JP2002332536A - THICK STEEL PLATE HAVING YIELD STRENGTH IN 500 TO 550 MPa CLASS AND EXCELLENT CTOD PROPERTY IN HEAT AFFECTED ZONE - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a thick steel plate which has yield strength in a 500 to 550 MPa class and, in which CTOD(crack tip opening displacement) at -10 deg.C in HAZ(heat affected zone) is >=0.2 mm, and which has the maximum plate thickness of <=76.2 mm (3 inches). SOLUTION: The thick steel plate having yield strength in a 500 to 550 MPa class and excellent CTOD properties in the heat affected zone has chemical components containing, by mass, 0.04 to 0.14% C, <=0.4% Si, 1.0 to 2.0% Mn, <=0.02% P, <=0.005% S, 0.001 to 0.01% Al, 0.005 to 0.03% Ti, 0.005 to 0.05% Nb, 0.0001 to 0.005% Mg, 0.001 to 0.005% O and 0.001 to 0.01% N, and the balance iron with inevitable impurities, and in which Ti/(Mg+Al)>=1 is satisfied. TiN of 0.01 to 0.5 μm including oxides of Mg and Al is present in >=10,000 pieces/mm<2> , and also, oxides of 0.5 to 10 μm are present in >=10 pieces/mm<2> .

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a heat affected zone (He).
at Affected Zone: HAZ) CTO
A steel sheet with excellent D characteristics and a yield strength of the class of 500 to 550 MPa, which is mainly used for offshore structures, but is also applicable to other welded structures requiring the same strength and HAZ toughness (CTOD characteristics). Applicable.

[0002]

2. Description of the Related Art Weld joints for offshore structures used in the North Sea are required to have CTOD characteristics at -10.degree. As a steel material requiring such strict HAZ toughness, for example, “Proceedings of 12th Inte
national Conference on O
MAE "1993. Glasgow. UK. A
SME. Volume III-A. pp. 207-
As described at 214, Ti oxide steel is used. Since the vicinity of the melting line of the HAZ is heated to 1400 ° C. or higher, the pinning effect of the TiN particles disappears, and austenite (γ) becomes extremely coarse.
The HAZ structure becomes coarse and the toughness deteriorates. The Ti oxide steel described above has been developed as a steel that solves such problems. This technology is disclosed in, for example, Japanese Patent Application Laid-Open No. 63-210235.
As described in Japanese Patent Application Laid-Open No. H06-075599, the thermally stable Ti oxide is used as a transformation nucleus in the coarsened γ grains in which the pinning effect of the TiN particles disappears. It is a steel whose HAZ structure has been refined by utilizing the generated needle-like ferrite. Coarse γ
This acicular ferrite, which effectively refines grains, is an intragranular transformed ferrite (Intra Granular Ferr).
item: IGF). However, this Ti
The yield strength of oxide steel is up to 420MPa class,
No thick steel plate has been developed that guarantees the CTOD characteristic of HAZ while having a higher yield strength. On the other hand, there is an increasing movement to reduce building costs by reducing the weight of offshore structures, and thick steel plates with high yield strength are required to reduce the weight of offshore structures. That is, a HAZ that can guarantee CTOD characteristics while having a yield strength of 500 to 550 MPa class, which is higher than conventional strengths.
There is a strong demand for thick steel plates having excellent toughness.

[0003]

SUMMARY OF THE INVENTION The present invention has a yield strength in the range of 500 to 550 MPa and a HAZ of -10.
The CTOD at ℃ is 0.2mm or more and the maximum plate thickness is 7
It is intended to provide thick steel plates up to 6.2 mm (3 inches).

[0004]

