tokyo             Main Steel Pipe Co., Ltd. (72) Inventor Ryuji Hirai             1-2-1, Marunouchi, Chiyoda-ku, Tokyo             Main Steel Pipe Co., Ltd. (72) Inventor Minoru Matsuda             1-2-1, Marunouchi, Chiyoda-ku, Tokyo             Main Steel Pipe Co., Ltd.

Claims (2)

[Claims] 1. C: 0.04 to 0.12% by mass%,
Si: 0.01 to 0.5%, Mn: 0.5 to 2%, S:
0.001-0.01%, sol. Al: 0.04 ~
0.08%, B: 0.0005 to 0.003%, O:
0.001-0.005%, N: 0.004-0.00
0.9% 1.3B ≦ N ≦ 1.2 × 1.3B at 7% (however
However, B is the content (%). ) Is included.
And the equivalent circle diameter is 0.5 μm or more and 3 μm or less
Al₂O₃1 x 10 precipitates^ThreePieces / mm² Residual content above
For welded structures with excellent low temperature toughness
steel. 2. Cu ≦ 0.5%, Ni ≦ 1.0%, Cr
≦ 0.5%, Mo ≦ 0.5%, V ≦ 0.1%, Nb ≦
The welded structural steel excellent in low-temperature toughness according to claim 1, containing one or more selected from the group of 0.03%.