SUMMARY OF THE INVENTION The present invention relates to a method for producing C by mass%.
: 0.04 to 0.14%, Si: 0.4% or less, M
n: 1.0 to 2.0%, P: 0.02% or less, S:
0.005% or less, Al: 0.001 to 0.01%, T
i: 0.005 to 0.03%, Nb: 0.005 to 0.
05%, Mg: 0.0001 to 0.005%, O:
0.001 to 0.005%, N: 0.001 to 0.0
1%, and further, if necessary, by mass% of Ca: 0.1%.
0005-0.005%, REM: 0.0005-0.
01%, Zr: 0.0005 to 0.01%, Cu: 0.
05 to 1.5%, Ni: 0.05 to 3.0%, Cr:
0.05-0.5%, Mo: 0.05-0.5%, V
: 0.005 to 0.05%, B: 0.0001 to
0.003% or more, and Ti /
(Mg + Al + Ca + REM + Zr) ≧ 1 and C
The sum of u, Ni, Cr, and Mo is 3.0% or less, and the balance has a chemical component consisting of iron and unavoidable impurities.
0.01 to 0.5 μg containing an oxide consisting of g and Al
m TiN is present at least 10,000 / mm ² , and
A thick steel plate having a CTOD characteristic of a weld heat-affected zone of 500 to 550 MPa and a yield strength of 500 to 550 MPa, characterized in that oxides of 0.5 to 10 μm are present in an amount of 10 / mm ² or more.

[0005]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be described below in detail with reference to FIG. FIGS. 1A to 1D are views schematically showing the concept of HAZ structure control in the present invention.

[0006] The yield strength of Ti oxide steel is increased from the current 420 MPa class to 500 PMa by adding an alloying element.
When the temperature is further increased to the 550 MPa class, the weld heat affected zone HAZ2 near the melting line 3 of the weld metal 1 hardens and it becomes difficult to secure sufficient CTOD characteristics. The HAZ structure at this time is schematically shown in FIG. HAZ
The primary cause of embrittlement is that even if the inside of the coarse γ grains is refined by the formation of intragranular ferrite IGF, the coarse grain ferrite (G) formed along the γ grain boundaries 4 of the coarse γ grains
rain Boundary Ferrite: GB
F) or ferrite side plate (Ferrite S)
This is because ide plate (FSP) increases the sensitivity to the occurrence of brittle fracture with hardening of the HAZ.
Therefore, it is necessary to reduce the susceptibility to the occurrence of brittle fracture by making these GBFs and FSPs finer.
The second cause of embrittlement is that the hardenability of the HAZ is increased by increasing the amount of alloying elements added to increase the strength, and the martensite-austenite mixed layer MA (Martensi) is used.
te-Austenite constituent)
This is because a large number of microscopic embrittlement phases, referred to as, are generated, which promotes the occurrence of brittle fracture. Therefore, 500-550M
Even when achieving a Pa-class yield strength, it is necessary to reduce MA as much as possible. From the above, in order to achieve good joint CTOD characteristics under high yield strength, it is necessary to eliminate the above two causes of embrittlement while maintaining the metallurgical effect (IGF effect) of Ti oxide steel. Be a guide. That is, the gist of the present invention is to simultaneously control the HAZ structure from the following three viewpoints. (1) G generated along the γ grain boundary of HAZ near the melting line
BF and FSP are miniaturized. (2) The inside of the γ grains of the HAZ near the melting line is refined by the generation of IGF. (3) Reduce the amount of MA generated in the HAZ near the melting line.

First, means for achieving (1) will be described. In order to refine coarse GBF or FSP which is harmful to the occurrence of brittle fracture, it is necessary to reduce γ grains. 1400 ° C
With the aim of strongly suppressing the γ-grain growth in the HAZ near the melting line heated beyond the limit, various steel components were intensively studied. As a result, by controlling Mg and Al appropriately, M
g and Al, a large number of ultrafine oxides of 0.01 to 0.1 μm are dispersed in the steel, and this is used as a nucleus to form 0.01 to 0.1 μm.
We have invented a technique for complex precipitation of 5 μm TiN. Since such composite precipitated TiN particles are thermally stable even near the melting line, they can strongly pin the movement of the γ grain boundary without growing or dissolving. Even if welding with a large welding heat input is performed, 100 μm of γ grains near the melting line
m. Furthermore, these pinning particles present on the γ grain boundary are themselves GBF and FS
It may function directly as the transformation nucleus of P, and contributes to miniaturization of GBF and FSP by increasing the number of transformation sites. Such composite precipitation of TiN particles is 1000
0 / mm ² or more, GBF and FSP
The size is reduced to a size that does not adversely affect the TOD characteristics. The number of such composite precipitated TiN particles is 10,000 /
If the diameter is less than mm ² , the γ-graining and the number of transformation nuclei on the γ-grain boundary become insufficient, so that GBF or FSP is not sufficiently refined and CTOD characteristics are deteriorated. This complex mobile TiN
Although sulfides may precipitate on the particles, they do not adversely affect the function as the above-described pinned particles or transformation nuclei. FIG. 1B is a schematic diagram of the HAZ structure when only the technique (1) described here is applied. GBF
However, with this technology alone, the inside of the γ grains is covered with an embrittlement structure including MA called upper bainite (Bu), and sufficient CTOD characteristics cannot be obtained. Therefore, the technique (2) described below must be used together.

Means for achieving (2) will be described. The present invention has sought to generate IGF using a relatively large oxide of about several μm under the condition that Mg is added to generate a large number of ultrafine oxides as described above. as a result,
The following three conditions were found to be important as IGF metamorphosis nuclei. There must be a minimum number. Be of appropriate size. The oxide composition is appropriate.

In view of the above, the IGF transformation nuclei are stably present in the HAZ near the melting line and have at least 10 nuclei / mm
^Two or more are required. If the number of IGF metamorphic nuclei is less than 10 / mm ² , the refinement of the HAZ structure is insufficient. Also, from the viewpoint of effective functioning as an IGF metamorphosis nucleus, 0.5 μm
m is required. Particle size 0.5μ
Below m, the ability as an IGF metamorphic nucleus is significantly reduced. In order to satisfy these conditions, in the present invention, 0.5 μm
The use of oxides of m or more as IGF transformation nuclei was studied. However, an oxide exceeding 10 μm is not preferable because it functions as a starting point of brittle fracture. From the viewpoint of, it has been found that for the oxide to effectively function as an IGF transformation nucleus, it is effective to contain 2% by mass or more of Ti in the oxide. For this purpose, the content of Mg, Al, Ca, REM, and Zr, which have stronger deoxidizing power than Ti, must be limited to increase the Ti content in the oxide. It has been discovered that by controlling the amount of these deoxidizing elements added to the range of the following formula [1] using mass%, the Ti content in the oxide of 0.5 to 10 μm can be controlled to 2 mass% or more. did. Ti / (Mg + Al + Ca + REM + Zr) ≧ 1 [1]

If these conditions are satisfied, 0.5 to 10 is satisfied.
The μm IGF transformation nuclei need not be oxide alone. Even particles having a similar size in a composite form in which sulfides or nitrides are precipitated on oxides effectively act as IGF transformation nuclei. FIG. 1C is a schematic diagram of the HAZ structure when the technique of (1) and the technique of (2) described here are used together. The generation of a large amount of IGF in addition to the miniaturization of GBF and FSP makes the HAZ structure fine. However, when the addition amount of the alloy component is inappropriate, the amount of generated MA increases and CT
OD characteristics become insufficient. Then, it is explained next (3)
It is necessary to stably improve the CTOD characteristics by using the technique described above.

The means for achieving (3) will be described. HAZ
It is known that the MA formation behavior of the steel greatly depends on hardenability and cooling rate. The hardenability of HAZ in the present invention is greatly affected by the γ particle size and the ability to generate IGF in addition to the steel component. In the conventional steel, the γ grain size and IGF generation are hardly taken into consideration for the hardenability of HAZ, but the steel of the present invention has a small γ grain and a high IGF generation ability. There is a feature that the transformation sites of ferrite are increased and the hardenability of HAZ is remarkably reduced with respect to the conventional steel having the same steel composition. For the steel of the present invention having such characteristics, the cooling rate (800
Based on the premise that the cooling time from 500 ° C. to 500 ° C. is approximately 15 s) and the range of C and Mn according to the present invention, the influence of the alloy component on the formation of MA was studied diligently. As a result, the following two points became clear. Even if Nb is made higher than before, the MA amount of HAZ is hard to increase. There is a strong discontinuous correlation between the sum of Cu, Ni, Cr and Mo and the MA content of HAZ.

From the viewpoint, it has been found that increasing Nb to 0.05% does not significantly affect the MA amount of HAZ. Nb actually used in a conventional steel plate (joint CTOD guaranteed steel) for marine structures is, for example, “Proc
eatings of 12th International
onal Conference on OMAE "1
993. Glasgow. UK. ASME. V
olume III-A. pp. 4 for 207-214
The upper limit is 0.02% Nb at a yield strength of the 20 MPa class, and "Proceedings of 12th In"
international Conference on
OMAE "1993. Glasgow. UK.
ASME. Volume III-A. pp. 19
For 9-205, a yield strength of 460 MPa class was 0.021.
% Of Nb is the upper limit, and "Proceedings o
f 13th International Conf
erence on OMAE "1994. Hou
stone. ASME. Volume III. pp.
For 307-314, a yield strength of 420 MPa class is 0.0
24% Nb. As described above, conventionally, the upper limit of the Nb content is about 0.02%, whereas the present invention has an advantage that Nb can be effectively used up to 0.05%. From the viewpoint, it was found that when the sum of Cu, Ni, Cr, and Mo exceeds 3.0%, the MA amount of HAZ sharply increases. From the above findings, as a component design when expanding the plate thickness to 76.2 mm while maintaining the yield strength in the range of 500 to 550 MPa, Nb is used as much as possible to increase the base material strength of the thick material, and on the other hand, The guideline is to reduce Cu, Ni, Cr, and Mo that promote the formation of MA. Reduction of Cu, Ni, Cr, and Mo is preferable also from the viewpoint of alloy cost. FIG. 1D is a schematic diagram of the HAZ structure when the technique of (3) described here is used in combination with the techniques of (1) and (2). Since the amount of MA is stably reduced in addition to sufficiently miniaturizing the HAZ structure, good joint CTOD characteristics are achieved even at high strength. in this way,
The present invention can be realized by simultaneously expressing the techniques (1), (2), and (3).

Next, the reasons for limiting the chemical components will be described.

C is required to be 0.04% or more in order to secure the strength and toughness of the base material and HAZ. However, 0.14%
If it exceeds 300, the toughness of the base material and the HAZ decreases, and the weldability deteriorates. Therefore, this is the upper limit.

[0015] Si can be added for deoxidation. However, if it exceeds 0.4%, HAZ toughness deteriorates. In the present invention, deoxidation is possible with Al, Ti, and Mg, and the smaller the amount of Si, the better from the viewpoint of HAZ toughness.
Si is an unfavorable element in the present invention because it promotes the formation of HAZ in MA.

Mn must be at least 1.0% in order to ensure the strength and toughness of the base material and HAZ. Mn is also important in forming sulfides constituting the IGF transformation nucleus. However, if Mn exceeds 2.0%, the base material and HAZ become brittle or the weldability deteriorates, so this is the upper limit.

P is an impurity element in the present invention, and is 0.02% in order to ensure a good base material and HAZ material.
It is necessary to reduce it below.

S is an impurity element in the present invention, and is 0.005% in order to secure a good base material and HAZ material.
% Or less.

Nb is extremely effective in minimizing HAZ toughness degradation and increasing base material strength. Nb is also effective in increasing the toughness through the refinement of the structure of the base material. 500MP at the maximum plate thickness of 76.2mm of the present invention
In order to achieve a more excellent base metal toughness while achieving a-class yield strength, 0.005% or more of Nb is essential.
However, if Nb exceeds 0.05%, the HAZ toughness deteriorates due to an increase in the amount of MA and precipitation hardening, so this is the upper limit. Nb is an element that should be actively used in forming the base material of the present invention, and it is preferable to effectively use 0.02% or more of Nb.

Al forms an ultrafine oxide of 0.01 to 0.1 μm together with Mg, and functions as pinning particles with TiN which is compositely deposited thereon, and also as a transformation nucleus of GBF or FSP, Refine HAZ structure. For that purpose, 0.001% or more is required. Al is 0.001
%, It is not possible to secure an ultrafine oxide of 10000 / mm ² or more, resulting in insufficient γ refinement and an insufficient number of transformation nuclei on the γ grain boundary, resulting in a sufficient GBF or FSP. The HAZ toughness is degraded without being refined. However, if the Al content exceeds 0.01%, even if the above formula [1] is satisfied, the Ti content in an oxide having a size effective as an IGF transformation nucleus becomes less than 2% by mass, and these oxides As a result of losing the ability as an IGF metamorphosis nucleus, 10 cells / m
It becomes difficult to stably secure an IGF transformation nucleus of m ² or more. When the number of IGF metamorphic nuclei is insufficient in this way, HA
Z toughness deteriorates. Therefore, the upper limit of Al is 0.01%.

Ti forms TiN to form ultrafine (Mg,
Al) A composite precipitate having a size of 0.01 to 0.5 μm is formed on the oxide, and as pinning particles, further, GBF or FSP
Functions as a transformation nucleus and refines the HAZ structure. Furthermore, IGF transformation is promoted by containing 2% by mass or more in an oxide of 0.5 to 10 μm. For that purpose, 0.005% or more is required. When the Ti content is less than 0.005%, such a composite form of TiN particles is reduced to 10,000.
Pieces / mm ² or more cannot be secured.
Is not sufficiently refined, and the HAZ toughness is deteriorated. When both Si and Al are close to the lower limits, the deoxidizing element may be insufficient, so that 0.01% or more is desirably added in the sense that Ti is deoxidized. However, if the Ti content exceeds 0.03%, the base material and the HAZ become embrittled due to precipitation of TiC and coarsening of TiN to several μm. For the above reasons, the upper limit of Ti is 0.03%.

Mg plays an important role in the present invention. Mg forms an ultrafine oxide of 0.01 to 0.1 μm together with Al, and functions as pinning particles with TiN which is compositely deposited thereon, and also as a transformation nucleus of GBF or FSP, and has a HAZ structure. It is to miniaturize. For that purpose, 0.0001% or more of Mg is required. Mg is 0.0
If the content is less than 001%, it becomes difficult to secure an ultrafine (Mg, Al) oxide of 10,000 / mm ² or more. However, even if Mg exceeds 0.005%, its metallurgical effect is saturated, so that the upper limit is set.

O forms an ultra-fine (Mg, Al) oxide to perform the pinning effect in HAZ,
It forms a 10 μm Ti-containing oxide and functions as an IGF transformation nucleus in HAZ. To fulfill these two roles, 0.001% or more of O is required. O is 0.001
%, It becomes difficult to secure an ultrafine oxide of 10,000 / mm ² or more and an oxide of 10 / mm ² or more of 0.5 to 10 μm. However, O is 0.005%
If more than 0.005%, a large amount of coarse oxides exceeding 10 μm is generated and this acts as a starting point of brittle fracture in the base material or HAZ, so the upper limit is 0.005%.

N forms TiN to form ultrafine (Mg, A
1) Complex precipitation is performed on the oxide with a size of 0.01 to 0.5 μm, which functions as pinning particles and further functions as a transformation nucleus of GBF or FSP and refines the HAZ structure. For that purpose, 0.001% or more is required. N is 0.001%
When it is less than 100, the TiN particles in such a composite form are reduced to 100
More than 00 pieces / mm ² cannot be secured. But,
If N exceeds 0.01%, solute N increases and the base material and HAZ
Embrittlement or the surface properties of the slab deteriorate.
This is the upper limit.

Next, the reasons for limiting the selected elements will be described.

Ca, REM and Zr can be added as a deoxidizing agent or a desulfurizing agent. It contributes to the reduction of the amount of O as a deoxidizing agent. At the same time as contributing to the reduction of S content as a desulfurizing agent,
Control sulfide morphology. In order to improve the material of the base material and the HAZ through these effects, it is necessary to use 0.000 respectively.
5% or more is required. However, if the amount of these strong deoxidizing elements is too large, even if the formula [1] is satisfied, the Ti content in an oxide having a size effective as an IGF transformation nucleus becomes less than 2% by mass, and these oxides Loses the ability as an IGF metamorphic nucleus, resulting in 10 or more IGFs / mm ²
It becomes difficult to secure a stable transformation nucleus. In this sense, the upper limit of each of Ca, REM, and Zr is 0.005.
%, 0.01% and 0.01%, and it is necessary to limit the sum of these three elements to 0.02% or less. REM here refers to lanthanoid elements such as La and Ce, and is the same even when a misch metal in which these elements are mixed is added.

Cu, Ni, Cr and Mo can be used to improve the strength, toughness, corrosion resistance and weldability of the base material. For that purpose, each element needs 0.05% or more. Conventionally, these elements have been actively used when simultaneously increasing the strength, toughness, and sheet thickness of the base material, but in the present invention, these elements are minimized from the viewpoint of securing the CTOD characteristics of the HAZ. It is preferable to reduce it. In this sense, the upper limits of Cu, Ni, Cr, and Mo are each set to 1.5.
%, 3.0%, 0.5%, 0.5%,
It must be adjusted so that the sum of these elements is 3.0% or less. If each element exceeds the upper limit or the sum of these elements exceeds 3.0%, the CTOD characteristic of the HAZ is significantly deteriorated.

V is effective for the strength of the base material and HAZ by precipitation strengthening. For that purpose, 0.005% or more is required. However, if V exceeds 0.05%, weldability and H
Since the AZ toughness deteriorates, this is set as the upper limit.

B is effective for improving the strength and toughness of the base material. For this purpose, 0.0001% or more is required. However, if B exceeds 0.003%, the weldability deteriorates remarkably, so this is made the upper limit.

The steel of the present invention is manufactured as a thick steel sheet by adjusting the chemical composition to a predetermined chemical composition in the steelmaking process of the steel industry, reheating a continuously cast slab, and controlling various processes of rolling, cooling and heat treatment in various ways. Is done. 50 for a 76.2 mm thick steel plate
In order to obtain a yield strength in the range of 0 to 550 MPa, it is effective to apply direct quenching or accelerated cooling after rolling in order to maximize the amount of Nb. Further, strength and toughness can be adjusted by tempering. It is also possible to perform hot charge rolling without cooling the slab once. H
AZ toughness is determined by the dispersion of pinned particles and IGF transformation nuclei in addition to steel components. The dispersion state of these particles does not change significantly during the manufacturing process of the base material. Therefore, the HAZ toughness does not largely depend on the manufacturing process of the base material, and any heating, rolling, or heat treatment process may be applied.

The dispersion state of the inclusions specified in the present invention is quantitatively measured, for example, by the following method.

Including oxide containing Mg and Al
The number of TiN of from 0.01 to 0.5 μm may be any number of base steel sheets.
An extraction replica sample is prepared from the location, and this is
10000 to 50,000 times magnification using a microscope (TEM)
At least 1000 μm ^TwoViewing over the above area
The number of TiN of the target size
Is the number per unit area (pcs / mm^Two). This
, The (Mg, Al) oxide and TiN are identified by TE
By energy dispersive X-ray spectroscopy (EDS) attached to M
Composition analysis and crystal structure solution of electron diffraction image by TEM
This is done by analysis. All that measures such identification
If it is complicated to perform for complex inclusions,
According to the following procedure. First, a square-shaped inclusion is made of TiN
And inclusions are present inside the target size TiN.
Measure the number of things present. Then, such a method
Of the composite precipitated TiN whose number was measured in
Detailed identification of 0 or more is performed as described above, and (M
g, Al) The ratio of the composite of oxide and TiN is determined. So
Then, the number of the composite deposited TiN measured first
Multiply by percentage. TEM observation of carbide in steel
If it disturbs the carbide, heat treatment at 500 ° C or less
Agglomerates and coarsens, allowing observation of target inclusions.
Can be easier.

The number of oxides having a size of 0.5 to 10 μm (in some cases, sulfides and nitrides) serving as IGF transformation nuclei can be measured by the following method. First, a small piece sample is cut out from an arbitrary position of the base steel sheet to prepare a mirror-polished sample, and elemental mapping of O is performed over an area of at least 1 mm ² using an X-ray microanalyzer (EPMA), and O is detected. The number of particles having a particle size of 0.5 to 10 μm is measured. The number per unit area (pcs / mm
² ) Alternatively, it can be measured by the following method. For the purpose of dissolving carbides, sulfides, and nitrides other than oxides, a small piece sample is heated at a high temperature (for example, 1400 ° C.), rapidly cooled, mirror-polished, and observed with an optical microscope.
Observe at least 1000 mm magnification over an area of at least 3 mm ² and measure the number of particles of 0.5 to 10 μm. This is converted into the number per unit area (pieces / mm ² ).

[0034]

EXAMPLES Table 1 shows the chemical composition of continuously cast steel, and Table 2 shows the thickness of the steel sheet, the manufacturing method, the number of pinned particles and IGF transformation nuclei, the base material, welding conditions, and HAZ toughness. The steel of the present invention has a thickness of 38.1 to 76.2 mm, a yield strength (YS) of the base metal of 510 to 570 MPa, and a heat input of 3.5 to 10.0 kJ / mm by submerged arc welding. At the spliced joint (CGHAZ)-
It has a good CTOD of more than 0.2 mm at 10 ° C.
On the other hand, the comparative steel had a bad chemical composition,
The base material or HAZ material is inferior at a plate thickness of mm.
Steel 11 is inferior in the toughness of the base metal and HAZ due to too much S. Steel 12 is inferior in strength and toughness of the base material because Nb is too small. Steel 13 has HA due to too much Nb
Poor Z toughness. Steel 14 is inferior in HAZ toughness due to insufficient number of pinned particles due to too little Al. Since steel 15 has too much Al and the balance of deoxidizing elements is poor, the composition of the oxide of 0.5 to 10 μm becomes inappropriate,
Their IGF transformation ability is reduced and HAZ toughness is inferior. In steel 16, the number of pinned particles was insufficient due to too small amount of Ti, and the balance of deoxidizing elements was poor.
The composition of the oxide of 5 to 10 μm becomes inappropriate, the IGF transformation ability is reduced, and the HAZ toughness is poor. Steel 17
Is inferior in the toughness of the base material and HAZ due to too much Ti. Steel 18 is inferior in HAZ toughness due to insufficient number of pinned particles due to too little Mg. Steel 19 is inferior in HAZ toughness due to insufficient number of pinned particles and IGF transformation nuclei due to too little O. Since the steel 20 has too little N, the number of pinning particles is insufficient and the HAZ toughness is inferior. Steel 21 is inferior in HAZ toughness because the sum of Cu, Ni, Cr and Mo is too large. Steel 22 has a poor balance of deoxidizing elements and an unsuitable composition of 0.5 to 10 μm oxide, resulting in a decrease in IGF transformation ability and poor HAZ toughness.

[0035]

[Table 1]

[0036]

[Table 2]

[0037]

According to the present invention, the joint CTOD characteristics of a high-strength and extremely-thick steel plate have been remarkably improved. This can greatly reduce the cost of building offshore structures and enable energy development in deeper waters.

[Brief description of the drawings]

FIG. 1 is a diagram schematically showing the concept of HAZ organization control in the present invention.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Weld metal 2 Heat affected zone (HAZ) 3 Welding line 4 γ grain boundary GBF grain boundary ferrite FSP ferrite side plate IGF intragranular transformation ferrite Bu Upper bainite MA Martensite / austenite mixed phase

 ──────────────────────────────────────────────────続 き Continuing on the front page (72) Inventor Akito Kiyose 20-1 Shintomi, Futtsu-shi Nippon Steel Corporation Technology Development Division (72) Inventor Joe Yoshida 1 Kimitsu, Kimitsu-shi Nippon Steel Corporation Kimitsu Works (72) Inventor Ryuji Uemori 20-1 Shintomi, Futtsu City Nippon Steel Corporation Technology Development Division

Claims (3)

[Claims] C: 0.04 to 0.14% by mass%,
Si: 0.4% or less, Mn: 1.0 to 2.0%, P:
0.02% or less, S: 0.005% or less, Al: 0.
001 to 0.01%, Ti: 0.005 to 0.03%,
Nb: 0.005 to 0.05%, Mg: 0.0001 to
0.005%, O: 0.001 to 0.005%, N
: Containing 0.001 to 0.01%, and Ti /
(Mg + Al) ≧ 1, the balance has a chemical composition of iron and inevitable impurities, and 0.01 to 0.5 μm of TiN containing an oxide of Mg and Al is 1000
0 / mm ² or more, and 0.5 to 10 μm of oxides are present at 10 / mm ² or more,
500-550M with excellent CTOD characteristics of welding heat affected zone
Thick steel plate with Pa-class yield strength. 2. In addition to the chemical components according to claim 1, Ca: 0.0005 to 0.005% by mass%, REM: 0.00
And at least one of Zr: 0.0005 to 0.01%, and Ti / (Mg + Al +
2. The steel according to claim 1, wherein the heat-affected zone toughness is excellent in CTOD characteristics, wherein Ca + REM + Zr) ≧ 1.
A thick steel plate having a yield strength of 00 to 550 PMa class. 3. Cu: 0.05 to 1.5% by mass.
Ni: 0.05 to 3.0%, Cr: 0.05 to 0.5
%, Mo: 0.05-0.5%, V: 0.005-
0.05%, B: contains one or more of 0.0001 to 0.003%, and the sum of Cu, Ni, Cr, and Mo is 3.
The steel sheet according to claim 1 or 2, wherein the heat-affected zone toughness is excellent in CTOD characteristics.
A steel plate having a yield strength of 0 to 550 PMa class.